Sol‐Gel Nano-/Micropatterning Process

Abstract

The nano-/micropatterning of sol‐gel-derived materials has been performed by means of embossing, molding, photo-assisted techniques, and so on. The research progress and some practical applications for each technique are reviewed, with embossing technique, for waveguides, optical gratings, memory disks, microlens arrays, and light-emitting diodes; molding technique including microcontact printing, replica molding, microtransfer molding, and micromolding in capillaries, for fluorophore-doped xerogels, photonic crystals, optical and bioactive glasses, and ionic conductors; and photo-assisted technique, for waveguides, gratings, antireflection coatings, and hologram recording films. Moreover, several advanced techniques are introduced such as hydrophobic–hydrophilic, photocatalytic, photolocking, and ink-jet patterning processes.

Introduction

Nano-/micropatterning techniques to form accurate, small structures on various substrates are essential in the scientific and technical fields of optics, photonics, electronics, optoelectronics, telecommunication, data storage, and biotechnology. Sol‐gel coating techniques provide promising and cost-effective nano-/micropatterning processes. In the sol‐gel nano-/micropatterning process, embossing technique using soft gel films, molding technique using sol‐gel precursors and patterned elastic replica, photo-assisted technique using photosensitive gel films, and some other advanced techniques have been developed so far (Uhlmann et al. 1990; Tohge, 1998; Xia et al. 1999; Menard et al. 2007).

Nano-/micropatterns are embossed in the soft gel films on various kinds of substrates using a stamp without expensive equipments such as exposure and developer apparatus, so that the embossing technique is promising for the fabrication of nano-/micropatterns on the substrates with large surface area due to the cost-effectiveness. When sol‐gel-derived inorganic–organic hybrid materials like organosilsesquioxane are applied to the embossing technique, the patterns with peak-to-valley values larger than several micrometers can be formed with a small shrinkage in the hybrid materials by adjusting the type and the amount of the organic groups. Moreover, the optical properties such as transmittance and refractive index of the hybrid materials can widely be controlled for the micro-optic devices by selecting the organic groups. The introduction of photopolymerizable organic groups such as methylene and epoxy into hybrid materials shortens the embossing time by irradiating corresponding light.

In the molding technique, an initially fluid sol‐gel precursor is allowed to acquire its final geometry by solidifying in a mold. This technique allows the reproduction of the fine details of the mold. Molds of structures with tens of nanometer-size features can be generated in elastic polymers such as polydimethylsiloxane (PDMS). As is the case with embossing technique, replica molding does not require photolithography and many replicas can be produced. In addition, it can be used to fabricate nano-/micropatterns even on curved substrates. It is important in the molding process that the precursor sol dewets the surface of the substrate and allows the contact of the elastic stamp with the substrate. Although gelation of the precursor sol occurs in a short period of time, the mold with stamp on substrates must be allowed to remain undisturbed for relatively longer period of time to consolidate the structures.

On the other hand, the photo-assisted technique using photosensitive gel films has a high potentiality to form precise nano-/micropatterns with high resolution, since the technique basically depends on the cutting edge of photolithographic process for semiconductor devices. Precise patterns with submicrometer periods of the gel films have been successfully fabricated by direct irradiation with an excimer laser, by exposure with high-pressure mercury lamp through a photomask, and by two-ultraviolet-beam interference method. The other advanced nano-/micropatterning techniques by the use of hydrophobic–hydrophilic patterns, photocatalytic effect of titania constituent, and ink-jet printing have also been proposed.

In this chapter, the research progress in the embossing, molding, and photo-assisted techniques for nano-/micropatterning is reviewed, and several practical applications of the sol‐gel-derived nano-/micropatterned films are described. In addition, advanced nano-/micropatterning processes based on hydrophobic–hydrophilic patterns and the photocatalytic effects of titania as well as ink-jet printing are reported.

Embossing Technique

Research Progress in Embossing Technique

Lukosz et al. have reported a pioneer work on embossed SiO₂–TiO₂ waveguide structures (Lukosz and Tiefenthaler 1983; Heuberger and Lukosz 1986). They embossed grating couplers with 120 line/mm in dip-coated gel films prepared from metal alkoxides. The grating retains the periodicity and spatial frequency after heat treatment and acts as an input–output coupler or a reflection grating.

Roncone et al. have successfully embossed sol‐gel-derived films with deep grating structures (Roncone et al. 1991). In this work, 50SiO₂ · 50TiO₂ films were prepared on the glass substrates and then prebaked at 70 °C. After prebaking, the films were embossed using a Au–Pt-coated photoresist grating as a master stamper. They documented that the changes in the embossing mechanism occurred as a function of the pre-emboss bake. At early prebake times, adhesion of the films to the master stamper is a problem; at intermediate times, the films are moldable and flow under the pressure of the master stamp; and at long times, the grating structure is not replicated but periodic stress cracks are produced. At the last stage, the film is sufficiently cross-linked to prevent significant plastic deformation.

Tohge et al. have developed a new process to form fine patterns in the sol‐gel-derived films applied on glass substrate with a large area (Tohge et al. 1988). The schematic illustration of the process is shown in Fig. 1. The sols containing an organic polymer such as poly(ethylene glycol), PEG, were used to coat glass substrates by the dipping–withdrawing method (a). A stamper with fine patterns was pressed against the resultant gel films while the films were still soft (b). After being dried, the patterned films were heat-treated in an electric furnace to decompose organic polymers added and densify the films (c). Through the above process, the glass substrates with negative patterns to the stamper were obtained (d). The key point of the fabrication of fine patterns is the control of the hardening of the gel films by adding PEG. The addition of PEG provides the gel films with malleability due to lowering of hardness and decelerating of hardening for the gel coating films, which permits embossing fine patterns. B₂O₃–SiO₂ system is selected for the coating films, since the gel films are densified at lower temperatures and show high transparency. The heat treatment of this patterned films produces the patterned 20B₂O · 80SiO₂ glass films. The addition of PEG has little effects on the texture and transparency of the resultant 20B₂O₃ · 80SiO₂ glass films, except for the large shrinkage in the direction vertical to the films.

