InGaAs/InP DHBTs with Emitter and Base Defined through

Electron-beam Lithography for Reduced Ccb

and Increased RF Cut-off Frequency

Evan Lobisser1,*, Johann C. Rode, Vibhor Jain2, Han-Wei Chiang, Ashish Baraskar3,

William J. Mitchell, Brian J. Thibeault, Mark J. W. Rodwell

Dept. of ECE, University of California, Santa Barbara, CA 93106, USA

(Now with 1Agilent Technologies, Inc., CA, 2IBM Corporation, VT, 3GlobalFoundries, NY)

Miguel Urteaga

Teledyne Scientific & Imaging, Thousand Oaks, CA 91360

Dmitri Loubychev, Andrew Snyder, Ying Wu, Joel M. Fastenau, Amy W. K. Liu

IQE Inc., Bethlehem, PA 18015

*evan.lobisser@agilent.com, +1 (707) 577-5629

International Symposium on Compound Semiconductors 2012

Outline

2

• Motivation

• HBT Design & Scaling

• Fabrication Process & Challenge

• Electrical Measurements

• Conclusion

High gain at microwave frequencies:

Precision analog design, high resolution ADCs, DACs

Digital logic for

optical fiber circuits

THz amplifiers for

imaging, communications

0.3- 3 THz imaging systems

0.1-1 Tb/s optical fiber links

Why THz Transistors?

3

Emitter:

n++ InGaAs cap

n InP Base:

p++ InGaAs

Doping grade Drift collector:

n- InGaAs/InAlAs grade

n- InP Sub-collector:

n++ InGaAs cap

n++ InP

Collector CP

Emitter BP

Base

z X

X’

XX’:

z

Semi-insulating InP substrate

C E B

Type-I InP DHBTs at UCSB

4

Surface prep

& doping

Lateral scaling

Epitaxial scaling

Parameter Change

collector depletion layer thickness decrease 2:1

base thickness decrease 1.41:1

emitter junction width decrease 4:1

collector junction width decrease 4:1

emitter contact resistivity decrease 4:1

base contact resistivity decrease 4:1

current density increase 4:1

Keep lengths the same, reduce widths 4:1 for thermal considerations

To double bandwidth of a mesa DHBT:

Keep constant all resistances and currents

Reduce 2:1 all capacitances and transport delays

HBT Scaling Laws

5

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

25 InGaAs 1-0.51020 : C Base

9.5 In0.53Ga0.47 As 11017 : Si Setback

12 InGaAs / InAlAs 11017 : Si B-C Grade

3 InP 5 1018 : Si Pulse doping

45.5 InP 11017 : Si Collector

7.5 InP 11019 : Si Sub Collector

5 In0.53Ga0.47 As 41019 : Si Sub Collector

300 InP 11019 : Si Sub Collector

3.5 In0.53Ga0.47 As Undoped Etch stop

Substrate SI : InP

Vbe = 1.0V, Vcb = 0.5V, Je = 0, 27 mA/m2

Thin (70 nm) collector for balanced fτ/fmax

High emitter/base doping for low Rex/Rbb

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0 50 100 150

En

erg

y (

eV

)

Distance (nm)

Epitaxial Design

6

Sub-200 nm Emitter Anatomy

7

TiW

W 100 nm

Mo

High-stress emitters fall off

during subsequent lift-offs

TiW W

Single sputtered metal has

non-vertical etch profile

Hybrid sputtered metal stack for

low-stress, vertical profile

W/TiW interfacial discontinuity

enables base contact lift-off

Interfacial Mo blanket-evaporated for low ρc

SiNx SiNx sidewalls protect emitter contact,

prevent emitter-base shorts

Semiconductor wet etch

undercuts emitter contact

Very thin emitter epitaxial layer

for minimal undercut

Positive i-line lithography Negative e-beam lithography

E-beam lithography needed to define < 150 nm emitters and for

< 50 nm emitter-base contact misalignment

Negative i-line lithography

Positive e-beam lithography

Lithographic Scaling and Alignment

8

Emitter Emitter

Base Mesa

Base

Contact

Web = 155 nm Wbc = 140 nm

Wbc = 150 nm

Tb + Tc = 95 nm

TiW

W

Pt/Ti/Pd/Au

SiNx sidewall

Measurement

10

RF measurements conducted using Agilent E8361A PNA from 1-67 GHz

DC bias and measurements made with Agilent 4155 SPA

Off-wafer LRRM calibration, lumped-element pad stripping used to de-embed device S-Parameters

Isolated pad structures used to provide clean RF measurements

0

5

10

15

20

25

109

1010

1011

Maso

n's

Unila

tera

l G

ain

(dB

)

Frequency (Hz)

Embedded

De-embedded

0

5

10

15

20

25

109

1010

1011

Maso

n's

Unila

tera

l G

ain

(dB

)

Frequency (Hz)

Embedded

De-embedded

β = 14 for 150 nm junction

VBceo = 2.44 V @ Je = 15 kA/cm2

Rex ≈ 2 Ω·µm2 (RF extraction)

Collector ρsheet = 14 Ω/□, ρc = 12 Ω·µm2

0

5

10

15

20

25

30

0 0.5 1 1.5 2 2.5

Je (

mA

/m

2)

Vce

(V)

Aje = 150 nm x 3 m

Ib,step

= 200 A

BVceo

= 2.44 V

25/30/35 mW/m2

Peak f, f

max

Vcb

= 0 V

10-8

10-7

10-6

10-5

10-4

10-3

10-2

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1

I c, I b

(A

)

