Self-aligned 0-level sealing of MEMS devices by a two layer thin film reflow process

Abstract

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 wafer-scale 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 re-flowed aluminium, germanium, and boron phosphorous silica glass. This allows for conducting as well as non-conducting sealing layers and for a variety of allowable thermal budgets. The proposed technique is therefore very versatile.