A method for forming a dielectric layer having a low dielectric constant and a method for forming copper wiring using the same are provided. In the method for forming a dielectric, an etch stop layer and a first dielectric are sequentially formed on a semiconductor substrate. Next, the first dielectric is selectively etched to form a pattern, and a second dielectric is formed thereon. Here, the second dielectric may be formed using a plasma enhanced chemical deposition method to have pores or voids therein. Then, the dielectric is planarized and a damascene copper wiring is formed. Since the dielectric includes pores or voids, it may have a very low dielectric constant, which results in an improvement in RC delay.