Improvement of anodic bond quality with the assistance of sputtered amorphous film

Abstract

This paper reports the development of a low temperature silicon-to-glass anodic bonding process. To improve the bonding quality, a hydrogen-free amorphous silicon layer of about 40 nm thickness is deposited on the silicon wafer. Without applying the amorphous film, both the bond efficiency and the bond strength are low. With the assistance of the amorphous film, the bond quality is obviously improved. The bubble-free interface can be achieved as long as the temperature is above 250 °C. Even at lower temperatures, the unbonded area can be less than 0.5% of the wafer area. The bubble size decreases with an increase in the bonding temperature. A similar effect is observed with the applied voltage. The bond strength obtained is typically 20 MPa or higher. In the destructive tests, fractures are found to occur mainly inside the glass wafer rather than at the interface. The interface is analyzed with scanning electron microscopy (SEM), Raman spectroscopy and secondary ion mass spectrometry (SIMS). The analyses show that SiO chemical bonds are formed at the interface. Silicon oxidation and hydrogen bonding are the two main mechanisms that generate the bonding between silicon and glass wafers.