EP3550589A1

EP3550589A1 - Ion guide comprising electrode plates and ion beam deposition system

Info

Publication number: EP3550589A1
Application number: EP18165950.9A
Authority: EP
Inventors: Hartmut Schlichting; Johannes Barth; Andreas Walz
Original assignee: Technische Universitaet Muenchen
Current assignee: Technische Universitaet Muenchen
Priority date: 2018-04-05
Filing date: 2018-04-05
Publication date: 2019-10-09

Abstract

Disclosed herein is an ion guide (42) for guiding an ion beam along an ion path, said ion guide having a centerline (46) corresponding to said ion path, and a plurality of electrodes extending along said centerline. The electrodes are formed by conductive electrode plates (44) which are radially arranged around said centerline (46), wherein each of said electrode plates (44) has a radially inner edge (48) that is closest to the centerline (46), and wherein an inner envelope of the radially inner edges (48) defines an ion guide volume. Said electrode plates (44) are connected or connectable with an RF voltage source for applying voltages collectively confining ions within said ion guide volume.

Description

FIELD OF THE INVENTION

The present invention relates to an ion guide and an ion guide assembly for guiding an ion beam along a path. In particular, the present invention relates to an ion guide for use in an ion beam deposition system, as well as to an ion beam deposition system comprising such ion guide or ion guide assembly, and to a method for guiding ions employing such ion guide.

BACKGROUND OF THE INVENTION

Ion beams have many uses in various fields of natural sciences and technology, including experimental physics, medical devices, electronic components manufacturing or life science, in particular mass spectroscopy, where electrically charged molecules (ions) are guided to, from or within a mass spectrometer or a collision cell. The general purpose of an ion guide is to confine an ion beam along its predetermined path, typically using a plurality of electrodes arranged around the ion path, which in combination generate an electrical potential guiding the ions. In the simplest case, the potential could be a static DC potential, which would typically be realized as an ion Einzel lens arrangement. This, however, demands a fixed correlation of the ions' radial and axial momentum to keep them on track. Any breaking of this correlation e.g. due to collisions with residual gas atoms makes the ions swerve and lose track. These conditions are very common at relatively high pressure in the first stages of a multistage ion guide system, or in collision cells or drift cells, but can also occur due to space charge effects in later stages.
To make an ion guide more resistant to such perturbations, systems of electrodes can be employed which are driven with radio frequency (RF) voltages having frequencies of about 0.5 to 5 MHz and amplitudes of some volts up to some 100 volts. When the amplitude and the frequency of the RF potential are properly chosen, ions will be effectively repelled from the RF electrodes by means of an effective potential or "pseudo-potential" which reflects the effect of the RF electric field on the ion averaged over a plurality of AC cycles. A repulsive force derivable from this pseudo-potential, the so-called "field gradient force", is proportional to the gradient of the square of the RF field strength, proportional to the square of the charge of the ion - and hence independent of its polarity - and inversely proportional to the ion mass and to the square of the RF frequency.
In most RF operated ion guide systems, adjacent electrodes are driven with sinusoidal voltages of opposite phase, i.e. with a phase shift of 180° in between. For example, in known multipole ion guides, four, six or eight rod electrodes may be arranged on a circle around and extending parallel to the ion path, thereby forming a quadrupole, hexapole or octopole structure, respectively.
While there are many purposes for ion guides in various fields of science and technology, and the present invention is not restricted to use in a specific one of them, the ion guide of the present invention is particularly suitable for use in ion beam deposition (IBD), mass spectroscopy (MS), such as triple quad, Orbitrap or quadrupole time-of-flight (Q-TOF) mass spectroscopy, in ion mobility spectroscopy (IMS) systems and for use as an injection module to a quadrupole mass spectrometer, collision cell or ion trap.
In IBD, ions are guided along an ion path through a series of pumping chambers with decreasing pressure prior to being deposited by means of so-called "soft landing" on a substrate or target. The purpose of the pumping chambers is to remove unwanted, neutral particles from the ion beam. Ion beam deposition has important advantages over conventional deposition techniques. For example, unlike sputtering, plasma spraying, physical vapor deposition (PVD) and atomic layer deposition (ALD), IBD is not restricted to the deposition of thermally stable molecules. Chemical vapor deposition (CVD) requires a chemical reaction between sometimes poisonous educts on the substrate, which can likewise be avoided using IBD. Finally, while spincoating is restricted to (on an atomic scale) large thicknesses, IBD allows for depositing layers of a defined atomic thickness.
Moreover, since an ion beam can be deflected using suitable electric fields, in IBD, it is possible to "write" structures on a substrate, in a way similar to mask free ion beam lithography. Accordingly, it is possible to position highly sensitive, thermolabile molecules with low masses, like amino acids up to molecules with high masses, like peptides, proteins or even DNA molecules with a layer thickness defined on an atomic scale in micro arrays for manufacturing assays, sensors or highly specific catalysts.
All of these advantages of IBD currently come at the price of a rather slow deposition speed, which is due to the limited yield of the IBD system in view of the comparatively low intensity of the ion beam in current IBD systems.

