Disclosed is a manufacturing method to fabricate Heterojunction Bipolar
Transistors (HBTs) that enables self-alignment of emitter and base metal contact
layers with precise sub-micron spacing using a dielectric-assisted metal lift-off
process. Such an HBT process relies on the formation of an "H-shaped" dielectric
(i.e., Si3N4/SiO2) mask conformally deposited
on top of the emitter contact metallization that is used to remove excess base
metal through lift-off by a wet chemical HF-based etch. This HBT process also uses
a thin selective etch-stop layer buried within the emitter layer to prevent wet
chemical over-etching to the base and improves HBT reliability by forming a non-conducting,
depleted ledge above the extrinsic base layer. The geometry of the self-aligned
emitter and base metal contacts in the HBT insures conformal coverage of dielectric
encapsulation films, preferably Si3N4 and/or SiO2,
for reliable HBT emitter p-n junction passivation. Thus, the disclosed HBT process
enables scaling of narrow emitter stripe widths down to sub-micron dimensions producing
transistors with cut-off frequencies in the range of several hundred GigaHertz.
