Submicron InP Bipolar Transistors: Scaling Laws, Technology Roadmaps, Advanced Fabrication Processes

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Submicron InP Bipolar TransistorsScaling Laws,
Technology Roadmaps, Advanced Fabrication
Processes
2002 SSDM Conference, September, Nagoya

• Mark Rodwell
• University of California, Santa Barbara

rodwell_at_ece.ucsb.edu 805-893-3244, 805-893-3262
fax
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Applications of InP HBTs
Optical Fiber Transceivers 40 Gb InP and SiGe
HBT both feasible ICs now available market has
vanished 80 160 Gb may come in time within
feasibility for scaled InP HBT world may not need
capacity for some time WDM might be better use of
fiber bandwidth
mmWave Transmission 60-80 GHz, 120-160 GHz,
220-300 GHzLow atmospheric attenuation (weather
permitting).High antenna gains (short
wavelengths).10 Gb/s transmission over 500
meters with 20 cm antennas needs 4 mW transmitter
power
Mixed-Signal ICs for Military Radar/Comms
direct digital frequency synthesis, ADCs,
DACshigh resolution at very high bandwidths
sought
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How Do We Improve the Bandwidth of Bipolar
Transistors ?
Thinner base, thinner collector higher ft ,
but higher RbbCcb , RexCcb what parameters are
really important in HBTs ?how do we improve HBT
performance ?
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How do we improve gate delay ?
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Scaling Laws for fast HBTs
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Challenges with Scaling
Collector-base scaling Mesa HBT collector under
base Ohmics. Base Ohmics must be one transfer
length ? sets minimum size for collector
Solution reduce base contact resistivity ?
narrower base contacts allowedSolution decouple
base collector dimensions transferred-substrate
, undercut-mesa, or buried SiO2 in junction
(SiGe) Emitter Ohmic Resistivity must improve
in proportion to square of speed
improvements Current Density self-heating,
current-induced dopant migration, dark-line
defect formation Loss of breakdownavalanche Vbr
never less than collector bandgap (1.12 V for Si,
1.4 V for InP) .sufficient for logic,
insufficient for power Yield !submicron HBT
processes have progressively decreasing yield
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Technology Roadmaps for 40 / 80 / 160 Gb/s
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Low Ccb InP HBT structures
transferred-substrate
Allows deep submicron collector scaling Problems
with heating at high J Low yield at deep
submicron scaling
undercut-collector
Popular approach Uncertain yield at submicron
geometries
Narrow-mesa with 1E20 carbon-doped base
Conservative approach Still not viable for gt
3000 transistors per IC
Need improved device structures for high yield
at 0.1 mm scaling
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Miguel Urteaga
Unbounded Power Gain in Submicron InAlAs/InGaAs
HBTs
Emitter
Unbounded 45-170 GHz Unilateral power gain
Power gain is high, but fmax cant be
determined
0.3 x 18 ?m2
Collector
Ic 5 mA, Vce 1.1 V
Int. Symp. Compound Semiconductors, Tokyo, Oct.
2001
0.7 x 18.6 ?m2
Int. Journal High Speed Electronics and Systems,
to be published
reduced
Capacitance modulation negative resistance
observed Gunn-like or IMPATT effects ?
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Collector velocity modulation Ccb cancellation
and negative resistance
Miguel Urteaga
If Ccb cancellation is observed, there must also
be an associated negative resistance
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2nd Hypothesis weak IMPATT effects in the
collector
Miguel Urteaga
IMPATT effect also produces both capacitance
cancellation and negative resistance
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UCSB
Deep Submicron Bipolar Transistors for 140-220
GHz Amplification
Miguel Urteaga
raw 0.3 mm transistor 6-11 dB power gain _at_ 200
GHz
1-transistor amplifier 6.3dB _at_ 175 GHz
3-transistor amplifier 8 dB _at_ 195 GHz
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Mattias Dahlstrom (UCSB) Amy Liu (IQE)
Wideband Mesa InP/InGaAs/InP DHBTs
UCSB / IQE
1 mm base contacts, 0.5 mm emitter junction
0.7 mm emitter contact
2000 Å InP collector 300 Å InGaAs base 8E19 to
5E19 graded C base doping InAlAs/InGaAs
base-collector grade. 500 Ohm/square base sheet
resistance Pd/Ti/Pd/Au base Ohmic contacts lt 10-7
Ohm-cm2 base contact resistance 7.5 V
Breakdown 282 GHz ft , gt 450 GHz fmax ,
operation to 500 kA/cm2 at 1.7 volts Rbb is low,
Ccb needs further reduction
282 GHz ft gt450 GHz fmax,
480 GHz
Vce1.7 V J3.7E5 A/cm2
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87 GHz HBT master-slave latch
UCSB
PK Sundararajan, Zach Griffith
InAlAs /InGaAs/InP MESA DHBT 400 Å base, 2000 Å
collector, 9 V BVCEO 200 GHz ft, 180 GHz
fmax 2.5 x 105 A/cm2 operation
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8 GHz S-D ADC
PK Sundararajan
Technology0.7 um InP MESA DHBT 400 Å base, 2000
Å collector, 9 V BVCEO, 200 GHz ft, 180 GHz
fmax2.5 x 105 A/cm2 operation
Design simple 2nd-order gm-C topology comparator
is 87 GHz MSS latch integration by capacitive
loads 3-stage comparator, RTZ gated DAC
Results133 dB (1 Hz) SNR at 74 MHzequivalent to
8.8 bits at 200 MS/s
975 kHz FFT bin size 8 GHz clock rate 65.5 MHz
signal641 oversampling ratio
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Very strong features of SiGe-bipolar transistors
High current density 10 mA/mm2 T-shaped
polysilicon emitter 0.25 mm junction
wide contact low resistance, high yield Thin
intrinsic base low tb Thick extrinsic base low
Rbb Low Ccb collector junction collector
pedestal CVD/CMP SiO2 planarization
regrown poly extrinsic base High-yield, planar
processing high levels of integration
LSI and VLSI capabilities