SUMMARY OF THE INVENTION

The problem underlying the invention is to provide an ion guide with improved properties, which in particular allows for increasing the yield of an IBD system, as well as an improved IBD system.
This problem is solved by an ion guide according to claim 1, ion guide assemblies according to claims 11 and 12, as well as by an IBD system according to claim 13 and by a method of guiding an ion beam according to claim 14. Favorable embodiments are defined in the dependent claims.
The ion guide of the invention is suitable for guiding an ion beam along an ion path, said ion guide having a centerline corresponding to said ion path, and a plurality of electrodes extending along said centerline. The electrodes are formed by conductive electrode plates which are radially arranged around said centerline. Each of said electrode plates has a radially inner edge that is closest to the centerline and an inner envelope of the radially inner edges defines an ion guide volume. The electrode plates are connected or connectable with an RF voltage source for applying voltages collectively confining ions within said ion guide volume.
The ion guiding potentials that can be generated with this type of ion guide is similar to potentials that could be generated using longitudinal rod electrodes located at positions corresponding to the radially inner edges of the electrode plates. However, the inventors found out that for the purpose of increasing the yield of an IBD system or related applications, an ion guide based on elongate rod electrodes arranged on a cylindrical surface around the ion beam path should preferably have a comparatively large number of electrodes that are arranged closely together and confine an ion guide volume that has a fairly small cross-section. In fact, the inventors have found that a preferable ion guide would require "electrode rods" that are so thin that they are formed as wires that need mechanical tensioning and straightening rather than ordinary rod electrodes. Corresponding ion guides and applications are the subject of the co-pending application Ion guide comprising electrode wires and ion beam deposition system.
The importance of closely spaced elongate electrodes, and hence the motivation of using "electrode wires" instead of "electrode rods" can be understood as follows. The yield of an IBD system is governed by the ion current that can be guided through the ion guide or ion guide arrangement, which is referred as the "current capacity" of the ion guide (arrangement) herein. The obvious way to increase the current capacity would be to increase the diameter of the ion guide as a whole. However, when the diameter of the ion guide increases, the diameters of apertures in separation walls separating adjacent pumping chambers likewise need to be made correspondingly larger. This in turn makes it more difficult to decrease the number of neutral particles in the ion beam by means of pumping. The flow of neutral particles in common with the ion beam is referred to as "gas load" in the following. In other words, the inventors noticed that when increasing the diameter of the apertures in the separation walls, eventually more pumping stages were necessary to reduce the gas load to a desired degree. A larger number of pumping chambers however increases the manufacturing and operating costs and extends the ion path, leading to an inherent increase of ion losses.
Accordingly, the inventors realised that it is not possible to optimise the current capacity in a straightforward way by simply increasing the diameter of the ion guide. The inventors have further found that, at a given ion guide diameter, the current capacity is increasing with increasing number of elongate electrodes. In addition, the inventors have found that optimum results can be achieved with a moderate diameter of the ion guide, but comparatively large numbers of elongate electrodes. Then, when also choosing optimum inter-electrode distances, the inventors found that in favourable ion guides, the elongate electrodes should be made thinner than conventional rod electrodes, and in fact be formed by electrode wires which are so thin (and hence flexible) that they need tensioning to be kept straight, as is described in the co-pending application Ion guide comprising electrode wires and ion beam deposition system.
While the wire-based ion guides disclosed in the co-pending application have proven to be highly advantageous, the mounting of the electrode wires is somewhat involved. It requires certain holding structures that both hold the electrode wires as well as apply mechanical tension to the electrode wires to keep them straight. Moreover, when devising the holding structures, care must be taken that any insulating parts of the holding structures are sufficiently far away from the ion guide volume such as to avoid that the holding structures are charged by stray ions from the ion beam, which would lead to a distortion of the electric field for guiding the ion beam and in consequence to a reduction of the current capacity.
The inventors however noticed that using the design of the present invention employing radial electrode plates allows for obtaining similar guiding potentials, since the radially inner edges of the electrode plates can be arranged similarly closely together than the electrode wires of the wire based ion guides, and this can be obtained with considerably less mechanical effort, because unlike the wire based ion guides, no tensioning mechanism is needed. Moreover, due to the radial arrangement, the electrode plates can be easily mounted at a radially outside portion which is sufficiently far away from the ion guide volume such that there is no risk of charging by stray ions. Accordingly, similar advantages can be obtained as in the case of the wire based ion guide of the co-pending application, but with less constructional and manufacturing effort. Furthermore the electrode plates can be modelled in ways that conical or more complex shapes of the inner envelope along the longitudinal axis can be generated easily.
Since the electrode plates employed in the ion guide of the invention tend to be rather thin, and since it is particularly the location of the radially inner edge of the electrode plates that dominates the generated ion guiding potential, the "electrode plates" are also referred to as "blades" herein, and the corresponding ion guide is referred to as a "Blade Ion Guide (BIG)".
In preferred embodiments, the aforementioned radial arrangement of the electrode plates or "blades" is radial in a strict sense, meaning that for each electrode plate, there exists a radius vector pointing radially outward from said centerline and lying within said electrode plate. This "precisely radial" arrangement is the preferred arrangement that has been employed in various embodiments of the present invention disclosed herein. Nevertheless, it may be possible to obtain similarly good or only moderately inferior results when slightly deviating from this "precisely radial" arrangement. Accordingly, when referring to electrode plates that are "radially arranged around the centerline", this is to be understood in the sense of "substantially radial", permitting some deviations from the "precisely radial" arrangement, as long as the ion guiding potential generated thereby is not significantly affected by the deviation from the "precisely radial" arrangement.
In preferred embodiments, said centerline is a straight line defining a longitudinal axis of said ion guide. However, in alternative embodiments, said centerline may be a curved line. By suitably forming the shape of the radially inner edges of the electrode plates, such curved centerlines can be easily obtained. This is another particular advantage over the wire based ion guides referred to above, where curved centerlines are much more difficult to achieve.
In each section plane along the length of and perpendicular to the centerline, the distances of the radially inner edges of the electrode plates from the centerline is preferably identical, or varies by less than 15%, preferably by less than 10%. If the distances are all identical, then the "inner envelope" of the radially inner edges of the electrode plates in each section plane could be regarded as the largest circle that touches the radially inner edges of all of the electrode plates. However, in order to also allow for embodiments where the distances vary to some extent, according to the present disclosure the "inner envelope" of the radially inner edges of the electrode plates will be regarded as a polygon having as many vertices as there are electrode plates, and wherein each of the vertices is located on a radially inner edge of a corresponding one of the electrode plates. Moreover, this "inner envelope" defines the "ion guide volume" as used herein.
In preferred embodiments, the ion guide further comprises a holding structure for holding the electrode plates, wherein a portion of said holding structure, if any, which is separated from said inner envelope by less than the local inter-plate distance, preferably by less than twice the local inter-plate distance, and most preferably by less than three times the local inter-plate distance is made from a material having an electrical resistivity of less than 10¹² Ohm-cm, preferably of less than 10⁹ Ohm·cm. A similar effect can be obtained if a portion of said holding structure, if any, which is separated from said inner envelope by less than the local inter-plate distance, preferably by less than twice the local inter-plate distance, and most preferably by less than three times the local inter-plate distance has a sheet resistivity of less than 10¹⁴ Ohm, preferably of less than 10¹⁰ Ohm on a surface facing said ion guide volume, preferably on any surface facing said ion guide volume. Herein, the local inter-plate distance is defined as the distance between the radially inner edges of adjacent electrode plates at a given axial position. If at some axial position the distances between the radially inner edges of adjacent electrode plates should be nonuniform, the "local inter-plate distance" corresponds to the average thereof.
Note that according to this embodiment, the holding structure may be of a type which in its entirety is located further away from the inner envelope than said multiples of the inter-plate distance, or in other words, of a type where there is no portion thereof which would be separated from the inner envelope by less than said multiples of the inter-plate distance. This variant is accounted for by the "if any" condition. In this variant, the material of the holding structure may be insulating, because it is sufficiently far away from the ion guide volume such that there is no risk that it is hit and consequently charged by stray ions.
In alternative variants of this embodiment, some portions of the holding structure may indeed be separated from the inner envelope by less than the aforementioned multiples of the inter-plate distance, which bears the risk that these portions could be hit by stray ions. However, in this case the resistivity of such portions is chosen to be less than 10¹² Ohm-cm, preferably less than 10⁹ Ohm-cm, such that no significant charging is caused even if this portion is hit by stray ions. Another way of providing for an effective draining of possible stray ions is by means of a sheet resistivity of less than 10¹⁴ Ohm, preferably less 10¹⁰ Ohm on any surface facing said ion guide volume. This can be achieved by a suitable coating. The coating may e.g. be a metal film having a thickness of 30 to 1000 nm, or a paste containing glass and metal oxides, wherein said paste preferably has a thickness of 5 to 1000 µm.
In preferred embodiments, the holding structure comprises ring-like elements having slots in which the electrode plates are received. Using ring-like elements, the electrode plates or "blades" can be mounted at a radially outside portion thereof, which is sufficiently far away from the ion guide volume such that there is no risk of being hit by stray ions.
In preferred embodiments, the electrode plates have one of

