A tantalum film capacitor has an .alpha.-tantalum as a lower electrode, a chemical conversion layer of .alpha.-tantalum as a dielectric and an upper electrode, an improvement involving forming a highly nitrogen-doped tantalum film between the .alpha.-tantalum and a substrate, and also, another improvement involving forming a transitional thin tantalum layer between said highly nitrogen-doped tantalum film and said .alpha.-tantalum. The nitrogen concentration of the highly nitrogen-doped tantalum film is from 14 to 30 atomic %. The electrical properties, especially leakage current, are improved over those of the prior art. A disadvantage of the conventional .alpha.-tantalum thin film capacitor, that is the necessity of using an expensive partial glazed Al.sub.2 O.sub.3 substrate, is eliminated, and a non-glazed Al.sub.2 O.sub.3 substrate can be used in the present invention.
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