1

SiGe(C) MOSFET Technology

Sanjay BanerjeeUniversity of Texas at Austin

� Issues in Scaled CMOS� Bandstructure, Transport and Strain� Enhanced Mobility Channels

- Strained Si and SiGe(C)� Multi-Gate and Novel MOSFETs� Process Integration Challenges

2

ITRS, 2003

3Streetman and Banerjee, Solid State Electronic Devices, Prentice Hall

4

Experimental output characteristics of n-channel and p-channel MOSFETs with 0.1 micron channel lengths. The curves exhibit almost equal spacing, indicating a linear dependence of ID on VG, rather than a quadratic dependence. We also see that ID is not constant but increases somewhat with VDin the saturation region. The p-channel devices have lower currents because hole mobilities are lower than electron mobilities.

( ) 20

12D X G T D D

WI C V V V VL

µ = − −

( ) 20

12DSAT X G T DSAT DSAT

WI C V V V VL

µ = − −

( ) satvD OX G TI WC V V≈ −

For SHORT channel,

For LONG channel

20

12DSAT X DSAT

WI C VL

µ =

( )[ ]TGDSAT VVVwhere −=

5For short �quasi-ballistic� MOSFETs, current is limited by source-to-channel injection of thermal carriers. Since, here the longitudinal field is low, this injection is limited by low field mobility. (Natori and Lundstrom)

)(11][ TG

c

cTODSAT VV

rrWvCI −

+−=

6

7

Compressive Strain in Si1-xGex

� Si1-xGex Bulk Properties.� 0 to 100% Ge possible� Lattice constant and bandgap

given by Vegard�s Law:

� Si1-xGex on Si� Up to 4.2% lattice mismatch� Strained epitaxial layers� Tetragonal distortion breaks

degeneracy in the bandstructure.

0 20 40 60100

101

102

103

104

Ge Conc. (%)

Cri

tical

Thi

ckne

ss (n

m)

Metastable

Relaxed

aSiGe x( ) = aSi 1 − x( ) + aGe x( )

8

Bandstructure effective mass, m*, is inversely related to curvature of bands, and depends on crystal orientation.Density of states m* is related to geometric mean of bandstructure m*. Must count number of �equivalent� valleys.Conductivity m* is harmonic mean of bandstructure m*.

m* = 3 1m1

* + 1m2

* + 1m3

*

−1

µ =e τm *

m* = m1*m2

*m3*( )

13

( ) ( )212

3

2

*

4 cde

c EEhmEg −

= π

=

2

2

2*

dkEd

m !“Effective mass”

9

Si-based Strained Materials

grow

th

dire

ctio

nE

k

HH

LH

grow

th

dire

ctio

n

E

k

HH

LH

CB

Bulk Si CS-SiGe

VB

CB

TS-Si Relaxed SiGe

VB

CS-SiGe TS-Si

10

Calculated Electron and Hole Mobility of Strained SiGeHole mobility with/without alloy scattering in plane and out-of-plane

Electron mobility with/without alloy scattering in plane and out-of-plane

Ge MOLE FRACTION

Fischetti and S.E. Laux, J. Appl. Phys. 80 (4), 1996

11

Monte Carlo calculations of minority hole mobilities in Si1-xGex for four doping levels (in cm-3) at 300 K: dot-dashed line is the vertical mobility of strained Si1-xGex, solid line is the mobility of unstrained Si1-xGex, dashed line is the planar mobility of strained Si1-xGex.

FM Bufler, P Graf, B Meinerzhagen, G Fischer, H Kibbel. Hole transport investigation in unstrained and strained SiGe. J. Vac. Sci. Technol. B 16(3), pp. 1667-1669, 1998.

