Fabrication of YBCO superconducting microarray by sol-gel process ...

SCIENCE CHINA Technological Sciences Progress of Projects Supported by NSFC

June 2013 Vol.56 No.6: 1409–1414 doi: 10.1007/s11431-013-5219-0

Fabrication of YBCO superconducting microarray by sol-gel process using photosensitive metal chelates ZHAO GaoYang1, 2*, JIA JiQiang1, JIANG Fen1, LEI Li2 & MA JunShan1 1 2

School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China; Advanced Materials Analysis and Test Center, Xi’an University of Technology, Xi’an 710048, China Received November 7, 2012; accepted April 7, 2013; published online April 30, 2013

We used yttrium acetate, barium acetate, and copper acetate as the starting materials, benzalacetone (BzAcH) as chemical modifier, and methanol (MEOH) as solvent to synthesize a stable fluorine-free YBCO precursor sol. The coated YBCO gel film using this precursor sol exhibited photosensitivity to UV irradiation at a wavelength of 330 nm. After the subsequent exposing, the YBCO gel film showed a decreased solubility in several organic solvents. Based on the photosensitivity of the YBCO/BzAcH gel film, YBCO superconducting microarray with the pitch of 5 μm was fabricated by irradiating the gel film with UV light through a mask, followed by leaching the unirradiated area in a mixture solvent of methanol and n-butyl alcohol with the volume ratio of 1:1. After proper heat treatment the x-ray diffraction result showed that the as-prepared arrays were highly c-axis oriented and with a high Tc by this new photosensitive sol-gel process. superconductors, YBCO, photosensitivity, sol-gel, microarray Citation:

Zhao G Y, Jia J Q, Jiang F, et al. Fabrication of YBCO superconducting microarray by sol-gel process using photosensitive metal chelates. Sci China Tech Sci, 2013, 56: 1409–1414, doi: 10.1007/s11431-013-5219-0

1 Introduction High-temperature superconducting electronic devices have attracted extensive attentions due to their notable advantages, such as small size, low loss, low noise, high sensitivity, and easy integration with other microwave solid-state circuits [1]. Compared with the traditional detectors, the infrared array detectors prepared by YBCO microarrays exhibit a higher sensitivity and wider spectral response range. Thus, it is considered as the best detector in far infrared and millimeter wave band at 65–90 K [2]. With the development of the microelectronics, the integration level of the array detectors has been greatly improved. Therefore, it is of great importance to prepare high-temperature superconducting microarrays for the array-detector application. *Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2013

At present, wet chemical etching (WCE) [3, 4] and ion beam etching (IBE) [5] are the developed methods for the fabrication of YBCO microarrays. For the WCE method, the water in the etching agent usually causes the deteriorated superconductivity of YBCO films and the sidewall-undercutting increases the processing difficulty for submicron and even nanoscale patterns and arrays. Although the IBE technique effectively improves the processing accuracy and avoids the contact of the water and YBCO films, the intense heat released during the etching process greatly suppress the superconducting properties of the films. Photosensitive solgel method has been adopted to fabricate several functional films (Al2O3-SiO2, ZrO2 and PZT) [6, 7] and their fine-patterns and microarrays by Zhao and Tohge et al. This lithography technique has a character that the film is photosensitive (without photoresist) and the patterning process of the film is achieved in the sol-gel stage, which has no effect on the tech.scichina.com

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properties of the oxide ceramic films. Therefore, this patterning process is quite suitable for the fabrication of YBCO microarray. In this paper, the photosensitive sol-gel technique is introduced into the patterning process of YBCO microarray. This will provide a novel approach for the fabrication of high-temperature superconductor microarrays.

2 Experiment

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patterned into micro-bridge or microarray, and then they were converted into YBCO superconducting micro-bridge and microarray after a proper heat-treatment [8]. The YBCO micro-bridge was utilized for critical transition temperature (Tc) by a standard four-probe method. The YBCO microarray was used for magnetization measurement under a multifunction vibrating sample magnetometer (VersaLab-VSM).

3 Results and discussions

The photosensitive YBCO precursor sol was prepared by adding a certain amount of benzoylacetone (BzAcH: C6H5COCH2COCH3) into the fluorine-free YBCO sol (the flow chart is shown in Figure 1). Thereinto, yttrium acetate [Y(OAc)3] (1.5964 g, 0.006 mol), diethylenetriamine (DETA, 0.9285 g, 0.009 mol) and methanol (2.8836 g, 0.09 mol) were added to a glass flask and stirred to dissolve at room temperature; after BzAcH (0.9732 g, 0.006 mol) was added, the mixture was stirred for 2 h and then a transparent yttrium sol was prepared. Copper sol was prepared by applying the same procedure to copper acetate [Cu(OAc)2] (3.5937 g, 0.018 mol), acrylicacid (AA, 2.5942 g, 0.036 mol), methanol (11.5344 g, 0.36 mol) and BzAcH (1.4598 g, 0.009 mol). At the same time, barium sol was prepared from barium acetate [Ba(OAc)2] (3.06504 g, 0.012 mol), lactic acid (LA, 3.24288 g, 0.036 mol) and methanol (7.6896 g, 0.24 mol) by stirring to dissolve at room temperature. Finally, the deepgreen photosensitive YBCO precursor sol was obtained by mixing the above sols and stirring for two hours. Using the above photosensitive precursor sol, YBCO gel films were prepared on silicon and quartz substrates by the dip-coating method. They were used for the patterning process and UV spectrum test to study their UV photosensitivity, respectively. The photosensitive YBCO gel films were

