Ultra High Vacuum (UHV)

The key-ingredient to surface science experiments is ultra-high vacuum. This means pressures in the 10 mbar range and below. Only such a low pressure will assure that a surface stays clean for a time long enough to do some experiments (we will see later how to obtain a clean surface). In the following, some important parts of UHV technology are described. To get a feel for it we will go down to the lab during the exercise and look at the hardware!

Vacuum requirements

We can quickly estimate the vacuum requirements for surface science. Let's imagine a surface in the vacuum. The number of gas molecules impinging on the surface is

 

where m is the molecular mass in kg and M is the molecular mass in units of the atomic mass constant. The usual units for the pressure in vacuum technology are torr or mbar (1 torr = 1.3332 mbar = 133.32 Pascal). For a pressure of mbar and a temperature of 300K we find

 

As an order of magnitude value a surface has atoms per square centimetre. This means that if every rest-gas molecule at the above conditions sticks to the surface the latter will only stay clean for a second or so. If we are not willing to tolerate more than, say, a percent of contaminating rest-gas molecules on the surface then the pressure has to be in the UHV region.

It is also interesting to calculate the mean free path   of the molecules at a given pressure, i.e. the mean distance before hitting another molecule. It is

 

where is the molecular diameter. For typical UHV pressures the mean free path of the molecules is many meters. This means that it is much more likely that the molecule hits the walls of the vacuum vessel than another molecule. We come back to this later.

Pumping and bakeout[7]

In order to achieve UHV conditions two stages of pumping are needed (see Fig ).

  
Figure: Pumping of a UHV system.

A roughing pump is used to pump the system down in the mbar region. A typical pump is an oil-sealed rotary vane pump   as shown in Fig. .

  
Figure: A oil-sealed rotary vane pump. Principle of operation: (a) gas from the vacuum system is expanded into the pump and (b) the gas is pushed through the pump exhaust.

The operation principle of this pump makes it obvious why there are two stages on the vacuum system: the rotary vane pump will work fine when the pressure is high enough to ensure a gas flow through the pump but hopeless when the mean free path of the molecules  is very long.

The pump of choice for the second stage is a so-called turbomolecular pump  (see Fig. ).

  
Figure: A turbomolecular pump. Picture: Leybold Vacuum GmbH, used with permisssion.

The rotor of the pump is build such that the gas molecules collide with the fast moving rotor and thereby attain an impulse in the direction of the roughing pump. In order to achieve efficient pumping the speed of the rotor has to be very high. Typical values are up to 80 krpm.

Once the low pressure has been achieved it can also be maintained by another type of pump, the so-called ion pump  shown in Fig. .

  
Figure: An ion pump.

The rest gas in that pump is ionized by a plasma discharge due to the high voltage between anode and cathode. The ionization probability is increased by the presence of the magnetic field which makes the electrons travel on spiral trajectories. When the ions hit the Titanium cathode they can be buried in it or react with it. Additionally, Ti is sputtered off the cathodes and deposited on other parts of the pump. This Ti can react with the rest gas and increases the pumping effect. The ion pump does not remove the rest gas from the system. It just binds it such that it can not contribute to the pressure any more. The current provided by the high-voltage power supply is proportional to the pressure in the system. Ion pumps  can therefore be used to estimate the pressure. The ion pump can be operated at pressures between 10 and 10 mbar.

In order to reach a low pressure in a short time it is necessary to perform a so-called bakeout of the whole vacuum system. During the bakeout the system is heated to at least 100-200 C for an extended period of time (24h or so). The heating causes a fast removal of the impurities adsorbed on the walls of the vacuum system (mostly water). The need for baking systems renders working with UHV chambers rather time-consuming. It also requires that the system is build only of components which can withstand high temperatures for a long period of time.

 

Pressure measurement[7]

For the roughing pump stage one can use a so-called Pirani gauge   to measure the pressure. The idea of this instrument is to measure the resistance of a heated wire placed into the vacuum system. The resistance depends on the temperature of the wire. At high pressures the wire will be cooled by collisions with the rest gas molecules, i.e. by warming up the rest gas. As the pressure is reduced this cooling mechanism gets less effective and the temperature of the wire rises. The resistance of the wire can be calibrated as function of the pressure. Pirani gauges work from ambient pressure down to about 10 mbar.

