Optical autofocus unit and method for autofocus

Abstract

A charged particle evaluation system may include: a column including an opening; an illumination unit configured to scan a region of a sample with an electron beam passing through the opening; and an optical autofocus unit configured to (i) illuminate the sample with a beam close to the electron beam during scanning of the region with the electron beam; (ii) receive a reflected beam from the sample; (iii) determine the focus state of the electron beam; and (iv) participate in compensation for electron beam defocusing.

Description

[0001] This application claims the benefit of U.S. Patent Application No. 17 / 893,836, filed August 23, 2022, the entire disclosure of which is incorporated herein by reference for all purposes. Background Technology

[0002] A scanning electron microscope (SEM) scans the sample surface with a raw electron beam. During the scan, the surface must be located at the focal point of the SEM.

[0003] Various samples (such as semiconductor wafers) have surfaces that are not perfectly flat and level—this requires fast and accurate autofocus solutions. Summary of the Invention

[0004] A charged particle evaluation system may be provided, the charged particle evaluation system comprising: a column including an opening; an illumination unit configured to scan a region of a sample with an electron beam passing through the opening; and an optical autofocusing unit configured to (i) illuminate the sample with a beam close to the electron beam during scanning of the region with the electron beam; (ii) receive a reflected beam from the sample; (iii) determine the focus state of the electron beam; and (iv) participate in compensation for electron beam defocusing.

[0005] A method for autofocusing can be provided, the method comprising: scanning a region of a sample by an illumination unit with an electron beam passing through an opening in a column; illuminating the sample with a beam close to the electron beam by an optical autofocusing unit during the scanning of the region with the electron beam; receiving a reflected beam from the sample by the optical autofocusing unit; determining the focus state of the electron beam by the optical autofocusing unit; and participating in compensation for electron beam defocusing by the optical autofocusing unit.

[0006] A non-transient computer-readable medium method may be provided, the non-transient computer-readable medium method storing instructions that, once executed by a charged particle evaluation system, cause the charged particle evaluation system to perform the following operations: scan a region of a sample by an illumination unit of the charged particle evaluation system with an electron beam passing through an opening in a column; illuminate the sample with a beam close to the electron beam by an optical autofocusing unit of the charged particle evaluation system during the scanning of the region with the electron beam; receive a reflected beam from the sample by the optical autofocusing unit; determine the focus state of the electron beam by the optical autofocusing unit; and participate in compensation for electron beam defocusing by the optical autofocusing unit. Attached Figure Description

[0007] The subject matter of the invention is specifically pointed out and clearly claimed in the conclusion of the specification. However, when compared with the appendix... Figure 1When reading this invention, the organization and operation methods can be best understood by referring to the following detailed description, together with the text, features, and advantages, as illustrated in the accompanying drawings:

[0008] Figure 1 Examples of the system and samples are shown;

[0009] Figure 2 Examples of the system and samples are shown; and

[0010] Figure 3 An example of the method is shown.

[0011] It will be understood that, for the sake of simplicity and clarity, the elements shown in the figures are not necessarily drawn to scale. For example, the dimensions of some elements may be exaggerated relative to others for clarity. Furthermore, where deemed appropriate, reference numerals may be repeated in the figures to indicate corresponding or similar elements. Detailed Implementation

[0012] In the following detailed description, numerous specific details are set forth to provide a thorough understanding of embodiments of the present disclosure.

[0013] However, those skilled in the art will understand that embodiments of this disclosure can be practiced without these specific details. In other instances, well-known methods, processes, and components have not been described in detail so as not to obscure the embodiments of this disclosure.

[0014] The subject matter of the embodiments of this disclosure is specifically pointed out and clearly claimed in the concluding section of the specification. However, when combined with the appendix... Figure 1 When reading this disclosure, reference to the following detailed description will best help to understand the embodiments, organization, and operation methods of the disclosure, as well as its purpose, features, and advantages.

[0015] It will be understood that, for simplicity and clarity, the elements shown in the figures are not necessarily drawn to scale. For example, the dimensions of some elements may be exaggerated relative to others for clarity. Furthermore, where deemed appropriate, reference numerals may be repeated in the figures to indicate corresponding or similar elements.

