By Applied Microengineering Ltd.
Solid Liquid Interdiffusion Bonding, also known as Transient Liquid-Phase bonding, is based on the rapid formation of Intermetallic Compounds (IMCs) between two metals: one low-melting point component (e.g. Sn) and one high-melting component (e.g. Cu, Au). The bonding is typically carried out at moderate temperatures between 250-350°C, and the reflow temperature is significantly higher than the original bonding temperature. The most common metal combinations are Cu-Sn, Au-Sn, Au-In. The AWB systems offer an in-situ oxide-removal process, effective for copper and indium. Prior to bonding, the wafers are mounted on separate upper and lower platens and are held apart by up to 30mm, and this allows chemical treatment of the wafer surfaces: formic acid vapour can be delivered into the bonding chamber for efficient removal of the oxide. This in-situ treatment can immediately be followed by vacuum pump-down, alignment of the wafers, and then contacting and bonding without any opportunity for regrowth of oxide. Advantages of SLID/TLP bonding: - Forms a hermetic seal - Maintains integrity of structures in substrates - Practical/achievable requirements for surface finish - Liquid phase allows for significant surface topography/roughness - Bond reflow temperature significantly higher than the bonding temperature Disadvantages: - A liquid phase is formed, and can flow into unwanted areas - Additional barrier layer on substrates is required - Lateral feedthroughs for electrical interconnections cannot be used Applications: - Wafer level packaging of MEMS e.g. inertial sensors, thermal imagers, pressure sensors
