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III-V/Ge Channel Engineering for Future CMOS
215th ECS Meeting, San Francisco, May 28, 2009 III-V/Ge Channel Engineering for Future CMOS Mark A. Wistey University of California, Santa Barbara Now at University of Notre Dame P. McIntyre, B. Shin, E. Kim Stanford University S. Bank University of Texas Austin Y.-J. Lee Intel U. Singisetti, G. Burek, A. Baraskar, V. Jain, B. Thibault, A. Nelson, E. Arkun, C. Palmstrøm, J. Cagnon, S. Stemmer, A. Gossard, M. Rodwell University of California Santa Barbara Funding: SRC
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Outline: Channels for Future CMOS
FET scaling requirements... and failures Motivation for Regrown MOSFETs III-V Benefits and Challenges Fabrication Process Flow Depletion-mode MOSFETs The Shape of Things to Come M. Wistey, Spring ECS 2009
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Future CMOS Priorities
High Performance High drive current Id / Wg for wiring Low gate delay CFET∆V/Id for local Low Parasitics Capacitance – already 1-2 fF/µm Resistance – already ~50% Rtot Leakage Currents – now 50% power Will give more detailed breakdown in a moment... VLSI needs all of these. Tempting to focus on just Compatible with Existing CMOS High packing density –width & contacts small Growable on Si substrates M. Wistey, Spring ECS 2009 Graphics: Mark Rodwell
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Simple FET Scaling ✓ ✓ ✓ ✓ ✓
Goal double transistor bandwidth when used in any circuit → reduce 2:1 all capacitances and all transport delays → keep constant all resistances, voltages, currents reduce 2:1 all lengths, widths, thicknesses gm , Id held constant ✓ (doubles mA/μm, mS/μm) ✓ held constant ✓ reduced 2:1 ✓ edge capacitances reduced 2:1 substrate capacitances reduced 2:1 ✓ must reduce ρc 4:1 M. Wistey, Spring ECS 2009 Slide: Mark Rodwell
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“External” Resistances are Critical
Coverlap Cfringing Lg Source Drain Gate Dielectric, εr tox n+ Channel (p or NID) xj n+ Rc Cox Back Barrier Rsp & Rif & Rq Raccess Resistances constant ⇒ Resistivities must scale as 1/Lg2: Contact resistance Rc Access & spreading resistance Interface resistance & source starvation Sharvin resistance (quantized conductance): These assume C’s & gm scale OK. Can’t get below 52Ω-µm unless ns>1E13/cm2. Sharvin: even for graphene & superconductors. Scales slowly. RTOT(Ω) = (2ρC/LPAD + 2Raccess + 2Rsp + 2Rit + Rq + Rch) / W M. Wistey, Spring ECS 2009
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Transconductance Scaling Challenges
gm ~ (1/Cg-ch)vthWg ...Should stay constant (double mA/µm) But voltage divider exists: Gate ☹ Cox scales as 1/EOT ...EOT scales poorly Top Barrier or Oxide ☹ Csemi ~ εchan.LgW/d ...Smaller Channel ☹ Cdos = Bottom Barrier ...Smaller E1 d tqw CB Wavefunction pushed away from gate Csemi scales poorly Thick channel Thin channel Buried channel = another capacitor Csemi = Cinversion in CMOS world. Another weakness of HEMTs. Must increase gm. Solution: Increase vth using new material. Similar problem with overlap & fringing capacitances. M. Wistey, Spring ECS 2009
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MOTIVATION FOR REGROWN FETs
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Device Choice: Why not HEMTs?
