Wafer Level Packaging of Compound Semiconductors

Abstract

The microelectronics industry has implemented a number of different wafer level packaging (WLP) technologies for high volume manufacturing, including: UBM deposition, solder bumping, wafer thinning, and dicing. These technologies were successfully developed and implemented at a number of contract manufacturing companies, and then licensed to many of the semiconductor manufacturers and foundries. The largest production volumes for these technologies are for silicon-based semiconductors. Continuous improvements and modifications to these WLP processes have made them compatible with the changes observed over the years in silicon semiconductor technologies. These industry changes include: the move from aluminum to copper interconnect metallurgy, increases in wafer size, decreases in pad pitch, and the use of Low-K dielectrics. In contrast, the direct transfer of these WLP technologies to compound semiconductor devices, like GaAs, SiC, InP, GaN, and sapphire; has been limited due to a number of technical compatibility issues, several perceived compatibility issues, and some business concerns From a technical standpoint, many compound semiconductor devices contain fragile air bridges, gold bond pads, topographical cavities and trenches, and have a number of unique bulk material properties which are sensitive to the mechanical and chemical processes associated with the standard WLP operations used for silicon wafers. In addition, most of the newer contract manufacturing companies and foundries have implemented mostly 200 and 300 mm wafer capabilities into their facilities. This limits the number of places that one can outsource the processing of 100 and 150 mm compound semiconductor wafers. Companies that are processing large numbers of silicon based semiconductor wafers at their facilities are reluctant to process many of these compound semiconductors because there is a perceived risk of cross contamination between the different wafer materials. Companies are not willing to risk their current business of processing silicon wafers by introducing these new materials into existing process flows. From a business perspective, many companies are reluctant to take the liability risks associated with some of the very high-value compound semiconductors. In addition, the volumes for many of the compound semiconductor devices are very small compared with silicon based devices, thus making it hard to justify interruption in the silicon wafer flows to accommodate these lower volume products. In spite of these issues and perceptions, the markets for compound semiconductors are expanding. Several high profile examples include the increasing number of frequency and power management devices going into cell phones, light emitting diodes, and solar cells The strategy for the work described in this paper is to protect all structures and surfaces with either a spin-on resist or a laminated film during each step in the process flow. These layers will protect the wafer from mechanical and chemical damage, and at the same time protect the fab from contamination by the compound semiconductor.