Note: Mechanical etching of atomic force microscope tip and microsphere attachment for thermal radiation scattering enhancement D. Brissinger, G. Parent, and D. Lacroix Citation: Review of Scientific Instruments 84, 126106 (2013); doi: 10.1063/1.4849575 View online: http://dx.doi.org/10.1063/1.4849575 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/84/12?ver=pdfcov Published by the AIP Publishing

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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 126106 (2013)

Note: Mechanical etching of atomic force microscope tip and microsphere attachment for thermal radiation scattering enhancement D. Brissinger, G. Parent,a) and D. Lacroix Université de Lorraine, LEMTA, UMR 7563, BP70239, 54500 Vandoeuvre-lés-Nancy, France

(Received 28 May 2013; accepted 3 December 2013; published online 19 December 2013) This Note describes a mechanical etching technique which can be used to prepare silicon tips used in atomic force microscopy apparatus. For such devices, dedicated tips with specific shapes are now commonly used to probe surfaces. Yet, the control of the tip morphology where characteristic scales are lower than 1 μm remains a real challenge. Here, we detail a controlled etching process of AFM probes apex allowing micrometer-sized sphere attachment. The technique used and influent parameters are discussed and SEM images of the achieved tips are given. Deceptive problems and drawbacks that might occur during the process are also covered. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4849575] Following Binning and Rohrer’s pioneering experiment,1 Scanning Probe Microscopes (SPM) are now widely used for several imaging applications.2 Devices such as the Scanning Tunneling Microscope (STM),1 the Atomic Force Microscope (AFM),3 and the Scanning Near field Optical Microscope (SNOM)4 are powerful tools that can overcome the Rayleigh criterion to produce high resolution images in surface science studies. Achieving ultra-high resolution images was an important objective for SPM development but its use has spread to various kinds of measurements in physics and chemistry. The tip is one of the imaging device’s main components in this context and needs to be carefully designed according to the application it is destined for. Its preparation is thus a very important issue that needs to be well-defined and described. The tip needs to be designed so as to achieve an efficient measurement of the local property studied without drastically decreasing the spatial resolution. Numerous procedures including coating,5 Focused Ion Beam (FIB) milling,6 nanoparticle or microsphere fixing,7–9 growing nanotubes,10 and chemical functionalization11 can be performed on microscale tips. They allow the shape to be modified with the physical and chemical properties of the tip and make them suitable for specific measurements. In the optical domain, SPM have taken advantage of their spatial resolution to perform electromagnetic near-field measurements from centimetre to visible wavelength scales.4, 12 During the last decade, particular attention has been paid to the near-field measurement of thermal radiation.13 With this kind of microscopy, called Thermal Radiation Scanning Tunneling Microscopy (TRSTM), the signal is very weak because the tip scattering cross section is very small as is the thermal radiation emitted by the sample. If only the far field radiation given by the Planck’s law is taken into account, the available signal would not be strong enough to be detected with classical infrared detectors. Fortunately, thanks to resonant surface waves, the thermal density of energy close to a surface is larger by several orders of magnitude than the far field a) Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0034-6748/2013/84(12)/126106/3/$30.00

density.14 Nevertheless, the signal remains very small and very near the noise equivalent power (NEP) of detectors in the mid-infrared domain (wavelength typically between 2 and 20 μm). Therefore tip optimization is a crucial issue, especially if we wish to collect a signal of larger magnitude. Recently, it has been demonstrated that an AFM tip with a single microsphere as the end significantly improves scattering efficiency compared with a classical tip.15, 16 Two effects can occur: an increase in the tip scattering volume and possibly some resonances of the spheres or “Fano resonances.” On the basis of the numerical simulation presented in Ref. 16, micrometer-sized spheres constitute an interesting compromise enabling the obtention of good signal enhancement taking advantage of Fano resonances without strongly decreasing the spatial resolution (the apex size remains smaller than the wavelength). Attachment of spheres have been studied17 but they were either nanometer-sized7, 8 or larger than the micrometer-size in which case the spheres are directly put on a cantilever without any tip.9 If we consider the whole optical collection device of a scattering aperture-less SNOM, the sphere cannot be fixed onto the cantilever and must be at the apex of the tip to avoid modulation of the far field thermal radiation and shadowing of the useful signal by the cantilever. Yet, for a micrometer sized particle, the attachment of a single microsphere at the apex of the tip is a real challenge as the spheres systematically move on the side of the tip cone. In this Note, we describe the modification of classical AFM cantilever tips in order to allow a single micro sized spherical particle to be attached at the tip apex. We propose a low-cost and easy method to perform mechanical truncation of the tip which provides a “plateau tip” with a size-controlled surface area to which a single microsphere can be attached. After this introduction, the Note is organized as follows. First, the experimental setup used to manufacture modified tips and the related procedure are described. Then, on the basis of the tips produced, the advantages and drawbacks of the etching and attachment procedure are discussed. Finally, recommendations and conclusions are given. In this study, a Park Systems AFM was used with classical silicon conical shaped AFM tip from Mikromasch

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FIG. 1. SEM image (a) presents a classical cone shape AFM tip. SEM image (b) shows the most likely position of the microsphere glued on a classical cone shape AFM tip.

