微图案BiFeO3薄膜的光刻自组装制备与表征

微图案BiFeO3薄膜的光刻自组装制备与表征
王艳;谈国强;苗鸿雁
【摘要】利用光刻自组装技术在玻璃基板上成功制备出图案化的BiFeO3薄膜.AFM和接触角测试表明,紫外光照射引起十八烷基三氯硅烷(OTS)单层膜改性,形成憎水的自组装单分子(SAM)区域和亲水的硅烷醇区域;XRD和XPS结果显
示,OTS单层膜和紫外照射处理的玻璃基板表面诱导吸附的薄膜为纯相六方扭曲钙钛矿结构的BiFeO3薄膜:SEM和EDS表明,SAM区域沉积的BiFeO3薄膜不连续,在超声波震荡下容易脱落,而硅烷醇区域沉积的BiFeO3薄膜致密均一,与基底结合牢固,边缘轮廓清晰.
【期刊名称】《无机材料学报》
【年(卷),期】2010(025)011
【总页数】5页(P1228-1232)
【关键词】光刻白组装技术;BiFeO3薄膜;微图案
【作者】王艳;谈国强;苗鸿雁
【作者单位】陕西科技大学,教育部轻化工助剂化学与技术重点实验室,西
安,710021;陕西科技大学,教育部轻化工助剂化学与技术重点实验室,西安,710021;陕西科技大学,教育部轻化工助剂化学与技术重点实验室,西安,710021
【正文语种】中文
【中图分类】O614;TB43
Bismuth ferrite (BiFeO3, BFO) is a rhombohedrally distorted perovskite. BiFeO3 is one of such few materials that belongs to the multiferroic material due to coexistence of ferroelectricity (TC,bulk=830℃) and antiferromagnetism (TN,bulk=370℃). The combination of ferromagnetism and ferroelectricity could employ the material in many potential applications, including information storage, the emerging field of spinotronics, sensors and so on[1-4].
Microfabrication and microelectronics are important to much of modern science and technology, which supports information technology and permeates society by the integration of microelectronic and optoelectronic functionalities within a very small area. Therefore, micropatterning techniques are essential. Now various novel nanolithographies such as solf lithography, dip-pen nanolithography[5-7], e-beam nanolithography and nanosphere lithography[8-12] have been used for the preparation of ordered nano-structure thin films. In contrast to the technology world, organisms develop abundant micro-patterned inorganic materials by a biomineralization process, which are usually under the assistance of a macromolecule template[13]. This macromolecule controls the nucleation, structure, morphology, crystal orientation and spatial confinement of the inorganic phase. The understanding of biomineralization is important for the design, control, and optimization of materials synthesis at both the molecular and the macroscopic levels[14]. In this article, BiFeO3 patterns were synthesized on glass substrates by employing a photolithography-
self-assembly technique.
1 Experimental
1.1 Materials
The starting materials were Bi(NO3)3·5H2O (Paini Chemical Reagent Factory, analytical grade), Fe(NO3)3·9H2O (Tianjin Zhiyuan Chemical Reagent Co, Ltd., analytical grade), glacial acetic acid (Xi'an Sanpu Fine Chemical Plant, analytical grade), ethanol absolute (Xi'an Sanpu Fine Chemical Plant, ≥99.7%), citric acid (Xi'an Chemical Reagent Factory, analytical grade), octadecyl trichlorosilane (Tianjin Xiangyu Technology Trading Company, 98%), acetone (Shanghai Chemical Reagent Factory,
≥99.5%), and toluene (Shanghai Chemical Reagent Factory, 99.9%). All chemicals were used without further purification.
1.2 Characterization
The crystalline phase and orientation of the annealed films were characterized by X-ray diffraction (XRD, D/Max2550VB+/PC, Japan ). X-ray diffraction data were collected with Cu Kα radiation at a step of 0.02°/min in the range of 2θ=15-70°. The surface microstructure and morphology of patterned BiFeO3 thin films were observed by scanning electron microscope (SEM, JSM6700F, JEOL, Japan) and atomic force microscope (AFM, SPA400-SPI3800N, Japan). Energy dispersive spectroscope(EDS) was utilized for element analysis of the thin films. X-ray photoelectron spectroscope (XPS) was used to identify the oxidation state of Fe. Surface wettability of the pre-treatment substrates was measured by Contact Angle Meter (SL200B, Solon Tech. (Shanghai) Inc, Ltd. Shanghai) with 1μL
water drop added each time.
