Pulse-field-assisted bonding for SOI devices

A technique for obtaining bulk-quality silicon layers on silicon dioxide by pulse-field assisted bonding of oxidized silicon wafers has been developed. Radiation-hardened 64 K silicon-on-insulator (SOI) DRAMs (dynamic random-access memories) were fabricated for the first time to the authors´ knowledge by a conventional bulk DRAM process. Void-free bonding is done by pressing the oxidized surfaces of two wafers together and applying an impulsive electrostatic force. The oxide thickness ranges from 0.05 to 4.0 μm. The pair of wafers is then placed on a SiC-coated carbon strip heater and heated to 800°C in a 0.1-Pa nitrogen atmosphere. A silicon strip electrode is placed on one wafer to make an electrical contact with the other. A bonding-pulse amplitude of 100 to 500 V is applied between the electrode and the lower heater. Voids are eliminated when the pulse is applied. Bonded wafers are then annealed and thinned by polishing and/or chemical etching to the desired thickness. The characteristics of the MOSFETs and DRAMs obtained by this method are briefly described