Optical device for generating an x-ray beam

Abstract

An optical device includes a photocathode and an optic configured to direct a light beam toward the photocathode, the light beam forming a submicron spot on the photocathode. The photocathode has air resistance, is made of a material selected from the group consisting of Na2O, K2O, and Rb2O, and is configured to convert the light beam into an electron beam.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to U.S. Provisional Patent Serial No. 63 / 492,204, filed March 24, 2023, the entire contents of which are incorporated herein by reference. BACKGROUND

[0003] Thermionic electron emitters are electron sources that use heat to give electrons enough energy to overcome the work function of the material and “boil” into the vacuum. However, thermionic electron emitters can work at very high temperatures (about 1550 °C) and the resulting electron beam has a high thermal emissivity that makes the beam difficult to focus with a Gaussian cross-section and can melt the anode used to produce X-rays. Currently, because the electron beam melts the anode surface, the focused spot at the anode must be moved every few minutes to keep the X-rays constant.

[0004] There is an increasing need to provide more durable electron sources. BRIEF DESCRIPTION OF DRAWINGS

[0005] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The organization structure of the invention, together with its objects, features, and advantages, can best be understood by reference to the following detailed description, taken in connection with the accompanying drawings in which:

[0006] Figure 1 An example of intrinsic thermal emissivity prediction is shown;

[0007] Figure 2 An example of components of an optical device is shown;

[0008] Figure 3 An example of components of an optical device is shown;

[0009] Figure 4 An example of a photocathode gun is shown;

[0010] Figure 5 An example of a photocathode electron gun with a back-on-anode is shown;

[0011] Figure 6 An example of a method is shown; and

[0012] Figure 7 An example of a position of a photocathode gun in a system. DETAILED DESCRIPTION

[0013] According to an embodiment, there is provided an optical device comprising a photocathode, and optics configured to direct a light beam towards the photocathode. The light beam forms a sub-micron spot on the photocathode. The photocathode is air resistant, made of a material selected from Na2O, K2O and Rb2O, and configured to convert the light beam into an electron beam. The electron beam can be sent to an anode configured to generate an X-ray beam after the electron beam is generated.

[0014] According to an embodiment, the optics comprise a beam shaping unit configured to convert an input light beam having a Gaussian beam profile into a converted light beam having an airy disk beam profile.

[0015] According to an embodiment, the optics further comprise a focusing unit configured to focus the converted light beam into a flattened top beam forming a sub-micron spot on the photocathode.

[0016] According to an embodiment, the light beam is a visible light beam having a wavelength in the range of 380 to 532 nanometers.

[0017] According to an embodiment, the optical device comprises a diamond amplification photocathode located downstream of the photocathode and configured to amplify the electron beam emitted by the photocathode to provide an amplified electron beam.

[0018] According to an embodiment, the amplified electron beam is sent to an anode configured to convert the amplified electron beam into an X-ray beam.

[0019] According to an embodiment, the diamond amplification photocathode comprises a conductive metal element, a diamond film and a hydrogen-terminated surface.

[0020] According to an embodiment, the photocathode and the optics are enclosed within a conductive shield comprising an output aperture.

[0021] According to an embodiment, at least a portion of the diamond amplification photocathode is located within the output aperture.

[0022] According to an embodiment, the optical device comprises a first biasing circuit for introducing an extraction potential between the photocathode and the diamond amplification photocathode.

[0023] According to an embodiment, at least a portion of the first biasing circuit is enclosed by the conductive shield.

[0024] According to an embodiment, the optical device comprises a second biasing circuit for introducing an acceleration voltage between the anode and the diamond amplification photocathode.

[0025] According to an embodiment, the optical device comprises a vacuum aperture.

[0026] An optical device is provided that includes an electron gun that uses a robust, air-tolerant, high quantum efficiency photocathode, such as a M2O group (M: Na, K, Rb) photocathode, that can provide a high current, low thermal emissivity electron beam that can be better focused. The low thermal emissivity allows for more efficient focusing of the electron beam and a uniform cross-sectional power distribution at the anode, allowing the anode to handle more power and thus more efficiently produce a higher brightness X-ray source.

