Both modeling and experimental results for the effects of plasma etching on photoresist line edge roughness and linewidth roughness (LER/LWR) and their transfer to underlayer films are presented and compared. In particular, we investigate the roughness formation on both photoresist and underlayer sidewalls during (1) isotropic trimming of photoresist, and (2) anisotropic plasma etching and LER transfer to substrate. The trimming process is modeled with an isotropic movement of the resist sidewall. In the anisotropic plasma etching process, the resist sidewall is used as a mask to anisotropically transfer the pattern to the underlying film. Experiments include trimming of a photoresist patterned with photolithography in plasma with no bias voltage and anisotropic etching of BARC and Si underlayers in and with bias. Both model and experimental results show that resist trimming causes reduction of resist LER and increase of the correlation length and roughness exponent with trimming time. This means that surface nano-protrusions versus trimming time become shorter and wider. In the case of anisotropic etching, the model predicts noticeable reduction of LWR, whereas correlation length and roughness exponent remain almost unaffected. The first experimental results seem to confirm these predictions.