Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J41/00—Discharge tubes for measuring pressure of introduced gas or for detecting presence of gas; Discharge tubes for evacuation by diffusion of ions
  • H01J41/12—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps
  • H01J41/18—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps with ionisation by means of cold cathodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J41/00—Discharge tubes for measuring pressure of introduced gas or for detecting presence of gas; Discharge tubes for evacuation by diffusion of ions
  • H01J41/12—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps
  • H01J41/18—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps with ionisation by means of cold cathodes
  • H01J41/20—Discharge tubes for evacuating by diffusion of ions, e.g. ion pumps, getter ion pumps with ionisation by means of cold cathodes using gettering substances

Definitions

• Muffin tin style cathode element for diode sputter ion pump Muffin tin style cathode element for diode sputter ion pump.
• the invention relates to ion pumps used primarily in high and ultra-high vacuum systems.
• Ion pumps are used in a variety of systems that require a high or ultra-high vacuum. Such systems include focused ion beam systems, electron microscopes, accelerators, molecular beam epitaxial deposition systems, and other analytical, fabrication and research systems and instruments. Ion pumps are typically used at pressures of between 10 "4 Torr and 10 " " Torr, with pressures of between 10 "7 Torr and 10 "9 Torr being common in, for example, focused ion beam systems.
• One type of ion pump is the diode sputter ion pump.
• FIG. 1 shows a typical diode sputter ion pump 10 consists of two cathodes 12, one on either side of an anode 14. FIGS.
• Each anode typically includes multiple anode cells 16, each having a longitudinal axis perpendicular to the planes of the cathodes.
• a positive voltage is applied to the anode 14
• a negative voltage or ground potential is applied to the cathodes 12
• a magnetic field is applied parallel to the longitudinal axes of the anode cells.
• electrons are trapped by the magnetic field, creating a stable electron cloud commonly known as a space charge cloud.
• the electron cloud is stable because the applied magnetic field constrains the electrons to travel in circular orbits each having a radius known as the cyclotron radius. Moreover, at higher pressures, individual electrons are shielded, through a phenomenon known as Debye screening, from the electric field of the anode by other electrons in the cloud.
• Debye screening a phenomenon known as Debye screening, from the electric field of the anode by other electrons in the cloud.
• the distribution of voltages and electrical charges in the system creates near the anode an area of steep potential gradient known as the anode sheath.
• the anode sheath tends to act as a boundary between the edge of the space charge cloud and the anode.
• the electrons tend to remain in the cloud until they migrate to the anode where they are counted as anode current.
• the sputtered titanium strikes and adheres to the anode, the cathodes, or elsewhere.
• the titanium is chemically active, gas molecules stick to and/or react with the titanium atoms, and are thereby fixed into a solid state and removed from the gas phase thus lowering the gas pressure in the vacuum chamber, essentially pumping gas from the chamber to create a better vacuum.
• Noble gas molecules that are not chemically active are removed from the gas phase by being buried under sputtered cathode material or by migrating into the crystal structure of the cathode after impact and being trapped within crystal structure defects in the cathodes.
• the pumping characteristics of an ion pump are determined primarily by the gas pressure in the vacuum chamber, the magnetic field, the voltages on the anode and cathodes, the shape of the anode cells, the distances between the anode cells and the cathodes, and the types of gases present.
• the pump cells are characterized by a sensitivity, which is defined as the ion current divided by the pressure and generally given in amps per Torr.
• the pump is generally characterized by a pumping speed which varies with the particular gas being pumped because of the different chemical reactions between the sputter cathode material and the particular gas molecule. The pumping speed is generally given in liters per second.
• the ion pump anodes of FIG. 2 is constructed as a series of rectangular cells. as described, for example, in U.S. Pat. No. 3,319,875 to Jepsen.
• the anode sheath does not conform well to the walls of a rectangular anode cell at the normal operating pressures of the ion pump, causing the anode sheath to be positioned away from the wall over much of the cell. Because the distance from the edge of the space charge cloud to the anode in many parts of its orbit is beyond the cyclotron radius, electrons in orbit around the edge of the space charge cloud do not have a high probability of striking the anode. Thus, the square cell anode is intrinsically inefficient, that is, has a low sensitivity, and square cell anodes have therefore been largely abandoned in favor of anodes that include a gathered cluster of cylindrical sectors as shown in FIGS. 3 and 4.
