Reducing Plasma Formation in an Ion Pump

Abstract

An ion pump controller is configured to alternate multiple times between increasing and decreasing the potential difference between the anode and the cathode of the ion pump during the start of pumping.

Description

BACKGROUND OF THE INVENTION

[0001] Ultra-high vacuum is a vacuum state characterized by a pressure below 10 -7 Pascals (10 -9 mbar, approximately 10 -9 Torr). Ion pumps are used in some environments to establish ultra-high vacuum. In an ion pump, an array of cylindrical anode tubes is arranged between two cathode plates such that the opening of each tube faces one of the cathode plates. A potential is applied between the anode and the cathode. At the same time, magnets on opposite sides of the cathode plates generate a magnetic field aligned with the axis of the anode cylinders.

[0002] The ion pump operates by trapping electrons within the cylindrical anode through a combination of potential and magnetic field comparable to a Penning cell setup. When gas molecules drift into one of the anodes, the trapped electrons strike the molecules, causing the molecules to ionize. The resulting positively charged ions are accelerated towards one of the cathode plates by the potential between the anode and the cathode, leaving the stripped electrons in the cylindrical anode for further ionization of other gas molecules. The positively charged ions are ultimately captured by the cathode and thus removed from the evacuated space. Typically, the positively charged ions are captured by a sputtering event in which the positively charged ions cause material from the cathode to be sputtered into the vacuum chamber of the pump. This sputtered material coats the surfaces within the pump and is used to capture additional particles moving within the pump.

[0003] The foregoing discussion provides only general background information and is not intended to assist in determining the scope of the claimed subject matter. The claimed subject matter is not limited to embodiments that solve any or all of the disadvantages recited in the background. SUMMARY OF THE INVENTION

[0004] An ion pump controller is configured to alternate multiple times between increasing and decreasing the potential difference between the anode and the cathode of the ion pump during the start of pumping.

[0005] According to a further embodiment, a method of operating an ion pump includes increasing and decreasing the voltage between the anode and the cathode of the ion pump and then determining that the state of the ion pump has changed. In response to the change in the state, a steady-state voltage is applied between the anode and the cathode.

[0006] According to yet a further embodiment, an ion pump controller is configured to automatically alternate between providing power to at least one of the anode and the cathode in the ion pump and not providing power to the at least one during startup.

[0007] This invention content is provided to introduce, in a simplified form, a selection of concepts further described below in the detailed description. This invention content is neither intended to identify key features or essential features of the claimed subject matter nor intended to be used to assist in determining the scope of the claimed subject matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 A cross-sectional view of an ion pump is provided.

[0009] Figure 2 A block diagram of a controller component according to one embodiment is provided.

[0010] Figure 3 Graphs of control signals, output voltage, and anode-cathode voltage are provided along a common timeline.

[0011] Figure 4 Flowcharts of methods according to various embodiments are provided. DETAILED DESCRIPTION

[0012] Figure 1 A cross-sectional view of an ion pump 100 attached to an ion pump controller 101 according to one embodiment is provided. The ion pump 100 includes a vacuum chamber 102 defined by a chamber wall 104, and the chamber wall 104 is welded to a connection flange 106 for connection to a system to be evacuated. Two ferrite magnets 108 and 110 are located outside the chamber wall 104 and are mounted on opposite sides of the ion pump 100. A magnetic flux guide 112 is positioned outside each of the ferrite magnets 108 and 110 and extends below and / or to the side of the ion pump 100 to guide magnetic flux between the outsides of each of the ferrite magnets 108 and 110, as indicated by arrows 130 and 132. The ferrite magnets 108 and 110 generate a magnetic field B that passes through the vacuum chamber 102.

[0013] Inside the vacuum chamber 102, an array of cylindrical anodes 114 is positioned between two cathode plates 116 and 118 such that the openings of the anode cylinders face the cathode plates.

[0014] The cylindrical anodes 114 and the chamber wall 104 are maintained at a positive potential, while the cathode plates 116 and 118 are maintained at ground potential. According to some embodiments, the potential difference between the cathode plates 116 and 118 and the cylindrical anodes 114 is 3 - 7 kV.