Figure 1

3.1

Photosensitivity analysis of YBCO gel films

It has been reported that BzAcH dissolved in the ethanol solution shows absorption bands at about 250 and 310 nm which correspond to the absorption of the phenyl group and →* transition in -diketonat ligands, respectively [7]. When some metal ions, such as Al3+ and Zr4+, are introduced, the BzAcH can react with the metal ions to form chelate rings which show the red-shift absorption bands of the enol form in BzAcH [9, 10]. In this work, BzAcH was added into Y-, Ba- and Cu-sols to form the corresponding metal chelates. The UV-visible absorption spectrums of these sols are shown in Figure 2. The absorption band of →* transition in the BzAcH-tailored Y-sol appears at around 330 nm, shifting by 20 nm to the side of the longer wavelength in comparison with the intrinsic absorption band of BzAcH, which implies that Y3+ ion reacts with BzAcH to form the Y/BzAcH chelate ring (as shown in formula (1)). The chelate reaction may also lead to the 6 nm blue-shift absorption band of the phenyl group, while the position of the absorption bands of the BzAcH-tailored Ba-sol is almost the same, indicating no chelate reaction occurred between Ba2+ ion and BzAcH. Like the BzAcHtailored Y-sol, the absorption band (at around 330 nm) of

Flow chart of photosensitive YBCO precursor sol.

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 Figure 2

UV absorption spectrums of several sols.

→* transition in the BzAcH-tailored Cu-sol has also a red-shift of 20 nm, which suggests that Cu2+ ion reacts with BzAcH to form the Cu/BzAcH chelate ring (as shown in formula (2)). The chelate reaction may also lead to the 4 nm red-shift absorption band of the phenyl group. In addition, the Cu/AA complex formed through the coordinating reaction between the hydroxyl oxygen atom in AA and Cu2+ ion shows an absorption band at around 252 nm which shifts by 7 nm to the side of the longer wavelength in comparison with the intrinsic absorption band (at around 245 nm) of AA. Therefore, it is believed that the absorption band at 252 nm may be a superposition of the bands of the phenyl group and the Cu/AA complex. The FT-IR spectra tests were performed on the Y-sol, Cu-sol, YBCO-sol and BzAcH + MeOH solution (as shown in Figure 3). For the BzAcH IR spectrum, the absorption peaks at 1565, 1485 and 1461 cm1 correspond to the stretching vibration of the phenyl group, while the peaks around 1598 and 1529 cm1 correspond to the stretching vibration of C=O and C=C of the enol form in BzAcH. When the metal ions (Y3+, Cu2+) were added into the sol, the C=O and C=C absorption peaks of the enol form shifted several wave numbers towards the long wavelength (as shown in the other three IR spectra). It is demonstrated that the H+ in BzAcH was replaced by Y3+ or Cu2+ to form Y/BzAcH or Cu/ BzAcH chelate rings (as shown in formulae (1) and (2)). This result is well consistent with the above UV spectra analysis.

Figure 3

IR spectra of several gel films.

(2) YBCO gel film is deposited on the quartz substrate using the BzAcH-modified YBCO (YBCO/BzAcH) precursor sol obtained by mixing the Y/BzAcH, Cu/BzAcH and Ba/BzAcHfree sols. The change of optical absorption spectrums of the YBCO gel film by irradiation with an UV light is shown in Figure 4. It can be seen that an UV characteristic absorption band appears at around 330 nm, indicating no change in the

(1) Figure 4 Change in optical absorption spectrums with UV irradiation for YBCO/BzAcH gel film.

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structure of the metal chelates and complexes after the mixture of the three sols. Figure 4 also shows the change of UV absorption spectrum of YBCO gel film irradiated by the UV light. It is obviously seen that the intensity of the absorption band, at around 330 nm, gradually decreases with the increase of irradiation time. This indicates that the chelate rings can be dissociated by the UV light and the longer the irradiation time, the less the amount of the metal chelates. The photodissociation will lead to the formation of the O-M-O structure. The photochemical reactions are shown in formulae (3)–(4).

