NANO LETTERS
High-Resolution Contact Printing with Dendrimers
2002 Vol. 2, No. 4 347-349
Hongwei Li, Dae-Joon Kang,† Mark G. Blamire,† and Wilhelm T. S. Huck* MelVille Laboratory for Polymer Synthesis, Department of Chemistry, UniVersity of Cambridge, Pembroke Street, Cambridge, CB2 3RA, United Kingdom Received January 8, 2002; Revised Manuscript Received February 5, 2002
ABSTRACT We have used amine-terminated polyamidoamine (PAMAM) dendrimer generation 4 as ink for contact printing. Periodic dendrimer lines with widths of 140 nm and interline widths of 70 nm were printed directly on a silicon substrate. Without obvious molecular diffusion during printing, the printed pattern is only determined by the conformal contact area, which is in turn determined by the mechanical properties of elastomeric stamp.
Microcontact printing (µCP) has developed rapidly into a robust printing tool over the past few years.1-5 Many applications, from printing proteins patterns,6 patterned polymer growth,7 to printed microelectrodes arrays,8,9 organic light-emitting diodes10 and organic thin-film transistors11 have been reported. It shows some great advantages over conventional photolithography because it requires no complicated, expensive facilities and is therefore available to a wide range of researchers. Contact printing is not diffraction limited and can in principle be used to pattern surfaces with sub-100 nm features. High-resolution, reliable replication and printing of ∼100 nm relief structure has been shown using siloxane polymers with improved mechanical properties.12,13 The process of contact printing consists of several crucial steps: (1) replication of the nanoscale master structures in an elastomeric stamp; (2) inking the elastomeric stamp with alkanethiols (or other surfactant-like molecules capable of forming self-assembled monolayers (SAMs) on the substrate) and (3) forming a conformal contact with a solid substrate.14 During the contact time with the solid surface, the “ink” reacts with the surface, and a monolayer of molecules is transferred from the elastomeric stamp. Printed patterns on gold with alkanethiols were the first demonstration of µCP.1 Subsequently, other inks were developed to extend its application to silicon and other oxide surfaces.2,15,16 However, most inks used so far are small molecules that contain long chains and can form SAMs on a solid surface. One of the disadvantages of this kind of ink is the obvious diffusion during printing, which will obscure the structure and limit the resolution seriously.17 The diffusion of the ink can be strongly minimized and higher * To whom correspondence should be addressed. E-mail: wtsh2@ cam.ac.uk. † Department of Material Science and Metallurgy, University of Cambridge. 10.1021/nl025503c CCC: $22.00 Published on Web 02/26/2002
© 2002 American Chemical Society
resolution can be achieved, when the stamp is inked under carefully controlled conditions.3,4 Reinhoudt et al. have shown that “heavyweight” molecules can be used as lowdiffusion inks.18 Here, we report our recent progress on highresolution contact printing on silicon substrates with a new type of ink: dendrimers. Our results show that the resolution of printed lines can reach sub-100 nm dimension without obvious influence of molecular diffusion. Dendrimers have received a great amount of interest in recent years. Their well-characterized highly branched, globular architecture provide a variety of applications.19,20,21 Due to their compact size and monodispersity, dendrimers could be suitable for applications in nanotechnology especially when the desired pattern size is at the scale of 10 nm. Monolayers of dendrimers have been fabricated and used as resists for scanning probe lithography.22,21 We have used “starburst” polyamidoamine (PAMAM) dendrimer, generation 4 (G4), as ink for contact printing. The results reveal that the resolution can be greatly enhanced compared to trialkoxy- or trichlorosilane inks. The procedures for replica molding and nanocontact printing are shown schematically in Figure 1. We prepared the elastomeric stamp by replica molding through casting the liquid prepolymer of the elastomer onto a solid master. After curing the elastomeric stamp was peeled off, yielding inverse structures.23,24 The solid master we use is a grating in InP substrate and its surface pattern structure is produced by interference photolithography and etching. Figure 2(a) shows the scanning electron microscopy (SEM) image of the grating structure. The lines are approximately 70 nm wide with a periodicity of ∼210 nm. The depth of the relief structure is also confirmed to be ∼130 nm by atomic force microscopy (AFM). To aid in the release of the polymer replica, we formed a SAM of perfluorinated thiol, (CF3(CF2)9-
Figure 2. (a) SEM image of the InP master produced by interference photolithography. (b) AFM image of the PDMS replica from InP master.
