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InGaAs/InP DHBTs in a planarized, etch-back technology for base contactsVibhor Jain, Evan Lobisser, Ashish Baraskar, Brian J Thibeault, Mark RodwellECE Department, University of California, Santa Barbara, CA 93106-9560

D Loubychev, A Snyder, Y Wu, J M Fastenau, W K LiuIQE Inc., 119 Technology Drive, Bethlehem, PA 18015

vibhor@ece.ucsb.edu, 805-893-3273

International Symposium on Compound Semiconductors 2011

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Outline

• HBT Scaling Laws• Refractory base ohmics• Fabrication• DHBT – Epitaxial Design and Results• Summary

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Ohmic contacts

Lateral scaling

Epitaxial scaling

Bipolar transistor scaling laws

To double cutoff frequencies of a mesa HBT, must:

(emitter length Le)

We

Tb TcWbc

Keep constant all resistances and currentsReduce all capacitances and transit delays by 2

RCf tr

21

exitbnbb vTDT 22

satcc vT 2

eex AR /contact

contacts

contactsheet 612 AL

WLWR

e

bc

e

ebb

cccb /TAC 2

cbmax, /)( cbieeffc TVAvI

effcbeffbb CRff

,,max 8

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InP bipolar transistor scaling roadmap

Emitter256 128 64 32 Width (nm)

8 4 2 1 Access ρ (Ω·µm2)

Base175 120 60 30 Contact width (nm)

10 5 2.5 1.25 Contact ρ (Ω·µm2)

Collector 106 75 53 37.5 Thickness (nm)

Current density 9 18 36 72 mA/µm2

Breakdown voltage 4 3.3 2.75 2-2.5 V

fτ520 730 1000 1400 GHz

fmax850 1300 2000 2800 GHzPe

rform

ance

Desi

gn

Rodwell, Le, Brar, Proceedings of IEEE, 2008

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• Pd contacts diffuse in base (p-InGaAs)• Contact resistance ↑ for thin base• Limits base thickness Scaling Limitation

100 nm InGaAs grown in MBE

15 nm Pd diffusion

Need for non-diffusive, refractory base metal

Contact diffusion TEM by E Lobisser

Ashish Baraskar

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Refractory base ohmics

Doping Metal Type c (-m2)1.5E20 Mo As deposited 2.51.5E20 Ru/Mo As deposited 1.31.5E20 W/Mo As deposited 1.21.5E20 Ir/Mo As deposited 1.02.2E20 Ir/Mo As deposited 0.62.2E20 Ir/Mo Annealed 0.8

Ashish Baraskar et al., EMC 2010

Refractory metal base contacts

Require a blanket deposition and etch-back process

Ashish Baraskar et al., Int. MBE 2010

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Emitter process flow

Mo contact to n-InGaAs for emitter

W/TiW/SiO2/Cr dep

SF6/Ar etch

SiNx Sidewall

SiO2/Cr removal

InGaAs Wet Etch

Second SiNx Sidewall

InP Wet Etch

W/TiW interface acts as shadow mask for base lift off

Collector formed via lift off and wet etch

BCB used to passivate and planarize devices

Mo

baseN- collector

InP substrate

sub collector

emitteremitter cap

n+ InGaAs

n InP

emitter capemitterbase

SiO2/Cr

TiW

MoW

SiN (SW)

emitterbase base

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W

MoInGaAs

p+ InGaAs Base

Ti0.1W0.9

InP

W

MoInGaAs

p+ InGaAs Base

Ti0.1W0.9

InP

Base process flow – I

PR

Blanket refractory metal PR Planarization

Isotropic Dry etch of metal

Removes any Emitter-Base short

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W

MoInGaAs

p+ InGaAs Base

Ti0.1W0.9

InP

W

MoInGaAs

p+ InGaAs Base

Ti0.1W0.9

InP

SiNx

Base process flow – IILift-off Ti/Au

Low base metal resistance

Blanket SiNx mask

Etch base contact metal in the field

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Base Planarization

Planarization: Emitter projectingfrom PR for W dry etch

Etch Back

Planarization Boundary

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Epitaxial Design

T(nm) Material Doping (cm-3) Description

10 In0.53Ga0.47As 81019 : Si Emitter Cap

15 InP 51019 : Si Emitter

15 InP 21018 : Si Emitter

30 InGaAs 9-51019 : C Base

4.5 In0.53Ga0.47 As 91016 : Si Setback

10.8 InGaAs / InAlAs 91016 : Si B-C Grade

3 InP 6 1018 : Si Pulse doping

81.7 InP 91016 : Si Collector

7.5 InP 11019 : Si Sub Collector

7.5 In0.53Ga0.47 As 21019 : Si Sub Collector

300 InP 21019 : Si Sub Collector

Substrate SI : InP

Vbe = 1 V, Vcb = 0.7 V, Je = 25 mA/m2

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

0 50 100 150 200

Ener

gy (e

V)

