presented by david q. kelly principal investigator: sanjay k. banerjee microelectronics research...
TRANSCRIPT
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Presented By David Q. KellyPrincipal Investigator: Sanjay K. Banerjee
Microelectronics Research CenterUniversity of Texas at Austin
Austin, Texas, U.S.A.
Germanium-Carbon Layers on Sifor Enhanced-Channel-Mobility MOSFETs
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Advantages of Germanium
• Bulk Ge has higher electron (2.5×) and hole (4×) mobility than Si– Buried channel PMOS has shown very high mobility
enhancements
• Compatible with high-κ gate dielectrics
• Lower temperature processing
Ge Sin (cm2/V·s) 3900 1500
p (cm2/V·s) 1900 450
Eg (eV) 0.66 1.12
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Background:Disadvantages of Germanium
Native GeO2 cannot be used as gate dielectric– Thermally desorbs above 420°C– Soluble in water
• Requires surface passivation for good interface with high-κ dielectrics
• Smaller energy bandgap– Increased subthreshold leakage current
• Poor NMOSFET Performance– Would require a separate approach
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Graded Si1-xGex Buffer Layers (MIT)
• Thick, relaxed, graded SiGe virtual substrates with compressively-strained Ge top layer
• Calls for a CMP step to remove surface roughness on the virtual substrate prior to strained Ge layer growth
Currie, et al. APL 1998
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Other Approaches to Ge on Si• Cyclical Thermal Annealing
– L. Kimerling (MIT)– Two-step growth following by cyclical
annealing at 900°C and 780°C
• Surfactant-mediated epitaxy– K. R. Hofmann (Germany)– Surfactant doping (Sb atoms)– Has only been demonstrated using
MBE
• Thermal annealing in hydrogen– Saraswat (Stanford)– Pure Ge layer grown on Si is annealed
in hydrogen to fully relax layer– Low-defect density Ge layer is re-
grown over relaxed layer
• Condensation of epitaxial SiGe– S. Takagi (Japan) / IBM– Could be promising for Ge-on-insulator– Requires SGOI substrate
Cyclical Annealing
Hydrogen Annealing
Ge Condensation
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CVD Ge1-xCx Grown Directly on Si
• Group at Arizona State University was the first to demonstrate Ge1-xCx films grown directly on Si (100) by UHVCVD– Appl. Phys. Lett. 68 (17) pp. 2407-2409, 1996.– Chem. Mat. 8 (10) pp. 2491-2498, 1996.
• Key to achieving efficient C incorporation is to use precursors with “pre-formed” Ge–C bonds– Methylgermane CH3GeH3
– Digermylmethane CH2(GeH3)2
– Trigermylmethane CH(GeH3)3
• MOS devices never reported until now
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Ge1-xCx Layer Growth by UHVCVD
• 4” n-type wafers cleaned using HF-last process
• Base pressure prior to deposition was 7.0×10-10 Torr
• Growth Temperature 450°C
• Mixture of GeH4 and CH3GeH3 precursors introduced at deposition pressure of 5 mTorr
• Final layer thickness ~ 30 nm
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XTEM Results (30 nm Ge1-xCx on Si)
Si Substrate
Ge 1-
xC x
Si c
ap
glue (sample preparation)
Si Substrate
Ge 1-
xC x
Si c
ap
glue (sample preparation)
• Larger area scan shows that Ge1-xCx layer is epitaxial and has no visible threading dislocations– Strain relaxation is thought to occur through misfit
dislocations confined at interface
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Ge1-yCy Surface Roughness Dependence on Growth Temperature and C Incorporation
• Film quality depends on both the growth temperature and the amount of methylgermane flow– AFM RMS surface roughness improves with decreasing
growth temperature– Higher carbon incorporation also leads to smoother film
11.9415.5831.03
0.641.01
0.10
1.00
10.00
100.00
0 0.02 0.04 0.06 0.08
CH3GeH3:GeH4 Ratio
AF
M R
MS
Ro
ug
hn
es
s (
nm
)
Temp. = 550C
Temp = 480C
3.862
1.724
0.911
0
1
2
3
4
5
400 450 500Growth Temperature (°C)
AF
M R
MS
Ro
ug
hn
es
s (
nm
)
CH3GeH3:GeH4Ratio = 0.04
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Very Low RMS Surface Roughness Measured by AFM
• Low growth temperature is important for achieving smooth Ge1-xCx film
• Nearly atomically flat
• RMS Roughness = 0.32 nm
• Large 3D islands• RMS Roughness = 31 nm
600°C Growth Temperature 450°C Growth Temperature
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Ge1-xCx
20 nmSi (001)
Substrate
glue (sample preparation)
20 nm
Pure Ge
Si (001) Substrate
