(Invited) MEMS Capping By Anodic Bonding of Evaporated Glass Thin Films

Abstract

Anodic bonding is a widely used MEMS process to join silicon and glass wafers. It has been utilized for many years in a large variety of devices and is valued for its possibility to create highly reliable and strong bonds at rather robust process parameters – e.g. not demanding extremely perfect surface conditions as in fusion bonding. However, the limitation in the choice of bond partners (typically borosilicate glass like Pyrex or Borofloat33 to silicon) and the rather harsh processing conditions (temperatures around 400°C and typical voltages of 600-1000V) has restricted the use of this process.  Extensive research has been done over the years to adapt the process conditions in order to enable anodic bonding at lower temperatures and voltages and reach a higher compatibility of the process. The quality of the anodic bond is characterized by the charge (alkaline ion content) moved in the glass substrate and the ability of the glass to form covalent bonds with the silicon wafer. It can be observed that the bonding result is mainly an interaction on three process parameters: temperature, voltage, and process time with a clear dependence on the surface roughness of the bond partners. In bulk anodic wafer bonding it is not p ossible to reduce all three of these parameters and receive a satisfactory result. In order to enhance the compatibility of the anodic bonding process, the limitation of bulk silicon glass bonding needs to be overcome. Work has been reported on replacing the bulk glass wafers by thinner glass structures e.g. formed on a silicon wafer by polishing a previously anodically bonded glass substrate to a thickness of 20-50µm to achieve a kind of glass film. Other approaches include sputtering of borosilicate glass and the evaporation of this glass type, which is the focus of this contribution. On part of the silicon wafer, anodic bonding has be verified to work with different metalization layers, esp. aluminum coatings. Our contribution gives an overview on the possibilities of bonding with evaporated glass thin films to silicon and aluminum structures, and it applications in e.g. MEMS capping or pressure sensors. We investigate the influence of the process parameters of the glass deposition on the bonding result and present concepts for enhancing the compatibility of the anodic bonding process using evaporated glass thin films.