Distance (nm)

Emitter

CollectorBase

Low Base doping

Good refractory ohmics not possible

Pd/W contacts used

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Results - DC Measurements

BVceo = 2.4 V @ Je = 1 kA/cm2

β = 26

JKIRK = 21 mA/m2

@Peak f,fmax

Je = 17.9 mA/m2

P = 30 mW/m2

Gummel plot

Common emitter I-V

0

10

20

30

0 0.5 1 1.5 2 2.5

J e (mA

/m

2 )

Vce

(V)

P = 25 mW/m2

Peak f/f

max

Ib = 200 A

Ib,step

= 200 A

10-7

10-5

10-3

10-1

0 0.2 0.4 0.6 0.8 1

I c, Ib (A

)

Vbe

(V)

Ic

Ib

Solid Line: Vcb

= 0 VDashed Line: V

cb = 0.7 V

nc = 1.76

nb = 3.29

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1-67 GHz RF Data

Ic = 22.4 mA

Vce = 1.67 V

Je = 17.9 mA/m2

Vcb = 0.7 V

Single-pole fit to obtain cut-off frequencies

0

10

20

30

109 1010 1011 1012

Gai

n (d

B)

freq (Hz)

U

H21

MSG

fmax

= 690 GHz

f = 410 GHz

Aje

= 0.22 × 5.7 m2

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Equivalent Circuit

Hybrid- equivalent circuit from measured RF data

Rex = 6 m2

Ccb,x = 4.48 fF

Ccb,i = 1.3 fF

Rcb = 17 k

Rc = 1.7

Rex = 4.7

Rbe = 42

Rbb = 24

Cje + Cdiff = (15 + 241) fF gmVbee-j

0.73Vbeexp(-j0.15ps)

Base

Emitter

Col

Ccg = 6.8 fF

Ccb,x = 4.48 fF

Ccb,i = 1.3 fF

Rcb = 17 k

Rc = 1.7

Rex = 4.7

Rbe = 42

Rbb = 24

Cje + Cdiff = (15 + 241) fF gmVbee-j

0.73Vbeexp(-j0.15ps)

Base

Emitter

Col

Ccg = 6.8 fF

freq (1.000GHz to 67.00GHz)

S(1,1

)

freq (100.0MHz to 67.00GHz)

S_pa

rame

ter_D

eemb

ed_P

NA..S

11d

S_pa

rame

ter_D

eemb

ed_P

NA..S

22d

S(1,2

)*5S(

2,1)/1

2S_

para

meter

_Dee

mbed

_PNA

..S12

d*5

S_pa

rame

ter_D

eemb

ed_P

NA..S

21d/1

2S(

2,2)

S21/12

S12x5

S11S22

--- : Measured x : Simulated

freq (1.000GHz to 67.00GHz)

S(1,1

)

freq (100.0MHz to 67.00GHz)

S_pa

rame

ter_D

eemb

ed_P

NA..S

11d

S_pa

rame

ter_D

eemb

ed_P

NA..S

22d

S(1,2

)*5S(

2,1)/1

2S_

para

meter

_Dee

mbed

_PNA

..S12

d*5

S_pa

rame

ter_D

eemb

ed_P

NA..S

21d/1

2S(

2,2)

S21/12

S12x5

S11S22

--- : Measured x : Simulated

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TEMLarge undercut in base mesa

Pd/W adhesion issue

High Rbb

Low fmax 0.1 m

Pd/W adhesion issueLarge mesa undercut

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Summary• Demonstrated a planarized, etch back process for refractory base

contacts• Demonstrated DHBTs with peak f / fmax = 410/690 GHz• Higher base doping, thinner base and refractory base ohmics

needed to enable higher bandwidth devices

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Questions?

Thank You

This work was supported by the DARPA THETA program under HR0011-09-C-006.

A portion of this work was done in the UCSB nanofabrication facility, part of NSF funded NNIN network and MRL Central Facilities supported by the MRSEC Program of the NSF under award No. MR05-20415