Generally, examples described herein relate to methods and processing systems for forming isolation structures (e.g., shallow trench isolations (STIs)) between fins on a substrate. In an example, fins are formed on a substrate. A liner layer is conformally formed on and between the fins. Forming the liner layer includes conformally depositing a pre-liner layer on and between the fins, and densifying, using a plasma treatment, the pre-liner layer to form the liner layer. A dielectric material is formed on the liner layer.