Formation of Black Silicon in a Process of Plasma Etching with Passivation in a SF

Langmuir probe diagnostics is a useful method for determination of plasma charged-particles related properties, which is important for kinetics understanding and radical concentration calculation. Measurements of I–V curves were made with ESPion Advanced probe (Hiden Analytical, Warrington, UK) in a range of 100–+20 V with 0. 1 V step. The cylindrical tungsten probe was 10 mm in length and 0. 3 mm in diameter. The measuring probe is mounted on a long (~200 mm) dielectric holder, which makes it possible to carry out measurements in the central region of the discharge. In order to prevent any kind of contamination on the probe surface, between the measurements, the probe was maintained at a potential of 15 V and was cleaned with an ion bombardment, which allows obtaining undisturbed probe characteristics [
56
,
57
]. Probe size measurements before and after plasma diagnostics prove invariability of feature size (no probe material etching takes place during measurements). The electron temperature, plasma potential and electron and positive ion concentration were calculated from the I–V characteristics. The ion current was determined using orbital-motion-limited (OML) theory [
58
]. The ion current branch of I–V characteristic was approximated by the expression I~V
α
. The deviation of α from 1/2, predicted by OML theory, was less than 20%, which indicates the validity of applying OML theory. Subtracting the ion current from I–V characteristics gives the electron current. The second derivative of electron current over potential is proportional to the electron energy distribution function and linear dependence of the natural logarithms of electron current with the probe potential corresponds to Maxwellian electron energy distribution. The electron temperature was calculated by the slope of the natural logarithm of electron current versus probe potential.