Abstract: A method for forming a dielectric layer with uniform thickness in a trench capacitor comprises providing a substrate structure. A trench device formed in the substrate structure is used as a capacitor and has sidewall and a bottom. Next, the sidewall of the trench device are treated by ion bombardment for forming amorphous structure thereon. Then a dielectric layer, such as an oxide layer, is formed on the sidewall and the bottom of the trench device by CVD or thermal oxidation. To be specific, because of amorphous structure of the sidewall and bottom of the trench device, the dielectric layer can have uniform thickness profile in the trench device.