Figure 1. Schematic diagram of the replica molding and nanocontact printing process.
(CH2)11SH)) on the surface of the InP master. The elastomer used was the modified poly(dimethylsiloxane) (PDMS) “Material A” as described by Schmid et al. in ref 12. After peeling off, the surface of the stamp was examined with AFM. Figure 2(b) gives the AFM image of the replicated PDMS stamp. We found no structural defects in the patterns on the surface over areas up to 1 square centimeter! The height of the lines is 120 nm, similar to those of InP master, indicating the high-quality of replication. The PAMAM dendrimer, 10 wt.% solutions in methanol, was obtained from Aldrich Chem. Co. It was diluted to 1 wt.% in ethanol before using. As indicated in Figure 1, we immersed the PDMS stamp into the dendrimer solution for two minutes to ink its surface. We then briefly rinsed the stamp with deionized water and dried it in a stream of nitrogen gas before printing. The dried stamp was placed on the surface of a cleaned silicon substrate and conformal contact formed between them. The stamp was in contact with the surface for ∼5 s and was then removed. Figure 3(a) and (b) are the AFM images of printed silicon surface. We have examined several positions of the same sample with a printed area of about 1 square centimeter and 348
the pattern was perfect over this whole area. No signs of collapsed or incomplete lines were found. The printed line width was about 140 nm. The distance between the printed dendrimer lines corresponds well with the original 70 nm features in the SEM picture, but there is a larger variation due to mechanical instability of the PDMS. Figure 3(c) gives the cross-section of AFM scan lines from the place indicated by arrow in Figure 3(b). The height profile of the printed dendrimer layer is no more than 1 nm. This means only a single layer of dendrimer molecules was deposited during printing. The high quality of the images indicates that the amine-terminated dendrimers form a very uniform layer on the surface of PDMS and can easily be transferred to the silicon surface, presumably through interaction with the primary amines and surface hydroxyl groups. Figure 3(b) is an AFM image of the patterned surface at higher magnification. It can be clearly seen that the edge of the lines is very sharp, which means that no serious molecular diffusion occurred during printing. The ease of printing with dendrimers indicates that this high-molecular weight ink provides a new route to patterning with sub-100 nm features. In Figure 3(b), individual dots can easily be observed inside the lines, with a diameter around 20 nm (roughly the size of a few dendrimer units). This indicates that, unlike the SAM-formation observed with alkanethiols and silanes, the printed layer formed by the dendrimer is not highly condensed. Because of the structure of the dendrimer molecules, the formation of dense, crystalline monolayers Nano Lett., Vol. 2, No. 4, 2002
lines excludes its usage as a resist. However, as the dendrimers have many functional amine end-groups, they are ideally suitable for further organic chemistry at nanometer scale. In summary, we reported nanocontact printing with modified PDMS stamps on silicon with amine-terminated dendrimer G4 as inks. Periodic lines with widths of 140 nm and interline widths of 70 nm were achieved. AFM measurements show that a single layer of dendrimer molecules can be transferred to silicon substrate without noticeable diffusion. The sharpness of the line edge is only determined by the conformal contact itself during printing. Acknowledgment. The authors thank the support from EPSRC. Dr. Wilfred Booij from Agilent Technologies, Ipswich, UK, is grateful acknowledged for providing us with the InP gratings. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Figure 3. (a) and (b) AFM images of printed dendrimer lines on silicon surface. Periodicity is 210 nm, line width is 140 nm. (c) The cross-section of dendrimer lines in (b) of the position indicated by the arrow.
is unlikely. The amine-terminated dendrimers form ionic bonds with silica surfaces. The strength of these bonds is dependent on pH, size (generation), and ionic strength of the solution.25 The adsorption of PAMAM dendrimers to silica and glass form aqueous solutions is well-understood.26 The adsorption of dendrimers from PDMS stamps will follow very different kinetics, but the formation of the ionic bonds will be the same. Because of their architecture the dendrimers are not able to fully stretch along the surface. The dendrimers can deform and flatten, but there will be dendrimer amines left that are not involved in binding. We are currently investigating a wider range of dendrimers with different surface functionality (amine and carboxylate groups) as well as studying their reactivity toward acid chlorides and isocyanates. The dendrimers probably aggregate and form clusters on the surface with heights up to 2 nm. Unlike the printed thiols on gold that can be used as resist in a subsequent etching process, the imperfection of dendrimer Nano Lett., Vol. 2, No. 4, 2002
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