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

Abstract

In this study, we have evaluated several interlevel dielectric materials for GaAs hetero-junction 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 <300oC, as device degradation can occur at higher temperatures. The dielectric materials evaluated are 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 lower dielectric constant than PECVD Si3N4. However, the polyimide has to be dry-etched, unlike the photosensitive PBO. Furthermore, the PBO has better gapfill and planarization capabilities than polyimide.