Results of recent Investigations uncovering significant errors in overlay (O/L) measurements are reported. The two major contributors are related to the failures of symmetry of the overlay measurement tool and of the mark. These may result In measurement errors on the order of 100 nm. Methodology based on the conscientious verification of assumptions of symmetry is shown to be effective in identifying the extent and sources of such errors. This methodology can be used to arrive at an estimate of the relative accuracy of the O/L measurements, even in absence of certified O/L reference materials. Routes to improve the accuracy of O/L measurements are outlined and some examples of improvements are given. Errors in O/L measurements associated with the asymmetry of the metrology tool can be observed by comparing the O/L measurements taken at 0 and 180 degree orientations of the sample in reference to the tool. Half the difference of these measurements serves as an estimate of such tool related bias in estimating O/L. This is called tool induced shift (TIS). Errors of this kind can be traced to asymmetries of tool components, e. g., camera, illumination misalignment, residual asymmetric aberrations etc. Tool asymmetry leads to biased O/L estimates even on symmetric O/L measurement marks. Its impact on TIS depends on the optical properties of the structure being measured, the measurement procedure and on the combination of tool and sample asymmetries. It is also a function of design and manufacture of the O/L metrology tool. In the absence of certified O/L samples, measurement accuracy and repeatability may be improved by demanding that TIS be small for all tools on all structures.

Date Published: 1 June 1990
PDF: 23 pages
Proc. SPIE 1261, Integrated Circuit Metrology, Inspection, and Process Control IV, (1 June 1990); doi: 10.1117/12.20042

Published in SPIE Proceedings Vol. 1261:
Integrated Circuit Metrology, Inspection, and Process Control IV
William H. Arnold, Editor(s)