A technique of thermocompressive gold-to-gold diffusion bonding at relatively low temperature has been devised to provide stable, uniform, strong bonds between struc- tural components of microelectromechanical systems. The technique can also be used for vacuum sealing of microscopic cavities. Unlike some other metal-to-metal diffusion bonding techniques, this technique does not entail significant outgassing or the formation of intermetallic compounds. The technique is suitable for bonding of parts made of silicon, quartz, low-thermal-expansion glass, and other materials that can withstand the relatively mild rigors of a low-temperature thermocompressive-bonding process. Two parts to be joined by this technique must have faying surfaces that are either flat or shaped to fit each other. In preparation for bonding, each of the faying surfaces is coated with a layer of chromium, then with a layer of gold (see figure). The coating is done by electron-beam evaporation. The coated substrates are cleaned, then clamped together with their gold layers touching in the desired final configuration in a press in a vacuum chamber.
The chamber is evacuated to a pressure of about 10–5 torr (about 1.3 × 10–3 Pa). While maintaining the clamping force and the vacuum, the coated parts are heated to a temperature in the approximate range of 100 to 350 °C for about 1 hour. The combination of heating and clamping pressure in the vacuum causes atoms to diffuse and mix between the touching gold layers of the two parts, forming a single gold layer that bonds the two parts together.
This work was done by Tony K. Tang and Roman Gutierrez of Caltech for NASA’s Jet Propulsion Laboratory.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, NASA Management Office–JPL (818) 354-7770. Refer to NPO-20076.