A statistical parameter study of indium tin oxide thin films deposited by radio-frequency sputtering

Thin Solid Films 476 (2005) 59 64 www.elsevier.com/locate/tsf A statistical parameter study of indium tin oxide thin films deposited by radio-frequency sputtering Seung-Ik Jun a, *, Timothy E. McKnight b, Michael L. Simpson a,b, Philip D. Rack a a Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA b Molecular Scale Engineering and Nanoscale Technologies Research Group, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Received 31 May 2004; received in revised form 18 August 2004; accepted 2 September 2004 Available online 30 October 2004 Abstract In order to optimize the electrical and optical properties of indium tin oxide (ITO) thin films, a statistical analysis called Taguchi design was employed. It is shown that the sheet resistance and transmittance are inversely proportional to each other as a function of the process parameters. Additionally, the preferred orientation of crystalline ITO film is distinguishably changed with the increase of sputtering temperature and oxygen fraction (O 2 /O 2 +Ar) in the sputtering ambient. The change in crystallinity results from the content of incorporated oxygen, which significantly affects the electrical and optical properties of ITO films and causes a rearrangement of atoms to form preferred closed-packed plane orientation. Finally, the microstructure of the ITO films becomes denser with the increasing oxygen fraction. As a result of this work, we have successfully achieved low sheet resistance (7.0 V/5) and high transmittance (~90%) for 300 nm thick films. D 2004 Elsevier B.V. All rights reserved. PACS: 73.50.-h; 78.20.-e Keywords: Indium tin oxide (ITO); Sputtering; Taguchi design; Transparent electrode 1. Introduction Indium tin oxide (ITO) thin films are widely used as transparent electrodes in electronic displays [1]. In recent years, ITO has also been implemented in light-addressed intracellular biological probes [2,3]. Normally RF magnetron sputtering is used for the deposition of the film because it provides a low temperature deposition process and higher process efficiencies, higher throughput, and process reliability [1]. When using ITO transparent electrodes, there is a well-known trade-off between the sheet resistance and the transmittance, which is dominated by the number of oxygen vacancies in the films [4,5]. Oxygen vacancies are donor levels in ITO; consequently, when ITO is slightly oxygendeficient, reasonable electrical transport can be realized. The transmittance however can be deleteriously affected if the * Corresponding author. Tel./fax: +1 865 974 0033. E-mail address: sjun3@utk.edu (S.-I. Jun). films are too oxygen-deficient as they become more metallike. Therefore it is important to determine the optimized deposition conditions in order to achieve high transmittance and low sheet resistance at the same time. In a typical RF magnetron sputtering system, normally there are four main factors that control the process; RF power, gas pressure, temperature, and gas composition. To optimize this wide parameter space in a serial manner is tedious and unwieldy and many parameter interactions can go undetected. To facilitate rapid process optimization and reduce the number of trials and analytical errors, statistical tools can be implemented. In this research, we describe a Taguchi design of experiment (DOE) for the purpose of optimizing the ITO deposition [6,7]. This DOE is normally used for optimizing a process condition that is completely unknown or in the initial stage of process development to determine the overall tendencies of process factors with less experiment trials [7]. The process targets for the ITO films are sheet resistance b10 V/5 and transmittance (at 488 nm) N85% for a nominal film thickness of 300 nm. 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.09.011