(NSC18 series, Figure 1(a)). The particles to paste are gold or SiO2 microspheres from the Alfa Aesar Company with diameters between 0.6 and 1.1 μm. Due to the microsphere size, attachment of a single sphere at the apex of the tip is a laborious and tricky task. Mechanically, this is the same challenge as positioning a sphere over a sharp cone vertex. As the microsphere weight is negligible compared to the adhesion forces between the particle and the tip, the microsphere remains on a stable equilibrium only on the side of the cone tip (see Figure 1(b)). To avoid this “off-axis” configuration and obtain a stable equilibrium “in axis” at the end of the tip it is necessary to modify its shape to obtain a plateau tip. In practical terms, the aim is to obtain a flat plateau-shaped tip apex with a surface area closed to 1 μm square. Different methods have been described in the literature to obtain a plateau-shaped AFM tip using FIB6 or ultra short laser pulses.18 Commercial solutions are also available but the method presented here does not require heavy or costly additional equipment. Furthermore, it can be used to obtain user-adjustable plateau sizes with a good level of reproducibility. Two methods were first considered to obtain the required shape: a pure mechanical abrasion or a wet-chemical etching (see Figures 2(a) and 2(b)). Finally, we combined both methods to obtain the plateau-shaped tip (Fig. 2(c)). The mechanical approach consists of grinding the tip apex on a SiO2 surface. In this case, even if a high force is applied between the tip and the surface, the abrasion of the tip is a slow process (the rate of abrasion remains low) with two main drawbacks. First, it is relatively difficult to accurately predict the height of the tip apex which will be removed due to grinding. Second, the material removed is not strictly speaking taken away but instead displaced to the tip side as shown in Figure 3(a). As a consequence, this technique does

FIG. 2. Techniques used to obtain a plateau-shaped AFM tip are considered: (a) pure mechanical abrasion, (b) pure wet-chemical etching in a droplet, and (c) combined wet-mechanical abrasion.

Rev. Sci. Instrum. 84, 126106 (2013)

FIG. 3. SEM images of the tip apex after a pure mechanical abrasion (a) and after a wet-mechanical abrasion (b).

not provide the expected shape and also microsphere gluing can be impossible. To overcome the previously discussed problems related to mechanical abrasion, wet-chemical etching was also considered. This chemical approach is based on the recognized ability of potassium hydroxide (KOH) to etch silicon.19 KOH solutions are commonly used for wet-chemical etching of Si materials in microelectronics. In our particular case, the main difficulty with this technique lies in achieving a selective etching of the tip apex without damaging the rest of the tip. Indeed, when a KOH solution is used directly, the meniscus caused by capillary forces leads to the whole cantilever immersion. To overcome this difficulty, a film of KOH just a few micrometers thick and diluted in ethanol was deposited onto a glass substrate. After evaporation of ethanol, some small clusters of KOH crystals remained. Then, due to the hydrophilic property of KOH, atmospheric water was absorbed and we observed the formation of very small droplets at the surface of the SiO2 substrate. KOH droplets are supposed to be close to saturation. The AFM images of the droplets show that their height can vary from a few hundreds of nanometres to some micrometers. Finally, the AFM was used to control the tip immersion in the droplets. Experiments were run to etch the tip in the KOH droplets without contact between the apex and the SiO2 substrate resulting in a pure wet-chemical etching process (Fig. 2(b)). In this configuration, only the tip apex was immersed in the KOH droplets. However, we did not observe significant modification of the tip. Finally, both mechanical and chemical approaches were combined. With this combined technique, the tip is driven down into a droplet and scans the substrate surface. A constant force was applied at the point of contact between the tip and the substrate. The force and the tip velocity (4 μm/s) were monitored by the AFM device. Under these new operating conditions, we clearly observed abrasion of the tip apex. Figure 3(b) shows the apex of an AFM tip after wet mechanical abrasion. The abraded height was measured for two constant forces (700 nN and 1600 nN) during the process. Measurements are shown in Figure 4 as a function of the processing time. The plateau size was determined from the SEM images. We observed that the higher the force applied, the higher the processing rate. We also observed that the abrasion velocity decreased with the height abraded (i.e., within the surface area of the plateau). The data obtained can be fitted with a power law in order to appraise the etching velocity. This constitutes an easy

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Rev. Sci. Instrum. 84, 126106 (2013)

FIG. 4. Abraded height of the tip as a function of time during the wet mechanical etching of the AFM tip.

