Sample Preparation Methodology for a CMOS+MEMS Device with Low Frequency and Seal Ring Short Failures

In this paper, the failure mechanisms of a CMOS+MEMS device with low frequency failure and seal ring short failure have been investigated. This CMOS+MEMS device consists of a CMOS chip bonded onto a MEMS piezoelectric sensor chip, with a MEMS cap sealing the vacuum in the cavities between the MEMS and CMOS chips. Due to the complexity of the MEMS device and the restricted information on the failed units available from our customer, limited tests and diagnostic analyses can be done at our site. This paper shares how our methodology approach on leveraging on our existing FA tools and techniques, using dry and wet etching, optical and SEM microscopy, as well as FIB milling helped us determine the failure mechanisms. With this method, we managed to conclude that the low frequency failure of this device was due to insufficient eutectic bond force and the seal ring short failure was a result of excessive bonding force at the MEMS to CMOS seal rings.