Device for processing a workpiece with an ion beam

Abstract

A device for processing a workpiece (14) with an ion beam (18) comprises a holder (12) for receiving the workpiece (14), an ion source (20) for generating ions, and an extraction grid (22), connected to a power source (24), which accelerates the ions generated by the ion source so that a directed ion beam (18) exits the extraction grid (22). According to the invention, a magnetostatic beam deflection unit (28) deflects the ion beam (18) exiting the extraction grid (22), preferably by a larger angle such as 90°. Atomized and generally electrically uncharged material, which is released by the impact of the ion beam on the workpiece or a target, is not deflected by the beam deflection unit (28) and thus cannot pass back to the extraction grid (22). This effectively prevents contamination and / or functional impairments of the extraction grid and the ion source.

Description

[0001] Device for machining a workpiece

[0002] with an ion beam

[0003] BACKGROUND OF THE INVENTION

[0004] 1. Field of the invention

[0005] The invention relates to a device for processing a workpiece with an ion beam that removes material from the workpiece, applies material to the workpiece or introduces material into the workpiece.

[0006] 2. Description of the state of the art

[0007] The surfaces of workpieces are treated in various ways using ion blasting. Ion blasting can be used to remove material from the surface of the workpieces, to apply material to the surface, or to introduce material into the surface.

[0008] Material removal is usually achieved through ion beam etching (IBE) or ion milling, in which accelerated ions are directed at the surface of the workpiece in a high vacuum. The kinetic energy of the ions causes atoms to be ejected from the surface, partially atomizing it. Ion beam etching is thus similar to sandblasting, except that significantly smaller particles are used instead of sand.

[0009] Ion etching processes include, for example, Ion Beam Figuring (IBF), in which particularly high-quality optical surfaces are post-processed in an ion beam etching system.

[0010] If the ions interact with the surface not only physically due to their kinetic energy, but also chemically, this is called reactive ion beam etching (Rl BE).

[0011] If, however, material is deposited onto the surface by the ion beam, this is called ion beam deposition, ion beam sputtering, or ion beam sputter deposition (IBS or IBSD). In this process, a broad ion beam strikes the surface of a target, causing individual atoms or molecules of the target material to be ejected. The released particles then land on the surface of the workpiece and form a thin coating. By using different targets in a single process chamber, multiple layers can be produced with very high quality.

[0012] The introduction of ions into a workpiece is called ion implantation and is used, for example, for doping semiconductors.

[0013] Devices used for workpiece processing or target atomization with ion beams often contain a wide-beam source from which a broad ion beam emerges. The diameter of the ion beams in these sources is typically between 5 and 500 mm, most commonly between 10 and 100 mm. The wide-beam source comprises the actual ion source for generating ions and ion optics that form the ions into a beam.

[0014] A Kaufman ion source is frequently used as the ion source. In this type of source, free electrons are generated using a heated cathode, which then ionize gas atoms or molecules through electron impact ionization. Inductively coupled RF ion sources are also commonly used, where ionization takes place in a high-frequency alternating magnetic field.

[0015] Ion optics extracts ions from the plasma and forms them into an ion beam. In wide-beam sources, the ion optics typically include an extraction grating to which an accelerating voltage is applied to accelerate the ions generated by the ion source. The extraction grating has numerous openings through which the ions emerge as individual beams that superimpose to form a wide beam. Often, several extraction gratings (e.g., screen gratings, accelerating gratings, and retarding gratings) are arranged close together, each subjected to different high voltages, forming a single extraction grating system.

[0016] Typically, the ion beam is directed perpendicularly or at small angles relative to the surface normal onto the workpiece surface. The incident ions atomize a portion of the material on the workpiece surface. The atomized material is then described according to a rotationally symmetric and degenerate COS. n The emission is distributed in such a way that a larger proportion of the atomized material is thrown back towards the wide-beam source. There, the atomized material settles on the extraction grid.

