<title>Nondestructive acoustic microimaging (AMI) analysis of MEMS materials, manufacturing, and packaging</title>

Abstract

The reliability of MEMS and MEOMS is of concern with regard to the base materials, manufacturing process and the packaging utilized to help protect them. It begins with the base materials, such as silicon, which is typically bonded to a second silicon or glass layer to form a SOI structure. Any particles or contaminates present during the bonding process can result in voids at that interface. Voids located in the wrong areas could cause a catastrophic failure of the MEMS/MEOMS device. Clearly, it is a benefit for both process development and quality control to have a method of evaluating these devices for internal, physical anomalies, which may relate to their premature failure prior to processing the bonded silicon further. Acoustic Micro Imaging (AMI) has demonstrated utility for the nondestructive analysis of materials, bonded wafers and electronic packaging. In general AMI uses high frequency ultrasound (5 to 500 MHz) to image the internal features in materials or components. The ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to air gaps (delamination, crack or void). However, it is necessary to understand the capabilities and limitations of acoustic methods relative to the construction of MEMS/MEOMS devices. This allows the analyst to determine which applications are viable and the best way approach for an application. The main type of AMI system utilized in this study was the C-Mode Scanning Acoustic Microscope (C-SAM), which is a reflection mode instrument. In this paper we describe the method and application of AMI at frequencies beyond 200 MHz, which reveals defects in the 5 micron size range at depths of 500 microns within silicon.