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Bubble-free bonding of 4-inch silicon wafers on either silicon or quartz wafers is achieved outside a cleanroom. Two wafers are stacked horizontally in a rack with the two mirror-polished surfaces facing each other. In order to avoid wafer contact during hydrophilization, cleaning, and drying, the wafers are separated in the rack by teflon spacers introduced at the wafer edges. After drying the wafers by a spin dryer, the spacers are removed and bonding occurs. Using this procedure we are also able to monitor the bonding process between quartz and silicon wafers at different temperatures. We find that the initial wafer bonding process at room temperature stops operating at temperatures above 200°C.

Gold gettering in directly bonded silicon wafers was investigated. Wafers of (100) orientation were rotationally misoriented against each other by 1° or 25° and bonded by annealing at 1100°C for 2 hours in nitrogen. Transmission electron micrographs had shown two different bonding interface structures depending on the rotational angle. The bonded wafers were gold-deposited on one side and annealed for 3 hours at 950°C or 1000°C in vacuum. The spreading resistance was measured and the result showed a double-U profile in both 1°-misoriented and 25°-misoriented samples. Our results show for the first time that the bonding interface acts as a gettering site and that the bonding interface might be used to artificially introduce gettering sites close to the regions in which devices would be fabricated.

Tunneling diodes with highly doped and abrupt pn-junctions, and p+nn+ diodes were fabricated by bonding 4 inch silicon wafers at room temperature. After low temperature annealing (300–800°C) the electrical properties of the bonding interface could be changed by applying high currents up to 500 A/cm2. For tunneling diodes stressed with high current densities negative resistance effects have been obtained. The p+nn+ diodes with bonded n+ and p+ emitters showed S-shaped current switching observed in p-n-I-M structures. It is shown that due to the presence of an oxide at the bonding interface the current is limited in bonded pn-junctions.