Fig. 1

Fine patterning process by the sol‐gel method; (a) formation of gel films on substrates, (b) patterning on the gel films, (c) heat treatment, (d) glass film fine patterns (Tohge 1988)

Matsuda and his colleagues have reported that pregrooves for optical memory disks were successfully fabricated in sol‐gel-derived SiO₂–TiO₂ coating films on glass substrates of 130 mm in diameter, using a sol‐gel micropatterning process (Matsuda et al. 1990; Matsuno et al. 1992; Mitsuhashi et al. 1992). The patterning was carried out under vacuum by pressing a stamper against the SiO₂–TiO₂ gel film containing PEG. The practical fabrication process for optical memory disk substrates is described in “optical memory disk substrates” in detail.

Pregrooves of 1.6 μm pitch for optical data storage have been embossed successfully by pressing a stamper against CH₃SiO_3/2–SiO₂ gel films on glass disk substrates of 130 mm in diameter (Matsuda 1998a). Atomic force microscopy (AFM) image of the pregrooves is shown in Fig. 2. When CH₃SiO_3/2 mol% is less than 40%, the resultant films are too hard to emboss patterns uniformly. The shrinkage of the patterns is around 4% for all the films containing 60–100 mol% CH₃SiO_3/2 even after a heat treatment at 350 °C, indicating that the nearly net negative shape of the stamper is preserved. The methyl groups in the films are decomposed at temperatures from 500 °C to 600 °C.

Fig. 2

AFM image of the pregrooves fabricated on a glass disk substrate for MeSiO_3/2 gel films derived from MeSi(OEt)₃ (Matsuda 1988a)

Menning et al. have reported a new synthesis and processing route for SiO₂ glass like micropatterns with heights up to 30 μm by embossing and thermal densification (Menning et al. 1997). An organically modified nanoparticulate sol was prepared from methyl- and phenyltriethoxysilanes (PTES) and tetraethoxysilane in combination with colloidal silica sol under acidic conditions. Flexible and low-cost silicone rubber stampers can be used due to low pressure required for embossing. The capability of this technique was demonstrated by the fabrication of light trapping structures with pyramids of 7 μm in height and 10 μm in width on an area of 20 × 20 mm² and microlens arrays of 30 μm in height and 600 μm in diameter on an area of 20 × 30 mm².

Shinmou et al. have successfully molded microlens arrays composed of organic–inorganic materials on glass substrates (Shinmou et al. 2000). Phenyltriethoxysilane and dimethyldiethoxysilane (DMDMS) were used for the starting materials. The preparation procedure and the evaluation of the microlens arrays are described in “Planar Microlens Array” in detail. In a related work, prismatic patterns with a pitch of 30 μm and a slant height of 30 μm were successfully embossed in phenyltriethoxysilane and methyltriethoxysilane-derived thick films on glass substrates by laminating an organic polymer sheet as a stamper with the patterns against the films (Matsuda et al. 2000). The embossed shape of the prismatic patterns precisely agrees with the negative shape of those of the stamper used. The resultant 70CH₃SiO_3/2 · 30C₆H₅SiO_3/2 (mol%) films are transparent and the refractive index of the films was adjusted to be 1.51, which matches with that of the glass substrate.

Krug et al. have reported an important embossing and UV-curing technique to fabricate surface relief in organically modified ceramics (Ormocer) (Krug et al. 1992). The coating materials for embossing were synthesized from methacryloxypropyltrimethoxysilane, methacrylic acid, and zirconium-n-propoxide. The buildup of the inorganic network takes place during sol‐gel synthesis, while the final polymerization occurs during embossing and UV curing. The refractive index of the materials can be varied by the composition. The composite has low optical loss, synthesis-controlled homogeneity, and low shrinkage, which allows structures with peak-to-trough values of some microns.

Guo has published a review paper on “nanoimprint lithography” whose concept is an analogue of embossing technique, but generally the scale is one or a few orders smaller than embossing technique (Guo 2007). Since nanoimprinting has advantages of high throughput, great precision, and low cost, a variety of research papers have recently been reported. For example, sol‐gel-derived hydrogen silsesquioxane/ZrO₂ nanoparticle composites were used for roll-to-roll nanoimprinting to fabricate rigid stamps on cylindrical substrates (Ryu et al. 2015). Byeon et al. have reported highly bright GaN-based light-emitting diodes could be fabricated through thermal nanoimprinting of ZnO nanoparticle-dispersed resin (Byeon et al. 2015).

Practical Application of Embossing Technique

Optical Memory Disk Substrate. An emboss-micropatterning technique to form pre-grooves or pit patterns on glass substrates for optical memory disks has been developed to a practical production level. The whole image of the sol‐gel optical memory disk glass substrate and the SEM image of the pit patterns of the sol‐gel CD-ROM (Matsuda 1998b; Yamaguchi et al. 1991) are shown in Figs. 3 and 4, respectively. The key point of the micropatterning technique developed is the embossing by pressing a stamper against the gel films, the hardness of which is controlled by the incorporation of poly(ethylene glycol), PEG (Tohge 1988).

