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1.0-THz fmax InP DHBTs in a refractory emitter and self-aligned base process for reduced base access resistanceVibhor Jain, Johann C. Rode, Han-Wei Chiang, Ashish Baraskar, Evan Lobisser, Brian J Thibeault, Mark RodwellECE Department, University of California, Santa Barbara, CA 93106-9560
Miguel UrteagaTeledyne Scientific & Imaging, Thousand Oaks, CA 91360
D Loubychev, A Snyder, Y Wu, J M Fastenau, W K LiuIQE Inc., 119 Technology Drive, Bethlehem, PA 18015
[email protected], 805-893-3273
Device Research Conference 2011
2
Outline
• Need for high speed HBTs
• Fabrication– Challenges
– Process Development
• DHBT – Epitaxial Design
– Results
• Summary
3
0
5
10
15
20
25
30
35
40
109 1010 1011 1012
Tra
nsi
sto
r P
ow
er G
ain
(d
B)
Freq (Hz)
Why THz Transistors?
High gain at microwave frequencies precision analog design, high resolution ADC & DAC, high performance receivers
THz amplifiers for imaging, sensing, communications
Digital Logic for Optical fiber circuits
4
HBT process requirements
• Refractory emitter contact and metal stack
– To sustain high current density operation
• Low stress emitters
– For high yield
• Low base access resistance
– For improved device fmax
• Thin emitter semiconductor
– To enable a wet etched emitter process for reliability and scalability
5
Fabrication Challenges – Stable refractory emitters
Emitter yield drops during base contact, subsequent lift-off steps
High stress in emitter metal stack
Poor metal adhesion to InGaAs
Need for low stress, high yield emitters
Fallen emitters
6
Fabrication Challenges – Base-Emitter Short
Undercut in thick emitter semiconductor
Helps in Self Aligned Base Liftoff
For controlled semiconductor undercut
Thin semiconductor
To prevent base – emitter short
Vertical emitter profile and line of sight metal deposition
Shadowing effect due to high emitter aspect ratio
Slow etch plane
InP Wet Etch
Fast etch plane
7
Fabrication Challenges – Base Access Resistance
contacts
contactgapsh,bcsh,esh, 2612 AL
W
L
W
L
WR
e
gap
e
bc
e
ebb
We
Wgap
Wbc
bcsh,esh,gapsh, ,
• Surface Depletion
• Process Damage
Need for very small Wgap
• Small undercut in InP emitter
• Self-aligned base contact
cbbbCR
ff
8max
8
Composite Emitter Metal Stack
TiW
W
• W/TiW metal stack
• Low stress
• Refractory metal emitters
• Vertical dry etch profile
W emitter
Erik Lind
Evan LobisserTiW emitter
9
TiW
W
Base Metal
BCB
SiNx
100nm
Vertical etch profile
Low stress
High emitter yield
Scalable emitter process
Vertical EmitterFIB/TEM by E Lobisser
10
InGaAs capMo contact
InP emitter
Dual SiN sidewall
Controlled InP undercut
Narrow BE gap50nm
Narrow Emitter UndercutFIB/TEM by E Lobisser
11
Epitaxial Design
T(nm) Material Doping (cm-3) Description
10 In0.53Ga0.47As 81019 : Si Emitter Cap
20 InP 51019 : Si Emitter
15 InP 21018 : Si Emitter
30 InGaAs 9-51019 : C Base
13.5 In0.53Ga0.47 As 51016 : Si Setback
16.5 InGaAs / InAlAs 51016 : Si B-C Grade
3 InP 3.6 1018 : Si Pulse doping
67 InP 51016 : Si Collector
7.5 InP 11019 : Si Sub Collector
5 In0.53Ga0.47 As 41019 : Si Sub Collector
300 InP 21019 : Si Sub Collector
Substrate SI : InP
