Formation of nanometer-thick delaminated amorphous carbon layer by two-step plasma processing of methacrylate-based polymer

Abstract

The authors show that extended He plasma pretreatment (PPT) of methacrylate-based 193 nm photoresist (PR) material in conjunction with a subsequent biased Ar plasma treatment can lead to blister formation at the polymer surface due to delamination of an ultrathin, ion-induced, dense, amorphous carbon (DAC) layer formed by low energy ion bombardment. For our experimental conditions, the delaminated layer is 1-2 nm thick and primarily composed of sp²-hybrized amorphous carbon. A He or Ar plasma process alone will not lead to this phenomenon, and so far the authors have only observed it for a methacrylate polymer. A possible mechanism of the formation of the ultrathin layer that is consistent with all observations is as follows: During He plasma pretreatment, volatile species are produced by ultraviolet/vacuum ultraviolet radiation-induced photolysis of the polymer pendant groups, e.g., adamantyl and chain-scissioning of the polymer backbone to a depth of greater than 100 nm. While volatile products formed close to the polymer surface can diffuse out during He PPT, those formed deep within the polymer bulk cannot and their concentration will become significant for extended He PPT. During the biased Ar plasma treatment step, a DAC surface layer is generated by Ar⁺ ion bombardment within the first seconds of plasma exposure. The thickness is dependent on ion energy and in the range of one to several nanometers. This layer appears to be impermeable to gaseous products formed in the PR material. Thus, volatile species diffusing to the surface can accumulate underneath the DAC layer, causing a loss of adhesion and subsequent delamination of this layer from the PR bulk film. The authors also report surface and electrical characterizations of the ultrathin DAC layer using optical microscopy, transmission electron microscopy, Raman and x-ray photoemission spectroscopy, and two-point probe techniques.