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<?xml version="1.0" ?> <tei> <teiHeader> <fileDesc xml:id="-1"/> </teiHeader> <text xml:lang="en"> <front>© 2003 The Royal Microscopical Society<lb/> Journal of Microscopy, Vol. 209, Pt 3 March 2003, pp. 267 – 271<lb/> Received 10 August 2002; accepted 25 October 2002<lb/> Blackwell Publishing Ltd.<lb/> Photoplastic near-field optical probe with sub-100 nm aperture<lb/> made by replication from a nanomould<lb/> G . M . K I M *, B . J. K I M , E . S. TE N H AVE , F. S E GE R I N K ,<lb/> N. F. VA N H U L S T & J. B R U GGE R *<lb/> * Microsystems Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland MESA Research Institute, University of Twente, The Netherlands<lb/> Key words. Microfabrication, nanomould, NSOM, self-assembled monolayer, SU-8.<lb/> Summary<lb/> Polymers have the ability to conform to surface contours<lb/> down to a few nanometres. We studied the filling of transpar-<lb/>ent epoxy-type EPON SU-8 into nanoscale apertures made in a<lb/> thin metal film as a new method for polymer/metal near-field<lb/> optical structures. Mould replica processes combining silicon<lb/> micromachining with the photo-curable SU-8 offer great<lb/> potential for low-cost nanostructure fabrication. In addition<lb/> to offering a route for mass production, the transparent<lb/> pyramidal probes are expected to improve light transmission<lb/> thanks to a wider geometry near the aperture. By combining<lb/> silicon MEMS, mould geometry tuning by oxidation, anti-<lb/>adhesion coating by self-assembled monolayer and mechani-<lb/>cal release steps, we propose an advanced method for near-field<lb/> optical probe fabrication. The major improvement is the<lb/> possibility to fabricate nanoscale apertures directly on wafer<lb/> scale during the microfabrication process and not on free-<lb/>standing tips. Optical measurements were performed with<lb/> the fabricated probes. The full width half maximum after a<lb/> Gaussian fit of the intensity profile indicates a lateral optical<lb/> resolution of ≈ 60 nm.<lb/> </front> <body>Introduction<lb/> Since the first aperture-type near-field experiment in the<lb/> microwave region (Ash & Nicholls, 1972) and in the optical<lb/> domain (Pohl et al ., 1984), the area of application for near-<lb/>field optical measurements has because they overcome the<lb/> diffraction limitation of optical resolution when using the<lb/> far-field method. One of the most reliable probes at present is a<lb/> tapered single-mode optical fibre probe (Betzig et al ., 1991).<lb/> The end of the fibre is tapered to a tip by heating and pulling,<lb/> the tip is subsequently coated with aluminium to create a<lb/> subwavelength aperture. The drawbacks of these optical fibre<lb/> tips are insufficient reproducibility and poor homogeneity of<lb/> both tip fabrication and aperture making. An alternative way<lb/> to make the tip is to use wet chemical etching (Hoffmann<lb/> et al ., 1995; Stöckle et al ., 1999). Chemically etched probes<lb/> have higher throughput due to a large tip angle, but they suffer<lb/> from large surface roughness, an asymmetrically shaped<lb/> apex, and also from problems caused by post metal deposition<lb/> on the free-standing probes. A fibre-type probe with improved<lb/> aperture definition using a focused ion beam (FIB) has<lb/> been shown to have better polarization and image quality<lb/> (Veerman et al ., 1998).<lb/> Recently, several studies on microfabricated near-field<lb/> scanning optical microscopy (NSOM) probes based on atomic<lb/> force microscopy (AFM) cantilever (Abraham et al ., 1998;<lb/> Eckert et al ., 2001) and photoplastic NSOM probes (Genolet<lb/> et al ., 2001; Kim et al ., 2001) have been presented.<lb/> Here, we present an improved manufacturing process for a<lb/> photopolymer NSOM probe using a nanomould technique.