in-situ iridium refractory ohmic contacts to p-ingaas
DESCRIPTION
In-situ Iridium Refractory Ohmic Contacts to p-InGaAs. Ashish Baraskar, Vibhor Jain, Evan Lobisser, Brian Thibeault, Arthur Gossard, Mark J. W. Rodwell University of California, Santa Barbara, CA Mark Wistey University of Notre Dame, IN. Outline. Experimental details Contact formation - PowerPoint PPT PresentationTRANSCRIPT
NAMBE 2010 Ashish Baraskar, UCSB 1
In-situ Iridium Refractory Ohmic Contacts to p-InGaAs
Ashish Baraskar, Vibhor Jain, Evan Lobisser,
Brian Thibeault, Arthur Gossard, Mark J. W. Rodwell
University of California, Santa Barbara, CA
Mark Wistey
University of Notre Dame, IN
NAMBE 2010 Ashish Baraskar, UCSB 2
Outline• Motivation
– Low resistance contacts for high speed HBTs
– Approach• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– Doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
NAMBE 2010 Ashish Baraskar, UCSB 3
Outline• Motivation
– Low resistance contacts for high speed HBTs
– Approach• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– Doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
NAMBE 2010 Ashish Baraskar, UCSB 4
Device Bandwidth Scaling Laws for HBT
To double device bandwidth:
• Cut transit time 2x
• Cut RC delay 2x
Scale contact resistivities by 4:1*
*M.J.W. Rodwell, CSICS 2008
bcWcTbT
eW
HBT: Heterojunction Bipolar TransistorRC
f in 2
1
effcbbb CR
ff
8max
NAMBE 2010 Ashish Baraskar, UCSB 5
InP Bipolar Transistor Scaling Roadmap
Emitter256 128 64 32 nm width
8 4 2 1 Ω·µm2 access ρ
Base175 120 60 30 nm contact width
10 5 2.5 1.25 Ω·µm2 contact ρ
Collector
106 75 53 37.5 nm thick
9 18 36 72 mA/µm2 current
4 3.3 2.75 2-2.5 V breakdown
ft 520 730 1000 1400 GHz
fmax 850 1300 2000 2800 GHz
Contact resistivity serious challenge to THz technology
Less than 2.5 Ω-µm2 base contact resistivity required for simultaneous THz ft and fmax*
*M.J.W. Rodwell, CSICS 2008
NAMBE 2010 Ashish Baraskar, UCSB 6
Approach - I
To achieve low resistance, stable ohmic contacts
• Higher number of active carriers
- Reduced depletion width
- Enhanced tunneling across metal-
semiconductor interface
• Better surface preparation techniques
- For efficient removal of oxides/impurities
NAMBE 2010 Ashish Baraskar, UCSB 7
• Scaled device thin base
(For 80 nm device: tbase < 25 nm)
• Non-refractory contacts may diffuse at higher temperatures through
base and short the collector• Pd/Ti/Pd/Au contacts diffuse about 15 nm in InGaAs on annealing
Approach - II
100 nm InGaAs grown in MBE
15 nm Pd/Ti diffusion
Need a refractory metal for thermal stability
TEM: Evan Lobisser
NAMBE 2010 Ashish Baraskar, UCSB 8
Outline• Motivation
– Low resistance contacts for high speed HBTs and FETs
– Approach• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– Doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
NAMBE 2010 Ashish Baraskar, UCSB 9
Semi-insulating InP Substrate
100 nm In0.52Al0.48As: NID buffer
100 nm In0.53Ga0.47As: C (p-type)
Epilayer growth by Solid Source Molecular Beam Epitaxy (SS-MBE)– p-InGaAs/InAlAs
- Semi insulating InP (100) substrate
- Un-doped InAlAs buffer
- CBr4 as carbon dopant source
- Hole concentration determined by Hall measurements
Epilayer Growth
NAMBE 2010 Ashish Baraskar, UCSB 10
100 nm In0.52Al0.48As: NID buffer
100 nm In0.53Ga0.47As: C (p-type)
