Patent Application
20120141731
Alkyl thiols and dendrimers have been used as resist molecules for wet chemical etching to generate arbitrary architectures with feature sizes varying from sub-100 nm up to several microns. See, for example, Geissler et al., Adv. Mater. 2004, 16, 1249; Perl et al., Langmuir 2006, 22, 7568; Rolandi et al., Nano Lett. 2004, 4, 889; Zhang, et al., Nano Lett. 2003, 3, 43; and Ducker et al., J. Am. Chem. Soc. 2006, 128, 392.
A basic principle behind this method is to first create a patterned monolayer of alkyl thiol molecule on substrates of interest, and then exposing this substrate to an etch solution. Etching can be achieved either by electrochemical methods, or by thiourea and iron nitrate based Etching, whereby different kinds of positive nanostructures could be generated. Alternatively, a thiol based molecule can be first patterned onto a metal substrate (which will be removed later on as a sacrificial molecule) and the unpatterned area backfilled with alkyl thiols. The backfill alkyl thiols act as etching resist due to their hydrophobicity after the first patterned monolayer is removed by electrochemical desorption. This backfilled alkyl thiol molecule will protect the covered area from being etched by the aqueous based etching solution, which generates the negatively etched nanostructures.
By using dip-pen nanolithography (DPN) printing, the size of the nanostructures can be minimized within 100 nm, and the patterned area can be as large as one square centimeter, while maintaining the sub-100 nm resolution of individual nanostructures. See, for example, Zhang, et al., Nano Lett. 2002, 2, 1389; Salaita et al., Angew. Chem. Int. Ed. 2006, 45, 7220; Salaita et al., Small 2005, 1, 940; Salaita et al., Nano Lett. 2006, 6, 2493.
In some cases, this method can involve strong chemical interactions between the thiol group and the noble metal substrates. However, in general, there can be two disadvantages of this method. A first is that even though the positively etched metal architectures are generated, the patterns are still covered by alkyl thiol molecules, which can negatively effect further studies of these nanostructures. Examples of such studies include plasma resonance and surface-enhanced Raman characterization. This alkyl thiol-Au chemical bond can be broken by exposing the sample to the UV light. However, this will create an extra experimental step. In the case of negative etching, electrochemistry is typically utilized in order to desorb the first patterned sacrifical thiol monolayer, which will also affect the density of the backfilled alkyl thiol monolayers, since the desorption potential for these two thiol-molecules are very close. This step negatively effects the thiol monolayer that was used as resist, unless special electrochemical instrument is employed in order to desorb the sacrificial thiol molecules.
A need exists for better, more versatile, and convenient resist processes for nanolithography and microlithography.
Described herein are, among other things, methods of making, articles, methods of using, and compositions.
For example, one embodiment provides a method comprising: disposing an ink composition on a tip, wherein the ink composition comprises poly(ethylene glycol) or poly(ethylene oxide), transferring the ink composition to a substrate surface to form a deposit, exposing the substrate to etching conditions.
One embodiment provides a method of forming nanostructures or microstructures comprising the steps of performing direct-write nanolithography to pattern a surface of a substrate with a patterning composition comprising an oligomeric or polymeric compound and to provide an exposed substrate, wherein the patterning composition is physisorbed to the substrate surface; and etching the exposed substrate on the patterned surface to form the nanostructures or microstructures.
Another embodiment provides a method of forming nanostructures or microstructures comprising the steps of performing microcontact printing to pattern the surface of a substrate with a patterning composition comprising an oligomeric or polymeric compound and to provide an exposed substrate, wherein the patterning composition is physisorbed to the substrate surface; and etching the exposed substrate on the patterned surface to form the nanostructures or microstructures.
Yet another embodiment provides a method of forming nanostructures or microstructures comprising the steps of performing direct-write nanolithography to pattern a surface of a substrate with a patterning composition comprising an oligomeric or polymeric compound, wherein the patterning composition is physisorbed to the substrate surface; coating the non-patterned region of the substrate surface with a passivating compound; removing the patterning composition to provide exposed substrate; and etching the exposed substrate to form nanostructures or microstructures.
