Vanadium Oxide Microbolometer Using ZnO Sandwich Layer

Research Paper Applied Science and Convergence Technology Vol.24 No.5, September 2015, pp.178 183 http://dx.doi.org/10.5757/asct.2015.24.5.178 Vanadium Oxide Microbolometer Using ZnO Sandwich Layer Myung-Soo Han a, Dae Hyeon Kim a, Hang Ju Ko a, and Heetae Kim b * a Medical Photonics Research Center, Korea Photonics Technology Institute, Gwangju 61007, Korea b Rare Isotope Science Project, Institute for Basic Science, Daejeon 34047, Korea (Received September 24, 2015, Accepted September 30, 2015) Optical, electrical and structural properties of VOx/ZnO/VOx thin film are studied. The VOx/ZnO/VOx multilayer is deposited by using a radio frequency (RF) sputtering system. The VOx/ZnO/VOx thin film shows the high temperature coefficient of resistance (TCR) of 3.12%/ o C and the low sheet resistance of about 80 kω/sq at room temperature. The responsivity and detectivity of the bolometer are measured as a function of modulation frequency. Keywords : IR detector, Microbolometer, Oxygen annealing, TCR I. Introduction Temperature of a body was measured by blackbody radiation and the size effect of the blackbody radiation was investigated [1-3]. The effective temperature of the body was shown when the temperature distribution of the body is not uniform [4-6]. An infrared technique was widely applied to medical field, industrial application and military purpose. Infrared detector devices were intensively developed [7,8]. In spite of high response speed and superior image reproduction, the cooledtype infrared detector requires a separate cooling device to operate at very low temperature. For this reason, an uncooled-type infrared detector device and an infrared image device that do not require a separate cooling device were developed. A bolometer which is uncooled infrared detector was intensively investigated because it has a high response feature and can be manufactured by using a conventional semiconductor manufacturing process [9-12]. Vanadium oxides were first observed by Morin in 1959 [13]. Vanadium oxides which include V 2 O 5, VO 2, V 2 O 3, V 6 O 13, V 4 O 7 and V 3 O 5 were made at various conditions [14]. The optical and electrical properties of the VOx thin films can be greatly influenced by their chemical structure. Vanadium oxides are considered as promising candidates in optoelectronic applications since they show metal-to-insulator transition (MIT) and high thermal sensitivity. VOx films were required to have high TCR and appropriate sheet resistance for applications in uncooled infrared detectors [15]. The VOx films with TCR of -2.15%/ o C and sheet resistance of 20 KΩ/square desirable for uncooled IR detectors were prepared by reactive sputtering [15]. They undergo transition from an insulator or semiconductor to a metal phase at a specific temperature. Single crystal VO 2 and V 2 O 5 had large TCR of above -4%/ o C [16]. However, the deposition of VO 2 thin film is very difficult and needs a high-cost ion-beam deposition method. V 2 O 5 is easily formed in high oxygen partial * [E-mail] kim_ht7@yahoo.com

Vanadium Oxide Microbolometer Using ZnO Sandwich Layer pressure, but its resistance at room temperature is very high. V 2O 3 has low formation energy and undergoes the phase transition from semiconductor to metal at -123 o C, so its resistance is very low at room temperature [17,18]. ZnO thin films were grown with radio frequency (RF) sputtering, which have the direct wide bandgap of 3.36 ev and have high binding excitation energy of 60 mev [19-22]. A vanadium oxides microbolometer was fabricated using multilayer thin film processing [23]. In this paper, the optical, electrical and structural properties of VOx/ZnO/VOx multilayers having VO 2 and V 2O 5 phase was studied. The VOx/ZnO/VOx multilayer is deposited by using RF sputtering system. The multilayer film is annealed with oxygen at 300 o C for 50 minutes. The electrical and structural properties for the device are then measured. II. Experiment VOx/ZnO/VOx thin film is deposited at room temperature by using conventional radio frequency (RF) sputtering. By using plasma enhanced chemical vapor deposition (PECVD) method, SiNx film is deposited onto the Si wafers with the thickness of 300 nm. Vanadium oxide films are deposited using radio frequency (RF) sputtering method with a vanadium metal target in argon and oxygen ambient. The VOx film depositions are carried out in a turbo molecular pump chamber evacuated previously to 3 10-6 torr. The flow ratio of O 2/(O 2+Ar) for VOx growth is 4.4%. The deposition rate is 2.5 nm/min at the RF power of 150 W. ZnO thin film is deposited in argon ambient at room temperature by using RF sputtering system. The lower layer of VOx in VOx/ZnO/VOx structures is deposited with 60 nm thickness to provide a sufficient oxygen source, which helps to make oxygen diffusion into the ZnO layer. The thickness of the ZnO layer is 10 nm. The upper layer VOx in the VOx/ZnO/VOx structures is deposited with 10 nm thickness to provide a short depth for oxygen diffusion from the surface into the ZnO thin film. The annealing temperature and time for the VOx/ZnO/VOx film in oxygen ambient are 300 o C and 50 minutes, respectively. Indium metallization is used for the electrical measurement. Sheet resistance is measured from 20 to 60 o C in steps of 1 o C by using a resistance tester. The TCR is obtained by linear curve fitting. The properties of the VOx/ZnO/VOx film are measured by atomic force microscope (AFM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Using RTB3000 (SBIR inc., USA), figures of merit is measured for the metal vacuum packaged microbolometer. The voltage signal of the device is measured by using a radiation test bench. The responsivity and detectivity of the device are obtained from the measured parameters. Figure 1. AFM images of VOx/ ZnO/VOx thin film. www.jasct.org//doi:10.5757/asct.2015.24.5.178 179