20 nm in-situ Ir
Semi-insulating InP Substrate
In-situ Ir contacts
In-situ iridium (Ir) deposition - E-beam chamber connected to MBE chamber- No air exposure after film growth
Why Ir? - Refractory metal (melting point ~ 2460 oC)- Easy to deposit by e-beam technique- Easy to process and integrate in HBT process flow
NAMBE 2010 Ashish Baraskar, UCSB 11
TLM (Transmission Line Model) fabrication
• E-beam deposition of Ti, Au and Ni layers
• Samples processed into TLM structures by photolithography and liftoff
• Contact metal was dry etched in SF6/Ar with Ni as etch mask, isolated by wet etch
100 nm In0.52Al0.48As: NID buffer
100 nm In0.53Ga0.47As: C (p-type)
20 nm in-situ Ir20 nm ex-situ Ti
50 nm ex-situ Ni500 nm ex-situ Au
Semi-insulating InP Substrate
NAMBE 2010 Ashish Baraskar, UCSB 12
• Resistance measured by Agilent 4155C semiconductor parameter
analyzer
• TLM pad spacing (Lgap) varied from 0.5-25 µm; verified from
scanning electron microscope (SEM)
• TLM Width ~ 25 µm
Resistance Measurement
W
RR ShC
C
22
0
1
2
3
4
5
0 0.5 1 1.5 2 2.5 3 3.5
Gap Spacing (m)
Res
ista
nce
()
NAMBE 2010 Ashish Baraskar, UCSB 13
• Extrapolation errors:– 4-point probe resistance
measurements on Agilent 4155C
– Resolution error in SEM
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 3 4 5 6
Res
ista
nce
()
Gap Spacing (m)
R
d
Rc
Error Analysis• Processing errors:
Lgap
W
Variable gap along width (W)
1.10 µm 1.04 µm
– Variable gap spacing along width (W)
Overlap Resistance
– Overlap resistance
NAMBE 2010 Ashish Baraskar, UCSB 14
Outline• Motivation
– Low resistance contacts for high speed HBTs and FETs
– Approach• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– Doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
NAMBE 2010 Ashish Baraskar, UCSB 15
1019
1020
40
50
60
70
80
0 10 20 30 40 50 60
hole concentration
mobility
Mob
ility
(cm
2 -Vs)
CBr4 foreline pressure (mtorr)
Hol
e C
once
ntra
tion
(cm
-3)
Doping Characteristics-I
Hole concentration Vs CBr4 flux
– Hole concentration saturates at high CBr4 fluxes– Number of di-carbon defects as CBr4 flux *
Tsub = 460 oC
*Tan et. al. Phys. Rev. B 67 (2003) 035208
NAMBE 2010 Ashish Baraskar, UCSB 16
2
4
6
8
10
10 20 30 40 50 60
Group V / Group III
Hol
e C
once
ntra
tion
(10
19)
cm-3
As V/III ratio hole concentration
hypothesis: As-deficient surface drives C onto group-V sites
Doping Characteristics-IIHole concentration Vs V/III flux
CBr4 = 60 mtorr
NAMBE 2010 Ashish Baraskar, UCSB 17
4 1019
8 1019
1.2 1020
1.6 1020
2 1020
300 350 400 450
Hol
e C
once
ntra
tion
(cm
-3)
Substrate Temp. (oC)
CBr4 = 60 mtorr
Doping Characteristics-IIIHole concentration Vs substrate temperature
*Tan et. al. Phys. Rev. B 67 (2003) 035208
Tendency to form di-carbon defects as Tsub *
NAMBE 2010 Ashish Baraskar, UCSB 18
4 1019
8 1019
1.2 1020
1.6 1020
2 1020
300 350 400 450
Hol
e C
once
ntra
tion
(cm
-3)
Substrate Temp. (oC)
CBr4 = 60 mtorr
Doping Characteristics-IIIHole concentration Vs substrate temperature
*Tan et. al. Phys. Rev. B 67 (2003) 035208
1019
1020
0 20 40 60 80 100
Tsub = 460 oC
Tsub = 350 oC
CBr4 foreline pressure (mtorr)
Hol
e C
once
ntra
tion
(cm
-3)
Tendency to form di-carbon defects as Tsub *
NAMBE 2010 Ashish Baraskar, UCSB 19
ρc lower than the best reported contacts to
pInGaAs (ρc = 4 Ω-µm2)[1,2]
