Spin Transfer in Magnetic Nanopillars

Transcription

1 Spin Transfer in Magnetic Nanopillars by Wenyu Chen A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Physics New York University September 2008 Advisor: Prof. Andrew D. Kent

2 c Wenyu Chen All Rights Reserved, 2008

3 Dedication To my parents, for their love, support and encouragement. iii

4 Acknowledgements It is a great pleasure for me to acknowledge all the help, support, and encouragement that I have obtained during my thesis research. First of all, my thesis advisor Prof. Andrew D. Kent is the one I would like to thank the most. I still remember the gentle smile I saw during the first time when I knocked at his door. His tender voice explaining on-going projects with patience evoked my interest in spin transfer. During the five years of my PhD study, his constant support and guidance have been a significant help for my fruitful and productive physics research. He is a great professor and scientist, as his knowledge and scientific intuition guided me into the correct path of experimental study. He has led me into the world of condensed matter physics research, which is likely to be my career. His advice during the past years is of great significance for my future. My PhD research is also in collaboration with the IBM T. J. Watson Research Center, under Dr. Jonathan Z. Sun s supervision. This knowledgeable scientist, who also came from Shanghai and Fudan University as undergraduate college (18 years earlier than I), has given me lots of guidance in sample fabricaiv

5 tion and device physics. I still remember so much time that he has spent for me during weekends and late nights. Many fruitful discussion with him broadened my views of scientific research. My projects could not have been accomplished without his help and support. I would like to thank Dr. Jean-Marc L. Beaujour and Dr. Grégoire de Loubens for their assistance. Jean-Marc works in our lab with a long time overlap with me. He helped me with the thin film deposition, and shared his data with me on extended magnetic films. Grégoire, a super knowledgeable and helpful postdoc, helped me a lot with the high frequency circuits and data analysis. I also want to thank Prof. Peter M. Levy for many useful discussions on spin transfer theories. Those conversations with him gave me a vivid understanding of the theoretical models behind phenomena. I appreciate his detailed explanation with patience and his constant interest in my experimental results. I would like to thank Enrique del Barco, Barbaros Özyilmaz and Gabriel D. Chaves. The first two helped me with measurement setups when I entered the lab, and I learnt many lab techniques from them. Gabriel gave me lots of assistance in software usage and numerical calculation. Meanwhile, I appreciate the help I received from Dr. Michael J. Rooks and Nina Ruiz as well, who did the e-beam lithography at IBM for my samples. Many thanks should also be delivered to Russell Allen, Steve Brown and John Connelly at IBM, who always provided kind assistance for fabrication setups. Sincere gratitude goes to those who worked closely with me in lab: Russell Ferri, Ning Xuan, Mariano Zimmler, Huanlong Liu and many others. They v

6 have made these years of my PhD study full of pleasant memories. Special thanks to technical support from Mark Ofitserov, Fred Hansen and Joseph Duke. They made many beautiful parts for my experimental setups. Last but not least, I am very grateful for those valuable discussions with so many famous visiting professors: Prof. Albert Fert (2007 Nobel Prize Winner in Physics), Prof. Arne Brataas, Dr. Thomas Silva, Prof. Sadamichi Maekawa, and Dr. Daniel Worledge. They brought many instructive advice and evocative suggestions to me and broadened my eyes in scientific research. Projects related to my PhD research are funded by NSF-FRG-DMS , NSF-DMR , ONR-N , NSF-DMR , ARO-W911NF and an NYU Research Challenge Fund Award. I was supported by the GSAS Dean s Dissertation Fellowship during the academic year. vi

7 Abstract This thesis describes experimental studies of spin-transfer induced magnetization switching and dynamics in bilayer magnetic nanopillars. Known as the spin transfer effect, a spin-polarized current can transfer its angular momentum to a magnetic layer, resulting in its reversal as well as microwave dynamics. It has become a major focus in both physics and technology. Our nanostructured spin valve junctions are fabricated using two different processes. The first one is the nanostencil process, where pillar structures are deposited into pre-defined stencil holes with submicron lateral dimensions, and extended leads are patterned using lithography. The other one is the subtractive process, where pillar structures are formed by etching pre-deposited magnetic films. Junctions were patterned so that current flows perpendicularly to the layer surface. Low frequency transport measurements were conducted on asymmetric Co/ Cu/ Co spin valve junctions with large perpendicular applied magnetic field to study the current needed to excite magnetization dynamics as a function of thin magnetic layer thickness. The critical current density decreases as the free layer vii

8 thickness decreases, but has a non-zero intercept at zero free layer thickness, which is in quantitative agreement with that expected due to a spin-pumping contribution to the magnetization damping. It might also be an indicative of decrease of spin transfer efficiency in thin layers. Fine structures in the differential resistance found in these measurements also indicate that the contacts play a role in the excitations. High frequency measurements were conducted by means of ferromagnetic resonance driven by spin torque interactions (ST-FMR). Here studies were made on spin valves that have a Co/Ni synthetic free layer with a perpendicular magnetic anisotropy. Resonance lines were measured under GHz excitation, and were compared with those on extended films with the same Co/Ni layer structure. The layers confined in spin valves have a lower resonance field, a narrower resonance linewidth and approximately the same linewidth vs frequency slope. The magnitude of spin transfer torque was estimated from both the zero dc bias resonance line and the change of the resonance linewidth with dc current. Some initial results on ST-FMR in a nonlinear regime are also included in this thesis. viii

9 Contents Dedication iii Acknowledgements iv Abstract vii List of Figures xiii List of Tables xxv 1 Introduction Giant magnetoresistance Spin transfer effects Principles Applications An overview of my thesis Fabrication of nanopillar devices Introduction Fabrication Techniques ix

10 2.2.1 Thin film deposition Lithography lateral shape definition Etching Nanopillars fabricated in a stencil process Stencils with standard size opening Nanopillar layer deposition Four-level electrode patterning Sample fabrication with a subtractive process Base layer and nanopillar layer deposition Double layer e-beam lithography Ion etching using the resist mask SiO 2 deposition, lift-off and metal capping Four-level processing Critical current as the function of free layer thickness Introduction and modeling prediction Measurement setup Experimental results Discussion and theoretical interpretation Effects of lateral confinement on nonuniform spin wave modes Effects of magnetic anisotropy Spin pumping induced nonlocal damping Decrease of spin torque efficiency Summary x

11 4 Effects of contact layers on bipolar excitations in bilayer spin valves Theoretical background Measurement setup Experimental results Discussion and summary Effective field interaction Motivation Method of measuring the effective field interaction Measurement setup and experimental results Discussion Comparison with model calculations of effective field interactions Magnetic film morphology Finite temperature effects Summary and some later works Spin torque driven ferromagnetic resonance studies on a Co/Ni synthetic layer confined in a spin valve Introduction Approaches of high frequency measurements Magnetic layers with perpendicular anisotropy Measurement setup and principle Measurement setup xi

12 6.2.2 ST-FMR measurement principle Experimental results DC transport measurements ST-FMR resonance lines Comparison with results on extended magnetic films and dc transport measurements Resonance fields Resonance linewidth Direct measurement of a spin transfer torque Spin torque driven ferromagnetic resonance studies in a nonlinear regime Introduction Measurement setup Experimental results Results with a modulated rf input Results with a constant (unmodulated) rf input and an rf amplifier Discussion Resonance shift foldover effect Nonlinear damping Summary and prospectives 191 A Anisotropic magnetoresistance measurements on extended Co films 194 xii

13 B Some detailed sample fabrication recipes 197 B.1 Wafer cleaning B.2 Pumping down and venting the UHV chamber B.3 A set of reference parameters for sputter deposition which I used at IBM B.4 Resist spinning, baking and exposure recipes Bibliography 202 xiii

14 List of Figures 1.1 Physics Nobel Prize Laureates of 2007, Profs. Albert Fert and Peter Grünberg First giant magnetoresistance curves measured by Grünberg s group [1] in (a) and Fert s group [2] in (b) (a) Density of states of up and down spin electrons in Cu and Co as the function of energy. Electron states are occupied up to the Fermi Energy at zero temperature, and electrons at the Fermi level transport the current in a metal. (b) Electron scattering schematic and equivalent two-channel circuit of up and down spins passing through a ferromagnetic layer. The small box represents a small resistance while the larger box is a larger resistance Schematics and equivalent circuits of spin dependent electron scattering in a bilayer structure, with two magnetic layers parallel in (a) and antiparallel in (b) xiv

15 1.5 Spin torque (small yellow arrows) acting on the magnetic layer when the spin current is flowing, from left to right in (a) and from right to left in (b), assuming that the spin-polarization to the left of the layer is fixed to be up (a) A bilayer structure that is used to measure spin transfer effects. (b) Analysis of the torques acting on the free FM layer, with the damping torque τ damping and the spin transfer torque τ ST labeled respectively. (Reprinted with permission from Ref. [3]. Copyright of John Wiley & Sons Limited 2007.) Interaction between a current and a magnetic layer, by a traditional charge current induced Oersted field in (a), or by a spin current induced spin transfer torque in (b). (Reprinted with permission from Ref. [3], copyright of John Wiley & Sons Limited 2007.) Cross-point array structure of magnetic random access memory using a charge current induced Oersted field as in (a), or a spin current induced torque as in (b) [4]. (This figure was originally printed in Barbaros Özyilmaz s PhD thesis. Reprint permitted by the author.) Point contact device structure A schematic of a nanopillar device UHV system in the cleanroom at NYU Step flow of stencil opening xv

16 2.5 A shallow angle SEM image of the stencil with an undercut. Reprinted from Ref. [5] with permission of J. Z. Sun Top view SEM image of an array of nm 2 stencil openings. (Courtesy of J. Z. Sun) Layer structure of the stencil being filled up with effective layer stack, shown with a schematic (a), shallow angle view (b) (reprinted from Ref. [5] with permission of J. Z. Sun) and a TEM image (c) (reprinted from Ref. [6] with permission of J. Z. Sun) (a): Linear motion shutter. (b): Schematic of layer deposition on a stencil with a wedge growth mechanism A sketch of the pillar junction in the four level processing. (a): a bottom level electrode patterning with the etching down to the Si substrate. (b): a meso-level etching to insulate adjacent junctions. (c): a SiO 2 deposition to prevent shorts between top and bottom electrodes. (d): a via-level opening through the SiO 2 layer. (e): a top level electrode patterning. Color legends: yellow: Cu; blue: the free FM layer; navy: the fixed FM layer; dark grey: Pt; light grey: SiO 2 ; light sky-blue: Si substrate Top view optical microscope image of one patterned stripe of regular junctions xvi

