Effect of BCl3 concentration and process pressure on the GaN mesa sidewalls in BCl3/Cl2 based inductively coupled plasma etching

Abstract

GaN mesa etching is investigated using BCl3/Cl2 based inductively coupled plasma at constant ICP/RF powers for HEMT fabrication. The effect of chamber process pressure (5–15 mTorr) and BCl3/Cl2 flow rate ratio >1 on mesa sidewall profile is studied in detail using less complex photoresist mask. Mesa sidewall sharpness varied strongly with chamber pressure and deteriorated at lower pressure ∼5 mTorr. The etched GaN mesas resulted in severely damaged sidewalls with significant sidewall erosion at BCl3/Cl2 ratio of <1, which reduced gradually as BCl3/Cl2 ratio was increased to values >1 mainly due to decreased Cl ion/neutral scattering at the edges. Finally, the smooth and sharp mesa sidewalls with angle of ∼80° and moderate GaN etch rate of ∼1254 Å/min are obtained at BCl3/Cl2 ratio of 2.5:1 and 10 mTorr pressure due to a better balance between physical and chemical components of ICP etching.