A proposed image 'intensity' expressing local irradiance

摘要

Authors would like to raise a discussion about image intensity for surface exposure, off course, including optical lithography. As a springboard for the discussion, a novel definition of image "intensity", which expresses local irradiance associating with optical image, is proposed. An experimental result, which strongly supports the proposed "intensity", is also obtained. To describe exposure dose, energy input for unit area with unit of J/m2, is applied as a measure of this amount. A phrase of "dose-to-clear" is frequently used to show sensitivity of a resist film. In contrast, conventional image intensity of optical image is defined as a value, which is proportional to volume energy density associating with image. The value is described with unit of J/m3. In some papers, it is mentioned that number of photochemical reactions in resist film is proportional to the volume energy density of electromagnetic filed, that is, conventional image intensity. It seems unclear what physical value is proper measure of surface exposure. We considered that, in optical lithography, energy flux is proper value to indicate degree of resist exposure from experience and some former reports. Then, a novel image "intensity", which expresses local irradiance associating with optical image, is proposed. The proposed image "intensity" is proportional to surface normal component of Poynting vector.