[0027] An air-tolerant photocathode with high quantum efficiency can also be used to control wafer charging in X-ray photoelectron spectroscopy (XPS) applications.

[0028] A high brightness, low thermal emissivity electron beam can enable a nanometer length scale X-ray imaging system that is needed for metrology and inspection in the semiconductor market.

[0029] To produce a higher brightness X-ray source, the electron beam needs to be focused to a smaller spot and have a uniform cross-section, allowing it to produce more X-rays per solid angle.

[0030] Current solutions use a new family of photocathodes called anti-fluorite materials (Na2O, K2O, Rb2O) that guarantee air stability because the presence of oxygen allows these materials to be stable to oxidation. The use of photocathodes is generally limited because the vacuum level required for operation is very low (10 -12 Torr or better, while the vacuum level for a normal gun is 10 -9 Torr, so it is difficult to achieve the operating conditions for a photocathode in a production tool for a long time.

[0031] In accelerators and free electron sources, the operating conditions are met and photocathodes have been successfully used to produce high brightness electron beams with long lifetime. In various metrology tools, the operating vacuum level is about 10 -7 Torr and is also contaminated by wafers that enter the chamber every minute with a layer of water that is released into the chamber vacuum creating an oxidizing environment that would rapidly degrade the performance of a normal photocathode. The anti-fluorite photocathodes are more robust in these conditions due to their oxidized state and thus allow the introduction of a photocathode in the operating field of various metrology tools.

[0032] The photocathode is air-tolerant because it can withstand operation at a vacuum level of about 10 -7 Torr (or better), a vacuum level that is also contaminated by wafers.

[0033] In the current solution, alkali antimonide photocathodes such as K2CsSb are replaced by M2O (M: Na, K, Rb) group. The M2O group includes Na2O, K2O, Rb2O.

[0034] It has been found that the M2O group exhibits comparable intrinsic thermal emissivity to K2CsSb, but with the additional advantage of better air stability Figure 1 Even though the presence of oxide anions in the M2O group allows them to be stable to oxidation, their ionic nature can be the reason for the M2O group to have a higher bandgap relative to K2CsSb.

[0035] It has also been found that the M2O (M: Na, K, Rb) anti-fluorite group exhibits low thermal emissivity when irradiated by visible light lasers in the 2.33 eV to 3.26 eV energy range (380 nm to 532 nm).

[0036] Since visible light driven lasers can be irradiated by the second harmonic of a Nd:YAG laser, and can produce a high spatial quality beam, photocathodes that can operate using visible light driven lasers are preferred. The Nd:YAG laser can also work in continuous wave (CW) or pulsed (Q-switched) mode.

[0037] Transmissive mode M2O photocathode films irradiated by a flat-top laser or LED beam in the 2.33 eV to 3.26 eV energy range (380 nm to 532 nm) can produce a flat-top electron beam with very low thermal emissivity (e.g. below 1.5 microns / mm) Figure 1 ).

[0038] According to embodiments, a laser beam shaping optical unit is used to convert a Gaussian-shaped laser beam profile into a flat-top beam profile. Refractive beam shapers enable uniformity of the intensity profile and a flat phase front.

[0039] According to embodiments, the beam shaper is a refractive ADLOPTICA FOCAL-ΠSHAPER Q FLAT TOP BEAMSHAPERS TM ) of EDMUND OPTICS INC. TM , Barrington New Jersy, which converts an input Gaussian beam into a collimated Airy disk profile. This forms a flat-top focal spot after lens focusing limited by diffraction. Their compact design and their close to 100% efficiency on these components make them easy to integrate into the recommended air-resistant photocathode electron gun.

[0040] Figure 2An optical train 80 is shown that includes a light source 81 that outputs an input beam 101 that is collimated by a collimator 82 to provide a collimated beam 102 having a Gaussian beam profile.

