Proc. SPIE. 8418, 6th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Design, Manufacturing, and Testing of Smart Structures, Micro- and Nano-Optical Devices, and Systems
With the shortening printing wavelength and increasing numerical aperture of lithographic tool, the depth of
focus(DOF) sees a rapidly drop down trend, reach a scale of several hundred nanometers while the repeatable accuracy
of focusing and leveling must be one-tenth of DOF, approximately several dozen nanometers. For this feature, this article
first introduces several focusing technology, Obtained the advantages and disadvantages of various methods by
comparing. Then get the accuracy of dual-grating focusing method through theoretical calculation. And the dual-grating
focusing method based on photoelastic modulation is divided into coarse focusing and precise focusing method to
analyze, establishing image processing model of coarse focusing and photoelastic modulation model of accurate
focusing. Finally, focusing algorithm is simulated with MATLAB. In conclusion dual-grating focusing method shows
high precision, high efficiency and non-contact measurement of the focal plane, meeting the demands of focusing in
193nm projection lithography.
A simple but effective phase analysis method for the fringe pattern that occurs in two superposed grating marks applied in the previously designed dual-grating-based alignment scheme for lithography is proposed. First, the fringe pattern is processed and analyzed using a frequency domain method based on two-dimensional (2D) Fourier transform, and the 2D notch filter is appropriately designed to select the useful spectrum to obtain the target phase information. Further, phase difference of two sets of fringes is computed to acquire the alignment offset. Numerical simulation and experiment are both performed to verify this method. Finally, certain analysis about the error of phase difference extraction in the fringe pattern and precision of alignment are also presented. The results indicate that the background and noise of the fringe pattern can be efficiently filtered and target phase information can be extracted with high accuracy through this method.
The relative position of mask and wafer can be reflected by the variation of the spatial phase in the alignment method
basing on spatial phase of fringe pattern, through spatial phase imaging of grating modulation. The relative displacement
of mask and wafer can be obtained from spatial phase demodulation of fringe patterns to carry out the alignment process.
Generally, the windowed Fourier transform (WFT) is representative method for spatial phase demodulation of the fringe
pattern. It is robust and accurate, but it is computational redundant for the alignment of nanolithography. This paper
presents a phase demodulation method which has good accuracy and can be carried out in real time to meet the need of
the alignment of nanolithography. This method is improved on the basis of the traditional WFT. The fast Fourier
transform (FFT) is introduced to WFT and the phase is computed by integrating the phase gradient which is extracted
directly from the fringe pattern through generation of the adaptive windowed Fourier elements. These two improvements
can individually reduce the computation time for spatial phase demodulation of fringe pattern. The theoretical
background and the principle of the algorithm of fringe pattern analysis are proposed. Numeric computation indicate that
this method is of high accuracy and computational efficient for fringe pattern analysis, which show high significance for
application of alignment of nanolithography.