Charged particle source for x-ray applications

Abstract

A charged particle source for use in x-ray applications, the charged particle source includes (a) a source electrode having an emission surface, the source electrode is configured to emit charged particles of a first polarity towards an extraction element to form a charged particle beam; and (b) an aperture lens that comprises an aperture that is concentric to the source electrode and is configured, due to a bias difference of a second polarity between the source electrode and the aperture lens, to limit an area of the source electrode from which the charged particle beam emanates, the second polarity is opposite to the first polarity.

Description

CHARGED PARTICLE SOURCE FOR X-RAY APPLICATIONSCROSS REFERENCE

[0001] This application claims priority from US provisional patent 63 / 637,366 filing date April 22,2024 which is incorporated herein in its entirety.BACKGROUND

[0002] There is a growing need to improve evaluation of samples.BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

[0004] FIG. 1 illustrates an example of a bias system and an electron source that includes an aperture lens having a truncated cone shaped aperture, a shield electrode, and a suppressing electrode;

[0005] FIG. 2 illustrates an example of an electron source that includes an aperture lens having a truncated cone shaped aperture, and a suppressing electrode;

[0006] FIG. 3 illustrates an example of an electron source that includes an aperture lens having a truncated cone shaped aperture;

[0007] FIG. 4 illustrates an example of an electron source that includes an aperture lens having a truncated cone shaped aperture, and a guard ring;

[0008] FIG. 5 illustrates an example of an electron source that includes an aperture lens having a cylindrical shaped aperture, and a suppressing electrode;

[0009] FIG. 6 illustrates an example of shank emission suppression by the aperture lens; and

[0010] FIG. 7 illustrates an example of a method.DETAILED DESCRIPTION OF THE DRAWINGS

[0011] The current application is related to a charged particle source. For simplicity of explanation some of the examples refer to electrons and / or an electron beam and / or to an electron source.

[0012] Any reference to electrons and / or an electron beam and / or to an electron source should be applied mutatis mutandis to ions and / or an ion beam and / or to an ion source and / or should be applied mutatis mutandis to positrons and / or a positron beam, and / or to a position source.

[0013] Any reference to a cathode (such as but not limited to a thermionic cathode) should be applied mutatis mutandis to a source electrode capable of emitting ions and / or positrons.

[0014] According to an embodiment, a polarity of bias voltages are reversed if the electron source is an ion source that emits ions or is a positron source that emits positrons instead of (negatively charged) electrons.

[0015] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

[0016] Metrology for semiconductor process control using X-ray requires high throughput. The process control may include evaluating and controlling film composition and thickness, dopant level, impurities, critical dimensions, nanostructures, surface roughness and potential defect levels in order to optimize the semiconductor manufacturing process.

[0017] The X-ray may be a soft X-ray for XRR (X-ray Reflectometry) and XRS (X- ray Scattering) metrology with wavelengths 1-20 nm, or x-rays with energies from 1- lOkeV for XPS and XRF metrologies.

[0018] There is a need to provide an electron source that generated a high flux electron beam that will impinge on an anode material or the anode metal target.

[0019] However, the current density and therefore x-ray flux density is limited by melting and / or deterioration of anode material or the anode metal film.

[0020] Existing electron guns typically provide a gaussian or gaussian type beam spot which has highest current density at the beam center.

[0021] The suggested solution uses an electron beam with a flat current density out to the edge of the beam, thereby improving the X-ray flux by a factor of two - according to thermal simulations.

[0022] Some thermionic cathodes are arranged with an electrode termed “Wehnelt” or “grid” which is biased negatively with respect to cathode potential. This limits the area of electron emission from the cathode surface and fully suppresses emission from the cathode shank but increases the space charge and reduces the available current density. It also causes a sharp focusing of the electron beam, causing high divergence farther along the focusing column.

[0023] It is possible to eliminate the Wehnelt electrode by using a guard ring placed around the cathode to delineate the emission area and to minimize the strong focusing. However, most thermionic cathodes operate at temperatures high enough to continuously evaporate cathode material at a different rate than the guard ring material evaporates. The distortion in the electrostatic potential causes an undesired focusing effect and reduces the cathode lifetime.

[0024] There is a need to generate an electron beam with high current density and a flat top beam profile using a thermionic cathode.

[0025] An aperture lens is provided with an aperture that is concentric with and positioned close to the cathode surface, defining the area of the cathode from which the electron beam originates.

[0026] According to an embodiment, the cathode surface is at least as large as the aperture, allowing recession while maintaining full current and current density.

[0027] According to an embodiment, the aperture diameter can vary from nearly zero up to one and a half the diameter of the cathode flat surface.

[0028] According to an embodiment, the aperture is larger in diameter than the cathode flat, and while it does not limit the emission area directly is does function as an aperture lens to minimize beam divergence.

[0029] According to an embodiment, the spacing of the aperture to the cathode flat can be nearly zero up to twice the aperture diameter.

