Investigation of Metallic Dust formed on Steel Substrates in Solar Cell Sputtering Chambers

Abstract: Investigations have been done as of why dust particles appear in a circular pattern on the backside of solar cells produced in sputtering chambers at Midsummer AB. An experimental approach was conducted, where solar cells were produced at standard conditions and their backside studied by material analytical methods. The solar cells dust particles were analyzed by energy-dispersive x-ray spectroscopy and x-ray diffraction, deducing that they consisted of iron selenide (Fe0.89Se). Furthermore, the dust particles appear due to formation of a thin iron selenide film that cracks and delaminate upon cooling from process temperature to room temperature. Iron selenide film thickness was found by energy-dispersive x-ray spectroscopy to occur in a pattern with radial symmetry with respect to the cell center, correlating with the film delamination pattern. The reason to the film formation was due to selenium reacting with the substrate steel at high temperatures (>400 ◦C) in deposition chambers having a selenium environment. The film delamination occurs at a critical film thickness at which stresses in the film is high enough for the film to yield and fracture. It was concluded that iron selenide film formation or delamination must be minimized in order to control dust particle formation. These two phenomena can be mitigated by protective substrate films, change of substrate material, selenium environment optimization or temperature profile optimization and should be researched further to find the most effective and viable solution.