Fig. 3

Whole image of the sol‐gel optical memory disk glass substrate (Matsuda 1998b)

Fig. 4

SEM image of the pit patterns of the sol‐gel CD-ROM glass substrate (Yamaguchi et al. 1991)

A model of the micropatterning process described from the viewpoint of gel film hardness is shown in Fig. 5. The process consists of the following steps: (a) formation of the gel films on a glass substrate, (b) pressing a stamper against the gel film, (c) removal of the stamper, and (d) heat treatment to decompose PEG in the film. It is essential in this process to press a stamper against the gel film while it is soft enough to deform by the press and to remove the stamper from the gel film after it becomes hard enough to keep the shape of the patterns formed. The specific values of hardness of the gel film in Fig. 5, H_d and H_r are defined as the maximum hardness to deform by the press and the minimum hardness to keep the shape of the pattern after removal of the stamper, respectively. In order to apply this process to the industrial fabrication, a shortening of the process time is necessary. Among the above steps (a) through (d), the period from (b) to (c) takes the longest time, since in this period each substrate must be pressed with a stamper individually. The preheat treatment during the pressing time is very effective for shortening the process time and it is expected that the process time decreases with an increase in the preheat treatment temperature. Moreover, it is obvious that there is a lower limit in the amount of PEG needed to deform the gel film by pressing a stamper and the lower limit of the PEG addition provides the largest hardening rate and hence the shortest pressing time to obtain the accurate patterns at a given preheat treatment temperature. After the gel film becomes hard, the stamper is removed and the glass disk substrate with patterned gel film is finally heat-treated at 350 °C to decompose PEG and to be densified. The process time is shortened to the practical fabrication level by optimizing the preheat treatment temperature and the amount of PEG added on the basis of the hardening properties of the gel films (Matsuno et al. 1992; Mitsuhashi et al. 1992). The pregroove depth and pitch of the glass disk substrates fabricated show almost constant values at a given radius. The mechanical characteristics of the glass disk substrates satisfy the international standard. The noise levels of the fabricated glass disk substrates are also low enough for the practical use. It is noted that the sol‐gel pregrooved glass disk substrates have excellent weathering resistance compared with even the conventional glass disk substrates by a photopolymerizable resin method or by a dry etching method. The pregrooved sol‐gel thin layer is considered to serve as an effective alkali-passivation layer for glass disk substrates.

Fig. 5

A model of the micropatterning process described from the viewpoint of gel film hardness (Matsuda 1998b)

Planar Microlens Array. Microlens arrays are important for the optical interconnections between optical fibers and light-emitting or detecting devices as well as for high-definition TV such as liquid crystal displays. Thermally stable microlens arrays have been formed on glass substrates at a practical fabrication level, so far (Shinmou 2000). The preparation procedure of a sol for micropatterning and fabrication process of microlens arrays on a glass substrate is illustrated in Fig. 6. Coating solutions were prepared from phenyltriethoxysilane and dimethyldiethoxysilane. The thick film was embossed inside a vacuum chamber using a stamper. Microlens arrays were obtained from these stamped gel films after heat treatment at about 350 °C. The structure of the lens arrays formed a very uniform pattern as shown in Fig. 7. The following observations were obtained. The heat-treated film exhibited low shrinkage due to methyl and phenyl groups in the film. The focal length is 216 μm at 633 nm. The spot width of 1/e² at the focal point was estimated to be 4.8 μm, which is nearly equal to the value of a theoretical diffraction limit. The microlens arrays show no change in appearance or focal length even after a heat shock test in which the sample is rapidly cooled from 350 °C to room temperature. No changes in transmittance or appearance are observed, and there is negligibly small weight loss (about 0.1%) even after keeping in 20% nitric acid or 20% hydrochloric acid for 2 h.

Fig. 6

Preparation of sol for micropatterning and fabrication procedures of microlens arrays (Shinmou 2000)

Fig. 7

SEM image of the structure of the microlens arrays formed (Shinmou 2000)

Optical Gratings for DWDM. A multichannel real-time power monitor module for dense wave division multiplexing (DWDM) using a novel sol‐gel glass grating and a photodetector array (PDA) has been developed to be coming onto the market (Shimmo 2001). The configuration of the DWDM channel power monitor module (DCMM) using the novel sol‐gel glass diffraction grating is illustrated in Fig. 8. All of the components are mounted on a package made from Pyrex glass frame of 25 mm × 82 mm × 6 mm in size. In order to achieve such downsizing, the Littrow mount is adopted. The package has an optical window so that the PDA can be mounted adjacent to the window. A single-mode fiber (SMF) held with a V-groove is arranged on the window. A beam with different wavelengths emitting from the SMF is collimated through a collimator lens with a focal length of 50 mm into a parallel beam. The grating is designed to propagate the diffracted light to the direction opposite to that of the incident beam. The lens focuses the beam onto the PDA. Because the diffraction angle is a function of the wavelength of the incident light, the position of focused beam on the PDA depends on the wavelength.

Fig. 8

DWDM channel power monitor module (DCMM) configuration using the novel sol‐gel glass diffraction grating (Shimmo 2001)

The novel sol‐gel diffraction grating has been formed on a glass substrate by embossing process. The process is as follows: a coating solution is prepared by mixing methyltriethoxysilane, tetraethoxysilane, ethanol, and acid. A glass substrate is spin-coated with the solution and heat-treated. A master grating is then embossed to the resultant gel film. Finally the mold film of the grating is obtained by heat treatment at 350 °C. The grating has various advantages; for instance, low thermal expansion coefficient substrate like silica glass can be used. It is to achieve low thermal drift, high reliability, and high-temperature stability (<300 °C). The grating can be heated up to 300 °C to form reflection coating by vacuum evaporation. The bandwidth of wavelength transmittance at −1 dB is 0.32 nm. The wavelength transmittance of the bandwidth at 0.8 nm is −20 dB. The temperature dependence of loss and the thermal drift of center wavelength from –20 °C to 60 °C are <0.01 dB/K and <0.003 nm/K, respectively.

Echelon gratings have beneficial property, such as low polarization dependent loss. Echelon gratings have been fabricated on glass substrate by embossing method using sol‐gel materials composed of phenyltriethoxysilane (PTES) and dimethyldiethoxysilane (DMDMS) (Yamamoto et al. 2001). The SEM image of the sol‐gel echelon grating is shown in Fig. 9. The sol‐gel echelon gratings show good optical properties: the diffraction efficiency is 37.5% and the polarization dependent loss is less than 0.1 dB. In addition, it shows high durability to heat shock and humidity. Briefly, the sol‐gel embossing method used with PTES and DMDMS should open a new way to fabricate various kinds of surface-relief type optical elements.

