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2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA1
High Doping Effects on In-situ andEx-situ Ohmic Contacts to n-InGaAs
Ashish Baraskar*, Mark A. Wistey, Vibhor Jain, Uttam Singisetti, Greg Burek,
Brian J. Thibeault, Arthur C. Gossard and Mark J. W. Rodwell
ECE and Materials Departments, University of California, Santa Barbara
Yong J. Lee
Intel Corporation, Technology Manufacturing Group, Santa Clara, CA
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA2
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
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA3
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
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA4
Device Bandwidth Scaling Laws for HBT
To double device bandwidth: • Cut transit time 2x:
– Reduce thickness 2:1 ☺– Capacitance increases 2:1 ☹
• Cut RC delay 2x– Scale contact resistivities by 4:1
*M.J.W. Rodwell, IEEE Trans. Electron. Dev., 2001
Uttam Singisetti, DRC 2007
bcWcTbT
eW
HBT: Heterojunction Bipolar Transistor
RCf in
2
1
effcbbb CR
ff
8max
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA5
Emitter: 512 256 128 64 32 width (nm) 16 8 4 2 1 access ρ, (m2)
Base: 300 175 120 60 30 contact width (nm) 20 10 5 2.5 1.25 contact ρ (m2)
ft: 370 520 730 1000 1400 GHzfmax: 490 850 1300 2000 2800 GHz
InP Bipolar Transistor Scaling Roadmap
- Contact resistance serious barrier to THz technology
Less than 2 Ω-µm2 contact resistivity required for simultaneous THz ft and fmax*
*M.J.W. Rodwell, CSICS 2008
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA6
Approach
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
- Ex-situ contacts: treatment with UV-O3, HCl etch
- In-situ contacts: no air exposure before metal
deposition
• Use of refractory metal for thermal stability
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA7
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
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA8
Epilayer Growth
Semi-insulating InP Substrate
150 nm In0.52Al0.48As: NID buffer
100 nm In0.53Ga0.47As: Si (n-type)
Semiconductor epilayer growth by Solid Source Molecular Beam Epitaxy (SS-MBE)– n-InGaAs/InAlAs
- Semi insulating InP (100) substrate
- Unintentionally doped InAlAs buffer
- Electron concentration determined by Hall measurements
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA9
Two Types of Contacts Investigated
In-situ contacts: Mo
- Samples transferred under vacuum for contact metal deposition
- no air exposure Ex-situ contacts: Ti/Ti0.1W0.9
- exposed to air
- surface treatment before contact metal deposition
150 nm In0.52Al0.48As: NID buffer
100 nm In0.53Ga0.47As: Si (n-type)
Contact metal
Semi-insulating InP Substrate
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA10
In-situ contactsIn-situ Molybdenum (Mo) deposition - E-beam chamber connected to MBE chamber
150 nm In0.52Al0.48As: NID buffer
100 nm In0.53Ga0.47As: Si (n-type)
20 nm in-situ Mo
Semi-insulating InP Substrate
Why Mo? - Refractory metal (melting point ~ 2623 C)- Work function ~ 4.6 (± 0.15) eV, close to the conduction band
edge of InGaAs - Easy to deposit by e-beam technique- Easy to process and integrate in HBT process flow
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA11
Ex-situ contacts
Ex-situ Ti/Ti0.1W0.9 contacts on InGaAs• Surface preparation - Oxidized with UV-ozone for 10 min - Dilute HCl (1:10) etch and DI rinse for 1 min each• Immediate transfer to sputter unit for contact metal deposition• Ti: Oxygen gettering property, forms good ohmic contacts*
ex-situ
150 nm In0.52Al0.48As: NID buffer
100 nm In0.53Ga0.47As: Si (n-type)
5 nm ex-situ Ti
in-situ
100 nm ex-situ Ti0.1W0.9
Semi-insulating InP Substrate
*G. Stareev, H. Künzel, and G. Dortmann, J. Appl. Phys., 74, 7344 (1993).
