Structure of Al/Al and Al/Si3N4 Interfaces Bonded at Room Temperature by Means of the Surface Activation Method

Al--Al joints and Al--Si3N4 joints were manufactured at room temperature by means of the surface activation method. In this procedure, the surfaces to be bonded are sputter-cleaned and activated by Ar fast-atom-beam (FAB) irradiation and then brought into contact with each other under a slight pressure. The primary concept of the method is based on the idea that clean metal surfaces are inherently active and react with other elements such as oxygen, nitrogen and carbon, and thus form a strong bonding with ceramics even at room temperature. Therefore it was expected that the method would be effective only in an ultra-high vacuum so that the sputtered surfaces remain clean until joining. It was found, however, that such a clean environment is not necessary for joining. Surfaces activated in a vacuum containing some residual gas can be bonded very strongly and do not lose the capacity of adhering even after exposure to the residual gases after sputter-cleaning. It is found by transmission electron microscopy that the interface structure depends remarkably upon the vacuum condition in which the surfaces to be bonded are sputter-cleaned. An intermediate layer consisting of a partially amorphous phase was found in Al--Al joints bonded under a vacuum pressure of 10 exp --5 Pa, while an Al--Al joint without an intermediate layer was formed by ultra-high vacuum bonding. It is assumed that O or water in the residual gas of the vacuum was absorbed on the Al (99.99%, 99.9995%) surface and dissolved into metal by the Ar irradiation. Thus, an amorphous Al--O solid solution or a kind of aluminum oxide is formed. Al--Si3N4 joints manufactured in a high vacuum from surface activated materials showed also an intermediate layer which was, however, entirely amorphous over the whole interface and was considered to have been formed predominantly on the Si3N4 surface.