On the Limits of Scanning Thermal Microscopy of Ultrathin Films
2. Materials and Methods
2.1. Atomic Force Microscopy (AFM) and Scanning Thermal Microscopy (SThM)
- AFM system: Bruker Dimension Icon ; operating software: NanoScope 9.1
- Thermal system: Bruker Vita Module  consisting of a power supply, a controller, a calibration box, a variable resistor box, the nano-TA calibration sample, and the software Bruker VITA studio v3.4
- Thermal probe: SThM probe Bruker VITA-DM-NANOTA-200 ; The thermal probe is made from crystalline silicon (Si) and can be heated repeatedly and reliably up to temperatures of 350 °C, according to manufacturer information. In the following, we use the term “probe” for the whole thermal probe (as it can be purchased), and the term “tip” only for the sharp area on the front side of the probe, which is directly touching the surface of the samples.
2.2. Samples and Materials under Investigation
- Test sample TGXYZ02 
- Copper iodide in zincblende structure (γ-CuI)
- Hexagonal boron nitride (h-BN).
3. Experimental Set-up
3.1. Probe Calibration
- Polycaprolactone (PCL; 55 °C)
- Polyethylene (PE; 116 °C)
- Polyethylene terephthalate (PET; 235 °C)
3.2. Useful Sample Positions of h-BN Sample
4. Results and Discussion
4.1. Demonstration of Topography Influences of SThM Using a γ-CuI Sample
4.2. Probe Temperature Dependency of the SThM Signal
- Mean thermal contrast: The thermal signals in each diagram in Figure 5 and Figure 6 were applied with an offset on the y-axis, so that the minimum is always zero. This is permitted, as the thermal signal represented by voltage is a potential unit. Hence, the mean value between the two maximum values (left and right of the step) is defined as “mean thermal contrast”. Those two measuring points define the statistic area and represent the local maxima of the thermal signal on top of the step where the difference quotients of first order change their signs. For the h-BN sample with 50 °C (red curve in Figure 5e) there are no corresponding maxima for which reason we use the same statistic area as the one from 100 °C.
- Standard deviation of the mean thermal contrast.
- Minimum thermal contrast: Minimum value within the statistic area.
- Median thermal contrast: Median value within the statistic area.
- Maximum thermal contrast: Maximum value within the statistic area.
4.3. Angle Dependency of the SThM Signal
- Touching angle: Angle between the tangent of the step and the middle plane of the tip, looking upside down according to Figure 8. We vary the touching angle by moving the middle plane of the tip by varying the y-positions of the cut lines within the software. The touching angle also increases with increasing y-position. The 90° case is the case of symmetry.
- Thermal contrast: Maximum value of the thermal signal for each curve in Figure 9c,d. The minimum value was set to zero, which is permitted, because the thermal signal might be interpreted as a potential represented by voltage.
- Absolute strength of super elevations: Highest value of each super elevation subtracted by the minimum value in the nearly constant area right of the super elevations. This evaluation was performed for all of the super elevations on the right side in Figure 9c,d.
- Thermal signals at vertical steps (especially occurring super elevations at the beginning and end of a vertical step) always have to be questioned critically as the thermal signal may not represent the correct local thermal conductivity here. We suggest ignoring the thermal signal near the vertical steps. The step heights in the present investigations varied from 15.5 nm (h-BN sample) to 120.0 nm (TGXYZ02 sample). In some measurements we also found that after passing a step the thermal signal is significantly greater than before, where it should be equal (h-BN sample: 0.2 V ( 50 °C), 0.3 V ( 100 °C), and 0.4 V ( 200 °C)). If present, this effect also has to be critically considered.
- SThM is a useful technique that is applied to ultra-flat surfaces but results according to samples with a high roughness (CuI sample had an Rq of 110 nm, whereas the h-BN sample had an Rq of 4.15 nm) must always be questioned critically. Samples with a super-flat surface without vertical steps are especially suitable for SThM technique. Between step angels of 90° and flat samples, in our current investigations, we were not able to estimate or deduce some kind of critical step angle with respect to the influence on the thermal signal. Further research needs to be done.
