Study of Cl2/BCl3 inductively coupled plasma for selective etching of GaAs

Inductively coupled plasma (ICP) etching has been replacing conventional reactive ion etching (RIE) for GaAs backside via etching to provide low inductance grounding in microwave devices like HEMT and MESFET. ICP tools provide higher throughput with faster etch rates in addition to better dimensional control, repeatability and reproducibility. Generally reported etch depths using ICP for via-hole etching applications in GaAs Monolithic Microwave Integrated Circuits are < 200mum using photoresist mask due to lower etch rate and poor mask selectivity. In this study, we are reporting the ICP etching of 60mum diameter via-holes for etch depth ~ 200mum, on 3-inch GaAs wafer with photoresist mask using C12/BC13 gases. ICP process parameters like pressure and platen power were varied at a fixed ICP coil power to achieve an etch depth ~ 200mum at a relatively very high etch rate with good etch surface morphology. The etch rate, etch depth, etch profile and surface morphology of etched holes were determined using scanning electron microscope. The average etch rate decreased rapidly with time, from 7mum/min for 10 minute of etching to 3.9mum/min for 45 minutes of etching on 3-inch GaAs wafer, mainly due to increased depth. Whereas etch rate increased with the increase in process pressure due to increased density of reactive species but anisotropy is maintained, as independent ion energy control is provided by RF biasing of the wafer platen in ICP systems. High etch rates ~ 4.4mum/min are obtained at 40mTorr pressure for an etch time of 45 minutes. Increasing platen power from 65W to 90W at pressure 30mTorr has resulted in reduction of etch rate from 3.9 mum/min to 3.6 mumm/min, probably due to increased physical etching component at 90W. The best mask selectivity obtained was >12:1.