US Patent 9768024 Multi-layer mask and method of forming same

A method includes forming a first insulating layer over a substrate, the first insulating layer having a non-planar top surface, the first insulating layer having a first etch rate. A second insulating layer is formed over the first insulating layer, the second insulating layer having a non-planar top surface, the second insulating layer having a second etch rate, the second etch rate being greater than the first etch rate. The second insulating layer is polished, the polishing partially removing the second insulating layer. The first insulating layer and the second insulating layer are non-selectively recessed.