Electon beam source for x-ray generation for sample evaluation

Abstract

An electron beam source for use in X-ray based sample evaluation, the electron beam source includes (a) an enclosure, (b) a photocathode that is configured to receive a laser beam, to convert the laser beam to a primary electron beam, and to output the primary electron beam within an inner space formed, at least in part, by the enclosure, (c) a diamond unit that comprises a diamond layer that is preceded by a conductive metal layer and is followed by a negative electron affinity surface that is configured to receive the primary electron beam from the inner space, to amplify the primary electron beam to provide an amplified electron beam, and to output the amplified electron beam to an anode aperture; and (d) a bias circuit configured to bias at least one of the photocathode and the diamond amplified photocathode.

Description

ELECTON BEAM SOURCE FOR X-RAY GENERATION FOR SAMPLEEVALUATIONCROSS REFERENCE

[0001] This application claims priority from US provisional patent applications serial number 63 / 587,698 filing date October 3, 2024 - which is incorporated herein by reference. BACKGROUND OF THE INVENTION

[0002] The throughput of X-ray evaluation systems is required to increase in order to cover samples (such as Semiconductor wafers) with smaller dimensions.

[0003] There is a growing to increase the throughput of X-ray evaluation systems while using x-rays that exhibit a high uniformity and smaller cross sections.BRIEF DESCRIPTION OF THE DRAWINGS

[0004] 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:

[0005] FIG. 1 illustrates an example of an electron beam source;

[0006] FIG. 2 illustrates an example of parts of the electron beam source within an evaluation system;

[0007] FIG. 3 illustrates an example of parts of the electron beam source and the environment of the electron beam source;

[0008] FIG. 4 illustrates an example of parts of the electron beam source and the environment of the electron beam source;

[0009] FIG. 5 illustrates an example of the integration of the heater within an inner space of a grounding pin; and

[0010] FIG. 6 illustrates an example of a method.DETAILED DESCRIPTION OF THE DRAWINGS

[0011] The NOVA VERAFLEX™ XPS system of Nova Measuring Instruments Inc. uses a LaBe source to generate an electron beam that is then focused onto a thin aluminum target to generate Al KaX-rays. These are subsequently filtered / focused by a monochromator onto the wafer, where the material characterization is measured. As the semiconductor industry moves to evaluating smaller critical dimensions (CD) nodes, thephysical density of features on the wafer architecture increases and more chip dies are printed onto a single wafer.

[0012] The throughput which at present requires 9 analysis sites at a rate of 20 wafer per hour (WPH) will increase to 19 sites at a 61 WPH. This demands an increase in the brightness of the X-ray source in order to shorten the analysis time per site.

[0013] In addition, a smaller spot size will be required in future nodes. The present electron source is a thermionic electron source, which means that in order for the electrons to have enough energy to overcome the work function of the source, needs to be heated to above 1570°C. For higher brightness we heat the LaB6 to 1800 C.

[0014] This allows the source to generate high electron currents density, but at a cost of high thermal and transverse emittance a measure of how well “bundled” the electrons emerge from the crystal. A high emittance beam can’t be focused tightly on the anode, and the beam shape becomes a clipped Gaussian with a strong power concentration at the center.

[0015] Using a UaBe source to achieve a high throughput requires to heat the UaBe source to very high temperatures (such as 1800 Celsius) which will result in an uneven x- ray beam that may melt the anode.

[0016] Using a thermally assisted photocathode UaBe source heated to 1400 K, below the threshold of thermionic emission will amount to a very low quantum efficiency (about 10'7) and the transverse emittance of the beam due to the emitting geometry of the UaBe source makes this solution very unattractive.

[0017] Using only a single photocathode was also be found to be inefficient as photocathode materials are chosen because they lose electrons easily, in other words they oxidize easily. This means that they are very sensitive to O2 and H2O contamination and must operate in extremely high vacuum levels (XHV) of about 10'12Torr which are not easily achieved outside of controlled laboratory conditions. Several approaches to making air-tolerant photocathodes have been suggested, such as III-N and M2O (M= Na, K, Yb) materials, and through the use of capping layers, such as graphene and hexagonal BN, but these materials are in the early experimental stage and are not currently available for this use.

