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Ellipsometry as optical metrology method for analysis of reference materials for nanoelectronics
(2024)
Electrical properties of materials at the nanoscale can be characterized using scanning microwave microscopes (SMM) and conductive atomic force microscopes (C AFM). However, the measurement results are difficult to compare since different setups and different reference standards are used. The development of new “out-of-lab” reference standards can contribute to the traceability and reliability of these scanning probe microscopy methods (SPM) and facilitate their broader industrial application.
In this study, we discuss the capability of optical methods such as ellipsometry for the characterization of existing and the development of new reference calibration samples for scanning microwave microscopy. Ellipsometry is a fast and non-destructive method, which enables very accurate determination of the layer thickness and the dielectric functions of the materials. Imaging ellipsometry is suitable for spatially resolved measurements when analyzing thin layers in microstructured samples.
We show how the electrical resistivity of indium tin oxide (ITO) layers in newly designed resistive calibration samples can be obtained from spectroscopic ellipsometric measurements. The extension of the measurement range into the mid-infrared region was necessary when analyzing ITO layers with low conductivity. This parameter was obtained by fitting a Drude function describing the absorption of the free carriers. The impact of the coating process conditions on the layer properties is discussed.
Imaging ellipsometry was applied for the characterisation of thin ITO and SiO2 layers in microstructured resistive and capacitance calibration kits. The uncertainties of determined layer thicknesses were specified according to standardized practice guides used in ellipsometry. We show how statistical fingerprint analysis of the measured ellipsometric transfer quantities can be used to validate the quality of potential reference materials for nano-electronics and to monitor the processing of structured samples.
Advanced Manufacturing and Advanced Materials have been identified by the European Commission as one of six Key Enabling Technologies (KETs), the full exploitation of which will create advanced and sustainable economies. Metrology is a key enabler for progress of these KETs. EURAMET, which is the association of metrology institutes in Europe, has addressed the vital importance of Metrology for these KETs through the support for the creation of a European Metrology Network for Advanced Manufacturing. The EMN for Advanced Manufacturing (AdvanceManu) was approved in June 2021 and held the formal kick-of meeting in October 2022. The EMN comprises both National Metrology Institutes (NMIs) from across Europe and other designated Institutes (Dis). The EMN is organized in three sections; Advanced Materials, Smart Manufacturing Systems and Manufactured components and products. The aim of the EMN is to engage with stakeholders in the field of Advanced Manufacturing and Advanced Materials (Large & SMEs, industry organisations, existing networks and academia) with the aim to prepare a Strategic Research Agenda (SRA) for Metrology for Advanced Manufacturing. In the shorter term, an orientation paper is aimed to be produce to in the context of the European Partnership for Metrology. In addition to the SRA, the EMN will establish knowledge and technology transfer and promotion plan. This includes leveraging the existing research results from the completed and running EMPIR JRP projects funded through EURAMET.
This presentation will outline the EMN for Advanced Manufacturing, describing the structures and goals, the route to the production of the SRA and the progress made to date identifying the key metrology challenges across the related Key Industrial Sectors (KICs). In particular, the presentation aims to inform the community on how to be involved in the shaping of the strategic research agenda for the future of Metrology for Advanced Manufacturing and Advanced Materials.
The progress of Advanced Manufacturing, which has been identified by the European Commission as a Key Enabling Technology (KET) for future economic and societal progress is strongly reliant on the development of metrology capabilities. EURAMET, the association of metrology institutes in Europe, has established metrology research programs to address the metrology requirements across a spectrum of different thematic areas. In order to leverage the benefits of these developments on the wider industrial landscape, a high-level coordination of the metrology community supporting the Advanced Manufacturing landscape is required. This coordination is aimed to be achieved by the establishment of European Metrology Networks (EMNs), which are intended by EURAMET to provide a sustainable structure for stakeholder engagement and support. The joint networking project 19NET01 AdvManuNet funded by EMPIR for 4 years, started in June 2020 and aims to accelerate the process of establishing an EMN to strengthen Europe’s position in Advanced Manufacturing. The AdvManuNet project aims to support the establishment of an EMN on Advanced Manufacturing via the following specific aims:
1. Creation of a single hub for stakeholder engagement across the landscape of various industrial sectors including relevant societies and standardization bodies.
