HYBRID OPTICAL-ELECTRICAL OVERLAY TEST STRUCTURE

This paper describes the exploratory use of electrical test structures to enable the calibration of optical overlay instruments of the type used to monitor semiconductor-device fabrication processes, Such optic al instruments are known to be vulnerable to hard-to-determine systema tic process- and instrument-specific errors known as shifts.(1) Howeve r, these shift errors generally do not affect electrical test-structur e measurements extracted from the same features.(2) Thus the opportuni ty exists to configure physical standards having overlay that can be c ertified by electrical means, thereby enabling estimates of the shifts prevailing in a particular application, In this work, a new hybrid te st structure, meaning one from which overlay measurements can be extra cted electrically, as well as by optical instruments, has been designe d and fabricated with built-in overlay values ranging from -60 to +60 nm, A selection of structures constituting a test chip has been patter ned in a single conducting film with CD (critical dimension) design ru les ranging from 1.0 mu m to 2.0 mu m and fabricated and tested, Elect rical overlay parameters, derived from multiple step-and-repeat die-si te measurements, generally match the corresponding optical measurement s to within several nanometers, subject to limitations of the pattern- replication process, This paper focuses on the extraction of overlay f rom the electrical measurements, the dependence of the measurements on CD design rules, and their comparison with the corresponding measurem ents made both by a commercial optical-overlay instrument and by a coo rdinate-measurement system having measurements traceable to absolute d imensional standards, It is presented as a first step toward the use o f electrical measurements for advancing shift management in optical ov erlay of features patterned in separate lithographic processes.