PHYS-E0424 Nanophysics Lecture 2: Lithography

PHYS-E0424 Nanophysics Lecture 2: Lithography 1

Course Overview 19/9 Nanophysics: An introduction 26/9 Nanofabrication: Lithography 3/10 Nanofabrication: Self-organization 10/10 Nanoscale characterization: Microscopy, diffraction etc. 17/10 Physics of nanoscale objects: Carbon nanostructures 24/10 No lecture 31/10 Visit to Murata (MEMS manufacturer in Vantaa) 7/11 Nanoelectronics 14/11 Nanomagnetism 21/11 Spintronics 28/11 Student presentations on nanophysics topic 5/12 Student presentations on nanophysics topic 2

Course Organization No exercises and exam Instead: - Attendance - Weekly article on lecture topic - Essay on a nanophysics topic - Topics will be provided, but can also be suggested by students - Submission deadline for essay: 23-12-2016 - Short presentation on nanophysics topic (about 15 minutes) 3

Weekly Article on Lecture Topic - Article will be sent to students on the lecture day (at the latest) - Students read article - Students send e-mail to sebastiaan.van.dijken@aalto.fi on Friday before the next lecture (at the latest) with: (a) A question about the lecture or article or (b) A comment or suggestion related to the lecture or article or (c) Additional material about the lecture topic For example: news item, article, video, cartoon, experiment, own research result In the beginning of the next lecture a selection of responses will be discussed 4

PHYS-E0424 Nanophysics Lecture 2: Lithography 5

Lithography Lithography (from Greek: λίθος lithos, stone + γράφω grapho, to write ) is a means for transferring patterns onto a substrate Lithography stone with Princeton University motif Kathryn Polk, The Legacy 6

Lithography methods Lithography is top down approach for patterning of micro- and nanostructures Bottom up approach : Self-organization (lecture 3) - Photolithography - Extreme UV lithography - X-ray lithography - Electron beam lithography - Focussed ion beam (FIB) lithography - Nanoimprint lithography - Scanning probe microscopy (SPM) lithography 7

Photolithography Patterns are transferred from a glass mask to a photoresist layer using exposure to UV light + removal of the weak areas in the photoresist layer by wet chemical development Typical photolithography steps 1. Wafer cleaning and drying 2. Wafer priming (promotes adhesion of resist coating) 3. Spin coating of photoresist 4. Soft bake (80-100ºC, reduces thickness of resist and decreases development rate) 5. Exposure to UV light through optical mask 6. Hard bake (optional) (removes more solvent) 7. Development of resist (removes weak areas in photoresist layer) 8. Hard bake (optional) (hardens photoresist, removes remaining traces of developer) 8

Resist 3 types of photoresist - Positive: Pattern in resist is the same as mask. Exposure to UV light degrades the polymer and this enhances the solubility of the photoresist in developers. - Negative: Pattern in resist is the inverse of the mask. Exposure to UV light polymerises the photoresist which makes it harder to dissolve in developers. - Combination: Resist that can be used for both positive and negative pattern transfers. light light positive resist negative resist 9

Positive Resist - Mostly used in ICT industry - Superior to negative photoresist - No swelling during development - Higher resolution - Resistant against plasma processing Primary components of positive photoresist - Non-photosensitive resin: glue-like compound that is solid in its undiluted state (usually novolac). - Solvent: Liquid chemical that is used to dissolve the resin. Allows the resin to be applied in a liquid state (spin-coating). E.g.: n-butyl acetate, xylene, 2-ethoxyethyl acetate. - Photoactive compound (PAC): The PAC inhibits dissolution in positive resists before exposure to light. After exposure, the PAC promotes dissolution of the resin. 10

Negative Resist - Unexposed areas of resist are dissolved during development. - Negative photoresist developers swell the resist, allowing for uncross-linked polymer chains to untangle and wash away. - Swelling of the resist during development is the largest contributor to loss of features and linewidth limitations. Primary components of negative photoresist - Non-photosensitive resin: About 80% of solid content. - Photosensitive cross-linking agent: About 20% of solid content. - Solvent: Liquid chemical that is used to dissolve the solid components. Usually a mixture of n-butyl acetate, n-hexyl acetate, and 2-butanol. Fraction varies. 11

