NANO LETTERS
Constructive Microlithography: Electrochemical Printing of Monolayer Template Patterns Extends Constructive Nanolithography to the Micrometer−Millimeter Dimension Range
2003 Vol. 3, No. 6 761-767
Stephanie Hoeppener, Rivka Maoz, and Jacob Sagiv* Department of Materials and Interfaces, The Weizmann Institute of Science, RehoVot 76100, Israel Received March 22, 2003; Revised Manuscript Received April 11, 2003
ABSTRACT Micrometer-scale patterns consisting of water-wetted hydrophilic features on the surface of a rigid metallic object (stamp) are successfully transferred to the hydrophobic surface of a highly ordered organosilane monolayer self-assembled on silicon upon the application of an appropriate voltage bias between the stamp and the silicon wafer substrate while the two are pressed against one another. The monolayer imprint is the result of a water bridge-mediated electrochemical oxidation process selectively converting surface exposed −CH3 groups of the monolayer to −COOH. This novel electrochemical printing process, analogous to the monolayer pattern inscription with a conductive scanning probe tip underlying constructive nanolithography (Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55−61. Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. Adv. Mater. 2000, 12, 424−429. Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. Adv. Mater. 2000, 12, 725−731.), paves the way to the possible advancement of a “bottom-up” nano−micro fabrication methodology applicable within the entire nanometer−millimeter dimension range.
The successful advancement of a comprehensive chemical approach to nanofabrication, the so-called bottom-up approach, depends on the availability of methods capable of providing subnanoscale precision in the 3D manipulation of molecular-sized objects so as to enable the assembly of a variety of planned architectural entities of nanometric dimensions. Self-assembly has invariably been invoked as a most appropriate approach to the problem of the effective rational handling of such minuscule objects. However, in the absence of an adequate mechanism of external control, there is no conceivable way by which spontaneous self-assembly processes alone would lead to supramolecular functional units that can perform diverse desired functions. External intervention is thus unavoidable if self-assembly is ultimately to become a viable approach to nanofabrication. The key problem we thus face centers on the question of how to exercise most effectively the necessary external control on various self-assembly processes in a manner that should also be compatible with the requirements of a practical bottomup nanofabrication methodology. The feasibility of a template-guided 3D self-assembly * Corresponding author. E-mail:
[email protected]. Phone: +972-8-9342309. Fax: +972-8-9344138. 10.1021/nl034176l CCC: $25.00 Published on Web 05/03/2003
© 2003 American Chemical Society
approach to nanofabrication has been addressed in a series of recent publications from this laboratory.1-5 It combines techniques of organized monolayer self-assembly and quantitative interfacial chemical modification (using robust organosilane mono- and multilayer film systems) with a newly invented lateral patterning method referred to as constructive nanolithography.2,3 The latter operates via site-selected nanoelectrochemical surface transformations carried out with the assistance of a conductive atomic force microscope (AFM) tip that “inscribes” the desired lateral distribution of chemically active sites on the outer surface of a highly ordered organosilane monolayer self-assembled on silicon.1-3 A predefined monolayer template pattern is thus created, which can guide the subsequent surface self-assembly of various organic and inorganic species that are in situ generated3,4 or prefabricated.5 The term “constructive” was used to emphasize the additive nature of the development process of the initial surface-inscribed chemical information, characteristic of this bottom-up approach,3 contrary to the usual downward transfer of the pattern produced in a resist layer, via destructive etching of the underlying substrate itself. In constructive nanolithography, the initial monolayer template is retained as part of the final fabricated structure, whereas the organic resist layer in top-down lithographic