[0016] Because most of the embodiments shown in this disclosure can be implemented using electronic components and circuits known to those skilled in the art, details will not be explained to any greater extent than is deemed necessary above in order to understand and grasp the basic concepts of the embodiments of this disclosure, and in order not to obscure or distract from the teachings of the embodiments of this disclosure.

[0017] Any references to methods in the specification shall be applied, with necessary modifications, to systems capable of performing the methods, and shall be applied, with necessary modifications, to computer program products storing instructions that, upon execution, result in the execution of the methods.

[0018] Any references to the system in the specification shall be modified as necessary to apply to methods that can be executed by the system, and shall be modified as necessary to apply to computer program products that store instructions that can be executed by the system.

[0019] Any reference to a computer program product in this specification shall be applied, with necessary modifications, to methods executable when executing instructions stored in the computer program product, and shall be applied, with necessary modifications, to systems configured to execute instructions stored in the computer program product.

[0020] A charged particle evaluation system can be provided that performs autofocus based on measurements relating to the beam of light near the electron beam on the object, thereby improving the accuracy of autofocus. Furthermore, beam-based autofocus is much faster than image-based autofocus.

[0021] Figure 1 The charged particle evaluation system and sample are shown.

[0022] Charged particle evaluation systems can use one or more charged particle beams (one or more electron beams and / or one or more ion beams) to evaluate samples. Examples of charged particle evaluation beams can include scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron beam imagers, ion beam imagers, and so on.

[0023] For simplicity of explanation, we will refer to SEM 10, which uses a single electron beam (such as electron beam 41).

[0024] Sample 90 may be a semiconductor wafer or may be different from a semiconductor wafer. The sample may include structural elements with nanoscale features, such as, but not limited to, transistors, memory cells, etc. Nanoscale features may, for example, have one or more dimensions (length, depth, width, critical size, distance between adjacent structural features) ranging from 0.1 nanometers to 500 nanometers.

[0025] The sample 90 includes a surface 91 irradiated by an electron beam 41. The surface 91 may be curved or otherwise not perfectly flat and level.

[0026] SEM 10 includes pillar 15, processing circuit 12, memory unit 14, controller 13, illumination unit, collection unit, one or more sensors, etc.

[0027] The processing circuitry can be configured to perform tasks such as image processing, sample evaluation, etc.—for example, determining critical dimensions of structural elements, performing any other measurements, inspecting for defects, detecting suspected defects, and so on.

[0028] The memory unit 14 can store measurement results, evaluation results, instructions to be executed by the processing circuit 12, and so on.

[0029] Controller 13 can control any aspect of the operation of SEM 10.

[0030] Column 15 includes opening 23 and window 17.

[0031] The illumination unit may include an electron beam source 16, a first deflector 31, a second deflector 32, a third deflector 33, a fourth deflector 34, and an electron optical lens 35—for example, a focusing lens. The illumination unit is configured to scan an area of ​​the sample 90 with an electron beam 41, which passes through an opening 23. The electron optical lens 35 may be configured to determine the focal point of the electron beam.

[0032] although Figure 1 Four deflectors and a double-deflected electron beam are shown, but there may be fewer or more than four deflectors, and the electron beam may be deflected once or more than twice within the column.

[0033] SEM 10 further includes an optical autofocus unit 18 configured to (i) illuminate the sample with a beam 42 close to (e.g., at a distance not exceeding 5 mm, 4 mm, 3 mm, 2 mm, 1 mm, or even less than one millimeter, e.g., between 0.8 mm and 1.2 mm) the electron beam 41 during electron beam scanning of the region; (ii) receive a reflected beam 43 from the sample; (iii) determine the focus state of the electron beam 41; and (iv) participate in compensation for electron beam defocusing.

[0034] The beam 42 can pass through the opening in the pillar. Alternatively, the beam 42 can propagate outside the pillar.

[0035] The focus state can be focused or unfocused. When unfocused, the state indicates the degree of defocusing to be compensated.