Myung-Soo Han, Dae Hyeon Kim, Hang Ju Ko, and Heetae Kim III. Results and Discussion An atomic force microscopy (AFM) is used to measure the surface roughness of the VOx/ZnO/VOx thin film. Fig. 1 shows the atomic force microscopy (AFM) images of the VOx/ZnO/VOx thin film. The root-mean square of surface roughness of the film [24] is 1.7 nm, which shows a smooth surface. The difference of the surface roughness between before and after annealing was negligible as we expected. Fig. 2 shows the XRD peaks of VOx/ZnO/VOx multilayer thin films in terms of annealing temperature. The device is annealed in oxygen ambient at 300 o C for 50 minutes. It is observed that as-deposited multilayer thin film exhibited V 2O 5 (111). After oxygen annealing above 250 o C for 50 minutes, VO 2 (211) was appeared. Fig. 3 shows the schematic cross-section views for as-deposited and oxygen-annealed of VOx/ZnO/VOx thin films. Fig. 4 shows the thermal resistance of the VOx/ ZnO/VOx thin films as a function of temperature. TCR is measured from 20 o C to 60 o C by controlling the temperature of the multilayer film. The resistance of the thin films decreases as temperature increases. From Fig. 4, the sheet resistance of the thin film is low with 80 kω/sq at 22 o C and the TCR is obtained to be -3.12%/ o C by linear curve fitting. These sheet resistance measurement suggests that the VOx/ZnO/ VOx multilayer thin film can be used for uncooled infrared detectors. Figure 2. XRD peaks of VOx/ZnO/VOx multilayer thin films in terms of annealing temperature. The device is annealed in oxygen ambient at 300 o C for 50 minutes. Figure 4. Sheet resistance measurement of the VOx/ ZnO/VOx thin films as a function of temperature. Figure 3. Schematic crosssection views for as-deposited and oxygen-annealed of VOx/ZnO/VOx multilayer thin films. 180 Appl. Sci. Conv. Technol. 24(5), 178-183 (2015)

Vanadium Oxide Microbolometer Using ZnO Sandwich Layer Figure 5. SEM image of microbolometer device. Figure 6. Responsivity and detectivity of microbolometer devices as a function of frequency. Fig. 5 shows the SEM image of the microbolometer fabricated in this experiment. The dimension of the microbolometer is 50 μm 50 μm. The bolometer device is mounted in a metal vacuum package to measure the optical properties of the microbolometer. The vacuum metal package is pumped and kept about 10-3 torr by using a rotary and turbo-molecular pump. Blackbody source of 500 o C radiates on the microbolometer at a given chopper frequency and the output voltages of the device are measured with a low noise preamplifier and a lock-in amplifier (Stanford Research 850 model). The responsivity and detectivity are shown as a function of frequency in Fig. 6. The responsivity and detectivity of the detector have the maximum value of 9 10 4 V/W and 1.4 10 8 cm Hz 1/2 /W, respectively. The responsivity and detectivity of the microbolometer decrease as the frequency increases. Compared to VOx bolometer detector, the TCR of the VOx/ZnO/VOx microbolometer is 4 or 5 times higher. The bolometer device shows high temperature coefficient of resistance (TCR) and low sheet resistance, which can be applied to microbolometer for uncooled infrared detector. IV. Conclusions We have shown the optical, electrical and structural properties of VOx/ZnO/VOx multilayer which shows the temperature coefficient of resistance (TCR) of -3.12%/ o C. The VOx/ZnO/VOx multilayer was deposited by using RF sputtering system. The multilayer film was annealed with oxygen at 300 o C for 50 minutes. XRD measurement showed that the multilayer film has V 2 O 5 and VO 2 phase. The microbolometer integrated from VOx/ZnO/VOx mutilayer showed the maximum responsivity of 9 10 4 V/W and the detectivity of 1.4 10 8 cm Hz 1/2 /W at 1 Hz and 1.5 V bias voltage. Acknowledgements This work was supported by the Rare Isotope Science Project of Institute for Basic Science funded by the Ministry of Science, ICT and Future Planning (MSIP) and the National Research Foundation (NRF) of the republic of Korea under contract 2013M7A1A10 75764. This work was also supported by Materials Components Technology Development Program funded by the Korea Ministry of Trade, Industry & Energy (MOTIE) under contract number G01201411010071. www.jasct.org//doi:10.5757/asct.2015.24.5.178 181

