Interlevel Dielectric Processes Using PECVD Silicon Nitride, Polyimide, and Polybenzoxazole for GaAs HBT Technology

Multiple factors need to be considered when selecting an interlevel dielectric material for GaAs semiconductor device fabrication
including what the electrical, mechanical, chemical, thermal, and cost requirements are and whether the material and the process
are compatible with GaAs processing. In this study, we evaluated several interlevel dielectric materials for GaAs heterojunction
bipolar transistor
HBT technology. This technology requires the material to have good gapfill and planarizing characteristics, as
the various device and interconnect structures can have significant topography. Additionally, the process typically can only have
a maximum temperature of
300°C, as device degradation can occur at higher temperatures. The dielectric materials evaluated
are plasma-enhanced chemical vapor deposition
PECVD, silicon nitride
Si3N4, polyimide, and photodefinable polybenzoxazole

PBO. The PECVD Si3N4 is mostly conformal when deposited. However, it has a high dielectric constant, cannot be used as
gapfill material, and does not planarize the underlying topography, which makes multilevel metallization challenging. Polyimide
and PBO, both of which need to be thermally cured, have a lower dielectric constant than PECVD Si3N4. However, the polyimide
in this study has to be dry-etched, unlike the photosensitive PBO. Furthermore, the PBO has better gapfill and planarization
capabilities than polyimide.