Material and Reliability Requirements for MEMS Packaging
Microelectromechanical Systems (MEMS) are the latest technology wave behind information revolution and microsystems. Miniaturization of mechanical systems promises new directions in science and technology. The major markets for MEMS are in optical communications, bio technology, car industry, and radio frequency (RF) applications. Also, many different MEMS sensors and actuators have been implemented. The rapid growth of MEMS requires a more thorough study of MEMS packaging. Although MEMS packaging uses many similar technologies to IC packaging, it is very difficult to follow standardized rules for MEMS packaging. One of the challenges that MEMS packaging faces is that the media, which MEMS devices operate in, varies drastically with applications. For example, the package that is designed for an electrostatic actuator (which is a moving device) follows different principles than a micro fluidic channel. Also, in many cases, the package should protect the structure from environment so that the device functions accurately. For example, all MEMS packages used for electrostatic actuators must provide protection against moisture. Moisture dampens the microscopic movement of the actuator. Also, many MEMS devices are fabricated with integrated circuits on a single substrate. Due to the heat generated by IC, the properties of the materials which MEMS 4 are built upon them, change. Therefore, the material selection for MEMS package should be done carefully. As with semiconductors dies, MEMS devices require to be attached to a package substrate. This requirement is very important in optical and sensor applications. Any die movement causes misalignment in optical packages. Also, sensing is usually done through use of electrostatic actuators which are very sensitive to movement. Any die movement causes faulty sensing. Therefore the die attach materials should be carefully chosen so that the CTE differences are minimized as much as possible. Also, as with semiconductors, the thermal expansion mismatch between MEMS and the substrate induces stress on the package which may cause reliability issues.