Pattern Recognition Automatic Fine Alignment

AbstractAs alignment tolerances and the associated overlay characteristics become more important to microlithography, automatic alignment has proven to be necessary for high yields and consistent throughput. This paper describes an Automatic Alignment System designed for, but not limited to, projection printers.In this scheme a vidicon is used to detect the images projected from special targets on the mask and wafer. The video information is procesed by a microcomputer system that performs a pattern recognition algorithm to determine target position and the necessary alignment correction. The system closely resembles operator alignment in concept, yet gives greater accuracy and more consistent performance.Operational principles and actual performance are discussed.IntroductionThe Perkin-Elmer M200 series projection mask aligner, which has become well established in the 1C industry, now moves closer towards fully automated operation with the addition of automatic fine alignment. Designing an automatic system to interface with an existing instrument presented a unique challenge. Any changes to the projection optics or viewing system that would compromise normal operation could not be tolerated. Calibration procedures must be kept to a minimum and be automatic where possible. The scheme chosen is based on pattern recognition of the alignment targets and closely emulates the manual alignment operation. The system stresses consistent alignments at high accuracy as well as alignment speed with no loss in overall throughput.Pattern recognition techniques inherently assure a high degree of fail-safe operation. For example, the system can be required to qualify the accuracy of alignment, the amount of valid data, and the quality of the data accumulated before alignment is considered achieved. Independent recognition of mask and wafer target positions allows easy implementation of alignment to a predetermined offset if desired. The computer system used for the recognition algorithm provides direct yet flexible operator interface through a keypad and display.Various state-of-the-art 1C fabrication processes make different demands on the automatic alignment system which, particularly in the early phase of development, must be flexible enough to offer solutions as the problems arise. This system has proven to be able to accommodate all but a few cases where, fortunately, the process itself could be changed to accommodate the alignment system. The system described in this paper has been developed and evaluated to the point where it is in production use at a number of 1C facilities. The basic concept and its implementation will be discussed and experimental results will be presented.General descriptionThe automatic fine alignment system is shown schematically in Figure 1. The mask and wafer targets are illuminated by a darkfield system implemented in the projection aligner condenser. These targets appear in the same plane when viewed through the beamsplitter in the primary prism assembly. The target images are relayed to a TV camera containing a vidicon pickup tube. The vidicon readout operates in a raster scan fashion providing a video signal proportional to the image intensity for each successive scan of the photosensitive faceplate. As the scanning takes place, the video signal is processed by a detection circuit that is designed to trigger on a waveform derived from the shape expected from the darkfield target image. The detection scheme is insensitive to amplitude and linewidth variations over a moderate range. The trigger pulse is fed into a digitizer circuit where it is converted into the coordinate number corresponding to its position in the scanned field. Both the vidicon scanning and the digitizer are synchronized by a master clock. All detected coordinate numbers are transferred to the microcomputer memory to await processing by the recognition algorithm. The algorithm recreates the patterns, determines the position of all target lines and computes the relative centers of the mask and the wafer targets. The alignment correction is performed by moving the wafer stage. This sequence is repeated until alignment within a preset tolerance is achieved. When all alignment criteria are satisfied, the projection aligner is permitted to expose the wafer.