CN118068551A

CN118068551A - Optical system and method of calibrating an optical system

Info

Publication number: CN118068551A
Application number: CN202310966789.5A
Authority: CN
Inventors: M·拉皮德
Original assignee: Applied Materials Israel Ltd
Current assignee: Applied Materials Israel Ltd
Priority date: 2022-11-23
Filing date: 2023-08-02
Publication date: 2024-05-24
Also published as: KR20240076365A; IL298509A; IL298509B1; IL298509B2

Abstract

The present disclosure relates to a method of calibrating an optical system, the optical system comprising: an objective lens arrangement for receiving reflected light from at least a portion of an object, and an imaging lens for projecting the light received from the objective lens arrangement to form an image of at least a portion of the object on a target plane, the method comprising: defining a target area on a target plane based on a predetermined size of an image of at least a portion of an object to be formed on the target plane; moving the imaging lens along its optical axis to a plurality of imaging lens positions that vary in distance from the target plane to vary the magnification of the image on the target plane; determining a first imaging lens position from the plurality of imaging lens positions when the image of at least part of the object fits within the target area on the target plane; and positioning the imaging lens at the operative position based on the first imaging lens position such that the image of at least part of the object is constrained within a target zone on the target plane.

Description

Optical system and method of calibrating an optical system

Technical Field

The present technology relates to optical systems (such as optical arrangements used in microscopes) and methods of calibrating such systems. The present technology further relates to, for example, an optical inspection system for inspecting a specimen or object, such as, but not limited to, inspection of a semiconductor wafer and/or mask, including the optical microscope system disclosed herein.

Background

There are a variety of systems for specimen inspection. These systems may include optical systems such as various microscopes (both conventional and digital) and microscope arrangements, and the specimens may include a range of objects such as semiconductor wafers and masks, food products, and organic specimens. U.S. patent No. 6,407,373, incorporated herein by reference, discloses, for example, a system for inspecting defects on an object, including optical microscopy and Scanning Electron Microscopy (SEM).

Conventional optical inspection systems typically include an objective lens for collecting light from the inspected sample. The objective lens may form part of an objective lens arrangement, which further comprises one or more additional lenses. The sample may be illuminated by a light source that reflects, transmits, and/or scatters light from the light source. Imaging the light collected from the sample allows analysis of the surface structure of the sample. The sample may be received and fixed on a stationary stage or moved on a stage mechanism that allows the sample to be moved in one dimension (e.g., changing the distance between the sample and the objective lens or the objective lens arrangement along the z-axis), two dimensions (e.g., along the z-axis and the scan direction orthogonal to the z-axis (x-axis or y-axis)) or three dimensions, as desired. The light source may be external to or provided as an integral part of the optical inspection system as desired, and may include aerial illumination, a single point light source (e.g., a laser), or an array of point light sources, with different wavelengths and intensities as desired. The objective arrangement continues to transmit the (reflected, transmitted and/or scattered) light collected from the sample, and then an imaging lens arranged along the optical axis of the objective arrangement forms an image of the sample (or a part of the sample) with the light from the objective arrangement at the image plane or back focal plane of the imaging lens. The magnified image of the sample (or portion of the sample) may then be detected using one of a variety of optical detector devices, including a conventional camera, an optical detector array (e.g., of a CCD detector), a photodiode or photomultiplier, or the like. Here, the "enlarged image" may include an enlarged image, a reduced image, or an image of the same scale as the sample (a part thereof); in other words, the magnified image may have a magnification of > 1, < 1, or equal to 1.

In general, the image magnification of the optics (e.g., objective lens or objective lens arrangement and imaging lens) at the optical detector between instruments may have a variation of a few percent. In the case where the image magnification is lower than the magnification specified for the instrument, the image does not fill the entire area of the optical detector device in use, and thus the detector arrangement is not efficiently utilized. In the case where the image magnification is higher than the magnification specified for the instrument, a portion of the optical signal may fall outside the area of the detector device and thus may not be detected. In general, if the image magnification deviates from the desired specification, vignetting may occur and the resulting non-uniformity along the field of view of the detector device may change the detection sensitivity. Furthermore, ghost images (weak second images caused by reflections within the optical components) and back reflections may also increase when light from the image falls on portions of the detector device where light is not intended to be received. The main reason for this magnification variation is the production process of the lenses used in the instrument, such as the polishing process. In some cases, there may also be minor variations due to environmental factors (such as temperature).

One approach to the magnification variation problem is to provide the instrument with a zoom capability using a conventional optical zoom system, which typically includes two or more lenses or optical modules, so that the magnification variation can be adjusted by operating the zoom system to the correct (specified) magnification of the optical detector. However, this approach is costly due to the additional optics involved, and may introduce additional uncertainties in optical performance, such as introducing field distortion, causing ghosts and back reflections, the visual axis, reducing transmission due to the introduction of additional optics, birefringence, etc.

It is therefore desirable to provide an improved method of calibrating an optical system to account for variations in magnification in microscope optics, particularly for use in semiconductor inspection and metrology equipment.

Disclosure of Invention

In view of the foregoing, an aspect of the present technology provides a method of calibrating an optical system, the optical system comprising: an objective arrangement for receiving reflected light from at least a portion of an object, and an imaging lens for projecting the light received from the objective arrangement to form an image of at least the portion of the object on a target plane, the method comprising: defining a target area on the target plane based on a predetermined size of the image of at least the portion of the object to be formed on the target plane; moving the imaging lens along its optical axis to a plurality of imaging lens positions that vary in distance relative to the target plane to vary the magnification of the image on the target plane; determining a first imaging lens position from the plurality of imaging lens positions when the image of at least the portion of the object fits within the target area on the target plane; and positioning the imaging lens at an operative position based on the first imaging lens position such that the image of at least the portion of the object is constrained within the target area on the target plane.