Wide recess→ okay DIBL, subthreshold slope, Cgd Wide contacts→ Rc okay even with poor contacts } Not scaled Injection thru barrier Low gate barrier = Limited carrier density Long recesses, lightly doped = high Raccess Be brief here. Iain Thayne mentioned these... need to understand true scaling needs. Very limited in future scaling of gm. This is a problem we’ll see. HEMTs obscure scaling issues. M. Wistey, Spring ECS 2009
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{ { { Scalable III-V FET Design Classic III-V FET (details vary):
Large Area Contacts { Large Raccess { Gap Source Drain Gate Advantages of III-V’s Large Rc Top Barrier or Oxide { Channel Disadvantages of III-V’s Bottom Barrier InAlAs Barrier Low doping III-V FET with Self-Aligned Regrowth: Channel In(Ga)P Etch Stop InAlAs Barrier High-k Gate n+ Regrowth High Velocity Channel Small Raccess Small Rc Self-aligned High doping: 1013 cm-2 avoids source exhaustion 2D injection avoids source starvation Low sheet resistance; dopants active as-grown High mobility access regions High barrier Not self-aligned. Doping limited by implant or delta-doping. Native oxide doesn’t passivate. Must provide carriers. Limited carrier density (HEMTs). Limited doping density. Geometry (gap). Heterobarriers. M. Wistey, Spring ECS 2009
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Big Picture: Salicide-like Process
Analogy: Self-aligned silicide (salicide) process: Salicides Barrier Source Contact High-k Metal Gate Channel n+ Regrowth Regrown Contacts .... Break from Silicon: No implantation Insufficient doping Surface damage Annealing is no panacea Encapsulate gate metals Arsenic capping Ship wafers for high-k Strain is cheap Advantages: Self-aligned No e- barrier CMOS-safe metals Metal Gate High-k .... Salicide Channel Take from Silicon: Unlike classic III-V devices: Avoid liftoff Dry etches Self-aligned processes Surface channels Do III-V fabrication in Si-like fashion. M. Wistey, Spring ECS 2009
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III-V Benefits and Challenges
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Channel Roughness Scattering
Challenge: Sixth power (!) scattering from interface roughness: µ ~ (1/Mscat)2 ~ 1/(∂E/∂W)2 ~ W6 (Gold SSC 1987) Trend weaker in shallow or narrow wells (Li SST 2005) Fit: ~W-1.95 Screening helps too Does not seem to be a problem for 5nm InGaAs channels. M. Wistey, Spring ECS 2009
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Drive Current in the Ballistic & Degenerate Limits
where eot includes the electron wavefunction depth Inclusive of non-parabolic band effects, which increase cdos , InGaAs & InP have near-optimum mass for nm EOT gate dielectrics Rodwell IPRM 2008
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No Shubnikov-de Haas Oscillations
High Mobility in Narrow InGaAs Channels Hall Measurements No Shubnikov-de Haas Oscillations 4K InAlAs mobility: 346 Electrons occupy multiple channels Preliminary simulations (difficult) by Dennis & Peter Mostly confined, not 98%. Need to verify simulations with FATFET etc. Remove ionized impurity scattering, much alloy scattering (InAlAs). But electrons aren’t in InAlAs...? Unclear whether high mobility is in narrow channel Unreasonable simulation difficulty. Nonparabolic bands, degenerate statistics, bandgap renormalization, screening... Need FET to remove uncertainty: eliminate doping altogether. M. Wistey, Spring ECS 2009
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Regrowth Interface Resistances
Interface resistances tested in separate blanket regrowths (no gates): In-situ Mo Contact ρc < 1 Ω - μm2 25 nm regrown InGaAs Rsh=70 Ω/sq Source Contact Drain Contact Metal Gate High-k n+ Regrowth InGaAs InP or InGaP etch stop InAlAs barrier Interfaces tested individually in separate blanket growths (No Gates): InGaAs-InGaAs re-growth resistance < 1 Ω - μm2. InGaAs-InP re-growth resistance = 6 Ω - μm2 (on thick InP).
