Abstract: In a semiconductor integrated circuit device, a polycrystalline silicon film is formed along an inner wall of a trench in which a capacitor is to be formed, and the polycrystalline silicon film and a lower electrode are contacted to each other on the entire inner wall of the trench. Oxygen permeated into the lower electrode during a thermal treatment of a tantalum oxide film is consumed at an interface between the polycrystalline silicon film and the lower electrode. Thus, oxygen does not reach the surface of a silicon plug below the lower electrode that would cause oxidation on the surface of the silicon plug and form a high-resistance oxide layer when a dielectric film formed on a lower electrode of a capacitor of a DRAM is subjected to a thermal treatment in an oxygen atmosphere.