Sequential Plasma Activated Process for Silicon Direct Bonding

Sequential plasma activated bonding (SPAB) process consisting of oxygen reactive ion etching (RIE) and nitrogen microwave radical plasma was developed for silicon direct bonding at room temperature. A strong influence of plasma time and gas pressure on voids was found both in the SPAB and O2 RIE processes. Tensile strength and surface roughness are functions of oxygen RIE, nitrogen radical time and gas pressure. Improved tensile strength was achieved in the SPAB process. High resolution transmission electron microscope (HRTEM) observations showed a thicker silicon oxide interfacial layer from the SPAB process than that from the O2 RIE process. The increase in thickness of interfacial oxide layers in both processes after annealing at 600{degree sign}C for 2 h in air is attributed to the oxygen concentration of silicon bulk wafers. The SPAB process can be explained by the reaction between two metastable surfaces, which allows water removal from interface, resulting in covalent Si-O-Si bonding.