17 2.11 Processing steps of the bottom level and the junction level etching. (a): layer structure before the four-level processing. (b): photolithography for bottom electrodes. (c): bottom level ion milling. (d): photoresist removal. (e): meso level photolithography. (f): meso level ion milling. Color legends are the same as those in Fig. 2.9, and the red color represents the photoresist Processing steps of via level etching and top metal deposition. (a): removal of the meso level photoresist. (b): deposition of the thick SiO 2 layer. (c): via level photolithography. (d): reactive ion etching to open up top of the SiO 2 layer. (e): removal of the via level photoresist. (f): deposition of the top level metal Top view optical microscope image of one patterned stripe of high speed junctions Base layer and nanopillar layer deposition HSQ e-beam lithography and NEB RIE etching SEM image (25 o off normal direction) of the double layer resist on top of a sample (taken under the guidance of J. Z. Sun) Ion milling to form the pillar structure SiO 2 deposition, lift off and metal capping steps SEM image of the pillars after the lift-off step Phase diagram of a nanopillar for η = 0.3, α = 0.01, and 4πM eff = 1.5 T, with a small (0.1 T ) in-plane uniaxial anisotropy. The threshold current densities for instability of the P state, J T P, and for instability of the AP state, J T AP, are indicated xvii

18 3.2 Low temperature setup for this study. (a): Oxford cryostat with a 12 T superconducting magnet located at the bottom. (b): Low temperature probe with the sample holder rotatable in the field (amplified in (c) Zero dc current in-plane magnetoresistance hysteresis loop for a nm 2 junction with t 2.8 nm (a) MR of all junctions as the function of t. (b) Zero dc current in-plane δr times lateral area A for all junctions as the function of t Positive current sweep hysteresis loop of the same junction as in Fig. 3.3 with perpendicular magnetic field set at 7 T Contour plot of dv/di minus a linear background as a function of both current density and magnetic field perpendicular to sample surface for decreasing current. Blue dots: The corresponding step in V/I for increasing current (a) V/I vs current bias hysteresis loops with fields set at 3, 5 and 7 T, in blue, green and red colors respectively. (b) δr/r at the critical current as the function of field. Solid line: Zero dc current in-plane MR xviii

19 3.8 (a) Solid points: critical-current density vs perpendicular magnetic field of 4 of the 50 nm series junctions with t=1.9, 2.8, 4.3, 5.3 nm (from left to right). Pink open circles: critical current densities of all 50 nm series junctions extrapolated to zero t. Crosses: those of nm 2 junctions. (b) Critical current densities as a function of free layer thickness. Squares: 7 T. Diamonds: 3 T. Red, blue and black data points are the ones of 50 50, and nm 2 junctions respectively. Solid lines: linear fits of critical current densities vs t of 50 nm series. Dashed lines: those of nm 2 junctions Zero-current in-plane magnetoresistance hysteresis loops of nm 2 junctions with thin Co layer thickness t 3.3 nm. Type A in (a) and Type B in (b) Positive current sweep dv/di hysteresis loops of the same junctions as in Fig Corresponding V/I current sweep hysteresis loops d 2 V/dI 2 current sweep contour plots of nm 2 junction with t 1.9 nm as the function of both current density and magnetic field perpendicular to the sample surface. (a): Type A sample. (b): Type B sample with currents swept from positive to negative dv/di current sweep curves of nm 2 junctions with the regions of excitations at positive currents magnified. (a): Type A sample with t 4.3 nm, H= 5 T; (b): Type B sample with t 1.9 nm, H= 2.3 T xix

20 5.1 Zero temperature switching phase diagrams. Solid lines: without the effective field term. Dashed lines: with the effective field term Low temperature probe used for this set of measurements, with the part enclosed in the black dashed box amplified in (b). (c): the probe closed up by a superconducting solenoid MR hysteresis curves of two junctions measured at a zero dc current. The free layer thickness t=1.0 nm in (a) and t=0.6 nm in (b) Magnetoresistance of all measured junctions as the function of free layer thickness Magnetic field sweep switching boundaries of the same junctions as in Fig H P AP in red triangles and H AP P in blue. H avg + in green circles The switching boundaries with the fixed layer aligned in the negative field direction. Havg are shown in green circles Spin transfer induced effective field and spin transfer torque as the function of free layer thickness, in (a) and (b) respectively AMR measurements on the extended Co film with applied magnetic field parallel to the current (in red) and perpendicular to the current (in blue). t=1.0, 0.7, 0.4 and 0.2 nm in (a) (b) (c) and (d) respectively Switching phase diagrams with a finite temperature correction shown in the red dashed curves xx

21 6.1 Spin transfer devices with (a) in-plane magnetization geometry, (b) perpendicular magnetization geometry, and (c) a perpendicular polarizer [7, 8, 9] (a): Spin valve layer structure, ST-FMR circuit and all key instruments used in the ST-FMR measurements. (b): Microscopic picture of the patterned high speed sample (a): The high frequency probe station with the probe and the sample mounted. (b): A microscope picture of the probe in contact with the electrodes of the nanojunction Field-driven FMR on same-stack extended films using the flipchip method Magnetization configuration of the free and fixed layers with the free layer on resonance Zero current in-plane MR hysteresis loop for a nm 2 spin valve junction with t= dv/di vs I of the same junction with a perpendicular magnetic field of 9.5 koe in a two-point geometry Contour plot of dv/di as a function of both dc current and perpendicular magnetic field xxi

22 6.9 (a): ST-FMR voltage signal (square points) as a function of applied perpendicular magnetic field together with a Lorentzian fit (solid line). The measurement was done on a nm 2 junction with an rf amplitude of I rf =560 µa at a frequency of 18 GHz. Inset: plot of V peak /Irf 2 vs I rf for measurements at this frequency but varied input rf power. (b): Zero dc current lock-in voltage signal as the function of applied magnetic field at different frequencies from 4 GHz up to 16 GHz in 2 GHz steps (a): ST-FMR voltage signal on another nm 2 junction at different rf frequencies from 3 up to 20 GHz in 1 GHz steps. (b): Comparison of resonance field as the function of frequency between the same spin valve junction as in (a) (black dots) and same-stack extended film (pink dots). Solid lines are linear fits. Green dashed line is the dispersion relationship of a 10 nm thick Co layer under this field geometry Broad peak structures of a nm 2 and t=0.2 junction. The ST-FMR signals were plotted as a function of applied perpendicular magnetic field at different rf frequencies from 2 up to 15 GHz in 1 GHz steps xxii

23 6.12 (a): A reiteration of Fig. 6.10(b) with an addition of a dashed line which represents the linear fit of resonance field with estimated dipolar fields corrected. (b): The dispersion of the lowest four spin wave modes on a nm 3 Co/Ni synthetic structure using OOMMF simulation (dots) and the analytical model discussed in the text (dashed lines). Solid line is the linear fit of H res of the extended film. Corresponding mode profiles are shown in the lower-right corner, in the order of (n x, n y )=(1, 1) (1, 2) (1, 3) (2, 1) from bottom to top H vs f for the spin valve junction shown in triangle symbols and the extended film shown in squares, together with their corresponding linear fits (a): ST-FMR signal as a function of applied field at different dc currents. The rf frequency was set at 18 GHz, and the rf amplitudes were 595, 595, 470, 470, 315 and 315 µa respectively for each dc current from -4 to 6 ma in 2 ma steps. Each adjacent curve is offset by 0.20 µv. Solid lines are Lorentzian fits of each data set. (b): H (full width at half maximum) vs dc current. Inset: H res vs dc current A reiteration of Fig. 6.8 with the addition of data points of critical currents determined from ST-FMR at three different fields and frequencies xxiii

24 6.16 A resonance line at 18 GHz. With all parameters shown in the figure given from the resonance line, the magnitude of the spin transfer torque is determined from this data Spin transfer torque per unit current as the function of rf frequency Spin transfer torque per unit current as the function of β (the angle between the free and fixed layer magnetizations). Red data points: those determined by competition between dc induced spin torque and damping Magnetization configuration of the free and fixed layers with the free layer on a large angle precession Measurement setup of the ST-FMR studies in a nonlinear regime, with a 20 db amplifier added to the circuit and a voltmeter replacing the lock-in amplifier Amplifier power calibration. (a) Power gains (b) power output as the function of input power at four different frequencies ST-FMR signals measured on a nm 2 junction with a modulated rf current. f=10 GHz, I dc =4 ma. Power input from the network analyzer is varied from -27 dbm to -12 dbm in steps of 3 dbm in (a), and from -15 dbm to 9 dbm in (b) ST-FMR signals at zero dc current with an unmodulated rf input. (a): f=6 GHz, power input is 0 dbm, (b): f=10 GHz, power input is 2 dbm xxiv

25 7.6 A series of single peak nonlinear ST-FMR signals measured at 16 GHz and zero dc current with an unmodulated rf input and a 20 db amplifier. The power input from the network analyzer was varied from -15 to 6 dbm Nonlinear ST-FMR signals at six selected powers from Fig. 7.6 with the nonresonant background subtracted. I rf was estimated to be 2.2, 3.4, 4.7, 6.3, 8.0, 9.3 ma respectively Plot of the foldover function at different b parameters Plot of V/I rf vs H with the same measurement as in Fig (a) V peak /I rf vs I rf, (b) H peak vs I rf Nonlinear resonance lines with different nonlinear damping parameters. (a): linear damping, (b): subcritical nonlinear damping, (c): supercritical nonlinear damping. (from Prof. A. N. Slavin) A.1 (a) Resistance as a function of angle between current and inplane magnetic field on an extended Co film with t=3.7 nm. Red curve: a sinusoidal fit. (b) AMR ratio as the function of Co layer thickness t B.1 Schematic of the UHV chamber pumping system xxv

26 List of Tables B.1 A reference of sputter rate of different 2-inch targets used at IBM Yorktown. Each target normally faces the substrate, which is 3 inches away xxvi

