Fabrication of a nano-scale pattern with various functional materials using electrohydrodynamic lithography and functionalization

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

Received 19th November 2015 , Accepted 5th January 2016

First published on 8th January 2016


Abstract

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.


Introduction

Lithography is a constantly developing field, and one that is essential in many advanced technologies. Establishing the ability to pattern materials with high fidelity levels and resolutions is of great importance in relation to technological applications. Traditional lithography techniques such as photolithography1,2 and electron beam (e-beam) lithography3,4 have been intensively studied to obtain micrometer- to nanometer-scale structures. Although many micro- to nano-scale patterns can be constructed with these lithographic techniques, drawbacks such as the optical diffraction limit in the case of photolithography and high cost and slow process issues in the case of e-beam lithography are also known to exist. Nanosize patterning by self-assembly has been developed to overcome the drawbacks of traditional lithography and patterning of sub-10 nm resolution has been reported using block copolymer (BCP).5,6 However, BCP self-assembly also has drawbacks in that it is not easy to control the process to obtain the pattern; there is also a limit in terms of the materials that can be utilized.

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.

Experimental

It is possible to obtain a fine pattern using two electrodes and a power supply, without special equipment. An external voltage was applied after stamp was on top of thin resist film coated on a substrate between two electrodes in order to obtain replicated pattern. 50–80 V was applied for 1–10 minutes and then line, hole, and pillar array structures were replicated over a large area.

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.

Table 1 Inorganic based resists and their compounds
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.

Results and discussion

An electric field laterally modulated by a topographically structured pattern is imposed to form controllable patterns. If a topographically structured upper substrate is used instead of a planar electrode, the electrode topography causes variation in the force field. Therefore, the instability of the region of the highest electric field, where the electrostatic pressure has a high magnitude, is locally enhanced. In addition, the time constant is much smaller underneath the structures such that the resist film is initially destabilized in the protrusion region. As a result, a positive replicated pattern, i.e., identical to a topographically structured electrode, is fabricated. In this work a thin inorganic based precursor film was liquefied and a constant voltage was applied across the two electrodes (Fig. 1(a)). Initial instabilities were coupled to the laterally varied electric field and further enhanced by the protruding structures underneath the upper stamp, with an increase in the time. The final structure was a positively replicated pattern of the master pattern as opposed to the negative replica patterns achieved by the conventional nanoimprint lithographic technique. The final replica patterns with various inorganic based materials achieved using EHL are shown in Fig. 1(b) and (c). Optical microscopy and SEM images show that well-ordered structures with various sizes and morphologies (lines, holes, and pillars) were replicated over a large area. These replicated patterns are formed over the entire area underneath the stamp that is exposed to the heterogeneous electric field. Moreover, they are highly reproducible.
image file: c5ra24493e-f1.tif
Fig. 1 Pattern replication process using the EHL method and images of various replicated patterns: (a) schematic illustration of the pattern replication process in a heterogeneous electric field. (b) Optical microscope and (c) SEM images of various replicated patterns. The images in (b) and (c) show that the pattern can be replicated with a variety of morphologies and sizes over a large area.

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.


image file: c5ra24493e-f2.tif
Fig. 2 SEM images of the hole pattern in hexagonal array replica (a) before and (b) after heat treatment. AFM images of pattern (c) before and (d) after annealing. (e) XRD and (f) Raman spectroscopy of TiO2 with increasing temperature. A and R indicate anatase and rutile phase, respectively. *Indicates the substrate peaks.

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.


image file: c5ra24493e-f3.tif
Fig. 3 (a) SEM image and (b) Raman spectra of a replicated VO2 line array pattern. (c) SEM image and (d) XRD data of a replicated Fe3O4 hole pattern in hexagonal array. The peaks in the Raman spectra indicate the polycrystalline VO2 and Fe3O4 phases. *In the Raman spectra and XRD denote peak from the silicon substrate.

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.


image file: c5ra24493e-f4.tif
Fig. 4 (a) SEM image of the replicated PZT line array pattern. (b) The EDX spectrum shows the elements in the replicated line pattern, including in this case lead (Pb), zirconium (Zr), and titanium (Ti). Pt detected in EDX results from a Pt coating of 3 nm on the surface of the sample.

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.


image file: c5ra24493e-f5.tif
Fig. 5 Change of current–voltage characteristics as a function of temperature for VO2 line array on SiO2 substrate.

Conclusion

In summary, we introduced and developed a pattern formation process with inorganic based materials using the EHL technique with a uniform pressure. Various sizes and shapes of nano-sized patterns of TiO2, VO2, Fe3O4, and PZT are successfully fabricated over a large area, demonstrating that the replicated pattern has crystallinity after the annealing process, also known as functionalization, through the use of a furnace. The functionalization process is explored one in which the transition from a pattern of amorphous phase to a pattern of polycrystalline phase without deformation of the pattern is observed. Functionalized patterns can be directly utilized in a wide range of applications; therefore, the results of this work highlight the possibility of investigating a controllable pattern formation process as well as the resulting inorganic pattern functionalities.

Acknowledgements

This work was supported by BioNano Health-Guard Research Center funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea as Global Frontier Project (Grant number H-GUARD_2013M3A6B2078).

