Process-window sensitive full-chip inspection for design-to-silicon optimization in the sub-wavelength era

Abstract

As lithographers continue to implement more exotic and complex Resolution Enhancement Techniques (RET) to push patterning further beyond the physical limits of optical lithography, full-chip brightfield inspections are becoming increasingly valuable to help identify random and systematic defects that occur due to mask tolerance excursions, OPC inaccuracies, RET design errors, or unmanufacturable layout configurations. PWQ, or Process Window Qualification, is a KLA-Tencor product* using brightfield imaging inspection technology that has been developed to address the need for rapid full-chip process window verification. PWQ is currently implemented at IBM's 300mm facility and is being used to isolate features that repeatedly fail as a function of exposure dose and focus errors. We will demonstrate how PWQ results have assisted in: 1) qualification of reticles and new OPC models; 2) identification of non-obvious lithographic features that limit common process windows; 3) providing input for long-term design for manufacturability (DfM), OPC, and/or RET modeling. PWQ allows full or partial chips to be scanned in far less time than a multi-point common process window collected on a SEM. PWQ findings supplement these traditional analysis methods by encompassing all features on a chip, providing more detail on where the process window truly lies. Examples of marginal features that were detected by PWQ methods and their subsequent actions will be discussed in this paper for an advanced 65nm and a 90nm CMOS process.

Optical proximity correction: A cross road of data flows

“…For a long time bright-field imaging "optical" inspection (using light in the DUV wavelength range) was used for process window qualification (PWQ) of new masks. 10,11) Even though the smallest hotspots cannot be resolved at DUV wavelengths, as long as they generate a pixel-intensity difference when compared to an image at the same position in a different die, they flag the presence of a hotspot (at least if the defect is not systematically the same in all dies). PWQ compares dies exposed at off-nominal dose-focus conditions with dies exposed at nominal conditions, thus identifying hotspots that occur at the edges of the process window.…”

Process Window Centering for 22 nm lithography

“…Defects that appear at conditions close to nominal get a high priority and are reviewed. As a result, PWQ is able to detect even small defects caused by mask or OPC (optical proximity correction) and process marginalities [3]. Both the optimization of the recipe and the analysis are done manually by an experienced engineer and are time-consuming.…”

“…The most common lithography learning methods are FEM (Focus Exposure Matrix) [1,2] and PWQ (Process Window Qualification) [2][3][4][5]. Traditionally FEM utilizes a few locations per die to define the best dose and focus based on CD (critical dimension) measurements analyzed on a Bossung plot.…”

Development and implementation of PWQ on patterned wafer darkfield inspection systems

“…Thus, PWQ typically utilizes a high-sensitivity broadband brighfield inspector, a methodology which has proven effective for identifying critical design errors. 2,3,4 Innovative techniques for darkfield patterned wafer inspection 5 have improved the defect detection capabilities of these tools on lithography layers. Recent studies showed that a darkfield imaging inspector provided the necessary sensitivity and noise suppression capabilities for litho process monitoring at high throughputs.…”