1.3 SAM Surface Functionalization
Clean glass substrates were sonicated in water, acetone and ethanol for 10min respectively. The wafers dried under an N2 atmosphere were exposed to UV light(PL16-110 Sen lights corporation) for 15min so that organics of the surface were cleaned up and the substrate surface achieved“atomic cleanliness”. The OTS-SAM was prepared by immersing the cleaned and dried glass substrates in an anhydrous toluene solution containing 1 vol % OTS for 30min. The substrates with the SAM were baked at 120℃for 5 min to remove residual solvent and promote chemisorptions of the SAM. The SAM on the glass substrates was exposed to UV light(λ=184nm)for 30min through a photomask. Selective photocleavage can decompose the exposed areas, creating new functional terminal difference from the original ones. UV-irradiation caused the hydroxylation of the head silane-based functional group. Thus UV-irradiated parts became hydrophilic because of silanol group formation, while the non-irradiated regions remained unchanged. Formation of the SAM and the modification to silanol groups by UV irradiation were verified using the static water drop contact angle.
1.4 Patterned BiFeO3 Films Preparation
Patterned BiFeO3 thin films were grown on glass substrates by liquid phase deposition technology. App ropriate portions of Fe(NO3)3·9H2O and Bi(NO3)3·5H2O were used as raw materials and dissolved in glacial acetic acid and distilled water. Citric acid was added to the solution as a
complexing agent. Patterned OTS-SAMs were immersed vertically into the BiFe O3 precursor solution at 70℃ for 8h. After immersed in the solution, the samples were rinsed with distilled water and air-dried at room temperature and then annealed in air.
1.5 Photolithography
Figure 1 describes the photolithography scheme employed to fabricate the micro-patterns of BiFeO3[14]. Octadecyltrichlorosilane hydrolyzed to form silanol groups by the liberation of HCl (Fig. 1(a)). The hydrolyzed organosilane was bonded to the glass substrate surface creating a SAM with a terminal CH3 functional group (Fig. 1(b)). Selective modification of SAM was carried out by exposing the sample to UV light through a photomask. UV-irradiation caused the hydroxylation of the head silane-based functional group. UV-irradiated parts became hydrophilic because of silanol group formation, while the nonirradiated regions remained unchanged (Fig. 1(c), (d)). OTS-SAM was soaked into the BiFeO3 precursor solution at a certain temperature for a period of time. BiFeO3 was site-selectively deposited on hydrophilic regions, while for the hydrophobic regions, BiFeO3 failed to deposit (Fig. 1(e)). A uniform, dense and crack-free film was selectively formed in the hydrophilic silanol regions (Fig. 1(f)). Fig. 1 Schematic description of photolithography
2 Results and discussion
2.1 OTS-SAM
Figure 2 shows AFM images of OTS monomolecular layer exposed to UV light for 30min through a photomask with scanning range of
500nm×500nm. Non-irradiated and irradiated regions display different morphologies. A smooth surface is obtained on the non-irradiated regions compared to the irradiated areas, indicating that UV irradiation has an obvious effect on the modification of OTS monomolecular layer. This result is confirmed by the water contact angle, changing from 115° to 5°[15]. As seen, UV-irradiated parts become hydrophilic, while the non-irradiated regions remaine original hydrophobic.