[0041] The collimated beam 102 is shaped by a beam shaper 83, such as a refractive ADL Optica FOCAL-HS HAPPER Q FLATTOP BEAM SHAPER, to provide a shaped beam 103 having an Airy disk profile that is focused by a focusing unit 84 to provide a focused beam 104 having a flat-top beam profile that forms a small uniform spot on the photocathode. Thus, a higher brightness X-ray source can be produced with the same anode structure. The photocathode can be integrated into the source assembly.

[0042] Figure 2 A conductive shield 121 and a cap 122 having an output aperture 93 are also shown.

[0043] Figure 2 A diamond amplification photocathode 92 downstream of the photocathode 91 is also shown. The diamond amplification photocathode 92 amplifies the electron beam 112 output from the photocathode 91 to provide an amplified electron beam 114 that passes through the output aperture 93 and impinges on an anode (not shown in Figure 2

[0044] In Figure 2 , the photocathode 91 is shown to include a fused silica optical element 91-1, a conductive metal element 91-2 such as a platinum underlayer, and a photocathode layer 91-3 made of M2O (M: at least one of Na, K, Rb, members of the fluorite group).

[0045] In Figure 2 , the diamond amplification photocathode 92 is shown to include a conductive metal element 92-1, a diamond film 92-2, and a hydrogen-terminated surface 92-3.

[0046] In Figure 2 , the photocathode 91 and the diamond amplification photocathode 92 are integrated together, and at least a portion of the diamond amplification photocathode 92 is within the output aperture 93.

[0047] Figure 2 A first biasing circuit 94 configured to introduce an extraction potential between the photocathode and the diamond amplification photocathode, and a second biasing circuit 95 configured to introduce an acceleration field between the diamond amplification photocathode and the anode are also shown.

[0048] Figure 3 ​Various optical components are shown, such as a voltage insulator 123 and optics (positioned within an intermediate region 143, which is located between a first plunge 141 and a second plunge 142) are enclosed by a conductive shield 121. Figure 3 A vacuum aperture 129 is also shown.

[0049] The modified high voltage pole assembly will allow flat top visible light driven laser or LED illumination from the back side, as shown in the lower portion of Figure 6

[0050] Figure 4 An example of a compact photocathode electron gun is shown, comprising a subunit 100 comprising at least a photocathode, optics, a conductive shield, and a cap. The photocathode electron gun further comprises an isolation valve actuator 126, and an ion / gas getter pump 125 forming a vacuum within a gun vacuum manifold 127. The vacuum in which the photocathode resides is set by the pressure level within the gun vacuum manifold 127.

[0051] The isolation valve actuator 126 is configured to keep the environment of the photocathode as clean as possible.

[0052] Figure 5 A photocathode electron gun is shown, comprising a subunit 100 comprising at least a photocathode, optics, a conductive shield, and a cap, an ion / gas getter pump 125 forming a vacuum within a gun vacuum manifold 127, an input cable 124, an isolation valve actuator 126, and a structural element 128 interfacing with the cap 122, a flat plate 130 connected with the structural element 138, and an amplified electron beam output optical element 132.

[0053] According to an embodiment, the photocathode is a transmissive photocathode.

[0054] According to an embodiment, the photocathode is a reflective photocathode.

[0055] According to an embodiment, the optical device is configured to generate an X-ray beam for various uses, e.g., for metrology applications and / or inspection applications in the semiconductor industry.

[0056] According to an embodiment, the optical device is used for controlling wafer charging in XPS applications.

[0057] According to an embodiment, the optical device is used as an X-ray beam source for electron beam metrology applications and / or for inspection applications in the semiconductor industry.

[0058] According to an embodiment, the optical device comprises a high-brightness electron source, which increases the throughput of a system comprising the optical device.

[0059] Figure 6 ​is an example of a method 200 for generating an X-ray beam.

[0060] According to an embodiment, the method 200 starts at step 210 by directing, by an optical device, a light beam toward a photocathode, wherein the light beam forms a sub-micron spot on the photocathode.

[0061] According to an embodiment, step 210 is followed by step 220 of converting, by the photocathode, the light beam into an electron beam, the photocathode being air resistant and made of a material selected from Na2O, K2O and Rb2O.

[0062] According to an embodiment, the method 200 then generates, after generating the electron beam, an X-ray beam by an anode.

