The present disclosure is directed to formation of a low-k spacer. For example, the present disclosure includes an exemplary method of forming the low-k spacer. The method includes depositing the low-k spacer and subsequently treating the low-k spacer with a plasma and/or a thermal anneal. The low-k spacer can be deposited on a structure protruding from the substrate. The plasma and/or thermal anneal treatment on the low-k spacer can reduce the etch rates of the spacer so that the spacer is etched less in subsequent etching or cleaning processes.