EP1095396A2

EP1095396A2 - Muffin tin style cathode element for diode sputter ion pump

Info

Publication number: EP1095396A2
Application number: EP00918829A
Authority: EP
Inventors: James B. Mcginn
Original assignee: FEI Co
Current assignee: FEI Co
Priority date: 1999-03-19
Filing date: 2000-03-17
Publication date: 2001-05-02
Also published as: WO2000057451A2; JP2002540563A; WO2000057451A3

Abstract

Diode sputter ion pumps display instabilities like current bursts, leakage currents and arcs typically following pumping exposure to gas doses greater than the ultimate pressure of the vacuum system in which the pump is operating. The instabilities are disruptive to the devices to which the sputter ion pump is attached. The invention provides an ion pump that exhibits improved stability and reduced leakage current. The instabilities are caused by explosive arc emission and electron emission from structures like dendritic protrusions that grow on the cathode plate, whose shape and placement give rise to high electric fields. According to the invention the cathode includes a sputterable material for removing gases from the environment of the ion pump and shaped so that during operation of the ion pump the electric field in a dendritic growth region is insufficient to cause field emission from the dendrites thereby reducing instabilities in the operation of the ion pump.

Description

Muffin tin style cathode element for diode sputter ion pump.

Technical Field

The invention relates to ion pumps used primarily in high and ultra-high vacuum systems. Related Applications This application claims priority from U.S. Provisional Patent Application

Numbers 60/125,317 and 60/125,318 both of which were filed March 19, 1999 are hereby incorporated by reference. Background of the Invention

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^"9Torr 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. 2, 3, and 4 show cross section of prior art anodes of differing design. Each anode typically includes multiple anode cells 16, each having a longitudinal axis perpendicular to the planes of the cathodes. In operation, a positive voltage is applied to the anode 14, a negative voltage or ground potential is applied to the cathodes 12, and a magnetic field is applied parallel to the longitudinal axes of the anode cells. Within each anode cell 16, 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. 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.

Electrons in the cloud collide with and ionize gas molecules that migrate into the cloud. The ionized gas molecules accelerate toward the cathodes 12, sputtering cathode material, typically titanium. The sputtered titanium strikes and adheres to the anode, the cathodes, or elsewhere. Because 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.

When a gas molecule is ionized by collision with an electron in the anode cell, one or more electrons are freed into the electron cloud. To maintain a steady state, electrons must leave the electron cloud at the same rate that new electrons are added to the cloud by the ionization of gases or by the arrival of secondary electrons due to ion bombardment of the cathode. An excess of electrons in the electron cloud will neutralize the gas ions before they have gained sufficient momentum to efficiently sputter material from the cathode.

By a phenomenon known as cross-field mobility, some electrons penetrate the anode sheath and impact the anode. Electrons in the space charge cloud within about two electron cyclotron radii of the sheath have a significant probability of striking the anode and leaving the cloud. The shape of the anode cell has a significant effect on the contour of the anode sheath and its distance from the anode wall, which contour and distance are also affected by the pressure, the magnetic field, and the applied voltages. 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.

In a cylindrical cell anode, 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. Thus, ion pumps having cylindrical diode cells are more sensitive than ion pumps having rectangular cells.

Diode sputter ion pumps having cylindrical cell anodes, however, 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. Summary of the Invention

Thus, it is an object of the invention to enhance the operational stability of ion pumps.

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. Generally, 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.

Several innovative aspects of an improved ion pump are described below. Depending upon the pump requirement for a particular application, the various innovative aspects can be applied individually or in combination to produce an improved ion pump.

In one aspect of the invention, 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. In one preferred embodiment, 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. By reducing the electric field where the ion sputtering occurs, applicant has reduced or eliminated instabilities, current bursts, leakage currents, and arcs which would result in prior art pumps from the high electric field at the dendritic protrusions.