27 Chapter 1 Introduction 1

28 Electronics has been widely used for several decades in information storage industry for several decades ever since the development of the semiconductor technologies. In that case, charge has been employed to control read and write in a computer storage/memory unit. With the discovery of Giant Magnetoresistance, spintronics has been developed into a new technology, where electron spin is manipulated to store information. Now spintronics has been studied in many different branches of condensed matter physics. Within those branches, an inverse effect to Giant Magnetoresistance, spin angular momentum transfer, was theoretically predicted by Slonczewski [10] and Berger [11] in 1996, and has been a forefront area of experimental research (see, for example, Refs. [12, 13, 14, 6, 15, 16, 17, 18]) during the past ten years. I have been studying different topics related to spin transfer during the years of my PhD research in Prof. Kent s group at NYU and also in a collaboration with the IBM T. J. Watson research center. This thesis describes the projects that I have worked on these years. Here I start with the Giant Magnetoresistance effect. 1.1 Giant magnetoresistance The Physics Nobel prize of 2007 was awarded to the French Scientist Albert Fert and the German Scientist Peter Grünberg (Fig. 1.1), who independently discovered the Giant Magnetoresistance (GMR) effect. It is thanks to the technologies developed based on this effect that made it possible to miniaturize hard disk of recent generations, where sensitive read-out heads are needed. 2

29 Figure 1.1: Physics Nobel Prize Laureates of 2007, Profs. Albert Fert and Peter Grünberg Magnetoresistance (MR) was measured by W. Thomson 150 years ago [19]. In that measurement, he found that the resistance of a ferromagnetic (FM) material (like Fe, Co, or Ni) changes with the orientation of the magnetization with respect to the current, in an effect known as the anisotropic magnetoresistance (AMR). It is a transport property that comes from electron spin-orbit coupling, where there is a larger probability of s-d scattering of electrons in the direction of the magnetization. Both calculation and measurements [20] showed that the electrical resistance reaches a maximum value when the direction of the current is parallel to the magnetization. As the angle θ between the current and the magnetization increases, the resistance decreases with cos 2θ, and reaches a minimum when θ = 90 [20]. The AMR ratio ( R/R) is usually very weak in 3

30 normal cases. (See Appendix A for my measurements of AMR on Co films.) Although AMR is typically less than 2%, it was already used in hard disk technologies [21, 22] before the discovery of GMR. However, since AMR arises from scattering throughout the FM layer, the AMR signal is reduced and becomes too small to be useful for recording heads with the decrease of layer volume, when the device unit density is above 5 Gbit/in 2 [4]. This became the bottleneck for the development of the recording/storage industry, as hard disks with higher and higher device density were required. The breakthrough was made by experiments from the groups of Albert Fert and Peter Grünberg in Instead of a single FM layer, magnetic multilayers were used, and this layer structure generated an MR signal at least one order of magnitude larger than AMR. Grünberg s group measured magnetoresistance of an Fe/Cr/Fe antiferromagnetically coupled layer structure and found a 1.5 % MR at 4.2 K in small applied fields [23, 1] (shown in Fig. 1.2(a)). This is a big enhancement compared with the AMR measured in a single Fe layer. Fert s group grew layer structures with many repetitions of Fe/Cr, and found MR as large as 50 % at 4.2 K in large magnetic fields, as indicated in Fig. 1.2(b) [2]. This increase in MR appeared to have revealed an entirely new phenomenon, which was caused by the change in the multilayered magnetic configuration that affects the scattering of conduction electrons. This phenomenon, which was named as the giant magnetoresistance (GMR), meets the requirement of large enough signal level with high device density. The physics of GMR can be understood by considering the difference in scattering rates between spin up and spin down electrons in electrical trans- 4

31 Figure 1.2: First giant magnetoresistance curves measured by Grünberg s group [1] in (a) and Fert s group [2] in (b). port through a magnetic layer. A basic GMR structure is composed of a FM/NM/FM asymmetric layer stack, where NM stands for a non-magnetic metal. The density of states (DOS) of Cu (an example of NM) and of Co (an example of FM) are shown in Fig. 1.3(a). Cu s band structure is symmetric in terms of spin orientation. Therefore, it is non-magnetic, since magnetization is proportional to the difference of total number of spin up and spin down electrons. Furthermore, conduction electrons with opposite spins are scattered at the same rate, since the scattering rate is proportional to DOS at the Fermi surface in a material without magnetic impurities. However, Co s band structure is asymmetric. Shown in Fig. 1.3(a), the number of spin up is more than that of spin down. Therefore spin up becomes the majority spin in Co and spin down 5

32 0 Cu [Ar].3d 10.4s 1 DOS[a.u.] 0 Co [Ar].3d 7.4s 2 0 E-E F (a) (b) Figure 1.3: (a) Density of states of up and down spin electrons in Cu and Co as the function of energy. Electron states are occupied up to the Fermi Energy at zero temperature, and electrons at the Fermi level transport the current in a metal. (b) Electron scattering schematic and equivalent two-channel circuit of up and down spins passing through a ferromagnetic layer. The small box represents a small resistance while the larger box is a larger resistance is the minority counterpart. On the other hand, the DOS at the Fermi surface of the up spin is much lower than that of the down spin. Therefore, the s-band conduction electrons with a majority spin are far less scattered into a d-band than those with a minority spin, as shown by the schematic in Fig. 1.3(b). It can also be interpreted with an equivalent circuit indicated in lower Fig. 1.3(b), where the conduction channels for majority and minority spins have different effective resistances. In the case where there is only a single ferromagnetic layer, one expects no significant change in total resistance once the magnetization is flipped to the opposite direction. In a FM/NM/FM bilayer structure, the two-channel electron scattering and equivalent circuit is shown schematically in Fig Compared with the single 6

33 layer case, resistance with a bilayer structure depends on the relative orientation of the two FM layers. Suppose both FM layers are aligned in the same direction, most of the electrons with majority spin go through both layers, while those with minority spin are mainly scattered, with a corresponding two-channel equivalent circuit shown in lower Fig. 1.4(a). However, if the FM layers are aligned in an antiparallel configuration, majority spins (for the first FM layer) mostly go through the first layer and get scattered at the second layer, and vice versa for the minority spins (Fig. 1.4(b)). As the result, the resistance with the FM layer magnetization aligned in an antiparallel state is higher than that under a parallel state. This is the origin of the giant magnetoresistance. The role of the NM layer in the bilayer structure is to adjust the exchange coupling between the FM layers. If the two FM layers are next to each other, strong exchange coupling would fix the relative orientation between them. If a thin NM layer (several Å to several nm thick) is inserted, an RKKY type of exchange interaction [24, 25, 26, 27] occurs between the FM layers. The strength and type (ferromagnetic or antiferromagnetic coupling) of the interaction can be adjusted by selecting material and thickness of the NM layer. In Grünberg and Fert s works [1, 2], they used Cr with a specific thickness as the NM layer such that the Fe layers are antiferromagnetically coupled to each other by the RKKY exchange interaction. Therefore, their devices had an antiparallel state at zero field, and switched to a parallel state at a finite applied magnetic field when the field overcomes the antiferromagnetic coupling. This switching field can be controlled by varying the thickness of the Cr layer, through which the strength of the exchange interaction is adjusted. 7

34 RKKY interaction oscillatorily decays as the thickness of the NM increases [28, 29, 30], and becomes negligible once the thickness is above a certain value d (for example, d 6 nm for a Co/Cu/Co bilayer structure [30]). GMR can still be observed in the absence of the RKKY interaction if the layer structure is asymmetric. In this case, the coercive fields of the two FM layers are different and it is possible to switch them independently. GMR devices with minimal exchange interaction between the layers are usually called spin valves. The thickness of the NM layer in a GMR device is usually designed to be much smaller than its spin diffusion length [31], which is the distance traveled by an electron before it experiences a spin-flip scattering event and loses its memory of its original spin orientation. In this way, the spin polarization of the current does not significantly decay before it enters the second FM layer. Similar two-channel circuits can be deduced for GMR with more than two magnetic layers, like in the original discovery of GMR [2]. Note that GMR is only a function of the relative angle between magnetic layers, and it is usually not related to magnetization direction of each individual layer. For example, in a spin valve junction, if both magnetic layers in Fig. 1.4 are aligned perpendicular to the layer surface, GMR between parallel and antiparallel configurations is still the same as that with an in-plane geometry [16], even though eigenfunctions that characterize spin orientation become those in the perpendicular direction. The only difference between the in-plane MR and the out-of-plane MR is due to the AMR, which is usually at least one order of magnitude lower than that of GMR. Therefore, GMR is an effect where spins of conduction electrons are con- 8

35 Figure 1.4: Schematics and equivalent circuits of spin dependent electron scattering in a bilayer structure, with two magnetic layers parallel in (a) and antiparallel in (b). trolled by relative orientation of local magnetization, and thus cause the change of transport properties. It is the basis for spintronics [32] which later developed into many branches of applied physics studies during the past two decades. It has also been widely applied in high density hard disks and magnetic random access memory technologies [33, 34, 35, 36]. The GMR effect can be widely applied in sensor, storage and memory industry, and a detailed discussion can be found in Chap. 5 of Ref. [4]. 9

36 1.2 Spin transfer effects Principles In a giant magnetoresistance effect, conduction electrons are spin dependently scattered or filtered, yielding a change in angular momentum of the spin current. At the same time, such a change of spin angular momentum is transferred to the background magnetization. This effect, known as the spin transfer effect, was theoretically predicted by Slonczewski [10] and Berger [11] in It is the inverse action of GMR on the collective magnetization order parameter. Due to the angular momentum conservation, it illustrates a counteraction from current to magnetization by exerting a torque onto it. This torque is called the spin transfer torque and can induce reversal/dynamics of the local magnetization. A simple picture can be established by considering the interaction between a spin polarized current and a magnetic layer when the spin current goes through the layer. Suppose the incoming current has the spin which is polarized in the direction pointing up, and the magnetization is driven out of equilibrium by an angle θ away from the in-plane direction, shown in Fig. 1.5(a). The spin of the incoming current can be written as a linear combination of the spin direction parallel and antiparallel to the magnetization. In a simple spin-filtering picture, the magnetic layer filters the spin component in the opposite direction, and the counterpart in the same direction goes through. As the result, the spin angular momentum of the current is changed from the original direction to the direction of the magnetization. Since the total spin angular momentum is conserved, the loss of the angular momentum L of the current is absorbed 10

37 Figure 1.5: Spin torque (small yellow arrows) acting on the magnetic layer when the spin current is flowing, from left to right in (a) and from right to left in (b), assuming that the spin-polarization to the left of the layer is fixed to be up. by the magnetization, and the absorption of the angular momentum per unit time is the spin transfer torque τ ST = dl/dt. Under this current direction, the torque stabilizes the magnetization of the layer back to the in-plane equilibrium, as indicated in Fig. 1.5(a). If the current flows in the opposite direction as in Fig. 1.5(b), the change in spin angular momentum of the current is reversed. Therefore, the direction of τ ST acting on the magnetic layer is also reversed. In this case, the spin torque tends to drive the magnetization further away from the up direction. Once the torque is large enough, it could drive the magnetization into large angle dynamics or even flip it to the opposite direction. 11

