Yasuyuki Unno

New Critical Exponent β of the XY Antiferromagnet on Stacked Triangular Lattice, CsMnBr3

The phase transition of the XY antiferromagnet on a stacked triangular lattice, CsMnBr 3 is studied by neutron diffraction. The critical exponent β is found to be β=0.25±0.01. The value is in excellent agreement with the predicted value by Kawamura for a new universality class, giving convincing experimental support to the phase transition characterized by the Z 2 ×S 1 or O(2) symmetry.

Distorted wave front produced by a high-resolution projection optical system having rotationally symmetric birefringence

The influence of birefringence caused by rotationally symmetric stress distribution in a high-resolution projection optical system is investigated. The general form of the pupil function is derived based on the Jones matrix calculation, expressing the wave front as a combination of the two orthogonal polarization components. Assuming a linearly polarized incident beam, it is found that the main polarization portion of the wave front exiting the projection lens has astigmatic aberration in the Seidel region and shows phase singularity at four pupil points at which the amplitude transmittance becomes zero.

Point-spread function for a rotationally symmetric birefringent lens

Imaging properties of a birefringent lens, in which the fast (or the slow) axis is distributed in the radial direction whereas magnitude of birefringence varies as a quadratic function of the pupil radius, are investigated by calculating a point-spread function. It is found that the point image is analytically described by using the Lommel function as well as the zero-order Bessel function, and a localized intensity null surrounded by bright regions in all directions can be realized at a geometrical focus under certain conditions. The magnitude of birefringence that is tolerable in image formations is also discussed, assuming that the lens is applied to microlithography.

Influence of birefringence on the image formation of high-resolution projection optics

The influence of birefringence on high-resolution projection optics used for microlithography is investigated theoretically. A formula for partially coherent imaging that is based on the scalar diffraction theory is extended to include the birefringent properties of lens materials. By the determination of the birefringent properties of lens materials by use of random numbers, data on 200 lenses, each composed of 10 birefringent elements, are generated to estimate statistically the degree of imaging-performance degradation. As a result, it was found that the image contrast of a five-bar pattern decreases as a quadratic function of the maximum magnitude of the birefringence that was set in common for all elements in each lens data set.

Impact of mask errors and lens aberrations on the image formation by a vortex mask

The images projected by the first vortex via masks, which comprised arrays of closely spaced dark spots that could pattern contacts with critical dimensions smaller than a third of an exposure wavelength in negative photoresist, showed several unexpected anomalies. Under certain conditions, the contact holes were elliptical (rather than round), displaced from their ideal locations, and had major axes oriented in directions that broke the expected symmetries. These effects have now been explained in terms of errors in the mask transmission and phase that give rise to unwanted Fourier components of the image combined with aberrations in the projection lens. Both effects must be present to break all the pattern symmetries. Distortions can be controlled by setting the numerical aperture of the projection lens to filter out four of the nine Fourier components and by proper design of the reticle.

Point-spread function for binary diffractive lenses fabricated with misaligned masks.

The influence of mask-alignment errors on the fabrication of four-level binary lenses has been investigated. The pupil function was derived as a function of the amount of misalignment between the two mask patterns based on scalar diffraction theory. It was found that the phase term in the pupil function affects only the location of the point-spread function, whereas the uneven light transmittance degrades the image quality in the direction of the misalignment. The Strehl intensity was found to decrease almost linearly with respect to the amount of error.

Anomalous Magnetization of RbFeCl3 around 31T in Pulsed High Magnetic Fields

Magnetizations of the triangular antiferromagnets RbFeCl 3 and RbFeBr 3 are measured in pulsed high magnetic fields up to 40T at liquid 4 He temperatures. In the case of RbFeCl 3 , the magnetization is found to increase almost linearly from the zero field, and to be flattened at some critical field of 14T. Further application of magnetic fields induces an anomalous double-step magnetization around 31T. The phenomenon is similar to the one which was found in CsFeCl 3 at some higher magnetic field, but noteworthy is the existence of a fine structure in the anomalous magnetization of RbFeCl 3 . The anomaly has thus far not been observed for RbFeBr 3 in the measured magnetic fields up to 85T.

Polarization analyses of aerial images produced by an optical lithography system.

The influence of polarization on the image formation of one-dimensional periodic patterns in a projection optical lithography system has been investigated. Assuming a linear polarizer at thelens pupil, I derived a simple expression representing the image intensity as the summation over spatial-frequency harmonics as well as three orthogonal polarizations. I calculated the coefficient for each image component as a function of the pattern frequency by independently varying the degree of partial coherence such that the image qualities of the two extreme polarization cases could be thoroughly compared.

Fabrication of N-level binary optical elements by use of m mask patterns with N in the range of 2(m-1) + 1 </= N </= 2(m).

Diffraction characteristics of N-level (5 </= N </= 8) binary gratings fabricated by repetition of three sets of photolithography and dry-etching processes have been investigated. After presentation of the general algorithm to determine a mask pattern and etching depth combination used in each fabrication process, all the methods for creating staircase structures with N = 5, 6, 7, 8 are derived. Then the diffraction efficiency achievable for the different level numbers, with the existence of mask alignment errors and for a fixed grating period, is compared, and it is found that an eight-level structure does not always give the best value.

Application of large scale binary optical elements to high resolution projection optics used for microlithography

Binary optical elements (BOEs) are very attractive because of their compactness and high aberration correction abilities. However, in their application to high resolution optics, some critical issues such as the influence of fabrication errors and the behavior of spurious order diffraction beams must be carefully examined. An eight-level BOE, ideally giving 95 % diffraction efficiency, is fabricated by lithographic techniques using a stepper for pattern delineation. A large element exceeding the size of stepper exposure area can be obtained by pattern stitching in the radial and the tangential directions. The performance of these BOEs confirm that it is fundamentally possible to apply BOEs fabricated with currently available techniques to high resolution projection optics used for microlithography.