12

• Base pressure: ~10-10 Torr• Cold wall, load locked system• Low deposition pressures

• 1 to 10 mTorr• ~500o C growth temperature

• Gases• Si2H6, GeH4, CH3SiH3

• B2H6, PH3

Ultra High Vacuum Chemical Vapor Deposition

� Can use hot wall UHVCVD or RTPCVD

13

14

Strained Si-on-SiGe Buffer

15

Inversion Layer Mobility

Strained Si mobility enhancement may be due to reduced surface roughness, rather than bandstructure(Fischetti)

16

Strained Si NMOSFET Monte Carlo Simulations

0.0 0.2 0.4 0.6 0.8 1.0 1.20

200

400

600

800

1000

MCUT, γ = 0.0 MCUT, γ = 0.2 MCUT, reduced SR, γ = 0.2

Takagi et al.

Rim et al.

Welser et al.

Normal Effective Field ( MV/cm )

!MOSFET (Tox= 2 nm) K. K. Rim et. al., IEEE Trans. on Elec. Dev., 47 (7), pp. 1406, 2000! MIT well-tempered device structure:http://www-mtl.mit.edu/Well/device50/topology50.html!1-D Schrödinger equation for quantum correction!As suggested by Fischetti et. al., J. Appl. Phys., 92 (12), 2002), surface roughness reduction may play a role in mobility increase in strained-Si devices.

17

Current enhancement in Strained Si NMOSFET

0.0 0.4 0.8 1.2 1.6 2.00

100

200

300

400

500

600

700 S-Si, γ=0.2 Bulk-Si S-Si, γ=0.2, reduced SR

Vg - VT = 1.0 V

Vg - VT = 2.0 V

Id (

µA/µ

m )

Vd (V)0.07 0.08 0.09 0.10 0.11 0.12 0.130

1

2

3

4

BSi = Unstrained SiSSi = Strained Si on Si0.8Ge0.2

Tox = 67 AL = 50 nmVG - VT = 0.5 V

Channel Position (um)

Ave

rage

Drif

t Vel

ocity

(107 c

m/s

)

BSi 0.1 V SSi 0.1 V BSi 0.3 V SSi 0.3 V BSi 0.6 V SSi 0.6 V

18

Strained Si p-MOSFETs

� µeff enhancement decreases with gate overdrive* for holes� No p-channel enhancement at Eeff = 1 MV/cm** for x = 0.28� Physical mechanism poorly understood

2.5x1012 5.0x1012 7.5x1012 1.0x10131.2

1.3

1.4

1.5

1.6

1.7

Mob

ility

Enh

ance

men

t ove

r Cz-

Si

Ninv per cm2

ε-Si on Si0.7Ge0.3

ε-Si on Si0.65Ge0.35

ε-Si on Si0.6Ge0.4

**Rim et al.Symposium on VLSI Technology(2002)

Performance benefits of Performance benefits of εε--Si Si primarilyprimarily from from nn--MOSFETMOSFET

Adapted from Leitz et al. J.Appl. Phys. 92, 3745 (2002)

*Rim et al. IEDM(1995).Nayak et al. IEEE Trans. on Elec. Dev. 43, 1709 (1996)

*Lee and Fitzgerald, MARCO, 2004

19

Mobility enhancement in tensile and compressively strained Si

20

Mobility with Strain (Courtesy: Yu)

21

SiGe strain- and band-engineered heterostructures

� Relaxed Si1-xGex virtual substrates allow � Si-rich alloys in biaxial tension (a║/ao > 1)

� Type-II band offset → Two-dimensional electron gas� Barrier for holes

� Ge-rich alloys in biaxial compression (a║/ao < 1)� Type-I band offset → Two-dimensional hole gas� Cannot confine electrons

Relaxed Si1-xGex

Ec

Ef

Ev

ε-Si

Increasing x

2DEG

Relaxed Si1-xGex

Ec

Ef

Ev

ε-Si1-yGey

Increasing y

22

Dual-channel heterostructures

� Cannot confine ψ in one channel or another� Different from traditional buried-channel device

� ε-Si1-yGey layer beneath ε-Si boosts µeff even at large Ninv, high Eeff

� ε-Si is more than just a �cap�� Enhanced hole mobility*, different from pseudomorphic SiGe devices

