Properties of aluminium oxide thin films deposited in high effective reactive pulsed magnetron sputtering process

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1 Materials Science-Poland, 30(4), 2012, pp DOI: /s Properties of aluminium oxide thin films deposited in high effective reactive pulsed magnetron sputtering process K. TADASZAK, K. NITSCH, T. PIASECKI, W.M. POSADOWSKI Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, Janiszewskiego 11/17, Wrocław, Poland Pulsed magnetron sputtering of metal targets in the presence of reactive gas is widely used to deposit compound materials. This method is very popular but still the aim of research is to obtain more stable and efficient processes. The standard procedure of compound thin film deposition is sputtering in so called reactive mode of magnetron work sputtering of the target surface covered with the formed compound. The authors postulate that the problem of low deposition rate of reactive compounds can be solved if the magnetron source operates in the metallic mode or near the border of metallic and transient mode. Aluminium oxide thin films were deposited using high effective reactive pulsed magnetron sputtering. The main purpose of the research was electrical characterization of metal-compound-metal structures in the wide range of frequencies and determination of deposition technique influence on the thin film properties. Keywords: impedance spectroscopy; aluminium oxide; reactive magnetron sputtering Wroclaw University of Technology. 1. Introduction Aluminium oxide thin films are widely used in many mechanical, optical and microelectronic applications because of their excellent properties [1], mechanical strength and hardness, transparency, high abrasion and corrosion resistance, as well as insulating and optical [2] properties. All these properties of alumina film depend on different parameters of sputtering system such as sputtering rate, distance between the target and substrate, pressures of reactive gases etc. The new method of magnetron sputtering, with in situ controlled power supply, introduces new possibilities of thin film deposition [3, 4]. Constant monitoring of process enables precise setting of the magnetron working point and synthesis of aluminium oxide with properties close to stochiometric compound. The preliminary works on Al Al x O y composites prepared using highly effective reactive magnetron sputtering technology were reported earlier [5]. This paper was presented at the 35 th International Microelectronics and Packaging IMAPS-IEEE CPMT Poland Conference, September 2011, Gdańsk-Sobieszewo, Poland katarzyna.tadaszak@pwr.wroc.pl The purpose of the research was to study dielectric properties of deposited thin film. The main aim of future investigations is modifying magnetron sputtering process for high quality insulator deposition. The electrical properties can be investigated using impedance spectroscopy [6], which was frequently applied as a useful diagnostic method for a wide range of materials [7]. 2. Experimental details Thin film structures containing ten capacitors (each was numbered for identification Fig. 1) were deposited on Corning 7059 glass substrates. The sandwich-type configuration (Fig. 1) consisted of the bottom and top aluminium electrodes (area 6.3 mm 2 ) and aluminium oxide film. The aluminum oxide layer thickness was nm, which was measured with a MII-4M optical microinterferometer. Al x O y was deposited in a highly effective pulsed magnetron sputtering process in the atmosphere of argon and oxygen mixture. The aluminium for the thin Al electrodes was sputtered in magnetron process in argon and the obtained contacts thickness was 100 nm. The purity of the Al film was not measured. It could contain thin aluminum oxide

2 324 K. TADASZAK et al. Fig. 1. A configuration of Al-aluminium oxide-al structures (each capacitor was numbered for identification). layer, due to oxidation in air. The magnetron source WMK-50 [8, 9] was equipped with a 50 mm diameter and 7 mm thick aluminium target. The oxygen partial pressure was 88 mpa, while the total gas pressure (Ar+O 2 ) was 0.53 Pa. The glass substrate was positioned at 7.5 cm distance from the unbalanced magnetron source. The process was carried out using the Dora Power Supply (DPS) [3] with an effective power of 1.5 kw. The available maximum target power density was about 1000 W/cm 2. Dora Power Supply (DPS 5 kw) was operating at a frequency of 100 khz with a 4 khz pulse quantity modulation (PQM) for the output power control. The maximum amplitude of output sine-shaped current pulses was 30 A. The power supply work for this DPS unit can be described by so called effective P E and circulating P C power [3]. The deposition rate is proportional to P E value. With a decrease of load impedance, the DPS power supply output changes from voltage type (1.2 kv) to current type (max. 8 A) and a part of stored energy in the oscillator resonance circuit is moved back to the supply block. The measure of this energy is the circulating power P C. The electrical properties were measured with the impedance spectroscopy method. The measurements were carried out using Solartron 1260 A impedance analyser with Keithley 428 current amplifier in the frequency range of 20 mhz to 4 MHz. The amplitude of voltage excitation was 100 mv. For the analysis of the impedance spectra by means of equivalent circuit modelling the Scribner ZView2 software was used. To investigate the temperature Fig. 2. Parameters of magnetron sputtering process of the Al x O y thin films circulating power P C and deposition rate S versus oxygen content. dependence of impedance spectra the temperature of the stage was increased to 483 K and the measurement was done during cooling. The temperature was controlled by the PID controller and resistive heater in the stage. A DC resistance was measured with Keithley Instruments Electrometer 610 C. 3. Results The highly effective reactive pulsed magnetron sputtering was based on the use of DPS power supply, which made it possible to control the sputtering process by observation of the changes in power supply parameter (circulating power P C ). This control method gave an opportunity to deposit thin film aluminium oxide in metallic mode of magnetron work, without target surface oxidation. For deposition of the presented in the paper material, the circulating power level was set to about 0.4 kw, which corresponded to the border between transient and metallic mode (Fig. 2). In effective reactive magnetron sputtering, the compound deposition rate was as high as that of metal, e.g. more than hundred nm/min instead of a few nm/min in classic sputtering in dielectric mode. However, the effective sputtering introduced new problems, such as composition of the thin film, which could be rich in metal atoms [5]. The working point of magnetron source was carefully cho-

