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1
InGaAs/InP DHBTs in a planarized, etch-back technology for base contacts
Vibhor 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
[email protected], 805-893-3273
International Symposium on Compound Semiconductors 2011
2
Outline
• HBT Scaling Laws
• Refractory base ohmics
• Fabrication
• DHBT – Epitaxial Design and Results
• Summary
3
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
2
1
exitbnbb vTDT 22satcc vT 2
eex AR /contact
contacts
contactsheet 612 AL
W
L
WR
e
bc
e
ebb
cccb /TAC 2
cbmax, /)( cbieeffc TVAvI
effcbeffbb CR
ff
,,max 8
4
InP bipolar transistor scaling roadmap
Emitter
256 128 64 32 Width (nm)
8 4 2 1 Access ρ (Ω·µm2)
Base
175 120 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
rfor
man
ceD
esig
n
Rodwell, Le, Brar, Proceedings of IEEE, 2008
5
• 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 diffusionTEM by E Lobisser
Ashish Baraskar
6
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
7
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
base
N- collector
InP substrate
sub collector
emitteremitter cap
n+ InGaAs
n InP
emitter capemitter
base
SiO2/Cr
TiW
MoW
SiN (SW)
emitterbase base
8
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
9
W
MoInGaAs
p+ InGaAs Base
Ti0.1W0.9
InP
W
MoInGaAs
p+ InGaAs Base
Ti0.1W0.9
InP
SiNx
Base process flow – II
Lift-off Ti/Au
Low base metal resistance
Blanket SiNx mask
Etch base contact metal in the field
10
Base Planarization
Planarization: Emitter projectingfrom PR for W dry etch
Etch Back
Planarization Boundary
11
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
En
erg
y (e
V)
Distance (nm)
Emitter
Collector
Base
Low Base doping
Good refractory ohmics not possible
Pd/W contacts used
12
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,
I b (
A)
Vbe
(V)
Ic
Ib
Solid Line: Vcb
= 0 V
Dashed Line: Vcb
= 0.7 V
nc = 1.76
nb = 3.29
13
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 (
dB
)
freq (Hz)
U
H21
MSG
fmax
= 690 GHz
f = 410 GHz
Aje
= 0.22 × 5.7 m2
14
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_p
aram
eter
_Dee
mbe
d_P
NA
..S11
dS
_par
amet
er_D
eem
bed_
PN
A..S
22d
S(1
,2)*
5S
(2,1
)/12
S_p
aram
eter
_Dee
mbe
d_P
NA
..S12
d*5
S_p
aram
eter
_Dee
mbe
d_P
NA
..S21
d/12
S(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_p
aram
eter
_Dee
mbe
d_P
NA
..S11
dS
_par
amet
er_D
eem
bed_
PN
A..S
22d
S(1
,2)*
5S
(2,1
)/12
S_p
aram
eter
_Dee
mbe
d_P
NA
..S12
d*5
S_p
aram
eter
_Dee
mbe
d_P
NA
..S21
d/12
S(2
,2)
S21/12
S12x5
S11S22
--- : Measured x : Simulated
15
TEM
Large undercut in base mesa
Pd/W adhesion issue
High Rbb
Low fmax 0.1 m
Pd/W adhesion issue
Large mesa undercut
16
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