Why Won't My Chamber Go Below 1 x 10-x Torr?
Although sometimes phrased differently, this is the major concern for vacuum users. Perhaps the questioner thought, given all the money he/she spent on the pumps, the pressure should be much lower. Maybe the chamber once reached a lower pressure. Possibly the user's work can't be done at the chamber's present base pressure.
And that's what we are discussing: base pressure: the lowest pressure reached when no gas is being (deliberately) added to the chamber. So, what limits the base pressure? If we wait long enough, why can't a pump pull the chamber to any vacuum level we care to name? Even more pertinent, why can't a particular pump pull the chamber to the ultimate pressure that the manufacturer claims for that pump?
There's a Catch 22 here. The answers to these questions are simple if you understand the terms: pumping speed, pump throughput, gas load, and outgassing rate. However, if you understand these terms, you already know the answers to these questions. Perhaps the right course of action is to link to those terms, read the explanations, and then return to this page.
Relationship between Gas Load and Pump Throughput
Qualitatively, we can predict what happens to the chamber pressure if we know the relative gas load and the pump throughput values. Some simple examples are listed in the table.
|Gas Load||Pump Throughput||When/Where Conditions Occur||Result|
|Moderate||High||Pumping chamber well above base pressure||Pressure falls|
|High||Low||Start of chamber bakeout||Pressure rises|
|Low||Low||Pumping empty chamber for many hours||Pressure steady|
|High||High||First stage of differential pumping system||Pressure steady|
This is quantified using gas load = pump throughput, when the pressure is stable. Or in more memorable words (not found in any vacuum text book):
At a fixed chamber pressure: Gas In = Gas Out
In other words, in any vacuum system the chamber pressure will not fall continuously no matter how long you wait. The chamber's gas load never drops to zero and the pump's throughput decreases with decreasing pressure. At some pressure they will be equal and that is the chamber's base pressure.
Estimating Base Pressure
We can use gas in = gas out to estimate the chamber's base (or working) pressure:
- Measure the area of metal exposed to vacuum inside the chamber.
- Find the outgassing rate for the chamber material from O'Hanlon's book (converting to units you understand).
- Multiple area times rate to estimate the gas load.
- From the pumping speed curve (at a guessed base pressure) and the conductance of the port connecting pump to chamber, calculate the effective pumping speed (using the volume/time units selected for the outgassing rate).
- Divide the gas load by the effective pumping speed to get the base pressure.
- If your first base pressure guess was wildly wrong (causing you to choose the wrong pumping speed), use the base pressure estimated in the bullet point above to select a new pumping speed and recalculate the new effective pumping speed.
Changing Gas Load and Pump Throughput
Can anything be done about gas load or pump throughput? Certainly! Gas load is addressed with vacuum technique and materials. Pump throughput responds to design and money.
Decreasing Gas Load
- Use only the lowest outgassing materials in construction
- Use appropriate vacuum design and welding techniques
- Physically and chemically clean the components and chamber walls
- Thoroughly leak check components prior to initial assembly
- Heat chamber walls to high temperature during initial pump-down
- Plasma clean the internal surfaces
- Freeze molecules to the wall with cryogen cooling
- Use a pump with minimum backstreaming (to reduce that contribution to gas load)
- Operate all pumps within a pressure range that minimizes backstreaming
- Avoid adding gases for which the existing pumps have low throughput
Increasing Pump Throughput
- Replace existing pump with another of larger pumping speed
- Increase effective pumping speed by increasing tubing diameter and reducing number of bends
- Add more pumps
- When the pump throughput matches the gas load, the pump removes gas molecules at the same rate as they enter the gas phase. The pumping action does not stop; the pressure simply reaches equilibrium.
- As the pressure decreases, the pump's throughput decreases.
- The chamber cannot reach the manufacturer's listed ultimate vacuum since the manufacturer measured it under the best possible conditions (which are never met in a real system).