FEATURE ARTICLE Soft lithographic methods for nano-fabrication Xiao-Mei Zhao, Younan Xia and George M. Whitesides* Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA Soft lithography is a low-cost, non-photolithographic strategy for carrying out micro- and nano-fabrication. This unconventional approach consists of techniques based on self-assembly and replica molding of organic molecules and polymeric materials.Four such techniques, microcontact printing (mCP), replica molding, micromolding in capillaries (MIMIC), and microtransfer molding (mTM), have been demonstrated for the fabrication of patterns and structures of a variety of materials with dimension30 nm. This review describes these techniques and their applications in fabrication and manufacturing at the sub-100 nm scale.The demand for fabrication techniques that are capable of of functionalized alkanes onto the surfaces of appropriate substrates.16,17 The thickness of a SAM is usually 2–3 nm, and forming nanometre-sized structures rapidly and economically is a major driving force in the development of nanoscience can be tuned with an accuracy of ca. 0.1 nm by varying the number of carbon atoms in the alkyl chain. The interfacial and nanotechnology.1 A wide range of techniques have been and are being developed for nano-fabrication: e.g., deep UV properties of a SAM-covered substrate can be easily modified by the incorporation of organic and inorganic functional (l=200–290 nm) and extreme UV (l<200 nm) photolithography, 2,3 phase-shift photolithography,4 electron-beam writ- groups into and/or at the end of the alkyl chain.SAMs of long-chain alkanethiolates [in particular, hexadecanethiolate, ing,5 focused ion beam (FIB) lithography,6 X-ray lithography,7 scanning probe lithography,8 and others.9 Despite the extra- CH3(CH2 )15S-] on gold represent one of the most developed and best characterized systems.They have been used as model ordinary success of these technologies, new strategies are still desired for manufacturing nanostructures; a major hurdle to systems for studying the properties of SAMs, such as structures, 18–20 wettabilities,21,22 and densities of defects.23–25 cross in the development of future technologies for nanofabrication is the enormous expense (both capital expense and We10,26–35 and others36–43 have demonstrated and developed SAMs of long-chain alkanethiolates and alkylsiloxanes as operating expense) of these technologies, and of the clean rooms and specialized reagents they usually require.ultrathin (2–3 nm) resists in lithography at the nanometre scale (100 nm). The formation of patterned SAMs is the key to This paper discusses soft lithography, a collective name for techniques based on self-assembly and molding,as a convenient such applications.Table 2 lists techniques that have been demonstrated for the generation of patterned SAMs with and low-cost approach to micro- and nano-fabrication (Table 1). Soft lithography uses soft, organic materials (e.g., feature sizes 100 nm. Other lithographic techniques (for example, photochemical oxidation,44,45 cross-linking,46 and functionalized alkanes and polymeric materials) to generate patterns and structures without the use of light or other high- generationof reactive groups47–50 ) are generally less convenient than mCP and have not been used with sub-mm structures, energy particles.Its strengths and weaknesses are very dierent from other microlithographic techniques.Although it is at an although they may still find some applications. Microcontact printing (mCP) is perhaps the most versatile very early stage of development, soft lithography has been shown to be a rapid and inexpensive way of forming and and cost-eective method for the generation of patterned SAMs with lateral dimension 100 nm. Fig. 1 shows the transferring patterns and structures (30 nm in dimension) onto or into other materials. This review focuses primarily on schematic procedure for mCP. It uses an elastomeric stamp (usually made from polydimethylsiloxane, PDMS) with a relief the procedures for four soft lithographic techniques, microcontact printing (mCP), replica molding, micromolding in capillaries (MIMIC), and microtransfer molding (mTM), as Table 2 Techniques for patterning SAMs at sub-100 nm scales well as their potential applications in the fabrication of patterns and structures having at least one dimension 100 nm.technique smallest feature size/nm ref. microcontact printing ca. 100 10, 26–29 Self-assembled monolayers and microcontact micromachining ca. 100 30–32 printing neutral atom lithography ca. 70 33–35 electron-beam writing ca. 5–6 36–38 Self-assembled monolayers (SAMs) are highly ordered molecu- STM writing ca. 10 39–42 lar assemblies that form spontaneously by the chemisorption Table 1 Four soft lithographic techniques smaller feature/nm largest patterned area technique (lateral dimension) (for this feature size) ref. microcontact printing (mCP) ca. 100 ca. 50 cm2 (ca. 0.5 mm) 10, 11 replica molding ca. 30 ca. 1 cm2 (ca. 0.2 mm) 12, 13 micromolding in capillaries (MIMIC) ca. 1000 ca. 1 cm2 (ca. 1 mm) 14 microtransfer