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Sacrificial aluminium etching enables micromechanical structures integrated with circuitry to be fabricated using standard IC processes followed by simple post-processing. In this paper, the etching characteristics of CMOS aluminium in four etch solutions are reported. The solutions are (A) a commercially available aluminium etchant, (B) Krumm etch, (C) diluted hydrochloric acid, and (D) diluted hydrochloric acid with hydrogen peroxide. The etching of narrow channels is studied as a function of time and temperature. Initially, the etching process is reaction-rate controlled and then crosses over to a diffusion-controlled regime with reduced etch rate. Underetching distances larger than are readily achieved with etchants `A', `B', and `D'. The commercially available aluminium etchant has a low initial underetch rate of at but offers best control. The initial etch rate of hydrochloric acid with hydrogen peroxide is at . However, irregular etch fronts are obtained. Reliable protection of aluminium pads against etchants `A', `B', and `D' is guaranteed by Shipley's photoresist S1828 spun at 3000 rpm and hardbaked at .

The electrical and optical properties of wafer bonded unipolar silicon-silicon junctions were investigated. The interfaces, both n-n type and p-p type, were prepared using wafers with hydrophilic surfaces. The current versus voltage characteristics, the current transients following stepwise changes in the applied bias, and the capacitance versus voltage characteristics as well as the temperature dependence of the current and capacitance were experimentally obtained and theoretically modeled. The proposed model assumes two distributions of interface states, one of acceptors and one of donors, causing a potential barrier at the bonded interface. It is argued that the origins of the interface states are impurities and crystallographic defects in the interfacial region. The capacitance of the bonded structures includes contributions from the depletion regions as well as from minority carriers. When bonded n-n type samples were illuminated with light of photon energies larger than the silicon band gap the current across the junction increased. This is caused by the photogenerated increase in the minority carrier concentration in the interfacial region, which results in a lowering of the potential barrier. Illumination of n-n type structures with light of photon energies lower than the band gap caused a considerable photocurrent at low temperatures. In this case the observed behavior cannot be explained by interaction with the interface states. Instead, the mechanism is the change in the occupancy of deep electron traps caused by the illumination. These traps are located in the silicon in a small volume around the bonded interface with energies close to the center of the band gap and with a peak concentration of about 1013 cm−3. Impurities present on the silicon surfaces before bonding and impurities gettered to the bonded interface are possible reasons for the increased concentration of deep electron traps in the vicinity of the bonded interface.

We report on two hydrogen-gold-related deep levels (G2 and 63) in gold-doped p-type silicon. The levels are formed after hydrogenation by wet chemical etching. Using deep-level-transient-spectroscopy depth profiling and capacitance-voltage analysis we demonstrate that the levels are caused by injection of hydrogen into gold-doped silicon. The etching treatment results in a decrease of the gold-donor concentration, which can be fully explained by a corresponding increase of the G2 trap concentration. The results indicate that the 62 level belongs to an electrically active gold-hydrogen (Au-H) complex that changes into an electrically inactive Au-H complex upon heat treatment between 150 and 200'C. Prolonged heat treatments within this temperature range result in dissociation of these electrically inactive Au-H complexes and the gold-donor concentration approaches its initial value.Hydrogen is able to neutralize both shallow and deep centers in silicon. ' It is normally introduced into silicon using remote hydrogen plasma, but it is even possible to introduce hydrogen to a smaller extent by wet chemical etching. ' Hydrogen passivation of the deep levels of