In another aspect of the invention, 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. In prior some art, cylindrical anode ion pumps, between each cylinder (Figure

3) and its nearest neighbors is an inter-cylindrical cell 18, typically having the shape of a hyper-extended square. In another aspect of the invention, applicant has discovered that the 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.

Additional objects, advantages and novel features of the invention will become apparent from the detailed description and drawings of the invention. Brief Description of the Drawings 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.

Detailed Description of Preferred Embodiments:

The applicant has shown that the primary cause of disruptions to clean, quiet, stable ion pump operation is the dendrites formed on the cathode. The formation of dendritic protrusions by ion bombardment has been studied by many authors. See for example, "Production of Microstructures by Ion Beam Sputtering" by W. Hauffe in Topics in Applied Physics, Vol. 64; Sputtering by Particle Bombardment III , Eds. R. Behrisch and K. Wittmaack, Springer-Verlag Berlin Heidelberg; 1991, and "Cone Formation on Metal Targets during Sputtering" by G. K. Wehner and D. J. Hajicek in J. Appl. Phys. Vol. 42, Number 3, March 1, 1971. These references discuss the formation of dendrites by ions bombardment in general, but do not address ion pump instability or show the type of, extent of, and formation conditions of the specific dendritic protrusions found on the cathode plates of ion pumps.

"Sputter Ion Pumps for Low Pressure Operation," Transactions of the National Vacuum Symposium of the American Vacuum Society (Nov. 10, 1963) by S. L. Rutherford suggests that field emission current leakage occurs from sharp points or "whiskers" that often form on the cathodes of sputter ion pumps, but does not provide a way for eliminating the instabilities and does not discuss a formation mechanism or the locations on the cathode plate of the dendrites.

Also M. Audi and M. Pierini in "Surface Structure and Composition Profile of Sputter-Ion Pump Cathode and Anode" in J. Vac. Sci. Technol A4(3), 303 (1986) mention needle-shaped formations, but their electron micrographs of the needle shaped formations do not show the sort of dendrites that have been found by applicant. The needle-shaped formations shown in the M. Audi, M. Pierini paper display decisively different topographical features from the dendrites found by the applicant and are believed to be insufficient to support field emission currents.

One of the applicants has found four major properties of the dendrites formed in ion pumps: 1) The dendrites 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. 3 are formed hyper-square inter-cylindrical cells 18 that support plasma densities greater than that within the standard cylindrical cells 16. The higher plasma density causes the cathode crater associated with the inter-cylindrical cells to have a greater dendritic population density than that found on the cathode plate opposite a standard cylindrical anode cell 16. Such dendrites cause explosive cathode arc emission and field electron emission from the cathode plate, which are responsible for such instabilities as current bursts and leakage currents. 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. The crater is about 1 mm in diameter and the zone about the crater which encloses the forest of dendrites is about 2.5 mm in diameter. 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 term "muffin tin cathode" is taken from the concept in solid state physics whereby an infinite series of potential wells along a crystal lattice is often described as a "muffin tin" potential. The depth, h, and the width, 2b, of each depression 34 are preferably related by h>=.5b. Also 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. Preferably, 2b=.7a, where a is the diameter of the anode cell A mathematical formalism for the electric field and potential near such a depression is published in J. Vac. Sci. Technol A. Vol 15, No. 4, Jul/Aug 1997 "Proposal for a new self-focusing . . .," pg. 2369. By enclosing the heavily bombarded region 48 into the electric potential well provided by the depression 34 of the muffin tin cathode 30, the electric field at the surface of the dendritic protrusions is reduced, and the dendritic protrusions will neither field emit electrons nor engage in explosive molten jet emission, thus eliminating, instabilities, current bursts, leakage currents and arcs.

Some measurements show that the 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.

If the anode design includes intercellular regions, like the inter-cylindrical cells 18 of FIG. 3, there may also be depressions corresponding to the central axis of each inter-cellular regions, with the depth and width of the depression being determined in a matter similar to that described above for the depressions aligned with the anode cells. 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. Further, the field emitted electrons and explosive molten jet emission phenomena that arise due to the said high electric field, lead to the macroscopically observable performance limitations of diode sputter ion pumps, namely instabilities, current bursts, leakage currents, and arcs.

