NANOTEXTURED SILICON SURFACES USING TAILORED VOLTAGE WAVEFORM PLASMAS: IMPACT OF ION BOMBARDMENT ENERGY ON ETCHING DYNAMICS AND PASSIVATION

摘要

Front surface nanotexturing of silicon is investigated using a capacitively coupled reactive ion etching SF_6/O_2 discharge at room temperature. The plasma is either powered with a standard RF voltage at 13.56 MHz, or an innovative multi-frequency approach named Tailored Voltage Waveforms (TVWs). It has been shown that by using TVWs, it is possible to significantly modify the ion bombardment energy (IBE) at the substrate while keeping the discharge power constant. Measurements suggest that increasing IBE is beneficial in achieving low reflectance values in a short process time, although the role of other mechanisms at play has still to be clarified. Nanotextured samples have then been coated by thermal ALD with a 7 nm thick Al_2O_3 passivation layer. The simultaneous effects of surface area enhancement and plasma induced damage of the passivation quality are successfully dissociated, and it is shown that increasing the IBE (in the range from 80 to 200 eV) strongly increases the surface recombination velocity (SRV). However, one can achieve both low reflectance (less than 5%) and good passivation quality (front SRV around 40 cm/s), without the need for an additional chemical damage removal etching. Furthermore, the possibility of recovering the passivation through a "dry" damage removal etching is examined.