Ion-energy Distributions At A Substrate In Reactive Magnetron Sputtering Discharges In Ar/h_2s From Copper, Indium, And Tungsten Targets

摘要

Ion-energy distributions from copper, indium, and tungsten targets were measured during reactive sputtering in argon-hydrogen sulfide (H_2S) mixtures, since reactive magnetron sputtering of sulfides from metallic targets is of increasing interest, especially for photovoltaic applications (buffer and absorber layers, i.e., CuInS_2, In_2S_3, or WS_2). The mass spectra of the ions show a wide range of molecules H_xS_n derived from H_2S by plasma-assisted attachment both for positive (n≤9) and for negative (n ≤ 6) ions. From the copper and the indium targets metallic ions (Cu~+, In~+) could be detected. While tungsten and indium sulfur compounds were found, copper does not form compounds with sulfur, caused by its lower chemical reactivity. Positive (Ar~+, S~+, W~+, Cu~+, In~+, etc.) as well as negative ions (S~-, InS~-, WS_3~-) were measured for dc and rf (27 MHz) plasma excitations. The positive ions originate mainly from the plasma in front of the substrate and exhibit energies of about 12 eV for the dc and 18 eV for the rf discharge for the substrate at floating potential. The energy difference is caused by the higher electron temperature in the rf compared to the dc discharge. The ion-energy distributions of negative ions exhibit two distinct peaks. The high-energetic peak can be attributed to ions accelerated in the cathode dark space to a high energy (up to more than 400 eV) corresponding to the cathode (target) voltage. The second peak has its maximum at zero energy decreasing steeply up to energies of about 100 eV. These ions are generated by charge-exchange collisions of energetic species from the target (reflected neutral argon, negative sulfur ions, etc.) on their passage from the target to the substrate caused by the high charge-exchange cross section, rf magnetron sputtering leads to significantly lower energies of negative ions from the target, caused by the lower discharge voltages, which could be advantageous for the deposition of active semiconducting sulfide films.