Upon exposure to plasma, the temperature of a material increases due to radiation, chemical reaction, and surface bombardment. Heat build-up is an essential concern for heat- or temperature-sensitive sample materials.
The temperature of components in a chamber is affected by a number of factors, including the orientation of the parts (are they in carriers or directly on the shelf), thermal conductivity of the material, and plasma process conditions such as gas flow rate, gases, plasma treatment time, input power, and the frequency of the generator.
To reduce heat build-up, it is necessary to optimize the process by balancing all of the aforementioned parameters. During the plasma process, the substrates are typically placed on the grounded electrode or the powered electrode. Therefore, the temperature of the substrate can be estimated using the electrode temperature.
This article discusses how the temperature of electrodes is influenced by plasma conditions, such as operating pressure, input power, and the distance between the electrodes.
March PX-1000 was used to run all of the experiments. The work surfaces or shelves for sample processing in this system are the electrodes. The temperature of the electrodes during the plasma process was measured using the Eight-Point Irreversible Temperature Indicators made by Cole- Parmer Instrument Company.
The size of the grounded and powered electrode was 16" × 18" and 13.5" × 16.5", respectively. The plasma processing gas used was argon and the distance between the electrodes was 6" unless indicated.
Results and Discussion
Electrode Temperature Comparison
The relationship between the temperature of the electrodes and plasma treatment time under various plasma conditions is shown in Figure 1. The results clearly show a faster increase in the temperature of the powered electrode compared to that of the ground electrode.
The faster rise in the powered electrode temperature is due to the DC bias on the powered electrode. The self DC bias causes strong ion bombardment on the powered electrode surface, increasing temperature faster than that on the ground electrode.
It should be noted that the time needed to attain significant temperatures on the powered and ground electrodes is normally much higher than the standard plasma processing times. Therefore, temperature problems caused by processing time are often not significant.
Figure 1. Electrode temperature comparison. Plasma conditions: 600 W, 400 mtorr
Effect of Plasma Input Power
Figure 2 shows the influence of input power on the powered electrode temperature. A faster rise in temperature is observed at a higher input power (600 W) compared to a lower input power (300 W). This faster rise is due to the fact that the ion density rises when the input power increases, causing high-density ion bombardment and energy transfer to the electrodes. As a result, higher power processes lead to higher temperature conditions.
Figure 2. Effect of power on the temperature of the electrode.
Effect of Operating Pressure
Figure 3 shows the influence of operating pressure on the temperature of the powered and the ground electrodes (G). The results clearly show a faster rise in temperature at high operating pressure compared to low operating pressure.
This faster rise is due to the plasma energy density. The volume of the plasma glow discharge reduces with increasing operating pressure. Therefore, the local plasma energy density rises between the electrodes, causing faster temperature rise on the electrodes.
Figure 3. Effect of system pressure on the temperature of the electrodes.
Effect of Electrode Distance
Figure 4 shows the influence of electrode distance on the electrode temperature. The distance between the electrodes is 2" and 6", respectively. The electrode temperature is higher when the electrode distance is narrower. A higher plasma energy density between the electrodes with narrower electrode gap is one of the reasons. Another reason is the narrower gap of the electrode causing a larger higher barrier to heat dissipation.
Figure 4. Effect of electrode distance on temperature of electrodes. The plasma conditions are 600 W and 200 mtorr.
Effect of Electrode Area
Figure 5b shows the effect of electrode size on the electrode temperature. This experiment used four work shelves, two of them were used as ground electrodes and the remaining two were used as powered (Figure 5a). The ground and powered electrodes were alternated with the top electrode being grounded. The distance between the electrodes was 2". The effects of the electrode area were evaluated by doubling the total electrode area (Figure 5a).
The results in Figure 5 reveal that the electrode temperature depends on the electrode area for a given total power input. The electrode temperature increases slowly when the power is distributed between multiple electrodes compared to a single pair of electrodes.
The observation is due to the increase in the energy density per unit area with decreasing electrode size. The high energy density indicates high ion bombardment and radiation, thus causing the temperature increase.
Figure 5. The influence of electrode area on temperature of electrodes. Plasma Conditions: 600W, 200mtorr. (a) Electrode configuration and (b) Result comparison.
The influence of plasma volume on the electrode temperature was also studied. The input power density was kept constant between the electrodes, but the number of electrodes pairs was changed from two to one (6 inch gap for single pair electrodes compared to a 2 inch gap for two pair electrodes).
Figure 6 shows a result similar to that displayed in Figure 5 - the energy density per unit area rises with decreasing electrode size. The high energy density indicates high ion bombardment and radiation, resulting in a temperature increase. The size of the electrodes is one of the key factors to control the electrode temperature.
Figure 6. The influence of electrode area on temperature of electrodes under the same volume between electrodes. Plasma conditions: 600 W, 200 mtorr.
The pressure, power, and operating time influence the temperature of the electrode (or substrate). Given that various systems are designed differently, all plasma systems cannot be generalized easily. The following is the rule of thumb regarding concern over plasma temperature.
Time: The longer the time, the higher the temperature.
Power: The higher the power, the higher the temperature.
Electrode size: The bigger electrode size, the lower electrode temperature.
Electrode distance: The larger electrode distance, the lower substrate temperature.
Most processes can be kept under 100°C by operating within 300 W of power. If a cooling system is not used, the process time can be kept under 20 minutes. Processing time should be kept to approximately 10 minutes and power to roughly 300 W if temperatures of under 60°C have to be maintained.
If low substrate temperature and longer plasma treatment time are required, the cooling system that circulates fluids to the surface of the work area may be required.
This information has been sourced, reviewed and adapted from materials provided by Nordson MARCH.
For more information on this source, please visit Nordson MARCH.