Ion Pumps Provide Clean UHV Environments.
Ion pumps provide contamination-free operation, pump all types of gases, and have long operating lives. They also provide high pumping speeds, fast starting, and stability; require no water or liquid nitrogen for operation; consume little energy; and have no moving parts to require maintenance. Additionally, an ion-pumped system does not lose vacuum during a power failure nor does it require rejuvenation or costly cleanup after power is restored. Another feature of the ion pump is that the current is a measure of pressure, often eliminating the need for an additional pressure gauge.
When pressure in a vacuum system has been reduced to less than [10.sup.-3] torr by means of a roughing pump, high voltage is applied between cathodic electrodes and tube-shaped anodes positioned between them. Since the electrodes are in a magnetic field, electrons that become accelerated toward the anodes are forced to take helical paths in the anode space. These longer paths increase the probability of the electrons colliding with gas molecules.
Positive ions formed in the collisions bombard the cathode plate, which consists of a chemically active metal (titanium). The ions combine with the active cathode material and eject more cathode atoms. These uncharged atoms, which are unaffected by electrical and magnetic fields, are mainly deposited on anode surfaces. In this way, a continuously replenished film of active cathode material is deposited on the anodes. Chemically active gas molecules that are present in the discharge region, such as nitrogen, oxygen, and hydrogen, combine with the active metal atoms to form solid compounds, thereby becoming "pumped" by the system.
The pumping mechanism buries inert gases in pump surfaces. When inert gases are ionized, they accelerate to the cathode and either penetrate several atomic layers upon impact and become trapped within the cathode lattice structure (but are re-emitted if the entrapping lattice atoms are sputtered away), or reflect as energetic neutrals and become embedded and trapped in the pump surfaces that see little or no sputtering, such as peripheral anode and cathode surfaces.
The magnetic field directly affects the pumping speed. The pumps use a barium ferrite magnet. This material exhibits a reversible field loss of 0.2% per degree Celsius and an irreversible field loss of 7% at 350 [degrees] C. This loss is non-cumulative (subsequent bakeouts to 350 [degrees] C do not cause an additional irreversible loss). Above 85 [degrees] C, the pumping speed declines with temperature. Ion pumps have difficulty maintaining operation above 250 [degrees] C because of magnetic field loss and the increased gas load as gases desorb from internal surfaces.
Conventional ion pumps provide the highest pumping speeds for air and active gases. Typically, the pumping speed for inert gases is an order of magnitude lower than for active gases. Because of the lower pumping speed of inert gases and other gases that do not readily combine with titanium, conventional ion pumps are best when inert gases are not intentionally introduced.
Some ion pumps experience large periodic pressure fluctuations while pumping mixtures that contain inert gases. These fluctuations, called "argon instabilities," occur both when pumping air at UHV (1% argon by volume) and pure argon or other inert gases. "Air-stable" is the term used to describe an ion pump that can pump against an air leak without becoming unstable. An "argon-stable" pump is one that can pump against a 100% argon leak without becoming unstable. The differential ion pump design provides both capabilities--air-stability and argon-stability--in a single pump.
Most inert gases are pumped on the anode structure and at the peripheral areas of the cathode where the sputtering rate is so low that total re-emission does not occur. Energetic reflected neutrals readily reach these peripheral areas and the anode surfaces because the neutrals are not affected by the magnetic field. Thus, at a higher rate of energetic, reflected neutral formation, inert gas pumping speed would be increased. To achieve high inert gas pumping speeds, differential pumping elements with one cathode chosen for good energetic neutral production (tantalum) and one chosen for its chemical reactivity (titanium) are used.
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|Title Annotation:||ultrahigh vacuum|
|Comment:||Ion Pumps Provide Clean UHV Environments.(ultrahigh vacuum)|
|Publication:||R & D|
|Article Type:||Brief Article|
|Date:||Jul 1, 2000|
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