ingaas/inp dhbts with emitter and base defined …...10 in 0.53 ga 0.47 as 8 1019: si emitter cap 15...
TRANSCRIPT
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
*[email protected], +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