TABLE OF CONTENTS As previously discussed in Chapter 1, MEMS is used in a broad range of applications. Accordingly, the selection criteria of the MEMS structural layer depend strongly on the function performed by each device and the operating environment. If we consider thermal infrared imagers, the structural layers should have high thermal insulation and should possess a physical property that is highly sensitive to the absorbed radiation. Furthermore, the structural material should provide good electrical contact to the substrate. On the other hand, if we consider MEMS mechanical resonators and switches, the structural layer should have high quality factor and low electrical resistivity to reduce insertion loses. For micromirrors, the structural layer should have high reflectivity to optimize the device functionality. Other applications might require that the electrical conductivity of the material vary by orders of magnitude from one part of the device to another, and in this case the use of a semiconductor material is mandatory. In some cases, the material is required to have good magnetic, piezoresistive or piezoelectric properties. Furthermore, the optical properties of the material might be a critical issue for specific applications.
In addition to the requirements of each specific application, there are common properties that should be always satisfied for most MEMS materials, such as the constraints imposed on mean stress and stress gradient. As the majority of MEMS devices consist of a suspended structure, the structural layer should ideally have zero stress and stress gradient or practically low tensile stress with the upper...
