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
1
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
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
Device Bandwidth Scaling Laws for HBT
To double device bandwidth:
We
• Cut transit time 2x
Tb
• Cut RC delay 2x
Wbc
Tc
Scale contact resistivities by 4:1*
1
  in  RC
2f
f
max

HBT: Heterojunction Bipolar Transistor
f
8    Rbb  Ccb eff
*M.J.W. Rodwell, CSICS 2008
NAMBE 2010
Ashish Baraskar, UCSB
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InP Bipolar Transistor Scaling Roadmap
256
128
64
32
nm width
8
4
2
1
Ω·µm2 access ρ
175
120
60
30
nm contact width
10
5
2.5
1.25
Ω·µm2 contact ρ
106
75
53
37.5
nm thick
9
18
36
72
4
3.3
2.75
2-2.5
V breakdown
ft
520
730
1000
1400
GHz
fmax
850
1300
2000
2800
GHz
Emitter
Base
Collector
mA/µm2 current
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
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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
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Approach - II
• 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
Need a refractory metal for thermal stability
15 nm Pd/Ti diffusion
100 nm InGaAs grown in MBE
TEM: Evan Lobisser
NAMBE 2010
Ashish Baraskar, UCSB
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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
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Epilayer Growth
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
100 nm In0.53Ga0.47As: C (p-type)
100 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
NAMBE 2010
Ashish Baraskar, UCSB
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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
20 nm in-situ Ir
100 nm In0.53Ga0.47As: C (p-type)
100 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
NAMBE 2010
Ashish Baraskar, UCSB
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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
50 nm ex-situ Ni
500 nm ex-situ Au
20 nm ex-situ Ti
20 nm in-situ Ir
100 nm In0.53Ga0.47As: C (p-type)
100 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
NAMBE 2010
Ashish Baraskar, UCSB
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Resistance Measurement
• 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
5
Resistance ()
4
3
2
2  RC 
1
0
0
0.5
1
1.5
2
2  C  RSh
W
2.5
3
3.5
Gap Spacing (m)
NAMBE 2010
Ashish Baraskar, UCSB
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Error Analysis
• Extrapolation errors:
• Processing errors:
– 4-point probe resistance
measurements on Agilent
4155C
– Resolution error in SEM
– Variable gap spacing along
width (W)
– Overlap resistance
3.5
Resistance ()
3
2.5
Variable gap along width (W)
1.10 µm
1.04 µm
dR
2
dd
1.5
Lgap
1
0.5
0
dRc
0
1
W
2
3
4
5
6
Overlap Resistance
Gap Spacing (m)
NAMBE 2010
Ashish Baraskar, UCSB
13
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
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Doping Characteristics-I
Hole concentration Vs CBr4 flux
80
70
Tsub = 460 oC
60
2
hole concentration
Mobility (cm -Vs)
-3
Hole Concentration (cm )
10
20
mobility
50
40
10
19
0
10
20
30
40
50
CBr foreline pressure (mtorr)
60
4
– Hole concentration saturates at high CBr4 fluxes
– Number of di-carbon defects as CBr4 flux *
*Tan et. al. Phys. Rev. B 67 (2003) 035208
NAMBE 2010
Ashish Baraskar, UCSB
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Doping Characteristics-II
19
Hole Concentration (10 ) cm
-3
Hole concentration Vs V/III flux
10
8
CBr4 = 60 mtorr
6
4
2
10
20
30
40
50
60
Group V / Group III
As V/III ratio
hole concentration
hypothesis: As-deficient surface drives C onto group-V sites
NAMBE 2010
Ashish Baraskar, UCSB
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Doping Characteristics-III
Hole concentration Vs substrate temperature
Hole Concentration (cm-3)
2 1020
1.6 10
20
1.2 10
20
8 1019
CBr4 = 60 mtorr
19
4 10
300
350
400
450
Substrate Temp. (oC)
Tendency to form di-carbon defects
as Tsub *
*Tan et. al. Phys. Rev. B 67 (2003) 035208
NAMBE 2010
Ashish Baraskar, UCSB
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Doping Characteristics-III
Hole concentration Vs substrate temperature
-3
Hole Concentration (cm )
Hole Concentration (cm-3)
2 1020
20
1.6 10
20
1.2 10
8 1019
CBr4 = 60 mtorr
19
4 10
300
350
400
1020
Tsub = 350 oC
o
Tsub = 460 C
1019
0
450
4
Substrate Temp. (oC)
Tendency to form di-carbon defects
20
40
60
80
100
CBr foreline pressure (mtorr)
as Tsub *
*Tan et. al. Phys. Rev. B 67 (2003) 035208
NAMBE 2010
Ashish Baraskar, UCSB
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Results: Contact Resistivity - I
Metal Contact
ρc (Ω-µm2)
ρh (Ω-µm)
In-situ Ir
0.58 ± 0.48
7.6 ± 2.6
15
Resistance ()
• Hole concentration, p = 2.2 x 1020 cm-3
• Mobility, µ = 30 cm2/Vs
• Sheet resistance, Rsh = 94 ohm/
(100 nm thick film)
10
5
p = 2.2 x1020 cm-3
ρc lower than the best reported contacts to
pInGaAs (ρc = 4 Ω-µm2)[1,2]
TLM width, W = 25 m
0
0
1
2
3
4
Pad Spacing (m)
1. Griffith et al, Indium Phosphide and Related Materials, 2005.
2. Jain et al, IEEE Device Research Conference, 2010.
NAMBE 2010
Ashish Baraskar, UCSB
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10
10
c
8
6
p = 5.7×1019 cm-3