<lb/> The major improvement with respect to our previous publica-<lb/>tion (Kim et al ., 2001) is the possibility of fabricating nano-<lb/>scale apertures directly onto a wafer-scale nanomould during<lb/> the microfabrication process rather than onto free-standing<lb/> tips (German Patent, DE19923444.2-42). An integrated<lb/> approach to making the nanoscale aperture has the potential<lb/> to reduce fabrication costs.<lb/> Fabrication process<lb/> Basically, the fabrication process follows the steps described<lb/> earlier (Kim et al ., 2001) with the essential difference that the<lb/> apertures are made directly into the metal layer in the mould<lb/> before forming the polymer probe. The probe consists of three<lb/> parts: a lower aluminium metal layer as an optical blocking<lb/> layer with a nanometre-scale aperture, the photoplastic main<lb/> body of the probe, and an optical fibre as a light guide from the<lb/> laser source to the polymer probe. The upper part of the main<lb/></body> <front>Received 10 August 2002; accepted 25 October 2002<lb/> </front> <front>Correspondence: G. M. Kim. Tel.: +41 21 693 6724; fax: +41 21 693 6670;<lb/> e-mail: gyuman.kim@epfl.ch </front> <front>Present address: School of Mechanical Engineering, Kyungpook National<lb/> University, 702–701 Daegu, Korea.<lb/></front> <page>268</page> <note place="headnote">G. M. KIM ET AL.<lb/></note> <front>© 2003 The Royal Microscopical Society, Journal of Microscopy , 209 , 267 – 271<lb/> </front> <body>probe is connected into the optical fibre. The lower side of the<lb/> probe including the tip is covered with metal except for the tip<lb/> end with the aperture. The process can be divided briefly into<lb/> the following steps:<lb/> (a) micromould fabrication for tip shape by KOH etching;<lb/> (b) mould shape control by low temperature oxidation;<lb/> (c) mould rounding with oxide etching;<lb/> (d) coating with a release layer of self-assembled monolayer<lb/> (SAM);<lb/> (e) deposition of metal (Al) layer;<lb/> (f) opening of nanometre-scale aperture by FIB to form a<lb/> 'nanomould in the micromould';<lb/> (g) filling the micro-and nanomould with polymer and<lb/> structuring the upper part of the probe;<lb/> (h) fibre bonding and probe releasing from mould.<lb/> Figure 1 shows the details of the fabrication process.<lb/> Micromould fabrication<lb/> Our fabrication sequence started with micromould fabrication<lb/> in a <100>-orientated p-type 75-mm Si wafer with resistivity<lb/> of < 10 Ω cm. Inverted shapes of pyramidal tips were made on<lb/> the Si mould by KOH (Merck, 25% wt at 75 ° C) wet etching<lb/> using a 500-nm thick thermal SiO 2 etching mask. In order<lb/> to make the tip shape with precise radius control, local non-<lb/>uniform oxide growth at low temperature (Marcus et al ., 1990;<lb/> Alkamine & Quate 1992) was used. This technique is nor-<lb/>mally used for tip sharpening, but recently we reported its use<lb/> for controlling the tip radius at the nanometre scale between<lb/> 10 and 250 nm (Kim et al ., 2002b). A 450-nm-thick SiO 2<lb/> layer was grown by low temperature wet oxidation at 900 ° C<lb/> for 5 h and removed by BHF in order to make the tip mould<lb/> round. The rounding defines an opening angle near the tip<lb/> apex, which is beneficial to the transmission of light near the<lb/> aperture. Besides that, a round probe is expected to be more<lb/> robust when operated and scanned near surfaces, and to be<lb/> able to withstand unexpected collisions into the sample sur-<lb/>face. Furthermore, a slightly rounded mould structure makes<lb/> the mechanical lift-off (see below) more reliable as sharp edges<lb/> in the mould are known to cause pinholes in the metal film<lb/> during lift-off.<lb/> Metal deposition and nanoscale aperture making<lb/> A self-assembled monolayer (SAM) of dodecyltrichlorosilane<lb/> was formed on the mould surface (Kim et al ., 2002a) by dip-<lb/>ping it into 1 m m solution of dodecyltrichlorosilane in distilled<lb/> toluene for 4 h. This step forms an ultra-thin (1.5 nm) SAM<lb/> layer, which is essential to allow for the probe replication<lb/> without affecting the geometry at a nanometre scale.