The present disclosure relates to a semiconductor device and a manufacturing method, and more particularly to a semiconductor device having an enhanced gap fill layer in trenches. The present disclosure provides a novel gap fill layer formed using a multi-step deposition and in-situ treatment process. The deposition process can be a flowable chemical vapor deposition (FCVD) utilizing one or more assist gases and molecules of low reactive sticking coefficient (RSC). The treatment process can be an in-situ process after the deposition process and includes exposing the deposited gap fill layer to plasma activated assist gas. The assist gas can be formed of ammonia. The low RSC molecule can be formed of trisilylamin (TSA) or perhydropolysilazane (PHPS).