- The combination of low film thickness (<50 nm), high surface roughness (Rq > 50 nm), vertical steps, and high in-plane thermal conductivity (>200 Wm−1 K−1) makes the results more unsatisfying and more difficult. These are the main limiting factors of useful SThM measurements.
- Our investigations show that the thermal contrast depends on the probe temperature. This was predominantly the case for the ultrathin h-BN sample (ultrathin sample with a film thickness at or below 23 nm with a high in-plane thermal conductivity up to 2000 Wm−1 K−1). Therefore, for similar h-BN samples, we suggest a probe temperature of 200 °C and for samples similar to TGXYZ02 (step height 120.0 nm consisting of Si and SiO2) we suggest probe temperatures of 100 °C. Greater probe temperatures may also increase the thermal contrast, but lead to a higher mechanical damage of tip and sample. For other samples, researchers need to be aware of possible saturation effects and it might be necessary to find some kind of optimal probe temperature, depending on the sample.
- There is no indicator that the thermal contrast depends on the touching angle. In theory, we would expect a small dependency of the thermal contrast on the touching angle, as the touching areas vary depending on the touching angle. Our investigation with TGXYZ02 shows that this effect obviously is negligible. The thermal contrast decreased from 4.5 V to 2.0 V (retrace curves of TGXYZ02, submerged circles) and from 4.6 V to 2.3 V (trace curves of TGXYZ02, submerged circles) with an increasing touching angle, but did not show signs of axis symmetry to the 90° line. Additionally, the absolute and relative strength of super elevations did not depend on the touching angle. The relative strength of the super elevations varied from 0.77 to 1.25 (retrace curves of TGXYZ02, submerged circles) and from 0.74 to 1.04 (trace curves of TGXYZ02, submerged circles) and did not show signs of axis symmetry to the 90° line.
- Some of our thermal and topography signals showed a kind of ripple after passing a vertical step. The IC-AFM measurements confirmed that the ripples in topography are artefacts and, hence, we also suppose the ripple in the thermal signals to be artificial. The frequency of those ripples can be estimated between 5 Hz (topography signal of TGXYZ02) and 6.7 Hz (thermal signal of TGXYZ02). We suppose the natural frequency of the cantilever, the surface hardness of the sample, and the proportional gain to influence this ripple most but unfortunately, we were not able to explain the origin of the ripples sufficiently and, thus, further research is necessary.
- The course of the thermal signal of the h-BN sample suggests the thermal conductivity to decrease with increasing film thickness in our investigations. Absolute numbers of the local thermal conductivities cannot be obtained with the SThM system used in this work.
- Last but not least, we were able to successfully apply the SThM method to h-BN films at or below 23 nm, which has not been reported before (general h-BN thickness: 6.075 nm ± 1.125 nm; h-BN thickness on top of the vertical step: 21.575 nm ± 1.125 nm).
Conflicts of Interest
- Moore, G.E. Cramming more components onto integrated circuits, Reprinted from Electronics, volume 38, number 8, April 19, 1965, pp.114 ff. IEEE Solid State Circuits Soc. Newsl. 2006, 11, 33–35. [Google Scholar] [CrossRef]
- ITRS. International Technology Roadmap for Semiconductors. Available online: https://www.semiconductors.org/resources/2015-international-technology-roadmap-for-semiconductors-itrs/ (accessed on 29 October 2019).
