Unlike optical lithography, there are multiple exposure limits, identified as the top dose, bottom dose, and critical dose, whose values must be determined experimentally for a proper exposure. The exposure must be sufficient to meet the requirements of the bottom dose, the exposure under which a photoresist residue will remain, and the top dose, the exposure over which the photoresist will foam. The critical dose is the exposure at which unexposed resist begins to be attacked. Due to the insensitivity of PMMA, a typical exposure time for a thick PMMA is six hours. During exposure, secondary radiation effects such as Fresnel diffraction, mask and substrate fluorescence, and the generation of Auger electrons and photoelectrons can lead to overexposure.