[0015] In operation, flange 106 is connected to the flange of the system to be evacuated. Once the flanges are connected, particles within the system to be evacuated travel into vacuum chamber 102 and ultimately move within the interior of one of cylindrical anodes 114. The combination of magnetic field B and the potential between anode 114 and cathode plates 116 and 118 causes electrons to be trapped within each of cylindrical anodes 114. While trapped within cylindrical anode 114, the electrons are in motion such that when a particle enters cylindrical anode 114, it is struck by the trapped electrons, causing the particle to ionize. The resulting positively charged ions are accelerated by the potential difference between anode 114 and cathode plates 116 and 118, causing the positively charged ions to move from the interior of cylindrical anode 114 toward one of cathode plates 116 and 118. The ions strike cathode plate 116 / 118, causing material from cathode plate 116 / 118 to sputter away from the plate and causing the ions to become embedded within cathode plate 116 / 118.

[0016] Ion pump controller 101 provides and monitors the current and voltage applied to anode 114 and cathode plates 116 / 118 via conductors 216 and 218. Ion pump controller 101 uses the measured current between anode 114 and cathode plates 116 / 118 to calculate the pressure within vacuum chamber 102. According to some embodiments, ion pump controller 101 includes a touch screen to receive control instructions and display the status of ion pump 100, including the current and voltage between anode 114 and cathode plates 116 / 118 and the pressure within vacuum chamber 102. Ion pump controller 101 also includes a network communication interface for communicating with various computing devices. Such a computing device may send command signals to ion pump controller 101 to control the operation of pump 100 and may receive values representing the current status of ion pump controller 101 and ion pump 100 from ion pump controller 101.

[0017] Prior art ion pumps have difficulty starting at pressures above 10 -5 millitorr. At such pressures, with a high voltage applied, a strong plasma forms within the pump, which conducts current between the cathode and the anode. This limits the magnitude of the potential difference that can be formed between the anode and the cathode, which in turn limits the amount of sputtering that occurs. Additionally, the formation of the strong plasma generates heat within the ion pump, which further increases the pressure. This pressure increase allows the plasma to conduct more current, further limiting the magnitude of the voltage between the anode and the cathode in the pump.

[0018] The embodiments described herein limit the formation of plasma during ion pump startup so that less electrical energy supplied to the pump is wasted as heat. In particular, instead of continuously applying power between the anode and the cathode, the embodiments apply pulses of the supply voltage between the anode and the cathode. Each pulse is sufficient to initiate sputtering within the pump while preventing or at least limiting the formation of a strong plasma within the ion pump. When power pulses are applied across the anode and the cathode, the pump monitors the state of the ion pump, such as the voltage between the cathode and the anode when power is supplied to the pump. When the monitored state reaches a threshold level, power is continuously applied between the anode and the cathode.

[0019] Figure 2 A circuit diagram of an ion pump controller 101 according to one embodiment is provided. The ion pump controller 101 receives power from a power supply 200. According to various embodiments, the power is 100 - 240 VAC; in other embodiments, the power is 12 or 24 VDC. The ion pump controller 101 is also connected to ground via the same plug that connects the ion pump controller 101 to the power supply 200.

[0020] Power from the power supply 200 is provided to a voltage regulation unit 202, which provides a regulated DC voltage to power the various circuits of the ion pump controller 101. The voltage regulation unit 202 also provides a regulated DC voltage output 204 to a switch 206. The switch 206 consists of one or more solid - state switches, such as power MOSFETs, controlled by a control signal 210 from a switch controller 212. The output 205 of the switch 206 is a pulsed signal that alternates between the voltage of the regulated DC voltage output 204 and ground based on the control signal 210.

[0021] The pulsed signal 205 is provided to a step - up transformer 208, which increases the voltage to produce a high - voltage AC signal 207. The high - voltage AC signal 207 is provided to a high - voltage multiplier 214, which produces a DC power output 209 having an open - circuit voltage that is a multiple of the magnitude of the high - voltage AC signal 207.

[0022] The DC power output 209 is connected to a voltage and current meter 220, which measures the voltage and current of the DC power output 209.