[0063] According to an embodiment, step 210 comprises converting, by a beam shaping unit of the optical device, an input light beam having a Gaussian beam profile into a converted light beam having an Airy disk beam profile, and focusing, by a focusing unit of the optical device, the converted light beam into a flattened top beam that forms a sub-micron spot on the photocathode.

[0064] According to an embodiment, step 220 comprises converting, by the anode, the electron beam into the X-ray beam.

[0065] According to an embodiment, the method 200 comprises step 225 of amplifying, by a diamond amplification photocathode, the electron beam to provide an amplified electron beam.

[0066] In this case, the method 200 then converts, by the anode, the amplified electron beam into the X-ray beam.

[0067] According to an embodiment, the diamond amplification photocathode comprises a conductive metal element, a diamond film and a hydrogen-terminated surface.

[0068] According to an embodiment, the photocathode and the optical device are enclosed within a conductive shield comprising an output aperture.

[0069] According to an embodiment, at least a portion of the diamond amplification photocathode is located within the output aperture.

[0070] According to an embodiment, the method 200 further comprises step 240 of biasing the optical device.

[0071] According to an embodiment, step 240 comprises introducing, by a first biasing circuit, an extraction potential between the photocathode and the diamond amplification photocathode.

[0072] According to an embodiment, step 240 further comprises introducing, by a second biasing circuit, an acceleration field between the diamond amplification photocathode and the anode. According to an embodiment, the extraction field is about 3Kv (other values can be applied) and the acceleration field is about 19Kv.

[0073] According to an embodiment, at least part of the first biasing circuit is surrounded by an electrically conductive shield.

[0074] According to an embodiment, the electrically conductive shield and the voltage insulator are intended to shield the environment of the optical device from the extraction and / or acceleration fields.

[0075] According to an embodiment, the optical device comprises a vacuum hole.

[0076] According to an embodiment, the light beam is a visible light beam having a wavelength in the range of 380 to 532 nanometers.

[0077] Figure 7 is an example of the position of the photocathode gun in the system, such as in the VERAFLEX® system of Nova Company in Ra'anana, Israel. TM

[0078] In the above detailed description, numerous specific details are set forth in order to provide a thorough understanding of the application. However, it will be understood by those skilled in the art that the present application can 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 application.

[0079] In the concluding portion of this specification, particularity is pointed out and distinctly claimed as subject matter of the present application. However, the organization structure of the application, the manner and process of operation thereof, as well as the objects, features and advantages thereof, can be better understood by reference to the following detailed description when read in conjunction with the accompanying drawings.

[0080] It will be understood that the elements shown in the figures are not necessarily to scale, for the sake of simplicity and clarity. For example, the dimensions of some of the elements can be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals can be repeated among the figures to indicate corresponding or analogous elements.

[0081] Since the illustrated embodiments of the present application can be implemented using electronic components and circuits known to those skilled in the art, for the purpose of understanding and recognizing the basic concepts of the present application, and in order not to obscure or confuse the teachings of the present application, the above illustrated content will not be explained in detail beyond what is necessary.

[0082] Any reference in this specification to a method should be applied mutatis mutandis to a system capable of carrying out the method.

[0083] Any reference in this specification to a system should be applied mutatis mutandis to a method that can be carried out by the system

[0084] ​In the foregoing specification, the application has been described with reference to specific examples of embodiments of the application. It is evident, however, that various modifications and changes can be made thereto without departing from the broader spirit and scope of the application as set forth in the appended claims.

[0085] Also, 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 and not necessarily for describing permanent relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the application described herein are capable of operation in other orientations than described or otherwise shown in the figures.

[0086] 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 can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermediate 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.

[0087] Also, the illustrated examples, which actual details of operation does not limit the scope of the application. Multiple operational are combinable into a single operation, a single operation is

[0088] Also, for example, in one embodiment, examples can be implemented as circuitry located on a single integrated circuit or within a same device.

[0089] However, other modifications, variations and alternatives are also possible. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive sense.

