Xenon difluoride etching of sacrificial layers for fabrication of microelectromechanical devices

Mechanical elements in microelectromechanical system (MEMS) structures require releasing in order to function correctly. Thus sacrificial layers must be etched away. Traditionally the etching of these sacrificial layers has been done with wet etching. However, this typically causes stiction related problems. One way to try to avoid stiction is to replace the use of liquids with dry vapor-based etch technologies. Xenon difluoride (XeF2) is a fluorine-based dry vapor etch that provides isotropic etching for e.g. silicon (Si). The purpose of this thesis is to present the characterization of the XeF2 etch process with various different materials typically used in MEMS. Firstly, the etch rates for the materials are determined. The results show that poly-Si and molybdenum (Mo) are reactive materials, Tungsten (W) is a conditionally reactive material, SiO2 and Si3N4 are low attack materials, Al2O3 and AlN are non-reactive materials. Secondly, the performed under etching tests provide a vertical etch rate of 3.8 - 4.9 µm / min for poly-Si sacrificial layers under photoresist mask and SiO2 hard mask. The achieved etch rates are high enough that successful etching of polysilicon sacrificial layers can be obtained. The final test in this thesis presents results obtained from a simplified self-supporting device structure. A successful release demonstrating the vast potential of XeF2 etching in microfabrication is obtained with a lateral etch rate of ~ 15 µm / min. The vertical and lateral etching tests presented in this thesis, both with the test and device structures, provide important information about the behavior of XeF2 in different etching environments. Based on the results it is possible to determine processes that are compatible with XeF2 etching. Furthermore, the results presented here provide valuable help in determining the suitable etching parameters for the processes. Thus the data collected for this thesis is a useful reference when considering the implementation of XeF2 etching.