When a material is exposed to plasma, its temperature increases due to chemical reaction, radiation, and surface bombardment. In the case of temperature- or heat-sensitive sample materials, accumulation of heat is an important concern.
Several factors affect the temperature of components in a chamber, such as thermal conductivity of the material, the frequency of the generator, the orientation of the parts (whether they are located directly on the shelf or in carriers), and plasma process conditions like gases, input power, plasma treatment time, and gas flow rate.
In order to reduce the accumulation of heat, it is essential to optimize the process by balancing all the parameters mentioned above. The substrates are usually placed on the powered electrode or on the grounded electrode at the time of the plasma process. Therefore, the substrate temperature can be measured using the electrode temperature.
In this article, the effect of plasma conditions — such as input power, the distance between the electrodes, and operating pressure — on the temperature of electrodes is discussed.
All the experiments were performed by using March PX-1000. In this system, the electrodes are used as the shelves or work surfaces for processing the sample. Using Cole-Parmer Instrument Company’s Eight-Point Irreversible Temperature Indicators, the temperature of the electrodes was measured during the plasma process.
The size of the powered and grounded electrode was 13.5″ × 16.5″ and 16″ × 18″, respectively. Argon was used as the plasma processing gas. The distance between the electrodes was 6″ unless specified.
Results and Discussion
Electrode Temperature Comparison
Figure 1 illustrates the relationship between the plasma treatment time and the temperature of the electrodes under different plasma conditions. It is evident from the results that there is a more rapid increase in the temperature of the powered electrode than that of the grounded electrode.
The faster increase in the temperature of the powered electrode is due to the DC bias on the powered electrode. The self DC bias leads to strong ion bombardment on the powered electrode surface, thereby increasing the temperature more rapidly than that on the grounded electrode.
It must be noted that the time required to achieve significant temperatures on the grounded and powered electrodes is usually much higher compared to the standard plasma processing times. Therefore, temperature problems due to the processing time are often insignificant.
Figure 1. Electrode temperature comparison. Plasma conditions: 600 W and 400 mTorr.
Effect of Plasma Input Power
The influence of input power on the powered electrode temperature is shown in Figure 2. When compared to a lower input power of 300 W, a faster increase in temperature is noted at a higher input power of 600 W. Such a faster increase is caused by the increase in ion density when there is an increase in the input power, leading to high-density ion bombardment and energy transfer to the electrodes. Consequently, higher power processes result in higher temperature conditions.
Figure 2. Effect of power on the temperature of the electrode.
Effect of Operating Pressure
The effect of operating pressure on the temperature of the grounded electrode (G) and the powered electrode is shown in Figure 3. It is evident from the results that there is a more rapid increase in temperature at high operating pressure compared to low operating pressure.
Such a faster increase is due to the density of plasma energy. There is a decrease in the volume of the plasma glow discharge with an increase in operating pressure. Therefore, there is an increase in the local plasma energy density between the electrodes, which leads to a faster increase in the temperature of the electrodes.
Figure 3. Effect of system pressure on the temperature of the electrodes.
Effect of Electrode Distance
The effect of electrode distance on the temperature of the electrode is shown in Figure 4. The distance between the electrodes is 2″ and 6″. When the electrode distance becomes narrower, the temperature of the electrode turns higher. One of the reasons is the higher plasma energy density between the electrodes with a narrower electrode gap. The other reason is the narrower gap of the electrode that causes a larger higher barrier to the dissipation of heat.
Figure 4. Effect of electrode distance on the temperature of electrodes. The plasma conditions are 600 W and 200 mTorr.
Effect of Electrode Area
The influence of electrode size on the electrode temperature is shown in Figure 5b. Four work shelves were used in this experiment. Two of the work shelves were used as grounded electrodes and the other two were used as powered electrodes (Figure 5a). The powered and grounded electrodes were alternated with the top electrode that was grounded. The distance between the electrodes was 2″. The total electrode area was doubled to assess the effects of the electrode area (Figure 5a).
The results in Figure 5 show that the electrode temperature is dependent on the electrode area for a specified total power input. When the power is distributed between multiple electrodes, the electrode temperature increases gradually than when the power is distributed between a single pair of electrodes.
The results are caused by the increase in energy density per unit area with a decrease in electrode size. The high energy density signifies high ion bombardment and radiation, which leads to an increase in temperature.
Figure 5. The influence of electrode area on the temperature of electrodes. Plasma conditions: 600 W and 200 mTorr. (a) Electrode configuration and (b) result comparison.
Moreover, the effect of plasma volume on the electrode temperature was analyzed. While the input power density between the electrodes was maintained constant, the number of electrodes pairs was varied from two to one (2 inch gap for two pair electrodes compared to a 6 inch gap for single pair electrodes).
Figure 6 illustrates a result similar to that shown in Figure 5 — the energy density per unit area increases with a decrease in electrode size. The high energy density signifies high ion bombardment and radiation, leading to an increase in temperature. One of the important factors that control the temperature of the electrode is the size of the electrodes.
Figure 6. The influence of electrode area on the temperature of electrodes under the same volume between electrodes. Plasma conditions: 600 W and 200 mTorr.
The temperature of the electrode (or substrate) is influenced by power, pressure, and operating time. Since different systems are designed in a different manner, it is not possible to generalize all plasma systems easily. The following is the rule of thumb when it comes to the concern over plasma temperature.
Electrode distance: The larger the electrode distance, the lower the substrate temperature.
Power: The higher the power, the higher the temperature.
Electrode size: The bigger the electrode size, the lower the electrode temperature.
Time: The longer the time, the higher the temperature.
The temperature of a majority of the processes can be kept less than 100 °C by operating within 300 W of power. The processing time can be kept under 20 minutes by eliminating the use of a cooling system. For maintaining temperatures of <60 °C, it is necessary for the processing time to be maintained to approximately 10 minutes and the power to approximately 300 W.
It may be necessary to use a cooling system that circulates fluids to the surface of the work area if there is a need for low substrate temperature and longer plasma treatment time.
This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.
For more information on this source, please visit Nordson MARCH.