Ayako Sugaya

Angular spectrum calculations for arbitrary focal length with a scaled convolution

Nyquist sampling theorem in an image calculation with angular spectrum method restricts a propagation distance and a focal length of a lens. In order to avoid these restrictions, we studied suitable expressions for the image computations depending on their conditions. Additionally, a lateral scale in an observation plane can be magnified freely by using a scaled convolution in each expression.

Analyses of alignment measurement error

Summary form only given. In this article, the alignment patterns that are subject to have some large alignment measurement errors are discussed. Secondly, FFO (FIA focus optimization) is discussed. FFO is an optical alignment technique, which greatly improves the accuracy of FIA (Field Image Alignment) sensor of Nikons NSR series.

Optical alignment optimizations for reducing wafer-induced shift

Detecting the position of wafers after chemical mechanical polishing (CMP) is a critical issue in current and forthcoming IC manufacturing. A wafer alignment system must be highly accurate for all processes. To satisfy such requirements, we have studied and analyzed factors that have made alignment difficult. From the results of the studies, we have developed new optical alignment optimizations that improve the accuracy of FIA (wafer alignment sensor of Nikons exposure system) and examined them. The approaches are optimizing the focus position based on new classification of measurement errors, developing an advanced algorithm for position determination, and selecting a suitable mark design. The new classification method classifies measurement errors into errors caused by light amplitude errors and errors caused by phase errors. In the experiment, we have fabricated special wafers that make it possible to evaluate the influence of CMP processes on the alignment accuracy. The simulation and experimental results show that overlay error decreases markedly with the new alignment optimizations. FIA with these new optimizations will be highly accurate and suitable alignment sensor for CMP and other processes of future-generation LSI production.

Method for designing phase-calculation algorithms for two-dimensional grating phase-shifting interferometry

We propose a design method of phase-analysis algorithms based on two-dimensional grating phase shifting for Talbot interferometry, Talbot-Lau imaging, or the Ronchi test. These algorithms are designed to separate the two orthogonal shearing wavefronts and eliminate error effects of unwanted diffraction orders, simultaneously. Taking the effect of multidiffraction into account, moving the two-dimensional grating along a certain pass leads to a series of phase-shifted interfrograms, from which two orthogonal shearing wavefronts are derived, for the tested wavefront to be retrieved. The designing process is demonstrated, and the residual errors are analyzed via simulation works and experimental comparison.

Tool-Induced Shift and Pupil Transmittance Distribution in Measurement Optics

To advance large scale integrated circuit (LSI), alignment optics of an exposure system must be highly accurate. To satisfy this requirement, amplitude error in the optics is studied and analyzed. The factor in amplitude error that causes measurement errors is the asymmetrical brightness distribution in numerical aperture (NA) of the beam at the detector surface. A method that measures the amplitude error and phase error separately is presented. The measurement errors caused by amplitude error are studied. For the pattern that is needed for progress on LSI, these errors tend to be large. In order to minimize these errors, our alignment technique is studied. We demonstrate that focus optimization of a field image alignment sensor (FIA). FIA focus optimization (FFO) is effective in reducing errors, especially errors dependent on focus, caused by pupil transmittance distribution. The advantages of FFO for future-generation LSI production are confirmed.

Alignment focus optimization and image contrast

In LSI mass production, an alignment sensor must be installed in an exposure system to detect the pattern position accurately even if the pattern condition is changed. To satisfy such a requirement, the defocus effect of improving the image contrast is studied. Particularly under optical conditions with large illumination σ, the theory of image profile change induced by defocus is clarified in this paper. For a field image alignment sensor (FIA) with large illumination σ, FIA focus optimization (FFO) is developed as a technique that realizes alignment with both high accuracy and better image contrast by optimized defocus. FFO makes this possible regardless of the pattern phase and/or pattern duty ratio. The effects are confirmed by simulations and experiments. The versatile advantages of FFO for LSI production are confirmed.

