Investigation on the effects of low-temperature anodic bonding and its reliability for MEMS packaging using destructive and non-destructive techniques

Abstract

Excellence in the performance of MEMS-based devices such as RF switches, microfluidics, and pressure sensors are well known and by now reported. Operations of these devices are very sensitive to the environmental factors such as contamination, humidity, vibrations etc. Thus, the integration of these micro-devices with the real-life systems could be challenging without a hermetic sealing. A very common practice for these sealing is to bond a recessed cap onto a micromachined wafer using low-temperature wafer bonding mechanism known as anodic bonding or high-temperature sealing techniques such as fusion bonding for vacuum packages. Considering the limit of high-temperature bonding due to thin-film metals like nickel and gold present on the wafer and the induced bow associated with this high-temperature, this paper reveals a devising electrode designed that successfully bonded the samples at a reduced temperature well below at 250 °C. The reliability and effects of this low-temperature bonding between the silicon and Pyrex glass using destructive and non-destructive mechanisms have been investigated in this paper. The tensile strength measurements indicated a superior bonding strength of 14.12 MPa for the sample bonded at 250 °C. The induced bow height reduced from 30.3 µm (at 450 °C) to 0.3 µm (at 250 °C) meaning a significant reduction of bow up to 80.2%. Elemental composition was studied at the interface using energy dispersive X-ray spectroscopy (EDAX). To evaluate the bond quality, infra-red (IR) imaging was performed on the bonded sample pair. The interfaces were examined and analysed by scanning electron microscopy (SEM). Finally, we implemented this technique for a MEMS based pressure sensor application to prove the feasibility of low-temperature anodic bonding.