2. Development of a Strategic Research Agenda (SRA) and roadmaps for Advanced Manufacturing metrology based on the stakeholder engagement activities, considering current gaps in metrological capabilities existing networks and roadmaps.
3. Establish a knowledge-sharing program for Advanced Manufacturing stakeholders, promoting the dissemination and exploitation of the results of the project, including those from previous EU funded research projects.
4. Development of a sustainable web-based platform and service desk for Advanced Manufacturing stakeholders to allow for easy access to European metrology capabilities and support the wider advanced manufacturing community with metrology-based requirements.
5. Develop a plan for a coordinated and sustainable European metrology infrastructure for Advanced Manufacturing via a European Metrology Network.
The project concept followed by the scope and definition of Advanced Manufacturing will be described. The analysis of the current capability of metrology for Advanced Manufacturing and the preliminary concepts for the strategic research agenda will be presented with a focus on dimensional metrology.
In this work, Mie resonances in single Si nanoparticles (NPs) of different sizes have been systematically studied, using dark field (DF) and cathodoluminescence (CL) spectroscopy. An analytical method has been developed to compare experiment with theory.
Experimental CL spectra are averaged over entire Si NPs, allowing for direct comparison to DF spectra of identical NPs. Theoretical spectra clarify the assignment of Mie resonances within the NP which contribute with different intensity in DF and CL, resulting in an apparent spectral shift. Furthermore, a substrate effect appears. A 100 nm-Si NP on 15 nm SiN results in a broad peak, spectrally in between that of the calculated electric and magnetic dipole, a NP on 50 nm SiN exhibits two separated peaks as theoretically predicted. High spatial resolution of electron beam excitation allows to study the spectral CL changes at varying beam impact parameters. Theory and experiment agree that depending on beam position within a small Si NP, relative intensity of electric and magnetic dipole change; electric dipole vanishing in the center of the NP. Similar results are found for larger (d = 210 nm) Si NPs although the mode assignment is challenging as higher order modes appear and overlap with others.
In conclusion, comparison of CL and DF spectra is not trivial, in fact, excitation/radiation of distinct Mie resonances within a single Si NP are dependent on beam placement. However, substrate effects need to be considered in CL.
Cathodoluminescence (CL) spectroscopy has become a powerful tool to study nanostructures due to its high spectral and spatial resolution down to sub-nanometer. More recently, CL technique has also been used for second order auto-correlation measurements (g(2)(t)) to identify different single photon emitters and photon bunching in different materials [1-2].
In this work, tungsten disulfide (WS2) monolayers encapsulated in hexagonal boron nitride (hBN) with and without monocrystalline Au nanodisks (NDs) have been studied, using CL and PL spectroscopy as well as g(2)-CL- and PL-measurements.
CL and PL maps of different WS2 monolayers before/after Au ND deposition show a narrow peak at ~625 nm without any background emission. In CL, the hBN not only protects WS2 from the electron beam but also acts as a charge carrier sink which substantially increases the CL signal [3]. A further CL enhancement is achieved by Au ND deposition, exhibiting the maximum at the center of the NDs without any size dependence. The PL intensity is unaffected. This indicates that Purcell enhancement cannot be the underlying mechanism.
Furthermore, a giant CL-photon bunching of the hBN-encapsulated WS2 monolayers is found which is independent of the applied voltage but highly dependent on the electron beam current. At the lowest current of ~2 pA, a CL bunching factor of up to 160 is observed. Varying thicknesses of the surrounding hBN increases the overall CL signal but does not affect the bunching factor, though it exhibits small local changes within the same flake. In contrast, there is no PL correlation (g(2)(0) = 1).
Interestingly, this photon bunching can be further increased by Au NDs, resulting in the highest ever observed bunching factor of close to 2200. Once again, this enhancement is independent of the Au ND’s diameter although some disks show higher bunching factors than others. Most likely, the Au acts as shield for the incoming primary electrons, resulting in an even further decreased current, and thereby, increased bunching.
