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:

1.- Vent your chamber using N2 instead of air;

2.- I like to warm up the chamber a little bit, using the heating tapes or an infrared lamp, prior to venting the chamber. This also helps reduce the adsorption of water;

3.- Try to keep your chamber vented as shortly as possible. If you need to work for a while in air before finishing (for instance, mounting new samples), you may want to close the chamber and start pumping it again, even with mechanical pumps only.

4.- Some people coat the inner surface of their chambers with a film of evaporated gold to passivate it and reduce the adsorption.

5.- Finally, and probably the most evident solution: can't you separate that access stage from the main chamber with a gate valve, and mount an additional pumping stage so that you can evacuate the introduction chamber before communicating it with the main vessel? If you open the valve only for a few minutes when the pressure there is already in the -7 range, introduce the sample and then close it back again you will be back in the -9 or -10 range in minutes.

Hope it helps

Hello Vijay.  Two solutions; 1) You need a load lock chamber.  A load lock (for short) lets you mount your sample without disrupting the vacuum in your main chamber.  When you have a load lock you also need a transfer system that lets you transfer your sample from the load lock to main chamber.  This will get you easy into uhv range you mention.  2)  Another more brutal solution is to add an ion pump connected to your main chamber and forget the load lock.  I am assuming your vacuum is done through a turbo pump.  Depending on the size of your main chamber there are commercial ion pumps that are small and can be easily mounted on 2 3/4 or 4 1/2 inches flanges.  The ion pump would get you in the 10-9mbar range but it will really depend on the size of your main chamber, in your case; the smaller main chamber the better.  Cheers.

The general ideas have been proposed  by Erie and Juan José. 

• Decrease the pumping volume
• Take care of the chamber while you breake the vacuum
• take care of the inner surface of the chamber (coating with gold,...)
• Increase the performence of pumping system 
• Add a trasfer chamber (load lock chamber)
• ....

The last proposition would be a good solution because, you can probably not to change everything on your main UHV chamber. If you analyze several samples at the same time, you can build a system with several sample holder etc. But the transfer system from the load lock chamber to main chamber needs some money.

Hello Vijay,

It is impossible to achieve UHV without a proper bake of the working chamber. One need to calculate the pumping speed necessary according to the base pressure needed and the gas load, especially the outgassing loads.

Very important is to reduce the outgas load from the chamber walls.

A bake to 70C is certainly too low a temperature, I would recommend to use at least 150C and higher for a period 24 - 48 hours and wait at least 24 hours till system comes to ambient temperature.

Cheers, JG

Hello Vijay,

there is a special pump-down technique with additional N2 flushings at pretty no additional costs. It is nicely described on: https://faebianbastiman.wordpress.com/2013/07/20/essential-maintenance-pump-down/

I never tried this on my own, but if you could succeed in improving you vacuum, please let me now.

Cheers

Hi!

Preliminary, multiple (3 times) venting and purging with pure nitrogen make the evacuation faster. To my experience using a fast, oil-free fore-vacuum pump (particularly a Scorll - type) speed up effectively evacuation of the system. Multiplying the number of turbopumps is not effective in this case. On the other hand I never reached vacuum better than 10e-7 mbar without bakeout (typically at 120-150 deg C, baking time from "overnight" up to 48 hours).

Cheers,

Jerzy L

Hallo again,

using S=RA/Pu, where S-pumping speed, R-outgasing rate for regular 304SS, Pu-desired ultimate base pressure.

Thenfor example for a regular working chamber of 100,000 cm^2, R(non treated nor baked SS) - 2x10^-8 (Torr-L/cm-s)and a desired base preassure of 2x10^10 torr, one gets 10ML/s!!

Such a monster pump does not exist, so baking of the UHV system is a must!!!

Cheers, JG

I think the suggestions from Juan José De Miguel are useful. A supplement for his suggestion#1 is "lower humidity of the venting nitrogen(

While the chamber is opened up, I find it helpful to keep N2 flowing out of any openings to minimize the air (with water) entering the chamber.  Doing that, I have achieved better than 10-9 Torr within a day without doing a bakeout IF the chamber was well baked before and IF the chamber was opened very briefly (few minutes).  I expect this will only work on small chambers with good pumping.

