Silicon wafer direct bonding at room temperature in a vacuum

Wafer direct bonding technique is now one of the key technologies to realize integrated micro electro mechanical devices. Most of the existing methods utilize surface modification by wet process and subsequent annealing at a high temprerature typically above 800°C. This high temperature annnealing may cause many problems when the methods are applied to the hooding of processed wafers such as IC chips or micro parts which contain various materials. To get over this problem, we have developed a new method to bond silicon wafers directly without any heat treatment. In this method silicon wafers washed by standard cleaning procedure are set in a vacuum chamber. After evacuation to high vacuum region, the surface of two wafers is etched by argon fast atom beam to generate clean and active surface. Then, two wafers are brought into contact at room temperature with a low pressure. Defects at the bonded interface were inspected by infrareed microscope and KOH etching method, and no void was detected in all the specimens tested. Measured bonding strength was above 10 MPa in the tensile test. Fracture occurred mainly in the bulk of silicon and adhesive glue used to fix the specimen to the testing apparatus. By TEM, no damaged area but a very thin amorphous-like intermediate layer was observed at the interface. From then results, it is concluded that the method realizes strong and tight bonding at room temperature and is promising to assemble small parts produced by the silicon wafer process.