Fig. 9

SEM image of the sol‐gel echelon grating (Yamamoto et al. 2001)

Molding Technique

Research Progress in Molding Technique

Whitesides’ research groups opened a new era in microfabrication and replication using sol‐gel method. They have explored so-called “soft lithography” techniques such as microcontact printing (μCP), replica molding (REM), microtransfer molding (μTM), and micromolding in capillaries (MIMIC), which have been described in their excellent review paper in detail (Xia and Whitesides 1998).

μCP is a flexible, non-photolithographic method that forms self-assembled monolayers (SAMs) containing regions terminated by different chemical functionalities with submicron lateral dimensions (Wilbur et al. 1994). An elastomeric polydimethylsiloxane (PDMS) stamp is used to transfer molecules like alkanethiols to the surface of the substrate by contact. The patterned SAMs act as passivating layers in selective deposition for sol‐gel precursors such as silica and titania in liquid phase as well as chemical vapor deposition.

In REM, the elasticity and low surface energy of the elastomeric PDMS mold allow it to be released from sol‐gel materials easily (Xia et al. 1996). Sol‐gel precursors are cast on the mold and the solidified sol‐gel materials are peeled off the mold after curing. An elastomeric PDMS mold also offers an opportunity to manipulate the size and shape of the patterns present on the mold by mechanical deformation.

In μTM, a drop of sol‐gel precursor is applied to the patterned surface of a PDMS mold and the excess solution is removed by scraping with a flat PDMS block or by blowing off with a stream of nitrogen (Zhao et al. 1996). The filled mold is then placed in contact with a substrate and irradiated or heated for transformation. After the sol‐gel precursor has been cured to a solid, the mold is peeled away carefully to leave or transform the patterned microstructures on the substrates.

MIMIC represents another non-photolithographic method that forms complex microstructures on both planar and curved surfaces (Kim et al. 1995). In this process, the PDMS mold is placed on the surface of a substrate and makes conformal contact with that surface. The relief structure in the mold forms continuous empty channels. When low viscosity sol‐gel precursor is placed at the open ends of the channels, the precursor solution spontaneously fills the channels by capillary action. After filling the channels and curing the sol‐gel precursor into a solid, the PDMS mold is removed, and a continuous sol‐gel material remains on the surface of the substrates. Yttria-stabilized zirconia has been fabricated with a combination of MIMIC and sol‐gel chemistry (Veldhuis et al. 2012). The final shape and dimensions of the patterned structures were controlled by modifying precursor solution concentration and mold geometry. The authors concluded that more study on the relationship between the precursor concentration, mold geometry, and observed coherent crack patterns in as-dried sol‐gel structures may lead to new techniques in sol‐gel MIMIC patterning.

Patterning of various phosphor films by sol‐gel soft lithography has been widely investigated. For example, Y₂O₃:Eu³⁺ (Pang et al. 2003a), Gd₂O₃:A (A = Eu³⁺, Dy³⁺, Sm³⁺, Er³⁺) (Pang et al. 2003b), YVO₄:Eu³⁺ (Wang et al. 2011), and fluorophore-doped xerogels (Carregal-Romero et al. 2012) were prepared and their geometry and luminescence properties were reported. These luminescent pixels are typically expected to be used as next-generation field-emission display devices.

Similar concepts that use colloidal crystal templates have been reviewed by Stein et al. (Stein, 2001). Inverse opal structures are fabricated by this method, and their potential applications include photonic crystal, optical, catalytic, and bioglasses.

Molding of Mesostructure Materials

The mesoporous films prepared by using a surfactant as a template are promising for applications such as photonic materials, ionic conductors, and optoelectronic devices.

Trau et al. have reported a micropatterning of oriented mesoscopic silica through guided growth (Trau et al. 1997). The method reported allows the direction of growth of the mesoporous silica tubules to be guided by infiltrating a reaction liquid into the microcapillaries of a PDMS mold in contact with the substrate. An electric field applied tangentially to the surface within the capillaries induces electroosmotic flow and also enhances the rates of silica polymerization around the tubules by localized joule heating. After removal of the mold, patterned bundles of oriented nanotubules remain on the surface. This permits the formation of orientated channels on a nonconducting substrate with an arbitrary microscopic pattern.

Yang et al. have reported the molding of hierarchically ordered oxides (Yang et al. 1998). Porous silica, niobia, and titania with three dimensionally ordered structures have been prepared by combining micromolding, polystyrene sphere templating, and cooperative assembly of inorganic sol‐gel species with amphiphilic triblock copolymers. These materials show hierarchal ordering over several discrete and tunable length scales ranging from tens of nanometer to a few microns. The respective ordered structures can be controlled by choosing different mold patterns, latex spheres, and block copolymers. The examples presented demonstrate the compositional and structural diversities that are possible with this simple approach.

Mesostructured silica waveguide arrays have been fabricated with a combination of sol‐gel precursors with block copolymer templating and soft lithography (Yang et al. 2000). Waveguiding is enabled by the use of low refractive index (1.15) mesoporous silica thin support. When the mesoporous structures are doped with the laser dye rhodamine 6G, amplified spontaneous emission is observed with low pumping threshold of 10 kW cm⁻². This is attributed to the mesostructure’s ability to prevent aggregation of the dye molecules even at relatively high loadings within the organized high surface area mesochannels of the waveguides. These highly processible, self-assembling mesostructured host media and cladding may have potential for the fabrication of integrated optical circuits.

Photo-Assisted Technique

Research Progress in Photo-Assisted Technique

One of the photo-assisted approaches for micropatterning of sol‐gel-derived films is based on the use of laser light to thermally densify local regions in the films. The undensified regions can be etched away to produce surface features.

