3. Experimental results The wafers

3. Experimental results The wafers were cut into 7.5 mm X 7.5 mm samples after annealing at 120°C for 1 h. Afterwards the pieces were further annealed at 120, 150 and 300°C for 2 h. A relatively high bond strength for the prebonded wafers is obtained. No careful handling is required. The bonding is strong enough to permit wafer cutting. After annealing, the strength of the bond was tested by using a draw-bench break-strength test set-up. In this experiment two steel cylinders were glued to either side of the bonded silicon and mounted in the draw-bench. The force required for breaking the sample is measured. The latest results of the tests are shown in Table 1. Other groups who presented silicon to silicon oxide bonding at low temperatures attained lower bond strengths [ 2,3]. We assume that we achieve a higher bond strength because in our process we are able to obtain a very low water concentration at the surface after the wet chemical treatment, which will be explained in the next section. Other causes for a lower bond strength could be a higher surface roughness or a lower density of OH-groups at the surface before prebonding. The maximum tensile strength reported in the literature for silicon bonded to thermal oxide at high temperatures is 200 MPa [4]. Although this very high bond strength is not reached with the low-temperature process, our bonding is nevertheless strong enough for sensor and actuator applications. Furthermore, infrared measurements with multiple internal reflection [5] were done. Therefore the prebonded wafers were cut into 1 cm X 5 cm samples and annealed at different temperatures. The shorter edges were bevelled at 45” to allow the infrared radiation to enter and exit the sample after 50 reflections. Fig. 1 shows the absorption spectrum of a prebonded sample. The shelved-down form is caused by the penetrationdepth-dependent absorption of the radiation by silicon. The visible peaks must be seen as additional absorption. Below 1500 cm-’ all radiation is absorbed by the bulk silicon and