Still, another embodiment provides a method of forming nanostructures or microstructures comprising the steps of performing microcontact printing to pattern the surface of a substrate with a patterning composition comprising an oligomeric or polymeric compound; coating the non-patterned region the substrate surface with a passivating compound, wherein the patterning composition is physisorbed to the substrate surface; removing the patterning composition and provide exposed substrate; and etching the exposed substrate to form nanostructures or microstructures.
In particular, material can be transferred from nanoscopic tips to a substrate by direct write methods.
Also in particular, the patterning composition may comprise oligomeric or polymeric compounds such as polyethylene glycol (PEG), polyethylene oxide and the like.
Advantages include for at least some embodiments excellent versatility and control. For positive etching, no passivation is needed. For negative etching, patterns can be easily removed. For both types, complicated electrochemical facilities, electrochemistry, or electrodes are not needed and etching is easy to operate.
In some embodiments, it has been discovered that PEG can be used as a novel and extremely useful resist material for generating both positive and negative structures in the context of DPN. In some embodiments, the PEG resist, when coupled with wet chemical etching, allows one to generate solid-state nanostructures in a manner that overcomes some of the limitations of the alkanethiol-based etching methods. Specifically, in some embodiments, the polymer-based approach requires only a simple washing step to desorb materials from a substrate surface and, in principle, can be used with many types of underlying substrates (there is no requirement of chemisorption). Furthermore, in some embodiments, the ability to generate a thick polymer layer and the elimination of the electrochemical desorption step associated with SAMs, results in less pitting of the surface due to pinholes. Finally, in some embodiments, the process works with parallel pen arrays allowing one to pattern over relatively large areas, and it is likely extendable to other polymeric materials and perhaps other lithographic techniques.
All references cited herein are hereby incorporated by reference in their entireties.
Present embodiments provide a facile and novel method for forming nanostructures and microstructures by patterning and etching a substrate surface.
Description of microfabrication, microstructures, nanofabrication and nanostructures is found for example in Madou, Fundamentals of Microfabrication, 2nd Ed., CRC, 2002. For example, chapter one describes lithography.
Nanolithography instrumentation can be obtained from Nanolnk, Chicago, Ill. or a conventional AFM instrument can be used. Direct-write methods are described in Direct-Write Technologies for Rapid Prototyping Applications, Ed. By Pique and Chrisey, 2002. See for example Chapter 1 overview and Chapter 10 for tip-based methods.
In one embodiment, a substrate surface is patterned using direct-write nanolithography. For example, Dip Pen Nanolithography printing techniques, which are described in various literature may be used.
For a more detailed description of Dip Pen Nanolithography printing, see, for example, U.S. Pat. Nos. 6,827,979; 6,635,311; and 7,102,656 as well published US appl. 20050191434A1 all hereby incorporated by reference as if fully set forth. Etching is described in for example US Patent Publications 200610014001 to Mirkin et al. and 200610081479 to Mirkin et al., which are hereby incorporated by reference in their entirety including etching procedures and characterization of microstructures and nanostructures. These references describe for example patterning, substrates, patterning compounds, and etching. Patterning compounds can be transferred from tip to substrate.
For tip based transfer methods, the tip can be a nanoscopic tip, scanning probe microscopy (SPM) tip, an atomic force microscopy (AFM) tip, a solid tip or a hollow tip. A channel can be present to conduct the patterning compound to the distal end of a tip.
In the context of tip based transfer methods, the substrate surface may be patterned by applying a patterning composition to the tip, and delivering said composition from said tip to the substrate surface.
In another embodiment, a substrate surface is patterned using microcontact printing, such as, for example applying a pattern using a stamp.
For a more detailed description of microcontact printing, see for example, in U.S. Pat. Nos. 6,951,818; 6,893,966; and 5,512,131 hereby incorporated by reference as if fully set forth.
Other methods include those described in for example US Patent Publication Nos. 2004/0228962 (Liu, Mirkin et al.) and 2004/0226464 (Mirkin, Zhang) for scanning probe contact printing.