molding (mTM) ca. 700 ca. 2 cm2 (ca. 1 mm) 15 J. Mater. Chem., 1997, 7(7), 1069–1074 1069Fig. 2 (A) Scanning electron micrograph (SEM) of an array of 2.5 mm wide lines of Au generated using the standard procedure of mCP, followed by chemical etching in a basic cyanide solution. (B) SEM of a gold pattern that was produced using mCP under water with the same PDMS stamp as in (A).The inked stamp was allowed to remain in contact with the gold surface for ca. 5 min. Fig. 1 Schematic illustration of the procedure for mCP. An elastomeric decanethiol from the edges of the pattern on the surface of the stamp is made by casting a prepolymer of PDMS against a master stamp.The resulting 100 nm wide lines were transferred into that is usually made by microlithographic techniques. The stamp is the thin film of gold by selective etching in an oxygen-saturated inked with a solution of hexadecanethiol in ethanol, dried in a stream cyanide solution. of N2, and then brought into contact with the gold surface.The Microcontact printing followed by selective chemical etching patterned SAMs can be used as resists in wet chemical etching to transfer patterns to the Au film. is capable of generating arrays of micro- and nano-structures of a variety of materials with controlled shapes and dimensions. This capability has direct applications in the fabrication of structure on its surface to transfer alkanethiol molecules (the custom-designed structures for studies of cell attachment,64–66 ‘ink’) to the surface of gold by contact.51 It is experimentally sensors,67 and other electrochemical and optical devices,68–70 simple and inherently parallel: it can form patterned sub-mm as well as in the fundamental studies of tribology such as features over an area of ca. 50cm2 in a single impression wetting and adhesion.71–74 The quality of the final products, within 30 s.11 Microcontact printing has been used to form however, has not yet met the requirements for the fabrication patterned SAMs of alkanethiolates on Au,10,26–29 Ag,52 Cu,53,54 of microelectronic devices for several reasons. First, the best and GaAs;55 and of alkylsiloxanes on Si/SiO2, glass and plasma- SAMs are formed on Au and Ag, and these metals are not treated polymer films.56–60 These processes are best understood acceptable as masks in the manufacturing of microelectronic for Au and Ag substrates, where the quality of SAMs is higher devices.Second, even high-quality SAMs have a relatively high relative to those on the other substrates. SAMs of long-chain density (5 pits mm-2) of defects.23 Third, the technology needed alkanethiolates [CH3(CH2)nS-, n12] with hydrophobic ter- to register patterns in multilevel fabrication has not yet been minal groups can eectively protect the underlying substrates developed.Nevertheless, the microlithographic techniques from dissolution in certain types of aqueous etchants.61–63 The based on mCP have attracted broad attention, and their patterns in printed SAMs can, therefore, be transferred into development is proceeding rapidly.We believe mCP will the underlying layers (e.g., Au, Ag and Cu) by selective chemical become an alternative method to conventional techniques for etching. The patterned structures of metals obtained this way micro- and nano-fabrication in the future.can be further used as secondary masks for the etching of underlying substrates of SiO2, Si, or GaAs. Patterns of SAMs with dimensions >200 nm are routinely Replica molding and related techniques for nano- generated using mCP. Smaller features (ca. 100 nm in dimen- fabrication sion) can also be generated using mCP by modifying the stamp and/or the printing procedure.10,26–29 For example, mechanical Photolithography,75 replica molding,76–78 embossing (or imprinting),79 and related techniques80–82 have been demon- compression of the stamp,28 controlled reactive spreading of hexadecanethiol under water,10 and casting stamps from blazed strated for the fabrication of micropatterns and microstructures of polymeric materials.Deep UV photolithography, electron- diraction gratings29 or masters prepared by anisotropic etching of Si(100)27 have been used to successfully fabricate features beam writing and X-ray lithography are the techniques commonly used in producing nano-structures (100 nm in with dimensions in the range of ca. 100 nm. Fig. 2 shows an example in which a reduction in feature size from ca. 2.5 mm dimension) with reasonably high wafer throughput. Cost of ownership issues and the requirements for fabrication space, to ca. 100 nm was accomplished by carrying out mCP under water, leaving the stamp in contact with the gold surface for however, have so far limited their applications, research and development. ca. 5 min. The reduction in dimension for the underivatized regions was caused by the reactive spreading of the hexa- Molding and embossing have been applied in generating 1070 J.Mater. Chem., 1997, 7(7), 1069–1074polymeric structures with feature sizes 100 nm. Functional making multiple (30) copies. This result demonstrates that replica molding against an elastomeric mold is capable of