A method of forming a semiconductor structure comprises forming sidewall oxide on a stack, by rapid thermal oxidation. The stack is on a substrate and comprises (i) a first layer comprising silicon, (ii) a second layer, comprising silicon and tungsten, on the first layer, and (iii) a capping layer, on the second layer. The sidewall oxide in contact with the second layer is at most 50% thicker than the sidewall oxide in contact with the first layer.