VERY-LOW DAMAGE ETCHING OF GAAS

A very low damage, anisotropic and selective reactive ion etching process has been developed using SiCl4 for etching GaAs which stops on extremely thin GaAlAs layer (4 monolayers 1.13 nm thick). Using low rf powers of less-than-or-equal-to 15 W, and hence low dc biases of 40 to less-than-or-equal-to 70 V, pressures of 8 mTorr and flow rates of 4-6 sccm, the damage was kept to a minimum value while maintaining very good anisotropy. Both the surface and sidewall damage were measured and the results were confirmed by evaluation of the performance of a metal-semiconductor field effect transistor (MESFET) with a recessed gate. Raman scattering studies of the etched surface of a heavily doped GaAs layer show that the surface damage thickness is only 3-4 nm after 2 min of etching. The damage depth increases and saturates at 9 nm after 4 min of etching (etch rate of approximately 100 nm/min). Conductance measurements [S. Thoms, S. P. Beaumont, C. D. W. Wilkinson, J. Frost, and C. R. Stanley, in Microcircuit Engineering, edited by H. W. Lehman and Ch. Bleicher (North Holland, Amsterdam, 1986), p. 249] of narrow wires formed in n+-GaAs by etching at dc biases of 40-70 V show that the sidewall damage is negligible (1 nm/sidewall) after the first 2 min of etching. This value increases to 5 nm/sidewall after 3 min etching and to 12 nm/sidewall after 5 min. This is far lower than the value 18 nm/sidewall obtained by etching for 2 min at dc biases of 200 V. Control of the transconductance of the GaAs MESFETs by wet recess etching is difficult. Dry etching to a thin stop layer is a better method, provided it is damage-free. A ratio of etching rates of GaAs: Ga0.7Al0.3As of > 10 000:1 on a 4 monolayer thick Ga0.7Al0.3As was obtained. On existing FET device wafers with a 5 nm GaAlAs stop layer, after 2 min etching which is sufficient to come to the stop layer, the transconductance was 4.02 mS, after 5 min etching was 3.67 mS and after 12 min was 2.05 mS. Optical emission spectroscopy revealed that the dominant emitting species in the plasma are Si, SiCl, SiCl2, and Cl. The variation of emission intensity of these species with power reveals clearly the presence of two distinct etch mechanisms, one below and one above 15 W (0.066 W/cm2) power density. The etch rate does not increase monotonically with power.