Quartz tapered sidewall etching using a two-step etching method was studied based on photoresist modification technology. First, the vertical sidewall of the photoresist was modified to a tapered profile using Ar/O₂/CF₄ plasma. Then, the tapered sidewall of the photoresist was transferred to quartz using the same etching chamber. The influences of gas flow, etching power, temperature, and etching time on the etching rate, selectivity, and morphology during the photoresist modification process were studied in detail. A tapered quartz profile with a sidewall angle of 60° and a smooth surface was prepared by optimizing the process parameters. This work provides a helpful guideline for the tapered sidewall etching of SiO₂, as well as other materials.