Vbe

(V)

Solid: Vcb

= 0.0 V

Ib

Ic

Dotted: Vcb

= 0.2 V

nc = 1.25

nb = 2.72

DC Data

11

Peak RF performance at >40 mW/μm2

Kirk limit not reached

0

5

10

15

20

25

30

109

1010

1011

1012

Ga

ins (

dB

)

Frequency (Hz)

H21

U

MAG/MSG

f = 530 GHz

fmax

= 750 GHz

Ic = 12.4 mA, V

ce = 1.5 V

Je = 27.6 mA/m

2, V

cb = 0.54 V

0

200

400

600

800

2.5

3

3.5

4

4.5

0 5 10 15 20 25 30

Cuto

ff fre

que

ncy (

GH

z)

Ccb (fF

)

Je (mA/m

2)

f

fmax

CcbV

cb = 0.5 V

RF Data

12

Lowest ρex to date due to Mo contact, highly doped epi

Ccb lower than 100 nm collector epi designs due to E-beam litho

ρex = 2 Ω·μm2

Ccb = 3.0 fF

Ajc = 1.86 μm2 ~ 450 nm x 4 μm

Ic = 12.4 mA

Vce = 1.5 V

(0.2 S)Vbeexp -jω(0.23 ps)

13

Equivalent Circuit Model

cbcex

c

Bje

c

Bcb CRR

qI

TnkC

qI

Tnk

f

2

1

230 fs 15 fs 45 fs

τec dominated by transit delays, high ideality factor reduces fτ ~ 10%

E

B

2.5 nm of Pt diffuses ~ 8 nm

Expected base ρc = 4 Ω·μm2 and Rsh = 800 Ω/□

yields fmax > 1.0 THz for same fτ

Epitaxial design, process damage explain

high ηb, Rbb

Rsh increased by base contacts reacting with

5 nm (20 %) of base

Performance Analysis

14

Conclusion

15

E-beam lithography used to define narrow emitter, narrowest base mesa reported to date

Narrow mesa, low emitter ρc enable 33% increase in fmax from previous UCSB results with 70 nm collector thickness

Epitaxial thinning increased fτ by 10% from 100 nm UCSB designs

1 THz bandwidth possible with improved base contact process

This work was supported by the DARPA CMO Contract No. HR0011-09-C-0060.

Portions of this work were done in the UCSB nanofabrication facility, part of the NSF-funded NNIN

network, and the MRL, supported by the MRSEC Program of the NSF under award No. MR05-20415.

Questions?

Extra Slides

Bipolar Scaling Laws eW

bcWcTbT

eLlength emitter

bc

bcegapbc

e

shbb

eecex

e

e

e

cbicbesatc

cccb

satcc

nbb

A

WWW

LR

AR

W

L

L

PT

TVVAvI

TAC

vT

DT

,

,

,

2

max,

2

1226

/

ln1

/)(

/

2/

2/

Wgap

Ti0.1W0.9

SiOx

Cr

n InGaAs, InP

EBL

PR

Cl2/O2 ICP etch

p InGaAs

W

Ti0.1W0.9

Cr

n InGaAs, InP

p InGaAs

W

SiOx

High power

SF6/Ar ICP etch

p InGaAs

Ti0.1W0.9

Cr

n InGaAs, InP

W

SiOx

Low power

SF6/Ar ICP etch

Mo

V. Jain

Fabrication: Emitter contact

1

9

p InGaAs

Ti0.1W0.9

Cr

n InGaAs, InP

W

SiNx PECVD deposition

CF4/O2 ICP etch

Ti0.1W0.9

W

InGaAs wet etch

Ti0.1W0.9

W

2nd SiNx sidewall

InP wet etch

Fabrication: Emitter mesa

2

0

Base Post Cap

Ccb,post does not scale with Le

Adversely effects fmax as Le ↓ Need to minimize the Ccb,post value

c

postr

postcbT

AC

0

,

Undercut below base post

0

2

4

6

0 1 2 3 4 5

Ccb (

fF)

Le (m)

y = 1.09x - 0.02

No contribution of Base post to Ccb

Transit time Modulation Causes Ccb Modulation

),(//1)(constant0 cbccbc

T

celectronsbase

holesbase

VIfTAVdxTxAxqnQQc

cb

f

c

cb

c

holesbase

f

cb

holesbase

cbVI

C

I

Q

V

QC

Camnitz and Moll, Betser & Ritter, D. Root holesbaseb

ΔQI ,

-2

-1

0

1

2

0 100 200 300 400

eV

nm

L

-2

-1

0

1

2

0 100 200 300 400

eV

nm

0 0

ccbcbf

ICV

:Modulation Velocity Collector

0 0

ccbcbf

ICV

: Effect Kirk

2

3

4

5

6

7

8

0 2.5 5 7.5 10 12.5

Ccb/A

e (

fF/

m2)

Je (mA/m

2)

-0.2 V

0.0 V

0.2 V

Vcb

= 0.6 V

cbb

cb

Vτ

C

by of modulation and-

collector into pushout base-

both to due is in Increase

100

200

300

400

500

0 2 4 6 8 10 12

f (

GH

z)

Je (mA/um

2)

f, -0.3 V

cb

0.0 Vcb

-0.2 Vcb

0.2 Vcb

0.6 Vcb

DHBTs InP in effect weak-

SHBTs InGaAs in effect strong-

reduced with in Increasecbcbc

CVτ