IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 6, DECEMBER 2011 3587 Characterization of a Transition-Edge Bolometer Made of YBCO Thin Films Prepared by Nonfluorine Metal–Organic Deposition Mehdi Hosseini, Ali Moftakharzadeh, Alireza Kokabi, Mohammad Ali Vesaghi, Helmut Kinder, and Mehdi Fardmanesh, Senior Member, IEEE Abstract—We present the results of a bolometric transition-edge sensor made of a high-T c superconductor YBCO thin film pre- pared by fluorine-free metal–organic deposition. The structure of the films was characterized by X-ray diffraction and scanning elec- tron microscopy, and the superconducting properties were deter- mined by R-T measurements. The applicability of the resulting film as an infrared sensor is reported here. The optical response in the range of near infrared and the noise characteristics of the patterned bolometer are measured and analyzed. The dependence of device sensitivity on the bias current and modulation frequency is also investigated. As it is presented in this paper, the results of the fabricated device are comparable to the devices made of high-quality pulsed laser deposition YBCO thin films. Index Terms—Bolometric device, metal–organic deposition (MOD) fabrication method, transition-edge sensor (TES). I. I NTRODUCTION C HEMICAL-solution-based processes such as metal– organic deposition (MOD) are known to be one of the best methods for mass production of large-area high-T c super- conducting films at low cost without the need of vacuum and precise composition control [1]. Large-area films are promising in both power engineering and microwave applications. The fluorine-free MOD method is particularly attractive because it is not hazardous. This method has been recently reported to yield both high critical current densities in excess of 10 MAcm −2 [2] and large-area films of high quality [3]–[6]. An RF surface resistance of about 0.8 mΩ at 77 K was also reported [7]. The issue of large-area films is also attractive for the fab- rication of optical detectors with a large number of integrated pixels. Patterned films as optical devices require reduced grain boundaries, resulting in better sensitivity and less noise. In addition, in this respect, the fluorine-free MOD method is well Manuscript received July 7, 2011; revised August 11, 2011; accepted August 12, 2011. Date of publication September 26, 2011; date of current version December 2, 2011. This paper was recommended by Associate Editor M. Mueck. M. Hosseini and M. A. Vesaghi are with the Department of Physics, Sharif University of Technology, Tehran 14588 89694, Iran. A. Moftakharzadeh, A. Kokabi, and M. Fardmanesh are with the Department of Electrical Engineering, Sharif University of Technology, Tehran 14588 89694, Iran (e-mail: fardmanesh@sharif.edu). H. Kinder is with the Department of Physics, Technische Universität München, 85748 Munich, Germany. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2011.2165319 suited because it yields highly oriented and uniform YBCO films [1], [4], [5], [7]. Superconductor transition-edge sensors (TESs) or bolome- ters are considered as very highly sensitive optical detectors in a wide range of wavelengths. The physical operation principle of these sensors is based on the steep fall in their resistance at a transition temperature. The sensitivity of these devices in- creases with increasing bias current [8]. However, the bolome- ter bias current is limited by joule heating and the critical current of the patterned YBCO film [8]. The high critical current density of fluorine-free MOD films [2] is therefore ideal for this application. Because of the named advantages, we have fabricated a su- perconductor TES using YBCO films prepared by the fluorine- free MOD method. First, we describe our process of film fabrication on lanthanum aluminate (LaAlO 3 ) substrates. Then, we present characterization data of the fabricated films and IR- response data of the structured bolometers. The obtained results are compared with those of the previously fabricated bolome- ters using YBCO films made by other deposition methods . II. EXPERIMENTAL DETAILS A. Film Fabrication The YBCO films were prepared in a similar way as reported in [1] by starting from the metal acetylacetonates (Sigma- Aldrich Company) with a molar ratio of Y:Ba:Cu =1:2:3. These were dissolved in a mixture of pyridine and propionic acid. The solution was then stirred at 40 ◦ C until the solvents were removed. The residuum was dissolved in methanol and filtered to obtain a green homogeneous solution. By adding or volatilizing methanol, the viscosity of the solution was properly adjusted for spin coating. As substrates, we used 10 mm × 10 mm LaAlO 3 (100) crystals without buffer layers. The substrates were preannealed to release most of the stresses arising at the ferroelastic twinning transition near 500 ◦ C. Onto these substrates, the solution was spin coated at 2500 r/min. Calcination was done in a quartz tube furnace at a slow ramp of 1 ◦ C/min up to 500 ◦ C and a dwell time of 10 h at this temperature under oxygen flow to remove most of the organic components. The coating and calcination processes could be repeated to increase film thickness. For crystallization, the calcined films were quickly heated up to 780 ◦ C in a 1051-8223/$26.00 © 2011 IEEE