Krchnavek et al. used an argon laser to perform maskless writing of SiO₂ coating film on Si substrates (Krchnavek et al. 1984). The heat source was the Si substrate beneath the SiO₂ film which adsorbed the laser light. The laser writing avoids the cracking and provides a way of controlling densification in the vertical direction.

Fabes et al. have reported a laser densification to generate channel waveguide structures in SiO₂, SiO₂–TiO₂ and Ta₂O₅ coatings (Fabes et al. 1990; Taylor and Fabes 1992). Here, SiO₂ substrates are firstly coated, covered with a thin metal layer, and then translated across a Nd:YAG laser beam. The laser energy is adsorbed by the metal film, which heats the underlying sol‐gel coating, causing local densification. Ridged waveguide structures are formed by etching away the undensified portion of the waveguide.

Another very important photo-assisted micropatterning process of sol‐gel-derived films has been demonstrated by Tohge et al. (1994) and Shinmou et al. (1994). The process is based on the chemical modification of metal alkoxides with β-diketones and permits micropatterning on oxide gel film with UV irradiation. β-Diketonato complexes have characteristic optical absorption band in UV range due to the π – π* transition of β-diketones. Thus the photoexcitation of this transition gives rise to the dissociation of the chelate bonds of the complexes. Zirconium tetra-n-butoxide and titanium tetra-n-butoxide are used as starting materials for ZrO₂ and TiO₂, respectively, β-diketones used were acetylacetone and benzoylacetone. The micropatterning process based on the photolysis of chemically modified gel films is shown in Fig. 10. Firstly, the gel films are irradiated with UV light through photomask. The irradiated films are then leached in appropriate solutions; e.g., HNO₃ aqueous solution for ZrO₂ and ethanol for TiO₂. The patterned gel films are heat-treated in air at around 450 °C for the elimination of residual organics.

Fig. 10

Micropatterning process based on the photolysis of chemically modified gel films (Tohge 1994)

Yogo et al. have reported a UV patterning process for alkoxy-derived lithium niobate film (Yogo et al. 1995). Benzoylacetone was selected as a ligand on an alkoxide precursor for the sensitive absorption of UV light. Lithium ethoxide and niobium ethoxide were used as starting materials. Patterned lithium niobate films with preferred c-axis orientation were crystallized on sapphire substrates by a heat treatment at 550 °C.

Kikuta et al. have reported the photoreaction of metal-organic compounds prepared by chemical modification of metal alkoxides for the patterning of ceramic films (Kikuta et al. 1999a, b). Alkanolamines such as diethanolamine and triethanolamine were found to be very effective for the titanium systems. The efficient photoreaction of the prepared precursor thin films was achieved by the selection of UV wavelength according to the absorption of the films. An excimer lamp was a useful light source for UV patterning. Similar chemical modification of tin(IV) isopropoxide with diethanolamine and/or N-phenyl-diethanolamine was applied for the patterning of the tin oxide films.

Tadanaga et al. have reported fine patterning of transparent, conductive tin oxide films by UV irradiation (Tadanaga et al. 2000a). The precursor films were prepared from SnCl₂ with acetylacetone. The precursor films were irradiated with UV light through a mask. The irradiated films were then leached in alkaline solution: aqueous NaOH or NH₃. The patterned films were heat-treated in air at 300–500 ° C. Patterns with a width of 3–50 μm and thickness of about 0.1 μm were formed with a pitch of 2–20 μm. The resistivity of the films heat-treated at 500 °C after UV irradiation was about 1 × 10⁻² Ω cm.

Takahashi et al. have investigated the effects of several hydroxy ketones such as acetol, acetoin, γ-ketobutanol and their combinations with monoethanolamine or ethylenediamine on the stabilization of titanium alkoxide (Takahashi et al. 2000). They have also examined and discussed the effects of UV irradiation to the gel films on crystallization of TiO₂ films. On the basis of the above results, Ohya and colleagues have demonstrated that patterning can be performed on ZrO₂ gel films stabilized with imines having C = N double bond which can adsorb near UV light (Ohya et al. 2002).

A new process using a phase mask for fabrication of diffraction gratings in photosensitive Al₂O₃ gel films has been proposed by Zhao et al. (1998) and Tohge et al. (1999). Photosensitive Al₂O₃ gel films were derived from Al(O-sec-Bu)₃ chemically modified with benzoylacetone. The irradiation system of an excimer laser by the use of a phase mask is schematically shown in Fig. 11: KrF (248 nm) and XeF (351 nm) are used. The laser beam is expanded through a lens and irradiated the gel film vertically by using a reflection mirror through a phase mask. The laser power density was varied from 0.4 through 1.3 mJ cm⁻²/pulse with a repetition of 40 Hz. Typical SEM image of Al₂O₃ thin film diffraction grating formed on Si wafer with XF laser, leaching, and heat treatment at 400 ° C is shown in Fig. 12. Good diffraction efficiency is confirmed using He–Ne laser (633 nm) for gratings in a reflection mode for silicon wafers and in a transmission mode for silica glass substrates.

Fig. 11

Schematic illustration of the irradiation system of an excimer laser by the use of a phase mask: KrF (248 nm) and XeF (351 nm) were used (Zhao 1998; Tohge et al. 1999)

Fig. 12

SEM image of Al₂O₃ film diffraction grating on Si substrate (Tohge et al. 1999)

Organically modified silicates (ormosils) with photopolymerizable functional groups, such as methylene or epoxy groups, have been studied extensively for optical applications, such as waveguide and gratings. Among them, ormosil coating films derived from methacryloxypropyltrimethoxysilane (MAPTMS), methacrylic acid (MMA), and zirconium-n-propoxide (Zr(OC₃H₅)₄) (Krug 1992) have been widely investigated for waveguides by many researches. This is because (1) films with thickness of more than several micrometers are easily obtained, (2) methacrylate group polymerizes under UV irradiation or thermal treatment, (3) the polymerization leads to an increase of refractive index to control waveguiding, and (4) selective UV irradiation and subsequent etching permit the formation of ridge waveguides.

