Optical device and focus correction method

Abstract

To provide an optical device capable of improving focus positioning, and a focus correction method.SOLUTION: The optical device comprises: a detector 20 detecting detection light L12 which is reflectance of illumination light L11 reflected on a sample 60; an optical system 10 irradiating the sample 60 with the illumination light L11 and guiding the detection light L12 reflected on the sample 60 to the detector 20; a displacement measuring part 40 measuring displacement drift indicating a drift amount of the position of an optical element included in the optical system 10; a storage part 52 storing correlation between the displacement drift and focus drift indicating a drift amount of distance between the sample 60 and the optical system 10 when the detection light L12 detected by the detector 20 focuses; and a prediction part 53 predicting focus drift from the measured displacement drift by using the correlation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an optical device and a focus correction method. [Background technology]

[0002] Patent Document 1 discloses a measurement system in which measurement light is transmitted through an optical element including a lens, etc., and the reflected light from a mirror attached on the opposite side to the incident direction is detected by a camera such as a CCD. In the measurement system of Patent Document 1, when the lens expands and the light path in the optical element changes, a change occurs in the interference pattern recorded by the camera, and based on this change, the shape change of the mirror is derived and the position of the optical element is adjusted using a manipulator.

[0003] Patent Documents 2 and 3 disclose an EUV exposure apparatus that uses a distance sensor to measure changes in the mirror spacing caused by thermal load and adjusts the mirror spacing based on the measurement results. The distance sensor in Patent Document 2 uses a laser interferometer or a capacitance sensor, and adjusts the position of the mirror attached inside the lens barrel using an actuator such as a piezoelectric element. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special table 2015-512145 publication [Patent Document 2] JP 2008-130850 A [Patent Document 3] JP 2000-286189 A Summary of the Invention [Problem to be solved by the invention]

[0005] For example, in defect inspection equipment that uses EUV light as illumination light for illuminating samples such as EUV masks, defocusing occurs even with thermal expansion due to a temperature change of less than 1°C due to the shallow focal depth. Images that include such focus drift tend to become blurred, leading to the problem that they are detected as false defects.

[0006] The present disclosure has been made in consideration of such problems, and provides an optical device and a focus correction method that can improve focus alignment. [Means for solving the problem]

[0007] An optical device according to one aspect of this embodiment includes a detector that detects reflected light of illumination light reflected by a sample, an optical system that illuminates the sample with the illumination light and guides the reflected light reflected by the sample to the detector, a displacement measurement unit that measures a displacement drift indicating an amount of drift in the position of an optical element included in the optical system, a memory unit that stores a correlation between the displacement drift and a focus drift indicating an amount of drift in the distance between the sample and the optical system when the reflected light detected by the detector is focused, and a prediction unit that predicts the focus drift from the measured displacement drift by using the correlation.

[0008] In the above optical device, the displacement drift may indicate the amount of drift of the optical element from a reference position, and the focus drift may indicate the amount of drift from a reference distance that serves as a reference between the sample and the optical system.

[0009] In the above optical device, the displacement measuring unit may include a mirror fixed to the optical element, a laser that emits laser light, and an interferometer that detects the position of the mirror from interference between the laser light emitted to the mirror and the laser light reflected by the mirror.

[0010] In the above optical device, the optical system includes a Schwarzschild optical system having a concave mirror and a convex mirror, the convex mirror is fixed to a holder including a first member extending in a first direction parallel to an optical axis of the convex mirror and a second member extending in a second direction perpendicular to the optical axis, the mirror is fixed to the optical element via the second member, and the displacement measuring unit may measure the position from interference between a laser light emitted along the second direction and the laser light reflected by the mirror in the opposite direction to the second direction.

[0011] The optical device may further include a control unit that controls a distance between the sample and the optical system, and the control unit may control the distance based on the focus drift predicted by the prediction unit.

[0012] A focus correction method according to one aspect of this embodiment is a focus correction method for an optical device including a detector that detects reflected light of illumination light reflected by a sample, and an optical system that illuminates the sample with the illumination light and guides the reflected light reflected by the sample to the detector, and includes the steps of: storing a correlation between a displacement drift that indicates an amount of drift in the position of an optical element included in the optical system and a focus drift that indicates an amount of drift in the distance between the sample and the optical system when the reflected light detected by the detector is focused; measuring the displacement drift; and predicting the focus drift from the measured displacement drift by using the correlation.