<lb/> A 150-nm thick layer of aluminium was then coated<lb/> directly onto the SAM layer by e-beam evaporation. The base<lb/> pressure was 1 × 10 − 7 mbar and the deposition rate of metal<lb/> was 0.4 nm s − 1 . This metal layer serves as a light-blocking<lb/> layer in the fabricated probe.<lb/> An aperture of 100 × 100 nm 2 was then drilled through the<lb/> metal layer in the mould using a FIB. Various apertures of sizes<lb/> down to 50 × 50 nm 2 can be made on the metal layer. In ear-<lb/>lier studies, direct FIB milling of sub-micrmetre scale aperture<lb/> on free-standing probes were reported (Veerman et al ., 1998;<lb/> Kim et al ., 2001). However as the set-up and alignment time<lb/> for modifying free-standing probes is long, and the number of<lb/> the loaded probes is limited, those solutions can not be used<lb/> for cost-efficient mass fabrication. Our new approach to FIB<lb/> milling at the wafer-scale is advantageous because thousands<lb/> of holes can be drilled using automatic FIB alignment. Figure 2<lb/> shows a typical 100 × 100 nm 2 aperture made by FIB drilling<lb/> in a 150-nm thick Al layer inside the mould.<lb/> SU-8 structuring and probe releasing<lb/> Two layers of SU-8 were then spin coated onto the metal layer<lb/> with the aperture, and structured by lithography and develop-<lb/>ment. An essential asset of SU-8 here is that the polymer fills<lb/> the mould and the previously made nano-apertures in the<lb/> Fig. 1. Schematic process diagram of novel SNOM probe fabrication. (a)<lb/> Inverted tip shape definition on mould. (b) Mould shape control by<lb/> oxidation. (c) Oxide layer removal (mould rounding). (d) SAM formation.<lb/> (e) Metal deposition. (f) Nano-aperture opening by FIB. (g) Polymer probe<lb/> structuring. (h) Optical fibre bonding and releasing from mould.<lb/> <note place="headnote">PHOTOPLASTIC NEAR-FIELD OPTICAL PROBE </note> <page>269<lb/> </page> <note place="footnote">© 2003 The Royal Microscopical Society, Journal of Microscopy , 209 , 267–271<lb/> </note> aluminium layer. Figure 3 shows a scanning electron micros-<lb/>copy (SEM) image of a cross-section of V-groove filled with SU-<lb/>8. The SU-8 was fully filled into an ultra-sharp pit in the mould<lb/> down to the 10 nm range. A cleaved optical fibre was then<lb/> assembled into the top of the fabricated probe and bonded<lb/> using optical glue under UV light. Finally, the photoplastic<lb/> probe together with nano-aperture on the light-blocking<lb/> metal layer was mechanically released from the micro-and<lb/> nanomould.<lb/> Figure 4 shows an SEM image of a NSOM probe released<lb/> from the mould. The probe was bonded using a tuning fork.<lb/> Shear-force distance control was used and the probe-to-<lb/>sample distance is kept below 10 nm (Ruiter et al ., 1997b).<lb/> The enlarged SEM image of the probe tip clearly shows the<lb/> released aperture. The aperture size was ≈ 55 nm although<lb/> the value on the other side of metal layer was 100 nm, which<lb/> explains the taper of the aperture.<lb/> The fabricated photoplastic probes were then used in a<lb/> home-built NSOM set-up for polarization sensitive single<lb/> molecule detection (Ruiter et al ., 1997a). The sample consisted<lb/> of single corbocyanine molecules (DiIC 18 ) in a polymer layer<lb/> (PMMA) which were excited at 514 nm with typically<lb/> 1 W cm − 2 excitation power. The fluorescence was collected<lb/> with a 1.3 NA objective, filtered with a 550 nm long-pass<lb/> filter, and directed onto two photon-counting avalanche pho-<lb/>todiodes by a broadband polarizing beamsplitter, so that two<lb/> orthogonal polarization directions were detected. Figure 5<lb/> shows the fluorescence images obtained