Mosfets

Department of Electrical and Computer Engineering
(i.e., A brief history of sand)
M. Fischetti
October 2, 2009
A (partial, biased?) history of the MOSFET
from a physicist’s perspective

2Department of Electrical and Computer Engineering MVF October 2, 2009
This talk: Void where prohibited, limitations and restrictions apply
 Technology (i.e., how do we make them?) vs. electronic operation (i.e., how do they work
and how do we make them better?)
 Too much to cover
 Talk about what I know
 Major omissions (that is, a disclaimer*):
• Doping: Diffusion (theory and technology), ion implantation, high-doping effects
• Lithography, possibly a “technology enabler”: Optical, contact, phase-shift, X-ray…
• Metallization: Deposition/growth, DAMASCENE, electromigration
• Etching: Wet vs. dry, RIE, plasma
• Film growth: Epitaxy, CVD, PE-CVD, MBE, ALD,…
• Contacts: Silicides, salicides, FUSI,…
• Layout issues: Isolation (deep/shallow trenches), cross-talk, latch-up, design rules,
DRAM/SRAM design, power,…
• …

History of the MOSFET? What’s that?
 Thanks to Jiseok Kim for having put me on the spot….
It’s OK… I wish him good luck in getting his PhD wherever ELSE he may wish to get it….
 I’m not sure what he meant by “history”… So, let’s start from the beginning….

The main character of our story: The MOSFET
 No other human artifact has been fabricated in larger numbers (except perhaps nails?)
 “…some consider it one of the most important technological breakthroughs in human
history…” (Wikipedia, the source of all human knowledge)

Timeline I
1925: Julius Edgar Lilienfeld’s MESFET patent
1935: Oskar Heil’s MOSFET patent
194?: Unpublished Bell Labs MESFET
1947: Ge BJT (Bardeen, Brattain, Shockley, Bell Labs)
1954: Si BJT (Teal, Bell Labs)
1960: MOSFET (Atalla&Khang, Bell Labs)
1961: Integrated circuit (Kilby, TI)
1963: CMOS (Sah&Wanlass, Fairchild)
1964: Commercial CMOS IC (RCA)
1965: DRAM (Fairchild)
1968: Poly-Si gate (Faggin&Klein, Fairchild)
1968: 1-FET DRAM cell (Dennard, IBM)
1971: UV EPROM (Frohman, Intel)
1971: Full CPU in chip, Intel 8008 (Faggin, Intel)
1974: Digital watch
1974: Scaling theory (Gänsslen&Dennard, IBM)
1978: Use of ion implanter
1978: Flotox EEPROM (Perlegos, Intel)
1980: Ion-implanted CMOS IC
1980: Plasma etching
1984: Scaling theory <0.25 μm (Baccarani, U. Bologna)
1986: 0.1 μm Si MOSFET (Sai-Halasz, IBM)
1991: CMOS replaces BJT also at high-end
1993: DGFET scalable to 30 nm (theory, Frank et al.)
2007: Non-SiO2 (HfO2–based) MOSFET (Intel)
1955: Si, Ge conduction band (Herring&Vogt)
Deformation-potential, high-field (Bardeen&Shockley)
1957: BTE in semiconductors – impurities (Luttinger&Kohn),
phonons (Price, Argyles)
1964: Band structure calculations (Hermann)
Monte Carlo for semiconductors (Kurosawa)
1965: Linear-parabolic oxidation model (Deal&Grove)
1966: Observations of 2DEG (Fowler, Fang, Stiles, Stern,..)
1967: Conductance technique (Nicollian&Goetzberger)
1974: DDE device simulator (Cottrell&Buturla)
1975: Quantum Hall Effect predicted (Ando)
1979: Quantum Hall Effect observed (von Klitzing)
1981: Identification of native Nit: Pb-centers (Poindexter)
Full-band MC (Shichijo&Hess)
1982: Fractional QHE observed (Störmer&Tsui, Laughlin)
1988: Full-band MC device simulator (MVF&Laux)
1992: NEGF device simulator (Lake, Klimeck, et al.)
Technology Physics/Simulations

Timeline II
1975: 20 μm (tOX≈250 nm)
1980: 10 μm (tOX≈150 nm)
1985: 5 μm (tOX≈70 nm)
1990: 1 μm (tOX≈15 nm)
1995: 0.35 μm (tOX≈8 nm)
2000: 0.18 μm (tOX≈3 nm)
2005: 65 nm (tOX≈1.4 nm)
2010: 32 nm (tOX≈1.2 nm?)
SiO2 growth and instability: Ions, traps, interface
SiO2 instability during operation: electron trapping, NBTI
Hot electron effects: oxide trapping, VT shift, breakdown
Scaling: Short-channel effects (SCE), oxide, dopants
….life is good…
Scaling: SCE, insulator
Leakage: Insulator
Power: Alternative devices
Feature size Main Problems
↑
↓

