An extremely thin SiN film formed with electron cyclotron resonance (ECR) nitrogen plasma has been measured by x‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), ellipsometry, and electron spin resonance (ESR). The XPS and AES results show that the SiN layer is easily formed on a silicon surface by ECR nitrogen plasma, and it has a structure with high nitrogen content similar to Si3N4only near the surface of the film. The ellipsometry and ESR results reveal that the SiN film is very thin, 2.5–4.0 nm, and has spin density in the order of 1013cm−2, which is reduced drastically by exposing it to hydrogen plasma. The SiN film can be applied to the inexpensive dual‐polycide‐gate complementary metal–oxide–semiconductor process.