Optimization of the Sputtering Process for Depositing Composite Thin Films

Transcription

1 Journal of the Korean Physical Society, Vol. 40, No. 3, March 2002, pp Optimization of the Sputtering Process for Depositing Composite Thin Films M. Farooq Pakistan Council of Renewable Energy Technologies 25, H-9, Islamabad, Pakistan Z. H. Lee Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejeon, (Received 8 October 2001) This paper presents optimization of the sputter deposition technique for fabricating composite thin film coatings. The thin films studied here consisted of metals (Al, Ni and V), dielectrics (SiO 2 and Al 2O 3), and their composites sputtered by D.C. and R.F. magnetron sputtering electrodes. The sputtering conditions, such as the chamber pressure, the sputtering voltage, and the current, at various sputtering powers have been optimized, and their effects are discussed. The growth rates and the volume fraction of the constituents have been calculated and are presented here. PACS numbers: 80 Keywords: Composites, Thin film, Sputtering, Vacuum I. INTRODUCTION Composite thin films deposited by Physical Vapour Deposition (PVD) methods have been extensively studied during the last two decades, to fulfil increasing demands by the coating industry. Visible and infrared selective coatings normally consist of composite materials. Film composition and thickness are critical to get optimum results. However, to obtain an accurate control of the composition and thickness is not an easy task, and this restriction has a severe impact on the selective sensitivity of the film. In this study, the composite films consisted of a metal and an insulator deposited by using a high vacuum (10 6 Torr) sputtering process. The chances of impurities in the films were limited due to the high vacuum. The sputtering process is versatile and provides good adhesion between the film and the substrate. Metal (Al, Ni or V) and dielectric (SiO 2 or Al 2 O 3 ) materials were sputtered by using DC and RF sources, respectively. It is quite difficult to sputter an insulating target (dielectric) using a DC power source due to an accumulation of positive ions on the front surface of the target, and the DC glow requires a conducting current to sustain the plasma. On the other hand, an RF system is beneficial because of its self-biased voltage. We have investigated and optimized the effect of chamber pressure, sputtering voltage, and current at various sputtering powers. The volume fractions of the constituents and the thicknesses of the mfarooq67@hotmail.com zhlee@hanbit.kaist.ac.kr composite films were calculated, and the results agreed very well (within 2 % error) with the experimental findings. II. EXPERIMENTAL To clean the glass substrates (3cm 3cm), which were used for measuring the film thickness, Decon-90 (Decon Laboratories England) was used. The substrates were ultrasonically cleaned in a 3 % Decon solution for 15 min., thoroughly rinsed with deionized (DI) water, and dried using nitrogen gas to avoid contamination. A Nordiko NS-3750 series magnetron sputtering system was used to experimentally deposit thin films of metal or dielectric. This system contained 1.25-kW RF and 6-kW DC generators with 4 12 electrodes 10 cm away from the target. To easily sputter dielectrics and metals, we designed RF and DC generators, respectively. The RF generator showed a 5 % 10 % reflected power during the deposition of the insulators. The substrate carrier was octagonal and was rotatable at various speeds for required co-sputtering deposition. The chamber was evacuated to below 10 6 Torr before the argon gas for sputtering was introduced. Here, we varied the sputtering pressure from 3.0 to 15 mtorr. Figure 1 shows a schematic of the Nordiko NS-3750 series magnetron sputtering system. The sputtering rates and the thicknesses of the films were determined using a Dektak IIA surface profilometer, which had been calibrated at the National Bureau of Standards. It is an instrument for producing -511-

