reference will now be made in detail to exemplary embodiments of the present invention which are illustrated in the accompanying drawings wherever possible the same reference numbers will be used throughout the drawings to refer to the same or like parts in the following embodiments of the present invention three exemplary methods are applied for reducing an aspect ratio a first method is for reducing the aspect ratio of a trench through an oxidation process according to impurity implantation the first method uses a difference between a first oxidation rate of pure silicon when an impurity diffusion area formed at a bottom of the trench forms a liner oxide film of a trench and a second oxidation rate of silicon to which impurities are implanted the oxidation rate of the silicon to which the impurities are implanted is several times to dozens of times higher depending on a dose amount accordingly the oxidation rate at the bottom of the trench is higher than that on a sidewall thereof therefore according to a feature that oxidation rates at top and bottom of silicon surfaces are a ratio of 0 46 0 54 when a trench with a same depth is formed there is an advantage that the aspect ratio of the trench at the top of the trench is reduced and thus.