As part of its bonded wafer inspection technology, Sonoscan has recently demonstrated acoustic imaging of defects in the seal that surrounds and protects the cavities in MEMS devices.

The defects most frequently take the form of voids (Device 1) within the seal, which may be direct Si, metallic, glass or polymer, depending on the reliability level of  hermetic seal required as per SEMI MS8-0309. In some locations on a wafer the seal may be breached (Device 2). Another frequent defect is delamination of the seal from one or both substrates, the result of poor wetting or contamination during fabrication.    
The defects are risky because thermal and mechanical stresses can cause them to grow until they cause a leak in the seal and the subsequent loss of the desired atmosphere within the cavity. The seal prevents outside particles, gases and humidity from reaching the cavity. Humidity, for example, can result in freezing up of moving parts within the cavity.
Defects in the seal may be only a few tens of microns in diameter and of sub-micron thickness, but can be imaged by Sonoscan’s C-SAM® systems because they represent a gap that reflects >99.99% of the VHF/UHF ultrasonic pulse.
In production, a percentage of MEMS devices may be imaged with C-SAM acoustic micro imaging systems in order to verify that process parameters are preventing the formation of voids. Where high reliability is essential, as in mil/aero or medical MEMS, 100% of devices may be inspected.