Low-Temperature Aluminum-Aluminum Wafer Bonding

Abstract

Thermo-compression wafer bonding is a key technology for the wafer-level production of hermetically sealed cavities, which are essential for the functioning of many microelectromechanical systems (MEMS). Aluminum, with its low material price, high thermal and electrical conductivities and its complementary metal oxide semiconductor (CMOS) compatibility, is a promising candidate for the fabrication of CMOS-MEMS, in which the sensor/actuator part is bonded to the electrical circuit. The chemically highly stable oxide layer on the Al surface cannot be removed by conventional methods. It acts as a thin diffusion barrier layer between the two aluminum metallization layers, and therefore inhibits successful low temperature Al-Al wafer bonding. So far, effective Al-Al wafer bonding has required extreme processing temperatures of >300°C and high contact pressures. Typically, 450°C-550°C led to bonded Al-Al wafers with incorporated alumina (Al2O2) precipitates and reasonable bonding quality. By using the EVG®580 ComBond®system, in which the aluminum oxide is first removed physically, followed by bonding of the two metallization layers – both performed in a high vacuum cluster – for the first time successful Al-Al wafer bonding was feasible at a temperature of 100°C. In the present work, the microstructure of structured and unstructured aluminum films on silicon substrates was characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). C-mode scanning acoustic microscopy (C-SAM), TEM and energy dispersive x-ray spectroscopy (EDXS) interface studies of Al-Al wafer pairs bonded with this novel method revealed oxide-free, atomic contact and grain growth across the original interface. Further, the bond strength was characterized based on dicing yield and pull tests.