Investigation of Silicon Surface Passivation by Microwave Annealing Using Multiple-Wavelength Light-Induced Carrier Lifetime Measurement

Abstract

A simple annealing method using a commercial 2.45 GHz microwave oven is reported to increase the minority carrier lifetime τeff for 4-in.-size 500-µm-thick 20 Ω cm n-type silicon substrates coated with 100-nm-thermally grown SiO2 layers. The microwave annealing was conducted with 2-mm-thick glass substrates, which sandwiched a silicon sample to maintain the thermal energy in silicon and realize gradual cooling. A 9.35 GHz microwave transmittance measurement system was used to measure τeff in the cases of continuous-wave 635 and 980 nm laser diode (LD) light illuminations. Radio-frequency Ar plasma irradiation at 50 W for 60 s to the top surface of a silicon sample markedly decreased τeff in the range from 6.0×10-6 to 2.4×10-5 s and from 4.2×10-5 to 6.4×10-5 s in the cases of 635 and 980 nm light illuminations, respectively, while τeff had the same distribution from 1.6×10-3 to 3.1×10-3 s for the initial samples. The finite element numerical analysis revealed that Ar plasma irradiation caused high densities of recombination defect states at the silicon top surface in the range from 1.3×1013 to 5.0×1013 cm-2. Microwave annealing at 700 W for 120 s markedly increased τeff in the range from 8.0×10-4 to 2.5×10-3 s, which were close to those of the initial samples. The density of recombination defect states was well decreased by microwave annealing to low values in the range from 7.0×1010 to 3.4×1011 cm-2. The high τeff achieved by microwave annealing was maintained for a long time above 5000 h.