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA12
TLM (Transmission Line Model) fabrication
• E-beam deposition of Ti, Au and Ni layers
• Samples processed into TLM structures by photolithography and liftoff
• Mo and Ti/TiW dry etched in SF6/Ar with Ni as etch mask, isolated by wet etch
150 nm In0.52Al0.48As: NID buffer
100 nm In0.53Ga0.47As: Si (n-type)
Mo or Ti/TiW20 nm ex-situ Ti
50 nm ex-situ Ni500 nm ex-situ Au
Semi-insulating InP Substrate
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA13
Resistance Measurement
• Resistance measured by Agilent 4155C semiconductor parameter analyzer
• TLM pad spacing varied from 0.6-26 µm; verified from scanning electron microscope
• TLM Width ~ 10 µm
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA14
Error Analysis• Error due to extrapolation*
– 4-point probe resistance measurements on Agilent 4155C
– For the smallest TLM gap, Rc is 40% of total measured resistance
• Metal Resistance
– Minimized using thick metal stack
– Minimized using small contact widths
– Correction included in data
• Overlap Resistance
– Higher for small contact widths
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)
Rd
Rc
*Haw-Jye Ueng, IEEE TED 2001
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA15
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
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA16
Results: Doping Characteristics
Tsub : 440 oC
n saturates at high dopant concentration
1018
1019
1020
1018 1019 1020
Total Si atoms (cm-3)
Act
ive
carr
iers
(cm
-3)
Enhanced n for colder growths
-hypothesis: As-rich surface drives Si onto group-III sites
4.5
5
5.5
6
6.5
7
7.5
400 410 420 430 440
Act
ive
Car
rier
s (x
1019
cm
-3)
Substrate Temperature, Tsub
(oC)
[Si]: 1.5 x1020 cm-3
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA17
Results: Contact ResistivityMetal Contact Active Carriers (cm-3) ρc (Ω-µm2)
In-situ Mo 6 x1019 1.1±0.6
In-situ Mo 4.2 x1019 2.0±1.1
Ex-situ Ti/Ti0.1W0.9 4.2 x1019 2.1±1.2
• Mo contacts: in-situ deposition;
clean interface
• Ti: oxygen gettering property
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA18
Results: Effect of doping-I
• Contact resistivity (ρc) ↓ with ↑ in electron concentration
0
2
4
6
8
10
12
14
16
0
1
2
3
4
5
0 2 4 6 8 10 12 14
in-situ Moex-situ Ti/Ti
0.1W
0.9
active carriers
Act
ive
Car
rier
s (x
1019
cm-3
)
Con
tact
Res
isti
vity
(
-m
2 )
Total Si atoms (x1019cm-3)
Tsub: 440 oC
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA19
Results: Effect of doping-II*
1exp
d
CN
ρTunneling
* Physics of Semiconductor Devices, SM Sze
Data suggests tunneling.
High active carrier concentration is the key to low resistance contacts
1
10
100
1 10-10 2 10-10 3 10-10 4 10-10
Co
nta
ct R
esis
tiv
ity
(lo
gρ c)
[Nd]-1/2, cm-3/2
ThermionicEmission ρc ~ constant*
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA20
Contacts annealed under N2
flow for 60 secs
• Mo contacts stable to at
least 400 C
• Ti/Ti0.1W0.9 contacts degrade
on annealing*
Results: Thermal Stability
1.5
2
2.5
3
0 100 200 300 400
in-situ Mo
ex-situ Ti/Ti0.1
W0.9
Anneal Temperature (C)
Con
tact
Res
isti
vity
(
-m
2 )
*T. Nittono, H. Ito, O. Nakajima, and T. Ishibashi, Jpn. J. Appl. Phys., Part 1 27, 1718 (1988).
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA21
Conclusion
• Extreme Si doping improves contact resistance
• In-situ Mo and ex-situ Ti/Ti0.1W0.9 give low contact resistance
- Mo contacts are thermally stable
- Ti/Ti0.1W0.9 contacts degrade
• ρc ~ (1.1 ± 0.6) Ω-µm2 for in-situ Mo contacts
- less than 2 Ω-µm2 required for simultaneous THz ft and fmax
Contacts suitable for THz transistors
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA22
Thank You !
Questions?
Acknowledgements
ONR, DARPA-TFAST, DARPA-FLARE
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA23
Extra Slides
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA24
Correction for Metal Resistance in 4-Point Test Structure
WLsheet /ρmetalR Wcontactsheet /)( 2/1ρρ
2/metalR
3///)( 2/1metalsheetcontactsheet RWLW ρρρ
Error term (-Rmetal/3) from metal resistanceEffect changes measured ρc by ~40% (@1.3 -m2)All data presented corrects for this effect
From hand analysis & finite element simulation
W
L
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA25
Doping Vs As flux
2
3
4
5
2 4 6 8 10 12 14
Dop
ing
dens
ity
(x10
19)c
m-3
As flux (x10-6) torr
Tsub: 440 C
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA26
0
1
2
3
4
5
800
1000
1200
1400
1600
1800
2000
2200
0 5 10 15
Active Carrier
Mobility
Mob
ility (cm2/V
s)
Act
ive
Ca
rrie
rs (
x101
9 cm
-3)
Total Si atoms (x1019cm-3)
Active Carrier, Mobility Vs Total Si
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA27
Strain Effects
[Si]=1.5 x1020 cm-3, n=6 x1019 cm-3
(asub – aepi)/asub = 5.1 x10-4
Van de Walle, C. G., Phys. Rev. B 39, 3 (1989) 1871
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA28
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
2009 Electronic Materials
Conference
Ashish BaraskarJune 24-26, 2009 – University Park, PA29
Accuracy Limits
• Error Calculations– dR = 50 mΩ (Safe estimate)– dW = 1 µm– dGap = 20 nm
• Error in ρc ~ 40% at 1.1 Ω-µm2