Andrews and Najafi have reported ridge waveguides and Bragg gratings by direct photoinscription and solvent-assisted lithographic development. Hybrid film was prepared from 10:4:4 molar ratio of MAPTMS:MMA:Zr(OC₃H₅)4(Andrews and Najafi 1997; Saravanamuttu et al. 1998). In their method, partial hydrolysis and polycondensation of MAPTMS is initiated with 0.75 equivalent of acidified H₂O (0.05 M HCl). A photoinitiator (1-hydroxycyclohexyl-1-phenyl ketone) is added to the coating solution. The fabrication process of ridge waveguides equipped with Bragg gratings is a multistep process that includes two different UV irradiation interventions. The waveguide is first defined as a channel by narrow band 4.9 eV UV illumination through a photomask. A propanol wet etching step is then used to remove hybrid from regions of the silicon wafer not exposed to the light source. Smooth-walled rectangular prism ridge waveguides exhibiting low 0.1 dB/cm loss at 1.55 μm are produced by limiting the UV exposure time to 30 s. A Bragg grating is inscribed as a volume element by generating a plus/minus first-order 6.4 eV interference pattern in the target ridge waveguide by excimer laser illumination through a precision phase mask. The 193 nm light from an ArF laser is used to deposit the grating in the waveguide. The structure is 6 μm wide × 2.5 μm high and has a step refractive index profile at 1.55 μm of 0.0585.

Yamada et al. have reported patternable inorganic–organic hybrid films synthesized from diethoxydimethylsilane and titanium tetraethoxide modified with ethylacetoacetate (Yamada et al. 1999). The films about 0.5 μm thick show high refractive index of 1.62–1.67 and small in-plane scattering. The decomposition of chelate complex of titanium tetraethoxide and ethylacetoacetate with UV irradiation causes the structural change of the hybrid film, leading to the solubility difference between UV irradiated and unirradiated regions. The changes in solubility permit the formation of patterns on the film by the use of a solution of 2-ethoxyethanol and 0.5 M HCl as an etchant.

Que et al. have reported the fabrication of optical channel waveguides and gratings made from composites of titania and organically modified silane (Que and Kam 2002). The composite is prepared using 3-glycidoxypropyltrimethoxysilane and titanium tetraisopropoxide stabilized with acetylacetone as precursors. Dense and porosity-free waveguide film is obtained at 100 °C. The refractive index of the film is varied from 1.44 to 1.55 at 633 nm by varying the titanium content. It is demonstrated that channel waveguide and grating structures are fabricated for these composite films by etching with dilute HF and by embossing.

Blanc et al. have reported the self-processing of surface-relief gratings in photosensitive hybrid sol‐gel glasses (Blanc et al. 1999). In this case, the acrylate-modified silica–titania films are prepared from combinations of methacryloxypropyltrimethoxysilane, titanium tetraisopropoxide, and methacrylic acid. The refractive index of the films at 632.8 nm is 1.505. This increases to 1.515 after irradiation with mercury lamp (10 mW/cm² for 10 min). Diffraction gratings with periods ranging from 1.4 to 4 μm are recorded by illuminating the film at quasinomal incidence, with a non-collimated 100 W UV mercury lamp, through a chromium amplitude mask. No further steps such as wet etching, thermal curing, and UV fixing are necessary to reveal and stabilize the surface relief. The profile in 4 μm period gratings is sinusoidal, which offers the advantage of reducing the intensity diffracted into the higher order. Tensile stress developed in the irradiated region should give rise to the surface-relief feature. Surface gratings with feature sizes as small as 0.26 μm are recorded by using a phase mask and UV light at 193 nm.

Kawamura et al. have recently reported a hologram formation in Ag-containing organic–inorganic nanocomposite films by interference exposure of laser beams (Kawamura et al. 2010). In this case, hologram is recognized as micro- or nanopatterning recorded inside the films. In their series of research, periodic Ag nanoparticle deposition caused by the interference exposure of laser beams was considered to be the origin of diffraction efficiency variation, which proved the formation of hologram in the films. The addition of ions of a halogen element was found to be effective to add a high sensitivity to visible light irradiation (Kawamura et al. 2011a). When TiO₂ (Kawamura et al. 2011b) or Cu (Kawamura et al. 2016) was added to the organic–inorganic nanocomposite films, the recorded hologram could be erased by irradiation of light with other wavelengths than the one used for recording. The recording and erasing cycle could be repeated as shown in Fig. 13 with a little deterioration of hologram formation ability of the films.

Fig. 13

(a) Schematic illustration and (b) Dynamics of diffraction efficiency of the film during interference exposure of blue laser beams for recording and green laser beam for erasure (Kawamura et al. 2016)

Brusatin et al. have reported that electron beam lithography or x-ray synchrotron radiation lithography could be used for sol‐gel nanopatterning (Brusatin et al. 2008). The newly developed epoxy-based sol‐gel hybrid materials showed lithographic resist-like characteristics without adding any photocatalysts. Stehlin et al. and Satyanarayana et al. have used deep and extreme UV lithography, respectively (Stehlin et al. 2012; Satyanarayana et al. 2014). With these short wavelengths, patterning with 100 nm or less periods can be fabricated only by using conventional lithographic techniques.

Applications of Photo-Assisted Technique

Low Fresnel reflection is very important for the practical application of optical elements. Antireflection coating with multilayers has been used to suppress the reflections so far. It was proposed that a periodic subwavelength structure formed by dry or wet etching can reduce the surface reflection. The most common structure is a two-dimensionally arrayed conical structure, which is known as “moth’s eye” because of the low reflection of the moth’s eye cornea. Such a structure has excellent optical characteristics, such as surface reflection in a wide wavelength region and polarization independence.

