Quantitative assessment of surface degradation in cycloolefin polymer films subjected to vacuum ultraviolet irradiation and oxygen plasma treatment using a micro slurry-jet erosion test

This study quantitatively evaluated the surface degradation of cycloolefin polymer (COP) films after vacuum ultraviolet (VUV) irradiation and oxygen plasma treatment. Surface degradation was quantified using the strength ratio, defined as the erosion resistance normalized to its bulk value, as measured by the micro slurry-jet erosion (MSE) test that enables depth-resolved evaluation of near-surface mechanical properties. VUV irradiation induced considerable surface degradation, reducing the strength ratios at the surface to approximately 28% and 32% at VUV irradiation doses of 600 and 1,200 mJ/cm2, respectively. However, strength ratios gradually recovered to the bulk value (100%) over a depth of approximately 1,200 nm, irrespective of the VUV irradiation dose. Conversely, oxygen plasma treatment caused moderate surface degradation, with the strength ratio at the surface decreasing to approximately 84% and 69% for oxygen plasma treatment times of 60 and 120 s, respectively. However, the strength ratio rapidly recovered to the bulk value within a depth of approximately 30 nm, regardless of the oxygen plasma treatment time. Previous peel tests between the COP film and the copper plating layer with a sputtered copper seed layer revealed substrate failure within 10 nm of the surface after VUV irradiation at 600 and 1,200 mJ/cm2, but not after oxygen plasma treatment for 60 and 120 s, indicating that a strength ratio of at least 69% is required to prevent substrate failure. Although further investigations are required to refine the critical threshold, the present framework provides a quantitative approach for evaluating the surface degradation characteristics.