Electrical isolation in non-volatile memory is provided by air gaps formed using sacrificial films of differing etch rates. A high etch rate material is formed in an isolation trench. Flowable chemical vapor deposition processes are used to form high etch rate films, and curing is performed to increase their etch rate. A low etch material is formed over the high etch rate material and provides a controlled etch back between charge storage regions in a row direction. A discrete low etch rate layer can be formed or the high etch rate material can be oxidized to form an upper region with a lower etch rate. A controlled etch back enables formation of a wrap-around dielectric and control gate structure in the row direction with minimized variability in the dimensions of the structures. At least a portion of the high etch rate material is removed to form air gaps for isolation.