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FABRICATION PROCESS FLOW
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Process Flow: Gate Deposition
High-k first on pristine channel. Tall gate stack. Litho. Selective etches to channel. SiO2 e r Metals Cr Critical etch process: Stop on channel with no damage NID InGaAs Channel No damage before high-k. For planarization later Thinner layers near channel InP/InGaP etch stop InAlAs barrier InP substrate M. Wistey, Spring ECS 2009
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Gate Stack: Multiple Layers & Selective Etches
Key: stop etch before reaching dielectric, then gentle low-power etch to stop on dielectric M. Wistey, Spring ECS 2009 Rodwell IPRM 2008
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Process Flow: Sidewalls & Recess Etch
SiO2, SiNx SiNx or SiO2 sidewalls Encapsulate gate metals Controlled recess etch Slow facet planes Not needed for depletion-mode FETs Metals e r Channel InP/InGaP etch stop InAlAs barrier InP substrate M. Wistey, Spring ECS 2009
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Surface Preparation Before Regrowth
SiO2, SiNx O3 (g) UV-ozone (20 min) HCl:H2O 1:10 etch (60 sec) H2O rinse, N2 dry Bake under ultrahigh vacuum Hydrogen cleaning 10-6 Torr, 30 min., 400°C (InP) or 420°C (InGaAs) Thermal deoxidation may work also. Metal e r Channel SiO2, SiNx HCl+H2O (l) Metal e r Channel UV Ozone: Remove carbon contamination, Sacrificial oxide. HCl: Remove oxide. Repeat H clean until RHEED reconstruction appears. SiO2, SiNx H2 + H (g) Metal e r Channel M. Wistey, Spring ECS 2009
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Clean Surface Before Regrowth
InP etch stop Clean, undamaged surface after Al2O3 dielectric etch & InGaAs recess etch. M. Wistey, Spring ECS 2009
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At Last: Regrowth & Metal
Regrow n++ InGaAs Doping: n~3.6x1019 cm-3 (Si~8x1019 cm-3) V/III ratio=30 Tsub = 460°C SiO2, SiNx Metals Mo e r n++ InGaAs n++ InGaAs Channel Lot of time to get here: starting surface is critical. Selective or nonselective growth OK InP etch stop RHEED before growth InAlAs barrier Blanket metallization: Either in-situ Mo or ex-situ TiW Singisetti APL, submitted, or Crook APL 2007 M. Wistey, Spring ECS 2009
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TEM of Regrowth: InGaAs on InGaAs
HRTEM InGaAs n+ regrowth Interface Interface HAADF-STEM` 2 nm InGaAs n+ Contamination levels undetectable by HAADF-STEM (<<1ML). Regrowth on processed but unpatterned InGaAs. No extended defects. M. Wistey, Spring ECS 2009
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Removing Excess Overgrowth
1. Spin on thick polymer Polymer Approach #1: Remove it Height-selective etch Wistey MBE 2008 & Burek JCG 2009 2. Mo & InGaAs Etch SiO2 SiO2 Polymer Metal Metal Oxide Oxide Regrowt h InGaAs Regrowth InGaAs InGaP etch stop InGaP etch stop InAlAs barrier InAlAs barrier Approach #2: Don’t Grow It Quasi-Selective growth Wistey EMC 2009 Sidewalls clean. Excess gets removed with gate cap. M. Wistey, Spring ECS 2009
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Regrowth on InP vs. InGaP
InP regrowth SEM InP regrowth RHEED SEM: U. Singisetti InAlAs barrier InGaAs InAs Conversion of <6nm InP to InAs Strain relaxation As P InGaAs InP etch stop InAlAs barrier As M. Wistey, Spring ECS 2009
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Regrowth on InGaP InGaP regrowth SEM Replace InP with InGaP
As Replace InP with InGaP Converts to InGaAs (good!) Strain compensation Converted from InGaP P InGaAs InGaAs InGaAs InGaP InGaP InGaP Wistey EMC 2008 InAlAs barrier InGaP regrowth SEM InGaP regrowth RHEED 2 M. Wistey, Spring ECS 2009
