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Reactive magnetron sputtering in argon-oxygen mixtures is widely used to deposit oxides for optical coatings (SiO2, TiO2), for protective coatings (SiO2, Al2O3), low emissivity coatings (ZnO, SnO2, Bi2O3) or transparent conductive oxides (ITO, ZnO SnO2). Since recently, also sulfides are of increasing interest, especially in thin film solar cells as buffer (CdS, In2S3) or for absorber layers (CuInS2, WS2). While for the first three applications of oxides the particle energies are of minor importance, the deposition of semiconductors requires a knowledge of the energy of the particles (ions, sputtered or reflected atoms) contributing or influencing the film growth. Defects, generated by energetic particle bombardment constitute either scattering centers in the case of TCOs or recombination centers in the case of absorber films, and have to avoided.
In this work ion-energy distributions were measured by a combination of an energy analyzer and a quadrupole mass spectrometer (PPM422, INFICON). Both, positive and negative ions were analyzed for different targets in Ar/O2 (Zn, In, Sn) and in Ar/H2S (Cu, In, W).
While positive ions exhibit an energy distribution centered at relatively low energies (10 to 30 eV) due to the acceleration across the substrate fall, the negative ions exhibit significantly different energy distributions. A low-energy component extending up to energies of about 100 eV is due to negative from the plasma. The high-energy component is caused by the acceleration of ions (O- or S-) from the target toward the substrate in the cathode fall. These ions aquire the full energy corresponding to the potential difference betwenn target and plasma (Vp-Vtarget). The excitation mode of the plasma (DC or RF) and the total sputtering pressure influence this high energy component through the DC target potential (decreasing with excitation frequency) and through thermalization of the energetic ions by gas collisions. |