LOW-TEMPERATURE PREPARATION OF SILICON SILICON INTERFACES BY THE SILICON-TO-SILICON DIRECT BONDING METHOD

Silicon/silicon interfaces were prepared by wafer bonding using the silicon-to-silicon direct bonding method at temperatures below 1000°C. The bonded samples were prepared using wafers with hydrophilic and hydrophobic surface properties. The density of voids at the bonded interface and the electrical properties of the interfaces were investigated. The density of voids in hydrophilic samples was mainly determined by the initial contacting of the wafer surfaces at room temperature. The influence of heat-treatments was weak. Compared to hydrophilic samples, hydrophobic samples were held together at a smaller fraction of the surface area before heat-treatment. During short heat-treatments, this fraction increased and a large number of point-shaped voids was generated. These voids, probably due to enclosed gas, disappeared with an activation energy of 2.3 eV when the annealing proceeded. After long annealing times, no significant difference in the density of voids between hydrophilic and hydrophobic samples was found. Silicon/silicon interfaces with excellent electric properties were prepared by using hydrophobic surfaces at temperatures in the range 700° to 1000°C. No influence of the bonded interface on the current-voltage characteristic of n-type/n-type samples prepared at 700°C for 5 min was found. Directly bonded p+n junctions prepared at 700°C for 5 min exhibited ideality factors n > 2 at small forward bias. A correlation between n factors and the density of states in the bandgap was found. Attempts to reduce the bonding temperature considerably below 700°C were thwarted by decreasing mechanical strength at the bonded interface. © 1990, The Electrochemical Society, Inc. All rights reserved.