SiGe clock rates up to 65 GHzMuch more complex
ICs than feasible in InP HBTInP HBT must reach
higher integration scales or will cease to compete
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InP vs Si/SiGe HBTs materials vs scaling
advantages
Advantages of InP201 lower base sheet
resistance, 51 higher base electron
diffusivity31 higher collector electron
velocity, 41 higher breakdown-at same ft.
Disadvantage of InP archaic mesa fabrication
processPresently only scaled to 1 um
(production)large emitters, poor emitter
contactlow current density 2 mA/um2high
collector capacitance nonplanar device - low
yieldlow integration scales
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InP HBT limits to yield non-planar process
Failure modes
Emitter contact
Etch to base
Liftoff base metal
Emitter planarization, interconnects
Yield degrades as emitters are scaled to
submicron dimensions
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MBE growth of Polycrystalline n InAs
Dennis Scott
SiGe HBT process extensive use of
non-selective-area poly-Si regrowthCan a similar
technology be developed for InP ?

• Polycrystalline InAs grown on SiN
• Doping 1.3 ?1019 cm-3, Mobility 620
  cm2/Vs
• doping-mobility product 8?1021 (V s cm)-1

• InGaAs lattice matched to InP
• Doping 1.0 ?1019 cm-3, Mobility 2200
  cm2/Vs
• doping-mobility product 22?1021 (V s cm)-1

Polycrystalline InAs has potential as an
extrinsic emitter contact
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Process Flow Single-poly-regrowth InP HBT
Dennis Scott
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Regrown-Poly-InAs-Emitter HBT
Dennis Scott
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Emitter Regrowth with Buried Base Contact Metal
Dennis Scott
Buried W/Au base metal under emitter ? further
reduced Rbb Similar to buried WSi base contact
process (SiGe, Washio)
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Submicron Scaling of InP HBTs
InP HBTs are a mixed-signal, not a MIMIC
technology for MIMICs, sub-0.1-mm InP HEMTs
are hard to beat mixed-signal is fiber ICs,
ADCs, DACs, digital frequency synthesis
these are 1000 -- 40,000 transistor ICs InP
HBTs are struggling to compete with SiGe HBT
application demands transistor counts near/beyond
yield limits large emitter junctions? high
current ? power near acceptable limits no
decisive speed advantage in relevant circuits
digital logic materials advantages being
squandered by inadequate scaling Critically
needed for InP HBTs highly scaled process
0.2 mm emitters, 0.4 mm collectors highly
planar and high-yield fabrication processes
small emitter junctions (0.2 mm x 0.5 mm) for
acceptable power