[0036] The distance on the sample may include the distance 88 between the center of the beam spot 41' formed by the electron beam 41 on the surface 91 and the center of the beam spot 42' formed by the beam 42 on the surface 91. Figure 1 An example is also shown in which the beam spot 42' covers the entire area 99 scanned by the electron beam.

[0037] Figure 1 Three examples of the spatial relationship between electron beam 41, beam 42 and reflected beam 43 are shown.

[0038] In the example above, electron beam 41, beam 42, and reflected beam 43 completely overlap.

[0039] In the intermediate example, electron beam 41, beam 42 and reflected beam 43 are slightly spaced apart from each other, electron beam 41 and beam 42 are parallel to each other and reflected beam 43 is tilted with respect to electron beam 41 and beam 42.

[0040] In the bottom example, the electron beam 41 is spaced apart from and parallel to the completely overlapping beam 42 and the reflected beam 43.

[0041] Figure 1 The electron sensor 22 located outside column 15 and the scattered electron beam 44 scattered from the sample due to irradiation of the sample by electron beam 41 are also shown. In addition to any external electron sensor (such as electron sensor 22), SEM 10 may also include one or more internal electron sensors.

[0042] The optical autofocus unit 18 can be configured to determine the length of the optical path of the reflected beam. The determination of the focus state is based on the length of the optical path.

[0043] The optical autofocus unit 18 can be configured to determine the length of the optical path of the reflected beam and the tilt angle of the reflected beam. The focus state is determined based on the length of the optical path and the tilt angle.

[0044] A non-zero tilt angle can affect defocusing calculations because it introduces the difference between the length (L) of the optical path of the reflected beam and the distance (Drp) from the reference point in the column to the sample.

[0045] The reference point is defined as follows: when there is a zero tilt angle, the distance between the reference point and the sample is equal to the length of the optical path of the reflected beam.

[0046] Therefore, Drp is equal to the square root of the sum of (i) the square of L and (ii) the square of the distance between the beam and the electron beam on the surface.

[0047] Figure 1 The optical autofocus unit 18 is shown as having a sensing unit 18(1) positioned outside the column. Window 17 optically couples the sensing unit of the optical autofocus unit 18 to one or more optical elements within the column. The one or more optical elements within the column may include a mirror 21 configured to deflect the beam 42 toward the sample and deflect the reflected beam 43 toward the window.

[0048] The optical autofocus unit 18 can be configured to participate in electron beam defocusing compensation by: (a) sending instructions or requests to an electro-optical element, such as an electro-optical lens 35, to change the focus of the electron beam, and / or (b) sending instructions or requests to a mechanical platform 19 to change the distance between the column and the sample 90. The electro-optical lens 35 may be located at various positions within the column and Figure 1 and Figure 2 Two locations are shown.

[0049] One or more instructions and / or one or more requests can be sent via an electro-optical controller and / or via a mechanical platform controller. For simplicity of explanation, Figure 1 The control lines from controller 13 to the electrostatic lens and to the mechanical platform are shown.

[0050] Figure 1 An optical autofocusing unit 18 is shown, which includes a sensing unit 18(1) of a laser interferometer. The laser interferometer includes a laser 61, a first beam splitter (BS) 44, a second BS 67, a third BS 65, a mirror 63, a Bragg unit 64, and a detector 66. The sensing unit may be different from the laser interferometer.

[0051] The laser beam from laser 61 is split by first BS 44 to provide beam 41 and reference beam, beam 41 passes through second BS 67 and propagates toward window 17.

[0052] The reference beam is reflected by mirror 63 to Bragg unit 64, and Bragg unit 64 outputs the reference beam that enters the third B3 65.

[0053] The reflected beam 43 is guided from the second BS 67 to the third BS 67.

[0054] The reflected beam 43 and the reference beam are mixed at the third BS 56 to provide an interference pattern on the detector 66.

[0055] The signal from detector 66 is sent to autofocus processing circuit 18(2), which can determine the focus state and how to compensate for defocus.

[0056] Figure 2 SEM 10' is shown. Figure 2 In this configuration, the sensing unit 18'(1) of the autofocus unit 18' is positioned within the post 15. Additionally, the post includes an in-lens electronic sensor 22'. The sensing unit may be partially located within the post and partially located outside the post. The sensing unit may be positioned within a shield or housing that prevents contamination of the sensing unit and / or protects it.