Relaxed Si1-xGex

Ec

Ef

Ev

ε-Si

ε-Si1-yGey

ψholes

ψ decays in ε-Si due to deep well in Si1-yGey

23

Ge, et. al, IEDM 2003

24

Sanuki, et. al. IEDM 2003

25

Electronic Properties of Strained Si1-xGex

HH & LHHH

∆Ev

∆Ec∆4

∆6

RelaxedSi

StrainedSi 1-x Ge x

Type-I (Compressive)

a||

a⊥

aSi

26

SiGe MOSFET Structure

� Typical MOS structure� Si1-xGex channel with Si cap

leads to buried channel, and lower gate capacitance

� Si cap� Used for oxidation� Also acts as a parasitic channel,

leading to gate operating �window�

Poly

SiGe

Si Substrate

Depth

Ene

rgy

Ec

Ev

Oxide

∆Ev

Z.Shi, ..S. Banerjee �Simulation and optimization of strained Si1-xGex buried channel p-MOSFETs,� Solid State Electronics, 44 (7): 1223, 2000.

27

Output, sub-threshold, and mobility-field characteristics of 70nm Si0.9Ge0.1-SiO2buried channel PHFET.

E. Quinones,..S.Banerjee �Design, Fabrication, and Analysis of SiGeCHeterojunction PMOSFETs,� IEEETrans.Elec.Dev., Sept. 2000.

28

Subthreshold and mobility-field characteristics for 180nm Si0.8Ge0.2-HfO2 PHFETsand control Si PMOSFET. Recovery of mobility degradation for high-k gate dielectrics with enhanced-mobility channels: (Onsongo,.., Banerjee)T.Ngai, J.Lee, S.Banerjee, �Electrical Properties of ZrO2 Gate Dielectric on SiGe,� Appl. Phys. Lett., 76(4), p. 502, Jan. 2000.

29

Addition of C to Si and Si1xGex

� Carbon has a smaller lattice constant� Strain compensation of SiGe (~8:1)� Tensile strained Si-C on Si

� Low solubility in Si� Need low temperature growth to incorporate

C due to low solubility (5x1017 cm-3)� Growth window for alloy growth

� Carbon ∆Ev=21-26meV/%C in SiGeC (Lanzerotti, EDL,1996)

� Ge ∆Ev=25 meV/3% Ge

� Carbon ∆Ec=75-90 meV/%C for SiC � (Faschinger, APL, 1995)

Si Si1-yCy

∆(6) ∆4

∆2

hh, lh lh

hh

CB

VB

Alloy

CarbideAmorphous

TwinnedIslanded

Temperature

Car

bon

Con

cent

ratio

n

A. R. Powell, K. Eberl, B. A. Ek, and S. S. Iyer, "Si1-x-yGexCy growth and properties of the ternary system," Journal of Crystal Growth Vol. 127, pp. 425, 1993.

K. Eberl, K. Brunner, W. Winter, “Pseudomorphic Si1-yCy and Si1-x-yGexCyalloy layers on Si,” Thin Solid Films, Vol. 294, pp. 98, 1997.

30

Carbon Strain Compensation

Red = after processing

Blue = as grownSi0.795Ge0.2C0.005

Si0.8Ge0.2

XRD Rocking Curves

-3 10-3

-2.5 10-3

-2 10-3

-1.5 10-3

-1 10-3

-5 10-4

0 100

-5-4-3-2-10

Bare Si0% C, 40% Ge1% C, 40% Ge1.5% C, 40% Ge

I D (A)

VD(V)

VG

-VT=-4V

VG

-VT=-2V

Si0.585Ge0.4C0.015 shows 55% drive current enhancement over bulk Si and 42% enhancement over Si0.6Ge0.4

W/L= 10/0.5 um; tox= 6 nm

S.Ray, .. S.Banerjee, �Novel SiGeC Channel Heterojunction PMOSFET,� Proc. of Int. Elec. Dev.Meet., 1996.

31

Electronic Properties of Strained Si1-xGex

� Strain splits the six fold degeneracy of conduction band valleys.

� The four-fold in-plane valleys are lowered, leading to less f-type scattering.

� Carriers have a lower out-of-plane and higher in-plane mass.

� Electron mobility dependent on directions: increase in ⊥⊥⊥⊥ , decrease in ||

� To enhance electron mobility for the planar NMOSFETs, tensile strained Sihas to be used.