a uniform thickness, and
a wedge-like profile with a thickness increasing in radially outward direction.

Accordingly, in the framework of the present invention, the term "plate" does not require a uniform thickness, but it also covers structures having nonuniform, wedge-like profiles. A wedge-like profile allows for a thin radially inner edge and concurrently provides more structural support by an increased thickness in radially outward direction. In case the electrode plates or "blades" have a wedge-like profile, in a cross-section perpendicular to the centerline, the wedge-like profiles may form angular sections with gaps in between, wherein at any given circle around the centerline, the ratio between the width of the angular sections in circumferential direction and the width of an adjacent gap is between 0.5 and 6.0, preferably between 0.8 and 4.0. This design leads to a constant ratio between the blade section and the gap section at the inner end of the wedge like blades, even if their inner envelope is not constant, particularly in case of a conical profile of the inner envelope along the longitudinal axis and thus leads to optimum current capacity of the ion guide.
In preferred embodiments, the electrode plates have a pointed tip formed by an acute angle between the radially inner edge of the electrode plates and an adjacent edge portion of said electrode plate on at least one of the longitudinal ends of the ion guide, wherein the acute angle is 70° or less, preferably 50° or less, and most preferably 30° or less. This pointed tip is particularly useful for receiving an ion beam from or transmitting an ion beam to an adjacent ion processing system, such as another ion guide, an ion separation system, an ion analysis system, an ion deposition system or an ion collision system. Herein, the pointed tip can be located closely adjacent to an entrance or exit of said further ion processing system, to thereby keep losses at the transitions between the ion guide and the further ion processing system at a minimum. The pointed tip is also useful for feeding an ion being through an aperture in a separation wall between two adjacent pumping chambers, as will be further illustrated below.
In a preferred embodiment, the radially inner edges of the electrode plates are, at least in a section along the length of the ion guide, conically converging or diverging from the centerline, wherein the average angle between the radially inner edges of the electrode plates and the centerline within said section is less than 45° preferably less than 5°, and most preferably less than 1°, and is 0.1° or more, preferably 0.2° or more, and most preferably 0.5° or more. For example, a wide end of a conical ion guide structure may facilitate feeding an ion beam into said ion guide and is less sensitive to slight misalignments of the ion guide with respect to an upstream component or allows for compressing the ion beam to a lower cross section. At the same time, keeping the angle between the radially inner edges of the electrode plates and the centerline below 5°, or even below 1° allows for keeping a repulsive force along the longitudinal axis due to the converging radially inner edges of the electrode plates in the direction of travel within acceptable bounds.
In a preferred embodiment, the number of electrode plates is 6 or more, preferably 8 or more, more preferably 10 or more, and most preferably 12 or more. With higher numbers of electrode plates, the current capacity of the ion guide for a given diameter of the ion guide volume can be increased. Note that due to the radial structure of the ion guide, the mounting of a comparatively large number of electrode plates with their radially inner edges arranged closely together can still be achieved with comparatively low mounting effort, at a high precision and without the risk that holding or mounting structures are inadvertently charged by stray ions
In preferred embodiments, the electrode plates are made from copper, molybdenum, tungsten, nickel, silver, gold, iron or alloys or compounds thereof or are covered with these materials.
In preferred embodiments, the thickness of each electrode plate close to the radially inner edges is 5.0 mm or less, preferably 1.0 mm or less, and more preferably 0.1 mm or less. Herein, the expression "close to the radially inner edge" accounts for the possibility that the radially inner edge is rounded, in which case the thickness is to be determined sufficiently away from the apex of the radially inner edge to be outside such possible rounded portion, such that a meaningful thickness can be determined. If the radially inner edges are not rounded, and a meaningful thickness can be determined at the radially inner edge, then the expression "close to the radially inner edge" may include the special case of "at the radially inner edge". It is emphasized that the comparatively small thicknesses of the electrode plates at or at least close to the radially inner edge allows for a comparatively large number of electrode plates at a comparatively small cross-section of the ion guide volume. When moving away from the radially inner edge, as mentioned above, the thickness of the electrode plate may increase, for example in favour of increased rigidity or structural support, to thereby lead to a wedge-like profile.
In a preferred embodiment, the ratio of the thickness of each electrode plate close to its radially inner edge and the inter-plate distance, at any given position along the centerline, is between 0.5 and 6.0, preferably between 0.8 and 4.0, wherein the inter-plate distance is defined as the distance between the radially inner edges of adjacent electrode plates at a given position along said centerline. These ratios of electrode plate thickness and inter-plate distance have been found to be beneficial for a high current capacity of the ion guide. Using electrode plates which have small thicknesses at or close to the radially inner edges of 5.0 mm or less, preferably of 1.0 mm or less and most preferably of 0.1 mm or less, these ratios can be achieved in spite of comparatively large numbers of electrode plates in combination with moderate ion guide diameters.
As explained above, the "inner envelope" may be confined, in each section perpendicular to said centerline, by a polygon having as many vertices as there are electrode plates, and wherein each of the vertices is located on a radially inner edge of a corresponding one of the electrode plates. Herein, the cross-section area of this inner envelope at the narrowest position along the centerline is preferably less than or equal to 200 mm², more preferably less than or equal to 20 mm², and most preferably less than or equal to 2.0 mm²; and is preferably larger than or equal to 0.1 mm², more preferably larger than or equal to 0.2 mm², and most preferably larger than or equal to 0.5 mm².
In a preferred embodiment, said electrode plates are connected to an RF driving source configured to drive adjacent two electrode plates with voltages of freely adjustable radiofrequency. In particular, said RF driving source may be configured to drive the electrode plates with an RF square wave signal, or a superposition of RF square wave signals, and preferably with a selectable duty cycle. A nonlimiting example of a "superposition of square wave signals" is a so-called "digital signal" which corresponds to a superposition of square waves with different amplitude and different duty cycle, but at the same base frequency.
Note that RF square wave driving signals or superpositions thereof are uncommon for conventional ion guides, where the electrodes are usually resonantly driven, using an LC circuit established by adding an inductive element and using the inherent capacitance of the electrodes for adjusting the resonance frequency. The inventors have noticed that the specific waveform (i.e. square wave digital waveform versus sinusoidal) has little bearing on the current capacity of the ion guide, but the square wave driving signal can be generated more easily with freely adjustable frequency than a sinusoidal driving signal. In fact, square wave signals can be generated by using switching circuits only, without having to provide for any resonant LC elements. Since the switching frequencies, the duty cycle and the superposition of square waves can be freely adjusted, the digital waveform or any other superposition of square waves can likewise be freely adjusted to thereby provide for optimum ion guiding performance.
In preferred embodiments, the electrode plates are connected to an RF driving source which supplies RF voltages having frequencies freely adjustable between about 0.05 to 20 MHz and/ or waveforms freely superimposed by square waves.
For applying a driving force on the ions in longitudinal direction of the ion guide, a DC electric field may be established along the centerline of the ion guide. For this purpose, in a preferred embodiment, at least some of the electrode plates are segmented, having conductive portions separated by intermediate portions of lower conductivity, in particular insulating portions, and different DC voltages are applied to different conductive portions, to thereby generate an electric field along the length of the electrode plate.
In a preferred embodiment, said ion guide is part of an ion beam deposition system, in which an ion beam is guided through a plurality of pumping chambers of decreasing pressure, wherein adjacent pumping chambers are divided by separation walls having an aperture for the ion beam to pass through.
A further aspect of the present invention relates to an ion guide assembly comprising two or more ion guides according to one of embodiments described above, wherein said two or more ion guides are arranged with their centerlines aligned with each other at the respective adjacent ends of said at least two ion guides, wherein said adjacent ends of the at least two ion guides are separated in a direction along said centerlines preferably by at least 0.01 mm and preferably by less than three times, more preferably by less than two times and most preferably by less than (one times) the square root of the cross-section area of the inner envelope of the corresponding end of one of the adjacent the ion guides. Herein, the "end" of a respective ion guide may be defined by the end of the respective electrode plates. Moreover, the adjacent ion guides may be separated by a gap, by an insulating material or by a material having an electrical resistivity of less than 10¹² Ohm-cm, preferably of less than 10⁹ Ohm·cm.
In a preferred embodiment of said ion guide assembly, adjacent ones of said two or more ion guides are arranged in adjacent pumping chambers which are separated by means of a separation wall, wherein an aperture is provided in the separation wall permitting ions guided by said adjacent ion guides to traverse from one pumping chamber into the other. The diameter of said aperture in the separation wall maybe 4.0 mm or less, preferably 3.0 mm or less, and more preferably 2.0 mm or less.
A further aspect of the invention relates to an ion guide assembly comprising

an ion guide according to one of the above-described embodiments having a pointed tip, and
a further ion processing system selected from a group consisting of another ion guide, an ion separation system, an ion analysis system, an ion deposition system and an ion collision system,

A further aspect of the invention relates to an ion beam deposition system comprising at least one ion guide or ion guide assembly according to one of embodiments described above.
A further aspect of the invention relates to a method of guiding an ion beam along an ion path, said ion guide having a centerline corresponding to said ion path, and a plurality of electrodes extending along said centerline, wherein said electrodes are formed by conductive electrode plates which are radially arranged around said centerline, wherein each of said electrode plates has a radially inner edge that is closest to the centerline, and wherein an inner envelope of the radially inner edges defines an ion guide volume, wherein each adjacent two electrode plates are driven with RF voltages of opposite polarity, in particular with an RF square wave drive signal, or a superposition of RF square wave drive signals, and wherein the method preferably further comprises a step of adjusting the frequency and the voltage amplitude of the drive signal depending on the type of ions to be guided by said ion guide. Herein, said ion guide is preferably an ion guide according to one of the embodiments recited above.

