Enabling on-device and target-free overlay measurement from CD-SEM contours

Bourguignon, Thibaut; Bouyssou, R.; Pradelles, Jonathan; Bérard-Bergery, Sébastien; Le-Gratiet, Bertrand; Bange, Romain; Schuch, Nivea G.; Figueiro, Thiago; Possémé, N.

doi:10.1117/12.2613327

Cited by 6 publications

(12 citation statements)

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“…Its working principle is briefly depicted Figure 2. 6 A good correlation with the overlay predicted by the optical targets located in the scribe lines has been previously demonstrated. However, a loss of sensitivity along one axis and a relatively high variability was observed when applied on SRAM patterns of production wafers.…”

Section: Introductionsupporting

confidence: 59%

“…The Figure 2: Global schematic of the developed metrology process for on-device target-less SEM overlay measurement. 6 process flow to generate the images is depicted Figure 4. The first step consists in generating a bi-layer layout file containing the patterns to be simulated.…”

Section: Generation Of Synthetic Images As a Referencementioning

confidence: 99%

“…In this section, synthetic SEM images are used to verify the hypothesis of the fixed edges as main contributors to the loss of sensitivity. 6 In both patterns studied in sections 3 and 4.1, fixed edges generated by gates were removed by a dilatation of the gates' contours followed by a boolean operation. This resulted in correlation slopes close to 1 for both cases.…”

Section: Fixed Edges and Measurement's Sensitivity On Sram Patternmentioning

confidence: 99%

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Contour based on-device overlay metrology assessment using synthetic SEM images

Bourguignon

Le-Gratiet²,

Pradelles³

et al. 2022

37th European Mask and Lithography Conference

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The shift of semiconductor industry applications into demanding markets as spatial and automotive led to high quality requirements to guaranty good performances and reliability in harsh environments. As reliability is directly related to a well controlled process, characterizing the local overlay and its variations inside the chip itself becomes a real asset. While most available in-chip overlay metrologies require dedicated target or dedicated tools, we developed a new method that aims to augment the current SEM tool park into measuring the local overlay directly on the product. In a previous proceeding, this on-device and target-free overlay measurement based on CD-SEM contours has been assessed on SRAM patterns and showed promising results. The work presented here pushes forward this assessment using SEM synthetic images generated from the open-source Nebula simulator of electron-matter interaction. From a layout, a 3D geometry of the measured pattern can be generated, with materials and interfaces carefully defined. Then, a GPU-accelerated Monte-Carlo model simulates in tens of seconds the SEM image. This fast generation of images enables the use of synthetic SEM images in a digital twin system : they can be used to characterize and to challenge the overlay metrology, before applying it to real products. Indeed, a known overlay can be programmed in these images. This way the performances of the measurement algorithm can be assessed with a ground truth reference. Firstly, imaging parameters such as pixel size and noise have been varied in a wide range. This demonstrated a good accuracy and precision inside a defined measurement window with a coefficient of correlation above 0.996 and an offset lower than 0.2nm. In a second part, the influence of the pattern measured has been investigated and experimental results on SRAM could be reproduced using synthetic images. The origin of the loss of sensitivity has been identified and improvements in the contour extractions and used template led to a correlation with a slope of 1.03, an offset of 0.1nm and a Root Mean Square Deviation of 1.36 nm. Finally, the developed digital twin already showed behaviors in the measurement that were hidden in the on-wafer experiments, that helped assessing the method and which will be used in the future to define guidelines for template based SEM-OVL measurements.

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Section: Introductionsupporting

confidence: 59%

Section: Generation Of Synthetic Images As a Referencementioning

confidence: 99%

Section: Fixed Edges and Measurement's Sensitivity On Sram Patternmentioning

confidence: 99%

See 1 more Smart Citation

Contour based on-device overlay metrology assessment using synthetic SEM images

Bourguignon

Le-Gratiet²,

Pradelles³

et al. 2022

37th European Mask and Lithography Conference

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“…The concept is depicted in the figure 3b. 5 After acquiring the images and retrieving the post-lithography or post-etch simulation, the contours of both layers of the desired pattern can be extracted. To do so, a dedicated algorithm has been recently developed for bilayer-contours extractions.…”

Section: Augmented Metrology: Cd-sem Overlaymentioning

confidence: 99%

“…Currently under development, it has been presented and assessed in previous works. 5,6 This paper investigates whether a dense overlay measurement highlights systematic overlay signatures that can be attributed to the layout. Non-zero overlay, local variability and overlay signatures at a chip level are discussed in order to better understand and optimize the process.…”

