# Temperature Effects in Packaged RF MEMS Switches with Optimized Gold Electroplating Process
## 3 Fabrication  
### Optimization of Gold Electroplating Process  
Among the fabrication processes of the switches, the gold electroplating process plays a decisive role in the mechanical as well as the RF properties of the switch, including the pull-in voltage, temperature stability, insertion loss, and lifetime. In order to achieve high-quality electroplating of gold, the electroplating parameters are studied and optimized. Commonly used electroplating methods include DC electroplating and pulse electroplating. Compared with DC electroplating, pulse electroplating can effectively reduce the difference in deposition rate and diffusion rate, thus improving the growth of the grain. The electroplating solution is based on potassium aurous cyanide. Figure 2 shows the relationship between the duty ratio of the pulse and the deposition rate of electroplating. From Figure 2, it can be seen that as the duty ratio increases, the deposition rate of the plating gradually decreases slightly. The reason is that as the duty ratio increases, the current shutdown time gradually decreases, and the solution does not have enough time to restore the metal ions near the cathode to their initial concentration, thereby reducing the current efficiency and causing a slight decrease in the deposition rate of the coating metal. The insets in Figure 2 show SEM images of electroplated gold with different duty ratios. From the images, it can be seen that the size of grains decreases first and then increases with the duty ratio. When the duty ratio is less than 50%, the size of grains decreases with the increase in duty ratio; when the duty ratio is greater than 50%, the size of grains actually shows an increasing trend. At 100% duty ratio (DC electroplating), the grain will grow on a constant grain nucleus for a long time, thus greatly increasing the size of grains. Then, the influence of current density on the electroplating is studied. Figure 3 shows the measured curve of deposition rate with current density. It can be seen that the deposition rate increases with the increase in current density. The insets in Figure 3 show that, under low current density conditions, electroplating grains have irregular shapes and larger sizes. As current density increases, grains have smaller sizes and more regular shapes. However, excessive current density can also lead to irregular crystal structures, resulting in increased roughness. Figure 2: Relationship between the duty ratio of the pulse and the deposition rate of electroplating. Electroplating condition: frequency is 1 kHz, current density is 0.5 A/dm\({}^{2}\), temperature is 50 \({}^{\circ}\)C, flow rate is 20 L/min. The insets show the grain size at different duty ratios. The temperature of the plating solution has a significant impact on the deposition rate and appearance of electroplating. Figure 4 shows the measured curve of the electroplating rate with temperature. From Figure 4, it can be seen that as the plating temperature increases, the deposition rate reaches its highest at 45 \({}^{\circ}\)C. At low temperatures, irregular small crystals are formed, and the surface roughness of the coating is relatively large. At high temperatures, fine and uniform crystals can be formed, and the coating has a brighter appearance. In addition, the influences of pulse frequency, and solution flow rate on the quality and deposition rate of gold electroplating are also studied. By optimizing and compromising the deposition rate and grain size, the optimized parameters of the gold electroplating process are shown in Table 2.  
Figure 4: Relationship between temperature and deposition rate. Electroplating condition: frequency is 1 kHz, duty ratio is 50%, current density is 0.5 A/dm\({}^{2}\), flow rate is 20 L/min. The insets show the electroplated gold surface at different temperatures. Figure 3: Relationship between current density and deposition rate. Electroplating condition: frequency is 1 kHz, duty ratio is 50%, temperature is 50 \({}^{\circ}\)C, flow rate is 20 L/min.