Potential causes of across field CD variation

The variation in critical dimension (i.e., CD) as a function of exposure field position is an important error component in total linewidth control. In this paper, we describe methods for uncovering root causes of the across field CD variation in a lithographic projection system. For example, using a special reticle artifact we form images of the exposure system source distribution at various points in the lens field. These field dependent source distributions provide important clues into the variation of partial coherence, dose and numerical aperture around the exposure field. We also present a novel technique for inferring the residual projection lens aberration signature from CD measurements in photoresist. Tests to understand the role of stray light in across field CD variation are also investigated. Exposure system specifications consistent with small variation in across field CD are provided on the basis of these test vehicles.