In addition to the above, and based on the same reasoning that any sharp point on the cathode plate or any other protrusion that may exist in the cathode to anode gap may field emit and lead to the problems thus described, the cathode to anode gap is also to be designed to be free of any protrusions. This preference dictates that 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.

In another aspect of a preferred ion pump, 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. For example, 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. For efficiency considerations, 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.

The embodiments described above are merely illustrative and skilled persons can make variations on them without departing from the scope of the invention, which will be defined claims in a utility patent application to be filed. The various aspects of the improved ion pump described above could be implemented separately or in any combination.

Claims

CLAIMS:

1. An ion pump, comprising: an anode including one or more anode cells; a source of a magnetic field for sustaining a plasma within the one or more anode cells; two cathodes, one cathode positioned on either side of and spaced apart from the anode; a source of an electric field between the anode and the cathode for accelerating particles to sputter the cathode; the cathodes including a sputterable material for removing gases from the environment of the ion pump and shaped so that during operation of the ion pump the electric field in a dendritic growth region is insufficient to cause field emission from the dendrites, thereby reducing instabilities in the operation of the ion pump.

2. The ion pump of claim 0 in which cathode includes a series of depressions.

3. The ion pump of claim 0 in which cathode the depressions are generally aligned with regions of high plasma density.

4. The ion pump of claim 0 in which cathode depressions are generally aligned with anode cells.

5. The ion pump of claim 0 in which cathode depressions are generally aligned with intracellular regions of the anode.

6. The ion pump of claim 0 in which operation of the ion pump creates craters in the cathode and in which the craters are contained within the depression.

7. The ion pump of claim 0 further comprising high voltage shields positioned between the areas of the cathode corresponding to projections from the anode cells.

8. The ion pump of claim 0 in which the high voltage shields are of a low sputter yield material.

9. An ion pump, comprising: an anode including a plurality of cells; a source of a magnetic field for sustaining a plasma within the plurality of cells; a cathode spaced apart from the anode and including a sputterable material for removing gases from the environment of the ion pump, the cathode including depressions generally aligned with the anode cells.

10. The ion pump of claim 0 in which the electric field in the depression in the cathode is reduced below the level that causes field emission from the cathode.

11. The ion pump of claim 0 in which the cathode further includes depressions generally aligned with intracellular regions of the anode.

12. The ion pump of claim 0 further comprising high voltage shields positioned between the depressions.

13. The ion pump of claim 0 in which the high voltage shield is of a low sputter yield material.

14. A method of producing an ion pump, comprising: providing an anode having at least one anode cell; providing a magnetic field for maintaining a plasma in the anode cells; providing a cathode including material for sputtering to remove gas from the ion pump environment; applying an electric field between the anode and cathode, the electric field accelerating ionized gas particles toward cathode into at least one impact area defined by the at least one anode cell; reducing the electric field at the impact area sufficiently to reduce instabilities while maintaining a sufficiently strong field to efficiently operate the ion pump.

15. The method of claim 0 in which reducing the electric field at the impact area comprises deforming the cathode away from the anode at the impact area.

16. The method of claim 0 in which the impact area is coaxial with the at least one anode cell and in which reducing the electric field at the impact area comprises depressing the cathode away from the anode at the impact area.

17. The method of claim 0 in which providing an cathode includes casting a cathode or machining a cathode from a sold block of metal.

18. A charged particle beam system exhibiting improved stability, comprising: a source of charged particles; charge particle optics for forming the charged particles into a beam; a vacuum system for creating an evacuated environment for the charged particle beam, the vacuum system including an ion pump including a cathode having depressions generally aligned with the anode cells to reduce instabilities of the vacuum system.

19. The charged particle beam system of claim 0 in which the source of charged particles comprises a source of ions.

20. The charged particle beam system of claim 0 in which the source of charged particles comprises a source of electrons.