1. Griffith et al, Indium Phosphide and Related Materials, 2005.
2. Jain et al, IEEE Device Research Conference, 2010.
Metal Contact ρc (Ω-µm2) ρh (Ω-µm)
In-situ Ir 0.58 ± 0.48 7.6 ± 2.6
• Hole concentration, p = 2.2 x 1020 cm-3
• Mobility, µ = 30 cm2/Vs• Sheet resistance, Rsh = 94 ohm/ (100 nm thick film)
Results: Contact Resistivity - I
0
5
10
15
0 1 2 3 4Pad Spacing (m)
Res
ista
nce
()
p = 2.2 x1020 cm-3
TLM width, W = 25 m
NAMBE 2010 Ashish Baraskar, UCSB 20
*
p1expρC
Tunneling
Data suggests tunneling
ThermionicEmission c ~ constant*
High active carrier concentration is the key to low resistance contacts
* Physics of Semiconductor Devices, S M Sze
Results: Contact Resistivity - II
0
2
4
6
8
10
5 10 15 20 25
Co
nta
ct R
esis
tivity
, c (
-
m2 )
Hole Concentration, p (x1019 cm-3)
1
10
5 10 15
[p]-1/2 (10-11 cm-3/2)
c (
m
2 )
p = 2.2×1020 cm-3
p = 5.7×1019 cm-3
NAMBE 2010 Ashish Baraskar, UCSB 21
Mo contacts annealed under N2 flow for 60 mins. at 250 oC
Thermal Stability - I
Before annealing After annealing
ρc (Ω-µm2) 0.58 ± 0.48 0.8 ± 0.56
0
5
10
15
0 1 2 3 4
Un-annealedAnnealed
Pad Spacing (m)
Res
ista
nce
()
TLM width, W = 25 m
p = 2.2 x1020 cm-3
TEM: Evan Lobisser
NAMBE 2010 Ashish Baraskar, UCSB 22
Summary
• Maximum hole concentration obtained = 2.2 x1020 cm-3 at a substrate temperature of 350 oC
• Low contact resistivity with in-situ Ir contacts lowest ρc= 0.58 ± 0.48 Ω-µm2
• Need to study ex-situ contacts for application to HBTs
NAMBE 2010 Ashish Baraskar, UCSB 23
Thank You !
Questions?
Acknowledgements:
ONR, DARPA-TFAST, DARPA-FLARE
NAMBE 2010 Ashish Baraskar, UCSB 24
Extra Slides
NAMBE 2010 Ashish Baraskar, UCSB 25
Correction for Metal Resistance in 4-Point Test Structure
WLsheet /metalR Wcontactsheet /)( 2/1
Error term (Rmetal/x) from metal resistance
L
W
I
I
V
V
xRWLW metalsheetcontactsheet ///)( 2/1
NAMBE 2010 Ashish Baraskar, UCSB 26
Random and Offset Error in 4155C
0.6642
0.6643
0.6644
0.6645
0.6646
0.6647
0 5 10 15 20 25
Res
ista
nce
(
)
Current (mA)
• Random Error in resistance measurement ~ 0.5 m
• Offset Error < 5 m*
*4155C datasheet
NAMBE 2010 Ashish Baraskar, UCSB 27
Accuracy Limits
• Error Calculations– dR = 50 mΩ (Safe estimate)– dW = 1 µm– dGap = 20 nm
• Error in ρc ~ 40% at 1.1 Ω-µm2