In conclusion, large CL-photon bunching is found in hBN-encapsulated WS2 monolayers which can be substantially enhanced by Au NDs.
References
[1] M.A. Feldmann, E.F. Demitrescu, D. Bridges, M.F. Chisholm, R.B. Davidson, P.G. Evans, J.A. Hachtel, A. Hu, R.C. Pooser, R.F. Haglund, B.J. Lawrie, Phys. Rev. B, 97, 081404(R) (2018)
[2] S. Meuret, L.H.G. Tizei, T. Cazimajou, R. Bourrellier, H.C. Chang, F. Treussartm M. Kociak, Phys. Rev. Letter, 114, 197401 (2015)
[3] S. Zheng, J.-K. So, F. Liu, Z. Liu, N. Zheludev, H.J. Fan, Nano Lett., 17, 6475-6480 (2017)
Influence of oxidation in the formation and performance of laser-induced periodic surface structures
(2022)
The fabrication of laser-induced periodic surface structures (LIPSS) has been reported for metallic, semiconductor and dielectric materials when irradiated with linearly polarized ultrashort laser pulses. Different sets of processing parameters have been identified and extensively studied, allowing the use of LIPSS for applications in medicine, tribology and surface wetting to name a few. One important parameter that is often not considered is the laser-induced oxidation that is produced especially when oxidation prone materials are irradiated in air environment. This surface oxidation can potentially influence the final structures
size, shape and orientation and modify their functionality in some specific applications.
In this work, we first study the influence of the laser-induced oxidation layer on the formation of LIPSS by characterizing the response of the oxidation prone
hard-coating material chromium nitride. Moreover, we employ complementary surface and depth-profiling analytic techniques to reveal morphological, chemical and structural features of different types of surface structures and LIPSS produced on titanium-based substrates. Finite-difference time-domain (FDTD) calculations finally explain how this superficial oxidation layer promotes the formation of a new type of low spatial frequency LIPSS. Also, we measure the depth-extent of the different oxides formed and demonstrate its beneficial influence for friction reduction and wear resistance applications.
Silicon is the material responsible for most of the technological developments during the past century, making it one of the most studied materials along different disciplines. However, there are still unturned stones regarding its superficial re-solidification after femtosecond laser-induced local melting. In this presentation, we report irradiation experiments with single femtosecond pulses (790 nm, 30 fs) with a spatially Gaussian distribution on two different types of silicon with orientations <111> and <100>. The surface modifications were studied in detail via different techniques, including optical microscopy, atomic force microscopy, spectroscopic imaging ellipsometry, energy dispersive X-ray spectroscopy and high-resolution transmission electron microscopy. We quantitatively estimate the resulting radial amorphous layer depth profiles with maximum thicknesses around some tenths of nanometers for fluences in between the melting and ablation thresholds. In particular, spectroscopic imaging ellipsometry (SIE) allowed fast data acquisition using multiple wavelengths to provide experimental measurements for calculating the nanometric radial amorphous layer thickness profiles with micrometric lateral resolution based on a thin-film layer model. SIE proved to be capable of detecting and measuring nanometric structural and chemical modifications (oxidation) on the studied laser spots. The accuracy of the SIE-based calculations is verified experimentally by characterizing an in-depth material lamella via high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (STEM-EDX). For completeness, we present a mathematical modelling for the melt layer thickness considering different optical absorption processes including one photon absorption, two photon absorption and free-carrier absorption, highlighting the relevance of the latter one in the femtosecond laser-induced melting of silicon.
Laser-induced oxide graded layers may contribute to the formation of a new type of embedded low-spatial frequency LIPSS with an anomalous orientation parallel to the laser polarization. In this contribution, we explore this effect experimentally with femtosecond laser pulses and numerically by finite-difference time-domain (FDTD) calculations.
Force-distance curves have been recorded on thin films of lubricants. The lubricants wet the AFM tip, which causes a capillary force. This force depends on the shape of the tip, as well as on physical properties like viscosity and surface tension, which have been determined additionally with other methods. This way, their influence on the shape of the curves could be analyzed.