Double-grating lateral shearing interferometer for extreme ultraviolet lithography

A lateral shearing interferometer (LSI) is one of the tools for measuring extreme ultraviolet lithograph project optics (EUVL PO) at-wavelength. In LSI, a grating is set behind the EUVL PO to shear the test wavefront. A point diffraction pinhole is used to correct the aberration of the light source. In LSI, there are some problems limiting measurement accuracy and dynamic range, for example, the fine fringe of the interferogram and the power change of the test beam. In order to solve all of these problems, we propose a new LSl for EUVL PO at-wavelength measurement. In the new LSI, the wavefront of the light source is sheared beforehand against the shearing direction of LSI by another grating set in front of the EUVL PO. After passing through the EUVL PO, the wavefront is sheared again as in a normal LSI. By two times of shearing, the wavefront of the light source is superposed on the imaging sensor, and therefore, the aberration of the light source can be canceled without using a point diffraction pinhole. On the other hand, the test wavefront is sheared on the imaging sensor and is measured as in a normal LSI. Because the point diffraction pinhole is removed from the interferometer, the test beam power becomes strong and stable. By setting the two gratings conjugated by the EUVL PO, a one-color interferogram is realized. We designed a new LSl with double gratings to realize our LSI in the at-wavelength EUVL PO test. The accuracy of the new interferometer is expected to be higher than 0.3 nm (RMS).