ELLIPSOMETRIC STUDY OF A-SI-H THIN-FILMS DEPOSITED BY SQUARE-WAVE MODULATED RF GLOW-DISCHARGE

Thin films of hydrogenated amorphous silicon (a-Si:H), deposited by square wave modulated (SQWM) rf silane discharges, have been studied through spectroscopic and real time phase modulated ellipsometry. The SQMW films obtained at low mean rf power density (19 mW/cm2) have shown smaller surface roughness than those obtained in standard continuous wave (cw) rf discharges. At higher rf powers (greater-than-or-equal-to 56 mW/cm2), different behaviors depending on the modulating frequency have been observed. On the one hand, at low modulating frequencies (< 40 Hz), the SQWM films have shown a significant increase of porosity and surface roughness as compared to cw samples. On the other, at higher modulating frequencies, the material density and roughness have been found to be similar in SQWM and cw films. Furthermore, the deposition rate of the films show more pronounced increases with the modulating frequency as the rf power is increased. Experimental results are discussed in terms of plasma negative charged species which can be relatively abundant in high rf power discharges and cause significant effects on the deposited layers through polymers, clusters, and powder formation.