Etching of 200 ϋm deep GaAs via holes with near vertical wall profile using photoresist mask with inductively coupled plasma

Metal semiconductor field effect transistors (MESFETs) and pseudomorphic high electron mobility transistors (pHEMTs) are used in high frequency microwave applications in monolithic microwave integrated circuits (MMICs). Ground via holes with low inductance to the source electrode of these devices is necessary for high frequency applications and also increasing the packing density of MMICs. Dry etching of via holes to the depth of 200 mum in GaAs using photoresist mask is particularly challenging due to its poor selectivity with masking material and the constraints of top and bottom dimensions with smooth sidewall profiles. The desired sidewall profile promotes easy coverage of the plated metallisation subsequently. Via holes of 200 mum depth with the desired profile and good uniformity over 3" GaAs wafers have been realized with inductively coupled plasma (ICP) etching using chlorine chemistry. The near vertical sidewall profile has been achieved by tailoring the photoresist mask profile, photoresist hardening process and the GaAs etching parameters. Optical and scanning electron microscopy (SEM) were used to characterize the via profiles.