[0018] There is provided an electron beam source that assists in generating a high brightness x-ray beam that supports high throughput evaluations of samples.

[0019] There is provided an electron beam source for use in X-ray based sample evaluation, the electron beam source includes (a) an enclosure, (b) a photocathode that is configured to receive a laser beam, to convert the laser beam to a primary electron beam, and to output the primary electron beam within an inner space formed, at least in part, by the enclosure, (c) a diamond unit that includes a diamond layer that is preceded by a conductive metal layer and is followed by a negative electron affinity surface that is configured to receive the primary electron beam from the inner space, to amplify the primary electron beam to provide an amplified electron beam, and to output the amplified electron beam to an anode aperture; and (d) a bias circuit configured to bias at least one of the photocathode and the diamond amplified photocathode.

[0020] Figure 1 illustrates an example of the electron beam source that includes: a. Enclosure 30 (including upper and lower insulation ceramics 31 and 32) to define inner space 33. b. Photocathode 40. c. Diamond unit 50 that includes conductive metal layer 51, diamond layer 52 and negative electron affinity (NEA) surface 53. d. Anode 60 having an anode aperture 61. e. Biasing unit 20 that includes extraction potential supply unit 21 configured to form extraction potential 24 between the photocathode and the diamond unit, and acceleration potential supply unit 22 configured to form an acceleration voltage between the anode and the diamond unit.

[0021] Figure 1 also illustrates laser beam 11, primary electron beam 12 and amplified electron beam 13.

[0022] Examples of vacuum levels - within the inner space there is a vacuum level of 10'12Torr, between the enclosure and the anode the vacuum level is and 10'9Torr. These are non-limiting examples of vacuum levels.

[0023] According to an embodiment, the enclosure is made of a ceramic material configured to withstand a bias voltage of at least one thousand volts.

[0024] According to an embodiment, the electron beam source includes a heater for selectively drying the negative electron affinity surface of the diamond unit.

[0025] According to an embodiment, the heater is located within a bias circuit aperture formed within a structural element of the bias circuit.

[0026] According to an embodiment, the bias circuit aperture is a longitudinal aperture.

[0027] According to an embodiment, the electron beam source includes laser beam optics that is configured to generate the laser beam to exhibit a flat top spot.

[0028] According to an embodiment, the laser beam optics is configured to convert a laser beam that has a Gaussian cross section to the laser beam that exhibits the flat top spot.

[0029] According to an embodiment, the electron beam source includes an anode that comprises the anode aperture.

[0030] According to an embodiment, the bias circuit is configured to bias the diamond unit to a first voltage level that differs from a voltage level of the anode and differs from a voltage level of the photocathode.

[0031] According to an embodiment, the electron beam source includes an indium seal for sealing the inner space.

[0032] According to an embodiment, the amplified electron beam is converted to an x- ray beam (for example using a target) and the higher intensity of the amplified electron beam provides a higher brightness x-ray beam - which speed up the evaluation of a sample - as it shortens the analysis time per evaluation site.

[0033] According to an embodiment the diamond unit is configured to amplify the primary electron beam by a factor of 100-200. The diamond layer is thin (for example 20pm) and made of a pure diamond crystal, that is flat - for example is polished to better than 5 nm flatness. The conductive metal layer may be thin and configured to provide the electrons. The negative electron affinity surface allows the electrons to escape to the vacuum.

[0034] When the primary electron beam strikes the conductive metal layer, it created electron-holes pairs in the diamond layer, for example- one pair per 13.3 eV of electron energy. Depending on the primary electron beam energy, this can lead to an amplification of the electron charge by a factor of up to 200.

[0035] The electrons then diffuse across the diamond layer under the influence of an extraction field (generated using the bias unit) and emerge through the negative electron affinity surface to the vacuum. The conversion process thermalizes the electrons very quickly, so regardless of the incident energy spread and emittance, the emerging beam is a uniform energy and has low emittance.

