Ion implantation Campbell, Chapter 5

Transcription

1 Ion implantation Campbell, Chapter 5 background why ion implant? elastic collisions nuclear and electronic stopping ion ranges: projected and lateral channeling ion-induced damage and amorphization basic components of ion implanters

2 Ion Implantation High Energy Accelerator Force on charged particle F qv x B Magnetic Field B 2mV qr 2 Implanted Dose Q 1 mqa T 0 I t dt 1. Ion Source 2. Mass Spectrometer 3. High-Voltage Accelerator (Up to 5 MeV) 4. Scanning System 5. Target Chamber m mass v = velocity V acceleration potential A = wafer area 2002 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Introduction to Microelectronic Fabrication, Second Edition by Richard C. Jaeger. ISBN

3 Why ion implantation? The depth distribution of the implanted species is adjustable and does not depend on equilibrium considerations, making it possible to create impurity atom concentration profiles that cannot be achieved in any other way Ion implantation is a low temperature process that replaces high temperature, long time diffusions The number and the chemical nature and purity of the ions that are implanted can be controlled to a very high degree A host of unusual interactions can occur as an energetic incident ion encounters the target atoms, resulting in new and useful materials modification processes Applicable to all materials semiconductors, ceramics, metals and polymers

4 Ion-solid interactions (1) In order to understand the ion-implantation process we must examine the physics of ion-solid interactions As the ion traverses the solid it will lose energy: Ion interaction with host atom nuclei -- nuclear stopping (the incident ion and target atom are viewed as hard spheres) Ion interaction with electrons -- electron stopping (the electrons in the target generate a viscous drag on the incident ion) The range is the total distance that an ion travels before it stops The range depends on the relative efficiencies of nuclear versus electronic stopping, so both need to be determined Stopping: de S Sn Se de Range: RE ( ) o dx n dx e 0 E o de de de dx S S 0 E o n e

5 Ion-solid interactions nuclear stopping Nuclear stopping assumes an elastic interaction between incident ion and substrate and dominates at low energies Atoms are treated as hard spheres with conservation of both energy and momentum before and after the collision b q (note that this simple picture is complicated if the projectiles have attractive or repulsive forces) f

6 Ion-solid interactions nuclear stopping Lindhard, Scharff and Schiott (LSS) performed the first quantitative calculation of nuclear stopping de Z Z M CN dx n M M Z1 Z2 1 2 (1= incident ion, 2 = target atom) ev/cm, where C LSS predicts a weak energy dependence with a broad maximum at intermediate energies (1 to 100 kev)

7 Ion-solid interactions electronic stopping Electronic stopping is much more complicated than nuclear stopping The motion of the incident ion is similar to a particle moving through a viscous liquid viscous drag LSS have also developed an expression for electronic stopping: de Z Z M Ce E k e E dx e M1M 2 Z 1 Z 2 M Electronic stopping dominates at high energies At low energies, log S e versus log E is approximately linear 32

8 Range and straggle incident ion R R p DR p DR l Range: the total distance that an ion travels RE ( ) o E o 0 Projected range (R p ): the distance the ion travels perpendicular to the surface Projected straggle (DR p ): uncertainty in R p de de de dx S S Lateral straggle (DR l ): uncertainty perpendicular to R p 0 E o n e

9 Conservation of momentum: p i p t p 0 Conservation of angular momentum: L i L t L 0 p 0 b p i Conservation of energy: p i p 0 p t 2m i 2m t 2m 0 p 0 DE E 0 1 sin 2 f E 0 sinf q where θ, φ functions of m s, b, E 0, etc. Scattering potential: Z V (r) q 2 1 Z 2 Z q 2 1 Z 2 e a 4b 4b where a = electron screening distance b 4m i m E t transfer m i m t p t E 0 Figure 5.6 Typical scattering problem. Figure inset shows electrostatic potential as a function of distance between the nuclei. The impact parameter is labeled b.

10 "Viscous"drag v so electronicstopping S e where k e E Z Z m m 3 2 i t i t m 3 m Z 2 3 Z 2 3 i t i t de dx e k e At high E, viscous model breaks down, & S e peaks E Neutron stopping : (Z s are charge numbers) Zi Zt S n 2.8x10 ev.cm Z 2 3 Z i t (In fact, S (E) is energy dependent) n Figure 5.7 The total distance that an ion travels in the solid is the range. The projection of this distance along the depth axis is the projected range, R p.