38 Figure 1.6: (a) A bilayer structure that is used to measure spin transfer effects. (b) Analysis of the torques acting on the free FM layer, with the damping torque τ damping and the spin transfer torque τ ST labeled respectively. (Reprinted with permission from Ref. [3]. Copyright of John Wiley & Sons Limited 2007.) Basically, in order to experimentally study the spin transfer effect, one needs two key elements: a source of spin current and a magnetic layer as the spin analyzer. Usually we consider an asymmetric bilayer spin valve structure where a thick FM layer has a fixed magnetization that generates a spin-polarized current and a thin (free) FM layer interacts with it. Fig. 1.6(a) shows such a layer structure. A regular charge current goes through a fixed FM layer and the current gets spin-polarized in the same direction as the fixed layer magnetization. A normal metal is inserted between the fixed and free layers in order to prevent a strong exchange coupling between each other (discussed in Section 1.1), while its thickness should still be much smaller than the spin 12

39 diffusion length [31] ( 300 nm for Cu at room temperature). Now let s analyze the dynamical motion of the free layer magnetic moment m. The free-layer dynamics in the classical limit can be described by the Landau-Lifshitz-Gilbert (LLG) equation, which states that d m/dt = γ m H + α ˆm d m/dt. The first term γ m H describes the magnetic precession in the field, where γ is the gyromagnetic ratio and H includes all field terms that act on the magnetic moment. As shown in Fig. 1.6(b), this term indicates that magnetization driven out of equilibrium does not come directly back to equilibrium, but precesses about the field. Dynamics of the magnetization in solids is also affected by an additional damping torque, so that the magnetization does not precess forever and gradually relaxes down to equilibrium [37]. A phenomenological description of the damping torque can be expressed as α ˆm d m/dt [38], which is the second term in the above equation. Here α is a material-dependent dimensionless parameter and ˆm is the unit vector in the direction of m. Without a spin current, the LLG equation determines dynamics of the magnetic moment. However, with the inclusion of the spin current that interacts with the magnetic moment, an additional spin transfer torque, which reflects the angular momentum transfer of the spin current to the magnetization per unit time, needs to be taken into account [10, 11]. The spin transfer torque is expressed as γa J m ( ˆm mˆ P ), where a J is a parameter proportional to the current density J and the spin polarization P of the current, and mˆ P is the unit vector in the direction of the fixed layer magnetization. It is in the direction transverse to the free layer magnetization and in the plane containing ˆm and 13

40 mˆ P. The complete equation of motion of the free layer is given as: d ˆm dt d ˆm = γ ˆm H + α ˆm dt + γa J ˆm ( ˆm mˆ P ). (1.1) The torque parameter a J has different expressions based on different models and assumptions. In Slonczewski s theory a J = JP 2eM s t, where M s magnetization density and thickness of the free magnetic layer. and t are The damping torque and the spin transfer torque are both orthogonal to the precession (Fig. 1.6(b)). In one direction of the current, the spin transfer torque is in the same direction as the damping torque. Therefore, the effective damping is enhanced and spin transfer assists the damping to stabilize the magnetic moment. On the other hand, if the current direction is reversed, the spin transfer torque competes with damping. As a result, the effective damping is reduced and it takes longer for the precession to attenuate. Particularly, once the spin transfer torque has the same magnitude as the damping torque at a critical injected current, the effective damping becomes zero and the magnetic layer is destabilized. In this case, an excitation of magnetization precession or switching will occur. Such an excitation could under certain conditions induce a reversal of the magnetic moment. In order to observe a spin transfer induced magnetization reversal, a current is essential to drive the magnetic layer into excitation. We could calculate the critical current density by equalizing the magnitude of the spin transfer torque to that of the damping torque shown in Eq In a Co/Cu/Co structure, the critical current will be at least 10 7 A/cm 2 (assuming α 0.01 and P 0.3), 14

41 and similar values are also expected in other metallic spin valve structures. This shows that lateral dimension of a spin transfer device needs to be reduced to a submicron scale, otherwise the total current applied will generate a large amount of heat dissipation in the device. Moreover, a charge current induced Oersted field is dominant in junctions with larger lateral dimensions, compared with effects induced by the spin current. This is another important reason why devices with smaller lateral dimension are required for studies on spin transfer. These will be discussed in detail in section 2.1. Thus, spin transfer opens up a new field for physics study. There have been other theoretical calculations in addition to Slonczewski s and Berger s original works, and they can be classified into several categories. One is based on a Landauer-Büttiker type formalism of ballistic transport, where Slonczewski s theory belongs. In Ref. [10], Slonczewski pointed out that spin transfer is an interfacial effect. Stiles et al. [39] discussed the length scale of the spin transfer interaction in detail. He concluded from his calculation that spin current decays quickly passing the NM/FM interface, and spin transfer takes place within only several atomic layers at the interface, consistent with Slonczewski s model. Circuit theory established by Brataas et al. [40] is another theory with a different approach. Their work considered the system as a two-channel circuit involving ferromagnetic elements based on the conservation of spin and charge current, and found that the spin transfer torque results from the real part of the mixing conductance G of the FM/NM interface [40]. This theory assumes that the transverse component of the spin current does not propagate in the FM layer, and therefore belongs to a ballistic-like category. It concluded with a 15

42 simple analytical expression of spin transfer, and offered a straightforward way for people to model multilayer circuits. There were also theories in a diffusive transport regime. Zhang, Levy and Fert [41] did this type of calculation based on the diffusive Boltzmann equation. Similar to the ballistic results, a spin torque term was also predicted, and it was shown to be related to the component of the spin polarization transverse to the magnetization of the FM layer. However, the diffusive model concluded that the presence of a transverse spin current inside the FM layer is essential for spin transfer effects [41]. As the result, there exists a propagating mode of spin current that penetrates into the FM layer at a transverse spin decay length λ J [42, 43], which is at the order of several nm. This conclusion is fundamentally different from those of ballistic models. Other than those introduced above, there have also been related theoretical/modeling works. For example, Refs. [44, 42, 43, 45, 46]. The first experimental studies on spin transfer were conducted with a currentvoltage measurement [12, 47, 13, 14, 16, 18], where a change in junction resistance by the full magnetoresistance indicated the magnetization switching induced by the current. In some later measurements, current-voltage measurement can also be applied to detect spin transfer induced nonuniform excitations in single magnetic layers [48]. This indicated that the key element of spin transfer is spin filtering, not necessarily spin dependent scattering, and as the result of this a magnetic layer can respond to a spin torque generated from the current polarized by itself. Using high frequency techniques, many groups were able to detect microwave (f GHz) power emission from the de- 16

43 vices [15, 17, 49, 50, 51, 52], where spin transfer induced dynamics were directly measured. These experiments suggested important results that a high frequency signal in a GHz regime can be generated with a steady dc current input. Time domain measurements of microwave voltage [53] and magnetization images [54] with the injection of current pulses were also used to reveal magnetization dynamics approaching reversal. Recently, ferromagnetic resonance technique has been used to drive magnetic layers into excitations using spin transfer interaction [55, 56], which enables quantitative measurements of magnetic properties of incorporated layers and spin transfer torque in magnetic nanopillar junctions. Although it is generally agreed that spin transfer is a mechanism by which a spin-polarized current changes the magnetic states of a nanoscale FM layer, there are still open questions that remain. A fundamental one is the length scale over which the spin transfer interaction takes place, since the ballistic model and the diffusive model gave different results. Another important issue is the efficiency of the spin angular momentum transfer and the magnitude of the spin transfer torque, as these two determine how difficult it is to alter magnetic states using the spin transfer mechanism. It is also of great interest to study material parameters that control the spin transfer process, such as the damping and anisotropy field, and results of this study can help optimizing designs of spin transfer devices applied in industry. The open key issues listed above are the motivation for my thesis research. 17

44 Figure 1.7: Interaction between a current and a magnetic layer, by a traditional charge current induced Oersted field in (a), or by a spin current induced spin transfer torque in (b). (Reprinted with permission from Ref. [3], copyright of John Wiley & Sons Limited 2007.) Applications Spin transfer was discovered as a new type of mutual interaction between current and magnetization. The traditional interaction is due to the Oersted magnetic field generated by a charge current, which was discovered in 1819 (Fig. 1.7(a)). The field strength is proportional to the current, and therefore the magnetization direction can be controlled by the current. However the Oersted field is distributed all over the space and its magnitude decreases as the distance increases. Therefore, it is not a local interaction. Meanwhile, the interaction due to spin transfer (shown in Fig. 1.7(b)) is much more efficient since the spin transfer interaction is localized within the transport channel. This idea trig- 18

45 Figure 1.8: Cross-point array structure of magnetic random access memory using a charge current induced Oersted field as in (a), or a spin current induced torque as in (b) [4]. (This figure was originally printed in Barbaros Özyilmaz s PhD thesis. Reprint permitted by the author.) gered the design of a new type of magnetic random access memory (MRAM) based on spin transfer. A cross-point array architecture of the memory cells is presented in Fig Each magnetic cell contains a GMR structure, whose parallel and antiparallel configuration represent 0 and 1 of the bit. The cells are located between a lower word line and an upper bit line, where electrical currents go through. To read the state of an MRAM cell, a current is sent through its word line into the cell and comes out through its bit line. Information of the bit state is reflected in its resistance value. To write a bit, traditional MRAM design uses a charge current induced Oersted field to change the magnetic state of the cell as in 19

46 Fig. 1.8(a). Such an Oersted field decays outside the conduction channel. With the requirement of the increasing device density, this approach is not feasible since the Oersted field also affects adjacent cells when the distance between each other is greatly reduced, and errors in writing occur. More importantly, with the concern of stability of magnetic elements in a CMOS unit, a larger anisotropy field of the elements are required in smaller bits. Therefore, higher applied fields are needed to switch the elements, and conventional MRAM designs do not satisfy this requirement for magnetic bits scaled down to a deep-sub-100nm region as modern CMOS technologies would demand. MRAM based on spin transfer can solve this problem, since the current through the cell induces switching of the magnetic state while the spin torque only take effects within the conduction channel. This approach provides an alternative way of effective bit writing with the requirement of a further enhanced cell density. Presently, a tunneling magnetoresistance (TMR) structure (see e.g. Refs. [57, 58]), where the two FM layers are separated by a tunnel barrier, has replaced the GMR structure in the conventional MRAM design. A TMR cell has a much higher MR ratio (>100 %), and is still good for spin transfer induced bit control [59, 60, 61]. More importantly, tunnel junctions have higher impedance which can be tuned over a wide range, making it feasible for impedance matching into CMOS circuits. This brings spin-torque-based memory circuit one step closer to being an economically viable technology. In a recent publication [62], a 20 ns high speed writing which is reliable for cycles was found on a magnetic tunnel junction with a CoFeB/MgO/CoFeB cell structure. This shows a strong promise of technologies for spin torque based MRAM with an ultrafast write 20