60 S.-I. Jun et al. / Thin Solid Films 476 (2005) 59 64 2. Experimental details The specified description of the DOE is described in detail in Table 1. Our initial DOE was a three-level design (low, medium, high) for the four process factors, which included RF power, temperature, pressure, and O 2 fraction (for a Ar+O 2 ambient). The responses for the DOE included deposition rate, transmittance, sheet resistance, and etching rate. For the DOE analysis, a high deposition rate, high transmittance, low sheet resistance, and high etch rate were desirable. Consequently, to qualify how the factors affected our responses, the following illustrates our interpretation of the results: dlarger is bettert for deposition rate, dlarger is bestt for transmittance, dsmaller is bestt for sheet resistance, and dlarger is bettert for etching rate. An AJA ATC2000 RF magnetron sputtering system was utilized for the deposition of ITO thin films on glass substrates. The sputtering target has a diameter of 50 mm and a thickness of 6 mm and consists of In 2 O 3 and 10 wt.% SnO 2. The base pressure prior to the sputtering deposition was below 5.310 6 Pa and the total flow rate of argon used in the sputtering was fixed at 25 sccm for all conditions. The substrate is heated by quartz lamps and the temperature is controlled within F1 8C. The transmittance of ITO thin films was measured using a calibrated Ar lamp and an Ocean Optics PC2000 spectrometer with a wavelength range 200 900 nm. The transmittance was fixed and normalized at 488 nm because our application is a light-addressed biological probe operated at that wavelength because silicon is the active layer and has a high absorption coefficient at 488 nm. The sheet resistance was analyzed via four-point probes (Veeco FPP-5000) and the reported value is an average of three measurements over the sample. The film thickness and etching rate were evaluated by using Surface Profiler (KLA Tencor Alpha-Step 500). The Fig. 1. Effects of RF power, temperature, pressure, and gas composition on the deposition rate, sheet resistance, and transmittance. etchant was a solution of HCl+CH 3 COOH+H 2 O (22%+ 6%+72%) and heated and maintained at 40 8C on a hot plate. The crystal orientation of ITO thin films was determined by a Phillips X-pert Pro X-ray diffraction Table 1 DOE for optimizing ITO thin film deposited by RF magnetron sputtering: Taguchi design No. Factors Responses RF power, W (W/cm 2 ) Temperature, 8C Pressure, Pa O 2, sccm D/R a, nm/min Tr b, % Rs c, V/5 E/R d, nm/min 1 75 (3.7) 30 0.7 0 2.63 35.1 262 N200 2 75 (3.7) 130 2.0 2 0.67 86.1 63,593 N200 3 75 (3.7) 230 3.3 4 0.14 89.5 391 b5 4 100 (4.9) 30 2.0 4 0.82 87.2 ~150,000 N200 5 100 (4.9) 130 3.3 0 0.93 85.8 231 N200 6 100 (4.9) 230 0.7 2 2.91 88.9 343 424 7 125 (6.2) 30 3.3 2 0.84 89.2 ~150,000 N2000 8 125 (6.2) 130 0.7 4 3.43 86.8 10,839 545 9 125 (6.2) 230 2.0 0 2.52 68.6 28 403 a D/R: deposition rate (nm/min). b Tr: transmittance at 488 nm (%). c Rs: sheet resistance (V/5) at 300 nm thickness films. d E/R: etching rate in HCl+CH 3 COOH+H 2 O (22%+6%+72%), 40 8C.

S.-I. Jun et al. / Thin Solid Films 476 (2005) 59 64 61 Table 2 Optimum levels for each factor and its priority in optimizing ITO deposition Targets RF power Temperature Pressure O 2 content Lower Rs Low High (1) Low Low (2) Higher Tr High High High (2) High (1) Higher D/R High (2) High Low (1) Low Optimized Low (b75 W) (XRD) and the microstructure was evaluated using a Hitachi S-4300 scanning electron microscope (SEM). 3. Results and discussion High (N230 8C) Low (b0.7 Pa) (1), (2): The first and second priorities effecting on process targets. Low (~0 sccm) The DOE used in this work and its results are illustrated in Table 1. As previously mentioned, the process factors are RF power, temperature, gas pressure, and oxygen fraction. The effects of the process factors on the deposition rate, sheet resistance, and transmittance at 488 nm, and etching rate following experiments are presented in Table 1 and tendencies are illustrated in Fig. 1. As shown in Fig. 1, the main factors that strongly effect on the deposition rate are pressure and RF power, with total pressure having the strongest effect. The significant lowering of the deposition rate with increasing pressure is a result of the shorter mean free path of the sputtered species as the pressure is increased. As pressure increases, the sputtered