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Fast, Non-Contact, Wafer-Scale, Atomic Layer Resolved Imaging of 2D Materials by Ellipsometric Contrast Micrography Philipp Braeuninger-Weimer, Sebastian Funke, Ruizhi Wang, Peter Thiesen, Daniel Tasche, Wolfgang Viöl, and Stephan Hofmann ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04167 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018
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ACS Nano
Fast, Non-Contact, Wafer-Scale, Atomic Layer Resolved Imaging of 2D Materials by Ellipsometric Contrast Micrography Philipp Braeuninger-Weimer1*+, Sebastian Funke2*, Ruizhi Wang1, Peter Thiesen2, Daniel Tasche3, Wolfgang Viöl3, Stephan Hofmann1+ 1 Department of Engineering, University of Cambridge, Cambridge CB3 0FA, UK 2 Accurion GmbH, Stresemannstraße 30, Göttingen, Germany 3 University of Applied Sciences and Arts, Faculty of Natural Sciences and Technology, VonOssietzky-Straße 99, Göttingen, Germany * These authors contributed equally to the manuscript
Abstract Adequate characterisation and quality control of atomically thin layered materials (2DM) has become a serious challenge particularly given the rapid advancements in their large area manufacturing and numerous emerging industrial applications with different substrate requirements. Here, we focus on ellipsometric contrast micrography (ECM), a fast intensity mode within spectroscopic imaging ellipsometry, and show that it can be effectively used for non-contact, large area characterisation of 2DM to map coverage, layer number, defects and contamination. We demonstrate atomic layer resolved, quantitative mapping of chemical vapour deposited graphene layers on Si/SiO2-wafers, but also on rough Cu catalyst foils, highlighting that ECM is applicable to all application relevant substrates. We discuss the optimisation of ECM parameters for high throughput characterisation. While the lateral resolution can be less than 1 µm, we particularly explore fast scanning and demonstrate imaging of a 4’’ graphene wafer in 47 min at 10 µm lateral resolution, i.e. an imaging speed of 1.7 cm2/min. Furthermore, we show ECM of mono-layer hexagonal BN (h-BN) and of hBN/graphene bilayers, highlighting that ECM is applicable to a wide range of 2D layered structures that have previously been very challenging to characterise and thereby fills an important gap in 2DM metrology.
Keywords: ellipsometry, graphene, h-BN, 2D material characterisation, chemical vapour deposition, wafer-scale mapping +
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Chemical vapour deposition (CVD) has become the dominant technique for the crystal growth of graphene and related 2D materials (2DM), driven by the emerging industrial demand for “electronic-grade” materials over large areas.1–4 Continuous graphene films are now routinely produced roll-to-roll by CVD and the development for large area manufacturing of other 2DM is progressing rapidly.5–8 Adequate characterisation and quality control of asgrown atomically thin materials has become a serious challenge for such scaled 2DM manufacturing and industrial applications. Given the limitations of each method in terms of resolution, signal, area and throughput, even basic structural 2DM characterisation relies on a combination of different techniques.9–12 A major constraint for integrated 2DM metrology is that most characterisation techniques require a specific substrate to characterise the 2DM layer adequately, thus usually 2DM transfer is required.11,13–15 However, in particular over large areas, 2DM transfer is a challenge in itself.3,4,16,17 This not only makes characterisation laborious, but the transfer and new substrate can significantly affect the 2DM properties.18–22
Optical microscopy is widely used to map 2DM coverage, layer number and contamination. However, its use is limited to contrast enhancing substrates.14,15,23,24 For a wide range of application relevant substrates the 2DM cannot be seen directly with sufficient contrast. Raman spectroscopy equally can be challenging on many substrates, such as widely used catalyst metals,25–27 and the comparatively slow acquisition times limit high-throughput mapping of large (> 1mm2) areas. In addition, 2DM like h-BN intrinsically do not exhibit a rich Raman signature, so the information gained can be limited and/or convoluted.15,28 Fluorescence quenching microscopy (FQM) is more flexible regarding the substrate, but requires coating of the 2DM with a fluorescent dye.29–31 Similarly limited is the analysis of 2DM film structure and domain size via the application of liquid crystal (LC) coatings.32–35 THz time-domain spectroscopy is emerging as a valuable tool to characterise the electrical properties of graphene over large areas. It has, however, a relatively large spot size (> 100 µm) and is not compatible with conductive substrates such as metal catalysts or highly doped Si wafers.11,13,36–38 Characterising 2DM is particularly challenging on polycrystalline metal foils which are widely used as CVD catalyst and whose surface can be macroscopically very rough (Ra ~300 nm) and not uniform.39–41 Currently there are no techniques to adequately characterise 2DM without further treatment over large areas directly on such CVD growth substrates. Hence, there is a clear need for high throughput, non-destructive, large-area 2DM characterisation that is compatible with any substrate, and that can be effectively used for every step of 2DM integrated manufacturing. Here we address this characterisation gap for 2DM by exploring the use of a fast intensity mode within spectroscopic imaging ellipsometry, referred to as ellipsometric contrast Page 2 of 20 ACS Paragon Plus Environment
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ACS Nano
micrography (ECM) mode. Ellipsometry is a widely used non-contact optical method that measures the change in light polarization upon reflection. Since the signal in ellipsometry uses the entire polarization state of light, of which intensity is only one parameter, it offers superior contrast to purely intensity based methods like optical microscopy.42,43 For 2DM this is of particular importance as the atomically thin layers only absorb a small fraction of incident light. Conventional spectroscopic ellipsometry has been widely used to explore the optical characteristics of flakes of graphene and other 2DM on flat wafer substrates42–49 and also as an in-situ monitoring tool.50 With imaging ellipsometry a lateral resolution of less than 1 µm can be obtained.51 Small area (4 layer areas.
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ECM can easily be extended to wafer scale mapping of 2DM. To demonstrate that ECM is suitable to characterise 2DM on wafer scale we transferred a graphene monolayer on a native 4’’ diameter Si wafer. We note that graphene on a native Si wafer only shows very little contrast and analysis by optical microscopy is unsuitable.14,23 ECM was performed with a 2X objective (λ = 550 nm, AOI = 60°, P = 180°, A = 125°). A resolution of 10 µm can be obtained at this magnification. The acquisition time to image the entire 4’’ wafer was 47 min. Figure 5 shows the resulting image which is comprised of several hundred micrographs. The graphene covered area in the centre can be clearly distinguished. The inset of Figure 5 shows an enlarged view, where even small holes (