[0036] The main disadvantage of photocathodes is that they are extremely sensitive to atmospheric contamination and must be used in an extreme high vacuum environment. However, using an enclosure to encapsulate the photocathode and the diamond layer, separated by a ceramic insulator, maintains an XHV environment inside the inner space surrounded by the enclosure and the photocathode operates within its optimum environment parameters.

[0037] According to an embodiment, the photocathode and the diamond unit are integrated within an electron gun - as illustrated in figures 2.

[0038] Figure 2 illustrates a portion 115 of the electron gun - that includes a diamond unit (referred to as a 20 micron diamond amplifier 112), and an enclosure (made of high voltage insulator ceramic for extraction field 114) and of the photocathode (referred to as fused silica optic with PC (photocathode) film 111). Figure 2 also illustrates the indium vacuum seal 113.

[0039] The electron gum of figure 2 is mounted on a high voltage insulator stem that serves as an atmospheric interface for the gun as illustrated in figure 3.

[0040] Figure 3 illustrates an example of laser window 127 and parts of the electron beam source and the environment of the electron beam source.

[0041] See, for example, conflat flange 121 , high voltage (HV) insulator ceramic 125, and the enclosure (referred to as PC bias insulator ceramic 122), PEEK insulator alignment bushing 123 and pogo pin HV contact for the diamond unit 124.

[0042] According to an embodiment, the photocathode is illuminated by a laser beam modulated by a Pi-shaper optic and focusing optic to deliver a flat top spot of the laser beam at the photocathode.

[0043] This produces a uniform primary electron beam which is the amplified at the diamond unit to produce the flat top, low emittance amplified electron beam. This can be assembled into a modified high voltage stem assembly that will allow laser illumination - as illustrated in figure 4. Figure 4 illustrates an example of parts of the electron beam source and the environment of the electron beam source. See, for example, laser light 111 illumination focused by pi-shaper optics (configured to convert a laser beam that has a Gaussian cross section to the laser beam that exhibits the flat top spot) followed by a high voltage (HV) stinger 131 that directs the laser beam to the photocathode that converts the laser beam to a primary electron beam that is amplified by the diamond unit 50 to provide amplified electron beam 12, and also illustrates a vacuum control element such as a vacuum isolation valve 133.

[0044] Although the negative electron affinity surface of the diamond unit is very stable, it will adsorb water molecules on exposure to air as the gun is being built. This will degrade its negative electron affinity characteristics.

[0045] However, the negative electron affinity effectiveness can be recovered by baking it out as a normal vacuum system.

[0046] This can be accomplished with having to bake out the entire system platform by internally heating ta HX stem via a heater in a modified HV stinger.

[0047] The HV stinger normally is used to deliver the HV and Bias voltages to the electrodes on the gun, as illustrated in figure 5.

[0048] Figure 5 illustrates an example of a part of a HV stinger that is configured to provide one or more bias voltages and includes a grounding pin 141, a high voltage electrode bias 142 and a photocathode bias voltage 143 (which is an electrode) that has a hollow core for laser beam delivery 144 - and a heater 145 that is inserted into the hollow core. The heater 145 may be integrated with a thermocouple 146.

[0049] The heating is made for about 125 degrees Celsius.

[0050] According to an embodiment, the electron beam source allows a more uniform power distribution on the anode, creating a x2 increase in the brightness of the x-ray source.

[0051] This will permit shorter analysis times per site, leading to an increase in analysis points / WFP.

[0052] In addition, the vacuum isolation valve will allow the preservation of the vacuum conditioning of the electron gun, leading to shorter recovery times from vacuum venting cycles. The electron source can be a one-for-one replacement for the standard LaBegun as a field replacement, but with the associated laser, pulsed HV power supplies and detector modifications.