Resist Sensitivity and Contrast The most important parameters characterizing a resist layer are sensitivity and contrast - Sensitivity: Intensity of radiation (dose) required to cause a sufficient modification of the resist to ensure that the desired pattern appears at the development stage. - Contrast: Measure of the solubility rate of the resist in its developer. Small features are transferred better when the contrast of the resist is large. Contrast ( ) 1 D log D 100 0 12

Photolithography Processing Two primary techniques to generate a pattern by photolithography - Etching: After exposure/development of the resist, the film/wafer is etched. Subsequent stripping of the resist layer leaves a pattern in the film/wafer. - Wet chemical etching - Plasma etching - Reactive ion etching - Ion milling - Lift-off: After exposure/development of the resist, a film is deposited onto the wafer. Subsequent stripping of the resist layer leaves a pattern in the film. 13

Etching resist film wafer thin film deposition and spin coating of resist film wafer exposure and development of resist wafer etching (dry/wet) wafer removal of resist - Photoresist has same polarity as final film - Resist never touches the substrate wafer 14

Lift-off resist wafer spin coating of resist exposure and development of resist wafer wafer deposition of film wafer removal of resist - Photoresist has opposite polarity as final film - Excess film never touches the substrate wafer 15

Etching Parameters Etching is characterized by - Etch rate (nm/minute) - Selectivity (S): Difference in etch rate of materials in a multilayer stack. High selectivity can be used as an etch stop. - Anisotropy (A): Difference in vertical and lateral etch rate. Lateral Etch Rate A 1 Vertical Etch Rate - Under cut: Difference between the line width in the resist and in film/wafer. If 0.8 m lines result from 1 m photoresist lines, the etch bias equals 0.1 m. 16

Etching Techniques - Wet chemical etching: Removes material by chemical reactions in a liquid (e.g. HF for SiO2) - Advantages: Cheap, limited damage due to pure chemical nature, high selectivity. - Disadvantages: Low anisotropy, temperature sensitivity, chemical disposal issues, difficult to use with small feature (bubbles etc). - Plasma etching: Removes material by the creation of a plasma. A plasma is a gaseous collection of ions, energetic molecules, and neutral gas species that is created by the application of electromagnetic fields. (e.g. CF 4 for Si) - Advantages: Can be selective, moderate anisotropy control. - Disadvantages: Ion damage, residue. - Reactive ion etching (RIE): Removes material by the creation of a plasma at lower pressure. - Advantages: Can be selective, high anisotropy control. - Disadvantages: High ion damage, residue. - Ion milling: Removes material by ion bombardment (e.g. Ar). - Advantages: Extremely anisotropic, independent of material composition. - Disadvantages: Very high ion damage, nonselective, residue. 17

Exposure 3 different systems for UV exposure - Contact: Mask is in contact with resist during exposure. - Advantages: Inexpensive equipment, moderately high resolution (0.5 m). - Disadvantages: Contact with resist wears mask, particles and dirt are directly imaged in the resist, loss of planarization results in non-uniform resolution, no magnification. - Proximity: Mask is almost in contact with resist during exposure. - Advantages: Inexpensive equipment, less wear on mask. - Disadvantages: Diffraction effects limit accuracy of pattern transfer (low resolution of 1-2 m, less repeatable than contact method, no magnification. - Projection: Mask image is projected and de-magnified to a smaller image (1:4 1:10). - Advantages: High resolution (0.065 m or better), almost no mask wear, mask defects or particles on mask are reduced in size on the wafer. - Disadvantages: Expensive and complicated equipment. 18

Resolution in Near Field (1) Near Field (contact or proximity configuration) Fresnel diffraction limit 2 W 1 g W W + W W W g D 19

Resolution in Near Field (2) Near Field (proximity configuration) Effect of increasing gap between mask and wafer If g 2 W The minimum feature size that can be resolved is W min k g where k is a constant that depends on the photoresist and development procedure ( 1). 20