2.2 Structure
Figure 3 is the X-ray diffraction spectra of BiFeO3 films deposited on glass substrates at 70℃ for 8h and annealed at 600℃ for 2h in air. From this, it is concluded that all BiFeO3 diffraction peaks are in good agreement with the JCPDS card for a rhombohedrally distorted structure BiFeO3 crystal. No other peaks are observed in this XRD pattern, indicating that these films are single phases. The signifcantly higher intensity of (101), (012) and (110) diffraction peaks illustrate that the BiFeO3 films have a high crystallinity. Fig. 2 AFM images of OTS-SAM
Fig. 3 XRD pattern of BiFeO3 film
Fig. 4 X-ray photoelectron spectrum of the Fe2p line of BiFeO3 film
XPS spectrum of the Fe2p peaks in the BiFeO3 films deposited on glass substrates is shown in Fig. 4. Due to the spin-obit coupling, the Fe2p core level is split into the 2p1/2 and 2p3/2 components. The 3/2 and 1/2 spin-obit doublet components are positioned at 710.9 eV and 724.4 eV respectively. For Fe3+, the value of Fe2p3/2 peak appeares between 710.6 and 711.2 eV, while for Fe2+ ion it appeares at 709.4 eV[16-17]. The XPS
results indicate that the BiFeO3 films are single phases and the oxidation state of Fe ion should be Fe3+ without detectable Fe2+, consistent with the XRD results.
2.3 Morphology
The surface microstructure and morphology of BiFeO3 patterns deposited on glass substrates at 70℃ for 8h and annealed at 600℃ for 2h in air were observed by SEM. Figure 5((a), (b) and (c)) are the SEM photographs of as-deposited BiFeO3 patterns without sonicated in water. BiFeO3 crystals are nucleated and grown on both hydrophobic SAM regions and hydrophilic silanol regions under the present conditions, but no continuous film is formed on hydrophobic regions. The signifcantly higher density and thickness of the deposited BiFeO3 crystals on silanol regions suggest that films on these surfaces has good uniformity and adherence.
The clear BiFeO3 micro-patterns were obtained after light sonication cleaning of the as-deposited BiFeO3 patterns in deionized water (Fig. 5(d), (e) and (f)). BiFeO3 crystals deposited on SAM areas can be easily peeled off by ultrasonication, giving patterns with a relatively thick BiFeO3 films in the hydrophilic areas. The boundaries between SAM and silanol regions can be observed clearly. In the deposition process, BiFeO3 crystals show different adhesions to the glass substrates due to different interface interactions. The surface functional groups impact some influence on the deposition behavior. Moreover, the nucleation energy in the hydrophilic silanol regions seems to be lower than that in the hydrophobic SAM regions because the induction period for nucleation in hydrophilic regions
is much shorter than that in hydrophobic regions[18]. The BiFeO3 patterns are therefore formed by preferable deposition in the UV-irradiated areas. The Fig. 5(f) is an enlarged image of BiFeO3 film in silanol regions. The
films are dense, uniform and smooth.
Fig. 5 SEM images of BiFeO3 patterns: ( a, b, c) non-sonicated in water; (d, e, f) sonicated in water
Table 1 reveals the elemental contents of site-selective deposited regions corresponding to Fig. 6(a). It can be seen that the contents of Bi and Fe are 2.83at% and 2.61at% respectively, closing to the ideal Bi/Fe atomic percent ratio 1:1. Table 2 exhibites the elemental contents of non-irradiated regions corresponding to Fig. 6(b). Thus little Bi and no Fe could be observed. Based on the results described above, it is confirmed that
BiFeO3 precursorparticles are absorbed more readily and strongly to hydrophilic silanol areas rather than hydrophobic SAM regions. These results are in good agreement with SEM.
Table 1 Elemental contents of EDS in the area of the BiFeO3 patternsElements wt% at% O K 67.26 94.57 Fe K 6.47 2.61 Bi M 26.27 2.83 Totals 100.00 -
Table 2 Elemental contents of EDS in the area of the SAMElements wt% at% O K 64.46 77.82 Na K 10.53 8.84 Mg K 1.83 1.45 Si K 15.80 10.87 Ca K 0.86 0.42 Bi M 6.52 0.60 Totals 100.00 -
Fig. 6 EDS patterns of BiFeO3 thin film patterns
3 Conclusion
In conclusion, micro-patterned BiFeO3 thin films on glass wafers were
prepared successfully by photolithography-self-assembly method. The modification of OTS monomolecular layer was achieved by UV-irradiation through a photomask, generating hydrophilic silanol areas and hydrophobic SAM regions. No continuous BiFeO3 particles deposited on SAM regions could be easily peeled off by ultrasonication, while a uniform and dense film was site-selectively formed in the hydrophilic silanol regions. Due to the excellent performance of the BiFeO3 thin films, the BiFeO3 patterns have the wide application prospect in the ferroelectric field. Moreover, this novel technique features simplicity, reproducibility, nonhazardousness, cost effectiveness and suitability for producing large area patterns.
