Theory of Operation
Sputter ion pumps operate by ionizing gas within a magnetically confined cold cathode discharge. The events that combine to enable pumping of gases under vacuum are:
- Entrapment of electrons in orbit by a magnetic field.
- Ionization of gas by collision with electrons.
- Sputtering of titanium by ion bombardment.
- Titanium gettering of active gases.
- Pumping of heavy noble gases by ion burial.
- Diffusion of hydrogen and helium into titanium.
- Dissociation of complex molecules into simple ones for pumping ease, e.g., CH4 breaks down into C and 2H2. Hydrogen is pumped separately. Carbon is no longer part of the residual gas and resides in solid form.
Burial is the basic means of pumping heavy noble gases. Argon ions neutralized via glancing collisions with a sputter cathode impact the pump wall and are coated with sputtered titanium. Triode pumps are specially designed to maximize the kind of collisions that produce energetic neutrals.
Argon is permanently pumped on the wall behind the cathode in these pumps. The wall area receives titanium for inert gas burial but, because of a retarding electrical field between the cathode and the wall, it is not subjected to ion bombardment and thus gases are not resputtered.
Standard Diode Pump
The figure above is a model of a diode sputter ion pump. The main elements are a vacuum-tight envelope external magnets and an element consisting of multiple anode cells and two cathodes. The application of a positive high voltage to the anode creates a plasma discharge. Ions are formed from the gas molecules present in the system. These ions are accelerated toward one of the cathodes. When they strike. they can be buried or reflected to be buried elsewhere. In addition. titanium is sputtered from the cathode to be deposited elsewhere in the pump where it acts as a getter for active gases. An ion pump, then. does not remove gas from the vacuum system. It binds gases down chemically and physically so they can no longer contribute to the pressure in the system. Diode pumps cover a very wide pressure range. The recommended starting pressure is 5 x 10-3 Torr or lower. The operation for extended periods of time at high pressure, other than starting, is not recommended because it shortens the pump life.
The ultimate pressure after bakeout is generally in the region of 2 x 10-11 Torr. Pumping speeds fall at these pressures because wall effects diminish discharge intensity. Between this extreme and 10-5 Torr, discharge intensity is proportional to pressure and thus the pump can be used as a gauge.
Triode Pumps
Triode Pumps the image above is a model of a triode sputter ion pump.
Triode sputter ion pumps include:
- External magnets which intensify the discharge.
- Stainless steel, internally welded pump envelope.
- Two-sputter cathodes consisting of multiple strips of titanium held at a negative high voltage.
- An array of stainless-steel anode cells which are at ground potential.
As in the diode. a plasma discharge is created within the anode cells upon the application of high voltage to the cathode grid. The ions impinge upon the sputter cathode and dislodge titanium atoms as in the diode. At this point, there is a significant difference. Because the cathode grids are open considerable titanium reaches the pump walls where it cannot be further disturbed by ion bombardment. This has at least two favorable results: Undisturbed deposits mean less regurgitation of previously pumped gases; and deposits at the pump wall mean that titanium compounds are kept cooler in the starting mode.
A further benefit of the open cathode grid structure is a higher production rate for energetic neutral atoms. These energetic neutrals are produced by glancing collisions at the cathode and are readily buried at the pump wall This burial without reemission accounts for the triode’s high speed for noble gases.
Thermionics manufactures a full line of sputter ion pumps.
These are:
- Standard Diode Pumps (IP Series)
- Noble Diode Pumps (NP Series)
- Hydrogen Diode (HP Series)
- Triode Pumps (TP Series)
Each pump Is
- Designed for a specific aoplicat1on
- Able to provide continuous and contaminant
- free pumping
- Reliable from 1 micron to 2 x 10-11 Torr (measured)
- Free from backstream1ng
- Bakeable to 300°C assembled
- Bakeable to 450°C magnets removed
- Able to act as its own vacuum gauge
Choosing the Right Pump for Your Application
As a full line vacuum manufacturer, TLI can recommend the best pump for your job without reservation or bias We cover the full line of ion pumps. The table below gives a comparison between pumps for different pumping applications.
Pump Characteristics Normalized to the Standard Diode
(% of Air) |
1 x 10-6 Torr |
||||
|---|---|---|---|---|---|
Light Duty Only |
10 μ |
hours |
|||
Recommended |
5 μ |
hours |
|||
10 μ |
hours |
||||
Light Duty Only |
50 μ |
hours |
Standard Diode Pump
Standard Diode Pump use two titanium cathodes in each pumping element. They are the pump of choice for most applications for their long life, reliability, and high speed per unit price. Not recommended when significant amounts of hydrogen or noble gases are to be pumped or where frequent high starting loads are encountered.