SHORT DESCRIPTION OF THE FIGURES

Fig. 1: is a schematic view of an ion beam deposition system employing two blade based ion guides (BIG) according to embodiments of the present invention.
Fig. 2a: is a perspective view of a BIG according to a first embodiment.
Fig. 2b: is a sectional view of the BIG according to the first embodiment.
Fig. 2c: shows an exemplary one of the electrode plates of the BIG of the first embodiment.
Fig. 3a: is a perspective view of a BIG according to a second embodiment.
Fig. 3b: is a view of an exemplary one of the electrode plates of the BIG according to the second embodiment.
Fig. 4a: is a perspective view of a BIG according to a third embodiment.
Fig. 4b: is an enlarged view of the encircled portion of Fig. 4a.
Fig. 4c: shows an exemplary one of the electrode plates of the BIG according to the third embodiment.
Fig. 5a: is a perspective view of a BIG according to a fourth embodiment.
Fig. 5b: is a perspective view similar to that of Fig. 5a, where however only a single electrode plate and a single extension element are shown.
Fig. 5c: is an enlarged view of the encircled portion of Fig. 5a.
Fig. 5d: is a perspective view of an exemplary one of the extension elements employed in the BIG according to the fourth embodiment.
Fig. 5e: shows an exemplary one of the electrode plates as used in the BIG according to the fourth embodiment.
Fig. 6a: is a perspective view of a BIG according to a fifth embodiment.
Fig. 6b: is an enlarged view of the encircled portion of Fig. 6a.
Fig 6c: shows an exemplary one of the electrode plates of the BIG according to the fifth embodiment.
Fig. 7a: is a perspective view of a BIG according to a sixth embodiment.
Fig. 7b: is a perspective view of a ring-like structure for holding electrode plates employed in the sixth embodiment.
Fig. 7c: is a sectional view of an end portion of the BIG according to the sixth embodiment.
Fig. 7d: shows an exemplary one of the electrode plates employed in the seventh embodiment.
Fig. 8: is a perspective view of a BIG according to a seventh embodiment.
Fig. 9: is a circuit diagram showing a driving circuit for driving the electrode plates of a BIG according to various embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to preferred embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated apparatus and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates.
In the figures described below, like elements will be designated with like reference signs, and the description thereof will not be repeated.
Figure 1 shows a schematic illustration of an ion beam deposition (IBD) system 10. The IBD system 10 comprises first to fourth pumping chambers 12 to 18 separated by separation walls 20. Each of the pumping chambers 12 to 18 is connected with a corresponding vacuum pump 22. While all of the vacuum pumps are designated with the same reference sign 22, they may be of different types. On the left end of the IBD system 10, an electrospray ionization (ESI) device 24 is provided, in which molecules are ionized such as to generate the molecular ions to be used for eventual deposition on a substrate 26 located in the fourth chamber 18 at the very right of the figure. The ESI method has first been described in Malcolm Dole, L.L.Mack, R.L. Hines, R.C.Mobley, D.Furgeson, M.B.Alice, Molecular Beams of Macroions, JChemPhys 49 p. 2240 (1968 ). A noble prize had been awarded to John B. Feen for this method, see John B. Fenn, Electrospray Wings for Molecular Elephants (Nobel Lecture), ). In the ESI device 24, charged droplets of an electrolyte are drawn by a very high voltage from a needle 28 which is operated at atmospheric pressure. Each droplet includes, in addition to the charged molecules to be deposited, a large amount of unwanted solvent/carrier gas that needs to be removed by means of the vacuum pumps 22 connected to the succession of pumping chambers 12 to 18. The ions and the solvent/carrier gas are guided into the first pumping chamber 12 by means of a heated capillary 30.
The first pumping chamber 12 exhibits a pressure of between 0.1 and 10 mbar. For forming an ion beam, a combined ion funnel and tunnel device 32 is employed, which extends from the first pumping chamber 12 through an aperture in the separation wall 20 into the second pumping chamber 14. The combined ion funnel and tunnel device 32 is referred to as a TWIN guide 32 herein and are described in more detail in the co-pending patent application "Partly sealed ion guide and ion beam deposition system".
An electrode wire based ion guide 36 is schematically shown, which extends from the second pumping chamber 14 through an opening in the separation wall 20 into the third pumping chamber 16. Wire based ion guides may be referred to as a "wire ion guide" (WIG) for short and are described in more detail in the co-pending patent application "Ion guide comprising electrode wires and ion beam deposition system". Herein, a portion of the WIG forms an aperture 34 through which neutral gas molecules can inadvertently pass from one chamber to the other.
In the third pumping chamber 16, a quadrupole mass separator 38 is provided, which comprises four rod electrodes 40. Also in the third pumping chamber 16, a first plate or "blade" based ion guide (BIG) 42 according to an embodiment of the invention is shown. As is seen in the schematic representation, the first BIG 42 has a conical ion guide volume with a large diameter at the upstream end facing the quadrupole mass separator 38 and a small diameter at the downstream end facing the separation wall 20 between the third and fourth pumping chambers 16, 18. Moreover, at the downstream end of the first BIG 42, the electrode plates or "blades" have a pointed tip, as will be further explained with reference to more detailed figures below. Finally, a second BIG 42 is provided in the fourth pumping chamber 18, having a conical ion guide volume with a small diameter at the upstream end facing the separation wall 20 between the third and fourth pumping chambers 16, 18, and a large diameter at the downstream end facing and fitting to the substrate 26. Moreover, at the upstream end of the second BIG 42, the electrode plates or "blades" have a pointed tip.
Figure 2a shows a perspective view and Fig. 2b a sectional view of a BIG 42 according to a first embodiment. The BIG 42 comprises 8 electrode plates 44 which are radially arranged around a centerline 46, which is not shown in Fig. 2a and 2b, but schematically shown in Fig. 2c, together with an exemplary one of said electrode plates 44.
