Suok Leea,
Sang Hee Junga,
Dae Joon Kang*c and
JaeJong Lee*ab
aNano Convergence and Manufacturing Systems Research Division, Korea Institute of Machinery and Materials, 156, Gajeongbuk-Ro, Yuseong-gu, Daejeon 305-343, Republic of Korea. E-mail: jjlee@kimm.re.kr; Fax: +82-42-868-7721; Tel: +82-42-868-7145
bDepartment of Nano Mechatronics, University of Science and Technology, 217, Gajeong-ro, Yuseong-gu, Daejeon 305-350, Republic of Korea
cDepartment of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea. E-mail: djkang@skku.edu; Fax: +82-31-290-5947; Tel: +82-31-290-5906
First published on 8th January 2016
Direct patterning with inorganic based materials has been studied using many lithographic techniques. Among lithographic methods, electro-hydrodynamic lithography (EHL) is a good candidate to obtain a fine inorganic pattern. Being a minimal contact patterning technique, our method is simple, versatile and inexpensive, and has the potential to become a powerful tool for realizing inorganic based nanostructures on a wafer scale. Inorganic precursor resists are exploited here as relatively high-speed resists compared to macromolecule resists in an effort to reduce the patterning time significantly. These resists are developed to have functionalities via a thermal annealing process which can be used in versatile applications. An amorphous inorganic precursor pattern fabricated by EHL is transitioned to a crystalline phase via an annealing process. Herein, functional material patterns such as TiO2, VO2, Fe3O4, and PZT are successfully fabricated and functionalized via EHL and the annealing process.
In recent years, lithographic techniques have extended to materials such as metals,7 metal oxides,8,9 and DNA,10–12 as resists, as well as polymers. Among the resists, metal oxides, due to their excellent optoelectronic and mechanical properties, are important for various applications such as photocatalysts, solar cells, sensors, and electronic devices.13–15 The conventional approaches to obtain metal oxide structures are a multi-step process including patterning with a polymer resist by lithographic methods, the deposition of the target material, and the lift-off process. Due to the complicated processes, there is the possibility of failure in the fabrication of the desired metal oxide structures or the existence of unexpected damages in the structures. Therefore, for the construction of nanostructures via a lower number of steps and with fewer defects than possible when using the multi-step process, direct patterning of metal oxides is an important issue.
Electro-hydrodynamic lithography (EHL),16–18 a promising non-contact soft-lithographic method that is attracting a great deal of attention recently due to its simple process, cost-effectiveness, and high-resolution patterning, is ideal for metal oxide patterning over large areas. EHL makes use of the instabilities in thin liquid films by imposing an external electric field. While thin liquid film is originally stable, the application of the external electric force provides control over the instabilities on the resist film resulting from the variation of the magnitude of the force. By imposing lateral variation of the force field, the instability is directed to replicate a master pattern with high precision. The various nanoscale structures of the master pattern in the stamp can be replicated with very high fidelity to the substrate using the instability of an inorganic-based resist that is controlled by a laterally modulated electric field between the stamp and the substrate.19 EHL uses the capillary film instability to construct patterns, meaning that it is possible to use any material films has been destabilized by electrostatic force and that is not necessary for the material to have sensitivity to a special source such as an e-beam or light wavelength. Therefore, this is a good lithographic technique for direct patterning of any inorganic materials, especially metal oxides.
In general, after pattern replication is performed using the EHL method, an annealing process is required to functionalize the inorganic-based materials using a furnace. The polycrystalline phase of the replicated patterns is changed to the polycrystalline phase via an annealing process; the crystallinity is dependent on the annealing conditions such as the temperature, gas atmosphere, and annealing time.
In this work, we studied the nanoscale pattern formation with various functional materials (e.g., TiO2, VO2, etc.) using the EHL technique and functionalization of the replicated patterns. The use of functional materials results in a considerable reduction of the time required to complete the EHL process to a few minutes over a large area and possibility of direct usage in applications. A functionalization process based on an annealing method suitable for obtaining functional structures of EHL-patterned inorganic based materials over a large area via the annealing process is presented. The fabricated pattern can be utilized in various applications via functionalization.
The most important technical aspect of the pattern formation process using the EHL method is the uniform control of the height of the air gap, which is the distance between the stamp and the thin resist film. Controlling the height of the air gap can be achieved by using SiO2 patterns as a spacer. The stamp with spacers is placed on the surface of the thin resist film, resulting in an air gap height identical to the height of the spacers.
