Effects of surface oxide on wafer
bonding of GaN and SiC
Jaeseob Lee, Robert F. Davis, and Robert J. Nemanich
North Carolina State University
Raleigh, NC 27695-8202 USA
February 12, 2002
Compact Power Supplies Based on Heterojunction
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Outline
Motivation
- GaN/SiC HBT
Introduction
- Wafer Bonding of GaN/SiC
Experiment
Results
- AES of GaN,SiC
- AFM of GaN/SiC
- I-V of GaN/SiC
Discussion
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GaN/SiC HBT
Advantage of GaN-SiC Device
; high temperature, high power,
high frequency operation
Al Emitter Contact
n-GaN Emitter
Al/Cr Base Contact
Larger bandgap emitter
-restrict the diffusion of hole from base to emitter
→ high electron injection efficiency
-heavily doped base
→ low base resistance
Indirect bandgap base
-longer carrier lifetime(longer diffusion length)
→ high base transport
Short base width
higher Emitter efficiency
Larger current gain
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p-SiC Base
n-SiC Collector
Al/Cr Collector Contact
GaN/SiC HBT proposed by J. Pankove, S.S. Chang,
H.C. Lee, R.J. Moustakas, B. Van Zeghbroeck
(Int. Electron Devices Meet Tech. Dig. ’94)
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GaN/SiC WB
Direct Growth
Nucleation Problem
Defect due to large mismatch
Waferbonding
GaN
Reduce defect formation at interface
No insulating buffer layer
between GaN and SiC
SiC
Buffer Layer(AlN) Growth
GaN
Buffer Layer acts as a insulator
SiC
GaN
AlN
SiC
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GaN/SiC WB
Electronegativity values
Element
N
C
Si
Ga
Electronegativity
3.0
2.5
1.8
1.6
Ga
N
Si
C
Si–N
C–Ga
C–N
Si–Ga
Bond and ionic character
Bond
N-Ga
N-Si
C-Ga
C-Si
N-C
Si-Ga
Difference in
Ionic
Electronegativity Character
1.4
39%
1.2
30%
0.9
19%
0.7
12%
0.5
7%
0.2
1%
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Schematic [1120] projection of
the GaN/SiC interface
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Processing Flow Chart
Ex situ Cleaning
Dicing into 12.8×6.5 mm2 pieces
Degreasing, HF(SiC)/HCl(GaN) dip
N2 blow dry
Characterization
(AFM,AES)
Ex situ Bonding
In situ Annealing
600, 800, 1000oC, 1hr
Characterization
(I-V)
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AFM of GaN/SiC
GaN
SiC
RMS roughness 20 ± 5 Å in 20×20 µm2 area of GaN and SiC
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AES of GaN
HCl dip 10min
(0001)Ga GaN
dN(E)/dE
200
0
-200
0
200
400
600
800
1000
1200
electron energy, eV
Surface atomic concentration:
Cl  0%, C  71%, N  33  8%, O  2  1%, Ga  57  9%
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AES of (0001)Si SiC
100
Surface atomic concentration(%)
SC1, SC2,
6H(0001)Si n-SiC
dN(E)/dE
HF(100:1) 1min
HF(10:1) 10min
0
100
200
300
400
500
600
O
50
Si
C
0
HF(100:1)
1min
electron energy, eV
HF(10:1)
10min
Surface atomic concentration:
Si  26  4%, C  20  4%, O  54  4% after HF(100:1) 1min dip
Si  21  3%, C  73  13%, O  7  1% after HF( 10:1) 10min dip
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IV of GaN/SiC(n-type to n-type)
Ti
n-SiC
n-SiC
100
Ti
HF(10:1) 10min
I(mA)
50
HF(100:1) 1min
0
-50
-100
-5.0
-2.5
0.0
2.5
5.0
V(V)
HF( 10:1) 10min dipped SiC to GaN pair shows more ohmic behavior
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Bonding result
Bonding process to 2H (0001)Ga GaN
6H SiC
surface
(0001)Si
HF
100:1
10:1
(000-1)C
HF dip
(min)
Annealing
Temp(oC)
Annealing
Time(min)
Heating
rate(oC/min)
Bonding
Results
Trials
1
1000,800
240,60
 10
10
1000,800
240,60
5
No
2
60
1000,800
60
5
Yes
2
10
1000,800
60
5
Yes
2
600
60
5
No
1
No
(1 partial bonding)
11
100:1
1
1000
240,30
5
Yes
5
10:1
10
1000
240,60
5
Yes
2
800
60
5
Yes
1
600
60
5
No
1
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Surface Conc. of SiC
HF(100:1) dip of (0001)Si/(000-1)C 6HSiC
100
100
6H(0001)Si n-SiC
80
70
O
60
50
40
Si
30
20
C
10
80
70
60
50
40
30
Si
20
10
0
6H(000-1)C n-SiC
C
90
Surface atomic concentration(%)
Surface atomic concentration(%)
90
O
0
0
20
40
60
80
100
120
140
160
0
HF(100:1) dip time(min)
20
40
60
80
100
120
140
HF(100:1) dip time(min)
Surface atomic concentration:
