With the machine learning breakthroughs in the past few years, the number of studies applying this principle to lithography steps is increasing constantly. In this article, the focus does not concern the learning models for OPC masks improvement, but the optimization of the data used for such learning. This part is essential for a good learning process, but has rarely been studied, despite its impact on the output results quality being as important as an improvement of the learning model. Several optimization methods are discussed, each with a specific objective: either reducing learning time, increasing the obtained results quality, or both. To evaluate these different results, classical optical proximity correction simulation tools are used, allowing for a complete evaluation in line with production standards.
To get better and more stable performance in the mix-and-match of scanners in the fab, the matching of the illumination between those scanners is a must-have for HVM ecosystems. “Traditional” methods have been developed throughout the past years to characterize and correct for any dematching of illumination between tools. In the case of a fast growing fab the latter is not viable anymore because the flow is not automated, and the measurement acquisition methods are not robust enough to mitigate long and fastidious manual intervention of the engineers. In the paper, after reminding the legacy method, we will explore the way of using contour-extracting software to improve quality, runtime, and automation of the full analysis flow.
The work will be divided in three parts:
- Improve data collection quality and get robust measurements
- Set an automated flow based on a contour-extraction software for post-treatment of SEM images and contour analysis
- Automatize all the flow to decrease the time between test wafer exposure and validation of the matching
This paper presents contour-based methods to assess mask variability. Mask certification depends on the measurement reliability and on criteria relevance. By now, ST and its maskshop partners rely mostly on CDSEM measurements for mask certification. However, this kind of metrology has limitations and, looking at the future, we think it would be timely to search for metrology which bypass those limitations. That is why we are looking at 2D metrology [1], especially to area and contour measurements [2] on SEM images using extracted contours. Thanks to the added value of 2D metrology, we expect to assess mask variability, mask uniformity and pattern fidelity. We also take the opportunity to compare the results on two FOVs (field of view) from the images provided by mask shops. Finally, we also intend to automate the whole measurement process to make it easier to use.
The idea to better use the thousands of CD-SEM (critical dimension scanning electron microscope) images that are acquired daily in a fab has been a driver since few years now [1,2]. In previous publications, we advocated for a remote image computing toolbox integration into the wafer fab. Such system would be based on an image computing environment (outside the metrology tools and outside the wafer fab) connected to databases and data-analytic tools so that CD-SEM image information could be enriched by the usage of more complex image treatment tool such as a CNN (convolutional neural network) for qualitative information or image analysis and contour extraction for quantitative and qualitative information. Contour based metrology offers a wide range of “new” measurements capability solutions [2,3,4] which are of interest for the process support (R&D or Production) but also for metrology equipment owners. Expressing the added value of a remote metrology toolbox on top of the existing one from metrology tool themselves is not necessarily an easy task. It is often related to use cases that will really show good return on investment. Many examples of what can be achieved with contour metrology have been published, even with thousands of images computed, but it remains far from being seen as production worthy if not experienced in real time. Based on encouraging preliminary results on valuable data extracted using remote metrology software, a proof-of-concept integration on manufacturing data was put in place in order to gather direct feedback to the fab. This paper will describe how an image computing software has been integrated to the data infrastructure of our wafer fab so that every CD-SEM image is being processed outside the CD-SEM equipment to monitor the image quality on one side (so the CD-SEM tool stability) and perform measurements on the other side. Results extracted from inline contour-based image computing over several months’ worth of data will be presented showing some of the benefits of integrating edge computing solutions in wafer fab production flow.
Overlay is a critical parameter for any semi-conductor foundry, with a direct impact on the fabrication yield, on the quality, and on the performances of the product. Being able to full the overlay constraints has also a direct in influence on the capacity to scale down and to integrate vertically. In addition, the shift of semiconductor applications into more demanding markets such as spatial and automotive leads to higher specifications for the process control. In the semiconductor manufacturing, the overlay is usually measured optically using dedicated targets in the scribe lines. However, targets differ from the product by their dimensions of an order of magnitude larger and by their position up to a few millimeters far from it. This can lead to residual errors and mismatches in the correction sent to the scanner, thus lowering the fabrication yield and the global product quality. For years, many SEM in-device overlay techniques have been published, however they are generally related to new generation of SEM imaging and metrology equipment and require a new dedicated target. In our case, the ambition is to extend the usage of our current CD-SEM tool park. To do so, an on-device and target-free overlay measurement process has been developed. It is based on sub-pixel contour extractions from CD-SEM images and on the use of the design. From a single image, contours of the two layers of interest are extracted from the SEM image and a custom algorithm calculates the overlay as a difference of the realigned design centroids. This dedicated algorithm allows extracting various overlay measurements from one image and enables a production friendly implementation. To evaluate the performances of this method, it has been applied on the patterns of a SRAM with scanner-programmed overlay. The measurements are compared to the conventional optical measurements. On a basis of thousands on-device measurements, the developed method showed a promising 95% rate of successful measurements. Good correlation between the optical model and the CD-SEM measured overlay on the SRAM has been demonstrated, reaching a coefficient of correlation of 0.99 on a pattern where conventional centroid or CD based overlay are limited. Finally, the flexibility of the method to measure various patterns with an ease of recipe creation is shown. Contour-based metrology offers the capability to extract highly valuable information from SEM images while keeping the layers differentiation. The proposed measurement process, being automated and requiring relatively low human inputs is a promising solution for a SEM on device overlay metrology suitable in a high-volume manufacturing environment.
Background: Stochastic effects stated as the ultimate limit of EUV lithography are widely studied by resist suppliers, mask and metrology tool fabricants. Aim: Although the phenomenon is less explicitly visible, we want to highlight its presence in DUV through variabilities in critical dimension (CD), in shape and in position. These manifestations are not yield killers but may become reliability killers, if combined with some local overlay. Approach: Contour extractions are done from CD-SEM images to enable in-depth computational analysis of the data, to characterize local variabilities, alternatively to massive metrology solutions. This augmented computational metrology multiplies the volume of accessible CD data without requiring any supplementary tool purchasing, as they are already included within conventionally stored SEM images. Results: Contour extractions allowed to detect 5σ deviations from normal law on CD distribution from 1000 images. A set of complementary metrics quantities shape and size uniformities, as well as pattern displacement. It leads to a narrower process window where stochastic effects are minimized. Conclusion: Contour-based metrology offers complementary metrics, useful to characterize or center a process, increasing reliability by doing an extensive analysis of a fair number of CD-SEM images. Are these metrics capable of detecting early signs of process instability?
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