Inductively Coupled BCl3/Cl2/Ar Plasma Etching of High Al Content AlGaN

III-nitride materials are researched extensively for high power electronics as well as for UV and visible optoelectronics. One of the many attractive qualities of III-nitride materials is their chemical stability. However, this quality can make material processing and device fabrication challenging. Though significant research has been carried out on processing III-nitride materials, comparatively few studies on high aluminum content AlGaN have been reported.

Al and Ga chlorides are volatile, making the gas chemistries of BCl₃ and Cl₂ attractive for use in dry etching. However, obtaining a smooth surface of AlGaN with a dry etch can be difficult due to two compounding effects: AlCl_x has a lower volatility than GaCl_x and the bond energy of Ga-N is lower than that of Al-N. This exploratory work aims to document some of the etch characteristics during inductively coupled plasma reactive ion etching (ICP-RIE) of high aluminum content AlGaN with a BCl₃/Cl₂/Ar plasma. Etch rate and surface morphology were investigated for Al_0.85Ga_0.15N. Etch rate lends insight into chemical tendencies of AlGaN alloys, while surface morphology is of interest in structures requiring subsequent processing steps such as gate recessing.

Our results show that lowering rf bias and increasing ICP power leads to smoother surfaces while maintaining an attractive etch rate. Also, the ratio of BCl₃ to Cl₂ significantly affects the surface roughness. In addition, we show significant improvement of surface roughness, even under sub-optimal etch conditions, by implementing a 1min BCl₃/Ar plasma pre-treatment dry etch step prior to introducing Cl₂ to the main etch using a BCl₃/Cl₂/Ar plasma.

This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.