Resolution in Far Field Far Field (projection configuration) Fraunhofer diffraction limit 2 W 1 g R 1.22 d f 0. 61 NA R is diameter of central peak d immersion technology R f 21

Depth of Focus Depth of Focus ( ): 2 NA large NA small NA - Increasing the numerical aperture (NA) will increase the resolution, but it decreases the depth of focus. - The depth of focus must be larger than the variations in surface height on the processed wafer. 22

Stepper Systems Projection optics - Can produce image smaller than object. - Lens does not have sufficient resolution to project image over whole wafer (300 mm diameter in ICT industry). - pixel count : field size/r min2. E.g. 1 cm 2 /(0.5 m) 2 = 4 x 10 8. - Requires mechanical translation steps of wafer under optics. wafer on stepping stage source mask condensing optics imaging optics 23

Comparison of Photolithography Systems 24

Resolution Enhancement R 1.22 d f 0. 61 NA Increase NA: NA nsin( max ) - Highly purified water between lens and wafer (emersion technology). Maximum NA is 1.0 for air and 1.3 for water. - Improved optics Decrease : - Mercury I line sources (365 nm) - ArF excimer lasers (193 nm) currently in use in industry - F 2 excimer lasers (157 nm) under development 25

Optical Lithography System parameters 1990 1995 1999 2002 2005 2008 NA 0.5 0.6 0.7 0.7 0.75 0.8 (nm) 365 248 248 248/193 248/193 193/157 node (nm) 500 250 180 130 90 65 field size (mm x mm) depth of focus ( m) 20 x 20 22 x 22 26 x 34 26 x 34 26 x 34 26 x 34 1.5 1.0 0.6 0.5 0.4 0.3 26

Extreme UV Lithography - = 13.4 nm - Reflective optics and masks - Low reflectivity of mirrors ( 70%) requires intense sources. - Requires resist that is sensitive to extreme UV radiation - Operation under vacuum conditions to reduce radiation absorption - High cost 27

Extreme UV Lithography 28

X-Ray Lithography - = 0.4 0.8 nm - Low level of diffraction - Not sensitive to dust or other organic contaminants - No suitable optics for projection lithography - Sources: proximity lithography instead - Laser-plasma, electron bombardment (point source, divergent beam) - Synchrotron (parallel beam) 29

X-Ray Sources Penumbra effect - Finite size of source results in a blurring effect due to beam divergence - Magnification of this effect depends on the gap between the mask and the wafer (typically 25 50 m) gs D : blur size S: source size D: distance between source and mask G: distance between mask and wafer 30

X-Ray Resist and Mask Resist - Low absorption coefficient - Resolution is determined by photoelectrons and/or Auger electrons that are emitted during absorption of photons in the resist - For 1-nm photons the emitted electrons have a mean free path of a few tens of nanometers Mask - Membrane mask that is transparent to x-rays - Combination of opaque (heavy elements such as Au) and transparent (e.g. BN) materials - Pattern written by e-beam lithography - Planarity and fragility problems 31

Summary - Photolithography - Resist - Lift-off/Etching - Extreme UV lithography ( = 13.4 nm) - Reflective optics - X-ray lithography ( = 0.4 0.8 nm) - Proximity lithography - Membrane mask 32

Books and Review Articles - The Science and Engineering of Microelectronic Fabrication Stephan A. Cambell - Fundamental Principles of Optical Lithography Chris Mack Video: ASML 33

Lithography methods Electron beam lithography 34

E-beam Lithography Lithography technique that uses an electron beam to write a pattern into a resist layer Setup consists of a scanning electron microscope (SEM) + e-beam lithography software (deflection control) 35

E-beam Sources and Spot Size E-beam sources - Thermionic emitters: Electrons are boiled off the surface by providing enough thermal energy to overcome the work function of the emitter - Field emitters: The application of a large electric field lowers the surface barrier and this results in quantum mechanical tunneling of electrons (field emission) - Photo emitters: Incident radiation provides enough energy to photo-electrons near the surface to initiate emission Electrons that are emitted are collimated or focused and accelerated to an energy of several tens of kv (typically 10 100 kv) Wavelength: (nm) 1.22 E(eV) E = 20 kev = 0.009 nm Spot size: 1 10 nm E-beam facility at Micronova 36