[0030] According to an embodiment, as the spacing increases, the aperture needs to be biased increasingly positive with respect to the cathode so that the electric field at the cathode is within about 30% of the field on the extractor side. According to an embodiment said maintaining of the field increases the power dissipation in the aperture if it intercepts some of the electrons emitted from the cathode.

[0031] According to one embodiment, a high-brightness cathode, such as but not limited to a lanthanum hexaboride (LaBe) cathode, is provided. The cathode is at least partially enclosed by a negatively biased aperture lens configured to suppress undesiredshank emission. The aperture lens includes an aperture having a diameter of approximately 300 micrometers (pm), positioned at a distance of approximately 200 micrometers (pm) from the emission surface of the LaBe cathode

[0032] According to an embodiment, the aperture lens is biased slightly positive with respect to the cathode so that it can function as an aperture lens. According to an embodiment, the aperture bias is adjusted so that the aperture lens is either diverging or converging as required.

[0033] According to an embodiment, the cathode shape can be either cylindrical (see figures 4 and 6) as illustrated or be a truncated cone (see figures 1-3 and 5), typically 45°. A truncated cone may minimize the power dissipation in the aperture while retaining the cathode face diameter during cathode recession.

[0034] According to an embodiment, the aperture bias is within 30% of the bias needed to maintain a uniform potential gradient. Actual voltage values are dependent on the aperture spacing and the extraction field.

[0035] According to an embodiment, the current density (of the electron beam) can be adjusted by changing the cathode temperature.

[0036] According to an embodiment, the suggested electron source: a. Provides high extraction fields while limiting the area of emission. b. Allows the profile of the electron beam to be an image of the cathode surface as limited by the aperture, thus achieving so-called critical illumination and a flat top beam profile. c. Be tolerant of cathode recession. d. Is configurable to provide a telescopic beam which reduces aberrations in the following lens(es). e. Provides an efficient way to generate a flat top electron beam. f. Provides (due to the implementation of a flat top electron gun) a more uniform power distribution on the anode, creating a brightness increment by a factor of two of the x-ray source. This will permit shorter analysis times per site, leading to an increase in analysis points / WFP.

[0037] According to an embodiment there is provided a charged particle source for use in x-ray applications, the charged particle source includes (a) a source electrode having an emission surface, the source electrode is configured to emit charged particles of a first polarity towards an extraction element to form a charged particle beam; and (b) an aperture lens that comprises an aperture that is concentric to the source electrode and isconfigured, due to a bias difference of a second polarity between the source electrode and the aperture lens, to limit an area of the source from which the charged particle beam emanates, the second polarity is opposite to the first polarity.

[0038] If, for example, the charged particles are negatively charged the aperture lens is positively biased in relation to the source electrode.

[0039] If, for example, the charged particles are positively charged the aperture lens is negatively biased in relation to the source electrode.

[0040] Figure 1 illustrates an electron source 10a for use in x-ray applications, the electron source 10a includes a thermionic cathode 14 having an emission surface 17, the thermionic cathode is configured to emit electrons towards an extraction element 19 to form an electron beam 90. The electron source also includes an aperture lens 20 that includes an aperture 21 that is concentric to the thermionic cathode and is positively biased in relation to the thermionic cathode to limit an area of the cathode from which the electron beam emanates.

[0041] Figure 1 also illustrates bias system 18 for biasing various components of the electron source. For simplicity of explanation the bias system is not shown in other figures.

[0042] According to an embodiment, the bias system is configured to provide bias signals that determine a focal length of the aperture lens.

[0043] According to an embodiment, the bias system is configured to provide a first bias signal to provide a negative focal length of the aperture lens and to provide a second bias signal to provide a positive focal length of the aperture lens.

[0044] According to an embodiment, a diameter of the aperture (denoted A 8) is smaller than a diameter of the emission surface.

[0045] According to an embodiment, a diameter of the aperture does not exceed one point five times a diameter of the emission surface.

[0046] According to an embodiment, a distance (denoted B 9) between the aperture lens and the emission surface does not exceed twice a diameter of the aperture.

[0047] According to an embodiment, a diameter (A 8) of the aperture and a distance (B 9) between the aperture and the emission surface range between one hundred and three hundred microns.

[0048] According to an embodiment, the aperture has a cylindrical shape (see figure 5).

[0049] According to an embodiment, the aperture has a truncated conical shape (see figures 1-4 and 6).

[0050] According to an embodiment, the electron source consists essentially of the extraction element, the thermionic cathode, and the aperture lens. See, for example, figure 3. A biasing system is not illustrated for simplicity of explanation.

[0051] According to an embodiment, the electron source includes a suppressor electrode (denoted 12 in figures 1, 2, 5 and 6) that is concentric to the thermionic cathode and is configured to suppress shank emission.

[0052] According to an embodiment, the electron source includes a guard ring that surrounds at least a portion of the thermionic cathode. See, for example guard ring 15 of figure 4.

[0053] According to an embodiment, the electron source includes a shield electrode that is concentric to the thermionic cathode. See, for example, shield electrode 11 in figure 1.

