Generally, this disclosure provides examples relating to tuning etch rates of dielectric material. In an embodiment, a dielectric material is conformally deposited in first and second trenches in a substrate. Merged lateral growth fronts of the first dielectric material in the first trench form a seam in the first trench. The dielectric material is treated. The treating causes a species to be on first and second upper surfaces of the dielectric material in the first and second trenches, respectively, to be in the seam, and to diffuse into the respective dielectric material in the first and second trenches. After the treating, the respective dielectric material is etched. A ratio of an etch rate of the dielectric material in the second trench to an etch rate of the dielectric material in the first trench is altered by presence of the species in the dielectric material during the etching.