E-beam Resolution Factors that determine the resolution - Electron energy (electron scattering, penetration depth, secondary electrons) - Minimum current ( 5-10 pa) - Sensitivity of resist ( C/cm 2 ) - Resolution of the D/A card (16 bits = 65536 points) - Speed of the D/A card (minimum dwell time, 5 10 s) For example: PMMA sensitivity: 300 C/cm 2 Current: 10 pa Dwell time: 10 s Then, the area per pixel is (10 pa x 10 s)/300 C/cm 2 = 3 x 10-12 cm 2 or 3 x 10-4 m 2 Step size: (3 x 10-4 m 2 ) 0.5 = 1.7 x 10-2 m or 17 nm 37

E-beam Writing Time For example: Write 100 m x 100 m area Current: 10 pa Dwell time: 10 s Area per pixel: 3 x 10-4 m 2 Then, total number of pixels is about 3 x 10 7 Total writing time: 3 x 10 7 x 10 s = 300 seconds Trade-off between resolution and writing time Small current provides high resolution, but long writing time (drift) Solution: write large area structures at high current (e.g. electrodes) write small structures with small current (e.g. actual nanodevice) 38

E-beam Resist Resist types: - Positive: Polymer chains are cut when exposed to the e-beam. Examples: PMMA (poly methyl methacrylate), EBR-9 (acrylate based resist), PBS (Poly butene-1-sulphone), ZEP (copolymer of chloromethacrylate and methylstyrene). - Negative: Interchain links are formed during e-beam exposure. Examples: COP (epoxy copolymer of glycidyl, methacrylate and ethyl acrylate), Shipley SAL (consists of 3 componenets; base polymer, acid generator, and crosslinking agent). After e-beam writing, (1) the resist is dissolved in a liquid solvent and (2) a film is lift-off or a film/substrate is etched (similar to photolithography processing). 39

Substrate Electron-Beam Interactions Exposed area e-beam spot size - Forward scattering: Broadening of electron beam in resist layer, film, and substrate due to elastic and in-elastic scattering events. - Backscattering: Electrons scatter back from the film or substrate into the resist layer. - Secondary electrons: High-energy electrons generate secondary electrons. D. Kyser and N.S. Viswanathan, J.Vac.Sci.Technol. 12, 1305 (1975) 40

Proximity Effect Proximity effect: The dose at a given point depends on the density of pattern features at that point. 41

Proximity Effects Strategies to limit proximity effects - Use of low electron energies: This can only be done at the expense of resolution, since the beam divergence due to forward scattering will increase. Also, due to chromatic aberrations, it is more difficult to focus a low energy beam. - Use of high electron energies: Backscattered electrons are diluted over a larger area, which limits their effect on the dose. - Dose calculations of patterns: Calculations can be used to locally correct the dose. Increases the complexity and required long computation times. - Use of resist that is only sensitive to high energies: Back-scattered and secondary electrons have lower energy than the initial beam. If the resist is not sensitive at lower energies, it will not suffer from exposure to back-scattered and secondary electrons. Resolution of e-beam lithography with PMMA resist is about 10 nm 42

Projection E-Beam Lithography Advantages of projection e-beam lithography: Much faster and larger writing areas than direct writing e-beam lithography Challenges of projection e-beam lithography Masking: Because of the high level of electron absorption in matter, it is difficult to find a suitable masking material that would have adequate contrast and transparency to allow reasonable exposures and still have the mechanical stability needed for repeated use. 43

Lithography methods Focussed ion beam (FIB) lithography 44

Focused Ion Beam (FIB) Lithography A high-energy ion beam is used to remove material from a film or substrate (resist not necessary). Direct writing (no mask) and ion projection lithography (stencil mask) techniques are in use. The ion beam is also used for high-resolution imaging. Here, the multichannel plate (MCP) is used to collect secondary particles. Ion sources: Liquid metal ion sources (LMIS). Available materials: Al, As, Au, B, Be, Cs, Cu, Ga, Ge, Er, Fe, H, In, Li, Ni, P, Pb, Pd, Pr, Pt, Si, Sn, U, and Zn. In FIB systems an large electric field (about 7 kv) is used to extract positively charged ion from a liquid Ga cone, which is formed on top of a W needle. Minimum spot size < 10 nm S. Reyntjens and R. Puers, J.Micromech. Microeng. 11, 287 (2001) 45