[0053] According to an embodiment there is provided a method 200 for generating an electron beam for use in X-ray based sample evaluation, the method includes: a. Step 210 of receiving by a photocathode a laser beam. b . Step 220 of converting, by the photocathode, the laser beam to a primary electron beam. c. Step 230 of outputting, by the photocathode, the primary electron beam within an inner space formed, at least in part, by the enclosure. d. Step 240 of receiving, by a diamond unit, the primary electron beam from the inner space; wherein the diamond unit includes a diamond layerthat is preceded by a conductive metal layer and is followed by a negative electron affinity surface. e. Step 250 of amplifying, by the diamond unit, the primary electron beam to provide an amplified electron beam. f. Step 260 of outputting, by the diamond unit, the amplified electron beam to an anode aperture. g. Step 201 of biasing, by a bias circuit, at least one of the photocathode and the diamond unit. Step 210 is executed in parallel to the previous steps (210-260) of the method.

[0054] According to an embodiment, the enclosure is made of a ceramic material configured to withstand a bias voltage of at least one thousand volts.

[0055] According to an embodiment, the method includes step 270 of selectively drying, by a heater, the negative electron affinity surface of the diamond unit.

[0056] According to an embodiment, the heater is located within a bias circuit aperture formed within a structural element of the bias circuit.

[0057] According to an embodiment, the bias circuit aperture is a longitudinal aperture.

[0058] According to an embodiment, the method includes generating, by a laser beam optics, the laser beam to exhibit a flat top spot.

[0059] According to an embodiment, the method includes converting, by the laser beam optics, a laser beam that has a Gaussian cross section to the laser beam that exhibits the flat top spot.

[0060] According to an embodiment, the anode aperture is formed within an anode.

[0061] According to an embodiment, the method includes biasing, by the bias circuit, the diamond unit to a first voltage level that differs from a voltage level of the anode and differs from a voltage level of the photocathode.

[0062] According to an embodiment, the method includes sealing, by an indium seal, the inner space.

[0063] According to an embodiment, there is provided a high brightness electron beam source for X-ray generation for metrology and inspection applications. An enclosure such as an hermetically sealed puck with a photocathode deposited on the inner surface of an optic window on one end and a diamond unit mounted on the opposite end, separated by an insulator ceramic to allow a high voltage bias to be applied between them.

[0064] The puck is maintained at extremely high vacuum levels (below 10'10Torr).

[0065] The extremely high vacuum environment allows a selection of the photocathode material based on an optimal quantum efficiency and lifetime characteristics.

[0066] According to an embodiment, a pi-shaper and focusing optic delivers a flat top laser beam to the photocathode to produce a uniform, flat top primary electron beam.

[0067] The diamond unit amplifies the primary electron beam by a factor of 100, delivering a low emittance, flat top amplified electron beam for optimal power density placement on the anode.

[0068] According to an embodiment, the uniform anode power loading will produce an increase of twice in the x-ray output.

[0069] In the foregoing 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.

[0070] 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.

[0071] 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.

[0072] Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits 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.

[0073] Any reference in the specification to a method should be applied mutatis mutandis to a system capable of executing the method.

[0074] Any reference in the specification to a system should be applied mutatis mutandis to a method that may be executed by the system.

[0075] 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.

[0076] 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 and not 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.

[0077] The connections as discussed herein may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise, the connections may for example be direct connections or indirect connections. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice versa. Also, plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed manner. Likewise, single connections carrying multiple signals may be separated out into various different connections carrying subsets of these signals. Therefore, many options exist for transferring signals.

[0078] Although specific conductivity types or polarity of potentials have been described in the examples, it will be appreciated that conductivity types and polarities of potentials may be reversed.

[0079] Each signal described herein may be designed as positive or negative logic. In the case of a negative logic signal, the signal is active low where the logically true state corresponds to a logic level zero. In the case of a positive logic signal, the signal is active high where the logically true state corresponds to a logic level one. Note that any of the signals described herein may be designed as either negative or positive logic signals. Therefore, in alternate embodiments, those signals described as positive logic signals may be implemented as negative logic signals, and those signals described as negative logic signals may be implemented as positive logic signals.

