M-TEST: A Test Chip for MEMS Material Property Measurement Using Electrostatically Actuated Test Structures

A set of electrostatically actuated microelectromechanical test structures is presented that meets the emerging need for microelectromechanical systems (MEMS) process monitoring and material property measurement at the wafer level during both process development and manufacturing. When implemented as a test chip or drop-in pattern for MEMS processes, M-Test becomes analogous to the electrical MOSFET test structures (often called E-Test) used for extraction of MOS device parameters. The principle of M-Test is the electrostatic pull-in of three sets of test structures [cantilever beams (CB’s), fixed–fixed beams (FB’s), and clamped circular diaphragms (CD’s)] followed by the extraction of two intermediate quantities (the S and B parameters) that depend on the product of material properties and test structure geometry. The S and B parameters give a direct measure of the process uniformity across an individual wafer and process repeatability between wafers and lots. The extraction of material properties (e.g., Young’s modulus, plate modulus, and residual stress) from these S and B parameters is then accomplished using geometric metrology data. Experimental demonstration of M-Test is presented using results from MIT’s dielectrically isolated wafer-bonded silicon process. This yielded silicon plate modulus results which agreed with literature values to within 4%. Guidelines for adapting the method to other MEMS process technologies are presented. [204]