In one embodiment, the patterning composition comprises an oligomeric or polymeric compound. In one example the composition may be formulated to also comprise a solvent. The solvent may be removed upon dispensing the patterning composition at the end of a tip or on a stamp surface. In another example, the patterning composition comprises a mixture of oligomeric and/or polymeric compounds. Said oligomers and/or polymers may have different molecular weights, functional groups, backbone structure or other variations. In another example, the patterning composition comprises co-oligomers and/or co-polymers.
In another embodiment, the patterning composition consists essentially of an oligomeric or polymeric compound. For example, the patterning composition may comprise one or more additional compounds in amounts to the extent that (a) do not interfere with transport of the polymeric or oligomeric compounds, (b) do not interfere with properties of the patterning composition as a resist, (c) do not substantially affect the removal of the pattern, or a combination thereof.
In yet another embodiment, the patterning composition consists of an oligomeric or polymeric compound.
In one embodiment, the oligomeric or polymeric compound comprises a hetero atom in the backbone such as, for example, an oxygen atom or a nitrogen atom. The backbone can function as a Lewis Base, and may or may not be linear. Preferably the oligomeric or polymeric compound comprises a polyether. Non-limiting examples of polyethers include polyalkylene oxides, polyalkylene glycols, and the like. Specifically, polyethylene oxide (PEO), polyethylene glycol (PEG) and polypropylene glycol (PPG) may be used.
The melting temperature of a suitable oligomeric or polymeric compound, when present, can vary, and may form a basis for selection of the compounds. For example, compounds may be chosen with a melting below about 60° C. so that in higher humidities the material flows well off of a tip to a substrate.
In general, the oligomeric or polymeric compound can have good solubility, compatible with water and organic solvents including polar and nonpolar solvents, good or reasonable transfer rates from tip to substrate, and form stable patterns in etching environment. The oligomeric or polymeric compound is preferably water soluble. It is also preferable that the formed pattern which comprises said oligomer or polymer, can be washed away with one or more rinses with water. Alternatively, the oligomer or polymer can be soluble in organic solvents such as for example dichloromethane. The oligomer or polymer can be soluble in both water and at least one organic solvent.
The molecular weight of the polymeric or oligomeric compound may vary. For example, the molecular weight may be between about 100 and about 1,000,000 or between about 500 and about 500,000 or between about 500 and about 100,000 or between about 1,000 and about 50,000 or between about 1,000 and about 10,000, or about 1,000 and 5,000, or about 1,000 and 3,000. In general, lower molecular weight compounds can facilitate faster transfer from tip to substrate.
Oligomers and polymers can be used as known to one skilled in the art of polymer chemistry. See for example Allock, Contemporary Polymer Chemistry, 1981; and Billmeyer, Textbook of Polymer Science, 3rd Ed., 1984.
Other compounds that may be used to pattern a substrate surface include for example, wax or polyethylene imine.
Patterning can be carried out at various humidity levels. For example, higher relative humidities such as at least 40%, or at least 60%, or at least 80% may be used. In some cases relative humidity of about 80% shows fast compound transfer rate from an AFM probe tip to a substrate.
Patterning conditions can be selected so that a good organic solvent solution can be used to coat evenly a tip surface.
The substrate surface can be a surface of any substrate although the surface can be adapted to function with the patterning composition and the application at hand. Substrates are generally preferred for providing high resolution patterns. For example, the substrate surface can be a surface of an insulator such as, for example, glass or a conductor such as, for example, metal, including gold. In addition, the substrate can be a metal, a semiconductor, a magnetic material, a polymer material, a polymer-coated substrate, or a superconductor material. The substrate can be previously treated with one or more adsorbates. Still further, examples of suitable substrates include but are not limited to, metals, ceramics, metal oxides, semiconductor materials, magnetic materials, polymers or polymer coated substrates, superconductor materials, polystyrene, and glass. Metals include, but are not limited to gold, silver, aluminum, copper, platinum and palladium. Other substrates onto which compounds may be patterned include, but are not limited to silica, silicon oxide SiOx, GaAs, InP and InAs.