[0013] In the above focus correction method, the displacement drift may indicate the amount of drift of the optical element from a reference position, and the focus drift may indicate the amount of drift from a reference distance that serves as a reference between the sample and the optical system.

[0014] In the above-mentioned focus correction method, the optical device further includes a displacement measurement unit, which includes a mirror fixed to the optical element, a laser that emits laser light, and an interferometer that detects a position of the mirror from interference between the laser light emitted to the mirror and the laser light reflected by the mirror, and in the step of measuring the displacement drift, the displacement drift may be measured by detecting the position of the mirror from interference between the laser light emitted to the mirror and the laser light reflected by the mirror.

[0015] In the above focus correction method, the optical system includes a Schwarzschild optical system having a concave mirror and a convex mirror, the convex mirror is fixed to a support member including a first member extending in a first direction parallel to an optical axis of the convex mirror and a second member extending in a second direction perpendicular to the optical axis, and the mirror is fixed to the optical element via the second member, and in the step of measuring the displacement drift, the position may be measured from interference between a laser light emitted along the second direction and the laser light reflected by the mirror in a direction opposite to the second direction.

[0016] The focus correction method may further include a step of controlling the distance based on the focus drift predicted in the step of predicting the focus drift. Effect of the Invention

[0017] According to the present disclosure, it is possible to provide an optical device and a focus correction method that can improve focus alignment. [Brief description of the drawings]

[0018] [Figure 1] 1 is a configuration diagram illustrating an inspection device according to a first embodiment. [Diagram 2] 4 is a configuration diagram illustrating a displacement measuring unit before and after displacement of an optical element in the inspection device according to the first embodiment. FIG. [Diagram 3]4 is a configuration diagram illustrating a displacement measuring unit before and after displacement of an optical element in the inspection device according to the first embodiment. FIG. [Figure 4] 2 is a block diagram illustrating a processing device in the inspection device according to the first embodiment. FIG. [Diagram 5] 1 is a graph illustrating an example of the correlation between displacement drift and focus drift in the inspection apparatus of embodiment 1, in which the horizontal axis represents displacement drifts dx and dy in an in-plane direction such as the X-axis direction or the Y-axis direction, and the vertical axis represents focus drift. [Figure 6] FIG. 4 is a flowchart illustrating a focus correction method according to the first embodiment. [Figure 7] FIG. 4 is a flowchart illustrating a method for acquiring the correlation between the displacement drift of an optical element and the focus drift in the focus correction method according to the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. The following description shows a preferred embodiment of the present disclosure, and the scope of the present disclosure is not limited to the following embodiment. In the following description, parts with the same reference numerals indicate substantially the same contents.

[0020] (Embodiment 1) An optical device according to embodiment 1 will be described. The optical device according to this embodiment is, for example, an inspection device that inspects a sample, such as an EUV mask, by illuminating the sample with illumination light such as EUV light. Note that the optical device is not limited to an inspection device, but may be other optical devices such as an exposure device, as long as it has an optical system that illuminates the sample with illumination light. Also, the sample is not limited to an EUV mask, as long as it is an object that is to be illuminated with illumination light.

[0021] Fig. 1 is a configuration diagram illustrating an inspection device according to embodiment 1. As shown in Fig. 1, the inspection device 1 includes a light source LS, an optical system 10, a detector 20, a stage 30, a displacement measuring unit 40, and a processing device 50.

[0022] The light source LS generates illumination light L11 including EUV light. Here, the light source LS generates EUV light of 13.5 nm, which is the same as the exposure wavelength of the sample 60 to be illuminated. The light source LS is, for example, a DPP (Discharge Produced Plasma) light source that uses discharge. Note that the illumination light L11 generated by the light source LS is not limited to light including EUV light, and may include light of other wavelengths, such as UV (UltraViolet) light, white light, and infrared light. The light source LS may be disposed inside the chamber 61.

[0023] The optical system 10 illuminates the sample 60 with illumination light L11 and guides the light reflected by the sample 60 to the detector 20. The optical system 10 includes optical elements and propagates the illumination light L11 such as EUV light. The optical system 10 includes a concave mirror 11, a concave mirror 12, a drop mirror 13, a Schwarzschild optical system 14, a plane mirror 17, and a concave mirror 18 as optical elements. The Schwarzschild optical system 14 includes a concave mirror 15 and a convex mirror 16. The optical system 10 may include other optical elements such as lenses and mirrors in addition to the above. Each optical element may be provided in the chamber 61. The optical system 10 is a bright-field optical system for imaging the sample 60. The optical system 10 may be a dark-field optical system for imaging the sample 60.

