Practical approach to full-field wavefront aberration measurement using phase wheel targets Lena V. Zavyalova *a , Bruce W. Smith a , Anatoly Bourov a , Gary Zhang b , Venugopal Vellanki c , Patrick Reynolds c , Donis G. Flagello d a Rochester Institute of Technology, 82 Lomb Memorial Drive, Rochester, NY, USA 14623; b Texas Instruments Inc., 13560 N Central Expwy, Dallas, TX, USA 75243; c Benchmark Technologies, 7 Kimball Lane, Bldg E, Lynnfield, MA, USA 01940; d ASML US Inc., 8555 S. River Pkwy, Tempe, AZ, USA 85284 ABSTRACT An automated aberration extraction method is presented which allows extraction of lithographic projection lens’ aberration signature having only access to object (mask) and image (wafer) planes. Using phase-wheel targets on a two- level 0/π phase shift mask, images with high sensitivity to aberrations are produced. Zernike aberration coefficients up to 9 th order have been extracted by inspection of photoresist images captured via top-down SEM. The automated measurement procedure solves a multi-dimensional optimization problem using numerical methods and demonstrates improved accuracy and minimal cross-correlation. Starting with a detailed procedure analysis, recent experimental results for 193-nm projection optics in commercial full field exposure tools are discussed with an emphasis on the performance of the aberration measurement approach. Keywords: Phase wheel monitor, aberrations, lithography, wavefront, Zernike polynomials 1. INTRODUCTION Aberration measurement is an integral part of lithographic projection system (exposure tool) characterization. State-of- the-art lithography lenses are of the highest quality, imposing new requirements on detection limits of the measurement methods; a method’s precision must be under λ/1000. Previously we reported on an aerial-image based method for extracting aberrations that approaches that that requirement. 1 We now extend this method to working with photoresist images. This paper discusses the details of our approach, the mathematical framework, the expected sensitivity, the optimization steps necessary, and the latest results. An overview of other wafer-based methods for measuring aberrations is outside the scope of this paper and has been well addressed in Reference 2. The phase wheel aberration monitor target is shown in Figure 1. The same target can be used to extract the aberrations from intensity images or from images in resist. The target is a transparent pattern of nine circular zones, each phase- shifted relative to the background, so that each prints as a ring in photoresist coated on a wafer (Figure 2). Image shape deviations will be the result of aberrations in that system. These patterns change uniquely with different aberration type and sign (negative and positive), which allows for the determination of the aberration causing the change. 3 Spatially such a target extends an advantage of having multiple sensing regions within the instantaneous field of view. The basis of the aberration retrieval method is the study of changes in phase wheel images with respect to defocus. Taking an in-focus image as well as the images recorded through focus makes use of defocus similarly to the phase diversity methods. 4 Defocusing an image perturbs it with an additional known phase error. Since the object (target) is known, forward calculations can predict image shape in the image plane using modeling. We compare modeled and measured image shape using numerical methods and decrease the difference through varying the pupil phase function. An iterative gradient-search algorithm finds the aberration signature that is consistent with in-focus and out of focus images. This method uses target images collected through focus at a single illumination setting (low but practical σ) at a maximum NA. * lvzemc@rit.edu; phone 1 585 475 7991; fax 1 585 475 5041; www.rit.edu/lithography