[001]

[010]

[100]

� Hole mobility increases with Ge fraction.� Valence band splitting with strain

results in reduced scattering.� Reduction of effective hole mass with

strain due to VB warpage� Increase in both ⊥⊥⊥⊥ and || mobilities

Chen XD,.. Banerjee SK, Hole and electron mobility enhancement in strained SiGe vertical MOSFETs, IEEE T ELECTRON DEV 48 (9): 1975-1980 SEP 2001

32

Hole mobility in Strained SiGe

33

Mobility in Relaxed SiGe

-4 0

-3 0

-2 0

-1 0

0

-2 . 5-2-1 . 5-1-0 . 50

S iR e l. S iG eS t r . S iG e

I D (m

A)

VD S

(V )

VG S

- VT

= - 1 .5 V

- 0 .5 V

0 V

W /L = 1 2 0 µµµµ m /7 0 n mT o x = 4 n m

Vertical Si, relaxed and strained Si1-xGeX PHFETsshowing enhancement of drive current over SiMOSFETs only in the presence of strain. NHFETs also enhanced! Jayanaran & Banerjee, EMC 2004

34

Ge MOSFETs

� Bulk Ge has higher electron (2.5x) and hole (4x) mobility than Si, and can potentially lead to faster MOSFETs and more balanced N vs. PMOSFETs.

� Germanium bulk substrates brittle, lower thermal conductivity (0.6W/cm-K vs. 1.5 for Si)� Smaller Ge bandgap than Si broadens absorption spectrum; optoelectronic integration

on CMOS?� Native oxide on Ge surface is not stable; GeO2 water soluble, GeO volatile at low T.

Deposited high-k gate dielectrics promising� Performance much worse than expected, especially for NMOSFETs, probably because

of poor interface between Ge and high-k gate dielectric, as well as poor dopantactivation and interface between metal- source/drain

� Higher junction leakage in Ge, especially at high T� Higher dielectric constant in Ge leads to worse electrostatics (DIBL, SS)

1.120.66Eg (eV)

4501900µp(cm2/Vs)

15003900µn(cm2/Vs)

SiGe

A. Ritenour et al., IEDM, p.433, 2003

Rosenberg et al. EDL 9, 639 (1988), Shang et al. IEDM(2002), Chui et al. IEDM (2002), Bai et al. VLSI (2003)

35

UHVCVD Ge-on-Si NMOS w/o SiGe buffer with PVD HfO2 and TaN gate

0

0.5

1

1.5

2

2.5

3

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Cap

acita

nce(

uF/c

m2 )

VG(V)

EOT=1.1nm

L=10um, W=100um

High frequency (1MHz) andcalculated low frequency

10-13

10-11

10-9

10-7

10-5

-1 -0.5 0 0.5 1 1.5 2 2.5 3

Vds=0.1VVds=0.6VVds=1.1VVds=1.6VVds=2.1V

I S(A/u

m)

VGS

(V)

L=2um, W=5um

SS=170mV/dec

Vt~0.3V

0

0.0001

0.001

0.01

0.1

1

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Ig(A)/cm2

Ig(A

)/cm

2

VG(V)

L=10um, W=100umEOT=1.1nm

0.0

2.0

4.0

6.0

8.0

10.0

0 0.5 1 1.5 2 2.5

Vg=0.1VVg=0.6VVg=1.1VVg=1.6VVg=2.1V

I S(uA

/um

)

VDS

(V)

L=2um, W=5um

Donnelly, .., Banerjee, SRC 2004

36

Effect of strain in Ge layer

2 4 6 8 10 12 14×10120

200400600800

10001200140016001800

Effe

ctiv

e m

obilit

y (c

m2 /

Vs)

Ninv per cm2

Si0.5Ge0.5

Si0.4Ge0.6

Si0.3Ge0.7

Si0.2Ge0.8

Si0.1Ge0.9

Ge Cz-Si

�� Higher strain in the Ge layer favors high Higher strain in the Ge layer favors high µµeffeff**