Timeline III
1975: 20 μm
1980: 10 μm
1985: 5 μm
1990: 1 μm
1995: 0.5 μm
2000: 0.25 μm
2005: 63 nm
2010: 32 nm
2015: 16 nm ?
Feature size Transport Physics
Drift-Diffusion
Hydrodynamic/
Energy transport
Boltzmann
Quantum?
↓
↕
↓

Transistor prehistory
1935 Heil’s patent 1947 First BJT 1960 Atalla’s MOSFET
Bardeen, Shockley, Brattain (Bell Labs)

IC Prehistory
1961 Kilby’s first IC 1962 Fairchild IC 1964 First MOS IC
(RCA)

Moore’s law prehistory
Gordon Moore 1965: Cost vs time Moore’s law 1960-1975

Moore’s law
Number of transistors/die & feature size vs time

Microprocessor prehistory
1965: Federico Faggin 1968: Fairchild 8-bit μP 1971: Intel 8080 μP

Memory prehistory: DRAM and EPROM
Bob Dennard (1-FET DRAM cell, 1968; 1971 Frohman’s UV-erasable EPROM
scaling theory with Fritz Gänsslen,1974) (written by avalanche injection)

More historical trends
J. Armstrong (ca.1989)

Timeline II once more
1975: 20 μm (tOX≈250 nm)
1980: 10 μm (tOX≈150 nm)
1985: 5 μm (tOX≈70 nm)
1990: 1 μm (tOX≈15 nm)
1995: 0.35 μm (tOX≈8 nm)
2000: 0.18 μm (tOX≈3 nm)
2005: 65 nm (tOX≈1.4 nm)
2010: 32 nm (tOX≈1.2 nm?)
SiO2 growth and instability: Ions, traps, interface
SiO2 instability during operation: electron trapping, NBTI
Hot electron effects: oxide trapping, VT shift, breakdown
Scaling: Short-channel effects (SCE), oxide, dopants
….life is good…
Scaling: SCE, insulator
Leakage: Insulator
Power: Alternative devices
Feature size Main Problems
↑
↓

SiO2 growth and instability
 Ionic contamination (K, Na): Unrecognized source of early problems
 Fixed traps (oxygen vacancies?), especially near Si-SiO2 interface
 Growth kinetics: Deal & Grove model: linear (reaction-limited) and parabolic (diffusion-limited)
regions; dry and wet oxidation rates
 Interface-state passivation: Al (with H) Post Metallization Anneal (PMA, Peter Balk):
 H2O → H+ + OH-
 Si- + H+ → Si-H
Andrew Grove (left), Bruce Deal (center)
and Ed Snow (left)
Ed Snow’s cartoon, ca. 1966
about SiO2 instabilities

SiO2 growth and instability, as-grown and during operation
 CV-plot instabilities (VFB or VT shifts):
 Ions (mainly Na+ and K+, contamination in chambers, handling, gases, etc…)
 Interface states generation (stretch-out, Lai, Feigl, Sandia group, Technion, Siemens,…)
 Electron and hole traps (DiMaria, Young, Feigl):
• Neutral: H2O-related (mainly OH-) in wet oxides, radiation induced in processing,
σ ≈ 10-15 to 10-17 cm2
• Charged-attractive: Ionic contamination, σ ≈ 10-13 cm2 , field-dependent
• Charged-repulsive: Radiation-induced, σ ≈ 10-19 cm2

SiO2 instability during operation
 Anomalous Positive Charge (APC):
 Caused by electron injection (Avalanche, Fowler-Nordheim) and also hole injection
 Related to Hydrogen: Boron deactivation in p-type substrates (Sah)
 Related to hole back-injection from anode? Dependent on gate-metal workfunction -
Au vs. Al vs. Mg (MVF&Weinberg, Chenming Hu)
 Occurring at Si-SiO2 interface even under negative bias: Neutral species such as
solitons, H2 diffusion…? (Weinberg).
 Connected to wear-out and breakdown (DiMaria, Stathis)
 Strongly correlated to interface traps (Pb-centers, Lenahan, Poindexter)
 Oxygen deficiency (Revesz)? Broken Si-H bonds (Si-D experiment, Lyding&Hess)?
 Negative Bias Temperature Instability (NBTI): No time to discuss, but big issue in high-κ
dielectrics

