A method includes providing a semiconductor structure including a plurality of devices; depositing a nitride cap over the semiconductor structure; forming an aluminum mask over the nitride cap, the aluminum mask including a plurality of first openings; converting the aluminum mask to an aluminum oxide etch stop layer; and performing middle-of-line fabrication processing, leaving the aluminum oxide etch stop layer in place. A semiconductor structure includes a plurality of devices on a substrate; a nitride cap over the plurality of devices; an aluminum oxide etch stop layer over the nitride cap; an inter-level dielectric (ILD) over the aluminum oxide etch stop layer; and a plurality of contacts extending through the ILD, the aluminum oxide etch stop layer and the nitride cap to the plurality of devices.