Noble Diode Pumps
Noble Diode Pumps use one titanium and one tantalum cathode to improve pumping speed for noble gases. Increased speed eliminates the pressure and speed instabilities shown by standard diodes when pumping against a prolonged air leak while retaining the long life and reliability of the standard diode. Most stable pump for noble gas loads.
Noble diodes were deve1oped to pump noble gases in every instance but one they are identical to a standard diode. The difference is the use of one tantalum cathode in place of one of the two titanium cathodes in the standard pump. The tantalum. because of its larger atomic number. produces a greater number of energetic neutrals that can now bury themselves in locations that are less subject to resputtering.
Tantalum is somewhat less effective than titanium as a getter. Therefore, the speed of the noble diode is approximately 5% lower than the standard diode. Another fact of note is the lessened solubility of hydrogen in tantalum at elevated temperatures. Since elevated temperatures will be encountered during prolonged starting, it would be wise to avoid applications that require this pump to handle large amounts of hydrogen. Naturally, applications differ. Should you have any questions about the applicability of a pump for your use, please give us a call. We have a wide variety of prior applications to draw upon.
Hydrogen Diode Pumps
Hydrogen Diode Pumps are also similar to standard diodes. There are two differences in construction. The first is a thicker cathode. Because hydrogen diffuses into titanium like water into a sponge, the more titanium the more capacity for hydrogen the pump has. This absorption of hydrogen can lead to a structural problem, however, Titanium swells and distorts as it absorbs large amounts of hydrogen. Therefore, hydrogen pumps have special structural modifications to prevent distortion from causing electrical shorts.
These extra construction details make the hydrogen diode a good choice for long pump life applications. Examples might be pumps operating in radioactive environments or operating at remote locations Recommended when major gas load is hydrogen or hydrogen-containing gases such as water vapor Also pumps other non-noble gases
Triode Pumps
Triode Pumps use reverse electrical polarity and a radically different cathode design to achieve two important advantages over the diode pumps: (1) an electrically isolated cathode allows the starting glow discharge to be confined at significantly higher pressures, resulting in shorter starting times; (2) the sputter cathode design allows noble and non-noble gases to be permanently buried without resputtering. Triodes have the highest speeds for noble gases and freedom from argon instability in the event of an air leak Disadvantage: because of the sputter cathode design, the triode pump requires more frequent service.
Starting Performance
For many applications, starting performance is not a criterion when choosing a pump because the system is seldom brought up to ai. In other applications, such as surface analysis equipment or process related equipment, the time lost waiting for pumpdown to operating pressures is a very important consideration. In some cases, the labor costs saved by shorter pumpdowns can earn back the differential in the price of a triode pump in less than a month. The figure below demonstrates the value of the higher triode speed at higher pressures. Two pumps, a 60 I/sec triode and a 440 I/sec diode, were attached to the same large vacuum chamber. The smaller triode pump first pumped the chamber down. It was then released to air and pumped down by the larger diode. The triode, despite a 7-to-1 speed disadvantage. showed a very significant advantage over the diode in reaching the 10-5 region. The diode, despite its larger rated speed at lower pressures, took more than three hours to match the pressure achieved by the smaller pump. Naturally, pumpdown performance depends on other variables such as the roughing system, prior chamber exposure. etc . but if pumpdown time is important to your ion pumped system, a triode is the pump of choice.
Summary
Any of the ion pumps will do a good job in a general pumping application. (1) The diode pump is usually the choice when there is no requirement for pumping noble gases and when the initial price is an important criterion. (2) The noble diode is designed for pumping noble gases with longer life, and does so at a lower price (3) The hydrogen diode is used not only where large amounts of hydrogen are expected, such as tube processing, but also in applications where a longer lifetime is needed. (4) The triode is the pump of choice when frequent pumpdowns are expected. It is also excellent for noble gas pumping.
Sizing the Pump to Your Applications
After you have selected the type of pump you need for your application; you must then determine its size. This section gives sample computations for three common pumping applications. They are: (1) pumping with no load other than wall out-gassing: (2) pumping with an additional load, and (3) pumping with an additional pump of another type. If you are accustomed to the large speeds required by diffusion pumped systems, you may be surprised at the capabilities of TLI ion pumps. Ion pumps do not need elaborate baffles to protect the system from contamination, hence effective speeds are not diminished by those conductance considerations.
System Information Needed
- What species of gases will be found in your system and what is the anticipated outgassing rate? (This, 1n large part, can be determined from your expected bakeout temperature.)
- What is the surface area of the system?
- What additional gas loads, if any, will be introduced as a result of the work done in the system?
- What is the conductance between the pump and the system?
- What is the desired process pressure and/ or desired ultimate pressure?