[0024] An optical system 10 for guiding illumination light L11 such as EUV light will be described. Illumination light L11 generated by light source LS advances while diverging. Illumination light L11 emitted from light source LS is reflected by concave mirror 11. Concave mirror 11 is, for example, an ellipsoidal mirror. Concave mirror 11 is a multilayer mirror in which Mo films and Si films are alternately stacked, and reflects EUV light. Illumination light L11 reflected by concave mirror 11 advances while being narrowed. After forming a focal point IF, illumination light L11 advances while diverging. Then, illumination light L11 is reflected by concave mirror 12.

[0025] The concave mirror 12 is, for example, an ellipsoidal mirror. The concave mirror 12 is a multilayer mirror in which Mo films and Si films are alternately stacked, and reflects EUV light. The illumination light L11 reflected by the concave mirror 12 travels while being narrowed down, and is incident on the drop-in mirror 13. The drop-in mirror 13 is a plane mirror, and is disposed above the sample 60. The illumination light L11 reflected by the drop-in mirror 13 is incident on the sample 60. The drop-in mirror 13 focuses the illumination light L11 on the sample 60. In this way, the inspection area of ​​the sample 60 is illuminated with the illumination light L11, which is EUV light. The sample 60 is placed on the stage 30.

[0026] Here, for convenience of explanation of the inspection device 1, an XYZ Cartesian coordinate system is introduced. The direction perpendicular to the upper surface of the stage 30 is defined as the Z-axis direction, and the plane parallel to the upper surface of the stage 30 is defined as the XY plane. The direction parallel to the XY plane is called the in-plane direction. For example, the X-axis direction and the Y-axis direction are in-plane directions.

[0027] The stage 30 is a movable stage such as an XYZ stage. The stage 30 moves in an XY plane perpendicular to the optical axis and in a Z-axis direction parallel to the optical axis. This causes the sample 60 to move in the XY plane and in the Z-axis direction. Since the illumination position of the sample 60 changes, any position of the sample 60 can be observed. Also, the inspection area where the sample 60 is illuminated can be changed.

[0028] As described above, the illumination light L11 illuminates the inspection area of ​​the sample 60. The light reflected by the sample 60 is defined as the detection light L12. The detection light L12 includes the EUV light reflected by the sample 60. The detection light L12 reflected by the sample 60 is incident on the Schwarzschild optical system 14. Specifically, the detection light L12 reflected by the sample 60 is incident on the concave mirror 15. The detection light L12 reflected by the concave mirror 15 is incident on the convex mirror 16. The convex mirror 16 reflects the detection light L12 from the concave mirror 15 toward the plane mirror 17. The detection light L12 reflected by the plane mirror 17 is reflected by the concave mirror 18 and is incident on the detector 20. The inspection area of ​​the sample 60 is enlarged and projected onto the detector 20 by the Schwarzschild optical system 14, the plane mirror 17, the concave mirror 18, and the like.

[0029] The detection light L12 is detected by the detector 20. The detector 20 detects the detection light L12 that is the illumination light L11 reflected by the sample 60. The detector 20 is an imaging device such as a CCD (Charge Coupled Device) sensor, a CMOS (Complementary Metal Oxide Semiconductor) sensor, or a TDI (Time Delay Integration) sensor, and images the sample 60. In other words, the detector 20 images an enlarged image of the inspection area of ​​the sample 60.

[0030] The displacement measuring unit 40 measures a displacement drift indicating the amount of drift in the position of an optical element included in the optical system 10. FIGS. 2 and 3 are configuration diagrams illustrating the displacement measuring unit 40 before and after the displacement of the optical element in the inspection device 1 according to the first embodiment. In FIGS. 2 and 3, the left side shows the optical element and the displacement measuring unit 40 before the displacement, and the right side shows the optical element and the displacement measuring unit 40 after the displacement. As shown in FIGS. 1 to 3, the displacement measuring unit 40 includes a mirror 41, a laser 42, and an interferometer 43.

