Methods of filling trenches/gaps defined by circuit elements on an integrated circuit substrate are provided. The methods include forming a first high-density plasma layer on an integrated circuit substrate including at least one trench thereon using a first reaction gas. The first high-density plasma layer is etched using an etch gas including nitrogen fluoride gas (NF3). A second high-density plasma layer is formed on the etched first high-density plasma layer using a second reaction gas including nitrogen fluoride.