Understanding the roughening and degradation of 193 nm photoresist during plasma processing: synergistic roles of vacuum ultraviolet radiation and ion bombardment

We have identified a synergistic roughening mechanism of 193 nm photoresist, where simultaneous ion bombardment, vacuum ultraviolet (VUV) radiation, and moderate substrate heating in a well-characterized beam system results in a similar level of surface roughness observed during conditions typical of plasma etching. VUV radiation (147 nm) results in bulk modification of the photoresist polymer, witnessed by the loss of carbon-oxygen bonds through transmission FTIR. Ion bombardment (150 eV) results in the formation of a densified surface layer on the order of a few nanometers in depth. We have shown that elevated levels of roughness are observed only during simultaneous exposure and that sequential exposure is not sufficient to produce surface roughness. In addition, through the use of transmission FTIR we have shown that an etching synergy does not exist and that etch rates are nearly independent of temperature. We propose that the observed roughness could be due to the drastically different mechanical properties of the ion-modified near-surface region and VUV-modified bulk photoresist, where the difference is exaggerated at elevated temperatures. A more complete understanding of plasma-induced surface roughness will require further study, resulting in the improvement of existing pattern transfer technologies and possibly novel new technologies as well.