processes serves as a removable etching barrier. The prefix nano, in nanolithography, refers to the nanometric dimensions of the features that may be routinely realized with a scanning AFM tip (typically, between several nanometers to several micrometers). As far as a bottom-up approach is concerned, constructive nanolithography can offer, beyond extreme miniaturization, some rather unique advantages thanks to its remarkable chemical versatility and flexibility in the selection and handling of a large variety of different desirable componentssorganic, inorganic, and biological. Furthermore, whereas the lateral positioning accuracy routinely achievable in AFM patterning is on the order of several nanometers, combining this scanning probe technique with molecular selfassembly offers unmatched capabilities for subnanometric accuracy in the vertical positioning of such components on a nanometrically defined pattern. The main drawback of constructive nanolithography has to do with the inherently slow nature of any serial patterning method. It is, for example, impracticable to employ a scanning AFM tip for the fabrication of connector features bridging the micrometermillimeter dimension gap, which are needed for communication with the macroscopic external world. Although such connector features may, in principle, be produced by suitable modifications of conventional top-down techniques, it would be highly desirable to avoid the expensive and inconvenient mixed use of different technologies in the development of a novel nanofabrication methodology. In this communication, we show that monolayer template patterns spanning the micrometer-millimeter dimension range can be produced in a straightforward manner by direct, one-step electrochemical printing of the entire desired pattern on the top surface of a high-quality OTS (n-octadecyltrichlorosilane) monolayer self-assembled on silicon. In the preliminary experiments reported here, some test patterns were created with conducting metal stamps consisting of transmission electron microscope (TEM) copper grids that were first exposed to a saturated water-vapor atmosphere (after being freshly cleaned to render them water wettable) and then immediately pressed against a monolayer-coated silicon wafer surface while applying a suitable electrical bias between the metal grid and the silicon substrate (Figure 1). On the basis of results previously obtained in similar electrochemical stamping experiments with copper grids,1,3 it was expected that top -CH3 functions of the OTS monolayer should be converted under such conditions to -COOH 3 (Figure 1). However, in the previously reported experiments,1,3 performed without exposure of the grids to a saturated water-vapor atmosphere prior to contacting the monolayer surface, no definite patterns could be observed. This led us to assume that only isolated surface sites where sufficiently close contact between the monolayer and the grid could be established were affected.3 Because large-area molecular-scale contact between a stamp and a hard substrate (i.e., conformal contact) requires that the stamp be made of a flexible soft material, like the elastomeric stamps currently employed in the many versions of soft lithography,6-10 the present intriguing results obtained with stamps that are both rigid and far from having molecularly smooth surfaces (vide 762
Figure 1. Schematic view of the pattern printing experiments: Rigid metal stamps consisting of TEM copper grids (SPI Supplies, West Chester, PA) mounted in a special Teflon holder were cleaned by ∼1 h Soxhlet extraction with toluene, followed by exposure to HNO3 vapor (twice 1 min on each side), sonication for several minutes in ultrapure water (Barnstead Nanopure system), and finally drying in a stream of clean nitrogen. A freshly cleaned grid attached to a piece of Scotch tape was placed for ∼15 s above a beaker filled with hot water and then pressed manually against a selfassembled OTS/Si monolayer specimen (prepared as described before)1,3 while applying (for ∼45 s) a voltage bias of 20 V between the grid (negative) and the silicon wafer substrate (positive). Electrical contact with the grid was realized through a wire lead immobilized together with the grid on the same piece of Scotch tape. Typical currents in the range of 150-300 µA were measured during the printing of the patterns.
