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Chapter 9: Sputter Ion Pumps

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Chapter 9: Sputter Ion Pumps Las Positas College Vacuum Technology 60A & 60B Chapter 9: Sputter ion pumps The invention of the ion pump did not occur until the 1950's when the Varian company exploited the pumping characteristics of the Penning cold cathode gauge. While it had been known that the sputtering effect caused by high voltage in the Penning gauge resulted in burial of ionized gas molecules, and that gettering of gases such as oxygen by reactive metals (titanium) were both occurring, the concept of using these mechanisms to remove gas molecules from a system was ignored. Soon after commercial sputter-ion pumps were made available, they were applied to the then new field of space environment simulation. Ion pumps were fitted to large carefully constructed vacuum vessels, and pressures as low as 10-11 Torr were obtained. This enabled evaluation of satellite components, space suits and rocket components. Currently sputter ion pumps are used in a variety of UHV applications including surface science techniques (study of the first few atomic layers of a surface), and ultra-high purity thin film deposition processes (e.g. molecular beam epitaxy). Sputter-ion pumps are gas capture type vacuum pumps that function without pump fluids or any moving parts. They offer a clean, quiet, and safe way to achieve ultra-high vacuum (10-11 Torr). Sputter-ion, or getter-ion pumps are often used on vacuum systems that are sensitive to oil contamination that is possible from oil diffusion pumps and turbo pumps. In general, sputter-ion pumps are used in systems in which pumping speed is less important than cleanliness and achieving an extremely low base pressure. Sputter-ion pump characteristics The operational characteristics of a sputter-ion pump may be simply described by the following three factors: 1. Pumping speed As with any high vacuum pump, the pumping speed will determine the ultimate base pressure for a given gas load. Ion pumps, however, exhibit pumping speeds that are a function of the gas specie being pumped. Hydrogen is pumped at a relatively high rate compared to argon. It is critical to match the ion pump to the application. 2. Starting pressure Ion pumps must be rough pumped to an acceptable pressure (2x10-2 Torr or lower) before being turned on. Typically, this is done with a cryo-sorption pump or dry pump to eliminate the possiblity of oil backstreaming into the vacuum vessel or ion pump body. If well-trapped, oil sealed mechanical pumps may be used for roughing the ion pump and vacuum vessel. 3. Operating principle Sputter-ion pumps may be single or multiple cell types, and can be of diode or triode design. For the purposes of this laboratory, we will describe the operation of a single cell diode type sputter-ion vacuum pump. As with all gas capture pumps, the sputter-ion pump requires no backing pump, and does have a limited lifetime, based on its Page 120 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B capacity. The pump reduces pressure in a vacuum vessel by three distinct mechanisms: chemisorption, ion burial, and physical burial. During operation, titanium metal is sputtered (energetically liberated by ion bombardment) from the cathode surfaces. Titanium, being a very reactive metal, will chemically combine with active gas molecules present (oxygen and hydrogen) to form stable compounds, thus removing the gases from the vacuum vessel. Additionally, gas molecules and atoms are ionized by electrons that are constrained to orbit in the anode tube by a strong external magnet. These ionized gases are accelerated to the cathode by high voltage from the pump power supply. On impact, gas ions become buried in the titanium cathode, and also sputter (or knock free by momentum transfer) titanium atoms that act as getters as explained earlier. On start-up the amount of sputtering that occurs is very high, resulting in an initially high electrical current in the pump. Sputter-ion pumps will be warm or even hot to the touch during this phase of operation. After the gas pressure reduces, the pump will draw much less current from the power supply, and the operating voltage will increase. The amount of current that a sputter-ion pump draws during operation may be used, along with conversion charts supplied by the vendor, to determine pressure in the pump. Table 9.1 Comparison of pumping speeds for various gases. Gas relative pumping speed Gas relative pumping speed [L/s] [L/s] hydrogen 270 oxygen 60 air 100 helium 50 nitrogen 95 argon 1 Sample problems: 9.1 Would you place a spinning rotor gauge in close proximity to a sputter-ion pump? why or why not? 9.2. Does a sputter-ion pump have a limited lifetime? Page 121 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B HV feedthrough Pump body Anode Cathodes gas from vacuum vessel Figure 9.1 Cutaway view of a single cell sputter-ion vacuum pump. High voltage applied between the anode and cathodes generates primary electrons that are constrained to spiral orbits within the anode. Collisions of these primary electrons with neutral gas atoms causes the atoms to become ionized. The positive gas ions are accelerated into the cathodes, resulting in burial of the gas ion and/or sputtering of the cathode material (titanium). Titanium atoms gas molecules or atoms positive gas ions electrons magnetic field A C B cathodeAnode tube cathode + HV Figure 9.2 Detail of the processes in a sputter-ion pump. Page 122 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B At location C a primary electron ionizes a gas atom; at B an ion impacts the cathode ejecting a titanium atom, and at A a gas ion is buried in the cathode. For further reading: Sputter-Ion pumps- Experimental Vacuum Science and Technology,Dillon, J.A., and Harwood,V.J., Prentice Marcel Dekker, Inc., NY, 1973. Answers to Chapter 9 sample problems 9.1 The strong magnetic field of a sputter-ion pump's permanent magnet may interfere with proper operation of a spinning rotor gauge. 9.2 Yes, the lifetime of a sputter-ion pump is limited. Some pump designs allow for replacement of internal components that are consumed. Laboratory Exercise 9.1: Pump Identification, Inspection and measurement of base presure. Select a sputter-ion vacuum pump to use for the next two exercises. A. Pump Identification: Who is the manufacturer? What is the pump model number? Locate the manufacturer's literature from the bookcase, and find the appropriate reference information. What is the advertised pump speed? What is the base pressure listed? What is the cathode material? Is it a single or multiple cell pump? Is it a diode or triode design? B. Physical Inspection of Sputter-Ion Pump: Inspect the pump for signs of damage or misuse. Check power supply electrical cables for cracks in insulation. Is the power supply appropriate for the pump? What is the input power requirement of the power supply? What is the power supply output voltage and current at start-up? What are theses values during operation at 10-6 Torr (approximately)? C. Measurement of ultimate pressure: assemble a system similar to that shown in figure 9.3. It would be preferable to use cryo-sorption pumps to rough the vacuum vessel and the sputter-ion pump to a pressure of less than 20 microns. A trapped mechanical pump will suffice if cryo-sorption pumps are unavailable. Page 123 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B IG1 TC1 Figure 9.3 Set-up for sputter-ion pump experiments. C. Measurement of ultimate pressure (cont.) Evacuate the sputter-ion pump and the vacuum vessel to a pressure of less than 20 microns (2 x10-2 Torr). Valve off the roughing pump, and start the sputter-ion pump. Record vessel pressure, and sputter-ion pump power supply voltage and current as a function of time. Data Table 9.1 Time Pressure Current Voltage Power [seconds] [Torr] [amps] [volts] [Watts] Page 124 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B Time Pressure Current Voltage Power [seconds] [Torr] [amps] [volts] [Watts] Calculate power for each of your readings (power = current*voltage). Plot the data you have collected as sputter-ion pump current, voltage and power as a function of time. Also plot vessel pressure vs. time. If the equipment and materials are available, isolate the sputter-ion pump from the evacuated chamber, and back fill the process chamber with an inert gas such as helium or argon and repeat the experiment. Page 125 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 .
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