Applied Mechanics and Materials Vol. 606 (2014) pp 51-54 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.606.51
Selective ZnO Nanorods Hydrothermal Growth through Resist Patterning Method Amirul Abd Rashid 1,2, a, Nor Hayati Saad 2,b , Daniel Bien Chia Sheng1,c , Lee Wai Yee 1,d and Freddawati Rashiddy Wong2,e 1
MIMOS Berhad, Technology Park Malaysia, Kuala Lumpur, Malaysia
2
Micro-Nano Electromechanical System Laboratory (MiNEMs), Faculty of Mechanical Engineering, Universiti Teknologi MARA, Shah Alam, Malaysia a
b
c
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected]
Keywords: Selective growth; ZnO nanowire; Hydrothermal synthesis; Resist patterning; Gas sensor
Abstract. Resist patterning method has been used to enable selective ZnO nanorods grown via facile hydrothermal process. The growth region of the ZnO nanorods was controlled by pre-coating the seed layer on the silicon base substrate. Using the plasma process, the seed layer which is not coated with a resist layer will be etched out. Therefore, when the samples completely undergo the hydrothermal process, there will be no nanorods grown in that specific area. The grown ZnO nanorods was in well array with flat hexagonal tip and wurtzite crystal structure. This technique can be applied for applications which require integration of nanostructure in specific critical areas such as interdigitated electrodes (IDE) for various gas sensor applications. Introduction Nanostrucutre technology has advanced from the basic process of top-down or bottom up to a more specific and controlled growth capability especially for sensor devices. In order to design and fabricate more selective, sensitive and sustainable product, the nanostructure need to be selectively grown on specified targets, so that subsequent packaging process can be proceed without much issue. One of the promising method is to control the seedlayer’s surface properties so that final morphology, shape, spacing and others can be easily controlled. In general, Metal oxide materials were widely used as the functional element which have capabilities to detect various types of gases. One of the metal oxide that receive much attention is Zinc Oxide (ZnO). This is due to its wide band gap energy, large excitation binding energy, and lack of central symmetry [1]. Given these properties, ZnO has been widely used in various applications, such as solar cells [2], photocatalysts [3], chemical sensors [4], gas sensors [5] and piezoelectric devices [6]. Current fabrication technology allows ZnO to be synthesized in a nanostructured size, allowing researchers to utilize the nanostructured ZnO material to improve device performance [7]. Several types of ZnO nanostructures exist, such as nanoparticles [8], nanorods and nanowires [9]. However, different applications may need different types nanostructure, size, shape, morphology as well as orientation. For example, piezoelectric devices typically require long and vertically aligned ZnO nanorods or nanowires for easy top and bottom electrode fabrication process, whereas such orientation and type may not be necessary for gas sensor applications [10, 11]. The properties of ZnO nanostructures are strongly dependent on their morphology and shape, so controlling the growth process so that the grown ZnO nanostructures will give the optimum effect for each particular application is important. For this purpose, a feasibility study has been conducted to selectively grow ZnO nanostructure at the specific location of the substrate by combing typical lithography process to the synthesis process of ZnO nanostructure. In this study, the ZnO in the form of nanowires been synthesized through the
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hydrothermal process. Hydrothermal process is considered one of facile methods because it doesn’t require complicated equipment set-up as well as relatively lower parameter setting. Methodology The source material of the ZnO nanorods was synthesized following the experimental procedure reported by [12]. All chemicals were of analytical reagent grade (Sigma-Aldrich) and used without further purification. The seed layer was prepared using a mixed solution of 0.2 M zinc acetate dehydrate and diethalonamine in ethanol. The solution was then spin coated onto the SiO2 surface at a speed of 3000 rpm for 30 s, followed by soft baking at 90 °C for 5 min. To demonstrate the selective growth of ZnO on the substrate, a simple rectangular mask pattern will be used as part of lithography process. The substrate will be coated with a resist layer before exposed to the UV light. The area which was exposed to UV will cure while the uncured area will be removed through the lift-off process. While lift off process on a typical bare silicon substrate is well established, the is a need to slightly change the lithogypahy process for resisting on seed layer. Figure 1 exhibit the differences between lift-off process of resisting on the seed layer compared to resist on silicon layer where the shape of the lift off was well defined compared to the irregular shape of seed layer substrate.
