Simple Cleaning and Conditioning of Silicon Surfaces with UV/Ozone Sources

Within this paper the applicability of three methods to clean and condition the surface of silicon wafers by an ultra-thin oxide layer were tested: two UV/O3 sources (an Hg vapor lamp and a high efficiency excimer module) as well as a wet chemical oxidation in HNO3. It has been demonstrated that organic residues resulting from wafering and masking the surface during alkaline texturing can be removed by UV/O3 (Hg vapor lamp) exposure. Moreover the use of UV/O3 (excimer) and HNO3 oxides result in an improvement of passivation quality and emitter saturation current in combination with Al2O3/SiNx or AlN/SiNx passivation stacks. For n-type silicon wafers with a 70 Ω/sq. boron emitter and Al2O3/SiNx passivation very low j0e values of 49 fA/cm2 are achieved by conditioning with UV/O3 or HNO3. Another important application of ultra-thin SiOx layers is as tunnel oxides for the rear side of the TOPCon (Tunnel Oxide Passivated Contact) cell concept [1,2]. High iVOC values exceeding 720 mV on planar samples and 710 mV on textured samples are obtained with UV/O3 grown tunnel oxides after annealing. The structural properties of the SiOx layers that were analyzed by XPS and compared to the iVOC data, indicate that a minimum oxide layer thickness of approx. 1.4 nm and a high amount of oxygen rich sub oxides species (Si2O3, close to the structure of stoichiometric SiO2) are required to obtain a good interface passivation stable up to annealing temperatures of 900 °C.