[0023] According to one embodiment, the voltage increase provided by boost transformer 208 is based in part on the frequency and / or pulse width of the pulses in pulse signal 205. Thus, switch controller 212 can change the voltage output by boost transformer 208 by modifying the frequency and / or pulse width of pulse signal 205. According to one embodiment, switch controller 212 modifies the frequency and / or pulse width based on the difference 229 between the target voltage 231 for DC power output 209 provided by microprocessor 222 and the measured voltage 233 of DC power output 209 provided by voltage and current meter 220. In Figure 2 this figure, this difference is shown as being generated by separate adder 228, but in other embodiments, the difference is determined within switch controller 212. When difference 229 indicates that measured voltage 233 is less than target voltage 231, switch controller 212 changes control signal 210 to adjust the switching of switch 206 such that the voltage on DC power output 209 increases. When difference 229 indicates that measured voltage 233 is greater than target voltage 231, switch controller 212 changes control signal 210 to adjust the switching of switch 206 such that the voltage on DC power output 209 decreases.

[0024] As further discussed below, when the pressure within the pump is above a certain threshold, such as at pump startup, the voltage of DC power output 209 is pulsed. During such pulses, switch controller 212 will either suspend adjusting the switching of switch 206 or will adjust the switching only based on the maximum voltage measured during each cycle of the pulsed DC power output 209.

[0025] Voltage and current meter 220 provides digital values representing the measured current and voltage of DC power output 209 to microprocessor 222 at regular intervals. Microprocessor 222 uses the current value to calculate the pressure in pump chamber 102 and changes the graphics on user interface 224 to display the values of current, voltage, and pressure. Microprocessor 222 also receives instructions for starting and stopping ion pump 100 via user interface 224 and / or via communication port 226.

[0026] The microprocessor 222 uses the measured voltage of the DC power output 209 to control the pulse switch 240, which alternately connects and disconnects the DC power output 209 from the conductor 216. According to one embodiment, the pulse switch 240 is a physical relay, while in other embodiments, the switch 206 consists of one or more solid-state switches such as power MOSFETs and high-voltage insulated gate bipolar transistors (IGBTs). According to one embodiment, the microprocessor 222 sets the control signal 241 to cause the pulse switch 240 to disconnect the DC power output 209 from the conductor 216 when the voltage of the DC power output 209 drops below a threshold voltage. After a time period, the microprocessor 222 changes the control signal 241 to cause the pulse switch 240 to reconnect the DC power output 209 to the conductor 216. These two steps are repeated, thereby generating voltage pulses on the conductor 216 that help prevent the formation of strong plasma when the pressure in the pump chamber is high, such as during pump startup. When the voltage on the DC power output 209 no longer drops below the threshold voltage when the pulse switch 240 is closed, the microprocessor 222 sets the control signal 241 to a constant value to maintain the pulse switch 240 in the closed position.

[0027] Figure 3 Three curves 302, 304, and 306 are provided along a common timeline 308. Curve 302 represents the control signal 241 and is shown transitioning between an off state 310 and an on state 312. The off state 310 represents that value of the control signal 241 that causes the pulse switch 240 to be off, and thus it does not connect the DC power output 209 to the conductor 216. The on state 312 represents that value of the control signal 241 that causes the pulse switch 240 to be on to connect the DC power output 209 to the conductor 216. Curve 304 is a curve of the voltage on the DC power output 209, and curve 306 is a curve of the voltage on the conductor 216, which voltage on the conductor 216 is also the potential difference between the anode 114 and the cathode plates 116 / 118.

[0028] Figure 4 A flowchart of a method for starting an ion pump according to one embodiment is provided. Before Figure 4 the method, such as at Figure 3 time point 314, no power is applied to the ion pump, and the ion pump is considered off. At Figure 3 time point 316 and Figure 4In step 400, the microprocessor 222 turns on the pump based on the input received through the user interface 224 and / or the instructions received through the communication port 226. At step 402, a target voltage is generated on the DC power output 209 while the microprocessor 222 issues a value on the control signal 241 such that the pulse switch 240 is open. Since the pulse switch 240 is open, the voltage on the DC power output 209 increases while the voltage on the conductor 216 remains at ground / neutral.

[0029] When the DC power output 209 reaches the target voltage, the microprocessor 222 sends a value on the control signal 241 at time point 318 to close the pulse switch 240, step 404. This causes the DC power output 209 to be connected to the conductor 216, resulting in a decrease in the voltage on the DC power output 209 and an increase in the voltage on the conductor 216 until the DC power output 209 and the conductor 216 reach voltage 319. The magnitude of voltage 319 is controlled by the amount of current flowing through the gas in the chamber 102 between the anode 114 and the cathode plates 116 / 118. Generally, for a higher gas pressure in the chamber 102, the current is higher. The current is associated with the flow of positive ions towards the cathode plates, resulting in the capture of ions at the cathode plates and / or the sputtering of other particles trapped in the chamber 102. Thus, the increase in the voltage on the conductor 216 causes a decrease in the pressure in the chamber 102.

