The etch rate and surface morphology of AlxGa1-xAs (x=0.3, 0.5, 0.65) versus composition of CrO3/HF solutions were studied using the dynamic etch mask technique. The selectivity, defined as a ratio of the etch rates, decreases from 179 to 8.6 for Al0.8Ga0.2As/Al0.3Ga0.7As with the volume ratio of HF (48 wt %)/CrO3(33 wt %) increasing from 0.01 to 0.138. The selective etching was experimentally applied to the fabrication of the vertical taper structures with angles ranging from 0.32degrees to 6.61degrees on an Al0.3Ga0.7As epitaxial layer. The surface roughness of AlxGa1-xAs (x =0.3, 0.5, 0.65), which was etched by CrO3/HF at a volume ratio of 0.028, were 1.8, 9.1, and 19.3 nm, respectively. The dependence of etch rate for AlxGa1-xAs (x=0.8, 0.85, 0.9) on composition of CrO3/HF has also been obtained. This technique has proven to be very useful in integrated optoelectronic device fabrication.