infra) point to an electrochemical pattern-transfer mechanism compatible with considerably wider gaps between the stamp and the printed surface. If properly understood and exploited, this may lead to a series of attractive further extensions of electrochemical printing. Some characteristic electrochemically printed patterns, produced with TEM grids having two different bar widths and hole sizes (mesh 400 and 1000), are shown in Figures 2 and 3. As expected, the patterns, produced by the local oxidation, under the grid bars, of the top -CH3 functions of OTS to -COOH (Figure 1), appear with high contrast in friction (lateral force) images only, because of the large difference between lateral forces exerted on the tip in polar (-COOH) and nonpolar (-CH3) surface regions.1,3 Although such nondestructive modification of the top monolayer surface does not affect the surface topography significantly,1,3 the resulting patterns are also weakly visible in the corresponding topographic images, albeit with a small negative contrast. (See the topographic images in Figures 2A and 3C and D.) This is an AFM artifact currently observed by us in the imaging of polar-nonpolar heterogeneous surfaces, probably caused by the large spot-to-spot force variations associated with the different hydration and adhesion properties of the polar and nonpolar regions in such systems.11 The apparent depth of patterned polar areas relative to the background of the unaffected monolayer surface depends on their hydrophilicity and the particular imaging conditions employed.12,13 Upon self-assembly at the surface-modified sites of a top NTS (nonadecenyltrichlorosilane) monolayer, the homogeneous nonpolar character of the entire imaged surface is restored so that a real topographic step corresponding to the expected height of the NTS monolayer1,3 Nano Lett., Vol. 3, No. 6, 2003
Figure 2. (A) Simultaneously acquired topography and friction (lateral force) contact mode AFM images of a wide bar crossing of the mesh 400 grid pattern printed on an OTS/Si monolayer. These images were acquired on a SOLVER P7LS scanning force microscope equipped with a 90-µm scanner (NT-MDT, Moscow, Russia) using standard silicon contact probes (MikroMasch, Tallinn, Estonia) with typical spring constants of 0.03-0.08 N/m. (B) Semicontact mode AFM topographic image of a limited portion of the boundary between a modified and an unmodified monolayer region in the vicinity of a bar crossing of the printed pattern (as in the marked square in image A above) after the assembly of a top NTS monolayer and (right side) the distance-height profile along the marked line in the image. This image was acquired on a SOLVER P47 scanning force microscope equipped with a 17-µm scanner (NT-MDT) using semicontact Si probes (MikroMasch) with typical spring constants of 4.5-14.0 N/m.
can be observed at boundaries between modified and unmodified surface regions. (See Figure 2B.) The widths of the grid bars vary between ca. 35 (mesh 400) and ca. 7 µm (mesh 1000), which allowed us to image, within the full 90-µm range of our AFM scanner, only a single bar crossing of the mesh 400 grid (Figure 2) and up to four such crossings of the mesh 1000 grid (Figure 3). In both cases and at different magnifications (Figure 3C and D), the obtained images appear to be homogeneous over the entire pattern-printed areas, being free of any visible intrabar structure that might be ascribed to discontinuities in the intimate contact between the surface of the grid and that of the patterned substrate. There is also a very good fit between the average size and the general appearance of the grid bars and holes, as revealed by SEM (scanning electron microNano Lett., Vol. 3, No. 6, 2003
scope) micrographs of portions of a grid (Figure 3A and B) and the AFM images of portions of the monolayer pattern produced with the same grid (Figure 3C and D). The broken bars pattern seen in Figure 3C (left side) represents a copy of actual broken bars in the grid itself, as deduced from a comparison with optical micrographs of this grid. Likewise, the separated small features visible on the lower left edge of the bar image in Figure 2A (indicated with an arrow) seem to be copies of real metal features present on bar edges of the grid, as suggested by the presence of similar features on the side of one of the grid holes in the SEM micrograph shown in Figure 3A (see arrow). From these observations, we may thus conclude that the printed patterns are faithful continuous copies, over length scales on the order of tens to at least ∼100 micrometers, of 763
