Effects of surface oxide on wafer bonding of GaN and SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors

NC STATE UNIVERSITY UCSB

Effects of surface oxide on wafer bonding of GaN and SiC

Jaeseob Lee, Robert F. Davis, and Robert J. NemanichNorth Carolina State University

Raleigh, NC 27695-8202 USA

February 12, 2002



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



Advantage of GaN-SiC Device; high temperature, high power, high frequency operation

Larger bandgap emitter-restrict the diffusion of hole from base to emitter → high electron injection efficiency-heavily doped base → low base resistanceIndirect bandgap base-longer carrier lifetime(longer diffusion length)→ high base transportShort base width

higher Emitter efficiency Larger current gain

Al Emitter Contact

n-GaN Emitter Al/Cr Base Contact

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)

GaN/SiC HBT



Direct GrowthNucleation ProblemDefect due to large mismatch

Buffer Layer(AlN) GrowthBuffer Layer acts as a insulator

Waferbonding Reduce defect formation at interface No insulating buffer layer between GaN and SiC

SiC

GaN

SiC

GaNAlN

SiC

GaN

GaN/SiC WB



Electronegativity valuesElement Electronegativity

N 3.0C 2.5Si 1.8Ga 1.6

Bond and ionic characterBond Difference in

ElectronegativityIonic

CharacterN-Ga 1.4 39%N-Si 1.2 30%C-Ga 0.9 19%C-Si 0.7 12%N-C 0.5 7%

Si-Ga 0.2 1%

Si

C

Ga

N

Si–N

Si–GaC–N

C–Ga

Schematic [1120] projection of the GaN/SiC interface

GaN/SiC WB



Dicing into 12.8×6.5 mm2 piecesDegreasing, HF(SiC)/HCl(GaN) dipN2 blow dry

Processing Flow Chart

Characterization(I-V)

Ex situ Cleaning

Ex situ Bonding

In situ Annealing

Characterization(AFM,AES)

600, 800, 1000oC, 1hr



GaN

SiC

AFM of GaN/SiC

RMS roughness 20 ± 5 Å in 20×20 µm2 area of GaN and SiC



0 200 400 600 800 1000 1200-200

0

200

HCl dip 10min(0001)Ga

GaN

dN(E

)/dE

electron energy, eV

Surface atomic concentration: Cl 0%, C 71%, N 33 8%, O 2 1%, Ga 57 9%

AES of GaN



0

50

100

HF(10:1) 10min

HF(100:1) 1min

O

Si

C

Sur

face

ato

mic

co

ncen

trat

ion(

%)

0 100 200 300 400 500 600

HF(10:1) 10min

HF(100:1) 1min

SC1, SC2,6H(0001)Si

n-SiC

dN(E

)/dE

electron energy, eV

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

AES of (0001)Si SiC



-5.0 -2.5 0.0 2.5 5.0-100

-50

0

50

100

HF(100:1) 1min

HF(10:1) 10min

I(m

A)

V(V)HF( 10:1) 10min dipped SiC to GaN pair shows more ohmic behavior

IV of GaN/SiC(n-type to n-type)

Ti

Ti

n-SiCn-SiC



Bonding process to 2H (0001)Ga GaN

6H SiC surface

HF HF dip

(min)

Annealing Temp(oC)

Annealing Time(min)

Heating rate(oC/min)

Bonding

Results

Trials

(0001)Si 100:1 1 1000,800 240,60 10 No(1 partial bonding)

11

10 1000,800 240,60 5 No 2

60 1000,800 60 5 Yes 2

10:1 10 1000,800 60 5 Yes 2

600 60 5 No 1

(000-1)C 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

Bonding result



0 20 40 60 80 100 120 140 1600

10

20

30

40

50

60

70

80

90

100

C

Si

O

6H(0001)Si n-SiC

Sur

face

ato

mic

con

cent

ratio

n(%

)

HF(100:1) dip time(min)

0 20 40 60 80 100 120 140 1600

10

20

30

40

50

60

70

80

90

100

C

Si

O

6H(000-1)C n-SiC

Sur

face

ato

mic

con

cent

ratio

n(%

)

HF(100:1) dip time(min)

Surface Conc. of SiC

Surface atomic concentration: SiC (0001)Si ; Si 274%, C 15 3%, O 575% after 1min dip Si 214%, C 7614%, O 31% after 60min dipSiC (000-1)C ; Si 234%, C 7313%, O 41% after 1min dip

