Contour detection of an object is a fundamental computer vision problem in image processing domain. The goal is to find a concrete boundary for pixel ownership between an OOI (object-of-interest) and its corresponding background. However, contour extraction from low SN SEM images is a very challenging problem as different sources of noise shadow the estimation of underlying structural geometries. As device scaling continues to 3nm node and below, the extraction of accurate CD contour geometries from SEM images especially ADI (after developed inspection) is of utmost importance for a qualitative lithographic process as well as to verify device characterization in aggressive pitches. In this paper, we have applied a U-Net architecture based unsupervised machine learning approach for de-noising CD-SEM images. Unlike other discriminative deep-learning based de-noising approaches, the proposed method does not require any ground-truth as clean/noiseless images or synthetic noiseless images for training. Simultaneously, we have also attempted to demonstrate how de-noising is helping to improve the contour detection accuracy. We have analyzed and validated our result by using a programmable tool (SEMSuiteTM) for contour extraction. We have de-noised SEM images with categorically different geometrical patterns such as L/S (line-space), T2T (tip-to-tip), pillars with different scan types etc. and extracted the contours in both noisy and de-noised images. The comparative analysis demonstrates that de-noised images have higher confidence contour metric than their noisy twins while keeping the same parameter settings for both data input. When the ML algorithm is applied, the contour extraction results would have higher confidence numbers comparing with the ones only applied the conventional Gaussian or Median blur de-noise method. The final goal of this work is to establish a robust de-noising method to reduce the dependency of SEM image acquisition settings and provide more accurate metrology data for OPC calibration.
CD-SEM images inherently contain a significant level of noise. This is because a limited number of frames are used for averaging, which is critical to ensure throughput and minimize resist shrinkage. This noise level of SEM images may lead to false defect detections and erroneous metrology. Therefore, reducing noise in SEM images is of utmost importance. Both conventional noise filtering techniques and recent most discriminative deep-learning based denoising algorithms are restricted with certain limitations. The first enables the risk of loss of information content and the later mostly requires clean ground-truth or synthetic images to train with. In this paper, we have proposed an U-Net architecture based unsupervised machine learning approach for denoising CD-SEM images without the requirement of any such ground-truth or synthetic images in true sense. Also, we have analysed and validated our result using MetroLER, v2.2.5.0. library. We have compared the power spectral density (PSD) of both the original noisy and denoised images. The high frequency component related to noise is clearly affected, as expected, while the low frequency component, related to the actual morphology of the feature, is unaltered. This indicate that the information content of the denoised images was not degraded by the proposed denoising approach in comparison to other existing approaches.
As we are stepping towards sub-10 nm nodes, process window monitoring for systematic defects is becoming more and more critical. In traditional process window excursion and control (PWEC) methods often optical defect inspection is done on a focus and dose modulated wafer first. Once the different systematic defects are detected in a particular focus/energy die, we flag the repeating defect locations as potential hotspots and rank them based on how early/late they fail in a focus/energy modulated columns. So, during this first pass we get a rough idea of which locations are failing. However, due to limited resolution of optical tools, the true process window can only be gathered during a second pass with an ebeam tool. The key idea to define a true process window demands a detailed analysis of CD and other underlying features. We have proposed a new method of analyzing the process window with an unsupervised machine learning approach. Our proposed algorithm will extract the underlying key features and encode these to latent feature vectors or latent vector space instead of the conventional CD, given a dataset of thousands of CD-SEM images, and then rank the images based on a similarity index and then to automatically determine the process window. This work addresses the following problems (1) with a defect inspection tool this task seems tedious and time consuming and often require human intervention to analyze a large number of features, (2) a CD-SEM based process window analysis might not always match with a defect inspectionbased process window. Our generalized variational auto-encoder based approach does this automatically. Also, we have analyzed and validated our result against conventional approach.
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