to etch through the thickness of the phosphorus doped oxide layer 240 may be calculated let this calculated time be designated with the letter t after the fluoride wet etch has etched through the phosphorus doped oxide layer 240 then the fluoride wet etch will start to etch the boron doped oxide layer 230 because the etch rate for the fluoride wet etch on the boron doped oxide layer 230 is comparatively very slow the etch rate almost seems to come to a u201c stop u201d compared to the previous etch rate through the phosphorus doped oxide layer 240 although the fluoride wet etch continues to etch the boron doped oxide layer 230 the rate at which the etch process proceeds is minimal this means that very little of the boron doped oxide layer 230 will be etched if the etch process is terminated very soon after the time t has elapsed the result of stopping the wet etch process after the time t has elapsed will be that the thickness of boron doped oxide layer 230 will be uniform and have a value at the desired thickness here five thousand u00e5ngstroms the result is illustrated in fig3 the portion of the phosphorus doped layer 240 that was not under resist mask 250 is etched away after time t the boron doped oxide layer 230 substantially u201c stops u201d the wet etch process after the wet etch process reaches boron doped oxide layer 230 as previously described the boron doped oxide layer 230 actually significantly slows the etch process because of the very slow etch rate of the boron doped oxide layer 230 the wet etch process is then terminated after time t and the resist mask 250 is removed fig4 illustrates a flow chart 400.