Understanding SiO2 degradation: Two approaches
MVF and DiMaria, INFOS 1989

SiO2 growth and instability: Injection techniques and damage generation

SiO2 growth and instability: Electronic transport in SiO2
 Electrons:
 Long-standing problems of high-field electron transport in polar insulators
(Karel Thornber’s 1970 PhD Thesis with Richard Feynman)
 LO-phonon scattering run-away connected to dielectric breakdown
 Experimental observations do not show predicted run-away at 2-3 MV/cm
 Umklapp scattering with acoustic phonons keeps electron energy under
control (MVF, DiMaria, Theis, Kirtley, Brorson, 1985)
 Holes: Small polaron (self-trapping) transport (Bob Hughes’ 1977 time-of-flight
experiments explained by David Emin’s 1975 theory).
MVF et al., PR B (1985)

Hot electron effects in constant-voltage-scaled MOSFETs
 Two problems:
 Understand origin/spectrum of hot carrier
 Understand nature/process of damage generation
 Practical problems:
 Unnecessary and expensive burn-in
 Wall Street “big glitch” in 1994
 Theory:
 Shockley’s “lucky-electron model” widespread in EE community in the ’80s
(publicized by Chenming Hu, UCB): Even the Gods can be wrong at times…
 Full-band models (Sam Shichijo & Karl Hess, MVF&Laux, then others)
 Basic physics of electron scattering, injection into SiO2, etc.
 The mid-1990s “pseudo-full-band” frenzy (Bologna, UNC, Udine, Lille, TU-
Vienna, Aachen,..): Gain without pain… didn’t work…

Electron injection into SiO2
MVF, Laux, and Crabbé, JAP (1996)

Timeline III once more
1975: 20 μm
1980: 10 μm
1985: 5 μm
1990: 1 μm
1995: 0.5 μm
2000: 0.23 μm
2005: 63 nm
2010: 32 nm
2015: 16 nm ?
Feature size Transport Physics
Drift-Diffusion
Hydrodynamic/
Energy transport
Boltzmann
Quantum?
↓
↕
↓

Electron transport in Si at 3 eV: A big headache
 Effective-mass approximation valid only for E ≈ a few kBT
 Scattering rates at E ≥ {a few kBT} totally unknown
 Moments of the BTE (DDE, Hydrodynamic) not sufficiently accurate

Electron transport in Si at 3 eV ca. 1992: A depressing picture…
The state-of-the art circa 1992

A good example of experiments-theory feedback
XPS (McFeely, Cartier, Eklund at the
Brookhaven IBM synchrotron line, 1993) Carrier separation (DiMaria, 1992)
Cartier et al. APL (1993)

Electron transport in Si at 3 eV ca. 1994: Much better…
The state-of-the art circa 1994
MVF et al., JAP (1996)

1980 → 1985 → 1988 → 1991 → 1994 → 1999 → 2003 …
LGATE = 10 μm 5 μm 2.5 μm 1.3 μm 0.63 μm 0.25 μm 130 nm ….
1980 → 1985 → 1988 → 1991 → 1994 → 1999 → 2003 …
1980 → 1985 → 1988 → 1991 → 1994 → 1999 → 2003 …
1980 → 1985 → 1988 → 1991 → 1994 → 1999 → 2003 …
1980 → 1985 → 1988 → 1991 → 1994 → 1999 → 2003 …
1980 → 1985 → 1988 → 1991 → 1994 → 1999 → 2003 …
1980 → 1985 → 1988 → 1991 → 1994 → 1999 → 2003 …
Scaling
 Shrink dimensions maintaining aspect-ratio
 Must shrink electrostatic features as well (depletion regions→ doping level and profiles)
↔
1 μm

Scaling
 Electrostatic integrity (Well-tempered MOSFET, Antoniadis): SOI, DGFETs, FinFETs, NW-FETs
 Reduce power, an example: The tunnel FET n (tFET)
 Reduced leakage: High-κ gate-insulators
 Improve (or, at least, maintain) performance: Alternative channel materials?

Scaling – Electrostatic integrity: SOI
22 nm strained-Si nFET: SOI to prevent punch- through,
strained Si to improve performance (B. Doris, IBM, 2006)

Scaling – Electrostatic integrity: Double gate FET
AIST (2003)

Scaling – Electrostatic integrity: Multibridge FETs (TEM, SEM)
Samsung Electronics Ltd. (2005)

Multibridge FETs: Process flow
Samsung Electronics Ltd. (2005)

Scaling – Electrostatic integrity: FinFETs
Freescale Semiconductors

Scaling – Electrostatic integrity: Si Nanowire Transistors
KAIST (2007)

