Unbonded areas or bubbles generated at the interface of bonded silicon wafers in the temperature range of 200-800°C have been investigated. Experiments described in this paper demonstrate that the desorption of hydrocarbon contamina­tion at the silicon wafer surfaces appears to be a necessary condition for the formation of these bubbles. SIMS data also indicate the existence of hydrocarbons at the bonding interface. It is speculated that hydrocarbon gas such as CH4 is re­quired for bubble nucleation and that either CH4 or H2 itself or a mixture of both gases is contained in these bubbles. Finally, methods to prevent the formation of these bubbles are presented. © 1991 The Japan Society of Applied Physics.