Focused Ion Beam (FIB) Imaging Milling Deposition S. Reyntjens and R. Puers, J.Micromech. Microeng. 11, 287 (2001) 46

Ion-Target Interaction Ion-target interactions can result in swelling, deposition, sputtering, redeposition, implantation, backscattering or nuclear reactions. For milling applications, sputtering of atoms near the surface is crucial. If the transfer of energy from the incoming ion to the surface atom exceeds the binding energy of the surface atom (3.8 ev for Au and 4.7 ev for Si), the atom is ejected as result. Sputtering yield (number of ejected atoms per incident ion (typically 1-50) depends on: - Mass of ion and target atom - Ion energy - Incident angle - Target temperature - Ion flux 30 kv As ions into Au 90 kv As ions into Au A.A.Tseng, J.Micromech. Microeng. 14, R15 (2004) 47

FIB Milling Electrical isolation in circuits by cutting metal lines It is possible to monitor the milling process using end point detection. The end point detector measures secondary electrons that are emitted during FIB milling. The yield of secondary electrons is material specific. Gas-assisted etching (GEA): Enhancement of the milling rate or material selectivity by the insertion of an etching gas S. Reyntjens and R. Puers, J.Micromech. Microeng. 11, 287 (2001) 48

Deposition Localized deposition of metals and insulators is possible using FIB instrumentation (CVDlike deposition technique) Process: (1) Precursor gas is sprayed onto the surface using a fine nozzle, (2) the ion beam decomposes the adsorbed precursor gas, (3) the volatile reaction products desorb from the surface and are removed through the vacuum system, while the desired reaction products (Pt, W, SiO 2 ) remain fixed on the surface as a thin film. The deposited material is not fully pure (contains organic contaminants and Ga + ion) The smallest features that can be deposited using FIB are of the order of 100 nm W line on a cylindrical surface S. Reyntjens and R. Puers, J.Micromech. Microeng. 11, 287 (2001) 3.3 m thick SiO 2 49

TEM Sample Preparation TEM samples prepared by FIB Kaito of Seiko Instruments Inc. S. Reyntjens and R. Puers, J.Micromech. Microeng. 11, 287 (2001) 50

FIB Milling of SPM Tips High-resolution AFM tips prepared by FIB 51

Ion Projection Lithography Ion projection lithography: - Much faster and larger writing areas than direct milling technique - Light ions such as H +, H 2+, or He + are used (limits mask erosion and heating) - Stencil mask consisting of a Si membrane - Resists like PMMA are used (similar processing steps as e-beam lithography) - Resist-less processing is possible with heavier ions - Disadvatages: Ion implantation, ion radiation can generate defects in the exposed film or substrate Ion proximity lithography Ion projection lithography J. Melngailis, J. Vac Sc. Technol. B 16, 927 (1998) 52

Lithography methods Nanoimprint lithography 53

Nanoimprint Lithography (NIL) High throughput, high-resolution parallel patterning method in which a surface pattern of a stamp is replicated into a material by mechanical contact and three-dimensional material displacement. Process: (1) Mold with nanoscale surface-relief features is pressed into a resist layer on a substrate at controlled temperature and pressure, (2) the mold is removed, (3) a plasma etch is used to remove residual layer and complete pattern definition, and (4) the substrate is patterned by standard lithography processes (etching/lift-off). NIL process steps Mold with 10 nm features Imprinted holes in PMMA S.Y. Chou et al., J. Vac. Sci. Technol. B 15, 2897 (1997) 54

