The contact wave velocity in silicon wafer bonding is experimentally found to decrease with wafer thickness and to be only weakly dependent on wafer diameter. Wafers of different thicknesses ranging from 270 to 5000 mu m, were dipped in HF:H2O before bonding to give the surfaces hydrophobic properties. A model based on energy conservation can explain the main characteristics of the experimental results. The contact wave velocity is determined by the amount of energy available as kinetic energy for the entrapped gas in the gap between the wafers. By increasing wafer thickness, the elastic energy stored in the material is increased, and the contact wave velocity is decreased. (C) 1996 American Institute of Physics.