On the Cl2 plasma etching mechanisms of AlN micropillars: Carrier wafer impact

Plasma etching of aluminum nitride (AlN) is a critical step in the fabrication of advanced optoelectronic and electronic devices, particularly deep-ultraviolet light-emitting diodes based on nanowire architectures. In research and development, small AlN samples are commonly mounted on carrier wafers (CWs) during plasma processing. Understanding how the CW influences the etching mechanisms is essential for developing a process transferable to full-size wafers. In this work, the impact of Si and Si3N4 CWs on AlN etching in Cl2 inductively coupled plasma was investigated using blanket-film and micropillar-patterning experiments. Optical emission spectroscopy revealed significant differences in plasma chemistry: Si CW generates plasmas enriched in SiClX species, whereas Si3N4 CW produces predominantly chlorine-based plasmas. Although the AlN etch rate was slightly affected by the CW, quasi-in situ x-ray photoelectron spectroscopy analysis suggested that SiClX-rich plasmas prevent the N depletion compared with ClX-rich plasmas. Pattern etching experiments showed distinct profile evolutions: vertical sidewalls were obtained with Si CW, while Si3N4 CW produced density-dependent tapered profiles. Scanning transmission electron microscope observations revealed a 20–50 nm passivation layer on pillar sidewalls only in the Si3N4 CW case. The potential deposited species on the AlN micropillar sidewalls were investigated by combining ellipsometry and XPS analysis. We demonstrated that Si3N4 CW etch by-products exhibit substantially higher sticking probabilities than those generated from Si, promoting passivation layer formation. Conversely, SiClX-rich plasmas enhance the removal of AlSiOCl-based deposits, preventing passivation accumulation. A comprehensive etching mechanism is proposed, highlighting the critical role of the CW material in plasma chemistry, by-product redeposition, passivation dynamics, and AlN profile evolution.