A method to bond silicon wafers directly at room temperature was developed. In this method, surfaces of two silicon samples are activated by argon atom beam etching and brought into contact in a vacuum. By the infrared microscope and KOH etching method, no void at the bonded interface was detected in all the specimens tested. In the tensile test, fracture occurred not at the interface but mainly in the bulk of silicon. From these results, it is concluded that the method realizes strong and tight bonding at room temperature and is promising to assemble small parts made by the silicon wafer process.

Using Ar beam etching in vacuum, strong bonding of Si wafers is attained at room temperature. With appropriate etching time, the bonding occurs spontaneously without any load to force two wafers together. However, surface roughness of the wafers increases during Ar beam etching. Because surface roughness has a strong influence on wafer bonding, long etching time degrades the bonding strength. Using atomic force microscope, we measured surface roughness enhancement caused by Ar beam etching, and investigated the relationship between surface roughness and bonding properties such as strength and interfacial voids. The results agree well with theoretical predictions using elastic theory and energy gain by bond formation. A guideline for successful room-temperature bonding is proposed from these results.

The residual stress originating form the thermal expansion mismatch has been a serious problem in the bonding of piezoelectric crystals onto silicon wafers. The room-temperature bonding method using argon-beam surface activation is applied to the bonding of lithium niobate and silicon. In this method, the surfaces of the specimens are etched by fast argon atom beam and bonded to each other in vacuum. Bonding strength equivalent to that of the bulk material is achieved without any heat treatment. Transmission electron microscope observations show intimate contact at the interface. This method is quite suitable for bonding dissimilar materials with thermal expansion mismatch, because the bonding is performed at room temperature throughout the whole process and no thermal stress is generated.