Each of the electrode plates 44 has a radially inner edge 48 (see Fig. 2b and 2c) that is closest to the centerline 46. The envelope of the radially inner edges 48 of all electrode plates 44 defines an ion guide volume. The electrode plates 44 are mounted by means of a holding structure comprising two ring-like elements 50 with slots 52 in which the electrode plates 44 are received. As can be seen in Fig. 2a and 2b, the ring-like elements 50 mount the electrode plates 44 at a radially outer portion, which is very far away from the ion guide volume defined by the envelope of the radially inner edges 48 of the electrode plates 44, such that there is no risk that they are hit by stray ions. Accordingly, here the ring-lilce elements 50 can be made from arbitrary insulating material.
As is seen in Fig. 2a, the electrode plates 44 have a plain or "flat" configuration and are radially arranged with regard to the centerline 46. However, in an alternative embodiment (not shown) it would be possible to twist the electrode plates 44 such as to acquire a slightly helical configuration. This can for example be achieved by rotating one of the ring-like elements 50 around the centerline 46 with respect to the other one. In each sectional plane perpendicular to the centerline 46, the electrode plate 44 would still be arranged radially, such that there is a vector having its origin on the centerline 46 and lying within the twisted plane of the electrode plate 44. The rationale of this twisted arrangement is that the ions tend to acquire less energy when interacting with the AC-field provided by the electrode plates 44, because the plane of oscillations of ions caused by the AC-field changes upon the ions' travel along the centerline 46.
Fig. 3a and 3b show a second embodiment of a BIG 42 of the invention, which is very similar to the first embodiment. More precisely, Fig. 3a shows a perspective view and Fig. 3b shows the center line 46 and an exemplary one of the electrode plates 44. The main difference between the first and the second embodiment is that in the second embodiment shown in Fig. 3a and 3b, the radially inner edges 48 of the electrode plates 44 are conically diverging from the center line 46, to thereby establish a conical ion guide volume. It is readily apparent, particularly from Fig. 3b, that this conical ion guide volume can be easily established by forming the shape of the radially inner edge 48 of the respective electrode plates, for example by suitable machining. In the embodiment of Fig. 3a and 3b, the shapes of all eight electrode plates 44 are identical, but this is not necessary. By individually designing the radially inner edges 48 of each of the electrode plates 44, arbitrary, not rotationally symmetric ion guide volumes can be formed, and in particular, volumes that are arranged around a center line that is curved (not shown).
Fig. 4a to 4c show a third embodiment of a BIG 42, which is again similar to the first and second embodiments. The third embodiment likewise comprises 8 electrode plates 44 arranged around the center line 46 (see Fig. 4c), where, similar as in the first embodiment, the radially inner edges 48 are parallel to the center line 46 at an identical distance therefrom. Accordingly, as in the first embodiment, all of the radially inner edges 48 of the electrode plates 44 lie on a cylindrical surface surrounding the center line 46. The difference between the third embodiment of Fig. 4a to 4c and the first embodiment of Fig. 2a to 2c is that in the third embodiment, the electrode plates 44 have a pointed tip formed by an acute angle α between the radially inner edge 48 and an adjacent edge portion 54, as seen in Fig. 4c. The advantage of such a pointed tip is that the BIG 42 can be brought very close to another ion guide, to an ion separation system such as the quadruple mass separator 38 provided in the third pumping chamber 16 shown in Fig. 1, to an ion analysis system, to an ion deposition system, to an ion collision system or to an aperture in a separation wall 20 between adjacent pumping chambers as is shown for the first and second BIGs 42 in Fig. 1, without further structures of the BIG 42 interfering.
A fourth embodiment of a BIG 42 is shown with reference to Fig. 5a to 5e. As can be discerned from these figures, the fourth embodiment can be regarded as an extended version of the first embodiment shown in Fig. 2a to 2c. Like the first embodiment, the fourth embodiment comprises two ring-like elements 50 with slots 52 in which rectangular electrode plates 44 are received. However, for each of the electrode plates 44, an extension element 56 made from metal is provided, which has the shape of a right-angled pyramid with a triangular base. The side 58 of the pyramid that is perpendicular to the triangular base is aligned with the radially inner edge 48 of the corresponding electrode plate 44, as can be seen in Fig. 5b. The function of the pointed extension element 56 is similar to that of the pointed end of the electrode plates 44 shown in the third embodiment of Fig. 4a to 4c. The key advantage of the pyramidal extension elements 56 as compared to the pointed ends of the electrode plates 44 of the third embodiment is that it is structurally more robust.
With reference to Fig. 6a to 6c, a fifth embodiment is shown, which is conceptually and structurally very similar to the third embodiment shown in Fig. 4a to 4c. In the fifth embodiment of Fig. 6a to 6c, the plate electrodes likewise have a pointed tip formed by an acute angle α (see Fig. 6c) between the radially inner edge 48 of the electrode plate 44 and an adjacent edge portion 54. However, while in the third embodiment of Fig. 4a to 4c the shape of the individual plate electrodes 44 was trapezoidal, in the fifth embodiment of Fig. 6a to 6c, the shape of the electrode plates is that of a polygon having five vertices. This shape can be thought of as a rectangular shape with a small triangular extension. This shape allows for a particularly small acute angle α, while at the same time the most part of the electrode plate may still be rectangular, which allows for a particularly easy and precise mounting and provides an improved stability.