Example: Speed Needed for Wall Outgassing Only
Internal surface area: A = 100,000 cm2
Bakeout temperature: T = 200°C
Outgassing rate of 304 SS*: R = 2 x 10-13Torr I/sec cm2
Desired base pressure: P = 2 x 10-10 Torr
Conductance between pump and chamber: C = 4800 I/sec**
The speed needed at base pressure is given by:
Inserting values:
This is the speed needed at the chamber. To adjust for conductance losses, use the formula:
Looking at the figure below, pumping speed as a function of pressure, we see that the speed at 2 x 10-10 is 70% of nominal rated speed. Dividing 102 by .7 yields a minimum nominal speed of 146 I/sec to achieve the desired base pressure. A 150 I/sec pump would, therefore, meet this requirement.
Example: Pumping with an Ion Pump and a Titanium Sublimation Pump
Ion pumps are often used in combination with titanium sublimation pumps. Sublimation pumps used with properly designed deposition shrouds can have very high speeds for getterable gases such as H2, H2O, CO. CO2, N2, and O2. They do not pump noble gases or saturated hydrocarbons, such as methane. In fact, sublimation pumps, like ion gauges, can produce methane at their hot surfaces. These non-getterable gases are pumped by ion pumps. For the purposes of speed calculation, it would be tempting to assume that the speeds are additive when applied to the partial pressures of the getterable and non-getterable gases. For several reasons too complex to consider here, this is not the case. As a practical matter, sublimation pumps are a major help in pumpdown at high pressure and will lower the pressure by a factor of ten at low pressures.
Thus, as a rule of thumb, you may reduce ion pump size by about half if a sublimation pump is added to a system. This will, in general, lower the initial cost of a system while still assuring the same base pressure. This rule of thumb may seem conservative in view of the high speeds achieved with sublimation pumping, but it can be qualitatively understood if it is realized that the ion pump is pumping a mixture of gases for which it has a lower speed at a lower pressure.
Air Pumping Speed as a Percentage of Rate Speed vs. Pressure
*Outgassing rates for 304 stainless steel:
2 x 10-13 Torr I/sec cm2 after an 850°C- 900°C bakeout in a vacuum furnace for 2 hrs. Ref: R.L Samuel, Vacuum. Vol 20. 1970, p 295.
3 x 10-12 Torr I/sec cm2 after a 250°C bakeout for 30 hrs. Ref: J.R Young. JVST. Vol 6, 1969, p 398
**This is the approximate conductance of a 6″ full opening gate valve. Use of a valve is good practice on ion pumped systems which are often brought up to atmospheric pressure.
Long-Term Pumping Speed as a Percentage of Rated Ion Pump Speed for Various Gases
| Carbon Monoxide | |||
| Water Vapor | |||
| Carbon Dioxide | |||
| Hydrogen | |||
| Misc. Hydrocarbons | |||
| Oxygen1 | |||
| Helium2 | |||
| Argon |
1 – Low oxygen speed is thought to be due to the formation of a sputter-resistant oxide film on the cathode. Speed for oxygen when other gases are present is undoubtedly much higher.
2 – In all ion pumps, the pumping mechanism for helium limits the long-term pumping speed for helium. When helium saturation occurs, which is a function of the load over time, pumping speeds for helium will be less than the percentages shown.
3 – Diodes are quite satisfactory for pumpdown of systems backfilled with argon. They do exhibit instabilities when pumping against air or pure argon leaks.
Performance Curves, Triode Pumps
Starting and Throughput
Pumpdown time is determined by net pumping speed in the 10-3 to 10-4 Torr range. Net pumping speed is rated speed minus outgassing due to power dissipation in the pump. Thus, net speed is time-dependent. Any factor such as a leak or a dirty system that prolongs the time spent in the high-power region can seriously lengthen pumpdown times.
30 Years Experience in Ion Pump Development and Manufacturing — 1000’s Sold!
- Most complete line of ion pumps, power supplies and accessories
- Fastest, most reliable ion pumps available
- Thermionics ion pumps are 100% compatible with your existing Varian or Perkin-Elmer power supply
Sizes
- 0.2 I/sec to 1,000 I/sec
Models
- Diode configurations
- Standard (IP Series)
- Nobile (D-I) (NP Series)
- Hydrogen (HP Series
- Triode Configurations
- Standard (TP Series)
- Greater (GT Series)
- Combo-Vac Pumps
- Combining a Nobile (D-I_ or Triode ion pumps, Titanium sublimation pump (TSP), and Cryo panel
Each pump is:
- Able to provide continuous and contaminant-free pumping
- Reliable from 1 micron to 2 x 10-11 Torr (measured)
- Free from backstreaming
- Bakeable to 300°C assembled
- Bakeable to 450°C magnets removed
- Able to act as its own vacuum gauge
Construction
- All pumps are manufactured to exacting UHV standards
- Pump bodies, flanges and element anodes are made of 304 stainless steel
- Double re-entrant electrical einsulators are made of high-quality aluminum oxide, with sputter shields