[0080] Furthermore, the terms "assert" or “set" and "negate" (or "deassert" or “clear”) are used herein when referring to the rendering of a signal, status bit, or similar apparatus into its logically true or logically false state, respectively. If the logically true state is a logic level one, the logically false state is a logic level zero. And if the logically true state is a logic level zero, the logically false state is a logic level one.

[0081] Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures may be implemented which achieve the same functionality.

[0082] 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.

[0083] 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.

[0084] Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner.

[0085] However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.

[0086] 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 usedherein, 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 in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

[0087] 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. An electron beam source for use in X-ray based sample evaluation, the electron beam source comprising: an enclosure; a photocathode that is configured to receive a laser beam, to convert the laser beam to a primary electron beam, and to output the primary electron beam within an inner space formed, at least in part, by the enclosure; a diamond unit that comprises a diamond layer that is preceded by a conductive metal layer and is followed by a negative electron affinity surface that is configured to receive the primary electron beam from the inner space, to amplify the primary electron beam to provide an amplified electron beam, and to output the amplified electron beam to an anode aperture; and a bias circuit configured to bias at least one of the photocathode and the diamond amplified photocathode.

2. The electron beam source according to claim 1, wherein the enclosure is at least partially made of a ceramic material configured to withstand a bias voltage of at least one thousand volts and is configured to insulate the photocathode and the diamond unit.

3. The electron beam source according to claim 1 , comprising a heater for selectively drying the negative electron affinity surface of the diamond unit.

4. The electron beam source according to claim 3, wherein the heater is located within a bias circuit aperture formed within a structural element of the bias circuit.

5. The electron beam source according to claim 4, wherein the bias circuit aperture is a longitudinal aperture.

6. The electron beam source according to claim 1, further comprising laser beam optics that is configured to generate the laser beam to exhibit a flat top spot.

7. The electron beam source according to claim 6, wherein the laser beam optics is configured to convert a laser beam having a Gaussian cross section to the laser beam that exhibits the flat top spot.

8. The electron beam source according to claim 1, further comprising an anode that comprises the anode aperture.

9. The electron beam source according to claim 8, wherein the bias circuit is configured to bias the diamond unit to a first voltage level that differs from a voltage level of the anode and differs from a voltage level of the photocathode.

10. The electron beam source according to claim 1, further comprising an indium seal for sealing the inner space.

11. A method for generating an electron beam for use in X-ray based sample evaluation, the method comprising: receiving by a photocathode a laser beam; converting, by the photocathode, the laser beam to a primary electron beam; outputting, by the photocathode, the primary electron beam within an inner space formed, at least in part, by the enclosure; receiving, by a diamond unit, the primary electron beam from the inner space; wherein the diamond unit comprises a diamond layer that is preceded by a conductive metal layer and is followed by a negative electron affinity surface amplifying, by the diamond unit, the primary electron beam to provide an amplified electron beam; and outputting, by the diamond unit, the amplified electron beam to an anode aperture; and biasing, by a bias circuit, at least one of the photocathode and the diamond unit.

12. The method according to claim 11, comprising insulating the photocathode and the diamond unit by the enclosure, wherein the enclosure is at least partially made of a ceramic material configured to withstand a bias voltage of at least one thousand volts.

13. The method according to claim 11, comprising selectively drying, by a heater, the negative electron affinity surface of the diamond unit.

14. The method according to claim 13, wherein the heater is located within a bias circuit aperture formed within a structural element of the bias circuit.

15. The method according to claim 14, wherein the bias circuit aperture is a longitudinal aperture.

16. The method according to claim 11, comprising generating, by a laser beam optics, the laser beam to exhibit a flat top spot.

17. The method according to claim 16, comprising converting, by the laser beam optics, a laser beam having a Gaussian cross section to the laser beam that exhibits the flat top spot.

18. The method according to claim 11 , wherein the anode aperture is formed within an anode .

19. The method according to claim 18, comprising biasing, by the bias circuit, the diamond unit to a first voltage level that differs from a voltage level of the anode and differs from a voltage level of the photocathode.

20. The method according to claim 11, comprising sealing, by an indium seal, the inner space.

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