TriQuint Semiconductor offers a low dielectric constant coating process for GaAs MMICs consisting of B-staged bisbenzocyclobutene(BCB). The BCB overcoat is applied on top of the existing Silicon Nitride(SiN) protective overcoat(PO) layer and is designed to encapsulate the MMIC surface and underfill the air bridges. BCB also provides a low-K dielectric spacer between the MMIC and any packing encapsulate. The addition of BCB allows for increase die protection from scratches and mishandling and underside mechanical support for air bridges.
In this paper we will cover the process issues we encountered during the process set up for the HV3S ad HVHBT technologies. Due to the difference in the overall device heights of the HV3S and HVHBT, two BCB thicknesses/processes have been set up; a 7um vs. 9um BCB processes. In this paper we will discuss about the details of process problems we encountered during the daily operation and solutions we implemented to eliminate the issues for 7um thick BCB (DOW 4022-35) process. The biggest challenge we encountered is the wrinkled BCB issue—see Fig 1.1 , the wrinkled BCB was an intermittent problem in the first two year that we were running the 7um BCB process, occasionally we see one wafer out of as many as 15 ( 6 wafer lot) lots that had the issue. However recently the wrinkled BCB has become a more predominate problem causing the entire BCB line was shut down. Together with the wrinkled BCB problem, also we are seeing bubbles at the metal line area, the bubbles (blisters) are very prevalent when they happened. See Fig 1.2.