etch through the large mask openings and d s is the average overall depth of the etch through the small mask openings in general it is desirable for both u03b4 and s to be small the results are summarized in table 1 as can be seen from table 1 when only the oxygen based plasma was used to etch the arc sample 1 the depth of the resulting overall etch was relatively uniform as indicated by the value of 8 for s the diameter of the small openings however changed significantly thereby making it difficult to control the processing of small critical dimensions on the other hand when only the fluorinated hydrocarbon based plasma was used to etch the arc sample 2 the size of the small openings increased by only about 10 nanometers nm however the depth of the overall etch was much less uniform due in part to the fact that the fluorinated hydrocarbon based plasma tends to etch through regions of the arc defined by the large mask openings somewhat more quickly than through regions of the arc defined by the small mask openings the fluorinated hydrocarbon based etch subsequently begins to etch exposed regions of sio 2 dielectric layer at several times the rate at which it continues to etch through the remaining regions of the arc in contrast when the arc was etched with an initial fluorinated hydrocarbon based plasma followed by an oxygen based plasma sample 3 the depth of the overall etch was relatively uniform as indicated by the value of 10 for s in addition the diameter of the small mask openings changed.