Examination of phase retrieval algorithms for patterned EUV mask metrology Rene A. Claus a , Yow-Gwo Wang b , Antoine Wojdyla c , Markus P. Benk c , Kenneth A. Goldberg c , Andrew R. Neureuther b , Patrick P. Naulleau c a Applied Science and Technology, University of California, Berkeley; b Electrical Engineering and Computer Sciences, University of California, Berkeley; c Center for X-ray Optics, Lawrence Berkeley National Laboratory ABSTRACT We evaluate the performance of several phase retrieval algorithms using through-focus aerial image measurements of patterned EUV photomasks. Patterns present a challenge for phase retrieval algorithms due to the high- contrast and strong diffraction they produce. For this study, we look at the ability to correctly recover phase for line-space patterns on an EUV mask with a TaN absorber and for an etched EUV multilayer phase shift mask. The recovered phase and amplitude extracted from measurements taken using the SHARP EUV microscope at Lawrence Berkeley National Laboratory is compared to rigorous, 3D electromagnetic simulations. The impact of uncertainty in background intensity, coherence, and focus on the recovered field is evaluated to see if the algorithms respond differently. Keywords: Extreme Ultraviolet Lithography, Phase Retrieval, Phase Shift Mask, Photomasks 1. INTRODUCTION Phase retrieval algorithms use aerial image measurements to calculate a scalar electric field representation of the object. This field can then be used to simulate aerial images under various imaging conditions including aberrations and different illumination conditions. In photomask inspection, this can be useful when evaluating the printability of defects and planning repairs. Since simulations using the recovered electric field can emulate the optics of the lithography system, phase retrieval may help to provide a more accurate estimate of printability. Alternatively, the process window impact of a defect can be computed by simulation instead of by imaging the defect through the entire process window, potentially saving time. An important challenge to the use of phase retrieval in photomask inspection is that some algorithms perform poorly on the high contrast, strongly diffracting patterns found on photomasks. 1 Low contrast patterns such as Extreme Ultraviolet (EUV) multilayer defects have already been successfully imaged. 2 There are many available methods for phase retrieval from defocused images; however, the phase retrieval problem is nonlinear, so the performance of a given algorithm generally depends on the object. To examine the performance on patterned masks, we examine three phase retrieval algorithms using different approaches. The Gerchberg-Saxton 3 (GS), Transport of Intensity 4 (TIE), and Weak Object Transfer Function 5 (WOTF) are applied to aerial image measurements of two different EUV photomasks. One mask uses a standard TaN absorber and the other is an etched multilayer mask. The measurements were performed on the actinic zoneplate microscope, SHARP, at Lawrence Berkeley National Laboratory. 6 Since the measurements are not perfect, we examine what impact three common measurement errors would have on the recovered field. We look at the impact of background intensity, coherence, and a focus offset. Further author information: (Send correspondence to Rene A. Claus) Rene A. Claus: E-mail: reneclaus@gmail.com, Telephone: 1 949 334 7363 Photomask Technology 2015, edited by Naoya Hayashi, Bryan S. Kasprowicz, Proc. of SPIE Vol. 9635, 96350F · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2197868 Proc. of SPIE Vol. 9635 96350F-1 DownloadedFrom:http://proceedings.spiedigitallibrary.org/on11/07/2015TermsofUse:http://spiedigitallibrary.org/ss/TermsOfUse.aspx