EVALUATION OF ENCAPSULATION AND PASSIVATION OF INGAAS INP DHBT DEVICES FOR LONG-TERM RELIABILITY/

Device encapsulation and passivation are critical for long-term reliab ility and stability. Several encapsulation techniques were evaluated i n terms of degradation of electrical characteristics, gap filling unde r the mesa structures, and adhesion to the semiconductor and metal sur faces. These included plasma enhanced chemical vapor deposited (PECVD) SiO2, electron cyclotron resonance CVD SiNx, spin-on glass, benzocycl obutene, and polyimide. Damage from plasma exposure caused gain degrad ation in the devices. Spin-on coatings cause little to no gain degrada tion, 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, howev er, they were poor at gap filling and had a great deal of shrinkage du ring curing. Device passivation was evaluated using double heterojunct ion bipolar transistor structures with either an abrupt or graded emit ter-base junction. Abrupt junction devices had the self-aligned base m etal directly on the p(+)InGaAs base. Graded junction devices had the base metal on top of graded InGaAsP layers, which the metal was diffus ed through, to make: contact to the base region. Abrupt junction devic es stressed at an initial J(E) of 90 kA/cm(2) at a V-CE of 2V at 25 de grees C degraded 20% within 70 h of operation, whereas, the graded jun ction devices show no degradation in de characteristics after operatio n 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 si ze devices.