[0030] In step 406, the microprocessor 222 detects that the voltage 319 of the DC power output 209 is lower than the threshold voltage 321 and, in response, sends a value on the control signal 241 at step 408 to open the pulse switch 240. This disconnects the connection between the DC power output 209 and the conductor 216, resulting in the voltage on the DC power output 209 returning to the target voltage and the voltage on the conductor 316 returning to ground / neutral.

[0031] In step 410, the microprocessor 222 waits for a time period such as 0.5 seconds and then returns to step 404 and recloses the pulse switch 240. When the pulse switch 240 is reclosed, the DC power output 209 is reconnected to the conductor 216, resulting in a decrease in the voltage on the DC power output 209 and an increase in the voltage on the conductor 216 until the DC power output 209 and the conductor 216 reach voltage 323. Voltage 323 is greater than voltage 319 because the pressure in the chamber 102 has been reduced by the voltage pulses on the conductor 216, thereby reducing the current between the anode 114 and the cathode plates 116 / 118.

[0032] At step 406, the microprocessor 222 again detects that the voltage 323 of the DC power output 209 is below the threshold voltage 321, and in response, sends a value on the control signal 241 at step 408 to turn off the pulse switch 240. This disconnects the connection between the DC power output 209 and the conductor 216, causing the voltage on the DC power output 209 to return to the target voltage and the voltage on the conductor 216 to return to ground / neutral. At step 410, the microprocessor 222 again waits for a time period such as 0.5 seconds and then returns to step 404 and recloses the pulse switch 240.

[0033] The microprocessor 222 continues to repeat steps 404, 406, 408, and 410, thereby generating a pulse sequence on the control signal 241 and corresponding voltage pulse sequences on the DC power output 209 and the conductor 216 during the time period 325. Thus, the microprocessor 222 alternates between providing power to the anode 114 and not providing power to it, thereby alternating between increasing and decreasing the potential difference between the anode and the cathode when starting the ion pump. Additionally, as the pressure in the chamber 102 drops, each successive pulse in the voltage pulse sequence on the conductor 216 has a slightly larger voltage.

[0034] Finally, at the time point 321, when the pulse switch 240 is closed, the voltage on the DC power output 209 does not drop below the threshold voltage 321. Thus, the microprocessor 222 does not re-open the pulse switch 240 after step 406, but instead closes the pulse switch 240 at step 412. This causes the voltages of the DC power output 209 and the conductor 216 to rise slowly until the voltages reach the target voltage at time 326.

[0035] In some embodiments, the microprocessor 222 opens and closes the switch 206 at regular intervals, where the length of time the pulse switch 240 is closed is equal to the length of time the pulse switch 240 is open. In other embodiments, the amount of time the pulse switch 240 is open is different from the amount of time it is closed. In additional embodiments, the amount of time the pulse switch 240 is closed during each pulse varies over time. According to various embodiments, the pulse switch 240 is closed for between 0.005 seconds and 2 seconds, and the pulse switch 240 is open for between 0.5 seconds and 2 seconds.

[0036] By applying voltage pulses at the start of the ion pump, this embodiment can limit or completely prevent the formation of plasma within the ion pump and thus reduce the amount of energy lost as heat when starting the ion pump. This is not only more efficient but also helps reduce damage to the ion pump due to overheating. Although the above embodiment describes applying voltage pulses during pump startup, in other embodiments, the voltage pulses can be applied at any time the voltage on the DC power output 209 is below the threshold voltage 321.

[0037] In the foregoing discussion, the pulse switch 240 is located between the high voltage multiplier 214 and the conductor 216. In another embodiment, the pulse switch 240 is located between the step-up transformer 208 and the high voltage multiplier 214. Moving the pulse switch 240 to a location before the high voltage multiplier 214 causes the pulse switch 240 to operate at a lower voltage, thereby reducing the cost of the pulse switch 240. However, placing the pulse switch 240 before the high voltage multiplier 214 also increases the delay between the switching of the pulse switch 240 and the resulting change in the voltage of the conductor 216. In other embodiments, the pulse switch 240 is located between the switch 206 and the step-up transformer 208. Again, this further reduces the voltage requirements of the pulse switch 240, thereby reducing the cost of the pulse switch 240 while further increasing the delay between the switching and the change in voltage on the conductor 216.