- Nick, C. Mikrointegrierte Nanostrukturen mit hohem Aspektverhältnis als neuronale Schnittstelle. Ph.D. Dissertation, TU Darmstadt, Darmstadt, Germany, 2015. [Google Scholar]
- Cahill, D.G.; Ford, W.K.; Goodson, K.E.; Mahan, G.D.; Majumdar, A.; Maris, H.J.; Merlin, R.; Phillpot, S.R. Nanoscale thermal transport. Phys. Rev. B Condens. Matter 2003, 93, 793–818. [Google Scholar] [CrossRef][Green Version]
- Cahill, D.G.; Goodson, K.; Majumdar, A. Thermometry and Thermal Transport in Micro/Nanoscale Solid-State Devices and Structures. Phys. Rev. B Condens. Matter 2002, 124, 223. [Google Scholar] [CrossRef]
- Cahill, D.G.; Braun, P.V.; Chen, G.; Clarke, D.R.; Fan, S.; Goodson, K.E.; Keblinski, P.; King, W.P.; Mahan, G.D.; Majumdar, A.; et al. Nanoscale thermal transport. II. 2003–2012. Appl. Phys. Rev. 2014, 1, 11305. [Google Scholar] [CrossRef][Green Version]
- Choi, S.R.; Kim, D.; Choa, S.-H.; Lee, S.-H.; Kim, J.-K. Thermal Conductivity of AlN and SiC Thin Films. Int. J. Thermophys. 2006, 27, 896–905. [Google Scholar] [CrossRef]
- Fladischer, K.; Leitgeb, V.; Mitterhuber, L.; Maier, G.A.; Keckes, J.; Sagmeister, M.; Carniello, S.; Defregger, S. Combined thermo-physical investigations of thin layers with Time Domain Thermoreflectance and Scanning Thermal Microscopy on the example of 500 nm thin, CVD grown tungsten. Thermochim. Acta 2019, 681, 178373. [Google Scholar] [CrossRef]
- Heiderhoff, R.; Haeger, T.; Dawada, K.; Riedl, T. From diffusive in-plane to ballistic out-of-plane heat transport in thin non-crystalline films. Microelectron. Reliab. 2017, 76, 222–226. [Google Scholar] [CrossRef]
- Pylkki, R.J.; Moyer, P.J.; West, P.E. Scanning Near-Field Optical Microscopy and Scanning Thermal Microscopy. Jpn. J. Appl. Phys. 1994, 33, 3785–3790. [Google Scholar] [CrossRef]
- Majumdar, A. Scanning Thermal Microscopy. Annu. Rev. Mater. Sci. 1999, 29, 505–585. [Google Scholar] [CrossRef]
- Hammiche, A.; Pollock, H.M.; Song, M.; Hourston, D.J. Sub-surface imaging by scanning thermal microscopy. Meas. Sci. Technol. 1996, 7, 142–150. [Google Scholar] [CrossRef]
- Price, D.M.; Reading, M.; Hammiche, A.; Pollock, H.M. Micro-thermal analysis: Scanning thermal microscopy and localised thermal analysis. Int. J. Pharm. 1999, 192, 85–96. [Google Scholar] [CrossRef]
- Lehermeier, W. Thermal Characterization of Thin Film Materials. Master’s Thesis, Deggendorf Institute of Technology, Deggendorf, Germany, 2018. [Google Scholar]
- Metzke, C.; Benstetter, G.; Frammelsberger, W.; Weber, J.; Kühnel, F. Temperature Dependent Investigation of Hexagonal Boron Nitride Using Scanning Thermal Microscopy. In Proceedings of the 6th Nano Today Conference, Lissabon, Portugal, 16–20 June 2019. [Google Scholar]
- Metzke, C.; Lehermeier, W.; Benstetter, G.; Frammelsberger, W. Evaluation of Topography Effects of SThM Measurements on Thin Thermoelectric Films. In Proceedings of the 4th Edition Smart Materials and Surfaces – SMS Conference, Venedig, Italy, 23–25 October 2018. [Google Scholar]