• the edge of the space charge cloud more closely follows the contour of the anode and therefore more electrons can be within the cyclotron radius of the anode while the parameters that determine the sheath, such as magnetic field, pressure, and voltage, are also conducive to effective pumping.
• the cylindrical cell maximizes the opportunity of the electrons to make their way to the anode itself, which is in close proximity to the space charge cloud.
• ion pumps having cylindrical diode cells are more sensitive than ion pumps having rectangular cells.
• Diode sputter ion pumps having cylindrical cell anodes display instabilities typically following pumping exposure to gas doses greater than the ultimate pressure of the vacuum system in which the pump is operating.
• the instabilities include current bursts, leakage currents, and arcs.
• the instabilities are disruptive to the devices to which the sputter ion pump is attached. For example, a current burst may stimulate a high voltage discharge that disrupts the electronic sub-systems of the system in which the pump is used. Such disruptions are a known cause of system failure.
• Another object of the invention is to enhance the stability of systems into which ion pumps are incorporated.
• a further object of the invention is to provide an ion pump that reduces or eliminates current leakage.
• Still another object of the invention is to provide an improved ion pump cathode that reduces the pump instabilities. Yet another object of the invention is to provide an improved anode that reduces ion pump instabilities.
• Yet a further object of the invention is to provide a charged particle beam system having improved stability. Still a further object of the invention is to provide a diode ion sputter pump that can pump noble gasses at an increased rate.
• the invention includes an ion pump that exhibits improved stability and reduced leakage current.
• Applicant has found that prior art ion pumps exhibit a continuous, unstable leakage current that persist throughout the lifetime of the typical ion pump.
• Applicant has determined that the instabilities, current bursts, leakage currents, and arcs are caused, to some degree, by explosive cathode arc emission and electron emission from structures whose shape and placement give rise to high electric fields.
• Such structures includes the various pump components, as well as dendritic protrusions that grow on the cathode plate, forming primarily at the edge of the cathode crater opposite to the regions of high plasma density.
• most of the surfaces that give rise to the electron emission processes are those that are cathodic and usually are a part of what is generally referred to as the cathode plate.
• a sputter ion pump includes a cathode in which the principal area of ion sputtering is in a region of low electric field, thereby preventing ion pump instabilities caused by high electric fields in such areas.
• the ion pump incorporates a cathode plate that includes a series of deformations or depressions directed away from regions of the anode having a high plasma density.
• the series of depressions typically centered on the axes of high plasma density regions, such as the anode cells or the anode intracellular regions, give the cathode an appearance like a muffin tin.
• the geometry of the cathode plate causes the area within the depression to be subject to an electric field significantly lower than that occurring near the surface of a flat cathode.
• a combination of the electrode shaping, the electrode attachment mechanism and the geometrical placement of the electrodes serves to reduce the current leakage in a diode sputter ion pump.
• the cathodic region is specifically geometrically designed and the cathode plate is specifically shaped to lower the high electric field in certain otherwise high electric field regions, thereby achieving a lower probability of leakage current and subsequent instabilities, current bursts, and arcs by comparison with diode sputter ion pumps than prior art pumps.
• This low leakage diode sputter ion pump is also relatively immune to instabilities, current bursts, and arcs.
• the low leakage diode sputter ion pump has the advantage in that it does not exhibit lifetime leakage current subsequent to gas dosing, such as occurs during the typical bakeout cycle of most system operation protocols.
• cylindrical anode ion pumps, between each cylinder Figure
• inter-cylindrical cell 18 typically having the shape of a hyper-extended square.
• inter-cylindrical cells 18 contribute to instabilities and are a liability to the clean and quiet operation of the diode sputter ion pump.
• the inter-cylindrical cells have been found, by applicant, to support a very dense plasma, which encourages the growth of dendrites on the cathode below the inter-cylindrical cell.
• the instabilities caused by the inter-cylindrical cells can also be eliminated by eliminating or minimizing the inter-cylindrical cell, or by altering the inter-cylindrical cells so that they do not support a dense plasma.