Nanoimprint Lithography Molds - The mold is a solid material with high strength and durability (Si, SiO 2, SiC, SiN, metals, sapphire, etc) - Thermal expansion coefficient of the mold material is important because the imprint step typically requires a temperature of 70-90ºC - Thermal mismatch between the mold and the substrate can result in pattern distortions or stress build-up during the cooling cycle (Si mold on Si substrate makes a good pair) - The mold is fabricated using other lithography techniques (e.g. photo, e-beam, FIB) or NIL L.J. Guo, Adv. Mater. 19, 495 (2007) 55

Nanoimprint Lithography Resist - The resist materials used in imprinting should be easily deformable under an applied pressure, should have sufficient mechanical strength, and should maintain their structural integrity during the de-molding process - The resist material should have a lower Young s modulus than the mold and the pressure required to perform the imprint should be higher than the sheer modulus of the polymer - To complete the imprinting process within a practical timeframe, the resist material should have sufficiently low viscosity - Thermal plastic resists: Low Young s modulus and viscosity are obtained by raising the temperature of the polymer above its glass transition temperature (70-90ºC) Poly(benzylmethacrylate Poly(cyclohexyl acrylate) 10 days after imprinting L.J. Guo, J. Phys. D 37, R123 (2004) 56

NIL with UV-Curable Resist Step-and-flash imprint lithography (SFIL) - Room temperature processing possible due to the use of UV-curable resist - The resist is a multicomponent solution containing a photoinitiator, Si to provide O 2 -RIE resistance, a monomer to allow crosslinking, and a low-weight monomer to reduce the viscosity - Process: (1) The resist is dispensed on the substrate, (2) a transparent mold is pressed into the resist, (3) UV light is used to polymerize the resist, and (4) the mold is removed. X. Cheng et al., Adv. Mater. 17, 1419 (2005) D. Stewart et al., J. Microlith, Microfabr, Microsyst 4, 011002 (2005) 57

Lithography methods Scanning probe microscopy (SPM) lithography 58

Scanning Probe Microscopy (SPM) Lithography Scanning tunnelling microscopy (STM) Heinrich Rohrer Gerd Binnig First tool to image surfaces with atomic resolution G. Binnig and H. Rohrer, Rev. Mod. Phys. 71, s324 (1999) 1986 Nobel Prize in Physics (developed in 1981) 59

Scanning Tunnelling Microscopy (STM) Tungsten STM tip vacuum ev F tip sub. F T Tunneling probability (Simmons formula) E) exp 2d 2 ( 2 2m F ev E 2 60

Scanning Tunnelling Microscopy (STM) Si(111)-7x7 surface 61

STM Lithography - Resist exposure - STM-induced oxidation - Material deposition - Material removal (decomposition) and etching (in acid solutions) - Manipulation of single atoms 62

STM Lithography Resist Exposure - STM current modifies resist locally (write pattern into resist layer (e.g. PMMA)) - Lift-off or etching step Au-Pd pattern after lift-off (positive resist) RIE-etched pattern in GaAs (negative resist) M.A. McCord et al., J.Vac.Sci.Technol. B 6, 293 (1988) R.K. Marrian et al., J.Vac.Sci.Technol. B 10, 2877 (1992) 63

STM Lithography Oxidation - Diffusion enhanced oxidation in the tip-substrate area - Electric field accelerates oxidation process in the presence of oxygen - Demonstrated for Si, Ti, Cr, and Si 3 N 4 Oxidation of a Ti line using STM under ambient conditions K. Matsumoto et al., J.Vac.Sci.Technol. B 14, 1331 (1996) 64

STM Lithography Material Deposition - STM tip acts as miniature emission source - Atoms or nanoparticles are transferred from the tip to the target surface by the application of a voltage Deposition of Pt dots and lines using a Pt STM tip and voltage pulses A. Houel et al., J.Vac.Sci.Technol. B 20, 2337 (2002) 65

STM Lithography Material Removal and Etching - Materials can be removed by e-beam induced thermal decomposition in the field emission regime (e.g. SiO 2 on Si) - Semiconductors can be etched in a controlled way on a nanometer scale by using STM in the presence of chemical solutions SiO 2 removal using field emission from a STM tip Etched pattern in GaAs using STM in a dilute (0.05%) HFsolution H. Iwasaki et al., Nanotechnology 14, R55 (2003) L.A. Nagara et al., Appl. Phys. Lett. 57, 270 (1990) 66