11 The implant profile Different implanted ions will go into a solid at different depths, even though the implant energy is the same This distribution arises due to the nature of the implant process multilple deflections of each incident ion angular distribution of the ion beam random variations in the implanter hardware A Monte-Carlo simulation (9999 ions) of a 50 kv P- implant into silicon; note the concentration at the surface is much less than the peak concentration of the distribution

12 Higher moments of the ion distribution We often approximate the impurity concentration as a Gaussian: 2 f x R p N x exp 2 DR 2DR 2 p p Further information comes from the higher moments of the distribution: i m i x Rp Nxdx 0 1 st moment: normalized dose 2 nd moment: (dose)(r p ) 2 3 rd moment:: skewness (related to asymmetry) 0 3 x R Nx p DR 3 p dx 4 th moment: kurtosis (related to Gaussian distortion) 0 4 x R Nx p DR 4 p dx

13 Ion Implantation Mathematical Model Gaussian Profile 2 Nx N p exp x R p 2 2DR p R p Projected Range DR p Straggle Dose Q = Nxdx 0 2 N p DR p 2002 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Introduction to Microelectronic Fabrication, Second Edition by Richard C. Jaeger. ISBN

14 Channeling While nuclear and electronic stopping serve as the mechanisms for reducing the depth that an implanted ion reaches, channeling causes the ion to go farther than expected oblique incidence aligned When the incident ion encounters a crystallographic direction with low atomic or electron density (a channel ) it may be steered down that channel by multiple grazing reflections

15 Aspects of channeling Channeling is especially severe in semiconductors because the covalent bonding results in a relatively open structure (packing efficiency of diamond ~34%) Depends on orientation of substrate surface (<100> versus <111>, exactly oriented versus misoriented) relatively easy to control Depends on the divergence of the ion beam relatively difficult to control Especially troublesome in shallow (low energy) implants of light ions (e.g. B into Si) Difficult to control, so it is best avoided: off-angle implant (~7º) implant through screen oxide to de-channel the beam pre-amorphize (example: Ge into Si)

16 Implantation damage (1) If Ed is the energy needed to dislodge an atom, then we have the following: 1.DE < E d : the target atoms are not displaced 2.DE ~ E d : simple displacement -- creation of an interstitial atom (the original target atom, now displaced) and a vacancy (where the original target atom used to be) 3.DE >> E d : damage cascade (multiple displacements by both ions and atoms)

17 Implantation damage (2) The amount of damage that occurs depends both on the energy of the incident ion and the relative masses of the ion and target 10 ev Au + (deposition) damage 1 kev Ar + (sputtering) 100 kev P + (implantation) 1 MeV He + (analysis)

18 Implantation damage (3) The ion implantation process generates a large number of both interstitials and vacancies (point defects) primary defects Upon subsequent annealing the defects may react with each other to form new combinations secondary defects End-of-range (EOR) dislocation loops are commonly observed after isolated point defects coalesce together Since diffusion in semiconductors is very sensitive to the number and type of point defects present, the diffusion of dopants and other materials may show anomalous behavior following ion implant

19 Implantation damage (4) light ion disordered regions heavy ion collision cascade light ion transfers a small amount of energy each collision deflected through a large scattering angle displaced target atom will have little energy imparted to it electronic stopping - relatively little damage heavy ion large amount of energy transferred incident ion deflected through a small scattering angle displaced target atoms can produce a large number of displacements nuclear stopping considerable lattice damage in a relatively small volume

20 The onset of amorphization Depending on energy and ion mass, low ion doses will generate different types of damage cascades Above a critical dose, the damage cascades will overlap and cause an amorphized region to form low dose individual damage cascades high dose a continuous amorphous region

21 Ion-assisted amorphization Ion implantation can lead to the athermal generation of amorphous materials (i.e. no need to rapidly quench from high temperature) Solid phase epitaxial regrowth of amorphized material (via crystallization from the underlying substrate) leads to a defect-free crystal -- much easier when the layer is fully amorphous and not just defective crystalline The question of what constitutes an amorphous implanted region is still subject to debate The quantitative structural characterization of very thin amorphous layers (ion implanted, SiO 2 ) remains a major problem in materials analysis

22 High resolution TEM micrograph of ion-amorphized silicon High resolution TEM image of as-implanted Si, 3.5 kv, boron dose cm -2

