Methods of silicon etch for trench sidewall smoothing are described. In one embodiment, a method involves smoothing a sidewall of a trench formed in a semiconductor wafer via plasma etching. The method includes directionally etching the semiconductor wafer with plasma generated from a fluorine gas to smooth the sidewall of the trench, the trench having a protective layer formed by plasma generated by a second process gas such as oxygen or a polymerization gas. In another embodiment, a method involves etching a semiconductor wafer to generate a trench having a smooth sidewall. The method includes plasma etching the semiconductor wafer with one or more first process gases including a fluorine gas, simultaneously performing deposition and plasma etching the semiconductor wafer with one or more second process gases including a fluorine gas and a polymerization gas mix, and performing deposition with one or more third process gases including a polymerization gas.