- Quality/strength/size of your pumps
- Size/surface area of the chamber
- Access of your pumps to the chamber (i.e. size of cross section)
- Pump time (hours vs days vs months)
The best thing to really ever do is a nice, long bakeout. However, with a small chamber, medium sized turbo pump, and a long pump time, I've been able to reach low 10^-9 torr without baking. Again, it depends on your system.
Good General Practice
"The problem with going down to UHV is the desorption of the water vapor that sticks to the chamber walls when you open it to the atmosphere. If you check the tables of vapor pressure, you will see that for water at room temperature it lies around 10-8 mbar. This is why you need to bake the system out, to force the quick desorption of that water.
So, if you need to speed up the pumping the only way is to prevent or reduce as much as possible the adsorption of water inside your chamber. There's a number of strategies to do that, although none will solve the problem completely" - Juan Jose de Miguel
To deal with this, we have a few steps of good general practice:
- Vent your chamber to N2 instead of air. If possible, you can even keep a low pressure of N2 gas flowing through your exposed system to reduce adsorbants. Ideally, you should use low humidity N2, but this may be out of your control.
- Heat a little bit as venting to reduce adsorption of air
- Keep your chamber vented for as short a time as possible. Get most of your work done before venting and/or keep your chamber at least scroll/rough pumped.
- Install a loadlock to have one subchamber with so-so pressure that accesses the chamber with really good pressure for a short amount of time. This is mostly used for sample transfer.
- Coat your chamber's inside with something (i.e. gold film) to passivate the surface, thus reducing adsorption.
- Use a carousel or garage inside your chamber to store several samples so that you don't have to vent.
- Use a copper gasket (rather than a rubber one) to really get a good seal from ambient. This my require your system to use different flanges, but is worth investing it. Some people use a copper gasket 2 or 3 times before replacing it, but for best pressure practices, a new copper gasket should be used every time.
More good discussion of this topic can be seen here.
Getting to 10^-7 Torr
Some systems have temperature limits, and bakes can be unnecessarily difficult. In this case, there are still a few tricks you can try. You should be able to access 10^-7 torr pressures with a decent turbo pump on your chamber. If not, you might have a leak or need stronger pumps.
Getting to 10^-8 Torr
From 10^-7, I've found that even a light bake at 80C (cheap system here, or a cheap variac with ebay heat tapes) and long pumping can move you into the 10^-8 range with just a turbo. Again, the caveat is.. it really depends on the 4 parameters mentioned at the very beginning of this post.
In addition, Dr. Faebian has an excellent blog, and in one post he explains some N2 magic for flushing your chamber and decreasing your pressure. In summary...
"Evaporation will transport moisture from the chamber moist inner surfaces into the dry N2 gas. Not all, but a significant fraction. The evaporation rate will depend on:
The concentration of water in the N2: which is zero at the beginning
Flow rate: initially high as the N2 penetrates the vacuum, but constatly dropping
Pressure: evaporation happens faster at lower pressure
Temperature: evaporation happens faster at higher temperature
Simply let the N2 in until ..500 mBar (remember the process happens faster at lower pressure). Wait a minute. Then pump the chamber down with the scroll pump again. Once it has reached 0.05 mBar, turn the pump off and flush it with dry N2 again. On the second pump down, open the As and P cracker’s needle valve manually a whole turn. The bulk should have been protected from atmosphere when you shut the valve during Essential Maintenance: System Venting. It should therefore be free from moisture, but there is always a chance the valve has a little leak. To be safe, we will pump down the bulk with the scroll pump now. To increase the effectiveness of the N2 flush you may consider heating the N2 gas to ~50°C. This can be highly regulated with an inline gas heater or poorly regulated by baking the SS N2 pipework to 50-70°C with heat wraps during the flushing phase"
After repeating the vent and (rough) pump 5 times, you should be able to get to 10^-6, but by adding a turbo, this should get you to 10^-7. Now, if you leave this overnight pumping, you should see 10^-8 by morning.
Getting to 10^-9 Torr and beyond
Once you've hit the 10^-8 range, you can start using ion pumps and TSPs to do some dirty work for you. Turning an ion pump should do quite a lot. For the TSP, you should probably run a few cycles. Some controllers allow you to program the TSP to cycle every hour or so for a set period of time. Both pumps are powerful, and over time, can get you to pressures as good at -11.
- "An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump capable of reaching pressures as low as 10−11 mbar under ideal conditions. An ion pump ionizes gas within the vessel it is attached to and employs a strong electrical potential, typically 3—7 kV, which allows the ions to accelerate into and be captured by a solid electrode and its residue." - Wikipedia
- "It consists of a titanium filament through which a high current (typically around 40 Amps) is passed periodically. This current causes the filament to reach the sublimation temperature of titanium, and hence the surrounding chamber walls become coated with a thin film of clean titanium. Since clean titanium is very reactive, components of the residual gas in the chamber which collide with the chamber wall are likely to react and to form a stable, solid product. Thus the gas pressure in the chamber is reduced. But after some time, the titanium film will no longer be clean and hence the effectiveness of the pump is reduced. Therefore, after a certain time, the titanium filament should be heated again, and a new film of titanium re-deposited on the chamber wall." -Wikipedia
Remember, even the best of systems can have a small leak. This can be extremely difficult to find, and typically you will need to use a mass spec to do a helium leak check. Without a completely sealed system, the rate of leak will counteract even the best of pumping. This post may make it seem like you never need to bake, but honestly to reach pressures of 10^-9 and better, you will typically need to bake. Sometimes, if you have an accident vent/leak, you can still recover good pressure, but a real vent for repair and such (without use of a load lock) will usually require a bake of a day or two at 120-150C.