- Sadeghi, M.M.; Park, S.; Huang, Y.; Akinwande, D.; Yao, Z.; Murthy, J.; Shi, L. Quantitative scanning thermal microscopy of graphene devices on flexible polyimide substrates. J. Appl. Phys. 2016, 119, 235101. [Google Scholar] [CrossRef]
- Shi, L.; Plyasunov, S.; Bachtold, A.; McEuen, P.L.; Majumdar, A. Scanning thermal microscopy of carbon nanotubes using batch-fabricated probes. Appl. Phys. Lett. 2000, 77, 4295–4297. [Google Scholar] [CrossRef]
- Ruiz, F.; Sun, W.D.; Pollak, F.H.; Venkatraman, C. Determination of the thermal conductivity of diamond-like nanocomposite films using a scanning thermal microscope. Appl. Phys. Lett. 1998, 73, 1802–1804. [Google Scholar] [CrossRef]
- Martinek, J.; Klapetek, P.; Campbell, A.C. Methods for topography artifacts compensation in scanning thermal microscopy. Ultramicroscopy 2015, 155, 55–61. [Google Scholar] [CrossRef][Green Version]
- Gomès, S.; Assy, A.; Chapuis, P.-O. Scanning thermal microscopy: A review. Phys. Status Solidi A 2015, 212, 477–494. [Google Scholar] [CrossRef]
- Mas-Ballesté, R.; Gómez-Navarro, C.; Gómez-Herrero, J.; Zamora, F. 2D materials: To graphene and beyond. Nanoscale 2011, 3, 20–30. [Google Scholar] [CrossRef]
- Balandin, A.A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C.N. Superior thermal conductivity of single-layer graphene. Nano Lett. 2008, 8, 902–907. [Google Scholar] [CrossRef]
- Sledzinska, M.; Graczykowski, B.; Placidi, M.; Reig, D.S.; Sachat, A.E.; Reparaz, J.S.; Alzina, F.; Mortazavi, B.; Quey, R.; Colombo, L.; et al. Thermal conductivity of MoS 2 polycrystalline nanomembranes. 2D Mater. 2016, 3, 35016. [Google Scholar] [CrossRef][Green Version]
- Zavaleyev, V.; Walkowicz, J.; Kuznetsova, T.; Zubar, T. The dependence of the structure and mechanical properties of thin ta-C coatings deposited using electromagnetic venetian blind plasma filter on their thickness. Thin Solid Films 2017, 638, 153–158. [Google Scholar] [CrossRef]
- Zubar, T.; Trukhanov, A.; Vinnik, D.; Astapovich, K.; Tishkevich, D.; Kaniukov, E.; Kozlovskiy, A.; Zdorovets, M.; Trukhanov, S. Features of the Growth Processes and Magnetic Domain Structure of NiFe Nano-objects. J. Phys. Chem. C 2019, 123, 26957–26964. [Google Scholar] [CrossRef]
- Bruker. Dimension Icon. Available online: https://www.bruker.com/products/surface-and-dimensional-analysis/atomic-force-microscopes/dimension-icon/overview.html (accessed on 29 October 2019).
- Bruker. Scanning Thermal Microscopy (SThM): The Resolution of AFM Coupled with Quantitative Thermal Analysis. Available online: https://www.bruker.com/products/surface-and-dimensional-analysis/atomic-force-microscopes/modes/modes/specialized-modes/sthm.html (accessed on 29 October 2019).
- Bruker. VITA-DM-NANOTA-200. Available online: https://www.brukerafmprobes.com/p-3701-vita-dm-nanota-200.aspx (accessed on 29 October 2019).
- Hui, F.; Lanza, M. Scanning probe microscopy for advanced nanoelectronics. Nat. Electron. 2019, 2, 221–229. [Google Scholar] [CrossRef]
- Bruker. RESP-20. Available online: https://www.brukerafmprobes.com/Product.aspx?ProductID=3929 (accessed on 29 October 2019).
