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. RodwellDept. of ECE, University of California, Santa Barbara, CA 93106, USA(Now with 1Agilent Technologies, Inc., CA, 2IBM Corporation, VT, 3GlobalFoundries, NY)

Miguel UrteagaTeledyne Scientific & Imaging, Thousand Oaks, CA 91360

Dmitri Loubychev, Andrew Snyder, Ying Wu, Joel M. Fastenau, Amy W. K. LiuIQE 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 capn InPBase:

p++ InGaAsDoping grade Drift collector:

n- InGaAs/InAlAs graden- InPSub-collector:

n++ InGaAs capn++ InP

CollectorCP

EmitterBP

Base

zX

X’

XX’:

z

Semi-insulating InP substrate

CE B

Type-I InP DHBTs at UCSB

4

Surface prep& doping

Lateral scaling

Epitaxial scaling

Parameter Changecollector depletion layer thickness decrease 2:1base thickness decrease 1.41:1emitter junction width decrease 4:1collector junction width decrease 4:1emitter contact resistivity decrease 4:1base contact resistivity decrease 4:1current 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 currentsReduce 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

0 20 40 60 80 100 120 140-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

Distance (nm)E

nerg

y (e

V)

Epitaxial Design

6

Sub-200 nm Emitter Anatomy

7

TiW

W100 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

EmitterEmitter

Base Mesa

Base Contact

Web = 155 nm Wbc = 140 nmWbc = 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 GHzDC bias and measurements made with Agilent 4155 SPA Off-wafer LRRM calibration, lumped-element pad stripping used to de-embed device S-ParametersIsolated pad structures used to provide clean RF measurements

0

5

10

15

20

25

109 1010 1011

Mas

on's

Uni

late

ral G

ain

(dB

)

Frequency (Hz)

Embedded

De-embedded

0

5

10

15

20

25

109 1010 1011M

ason

's U

nila

tera

l Gai

n (d

B)

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

J e (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, Ib (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

Gai

ns (d

B)

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/m2, 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

Cut

off f

requ

ency

(GH

z)

Ccb (fF)

Je (mA/m2)

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 mAVce = 1.5 V

(0.2 S)Vbeexp -jω(0.23 ps)

13

Equivalent Circuit Model

cbcexc

Bje

c

Bcb CRR

qITnkC

qITnk

f

21

230 fs 15 fs 45 fs

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

EB

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 explainhigh 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

AWWW

LR

AR

WL

LPT

TVVAvI

TACvTDT

,

,

,

2max,

2

1226

/

ln1

/)(

/2/2/

Wgap

Ti0.1W0.9

SiOx

Cr

n InGaAs, InP

EBLPR

Cl2/O2 ICP etch

p InGaAs

W

Ti0.1W0.9

Cr

n InGaAs, InPp InGaAs

W

SiOx

High powerSF6/Ar ICP etch

p InGaAs

Ti0.1W0.9

Cr

n InGaAs, InP

W

SiOx

Low powerSF6/Ar ICP etch

Mo

V. Jain

Fabrication: Emitter contact

19

p InGaAs

Ti0.1W0.9

Cr

n InGaAs, InP

W

SiNx PECVD depositionCF4/O2 ICP etch

Ti0.1W0.9

W

InGaAs wet etch

Ti0.1W0.9

W

2nd SiNx sidewallInP wet etch

Fabrication: Emitter mesa

20

Base Post Cap

Ccb,post does not scale with Le

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

c

postrpostcb T

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 VIfTAVdxTxAxqnQQ c

cb

f

c

cb

c

holesbasef

cb

holesbasecb VI

CI

QV

QC

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

E

drift collectorbase

-

+

+

+

+

-

-

-

--

-

-

-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

C cb/A

e (fF/m

2 )

J e (mA/m2)

-0.2 V

0.0 V

0.2 V

Vcb

= 0.6 Vcbb

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/um2)

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 Increase cbcbc CVτ