Scaling – Reduce power: The tunnel-FET (tFET)
InAs Tunnel-FET: structure
(M. Haines, UMass 2009)
InAs Tunnel-FET: pair generation rate
(M. Haines, UMass 2009)
 Stand-by power dissipation approaching “on”
power dissipation… Cannot continue like this!
 60 mV/dec → ΔVG ≈ 250 mV for Ioff/Ion ≈ 10-4
 VT + ΔVG ≥ 0.45 V at 300 K (nFETs)
 Must increase slope (i.e., go below 60 mV/dec) if
we want the `Green’ FET (term coined by C. Hu)
 Problem: Ion too low in all attempts (DARPA to
IBM, UCB, Stanford,…) so far

Scaling – Reduce leakage
 Off-leakage:
 Accepted value increasing: Ioff/Ion ≈ 10-4 for the 32 nm node (used to be 10-6 or lower!)
 Connected to electrostatic integrity (punch-through, junction leakage, gate leakage)
 Gate leakage:
 C = εox/tox, so if tox has reached its limit (≈ 1nm, too aggressive so far), scale εox:
High-κ insulators such as HfO2, ZrO2, Al2O3, etc.
 Problem: Low mobility in high-κ MOS systems (scattering with interfacial optical phonons)
 Metals with different workfunction needed!
Hi-res TEM from
Susanne Stemmer,
UCSB MVF et al., JAP (2001)

Scaling – Reduce leakage: Gate oxide scaling at Intel
C. Hu et al., IEDM (1996)

Scaling – Improve performance
 Taken for granted early on (ca. 1986)
 Slow realization that early optimism was unjustified
MVF and S. Laux, EDL (1987)

Scaling – Improve performance
 Look for high-velocity, low-effective mass semiconductors… or should we?
 Problems:
 High-energy (≈ 0.5 eV ≈ 20 kBT) DOS and rates identical in most fcc semiconductors
 Low DOS → loss of transconductance
 Low DOS → smaller density in quasi-ballistic conditions → lower Ion
 Low DOS → less scattering in source → source starvation

Scaling – Improve performance: DOS bottleneck
MVF and S. Laux, TED (1991)

Scaling – Improve performance: Strained Si
MVF and S. Laux, JAP (1996)

Scaling – Improve performance: Strained Si
IBM 32 nm strained (tensile)
Si nFET on SiGe virtual substrate
Intel 45 nm strained (compressive)
Si pFET with regrown SiGe S/D

Why are sub-40 nm devices getting slower?
 Power dissipation → reduce frequency or fry!
 Parasitics play a bigger role (Antoniadis, MIT)
 Higher oxide fields squeeze carriers against interface → increased scattering
(Antoniadis, MIT)
 Intrinsic Coulomb effects!
MVF and S. Laux, JAP (2001)

Why are sub-40 nm devices getting slower? The effect of e-e interactions

Sub-32 nm Si CMOS devices: Where do we stand?
 22 nm: Planar (Intel), SOIs (IBM), FinFETs doable but too expensive.
 16 nm: Possibly FinFETs, still Si
 Below 16 nm:
 Ge pFETs and III-V nFETs (IMEC)? A pipedream…
 Ge nFETs still lousy, improvements promised at Dec 2009 IEDM, we’ll see
 III-Vs in the works:
• MIT (del Alamo): Great HEMTs, but huge S/D-gate gap not easily scalable
• SRC/UCSB MOSFETs: Wait and see…

The future and “post Si CMOS” devices: What do we need?
 Three terminal devices (Josephson computers taught us something…!)
 At least some gain (preserving signal over billions of devices, beating kBT)
 At least a few devices must have high Ion to charge external loads
 On/off behavior (Landauer’s water faucet analogy)
 Low power, possibly non-charge-switching (spins, QCA,…). BUT: If we use ≈ kBT to
switch, the heat bath will switch for us even if we do not want to…
 Notable historic failures:
 Josephson: Excessively strict tolerances (on insulators), complicated 2-terminal logic
 SETs: No output current (`a slight impedance matching issue’, as someone kindly put
it….)
 Optical computers: Photons are huge! Clumsy 3-terminal devices
 Resonant tunneling diodes and multi-state logic: Non off-off switches, impossible to
control manufacturing tolerances
 High hopes:
 Nanowires: They are just thin and narrow FinFETs
 Long shots:
 Spins and QCA: Low power but no gain
 CNT: No current in single tube, must use many in parallel
 III-Vs: Battle already lost in 1991 (DOS bottleneck),,, why bother again?

And by popular demand… The future of “post-Si CMOS” logic technology

More slides about the lunatic fringe….

The lunatic fringe: Exploratory devices

Carbon NanoTube (CNT) FET
IFF-Jülich, Germany (2004)

CNT Transistors
IFF-Jülich, Germany (2004)

CNT FET inverter
J. Appenzeller, IBM

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