2 -512- Journal of the Korean Physical Society, Vol. 40, No. 3, March Effect of Chamber Pressure on Sputtering Rate Fig. 1. Schematic of the Nordiko NS sputter coater. a cross-sectional plot of non-uniform surfaces by moving the sample surface beneath a diamond-tipped stylus [6]. The data are stored in a digital computer and can be displayed on a screen (CRT). The variation in the uniformity of the film across the substrate was observed to be ± 54 Å and precision was ± 21 Å for 300 nm thick films. III. RESULTS AND DISCUSSION The selection of the deposition conditions is essential for fabricating composite thin films. The most important conditions are the deposition rate, the current, the voltage and the chamber pressure. An aluminium target was selected for investigating the optimum sputtering conditions for metals. The sputtering power was varied from 100 watts to 1400 watts. The effect of the chamber pressure on deposition rate was observed at different sputtering powers. The influence of the current and the voltage on the sputtering rate was also investigated. The deposition rate efficiency is measured by the slope of the linear deposition rate plotted as a function of the power density. The sputtering rate for low to medium ion energy is given by dn Adt = Y I e (1) where N is the number of atoms sputtered, A is the area of the target, Y is the sputtering yield, I is the ion current, and e is the electronic charge. The growth rates of metals and insulators were determined for DC and RF sputtering, respectively, for time intervals of 30 to 60 min, and for various sputtering powers. The sputtering chamber pressure is inter-linked to the sputtering power [7]. For low sputtering power, a high chamber pressure reduces the energy of the sputtered atoms and covers the substrate with charged particles [8]. If the power is kept constant, with an initial increase in the chamber pressure, the ion density increases. The sputtering rate increases with increasing chamber pressure. If the chamber pressure is further increased, the sputtering rate decreases owing to back diffusion. The chamber pressure was varied from 3 to 15 mtorr. At low power like 100 watts, with increasing chamber pressure, the sputtering rate increased and then decreased. The optimum pressure for sputtering at 100 watts was 7.5 mtorr. In other words, the sputtering rate increased due to the increase in the ion density at chamber pressures up to 7.5 mtorr for 100 watts, as shown in Fig. 2. A further increase in the chamber pressure caused back diffusion and collisions with argon ions. When the DC power was increased to 200, 300, and 400 watts, the ionized and sputtered particles became more energetic, and the sputtering rate increased. On the other hand, collisions of sputtered particles with chamber particles (argon gas and ions) increased. The saturation point at 200, 300, and 400 watts was reached at a lower chamber pressure than at 100 watts due to back diffusion. Figure 2 shows the effect of chamber pressure on the sputtering rate where the optimized chamber pressure was 6 mtorr, 4.5 mtorr, and 3 mtorr for 200, 300, and 400 watts, respectively. A drastic fall in the sputtering rate for high chamber pressure at high power sputtering is seen. The line AB represents the optimum chamber pressure for maximum sputtering rate at a specified power. The current density is a factor, which affects the maximum sputtering rate at different powers for different chamber pressures. As the power increases, the current density also increases. This results in a decrease in the gas density near the cathode while the gas density in the rest of the chamber remains the same [9]. The reduction in the gas density is due to the rise in temperature caused by the sputtered flux of incident ions and ions reflected from the cathode surface [10]. The gas density reduction is also a function of the inert gas mass and the sputtering yield of the target. The higher the sputtering yield of the target and the mass of the gas, the greater is the gas density reduction near the cathode. 2. Sputtering Current and Voltage The sputtering current and voltage play important roles in ionizing the plasma discharge. The deposition rate increases with an increase in current, which is the flow of ionized particles. The deposition rate increases