species get scattered while traversing the ~130 mm target-to-substrate distance. According to the kinetic theory of gas, the thermal mean free paths at 0.7, 2.0, and 3.3 Pa are ~54, 18, and 11 mm, respectively. Another interesting observation from Fig. 1 is that the deposition rate decreases with increasing oxygen flow. We also surveyed XRD results and concluded that the deposition rate was lowered by oxygen addition due to the change from amorphous to crystalline ITO and the transformation of its preferred crystallinity [8]. A detailed explanation of this phenomenon will be discussed later. Fig. 1 also shows the effects that the four factors have on sheet resistance and illustrates that temperature is the most dominant factor affecting the sheet resistance of the ITO films. As the temperature is increased, the sheet resistance can be lowered significantly. The reason for the observed decrease in the sheet resistance with increasing temperature is a structural amorphous-to-crystalline transformation in the ITO films as will be discussed in detail below. The observed relationship of decreasing sheet resistance with decreasing RF power can also be correlated to the crystallinity of the ITO films as determined from the XRD data [9]. The effect that the four factors have on the transmittance of ITO films is one of the critical responses of the DOE and the prevailing factor is the flow rate of oxygen. In the process of Ar sputtering (no oxygen), the oxygen concentration of the ITO film is lower than that of the target because not all the oxygen sputtered from the target will be incorporated into the film. That is, the films without oxygen flow show metal-like characteristics with low transmittance, low sheet resistance, and higher oxygen vacancy concentration. On the other hand, as the oxygen fraction is increased in sputtering gas, the concentration of oxygen vacancies is drastically decreased and it results in higher transmittance and higher sheet resistance like an oxide insulating thin film [10,11]. The optimum levels of each factor and its priority are summarized in Table 2. Optimized levels from the Taguchi DOE are lower RF power, higher temperature, lower pressure, and lower oxygen contents in order to minimize sheet resistance and to maximize transmittance of ITO films at the same time. In order to optimize the dominant factors of temperature and oxygen fraction, a second experiment was designed as shown in Table 3. For this experiment, the total pressure and Ar flow rate were fixed at 0.7 Pa and 25 sccm, respectively, for all conditions. Finally, we compared the optimized Table 3 Additional experiment for optimizing dominant factors: temperature and O 2 /(O 2 +Ar) ratio No. Factors Responses RF power, W (W/cm 2 ) Temperature, 8C O 2 /(O 2 +Ar) D/R, nm/min 1 75 (3.7) 400 0.00 3.23 87.3 7.0 405 2 75 (3.7) 400 0.01 3.29 82.7 97.2 305 3 75 (3.7) 400 0.02 3.19 81.5 242.1 295 4 75 (3.7) 400 0.04 2.81 92.8 201.5 334 5 75 (3.7) 400 0.06 2.56 98.1 339.8 804 6 75 (3.7) 400 0.08 2.39 96.6 479.3 954 7 125 (6.2) 400 0.00 4.70 68.2 8.9 440 8 125 (6.2) 30 0.00 3.22 75.8 206.0 164 9 b 125 (6.2) 30 0.00 3.22 85.9 8.2 369 All films are deposited at 0.7 Pa pressure. a (222) intensity. b Post-annealed after room temperature deposition (400 8C, 30 min in the vacuum). Tr, % Rs, V/5 XRD intensity a

62 S.-I. Jun et al. / Thin Solid Films 476 (2005) 59 64 Fig. 2. XRD pattern for sintered ITO target containing In 2 O 3 and 10 wt.% SnO 2. Fig. 4. XRD patterns for two different applied RF powers. condition from the DOE with a film deposited at room temperature and post-annealed at 400 8C, 30 min (experiment no. 9). Fig. 2 shows that XRD pattern of a sintered ITO sputtering target, which shows that the preferred orientation is (222). Additionally, the other dominant orientations in this pattern are (211), (400), (440), and (622). The XRD patterns of the ITO films at different substrate temperature are shown in Fig. 3. All were done under 0.7 Pa of pressure, 75 W (3.7 W/cm 2 ) of RF power, and 25 sccm of Ar flow rate. From Fig. 3, it is seen that the crystallinity of the ITO film deposited at room temperature is