47 speed. Other than the storage and memory applications, spin transfer devices can also be used in high frequency techniques. At the critical current, the spin transfer torque completely overcomes damping and a stable precessional state can be observed. Similar precessional states occur under other circumstances based on model calculations (see e.g. Ref. [63]). This precessional motion generates a GHz oscillating voltage across the device with a microwave power output [15, 17]. This property can be used to design a microwave oscillator. 21

48 1.3 An overview of my thesis In this chapter, I have already briefly discussed the basic physics of spin transfer, as well as topics of interest and applications in industry. Beginning from the next chapter, I will focus on all the projects that I have done. In Chapter 2, I will talk about all techniques and steps involved in the nanopillar sample fabrication. It includes a basic technique introduction and detailed steps of two fabrication processes that I have used nanostencil and subtractive processes. Chapter 3 includes my main project that employs a low frequency transport measurement. It is the study of critical current of spin transfer induced magnetization switching as the function of free layer thickness. This work was published in Phys. Rev. B 74, (2006), and presented in American Physical Society (APS) March Meeting, Los Angeles CA, Mar. 2005, 50 th MMM Conference, San Jose CA, Nov. 2005, and APS March Meeting, Baltimore MD, Mar Chapter 4 describes a side project of the work in Chap. 4, the effects of contact layers on nonuniform excitations in spin valve junctions. This work was published in J. Appl. Phys. 99, 08G511 (2006), 50 th MMM Conference Proceeding. In Chapter 5, I will discuss the measurement of the spin transfer induced effective field interaction, which was my first experimental project in this lab. This work still remains unpublished, and was presented in 49 th Magnetism and Magnetic Materials (MMM) Conference, Jacksonville FL, Nov A preceding work with my co-authorship was published in Phys. Rev. B 70, (2004). 22

49 Starting from Chapter 6, I will shift the discussion to high frequency measurements. I will talk about spin torque driven ferromagnetic resonance on spin valves with a Co/Ni synthetic free layer. Part of this work was published in Appl. Phys. Lett. 92, (2008) and J. Appl. Phys. 103, 07A502 (2008), proceeding of the 52 nd MMM Conference. It was also presented at the APS March Meeting, Denver CO, Mar. 2007, MMM Conference, Tampa FL, Nov. 2007, and APS March Meeting, New Orleans LA, Mar In Chapter 7, I will briefly talk about preliminary results of the spin torque driven ferromagnetic resonance in a nonlinear regime. This work is also unpublished. The thesis ends up with a summary and prospective in Chapter 8. 23

50 Chapter 2 Fabrication of nanopillar devices 24

51 2.1 Introduction As is mentioned in the previous chapter, a large current density (> 10 6 A/cm 2 ) is required for spin transfer effects. As a result, a regular CPP (current perpendicular to the film plane) device with mm lateral dimension would require a large current input (of the order of tens of ka). For a confined structure of submicron scale, the total current may be several ma, which significantly lowers the heat dissipation. In addition to this, the effects of the charge current induced Oersted field also needs to be taken into consideration. Assuming that a charge current flows uniformly through a cylinder, the magnitude of the Oersted field under a fixed current density is proportional to its radius (H r). A 10 6 A/cm 2 current density at 1 µm radius results in a 60 Oe field, and that is comparable to coercive field of the layers in a device. The Oersted field can be minimized by reducing the sample size. In addition to the two main reasons listed above, a consideration of magnetic domain formation also favors devices with a small lateral dimension. A magnetic film can split into domains. The formation of domains is due to a competition between exchange energy, magnetostatic energy and magnetocrystalline energy [64]. The average size of the magnetic domain varies between tens of nm up to mm, depending on the film s layer structure as well as geometric shape, external field, and even the history of being magnetized. The smaller the lateral size of the device, the less chance for the magnetic film to be separated into domains, and a device with a multi-domain structure adds complexity to data understanding. Therefore, a magnetic multilayer structure, which has a small lateral dimen- 25

52 Normal Metal Top Leads Insulating Layer Free FM Layer Normal Metal Spacing Fixed FM Layer Normal Metal Bottom Leads Figure 2.1: Point contact device structure sion and enables a localized high current density to go through, is necessary for our spin transfer study. However, the capability of fabricating devices with relevant length scales strongly influences discovery of phenomena and the physics behind them. Samples used in initial experiments of spin transfer were point contacts [12], and they are still being widely used [17, 50, 52, 65]. In such an approach, the magnetic layers are not patterned, but the current is restricted in a small point-contact region and injected into the magnetic layers right underneath. Fig. 2.1 shows the geometry in Refs. [17, 50, 52, 65]. Both the thin and thick ferromagnetic (FM) layers are extended. An insulating layer (usually SiO x or Al 2 O 3 ) covers top of the multilayer stack. E-beam lithography and chemical etching are used to open up a sub-µm small hole in the insulator. Normal metal is deposited to fill up the hole and form the top electrode contact. A current flowing in the top electrode is injected into the point contact forming a local 26

53 high current density. Although the current spreads out after entering the film, the current density near the contact is still very large. Because the resistance of the thick non-magnetic bottom electrode is far less than that of the magnetic layer stack, the current flows approximately perpendicularly to the FM layer film surface. As the first device fabrication technique used in spin transfer experiments, the point contact processing is relatively easy compared with those later developed as the result of which the FM layers have submicron lateral sizes. This is because the point contact method avoids the more lithographically challenging step of patterning a FM thin film. Spin transfer features were successfully observed in devices fabricated in this way [17, 50, 52, 65]. However, it is not so ideal for quantitative studies, since it is not easy to estimate the real current density. All devices studied in this thesis have well-defined submicron lateral dimensions. The fabrication methods are classified into two: nanostencil process and subtractive process. The former one was used to fabricate most of the devices I measured. The latter was developed at IBM first for fabricating magnetic tunnel junctions, and I adapted this method for processing metallic spin valve junctions, which involved a few trial-and-error experiments specifically in etching different materials in spin valves. I will introduce them in sections 2.3 and 2.4 respectively. In the next section, I will focus on all the fabrication techniques involved. 27

54 2.2 Fabrication Techniques Compared to the point contact method introduced in Chap. 2.1, a nanopillar with a schematic picture shown in Fig. 2.2, which is composed of a device unit with FM multilayers etched into a submicron lateral dimension, is more widely used in spin transfer experiments [13, 6, 16, 18]. Other than the central layers, this structure also includes an insulator around the pillar for necessary chemical protection, and top and bottom electrodes where both dc and ac currents can be injected into the pillar perpendicular to the film plane (CPP geometry). The main reason for having the nanopillar structure rather than a point contact is to obtain a well defined lateral geometry such that quantitative measurements can be conducted. The entire process of nanopillar fabrication includes a number of steps. In each step, many fabrication techniques are involved. For example, we grow metals and insulators onto a substrate using thin-film depositions, define lateral shapes of devices and electrodes using lithography, and remove materials using etching. In this chapter, I focus on those fabrication techniques Thin film deposition The purpose of thin film deposition is to grow material layers onto a substrate, usually an oxidized and polished Si wafer. Before loading a sample substrate into a thin film deposition system, a cleaning treatment to the substrate is essential, and the details of the sample cleaning is shown in Appendix B.1. In a thin film deposition system, materials which are used to deposit films 28

55 Figure 2.2: A schematic of a nanopillar device change from their original solid phase to the gas phase in order to accumulate on the surface of the substrate. It is composed of the following units introduced below. Ultra high vacuum system The ultra high vacuum (UHV) system is a key unit for thin film deposition. In a thin film deposition run, as source materials are heated up/bombarded, their atoms are excited to a higher energy state with a strong tendency to react. An ultra high vacuum (UHV) system, provides the best way to isolate samples and 29

56 Figure 2.3: UHV system in the cleanroom at NYU sources from air impurities and moisture. Those gas molecules, e.g. nitrogen, oxygen, water vapor, organic substances, should be removed or at least reduced to a sufficiently low concentration in the chamber, in order to reduce the chance of chemical reaction with source materials. Generally, a base pressure lower than 10 6 Torr was required such that the materials being deposited are pure enough. In addition, we also require the mean free path λm = kt /πd2 P of the source material atoms to be much longer than the distance between the material crucible and the substrate. Here d is the diameter of the evaporated atoms, T is the temperature and P is the pressure. For a deposition using evaporation, λm is at the order of hundred of meters at a typical evaporation pressure P = 10 6 Torr, therefore scattering due to gas molecule collision is not an issue. However in a sputter deposition, a typical pressure at several mili-torr of Ar would lead to the fact that λm is at the order of tens of centimeter. However, scattering 30

57 by noble gases like Ar also helps to thermalize atoms and result in denser deposited films with higher quality. Therefore, an optimization of Ar pressure regarding gas molecule scattering for a sputter deposition should be taken into consideration. Fig. 2.3 shows a photograph of the UHV system of NYU. All parts related to the UHV chamber must be sealed with Cu gaskets. The structure of the chamber is shown in Fig. B.1. The chamber is connected to a roughing pump, which pumps the chamber from room pressure down to a sub-torr regime, by removing the air mechanically. To get to a lower pressure, a Turbo molecular pump is used. The sieves of the molecular pump rotates at a high angular velocity ω, and at a certain radius r, molecules with the motion speed ωr are pumped out of the chamber. A molecular pump with this principle can continuously lower down the chamber pressure till the 10 8 Torr regime. Detailed steps of venting and pumping down the chamber are included in Appendix B.2. Thermal evaporation Thermal evaporation is the easiest and most straightforward way to deposit materials onto a substrate. By applying a high current into a thermal evaporator which is composed of a metallic boat that holds the material, Joule heating is generated in the boat. When the boat temperature reaches the melting/sublime temperature of the material inside, the material is evaporated onto the substrate above (Ref. e.g. [66]). The thermal evaporator is located at the back of the chamber shown in Fig Its leads need to be cooled down especially when the melting temperature 31

