Si/SiGe heterostructures are gaining traction as a starting template in applications such as Gate-All-Around Field-Effect Transistor (GAAFET), complementary FET (CFET), and 3-dimensional dynamic random access memory (3D-DRAM), where the SiGe alloy plays the role of sacrificial material for channel release. However, the formation of crystalline defects (e.g. crosshatch) in the epitaxially grown layers plays a critical part in determining the overall device performance. As such, it is key to be able to control the defectivity level using large surface area inspection techniques. The challenge of such inspection is that it must combine a high enough throughput to detect low-density defects together with sensitivity to nanometer size defects. In addition, the technique should also be able to distinguish these elongated one-dimensional crystalline defects from other types of defects. In this study, we investigate the impact of the number of Si/SiGe bilayers on the crystal defect distribution utilizing a combined approach of optical inspection and extensive e-beam review for both qualitative and quantitative defect characterization. In-line optical inspection techniques revealed that the crosshatch density and distribution varied significantly with the number of Si/SiGe bilayers. These observations were then confirmed by high-resolution e-beam review coupled with image analysis and signal processing to enable crosshatch quantification. Our approach considers an initial investigation on thin Si/SiGe bilayers (up to ~5x bilayers) and is further extended to thick stacks (up to 60x bilayers) to evaluate the capability of optical inspection as the high-throughput reference technique. In conclusion, this study aims to develop a methodology to investigate the crosshatch density in Si/SiGe superlattices, using optical inspection and e-beam review as main characterization tools. These techniques offer valuable insights in terms of defect distribution at the wafer level for the design and fabrication of next-generation semiconductor devices.
In the manufacturing of CMOS devices, the golden standard for determining yield at various stages of production is electrical testing. This allows for the identification of failing devices or early yield failures before proceeding to subsequent steps. However, the failure of electrical tests can occur due to various reasons inherent to the structures of the devices. Additionally, a comprehensive analysis is necessary to ascertain the root cause of the failure mechanism once a device does not pass the electrical tests.
We have used large-field-of-view voltage contrast metrology to determine the design rules on a pitch 28 nm single-exposure extreme ultra violet dual damascene process, and to study a use case in which two design parameters, metal tip-to-tip critical dimension and via-to-line placement, interact nontrivially in the yield determination. By designing proper test structures, it is possible to determine the different failure mechanisms for the given process integration and determine the patterning cliffs and design rules.
Depth of focus reduction due to the increasing numerical aperture (NA) for High NA Extreme Ultraviolet (EUV) lithography and decreasing feature sizes of the latest process nodes necessitate smaller resist thicknesses. Reduced resist thickness degrades scanning electron microscope (SEM) image contrast significantly due to a lower signal-to-noise ratio (SNR). It is possible to improve SNR by changing the number of frames averaging or using higher resolution SEM images. However, these techniques limit high-throughput defect screening and can potentially impact the measurements due to electron beam damage. In this work, we present a deep-learning-based denoising method for sub-nm metrology. Power spectral density analysis of artificial intelligence (AI) reconstructed images shows the developed AI model is capable of denoising SEM images to provide comparable measurements such as line width roughness (LWR) that are only attainable with SEM images with higher SNR.
Tail CD was previously introduced as an empirical metric that correlates the CD distributions rather than mean CD to observed defect failure rates. It was found useful for the prediction of defect process windows, but also showed its limitation on the “merge” defect in dense hexagonal contact hole patterns. Results of a follow-up study are shown here to address this limitation by exercising the Tail CD concept on a new metric called ‘Wall CD’. The Wall CD showed orientation-depended performance that could be traced back to the illumination shape explaining the predominance of diagonal merges in the contact hole failure rates. Verification of the new approach demonstrated a significant improvement in the accuracy of the predicted defect process window. Furthermore, we utilized data analytics techniques to investigate the impact of additional parameters collected during the Wall CD extraction. This analysis demonstrated, in addition to the mean and variance, the importance of the higher statistical moments (skew, kurtosis) of the distributions for the prediction of defects and the relevance of incorporating further parameters into defect models.
