like elements throughout the specification fig1 a to 1 d are cross sectional views illustrating a crystallization process according to the present invention first fig1 a is a cross sectional view illustrating a process of sequentially forming a buffer layer an amorphous silicon layer and a filtering oxide layer on a substrate as shown in fig1 a a buffer layer 102 is formed on an insulating substrate 101 made of a plastic or a glass using a chemical vapor deposition u201c cvd u201d technique or a physical vapor deposition u201c pvd u201d technique the buffer layer 102 has a single layer or dual layer structure of a silicon oxide layer or and silicon nitride layer here the buffer layer 102 serves to prevent a diffusion of moisture or impurities which may occur on the substrate 101 or to control a heat transfer speed during a crystallization process thereby helping a crystallization of the semiconductor layer subsequently an amorphous silicon layer 103 is formed on the buffer layer 102 typically the amorphous silicon layer 103 is formed using a cvd technique the amorphous silicon layer formed using a cvd technique has a gas such hydrogen which decreases an electron mobility and thus a dehydrogenation process is performed so that hydrogen does not remain in the amorphous silicon layer then a filtering oxide layer 104 is formed on the amorphous silicon layer 103 the filtering oxide layer 104 is formed of a silicon oxide layer in which a metal catalyst is not easy to.