With reference to Fig. 7a to 7d, a sixth embodiment of a BIG 42 is shown. Fig. 7a shows a perspective view of the BIG 42 including again 8 electrode plates 44, of which an exemplary one is shown in Fig. 7d. The electrode plate 44 shown in Fig. 7d has a generally rectangular shape, with triangular extensions at its ends each forming an acute angle α between the radially inner edge 48 and an adjacent edge 54. Moreover, at the respective ends and close to the radially outer edge 60, two nose-like protrusions 62 are formed.
Similar to the previous embodiments, the BIG 42 of the sixth embodiment comprises two ring-like elements 50, comprising slots 52 for receiving the plate electrodes 44. A perspective view of one ring-like element 50 is shown in Fig. 7b. As is seen therein, at the radial outer ends of each slot 52, a through hole 64 is formed. The through holes 64 serve to radially fix the electrode plates 44 in the slots 52, for example by injecting glue into these holes 64, or by bending a portion of the electrode plate 44 close to the radially outer edge 60 within the through hole 64. Moreover, on the left end of the BIG 42 of the sixth embodiment as shown in Fig. 7a and 7c, an end ring 66 is provided. As can be seen in in the sectional view of Fig. 7c, the end-ring 66 comprises recesses 68 into which the nose-like protrusion 62 may engage. While in the sectional view of Fig. 7c the recesses 68 appear to be separate, they may in some embodiments also be part of a same annular recess 68.
While difficult to discern with the bare eye in Fig. 7d, a radially inner edge 70 of the nose-like protrusion 62 is slightly inclined. Accordingly, when the nose-lilce protrusion 62 is inserted into the recess 68 in the end-ring 66, the plate 44 is moved in a radially outer positon, until it acquires a predetermined radial rest position. Accordingly, by attaching the end-ring 66 at one or both of the ends of the electrode plates 44 received in the slots 52 of the ring-like element 50, the electrode plate 44 is moved to and fixed in the pre-determined radial position. The end-ring 66 may for example be attached to the ring-like elements 50 by gluing. If the end-ring 66 is employed, no further fixation of the plate electrodes 44 of the kinds described before, i.e. by means of injecting glue into the holes 64 or bending the radially outer portion of the plate electrode 44 maybe necessary.
Finally, with reference to Fig. 8, a seventh embodiment of a BIG 42 is illustrated. As before, the BIG 42 comprises eight electrode plates radially arranged around a center line (not shown in Fig. 8). However, instead of using ring-like elements for holding the plate electrodes 44, in the seventh embodiment shown in Fig. 8, the plate electrodes 44 are embedded within an embedding material 72, such as a molding material which can for example be applied by injection molding. In the embodiment shown, the radially inner edges 48 of the electrode plates 44 are arranged on a cylindrical surface around the center line 46, which is not shown in Fig. 8 for clarity, but corresponds to the symmetry axis of the cylindrical structure shown. A bore is provided in the embedding material 72, which likewise coincides with the cylindrical surface on which the radially inner edges 48 of the electrode plates 44 are arranged. This can for example be achieved by inserting a cylindrical pin having a diameter that is just large enough to simultaneously contact each of the radially inner edges of the electrode plates 44 prior to adding a moldable embedding material 72, and by removing said pin after the moldable material 72 is solidified. In an alternative, the entire space between the electrode plates 44 may be filled with a molding material 72 and may then be removed from the cylindrical area confined by the radially inner edges 48 of the electrode plates 44 by a high precision drilling operation.
Note that in this embodiment, the embedding material 72 extends all the way up to the inner envelope of the radially inner edges 48 of the electrode plates 44. Accordingly, there is a high risk that the embedding material 72 will be hit by stray ions when the BIG 42 is in use. In order to avoid an inadvertent charging of the embedding material 72, in the embodiment shown this embedding material 72 is an intermediate resistivity material having an electrical resistivity of between 102 Ohm * cm and 10¹² Ohm * cm, preferably of between 3 * 105 Ohm * cm and 109 Ohm * cm. Such intermediate resistivity material can be a plastic material or a ceramic material including or mixed with conductive particles, in particular metal or graphite particles. In an alternative, the embedding material 72 could be a ferrite-based material.
In operation, high-frequency AC voltages are applied to the electrode plates 44 with frequencies on the order of 0.05-20 MHz and amplitudes of some 0.1-100 V. For clarity of illustration, the corresponding high-frequency driving source is omitted in Fig. 1 to 8. A circuit diagram of a suitable driving source is shown in Fig. 9. The driving source comprises a DC voltage source 104, four switches 100 and a control unit 106 for controlling the switching states of the switches 100. Between the switches 100 and the control unit 106 potential separating elements 102 are provided. The RF output voltage is supplied at the terminals 108 and 110. The control unit 106 controls the switches 100 to alternate between two switching states, a first switching state, in which the upper left and the lower right switch 100 are closed and the remaining switches 100 are open, and a second, opposite state, in which the lower left and the upper right switch 100 are closed and the remaining switches 100 are open. In the first switching state, the RF terminal 108 has positive voltage and the RF terminal 110 has negative voltage, while in the second switching state, the voltages are reversed. Accordingly, by alternating between the first and second switching states, under the control of the control unit 106, a square wave RF output voltage at the terminals 108, 110 is provided. Moreover, under the control of the control unit 106, the output RF frequency can be freely adjusted.
Although a preferred exemplary embodiment is shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiment is shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of protection of the invention as defined in the claims.