glue (sample preparation)
XTEM Comparison of Ge1-xCx on Si with pure Ge on Si
• Pure Ge grown at low temperature directly on Si shows large number of threading dislocations
• Not present in Ge1-xCx layer
Ge1-xCx Pure Ge
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• Threading dislocation densities cannot be calculated using conventional etch-pit techniques because Ge1-xCx is too thin– Use a diluted etch pit solution in conjunction
with atomic force microscopy (AFM)– 30µm×30µm measurement window
– Estimated density for Ge1-xCx ~3×105 cm-2
Bulk Ge Ge1-xCx on Si Pure Ge on Si
No etch pits
3 etch pits Numerous etch pits
EPD technique was developed by UT-Austin Master’s student Isaac Wiedmann
2.1×108 cm-2
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Si (001) Substrate
Ge1-xCx
in-plane mismatch
perpendicular mismatch
Lattice Parameter and Strain Relaxation (XRD)
aGe
5.658 Å
a||
5.598 Å a ⊥
5.679 Å ar
5.643 Å
Ge1-xCx
Reciprocal space map of (224) reflection78% relaxed
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Carbon Segregation Effects
1E+17
1E+18
1E+19
1E+20
1E+21
1E+22
1E+23
1E+24
0 10 20 30 40 50 60 70 80
Depth (nm)
Co
nc
en
tra
tio
n (
ato
ms
/cm
-3)
12C
28Si
Interface withSi Substrate
SIMS EFTEM
• Brighter regions correspond to higher C concentration
• Suggests chemical reaction of C with Si at the GeC/Si interface
• SIMS measured using standard prepared by ion implantation
• Higher C level at interface
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EELS Data
• Energy of the C plasmon peak energy increases as we get closer to the Ge1‑xCx/Si substrate interface
• This is evidence for the higher sp3 character of the C atoms located near this interface
• This higher sp3 character could indicate the presence C-containing interstitial complexes or substitutional C in Ge near the interface. Both of these are mechanisms for strain relaxation, which helps to explain the low density of threading dislocations in the films
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0.2
0.25
0.3
0.35
0.4
0.45
0.5
No Anneal 600C 700C 800C
Anneal Temperature (1 min. RTA)
RM
S S
urf
ac
e R
ou
gh
ne
ss
(n
m)
Thermal Stability:AFM and XRD Rocking Curves
-8000 -6000 -4000 -2000 0 2000
Δθ (arcseconds)
Inte
ns
ity
(a
rb. u
nit
s)
Ge1-xCx
Si
as grown
600 °C
700 °C
800 °C
• Layers were 30nm Ge1-xCx with 5nm Si cap
• Ge1-xCx peak in XRD rocking curve is shifting toward the Si peak
– Lattice constant decreasing due to relaxation and Si diffusion
• RMS roughness measured by AFM increases slightly but remains smooth
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Effect of Annealing on Lattice Parameter(Si Diffusion, Relaxation)
1.0303
1.04571.0468
1.0398
1.0261.0281.0301.0321.0341.0361.0381.0401.0421.0441.0461.0481.050
no anneal(as grown)
600C1 min. RTA
700C1 min. RTA
800C1 min. RTA
Anneal Condition
d4
00 r
atio
GeC
fully relaxed bulk Ge• d400 ratio defined as ratio of Si
and Ge1-xCx d-spacings measured using rocking curves with (004) reflection
• Shows that lattice parameter decreases below the value for fully-relaxed Ge
– Most likely due to diffusion of Si atoms during annealing
– Also due to strain relaxation
• Need to use reciprocal space maps for more precise lattice parameter measurement
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Step Process Notes
1Si Substrate
Cleaning2:1 H2SO4:H2O2 piranha followed by 40:1 DI:HF
2 Ge1-xCx Growth5 mTorr, GeH4 and CH3GeH3, 450C
3Surface
pretreatment
Si control - 40:1 DI:HFBC Ge1-xCx - 40:1 DI:HFSC Ge1-xCx - None
4 PVD HfO2/TaN ~7nm HfO2, 200nm TaN
5 Gate PatternLithography (Ring-type gates), RIE w/ CF4
6 Ion Implant BF2, 5×1015 cm-2, 25 keV
7 Contact LTO 530C, 2 hrs., 200nm
8 Contact/Metal Lithography, sputtered Al
9 Forming gas 6 slm, 450C, 30 min.
R2
R1
R1 = 75 µm
R2 = 85 µm
Leq ~ 10 µm
Weq ~ 500 µm
PMOSFET Fabrication Process
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PMOSFET Device Structures(Buried- and Surface-Channel Devices)
Ge1-xCx BC Gate Stack
n-type Si (001) Substrate
Ge1-xCx (30 nm)
HfO2
TaN
Si cap layer (6 nm)
Ge1-xCx SC Gate Stack
n-type Si (001) Substrate
Ge1-xCx (30 nm)
HfO2
TaN
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Ge1-xCx HfO2/TaN MOS CapacitorC-V Characteristics
• No Si cap layer• EOT
2.3 nm• Leakage J @ 1V
3.3×10-5 A/cm2
• Dit
4.8×1011 eV-1cm-2
• High VFB could be due to fixed negative charged introduced by diffusion of C atoms
