A comparative study of wet and dry selective etching processes for GaAs/AIGaAs/lnGaAs pseudomorphic MODFETs

The etching characteristics of Al_xGa_1-xAs in citric acid/H₂O₂ solutions and SiCl₄/SiF₄ plasmas have been studied. Using a 4:1 solution of citric acid/H₂O₂ at 20° C, selectivities of 155, 260, and 1450 have been obtained for GaAs on Al_xGa_1-xAs with x = 0.3, x = 0.45, and x = 1.0, respectively. Etch rates of GaAs in this solution were found to be independent of line widths and crystal orientations for etched depths up to 1000Å. GaAs etch profiles along [110] and [110] directions displayed different slope angles as expected. Selective reactive ion etching (SRIE) using SiCl₄/SiF₄ gas mixtures at 90 mTorr and -60 V self-biased voltage yielded selectivities between 200 and 500 for x values ranging from 0.17 to 1.0. SRIE etch rates for GaAs were relatively constant for etch depths of less than 1000Å. At greater etch depths, etch rates varied by up to 76% for line widths between 0.3 and 1.0μm. Both selective wet etch and dry etch processes were applied to the fabrication of pseudomorphic GaAs/AIGaAs/lnGaAs MODFETs with gate lengths ranging from 0.3 to 2.5 μm on heterostructures with an embedded thin AlAs etch stop layer. A threshold voltage standard deviation of 13.5 mV for 0.3 μ gate-length MODFETs was achieved using a 4:1 citric acid/H₂O₂ solution for gate recessing. This result compares favorably with the 40 mV obtained using SRIE, and is much superior to the 230 mV achieved using the nonselective etch of 3:1:50 H₃PO₄: H₂O₂: H₂O. This shows that selective wet etching using citric acid/H₂O₂ solutions in conjunction with a thin Al_xGa_1-xAs (x ≥ 0.45) etch stop layer provides a reasonably simple, safe, and reliable process for gate recessing in the fabrication of pseudomorphic MODFETs.