Bubble formation during silicon wafer bonding: causes and remedies

Several causes of bubble formation are reported. The role of surface contamination and methods of reducing bubble formation are discussed. In the experiments unstructured, polished single-crystal silicon wafers are used although for actual micromechanical applications, wafers with grooved surfaces frequently have to be bonded. Problems associated with wafer flatness, trapped air, and particles are considered and solutions proposed. The remaining problem of bubble formation in an intermediate temperature range is also discussed. It is proposed that the cause of wafer bonding bubbles formed during annealing is the presence of hydrocarbons contaminating silicon surfaces. Bubbles are generated when hydrocarbons are evaporated from the surface in the 200-800 degrees C temperature range. At higher temperatures, bubbles are dissolved into the bulk silicon or interface oxide layers. There are two different bubble sizes. The large one may be deducted by infrared, whereas the smaller one requires the use of X-ray topography. Preheating wafers at 600-800 degrees C prior to bonding can evaporate all the absorbed hydrocarbons on the surface and prevent bubble formation after bonding and annealing.<>