[0031] The mirror 41 is fixed to an optical element. For example, the mirror 41 is fixed to the convex mirror 16. The mirror 41 may be fixed to the convex mirror 16 via a holder 45 that supports the convex mirror 16.

[0032] The holder 45 includes a first member 46 and a second member 47. The first member 46 extends in the Z-axis direction parallel to the optical axis C of the convex mirror 16. The second member 47 extends in an in-plane direction (for example, the X-axis direction or the Y-axis direction) perpendicular to the optical axis C. The convex mirror 16 is fixed to the holder 45. The mirror 41 is fixed to the convex mirror 16 via the second member 47 of the holder 45. The first member 46 and the second member 47 preferably include materials having the same thermal expansion coefficient. For example, the first member 46 and the second member 47 preferably include the same material. The first member 46 and the second member 47 preferably have the same thickness. The length of the first member 46 in the Z-axis direction and the length of the second member 47 in the in-plane direction are preferably approximately the same.

[0033] The laser 42 emits a laser beam L13. Specifically, the laser 42 emits the laser beam L13 toward the mirror 41. The interferometer 43 detects the position of the mirror 41 from the interference between the laser beam L13 emitted toward the mirror 41 and the laser beam L13 reflected by the mirror 41. In this way, the displacement measuring unit 40 measures the position of the convex mirror 16 from the interference between the laser beam L13 emitted along an in-plane direction perpendicular to the optical axis C and the laser beam L13 reflected by the mirror 41 in the opposite in-plane direction.

[0034] The displacement drift, which indicates the amount of drift in the position of the optical element, indicates the amount of drift from the reference position of the optical element. For example, a predetermined position of the optical element when the detection light L12 detected by the detector 20 is focused may be the reference position. Also, the position of the optical element when the detection light L12 detected by the detector 20 is focused after illuminating the reference sample with the illumination light L11 may be the reference position. Also, the position of the optical element when the detection light L12 detected by the detector 20 is focused when the inspection device 1 is at a predetermined temperature may be the reference position.

[0035] Due to the propagation of illumination light L11 such as EUV light and detection light L12, a portion where the density of EUV light increases, such as the periphery of convex mirror 16, is heated. This causes the temperature around convex mirror 16 to rise. This temperature rise causes thermal expansion of convex mirror 16 and holder 45, etc., and the position of convex mirror 16 changes. That is, the position of convex mirror 16 drifts. This causes the focus position of detection light L12 detected by detector 20 to drift. Specifically, the relative distance between concave mirror 15 and convex mirror 16 changes as the holder 45 of convex mirror 16 thermally expands due to the temperature rise, which causes the distance between sample 60 and optical system 10 to change, causing the focus position to drift.

[0036] In this way, the drift amount of the distance between the sample 60 and the optical system 10 when the detection light L12 detected by the detector 20 is focused is called focus drift. Focus drift indicates the amount of drift from a reference distance that serves as a reference between the sample 60 and the optical system 10. For example, the distance between the sample 60 and the optical system 10 when the optical element is located at a reference position and the detection light L12 detected by the detector 20 is focused may be the reference distance. Also, the reference sample may be illuminated with the illumination light L11, and the distance between the sample 60 and the optical system 10 when the detection light L12 detected by the detector 20 is focused may be the reference distance. Also, the distance between the sample 60 and the optical system 10 when the detection light L12 detected by the detector 20 is focused when the inspection device 1 is at a predetermined temperature may be the reference distance.

[0037] The inspection device 1 of this embodiment uses the mirror 41 attached to the bottom of the holder 45 to measure the displacement drift of the convex mirror 16 caused by thermal expansion in the in-plane direction of the holder 45. Then, from the measurement result, the processing device 50 predicts the focus drift when the detection light L12 is focused on the detector 20.

[0038] Here, the thermal expansion around the convex mirror 16 is considered to be isotropic. Therefore, the amount of thermal expansion in the Z-axis direction, which is the optical axis C direction (focus direction), can be predicted from the amount of thermal expansion in the in-plane direction (for example, the X-axis direction and the Y-axis direction) of the holder 45. Therefore, the focus drift when the detection light L12 is focused can be predicted from the displacement drift in the in-plane direction of the holder 45. Then, the distance in the focus direction between the sample 60 and the optical system 10 is adjusted according to the predicted focus drift. For example, the position of the sample 60 on the stage 30 is adjusted by moving the stage 30. This allows the detector 20 to capture an image with reduced blur. When adjusting the distance in the focus direction between the sample 60 and the optical system 10, the optical system 10 may be moved instead of moving the stage 30.

