Gate structures, comprising a first insulation film, a first gate material and
a gate oxide film, are formed. A second insulation film is formed on side surfaces
of the gate structures in the peripheral region. Trenches are formed at a surface
of the semiconductor substrate by etching the semiconductor substrate with the
first and the second insulation films used as masks. The second insulation film
formed on side surface of the gate structures is removed, exposing the surface
of the semiconductor substrate in the vicinity of the gate structures on both sides
of the trenches. Element-isolating insulation films are formed in the trenches
and on the exposed substrate. The gate structures in the peripheral region are
removed. Gate structures of peripheral transistors are formed between the element-isolating
insulation films in the peripheral region.
