A two layer hermetic-like coating process for on-wafer encapsulation of GaAs MMICs

We have developed a low-cost, manufacturable, 2-layer coating process for on-wafer encapsulation of GaAs MMICs. This packaging approach takes advantage of the low dielectric permittivity of polymers such as benzocyclobutene (BCB) and the sealing properties of ceramics such as SiC to provide both mechanical protection to MMICs during handling and also hermetic-like equivalence to moisture with predictable changes in the electrical performance of the coated MMICs. The effects of coatings on FET parameters, spiral inductors and a two stage X-Band LNA have been investigated. Results on FETs indicate that the internode capacitances Cgs and Cgd exhibited the same incremental change of 0.035 pF/mm (3 and 25% increase respectively), while Cds changed by 0.051 pF/mm (27% increase) with very minimal changes in the other FET parameters. The only observed change in spiral inductors was a 112% increase in Cp from 0.006 pF to 0.013 pF. The LNA exhibited a 1 GHz shift in frequency response from 7 to 11 GHz to 6 to 11 GHz with no substantial changes in gain and noise figure. Preliminary reliability investigations on coated devices did not show any failures after 150 hours in autoclave (120 C, 100% humidity).