[0017] Contamination from the atomized material leads, among other things, to changes in the properties of the ion beam with increasing processing time. Furthermore, the frequency of discharges in the ion optics (so-called "arcing") increases significantly, which, for example, contributes to an increase in defect density in ion beam deposition. The layer of atomized material that has grown on the extraction grating eventually begins to flake off, leading to process contamination and potentially causing unwanted discharges. Since the atomized particles sometimes possess very high kinetic energies, the ion optics can also be damaged by material removal.

[0018] These problems are not limited to material-removing ion beam processing, because in all types of ion beam processing, high-energy ions collide with surfaces and cause sputtering. For example, the angle of incidence of the ion beam on the target in ion beam deposition is approximately 45° to 65°. A higher angle of incidence asymmetrically deforms and tilts the distribution of the sputtered material and thus the emission characteristics. Furthermore, albeit to a lesser extent, the emission characteristics described above result in a detrimental material flow back towards the ion optics.

[0019] Some of the aforementioned problems can be resolved by replacing the ion optics in a timely manner. However, these replacements are very expensive, which negatively impacts the machining costs per workpiece. Even replacement does not always solve the problems described. Often, the processing time for a single operation can be several hours or even days. During this time, noticeable degradation of the ion optics can occur due to the atomized material, thus impairing the machining result. SUMMARY OF THE INVENTION

[0020] The object of the invention is to provide a device for processing a workpiece with an ion beam in which the extraction grating or other parts of the ion optics are not or less likely to be contaminated or damaged by atomized workpiece or target material.

[0021] According to the invention, this problem is solved by a device for processing a workpiece with an ion beam, which has a holder for receiving the workpiece and an ion source configured to generate ions. An extraction grid connected to a voltage source, which may be part of an extraction grid system, is configured to accelerate the ions generated by the ion source such that a directed ion beam emerges from the extraction grid. A magnetostatic beam deflection unit is configured to deflect the ion beam emerging from the extraction grid and, for this purpose, includes a yoke-free cylindrical coil.

[0022] The invention is based on the consideration that the atomized workpiece or target material is generally uncharged. As a result, the atomized material does not interact with the magnetic field generated by the magnetostatic beam deflection unit and is consequently not deflected. The beam deflection unit thus acts like a kind of switch, deflecting the ion beam in a different direction, but preventing the atomized material propagating in the opposite direction from reaching the extraction grid. In this simple way, it very efficiently prevents atomized material from reaching the extraction grid in significant quantities and causing contamination or damage there.

[0023] If the beam deflection angle caused by the beam deflection unit is very small, atomized material moving away from the workpiece at a larger angle to the surface normal may still reach the extraction grid despite the lack of deflection. Therefore, to achieve a noticeable effect, the beam deflection angle should be at least 5°. An ideal beam deflection angle is 90°, as this prevents any atomized material from traveling directly from the workpiece to the extraction grid. The beam deflection angle can also be greater than 90°, up to 180°. In this case, the ion beam is deflected along a U-shaped path and, after deflection, runs antiparallel to the original emission direction.

[0024] The beam deflection unit is magnetostatic, since the desired effect cannot be achieved with electrostatic deflections, such as those disclosed in US 4,381,453. The deflection electrode and the upstream grounded grid disclosed therein lie entirely within the propagation path of the atomized material and would therefore quickly become contaminated or damaged. The problem solved by the invention is thus merely shifted from the extraction grid to the electrode and the grounded grid.

[0025] In contrast, with magnetostatic deflection, the components that generate the magnetic field can be arranged so that they are not in the path of the atomized material.

[0026] The beam deflection unit can be configured to deflect the ion beam exiting the extraction grid by a deflection angle between 5° and 180° and preferably between 45° and 135°.

[0027] Even with a deflection angle of just 5°, contamination from atomized material is significantly reduced, as most of the atomized material is flung back in the direction of travel (i.e., 0°) and, without deflection, would narrowly miss the extraction grid. The effect of deflection on contamination is maximum when the deflection angle is greater than or equal to 90°, because then practically no atomized material can reach the extraction grid. However, smaller deflection angles of 70° or 80° are sufficient in many practical cases, since very little material is flung away at very shallow angles such as 10° or 20° relative to the workpiece surface.