For lower pressures, a so-called ion gauge   can be used. It is shown in Fig. .

  
Figure: An ion gauge.

The filament emits electrons which are accelerated inside the cylindrical cage. In the cage the electrons hit rest gas molecules and ionize them. The ions are collected at the wire in the middle of the cage and the current from that wire is a measure for the pressure. A problem is that the sensitivity of the gauge depends on the chemical identity of the rest gas. The ion gauge  can be used in for pressures between 10 and 10 mbar. The low pressure limit is given by an unwanted effect: when an electron hits the cage it produces a photon which then can cause the emission of an electron from the centre wire. While this effect sounds extremely unlikely it becomes the dominant contribution to the current at very low pressures.

Materials and construction[7]

We have already seen above that all materials in the vacuum system have to be able to withstand high temperatures during the bakeout. Another important criterion is that the vapour pressure   of the materials has to be very low at the normal operating temperature of the vacuum system. The following table gives the temperatures in K to generate a certain vapour pressure.

Obviously, the first materials in the table are very undesirable in a vacuum chamber! The last two are important for the actual construction: most chambers are made of stainless steel and almost all the filaments (for example in the ion gauges ) are made of W. Electrical insulators are made of ceramics with a low vapour pressure.

A vacuum vessel has many circular flanges where windows or equipment with the same flange type can be bolted on. These flanges have sharp knife-edges profiled on their operating sides. A leak-tight seal between the two flanges is achieved by placing a copper gasket between the knife edges and pressing it until it is tight. It is necessary to use copper (instead of simply ``rubber'') because of the bakeout.

When putting the pumps on the chamber one has to consider the concept of conductance. Keep in mind that in the low pressure region the mean free path of the molecules is much longer than the dimensions of the vacuum system. It is therefore clear that it does not make sense to connect a very powerful pump to the vacuum system via a small diameter tube with many right angles. The rest-gas molecules will never find their way to the pump.

Another important requirement is to linearly move items inside the vacuum or to rotate them. Most of the motions are transferred from outside via stainless steel bellows. We will look at some linear and rotational feedthroughs in the lab.

Chemical analysis of the rest gas[7]

Finally, it is desirable to measure the chemical compositions of the rest gas in the system. This can be used to see if there is a leak in the vacuum chamber, to check the purity of gases used for the experiment and to leak-test the system. To do this, one directs a stream of He gas on the suspected leak from outside the system. If one observes a increase in the partial pressure of He inside, the leak has been found.

A so-called quadrupole mass spectrometer   is shown in Fig. . It consists of three sections. In the first, rest-gas atoms are ionized just as in the ion gauge. These ions are then accelerated and focused into the second section, the actual mass filter. It consists of four bars which set up an electrical quadrupole field, driven by the superposition of a DC and a radio frequency AC voltage. The filter works such that only ions with the same m/e can pass it. The last element of the spectrometer is the ion detector which contains an electron multiplier leading to an amplification of the signal by a factor 10 -10 . The lower limit of the partial pressures which can be detected with such a spectrometer is about 10 mbar.

  
Figure: A quadrupole mass spectrometer

A typical mass spectrum is shown in Fig. . The main residual gases in the vacuum are hydrogen, water, carbon monoxide and some carbon dioxide. It is important to note that the criterion is m/e and not simply m! It means that doubly charged ions appear as particles with half the mass in the spectrum. CO does for example   not only give a peak at m=28 but also one at m=14 due to double ionization. The gases are also dissociated in the spectrometer such that one does not only find one peak for single-ionized water at m=18 but also peaks at m=16, 17 and 2 for the fragments.

  
Figure: A typical mass spectrum.

After the bakeout of the system, the partial pressure of water will be strongly reduced and the total pressure will be determined by CO and atomic hydrogen. If there is an air leak in the system this would show up as peaks of 28 and 32.

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