3 Properties of aluminium oxide thin films deposited in high effective reactive pulsed magnetron sputtering process 325 Fig. 3. Impedance spectra of the aluminium oxide capacitor impedance modulus Z and phase θ (capacitors numbered form 2 to 9 according to Fig. 1). sen to avoid metal grains in the deposited thin film. The higher content of metal in the layer caused the change of transparency and colour, which was marked in the graph, Fig. 2, with different point colours (white points refer to the transparent film). The main purpose of the research was the investigation of electrical properties of Al (Al x O y ) Al capacitor structures. It was very important to confirm the usefulness of the deposited dielectric film as insulator and assess its quality. The measurement results were plotted at the Bode graph the phase and the logarithm of impedance modulus versus the logarithm of frequency (Fig. 3). The graphic representation of Z, Y, C(ω) [10] simplified the analysis of impedance spectra and helped to determine the electrical properties and capacitance dispersion (Fig. 4). The leads inductance (about 280 nh) was taken into account and subtracted prior to the analysis of impedance spectra. Scatter of data points in low frequency range was a result of the measurement system limitations. The significant changes in C(ω) characteristics for each capacitor could be observed. It might be a result of the chosen deposition method. The glass substrate was immobile during sputtering, which could cause fluctuations of composition and thickness of deposited thin film. However, no simple dependence between capacitance and position of measured capacitor on the substrate could be seen. Calculated relative permittivity at 1 khz was in the range of for each capacitor, which was a proper value for thin film aluminum oxides [11]. Based on presented characteristics, the serial equivalent circuit (Fig. 5) was developed. The application of the impedance spectroscopy for the investigation of the electrical properties of the materials and systems enabled the direct comparison of the behavior of the real object and its equivalent circuit. The developed circuit contained two resistances: R S series resistance and R DC direct current resistance. The capacitance C of the structure was replaced with the constant-phase element (CPE), which better described the losses in capacitance in the real characteristics. The CPE element modelled the constant, frequency independent phase-shift between voltage and current; its admittance is described with: Y CPE = Q( jω) n (1) where ω frequency, Q, n frequency independent parameters; for diffusion 0.5 < n < 1 [8]. The values of n factor in the CPE element tended to 1. However, it could not be replaced by capacitance, because of incompatibility of the fitted and experimental curves. The CPE element much better described loses in the investigated thin film, which can be seen in Fig. 4d. Some relations between the values of electrical equivalent circuit and capacitor position (number) could be seen (Table 1, Fig. 6). Therefore, the influence of the target-surface configuration could not be completely excluded and required further investigation. The value of series resistance was significant and it changed with the position of capacitor. It could be related to the electrode length as well as deposition method. The highest R S was determined for the longest bottom electrode. The lowest value was obtained for capacitor number 2 with the shortest bottom electrode. However, for the capacitor number 9, with the same electrode length, the resistance was much higher. This is an evidence that the parameters of magnetron sputtering process and the position of the substrate influence the capacitor properties.