Notes and references

  1. N. Tohge, K. Shinmou and T. J. Minami, J. Sol-Gel Sci. Technol., 1994, 2, 581 CrossRef CAS.
  2. N. Tohge, K. Shinmou and T. J. Minami, J. Sol-Gel Sci. Technol., 2000, 19, 119 CrossRef CAS.
  3. K. R. V. Subranmanian, M. S. M. Saifullah, E. Tapley, D. J. Kang, M. E. Welland and M. Butler, Nanotechnol., 2004, 15, 158 CrossRef.
  4. M. S. M. Saifullah, K. R. V. Subranmanian, D. J. Kang, D. Anderson, W. T. S. Huck, G. A. C. Jones and M. E. Welland, Adv. Mater., 2005, 17, 14 CrossRef.
  5. J. W. Jeong, W. I. Park, L. M. Do, J. H. Park, T. H. Kim, G. Chae and Y. S. Jung, Adv. Mater., 2012, 24, 3526 CrossRef CAS PubMed.
  6. J. G. Son, M. Son, K. J. Moon, B. H. Lee, J. M. Myoung, M. S. Strano, M. H. Ham and C. A. Ross, Adv. Mater., 2013, 25, 4723 CrossRef PubMed.
  7. B. Radha, S. H. Lim, M. S. M. Saifullah and G. U. Kulkarni, Sci. Rep., 2013, 3, 1078 Search PubMed.
  8. M. S. M. Saifullah, K. R. V. Subranmanian, E. Tapley, D.-J. Kang, M. E. Welland and M. Butler, Nano Lett., 2003, 3, 11 CrossRef.
  9. S. S. Dinachali, M. S. M. Saifullah, R. Ganesan, E. S. Thian and C. He, Adv. Funct. Mater., 2013, 23, 2201 CrossRef CAS.
  10. S. P. Surwade, S. Zhao and H. Liu, J. Am. Chem. Soc., 2011, 133, 11868 CrossRef PubMed.
  11. Z. Jin, W. Sun, Y. Ke, C.-J. Shih, G. L. C. Paulus, Q. H. Wang, B. Mu, P. Yin and M. S. Strano, Nat. Commun., 2013, 4, 1663 CrossRef PubMed.
  12. W. Sun, E. Boulais, Y. Hakobyan, W. L. Wang, A. Guan, M. Bathe and P. Yin, Science, 2014, 346, 6210 Search PubMed.
  13. J. W. Reiner, A. M. Kolpak, Y. Sega, K. F. Garrity, S. Ismail-Beige, C. H. Ahn and F. J. Walker, Adv. Mater., 2010, 22, 2919 CrossRef PubMed.
  14. T. Kamiya, H. Hosono and H. NPG, NPG Asia Mater., 2010, 2, 15 CrossRef.
  15. G. R. Patzke, Y. Zhou, R. Kontic and F. Conrad, Angew. Chem., Int. Ed., 2011, 50, 826 CrossRef PubMed.
  16. S. Y. Chou, L. Zhuang and L. Guo, Appl. Phys. Lett., 1999, 75, 1004 CrossRef.
  17. E. Sch€affer, T. Thurn-Albrecht, T. P. Russell and U. Steiner, Europhys. Lett., 2001, 53, 518 CrossRef.
  18. Z. Lin, T. Kerle, S. M. Baker, D. A. Hoagland, E. Sch€affer, U. Steiner and T. P. Russell, J. Chem. Phys., 2001, 114, 2377 CrossRef CAS.
  19. E. Sch€affer, T. T. Albrecht, T. P. Russell and U. Steiner, Nature, 2000, 403, 874 CrossRef PubMed.
  20. W. F. Zhang, Y. L. He, M. S. Zhang, Z. Yin and Q. Chen, J. Phys. D: Appl. Phys., 2000, 33, 912 CrossRef CAS.
  21. S. Jianying, C. Jun, F. Zhaochi, C. Tao, L. Yuxiang, W. Xiuli and L. Can, J. Phys. Chem. C, 2007, 111, 693 Search PubMed.
  22. J. C. Parker and R. W. Siegel, J. Mater. Res., 1990, 5, 1246 CrossRef CAS.
  23. G. R. Hearn, J. Zhao, A. M. Dawe, V. Pischedda, M. Maaza, M. K. Nieuwoudt, P. Kibasomba, O. Nemraoui and J. D. Comins, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 70, 134102 CrossRef.
  24. F. J. Morin, Phys. Rev. Lett., 1959, 3, 34 CrossRef CAS.
  25. N. F. Mott and L. Friedman, Philos. Mag., 1974, 30, 2 Search PubMed.
  26. P. Schilbe, Phys. B, 2002, 316, 600 CrossRef.
  27. P. Schilbe and D. Maurer, J. Mater. Sci. Eng., 2004, 370, 449 CrossRef.
  28. D. O. Smith, Phys. Rev., 1956, 102, 4 Search PubMed.
  29. J. J. Versluijs, M. A. Bari and J. M. D. Coey, Phys. Rev. Lett., 2001, 87, 2 CrossRef.
  30. S. Biermann, A. Poteryaev, A. I. Lichtenstein and A. Georges, Phys. Rev. Lett., 2005, 94, 026404 CrossRef CAS PubMed.
  31. J. Wu, Q. Gu, B. S. Guiton, N. Leon, L. Ouyang and H. Park, Nano Lett., 2006, 6, 2313 CrossRef CAS PubMed.

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