Choosing a rough vacuum pump for a new vacuum chamber can sometimes be challenging. Chamber suppliers sometimes provide the rough pump by default as part of their scope of supply: How might their choice impact you as the end user? Other suppliers want absolutely nothing to do with the pumps and leave that decision completely to the buyer: How then are you to make a selection, do you simply pick a pump that meets the pumping speed and ultimate pressure required for your application?
Over the next few months we will discuss a range of considerations on this topic such as your application, pump technologies and cost. We will assume pumping speed and ultimate pressure can be ignored as most rough pump lines are offered in standard pumping speeds and operate within a very tight pressure regime. The object is to help educate you, the end user, in making a sound pump decision that is likely to affect you for years to come.
Your application is probably the most critical component of your rough pump decision. If you choose the wrong pump for your application, it will affect your cost of ownership, reliability and chamber uptime. The good news is your chamber supplier will almost certainly recommend a pump that is ideally suited for your application, so you should give their choice consideration. However, their recommendation will typically be limited to currently-manufactured new product or currently-available refurbished product, which significantly impacts cost, limits the range of available options, and creates supportability issues as your facility expands and changes over time. Ideally, you should consider options that are proven to be reliable on your application, while also being mindful of initial and ongoing cost, supportability and maintaining spare inventory.
Ensuring rough pump reliability on your application may seem complicated on the surface, however it can be very simple with a proper understanding of vacuum principles. Essentially, overall pump application reliability comes down to the characteristics of the gases used in your process/vacuum chamber. Inert gases are generally easy on rough pumps and present few challenges to reliability. However, many gases used in your chamber can be corrosive, explosive, combustible or otherwise potentially harmful to the pump. In addition, with the pressure, volume and temperature changes that occur during rough pumping, these and other gases may also condense (gas to liquid to solid) or react in a way to form other challenging substances in your vacuum lines or the pump itself. Due to these issues, rough pumping can become a big challenge and potentially unsafe if not given proper consideration. However, by selecting the appropriate pump technology and configuration for your application you can easily mitigate these concerns.
In modern semiconductor vacuum processing, applications are typically defined as one of the following three categories:
- Light Duty: e.g. load lock, evaporation, sputtering, and microscopy chambers
- Medium Duty: e.g. photoresist stripping/ashing, polysilicon and oxide etch chambers
- Harsh Duty: e.g. PECVD, LPCVD, ALD, MOCVD, epitaxy and metal etch chambers
It is the harsh duty range of pumping where the non-inert and condensable gases are present including such potentially dangerous substances as Chlorine, Ammonia, Hydrogen, Phosphine, Silane and many others. Very careful consideration should be given on these applications as specific pumping types and configurations are superior for ensuring reliability and avoiding a visit from your Environmental, Health and Safety team.
In our next segment, we will review different types of pumping technology such as oil sealed rotary vane pumps, dry pumps and mechanical blowers and how each can be configured and employed to optimize reliability and ensure safety on your application.