For a controllable experiment, the height of the air gap should be uniformly maintained. However, the height of the air gap varies due to unintended factors such as dust, particles, and defects on the thin film. These unintended factors prevent a perfect replication process. The initial thickness of the air gap in the EHL system can be maintained over the entire surface by applying uniform pressure.
The following materials are used as precursor resists here: titanium dioxide (TiO2), vanadium dioxide (VO2), iron oxide (Fe3O4), and lead zirconium titanate (PZT). The transferred pattern with these inorganic materials can be directly exploited in various optical, electrical, magnetic, and piezoelectric devices, instead of the pattern acting as an intermediate pattern to create functional nanostructures. Table 1 shows the inorganic-based resists and their compounds as used in the experiment. To formulate the TiO2, VO2, and PZT precursor solutions, alkoxide materials are used as base materials. These alkoxides are very sensitive to hydrolysis; therefore, the synthesis of the resist solutions with alkoxides is done in a glove box (relative humidity of less than 5%), and stabilizers are used to slow the reaction with moisture when processing the resist solutions. For the Fe3O4 precursor solutions, the base materials are relatively resistant to moisture; hence, a stabilizer is not necessary.
Compound material | Base material | Solvent | Stabilizer |
---|---|---|---|
TiO2 | Titanium(IV) n-butoxide | Methanol | Benzoylacetone |
VO2 | Vanadium oxyisopropoxide | Isopropanol | Benzoylacetone |
Fe3O4 | Iron naphthenate | Toluene | |
PZT | Lead acetate, zirconium acetate, titanium isopropoxide | Ethanol, isopropanol | Acetic acid |
After pattern replication process, thermal annealing process carried out. Replicated pattern sample was loaded into the furnace to functionalize and heat-treatment was carried out. Annealing process was performed under various atmosphere due to materials.
All characterizations are carried out with optical microscopy (OM), field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDX) and Raman spectroscopy. Electrical property was measured by probe station.
After the fabrication of various inorganic-based material patterns using the EHL method, a functionalization process was carried out. Functionalization is expressed when the amorphous phase of the thin film or pattern including an inorganic-based material (i.e., titanium, vanadium, iron, etc.) is changed to the polycrystalline phase of the thin film or pattern (TiO2, VO2, Fe3O4, etc.). To functionalize the thin film or pattern, thermal annealing was performed using a furnace. Under a certain experimental condition, the amorphous phase of the structures was transitioned to the polycrystalline phase. This process was predominantly carried out at a relatively high temperature (above 400 °C) and lasts several hours.
First, patterning process and the crystallization characteristics of the TiO2 precursor material are studied. Fig. 2(a) and (b) show SEM images of TiO2 hole array pattern structures before and after the heat treatment. The average width and depth of the TiO2 replica pattern after the heat treatment is slightly reduced. Width and depth of the pattern were reduced by 40 nm (from 600 to 560 nm) and 100 nm (from 180 to 80 nm), respectively (see Fig. 2(c) and (d)). This volume contraction of the pattern results from vaporization of the solvent during the annealing process. TiO2 is known to exist in several crystalline phases, such as the anatase, brookite and rutile phases.20 Its crystallization characteristics are investigated upon a heat treatment using XRD and Raman spectroscopy. The TiO2 precursor pattern on silicon substrates was annealed at various temperatures (400–800 °C) for several hours in an air atmosphere inside a furnace in order to determine the effects annealing temperature have on the crystallization process. It was found that the TiO2 precursor pattern was transformed to the anatase phase when the heat treatment exceeded 400 °C. A further increase in the annealing temperature resulted in the appearance of the rutile phase. XRD is an effective method to distinguish different crystalline phases of TiO2. Using this technique, the formation of the anatase (A) and rutile (R) phases were verified (Fig. 2(e)). Anatase phase was firstly appeared at above 400 °C and rutile phase was detected at 600 °C. Rutile phase was existed mixed with anatase phase from 600 to 800 °C and it was only detected at above 800 °C. Raman spectroscopy is another characterization tool for confirming the crystalline phase. When examining the samples with Raman spectroscopy, the Raman modes of the anatase phase are identified at 141, 193, 395, and 635 cm−1, denoted as Eg, Eg, B1g, and Eg, respectively, in Fig. 2(f).21–24 Note that B1g at 513 cm−1 and the A1g modes at 518 cm−1 are not clearly seen here due to the overlap of the Si substrate at 520 cm−1. It was found that the intensity of the Raman spectra increases with an increase in the annealing temperature, indicating an improvement in the crystallinity of the pattern. This should be explained from larger crystalline size. It was also noted that a further increase in the temperature introduces other crystalline phases and eventually results in the rutile phase only, which is in good agreement with the results of the XRD analysis.
