A semiconductor structure includes a base layer with a top surface and a plurality of processed areas. A primary pattern is disposed on the top surface of the base layer, wherein the primary pattern has a pattern top surface, a processed area on the pattern top surface, and a sidewall, and the primary pattern has a first critical dimension, and the processed areas are on the part of the top surface of the base layer exposed by the primary pattern. A secondary pattern is disposed on the sidewall of the primary pattern, wherein the secondary pattern has a second critical dimension, and the second critical dimension is smaller than the first critical dimension.