Section: Introductionmentioning

confidence: 99%

High-spatial frequency on-device overlay characterization using CD-SEM contours

Bourguignon

Gratiet

Pradelles

et al. 2023

Metrology, Inspection, and Process Control XXXVII

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The on-device and target-less overlay (OVL) measurement adds various challenges to the conventional target-based one.1 Most of the common solutions are based on the use of high voltage SEM to have a high resolution and see-through capabilities to see both layers entirely. To ensure repeatability and robustness the measurement is also generally realized on dedicated targets.2–4 Another approach is explored in this work : it consists of a direct measurement on the device itself, without any dedicated target and using a CD-SEM that do not have see-through capabilities. To avoid any damage on the final device and to ensure a partial vision of the bottom layer, the measurement is realized post etch. What can a true on-device, target-less, SEM overlay metrology reveal of the process? To answer this question, a custom design-based on-device CD-SEM overlay metrology was developed.5 It allows measuring the overlay on any bilayer 2D structure, even partially hidden, provided that sufficient edges are visible. Thanks to this post-processing capacity to measure on partially hidden patterns, the CD-SEMs from the current tool park can be used and non-transparent layers can be investigated. After demonstrating the capability of the previously mentioned solution on synthetic SEM images,6 by taking advantage of the versatility of the method and of an improved bilayer contour extraction algorithm, one product wafer was measured densely at different steps of the process on several patterns. These extensive measurements aim to reveal overlay signatures at different spatial frequencies: intra-field, intra-chip and even inside a random logic array. This paper details and interprets the observed overlay measurement with models while investigating the matching between the different measurement patterns and the mismatches with the optical overlay. To conclude, the contour-based metrology offers the capability to extract highly valuable information from CD-SEM images and represents a great opportunity for on-device overlay measurement. By measuring directly the products’ patterns, the developed metrology shows a mismatch between optical and SEM measurements up to a couple of nanometers. A hybrid model, mixing SEM and optical measurements reduced by 15% the on-product residuals. Additionally, the local overlay variability could be evaluated and low amplitude chip signatures have been observed. This new information will help process engineers to improve the process characterization, leading to a better process control that results in an improved reliability.

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Can remote SEM contours be used to match various SEM tools in fabs?

Pradelles

Perraud

Sezestre

et al. 2023

Metrology, Inspection, and Process Control XXXVII

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CD-SEM is one of the workhorse metrology tools for evaluating the patterning performances in the semiconductor industry. With the continuous shrink of device feature sizes, measurement uncertainty, of such devices should improve accordingly, including tool-to-tool matching. Great efforts are made in fabs to ensure matching between CD-SEMs. Basically, there are two approaches : tool matching based on statistical CD measurements and tool matching based on physical tool parameters. The first methods require long recipe run on each tool for data collection while the second methods require dedicated maintenance tool time. In parallel, the emergence of SEM contour metrology brought new opportunities, for example in OPC modeling, process assessment or overlay metrology. Recently, a step has been made to demonstrate that offline SEM contour metrology can be matched with standard CD-SEM. In this paper we evaluate the possibility to use SEM contour metrology to perform a remote CD matching on images coming from two different CD-SEM. A large set of patterns of various dimensions and shapes (both 1D and 2D) is imaged with two different CD-SEM after etch process step. For each image the contours are extracted using a remote software. A matching model is calibrated to match contour measurements of the same pattern imaged with the two CD-SEMs. The matching model can be calibrated using either one or both inline SEM measurement or using additional metrology data (typically AFM measurement or any reference metrology measurement). The matching model parameters include both parameters for the contour extraction calibration and measurement matching parameters. Two modeling approaches are compared : one matching model averaging multiple measurement matching sites and a multiple (per pattern type) contour calibrations/ measurement matching sites model. These different matching models are assessed using as main metrics the remaining CD difference between SEM contour measurements on images coming from the two CD-SEM (ΔCDmatching) and TMU analysis. We show that an overall ΔCDmatching variability of 0.6 nm and a TMU below 2 nm can be obtained with the dataset. We evaluated a strategy to achieve matching of two CD-SEM using a remote contour extraction solution and a matching model.

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Enabling on-device and target-free overlay measurement from CD-SEM contours

Cited by 6 publications

References 0 publications

Contour based on-device overlay metrology assessment using synthetic SEM images

Contour based on-device overlay metrology assessment using synthetic SEM images

High-spatial frequency on-device overlay characterization using CD-SEM contours

Can remote SEM contours be used to match various SEM tools in fabs?

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