[0054] According to an embodiment, the aperture lens is positively biased to maintain an electric field at the cathode that is within thirty percent of an electric field at the extraction element.

[0055] According to an embodiment, the aperture can be varied to change the divergence or convergence of the electron beam.

[0056] According to an embodiment, the bias provided to any of the components of the electron source can be varied to defined an electric field local to the cathode. According to an embodiment, a telescopic beam is desirable, because it minimizes the aberrations of the subsequent lens systems.

[0057] According to an embodiment, the focal length of the aperture lens is equal to approximately 4*E / (F1-F2) where E is the electron energy as it passes through the aperture, and Fl and F2 are the electric field strengths on the cathode and the extractor sides of the aperture, respectively. Accordingly - the aperture lens can have a negative focal length (a diverging lens) or positive focal length (converging) dependent on the aperture lens bias.

[0058] Figure 7 illustrates an example of a method 100 for providing an electron beam for use in x-ray applications.

[0059] According to an embodiment method 100 includes step 110 of emitting charged particles of a first polarity from an emission surface of a source electrode towards an extraction element to form a charged particle beam.

[0060] According to an embodiment step 110 is followed by step 120 of limiting an area of the source electrode from which the charged particle beam emanates by an aperture lens that comprises an aperture that is concentric to the source electrode and is biased to introduce a bias difference of a second polarity between the source electrode and the aperture lens, the second polarity is opposite to the first polarity

[0061] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings.

[0062] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

[0063] Because the illustrated embodiments of the present invention may for the most part, be implemented using electrooptic components known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.

[0064] Any reference in the specification to a method should be applied mutatis mutandis to a system capable of executing the method.

[0065] Any reference in the specification to a system should be applied mutatis mutandis to a method that may be executed by the system

[0066] In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims.

[0067] Any reference to “comprising” or “Having” or “including” should be applied, mutatis mutandis to “consisting” and / or should be applied, mutatis mutandis to “consisting essentially of.”

[0068] Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes andnot necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

[0069] Any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected," or "operably coupled," to each other to achieve the desired functionality.

[0070] Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.

[0071] The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.

[0072] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an." The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first" and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited inmutually different claims does not indicate that a combination of these measures cannot be used to advantage.

[0073] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

WE CLAIM1. A charged particle source for use in x-ray applications, the charged particle source comprising: a source electrode having an emission surface, the source electrode is configured to emit charged particles of a first polarity towards an extraction element to form a charged particle beam; and an aperture lens that comprises an aperture that is concentric to the source electrode and is configured, due to a bias difference of a second polarity between the source electrode and the aperture lens, to limit an area of the source electrode from which the charged particle beam emanates, the second polarity is opposite to the first polarity.

2. The charged particle source according to claim 1, wherein a diameter of the aperture is smaller than a diameter of the emission surface.

3. The charged particle source according to claim 1, wherein a diameter of the aperture does not exceed one point five times a diameter of the emission surface.

4. The charged particle source according to claim 1, wherein a distance between the aperture lens and the emission surface does not exceed twice a diameter of the aperture.

5. The charged particle source according to claim 1, wherein a diameter of the aperture and a distance between the aperture and the emission surface range between one hundred and three hundred microns.

6. The charged particle source according to claim 1, wherein the aperture has a cylindrical shape.

7. The charged particle source according to claim 1, wherein the aperture has a truncated conical shape.

8. The charged particle source according to claim 1, consisting essentially of the extraction element, the source electrode, and the aperture lens.

9. The charged particle source according to claim 1, further comprising a suppressor electrode that is concentric to the source electrode and is configured to suppress shank emission.

10. The charged particle source according to claim 1, further comprising a guard ring that surrounds at least a portion of the thermionic cathode.

11. The charged particle source according to claim 1, further comprising a shield electrode that is concentric to the thermionic cathode.

12. The charged particle source according to claim 1, wherein the aperture lens is biased to maintain an electric field at the source electrode that is within thirty percent of an electric field at the extraction element.

13. The charged particle source according to claim 1 comprising a bias system for biasing at least the aperture lens and the source electrode.

14. The charged particle source according to claim 13, wherein the bias system is configured to provide bias signals that determine a focal length of the aperture lens.

15. The charged particle source according to claim 13, wherein the bias system is configured to provide a first bias signal to provide a negative focal length of the aperture lens and to provide a second bias signal to provide a positive focal length of the aperture lens.

16. The charged particle source according to claim 1 further comprising a converter that is configured to convert the charged particle beam to an x-ray beam.

17. A method for providing a charged particle beam for use in x-ray applications, the method comprising: emitting charged particles of a first polarity from an emission surface of the source electrode towards an extraction element to form a charged particle beam; and limiting an area of the source electrode from which the charged particle beam emanates by an aperture lens that comprises an aperture that is concentric to the source electrode by maintaining a bias difference of a second polarity between the source electrode and the aperture lens, the second polarity is opposite to the first polarity .

18. The method according to claim 17, further comprising converting, by a converter, the charged particle beam to an x-ray beam.

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