Chlorine is incorporated into pad oxide (110) formed on a silicon substrate (120) before the etch of substrate isolation trenches (134). The chlorine enhances the rounding of the top corners (140C) of the trenches when a silicon oxide liner (150.1) is thermally grown on the trench surfaces. A second silicon oxide liner (150.2) incorporating chlorine is deposited by CVD over the first liner (150.1), and then a third liner (150.3) is thermally grown. The chlorine concentration in the second liner (150.2) and the thickness of the three liners (150.1, 150.2, 150.3) are controlled to improve the corner rounding without consuming too much of the active areas (140).