3 Optimization of the Sputtering Process for Depositing Composite Thin Films M. Farooq and Z. H. Lee Fig. 2. The optimized chamber pressure for sputtering at various sputtering powers. Fig. 3. Effect of chamber pressure on the discharge current at various sputtering powers. slowly with an increase in the energy of the ionized particles, which is due to the potential difference or voltage [11]. The current is more important than the voltage for increasing the sputtering yield. This study suggests the use of the current as a basic parameter in thin film sputtering process, rather than the voltage or the power. The voltage and the current were studied for 100-watt, 200-watt, 300-watt and 400-watt sputtering powers under various deposition conditions. When the argon density is increased in the chamber, the current increases, as more particles are available to move towards the target. The variation in the deposition current caused by the argon chamber pressure is shown in Fig. 3, where the current is shown to be increasing with chamber pressure for all sputtering powers. On the other hand, the voltage decreases with an increase in pressure at a constant power. At higher pressures, the ionized particles are not so energetic as they are at low pressure at a constant power [12]. As not all the particles are sufficiently energetic to sputter the target material, the sputtering rate falls with increasing pressure. If we keep on increasing the power under specific sputtering conditions, then at a certain stage, the increase in voltage with increasing power will become very slow, and all the energy supplied will be utilized to increase the current. This effect is presented in Fig. 4, where it can be seen that above a 300-watt sputtering power, the voltage is almost constant. On the other hand, in Fig. 3, the current increases with increasing power, even at 300 watts and 400 watts. At constant power, with changing chamber pressure, the current and the voltage are related as I = V (2) P where I is the change in current, V is the change in voltage, and P is the sputtering power. The negative sign indicates a decrease in voltage under specific conditions. For optimized deposition conditions, nickel and vanadium were sputtered onto cleaned glass substrates. The sputtering current was varied from 0.1 A to 1.2 Å, depending on the nature of the material. The sputtering yield of vanadium was lower than that of nickel, so higher power was required to sputter vanadium at the same rate as that of nickel. The slopes of the deposition rate plotted as a function of sputtering current were Å/A and Å/A for nickel and vanadium, respectively. The slopes of the lines showed that the deposition rate of nickel was significantly higher than that of vanadium. The intercepts of the sputtering rates for nickel and vanadium lie at -5.4 Å and Å, respectively. The deposition rates of both materials are shown in Fig. 5. It is very difficult to deposit dielectric materials by evaporation due to their higher evaporation points. In RF sputtering, higher power is utilized to sputter the materials from the target due to their lower sputtering yields compared to metals. Cooling of the target is essential to avoid damaging it. SiO 2 and Al 2 O 3 have been sputtered at powers ranging from 1000 W to 1400 W. The sputtering yield of SiO 2 seems to be higher than that of 3. Growth Rates of Other Metals and Dielectrics Fig. 4. Effect of chamber pressure on the sputtering voltage at various sputtering powers.

4 -514- Journal of the Korean Physical Society, Vol. 40, No. 3, March 2002 Fig. 5. Deposition rates of vanadium and nickel. Al 2 O 3 due to the bonding forces of the individual materials. The slopes and the intercepts of the deposition rate, indicating the sputtering efficiencies, were 0.05 Å/A and 0.06 Å/A, and -35 A and -41 A for Al 2 O 3 and SiO 2, respectively. The deposition rates for these dielectrics are plotted against sputtering power in Fig Production of Composite Films The metallic to dielectric material ratio is initially determined by the sputtering rates of both the materials from which the composite film is fabricated. In the sputtering system, the substrate holder is rotatable and hexagonal in shape. The sputtering rate of each material is determined using a stationary substrate. While depositing a composite film, the substrate holder is kept rotating, as the targets are on opposite sides of the substrate. The deposition rate is assumed to be one sixth of the rate at which the material is deposited on a stationary substrate. In practice that is not true, and composite films are thicker than estimated. The sputtering rate of each material was determined while the substrate holder was kept rotating. A comparison of both the rates was made. It was found that one third more material than calculated was deposited on the film while the substrate was not exactly in front of the target. This material was deposited on the film while the substrate was moving towards and away from the target. The actual sputtering rate was determined while the substrate was rotating. The film thickness (measurement error ± 0.1%) was calculated using the following relation for a stationary substrate: (A+B) (A + B) + 3 C = (3) 6 where the sputtering rate of a metal onto a stationary substrate is A, the sputtering rate of an insulator onto a stationary substrate is B, and the deposition rate of composite film is C. 5. Determination of VF by Sputtering Rates The sputtering rates of all materials were determined at a single chamber pressure and various powers to make a film of the required volume fraction (VF). In order to get the specific VF of a metal in each layer for particular DC and RF powers, we determined the VF as V F = A A + B (4) where A is the sputtering rate of the metal, which is a known quantity, and B is the calculated sputtering rate of the insulator for the required VF. The time T for sputtering the thickness of each layer is T = D (5) C where D is the required thickness of each layer. This process is repeated for each layer. The thickness and the VF of the fabricated films were determined empirically. The thickness measurements of the composite films validated this method. In the case of metals, increasing the sputtering current in small increments controlled the sputtering rate so as to achieve the desired graded composite film. The selected current points at which the sputtering rates of all the metals were determined were 0.06 to 1.0 Å. Such low discharge currents are needed in these experiments. IV. CONCLUSIONS Fig. 6. Deposition rates of SiO 2 and Al 2O 3. The chamber pressure at which maximum sputtered deposition rate was achieved for various deposition powers was optimized. The deposition current was the main factor influencing the deposition rate. A method was developed to deposit composite films with an accurate thickness and volume fraction of the constituent.

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