Low temperature covalent wafer bonding for X-ray imaging detectors

We developed a process for covalent semiconductor wafer bonding carried out at low temperature and its application to monolithic CMOS integrated pixel detectors for medical and industrial applications. This process is based on the combination of wet chemical etching of the oxide, resulting in hydrogen-terminated, hydrophobic surfaces, and the subsequent removal of the H-passivation by a low-energy plasma. It permits room temperature wafer bonding with electrical properties close to the ones of single crystal surfaces bonded in ultra-high vacuum. The bond strengths achieved by our process are on the order of 1.5 J m(-2). High resolution transmission electron microscopy indicates a nearly perfect crystalline bonding interface with an amorphous layer thickness typically around 0.5 nm. This opens the way for numerous applications of which monolithic CMOS processed pixel detectors is but a first successful example. (C) 2019 The Japan Society of Applied Physics