Characterization of transmission laser bonding (TLB) technique for microsystem packaging

In micro-fabrication and packaging process, the wafer bonding technique offers a unique opportunity to combine different materials for micro-assembly and encapsulation purposes. This research presents the studies conceiving transmission laser bonding (TLB) technique. In order to demonstrate the versatility of this technique, tests were conducted on the bonding of glass wafer to silicon wafer. The pair of the Pyrex glass and silicon is the most popular combination in wafer bonding technique. During the TLB process, a focused laser beam is transmitted through a transparent glass wafer and absorbed by the surface of an opaque silicon wafer. The absorbed laser energy melts a thin layer of the opaque substrate as well as the transparent material near the interface. This melting results in the formation of a strong chemical bond. In the present study, a TLB workstation with the necessary apparatus was developed for experimentally establishing the correlation of the bonding process and its material parameters with the resulting quality of the bond. The parameters of contact pressure, surface roughness, the thickness of the intermediate oxide layer and bonding geometry were examined for their influence on the bond strength. The typical bond strength achieved by TLB technique is 10 MPa. This strength, as measured by a tensile test, is comparable to the strength of bonds obtained using other major wafer bonding techniques including fusion bonding and anodic bonding. The bonded interfaces were analyzed using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) to define the mechanism of the wafer bonding through evaluation of the migration and diffusion of different atoms among the glass and silicon substrates during the bonding process. The main advantage of using the TLB technique is that the bonding process can be performed at room temperature with a relatively low contact pressure and a reduced bonding time. There is also no need of applying a high electrical potential. TLB can be easily integrated into an existing semiconductor production line without adding a vacuum environment.