23 Sidebar -- Rutherford Backscattering Spectrometry (RBS) RBS is one of the most widely used methods for monitoring ion implant-induced damage generation mass analyzer MeV accelerator He + ions at E o ion source multichannel analyzer sample backscattered energetic He + ions at E 1 ion detector E E 1 2 o L NM M M1 sin q M1 cosq E M M E 2 1 O QP 2 at 180º M M M M o F HG 2 1 I KJ 2

24 RBS analyses of ion implanted materials The energies of the backscattered peaks identifies the chemical composition of the sample and gives information on layer thicknesses The change in backscattering yield due to the presence of an amorphous layer gives information on crystalline perfection in an implanted layer Normalized yield kv, 180 s, as-implant. 3.5 kv, 60 s, as implant. 3.5 kv, 60 s, annealed unimplanted Channel number

25 Typical RBS instrumentation It s so big!!

26 Basic components of ion implant systems ion source acceleration column beam extraction and mass analyzer ion accelerator beam manipulation end station sample manipulation beam monitoring

27 A full schematic of an ion implanter

28 Ion sources The Freeman ion source uses a heated filament (sometimes with RF assistance) to ionize a gaseous species and produce ions: gas in perpendicular magnetic field RF antenna ion beam gas out extraction slit If the desired ion does not exist in a gaseous form, it can be heated in an oven inside the ion source to generate a vapor A perpendicular magnetic field causes the electrons from the filament to follow helical paths, thus increasing the probability of ionization

29 Mass analyzer The ion beam is passed through a magnetic sector that selects a particular ionic species An ion of mass M and charge q B (in) moving at a velocity v in a circular path will experience a force ion in M,v r Mv r 2 qvb The kinetic energy of the ion is given by its extraction voltage: 1 E qvext Mv v 2 2qV M and 2 ext r Mv M qb 1 B 2 q Vext

30 Ion acceleration column low energy ions in high energy ions out The mass-analyzed ions are accelerated by an electrostatic linear accelerator A precision high voltage divider network applies a potential to a series of accelerator rings Since variations in the acceleration potential will change the ion energy and hence its penetration depth, precision high voltage engineering is needed Low energy implants may need both acceleration and deceleration stages HV

31 Beam manipulation V H V v Horizontal and vertical electrostatic deflection plates are used to raster the beam across the sample to improve implant uniformity Other electrostatic lenses (i.e. quadrupole lens) may be added to alter or improve the beam profile

32 A typical implanter end stage

33 End station Besides providing the means to insert and remove wafers from an ion implanter, the end station of a production system will have two important components: a multiple wafer rotation stage that will allow many wafers to be implanted simultaneously, thus both increasing throughput and improving uniformity a Faraday cup that will permit a precise determination of the implant dose by determining the charge carried by the ion beam per unit time multiple wafer rotation platform in an Eaton implanter

34 Various types of commercial implanters Implanter type Focussed ion beam Maximum ion current (ma) Maximum energies (kev) < Low energy 0.1 to 2 30 Use direct writing implants shallow junction doping Medium current 2 to diffusion implants High current High energy < 0.05 several MeV buried layer formation deep doping at low temps.

35 Doping by ion implantation By far the major application of ion implantation in semiconductors is for doping purposes Two main processes are used: the desired doping concentration profile is provided by controlling the implant profile the implant provides a fixed amount of dopant that is subsequently diffused in The deeper the profile, the higher the energy needed The total dose depends on beam current and implant time

36 Post-implant annealing An as-implanted semiconductor such as silicon is virtually always electrically inactive: the crystal will have a high density of point defects (or may even be amorphous) the implanted dopants will not be located on substitutional lattice sites The implanted wafer must go through a post-implant anneal: the crystallographic damage can be repaired the dopants can diffuse to substitutional sites One of two anneals is usually used: furnace anneal (typically 30 minutes at 600ºC) rapid thermal anneal (typically 10 seconds at 1100ºC)

37 Typical post-implant anneal results Table 14.1 Sheet resistance of Boron-implanted Silicon following annealing Anneal time, temperature Sheet resistance as implanted 5 sec., 950C 20 sec, 950C 30 sec., 1050C 30 sec., 1100C 1215 /sq. 885 /sq. 325 /sq. 209 /sq.

