Quantifying the capability of a new in-situ interferometer

As design specifications push the limits of optical lithography deep into sub-λ imaging requirements, characterization of imaging tools has become increasingly more important and difficult. The customary Usable Depth of Focus (UDOF) and Distortion testing no longer provide all information necessary to predict the capability of the imaging tools. Process variations and sensitivities convolute the findings of many tool acceptance tests and can mask the true capability of todays High NA imaging tools. A new interferometer imaging tool developed by Litel Instruments (InspecStep^TM in-situ Interferometer) has been shown to be able to measure the distortion coefficients which are as accurate as conventional Phase Measurement Interferometer (PMI) techniques used by lens manufacturers. We will collect and present data quantifying the capability of this new tool. Repeatability and Reproducibility will be presented in terms of spherical wavefront aberration (rms) as well as comparisons of Zernike coefficients to vendor PMI data. Output configuration compatibility for commercially available lithographic simulators will be demonstrated. Finally a matching study will be presented in which we compared the predictions and calculations of interferometers