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An overview is given on measurement techniques and results obtained for the characterization of bonded St-St, Si-SiO~, and SiO~-SiO2 interfaces. The electrical properties of St-St interfaces are similar to those of grain boundaries, which suggests a model where the current across the interface is limited by a potential barrier determined by charge .carriers captured into electron states at the bonded interface. From current-voltage and capacitance-voltage measurements, the interface charge and its energy distribution can be determined. For bonded Si-SiO2 interfaces, energy distributions of interface states are obtained by capacitance-voltage technique with midgap densities in the region at or above 5 • 10 TM eV -1 cm -2. Interfaces between two bonded SiO2 layers exhibit charge trapping phenomena.

The degradation of wafer bonded silicon dioxides as a result of Fowler-Nordheim electron injection has been studied. The samples were metal oxide semiconductors (MOS) capacitors with wafer-bonded SiOz-SiO2 interfaces at the oxide center. The charge trapping in the oxide and the Si-SiO2 interface state generation were monitored as a function of injected charge and compared to reference MOS capacitors without bonded interfaces. A larger change in the oxide charge was found in the bonded capacitors as compared to the reference structures. The centroid of trapped negative oxide charge was found to be located close to the SiOz-SiOa interface in the bonded structures, while the reference structures exhibited centroids close to the injecting contact. The electron injection caused approximately the same generation of interface states in both groups of capacitors.

The choice of surface treatment prior to silicon wafer bonding is crucial both for the reliability of the bonding process and the electrical properties of the bonded structure. In the case of silicon direct bondingfor manufacturing of buried electrical junctions, the bonding step is particularly critical for accurate and reproducible results. In this work, the attraction between IIF-etehed surfaces and the effect of a following water rinse were investigated by studying the bonding spontaneity, velocity of the initial bonding wave, and the electrical properties of the bonded junctions. Spontaneous bonding occurred when no subsequent water rinse was made after the HF-etch, while water-rinsed wafers did bond only with the help of an applied pressure and also ended up with more voids. The highest contact wave velocit~r together with good electrical properties, was obtained for surfaces treated in a 10% solution of HF in water, without subsequent water rinse. For unipolar bonded junctions annealed at 600~ no influence of the bonded interface was found on the electrical characteristics. In contrast, samples annealed at 800~ exhibited slightly nonlinear current vs. voltage characteristics and a larger dynamic resistance. For water-rinsed samples, annealed at 800~ the dynamic resistance showed a larger spread, indicating a lower reproducibility as compared to nonrinsed samples.