Figure 6.6 Etch profiles with varying concentrations of acid and peroxide in the H2SO4:H2O2system. The profiles are similar with HCl but at higher HCl concentrations. Figure 6.7 shows the isoetch diagram. In region B, which has a high concentration of H3PO4, the etch rate is not linear with time but varies as its square root. In regions A, D, and C the etch rates for all principal planes are equal except for the (111) Ga plane, which etches at about half the rate. Anisotropic etching occurs in region C, which has the fastest etch rate. In large-volume production, the concentration of peroxide in the bath will change over time, so the etch bath life must be determined and the bath changed after that time. Agitation is needed to avoid the effect of bubble formation on small features. Bath temperature control of +/– 1°C may be required. 18.104.22.168 Citric acid system (C3H4(OH)(COOH)3 H2O:H2O2:H2O)This system has the advantage of good etch depth control for shallow etches, because the etch rate can be varied over a wide range by changing the composition. A mixture of 50% by weight of citric acid with H2O2 has been shown to give etch rates from 1 to 100 Å/sec . At low etch rates, the rate is sensitive to agitation and spray etching may be preferred in practice. 22.214.171.124 Ammonia peroxide system (NH4OH:H2O2:H2O) This system with a very low concentration of NH4OH etches all faces at about the same rate (0.3 mm/min) and is closest to an isotropic etch. This bath also ages less in comparison to bromine-
methanol polishing etches. Other formulations of this system can be anisotropic.