Gombert et al. have reported subwavelength structured antireflective surfaces prepared on glass by embossing a nickel master shim on acrylic siloxane layer with UV irradiation (Gombert et al. 1999). Hemispherical reflectance values of <1% are achieved for nonabsorbing planar sheet.

Nishii et al. have successfully prepared low-reflection microstructures by the use of a two-beam interference patterning on a photosensitive ZrO₂ gel film (Nishii et al. 2002; Tohge et al. 2003). In this method coating films on SiO₂ glass are exposed to an interference fringe of 325 nm He–Cd laser beams. The substrate is rotated by 90° between the first and second irradiation steps. Island and lattice type periodic two-dimensional structures with a pitch of 1 μm are formed after the irradiation followed by leaching with ethanol. The examples of AFM images of the island and lattice types of the microstructure of ZrO₂ gel films are shown in Fig. 14. Small protuberances are regularly arranged with a period of about 1.0 μm in the island type (a) and hollows in the lattice type (b). The spectra in a near infrared region of silica glass substrates with ZrO₂ coatings before and after patterning are shown in Fig. 15. The absorption bands observed with the silica glass substrate at 2.7, 2.2, and 1.4 μm are associated with SiOH: the stretching vibration, the combination of stretching and deformation vibrations, and the first overtone of the stretching, respectively. The transmittance of the substrate coated with not patterned ZrO₂ film (b) is lower than that of the silica substrate itself (a), because of the higher refractive index of the ZrO₂ film than SiO₂ substrate. On the other hand, the transmittance of the substrate coated with two-dimensionally patterned ZrO₂ films of island type (c) is increased as compared with that of the substrate in a wavelength of range from 1.6 to 2.6 μm owing to the reduction of reflectance. The steep decrease in transmittance from 1.3 toward 1.0 μm is due to diffraction effect of the patterned structure as a grating. Reflectance of 0.1% for an optically polished SiO₂ surface was attained at around a 1750 nm wavelength by the formation of an island-type microstructure. The 2D gratings of the lattice type also showed similar antireflection effect, whereas its performance was inferior to that of the island type. The microstructures can be heated at 200 °C in an ambient atmosphere without a conspicuous increase in reflectance.

Fig. 14

Examples of AFM images of the island and lattice types of the microstructure of ZrO₂ gel films. Small protuberances are regularly arranged with a period of about 1.0 μm in the island type (a) and hollows in the lattice type (b) (Tohge et al. 2003)

Fig. 15

Optical transmission spectra in a near infrared region, (a) is for a silica substrate, (b) for the substrate coated with not patterned ZrO₂ film, and (c) for the substrate coated with a two-dimensionally patterned ZrO₂ film of island type (Tohge et al. 2003)

Advanced Micropatterning

Hydrophobic–Hydrophilic Micropatterns

Patterning of a surface into regions of different surface energies using self-assembled monolayers (SAMs) has been reported. Widespread examples are alkylsiloxanes, fatty acids and alkanethiolates. Among them, alkylsilanes like fluoroalkylsilane and long-chain alkylsilane are often used to form SAMs for glass and metal substrates. Patterned SAMs can be formed on substrates by microcontact printing (μCP) of alkylsilanes using PDMS stamps. SAMs derived from alkylsilane can be also patterned by UV irradiation through a photomask because the UV-irradiated area becomes hydrophilic due to the cleavage or decomposition of alkyl chains to form silanol groups.

Jeon et al. reported a selective deposition of Ta₂O₅ as well as LiNbO₃ and (Pb,La)TiO₃ thin films on patterned SAMs (Jeon et al. 1995). The surface of the substrates was selectively functionalized with hydrophobic SAMs of octadecyltrichlorosilane by μCP. Sol‐gel deposition of oxideon these functionalized substrates, followed by mild, nonabrasive polishing, yielded high quality, patterned oxide thin layers only on the unfunctionalized regions.

Low temperature deposition of patterned TiO₂ thin films using photopatterned SAMs has been reported by Collins et al. (1996). In this study, thioacetate-terminated trichlorosilanes having SCOCH₃ group are deposited, providing modestly hydrophobic monolayer films. Upon photolysis through a mask, the exposed thioacetate groups are transformed into acidic sulfonate moieties like SO₃H, making the monolayer hydrophilic in those regions. Upon immersion into an aqueous medium, the sulfonate groups deprotonate to become negatively charged, whereas the nonphotolyzed regions remain uncharged. TiCl₄-derived TiO₂ is deposited selectively on the photolyzed hydrophilic region of the monolayer.

Koumoto et al. have reported a new patterning process for TiO₂ deposition that utilizes a phenyltrichlorosilane-based SAM as a patterning template (Koumoto et al. 1999). The strong deep UV absorption of the phenyl chromophore rendered PTCS films susceptible to patterning via Si–C bond photocleavage at lower exposure doses than those obtained for simple alkylsilanes. The method employs a liquid-phase deposition technique that directly deposits crystalline TiO₂ films near room temperature. Masuda with Koumoto’s group have further evolved these techniques for the arrangement of nanosized ceramic particles of SAMs (Masuda et al. 2000). Silica spheres were arranged on silanol surfaces of SAMs selectively in the presence of an acid or base. A two-dimensional close-packed arrangement of spheres was achieved by using hydrochloric acid to form siloxane bonds among silica spheres and SAMs. Site-selective immersion has been also achieved using air bubbles which move continuously across a substrate with a hydrophilic/hydrophobic patterned surface (Masuda et al. 2002). A micropattern of anatase type TiO₂ thin films having no cracks and high feature edge acuity is successfully fabricated by this process.