[0028] Particularly for reasons of installation space, it can be advantageous in certain cases to deflect the beam by 180°, resulting in a U-shaped trajectory for the ion beam. However, very large deflection angles tend to lead to greater beam expansion due to residual inhomogeneities in the magnetic field, which is often undesirable. The particularly preferred deflection angle range is therefore between 70° and 110°, and especially between 80° and 100°. In the simplest case, the magnetic field required for deflection could be generated by a simple permanent magnet, which could, for example, be horseshoe-shaped. However, the magnetic field generated by such simple magnets is very inhomogeneous, leading to varying deflection angles and thus to an unwanted expansion of the ion beam.

[0029] Electromagnets are more economical because they allow for the generation of stronger and more homogeneous magnetic fields. In electromagnets, the coil is often surrounded by a yoke made of an iron core or another ferromagnetic material. However, it is difficult to position the yoke so that it is not in the path of the atomized material. According to the invention, the beam deflection unit therefore contains a yoke-free cylindrical coil.

[0030] The cylindrical coil can, in particular, be a Helmholtz coil having a first axis of symmetry that forms a first angle with the ion beam emerging from the extraction grating, the first angle preferably being 90°. A Helmholtz coil consists of two short circular coils with a large radius R, preferably positioned at a distance R along the axis of symmetry and carrying current in the same direction. Although the field of each individual coil is inhomogeneous, the combined field inside the two coils produces a very homogeneous magnetic field. An ion beam entering the space between the two coils therefore experiences a uniform deflection due to the Lorentz force, which does not lead to any significant beam expansion. The deflection angle can be determined within very wide limits by the strength of the magnetic field and the size of the coils.

[0031] The two coils of the Helmholtz coil do not necessarily have to be circular; they can also have other cross-sections, such as elliptical, triangular, square, or rectangular. Furthermore, the coils do not necessarily have to be exactly the same size. Such deviations from the ideal shape have little effect on the homogeneity of the magnetic field but can, under certain circumstances, offer design advantages.

[0032] The beam deflection unit can include a second Helmholtz coil with a second axis of symmetry. This second axis forms a second angle relative to the ion beam exiting the extraction grid and a further angle relative to the first angle, with the second and further angles preferably each being 90°. With two Helmholtz coils arranged in this way, it is possible to deflect the ion beam in two different directions. While this generally offers no advantage with regard to the degradation caused by the atomized material, it is particularly useful when the beam deflection unit is configured to generate a variable magnetic field to deflect the ion beam exiting the extraction grid by different angles. In such cases, two Helmholtz coils allow the ion beam to be guided slowly over the workpiece within certain limits.For larger workpieces, this eliminates the need for a movable workpiece holder.

[0033] Such a variable magnetic field is still considered magnetostatic, since the change in magnetic field strength is below 1 Hz and is therefore an order of magnitude slower than in alternating electromagnetic fields, such as those used in electron tubes to guide the electron beam.

[0034] Instead of Helmholtz coils, other yoke-free cylindrical coils can also be used, e.g., Maxwell, Braunbeck, or Barker coils. These coils consist of three or more individual coils and, compared to Helmholtz coils, have larger areas where the magnetic field is homogeneous.

[0035] In one embodiment, the device includes an absorber positioned in the propagation path of particles carried along in the ion beam. These particles are uncharged and therefore not deflected by the beam deflection unit. The underlying principle is that, even under a good vacuum, charge transfer collisions always occur during the propagation of an ion beam through a process atmosphere to the workpiece. In these collisions, the ion beam transfers its charge to neutral particles in the residual gas, depending on the propagation length. This effect is independent of any magnetic field. With typical parameters, after a propagation length of approximately 600 mm, about 10% to 30% of the ions have transferred their charge and are electrically neutral. These particles continue to travel with the ion beam, but due to their lack of charge, they are not deflected by the magnetic field of the beam deflection unit and therefore do not reach the workpiece.The absorber captures these uncharged particles in such a way as to prevent atomization, which would lead to contamination or damage of the extraction grid.

