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ReViews Patterning Techniques for Mesostructured Films† Plinio Innocenzi,*,‡ Tongjit Kidchob,‡ Paolo Falcaro,§ and Masahide Takahashi‡ DAP, Laboratorio di Scienza dei Materiali e Nanotecnologie, CR-INSTM and Nanoworld Institute, UniVersità di Sassari, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (Sassari), Italy, and Associazione CIVENsNano Fabrication Facility, Via delle Industrie 9, 30175 Marghera, Venezia, Italy ReceiVed July 6, 2007. ReVised Manuscript ReceiVed October 30, 2007
Patterning mesostructured films is becoming an important area of research for its technological implications. Self-assembled mesoporous materials synthesized through templating of nano-objects offer, in fact, the possibility of hosting active organic molecules or nanoparticles, and several applications in photonics and microelectronics are now emerging. At the same time, new “unconventional” lithographic techniques are also available to design nano- or microscale patterned mesoporous structures. We discuss the recent developments of lithographic methods for patterning mesoporous materials; some applications and future trends are also envisaged in the present review.
Introduction Mesoporous and mesostructured films are an important “bottom-up” example of porous materials for nanotechnology. The combination of sol–gel and supramolecular chemistry allows the elaboration of complex systems that selfassemble into organized structures.1 The formation process of these films is generally well described in terms of evaporation-induced self-assembly (EISA)2 and several examples of silica, metal oxide,3 and hybrid organic–inorganic4 films have been reported in the literature so far. Different nanoscale morphologies have also been observed; porous organization has been achieved in different “crystallographic” systems,5 and hexagonal,6 cubic,7 tetragonal,8 orthorhombic,9 and bicontinuous10 ordered phases have been obtained. Microelectronics and photonics are important fields of applications for mesostructured thin films, and micro- and nanofabrication technologies need to be developed for a practical exploitation of these new materials. The limit to define a nanofabrication process11 is the capability of patterning structures below 100 nm; beyond this dimension, the process can be classified as microfabrication. Lithography processing because of its high commercial interest is, therefore, the object of intense research activity. The two main methods to obtain nano- to microscale patterns are commonly characterized as “bottom-up” and “top-down”. In the top-down approach, different types of lithographic techniques are employed to pattern the film; in the bottomup route, nano- to microscale patterned structures are assembled using interactions between molecules or colloidal particles. There is another classification of the lithographic †
Part of the “Templated Materials Special Issue”. * To whom correspondence should be addressed. E-mail:
[email protected]. Università di Sassari. § Associazione CIVENsNano Fabrication Facility. ‡
techniques that is commonly used: “conventional” and “unconventional” lithographies. This classification is generally based on the level of commercial development of the technique. Photolithography and particle-beam lithography are the most widely used conventional techniques for micropatterning. Molding, embossing, scanning-probe lithography,12 and nanoimprint lithography13 are some examples of nonconventional methods. The possibility of designing patterns of porous materials that also exhibit a highly controlled pore-size distribution and pore topology is of high technological interest. The organized mesopores can be used as hosts for functional organic molecules or nanoparticles that allow for the design of materials for different types of advanced applications. The mesostructured films have several specific properties that make them a very challenging material to be patterned. It is, in fact, possible to use top-down lithographic techniques that are based on the production of a mesophase change in the film porous structure. On the other hand, the porosity can be used to introduce guest molecules that are photosensitive and writing on–off systems can be easily realized. Several examples of lithography techniques have been used to fabricate patterned mesoporous structures and are reviewed in this article. Some examples of these lithographic techniques are just an extension to mesoporous materials of wellestablished processes. A few others are, instead, an original development of patterning processes that have been specifically designed for self-assembly mesoporous-mesostructured materials. We have simply ordered the article as a function of the different techniques that have been reported so far for mesoporous patterning, preferring this approach to the division of the methods into nano- or microfabrication or other possible criteria.
10.1021/cm071784j CCC: $40.75 2008 American Chemical Society Published on Web 12/15/2007
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Figure 1. Scheme of the production process of a topographically patterned stamp (or mold) and its replica. The fabrication process is realized in different steps: (1) a silicon substrate is patterned with a photoresist; (2) a PDMS prepolymer is used to coat the photoresist-patterned silicon; (3) the sample is cured, the PDMS mold is peeled off, and a master is generated; (4) the master is used to generate a replica; (5) after curing and peeling off of the mold, a replica is generated.
Replica Molding Techniques Micromolding in capillaries (MIMIC) is a soft lithographic technique14,15 that uses capillary forces to fill patterned channels in a mold. A soft lithography is generally based on the preparation of a stamp from a liquid polymeric precursor by using a prepatterned master.16 By this method, patterned molds with relief structures are created using a master to produce a polymeric stamp that is then used to obtain a replica of the pattern (Figure 1). The most popular material to prepare the molds is poly(dimethylsiloxane) (PDMS). The main drawback of a MIMIC technique is the long processing time, but, on the other hand, the fabrication of the stamp is very cheap. It is, in fact, possible to produce up to 100 samples from a PDMS replica. Soft lithography is particularly feasible when applied to materials whose precursor is a low-viscosity solution. It results, therefore, in one of the simplest routes to fabricating patterned mesostructured materials. A MIMIC soft-lithographic method was used to fabricate a photonic device based on mesostructured films.17,18 This is an interesting example of exploitation of the properties of mesoporous films to fabricate a complex device. The organized porosity of the films can be used for different purposes such as to prepare a low refractive index layer or an active waveguiding layer/structure via doping of the pores with a proper photonic dye. An important property of mesoporous silica films is the low refractive index, typically in the 1.1–1.3 range,19 which allows the preparation of a cladding layer on silicon wafers. To propagate the light in the waveguide, it is, in fact, necessary that the refractive index of the cladding layer be lower than tha of the guiding layer. In a second step, waveguide structures can be fabricated via MIMIC on this mesoporous silica cladding layer. In the example reported by Yang et al.,18 the MIMIC process performed using a silicon substrate with a mesoporous silica cladding layer has allowed the obtainment of patterned waveguide arrays with lengths of several centimeters; typical dimensions that have been reported for the patterns are 1–3 µm width, 1–2 µm height, and 8 µm interspace between the waveguides. The patterned waveguides have exhibited a good capability of guiding optical signals. An advantage to preparing this type of device, based on patterned mesoporous waveguides, is the possibility of doping the patterns with optically active molecules or nanoparticles. This allows the fabrication of advanced optical components; amplified spontaneous emission (ASE) has been observed when the mesostructured waveguides have been doped with the laser dye rhodamine 6G. It is important to observe that “one-pot” synthesis has been used to introduce the dye in the mesoporous waveguides. This has the
advantage that the block copolymers stabilize the dyes, avoiding dimerization, which reduces their lasing capability.20 While the previous example has demonstrated that mesostructured waveguides show ASE, a laser device requires the incorporation of the gain material into a cavity with a resonant feedback. The same authors21 fabricated, by MIMIC soft lithography, a rhodamine 6G doped silica grating mesostructure that worked as a distributed feedback laser. These in-plane lasers require for operation reflection of light in a waveguide through a periodic modulation of the refractive index. From holographic photoresist-patterned masters, molds have been obtained by casting and curing PDMS onto the relief structures. Grating periodic structures with a periodicity of 190–210 nm and a depth of 40 nm have been produced. The multimode pulsed lasing has been demonstrated with fwhm of
