<title>New alignment sensors for wafer stepper</title>

Abstract

Two new alignment sensors for wafer steppers are developed to attain high alignment accuracy on all layers by targeting layers that are difficult to align using the existing alignment sensor, which is based on a laser beam scanning system. The Field Image Alignment (FIA) is a bright- field TV image processing alignment system using broadband illumination. The major advantage of FIA is that due to the broadband light source used for illumination, the edges of the alignment mark can be detected without being influenced by the interference fringes formed by the photoresist. Additionally, even if the cross section of the alignment mark is asymmetrical, the asymmetry can be accurately captured and alignment at the proper position can be achieved. The Laser Interferometric Alignment (LIA) is a grating alignment system based on an optical heterodyne interferometry technique. The advantage of this sensor is that is not affected by surface irregularities such as grains because it will process only specific spatial frequency components diffracted from the alignment mark. Therefore, the spatial frequency components which are diffracted from the metallic grains will be disregarded and will not influence alignment. This allows the alignment to be successful even for low step height or deformed marks. With the development of FIA and LIA, the authors have successfully complemented the existing sensor, so that a high alignment accuracy for the mass production of VLSI with 0.5-0.35 micron rules can be achieved on most layers.