In-situ characterization of mechanical properties of MEMS structural layers using different test approaches, with application to thick poly-SiGe

Abstract

This doctoral thesis deals with a number of characterization techniques for the determination of the Young's modulus. the residual stress and stress gradient of surface microfabricated MEMS structural layers. A special manufacturing process was developed that allows to predict the behavior of the used test structures in a more simple way. In a final phase of this work, a number of polycrystalline silicon germanium structural layers are characterized using different approaches based on models and methods developed in this thesis. When a machine part or any other object is designed, it is important to have knowledge of the mechanical properties of the materials used. In order to function properly the designed part needs to fulfill requirements regarding strength and distortion. Large constructions such as for example bridges may not bend too much and must be strong enough so that they do not collapse. Also smaller parts or utensils such as a table. a tennis racket. or a cable need to have a certain stiffness and strength. In the world of micro electromechanical systems (MEMS). consisting of sensors and actuators a lot smaller than the human hair (on average 0.1 millimeters thick). knowledge of material properties is also needed. A popular example of a MEMS is the accelerometer. This is a moving mass using electrostatically loaded fingers that detects accelerations or decelerations. An accelerometer can be found in each airbag sensor but also finds its application in the remote controls of game consoles and in smart phones. More performant and cheaper MEMS can be obtained through integration whereby the MEMS is fabricated above the electronic chip instead of beside it or separately. Polycrystalline silicon germanium (poly-SiGe) as structural layer for MEMS is a very good alternative to the traditional polycrystalline silicon (poly-Si) because it has the desi red mechanical properties at compatible CMOS back-end process temperatures. Using poly-SiGe as structural layer makes therefore the fabrication of MEMS on top of the electronic chip possible. Research on the mechanical properties of poly-SiGe as structural layer for MEMS is still very limited and will therefore be further investigated in this thesis. In this work a number of mechanical properties of poly-SiGe are determined using a number of test structures that are fabricated with an special manufacturing process. The manufacturing process. consisting of a sequence of deposition, exposure, etching and planarization steps, allows the assumption of a perfect clamping of the test structures. This results in less complex mathematical modeling and more predictable behavior of the test structures. Moreover. the developed process allows the fabrication of a multitude of types of test structures.