4 326 K. TADASZAK et al. (a) (b) (c) (d) Fig. 4. Impedance spectra of the Al (Al x O y ) Al capacitors (numbered form 2 to 9, according to Fig. 1.): (a) Nyquist diagram Y (Y ); (b) conductance; (c) capacitance; (d) dissipation factor. The DC resistance could not be derived from the characteristics, because it was to high to be fitted properly. Therefore, its value of W was measured with Keithley Instruments Electrometer 610 C. It was marked with a green line in Fig. 7. The measured aluminium oxide thin film had good DC insulating properties. The high slope of the conductance characteristics at higher frequencies indicated the presence of some irregularity in the insulator film. However, the slope was too high to be related to hopping conductance of amorphous thin film [13]. At the frequency about 10 MHz, the characteristics responded to serial resistance, R 1/ω 2. The changes in dielectric

5 Properties of aluminium oxide thin films deposited in high effective reactive pulsed magnetron sputtering process 327 Fig. 5. The serial equivalent circuit of Al Al x O y Al capacitor structure. Table 1. Parameters of equivalent circuit describing impedance spectra of Al x O y capacitor. N o R S [W] CPE Q [W 1 s n ] [12] ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± n Fig. 7. Analysis of the frequency dependence of electrical conductivity of Al x O y capacitor. functions with rising frequency could be caused by inhomogeneities in the film structure. Different phases of the thin film, characteristic of the used deposition method, could introduce loses in higher frequencies. The impedance spectra of the capacitor were measured at different temperatures. After heating up to 483 K, the properties were examined during cooling and are presented in Fig. 8. The observed changes in conductance were insignificant and could be caused by the changes in geometry. The capacitor was stable in the tested Fig. 8. The frequency dependence of conductivity of Al x O y capacitor at temperatures 303 K 483 K. Fig. 6. The values of CPE element n and Q parameters of the electrical equivalent circuit of Al x O y capacitor. temperature range and neither further oxidation nor other temperature dependent effects could be observed. The DC resistance was dropping linearly with temperature (Fig. 9). It was also measured with an electrometer, because it was too small to calculate using ZView2. The estimated TCR (Temperature Coefficient of Resistance) was about 5000 ppm/k. the values of the CPE element of electrical equivalent circuit changed monotonously.

6 328 K. TADASZAK et al. (a) Fig. 9. a) Temperature dependence of DC resistance b) the value of n and Q parameter of CPE element of electrical equivalent circuit presented in Fig Conclusions The deposited aluminum oxide films had satisfactory dielectric properties, relative permittivity of about , dissipation factor less than 10 2 (for frequency below 10 MHz). The capacitance was not constant and changed with frequency. Dielectric losses were simulated with the CPE element, instead of an ideal capacitor. The drop of capacitance with rising frequency was significant. It was much higher than the reported for Al Al 2 O 3 Al capacitors, formed by low temperature anodization method [14]. It was observed that the change of the parameters of the effective reactive magnetron sputtering process could have a significant influence on the deposited layers. This technology could introduce inhomogeneities caused by columnar growth and variation of film thickness (deposition profile). In the next step of our research we are going to carry out the deposition on rotating substrates. This should significantly improve uniformity of the thin film. This influence of technology on the microstructure of thin film could be used in deposition of composites or materials with gradient composition. Impedance spectroscopy was shown to be an excellent tool for thin film characterization, which allowed controlling not only electrical properties but also the structure of the film. (b) Acknowledgements This work was financed by Statutory Grant from the Wrocław University of Technology. References [1] DÖRRE E., HÜBNER H., Alumina: Processing, Properties and Applications, Springer-Verlag, Berlin, [2] EDLOU S.M., SMAJKIEWICZ, AL-JUMAILY G.A., Appl. Optics, (1993), [3] POSADOWSKI W.M., WIATROWSKI A., DORA J., RADZIMSKI Z., Thin Solid Films, 516 (2008), [4] Krówka K., Wiatrowski A., Posadowski W.M., 8 th International Conference on Coatings on Glass and Plastics ICCG. Proceedings, Braunschweig, June 2010, 179. [5] TADASZAK K., NITSCH K., PIASECKI T., POSAD- OWSKI W.M., Microel. Reliab., 51 (2011), [6] BARSOUKOV E., MACDONALD J.R., Impedance Spectroscopy, Theory Experiment and Applications, 2 nd ed.; Wiley-Interscience:Hoboken, NJ, [7] PIASECKI T., NITSCH K., Surf. Coat. Tech., 205 (2010), [8] POSADOWSKI W.M., Thin Solid Films, 392 (2001), 201. [9] POSADOWSKI W.M., Polish patent, No , [10] MACDONALD J.R., Solid State Ionics, 13 (1984), 147. [11] ARGALL F., JONSCHER A.K., Thin Solid Film, 2 (1968), 185. [12] HSU C.H., MANSFELD F., Corrosion, 57 (2011), 747. [13] JONSCHER A.K., J. Phys. D: Appl. Phys., 32 (1999), R57. [14] RAO M.K., JAWALEKAR S.R., Thin Solid Films, 51 (1978), 185. Received Accepted