• a preferred anode cell design reduces or eliminates the inter-cylindrical cells entirely, while maintaining conformance of the electron cloud to the anode to allow electrons to leave the electron space charge cloud.
• FIG. 1 shows a typical diode ion sputter pump.
• FIG. 2 shows a cross section of a rectangular cell prior art anode for a diode ion pump such as the one shown in FIG. 1.
• FIG. 3 shows a cross section of cylindrical cell prior art anode for a diode ion pump such as the one shown in FIG. 1.
• FIG. 4 shows a cross section of close-packed cylindrical cell prior art anode for a diode ion pump such as the one shown in FIG. 1.
• FIG. 5 is an ion micrograph showing on an ion pump cathode dendrites in a region across from an intercylindrical anode cell as shown on FIG. 3.
• FIG. 6 is an ion micrograph showing the dendrites of FIG. 5 using increased magnification.
• FIG. 7 is a cross section of an ion pump having a muffin tin cathode in accordance with an embodiment of the invention.
• FIG. 8 is a cross sectional view variation of the muffin tin cathode of FIG. 7.
• FIG. 9 is a partial, cross-sectional view of the ion pump of FIG. 8.
• FIG. 10 shows another ion pump that illustrates various aspects of the present invention.
• FIG. 1 1 shows an anode having non-rectangular cells and a no intercylindrical cells.
• the dendrites formed in ion pumps are formed both inside of and within near proximity of the well known cathode crater, that is formed by ion sputtering 2) The dendrites do not exist outside of the visible zone of the cathode crater, 3) The dendrites are of sufficient aspect ratio to provide the field enhancement necessary to the achievement of field emission of electrons from the dendrites, and 4) The dendritic population density appears to be directly related to the plasma density of the cell. At the junctions between linked cylindrical anode cells 16 of an ion pump anode 14 of FIG.
• FIG. 5 is an ion micrograph that shows the field of dendrites clustered in a zone directly about the cathode crater formed by the plasma of an intercylindrical cell.
• FIG. 6 is a higher magnification ion micrograph of the dendrites of FIG. 5.
• the dendrites shown in this photo are markedly different than any feature shown in the M. Audi paper.
• Dendritic protrusions like those shown in FIG. 5 will field emit electrons under the applied field of the anode, particularly at lower operating pressures where the electric field at the cathode surface is greatest. The field emitted electrons lead to the macroscopically observable performance limitations of diode sputter ion pumps, namely instabilities, current bursts, leakage currents, and arcs.
• FIG. 7 shows a cross section of an ion pump 28, which includes two cathodes 30, typically comprised of titanium and sometimes tantalum or other metal.
• Each cathode 30 includes a flat area 32 and depressions 34 in the direction away from an anode 36, which includes multiple anode cells 38, each having a longitudinal axis 42.
• the depressions 34 correspond to regions 48 that are subject to heavy ion bombardment and where cathode craters are formed.
• the shape of depressions 34 create electrical potential wells that partially occludes the applied electric field from regions 48. Inside the electric potential well, the electric field strength is much lower than it is near flat area 32, so there will be little or no electric field on the dendritic protrusions that form in regions 48.
• the dendritic protrusions will not, therefore, be capable of electron field emission or explosive molten jet emission. Because ion bombardment regions 48 are typically aligned with the anode cells axes 42, which are in a regular pattern, the depressions 34 are also preferably in a regular pattern.
• Metal cathode plate 30, therefore, is shaped similar to a muffin tin where there are periodic depressions 34 centered on the longitudinal axis 42 of each anode cell 38.
• the muffin tin cathode could similarly have been named a Gaussian well cathode.
• the depression width, 2b should be at least twice the size of the typical cathode crater formed by ion bombardment, but less than the diameter of the anode cell.
• 2b .7a, where a is the diameter of the anode cell
• ion pump of FIG. 7 having a muffin tin cathode is not only is more stable than prior art diode ion pumps, it is also more effective than prior art diode pumps for pumping noble gases, particularly argon, possibly pumping argon at speeds that equal or exceed those of triode pumps typically used to pump noble gasses.