One of the advantages of the present method is that it does not require for a substrate surface to be chemical or physical modified prior to forming the pattern. Accordingly, in some embodiments, the substrate surface can be an unmodified substrate surface, i.e. a substrate surface, which was not chemically or physically modified prior to being patterned.
The chemical or physical medication of the substrate surface is usually used in the prior art methods to promote ink transport from the tip or stamp to the substrate surface, to enhance ink adhesion to the substrate surface or to covalently modify the substrate surface. Examples of physical or chemical modification of the substrate surface include but not limited to base treatment of a charged surface of silicon oxide, silanization with amino or mercaptosilinizing agents or modification with polymers carrying chemically reactive groups.
Another advantage of the present method that it does not require prepatterning of the substrate surface.
The substrate can be monolithic, or comprise multiple materials including multiple layers. In a preferred embodiment, the substrate surface is a semiconductor or metal substrate surface.
The substrate surface can comprise conductive portions, insulative portions, or both. The conductive portions can be electrodes for example. The patterning composition can be used for example to form an etch resist for eventually fabricating electrodes or to modifying existing electrodes.
In one embodiment, the patterning composition is weakly bonded to the substrate surface. Preferably, the patterning composition physisorbs to the substrate surface. As used herein “physisorb” denotes an interaction that does not involve substantial chemisorption or covalent linkage. A physiosorbed patterning compound can be removed from the substrate surface with a suitable solvent.
The present embodiments relate to both positive and negative etching. In the present embodiments, the pattern(s) formed on a substrate surface function as an etch resist. In the preferred embodiments, said pattern(s) function as a wet chemical etch resist. However, certain embodiments may include a non-wet etching step.
The nanostructures and microstructures formed according to embodiments of the present invention can be comparable to those formed using alkyl thiol molecules as etching resist, in terms of both the etching quality and the resolution of features.
For example, dots and lines can be prepared. Lines can be straight or curved. Lateral dimensions such as line width and diameter can be for example about 10 microns or less, or about one micron or less, or about 500 nm or less, or about 250 nm or less, or about 100 nm or less. Minimum lateral dimension can be for example about 2 nm or about 5 nm or about 10 nm.
The embodiments claimed herein are further illustrated with the use of non-limiting working examples.
An embodiment is shown in
Alternatively, as shown in
Dip-pen nanolithography (DPN)[1,2] has emerged as a powerful tool for printing soft and hard matter on surfaces with sub-50 nm to many micrometer resolution. Indeed lithographic patterns of various small organic molecules,[3-5] polymers,[6-8] proteins,[9-12] sol gels,[1,3] nanoparticles,[14, 15] high melting temperature molecules,[16] and viruses[17] have been generated on a wide variety of substrates, including Au,[2, 18, 19] Ag,[20] GaAs,[21] and SiOx.[4, 22] With the development of cantilever arrays (linear A-26 pen[23] and 2D 55,000 pen array systems[24]) the technique has evolved into a parallel methodology[25] that, in certain cases, exceeds the throughput capabilities of serial nanolithographic techniques such as e-beam lithography. Indeed, it has been shown that by using a 2D 55,000 pen array in conjunction with wet-chemical etching protocols, one can generate millions of solid-state nanostructures over a square centimeter area in less than 30 min.[24]
A variety of etching protocols in combination with etch resist materials have been utilized to generate solid-state metal structures for applications in electronics, catalysis, and optics.[26] For example, alkanethiols have been used extensively as etching resists because they form self assembled monolayers (SAMs) that can protect an underlying metal surface from chemical or electrochemical oxidation and dissolution.[27-31] In fact, alkanethiols as DPN inks combined with wet-chemical or electrochemical etching protocols have been used to produce solid-state nanostructures with feature sizes ranging from 12 nm to many microns.[30-33] Typically, lithographic patterns of 1-octadecanethiol (ODT) or 16-mercaptohexadecanoic acid (MHA) are generated via DPN. Exposing the substrate containing the alkanethiol SAMs to etching solutions produces positive solid-state nanostructures. On the other hand, hole features (negative nanostructures) can be generated through the fabrication of MHA lithographic features using DPN, subsequently backfilling the exposed gold regions with ODT, electrochemically desorbing the MHA SAMs, and incubating the substrate in an etching solution. [33]
Although alkanethiols can be excellent etch resist materials for many surfaces (e.g., Au, Pd, and Ag), they possess certain limitations. In generating negative features, two different SAMs are required (e.g. ODT and MHA), and an electrochemical set-up can be necessary to selectively desorb one SAM in the presence of the other.[33, 34] Pinholes can lead to non-uniform etching and lower quality structures. Finally, one can use chemical protocols to eliminate the SAM resist from the surface once the desired solidstate structures have been made.