10: IBD system
12: first pumping chamber
14: second pumping chamber
16: third pumping chamber
18: fourth pumping chamber
20: separation wall
22: vacuum pump
24: electrospray ionization (ESI) device
26: substrate
28: needle
30: heated capillary
32: combined tunnel and funnel
34: aperture
36: wire based ion guide (WIG)
38: quadrupole mass separator
40: rod electrode
42: blade ion guide (BIG)
44: electrode plate
46: centerline of BIG
48: radially inner edge of electrode plate 44
50: ring-like holding element
52: slot in ring-lilce holding element 50
54: adjacent edge forming acute angle with radially inner edge 48
56: pyramidal extension
58: edge of pyramidal extension 56
60: radially outer edge of electrode plate 44
62: nose-like protrusion
64: hole in ring-like holding element
66: endring
68: recess in endring 66
70: radially inner edge of nose-lilce protrusion 62
72: embedding material
100: switch
102: potential separating element
104: DC voltage source
106: control unit
108: RF terminal
110: RF terminal

Claims

An ion guide (42) for guiding an ion beam along an ion path, said ion guide having a centerline (46) corresponding to said ion path, and a plurality of electrodes extending along said centerline, characterized in that
said electrodes are formed by conductive electrode plates (44) which are radially arranged around said centerline (46),
wherein each of said electrode plates (44) has a radially inner edge (48) that is closest to the centerline (46), and wherein an inner envelope of the radially inner edges (48) defines an ion guide volume,
wherein said electrode plates (44) are connected or connectable with an RF voltage source for applying voltages collectively confining ions within said ion guide volume.
The ion guide (42) of claim 1, wherein for each electrode plate (44), there exists a radius vector pointing radially outward from said centerline (46) and lying within said electrode plate (44), and/or
wherein said centerline (46) is a straight line defining a longitudinal axis of said ion guide, or
wherein said centerline (46) is a curved line.
The ion guide (42) of one of the preceding claims, wherein in each section plane along the length of and perpendicular to the centerline (46), the distances of the radially inner edges of the electrode plates from the centerline (46) is identical, or varies by less than 15%, preferably by less than 10%.
The ion guide (42) of one of the preceding claims, further comprising a holding structure (50, 72) for holding the electrode plates, wherein a portion of said holding structure (50, 72), if any, which is separated from said inner envelope by less than the local inter-plate distance, preferably by less than twice the local inter-plate distance, and most preferably by less than three times the local inter-plate distance is made from a material having an electrical resistivity of less than 10¹² Ohm-cm, preferably of less than 10⁹ Ohm-cm, or has a sheet resistivity of less than 10¹⁴ Ohm, preferably of less than 10¹⁰ Ohm on a surface facing said ion guide volume (128), wherein the local inter-plate distance is defined as the distance between the radially inner edges (48) of adjacent electrode plates (44) at a given axial position,
wherein the holding structure preferably comprises ring-like elements (50) having slots (52) in which the electrode plates (44) are received.
The ion guide (42) of one of the preceding claims, wherein the electrode plates (44) have one of
- a uniform thickness, and

