Evaluation of encapsulation and passivation of InGaAs/InP DHBT devices for long-term reliability

Device encapsulation and passivation are critical for long-term reliability and stability. Several encapsulation techniques were evaluated in terms of degradation of electrical characteristics, gap filling under the mesa structures, and adhesion to the semiconductor and metal surfaces. These included plasma enhanced chemical vapor deposited (PECVD) SiO2, electron cyclotron resonance CVD SiNx, spin-on glass, benzocyclobutene, and polyimide. Damage from plasma exposure caused gain degradation in the devices. Spin-on coatings cause little to no gain degradation, provided that there is minimal stress in the cured film. SOG and BCB films have acceptable adhesion properties and were excellent for gap filling. Polyimide films have excellent adhesion properties, however, they were poor at gap filling and had a great deal of shrinkage during curing. Device passivation was evaluated using double heterojunction bipolar transistor structures with either an abrupt or graded emitter-base junction. Abrupt junction devices had the self-aligned base metal directly on the p(+)InGaAs base. Graded junction devices had the base metal on top of graded InGaAsP layers, which the metal was diffused through, to make: contact to the base region. Abrupt junction devices stressed at an initial J(E) of 90 kA/cm(2) at a V-CE of 2V at 25 degrees C degraded 20% within 70 h of operation, whereas, the graded junction devices show no degradation in de characteristics after operation for over 500 h. Typical common emitter current gain was 50. An f(t) of 80 and f(max) of 155 GHz were achieved for 2 x 4 mu m(2) emitter size devices.