microstructures such as diraction gratings,77,83 compact discs (CDs),76,84 and microtools,85 are routinely manufactured at providing multiple copies of nano-structures from a single master: that is, it is capable of manufacturing nano-structures.the mass-production scale. Fabrication in the nanometre scale using these techniques has begun to attract attention. In Replica molding against an elastomeric mold is an extended form of the conventional technique based on rigid molds.The particular, we have introduced a range of techniques, replica molding against an elastomeric master,13 micromolding use of elastomeric molds allows the sizes and shapes of the features on the final replicas to be controlled by using mechan- in capillaries (MIMIC),14,86,87 and microtransfer molding (mTM),15,88 for forming micro- and nano-structures of polymers ical compression, bending, stretching, or a combination of these techniques, and thus, adds flexibility to the replica and sol–gel materials, and Chou et al.89,90 have demonstrated excellent results with embossing.Replica molding, MIMIC, molding technique. Replica molding against a deformed elastomeric mold provides a unique new route to fabricate complex and mTM were initially developed to make microstructures of polymers with controlled shapes on planar and/or contoured micro- and nano-structures with shapes, sizes, and periodicities that are significantly dierent from those on the original surfaces; they are now being extended to the fabrication and manufacturing of nano-structures.12 master.Fig. 4D gives a representative AFM image of an array of 30 nm wide lines fabricated by replica molding against a bent PDMS mold.Comparison of the nano-features on the Replica molding replica (ca. 30 nm wide) to those on the original Au master The form of replica molding which we have developed diers (Fig. 4C) establishes that the dimensions of certain features on from the conventional molding techniques in the use of an elastomeric PDMS mold.Fig. 3 illustrates the general procedure we used in the replication.12,13 The use of an elastomeric (rather than rigid) mold simplifies the separation between the replica and the mold, and greatly reduces the possible damage to the mold and the fragile structures on the surface of the replica. Fig. 4A and B show AFM images of a master having an array of ca. 60 nm wide and ca. 50 nm high lines, and one of its replicas of polyurethane (PU). It is evident that replica molding against an elastomeric mold faithfully replicates the original master having delicate features. We have also monitored the changes in quality of the nano-structures on the original master and the PU replicas vs. the number of replications conducted. No observable reduction in quality was found either on the original Au master or on the replicas after Fig. 3 Schematic procedure for carrying out replica molding against an elastomeric PDMS mold. The PDMS mold is fabricated by casting against nanometre-sized relief structures fabricated using X-ray lith- Fig. 4 (A, B) AFM images of a master with an array of 60 nm wide ography or electron-beam writing. The test pattern shown here is an array of ca. 50 nm lines. Replica molding can also be conducted while lines of Au on Si/SiO2 and a PU replica generated from the PDMS mold cast from this Au master; (C, D) AFM images of another Au the PDMS mold is deformed, for example, by mechanical bending (B). The dimensions of the lines were reduced from ca. 50 nm to ca. 30 nm master having an array of 50 nm wide lines and a PU replica generated from a bent PDMS mold cast from this Au master in this process while the spacings between the lines increased slightly.J. Mater. Chem., 1997, 7(7), 1069–1074 1071the master have been reduced significantly from ca. 50 nm to (Fig. 7).15 It is also capable of generating isolated structures. Fig. 8 shows an array of submicron-wide pyramids having ca.ca. 30 nm by casting against a mold deformed by mechanical bending. We have also demonstrated that replica molding 100 nm size tips made using mTM. A disadvantage of mTM is that the features that are formed usually rest on a continuous, against a PDMS mold is capable of generating microstructures on curved surfaces,13 and producing functional microdevices thin (100 nm) film of the polymer.Micromolding in capillaries and microtransfer molding are with changing periodicities (e.g., chirped diraction gratings).13 the two new techniques capable of generating microstructures of polymers, inorganic salts, and sol–gel materials on Micromolding in capillaries and microtransfer molding substrates of completely dierent materials. Fabrication of We recently developed a new technique, micromolding in free-standing polymeric webs (using MIMIC),86 multilayer capillaries (MIMIC), for the fabrication of microstructures of structures (using mTM),15 and functional devices (e.g., polymers and other materials (Fig. 5).14,86–88 The PDMS polymeric waveguides,15,91 waveguide couplers,92,93 and master used in MIMIC is cast from an original master (for interdigitated carbon capacitors and suspended carbon