Magnetic Field Strength as a Variable in Optimizing Magnetron Sputtering Processes FRANCISCO JIMENEZ*^, MARTIN DAVIS^, DAVID FIELD^, TERRY HUNT^ AND STEVEN DEW * *University of Alberta ^Smith and Nephew (Alberta) Inc. Abstract As manufacturers seek to optimize their thin film deposition systems for greater throughput and lower operating cost, they most frequently do this through models including cathode power or current, gas pressure and mixture, and throw distance. In this work the magnetic field strength is examined as an important variable. Data is presented showing how the magnetic field influences the cathode I-V relationship, deposition rate, and heat flux to the substrate with obvious implications on throughput and film quality. I. Introduction The magnetron sputter deposition technique has long been used to coat a great variety of substrates. Depending on requirements, more or less heat flux relative to deposition rate may be desired in order to synthesize films with the required properties and at the most efficient rates. For example, efficiently coating films onto heat sensitive substrates may require maximizing deposition rate and minimizing heat flux. The total heat flux to the substrate has many components, the contribution from each component depending on process conditions. For instance, heat due to neutrals bombardment may represent most of the heat flux at low pressures, while at higher pressures plasma species tend to dominate[1,2]. As magnetic field strengths are modified, the plasma density adjusts, affecting the neutral:plasma density ratio at the substrate, and an effect on heat is seen. In this report this effect is investigated with respect to pressure and throw distance. II. Methods A 12.5 cm x 37.5 cm rectangular silver target was sputtered in an inert environment (argon). Details of the chamber and cathode design are available elsewhere [3]. The working pressure was controlled using downstream pressure control. This work only reports on two pressures: 1 and 4.3 Pa (flow rates were set to 120 and 300 sccm respectively). The cathode was operated in the constant current mode of operation (1 A). Figure 1 shows the magnetic fields strengths used in this study (modeled using COMSOL Multiphysics software [4] and calibrated by measuring the magnetic field using a Lakeshore 410 Gaussmeter). The figure shows the component of the magnetic field parallel to the target at 0.8 mm from the surface Figure 1. Magnetic field strengths used in this work. The total heat flux at the substrate was measured using a thin film sensor available commercially (RdF corporation, p/n 27134-3). The sensor was mounted on a water cooled copper base and protected from fouling by metal flux using a thin aluminum foil mask (thickness=0.07 mm). Sensor outputs were processed using an isolated amplifier module and a USB data acquisition system (both from dataQ corporation) connected to desktop computer. The deposition rate was measured using a crystal microbalance at the same locations where the heat flux was obtained. The target temperature was measured using two resistance temperature detectors (pt100) mounted within cavities on either end of the silver target. Signals were read using an isolated amplifier module installed on the same DAQ system used for the heat flux measurements. A computer-controlled motorized platform with three degrees of freedom was used to vary the location of the heat flux sensor and crystal microbalance without breaking the vacuum. Due to geometrical constraints, it was not possible to sample the entire substrate region, and only results on a portion of it are reported. The sampled area was a 12 x 12 cm2 square region aligned with one of the quadrants of the substrate, one corner being located at 3.5 cm from the geometrical center of the substrate in the longitudinal direction. The etch track turn-around begins within, and extends ~ 1 cm beyond the end of, the sampling region. III. Results The I-V curve of the experiment is shown in Figure 2. As expected, the voltage increases for lower magnetic fields as confinement efficiency is reduced. A similar trend is observed for lower pressures. The mechanisms behind this voltage response as a function of these two variables are well understood [5,6]. Figure 2. I-V characteristic of the geometry used in this work. Figure 3 shows the main findings of this work. In the figure, the integrated heat and particle fluxes at the substrate are shown as a function of the magnetic field strength for three different throw distances. The integrated area was the same measured region as described at the end of Section II. In general, the overall trend is a reduction in heat flux for a reducing magnetic field strength and a slight increase in the deposition rate. This trend was also observed for the 1 Pa case in agreement with a previous experiment [3]. Note how the effect is observed at all three throw distances and both pressures reported in this work, implying that the magnetic field strength can be used as a variable to tailor the heat flux for a specific application. However, the change in magnetic field strength may affect positively or negatively the uniformity of the deposition rate at the substrate (not shown) depending on the throw distance. One of the mechanisms responsible for this effect is due to the ion distribution in front of the target being modified, affecting the neutral particle transport mechanisms. These factors should be considered for a particular application if the efficiency of the process is to be optimized. Figure 3. Average heat and particle flux over the sampled region for two pressures. a) Average deposition rate for 4.3 Pa, b) Average heat flux for 4.3 Pa, c) Average deposition rate for 1 Pa, and d) Average heat flux for 1 Pa. The temperature of the target only fluctuated about 5 degrees for all magnetic fields used in this work with maximum temperatures (~ 41 °C) measured at intermediate magnetic field strengths. These results strongly suggest that the target temperature does not play a significant role in the measured heat flux. However, other geometries and target/gas combinations should be investigated to further elucidate the implications of target temperature on the total heat flux. IV. Conclusions The magnetic field strength can be optimized to change the heat flux relative to deposition rate at the substrate. It was found that the deposition rate is not significantly impacted and may be improved by reducing the magnetic field within the pressure range investigated in this work. The target temperature was determined to have a negligible impact on the measured heat flux based on the small temperature variation of the target during deposition. These findings have significant applications for heat sensitive substrates, in particular when one is looking to increase throughput without damaging the substrate. V. References 1. Bedra et al., J. Phys. D: Appl. Phys. 43, 033501 (2006) 2. Ekpe et al. J. Vac. Sci. Technol. A 20, 1877 (2002) 3. Ekpe et al. J. Vac. Sci. Technol. A 27, 1275 (2010) 4. COMSOL Multiphysics v 3.5b (www.comsol.com) 5. Sheridan et al. J. Vac. Sci. Technol. A 8, 30 (1989) 6. Miranda et al. J. Vac. Sci. Technol. A 8, 1627 (1990)