with two probes with<lb/> a 100 nm aperture on the mould; the colour scheme shows<lb/> the orientation of the molecules (red, vertical direction; green,<lb/> horizontal direction; yellow, in-between horizontal and verti-<lb/>cal) (Veerman et al ., 1999). The images show that the probes<lb/> do not have a preferential direction of polarization for the<lb/> transmission of light. The typical feature size of the single<lb/> molecules is ≈ 100 nm for both probes (molecules 1, 2 and 3),<lb/> although certain molecules (e.g. molecule 4) measured with<lb/> the probe of image 4b are substantially smaller ( ≈ 60 nm,<lb/> Fig. 6). The fabricated probes showed better optical resolution<lb/> Fig. 2. Focused ion beam (FIB) image of aperture made inside the mould.<lb/> An aperture of 100 × 100 nm 2 is drilled through a 150-nm-thick Al layer<lb/> on the mould by FIB milling. Step (f ) in Fig. 1.<lb/> Fig. 3. Scanning electron microscopy image of cross-section of V-groove<lb/> filled with photoplastic SU-8. The SU-8 is fully filled into ultra-sharp<lb/> mould down to 10 nm range. The white particles on the surface are gold<lb/> particles coated to prevent charging of SEM images.<lb/> Fig. 4. SEM image of aperture on the tip of released SNOM probe.<lb/> Aperture made on metal layer of mould was released successfully from<lb/> the mould together with the probe. Step (h) in Fig. 1.<lb/> <page>270</page> <note place="headnote">G. M. KIM ET AL.<lb/></note> <note place="footnote">© 2003 The Royal Microscopical Society, Journal of Microscopy , 209 , 267 – 271<lb/> </note> because the aperture of the released probe was smaller than<lb/> the expected drilled size due to the taper of the aperture. The<lb/> optical throughput of the probes is 8.5 × 10 − 5 and 4.9 × 10 − 5 ,<lb/> respectively.<lb/> Conclusions<lb/> We have demonstrated the microfabrication of NSOM probes<lb/> using a new nanomould technique. This process has several<lb/> advantages: first, mass fabrication of probes using an inte-<lb/>grated moulding process enables low cost and high reproduci-<lb/>bility. Second, the SAM anti-adhesion layer and rounded<lb/> mould edge allow mechanical releasing of the probe without<lb/> pinhole formation. The combination of a full mould-scale<lb/> aperture making process, conformal filling of polymer into the<lb/> nanomould, and the new releasing method allow low cost and<lb/> wafer-scale NSOM probe fabrication.<lb/> Fig. 5. SNOM images of single molecules taken by two fabricated probes. (a) Optical and topography images taken by probe 1 (scanning area 3 × 3 µm 2 ).<lb/> (b) Optical and topography images taken by probe 2 (scanning area 1.8 × 1.8 µm 2 ). The topography images show the surface of the spin-coated PMMA,<lb/> and the optical images show the fluorescence of the molecules embedded in the PMMA.<lb/> Fig. 6. Intensity profile of molecule 4 in Fig. 5(b). The signal-to-noise<lb/> ratio is ≈ 2 and the FWHM of the intensity profile after Gaussian fit is<lb/> ≈ 60 nm.<lb/> <note place="headnote">PHOTOPLASTIC NEAR-FIELD OPTICAL PROBE</note> <page>271<lb/> </page> <note place="footnote">© 2003 The Royal Microscopical Society, Journal of Microscopy , 209 , 267–271<lb/> </note> </body> <back> <div type="acknowledgement">Acknowledgements<lb/> The authors would like to thank J. Huskens and E. Speets<lb/> for fruitful discussion on the SAM work. This research was<lb/> financially supported by the Strategic Research Orientation<lb/> NanoLink of MESA +, University of Twente, and by EPFL.<lb/></div> <listBibl>References<lb/> Abraham, M., Ehrfeld, W., Lacher, M., Mayr, K., Noell, W., Guthner, P. &<lb/> Barenz, J. (1998) Micromachined aperture probe tip for multifunc-<lb/>tional scanning probe microscopy. Ultramicroscopy , 71 , 93–98.<lb/> Akamine, S. & Quate, C.F. (1992) Low-temperature thermaloxidation<lb/> sharpening of microcast tips. J. 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