Floating zone silicon wafer bonded to Czochralski silicon substrate by surface-activated bonding at room temperature for infrared complementary metal-oxide-semiconductor image sensors

Abstract

We propose to use a bonding wafer as an alternative epitaxial wafer with an extra thick epitaxial layer of more than 100 μm thickness to fabricate vertical time-of-flight (TOF) complementary metal-oxide-semiconductor (CMOS) imaging sensors that can detect infrared (IR) radiation of wavelength greater than 1120 nm. This bonding wafer comprises a floating zone (FZ)-grown silicon wafer bonded to a Czochralski (CZ)-grown silicon substrate by room-temperature surface-activated bonding. Because the device-fabricating region is formed by bonding the FZ-grown wafer to the CZ-grown silicon substrate at room temperature, the oxygen concentration in this region is decreased to less than that in an epitaxial wafer. In addition, our bonded wafer can have a strong gettering capability for oxygen and transition metals (nickel, copper, and iron) in the bonding interface. Furthermore, the bonded wafer can inhibit out-diffusion of oxygen or transition metal to the device-fabricating region from the CZ-grown silicon substrate, and the device-fabricating region can have fewer impurities after fabricating the devices in the bonded wafer. Therefore, we consider that this bonded wafer can be fabricated by the simple processes of bonding and grinding (polishing) at room temperature without thermal stress, and this method is effective for decreasing the dark currents and white-spot defects generated owing to the presence of oxygen or transition metal, which are undesirable in advanced TOF-CMOS imaging sensors of a vertical structure.