Technique for optical characterization of exposure tool imaging performance down to 100 nm

A new technique for accurate and fast characterization of exposure tool imaging performance is presented. It is based on optical measurements of a macroscopically large length of test marks printed in a photoresist in a two-step process. In the first step, the line of the specific critical dimension (CD) is exposed with energy of one-half the nominal energy for a fully formed image. In the second step, the line of the same CD but with a slightly different orientation is exposed with the same energy as in the first step in such a way that its image superimposes the image exposed in the first step. The length of the resulting test mark is linearly proportional to the width of a line printed in a regular exposure process and is inversely proportional to sin(α), where 2α is an angle between two orientations. The coefficient of proportionality is found to be constant for a given resist process over broad ranges of CD variations caused by defocusing, aberrations, exposure dose change, etc. The mark length is measured rapidly with high accuracy by an optical scanning system and the result is then converted into the CD value. A measurement precision (3σ) of 0.5 nm is achieved for sub-150 nm CDs. Results of deep ultraviolet exposure tool characterization at CDs down to 100 nm are presented.