Myung-Soo Han, Dae Hyeon Kim, Hang Ju Ko, and Heetae Kim References [1] S. J. Yu, S. J. Youn, H. Kim, Size effect of thermal radiation, Physica B. 405, 638 (2010). [2] H. Kim, S. J. Youn, S. J. Yu, Finite Size Effect of One-dimensional Thermal Radiation, J. Korean Phys. Soc. 56, 554 (2010). [3] H. Kim, S. C. Lim, Y. H. Lee, Size effect of twodimensional thermal radiation, Phys. Letts. A. 375, 2661 (2011). [4] H. Kim, M.S. Han, D. Perello and M. Yun, Effective temperature of thermal radiation from non-uniform temperature distributions and nanoparticles, Infrared Physics & Technology 60, 7 (2013). [5] H. Kim, C. S. Park, M.S. Han, Effective temperature of two dimensional material for non-uniform temperature distribution, Optics Communications 325, 68 (2014). [6] H. Kim, W. K. Kim, G.T. Park, C.S. Park, H. D. Cho, Size effect of the effective temperature in one-dimensional material, Infrared Physics & Technology 67, 49 (2014). [7] A. Tanaka, S. Matsumoto, N. Tsukamoto, S. Itoh, K. Chiba, T. Endoh, A. Nakazato, Infrared focal plane array incorporating silicon IC process compatible bolometer, IEEE Transactions on Electron Devices, 43, 1844 (1996). [8] D. Manno, A. Serra, M. Di Giulio, G. Micocci, A. Taurino, A. Tepore, D. Berti, Structural and electrical properties of sputtered vanadium oxide thin films for applications as gas sensing material, J. Appl. Phys. 81, 2709 (1997). [9] Y. Shimizu, K. Nagase, N. Miura, N. Yamazoe, New preparation process of V2O5 thin films based on spin coating from organic vanadium solution, Jpn. J. Appl. Phys. 29, 1708 (1990). [10] Y. Zhao, Z. C. Feng, Y. Liang, H. W. Sheng, Laser-induced coloration of WO3, Appl. Phys. Lett. 71, 2227 (1997). [11] D. Barreca, Vanadyl precursors used to modify the properties of vanadium oxide thin film obtained by chemical vapor deposition, J. Electrochem. Soc. 146, 551 (1999). [12] F.C. Case, Modifications in the phase transition properties of pre-deposited VO2 films, J. Vac. Sci. Technol. A 2, 1509 (1984). [13] F.J. Morin, Oxides which show a metal-to-insulator transition at the Neel temperature, Phys. Rev. Lett., 3, 34 (1959). [14] V.N. Ovsyuk, Uncooled microbolometer IR FPA based on sol-gel VO, Proc. of SPIE, 5834, 47 (2005). [15] H. G. Li, Preparation of VOx Films for Uncooled Infrared Detecors, Semi. Optoelectron. 22, 38 (2001). [16] J. C. Yang, Vanadium Oxide-based Bolometric Infrared Spectrometer, Proc. of SPIE, 6759, 675905 (2007). [17] M. Ghanashyam Krishna, Y. Debauge and A. K. Bhattacharya, X-ray photoelectron spectroscopy and spectral transmittance study of stoichiometry in sputtered vanadium oxide films, Thin Solid Films 312, 116 (1998). [18] P. Jin, M. Tazawa, K. Yoshimura, K. Igarashi, S. Tanemura, K. Macak, U. Helmersson, Epitaxial growth of W-doped VO2/V2O3 multilayer on α- Al2O3(110) by reactive magnetron sputtering, Thin Solid Films 375, 128 (2000). [19] A. Shimizu, M. Kanbara, M. Hada, M. Kasuga, ZnO Green Light Emitting Diode, Jpn. J. Appl. Phys., Part 1 17, 1435 (1978). [20] C. Klingshirn, The Luminescence of ZnO under High One- and Two-Quantum Excitation, Phys. Status Solidi B 71, 547 (1975). [21] S. Cho, H. Kim, Effects of Rapid Thermal Annealing on the Photoluminescent Properties of ZnO Thin Films, Journal of the Korean Physical Society 53, 1987 (2008). [22] S. Cho, H. Kim, Effect of deposition temperature 182 Appl. Sci. Conv. Technol. 24(5), 178-183 (2015)

Vanadium Oxide Microbolometer Using ZnO Sandwich Layer on the properties of nitrogen-doped AZO thin films grown by rf reactive magnetron sputtering, Material Science and Engineering B, 172, 327 (2010). [23] M.S. Han, D.H. Kim, H.J. Ko, J.C. Shin, H.J. Kim, D.G. Kim, A fabrication and characterictics of microbolometer detectors using VOx/ZnO/VOx multilayer thin film processing, Proc. SPIE 9070, Infrared Technology and Applications XL, 90701X (2014); doi:10.1117/12.2049513. [24] J.K. Lee, C.S. Park, H. Kim, Sheet resistance variation of graphene grown on annealed and mechanically polished Cu films, RSC Advances 4, 62453 (2014). www.jasct.org//doi:10.5757/asct.2015.24.5.178 183