Effect of oxygen addition to an argon plasma on etching selectivity of poly(methyl methacrylate) to polystyrene

The effect of oxygen addition to an argon plasma on the etching selectivity of poly(methyl methacrylate) (PMMA) to polystyrene (PS) (hereafter “PMMA/PS etching selectivity”) was investigated. The PMMA/PS etching selectivity was evaluated by using inductively coupled plasmas composed of argon and oxygen. The etching selectivity in the case of argon plasma was estimated to be 3.9, which is higher than that of oxygen plasma, which is 1.7. The time dependence of etching depth shows that the etching rate of PMMA is reduced to less than one half of its initial value after the etching depth exceeds 15 nm. X-ray photoelectron spectroscopy of the PMMA surface revealed that the reduction of etching rate is caused by a depletion of oxygen concentration by argon-ion bombardment. To compensate the oxygen-concentration depletion, 1% oxygen was added to the argon plasma. As a result, the reduction of PMMA etching rate was suppressed, and constant etching rate was obtained even when etching depth exceeded 50 nm. The mixed argon-oxygen plasma was used to fabricate a PS mask pattern with a full pitch in the range of 25.5 to 77 nm.