[0090] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude other elements or steps than those listed in a claim. Furthermore, the term "a" or "an" as used in this document does not exclude a plurality of things. Also, the use of introductory phrases such as "at least one" and "one or more" in the claims should not be construed as implying that the invention is limited to only those implementations that explicitly state such phrase. Similarly, it is recognized not to render equal to the expressions "comprising a" or "comprising one of" followed by a list of elements or options "only" one of those elements or options can be present in a claim. It is intended to be understood that such phrasing as "one or more" preceding a list of items, such as interpreted the fresh claim limitation to exclude other claim limitations than following the phrasing "one or more" preceding the list of items. The same applies to phrasing "at least one" preceding a list of items. As used herein, the indefinite articles "a" and "an" are defined as one or more unless specified otherwise. The same applies to the indefinite article "the" and the definite article "the". Unless otherwise specified, terms such as "first" and "second" are used for any arbitrary identification and are not intended to signify a chronological or other priority. The fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0091] While certain features of the application have been illustrated and described, 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 application.

Claims

1. An optical apparatus for generating X-rays, the optical apparatus comprising: a photocathode; and optics configured to direct a light beam toward the photocathode, wherein the light beam forms a sub-micron spot on the photocathode; wherein the photocathode is air-resistant, made of a material selected from Na20, K20, and Rb20, and configured to convert the light beam into an electron beam.

2. The optical device of claim 1, wherein, The optics include a beam shaping unit configured to convert an input light beam having a Gaussian beam profile into a converted light beam having an Airy disk beam profile.

3. The optical device of claim 2, wherein, The optics further include a focusing unit, The focusing unit is configured to focus the converted light beam into a flattened top beam that forms the sub-micron spot on the photocathode.

4. The optical device of claim 1, wherein, The light beam is a visible light beam having a wavelength in a range of 380 to 532 nanometers.

5. The optical apparatus of claim 1, further comprising a diamond amplification photocathode located downstream of the photocathode and configured to amplify the electron beam to provide an amplified electron beam.

6. The optical device of claim 5, wherein, The diamond amplification photocathode includes a conductive metal element, a diamond film, and a hydrogen-terminated surface.

7. The optical device of claim 5, wherein, The photocathode and the optics are enclosed within a conductive shield including an output aperture.

8. The optical device of claim 7, wherein, At least a portion of the diamond amplification photocathode is located within the output aperture.

9. The optical apparatus of claim 7, further comprising a first biasing circuit for introducing an extraction potential between the photocathode and the diamond amplification photocathode.

10. The optical device of claim 9, wherein, At least a portion of the first biasing circuit is enclosed by the conductive shield.

11. The optical apparatus of claim 1, including a vacuum aperture.

12. A method for generating an electron beam for generating X-rays, the method comprising: directing, by optics, a light beam toward a photocathode, wherein the light beam forms a sub-micron spot on the photocathode; and converting, by the photocathode, the light beam into an electron beam, the photocathode being air-resistant and made of a material selected from Na20, K20, and Rb20.

13. The method of claim 12, further comprising converting, by a beam shaping unit of the optics, an input light beam having a Gaussian beam profile into a converted light beam having an Airy disk beam profile.

14. The method of claim 13, further comprising focusing, by a focusing unit of the optics, the converted light beam into a flattened top beam that forms the sub-micron spot on the photocathode.

15. The method of claim 12, further comprising amplifying the electron beam by a diamond amplification photocathode to provide an amplified electron beam, wherein, Generation of an X-ray beam includes converting, by an anode, the amplified electron beam into the X-ray beam.

16. The method of claim 15, wherein, The diamond amplification photocathode includes a conductive metal element, a diamond film, and a hydrogen-terminated surface.

17. The method of claim 15, wherein, The photocathode and the optics are enclosed within a conductive shield including an output aperture.

18. The method of claim 17, wherein, At least a portion of the diamond amplification photocathode is located within the output aperture.

19. The method of claim 15, further comprising introducing an extraction potential between the photocathode and the diamond amplifying photocathode by a first biasing circuit.

20. The method of claim 12, wherein, The light beam is a visible light beam having a wavelength in the range of 380 to 532 nanometers.

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