NANO LETTERS
Control of Droplet Size in Liquid Nanodispensing
2006 Vol. 6, No. 10 2368-2374
Aiping Fang, Erik Dujardin, and Thierry Ondarc¸ uhu* NanoScience group, CEMES-CNRS, 29 rue Jeanne MarVig, 31055 Toulouse Cedex 4, France Received July 21, 2006; Revised Manuscript Received August 23, 2006
ABSTRACT In this Letter, the phenomena and parameters governing the nanoscale dispensing of liquid through an apertured atomic force microscope probe milled by focused ion beam lithography are characterized in detail. We show that the size of deposited droplets can be reproducibly defined by controlling the aperture size on the probe and the surface energies of both tip outer wall and substrate surface. On the basis of these findings, tips with aperture diameter as small as 35 nm enabled the deposition of regular arrays of nanodroplets with diameter down to 70 nm on an alkylamine-modified surface. The fine control of droplet volumes down to a few tens of zeptoliters (10-21L) provides a unique tool for creating devices and probing the fundamentals of wetting at the nanometer scale.
Introduction. Mastering manipulation and deposition of liquids on surfaces in the nanometer-scale regime are major challenges faced by numerous and varied technologies as well as scarcely explored academic topics despite a number of open questions. Indeed, while surface patterning at the micrometer scale is now routinely performed by methods such as microcontact printing,1 microfluidics,2,3 inkjet printing,4 or microlever plotting,5 recently developed tools, such as dip-pen nanolithography (DPN),6 are pushing the resolution limit of nonlithographic techniques down to a few tens of nanometers. This method relies on sharp tips to reach highresolution deposit or surface functionalization. The liquid is dried on an atomic force microscope (AFM) tip before transferring the molecules onto the substrate by diffusive transport. If DPN reaches the best lateral resolution so far (a few tens of nanometers), it suffers from three main limitations: (i) it needs periodic refilling, (ii) it is restricted to a limited number of transferable molecules, and (iii) it is dependent on the establishment of an uncontrolled water meniscus between the tip and the substrate.7 The first issue was addressed by adding a liquid reservoir connected to DPN tips in the micromachined fountain pen techniques.8,9 The second limitation could be obviated by the direct manipulation of liquid solution with a nanopipet;10,11 however the spatial resolution of this approach is currently limited to 200 nm. Combining the high resolution potentials of DPN with the flexibility of manipulating liquids, the newly developed nanoscale dispensing system (NADIS)12 is based on the direct transfer of liquid from a hollow AFM tip to the substrate * Corresponding author: e-mail,
[email protected]; tel, +33 5 62 25 78 38; fax, +33 5 62 25 79 99. 10.1021/nl061694y CCC: $33.50 Published on Web 09/09/2006
© 2006 American Chemical Society
Figure 1. Schematic representations of (a) the cross section of a liquid-loaded NADIS tip and (b) the deposition procedure.
trough a small opening at its apex by direct contact (Figure 1). When the cantilever is fitted with an ink reservoir, a large number of droplets with a volume in the femtoliter to subattoliter range can be deposited on a substrate with a high spatial density. Compared to DPN, NADIS provides a more versatile technique since (i) a single cantilever can carry a volume equivalent to typically millions of spots, (ii) it can deposit any soluble molecule, and (iii) the liquid transfer is entirely controlled by aperture size and surface energies. In this work, we explore the parameters influencing the size of the spots and show that a fine control of the liquid deposition on various surfaces is possible. Besides the hole diameter, the size of the deposit spot is determined by the wetting properties of the solvent on the tip and substrate surfaces and by the dynamical parameters used in the deposition process. In an optimized configuration, we
obtained deposition of droplets with diameter as small as 70 nm suggesting that the NADIS concept could be used to produce miniaturized biological assays and nanodevice fabrications. Methods. (A) FIB Nanofabrication. NADIS tips were fabricated by modification of commercially available AFM tips (Olympus OMCL-RC800RP) by focused ion beam (FIB) lithography. The silicon nitride AFM tips were gold coated on both inner and outer sides and had a hollow square pyramidal shape with a wall thickness of about 800 nm. FIB lithography was performed on a ZEISS 1540XB Crossbeam with a Gemini SEM and a gallium source FIB both operated at 30 kV. Large square apertures (>200 nm) were directly milled through the apex from the inner side at a nominal dose of 4 × 1017 ions‚cm-2. However, this approach could not be reproducibly applied to sub-200 nm apertures because of the rapid degradation of the aperture quality when its aspect ratio (tip wall thickness:diameter) exceeded 4:1. This limitation was obviated by the development of a two-step protocol as illustrated in Figure 2. First, the tip was locally thinned down by about 85-90% in three steps of decreasing area centered on the apex (Figure 2a). The resulting funnel with the shape of an inverted step pyramid is shown in Figure 2c. Second, for hole size ranging between 200 and 100 nm, the aperture itself was drilled through from the inner side leaving the outer surface of the NADIS tip intact (Figure 2b). The irradiation dose used for the thinning step (typically 4 × 1017 ions‚cm-2) was reduced by 1 order of