[0057] Figure 2 The sensing unit 18'(1) is also shown. The sensing unit 18'(1) includes a laser source 71, an optical device 72 for guiding one or more beams such as a light beam, a reflected beam, etc., an optical path length detection unit 73 (for detecting the length of the optical path), and a tilt angle detection unit 74 (for detecting the tilt angle of the reflected beam).

[0058] Figure 3 A method 100 for autofocus is shown.

[0059] Method 100 can begin with steps 110 and 120.

[0060] Step 110 may include scanning a region of the sample by an illumination unit using an electron beam passing through the column opening.

[0061] Step 120 may include illuminating the sample with a beam close to the electron beam by an optical autofocusing unit during electron beam scanning of the region. The region may be millimeter in size. The beam may scan another region of the sample—this other region may be close to the region scanned by the electron beam. The other region and the region scanned by the electron beam may overlap or may partially overlap. The beam may form a beam spot on the sample, and the beam spot may cover the entire region scanned by the electron beam, or it may cover only some areas of the region scanned by the electron beam, or it may be outside the region scanned by the electron beam.

[0062] Step 120 can be followed by step 130, where the optical autofocus unit receives the reflected beam from the sample.

[0063] Step 130 can be followed by step 140, where the focusing state of the electron beam is determined by the optical autofocusing unit.

[0064] Step 140 can be followed by step 150, in which the optical autofocus unit participates in the compensation of electron beam defocus.

[0065] Step 140 may include determining the length of the optical path of the reflected beam. The determination of the focus state is based on the length of the optical path.

[0066] Step 140 may include determining the length of the optical path and the tilt angle of the reflected beam by an optical autofocus unit. The determination of the focus state is based on the length of the optical path and the tilt angle.

[0067] At least step 120 can be performed by an optical autofocus unit including a sensing unit positioned outside the column, wherein the column includes a window that optically couples the sensing unit to one or more optical elements within the column. The one or more optical elements within the column may include mirrors configured to deflect a beam toward the sample and to deflect a reflected beam toward the window.

[0068] Step 150 may include at least one of the following: performing the entire compensation, performing a portion of the compensation, triggering compensation, requesting compensation, commanding compensation, communicating with the controller of the electro-optical lens, communicating with the electro-optical lens, communicating with the controller of the mechanical platform, communicating with the mechanical platform, etc.

[0069] The optical autofocus unit may include a laser interferometer vibrometer.

[0070] At least step 120 can be performed by an optical autofocus unit, which includes a sensing unit positioned within the column.

[0071] Step 110 can be followed by step 160, which involves detecting electrons emitted from the sample.

[0072] Step 160 can be followed by step 170, which evaluates the sample based on the detected electrons. Evaluation may include defect inspection, defect detection, metrology, critical size measurement, etc.

[0073] In the foregoing description, embodiments of the present disclosure have been described with reference to specific examples of examples. However, it will be apparent that various modifications and changes can be made without departing from the broader spirit and scope of the appended claims.

[0074] Furthermore, the terms “front,” “rear,” “top,” “bottom,” “above,” “below,” etc. (if any) used in the specification and claims are for descriptive purposes and are not necessarily used to describe permanent relative positions. It should be understood that the terms thus used are interchangeable where appropriate, such that embodiments of the disclosure described herein can, for example, operate in other orientations different from those shown or otherwise described herein.

[0075] The connections discussed herein can be of any type suitable for transmitting signals from or to a corresponding node, unit, or device, for example, via an intermediate device. Thus, unless implied or otherwise stated, the connection can be, for example, a direct connection or an indirect connection. Connections can be described or illustrated with reference to a single connection, multiple connections, unidirectional connections, or bidirectional connections. However, different embodiments can vary the implementation of the connection. For example, separate unidirectional connections can be used instead of bidirectional connections, and vice versa. Furthermore, multiple connections can be replaced by a single connection that transmits multiple signals serially or in a time-multiplexed manner. Similarly, a single connection carrying multiple signals can be divided into various different connections carrying subsets of these signals. Thus, there are many options for transmitting signals.