[0039] Although the displacement drift of the convex mirror 16 is used as the displacement drift of the optical element, the present invention is not limited thereto, and the displacement drift of other optical elements such as the concave mirror 15 and the drop-in mirror 13 may be used. Furthermore, although the mirror 41, the laser 42, and the interferometer 43 are used as the displacement measuring unit 40 for measuring the displacement drift of the optical element, the present invention is not limited thereto, and the displacement drift of the optical element may be measured using a distance sensor or the like.

[0040] Furthermore, when measuring the displacement drift of an optical element, not only the displacement drift in the in-plane direction but also the displacement drift in the Z-axis direction may be measured. However, the convex mirror 16 in the Schwarzschild optical system 14 is smaller than the concave mirror 15. In addition, the upper part of the convex mirror 16 is covered by the concave mirror 15. The lower part of the convex mirror 16 is occupied by the sample 60. Therefore, it is difficult to secure a space for measuring the displacement drift in the Z-axis direction of the convex mirror 16, which has a large temperature change. Therefore, in this embodiment, the thermal expansion in the Z-axis direction is converted into thermal expansion in the in-plane direction by utilizing the fact that the thermal expansion of the holder 45 is isotropic. Therefore, since the displacement drift of the convex mirror 16, which has a large temperature change, can be measured, the focus alignment can be improved. The same can be said about the drop-in mirror 13 as about the convex mirror 16.

[0041] The image of the sample 60 captured by the detector 20 is output to the processing device 50. The processing device 50 is an arithmetic processing device having a processor, a memory, etc., and performs an inspection based on the image of the sample 60. The processing device 50 may be, for example, an information processing device such as a PC (Personal Computer) or a server.

[0042] 4 is a block diagram illustrating a processing device 50 in the inspection device 1 according to the embodiment 1. As shown in FIG. 4, the processing device 50 includes a processing unit 51, a storage unit 52, a prediction unit 53, and a control unit .

[0043] The processing unit 51 performs image processing. The processing unit 51 performs defect inspection by comparing the brightness of the image of the sample 60 with a threshold value. The processing unit 51 also associates the coordinates of the stage 30 with the image. This makes it possible to identify defect coordinates of the sample 60. The processing unit 51 also displays an image of the defect on a monitor. This allows the user to check the defect.

[0044] The memory unit 52 stores defect coordinates and defect images. The memory unit 52 also stores the position of the optical element measured by the displacement measurement unit 40 and the displacement drift. The memory unit 52 also stores the distance between the sample 60 and the optical system 10 when the detection light L12 detected by the detector 20 is focused, and the focus drift. The memory unit 52 also stores the correlation between the displacement drift and the focus drift.

[0045] 5 is a graph illustrating the correlation between the displacement drift and the focus drift in the inspection device 1 according to the embodiment 1, where the horizontal axis indicates the displacement drifts dx and dy in the in-plane direction such as the X-axis direction or the Y-axis direction, and the vertical axis indicates the focus drift. As shown in Fig. 5, for example, the storage unit 52 stores an approximation of the following formula (1) and / or formula (2) as the correlation between the displacement drift and the focus drift.

[0046] (Focus drift)=(k dx) (1) (Focus drift)=(k·dy) (2)

[0047] The storage unit 52 may store, as the correlation, a linear approximation of the above formulas (1) and (2), or may store another approximation such as a quadratic formula. The storage unit 52 may also store, as the correlation, a table in which a plurality of displacement drifts are associated with a plurality of focus drifts. The prediction unit 53 predicts the focus drift from the displacement drift measured by the displacement measurement unit 40 by using the correlation.

[0048] The control unit 54 controls the distance between the sample 60 and the optical system 10. For example, the control unit 54 controls the driving of the stage 30. As a result, the control unit 54 controls the distance between the sample 60 and the optical system 10 based on the focus drift predicted by the prediction unit 53. Note that the control unit 54 may control the distance between the sample 60 and the optical system 10 by moving the stage 30, or may control the distance between the sample 60 and the optical system 10 by moving the optical system 10.

