Investigation of organic precursors and plasma mixtures allowing control of carbon passivation when etching HgCdTe in hydrocarbon-based inductively coupled plasmas

Our interest in introducing oxygen and/or nitrogen atoms in CH₄/H₂/Ar plasma mixtures by means of organic precursors in the place of CH₄ or by partly substituting O₂ or N₂O for H₂ and thereby controlling carbon deposition and surface passivation when etching HgCdTe under low substrate bias conditions is investigated in this study. Using in situ ellipsometry, in situ x-ray photoelectron spectroscopy, atomic force microscopy, and secondary electron microscopy (SEM), we show that nitromethane as a precursor and N₂O substitution are attractive alternatives for the following: the absence of deposition although no bias is applied, equivalent postetch surface composition, and comparable performances in terms of etch rate and surface roughness for the latter. Such an approach could provide the soft dry etching conditions needed to reduce the pixel pitch of HgCdTe infrared photovoltaic detectors. The mechanisms that allow suppressing carbon deposition upon O₂ or N₂O substitution in CH₄/H₂/Ar plasmas are studied by combining plasma diagnostics (optical emission spectroscopy, actinometry, and ion flux measurements) and surface characterizations. First, using Si as a reference substrate, it is shown that deposition is suppressed whenever the O₂ and N₂O flow rate is half and twice that of CH₄, respectively. Moreover, the study reveals that oxygen is the true etch reactant controlling the competition between film growth and etching in both substituted processes, whereas incorporation of nitrogen in the film upon N₂O substitution favors its etching kinetics simultaneously. Second, on HgCdTe, deposition of a hydrocarbonitride film is observed at a low N₂O/CH₄ ratio (∼0.6). SEM and SEM-energy dispersive x-ray spectroscopy imaging suggest that the film nucleates and grows preferentially on regions where the Cd removal mechanisms happen to be less efficient. At a high N₂O/CH₄ ratio (∼4), oxidation of the HgCdTe surface is observed.