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The use of narrow band illumination for automatic alignment can result in thin film interference effects that can make optical alignment difficult. Depending upon the thickness of the photoresist and the types and thicknesses of the underlying film structure, the reflectivity of the alignment marks are enhanced or decreased. This can also produce fringes of alternating light and dark areas that interfere with the system's ability to recognize the alignment marks accurately. Small changes in the thicknesses of the photoresist or the underlying films can result in large changes in the quality of the alignment marks. In the past, most efforts to overcome these problems have relied upon efforts to accurately control the thickness of the optical films, bleaching of the photoresist, or changing the wavelength of the alignment illumination. This paper describes a technique that is used to assure that good alignment mark images are obtained under a wide range of film thickness variations. This technique has been developed specifically for use with Eaton-Optimetrix wafer steppers, but the principles are applicable to other types of steppers. The technique that has been developed takes advantage of the fact that small changes in film thickness will often cause a poor optical image to become good. The alignment marks for this wafer stepper have been divided into two separate areas. Through the use of a simple technique, that does not necessarily require any additional process steps, the optical thickness and/or structures of these two areas are made to differ from each other. In most cases, at least one of the two areas will produce an alignment image of good quality. During alignment, the better of the two alignment mark areas is used by the stepper for aligning the wafer to the projected image of the reticle. This paper discusses the theory and application of the new alignment mark (Double Structured Alignment Mark). The characteristics of the alignment marks are evaluated by comparing the theoretical results with plots of actual data. Test results and photographs of the actual alignment mark images are shown to verify the effectiveness of the technique.
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