a
b
c
Figure 1 Lithography result comparison for resist on typical silicon and resist on seed layer. Bottom half of the substrate was covered by tape during the seed layer coating (a). The whole substrate then coated with resist and exposed to UV light (b) and after lift off process, the resist pattern for surface without the seed layer shows better shape compared to surface with a seed layer (c). The seed layer which was exposed after lift-off process will be removed by oxygen plasma etching process using PECVD equipment. The flow rate of O2 gas parameter was varied during the plasma process to understand the effect of the O2 flow to the final ZnO morphology after the process completed. This plasma operated at a constant 200 watt with process duration up to 1 hour. Theoretically, O2 plasma was used to remove and modify the seed layer surface prior the hydrothermal process. For this facile hydrothermal, the samples were suspended inside a sealable glass beaker containing a mix of 0.04M zinc nitrate hexahydrate and hexamethylenetetramine. The aqueous solution then will be heated to 80 °C for 4-5 hours in typical convection oven. After the samples cool down to room temperature, the sample was washed by dipping into DI water and dried under ambient for the morphological analysis. Results and Discussion The overall process to selectively grow ZnO nanorods is shown in Figure 2. The additional lithography process for this method is to prepare a specific to protect the area of interested to grow the ZnO during the template so the unwanted area of the seed layer can be removed, hence nanorods will not able to grow in that region because the seed layer have been etched out by the plasma process. It was also found that the distribution of ZnO nanorods diameter and length been
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influenced by the plasma temperature and O2 gas flow rate setting. The higher flow rate of 200 s.c.c.m seems to produce longer nanorods but the diameter of the nanorods was increased. This is similar finding, reported by Rashid [13] where O2 plasma significantly influences the final morphology of ZnO nanostructure grown via hydrothermal process.
Figure 2 Shows the process to selectively grow ZnO nanorods on specific area of the substrate by incorporating simple lithography process. SEM analysis of the selective grown ZnO nanorods was shown in Figure 3. It can be seen that the area which was exposed to plasma cleaning show almost no nanorods growth. In contrast, the area which was protected by resist during the plasma process produce very well aligned of ZnO nanorods. Nevertheless, the is very important to establish process parameters for lithography process on surface that contains the seed layer otherwise the resist pattern is not very well in control hence will not function to protect the seed layer during plasma cleaning.
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b
Figure 3 SEM image of the grown nanorods area and non-grown nanorods area after the seed layer exposed to O2 plasma process (a). The insert shows the un-protected area produces very dense nanorods while (b) is the cross sectional SEM image of the grown nanorods.
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Conclusions This paper reports the feasibility study of selectively grow ZnO nanorods array at specific area by integrating lithography process with hydrothermal synthesis route. By properly controlled the resist pattern during lift-off process, the exact location of the nanostructure to grow can be determined. However, the characteristic of resist on seed layer require further optimization particularly on the resist thickness, UV exposure time as well as the lift-off duration to ensure the pattern is well shaped hence the nanostructure grown area can be precisely controlled. Acknowledgements The authors wish to thank MIMOS Berhad for facility accessibility to perform this experiment,University Teknologi MARA (UiTM) as well as Ministry of Higher Education Malaysia (MOHE) for the financial support under RIF grant (600-RMI/DANA 5/3/RIF (390/2012)). References [1] Z.L. Wang, Zinc oxide nanostructures: growth, properties and applications, Journal of Physics: Condensed Matter 16.25 (2004): R829. [2] S.M. Lukas and L.M.D. Judith, ZnO–nanostructures, defects, and devices, Materials today 10.5 (2007): 40-48. [3] B.Pal and S. Maheshwar, Enhanced photocatalytic activity of highly porous ZnO thin films prepared by sol–gel process, Materials chemistry and physics 76.1 (2002): 82-87. [4] G.C. Yi,W. Chunrai and P. Won, ZnO nanorods: synthesis, characterization and applications, Semiconductor Science and Technology20.4 (2005): S22. [5] X. Jiaqiang et al., Hydrothermal synthesis and gas sensing characters of ZnO nanorods, Sensors and Actuators B: Chemical 113.1 (2006): 526-531. [6] U.Ozgur et al., A comprehensive review of ZnO materials and devices, Journal of applied physics 98.4 (2005): 041301-041301. [7] V. G. Irene an L.C. Monica, Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review, Energy & Environmental Science 2.1 (2009): 19-34. [8] S.Talam, S.R. Karumuri and N. Gunnam, Synthesis, Characterization and spectroscopic properties of ZnO nanoparticles, ISRN Nanotechnology, Volume 2012, Article ID 372505, DOI: 10.5402/2012/372505. [9] N.D. Tam,S. Karandeep, M. Meyyappan and M.O. Michael, Vertical ZnO nanowire growth on metal substrates, Nanotechnology 23 (2012) 194015. DOI 10.1088/0957- 4484/23/19/194015. [10] R. Mohammed,S. Jinhui,N. Omer, L.W. Zhing and W. Magnus, Study of the piezoelectric power generation arrays grown by different methods, Advanced Functional Materials, 2010, xx,1-6, DOI: 10.1002/adfm.201001203. [11] J.H.Ting, L.H. Cheng,J.C. Shoou, I.C. Chen, Laterally grown ZnO nanowire ethanol gas sensor, Sensors and actuators B 126 (2007) 473-477. [12] K. Anuar,W.Y.Lee, H.W. Lee. A.S.Teh, D.C.S. Bien, I.A. Azid, Effect of Seed Annealing Temperature and Growth Duration on Hydrothermal ZnO Nanorod Structures and their Electrical Characteristics, Applied Surface Science (2013), http://dx.doi.org/10.1016/j.apsusc.2013.06.159 [13] Abd Rashid Amirul et al.Effect of Oxygen Plasma Conditioning on Zinc Oxide (ZnO) Nanostructure Morphology in Hydrothermal Growth Method, International Conference On Nanomaterials (ICNM 13), Chennai, India.
