Etching interleaved structures of semiconductor material forming fins of finFETs and local isolation material interposed between the fins is performed alternately and cyclically by alternating etchants cyclically such as by alternating gases during reactive ion etching. Etchants are preferably alternated when one of the semiconductor material and the local isolation material protrudes above the other by a predetermined distance. Since protruding surfaces are etched more rapidly than recessed surfaces, the overall etching process is accelerated and completed in less time such that erosion of other materials to which the etchants are less than optimally selective is reduced and allow improved etching of trenches for improved isolation structures to be formed.