mems and chip to chip bonding

MEMS are finding applications in products or systems that require reliable operation over extended periods of time. The reliability requirements for the final product encompass both the mechanical behaviors and the electrical characteristics of the overall system. One critical element in many MEMS applications is chip-to-chip bonding (component bonding), for which long-term operation and storage reliability needs to be understood. MEMS packages are likely to have a large number of bond layers because of multiple interfaces inside the package. The bond layers in MEMS devices must often maintain precise chip alignment in addition to withstanding loading from the macro-environment and loading within the package. A primary indicator of failure (or impending failure) in a chip-to-chip bonded system is delamination between the chip and the material used to bond the chips together. In spite of its importance in MEMS packaging, previous work on bonding in MEMS structures is limited. Very little MEMS-specific work on the reliability of the chip-to-chip bonds exists, let alone, non-destructive methods for determining the reliability of chip-to-chip bonded MEMS.
The Use of Acoustic Micro Imaging (AMI) in non-destructive reliability assessment of microelectronic devices is common. AMI techniques to assess the delamination in plastic encapsulated microcircuits. AMI techniques have been used to asses the cracks in solder joints in flip-chip assemblies. In MEMS structures bonding has been studied previously using destructive testing. AMI has been used to assess failures in tunneling accelerometers but not applied to the assessment of bond reliability. Others have proposed the use of ultrasonic techniques using AMI to check bond quality.