Yoshinobu Takeuchi

Resolution Enhancement by Oblique Illumination Optical Lithography Using a Transmittance-Adjusted Pupil Filter

This paper describes the resolution enhancement by oblique illumination optical lithography using a transmittance-adjusted pupil filter that has a conjugate shape to the secondary light source. The resolution enhancement depends on the amplitude transmittance distribution of the pupil filter in the lens aperture. Thus, the optimum transmittance distribution attaining the highest resolution at a practical depth of focus is investigated, and superior performance is verified experimentally using an actual i-line stepper, that is, the actual resolution improvement is comparable to the simulation results. Despite this improvement, two problems must be addressed. Pattern profiles degrade at the ends of periodical patterns, and the opaque pattern widths of middle-pitch patterns become a little narrow. These two problems are solved by optimizing the transmittance distribution of the filter such that there is no significant deterioration in the high-resolution performance.

Patterning Characteristics of Oblique Illumination Optical Lithography

The patterning characteristics of oblique illumination lithography are investigated. When the reticle is illuminated obliquely, the pattern widths corresponding to the opaque parts of the reticle are apt to become thinner than those replicated by conventional optical lithography. This characteristic is verified by both analysis of the projected image intensity distribution and some patterning experiments. The double harmony of the periodical pattern image plays an important role in defining the pattern widths. We concluded that the lack of the double harmonic light waves caused the pattern shrinkage in the case of oblique illumination lithography. In recognizing the above differences, the exposure dose can be suitably controlled so as to attain high resolution and large focus latitude.

Analytical Method for Image Characteristics of Annular Illumination with a Spatial Filter in Optical Projection Lithography

Annular illumination with a spatial filter, or SSBL (single-sideband optical lithography), has performance equivalent to phase shifting in optical column technology. This paper presents an analytical method for examining image deformation characteristics in optical projection systems including SSBL. We express image intensity as the sum of a symmetric and an antisymmetric component. Image deformation can be clearly observed by using the antisymmetric component. From the point of view of proximate interaction, we show that the real part of mutual intensity is useful in interpreting image deformations. Optical image deformations are unavoidable in the range of 0.2-0.3 µm at the KrF-line wavelength. We present a simplified correction method for mask data which reduces computational time.

Investigation and improvement of patterning characteristics for annular illumination optical lithography at the periodical pattern ends

This article describes particular patterning characteristics of annular illumination lithography and a method to improve them. Annular illumination lithography is one of the most practical methods to enhance resolution and enlarge focus latitude. However, improving the patterning characteristics is not sufficient at the ends of periodical patterns in spite of superior performance at the periodical parts. Here, the degradation of the patten profiles at the periodical ends are investigated in detail, and size-modification of the end patterns is proposed. By making the reticle pattern widths a little wider only at the ends, end-pattern degradation is greatly improved, and practical depth-of-focus is favorably extended.

Optical Image Simulator Using the Real-Space Expression of the Object Pattern and a Differential Operator for Optical Proximity Effect Estimation

The new optical image simulator described here can accurately estimate the optical proximity effect that causes pattern distortion in systems using off-axis illumination or oblique illumination and in systems using phase-shifting masks. To prevent an undesirable proximity effect from being influenced by the repeated object pattern that occurs in simulations using the conventional fast Fourier transform (FFT) routine, we calculate the image intensity using the real-space expression of the object pattern. A differential operator and Taylor series expansion are introduced to accelerate the calculation. The simulation results are compared here with those of a commercially available FFT simulator, and this comparison confirms that the new simulator evaluates the proximity effect correctly and that the FFT simulator can produce incorrect image intensity at the circumference of the object pattern. We also show that the differential operator and Taylor series expansion are effective in speeding up the optical image calculation using the real space expression.