0.0
0.5
1.0
1.5
0.0 1.0 2.0 3.0 4.0
1 MHzCalc. Low-Freq.
Cap
acit
ance
(µ
F/c
m2)
Voltage (V)
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Ge1-xCx HfO2/TaN MOS CapacitorC-V Hysteresis
• Measured using forward and reverse voltage sweeps
• 78 mV at 1 MHz• 85 mV at 500 kHz• About 250 mV
dispersion between two measurement frequencies
0.0
0.5
1.0
1.5
0.0 1.0 2.0 3.0 4.0
1 MHz500 kHz
Ca
pac
itan
ce
(µ
F/c
m2 )
Voltage (V)
78 mV
85 mV
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Ge1-xCx BC and SC pMOSFETsGate Leakage Current
• Higher gate leakage for surface-channel device
• Could be due to inadequate surface passivation prior to HfO2 deposition and/or HfO2/Ge1-xCx interdiffusion10-9
10-810-710-610-510-410-310-210-1100101
-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0
Si ControlBC GeCSC GeC
Lea
kag
e C
urr
ent
(A/c
m2)
Voltage (V)
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Output Characteristics forBuried-Channel Ge1-xCx pMOSFET
• Good saturation behavior
• IDsat = 10.8 µA/µm
@ VGS-VT = 1.0 V
• 2× enhancement over Si control
• EOT = 1.9 nm• W ~ 500 µm• L ~ 10 µm
0
2
4
6
8
10
12
-1.5 -1.0 -0.5 0.0
BC GeCSi Control
I D (
µA
/µm
)
VDS
(V)
~2X
VGS
-VT (V) = -0.2, -0.4, -0.6, -0.8, -1.0
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Gate C-V Characteristic for BC pMOSFET
• Gate C-V shows buried-channel behavior
• “Kink” is due to valence band offset between Ge1-xCx and Si cap layer
• Gate leakage (inset) is 2.6×10-6 A/cm2 @ -1V
0.0
0.5
1.0
1.5
2.0
-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0
Gat
e C
ap
acit
anc
e (µ
F/c
m2 )
Gate Voltage (V)
EOT = 1.9 nm
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Linear ID and Gm Characteristics forBuried-Channel Ge1-xCx pMOSFET
Si ControlBC GeC
• 1.8× enhancement in both IDlin and Gm over Si control
• Ion/Ioff = 5×104
0.0
0.2
0.4
0.6
0.8
1.0
1.2
-1.0 -0.5 0.0 0.5
Gm
(µ
S/µ
m)
VGS
- VT (V)
1.8X
0.0
0.2
0.4
0.6
0.8
1.0
-1.0 -0.5 0.0 0.5
I D (
µA
/µm
)
VGS
- VT (V)
1.8X
VDS
= 50 mV
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Output Characteristics forSurface-Channel Ge1-xCx pMOSFET
• IDsat = 15.2 µA/µm
@ VGS-VT = 1.0 V
• 3× enhancement over Si control
• EOT = 1.9 nm• W ~ 500 µm• L ~ 10 µm
02468
10121416
-1.5 -1.0 -0.5 0.0
SC GeCSi Control
I D (
µA
/µm
)
VDS
(V)
~3X
VGS
-VT (V) = -0.2, -0.4, -0.6, -0.8, -1.0
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Linear ID and Gm Characteristics forSurface-Channel Ge1-xCx pMOSFET
Si ControlSC GeC
• 2× enhancement in both IDlin and Gm over Si control
• Ion/Ioff < 102
0.0
0.2
0.4
0.6
0.8
1.0
-1.0 -0.5 0.0 0.5
I D (
µA
/µm
)
VGS
- VT (V)
~2X
VDS
= 50 mV
0.00.20.40.60.81.01.21.41.6
-1.0 -0.5 0.0 0.5
Gm
(µ
S/µ
m)
VGS
- VT (V)
~2X
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Subthreshold Characteristics forBC and SC Ge1-xCx pMOSFETs
10-710-610-510-410-310-210-1100101
-1.0 -0.5 0.0 0.5 1.0
I D (
µA
/µm
)
VGS
- VT (V)
VDS
= 50 mV
Si ControlGeC
• High subthreshold leakage makes SS calculation difficult
• Ion/Ioff BC = >5×104, SC = >102
10-710-610-510-410-310-210-1100101
-1.0 -0.5 0.0 0.5 1.0
I D (
µA
/µm
)
VGS
- VT (V)
VDS
= 50 mV
BC
SC
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Effective Hole Mobility Comparisons
• BC and SC pMOSFETs exhibit 1.5× and 2.5× enhancement over universal Si, respectively
0.0
100.0
200.0
300.0
400.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Universal SiSi ControlBC GeCSC GeCStrained Ge onrelaxed SiGe(Ritenour, et al.)
µef
f (c
m2 V
-1s-1
)
Eeff
(MV/cm)
2 12
lnDsat
effox TGS
R RIC V V
1
3i
eff dGe
QE Q
( )i ox GS TQ C V V 4d Ge a bQ qN