The present invention provides methods for forming images in positive- or negative-tone chemically amplified photoresists. The methods of the present invention rely on the vertical up-diffusion of photoacid generated by patternwise imaging of an underlayer disposed on a substrate and overcoated with a polymer containing acid labile functionality. In accordance with the present invention, the vertical up-diffusion can be the sole mechanism for imaging formation or the methods of the present invention can be used in conjunction with conventional imaging processes.