Kyoichi Suwa

Automatic laser-scanning focus detection method using printed focus pattern

An innovative method of determining best focus with an optical exposure tool has been developed. The method uses wedge shaped marks in photoresist that can be measured automatically. The results show that best focus can be measured with a repeatability of 20 nm. The automatic focus measurement system can be used to characterize lens astigmatism, field curvature and tilt. Data shows good correlation with conventional methods using SEM linewidth measurement.

Characterization of spatial coherence uniformity in exposure tools

A novel technique to characterize variations of the spatial (partial) coherence (sigma) across the image field in modern steppers and scanners has been developed and experimentally tested. It is based on the high sensitivity of the length L of macroscopically large diamond-shaped marks printed in photoresist to (sigma) variation. Variations in the (sigma) value across the image field lead to variations in the length of marks printed at different image field locations. The mark lengths are measured rapidly with high accuracy by a built-in optical system and then converted into (sigma) values using the calibration dependence L((sigma) ) measured in the same exposure tool. Simulation and experimental studies show that the level of projection lens aberrations in modern Nikon tools have practically no effect on (sigma) measurements obtained with this technique. Our results demonstrate that in the conventional illumination scheme, (sigma) distribution can be measured with an accuracy of 2.5%. The main advantage of the presented method is that (sigma) variation over the image field is characterized by the exposure tool itself, avoiding expensive and time-consuming SEM measurements. Moreover, since the measurement procedure is based on the wedge-shaped marks and laser scanning system currently used in Nikon tools for automated focus detection, implementation of the technique does not require any hardware or software modification.

The Optical Stepper With A High Numerical Aperture I-Line Lens And A Field-By-Field Leveling System

A high N.A. i-line lens and a field-by-field leveling system have been developed. Resolution of the lens is better than 0.65 μm. The leveling system has achieved ±0.3 uμ/field on an experimental basis.

New methodology for through silicon via array macroinspection

Abstract. A new methodology for inspection of through silicon via (TSV) process wafers is developed by utilizing an optical diffraction signal from the wafers. The optical system uses telecentric illumination and has a two-dimensional sensor for capturing the diffracted light from TSV arrays. The diffraction signal modulates the intensity of the wafer image. The optical configuration is optimized for TSV array inspection. The diffraction signal is sensitive to via-shape variations, and an area of deviation from a nominal via is analyzed using the signal. Using test wafers with deep via patterns on silicon wafers, the performance is evaluated and the sensitivities for various pattern profile changes are confirmed. This new methodology is available for high-volume manufacturing of future TSV three-dimensional complementary metal oxide semiconductor devices.

Optical displacement metrology using alternating direction Moire

Abstract. We develop a new double exposure Moire method for an optical registration metrology system in photolithography. Our method enables us to achieve at least a factor of 10 improvements in precise displacement metrology using a conventional optical sensor. We utilize a new registration mark printed to the photoresist on a bare silicon wafer using a double exposure of the gratings. The mark consists of two types of Moire with opposite phases. The two types of Moire are oriented in alternate directions. Displacement is measured from the distance between the positions of the two types of Moire in analogy with the conventional registration method. This concept is called alternating direction Moire. Performance is experimentally confirmed using an i-line wafer exposure apparatus. Precision is improved by up to 32 times as compared with the conventional method and can be applied to other Moire metrologies.

Macroinspection methodology for through silicon via array in three-dimensional integrated circuit

Abstract. We are developing a new macroinspection technology for through silicon via (TSV) process wafers. We present new simulation results obtained with a fine TSV model and new optics. The optical system includes not only diffraction optics, but also polarization optics, by which we can detect changes in the profile (cross-sectional shape) of repeated patterns by detecting changes in the polarization status of reflected light. We confirmed the performance of the methodology by optical simulation using a model of via patterns with 1 μm diameter and 10 μm depth as a typical intermediate-interconnect-level TSV.

Theoretical analysis of focus wedge mark for best-focus determination in photolithography

The focus wedge mark (FWM), a periodic array of wedge shaped marks, is a tool used to determine the best-focus position in photolithography. Here, we develop a one-dimensional physical model for the FWM and analyze the focus dependence of the wedge length under coherent imaging conditions. The amplification factor of the FWM, which describes the superiority of the FWM compared to measurements based on line and space marks, is determined by the geometrical magnification of the wedge. We show that around the exposure dose energy typically used in lithography the amplification factor is much higher than the geometrical magnification. Numerical lithography simulations confirm that the focus sensitivity of the FWM is much higher than that of a line and space pattern.

Accuracy of focus wedge mark measurement using fitting function for partially coherent illumination

The theory of best focus determination in focus wedge mark (FWM) metrology is extended from coherent illumination to partially coherent illumination. Through a theoretical analysis, we propose a focus curve fitting function suitable for the FWM, which is expressed by the sinc function. The experimental results for best focus estimations using two types of fitting functions, which are a conventional quadratic polynomial and the proposed sinc function, are reported for various defocus regions to be fitted. When the first inflection points of the sinc function are included in the fitting defocus region, we can achieve stable best focus estimation within 3 nm in our i-line exposure experiments.

A survey of advanced excimer optical imaging and lithography

The first item discussed in this paper is to estimate the future trend regarding minimum geometry and the optical parameters, such as NA and wavelength. Simulations based on aerial images are performed for the estimation. The resolution limit is defined as a minimum feature size which retains practical depth of focus (DOF). Pattern geometry is classified into two categories, which are dense lines and isolated lines. Available wavelengths are assumed to be KrF excimer laser (λ=248 nm), ArF excimer laser (λ=193 nm) and F2 excimer laser (λ=157 nm). Based upon the simulation results, the resolution limit is estimated for each geometry and each wavelength. The second item is to survey ArF optics. At present, the ArF excimer laser is regarded as one of the most promising candidates as a next-generation light source. Discussions are ranging over some critical issues. The lifetime of ArF optics supposedly limited by the radiation compaction of silica glass is estimated in comparison with KrF optics. Availability ...