References:
【相关文献】
[1] Hill N A. Why are there so few magnetic ferroelectrics? J. Phys. Chem. B, 2000, 104(29): 6694-6709.
[2] Wang J, Neaton J B, Zheng H, et al. Epitaxial BiFeO3 multiferroic thin film feterostructures. Science, 2003, 299(5613): 1719-1722.
[3] Yang S Y, Zavaliche F, Mohaddes-Ardabili L. Metalorganic chemical vapor deposition of lead-free ferroelectric BiFeO3 films for memory applications. Appl. Phys. Lett., 2005, 87(10): 2903-2905.
[4] Yun K Y, Noda M, Okuyama M. Structural and multiferroic properties of BiFeO3 thin films at room temperature. J. Appl. Phys., 2004, 96(6): 3399-3403.
[5] Liu G Y, Xu S, Qian Y L. Nanofabrication of self-assembled monolayers using scanning probe lithography. Acc. Chem. Res., 2000, 33(7): 457-466.
[6] Xu S, Liu G Y. Nanometer-scale fabrication by simultaneous nanoshaving and molecular self-assembly. Langmuir, 1997, 13(2): 127-129.
[7] Kramer S, Fuierer R R, Gorman C B. Scanning probe lithography using self-assembled monolayers. Chem. Rev., 2003, 103(11): 4367-4418.
[8] Hulteen J C, Van Duyne R P. Nanosphere lithogaphy: a materials general fabrication process for peri odic particle array surfaces. J. Vac. Sci. Technol. A, 1995, 13(3): 1553-1558.
[9] Hulteen J C, Treichel D A, Smith M T, et al. Nanosphere lithography: size-tunable silver nanoparticle and surface cluster arrays. J. Phys. Chem. B, 1999, 103(19): 3854-3863. [10] Kuo C W, Shiu J Y, Chen P. Size and shape-controlled fabrication of large-area periodic nanopillar arrays. Chem. Mater., 2003, 15(15): 2917-2920.
[11] Bullen H A, Garrett S J. TiO2 nanoparticle arrays prepared using a nanosphere lithography technique. Nano Letters, 2002, 2(7): 739-745.
[12] Frey W, Woods C K, Chilkoti A. Ultraflat nanosphere lithography: a new method to fabricate flat nanostructures. Adv. Mater., 2000, 12(20): 1515-1519.
[13] Aizenberg J, Muller D A, Grazul J L. Direct fabrication of large micropatterned single crystals. Science, 2003, 299(5610): 1205-1208.
[14] Gao Y F, Koumoto K. Bioinspired ceramic thin film processing: present status and future perspectives. Crys. Growth Des., 2005, 5(5): 1983-2017.
[15] Tan G Q, Song Y Y, Miao H Y, et al. Micropatterning BiFeO3 thin film on self-assembled monolayers. Chinese J. Inorg. Chem., 2009, 25(11): 1-5 (in Chinese).
[16] Zhang Y, Pang L H, Lu M H, et al. Microstructures and multiferroic properties of textured Bi0.8La0.2FeO3 thin films. App. Sur. Sci., 2008, 254(21): 6762-6765.
[17] Xu J M, Wang G M, Wang H X, et al. Synthesis and weak ferromagnetism of Dy-doped BiFeO3 powdes. Mater. Lett., 2009, 63(11): 855-857.
[18] Masuda Y, Gao Y F, Zhu P X, et al. Site-selective deposition of ceramic thin film using self-assembled monolayers. J. Ceram. Soc. Jpn., 2004, 112(5): 1495-1505.。