Diffraction order control in overlay metrology: a review of the roadmap options

Abstract

Resolution enhancement in advanced optical lithography will reach a new plateau of complexity at the 32 nm design rule manufacturing node. In order to circumvent the fundamental optical resolution limitations, ultra low k 1 printing processes are being adopted, which typically involve multiple exposure steps. Since alignment performance is not fundamentally limited by resolution, it is expected to yield a greater contribution to the effort to tighten lithographic error budgets. In the worst case, the positioning budget usually allocated to a single patterning step is divided between two. A concurrent emerging reality is that of high order overlay modeling and control. In tandem with multiple exposures, this trend creates great pressure to reduce scribeline target real estate per exposure. As the industry migrates away from metrology targets formed from large isolated features, the adoption of dense periodic array proxies brings improved process compatibility and information density as epitomized by the AIM target 1 . These periodic structures enable a whole range of new metrology sensor architectures, both imaging and scatterometry based, that rely on the principle of diffraction order control and which are no longer aberration limited. Advanced imaging techniques remain compatible with side-by-side targets while scatterometry methods require grating-over-grating targets. In this paper, a number of different imaging and scatterometry architectures are presented and compared in terms of random errors, systematic errors and scribespace requirements. It is asserted that an optimal solution must combine the TMU peak performance capabilities of scatterometry with the cost of ownership advantages of target size and multi-layer capabilities of imaging. INTRODUCTIONBrightfield imaging has been the mainstay sensor architecture of overlay metrology since the inception of automated methods in the 1980s. Throughout this period, the vast majority of measurements have been performed on so called boxin-box targets, which can be conveniently described as "large and isolated" test structures. By virtue of their smoothly varying centralized pupil distribution functions, such structures present well understood imaging challenges to the metrology tool designer. However, the features of interest for which today's advanced processes are optimized and the distances between them are rapidly shrinking away from the lumbering dimensions of these structures. This is burdening the process engineer with major compatibility challenges between metrology and device structures.At the outset of the 32 nm design rule era, these trends are driving the emergence of a whole new generation of overlay metrology structures, which are at various stages of industry adoption. The majority of these new structures differ fundamentally from their box-in-box predecessors in the crucial metrics of information content and feature density. They are enabled by replacing isolated structures from two patterning steps with periodic ones, either side-by-side or gratin...

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