Integration of Photodefinable Polybenzoxazole as Intermetal Dielectric for GaAs HBT Technology

Polybenzoxazole (PBO) is a polymer dielectric that has excellent mechanical, physical, and electrical characteristics and that is used for various semiconductor thin film applications.  The PBO has low dielectric constant, and has good thermal stability and thermo-oxidative resistance.  Furthermore, the PBO material can be applied by spin-coating and can be cured at temperature of <300^oC, which is significantly lower than many other spin-on dielectric materials.  These characteristics make PBO suitable for use in many semiconductor technologies and applications that require the processing temperatures not to exceed 300^oC, such as GaAs hetero-junction bipolar transistor (HBT) technology. The PBO material is easily processed, and can be made photodefinable by adding photoactive compounds, and therefore be patterned on semiconductor wafers.  By using a photodefinable PBO dielectric material, there will not be any need to perform patterning by dry-etch process steps that are typically performed in GaAs technologies.

In this study, photodefinable PBO has been integrated as intermetal dielectric in GaAs HBT technology.  Process module optimization and integration studies were performed and the PBO results and process flow were compared to those of dry-etch polyimide, which is typically used as intermetal dielectric in GaAs technology.  The photodefinable PBO is shown to allow the intermetal dielectric process flow to be simplified and the number of processes and equipments to be reduced.  Results show that PBO has more planarity than polyimide, when coated on GaAs HBT wafers, which typically have significant topography.  The PBO photolithography coat, expose, and develop processes were optimized in order to obtain the desired dielectric thickness and the required via dimension and sidewall profile, in addition to removing the PBO from the die street.  The PBO thermal curing process was performed at 300^oC, while ashing was performed to remove any PBO residue in the vias and streets.  Figure 1 shows the FIB/SEM images of GaAs wafer after deposition of Metal 1 and coated with photodefinable, after via expose and develop, after cure, and after ash.  Figure 2 shows the GaAs HBT wafer with (a) dry-etch polyimide and (b) photodefinable PBO used as intermetal dielectric.  These results show that the photodefinable PBO film is compatible with, is suitable for, and can be integrated as intermetal dielectric in GaAs HBT technology.