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Impact of Mask Errors on Optical Lithography by Chris A. Mack, FINLE Technologies F. M. Schellenberg, Mentor Graphics
This paper presents the characterization of the Mask Error Enhancement Factor (MEF) for a variety of feature types under a variety of processing conditions. For example, contact holes are found to exhibit significantly higher MEF values than the comparable dense or isolated lines. Errors in processing, such as focus and exposure errors, also affect the value of the MEF. Thus, another approach to evaluating the impact of mask errors is to look at the reduction in the process window caused by these errors. Using simulation, a study of the impact of mask CD errors on the overlapping process windows is presented.
Introduction
As optical lithography pushes to smaller and smaller dimensions, patterned features smaller than the wavelength of light must be routinely manufactured. In this regime mask errors take on an increasingly large share of the sources of critical dimension (CD) errors. The Mask Error Enhancement Factor (MEF), first discussed by Wilhelm Maurer1, 2 , serves to amplify reticle errors due to the highly non-linear nature of imaging near the resolution limit. Thus, CD control requirements on the mask are shrinking faster than the requirements on the wafer. An understanding of the MEF, and what processing variables affect it, is essential if the CD control goals of future lithographic generations are to be met.
of the imaging tool). One way to define the MEF of an array of line/space patterns is to assume a CD error for all the lines (dark features) keeping the pitch constant, then measure the resulting resist CD. A MEF of 1.0 is the definition of a linear imaging result. Although a MEF less than one can have some desirable consequences for specific features, in general a MEF of 1.0 is best. The MEF is not a constant value for a given process. It is a strong function of feature size and type. Also, processing errors can affect MEF, usually negatively. Focus errors, in particular, can make the MEF significantly greater. It is important to characterize the MEF for all feature types and sizes, and under a reasonable range of processing conditions, in order to properly specify the allowed reticle CD errors.
The MEF (also called MEEF by some authors) can be defined quite simply as the ratio of the change in resist feature width to the change in mask feature width assuming everything else in the process remains constant. In mathematical terms, MEF =
∂CDresist ∂CDmask
(1)
where the mask CD is in wafer dimensions (that is, already scaled by the magnification
F i g u re 1. The “i mage CD” can be defined as the width of the aerial image measured us ing a pre d e t e rmined aerial image t hreshol d va lue.
Spring 2000
Yield Management Solutions
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