• FIG. 8 shows schematically a cross section of another ion pump 50 of the present invention.
• FIG. 8 shows a cathode 52 and walls 54 of anode cells 56.
• Cathode 52 includes depressions 34 concentric with the anode cells and also depressions 58 concentric with the inter-cylindrical cells.
• Ion pump 50 may also include optional high voltage shields 64 between the depressions.
• High voltage shields 64 are raised areas composed of a low sputter yield material, such as molybdenum, and serve to further reduce the eiectric field in depressions.
• FIG. 9 shows another, partial view of an ion pump 66 using optional high voltage shields 68 opposite anode walls 72 on a cathode 70.
• High voltage shields can extend partially or completely around the projection of anode walls 72 or cathode 70.
• a muffin tin cathode in accordance with the invention can be manufactured by stamping and drawing methods, but such manufacturing methods may leave residual stresses and or macroscopic or microscopic cracks in the cathode plate that would serve as sites of instability initiation.
• a preferred method of manufacturing the muffin tin cathode may be either a casting method or direct machining from a solid block of intended cathode material, with casting preferred.
• FIG. 10 shows a preferred embodiment of an ion pump 80 in accordance with the second aspect of the invention.
• Ion pump 80 includes two cathode plates 82 and anode cells 84.
• Cathode plate 82 extends at least one half of one anode cell diameter beyond the furthest extent of the grouping of anode cells 84.
• the edge 86 of the cathode plates 82 that is on the anode side is rounded away from the anode and polished to a smooth finish.
• the sharp edges of prior art cathodes near the edge of an anode cell are thought to field emit electrons under the applied field of the anode and more so at lower operating pressures.
• the cathode to anode gap is also to be designed to be free of any protrusions.
• the elemental support structures 88 that is, the electric contacts, are connected to cathode plate 82 on the side of the cathode plate that is away from the anode, thereby creating connections only outside of the cathode to anode gap.
• FIG. 10 shows the elemental support structure connections away from the anode side of the ion pump and that there is no intervening item in the cathode to anode gap.
• the anode cells are of a shape and arrange to eliminate or minimize the intercylindrical cell while permitting the contour of the anode sheath to follow the cell wall throughout most of the cell.
• FIG. 11 shows a cross section of an anode 90, having a quasi-cylindrical anode cell 92, that is, one that approximates a cylinder to the extent consistent with eliminating the inter-cylindrical cell.
• the diameter of the quasi-cylinder should vary less than approximately two electron cyclotron radii, typically about 4mm from the minimum diameter throughout most of the cell 82, although the diameter will have a greater variation along its long axis.
• the curved walls of the present invention allow the electron cloud to conform sufficiently to the anode wall so that electrons can efficiently leave the electron space charge cloud and strike the anode, while the quasi-cylindrical shape allows the anodes to fill the space of the anode without creating inter-cylindrical cells.
• the anode cells is preferably non- rectangular, thereby eliminating the inefficiencies inherent in prior art rectangular cell anodes.
• the anode of FIG. 11 was described in more detail in U.S. Provisional Pat. App. No. 60/125,317.
• a typical operating condition for use of a pump of the present invention in a focused ion beam system include a cathode voltage of 0 Volts ( held at ground potential, an anode voltage of 5000 Volts, a magnetic field value of 1200 Gauss, a gas pressure of 3 x 10-8 Torr, and an anode-to-cathode spacing of 14mm.
• the operating parameters vary with the application.
• Cathode voltages are typically at 0 Volts, anode voltages ranging from 3000 to 7500 Volts, magnetic field values ranging from 1000 to 1300 Gauss, pressures ranging from 10-3 to 10-11 Torr, and anode-to-cathode spacing ranges from 5 to 18 mm. Skilled persons can determine the proper setting for any particular application without undue experimentation.

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• 2000
  • 2000-03-17 JP JP2000607245A patent/JP2002540563A/ja not_active Withdrawn
  • 2000-03-17 WO PCT/EP2000/002546 patent/WO2000057451A2/en not_active Application Discontinuation
  • 2000-03-17 EP EP00918829A patent/EP1095396A2/de not_active Withdrawn

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