A method of forming a pre-metal dielectric (PMD) layer is disclosed. In the method, after a nitride liner layer is formed on a substrate having a transistor, a USG layer is deposited thereon and then planarized. Next, ion implantation and annealing are performed for gettering, first in a gate region and then in a non-gate region of the USG layer. The USG layer is generally free from plasma damage and has a good gap-fill capability. Further, ion implantation and annealing after deposition of the USG layer may enhance a gap-fill capability, a gettering capability, and electrical properties of a transistor.