1
Tunneling  ρC  exp p
c (m2)
2
Contact Resistivity,  (-m )
Results: Contact Resistivity - II
*





1
4
p = 2.2×1020 cm-3
5
10
-1/2
2
[p]
-11
(10
15
-3/2
cm
)
Thermionic
*

~
constant
Emission  c
Data suggests tunneling
0
5
10
15
20
25
19
-3
Hole Concentration, p (x10 cm )
High active carrier concentration is the key to low resistance contacts
* Physics of Semiconductor Devices, S M Sze
NAMBE 2010
Ashish Baraskar, UCSB
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Thermal Stability - I
Mo contacts annealed under N2 flow for 60 mins. at 250 oC
Before annealing
After annealing
0.58 ± 0.48
0.8 ± 0.56
ρc (Ω-µm2)
15
Resistance ()
Un-annealed
Annealed
10
5
p = 2.2 x1020 cm-3
TLM width, W = 25 m
0
0
1
2
3
Pad Spacing (m)
NAMBE 2010
Ashish Baraskar, UCSB
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TEM: Evan Lobisser
21
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
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Thank You !
Questions?
Acknowledgements:
ONR, DARPA-TFAST, DARPA-FLARE
NAMBE 2010
Ashish Baraskar, UCSB
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Extra Slides
NAMBE 2010
Ashish Baraskar, UCSB
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Correction for Metal Resistance in 4-Point Test Structure
Rmetal
I
W
L
(  sheet  contact )1 / 2 / W
 sheet L / W
V
V
I
(  sheet  contact )1/ 2 / W   sheet L / W  Rmetal / x
Error term (Rmetal/x) from metal resistance
NAMBE 2010
Ashish Baraskar, UCSB
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Random and Offset Error in 4155C
0.6647
• Random Error in
resistance
measurement ~ 0.5
m
• Offset Error < 5 m*
Resistance ()
0.6646
0.6645
0.6644
0.6643
0.6642
0
5
10
15
20
25
Current (mA)
*4155C datasheet
NAMBE 2010
Ashish Baraskar, UCSB
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Accuracy Limits
• Error Calculations
– dR = 50 mΩ (Safe estimate)
– dW = 1 µm
– dGap = 20 nm
• Error in ρc ~ 40% at 1.1 Ω-µm2
NAMBE 2010
Ashish Baraskar, UCSB
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