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DEPLETION-MODE MOSFETS
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Scalable InGaAs MOSFETs
300 nm SiO2 Cap SiNx 50 nm Cr Mo 50 nm W Mo 5 nm Al2O3 n++ regrowth InGaAs Channel, NID n++ regrowth 3 nm InGaP Etch Stop, NID 10 nm InAlAs Setback, NID 5 nm InAlAs, Si=8x1019 cm-3 200 nm InAlAs buffer Semi-insulating InP Substrate Oblique view Top view SEM Quasi-selective M. Wistey, Spring ECS 2009
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Scalable InGaAs MOSFETs
300 nm SiO2 Cap SiNx 50 nm Cr Conservative doping design: [Si] = 4x1013 cm-2 Bulk n = 1x1013 cm-2 >> Dit Large setback + high doping = Can’t turn off Mo 50 nm W Mo 5 nm Al2O3 n++ regrowth InGaAs Channel, NID n++ regrowth 3 nm InGaP Etch Stop, NID 10 nm InAlAs Setback, NID 5 nm InAlAs, Si=8x1019 cm-3 200 nm InAlAs buffer Semi-insulating InP Substrate 0.5µm 0.6µm 0.7µm 0.8µm 0.9µm 1µm 10µm High R independent of Lg. Despite the high series resistance, the transconductance of these transistors was as high as 1.7 mS (or 0.24 mS/µm) for 0.9 µm long devices at Vds=2V and Vgs=1V. Many devices show comparable gm, as shown in Figure 9(c). These results are promising for future MOSFETs. M. Wistey, Spring ECS 2009
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Series Resistance Rch indept of Lg below 1µm: large series resistance.
300 nm SiO2 Cap Possible causes: Poly nucleation at gate Thinning near gate Strain-induced dislocations Incomplete underfill Insufficient doping under sidewall SiNx Rch indept of Lg below 1µm: large series resistance. Now known to be a growth issue. See DRC & EMC 2009. 50 nm Cr Mo 50 nm W 5 nm Al2O3 n++ regrowth InGaAs Channel, NID 3 nm InGaP Etch Stop, NID 10 nm InAlAs Setback, NID 5 nm InAlAs, Si=8x1019 cm-3 InAlAs buffer Now known to be a growth issue. See DRC & EMC 2009 for solution. M. Wistey, Spring ECS 2009
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The Shape of Things to Come
Generalized Self-Aligned Regrowth Designs: Recessed Raised Gate Gate Top Barrier Top Barrier n+ Regrowth Channel Channel Back Barrier Back Barrier Substrate Substrate Self-aligned regrowth can also be used for: GaN HEMTs (with Mishra group at UCSB) GaAs pMOS FETs InGaAs HBTs and HEMTs All high speed III-V electronics M. Wistey, Spring ECS 2009
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Conclusions Scaled III-V CMOS requires more than reduced dimensions
InGaAs offers a high velocity channel, high mobility access Self-aligned regrowth: a roadmap for scalable III-V FETs Provides III-V’s with a salicide equivalent Can improve GaN and GaAs FETs too DFETs show peak gm = 0.24mS/µm High resistance (a growth problem) limited FET performance M. Wistey, Spring ECS 2009
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Acknowledgements Rodwell & Gossard Groups (UCSB): Uttam Singisetti, Greg Burek, Ashish Baraskar, Vibhor Jain... McIntyre Group (Stanford): Eunji Kim, Byungha Shin, Paul McIntyre Stemmer Group (UCSB): Joël Cagnon, Susanne Stemmer Palmstrøm Group (UCSB): Erdem Arkun, Chris Palmstrøm SRC/GRC funding UCSB Nanofab: Brian Thibeault, NSF M. Wistey, Spring ECS 2009
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Conclusions Scaled III-V CMOS requires more than reduced dimensions
InGaAs offers a high velocity channel, high mobility access Self-aligned regrowth: a roadmap for scalable III-V FETs Provides III-V’s with a salicide equivalent Can improve GaN and GaAs FETs too DFETs show peak gm = 0.24mS/µm High resistance (a growth problem) limited FET performance M. Wistey, Spring ECS 2009
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