Figure 3. (A, B) Scanning electron micrographs of portions of the slim bar grid (mesh 1000) acquired on an E-SEM Philips XL30 instrument (without any additional coating on the specimen). (C, D) Topography and friction contact mode AFM images (acquired as in Figure 2,A) of printed monolayer patterns produced with the grid from which the SEM pictures shown in A and B were taken. (E) Semicontact mode AFM topographic image (acquired as in Figure 2, B) of a portion of the smoother (shiny) side of a bar from the same mesh 1000 grid and (right side) distance-height profile along the marked line in the image. 764
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the actual geometrical features of the grids used as stamps in these experiments. The AFM image of a portion of the surface of the mesh 1000 grid used in the present printing experiments (Figure 3E) shows a grid bar with a characteristic sharp edge boundary and a corrugated surface displaying peak-to-valley features in the range between ca. 20 to 50 nm. This surface corrugation alone should prevent continuous direct charge transfer to a rigid substrate over large geometric contact areas, as suggested by the reported failure of some electrical microcontact printing experiments performed with rigid stamps exposing smoother contact surfaces.9 Moreover, the two sides of the present employed TEM grids are not identical, one side being considerably rougher (peak-to-valley features up to ca. 230 nm) and having a curved lateral surface profile of the bars. Printing attempts with this rougher side touching the monolayer surface also produced continuous patterns over multimicrometer areas, the bars appearing to be narrower because of their macroscopic surface curvature. Because the printing process described here relies on an electrochemical oxidation mechanism mediated by water,1-3 these results point to the presence of a conformal water bridge of considerable thickness between the rigid metal stamp and the rigid monolayer surface. Under such conditions, an uninterrupted flux of electrogenerated reactive species would effect the uniform oxidative conversion of surface exposed -CH3 groups to -COOH over the entire monolayer area covered by the stamp. The essential role played by an interfacial water bridge in the present process is consistent with the interesting observations of the formation of tip-surface water bridges in a related electrochemical patterning process of silicon surfaces carried out with conductive AFM tips operated in the noncontact mode.14 However, whereas the tip-surface water bridges in the reported AFM patterning experiments extend to less than ∼20 nm,14 the present results provide evidence for the possible formation of continuous interfacial water layers with thicknesses reaching tens and even hundreds of nanometers.15 Interesting complementary information on the nature of the patterns that were produced and their appearance on scales larger than 90 µm was obtained from optical micrographs of the water condensation patterns produced on highly hydrophobic OTS monolayer surfaces on which grid patterns were printed by the present procedure (Figure 4). The formation of a continuous water layer covering large portions of the geometrical area of each of the surface-modified regions of the printed grid patterns confirms the high local surface hydrophilicity generated by this printing process, thus providing further supporting evidence for the conversion of top monolayer -CH3 groups to -COOH. As expected, no water condensation patterns could be produced after the modified OTS sites were covered with a hydrophobic NTS monolayer (as in Figure 2B), which erases the wettability contrast between the modified and unmodified surface regions. However, from the images shown in Figure 4, it is apparent that besides uninterrupted wet features extending to more than 0.5 mm in length there are also discontinuities, pattern regions on which less water was adsorbed (lower contrast in the image), as well as entire portions of the grid Nano Lett., Vol. 3, No. 6, 2003
Figure 4. Optical micrographs of water condensation patterns recorded from OTS/Si monolayer specimens with printed patterns of the mesh 400 and mesh 1000 grids. To induce the condensation of water vapor on the monolayer surface, the examined specimen was cooled by placing it on top of a piece of ice while being covered with an inverted funnel through which the surface could be viewed and micrographs of it recorded. Before covering the specimen, water vapor was allowed to condense on the inner surface of the funnel. Hydrophilic surface areas retaining condensed liquid water appear darker because of their diminished reflectivity compared to the shiny appearance of the dry silicon surface. The wide dark regions in the upper picture and the region covered by water droplets of variable size in the lower picture represent surface areas that were contacted by the rims of the grids.