Si 214%, C 7614%, O 31% after 60min dip

HF(100:1) dip of (0001)Si/(000-1)C 6HSiC



6.5 mm

12.8 mm

406 µm pSiC 3.5E18

1 µm GaN <1E170.1 µm AlN Conductive260 µm nSiC 4~6E18

Ti

Pt or Ti

1 mm

1 mm

p-6H SiC(0001)Si ;Degreasing, HF(10:1) 10min dip

2H GaN(0001)Ga ;Degreasing, HCl 1min dip

600oC, 800oC, 1000oC 1hr annealing for WB

WB of GaN/SiC



-5.0 -2.5 0.0 2.5 5.0-100

-50

0

50

100

Ti/p-SiC

Pt/p-SiC

Ti/n-SiC

I(m

A)

V(V)

IV of metal/SiC

Ohmic behavior of Metal/SiC

800oC,20min

Tin-SiC

Tin-SiC

Ptp-SiC

800oC 20min

MetalSiC

Metal/polished side

Metal/unpolished side Ohmic contact



(0001)C SiC/(0001)Ga GaN pair shows low resistance than(000-1)Si SiC/(0001)Ga GaN pair

IV of GaN/SiC(n-type to n-type)

-5.0 -2.5 0.0 2.5 5.0-100

-50

0

50

100

Si-Ga

C-Ga

I(m

A)

V(V)

(0001)Si, (000-1)C SiC; HF(10:1) 10min dip(0001)Ga GAN; HCl 1min dip

Ti

Ti

n-SiCn-SiC



IV of GaN/SiC(n-type to p-type)

-5.0 -2.5 0.0 2.5 5.0-100

-50

0

50

100

Si-Ga

C-Ga

I(m

A)

V(V)

(0001)C SiC/(0001)Ga GaN pair and (000-1)Si SiC/(0001)Ga GaN pair show rectifying behavior

1000ºC 1hr in-situ annealing Ti

Pt

n-SiCp-SiC

Ti

Ti

n-SiCp-SiC




-5.0 -2.5 0.0 2.5 5.0-100

-50

0

50

100

Si-Ga

C-Ga

I(m

A)

V(V)

(0001)C SiC/(0001)Ga GaN pair shows rectifying behavior But (000-1)Si SiC/(0001)Ga GaN pair shows ohmic behavior

800ºC 1hr in-situ annealing Ti

Pt

n-SiCp-SiC

Ti

Ti

n-SiCp-SiC




(0001)C SiC/(0001)Ga GaN pair shows rectifying behavior But (000-1)Si SiC/(0001)Ga GaN pair shows ohmic behavior

800ºC 1hr in-situ annealing

-20 -10 0 10 20-100

-50

0

50

100

Si-Ga

C-Ga

I(m

A)

V(V)

Ti

Pt

n-SiCp-SiC

Ti

Ti

n-SiCp-SiC



(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

600ºC 1hr in-situ annealing

5mm

(a) (0001)Si SiC/(0001)GaGaN (b) (000-1)C SiC/(0001)GaGaN

5mm

IR image of GaN/SiC



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.



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



Fabrication of HBT

Substrate ; 300µm nSiC 4~6E18

Epi 1 ; 12.0µm nSiC 6.9E15

Epi 2 ; 0.2 µm pSiC 3.5E18

Substrate ; 260µm nSiC 4~6E18

0.1µm AlN cunductive1 µm nGaN 4~6E18

From Dr. Davis group

From Cree Research Inc.

2in



After bonding & Polishing

6.5 mm

6.5 mm

0.2 μm pSiC 3.5E18

12 μm nSiC 6.9E15

substrate nSiC 4~6E18

1 μm nGaN 4~6E180.1 μm AlN Conductive1 μm nSiC 4~6E18

2 μm

300 μm

Polishing SiC substrate; 260µm to 50 µm BondingPolishing SiC substrate: 50 µm to 2 µm

by Diamond lapping film

GaN on Si wafer is easy for layer transfer



0.2 μm pSiC 3.5E18

12 μm nSiC 6.9E15

substrate nSiC 4~6E18

0.5 μm nGaN 4~6E18

Al Al/Cr

Al/Cr

RIE/metallizing Plan

SF6 500Å/min SiC20min for 1μm

Cl2/Ar 4000Å/min GaN2.5min for 1μm

Al deposition Al/Cr deposition

RIE RIE

By Parallel-plate RIE

Effects of surface oxide on wafer bonding of GaN and SiC

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