The series of vacuum plasma treatment systems (Nebula and HPT models) from Henniker Plasma feature an appropriate vacuum pump which is necessary for the low pressures required during plasma operation.
There are many kinds of vacuum pumps on offer to customers, and the optimal vacuum pump can be found by having a consultation with Henniker’s technical sales department, who have several years of expertise in plasma and process vacuum technology.
This article aims to outline the differences between the range of pump choices and offers guidance on how to determine which pump is best suited to the specific application.
The key factors regarding the choice of vacuum pumps are (i) process compatibility, (ii) pumping speed, (iii) laboratory or work area environment and (iv) cost.
(i) Process Compatibility
Image Credit: Henniker Plasma
Oxidants
Various gases can be utilized to perform plasma processing which will depend on the desired result of the process and the material being handled. The standard process gases are hydrogen, argon, oxygen and air, either mixed or in pure form.
Oxygen and air are by far the most popular gases employed for surface activation and routine cleaning. There are typically no compatibility problems to be addressed when working with inert gases or air.
Vacuum pumps that have been lubricated with hydrocarbon oil should not be utilized due to the flammability risk if oxygen is employed in pure form or as part of a mixture where the percentage of oxygen is greater than 25% by volume.
It is vital to note that this is still relevant if the oxygen percentage is usually less than 25% by volume but has the potential to increase above this percentage if the condition is faulty.
The oxidant can be diluted to a harmless concentration using the gas ballast port of a hydrocarbon oil rotary vane pump, but this must only be performed with an inert gas (for example, dry nitrogen).
This should be performed under rigid monitoring constraints and a safety design must be used, which can halt the oxidant flow in the event that either the oxidant flow increases or the inert gas flow is impeded.
This technique is only appropriate when the flow rate of oxidant gases is low or else the pump will not withstand the extra flow of gas from the diluent.
When utilizing oxygen greater than 25% volume, a safer technique is to employ a dry vacuum pump that has no lubricants at all or a PFPE lubricated vacuum pump. PFPE oil is chemically inert and will not form a reaction to oxygen. Dry vacuum pumps do not contain any oil and utilize a rotating tip seal.
Flammable Gases
Several flammable gases, for example, hydrogen, are explosive in specific concentrations where an oxidant and a source of ignition is present.
All types of vacuum pumps can pump hydrogen, but in all applications, it should be diluted to a percentage under the safe level in the vacuum pump before being compressed at the vacuum pump exhaust port back to atmospheric pressure.
ATEX advice should be obtained where necessary, and flame arresters must also be utilized as a part of the general system safety design.
Corrosive Gases
Certain gases, for example, ammonia, are corrosive to the surfaces and internal parts of regular vacuum pumps. Each kind of pump has corrosion-resistant versions that can be bought and should always be selected where required.
Special consideration should also be given to eliminating moisture ingress, which can quicken corrosive reactions, and an inert gas purge must be employed as part of the shut-down process for pump flushing.
Image Credit: Henniker Plasma
(ii) Pumping Speed
As the term suggests, the pumping speed is the rate at which gas is eliminated by the vacuum pump from the chamber of the vacuum.
Eventually, all pumps achieve an equal ultimate vacuum level, but a pump with a slower pumping speed will reach the point slower than one with a higher pumping speed.
As such, a more efficient process time overall is achieved by a higher pumping speed which causes faster pump downtime.
An efficient process may not be particularly vital if the plasma system is being employed in a laboratory where the sample throughput is relatively small, for example.
If being utilized in a production environment that requires high throughput, the pump downtime can be critical if the plasma stage (the time for which the plasma is on) is short, as the downtime of the pump becomes the lengthiest stage of the process overall.
Figure 1 presented below demonstrates the pump down curves for the HPT-200 benchtop plasma system for pumps with respective nominal pumping speeds of 3 m3 per hour and 5 m3 per hour.
Figure 1. Pump down curves for the HPT-200 with two different pump speeds. Image Credit: Henniker Plasma
0.1 mbar is achieved by the 5 m3 per hour pump in 1 minute 12 seconds, compared to the 1 minute 30 seconds it takes for the 3 m3 per hour pump to achieve an equal vacuum level.
This is a comparatively small difference which might not be an issue in several applications. The more efficient pump should be selected if overall throughput and process time are important.
The corresponding data for the bigger HPT-500 benchtop plasma system is depicted in figure 2.
It takes the 8 m3 per hour pump a duration of 2 minutes 50 seconds to achieve 0.1 mbar compared to the 12 m3 per hour pump, which attains this level around one minute sooner. The larger pump would be the ideal choice in the majority of cases in this example.
Figure 2. Pump down curves for the HPT-500 with two different pump speeds. Image Credit: Henniker Plasma
No samples were present in the plasma chamber when the above data was quantified. Most glass, polymers, ceramics and metals are low porosity and do not outgas in the vacuum chamber.
When samples of a high porosity are plasma-treated, they can introduce a strong gas load to the vacuum pump as a result of the high outgassing rate.
It will take longer for the pump to eliminate this extra load of gas which means the pump downtime will be prolonged. In these examples, it is recommended to use a larger pumping speed.
(iii) Lab or Work Area Environment
When evaluating pump choices, the setting in which the vacuum pump will be used is also vital.
As an example, if the vacuum pump and plasma system will be located in a clean room, a dry vacuum pump would be the ideal choice to reduce the influence on air quality caused by oil mist being released from the exhaust of the vacuum pump.
An appropriate vacuum pumps exhaust filter can also be used to solve this issue which is delivered as standard on all pumps supplied by Henniker plasma systems.
If the vacuum pump is to be located externally to the cleanroom, for example, through a bulkhead, then the extra length of the vacuum line between the pump and the plasma unit should be evaluated.
A conductance restriction may be created, which can decrease the required pumping speed. Where necessary, larger bore vacuum lines and/or larger pumps should be utilized.
Image Credit: Henniker Plasma
(iv) Pump Price
Vacuum pump models with higher pumping speeds have a higher cost. Hydrocarbon oil pumps are less expensive than PFPE pumps of an equal pumping speed.
Dry vacuum pumps are usually more expensive compared to PFPE pumps. This is also seen for the corrosion-resistant type of any pump with an equal nominal pumping speed.
Image Credit: Henniker Plasma
Final Words
The most appropriate vacuum pump is one that is chemically compatible and will achieve the throughput demands required while being appropriate for the workspace or laboratory environment and also inside the assigned budget.
This information has been sourced, reviewed and adapted from materials provided by Henniker Plasma.
For more information on this source, please visit Henniker Plasma.