Development Of Sem-Based Dedicated Ic Metrology System

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Abstract
IC metrology requirements, as feature size reaches one micron or less, are beginning to exceed the capabilities of light optical systems. It is well known that scanning electron microscopy provides sub-micron measurement capabilities. Thus, SEM based techniques are often considered as an alternative to the more conventional metrology techniques. It is possible to modify a commercial, general purpose SEM to perform basic metrology functions. However, in the modification of an instrument designed for other purposes, some trade-offs of performance are inherent. We feel that the most important features of an SEM based metrology system are reliability, reproducibility, throughput and the ability to perform automated sample alignment and that these parameters can only be optimized by the design of a totally dedicated IC metrology SEM. In this paper we will describe the design philosophy, development, and performance of such a dedicated system. The major design objective includes totally automated, non-destructive operation. This required the development of a low keV electron beam column controlled by digital, microprocessor controlled electronics. Fully automated operation, including site location, rotation and height correction, auto-focus, contrast/brightness and astigmatism control required extensive software development; as did the long term accuracy and reproducibility requirements. A hardware description and performance summary of the first generation commercial metrology SEM resulting from this work will be given, along with an outline of current developments towards a second generation system with high throughput and larger wafer handling capabilities.

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