Tolerances and alignment method for high-aperture hybrid diffractive/reflective objective

For a laser scanning nanolithographic system operating at a deep ultraviolet wavelength of 266 nm, a hybrid highnumerical-aperture objective is proposed. The most important focusing part of such an objective is a combination of a diffractive optical element (DOE) and a spherical annular reflector (SAR). In the considered version, the proposed objective has the numerical aperture of NA = 0.95, the working distance of 1 mm and the size of a sub-wavelength focused spot of 105 nm (FWHM). However, such performance was found to require a high quality of optical components, as well as severe mechanical tolerances and a high precision of their alignment. The problem of a precision alignment of the optical components is additionally complicated by the fact that the wavelength of 266 nm is not visible and it is therefore not possible to use conventional alignment methods. To alleviate this problem, we propose to use an auxiliary brightening laser with a visible doubled wavelength equal to 532 nm. A dichroic mirror is used to combine the visible and invisible laser beams. In order to apply the proposed method, it is required to optimize the diffraction structure of the DOE for the simultaneous operation in the second order of diffraction at 266 nm and in the first order at 532 nm. An example of the optimized binary-phase structure of a two-wavelength DOE is given alongside with the optical response of this structure at the visible and invisible wavelengths. Also presented are the results of a computer simulation of the main aberrations of the focusing system under the displacement of its components. It is shown that the required accuracy of assembly can be achieved by the use of only a few of alignment methods at the visible wavelength.