3D topography imaging systems such as Atomic Force Microscopy (AFM) are used for surface characterization and metrology in numerous contexts especially when nanometer resolution is required (e.g. semiconductor industry & research). During the acquisition of an AFM image often a drift is present in vertical direction that is superimposed on top of the topography signal. This represents an artefact that cannot be removed with a single one-size-fits-all algorithm and typically requires manual input and expert assessment whether the correction is done appropriately. Hence, the final result is operator dependent.
In this work we propose a method to correct various artifacts that arise from vertical (Z) drift that can be regarded a superimposed envelope (ENV) on top of the true topography of the sample. We remove this envelope with the help of processing the raw image data with the help of Deep Neural Networks. Moreover, we employ a normalization scheme for pixel intensities for the preservation of absolute vertical height values for corrected images thus allowing for quantitative measurements of topography for metrology needs. Our approach allows for automatic and operator independent data correction, leading to more robust data analysis and interpretation, enabling faster speed of learning
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.
A method is proposed for fast CDSEM screening of defect process windows. The concept of Tail CD is used to build defect correlation trends from limited sampling. The trends are extrapolated towards larger ensembles to overcome the throughput limitation of the CDSEM. Application of the methodology on 3 different use cases (square and hexagonal contact hole arrays and L/S pattern) was demonstrated and limitations discussed. The impact of experimental conditions such as illumination, resist choice and etch was investigated. The predicted process windows were verified for selected dies to check the accuracy of the prediction, which showed good agreement.
CD-based process windows have been an analysis workhorse for estimating and comparing the robustness of semiconductor microlithography processes for more than 30 years. While tolerances for variation of CD are decreasing in step with the target CD size, the acceptable number of printed defects has remained flat (Hint: Zero) as the number of features increases quadratically. This disconnect between two key process estimators, CD variability and defect rate, must be addressed. At nodes that require EUV lithography, estimating the printed defects based solely on a Mean CD (“Critical Dimension”) process window is no longer predictive. The variability / distribution of the printed CDs must be engineered so that there are no failures amongst the billions of instances, rendering the Mean CD, often measured on just hundreds or thousands of instances, a poor predictor for outliers. A “defect-aware” process window, where the count of printed defects is considered in combination with more advanced statistical analysis of measured CD distributions can provide the needed predictability to determine whether a process is capable of sufficient robustness. Determining process robustness where stochastics and defects are taken into account can be simplified by determining the CD process margin. In this work we study dense contact hole arrays exposed with 0.33NA single exposure EUV lithography after both the lithography and etch steps. We describe a methodology for expanding the analysis of process windows to include more than the mean and 3σ of the data. We consider the skew and kurtosis of the distribution of measured CD results per focus-exposure condition and compare / correlate the measured CD process window results to the CD process margin.
Deep-learning-based SEM image denoiser
Dorin Cerbu1, Sandip Halder1, Philippe Leray1
1IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
We report the development of a new method to denoise SEM images with the help of artificial neural networks. Upon using a preprocessing and training scheme tailored for SEM images of structures, most often encountered in semiconductor manufacturing, we can efficiently denoise images affected with varying degrees of noise severity and origin. In the figure below, we show an example of how we can use this filter efficiently to treat noisy images and improve the image quality. This can help in acquisition of more stable and better metrology data.
Fig1(a) original image (b) Image which has been denoised using deep-learning based algorithms
This development is of utmost importance for the case of post-litho processing step where resist nanostructures when SEM inspected are usually impacted by the electron beam and shrink, hence skewing critical dimension measurements. This is especially true as we push towards sub N-10 nm nodes. Application of our deep-learning processing scheme allows efficient noise reduction on SEM inspection images and helps us discern minor details previously shadowed by noise. This is extremely important as we move towards using EUV in high volume manufacturing. Small details can be crucial to understand the root-cause of stochastic and process defects. In previous work, we have already shown different approaches to understand stochastic defects [1-2]. The goal of this work is to enhance the image quality as much as possible to gain further fundamental understanding on nano-defects.
[1] S. Halder et. al., ‘Using machine learning techniques to understand EUV stochastics, SPIE Photomask Technology + Extreme Ultraviolet Lithography, 2018
[2] K. Sah et.al., ‘EUV stochastic defect monitoring with advanced Broadband optical wafer inspection and e-Beam review systems’, SPIE Photomask Technology + Extreme Ultraviolet Lithography, 2018
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