International Symposium on Compound Semiconductors 2011
InGaAs/InP DHBTs in a planarized,
etch-back technology for base contacts
Vibhor Jain, Evan Lobisser, Ashish Baraskar, Brian J Thibeault,
Mark Rodwell
ECE Department, University of California, Santa Barbara, CA 93106-9560
D Loubychev, A Snyder, Y Wu, J M Fastenau, W K Liu
IQE Inc., 119 Technology Drive, Bethlehem, PA 18015
vibhor@ece.ucsb.edu, 805-893-3273
1
Outline
• HBT Scaling Laws
• Refractory base ohmics
• Fabrication
• DHBT – Epitaxial Design and Results
• Summary
2
Bipolar transistor scaling laws
1
  tr  RC
2f
f max 
f
8Rbb,eff Ccb ,eff
We
Tb
Wbc
Tc
To double cutoff frequencies of a mesa HBT, must:
Keep constant all resistances and currents
Reduce all capacitances and transit delays by 2
 b  Tb2 2 Dn  Tb vexit
 c  Tc 2v sat
Ccb  Ac /Tc
(emitter length Le)
Epitaxial scaling
I c,max  veff Ae (Vcb  bi ) / T
Rex  contact /Ae
2
c
 We Wbc  contact
 
Rbb  sheet 

 12 Le 6 Le  Acontacts
Lateral scaling
Ohmic contacts
3
InP bipolar transistor scaling roadmap
256
128
64
32
Width (nm)
8
4
2
1
Access ρ (Ω·µm2)
175
120
60
30
Contact width (nm)
10
5
2.5
1.25
Contact ρ (Ω·µm2)
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
730
1000
1400
GHz
1300
2000
2800
GHz
Design
Emitter
Base
Performance
Collector 106
fτ 520
fmax 850
Rodwell, Le, Brar, Proceedings of IEEE, 2008
4
TEM by E Lobisser
Contact diffusion
15 nm Pd diffusion
Ashish Baraskar
• Pd contacts diffuse in base (p-InGaAs)
• Contact resistance ↑ for thin base
• Limits base thickness
 Scaling Limitation
100 nm InGaAs grown in MBE
Need for non-diffusive, refractory base metal
5
Ashish Baraskar et al., EMC 2010
Ashish Baraskar et al., Int. MBE 2010
Refractory base ohmics
Doping
1.5E20
1.5E20
1.5E20
1.5E20
2.2E20
2.2E20
Metal
Mo
Ru/Mo
W/Mo
Ir/Mo
Ir/Mo
Ir/Mo
Type
As deposited
As deposited
As deposited
As deposited
As deposited
Annealed
c (W-mm2)
2.5
1.3
1.2
1.0
0.6
0.8
Refractory metal base contacts
Require a blanket deposition and etch-back process
6
Emitter process flow
SiO2/Cr
Mo
SiN (SW)
n+ InGaAs
TiW
W
Mo
emitter cap
n InP
emitter
base
N- collector
sub collector
InP substrate
Mo contact to nInGaAs for emitter
emitter cap
emitter
base
emitter
base
W/TiW/SiO2/Cr dep
SiO2/Cr removal
SF6/Ar etch
InGaAs Wet Etch
SiNx Sidewall
base
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
7
Base process flow – I
Blanket refractory metal
PR Planarization
Isotropic Dry etch of metal
Removes any Emitter-Base short
Ti0.1W0.9
Ti0.1W0.9
W
W
Mo
InGaAs
InP
p+ InGaAs Base
PR
Mo
InGaAs
InP
p+ InGaAs Base
8
Base process flow – II
Lift-off Ti/Au
Blanket SiNx mask
Low base metal resistance
Etch base contact metal in the field
SiNx
Ti0.1W0.9
Ti0.1W0.9
W
W
Mo
InGaAs
InP
Mo
InGaAs
InP
p+ InGaAs Base
p+ InGaAs Base
9
Base Planarization
Planarization: Emitter projecting
from PR for W dry etch
Etch Back
Planarization Boundary
10
Epitaxial Design
1.5
Material
Doping (cm-3)
Description
10
In0.53Ga0.47As
81019 : Si
Emitter Cap
InP
51019
15
: Si
Emitter
15
InP
21018
: Si
Emitter
30
InGaAs
9-51019 : C
Base
4.5
In0.53Ga0.47 As
91016 : Si
Setback
10.8
InGaAs / InAlAs
91016 : Si
B-C Grade
3
InP
6 1018 : Si
Pulse doping
81.7
InP
91016 : Si
Collector
7.5
InP
11019 : Si
Sub Collector
7.5
In0.53Ga0.47 As
21019 : Si
Sub Collector
300
InP
21019 : Si
Sub Collector
Substrate
SI : InP
1
0.5
Energy (eV)
T(nm)
0
-0.5
-1
-1.5
Emitter
Base
-2
Collector
-2.5
0
50
100
Distance (nm)
150
200
Vbe = 1 V, Vcb = 0.7 V, Je = 25 mA/mm2
Low Base doping
 Good refractory ohmics not possible
 Pd/W contacts used
11
Results - DC Measurements
Common emitter I-V
2
P = 25 mW/mm
I
b,step
= 200 mA
@Peak f,fmax
2
Je (mA/mm )
30
Je = 17.9 mA/mm2
Peak f /f
20
 max
P = 30 mW/mm2
10
I = 200 mA
Solid Line: V
b
0
0.5
1
V
2
2.5
cb
Dashed Line: V
-3
10
cb
=0V
I
= 0.7 V
c
BVceo = 2.4 V @ Je = 1
kA/cm2
-5
10
b
n = 3.29
b
n = 1.76
β = 26
JKIRK = 21 mA/mm2
c
I
b
ce
1.5
(V)
I , I (A)
0
-1
10
c
-7
10
0
Gummel plot
0.2
0.4 0.6
V (V)
be
0.8
1
12
1-67 GHz RF Data
MSG
H
Gain (dB)
30
f = 410 GHz

21
f
U
max
= 690 GHz
Ic = 22.4 mA
20
Vce = 1.67 V
Je = 17.9 mA/mm2
10
A = 0.22 × 5.7 mm
2
Vcb = 0.7 V
je
0
9
10
10
10
10
freq (Hz)
11
10
12
Single-pole fit to obtain cut-off frequencies
13
Equivalent Circuit
Ccb,x = 4.48 fF
Rex = 6 Wmm2
Rcb = 17 kW
Rbb = 24 W
Ccb,i = 1.3 fF
Rc = 1.7 W
Base
Col
Rbe = 42 W
S21/12
Ccg = 6.8 fF
Cje + Cdiff = (15 + 241) fF
S12x5
Rex = 4.7 W
gmVbee-j
0.73Vbeexp(-j0.15ps)
Emitter
--- : Measured
x : Simulated
S11
S22
Hybrid- equivalent circuit from measured RF data
14
TEM
Large undercut in base mesa
Pd/W adhesion issue
 High Rbb
 Low fmax
0.1 mm
Pd/W adhesion issue
Large mesa undercut
15
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
16
Thank You
Questions?
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
17