A method of reducing substrate
step height. The method includes providing a substrate having a low-
voltage device area and high-
voltage device area divided by an isolation structure, forming an oxidation
mask at least approximately 500 Å thick over the low-
voltage device area and parts of the isolation structure, forming a first
oxide layer on the exposed high-voltage device area and isolation structure using the oxidation
mask as a
mask, removing the oxidation mask, and forming a second
oxide layer, thinner than the first
oxide layer, on the low-voltage device layer.