Innovative optical alignment technique for CMP wafers

Detecting position of the wafers such as after CMP process is critical theme of current and forthcoming IC manufacturing. The alignment system must be with high accuracy for any process. To satisfy such requirements, we have studied and analyzed factors that have made alignment difficult. From the result of the studies, we have developed new optical alignment techniques which improve the accuracy of FIA (alignment sensor of Nikons NSR series) and examined them. The approaches are optimizing the focus position, developing an advanced algorithm for position detection, and selecting a suitable mark design. For experiment, we have developed the special wafers that make it possible to evaluate the influence of CMP processes. The experimental results show that the overlay errors decrease dramatically with the new alignment techniques. FIA with these new techniques will be much accurate and suitable alignment sensor for CMP and other processes of future generation ULSI production.

Advanced alignment optical system for DUV scanner

Advanced scanners need an extremely high accuracy wafer alignment system, and nowadays it is also necessary that the alignment marks occupy a smaller area in order to expand the available area for IC patterns. Therefore, narrower lines with a smaller pitch must form the alignment marks. In this paper, a higher Numerical Aperture (NA) and lower aberration alignment optical system are studied for these requirements. At first the small alignment marks are shown, and suitable NA in the optical system is then discussed. As a result, the necessity for higher NA is shown. As for low aberration, the necessary specification of wavefront aberration is discussed. Assuming it is possible to suitably select the NA and the illumination NA in the optical system, the results of simulation -- that simulate image signals and perform image processing -- are reported. These results show the optical system that has aberration causes position shift, so that the specification of wavefront aberration is estimated in order that the position shifts may be sufficiently small. To make sure that with such a strict specification the system will be possible, a trial optical system has been made. Finally the techniques of manufacturing and the results of evaluation are reported.

TIS & pupil transmittance distribution in measurement optics

The progress of semiconductor requires imaging optics with the highest accuracy of an exposure system. Miniaturization of LSZ needs the projection lenses with high accuracy. And it also needs the alignment optics with high accuracy. The alignment system with the alignment optics must measure a pattern position within one-third of the fine pattern width. Therefore the required performance of the optics is very high. In this paper, an imaging type alignment system is discussed. The alignment system has illumination optics, imaging optics, image pickup devices, and an image processing system. First of all, the error factors of illmination optics and imaging optics are analyzed theoretically. Errors of optics are classified into the phase error( Y error) and the amplitude error( Am error) of a light’). Y error and/or Am error in measurement optics causes the measurement errors that is called TIS. In this paper, TIS caused by Am error in measurement optics (Am error-induced shift) are discussed. The factors of Am error-induced shift that have been reported are the aperture stop eccentricity to the optical axis in the imaging optics (AEO)” and illumination inconsistency in the illumination optics3). These reports are not enough to show Am error-induced shift as a whole. In this paper, we propose the panoptic grasp and analysis of Am error-induced shift. If the pupil transmittance distribution in a whole optics is not uniform, the amplitude of a diffraction ray is modulated. And if the amplitude distribution is asymmetry of the optical axis, the image becomes asymmetry, and the asymmetrical image causes measurement error. This Am error-induced shift is lead by the pupil transmittance distribution not only in the illumination optics but also in the imaging optics. That is to say, the factor of Am error-induced shift is the nonuniform brightness in NA of the beam at the detector surface. This is the new panoptic propose. The conventional have proposed only narrow meanings of that. Although the Am errorinduced shift can be also caused by a pattern condition, this paper assumes the uniform pattern and discusses only TIS. Secondly, we propose a measurement method to distinguish AEO from the random nonunifonnity. Furthermore, we propose a method and theory to distinguish the difference of the pupil transmittance distribution between in the illumination optics and the imaging optics. In order to reduce Am error-induced shift mentioned above, a method that gives the x / 2[rad] phase between Oth ray and other diffraction rays‘) is very effective. FFO realizes it by focus optimizatidn to the wafer alignment sensor of Nikon exposure system (FIA)4’. FFO is effective in reducing TIS, especially TIS dependence on focus ( A TIS), caused by pupil transmittance distribution. One example shows that 80% decrease of TIS dependence on focus with FFO from that with the conventional FIA (Fig. 1). The position detection focus with the conventional FIA is the focus position where optics aberration is at a minimum. The FFO effect is especially powerful in the case of patterns with low step height and narrow width. Of course the tool errors have