58 of the source material is relatively high. One needs to pay attention to the chemical reactivity between the boat and the material. We usually use a W boat to thermally evaporate Cu, and with our thermal evaporator, we can precisely control the evaporation of Cu with the rate between 0.5 and 3 Å/s. But W boat is not good to use with a Ni source since they can gradually intermix with each other at high temperature. If the required temperature is too high (>thousands of Celsius), thermal evaporation is not an efficient deposition method, and we need to use other ways. Furthermore, if the source material does not have a stable melting temperature (like some organic compounds), an automatic feedback control would be necessary. E-beam evaporation E-beam evaporation is another way of depositing materials by melting/subliming the materials, similar to thermal evaporation. However, the method to heat up the source in e-beam evaporation is quite different. High voltage ( kv) accelerates an electron beam emitted from the cathode to a high energy state, and a local magnetic field deflects and focuses the electron beam onto the material crucible to bombard the material. Such a process creates a local high temperature at the crucible and evaporates the material onto the substrate. E-beam evaporation only heats up the material locally. Therefore, it is ideal for material with relatively higher melting temperature. When operating the e- beam source, one needs to be very cautious and always keep away from the high voltage input for the e-beam evaporator. The two e-beam sources are located 32

59 on both sides of the chamber shown in Fig E-beam evaporation is ideal for most metals, like Co, Pt, Ni, Py, Fe and Au. For materials with lower melting temperatures or high vapor pressures, e-beam evaporation is not needed. E-beam evaporation can also deposit oxides, but it is not an optimal choice since a very stable rate is hard to reach and sputtering introduced next would be a much better method. One also needs to notice that the material should not chemically react or physically intermix with the crucible at high temperature. Professional information can be found at a Kurt Lesker handbook [67]. dc and rf sputtering Sputtering is a type of bombardment process by means of high energy ions [68, 66]. An ion can hit the target, and with a momentum and energy transfer from ion to target atoms, the latter can be dislodged from the source onto the substrate. An inert gas is used in sputtering as an ionized bombardment source in order to prevent chemical reactions. Since bombardment is a momentum and energy transfer process and the transfer efficiency is maximized when the masses of ion and target atom are equal to each other, ideally a gas with the molecule mass close to that of the sputtered material is chosen. Argon is widely used for this purpose, while helium is not preferred since its mass is too low compared with those of most source materials. dc sputtering is used to deposit conducting materials such as metals. In a dc sputter target, the material itself can be used as a cathode where a kv voltage 33

60 is applied with respect to the enclosing ground electrode. However, if the target is an insulating material, the target can no longer conduct steady state current, and an rf power (f MHz) is used. rf sputtering can be used for both metal and insulator deposition. Compared with dc, an oscillating rf potential applied between anode and cathode produces a better energy coupling to the electrons as well as a higher plasma density. The deposition rate of sputtering varies with the target geometry. Typically it drops rapidly in the first several runs after a new source is installed. It reaches a relatively stable regime afterwards, and the rate calibration can be used for a continual series of runs without repeated checks, while it is always good to check the rate change especially after the target has not been used for a while. Sputter rates of targets that I have used at IBM calibrated within the stable regimes are listed in Appendix B.3. These are all the methods of thin film deposition that I have used in my PhD research. There can be many other ways that one could establish in similar fabrication processes Lithography lateral shape definition Optical lithography and e-beam lithography are the two methods used in lateral shape definition. Optical lithography is a method which defines the shape through the alignment between the sample and a predefined pattern on a glass plate, known as an optical mask. It can be operated in many different ways. The one used in the optical lithography step that I have been involved in is a contact mask method, 34

61 which is conducted through a direct contact of the substrate and the mask. Photoresist, an organic chemical compound that is sensitive to UV lights, is evenly covered on the substrate, by spinning resist followed by hot-plate baking. Then the substrate is put underneath the mask. For a multi-step lithography, one needs to do the optical alignment between the mask and the features on the sample before they are completely in contact. Once the mask and the substrate is in contact, an ultraviolet (UV) light is turned on above the mask. The part of the photoresist which is exposed to the UV becomes cross-linked. The lateral shape of the photoresist is defined by developing the resist in a developer solution. For a regular positive resist, the part which was exposed to the UV light is dissolved and the unexposed part remains; while a negative resist works the other way around. In this way, a resist mask is formed and is used for succeeding deposition/etching steps. Regular contact-mask-based optical lithography creates features down to half a micron in size, while modern optical lithography employing a phase shift optical mask can create a resolution even in a sub-100 nm regime [69]. A commonly used way to generate sub-micron features (like the lateral dimension of the nanopillars) is electron-beam (e-beam) lithography, where e-beam resists that changes their chemical/physical form under an high energy electron exposure are used for pattern definition. An electron beam with the de Broglie wavelength (typically several nm) much shorter than that of UV, is focused on the sample to expose the resist, such that the feature resolution is not limited by diffraction in a sub-micron regime. Similarly, e-beam resists are also classified into positive and negative resists depending on their e-beam sensitive character- 35

62 istics. The pattern of the e-beam lithography is produced by developing exposed e-beam resist in appropriate developers. The recipes of optical lithography that I have used are listed in Appendix B Etching Many different methods can be used for etching thin films, based on the requirement of a specific processing step. Here I start with ion milling. Ion milling Similar to sputtering, ion milling is also an ion-beam bombardment process. By shooting high energy Ar ions onto the sample surface, atoms on the surface are dislodged after a momentum-energy transfer. Ion-milling could stop at any point as one wants. However, one needs additional approaches in order to control the end point. One simple solution could be visual inspection. During the deposition prior to this ion milling step, a glass coupon can be placed next to the substrate so that all layer stacks intended for etching in the ion milling is also deposited onto the coupon. During the ion milling, the etching can be stopped immediately when the coupon sample clears up. A more sophisticated way is to use a secondary ion mass spectroscopy (SIMS) detector in the ion mill chamber so as to know which layer the mill is etching through instantaneously and stop at the desired end point. Ion milling is an efficient etching technique for metals and oxides, but very slow for carbon and organic materials. Therefore photoresist is a material that 36

63 has a very low rate being etched by ion-mill. It therefore can be used as a mask to protect an underlying film from etching. Photoresist protects the underlying film, while the exposed part is etched away, leaving the structure with the designed lateral dimension. Ion milling is a type of directional etching, which could form a good edge shape. However materials being etched away redeposit at any location in the chamber. It might stay on the edge of the resist, as well as the sidewall of the structure being etched. For a tunnel junction with large resistance, a shallow angle milling step (with respect to the film plane) that cleans up the side wall of the etched structure is important in order to prevent a short circuit that goes along the edge of the junctions. In the pillar formation step of a subtractive process (to be introduced in section 2.4), I used a Veeco ion mill at IBM Yorktown that has a spectroanalyzer to control the stop layer. In a later electrode patterning process (to be introduced in details in section 2.3), I used the Oxford ion mill with a coupon film deposited on a piece of transparent glass as a reference for the end point. Wet etching Wet etching is another etching method, which has a relatively easier processing principle. In this method, a sample is dipped into a chemical solution, and the part dissolved by the solution is etched away. Developing, as mentioned in lithography, is also a wet etching for cross-linked photoresist. For example, HF is a common etchant for SiO 2, and the corresponding 37

64 chemical reaction equation is given in Eq. 2.1: SiO 2 + 4HF 2H 2 O + SiF 4 (2.1) By such a chemical reaction, SiO 2 is dissolved by the HF solution. The solution content, concentration and etching time are very important parameters. As most chemical solutions wet the sample, the sample should be taken out of the solution and rinsed in a flowing deionized (DI) water right after the etching stop point, otherwise the etching still continues with the solution left on the sample surface. A photoresist mask is ideal in ion milling, but it might not be as stable in wet etching, and since it can sometimes react with or become physically soft in chemical solutions. Therefore a wet etching step is sometimes combined with the ion milling. In the nanostencil process [5, 6] (to be introduced in section 2.3), a wet etching is preceded by an ion milling which forms a metal mask. In contrast to ion-milling, wet etching is usually isotropic. At the very beginning, the chemical solution etches the exposed part downward. As the etching goes deeper and deeper, it also etches the sidewall and creates an undercut. This is an important phenomenon utilized in the nanostencil process. Reactive ion etching (RIE) Reactive ion etching (RIE) is a process which is similar to both ion milling and wet etching. On one hand, there is ion bombardment, while on the other hand chemical reactions take place. The etching directionality is also between ion 38

65 milling and wet etching. If we need to etch SiO 2 without a metal mask on top, the best way is by RIE where the gas flow is composed of Ar as an ion source and CF 4 as a chemical reactant. Eq. 2.2 describes the chemical reaction involved: SiO 2 + CF 4 CO 2 +SiF 4 (2.2) In this way, both CO 2 and SiF 4 can be pumped away from the chamber, yielding a more efficient etching. In a similar example, RIE is also used as a touch-up step to etch away remanent Cr at bottom of nanostencil openings. The chemical equation is shown in Eq Cr + 2O 2 + CF 4 CO 2 +2CrOF 2 (2.3) Here, I also introduce two commonly used RIE methods for etching organic residuals: O 2 ashing and CO 2 RIE. They both help oxidize organic residuals left on the sample surface very quickly without affecting inorganic compounds. The former is often used in sample cleaning, and the latter is widely used in e-beam resist mask formation of a subtractive process. 39

66 2.3 Nanopillars fabricated in a stencil process Nanopillars may be fabricated using a nanostencil or a subtractive process. We focus on the nanostencil process [5, 6] in this section Stencils with standard size opening The key idea of forming a nanopillar structure using a nanostencil process is to deposit layers into a pre-defined stencil opening. It is a practical approach since one can pre-fabricate the stencil structure, and then do subsequent film deposition and electrode patterning using only optical lithography. This significantly reduces the material exploration s cycle time related to fabrication. The original step in such a process is to make a stencil with submicron sized holes. As the current flows from the top leads into the pillar junction and comes out through the bottom lead, it is necessary to deposit base layers first, as is shown in Fig. 2.4(a). Since the sample will experience many fabrication steps, it is essential to start deposition with an adhesion layer on a clean Si substrate that has an oxidized layer on top. A cleaning recipe is listed in detail in Appendix 2, and ideal adhesion materials include Ti, Ta and evaporated Pt. A base layer is grown on top of the adhesion metal. The base layer is required to be a good conducting layer, since a uniform current distribution in the pillar is needed. A thick Cu layer (>150 nm) is usually used as a base layer, and it is capped with an inert Pt layer for protection. An insulating layer deposition is deposited directly on this base layer. Usually SiO 2 or Al 2 O 3 is chosen for layer isolating the pillar from the metals. Shown 40