[0049] Next, a focus correction method according to the present embodiment will be described. The focus correction method according to the present embodiment is a focus correction method for the inspection device 1. FIG. 6 is a flow chart illustrating the focus correction method according to the first embodiment. As shown in step S10 in FIG. 6, first, a correlation between a displacement drift of an optical element included in the optical system 10 and a focus drift is stored. For example, the correlation is stored in the storage unit 52.

[0050] FIG. 7 is a flow chart illustrating a method for acquiring the correlation between the displacement drift of the optical element and the focus drift in the focus correction method according to the first embodiment. As shown in step S11 of FIG. 7, the displacement drift of the optical element and the focus drift are acquired simultaneously. First, the displacement of the position of the optical element is measured. For example, the displacement measuring unit 40 is caused to measure the displacement of the position of the optical element. Then, the focus drift at that time is acquired. For example, when the position of the optical element changes, the control unit 54 is caused to move the stage 30 so that the image detected by the detector 20 is focused when the position of the optical element changes. From the position of the stage 30 at this time, the best focus position when the position of the optical element changes is acquired.

[0051] When the position of the optical element is changed to a plurality of positions, each displacement at each position is measured. Then, the best focus position at each displacement is obtained. In this manner, by obtaining a plurality of best focus positions at a plurality of positions, the accuracy of the correlation can be improved. Then, the processing unit 51 is caused to calculate the displacement drift of each displacement using a predetermined position of the optical element as a reference position, and to calculate the focus drift of each best focus position using the distance between the sample 60 and the optical system 10 at a predetermined focus position as a reference distance.

[0052] Next, as shown in step S12, the correlation between the displacement drift of the optical element and the focus drift is obtained. For example, an approximation formula is derived by having the processing unit 51 fit a graph such as that shown in FIG. 5. Note that the processing unit 51 may create a table instead of the approximation formula. In this manner, the correlation between the displacement drift of the optical element and the focus drift is obtained.

[0053] Next, the displacement drift of the optical element is measured as shown in step S20 of Fig. 6. For example, when measuring the displacement drift of the optical element, the displacement drift of the optical element may be measured by detecting the position of the mirror 41 from the interference between the laser light L13 emitted to the mirror 41 in the displacement measuring unit 40 and the laser light L13 reflected by the mirror 41.

[0054] Furthermore, when the optical system 10 includes the Schwarzschild optical system 14 having the concave mirror 15 and the convex mirror 16, the convex mirror 16 may be fixed to a holder 45 including a first member 46 extending in the Z-axis direction parallel to the optical axis and a second member 47 extending in the X-axis direction or Y-axis direction perpendicular to the optical axis. In this case, the mirror 41 may be fixed to the optical element via the second member 47. Then, the position of the optical element may be measured from the interference between the laser light L13 irradiated from an in-plane direction including the X-axis direction or the Y-axis direction and the laser light L13 reflected in the opposite direction by the mirror 41.

[0055] Next, as shown in step S30, the focus drift is predicted from the measured displacement drift by using a correlation.

[0056] As shown in step S40, the distance between the sample 60 and the optical system 10 may be controlled based on the focus drift predicted in step S30 for predicting the focus drift. Specifically, the control unit 54 controls the distance between the sample 60 and the optical system 10 by moving the stage 30 on which the sample 60 is placed.

[0057] Next, the effects of this embodiment will be described. The inspection device 1 of this embodiment stores in advance in the storage unit 52 the correlation between the displacement drift and the focus drift of the optical element. Then, the correlation is used to predict the focus drift from the displacement drift of the optical element displaced due to a temperature rise. This makes it possible to improve focus alignment. Also, since the correlation is stored in advance, the focus can be quickly adjusted.

[0058] The inspection device 1 also includes a displacement measuring unit 40 that measures the displacement drift of the optical element. This allows for rapid response to environmental changes such as temperature rise. The displacement measuring unit 40 includes an interferometer 43, allowing for highly accurate measurement of the displacement drift of the optical element. The displacement measuring unit 40 also measures the displacement drift of the convex mirror 16 of the Schwarzschild optical system 14. This allows for measurement of the displacement drift of an optical element on which the detection light L12 is concentrated and which is subject to large temperature changes, thereby improving the accuracy of predicting focus drift.

[0059] Furthermore, the displacement measuring unit 40 measures the displacement drift using the mirror 41 fixed to the first member 46 and the second member 47. This makes it possible to convert the displacement caused by thermal expansion in the optical axis direction into an in-plane direction, and to measure the displacement drift of the convex mirror 16, which is subject to large temperature changes. This makes it possible to improve the accuracy of predicting the focus drift.

