Tensile strength characterization of low-temperature fusion-bonded silicon wafers

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Abstract
Silicon fusion bonding was investigated with emphasis on the low temperature regime. For (100) p-type wafers with and without thermally oxidized surfaces the optimum process conditions with respect to surface preparation and gas ambient were established. H2SO4/H2O2 and hot HNO3 were found to be best suited for bonding blank/blank (without thermal oxide) and oxidized wafers, respectively. Direct measurements of bond tensile strength were made on a computer-controlled test machine. The temperature and time dependence of the bond tensile strength was determined. If the best possible clean-room conditions during processing are observed, wafers without thermal oxide, i.e. with only a chemical oxide of 25 AA thickness due to surface preparation, exhibit a measured bond tensile strength in excess of 240 kp cm-2 at a bonding temperature of only 200 degrees C and a bonding time of 4 h. This value cannot be improved by increasing the bonding temperature. If one or both surfaces are thermally oxidized the bond tensile strength is only 20% of this value. Bond tensile strength for oxidized surfaces, however, can be improved at higher temperatures to reach the high values as found for blank wafers.