magnitude for the drilling of the smallest apertures. In Figure 2d, a 150 × 180 nm rectangular aperture drilled at the bottom of a funnel is viewed from the inner (top) side, the walls of the funnel appearing roughened. The centering of the aperture on the tip apex is crucial to ensure efficient liquid transfer from the tip to the substrate. The two-step protocol ensured a satisfactory centering precision for aperture size down to 100 nm as shown in Figure 2e where the NADIS tip is viewed from the outer (tip) side. However, the sharp AFM tip apex profile and the residual misalignment between the inner and outer apexes frequently resulted in off-centered apertures for sub-100-nm holes. Consequently the second step of the protocol was modified for the ultimately small apertures as illustrated in Figure 2b (sketch 2′). If the thinning was still done from the inner side, the final aperture drilling was performed from the outer side, thus allowing a precise positioning of the targeted milled area. This second step was performed as rapidly as possible to minimize any FIB damage on the outer surface of the AFM tip. Considering a minimal tip thickness after thinning of 100 nm and a maximal typical aspect hole ratio of 3:1 to 4:1, one can consider that the smallest attainable aperture diameter is about 25-30 nm. Proceeding with this method, we could indeed fabricate perfectly centered NADIS tips with aperture diameter as small as 35 nm as shown in Figure 2f. It should be noticed that larger square apertures were performed by rastering the ion beam so that the truncated pyramid tip would be as flat as possible. On the contrary, the smallest apertures were obtained in spot mode where the ion beam is static yielding Nano Lett., Vol. 6, No. 10, 2006
Figure 2. Nanofabrication of NADIS tips by FIB lithography. Schematics of the two-step milling protocol, namely, (a) thinning from the inner side and (b) aperture drilling. Apertures larger than 100 nm in diameter can also be drilled from the inner side (2) while smaller apertures have to be perforated from the tip side (2′). (c) Tilted view of the funnel resulting from the thinning step. (d) Rectangular 150 × 180 nm hole at the bottom of a funnel with the shape of an inverted three-step pyramid (1.0, 0.5, and 0.2 µm). (e) Side view of a 120 × 70 nm aperture. (f) Axial view of a centered 35-nm round aperture. (g) NADIS tip with an intact gold coating on the outer surface (hole size, 280 nm). (h) NADIS tip with a silicon nitride area exposed by locally removing the gold coating (hole size, 215 nm).
a circular hole shape. Moreover, four small protrusions were invariably observed at the corners of larger apertures, which result from the fact that the AFM tip edges bulge slightly above the tip faces (parts e, g, and h of Figure 2). However, these protrusions remain small compared to the aperture diameter and did not affect the liquid transfer. Finally, FIB exposure was also used to modify the NADIS tip surface properties since the AFM tip is made of silicon nitride coated with 30 nm of gold. Hence, the outer surface of the tip could be left intact and covered with gold for further chemical functionalization by thiol chemistry (Figure 2g). Alternatively, since the sputtering rates of gold and silicon nitride (SixNy) are significantly different, the gold layer could be removed over a desired area around the aperture thus 2369
Table 1. Advancing and Receding Contact Angles of Glycerol Measured with a Goniometer on the Substrates Studied in the Experiments Using Surface Functionalization samples
θadv (deg)
θrec (deg)
SiO2, freshly cleaned with piranha (SiO2) SiO2, modified with (aminopropyl)trimethoxysilane (amine) SiO2, modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (fSiO2)
30 ( 2 53 ( 2 95 ( 2
19 ( 2 27 ( 2 85 ( 2
exposing the underlying nitride (Figure 2h). SixNy is intrinsically hydrophilic upon oxidation in ambient conditions but it can also be chemically functionalized orthogonally to gold by using silane and siloxane chemistry. (B) Surface Chemistry. Surface chemistry was used to functionalize tip and substrate surfaces with self-assembled monolayers in order to control their wetting properties. All surface treatments were operated in gaseous phase in a homebuilt vacuum line. Substrates with different surface energies were obtained from 100 nm thick, thermally grown SiO2 on silicon wafer chips (∼5 mm × 5 mm) as follows: The samples were first cleaned in a freshly prepared Piranha solution (a 3:1 mixture of concentrated sulfuric acid with 30% hydrogen peroxide, v/v; Caution: hazard) for 15 min, followed by copious rinsing with deionized water and drying under a stream of pure argon. The samples were immediately transferred into the vacuum chamber, where an atmosphere of either 1H,1H,2H,2H-perfluorodecyltrichlorosilane (Lancaster Synthesis) or (aminopropyl)triethoxysilane (Aldrich) was maintained for 2 or 20 min, respectively. The surface treatment of the gold-coated NADIS tips was achieved in a similar way but in an atmosphere of dodecanethiol (Aldrich) and with a reaction time of 30 min. All samples and NADIS tips were stored in dry ambient conditions. These surface treatments allow varying surface properties to a large extent, from hydrophilic silica to moderately hydrophobic amine-covered and highly hydrophobic fluorinated surfaces. The advancing and receding contact angles of glycerol on such surfaces are reported in Table 1. (C) Deposition Process. All the experiments were performed with glycerol-based liquids. Owing to its very low volatility (boiling point, 290 °C; vapor pressure (25 °C),