micro- example, a photoresist master made using photolithography). resonators94,95) have also been demonstrated.MIMIC and Fig. 6 shows a test pattern having regions which are <100 nm mTM attract attention because of their abilities to fabricate high but ca. 2 mm wide.14 We have not applied MIMIC to complex topologies and structures with a broad range of smaller structures.Although it should, in principle, be appli- materials and to accept non-planar surfaces as substrates. cable to such structures, in practice, the very slow filling of Until now, MIMIC and mTM have primarily been applied to very small capillaries may limit its usefulness. the fabrication of features at micrometre scales. Their utility Microtransfer molding (mTM) oers a procedure, for rep- in forming nano-structures has begun to be explored with licating microstructures from an elastomeric mold, that is more promising initial results (Fig. 6 and 8). rapid than MIMIC, and applicable to larger areas; it has been applied successfully to both planar and contoured surfaces Conclusions and future work Nano-structures (100 nm in dimension) are an important set of targets in materials science.In the past, they have been fabricated mainly using electron-beam and ion-beam writing; deep UV, X-ray, and scanning probe lithographies. Although these technologies are very capable of generating a broad range of structures, they have a number of disadvantages that may limit their applications in manufacturing: for example, they are restricted in the types of materials that can be used as resists; they are not easily applicable to curved surfaces; and most importantly, they require high capital and operating costs.Soft lithography, in contrast, represents a class of largely unexplored, non-photolithographic techniques that oer a cost-eective strategy for fabricating and manufacturing nano- Fig. 5 Schematic procedure for MIMIC. This technique relies on a conformal contact formed between a support and an elastomeric (PDMS) mold with relief features on its surface to create a network Fig. 6 SEM (A) and AFM (B) images of patterned microstructures of of microchannels.A low-viscosity, liquidprepolymer fillsthese channels by capillary action. Solidification of the precursor in situ, followed by PU on a Si/SiO2 surface generated using MIMIC.The arrow in (A) indicates a line that is <100 nm in height. (C) A cross-sectional SEM removal of the PDMS mold, results in the formation of polymeric structures on the surface of the support. image of the fractured sample. 1072 J. Mater. Chem., 1997, 7(7), 1069–1074conditions, and we have, in fact, been able to generate patterned features 30 nm using a number of soft lithographic techniques, in a chemical laboratory, without using clean room facilities.Several issues remain to be solved before soft lithographic techniques find applications in the fabrication of complex, functional nano-structures. For example, the densities of defects23 in the structures formed by chemical etching using printed SAMs as resists are still too high to be used for the fabrication of microelectronic devices.A lack of tools for registration with nanometre accuracy limits its use in multilayer fabrication. Replica molding, MIMIC, and mTM may suer from artifacts due to deformation of the molds.97 The levels of defects in these structures (and in structures produced using techniques such as embossing) have only begun to be characterized.All these techniques for soft lithography are still in their early stages of development. Their opportunities and limitations in nano-fabrication and nanomanufacturing are still being defined. It is clear, however, that they oer exceptional convenience and economy in making certain kinds of structures, and the most probable strategy for their use will be to produce copies of master structures prepared by conventional but more expensive techniques (for example, X-ray lithography and electron-beam writing).The applicability of soft lithography to more complex structures will be defined as it is developed further. Fig. 7 Schematic diagram for mTM. A drop of prepolymer is applied This work was supported in part by the Oce of Naval on the patterned surface of a PDMS mold.The excess prepolymer is Research, the Advanced Research Projects Agency, and the scraped away using a piece of flat PDMS, leaving a filled PDMS National Science Foundation (PHY 9312572). This work made mold. The filled mold is then brought into contact with a substrate use of MRSEC Shared Facilities supported by the National and the prepolymer is allowed to solidify in situ.Patterned microstructures are obtained after the PDMS mold is removed. The process can Science Foundation under Award Number DMR-9400396. We be repeated on a substrate whose surface has already been patterned thank Dr. Hans Biebuyck for helpful discussions, and Dr. Je with a layer (or layers) of relief structures to build multilayer structures Carbeck, Andrew Black, and Joe Tien for their help in editing layer by layer.this manuscript. References 1 F. Cerrina and C. 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