A method of manufacturing a semiconductor device is provided. A pattern layer is formed on a substrate defined to include a main pattern region and a dummy pattern region. A preliminary main pattern and a preliminary dummy pattern may be formed by patterning the pattern layer so that an upper surface area of the preliminary dummy pattern facing away from a surface of the substrate is less than an entire area of the dummy pattern region that is be subjected to subsequent planarization. The preliminary main pattern and the preliminary dummy pattern are partially etched to form a main pattern and a dummy pattern.