[0036] Preferably, the absorber is made of graphite because this material exhibits particularly low atomization by incident uncharged particles. To further reduce the number of absorber particles flying towards the extraction grid, the absorber can have a surface facing the uncharged particles with a surface normal inclined to the direction of propagation of the incident uncharged particles. Due to the inclination of the absorber surface, less atomized absorber material spreads out in such a way that it can reach the extraction grid. The inclination can be, for example, between 50° and 70°, and preferably between 45° and 55°. This reduces the amount of absorber material reaching the extraction grid by up to 80%.

[0037] In one embodiment, the device has a pivot joint designed to pivot the ion source, the extraction grid, and the beam deflection unit together about a mechanical axis. This makes it possible to move the ion beam over the workpiece without having to change the magnetic field.

[0038] Preferably, the extraction grating is shaped such that the emerging ion beam converges at a focal point and the beam deflection unit is located in the region of the focal point. This allows the beam deflection unit to be particularly small because the ion beam has its smallest diameter at the focal point. For focusing purposes, the extraction grating can have a curvature, as is known in the prior art.

[0039] In one embodiment, the device has a replaceably mounted dirt collection plate, with the jet deflection unit located along a line of sight between the workpiece or target and the dirt collection plate. The atomized material emanating from the workpiece or target, which is not deflected by the jet deflection unit due to a lack of charge, is deposited on the dirt collection plate. Therefore, the dirt collection plate is preferably arranged parallel to the surface of the workpiece or target. Replacing such a separate dirt collection plate is generally easier than replacing a housing wall, which the atomized material would otherwise strike without a dirt collection plate.

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. These show:

[0042] Figure 1 shows a machining device according to the invention in a first embodiment in a schematic top view;

[0043] Figure 2 shows a section of the machining device in a perspective view;

[0044] Figure 3 shows a variant of the processing device shown in Figures 1 and 2 in a schematic top view, in which the ion beam is focused and an absorber is arranged obliquely to the direction of flight of neutral particles;

[0045] Figure 4 shows a second embodiment of a machining device according to the invention, in which essential parts can be pivoted together; and

[0046] Figure 5 shows a section of a machining device according to a further variant with two orthogonally arranged Helmholtz coils in a perspective view.

[0047] DESCRIPTION OF PREFERRED EXAMPLES

[0048] 1. First embodiment

[0049] Figure 1 shows a schematic top view of a machining device according to the invention, designated as 10. The machining device 10 comprises a housing 11 in which a vacuum is maintained, and a holder 12 for receiving a workpiece 14. The workpiece 14 can be moved by means of the holder, as indicated in Figure 1 by a double arrow 16. The holder 12 can, for example, be designed as a traversing table with two orthogonal axes of travel. Such traversability is particularly advantageous for large workpieces 14 that are to be machined successively using an ion beam 18, indicated by several lines.

[0050] The ions that make up the ion beam 18, and which may be, for example, argon ions, are generated by an ion source 20, which is only schematically indicated in Figure 1.

[0051] The processing device 10 also includes an ion optic in the form of an extraction grating 22, which is connected to a high-voltage source 24. Typically, not just one, but several extraction gratings 22 are provided, to which different voltages are applied, forming an extraction grating system. The extraction grating 22 shown in Figure 1 then corresponds to the outer extraction grating of such a system, i.e., the one facing away from the ion source 20.

[0052] The extraction grid 22 accelerates the ions generated by the ion source 20 such that the ion beam 18 emerges directed from the extraction grid 22. Contrary to what the illustration in Figure 1 suggests, the partial beams emerging from the openings 26 of the extraction grid 22 superimpose in such a way that a broad and largely homogeneous ion beam 18 is formed at a short distance behind the extraction grid 22.

[0053] The processing device 10 also includes a beam deflection unit 28, which deflects the ion beam 18 exiting the extraction grid 22 by 90°.

[0054] In the perspective, but also schematic, representation of Figure 2, it can be seen that the beam deflection unit 28 contains a Helmholtz coil 30, which in turn comprises two individual coils 32a, 32b. The two individual coils 32a, 32b are arranged concentrically and define an axis of symmetry 34, which runs at an angle of 90° to the direction 36 of the ion beam 18 emerging from the extraction grid 22. A largely homogeneous magnetic field 38 is formed between the individual coils 32a, 32b, which is indicated in Figure 2 by magnetic field lines.