- Born, A. Nanotechnologische Anwendungen der Rasterkapazitätsmikroskopie und verwandter Rastersondenmethoden. Ph.D. Thesis, Universität Hamburg, Hamburg, Germany, 2000. [Google Scholar]
- Frammelsberger, W. Improved Atomic Force Microscopy Based Techniques for Electrical and Structural Charcterisation of Thin Dielectric Films. Ph.D. Thesis, University of the West of England, Bristol, UK, 2006. [Google Scholar]
- Zhang, Y.; Zhu, W.; Hui, F.; Lanza, M.; Borca-Tasciuc, T.; Muñoz Rojo, M. A Review on Principles and Applications of Scanning Thermal Microscopy (SThM). Adv. Funct. Mater. 2019, 107, 1900892. [Google Scholar] [CrossRef][Green Version]
- Altes, A.; Heiderhoff, R.; Balk, L.J. Quantitative dynamic near-field microscopy of thermal conductivity. Meas. Sci. Technol. 2004, 37, 952–963. [Google Scholar] [CrossRef]
- Fiege, G.B.M.; Altes, A.; Heiderhoff, R.; Balk, L.J. Quantitative thermal conductivity measurements with nanometre resolution. Jpn. J. Appl. Phys. 1999, 32, L13–L17. [Google Scholar] [CrossRef]
- MikroMasch®. TGXYZ Series. Available online: https://www.spmtips.com/test-structures-TGXYZ-series.html (accessed on 29 October 2019).
- Yang, C.; Souchay, D.; Kneiß, M.; Bogner, M.; Wei, H.M.; Lorenz, M.; Oeckler, O.; Benstetter, G.; Fu, Y.Q.; Grundmann, M. Transparent flexible thermoelectric material based on non-toxic earth-abundant p-type copper iodide thin film. Nat. Commun. 2017, 8, 16076. [Google Scholar] [CrossRef]
- Yang, C.; Kneiβ, M.; Lorenz, M.; Grundmann, M. Room-temperature synthesized copper iodide thin film as degenerate p-type transparent conductor with a boosted figure of merit. Proc. Natl. Acad. Sci. USA 2016, 113, 12929–12933. [Google Scholar] [CrossRef][Green Version]
- Grundmann, M.; Schein, F.-L.; Lorenz, M.; Böntgen, T.; Lenzner, J.; von Wenckstern, H. Cuprous iodide: A p-type transparent semiconductor, history, and novel applications. Phys. Status Solidi A 2013, 210, 1671–1703. [Google Scholar] [CrossRef]
- Khan, M.H.; Liu, H.K.; Sun, X.; Yamauchi, Y.; Bando, Y.; Golberg, D.; Huang, Z. Few-atomic-layered hexagonal boron nitride: CVD growth, characterization, and applications. Mater. Today 2017, 20, 611–628. [Google Scholar] [CrossRef]
- Sevik, C.; Kinaci, A.; Haskins, J.B.; Çağın, T. Characterization of thermal transport in low-dimensional boron nitride nanostructures. Phys. Rev. B 2011, 84. [Google Scholar] [CrossRef][Green Version]
- Kumar, A.; Pal, D. Lattice Thermal Conductivity of Boron Nitride Crystals at Temperatures 1.5 to 300 K. Phys. Status Solidi B 1985, 129, K9–K12. [Google Scholar] [CrossRef]
- Ouyang, T.; Chen, Y.; Xie, Y.; Yang, K.; Bao, Z.; Zhong, J. Thermal transport in hexagonal boron nitride nanoribbons. Nanotechnology 2010, 21, 245701. [Google Scholar] [CrossRef] [PubMed]
- Golberg, D.; Bando, Y.; Huang, Y.; Terao, T.; Mitome, M.; Tang, C.; Zhi, C. Boron nitride nanotubes and nanosheets. ACS Nano 2010, 4, 2979–2993. [Google Scholar] [CrossRef] [PubMed]