� x ≤ 0.7� reasonable agreement with theoretical calculations**

� x ≥ 0.8 � rampant defect nucleation in Si cap, µeff depressed

x ↓, εGe ↑,µeff ↑

**M. V. Fischettiand S. E. Laux, J. Appl. Phys. 80,(1996).

0.5 0.6 0.7 0.8 0.9 1.00

2

4

6

8

10

122 1.6 1.2 0.8 0.4 0

Hol

e m

obilit

y en

hanc

emen

t

Ge fraction in virtual substrate

Experimental Theoretical

% Compressive strain in Ge

Experimental values computed at Ninv = 8×1012/cm2

*Lee et al. Appl. Phys. Lett. 79, 3344 (2001). Lee et al. inMater. Res. Soc. Symp. Proc., vol. 686, A1.9.1(2002).

37

3-D Transistor Structures

S

G

D

S DG

S G D

source

top gate

bottom gate drain

100 nmIBM �97

Berkeley �99

Lucent �99

Mark Bohr, ECS Meeting PV 2001-2, Spring, 2001

38

Si0.9Ge0.1 (340 nm)

Buried-Oxide (100 nm)

Strained-Si (20 nm)

SiO2 (9 nm)

n+-Poly Gate(200 nm)

Strained-Si PMOSFET on SiGe-on-Insulator

T. Mizuno et al., IEDM, 934-936 (1999)

Strained SiRelaxedSi1-xGex

SiGe Buffer

Si Substrate

Gate

p+ p+

SiO2

S

G

D

Tensile Strain

Buried SiO2

39Rim, et. al. & Sturm et. al. (IEDM, 2003)

SSOI with thin SiGe buffers w/o misfits using BPSG compliant substrates

40

High mobility heterojunction transistor (HMHJT)

� A Si/SiGe/Si quantum well is used to increase the drive current.

� The bulk punchthrough, DIBL and floating body effect are still suppressed due to heterojunction in the deep source/drain region.

Q.Ouyang, X.Chen, ..A.Tasch, S.Banerjee, �Bandgap Engineering in Deep Submicron Vertical PMOSFETs,� Dev. Res. Conf., 2000.

41

Energy Band Diagram and Hole Concentration of HMHJT in the Channel

-0.5

0

0.5

1

1.5

2

2.5

-0.05 0 0.05 0.1 0.15

E v (eV)

Distance along the channel (µµµµm)

HJMOSFETHMHJTSi

VDS

=VGS

=-2V

1nm from the surface

Source Channel

Drain

1018

1019

1020

1021

-0.05 0 0.05 0.1 0.15

Hol

e C

once

ntra

tion

(cm

-3)

Distance along the channel(µµµµm)

HJMOSFETHMHJTSi

VDS

=VGS

=-2V

42

Subthreshold and Output Characteristics for HMHJT

10-11

10-9

10-7

10-5

10-3

-3 -2.5 -2 -1.5 -1 -0.5 0

I D (A

)

VGS

(V)

Si controlHMHJT

VDS

= -0.1, -0.6, -1.1, -1.6 V

-8

-7

-6

-5

-4

-3

-2

-1

0-2 -1.5 -1 -0.5 0

I D (m

A)

VDS

(V )

Si control deviceHMHJT

VG-V

T=0, -0.5, -1, -1.5, -2V

43

Process Issues

� STI edge leakage can be increased by SiGe buffer.

� B diffusion enhanced by tensile strain; compressive retards it. Ge retards B and enhances As/P diffusion in SiGe buffer.

� Need higher doping in channel due to reduced bandgap-negates some advantages of strain

44

Defects in Strained Si

45

Takagi, et. al, IEDM 2003

46

Device Metrics� Speed

� τ = Cload VDD / Id� Power

� P = f Cload VDD2

� Saturation current� IDSAT = (W/2L) (krkoA) (TEOT,INV)-1 µ (VG-VT)2

� Consider VG ⇒ VDD

� Transconductance� gm = (W/L) (krkoA) (TEOT,INV)-1 µ VDSAT

� Off-state power� Subthreshold swing and source/drain junction leakage