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Hydrogen-plasma etching of thin amorphous silicon layers for heterojunction interdigitated back-contact solar cells

  • Stefano. N. Granata (a1) (a2), Twan Bearda (a1), Ivan Gordon (a1), Jef Poortmans (a1) (a2) and Robert Mertens (a1) (a2)...

Abstract

In this study, A H2-plasma is studied as a dry method to etch thin layers of amorphous silicon aSi:H(i) deposited on a crystalline wafer. It is found that H2-plasma etches aSi:H(i) selectively toward silicon nitrides hard masks with an etch rate below 3nm/min. Depending on power density and temperature of the substrate during the H2-plasma, the energy bandgap, the hydrides distribution and the void concentration of the aSi:H(i) layers are modified and the amorphous-to-crystalline transition is approached. At high temperature (>250C) and low plasma power (<20mW/cm2), the dihydride (SiH2) content increases and the bandgap widens. The etch rates stays below 0.5 nm/min. At low temperature (<150°C) and high power (>70mW/cm2), the void concentration increases significantly and etch rates up to 3nm/min are recorded.

These findings are supported by a theoretical model that indicates formation of Si-H-Si precursors in the layer during exposure to H2-plasma. According to the experimental conditions, these precursors either diffuses and forms Si-Si strong bonds or are removed from the film, causing layer etching.

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