67 Base Layer Deposition SiO 2 and Pt Layers PMMA Spin (a) Wet Etching Form Undercut (b) Ion Milling (c) E-beam Lithography (f) (e) (d) Figure 2.4: Step flow of stencil opening in Fig. 2.4(b), an inert metal layer (Pt) is grown on top of SiO 2. Then after a spin and bake of PMMA as the e-beam resist, a e-beam lithography step shown in Fig. 2.4(c) and (d) opens PMMA layer with accurate sizes that define the lateral dimensions of the pillars. Ion milling shown in Fig. 2.4(e) is done after the PMMA mask is formed. As the ion mill etches PMMA much more slowly than metals and oxides, the part underneath PMMA is protected, while the exposed part is etched in the same direction as the ion beam. This step etches through Pt and stops inside SiO 2 layer. 41

68 Figure 2.5: A shallow angle SEM image of the stencil with an undercut. Reprinted from Ref. [5] with permission of J. Z. Sun. A wet etching step shown in Fig. 2.4(f) is used to open the stencil holes. After the wet etching using a mixed solution of HF and HCl that dissolves SiO 2, the remaining SiO 2 is etched away. An undercut is formed, and the lateral dimension of the pillars are defined by the sizes of the Pt opening on top. A wet etching is followed by a DI water rinse to remove the remaining solution. A N 2 dry blow and a vacuum dry step are used to remove the moisture that may contaminate the stencil surface. Fig. 2.5 shows an SEM image of a stencil taken from a shallow angle. The undercut can be seen from the side of the broken edge. Fig. 2.6 gives an SEM top view of the stencil openings. The lateral dimensions of the openings are very accurate and they all have the square shape. Pt layer grains both on top and at the bottom of the openings can also be clearly seen. 42

69 Figure 2.6: Top view SEM image of an array of nm 2 stencil openings. (Courtesy of J. Z. Sun) Nanopillar layer deposition After the stencil is ready, we deposit the multilayers pillar structure into the stencil opening. There are several crucial things involved: The first metallic layer As Pt is on the top of the base electrode contact, we start with a thin Cu layer. This is because Pt/FM interface generates a perpendicular magnetic anisotropy to the FM layer magnetization. Therefore, a 10 nm Cu layer is usually inserted between the FM and Pt. 43

70 Figure 2.7: Layer structure of the stencil being filled up with effective layer stack, shown with a schematic (a), shallow angle view (b) (reprinted from Ref. [5] with permission of J. Z. Sun) and a TEM image (c) (reprinted from Ref. [6] with permission of J. Z. Sun). Total thickness The total thickness of the multilayer to fill the stencil holes is very critical, as the stencil openings on samples that we use are typically only 75 nm in depth. For a usual Co/Cu/Co bilayer structure, the thicknesses of the free and fixed layers are 3 and 12 nm respectively. A 10 nm Cu intermediate layer is deposited to prevent strong exchange coupling between two FM layers, as introduced in Chap. 1. Including the 10 nm Cu underlayer, the total thickness of the simplest spin valve structure already adds up to 35 nm. For devices that incorporate complicated layer structures, for example antiferromagnetically coupled exchange bias layers or synthetic free and fixed layers, the total thickness which 44

71 is needed might be even more. Therefore, one needs to plan the layer stack design such that it matches with the stencil hole depth. Fig. 2.7(b) shows the picture when effective layers are deposited into the stencil holes. Magnetic residuals out of stencil holes Layers are grown in stencil openings as well as on top of the Pt layer outside. Therefore magnetic layer residuals remain there even after the sample is completely processed. Once the effective stack is deposited, one needs conducting material such as Cu to fill up the stencil holes. As the Cu resistance is much less than that of the magnetic residuals, most of the current flows in the Cu rather than the magnetic residuals. Therefore, the magnetic residuals do not greatly affect the device resistance. However, magnetic residuals affect the local field distribution through a dipolar field they generate. As a result, this dipolar field also contributes to the field acting on the magnetic layers in the pillar structure. This field acting on the free layer is important because this layer has a relatively small coercive field. Therefore, we usually deposit the free layer closer to the bottom of the stack. Fig. 2.7(a) shows schematic of the structure when the stencil is filled up. Fig. 2.7(c) shows a TEM image of the structure. Wedge growth mechanism We can vary the thickness of a particular layer systematically across a wafer. This is important because the physical properties of stencils may vary slightly. So here I introduce a wedge growth mechanism that I have used which enables 45

72 Figure 2.8: (a): Linear motion shutter. (b): Schematic of layer deposition on a stencil with a wedge growth mechanism. this type of deposition. Fig. 2.8(a) shows a linear motion shutter, the crucial part in the wedge growth mechanism. Different from regular shutter which can open or close which determines whether the wafer is exposed to or shadowed from the deposition beam, the linear motion shutter can be engaged with a linear motor to move at a constant speed. Suppose the deposition rate is stable and the linear motion shutter gradually opens up until the entire wafer is exposed to the beam. The wafer on one side consequently has the thickest deposition while the other side has the thinnest. A uniform gradient of the layer thickness is produced on the wafer. A drawing of this type of growth is shown in Fig. 2.8(b). 46

73 2.3.3 Four-level electrode patterning After the effective layer deposition, the nanopillar is already formed. However, the sample is still far from that needed for transport measurements, where patterned bottom and top electrodes are required. Furthermore, individual pillars must be isolated so that current flows only through one pillar. Although only the two-layer electrodes are patterned, the processing steps involve four levels, which are called bottom level patterning, meso level metal insulation, via level insulator opening, and top level patterning. These levels are introduced in the next section. Fig. 2.9 describes the basic flow of the four-level patterning. The bottom electrode is patterned first, such that junctions with different bottom leads are isolated. However junctions sharing the same bottom electrode are still connected through the Cu top layer. A next meso-level etching should stop at the SiO 2 layer (Fig. 2.9(b)) in order to separate each junction. Then a thick SiO 2 layer grows on top of it to prevent shorts between top and bottom electrodes (Fig. 2.9(c)). A via-level etching through the thick SiO 2 layer into the top of the pillar makes available the future contact between the pillar and the top metal to be deposited (Fig. 2.9(d)). A top metal is grown on top of it (Fig. 2.9(e)), followed by a top-level electrode etching. Fig is a mm 2 image of a fully patterned regular site with the electrode layout. Each stripe contains 14 junctions that share the same bottom electrode (vertical electrode in Fig. 2.10). Each junction is located at the crossing of the top and the bottom electrode. This layout is ideal for a four-point resistance measurement since current flows from one top lead into the 47

74 (a) (b) (c) (d) (e) Figure 2.9: A sketch of the pillar junction in the four level processing. (a): a bottom level electrode patterning with the etching down to the Si substrate. (b): a meso-level etching to insulate adjacent junctions. (c): a SiO 2 deposition to prevent shorts between top and bottom electrodes. (d): a via-level opening through the SiO 2 layer. (e): a top level electrode patterning. Color legends: yellow: Cu; blue: the free FM layer; navy: the fixed FM layer; dark grey: Pt; light grey: SiO 2 ; light sky-blue: Si substrate. junction and comes out through one bottom lead, and the two other electrodes are used for voltage measurement. Here I introduce the detailed steps of the four-level processing. Bottom level and meso level etching Etching of bottom and meso level are relatively easy, and a schematic in Fig (from (a) to (f)) shows those steps. A standard developing duration time ( 7 sec after the photoresist is visually dissolved) was used in the bottom level 48

75 Figure 2.10: Top view optical microscope image of one patterned stripe of regular junctions. lithography. The ion-milling step in base level will etch the sample all the way down through the bottom layer to the Si substrate. The vertical electrode in Fig is etched in this step, and the typical width of the bottom electrodes of junctions shown in Fig is 40 µm. Lithography of the meso level is slightly more tricky, since the resist mask in this step is composed of some isolated 20 µm sized square islands. Therefore adhesive promoter is needed so that those islands stick well to the sample without floating away from their original positions. An organic compound called fast HMDS was used before spinning resist. Or else, we need to make sure the sample is completely cleaned up before resist spinning. The developing time needs to be slightly shorter than that in the base level lithography, because if 49

76 Photoresist 1811 Bottom level lithography Ion Milling (a) (b) (c) Photoresist 1811 Resist Strip Meso level lithography Ion Milling (d) (e) (f) Figure 2.11: Processing steps of the bottom level and the junction level etching. (a): layer structure before the four-level processing. (b): photolithography for bottom electrodes. (c): bottom level ion milling. (d): photoresist removal. (e): meso level photolithography. (f): meso level ion milling. Color legends are the same as those in Fig. 2.9, and the red color represents the photoresist. the corners of these meso level islands are etched too much, the optical alignment of the succeeding via level lithography that is defined inside those squares would become very challenging. The ion-milling step of the junction level needs to stop on top of the SiO 2 layer. It is OK to overetch slightly, but the etching cannot go as deep into the base layer metal, since conductivity of the bottom electrode would otherwise be reduced. It might be hard to see the meso level islands in Fig. 2.10(a), however 50

77 Resist strip SiO 2 deposition Photoresist 1811 Via level lithography (a) (b) (c) RIE Etching with CF 4 Resist strip Top metal deposition (d) (e) (f) Figure 2.12: Processing steps of via level etching and top metal deposition. (a): removal of the meso level photoresist. (b): deposition of the thick SiO 2 layer. (c): via level photolithography. (d): reactive ion etching to open up top of the SiO 2 layer. (e): removal of the via level photoresist. (f): deposition of the top level metal. they can be clearly seen in Fig. 2.10(b) when the central area is amplified. At each crossings of the top and bottom electrodes, there are two squares with one enclosing the other. The larger squares are ones formed by the meso level etching. The size of the junction level square defines the size of the magnetic residuals extended out of the stencil openings. We will use this size to estimate the dipolar fields contribution from these residuals. 51

78 SiO 2 deposition and via level etching rf sputtering is then used to deposit SiO 2. This step, shown in Fig. 2.12(b), is made to further insulate the top and bottom electrodes, particularly from the metallic contact at the edges of the bottom electrodes. It is necessary to have a very thick SiO 2 layer ( 200 nm) so that the whole stack is submerged in the SiO 2. Via level lithography (Fig. 2.12(c)) is the most challenging part, since we use the same resist and an opaque metal optical mask in this step. Therefore, only the part to be etched, which is extremely small, are visualized in the optical alignment, and all the lithographic structures need to be located in their corresponding meso level squares. A position mismatch of the via level lithography should not be more than several microns across the whole cm wafer that contains hundreds of junctions. There is no trick to do this step. It is just necessary to make accurate enough alignments with marks located at the corners of each site. Furthermore, a perfect alignment in the meso level lithography also makes the via level lithography easier. Via level developing needs to last several seconds longer than standard developing step. This is necessary because we need to make sure that all via holes are opened up. This step is especially critical to high-speed sties which I will show in the next paragraphs, since the via holes of the high-speed sites are only 4 µm in size. Via level lithography is followed by a RIE etching of SiO 2 (Fig. 2.12(d)) using CF 4. After this step, SiO 2 is opened up on top of the junctions, creating a channel ready for metallic contacts. Via openings can be seen in Fig. 2.10(b) 52