[0076] Although specific conductivity types or potential polarities have been described in the examples, it should be understood that conductivity types and potential polarities can be reversed.

[0077] Each signal described herein can be designed as either positive or negative logic. In the case of a negative logic signal, the signal is active low, where a true logic state corresponds to logic level zero. In the case of a positive logic signal, the signal is active high, where a true logic state corresponds to logic level one. Note that any signal described herein can be designed as either a negative or positive logic signal. Therefore, in alternative 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.

[0078] Furthermore, when it comes to presenting a signal, status bit, or similar device as its logical true or logical false state, this document uses the terms "assert" or "set" and "deny" (or "deassert" or "clear"). If the logical true state is logic level one, then the logical false state is logic level zero. And if the logical true state is logic level zero, then the logical false state is logic level one.

[0079] Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative, and alternative embodiments may combine logic blocks or circuit elements or impose alternative functional decompositions on various logic blocks or circuit elements. Therefore, it should be understood that the architectures depicted herein are merely exemplary, and many other architectures that achieve the same functionality can actually be implemented.

[0080] Any arrangement of components that perform the same function is effectively “associated” to achieve the desired function. Therefore, any two components combined in this paper to achieve a specific function can be considered “associated” with each other to achieve the desired function, regardless of the architecture or intermediate components. Similarly, any two components that are so associated can also be considered “operably connected” or “operably coupled” to each other to achieve the desired function.

[0081] Furthermore, those skilled in the art will recognize that the boundaries between the above operations are merely illustrative. Multiple operations can be combined into a single operation, a single operation can be distributed among additional operations, and operations can be performed with at least partial overlap in time. Additionally, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be varied in various other embodiments.

[0082] Furthermore, for example, in one embodiment, the illustrated example may be implemented as circuitry located on a single integrated circuit or within the same device. Alternatively, the example may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner.

[0083] Furthermore, for example, an example, or a portion thereof, may be implemented as a physical circuit or a soft or code representation of a logic representation that can be converted into a physical circuit, such as in any suitable type of hardware description language.

[0084] However, other modifications, alterations, and substitutions are also possible. Therefore, the specification and drawings should be considered illustrative rather than restrictive.

[0085] In the claims, any reference symbols in parentheses should not be construed as limiting the claims. The word “comprising” does not exclude the presence of other elements or steps besides those listed in the claims. Furthermore, the terms “a” or “an” as used herein are defined as one or more. Moreover, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed as implying that any particular claim containing such an introduced claim element, introduced by the indefinite article “a” or “an,” limits any particular claim to containing only one embodiment of that element, even if the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” are included in the same claim. The same applies to the use of definite articles. Unless otherwise stated, terms such as “first” and “second” are used to arbitrarily distinguish the elements described by these terms. Therefore, these terms are not necessarily intended to indicate the time or other priority of such elements. The fact that certain measures are stated in mutually different claims does not mean that a combination of these measures cannot be used advantageously.

[0086] While certain features of the embodiments have been described and illustrated herein, many modifications, substitutions, alterations, and equivalents will now occur to those skilled in the art. Therefore, it should be understood that the appended claims are intended to cover all such modifications and alterations falling within the true spirit of the invention.

Claims

1. A charged particle evaluation system, comprising: A column, the column including an opening; An illumination unit configured to scan a region of the sample with an electron beam passing through the opening; and An optical autofocusing unit is configured to: (i) illuminate the sample with a beam close to the electron beam during scanning of the region with the electron beam; (ii) receive a reflected beam from the sample; (iii) determine the focus state of the electron beam; and (iv) participate in compensation for electron beam defocusing. The optical autofocus unit includes a mirror and a sensing unit, the mirror being positioned within the column, and the mirror being configured to deflect the light beam toward the sample and toward the sensing unit.

2. The charged particle evaluation system of claim 1, wherein the optical autofocus unit is configured to determine the length of the optical path of the reflected beam, and wherein the determination of the focus state is based on the length of the optical path.

