9. Intrinsic Stress Effects in Deposited Thin Films

THIN FILM DEPOSITION is a widely used technique for the fabrication of MEMS (Micro-Electro-Mechanical Systems). This technique is required to manufacture free-standing structures which can induce or sense a mechanical movement. During the deposition process of thin layers and aftermath an intrinsic stress is generated. In subsequent process steps, after removal of the underlying sacrificial layer, the (stressed) deposited layer which is an important component of the desired MEMS device, is left free-standing. As a consequence the process induced stress can relax and deform the deposited layer in an undesirable way.

Polycrystalline silicon-germanium (poly-SiGe) has been promoted as an attractive material suitable as structural layer for several MEMS applications [132]. Poly-SiGe is a good alternative to polycrystalline silicon (poly-Si), because it has similar properties. The same good mechanical and electrical properties can be obtained with poly-SiGe at much lower temperatures (down to 400 $^\circ$C) compared to poly-Si (above 800$^\circ$C). These low processing temperatures enable MEMS post-processing on top of MOS without introducing significant changes in the existing MOS fabrication processes. The sacrificial layer is normally made of silicon dioxide (SiO$_2$), because this material can then be etched with a high selectivity towards the structural layer by the use of hydrogen fluoride (HF).

Different aspects of the connection between microstructure and stress have been investigated in the past 30 years. The focus was mostly on some specific grain-grain boundary configurations in early or mature stages of microstructure evolution [133]. As a result there exist numerous models derived on the basis of continuum mechanics, which are applicable only for highly simplified situations. On the other side a group of researchers, mostly mathematicians, has developed complex models for describing morphology of the microstructural evolution, a development which culminates in multi-level set models of grain evolution [134,135]. These models can reproduce the realistic grain boundary network in a high degree, but they do not include stress [135]. The goal of this work is the integration of microstructure models which describe strain development due to grain dynamics in a macroscopic mechanical formulation. This strain loads the mechanical problem which provides a distribution of the mechanical stress and enables the calculation of displacements in the MEMS structure.