79 with the squares enclosed by the meso level squares. Each via level square needs to be inside the corresponding junction level square, otherwise electrical short at the side of the layer structure will occur. Top metal deposition and top level etching A top metal deposition follows the via etching shown in Fig. 2.12(f). Top layer lithography and etching are not shown in Fig since the lithography pattern is in an orthogonal direction. But the steps are basically the same as those in a bottom level. A glass coupon with the same thickness as the top layer is used in the top level ion etching step. After the coupon is cleared, an additional 5 min normal angle etching is needed to clean up the metal that aggregates on the sidewall shown in Fig. 2.12(f). After a resist strip, the sample is basically ready for measurements. High speed sites Fig shows a sample with regular electrode design used in low frequency transport measurements. However, if we want to do high frequency measurements (f > GHz), power loss would occur in electrodes before the rf power reaches the junction. Therefore, an electrode layout that meets the impedance matching requirements at high frequencies needs to be designed. Fig shows the sample layout with this design. The two pads on the right are the bottom leads, and the top leads located on the left go through a wiggle on top of the junction. The electrodes are much shorter and each 53

80 Figure 2.13: Top view optical microscope image of one patterned stripe of high speed junctions. junction has its separated bottom and top electrodes. Junction level and via level squares are even smaller (designed as 8 µm for junction and 4 µm for via level). They are more like circles rather than squares because of the limitation set by the optical lithography. I usually pattern half of a cm sized wafer with regular sites and the other half with high speed sites for my measurements, so that transport measurements at low frequency and high frequency can be compared. 54

81 2.4 Sample fabrication with a subtractive process In the previous chapter, I discussed nanopillar fabrication with a nanostencil process in detail. This is a very powerful method and most of the samples I used were fabricated that way. There is another widely used subtractive method, which was first presented in Ref. [13]. In the final years of my PhD research, I made efforts in adapting an IBM subtractive recipe (pending patent YOR US1) to fabricating metallic spin valve nanojunctions for my measurements. Compared with the nanostencil process, the biggest benefit of fabricating samples with a subtractive process is that the extended magnetic residuals are completely removed. dipolar field effects form the residuals are no longer an issue. As a result, Therefore, a subtractive process may be preferred for this concern. The basic idea of the subtractive process is to deposit the magnetic multilayers first and then etch those layers into structures with lateral confinement, followed by electrode patterning. So here I begin with the layer deposition Base layer and nanopillar layer deposition Layer deposition in a subtractive process should also begin with the base layer. On one hand, to create a good conductor under the nanopillar, a thick enough bottom layer is needed; on the other hand, a thick base layer leads to a rough surface. Therefore, a laminated base layer is grown as shown in Fig. 2.14(a). The layer sequence should start with the Ti or Ta adhesion layer, while Ti or 55

82 Base layer deposition (Laminated layers) Nanopillar layer deposition (a) (b) Figure 2.14: Base layer and nanopillar layer deposition Ta also reduces the roughness of the metallic layers. A laminated bottom metal with a repetition of [5 nm Ta 30 nm Cu] for 5 times is usually used in my samples. Different from those in the nanostencil process, magnetic multilayers are deposited right after the base layer deposition in a subtractive process (Fig. 2.14(b)). We also want the nanopillar layers to be as thin as possible, since the less material that needs to be etched, the thinner the resist that is required in the pillar definition. A too high resist of a submicron lateral dimension has more chances of falling over. 56

83 For example, in a Co/Cu/Co bilayer junction, the thickness of each layer still needs to be 3 nm, 10 nm and 12 nm respectively. The thick Co layer can now be on the bottom, which provides the option either to etch through the thick FM layer or not. The advantage of not etching the thick FM layer is to make the pillar formation step easier, while the thick FM layer also remains extended all over the bottom electrode. The Co layer is very easily oxidized in air, therefore one must put an inert layer on top. Therefore, a thin Cu layer is deposited on top of the Co layer followed by Pt, to protect the stack. As the result, we deposit 7 nm Cu and 2 nm Pt on top of the Co layer. Still, the general rule is the thinner the etched layers, the easier the processing. The total thickness of the stack to be confined through etching is also critical for some later processing. It needs to be at least 10 nm lower than the thickness of the SiO 2 to be deposited around the etched pillar. If larger, part of the pillar will be exposed to air and get oxidized in later processing steps. My trials of adapting the subtractive process recipe to spin valve fabrication met with drawbacks at this point. Thus, this is very important. If they are too close to each other, there will not be any height contrast for one to do optical alignments Double layer e-beam lithography Beginning from here I no longer show the lamination of the base layer indicated by different colors in figures. If we use the same e-beam pattern as in the stencil process, we need to use negative e-beam resist. The commonly used negative e- beam resists are NEB and HSQ. NEB is an organic compound which is soft and 57

84 NEB Spin and Bake HSQ Spin (a) (b) E-beam lithography of HSQ Uniaxial RIE Etching of NEB (c) (d) Figure 2.15: HSQ e-beam lithography and NEB RIE etching easily dissolved in solvents. But it is extremely sensitive to the condition of the e-beam exposure, therefore the lateral sizes defined through e-beam lithography with NEB might differ between different runs. Lateral definition of HSQ is much more stable. However it forms solid structure after being dried. No solvent can remove it and the contact with metals could cause fracture at the interface which tends to peel off the entire film underneath. Therefore, we use a combination of NEB and HSQ in our e-beam lithography. Shown in Fig. 2.15(a) and (b), we firstly spin NEB. Diluted NEB 31A3 was used and the total thickness was 160 nm after baking. On top of NEB, we spin 58

85 Figure 2.16: SEM image (25 o off normal direction) of the double layer resist on top of a sample (taken under the guidance of J. Z. Sun). HSQ ( 30 nm). Then the sample is loaded into the e-beam lithography system for exposure. After this step is done, the sample is dipped into the LDD T M -26M developer to form the HSQ mask only, as in Fig. 2.15(c), (NEB is not developed in this solution). A DI water rinse and a N 2 dry blow follow that step. The next step is to etch NEB with the HSQ mask. Here, we used a uniaxial RIE system. It is important to use a uniaxial RIE chamber since an isotropic RIE could create an undercut beneath HSQ, which causes instability of the double layer resist. Since NEB is an organic compound, CO 2 is used as the reactant and it does not affect metals. A roughly 6 min CO 2 RIE etches the NEB away into the shape presented in Fig. 2.15(d). Using this double layer recipe, the shortcomings of both HSQ and NEB are 59

86 avoided. Here the lateral shape is defined by HSQ lithography, which is robust compared to NEB; the contact layer to metal is NEB, which is soft and does not cause interface fracture like HSQ does. Fig shows an SEM image of the double layer resist on top of a sample with metal depositions. This photo is taken at 25 away from the normal direction, and the elongated shape is due to this angle Ion etching using the resist mask Since the layer to be etched by ion-milling to form a nanopillar structure is very thin, it is not recommended to use a coupon film on glass. The visual judgement of whether the coupon is cleared would be tricky, and it is very easy to over/under-etch by 10 nm. 10 nm makes a big difference in this step. Furthermore, it is not feasible to prepare a coupon like that, since one might need to grow the stack on the glass coupon in a separate run provided the deposition rate is well calibrated. Therefore, if possible it will be better to use an ion mill with a secondary ion mass-spectroscopy (SIMS) end point detector, which could analyze what material is being etched at any time. Normal angle ion-milling should stop at the planned layer (Fig. 2.17(b)) shown by the detector. As was mentioned in the previous chapter, material removed by ion milling is very easy to aggregate on the sidewall of the formed pillar, shown with orange color in Fig. 2.17(b). This effect creates additional conduction channel between top and bottom leads, and it is even more serious in a magnetic tunnel junction since a metallic contact creates a short circuit. To prevent this, we need to 60

87 Load sample with resist mask into ion mill (a) Normal angle milling till the stop layer Shallow angle milling to clean up sidewalls (b) (c) Figure 2.17: Ion milling to form the pillar structure add a shallow angle ion milling as in Fig. 2.17(c). The estimated shallow angle milling time should be at least half of the normal angle milling time and no more than twice of it. Too long a shallow angle ion milling time would result in a shape distortion of the pillar SiO 2 deposition, lift-off and metal capping An immediate loading into a SiO 2 sputter chamber must follow the ion-milling step. The etched pillar cannot be exposed to the atmospheric pressure with moisture for more than 10 min. After the chamber is pumped down, and rf 61

88 Load into sputter chamber SiO 2 Deposition (a) (b) Lift off Metal contact deposition (c) (d) Figure 2.18: SiO 2 deposition, lift off and metal capping steps sputtering of SiO 2 needs to be done to isolate and protect the pillar. Shown in Fig. 2.18(b), the thickness of the deposited SiO 2 layer needs to be 10 nm higher than the thickness of the etched layer. This is because we need height contrast for later optical alignments. However, it also should not be much higher, since the thicker the SiO 2 layer, the more difficult the following lift-off step will be. A lift-off step (Fig. 2.18(c)) comes immediately after the SiO 2 deposition. The detailed recipe includes a cotton wiping rinsed by pure acetone and an half an hour acetone ultrasonic buzz. Here, the NEB layer is dissolved by the 62

89 Figure 2.19: SEM image of the pillars after the lift-off step acetone, and HSQ together with SiO 2 on top of it is removed. Only the top of the pillar is exposed. It is not recommended to use any type of alcohol solvent in this step, since alcohol strongly absorbs moisture. Any moisture left on top of the pillar may cause fast oxidization of the pillars. After the acetone buzz, one can do a vacuum drying by placing the sample into an O 2 asher chamber and pumping it down for 5 min. An SEM inspection might be necessary to see whether the resist is completely lifted off. Fig shows an SEM image of a sample after a successful lift-off. After the SEM inspection, it is recommended to do a 150 W 2 min O 2 ashing step to remove all of the organic residual which tend to stay on the sample. In this way, there is a good contact on top of the pillars. 63