almost amorphous ITO. However, as the substrate temperature is increased during deposition, the crystallinity increases with (400) preferred orientation mixed with small amounts of (222), (440), and (620). These results are similar to those observed in the initial DOE. The reason for (400) preferred orientation is that the concentration of oxygen vacancies in the ITO film at high temperature sputtering is higher than that of room temperature sputtering because of the reduced sticking coefficient of oxygen at higher temperatures. Therefore crystalline ITO deposited high temperature typically grows (400) preferentially to accommodate the oxygen vacancies on these planes. On the other hand, the post-annealed ITO film at 400 8C/30 min that was deposited at room temperature shows (222) preferred plane that is a close-packed plane in In 2 O 3 body-centered cubic structure. This plane does not accommodate vacancies very well and is stabilized when there are fewer oxygen vacancies. Fig. 4 shows the dependence of the RF power on the XRD patterns. Both are processed under 0.7 Pa of pressure, 400 8C of temperature, and 25 sccm of argon flow rate (without oxygen). The slight increase of intensity explains that the increase of RF power results in wellcrystallized structure due to the impinging of higher energy ions onto substrate. Fig. 5 shows the XRD results according to the fraction of oxygen in sputtering gas. The sputtering conditions were 0.7 Pa, RF power of 75 W (3.7 W/cm 2 ), 25 sccm of argon flow rate, and substrate temperature of 400 8C. In argon alone, there are two dominant planes in the film, (222) and (400). As the fraction of oxygen is increased, the (400) peak disappears and (222) peak becomes the dominant plane. We can speculate from these results that the driving forces of this change in preferred orientation are the concentration of oxygen vacancies and their subsequent diffusion. If the deposition is processed with zero or low oxygen, more oxygen vacancies are incorporated in the ITO film versus those films processed at higher oxygen fractions in the sputtering gas. The vacancies play an important role in atomic diffusion and the atoms can diffuse through these vacancies. Therefore we can infer that various preferred planes are shown in argon sputtering without oxygen. On Fig. 3. XRD results according to substrate temperature and post-annealing. Fig. 5. XRD results as a function of fraction of oxygen in sputtering gas.

S.-I. Jun et al. / Thin Solid Films 476 (2005) 59 64 63 the other hand, as the fraction of oxygen is increased, the concentration of oxygen vacancies is decreased. As a result, the (222) peak will dominate in higher oxygen content in the sputtering ambient since the close-packed plane is (222) in the body-centered cubic In 2 O 3 crystal. The relationship between transmittance and sheet resistance is illustrated in Fig. 6. All were deposited at 0.7 Pa pressure, 400 8C temperature, and 75 W RF power. The trend done in the first DOE is observed in the second experiment: namely, the transmittance and sheet resistance both have increasing relationship as the fraction of oxygen is increased. Specifically, the transmittance decreases when the sheet resistance is lowered. As the fraction of oxygen increased, the incorporated oxygen in the film will contribute to the enhancement of transmittance but the sheet resistance will be increased and deteriorated by the oxygen contrariwise. It results from the incorporated oxygen which decreases the donor level concentration and increases the sheet resistance and concomitantly increases the transmittance. It was previously concluded from original DOE that this results from the formation of oxygen vacancies in the film when it is deposited in argon gas only. In Fig. 6, we also can see that the deposition rate is lowered by addition of oxygen gas because the preferred plane of the film is clearly changed from the (400) plane that is perpendicular to substrate into (222) plane with the increase of oxygen fraction in gas. In lower oxygen ambient, the ITO film was grown as (400) preferentially with higher oxygen vacancies Fig. 7. SEM image of