Tadanaga et al. have succeeded in the preparation of superhydrophobic–superhydrophilic micropatterns by the use of unique flowerlike Al₂O₃ coatings (Tadanaga et al. 2000b). The superhydrophobic coating film which consisted of three layers, a flowerlike Al₂O₃ gel film, a thin titania layer, and a fluoroalkylsilane (FAS) layer, was irradiated with UV light to cleave the fluoroalkyl chain in FAS selectively. It was found that well-defined superhydrophobic and superhydrophilic regions are formed. The contact angles for water of the superhydrophobic and superhydrophobic regions were 165° and less than 5°, respectively. As shown in Fig. 16, convexly shaped silica micropatterns are beautifully formed using the superhydrophobic–superhydrophilic patterns owing to a large difference in surface free energy, i.e., contact angles for the sol‐gel-derived silica sols (Tadanaga et al. 2002). This superhydrophobic–superhydrophilic pattern has a wide variety of applications, such as fabrication of micro-optical components, micropatterned oxide thin films, and stamps for printing.

Fig. 16

Convexly shaped silica micropatterns formed using the superhydrophobic–superhydrophilic patterns owing to a large difference in contact angles for the sol‐gel-derived silica sols (Tadanaga et al. 2002)

Photocatalytic Micropatterning

Photocatalytic micropatterning of transparent ethylsilsesquioxane–titania (EtSiO_3/2–TiO₂) hybrid films has been reported by Matsuda et al. (2002). This process can be categorized to photo-assisted techniques, whereas photocatalytic effect of titania constituent is used to induce structural changes in the hybrid film due to cleavage of Si–C bonds with UV irradiation. Refractive index of 80EtSiO_3/2 · 20TiO₂ hybrid films increases from 1.50 to 1.55 accompanied by a decrease in film thickness by about 30% after UV light irradiation. The contact angle for water of the films decreases from 95° to about 45° and the dynamic hardness increases with UV irradiation. These changes in physical and chemical properties of the films with UV irradiation are caused by the cleavage of Si–C bonds and elimination of ethyl groups in the films due to the photocatalytic effect of the titania component. It is noteworthy that cleavage of Si–C bonds with UV light irradiation occurs owing not to titania crystals but to amorphous titania component in which titanium ions are incorporated in four-coordinated state in silsesquioxanes. Micropatterning is successfully performed on the films applied on the substrates by UV irradiation through a photomask. AFM image of the surface of the film after the UV light irradiation through a photomask is shown in Fig. 17. Arrayed dark squares are concave areas, which correspond to the through holes of the metal mesh used as a photomask. The depth of the concave areas is 0.11 μm, which is 18% of the initial thickness (0.62 μm) of the film before UV irradiation. This micropatterning process has a great potential for fabrication of the micro-optics and photonics components as well as for the application to microprinting.

Fig. 17

AFM image of the surface of 80EtSiO_3/2· 20TiO₂ hybrid film after the UV light irradiation through a photomask (Matsuda et al. 2002)

Photolocking of Dopant and Photoinduced Reduction of Refractive Index

Increases in refractive index and in film thickness with UV light exposure due to photolocking of benzyldimethylketal (BDK) as a photoinitiative dopant in the films have been reported by Bae et al. (2001). As polymerization proceeds, the mobility and volatility of the radicals are reduced, and BDK is ultimately locked into the hybrid materials. This causes an increase in refractive index of the hybrid film. On the other hand, BDK remaining in the unexposed region is removed by annealing, which results in lowering the refractive index and the film thickness. Thus, single-mode ridge waveguides can be fabricated by direct laser exposure without using photomask and development processes.

A new hybrid material, which shows photoinduced reduction of refractive index as well as volume contraction, has been prepared (Park et al. 2003). Whereas conventional photosensitive hybrid materials show photoinduced increases in refractive index, this hybrid material derived from 3-trimethoxysilylpropyl methacrylate and (heptadecafluorodecyl)trimethoxysilane shows a significant decrease in refractive index via decomposition of methacrylate on UV irradiation. Direct imprinting of the hybrid material without any further steps, such as etching, thermal curing, and UV fixing, is performed utilizing the volume contraction on light irradiation.

Ink-jet Printing

Ink-jet printing is a promising technique to form micropatterns (Calvert 2001). Danzebrink and Agerter have reported microlenses of hybrid organic–inorganic materials fabricated on glass substrates by using a drop-on-demand ink-jet printing system with a 50 μm diameter nozzle driven by a piezoelectric device and subsequent UV irradiation (Danzebrink and Agerter 1999). The organic–inorganic sols employed in this work consisted of methacryloxypropyltrimethoxysilane, tetraethylene glycol dimethacrylate, and a photoinitiator. Microlenses obtained are transparent, having a refractive index of n – 1.5 in a region between 375 and 2700 nm. The printing technique allows to obtain plano-convex spherical microlenses with diameter varying from 50 to 300 μm, focal length from 70 to 3 mm. Arrays of spherical fluorescent and colored lenses of similar shape are also obtained.

Concluding Remarks

In this chapter, sol‐gel nano-/micropatterning processes based on embossing, molding, and photo-assisted techniques are described. In addition, some advanced techniques by the use of surface wettability, photocatalytic effects of titania moiety, photolocking of dopant, and ink-jet printing are presented. The sol‐gel nano-/micropatterning process offers a lot of notable advantages: (a) high purity and high homogeneity of the resultant materials, (b) design possibility for physical and chemical properties of the components, (c) high-thermal and weathering reliability of the resultant devices, and (d) cost-effectiveness owing to the high-productivity and simple manufacturing facilities. In order to achieve high definition and high resolution of micropatterning as well as to lead better productivity, the precise control of elasticity and plasticity of the gel materials for embossing and the further improvement of sensitivity and light selectivity of photosensitive gel materials are very important. Several promising optical, electronic, and optoelectronic components and devices are nearing commercialization. It is evident that the sol‐gel nano-/micropatterning promises to be a key technology in the widespreading fields of optics, photonics, optelectronics, and biomedicine.