I have investigated the humidity of the venting gas and environment on the outgassing rate of the vacuum chamber. If the humidity of the venting gas and environment is low enough, the pressure lower than 1E-9 Torr without 1 d can be achieved for a 2 m long vacumm chamber(diameter 100 mm). For more details, pls refer to the paper titled "Effect of environmental humidity on rate of thermal outgassing."

Vijay,

you have already many good suggestions, did something work?

From my experience, water only should not prevent you from going down to mid or low 10-8 mbar if your pumping speed is high and your vacuum chamber is clean.

- Leak check with Helium gas your chamber. This time use a copper gasket if you can instead of a Viton ring (if that is the case). Once done, do an RGA to see if there are high masses and dirt, besides water.

- Estimate desorption rates and virtual leak: I do not remember the full details but if you switch off your pumping, record pressure increase, you can characterize it. That will help you to review if your pumping speed is well dimension.

- Determine if your quick door (if you have one) has the Viton seals in good condition, consider replacing that for copper gaskets. Another option is to use a quick door with a differential pumping, so you still have the re-utilizable Viton but minimize its leak (a proper Viton seal in good condition should not have a leak rate much worse than mid 10-9mbar*l/s). The only company I know selling those is McAllister Technical services.

- When venting, keep N2 flow and only open the chamber door for as short as you can.

- Consider a cold trap: this will be very efficient trapping your water. This can simply use LN2 but I guess there might be already cryogen free solutions if that needs to be running permanently.

- Check the status of your primary pump, is it condensing water vapor? You may want to use gas ballast (or several times vent and pump dry gas) . Same for turbo, you could do a mild bakeout of the turbo. When you vent, do you valve off your chamber from the turbo? and turbo from your primary pump?

- Summary: vent as clean as possible, open as short as possible. Leak test, RGA test, check/replace Viton seals, confirm pumping power was well dimension and consider a Meissner (cold) trap either commercial or inhouse made.

Good luck!

Basically, as you can tell from all other responses the answer is not the way you are doing it. First a bakeout at 70 °C is basically not a bakeout, you should bake at least at 120 °C (you are trying mostly to desorb water).

Now, there are alternatives to external heating bakeouts (i.e. heating tapes, jackets, tents...), which should be pretty quick and might be what you are looking for, though might be expensive.

Option A: using internal quartz lamps, although since you have delicate optics in the chamber this might not work. Check this out: http://mcallister.com/shop-2/heating-cooling/bakeout-heating/

Option B: Internal UV water desorption. I think someone else mentioned it briefly. I think this might be your best bet. Check it out: http://www.rbdinstruments.com/products/uvb-100-water-desorption-system.html

Good luck!

Vijay,

the answer is yes, and now I  know what is your solution and as you are at ETH maybe you can afford it :)

If you have an STM close or something like that you may need to be careful to ensure decoupling of vibration.

Cryopump.

If you swim in money, you can go for a BROOKES CTI cryompump. There are others, i.e. oerlikon. You should probably consider a 4000 H2O l/h or so.

If you don´t, you can try refurbished or even try ebay. I have done.

Once I got mine finally operating I let you know, but it seems that is going to be as well a perfect solution for you.

You owe me a beer if that works for you. :)

Hi Vijay.

I have been doing daily in my XPS/Auger system UHV at 10E-9 till 10E-11 torr.

Read the suggestion from Daniel Velazquez above which is a very good suggestion!

I would say it will be also very good if you can gat and RGA and see you do not have an air leak in your UHV system.

If everything will be fine you should get around 10E-10 Torr in 48 hours after closing your heating!

Good Luck!

Edward, would you happen to have a source for that temperature figure?

I'm interested to read more about desorption during baking.

Hi Vijay,

I would not change my views who I wrote on April 14, it continue to be vigent. In summary:

1. vent the system to atmospheric pressure using dry N2.

2. Open the chamber using a positive dry N2 flow from the chamber into the lab.

3. Introduce your samples into the chamber, close the port and start pumping down.

4. continue pumping down + bake at least 180 deg C for 24 hours.

5.Let the system cool down to room temperature for 48 hours and the pressure will go down to 10E(-10) Torr.

6. of course do an RGA scan 1 till 50 amu to be sure there is no air leak into your system.

Hope this helps

do you have turbomolecular pump? wich material for vacuum chamber?