3. The charged particle evaluation system of claim 1, wherein the optical autofocus unit is configured to determine (a) the length of the optical path of the reflected beam and (b) the tilt angle of the reflected beam, and wherein the determination of the focus state is based on the length of the optical path and the tilt angle.

4. The charged particle evaluation system of claim 1, wherein the optical autofocus unit is configured to participate in the compensation for electron beam defocusing by sending instructions to the mechanical platform.

5. The charged particle evaluation system according to claim 1, wherein the optical autofocus unit comprises a laser interferometer.

6. The charged particle evaluation system of claim 1, wherein the optical autofocus unit comprises a sensing unit positioned within the column.

7. The charged particle evaluation system of claim 1, wherein the optical autofocus unit is configured to participate in the compensation for electron beam defocus by sending instructions to the electro-optical elements.

8. A charged particle evaluation system, comprising: A column, the column including an opening; An illumination unit configured to scan a region of the sample with an electron beam passing through the opening; and An optical autofocusing unit is configured to: (i) illuminate the sample with a beam close to the electron beam during scanning of the region with the electron beam; (ii) receive a reflected beam from the sample; (iii) determine the focus state of the electron beam; and (iv) participate in compensation for electron beam defocusing. The optical autofocus unit includes a sensing unit positioned outside the post, wherein the post includes a window that optically couples the sensing unit to one or more optical elements within the post; and The one or more optical elements within the column include mirrors configured to deflect the light beam toward the sample and toward the window.

9. A method for autofocusing, the method comprising: The region of the sample is scanned by an electron beam passing through an opening in the column by an illumination unit; During the scanning of the region with the electron beam, the sample is illuminated by an optical autofocusing unit with a beam close to the electron beam; The reflected light beam from the sample is received by the optical autofocus unit; The focusing state of the electron beam is determined by the optical autofocusing unit; as well as The optical autofocus unit participates in the compensation for electron beam defocusing; The optical autofocus unit includes a mirror and a sensing unit, the mirror being positioned within the column, and the mirror being configured to deflect the light beam toward the sample and toward the sensing unit.

10. The method of claim 9, further comprising determining the length of the optical path of the reflected beam by the optical autofocusing unit, wherein the determination of the focus state is based on the length of the optical path.

11. The method of claim 9, comprising determining the length of the optical path of the reflected beam and the tilt angle of the reflected beam by the optical autofocusing unit, wherein the determination of the focus state is based on the length of the optical path and the tilt angle.

12. The method of claim 9, wherein the compensation for electron beam defocusing comprises sending a command to the electron optical lens.

13. The method of claim 9, wherein the optical autofocus unit comprises a laser interferometer.

14. The method of claim 9, wherein the optical autofocus unit comprises a sensing unit positioned within the column.

15. A method for autofocusing, the method comprising: The region of the sample is scanned by an electron beam passing through an opening in the column by an illumination unit; During the scanning of the region with the electron beam, the sample is illuminated by an optical autofocusing unit with a beam close to the electron beam; The reflected light beam from the sample is received by the optical autofocus unit; The focusing state of the electron beam is determined by the optical autofocusing unit; as well as The optical autofocus unit participates in the compensation for electron beam defocusing; The optical autofocus unit includes a sensing unit positioned outside the post, wherein the post includes a window that optically couples the sensing unit to one or more optical elements within the post; and The one or more optical elements within the column include mirrors configured to deflect the light beam toward the sample and toward the window.

16. A non-transitory computer-readable medium storing instructions, which, once executed by a charged particle evaluation system, cause the charged particle evaluation system to: The illumination unit of the charged particle evaluation system scans a region of the sample with an electron beam passing through an opening in the column; During the scanning of the region with the electron beam, the sample is illuminated by the optical autofocusing unit of the charged particle evaluation system with a beam close to the electron beam; The reflected light beam from the sample is received by the optical autofocus unit; The focusing state of the electron beam is determined by the optical autofocusing unit; as well as The optical autofocus unit participates in the compensation for electron beam defocusing; The optical autofocus unit includes a mirror and a sensing unit, the mirror being positioned within the column, and the mirror being configured to deflect the light beam toward the sample and toward the sensing unit.

Images