[0038] In the foregoing discussion, the cathode plates 116 / 118 are described as being grounded while the anode 114 is at a positive voltage. In other embodiments, the anode 114 is maintained at ground while a negative potential is applied to the cathode plates 116 / 118 with each pulse. The choice of whether to apply a negative voltage to the cathode plates 116 / 118 or a positive voltage to the anode 114 is a matter of design preference. Thus, power can be applied to either the cathode plates 116 / 118 or the anode 114. Herein, regardless of the polarities of the anode 114 and the cathode plates 116 / 118, the magnitude of the voltage between the anode 114 and the cathode plates 116 / 118 is referred to as the potential difference between the anode 114 and the cathode plates 116 / 118.

[0039] Although the elements have been shown or described above as separate embodiments, several parts of each embodiment may be combined with all or part of other embodiments described above.

[0040] Although the subject matter has been described in language specific to structural features and / or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. On the contrary, the specific features and acts described above are disclosed as example forms for implementing the claims.

Claims

1. An ion pump controller configured to alternate multiple times between increasing and decreasing the potential difference between the anode and cathode of an ion pump during the start of pumping to limit the formation of plasma in the pumping chamber of the ion pump while causing ions near the anode to move towards the cathode, wherein the ion pump controller is configured to alternate between increasing and decreasing the potential difference between the anode and cathode until a condition is met, and then increase the potential difference between the anode and cathode until the potential difference between the anode and cathode reaches a target potential difference, the condition being whether the potential difference between the anode and the cathode is greater than a threshold value.

2. The ion pump controller according to claim 1, wherein each successive increase in the potential difference between the anode and the cathode results in a greater potential difference between the anode and the cathode than the previous increase in the potential difference.

3. The ion pump controller according to claim 1, wherein the ion pump controller pauses between decreasing and increasing the potential difference.

4. The ion pump controller according to claim 1, wherein the ion pump controller increases the potential difference by controlling a switch to cause the switch to close, and the ion pump controller decreases the potential difference by controlling the switch to cause the switch to open.

5. A method of operating an ion pump, the method comprising: increasing and decreasing the potential difference between the anode and cathode of the ion pump; determining that the state of the ion pump has changed; and in response to the change in the state, increasing the potential difference between the anode and cathode to a target potential difference, wherein increasing and decreasing the potential difference includes applying voltage pulses between the anode and cathode to limit the formation of plasma in the ion pump, wherein determining that the state of the ion pump has changed includes determining that the potential difference between the anode and cathode is higher than a threshold voltage during a voltage pulse.

6. The method according to claim 5, wherein each voltage pulse is formed by closing a switch and then opening the switch.

7. The method according to claim 6, wherein each voltage pulse provides a greater potential difference than all previous voltage pulses.

8. The method according to claim 5, wherein each voltage pulse prevents the formation of plasma in the ion pump.

9. The method according to claim 5, further comprising, after decreasing the potential difference: pausing for a time period; and after the pause, increasing the potential difference between the anode and cathode and then decreasing the potential difference between the anode and cathode.

10. An ion pump controller configured to automatically alternate between supplying power to at least one of the anode and cathode in an ion pump and not supplying power to the at least one during startup of the ion pump to limit the formation of plasma in the ion pump, Wherein the ion pump controller is configured to determine the state of the ion pump and, in response to the determined state, stop alternating between providing power and not providing power and instead continuously provide power. The ion pump controller is configured to determine the state of the ion pump by determining a voltage between the anode and the cathode when power is provided. Wherein, the ion pump controller stops alternating between providing power and not providing power and instead continuously provides power in response to the voltage between the anode and the cathode becoming greater than a threshold.

11. The ion pump controller according to claim 10, further configured to pause for a time period of less than two seconds between not providing power and providing power.

12. The ion pump controller according to claim 10, comprising a solid state switch that provides power to the ion pump when closed and does not provide power to the ion pump when open.

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