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The fundamentals of overlay metrology
Abstract
A study on the fundamentals of overlay metrology is performed. The overlay measurement equipment performance is found to be better than critical dimension (CD) metrology with respect to error budget allocations and relative stability of the overlay measurement target design. The combination of CD and overlay excursions are used for the determination of overlay requirements for device design. A variant of the box-in-box test structure is used for the measurement of overlay errors.
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This paper describes a methodology based on advanced metrology to quantify the overlay performance of individual wafer steppers and a means to optimize their matching. Use of accurate two point through the lens alignment directly referenced to the reticle and a three axis interferometrically controlled X-Y-0 stage permits full characterisation of the individual machine overlay errors. With the help of parameters determined in an extended metrology model of the microlithographic lens distortions and the wafer stage grid distortions one can automatically adjust corresponding machine servo-mechanisms for optimum matching. The model can be used to calculate and predict the systematic errors between random pairs of machines rather thav having to directly measure all pairs, so ensuring efficient, high quality matching. Results of an eight machine matching experiment demonstrating the model's validity and effective use on PAS 2500 wafer steppers are described.
Measurement precision, especially the offset error in the automatic measurement technique of overlay, was studied for sub-half-micron device manufacturing. Experimental data showed that the offset error depended on the cross-sectional structure rather than the reflectivity or the roughness of the measurement marks. Dependence of the offset error upon equipment factors, e.g., the incident angle of illumination, was also studied. The depth of focus was considerably increased up to 1.5 mum on the LSI wafers with well-aligned light. This paper also shows an offset error on critical levels in sub-half-micron device manufacturing.
Definitions of centerline and centerline overlay (O/L) are built on the
implicit assumptions of certain symmetries. Verification of these
symmetries, and the use of redundances built into the design of O/L
measurement marks, can estimate the relative accuracy of O/L
measurements, even in the absence of certified reference materials
(standards). Verification carried out, as of paired comparison, is shown
to be very effective; in addition, they constitute the basis for data
culling. Several sources of O/L measurement inaccuracy, associated with
O/L measurement tools and with O/L measurement inaccuracy, associated
with O/L measurement tools and with O/L measurement marks, are pointed
out and some are illustrated using modeling. A simple measure of
tool-related inaccuracy, tool-induced shift (TIS), is proposed and
utilized as a performance estimate of O/L measurement tools relative to
accuracy. Inaccuracies of O/L measurements, caused by O/L measurement
tools and marks, on the order of 100 nm, are observed and their causes
identified. Examples are given of significant improvements in accuracy.
A roadmap toward accurate O/L measurement is outlined. This approach is
also applicable to alignment systems.