SiC (0001)Si ; Si  274%, C  15 3%, O  575% after 1min dip
Si  214%, C  7614%, O  31% after 60min dip
SiC (000-1)C ; Si  234%, C  7313%, O  41% after 1min dip
Si  214%, C  7614%, O  31% after 60min dip
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160
WB of GaN/SiC
600oC, 800oC, 1000oC 1hr
annealing for WB
12.8 mm
1 mm
6.5 mm
1 mm
Ti
260 µm nSiC 4~6E18
0.1 µm AlN Conductive
1 µm GaN <1E17
406 µm pSiC 3.5E18
Pt or Ti
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2H GaN(0001)Ga ;
Degreasing, HCl 1min dip
p-6H SiC(0001)Si ;
Degreasing, HF(10:1) 10min dip
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IV of metal/SiC
Metal
SiC
100
Metal/polished side
Ti/n-SiC
50
Pt/p-SiC
I(mA)
800oC,20min
0
Ti/p-SiC
n-SiC
Ti
-50
n-SiC
Ti
-100
-5.0
-2.5
0.0
2.5
V(V)
5.0
p-SiC
Pt
Metal/unpolished side
 Ohmic contact
Ohmic behavior of Metal/SiC
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800oC 20min
UCSB
IV of GaN/SiC(n-type to n-type)
Ti
n-SiC
n-SiC
100
Ti
C-Ga
I(mA)
50
Si-Ga
0
-50
-100
-5.0
-2.5
0.0
2.5
(0001)Si, (000-1)C SiC
; HF(10:1) 10min dip
(0001)Ga GAN
; HCl 1min dip
5.0
V(V)
(0001)C SiC/(0001)Ga GaN pair shows low resistance than
(000-1)Si SiC/(0001)Ga GaN pair
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IV of GaN/SiC(n-type to p-type)
1000ºC 1hr in-situ annealing
Ti
n-SiC
p-SiC
100
Pt
C-Ga
I(mA)
50
Ti
n-SiC
p-SiC
Si-Ga
0
Ti
-50
-100
-5.0
-2.5
0.0
2.5
5.0
V(V)
(0001)C SiC/(0001)Ga GaN pair and
(000-1)Si SiC/(0001)Ga GaN pair show rectifying behavior
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IV of GaN/SiC(n-type to p-type)
800ºC 1hr in-situ annealing
Ti
n-SiC
p-SiC
100
Pt
C-Ga
I(mA)
50
Ti
n-SiC
p-SiC
Si-Ga
0
Ti
-50
-100
-5.0
-2.5
0.0
2.5
5.0
V(V)
(0001)C SiC/(0001)Ga GaN pair shows rectifying behavior
But (000-1)Si SiC/(0001)Ga GaN pair shows ohmic behavior
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IV of GaN/SiC(n-type to p-type)
800ºC 1hr in-situ annealing
Ti
n-SiC
p-SiC
100
Pt
C-Ga
I(mA)
50
Ti
0
n-SiC
p-SiC
Si-Ga
Ti
-50
-100
-20
-10
0
10
20
V(V)
(0001)C SiC/(0001)Ga GaN pair shows rectifying behavior
But (000-1)Si SiC/(0001)Ga GaN pair shows ohmic behavior
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IR image of GaN/SiC
600ºC 1hr in-situ annealing
5mm
5mm
(a) (0001)Si SiC/(0001)GaGaN
(b) (000-1)C SiC/(0001)GaGaN
(0001)C SiC/(0001)Ga GaN pair and (000-1)Si SiC/(0001)Ga GaN pair
do not bond at 600ºC 1hr in-situ annealing
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Conclusion
Polarity is important factor in WB of SiC/GaN
(000-1)C n-SiC/(0001)Ga n-GaN pair has the low resistance
(nearly Ohmic)
(000-1)C p-SiC/(0001)Ga n-GaN pair keep good rectifying
behavior at lower bonding temperature(800oC)
No bonding happened at 600oC with wafer surface having
RMS roughness 20Å in 20×20 µm2.
Compact Power Supplies Based on Heterojunction
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Future Research
Characterize the bonded interface with FiB-TEM
Bond patterned GaN structures appropriate for HBT
Explore improved polishing of SiC and GaN surface to get
low T bonding
Compact Power Supplies Based on Heterojunction
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Fabrication of HBT
2in
1 µm nGaN 4~6E18
0.1µm AlN cunductive
Substrate ;
260µm nSiC 4~6E18
From Dr. Davis group
Epi 2 ; 0.2 µm pSiC 3.5E18
Epi 1 ; 12.0µm nSiC 6.9E15
Substrate ;
300µm nSiC 4~6E18
From Cree Research Inc.
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After bonding & Polishing
6.5 mm
6.5 mm
2 μm
1 μm nSiC 4~6E18
0.1 μm AlN Conductive
1 μm nGaN 4~6E18
0.2 μm pSiC 3.5E18
Polishing SiC substrate
; 260µm to 50 µm
Bonding
Polishing SiC substrate
: 50 µm to 2 µm
12 μm nSiC 6.9E15
by Diamond lapping film
300 μm
substrate nSiC 4~6E18
GaN on Si wafer is
easy for layer transfer
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RIE/metallizing Plan
By Parallel-plate RIE
RIE
RIE
Al
Al/Cr
0.5 μm nGaN 4~6E18
0.2 μm pSiC 3.5E18
12 μm nSiC 6.9E15
substrate nSiC 4~6E18
Al/Cr
SF6
500Å/min SiC
20min for 1μm
Al deposition
Cl2/Ar
4000Å/min GaN
2.5min for 1μm
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Al/Cr deposition
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