Hi Vishesh,

The UHV condition is usually obtained by a series of pumps connected to the STM/SEM chambers. The pressure could be ~ 10-3 Torr using roughing pump (rough vacuum), ~ 10-8-10-9 Torr (UHV) by turbo pump, or below 10-10-10-11 Torr using ion pump. For UHV-STM, usually all three pumps are needed to obtain really UHV of < 5x10-11 Torr. For SEM, usually a roughing and a turbo is good enough.

Hope it help!

Duc

Thank you very much Duc for your answer

Thanks Mohammad panahi for your answer

Dear Vishesh,

as a student, I was lucky to learn UHV technique just by doing it in the lab, under the guidance of an experienced advisor (M. Bode, now professor at U. of Würzburg, Germany), which certainly is the most effective way to pick up all the tricks of that business. If you lack such an opportunity you can still do it the straight way. There is one authoritative book on the matter, Redhead, P. A., Hobson, J. P. and Kornelsen, E. V., The Physical Basis of Ultrahigh Vacuum, Chapman & Hall, London (1968), discussing in great detail all aspects of UHV technique such as vacuum chambers, types of pumps, measurement equipment, leak detection, vacuum valves and other components etc.

As a good starting point, you may want to consult https://www.pfeiffer-vacuum.com/en/know-how/, an interesting page provided by one of the great companies in the vacuum branch. 

Another interesting page is this one, http://www.idealvac.com/files/literature/06_Varian_2011_Ion_Pumps.pdf,

focussing on ion getter pumps. This type of pump was already mentioned by Duc and Mohammad. Such a pump allows you to advance from high vacuum (HV) to ultra high vacuum (UHV). Equally important: It operates without any moving mechanical part, hence it doesn't generate any noise that otherwise would disturb your STM experiment.

As you can imagine, UHV equipment is quite costly.

Oswald

"ion getter pumps"

Ionizing devices may be disadvantageous in STM surface analysis under UHV conditions, see references below.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.81.1255

Article Residual gas: Traditional understanding and new experimental...

Thank you Johan sir

Dear Mr. Vishesh Saxena,

Generally UHV is achieved in the following steps:

Hope, it will be helpful for you.

Best wishes

Anjan

Thank you Anjan

Idea behind achieving UHV condition is to reduce the adsorption of residual gas molecules on the surfaces. The usual thumb rule is that, at 10-6mbar, it takes 1 second for gas molecules to adsorb on the surfaces. ( For 10-7mbar it will be 10 secs, 10-8mbar it will be 100 sec so on and so forth). Therefore, you generally end up getting very good vacuum conditions when you want to work with STM and AFM, especially if you are concerned about clean surfaces and things like that.

Achieving UHV conditions is via various pumping systems. By using sequentially, the Rotary pumps, Turbomolecular pumps, Ion Pumps, you can achieve ultra high vacuum conditions ( pumping will remove all the residual gasses in inside your chamber). This has to be followed by BAKING the whole chamber so that even the adsorbed H2O will be taken out. Generally UHV systems will be attached with a Residual Gas analyzers and if the RGA spectra should show you only Hydrogen peak, then it is guaranteed that you have a ultra high vacuum conditions inside your system.

For SEM, you do not use the UHV conditions in general. Although in special cases, SEM's are used in UHV conditions too.

hope this helps,

best,

Ramesh

To obtain UHV it is not a matter of how large the pumping speed of your pums are, it is the degasing of water and adsorbates from the walls and other infernal parts of the system. To get low outgasing rates you have to take care on the materials you are using inside your system e.g. no plastic, no brass. All parts must survive a bakeout at 150-250°C. Flanges should be metal sealed (normally OFHC copper). Turbopumps can achieve vacuum down to 3.10-11 mbar if properly treated (never vented from the prevacuum side) and a prolonged bakeout after the first Installation of the system. If you are using oil sealed pre pumps install a forline trapp between rotary and Turbo. Inner parts should not be touched by bare fingers, no grease on moving parts. All new parts should be cleaned before installation in ultra sonic bathes, first step Aceton second step Ethanol. The system should be vented with very dry nitrogen (we use a dewar with liquid nitrogen fed by a hose to a valve.

Dear Mr. Vishesh Saxena ,

During bake-out of the STM chamber, you should be well aware about the maximum allowable temperature for the internal electronics and magnets. You should check it from the respective manuals and keep the bake-out temperature (and duration) moderately below of allowable temperature. Otherwise, it would damage the in-vacuum components and piezos.

With regards,

Anjan

From atmosphere:

1. Rough pump (rotary pump or scroll pump)

2. Turbo pump (+ion pump optional). You will reach a limit of about 1e-8 mbar. Water from the walls will continously degass unless you bake (120C for 2 days) while pumping.

3. Let it cool, and degass filaments while cooling. Switch ion pump on (or getter pump if applicable) and fire TSP filaments every 6-8 hours.

Oswald Pietzsch Thanks a lot for your response. I was going through these questions again and I realized I did not answer you! Coincidentally, I will be joining the Wiesendanger group for my PhD!

Ultrahigh vacuum (UHV) is achieved by three stage pumping process.

First, you need to create a rough vacuum (~E-3 Torr) using any scroll/rotary pump. Then you create a high vacuum (~E-8 Torr) using Turbomolecular pump (TMP). At the final stage, you use Ion and Titanium sublimations Pumps to achieve UHV (~E-11 Torr).

Subsequently, you use three differnt types of guages, that is, Pirani, Penning and Ion guage to measure rough, high and ultrahigh vacuum.

But, Since, TMP creates a lot of vibrations, you can't use it during STM/AFM measurments.. In that case you create the UHV first and then turn off the TMPs. The system can easily UHV on Ion pump if it is in healthy states and you don't do any excessive heating/spuettrring of the sample.

I hope it helps.

Best,

Monu

Monu Mishra: "The system can easily UHV on Ion pump"

Please note that ion pumps and gauges are sources of CO contamination on surfaces, so I would recommend mechanically uncoupled turbo and/or Ti sublimation pumps.