Phenomenology of electron-beam-induced photoresist shrinkage trends

Abstract

For many years, lithographic resolution has been the main obstacle in keeping the pace of transistor densification to meet Moore's Law. For the 45 nm node and beyond, new lithography techniques are being considered, including immersion ArF (iArF) lithography and extreme ultraviolet lithography (EUVL). As in the past, these techniques will use new types of photoresists with the capability to print 45 nm node (and beyond) feature widths and pitches. In a previous paper [1], we focused on ArF and iArF photoresist shrinkage. We evaluated the magnitude of shrinkage for both R&D and mature resists as a function of chemical formulation, lithographic sensitivity, scanning electron microscope (SEM) beam condition, and feature size. Shrinkage results were determined by the well accepted methodology described in ISMI's <i>CD-SEM Unified Specification</i> [2]. A model for resist shrinkage, while derived elsewhere [3], was presented, that can be used to curve-fit to the shrinkage data resulting from multiple repeated measurements of resist features. Parameters in the curve-fit allow for metrics quantifying total shrinkage, shrinkage rate, and initial critical dimension (CD) from before e-beam exposure. The ability to know this original CD is the most desirable result; in this work, the ability to use extrapolation to solve for a given original CD value was also experimentally validated by CD-atomic force microscope (AFM) reference metrology. Historically, many different conflicting shrinkage results have been obtained among the many works generated through the litho-metrology community. This work, backed up by an exhaustive dataset, will present an explanation that makes sense of these apparent discrepancies. Past models for resist shrinkage inherently assumed that the photoresist line is wider than the region of the photoresist to be shrunk [3], or, in other words, the e-beam never penetrates enough to reach all material in the interior of a feature; consequently, not all photoresist is affected by the shrinkage process. In actuality, there are two shrinkage regimes, which are dependent on resist feature CD or thickness. Past shrinkage models are true for larger features. However, our results show that when linewidth becomes less than the eventual penetration depth of the e-beam after full shrinkage, the apparent shrinkage magnitude decreases while shrinkage speed accelerates. Thus, for small features, most shrinkage occurs within the first measurement. This is crucial when considering the small features to be fabricated by immersion lithography. In this work, the results from the previous paper [1] will be shown to be consistent with numerically simulated results, thus lending credibility to the postulations in [1]. With these findings, we can conclude with observations about the readiness of SEM metrology for the challenges of both dry and immersion ArF lithographies as well as estimate the errors involved in calculating the original CD from the shrinkage trend.