ITO film with the increase of oxygen in gas: (a) O 2 / (O 2 +Ar): 0.00; (b) 0.02; (c) 0.08. and higher deposition rate. On the other hand, in higher oxygen gas, the film was arranged to (222) plane that is most close-packed plane in In 2 O 3 having a BCC structure. Therefore the deposition rate decreases with the increase of oxygen addition in plasma because the (222) close-packed plane is denser than the (400) plane which accommodates more vacancies. Fig. 7 shows the SEM images as the oxygen fraction is increased from 0.00 to 0.08. This result shows that the ITO grains become coarser and denser with the increase of oxygen. 4. Conclusions Fig. 6. The characteristics of transmittance, sheet resistance, deposition rate, and XRD intensity (222) as a function of oxygen fraction in sputtering. To improve the electrical properties, lower sheet resistance, ITO deposition should be processed under higher temperature and low oxygen fraction. A tradeoff between

64 S.-I. Jun et al. / Thin Solid Films 476 (2005) 59 64 low sheet resistance and high transmission exists due to the role that the oxygen vacancies have in the conduction and transmission. A high number of oxygen vacancies facilitate conduction; however, the film becomes less transparent. Conversely, few vacancies facilitate high transmission; however, the conduction decreases significantly. To rapidly determine an optimum process for ITO films, a Taguchi design of experiments was implemented to investigate the effects that RF power, substrate temperature, total pressure, and oxygen partial pressure have on the sheet resistance, transmission, deposition rate, and etch rate. The effects that each process parameter had on each response were shown. The oxygen partial pressure was determined to be the dominant factor that controlled the sheet resistance and transmission so a subsequent experimental design was run to closely determine the effect of oxygen partial pressure on the ITO films. The properties were correlated to the crystal structure and microstructure of the ITO films. It was discovered that the optical and electrical properties are strongly correlated to the preferred orientation of the films which varies from (400) to (222) as the fraction of oxygen in sputtering increases. We speculate that the preferred orientation is a result of the concentration of oxygen vacancy and rearrangement of atoms in close-packed plane of In 2 O 3 having a body-centered cubic structure. As shown in the above results, the crystallinity of ITO films strongly depends on temperature, RF power, and oxygen fraction in sputtering. Specifically, high temperature, energetic ion bombardment due to higher RF power, and less oxygen vacancy due to addition of oxygen gas in sputtering ambient all contribute to the enhancement of the crystallinity of the films. Acknowledgements This work was supported in part by the National Institute for Biomedical Imaging and Bioengineering under assignment R01EB000433 and through the Laboratory Directed Research and Development funding program of the Oak Ridge National Laboratory, which is managed for the U.S. Department of Energy by UT-Battelle, LLC. References [1] M. Katayama, Thin Solid Films 341 (1999) 140. [2] V. Bucher, B. Brunner, C. Leibrock, M. Schubert, W. Nisch, Biosens. Bioelectron. 16 (2001) 205. [3] V. Bucher, M. Schubert, D. Kern, W. Nisch, Microelectron. Eng. 57 (2001) 705. [4] V. Teixeira, H.N. Cui, L.J. Meng, E. Fortunato, R. Martins, Thin Solid Films 420 (2002) 70. [5] Ho-Chul Lee, O Ok Park, Vacuum 72 (2004) 411. [6] Phillip J. Ross, Taguchi Techniques for Quality Engineering, McGraw-Hill, New York, 1996. [7] Ranjit K. Roy, Design of Experiments Using the Taguchi Approach, Wiley, New York, 2001. [8] Y.S. Kim, Y.C. Park, S.G. Ansari, B.S. Lee, H.S. Shin, Thin Solid Films 426 (2003) 124. [9] E. Terzini, P. Thilakan, C. Minarini, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 77 (2000) 110. [10] I. Baia, B. Fernandes, P. Nunes, M. Quintela, R. Martins, Thin Solid Films 383 (2001) 244. [11] C.G. Choi, K. No, W.J. Lee, H.G. Kim, S.O. Jung, W.J. Lee, W.S. Kim, S.J. Kim, C. Yoon, Thin Solid Films 258 (1995) 274.