Quartz glas is a high-puridity glas consisting from nothing else but SiO2.
In some cases stainless steel is not sufficiently resistant against plasma processes. In such cases the material of the plasma chamber is quartz glass which is almost completely resistant against most plasma processes.
In addition all screens and windows of a recipient are from quartz glass as well as the tracks to carry substrates in case of some special etching processes. Silicon wafers are positioned into quartz glass boots for plasma etching.