Abstract: The measurement of overlay error by optical instruments is subject to systematic error, sometimes known as tool induced shift, or TIS. Some investigations into the relationship between the mark structure and the properties of the instrument have been conducted, but the effects are still poorly understood. In this paper we report on experiments designed to investigate further the relationship between the mark structure, the alignment of the instrument and the resultant TIS error in the measurements. The optical images of wafers manufactured with overlay marks of known offsets and step heights look the same and enable us to distinguish between geometrical and optical effects. By varying the alignment of various optical elements in the tool, the resultant TIS data allows determination of the most critical instrument parameters which must be controlled before accurate measurements can be made. !4
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