Photoresist (PR) materials undergo significant physical and chemical modification from the ions, vacuum ultraviolet (VUV) photons, and reactive species that comprise a plasma etch process. The modifications of these materials, which are an integral component in the manufacturing process of semiconductor devices, has key implications on the control of the etching and roughening behaviors that are vital for establishing selective pattern transfer processes that maintain feature fidelity at increasingly smaller feature sizes and pitches. In the initial chapters of this dissertation, we focus on establishing a fundamental understanding of the relationship between PR modification and the resulting etching behavior under an inert argon plasma process. To establish the key relationships, we utilize a combination of in situ ellipsometry supported by x-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to develop an ellipsometric model that interprets the correlation between the PR layer structure, which develops due to energetic ion bombardment in a plasma process, and the corresponding etching behavior. From this analysis, we find that energetic ion bombardment produces a dense amorphous carbon (DAC) layer on the PR surface that reduces the overall PR etch rate with increasing thickness. Secondary characterization via atomic force microscopy (AFM) also shows corresponding development of surface roughness. Expanding the scope to reactive plasma chemistries containing fluorocarbon (FC) species, we find that the FC species interact with the DAC layer to produce an FC-rich modified layer on the surface. In the latter part of this dissertation, we apply our findings regarding the PR surface modification to address an industrial need to improve the etch selectivity of silicon dioxide (SiO2) to PR by minimizing the thickness loss of an extreme ultraviolet (EUV) PR under an atomic layer etching (ALE) process by systematically evaluating the ALE processing parameters. We find that cyclic ion bombardment of a deposited FC layer leads to the development of a modified layer that significantly reduces PR loss while simultaneously maintaining SiO2 etching, thus producing a high SiO2/PR etching selectivity. Lastly, we examine another industrial challenge concerning the extent of off-normal ions affecting the etching uniformity of PR samples.