Smooth etching of epitaxially grown AlN film by Cl₂/BCl₃/Ar-based inductively coupled plasma

Inductively coupled plasma dry etching of epitaxially grown aluminum nitride (AlN) film using Cl₂/BCl₃/Ar mixture has been investigated. The etch rate of AlN increases significantly with the addition of BCl₃, and an etch rate as high as 258 nm/min has been demonstrated. High selectivity of AlN over Ni mask has been realized, and the etch rate of Ni is almost zero for lower bias voltage. By optimizing the etching parameters, grain formation due to crystallographic orientation dependent etching has been suppressed, and smooth etched surfaces and nearly vertical sidewalls are obtained. The root-mean-square roughness of the etched surface is measured to be 0.77 nm, which is almost the same as that of the as-grown surface. These results are suitable for the fabrication of low-loss AlN waveguides and optoelectronic devices.