Many micro electromechanical systems (MEMS) require a vacuum or controlled atmosphere encapsulation in order to ensure either a good performance or an acceptable lifetime of operation. Two approaches for waferscale zero-level packaging exist. The most popular approach is based on wafer bonding. Alternatively, encapsulation can be done by the fabrication and sealing of perforated surface micromachined membranes. In this paper, a sealing method is proposed for zero-level packaging using a thin film reflow technique. This sealing method can be done at arbitrary ambient and pressure. Also, it is self-aligned and it can be used for sealing openings directly above the MEMS device. It thus allows for a smaller die area for the sealing ring reducing in this way the device dimensions and costs. The sealing method has been demonstrated with reflowed aluminium, germanium, and boron phosphorous silica glass. This allows for conducting as well as nonconducting sealing layers and for a variety of allowable thermal budgets. The proposed technique is therefore very versatile.
Rusu, C. R., Jansen, H. V., Gunn, R., & Witvrouw, A. (2004). Self-aligned 0-level sealing of MEMS devices by a two layer thin film reflow process. Microsystem technologies, 10(5), 364-371. https://doi.org/10.1007/s00542-004-0415-2