[0055] When positively charged ions propagating along direction 36 pass through the magnetic field 38, they are deflected away from the viewer according to the left-hand rule in Figure 2 due to the Lorentz force. With suitable geometry and current in the individual coils 32a, 32b, the deflection angle can be approximately 90°, as can be clearly seen in the top view of Figure 1. Possible parameters are, for example: coil radius = 140 mm, coil length = 80 mm, number of turns = 2400, coil current = 14.9 A, magnetic flux = 0.159 T. This results in a deflection radius of 180.5 mm.

[0056] Since the magnetic field 38 is largely homogeneous, the ion beam 18 is not significantly expanded in the beam deflection unit 28. In the illustrated embodiment, the ion beam 18 strikes the workpiece 14 perpendicularly. It is assumed here that the workpiece 14 is to be etched with the ion beam 18 in order to remove material from its surface. The kinetic energy of the ions causes material to be ejected upon impact with the surface of the workpiece 14, as indicated in Figure 1 for a point in the center of the workpiece 14. The emission directions 40 of the atomized material are indicated by arrows in Figure 1; the length of the arrows represents the percentage of material emitted in the respective emission direction 40. It can be seen that most of the material is emitted in the opposite direction to the incidence of the ion beam 18 or at small angles to it.However, the proportion of material that is ejected at an angle of 45° to the surface normal of the workpiece 14 is comparatively small.

[0057] The atomized material, propelled in the beam directions 40, is predominantly uncharged. Since the Lorentz force only acts on charged particles, this material is not deflected in the beam deflection unit 38. Dotted lines 42 in Figure 1 indicate how the atomized material falls onto and settles on a replaceably mounted spray plate 44 after passing through the beam deflection unit 28 without deflection. The beam deflection unit 28 thus ensures that the atomized material settles exclusively on the dirt collection plate 44 and does not reach the extraction grid 22 or penetrate the ion source 20 through its openings 26.

[0058] In Figure 1, the dotted lines indicate the trajectories 46 of neutral particles that, coming from the extraction grid 22, have passed through the beam deflection unit 28 but have not been deflected due to their neutrality. Such neutral particles are formed when ions interact with residual gas present in the processing device 10. The ions transfer their charge to atoms or molecules in the residual gas, which is thus ionized. Since the ionized residual gas particles are not accelerated by the extraction grid 22, they are diffusely deflected by the beam deflection unit 28 without significantly influencing the process.

[0059] The aforementioned neutral particles, which propagate along the trajectories 46, have high kinetic energy and regularly cause atomization upon impact with surfaces. Therefore, the processing device 10 includes an absorber 48, which is positioned in the trajectory 46 such that the neutral particles impact it. The material of the absorber 48 is selected to minimize atomization upon impact of the neutral particles. For this purpose, the absorber 48 can, for example, be made of graphite. Minimal atomization is necessary because the material atomized by the neutral particles at the absorber 48 can pass unhindered through the beam deflection unit 28 and reach the extraction grid 22.

[0060] 2. Variants and further examples of implementation

[0061] Figure 3 shows a variant of the machining device 10 shown in Figure 1 in a similarly schematic top view.

[0062] In this variant, the absorber 48 is not arranged perpendicularly, but at an angle to the trajectories 46 of the neutral particles. In the illustrated embodiment, the surface normal 50 of the absorber 48 runs at an angle of approximately 45° to the mean direction of propagation 52 of the neutral particles. As a result of this inclination, the material atomized at the absorber 48 is predominantly deflected in such a way that it cannot return to the beam deflection unit 28, but is deflected by a total of approximately 90° away from the direction of propagation 52 of the neutral particles.

[0063] Furthermore, in the processing device 10 shown in Figure 3, the extraction grid 22 is concavely curved. In this way, the partial beams emerging from the openings 26 converge, creating a focusing effect. The focal point 54, where the ion beam 18 has its smallest cross-section, is located in the center of the beam deflection unit 28. Due to the particularly small beam cross-section there, a compact Helmholtz coil 30 can be used and / or the homogeneity of the magnetic field can be improved.