that have not generated monolayer replicas sufficiently hydrophilic to allow the formation of a visible water condensation pattern. At these locations, the stamp-monolayer gap was probably too large for the formation of a continuous water bridge that can mediate an electrochemical pattern transfer process. This may be caused by structural imperfections and geometrical deformations of the TEM grids that cannot be assessed by the presently employed means. Another less probable possibility would be that the applied grid-cleaning procedure did not render the entire grid surface 765
sufficiently hydrophilic for the formation of a continuous interfacial water layer. The possible electrochemical generation of planned monolayer patterns that span the entire nanometer-millimeter dimension range was demonstrated in a series of preliminary experiments in which a scanning AFM tip was used to inscribe arbitrarily chosen geometric shapes of nanometer dimensions at arbitrarily selected sites within unaffected monolayer areas of a preprinted TEM grid pattern (Figure 5). Like the much larger grid features produced by printing (Figures 2A and 3C and D), the tip-inscribed nanometric shapes shown in Figure 5 give rise to the same characteristic negative contrast in the topographic image and concomitant positive contrast in the corresponding friction image, which points to the identical chemical nature of the patterns realized by the two different patterning methods. In conclusion, using an improvised metal stamp and a very simple and straightforward experimental procedure, we have been able to demonstrate the feasibility of a micropattern printing method based on a nondestructive electrochemical process analogous to that underlying the nanoelectrochemical patterning of monolayer surfaces with conducting AFM tips.3 By analogy with the tip-induced nanopatterning method described before,3 the present printing method will be referred to as constructive microlithography. The success of the preliminary experiments described here, despite the simple experimental setup that was used, points to the remarkable unique advantage of constructive microlithographysthe possible utilization of both rigid stamps and rigid substrates, with significantly relaxed requirements as to the uniformity of the intimate stamp-substrate contact necessary for the production of high-fidelity replicas of the stamp.6-8 This is achieved as a result of the formation of a liquid-water bridge of considerable thickness at the stamp-substrate interface, which plays the role of a conformal flexible medium through which the to-be-printed information is electrochemically rather than electrically9,10 transferred from the stamp to the substrate. Compared to the serial pattern inscription by a scanning probe,3 the present one-step printing method offers the obvious high-speed advantage of parallel lithographic techniques without sacrificing any of the advantages inherent in the additive nature of the pattern-development process characteristic of a bottom-up approach. Thus, constructive microlithography provides a practical means for the direct realization of monolayer template patterns extending beyond the limited range (nanometer-micrometer) normally accessible to a scanning probe.16 At this stage, many questions with important implications for further attractive extensions of this novel printing method remain unanswered. For example, what is the optimal water-bridge thickness for the effective electrochemical transfer of individual stamp features with variable size and lateral spacing, and how could the thickness of such a liquid bridge be reproducibly adjusted?15 What is the ultimate lateral resolution and feature size limit for pattern transfer by such printing processes? Preliminary results obtained just before the submission of the present report point to the possible one-step duplication of micropatterns printed on OTS/Si monolayers, using such printed 766
Figure 5. Contact mode AFM images of some tip-inscribed nanofeatures on OTS/Si in the vicinity of a preprinted grid bar edge (covered with a top NTS monolayer). Both the pattern inscription and the acquisition of the images were done on the SOLVER P47 instrument (see also the description in Figure 2) with the same conductive tips: TiO2-coated silicon contact probes (MikroMasch) with typical spring constants of 0.03-0.08 N/m. The inscription of the star-shaped nanofeatures was done in a controlled-humidity atmosphere (∼20% relative humidity). Each feature was produced by locally defining a 400 × 400 raster-scanned dot matrix, within which dots were written by applying electrical pulses of 0.05 ms/ dot at a positive surface bias of 4.0 V relative to the tip.
