Background: Stochastic effects in DUV lithography are manifested by variabilities in critical dimension (CD), in placement or in shape. A combination of these very local variabilities can lead to yield killer open contacts. Traditionally, opens are massively measured with Voltage Contrast (VC) tools, returning the defects density after etching and metal filling. Aim: A set of contour-based metrics for the quantification of stochastic effects in DUV has already been presented. In this paper, we correlate these metrics and open count to predict failure risk. Approach: With an in-depth analysis of post-lithography CD-SEM images, we investigate if variabilities inside the metrology target are forerunners of open risk inside the product. It is challenging because of the difference between the surface inspected with defectivity tools and the one measured with CD-SEM. Results: We applied the methodology on contacts of a 28 nm node technology, on a Focus Exposure Matrix (FEM) wafer, to obtain post-lithography contour-based metrics mappings. A new metric has been computed: the classification of shapes inside the image. After post-processing, the correlations between contour-based metrics and the log value of open count are presented. A threshold value of size variability emerges above which open risk is too high, enabling process monitoring. Conclusion: As contour-based metrology offers complementary metrics not only related to CD metrology, we can now predict open probability with new indicators coming from traditional CD-SEM images. This early detection of an atypical situation allows the process assessment.
Over the past few years, patterning edge placement error (EPE), which combines information on variability of pattern sizes and placement between adjacent device layers, has been established as the key metric for patterning budget generation and holistic patterning control. More recently, the emergence of high-throughput SEM tools that provide inspection and large-volume CD metrology capabilities has enabled unprecedented statistical analysis of on-product pattern variability.
In the current paper we address edge placement budget generation as well as potential for improved patterning control for an HVM use case at the 28nm litho node. Edge placement and possible related defect mechanisms arise most critically at the contact layer, where contact hole patterning and EPE, with respect to both underlying gate and active layers need to be well controlled. At the 28nm node and for automotive applications, variability control within 5-sigma, i.e. to failure rates below 1 ppm, is generally required to ensure device reliability.
To support generation of an EPE budget by wafer data that captures inter and intra-field components, including local stochastic variations, we use a high-throughput, large field-of-view SEM tool from Hermes Microvision, at all three process layers of interest, as well as YieldStar metrology for overlay characterization. The large volume of data being made available -tens of millions of individual CD measurements- allows mapping out the low-probability ends of variability distributions and detecting non-Gaussian ‘fat tails’ indicative of defect rates that would be underestimated by 3-sigma estimates. Data analysis includes decomposing the total pattern variations into sources of variability, such as global CDU, mask variations and local stochastics. In addition to established CD metrology, we apply novel SEM image based analysis of repetitive patterns in SRAM arrays to generate 2-dimensional process variability bands, including estimates of pattern placement. This approach allows to investigate in detail the probabilistic interaction between active, gate and contact layers.
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