[0064] A circular aperture 53 ensures that no ions can reach locations outside the workpiece 14.

[0065] Figure 4 shows a variant of the processing device 10 in a representation based on Figures 1 and 3. In this variant, the processing device 10 has a pivot joint 56, by means of which the ion source 20, the extraction grid 22, the beam deflection unit 28, the absorber 48 and the dirt collection plate 44 can be pivoted together about a mechanical axis 58. By such a pivoting motion, the ion beam 18 is also pivoted, so that it can sweep over the workpiece 14 in a direction perpendicular to the plane of the paper.

[0066] If the strength of the magnetic field 38 generated by the Helmholtz coil 30 is also varied, the ion beam 18 can additionally be pivoted in a plane perpendicular to the magnetic field lines 38. Different trajectories of the ions are indicated by solid and dashed lines in Figure 4.

[0067] Finally, Figure 5 shows another variant of a machining device 10 in a representation based on Figure 2. In this variant, the beam deflection unit 28 has not just one, but two Helmholtz coils 30, 60, whose axes of symmetry 34 and 60, respectively, are oriented as follows:

[0068] 62 run perpendicular to each other. In this way, it is possible to deflect the emerging ion beam variably in two orthogonal directions by changing the magnetic fields in the Helmholtz coils 30, 60. This allows even larger workpieces 14 to be processed by the ion beam 18 without having to move the workpiece 14 from the holder 12 and / or mechanically pivot parts of the processing device 10, as is the case in the embodiment shown in Figure 4.

Claims

PATENT CLAIMS 1. Device for machining a workpiece (14) with an ion beam (18), with a holder (12) for receiving the workpiece (14), an ion source (20) which is configured to generate ions, an extraction grid (22) connected to a voltage source (24), which is configured to accelerate the ions generated by the ion source (20) such that a directed ion beam (18) emerges from the extraction grid (22), and with a beam deflection unit (28) which is configured to deflect the ion beam (18) exiting the extraction grid (22), characterized by the fact that the beam deflection unit (28) is magnetostatic and contains a yoke-free cylindrical coil (30, 60).

2. Device according to claim 1, characterized in that the beam deflection unit (28) is configured to deflect the ion beam (18) exiting the extraction grid (22) by a deflection angle which is between 5° and 180°.

3. Device according to claim 1 or 2, characterized in that the beam deflection unit (28) has a Helmholtz coil (30) having a first axis of symmetry (34) which encloses a first angle to the ion beam (18) exiting the extraction grid (22), wherein the first angle is preferably 90°.

4. Device according to claim 3, characterized in that the beam deflection unit (28) has a second Helmholtz coil (60) having a second axis of symmetry (62) which forms a second angle to the ion beam (18) emerging from the extraction grid (22) and a further angle to the first angle, wherein the second angle and the further angle are preferably each 90°.

5. Device according to any one of the preceding claims, characterized by an absorber (48) arranged in a propagation path of particles carried along in the ion beam (18) which are uncharged and therefore not deflected by the beam deflection unit (28).

6. Device according to claim 5, characterized in that the absorber (48) has a surface facing the uncharged particles with a surface normal (50) which is arranged inclined to a propagation direction (52) of the incident uncharged particles.

7. Device according to one of the preceding claims, characterized in that the beam deflection unit (28) is configured to generate a variable magnetic field (38) in order to deflect the ion beam (18) emerging from the extraction grid (22) by different deflection angles.

8. Device according to one of the preceding claims, characterized by a pivot joint (56) which is configured to pivot the ion source (20), the extraction grid (22) and the beam deflection unit (28) together about a mechanical axis (58).

9. Device according to one of the preceding claims, characterized in that the extraction grid (22) is shaped such that the emerging ion beam (18) converges at a focal point (54), and that the beam deflection unit (28) is arranged in the region of the focal point (54).

10. Device according to claim 2, characterized by a replaceably attached dirt collection plate (44), wherein the jet deflection unit (28) is located along a line of sight between the workpiece (14) or a target and the dirt collection plate (44).

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