MOCVD Emitter Regrowth Technology for Scaling InGaAs/InP HBTs to Sub-100nm Emitter Width
- Author(s): Choudhary, Prateek
- Advisor(s): Rodwell, Mark JW
- et al.
By scaling semiconductor thicknesses, lithographic dimensions, and contact
resistivities, the bandwidth of InGaAs/InP Hetero-junction Bipolar Transistors
(HBTs) has reached 550/1100 GHz ft/fmax at 128 nm emitter width (wE). Primary challenges faced in scaling the emitter width are: developing high aspect
ratio emitter metal process for wE < 100nm, reducing base contact resistivity
ρb,c, and maintaining high DC current gain β.
The existing W/TiW emitter process for RF HBTs cannot scale below 100
nm. Process modules for scaling the emitter width to 60 nm are demonstrated.
High aspect ratio trenches are etched into a sacrificial Si layer and then filled
with metal via Atomic Layer Deposition (ALD). Metals with high melting points
are chosen to withstand high emitter current densities (JE) at elevated junction
temperatures without suffering from electromigration or thermal decomposition
and is thus manufacturable. ALD deposition of TiN, Pt, and Ru are explored.
Novel base epi designs are proposed for reducing Auger recombination current
(IB,Auger). A dual doping layer in the base is proposed with a higher doping in the
upper 5 nm of the base for lower ρb,c and a lower doping in the remainder of the
base for reducing IB,Auger. Presence of a quasi-electric field (4EC) in the upper
doping grade accelerates electrons away from the region towards the collector,
thus further reducing IB,Auger.
Selective regrowth of the emitter semiconductor via Metal-Organic Chemical
Vapour Deposition (MOCVD) is proposed for decoupling the extrinsic base region
under the base metal from the intrinsic region under the emitter-base junction,
for increasing β,ft, and improving ρb,c. Carbon p-dopants in the InGaAs base are
passivated by H+ during regrowth. Annealing to reactivate carbon induces surface
damage and increases base sheet resistance (Rb,sh) and ρb,c. Process techniques
for minimizing Rb,sh and ρb,c in an emitter regrowth process are demonstrated
and compared. ρb,c of 5.5 Ω.µm2 on p-InGaAs is demonstrated on Transmission
Line Measurement (TLM) structures after regrowth and anneal, by protecting the
semiconductor surface with tungsten. This is comparable to 2.9 Ω.µm2 measured
on TLM structures that do not undergo regrowth and anneal.
Feasibility of emitter regrowth is demonstrated on Large Area Devices (LADs)
with SiO2 as regrowth mask, and W cap during anneal. Emitter-regrowth and
non-regrowth devices of identical dimensions and epi design are compared. Emitterregrown HBTs yield higher β of 28 as compared to 13 for non-regrowth devices.
Benefits of emitter regrowth cannot be ascertained on LADs due to high series
resistance and large gap spacings between base metal and emitter-base junction.
A process flow is proposed for scaling regrown HBTs to 60 nm emitter widths.
The process incorporates ALD emitter metal technology that is demonstrated in
the first half of the dissertation. New epi designs for regrown-emitter HBTs are
optimized for maximizing β, ft. Scaling regrown-emitter HBTs is essential for
realizing their benefit over non-regrowth HBTs.