1 u00e5 through 4 u00e5 per minute referring to fig6 through 8 the buffer layer 130 is removed by wet etching to expose the semiconductor substrate 1 00 of the source region 105 a and the drain region 105 b step s 3 here an etching solution is used to etch the silicon oxide film buffer layer and is an ammonium hydroxide nh 4 oh solution at a temperature of 30 u00b0 c to 80 u00b0 c thus an etching rate of the silicon oxide film increases and the time required by the entire process can be greatly reduced while an etching process is in progress the first interlayer insulating film 150 which is a silicon oxide film formed by hdp cvd is also etched however an etching rate of the first interlayer insulating film 150 is about 2 u00e5 per minute since the etching rate of the first interlayer insulating film 150 is slower than that of the buffer layer 130 as the etching rates of a mid temperature oxide mto film the buffer layers 130 and the first interlayer insulating film 150 are about 5 u00e5 and hundreds of u00e5 respectively while thousands of u00e5 of the first interlayer insulating film 150 is etched per minute therefore this wet etching hardly causes damage the contact hole morphology considering.