Herein, it is shown how polyethylene glycol (PEG), coupled with the high-resolution of DPN and wet-chemical etching methods, can be used as a novel physisorbed resist to generate high quality positive and negative nanostructures (
DPN templates of PEG features are used either as a protective or sacrificial layer to generate raised or recessed structures on surfaces,
One of the attributes of DPN is the ability to tailor feature size by varying the scan rate of the tip array and tip-substrate contact time. There is sometimes a feature size dependence that correlates with the square root of tip-substrate contact time.[16, 38-40] The PEG exhibits a similar dependence when deposited on a 30 nm thick Au film thermally evaporated on a SiO2 substrate. Scan rates of 0.05, 0.10, and 0.75 μm/sec gave 175, 105, and 70 nm wide line features, respectively,
In addition to working out the protocol for patterning PEG, the potential for using the PEG resist and wet chemical etching to generate positive solid-state features was evaluated. A 26-pen parallel array was used to generate twenty six 15×20 PEG dot arrays on a gold thin film surface. Each array consists of dots with deliberately generated 200, 300, 400 and 500 nm diameter features. The total time needed to generate the 26 identical PEG dot arrays was about 1 hr. The patterned substrate was subsequently etched using an aqueous solution of 20 mM thiourea and 30 mM iron nitrate monohydrate to generate positive Au nanostructures with dot diameters of 205, 289, 400, and 517 nm (±10 nm), respectively,
The DPN technique coupled with the novel PEG resist is quite versatile and allows one to generate very sophisticated structures, including complex shapes and patterns. As further demonstration, a digitized image was used of the Northwestern University logo, and a PEG replica of it at 80 nm dot size resolution in dot matrix form (about 12,000 features) was generated on an Au thin film substrate in 50 min. This structure was etched as described above for 45 min, rinsed with dichloromethane, and characterized by SEM (
Interestingly, the PEG not only can be used to generate positive features but also negative ones. To generate negative features, the PEG was used as a sacrificial template (
PEG templates were generated using DPN. A-26 tip-array AFM probe was dip-coated in a 5 mg/ml polyethylene glycol acetonitrile solution for 10 seconds. The polymer coated cantilever probes were then mounted on an NSCRIPTOR™, and polyethylene glycol features were written on a thin layer of Au thermally evaporated on a SiO2 substrate with a 10 nm Cr adhesion layer. PEG templates that were first passivated with 1 mM 1-octadecanethiol for 15 min and rinsed with a dichloromethane solution, prior to incubation in an etching solution containing 20 mM thiourea and 30 mM iron nitrate nonahydrate, were used to generate negative nanostructures. Positive solid-state nanostructures were generated upon direct incubation of the substrate in the same aqueous etching solution. The substrates were then rinsed with copious amounts of water to remove the PEG.
The following sixty embodiments are described in U.S. priority provisional application Ser. No. 60/886,839 filed Jan. 26, 2007.
This concludes the sixty embodiments.
This application claims the benefit of priority to provisional application Ser. Nos. 60/875,447 filed Dec. 18, 2006 and 60/886,839 filed Jan. 26, 2007, each of which are incorporated herein by reference in their entireties.
These inventions were developed in part with support from the Army Research Office (ARO) under contract no. 28065-3-A2//W9 1 1NF-04-1-07 1 and the National Institute of Health (NIH) under contract no. DPI OD 000285-02. The U.S. government has certain rights in the inventions.
| Number | Date | Country | |
|---|---|---|---|
| 60875447 | Dec 2006 | US | |
| 60886839 | Jan 2007 | US |