[0060] Although the embodiments of the present disclosure have been described above, the present disclosure includes appropriate modifications that do not impair the objects and advantages thereof, and is not limited to the above-described embodiments. [Explanation of symbols]

[0061] 1. Inspection equipment 10 Optical system 11, 12 concave mirror 13. Recessed mirror 14 Schwarzschild Optical System 15 concave mirror 16 Convex mirror 17 plane mirror 18 concave mirror 20 Detector 30 Stages 40 Displacement measurement section 41 Mirror 42 Laser 43 Interferometer 45 Holder 46 First member 47 Second member 50 Processing equipment 51 Processing section 52 Storage section 53 Prediction Department 54 Control section 60 samples 61 Chamber C optical axis IF focus L11 Illumination light L12 Detection light L13 Laser light LS light source

Claims

1. a detector for detecting light reflected by the sample from the illumination light; an optical system that illuminates the sample with the illumination light and guides the reflected light reflected by the sample to the detector; a displacement measurement unit that measures a displacement drift indicating an amount of drift of a position of an optical element included in the optical system; a storage unit that stores a correlation between the displacement drift and a focus drift that indicates a drift amount of a distance between the sample and the optical system when the reflected light detected by the detector is focused; a prediction unit that predicts the focus drift from the measured displacement drift by using the correlation; An optical device comprising:

2. the displacement drift indicates the amount of drift of the optical element from a reference position; The focus drift indicates the amount of drift from a reference distance between the sample and the optical system.

2. The optical device of claim 1.

3. The displacement measuring unit is A mirror fixed to the optical element; A laser that emits laser light; an interferometer that detects a position of the mirror based on interference between the laser light emitted to the mirror and the laser light reflected by the mirror; Including, 2. The optical device of claim 1.

4. the optical system includes a Schwarzschild optical system having a concave mirror and a convex mirror; the convex mirror is fixed to a holder including a first member extending in a first direction parallel to an optical axis of the convex mirror and a second member extending in a second direction perpendicular to the optical axis; the mirror is fixed to the optical element via the second member, the displacement measuring unit measures the position based on interference between a laser beam emitted along the second direction and the laser beam reflected by the mirror in a direction opposite to the second direction.

4. The optical device according to claim 3.

5. A control unit that controls a distance between the sample and the optical system; Further equipped with The control unit controls the distance based on the focus drift predicted by the prediction unit.

2. The optical device of claim 1.

6. a detector for detecting light reflected by the sample from the illumination light; an optical system that illuminates the sample with the illumination light and guides the reflected light reflected by the sample to the detector; A focus correction method for an optical device comprising: A step of storing a correlation between a displacement drift indicating an amount of drift in the position of an optical element included in the optical system and a focus drift indicating an amount of drift in the distance between the sample and the optical system when the reflected light detected by the detector is focused; measuring the displacement drift; predicting the focus drift from the measured displacement drift by using the correlation; The focus correction method includes:

7. the displacement drift indicates the amount of drift of the optical element from a reference position; The focus drift indicates the amount of drift from a reference distance between the sample and the optical system. The focus correction method according to claim 6 .

8. The optical device further includes a displacement measuring unit, The displacement measuring unit includes: A mirror fixed to the optical element; A laser that emits laser light; an interferometer that detects a position of the mirror based on interference between the laser light emitted to the mirror and the laser light reflected by the mirror; Including, In the step of measuring the displacement drift, measuring the displacement drift by detecting a position of the mirror based on interference between the laser light emitted to the mirror and the laser light reflected by the mirror; The focus correction method according to claim 6 .

9. the optical system includes a Schwarzschild optical system having a concave mirror and a convex mirror; the convex mirror is fixed to a support member including a first member extending in a first direction parallel to an optical axis of the convex mirror and a second member extending in a second direction perpendicular to the optical axis; the mirror is fixed to the optical element via the second member, In the step of measuring the displacement drift, measuring the position based on interference between a laser beam emitted along the second direction and the laser beam reflected by the mirror in a direction opposite to the second direction; The focus correction method according to claim 8.

10. The method further comprises controlling the distance based on the focus drift predicted in the step of predicting the focus drift. The focus correction method according to claim 6 .

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