Structure and method for replacement gate mosfet with self-aligned contact using sacrificial mandrel dielectric

A method of forming a semiconductor device comprising:

• forming a protective dielectric layer on sidewalls of a replacement gate structure that is present over a channel region of a substrate;
• forming a source region and a drain region on opposing sides of the replacement gate structure;
• forming a mandrel dielectric layer overlying the source region and the drain region of the substrate, wherein the mandrel dielectric layer has an upper surface that is coplanar with an upper surface of the replacement gate structure;
• removing the replacement gate structure to provide an opening exposing the channel region of the substrate;
• forming a functional gate structure over the channel region including a work function metal layer;
• forming a protective cap structure over the functional gate structure;
• etching at least one via through the mandrel dielectric layer selective to the protective cap structure and the protective dielectric layer to expose a portion of at least one of the source region and the drain region;
• and
• forming a conductive fill in at least one via to provide a contact to the at least one of the source region and the drain region.