(3)

(4) 3.2

Fabrication of YBCO fine-pattern and microarray

The photodissociation products obtained from the reactions (3) and (4) are usually insoluble in the organic solvent. Thus with the increase of irradiation time, the photosensitive metal chelates are gradually decomposed, resulting in the solubility of YBCO/BzAcH gel film is distinctly decreased. This provides us a new approach for fabricating YBCO finepattern and microarray. When the photosensitive YBCO/BzAcH gel film is irradiated by the UV light through a mask, the unirradiated area in the gel film can be dissolved in organic solvent while the irradiated area is retained. Thus the patterned YBCO/BzAcH gel film which has the same pattern as the negative image of the mask is obtained. It is learned from Figure 4 that the photodisociation process is almost over after irradiation for 40 min. So the different solvents are adopted to pattern the

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YBCO/BzAcH gel film with the irradiation time of 40 min. Figure 5 shows the optical microphotographs of the YBCO/ BzAcH gel film patterns leached in different solvents. It is found that the unirradiated area is not completely dissolved in n-butyl alcohol (as shown in Figure 5(a)) while the irradiated area is slightly corroded in MeOH and shows a rough surface (as shown in Figure 5(b)). This indicates that the solubility of YBCO/BzAcH gel film in MeOH is stronger than that in n-butyl alcohol for the same irradiation time. Therefore, the mixing solvent of n-butyl alcohol and MeOH with the volume ratio of 1:1 was used to obtain the highquality pattern with the smooth surface and clear edge (as shown in Figure 5(c)). For the same solvent, the exposure time is a quite crucial factor to obtain the high-quality patterns. The optical microphotographs of YBCO/BzAcH solar-like fine-patterns leached in the mixture solvent and exposed under the UV light for different lengths of time are shown in Figure 6, where the patterns exposed for 1030 min show a rough surface and the roughness gradually decreases with the increase of the exposure time. This suggests that the longer the exposure time, the solubility difference between the exposed and unexposed areas in the YBCO/BzAcH gel films. The pattern has smooth surface and clear edge when the exposure time is increased to 40 min (as shown in Figure 6(d)). The fine-pattern has almost no change when increasing the exposure time of more than 40 minutes. According to the above discussions, YBCO/BzAcH gel film can be patterned to form the microarray after being exposed under the UV light for 40 min and leached in the mixture solvent. Figure 7 shows the laser confocal scanning microscopic images of the YBCO/BzAcH microarray. The white spots in Figure 7(a) are the microarray and the area around them is the silicon substrate. The 3D view of Figure 7(a) is shown in Figure 7(b). It is learned that the pitch of the microarray is about 5 m and the thickness is about 83 nm (as shown in Figure 7(c)). The YBCO/BzAcH gel film microarray can be converted to the oxide ceramic YBCO microarray with excellent superconductivity after a proper heat treatment process. Usually, the microstructure and superconductivity of YBCO

Figure 5 Optical microphotographs of the YBCO/BzAcH gel films leached in (a) n-butyl alcohol, (b) MeOH and (c) mixture of n-butyl alcohol and MeOH (volume ratio is 1:1).

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Figure 6 Optical microphotographs of YBCO/BzAcH gel films after being exposed for (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min and leached in the mixture solvent.

Figure 7 profile.

Laser confocal scanning microscopic images and cross-sectional profile of YBCO microarray (a) top-view image (b) 3D image (c) cross-sectional

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fine-patterns fabricated by the traditional patterning methods (such as WCE and IBE) may be destroyed or suppressed. In this work, however, the YBCO microarray obtained by the photosensitive sol-gel method shows good c-oriented epitaxial growth on LAO substrate after a proper heat-treatment (as shown in Figure 8). A dependence of resistivity on temperature (R–T curve) is shown in Figure 9. The R–T curve was acquired through carrying out an electrical transport measurement on an YBCO micro-bridge prepared by the same patterning process (as shown in the inset of Figure 9). It can be seen from the R–T curve that the patterned YBCO film exhibits well superconductivity with a critical transition temperature of about 89.4 K. Furthermore, the magnetization measurement was adopted to confirm the superconductivity of the above YBCO microarray. A starlike magnetic loop (M-H curve) is shown in Figure 10. This starlike M-H curve implies the superconducting nature. According to the above the analysis, it can be learned that YBCO patterned films with good c-oriented epitaxy and superconductivity can be fabricated by the photosensitive sol-gel process.

4 Conclusion YBCO/BzAcH sol can be synthesized using yttrium acetate,

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Figure 10

barium acetate, and copper acetate as the starting materials, benzalacetone (BzAcH) as chemical modifier, and methanol (MEOH) as solvent. BzAcH, as a chemical modifier, can react with the metal ions to form the photosensitive metal chelate rings which have good chemical stability in the YBCO/BzAcH sol. The photosensitive YBCO/BzAcH gel film can be patterned to form the fine-patterns and microarray with a pitch of 5 m by the UV light irradiating through a mask. The YBCO fine-patterns exhibit good c-oriented epitaxy and superconductivity after a proper heat-treatment. This work was supported by the National Natural Science Foundation of China (Grant No. 51072163), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20096118110002), the Special Funds for the Construction of Key Disciplines Project of Shanxi Province. 1

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4 Figure 8

XRD -2 scan analysis of YBCO microarray. 5 6

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Figure 9 R-T curve of YBCO micro-bridge (the inset: micrograph of YBCO micro-bridge).

Magnetic loop (M-H curve) of YBCO microarray.

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