STM - Manipulation of Single Atoms Tip - sample interaction modes - Pulling: The tip is positioned above an adatom and the brought down towards the surface, which increases the tunnel current. The tip is then moved horizontally, which decreases the tunnel current until the adatom hops towards the tip while remaining on the surface. - Pushing: Similar to pulling, but now the tip approaches the adatom horizontally. The tunnel current increases until the tip repels the adatom forward. - Sliding: The forces between the tip and the adatom are attractive. As the tip approaches the adatom, the tunnel current increases and the adatom jumps onto the tip and remains there. The adatom can be dropped at an accurately defined position by retraction of the tip. Pulling Pushing Sliding 67

STM - Manipulation of Single Atoms Xe on Ni(110) Fe on Cu(111) http://www.almaden.ibm.com/vis/stm/gallery.html 68

STM - Manipulation of Single Atoms Kanji characters for atom : Fe on Cu(111) http://www.almaden.ibm.com/vis/stm/gallery.html CO on Pt(111) 69

Scanning Probe Microscopy (SPM) Lithography Atomic force microscopy (AFM) normal force Contact mode: - Tip is in contact with the surface. - Measures the surface topography by keeping the deflection z constant, related to the force by F=k z - Allows access to lateral force by the deflection x. lateral force 70

AFM Lithography - Resist exposure (similar to STM lithography) - Thermally induced modification - Tip-induced oxidation (similar to STM lithography) - Material deposition (direct transfer or precursor decomposition) - Material removal by scratching - Dip-pen nanolithography 71

AFM Lithography Thermally Induced Modification - Imprint technique using heated AFM tips (150-200ºC) IBM Millipede computer memory based on nanoscopic pits burned into a thin polymer layer H.J. Mamin, Appl. Phys. Lett. 69, 433 (1996) 72

AFM Lithography Material Removal by Scratching - Mechanical material removal by direct tip scratching (metals, oxides, semiconductors) - In some cases the scratch step is followed by an etch step to prevent damage to the substrate Scratches in GaSb/InAs heterostructure Electrodeposited Pd lines on scratched + HF-etched SiO 2 /Si R. Magno, Appl. Phys. Lett. 70, 1855 (1997) J. Michler et al., Electrochem. Solid-State Lett. 7, A41 (2004) 73

AFM Lithography Dip-Pen Lithography - AFM tip is coated with a thin film of ink molecules - A water meniscus between the tip and the substrate acts as a bridge over which the molecules migrate R.D. Piner, Science 283, 661 (1999) Examples of molecular printing on a Au substrate 74

Summary - Electron beam lithography - Direct writing into resist and projection lithography - Proximity effects - Focussed ion beam (FIB) lithography - Direct writing and projection lithography - Ion implantation (defects) - Nanoimprint lithography - Scanning probe microscopy (SPM) lithography 75

Books and Review Articles - Focused Ion Beam Applications to Solid State Devices Shinji Matsui and Yukinori Ochiai, Nanotechnology 7, 247 (1996) - A Review of Ion Projection Lithography J. Melngailis, J. Vac. Sci. Technol. B 16, 927 (1998) - A Review of Focused Ion Beam Applications in Microsystem Technology, J. Micromech. Microeng. 11, 287 (2001) - Recent Developments in Micromilling Using Focused Ion Beam Technology Ampere A. Tseng, J. Micromech. Microeng. 14, R15 (2004) - Nanoimprint Lithography: Methods and Material Requirements L. Jay Guo, Adv. Mater. 19, 495 (2007) - Nanoimprint Lithography: An Old Story in Modern Times? A Review Helmut Schhift, J. Vac. Sci. Technol. B 26, 458 (2008) - Nanofabrication by Scanning Probe Microscopy Lithography: A Review Ampere A. Tseng et al., J. Vac. Sci. Technol. B 26, 458 (2008) 76

This Week s Articles 77

Next Lecture 3/10 Nanofabrication: Self-organization 78