- a wedge-like profile with a thickness increasing in radially outward direction,
wherein in a cross-section perpendicular to the centerline (46), the wedge-like profiles preferably form angular sections with gaps in between, wherein at any given circle around and along the centerline (46), the ratio between the width of the angular sections in circumferential direction and the width of an adjacent gap is between 0.5 and 6.0, preferably between 0.8 and 4.0.
The ion guide (42) of one of the preceding claims, wherein the electrode plates (44) have a pointed tip formed by an acute angle between the radially inner edge (48) of each electrode plate (44) and an adjacent edge portion (54) of said electrode plate (44) on at least one of the longitudinal ends of the ion guide, wherein the acute angle is 70° or less, preferably 50° or less, and most preferably 30° or less, and/or
wherein the radially inner edges (48) of the electrode plates (44) are, at least in a section along the length of the ion guide (42), conically converging or diverging from the centerline (46), wherein the average angle between the radially inner edges (48) of the electrode plates (44) and the centerline (46) within said section is less than 45°, preferably less than 5°, and most preferably less than 1°, and is 0.1° or more, preferably 0.2° or more, and most preferably 0.5° or more.
The ion guide (42) of one of the preceding claims, wherein the number of electrode plates (44) is 6 or more, preferably 8 or more, more preferably 10 or more, and most preferably 12 or more, and/or
wherein the electrode plates (44) are made from copper, molybdenum, tungsten, nickel, silver, gold, iron or alloys or compounds thereof or have a coating of these materials, and/or
wherein the thickness of each electrode plate (44) close to the radially inner edges (48) is 5.0 mm or less, preferably 1.0 mm or less, and more preferably 0.1 mm or less.
The ion guide (42) of one of the preceding claims, wherein the ratio of the thickness of each electrode plate (44) close to its radially inner edge and the inter-plate distance, at any given position along the centerline (46), is between 0.5 and 6.0, preferably between 0.8 and 4.0, wherein the inter-plate distance is defined as the distance between the radially inner edges (48) of adjacent electrode plates (44) at a given position along said centerline (46), and/or
wherein the inner envelope is confined, in each section perpendicular to said centerline, by a polygon having as many vertices as there are electrode plates (44), and wherein each of the vertices is located on a radially inner edge (48) of a corresponding one of the electrode plates (44), wherein the cross-section area of this inner envelope at the narrowest position along the centerline (46) is less than or equal to 200 mm², preferably less than or equal to 20 mm², and most preferably less than or equal to 2.0 mm²; and is larger than or equal to 0.1 mm², preferably larger than or equal to 0.2 mm², and most preferably larger than or equal to 0.5mm².
The ion guide (42) of one of the preceding claims, wherein said electrode plates (44) are connected to an RF driving source configured to drive adjacent two electrode plates with voltages of opposite polarity and freely adjustable radiofrequency,
wherein said RF driving source is preferably configured to drive the electrode plates (44) with an RF square wave signal, or a superposition of RF square wave signals, preferably with a selectable duty cycle.
The ion guide (42) of one of the preceding claims, wherein at least some of the electrode plates (44) are segmented, having conductive portions separated by intermediate portions of lower conductivity, in particular insulating portions, and wherein different DC voltages are applied to different conductive portions, to thereby generate an electric field along the length of the electrode plate (44), and/or
wherein said ion guide (42) is part of an ion beam deposition system (10), in which an ion beam is guided through a plurality of pumping chambers (12-18) of decreasing pressure, wherein adjacent pumping chambers (12-18) are divided by separation walls (20) having an aperture for the ion beam to pass through.
An ion guide assembly comprising two or more ion guides (42) of one of the preceding claims, wherein said two or more ion guides (42) are arranged with their centerlines (46) aligned with each other at the respective adjacent ends of said at least two ion guides (42), wherein said adjacent ends of the at least two ion guides (42) are separated in a direction along said centerlines (46) preferably by at least 0.01 mm and preferably by less than three times, more preferably by less than two times and most preferably by less than the square root of the cross-section area of the inner envelope of the corresponding end of one of the adjacent the ion guides (42),
wherein adjacent ones of said two or more ion guides (42) are preferably arranged in adjacent pumping chambers (12-18) which are separated by means of a separation wall (20), wherein an aperture is provided in the separation wall (20) permitting ions guided by said adjacent ion guides to traverse from one pumping chamber (12-18) into the other,
wherein the diameter of said aperture in the separation wall (20) is preferably 4.0 mm or less, preferably 3.0 mm or less, and more preferably 2.0 mm or less.
An ion guide assembly comprising
- an ion guide (42) of one claims 10 to 19 having a pointed tip, and

- a further ion processing system selected from a group consisting of another ion guide (42), an ion separation system, an ion analysis system (38), an ion deposition system and an ion collision system,
wherein said pointed tip is located adjacent to an entrance or exit of said further ion processing system.
An ion beam deposition system (10) comprising at least one ion guide (42) or ion guide assembly of one of the preceding claims.
A method of guiding an ion beam along an ion path, said ion guide (42) having a centerline (46) corresponding to said ion path, and a plurality of electrodes extending along said centerline (46), characterized in that
said electrodes are formed by conductive electrode plates (44) which are radially arranged around said centerline (46),
wherein each of said electrode plates (44) has a radially inner edge (48) that is closest to the centerline (46), and wherein an inner envelope of the radially inner edges (48) defines an ion guide volume,
wherein each adjacent two electrode plates (44) are driven with RF voltages of opposite polarity, in particular with an RF square wave drive signal, wherein the method preferably further comprises a step of adjusting the RF frequency and the voltage amplitude of the drive signal depending on the type of ions to be guided by said ion guide (42).
The method of claim 14, wherein said ion guide is an ion guide of one of claims 1 to 10.