Ex situ metrology of x-ray diffraction gratings - eScholarship

Ex situ metrology of x-ray diffraction gratings V. V. Yashchuk, W. R. McKinney, and N. A. Artemiev Advanced Light Source, Lawrence Berkeley National Laboratory, One Cyclotron Rd., Berkeley, CA 94720, USA Email: vvyashchuk@lbl.gov The idea of measurements of groove density distribution of diffraction gratings suggested and first realized in Ref. [1] consists of determination of the spatial frequency of the first harmonic peak appearing in the power spectral density (PSD) distribution of the grating surface profile observed with a microscope. Using a MicroMap TM -570 interferometric microscope, it was experimentally proven that this technique is capable for high precision measurements with x-ray gratings with groove density of about 250 grooves/mm varying along the grating by ±5%. In the present work, we provide analytical and experimental backgrounds for reliable application of PSD characterization of groove density of diffraction gratings. We analyzed the shape of the first harmonic peak and derive an analytical fitting function suitable for fitting of the PSD peaks obtained with gratings with a variety of groove shapes. In the case of a gratings with an rectangular groove shape, a reliable fitting function with a limited number of parameters is Fit ( f )  A  B  Log   ( f  f 1 ) 2     w 2  ( f  f 1 ) 2   w 2       C  ( f  f ) , where f is the spatial frequency, and fitting parameters are the peak position f 1 , the peak width w , and the constants  , A , B , and C . Figure 1 presents a result of fitting with the function (1) of a PSD peak profile of a diffraction grating with 300 grooves/mm, measured with a ZYGO TM NewView-7300 interferometric microscope at 20× magnification. The estimated accuracy for the best-fit