The acid-diffusion effect on line edge roughness (LER) in a negative-type chemically amplified resist of electron beam lithography was evaluated experimentally.. The edge roughness evaluation method was utilized to quantify the acid-diffusion length. Adjusting the prebaking temperature controlled the acid-diffusion coefficient in the range from 8.1 nm(2)/s at 70degreesC to 0.8 nm(2)/s at 130 degreesC. Acid diffusion reduced the LER of a 100 nm isolated line from 6.9 to 4.0 nm while increasing the diffusion length from 15 to 44 nm, but a too-long diffusion,length degraded line and space patterns. These results suggest that acid diffusion has two opposing effects on LER: one is the smoothing effect reducing LER and the other is the degrading image contrast effect increasing LER. It is concluded that LER occurs as a consequence of the amplification of the so-called uncertainty band by the slope of the quasibeam profile. The bandwidth signifies the effect of molecular structure, the spatial dispersion of photochemical events caused by shot noise, the smoothing effect of acid diffusion, and development uncertainty at the pattern edge. The magnitude of the amplification is determined by the slope of the quasibeam profile that comprises aerial-image quality, acid-diffusion length, and resist contrast.