(May 13, 2010) ELK GROVE VILLAGE, IL — As part of its bonded wafer inspection technology, Sonoscan 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 silicon (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 result in a leak in the seal and subsequent loss of the desired cavity atmosphere. 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 submicron 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 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.

For more information on inspection services, contact SonoLab manager Ray Thomas at (847) 437-6400 x245. For more information on inspection systems, contact Sonoscan’s technical marketing manager Steve Martell at (847) 437-6400 x240.

The images show Sonoscan acoustic images of voids (Device 1) and a breached seal (Device 2) in MEMS devices before wafer dicing.