<title>Laser Scanning Autoalignment In Projection System</title>

Abstract

Automatic alignment by laser beam scanning method is known as a useful technique for both contact and proximity printing, and is now put in practical use. This method utilizes a laser spot as an optical probe which detects the information of a mask and a wafer simultaneously. Laser beam forms a small spot on the objects, and scans them at a constant velocity by a rotating polygonal mirror and F-θ optical systems. The alignment marks on the objects are placed along the scanning line. In order to increase the signal-to-noise ratio of the object information, a part of the scattered light from the marks is selectively detected by optical filtering method. As a result, high detectability of wafers information and highly accurate automatic alignment can be achieved. This laser spot scanning method is easily applied to the projection printing aligners too, such as lens or mirror system. It has an advantage to make full use of the characteristics of the projection system. For example, the interference phenomena between a mask and a wa-fer can be eliminated by the use of a quarter-wave plate or phase-shift mirrors in combination with the polarization characteristics of a laser. Cosequently, stable output signals are obtained, and the alignment accuracy is improved to 0.3μm (2σ).