- Giovannetti, G.; Khomyakov, P.A.; Brocks, G.; Kelly, P.J.; van den Brink, J. Substrate-induced band gap in graphene on hexagonal boron nitride: Ab initio density functional calculations. Phys. Rev. B 2007, 76. [Google Scholar] [CrossRef][Green Version]
- Ji, Y.; Pan, C.; Zhang, M.; Long, S.; Lian, X.; Miao, F.; Hui, F.; Shi, Y.; Larcher, L.; Wu, E.; et al. Boron nitride as two dimensional dielectric: Reliability and dielectric breakdown. Appl. Phys. Lett. 2016, 108, 12905. [Google Scholar] [CrossRef]
- Matović, B.; Luković, J.; Nikolić, M.; Babić, B.; Stanković, N.; Jokić, B.; Jelenković, B. Synthesis and characterization of nanocrystaline hexagonal boron nitride powders: XRD and luminescence properties. Ceram. Int. 2016, 42, 16655–16658. [Google Scholar] [CrossRef]
- Balmain, W.H. Bemerkungen über die Bildung von Verbindungen des Bors und Siliciums mit Stickstoff und gewissen Metallen. J. Prakt. Chem. 1842, 27, 422–430. [Google Scholar] [CrossRef]
|Temperature (°C)||55 ( of PCL)||116 ( of PE)||235 ( of PET)|
|Heating voltage (V)||2.90 ± 0.03||4.80 ± 0.03||6.78 ± 0.03|
|Probe Temperature (°C)||Statistic Area|
|Mean Thermal Contrast|
|Standard Deviation (V)||Minimum Thermal Contrast (V)||Median Thermal Contrast (V)||Maximum Thermal Contrast (V)|
|50||2.070–3.203 (n = 59)||0.184||0.041||0.087||0.202||0.227|
|100||2.070–3.203 (n = 59)||0.523||0.055||0.402||0.534||0.625|
|200||2.090–3.320 (n = 64)||1.195||0.145||1.027||1.138||1.647|
|Probe Temperature (°C)||Statistic Area|
|Mean Thermal Contrast (V)||Standard Deviation (V)||Minimum Thermal Contrast (V)||Median Thermal Contrast (V)||Maximum Thermal Contrast (V)|
|50||1.094–4.023 (n = 76)||0.667||0.410||0.145||0.512||1.499|
|100||1.094–4.063 (n = 77)||1.964||0.505||1.438||1.768||3.793|
|200||1.289–4.180 (n = 75)||2.208||0.246||1.756||2.235||2.819|
|y-Position of Cut Line (µm)||0.85||1.0||1.4||1.7||2.2 (angle = 90°)||2.7||3.0|
|Retrace: Thermal Contrast (V)||4.5||4.3||4.2||3.8||3.3||2.5||2.0|
|Trace: Thermal Contrast (V)||4.6||4.7||4.4||4.1||3.5||2.7||2.3|
|Retrace: Absolute Strength of Super Elevations (V)||1.97||1.87||1.63||1.44||1.34||1.05||0.83|
|Trace: Absolute Strength of Super Elevations (V)||1.96||1.90||1.60||1.43||1.12||0.90||0.73|
|Retrace: Relative Strength of Super Elevations (1)||0.90||0.90||0.77||0.78||1.04||1.25||1.10|
|Trace: Relative Strength of Super Elevations (1)||0.85||0.81||0.74||0.76||0.85||1.04||0.94|
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Metzke, C.; Frammelsberger, W.; Weber, J.; Kühnel, F.; Zhu, K.; Lanza, M.; Benstetter, G. On the Limits of Scanning Thermal Microscopy of Ultrathin Films. Materials 2020, 13, 518. https://doi.org/10.3390/ma13030518
Metzke C, Frammelsberger W, Weber J, Kühnel F, Zhu K, Lanza M, Benstetter G. On the Limits of Scanning Thermal Microscopy of Ultrathin Films. Materials. 2020; 13(3):518. https://doi.org/10.3390/ma13030518Chicago/Turabian Style
Metzke, Christoph, Werner Frammelsberger, Jonas Weber, Fabian Kühnel, Kaichen Zhu, Mario Lanza, and Günther Benstetter. 2020. "On the Limits of Scanning Thermal Microscopy of Ultrathin Films" Materials 13, no. 3: 518. https://doi.org/10.3390/ma13030518