Mechanical considerations in the design of high-precision surface micromachined devices

Abstract: Many mechanical properties of surface micromachined devices, such as strength and elastic properties, depend on the manufacturing process and are therefore unique to the device. In order to achieve higher precision, it is essential that correct mechanical characteristics are used in the work of designing devices. Therefore, it is necessary to test the mechanical properties with devices micromachined in a relevant process.In the present thesis, a test unit mounted on a micromanipulator adapted to an SEM was used for in situ testing of micromachined beams. Both tensile and bending tests were performed. The strength measurements of the brittle materials silicon and indium phosphide were treated with Weibull analysis. The strength of an accelerometer was calculated with Weibull analysis from the results of strength testing. Young's modulus of textured polycrystalline silicon, single crystalline silicon as well as electrodeposited nickel and nickel-iron alloy was measured, and the yield strength of electrodeposited nickel and nickel-iron alloy was evaluated.The internal stress was measured with two sets of test devices and finite element analysis was used to evaluate the effects. The homogeneous stress component was measured with deflecting indicators while the stress gradient component was measured from the curvature of cantilever beams. A homogeneous stress causes an out-of-plane deflection in a device due the asymmetry of its support. A stress gradient deflects the rim of a support undercut by the sacrificial layer etch and the device attached to it. Thisdeflection can be of major importance in e.g. optical components.Finite element analysis was used to investigate electrostatic actuation, thermal effects, dynamic properties and internal stress effects of a micromachined, tuneable Fabry-Perot filter.