Vbe = 1 V, Vcb = 0.7 V, Je = 24 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
Thin emitter semiconductor
Enables wet etching
12
Results - DC Measurements
BVceo = 3.7 V @ Je = 0.1 mA/cm2
β = 17
@Peak f,fmax
Je = 20.4 mA/m2
P = 33.5 mW/m2
Gummel plot
Common emitter I-V
0
5
10
15
20
25
30
0 1 2 3 4 5
J e (
mA
/m
2 )
Vce
(V)
P = 30 mW/m2
Ib,step
= 200 A
BV
P = 20 mW/m2
Aje
= 0.22 x 2.7 m2
10-9
10-7
10-5
10-3
10-1
0
5
10
15
20
0 0.2 0.4 0.6 0.8 1
I c, Ib (
A)
Vbe
(V)
Solid line: Vcb
= 0.7V
Dashed: Vcb
= 0V
nc = 1.19
nb = 1.87
Ic
Ib
13
TEM – Wide, misaligned base mesa
0.5 m
50 nm
Small EB gap
Misalignment
FIB/TEM by H. Chiang
• 220 nm emitter-base junction
• 1.1 m wide base-collector mesa
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RF Data
Ic = 12.1 mA
Je = 20.4 mA/m2
Vcb = 0.7 V
P = 33.5 mW/m2
0
5
10
15
20
25
30
35
109 1010 1011 1012
Ga
ins
(dB
)
freq (Hz)
Aje
= 0.22 x 2.7 m2
f = 480 GHz
fmax
= 1.0 THz
H21
U
W-Band measurements to verify f/fmax
15
Base Post Cap
Ccb,post does not scale with Le
Adversely effects fmax as Le ↓
Need to minimize the Ccb,post value
c
postr
postcb T
AC
0,
16
Base Post Cap
Ccb,post does not scale with Le
Adversely effects fmax as Le ↓
Need to minimize the Ccb,post value
c
postr
postcb 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
17
Base Metal Resistance
bc
emetalbb W
LR
6metalsh,, Base
Ib
BP
• Rbb,metal increases with emitter length
fmax decreases with increase in emitter length
18
Base Metal Resistance
bc
emetalbb W
LR
6metalsh,, Base
Ib
BP
• Rbb,metal increases with emitter length
fmax decreases with increase in emitter length
200
300
400
500
0 5 10 15 20 25
Le = 3m
Le = 4m
Le = 5m
f (G
Hz)
Je (mA/m2)
200
400
600
800
1000
0 5 10 15 20 25
Le = 3m
Le = 4m
Le = 5m
f ma
x (G
Hz)
Je (mA/m2)
19
Parameter Extraction
Jkirk = 23 mA/m2 (@Vcb = 0.7V)
200
600
1000
200
300
400
500
600
0 5 10 15 20 25 30
f
f max
(G
Hz) f (G
Hz)
Je (mA/m2)
Vcb
= 0V
Vcb
= 0.7Vfmax
2
3
4
5
6
0 5 10 15 20 25 30
Ccb
(fF
)
Je (mA/m2)
Vcb
= 0.7 V
Vcb
= 0.5 V
Vcb
= 0 V
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Equivalent Circuit
Hybrid- equivalent circuit from measured RF data
Rex ~ 4.2 m2
freq (1.000GHz to 67.00GHz)
S(1
,1)
S(2
,2)
S(1
,2)*
5S
(2,1
)/10
freq (100.0MHz to 67.00GHz)
S_p
aram
eter
_Dee
mbe
d_P
NA
..S12
d*5
S_p
aram
eter
_Dee
mbe
d_P
NA
..S21
d/10
S_p
aram
eter
_Dee
mbe
d_P
NA
..S11
dS
_par
amet
er_D
eem
bed_
PN
A..S
22d
S21/10S12x5
S11
S22
--- : Measured x : Simulated
freq (1.000GHz to 67.00GHz)
S(1
,1)
S(2
,2)
S(1
,2)*
5S
(2,1
)/10
freq (100.0MHz to 67.00GHz)
S_p
aram
eter
_Dee
mbe
d_P
NA
..S12
d*5
S_p
aram
eter
_Dee
mbe
d_P
NA
..S21
d/10
S_p
aram
eter
_Dee
mbe
d_P
NA
..S11
dS
_par
amet
er_D
eem
bed_
PN
A..S
22d
S21/10S12x5
S11
S22
--- : Measured x : Simulated
Ccb,x = 2.72 fF
Ccb,i = 0.52 fF
Rcb = 27 k
Rc = 3.4
Rex = 7
Rbe = 86
Rbb = 27
Cje + Cdiff = 8.8 + 59.2 fF gmVbee-j
0.234Vbee(-j0.14ps)
Base
Emitter
Col
Ccg = 3.2 fF
21
Summary
• Demonstrated DHBTs with peak f / fmax = 480/1000 GHz
• Small Wgap for reduced base access resistance High fmax
• Undercut below the base post to reduce Ccb
• Narrow sidewalls, smaller base mesa and better base ohmics needed to enable higher bandwidth devices