A method for etching a dielectric layer disposed below an antireflection layer (ARL) is provided. The method comprises (a) forming a patterned mask with mask features over the ARL, the mask having isolated areas and dense areas of the mask features, (b) trimming and opening, and (c) etching the dielectric layer using the trimmed mask. The trimming and opening comprises a plurality of cycles, where each cycle includes (b1) a trim-etch phase which etches the ARL in a bottom of the mask features and selectively trims the isolated areas of the mask with respect to the dense areas, and (b2) a deposition-etch phase which deposits a deposition layer on the mask while further etching the ARL in the bottom of the mask features. The trimming and opening result in a net trimming of the mask in the isolated areas.