way to control the size of the plateau obtained at the end of the tip. To identify the chemical contribution of the KOH, we also ran the same experiments using another hydrophilic salt (CaCl2 ). The droplets obtained were similar and the plateau sizes observed very comparable to those obtained with the KOH droplets. This complies with the fact that no significant modification of the AFM tip was observed with the pure wet-chemical etching process in the KOH droplets. Indeed, during the wet-mechanical process, the removed particulates were dispersed in the fluid phase rather than only displaced to the tip side as with the dry mechanical process. Here, the key property of KOH or CaCl2 is their hydrophilic property which enables selective wetting of the very end of the tip. The last stage of the manufacturing process is the microsphere attachment. First, the microspheres were dispersed in pure ethanol and deposited on a glass substrate. After ethanol evaporation spheres are no longer packed and can be individually approached by the AFM tip. We then rubbed the plateau-shaped AFM tip with the silicone glue. Capillary action draws the glue around the tip. Finally, the tip was brought close to the microsphere using the AFM location. The AFM tip scanned over the microsphere at a constant height. We gradually decreased the height of the scan until the AFM detected the forces between the tip and the microsphere. The very top of the microsphere was then clearly observed on the AFM image. Finally, the tip was centred at the top of the microsphere and we picked it up. We systematically confirmed the location and the diameter of the microsphere with SEM images. Figure 5 shows the SEM image of a SiO2 microsphere (with a 900 nm diameter) at the edge of an AFM tip. As one can see the plateau shape of the tip now allows the attachment the microsphere “on axis” at the very end of the tip. In conclusion, a wet-mechanical abrasion of classical silicon cone shaped AFM tip has been presented and discussed. This method is low cost and easy to use. It can be used to obtain a plateau at the end of an AFM tip with controlled size and shape. The size of the plateau depends on the processing time spent and force applied during the tip etching. Among others finding, we have shown that the processing time and the force applied between the apex of

FIG. 5. SEM image of a microsphere attach “on axis” at the apex of an AFM tip modified by wet-mechanical etching. The microsphere diameter is close to 900 nm.

the AFM tip and the sample surface are key parameters which regulate and define the etching rate. Finally, the particular shape obtained means single microspheres with diameter around 1 μm can be attached at the very end of the tip. This procedure is independent of the microsphere material and is particularly interesting for applications that require keeping a large distance height ratio between the particle and the cantilever like in TRSTM measurement.16

1 G.

Binnig and H. Rohrer, Helv. Phys. Acta 55, 726 (1982). Probe Microscopy in Nanoscience and Nanotechnology, NanoScience and Technology, edited by B. Bhushan (Springer, 2010). 3 G. Binnig, C. F. Quate, and Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986). 4 D. W. Pohl, W. Denk, and M. Lanz, Appl. Phys. Lett. 44, 651 (1984). 5 T. Kim, D. S. Kim, B. Y. Lee, Z. H. Kim, and S. A. Hong, Adv. Mater. 21, 1238 (2009). 6 C. Menozzi, L. Calabri, P. Facci, P. Pingue, F. Dinelli, and P. Baschieri, J. Phys.: Conf. Ser. 126, 012070 (2008). 7 Q. K. Ong and I. Sokolov, J. Colloid Interface Sci. 310, 385 (2007). 8 T. Kalkbrenner, M. Ramstein, J. Mlynek, and V. Sandoghar, J. Microsc. 202, 72 (2001). 9 L. H. Mak, M. Knoll, D. Weiner, A. Gorschlüter, A. Schirmeisen and H. Fuchs, Rev. Sci. Instrum. 77, 046104 (2006). 10 H. J. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert, and R. E. Smalley, Nature (London) 384, 147 (1996). 11 C. D. Blanchette, A. Loui, and T. V. Ratto, “Tip functionalization: Applications to chemical force spectroscopy,” Handbook of Molecular Force Spectroscopy, edited by A. Noy (Springer, New York, 2008), pp. 185–203. 12 E. A. Ash and G. Nicholls, Nature (London) 237, 510 (1972). 13 Y. De Wilde, F. Formanek, R. Carminati, B. Gralak, P.-A. Lemoine, K. Joulain, J.-P. Mulet, Y. Chen, and J.-J. Greffet, Nature (London) 444, 740 (2006). 14 A. V. Shchegrov, K. Joulain, R. Carminatti, and J.-J Greffet, Phys. Rev. Lett. 85, 1548 (2000). 15 K. Joulain, P. Ben-Abdallah, P.-O. Chapuis, Y. De Wilde, A. Babuty, and C. Henkel, “Strong tip-sample coupling in thermal radiation scanning tunneling microscopy,” J. Quant. Spectrosc. Radiat. Transfer (to be published) [preprint arXiv:1311.1983 (2013)]. 16 J. Muller, G. Parent and D. Lacroix, J. Opt. 14, 075703 (2012). 17 Y. Gan, Rev. Sci. Instrum. 78, 081101 (2007). 18 P. Biagioni, J. N. Farahani, P. Mühlschlegel, H.-J. Eisler, D. W. Pohl, and B. Hecht, Rev. Sci. Instrum. 79, 016103 (2008). 19 K. Sato, M. Shikida, Y. Matsushima, T. Yamashiro, K. Asaumi, Y. Iriye, and M. Yamamoto, Sens. Actuators, A 64, 87 (1998). 2 Scanning

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Note: Mechanical etching of atomic force microscope tip and microsphere attachment for thermal radiation scattering enhancement.

This Note describes a mechanical etching technique which can be used to prepare silicon tips used in atomic force microscopy apparatus. For such devic...
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