Tadashi Nagayama

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.

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.

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.