monolayers as stamps with which direct electrochemical pattern transfer to another OTS coated substrate is realized.17 Would it be possible to similarly duplicate nanopatterns produced by the serial AFM process? This would open exciting possibilities for the extension of the parallel printing process down to nanometer dimensions using flat rigid stamps and a pattern-transfer mechanism free of the problems caused by the lateral diffusion of surface-deposited chemical Nano Lett., Vol. 3, No. 6, 2003
inks.7 All of these issues now wait to be elucidated in future research carried out with purpose-designed stamps and under improved experimental conditions. Among others, an interesting exploration path that we are presently contemplating would be to coat the hydrophilic features of rigid stamps with hydrogel layers so as to create surface-immobilized quasi-liquid bridges, the thickness and conformal contact performance of which can be adjusted and optimized for specific electrochemical printing tasks. Coating wettable surface features of a rigid stamp with a gel layer carrying a chemical reactant or a catalyst could also enable pattern transfer via suitable surface chemical modifications. Acknowledgment. We thank Dr. Asa H. Barber for providing the SEM micrographs shown in Figure 3. This research was supported by The Israel Science Foundation and the Minerva Foundation (Germany). S. H. is grateful to the Minerva Foundation for a postdoctoral fellowship at the Weizmann Institute. References (1) Maoz, R.; Cohen, S. R.; Sagiv, J. AdV. Mater. 1999, 11, 55-61. (2) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 424-429. (3) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 725-731. (4) Hoeppener, S.; Maoz, R.; Cohen, S. R.; Chi, L. F.; Fuchs, H.; Sagiv, J. AdV. Mater. 2002, 14, 1036-1041. (5) Liu, S.; Maoz, R.; Schmid, G.; Sagiv, J. Nano Lett. 2002, 2, 10551060. (6) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. DeV. 2001, 45, 697-719. (7) Geissler, M.; Bernard, A.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. J. Am. Chem. Soc. 2000, 122, 6303-6304. (8) Loo, Y.-L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. Appl. Phys. Lett. 2002, 81, 562-564.
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(9) Jacobs, H. O.; Whitesides, G. M. Science 2001, 291, 1763-1766. (10) Schmid, H.; Wolf, H.; Allenspach, R.; Riel, H.; Karg, S.; Michel, B. AdV. Funct. Mater. 2003, 13, 145-153. (11) Lindsay, S. M. In Scanning Probe Microscopy and Spectroscopy, 2nd ed.; Bonnell, D. A., Ed.; Wiley-VCH: New York, 2001; pp 289335. Burnham, N. A.; Colton, R. J. In Scanning Probe Microscopy and Spectroscopy, 2nd ed.; Bonnell, D. A., Ed.; Wiley-VCH: New York, 2001; pp 337-369. Ebenstein, Y.; Nahum, E.; Banin, U. Nano Lett. 2002, 2, 945-950. (12) Highly hydrophilic phosphate-terminated patterns, produced by the in situ reduction of initial carboxylic acid terminal functions to alcohol followed by their conversion to esters of phosphoric acid, were found to be particularly susceptible to this artifactal effect, displaying apparent depths up to ca. 5 nm below the unpatterned OTS background. It was also found that standard contact tips usually give rise to larger topographical differences of this kind, compared to the coated (conductive) tips used for patterning (Hoeppener, S.; Maoz, R.; Sagiv, J. To be submitted for publication). (13) It should be noted that exceeding the bias range under which nondestructive patterning of the top monolayer surface is possible will result in the growth of silicon oxide, with the consequent development of significant positive contrast (i.e., higher topographic features at surface-affected sites).3 (14) Garcia, R.; Calleja, M.; Pe´rez-Murano, F. Appl. Phys. Lett. 1998, 72, 2295-2297. Garcia, R.; Calleja, M.; Rohrer, H. J. Appl. Phys. 1999, 86, 1898-1903. Calleja, M.; Tello, M.; Garcia, R. J. Appl. Phys. 2002, 92, 5539-5542. (15) Successful pattern-transfer experiments could not be carried out in the absence of a sufficiently thick water layer adsorbed on the freshly cleaned TEM grid surface upon exposure to the saturated water vapor above a vessel filled with hot water. However, a printing experiment with a grid on which a drop of water was placed before contacting it with the monolayer surface produced a featureless surface spot, thus confirming the liquid-mediated, long-range electrochemical nature of the printing mechanism in these experiments. (16) Studies presently under way with printed grid patterns on top of which an NTS monolayer was assembled (as in Figure 2B) demonstrate the selective self-assembly of a metallic silver film on thiolated pattern features produced by the in situ chemical modification of the terminal ethylenic functions of NTS.4 In this manner, template-guided metallization of both tip-inscribed and printed features, over the entire nanometer-millimeter range, can be simultaneously achieved. (17) Hoeppener, S.; Maoz, R.; Sagiv, J. To be submitted for publication.
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