38 Implant and annealing uniformity A sheet resistance map of a wafer can be recorded after implantation and post-implant anneal Variations due to both the implant and the anneal can be observed A standard deviation of 2% across the wafer is normal; less is better Large variations will mean that devices fabricated at different locations will have different electrical properties

39 High resolution TEM micrograph of ion-amorphized and annealed silicon High resolution TEM image of as-implanted Si, 3.5 kv, B dose cm -2, annealed 850 C 5 sec C 10 sec

40 Post-implant annealing The annealing behavior of implanted materials can be very complex interaction of implanted atoms, vacancies and interstitials B-implanted Si can show a reverse annealing behavior as point defects agglomerate into extended defects defects serve as a sink for boron atoms; the substitutional boron concentration decreases Only at high temperatures is 100% activation achieved

41 Ion implantation in silicon device processing Ion implant is widely used as a low-temperature doping process in silicon device fabrication Masks of SiO 2, Si 3 N 4 or polymer photoresist can define specific regions to be implanted Example: p-well implant in CMOS devices (n-type substrate): boron ion implant photoresist (a) 5 -cm n-si substrate selective boron implant SiO 2 (b) p n drive-in diffusion

42 (c) p n open p-mos source-drain windows (d) p n p + p + source-drain junctions (e) n + n p + n p + n + source-drain junctions

43 (f) n + n + p n p + grow gate oxides (g) n + n + p n p + open source-drain contacts (h) n + n + p n p + metallization

44 Shallow junction doping The junction depth beneath the source and drain electrodes in an MOS transistor are one of the most critical features in device design source gate drain S/D junction depth For a 0.18 mm gate length, the S/D junction depth must be 400Å to 600Å Problems include boron channeling, transient-enhanced diffusion, and low throughput (low beam current) at low acceleration voltages

45 Typical SIMS profiles of boronimplanted shallow junction profiles Concentration(cm -3 ) khz, 1.8E+15 1 khz, 8.9E Hz, 1.8E+16 Concentration(cm -3 ) khz, khz, Hz, 1.8E Depth (Angstroms) Depth (Angstro 2 kev, as implanted 2 kev, annealed

46 Transient enhanced diffusion (TED) TED is the observed increase in the diffusion coefficient of dopants (mainly B) at the early stages of an anneal following ion implantation TED causes the tail of the implant distribution to move to greater depths terrible for shallow junction formation Recent (1996) work has shown that TED is due to reactions between silicon interstitials

47 Buried dielectric layer The formation of a buried electrical insulating layer underneath CMOS devices can lead to a significant improvement in performance This can be accomplished by SIMOX -- Separation by Implantation of Oxygen high dose oxygen implant buried high oxygen layer anneal to form buried SiO 2

48 IBM commercializes a SIMOX CMOS process (4Q98)

49 Defect engineering -- Smart-Cut process In many applications it is desirable to have an electricallyinsulating layer buried underneath the surface Problem: how do you bury an amorphous SiO 2 layer under a single crystalline layer? Solution 1: SIMOX (expensive!) Solution 2: the Smart-Cut process The recipe: Ion implant H 2 or He into (and under) a Si surface Compression bond to SiO 2 /Si wafer Anneal to form voids (gas bubbles ) that delaminate a thin Si layer Touch-up polish to smooth surface

50 The Smart-Cut process starting silicon wafer H 2 ion implant to form voids under the surface SiO 2 Si compression bond to SiO 2 /Si wafer anneal to grow voids for delamination; touch polish

51 Limitations to current ion implanters dose accuracy and uniformity ion damage to wafers wafer charging and heating impurity contamination ion damage to masking materials instrument footprint cost

52 Beamline vs. plasma implantation systems Conventional beamline ion implanter (left) Plasma source ion implantation system (right)

53 Advantages of PSII over beamline implantation systems Non-line-of-sight; can treat large, heavy, and complicated shapes Process time independent of surface area; capable of treating multiple workpieces simultaneously Compatible with more traditional plasma tools and processes High ion doses at low implant voltages Low temperature process

54 Shallow junction formation in BF 3 plasma-implanted silicon BF 3 implants were performed at kv and -0.7 kv RTA: 800C, 5s+1050C, 10s Define junction depth at cm -3 As-implanted junction depths 26 nm /36 nm (-0.5 /-0.7 kv) Annealed junction depths 50 nm /67 nm (-0.5 /-0.7 kv)

55 Beamline versus plasma implant profiles (as-implanted and annealed) As-implanted junction depth of 3.5 kv plasma implant is comparable to 5 kv BF 2 + beamline implant Plasma implant profile seems to diffuse less (10 to 20 nm shallower) than beamline Transient enhanced diffusion may be suppressed in plasma implant processes