Second, a VO2 precursor solution was used as a resist material and a thin film was coated by the spin-coating method. With external electric force, the VO2 precursor thin film was deformed and finally changed to a line array pattern via a replication process. Fig. 3(a) shows line array pattern of 250 nm in width.
VO2 is a material which undergoes a metal insulator transition (MIT) in which the electrical property is changed from an insulator (monoclinic phase) to a metal (tetragonal phase) at 68 °C.25 The fabricated amorphous VO2 precursor pattern was functionalized via a heat-treatment process. The amorphous phase of the VO2 pattern was transitioned into the monoclinic insulating phase above 450 °C for over 1 hour under a mixed atmosphere of argon and 5% hydrogen gas. The crystalline phase of the VO2 pattern was characterized by Raman spectroscopy. Group theory predicts that there are 18 Raman active modes (9Ag + 9Bg) in the low-temperature monoclinic insulating phase.26,27 Ag modes at 191, 226, 339, and 618 cm−1, and Bg modes at 264 and 395 cm−1 are detected from the functionalized VO2 pattern. Other Raman peaks are not detected due to the overlapping with the considerably large silicon peaks and the non-deconvoluted peaks.
Third, the Fe3O4 precursor resist was used to create the patterns. In this case, iron naphthenate was used as a precursor material and a hole pattern in a hexagonal array was fabricated. Fig. 3(c) shows an SEM image of the replicated Fe3O4 precursor pattern.
The hole array pattern, with a diameter and periodicity of 400 nm and 1 μm, respectively, is replicated. This pattern was then annealed at 350 °C for more than 3 hours using a furnace to obtain the Fe3O4 crystalline phase. The sample was annealed under an Ar + H2 (5%) mixed gas condition, and total four peaks were detected in the XRD (Fig. 3(d)). Iron ion in iron naphthenate which is in the resist pattern was initially converted into the Fe2O3 phase and was then reduced from Fe2O3 to Fe3O4 by the hydrogen of the Ar + H2 (5%) mixed gas during annealing process. Peaks in XRD indicate the polycrystalline phase of magnetite Fe3O4. Magnetite is a ferromagnetic material of spinel ferrite with the chemical formula Fe3O4; it has been studied intensively given its several interesting properties. It has a high Curie temperature of 858 K, and is electronically conducting with highly spin-polarized conduction electrons.28,29 As a result, it is an intriguing candidate for magnetic recording media or spin-valve applications.
Thus far, solutions including only one inorganic material have been used as a resist. We studied lead zirconium titanate (PZT), which consists of three different materials – lead (Pb), zirconium (Zr), and titanium (Ti) – as well and achieved a successful nanoscale replicated pattern using a PZT precursor solution. A line array pattern of 200 nm was successfully formed (Fig. 4(a)), and the elements of Pb, Zr, and Ti in the pattern could be observed in the EDX spectrum (Fig. 4(b)). This result demonstrates the possibility of pattern formation with a resist consisting of multiple elements as well. PZT is a good material to show piezoelectric effect and the replicated pattern can be directly used in piezoelectric devices after functionalization process.
These examples of successful pattern formation with various inorganic-based materials are worthwhile, as these patterned materials can be utilized in practical research and in industry through the functionalization process. The electrical measurement was studied to investigate the possibility of direct usage in application. As mentioned previously, VO2 is known to show MIT behaviour around 68 °C. Here, temperature dependence of electrical property was measured with VO2 line array fabricated by EHL method in four point measurements. Fig. 5(a) and (b) show the current–voltage characteristics as a function of temperature from 25 to 95 °C. MIT effect was not abrupt, rather gradual MIT behaviour was observed in our measurement. This result is presumably due to a local oxygen content ratio in VO2 lines,30 or a strain effect between the substrate and the VO2 line array.31 From the result of temperature dependence of electrical property, we expect that VO2 line array device can be used in optical and electrical switching devices. Other patterns with inorganic based material are able to be utilized in various applications through the functionalization process. It is essential to investigate the pattern formation process with various sizes and shapes and to test the fabricated patterns in versatile applications.
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Fig. 5 Change of current–voltage characteristics as a function of temperature for VO2 line array on SiO2 substrate. |
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