According to an embodiment of the present technology, there is provided a method in which magnification deviation of an optical device of an optical system is calibrated by a simple operation of adjusting a position of an imaging lens along an optical axis (z-axis) thereof with respect to a target plane on which a magnified image is formed. In particular, a target area is defined on the target plane within which the magnified image is to be constrained. The target area or the size of the target area may be considered as the desired magnification specified during production. For example, the target area may represent the FOV of the optical detector device (e.g., camera) used, a predetermined area within the imaging area of the optical detector device, and/or a predetermined number of imaging areas (e.g., pixels) of the optical detector device. By simply adjusting the axial position of the imaging lens and positioning the imaging lens in a position that constrains the resulting magnified image within the target area on the target plane, the optical system can be calibrated to produce an image with a specified magnification. Thus, embodiments of the present technology enable the magnification to be calibrated without the need for expensive additional optics, rather than accommodating variations in magnification.

There may be various suitable ways to detect or otherwise determine when images fit within a target area on a target plane. In some embodiments, the method may further include detecting an image of at least a portion of the object using one or more optical sensors located substantially at a position on the target plane.

In some embodiments, the photodetector device may include an imaging region formed by a plurality of imaging regions (e.g., pixels), and the target region may be defined as a predetermined number of imaging regions on the photodetector device. For example, a calibration target may be used as the object, and the first imaging lens position may be determined when the image of the calibration target fits within an imaging region that includes all pixels or within a given number (one or more) of pixels on the camera.

In some embodiments, the object may be a calibration object of known size such that the magnification of the target area defining object is defined on the target plane based on a predetermined size of an image of at least a portion of the object to be formed on the target plane. Using a calibration object with a known size or dimension means that the size of the target area can be defined in order to achieve the desired magnification.

After adjusting the imaging lens position, the image formed on the target plane may be out of focus. Thus, in some embodiments, the method may further comprise: adjusting an axial position of the object along the optical axis to change a distance between the object and the objective lens arrangement; and determining an object axial position of the object at which an image of at least a portion of the object is focused on the target plane; the object is disposed at an object axial position. In so doing, the image formed on the target plane may be refocused directly. In embodiments where the objective lens is arranged telecentric on the object side, the image formed on the object plane can be refocused by adjusting the axial position of the object along the optical axis without affecting the magnification of the image.

There may be situations where the image covers only a portion of the target area when the imaging lens is in the first imaging lens position, in which case any detection or sensing equipment for detecting the image is not fully utilized. Thus, in some embodiments, the method may further comprise: the second imaging lens position is determined from the plurality of imaging lens positions when the image extends beyond the target area on the target plane, and the operating position is set to a position between the first imaging lens and the second imaging lens position.

In some embodiments, the operational position may be disposed substantially midway between the first imaging lens position and the second imaging lens position.

Another aspect of the present technology provides a non-transitory computer readable medium comprising machine readable code which, when executed by a processor, causes the processor to perform the method as described above.

A further aspect of the present technology provides an optical microscope system comprising: an objective lens arrangement configured to receive reflected light from at least a portion of the object; an imaging lens disposed at an operative position spaced apart from the objective lens arrangement, the imaging lens configured to receive light from the objective lens arrangement and project the received light from the objective lens arrangement to form an image of at least the portion of the object on a target plane; and a control unit configured to determine the operation position by: defining a target area on the target plane based on a predetermined size of the image of at least the portion of the object to be formed on the target plane; moving the imaging lens along its optical axis to a plurality of imaging lens positions that vary in distance relative to the target plane to vary the magnification of the image on the target plane; determining a first imaging lens position from the plurality of imaging lens positions when the image of at least the portion of the object fits within the target area on the target plane; and setting the operating position based on the first imaging lens position such that the image of at least the portion of the object is constrained within the target area on the target plane.

In some embodiments, the objective arrangement is configured such that an exit pupil of the objective arrangement is positioned outside the objective arrangement on a back focal plane of the objective arrangement.

Still a further aspect of the present technology provides an inspection system for inspecting an object, comprising: a light source arranged to illuminate the object; an optical microscope system as described above, the optical microscope system being arranged to transmit reflected light from the object onto the target plane; and at least one light detector device arranged to detect light transmitted through the optical system.

In some embodiments, the at least one light detector device comprises a light detector array.

In some embodiments, the at least one light detector device may include a plurality of imaging regions.

In some embodiments, the inspection system may further comprise a platform configured to receive the object, wherein a position of the platform is adjustable relative to a distance from the objective lens arrangement of the optical system.

Each embodiment of the present technology has at least one, but not necessarily all, of the above-described objects and/or aspects. It will be appreciated that some aspects of the present technology resulting from an attempt to achieve the above-described objects may not meet the objects and/or may meet other objects not specifically recited herein.

Additional and/or alternative features, aspects, and advantages of embodiments of the present technology will become apparent from the following description, the accompanying drawings, and the appended claims.

Drawings

For a better understanding of the subject matter disclosed herein and to illustrate how the subject matter disclosed herein may be practiced, embodiments will be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates an exemplary inspection system for inspecting a specimen;

FIG. 2 schematically illustrates an exemplary optical microscope system for use in the inspection system of FIG. 1;

FIG. 3 shows a representation of a target zone;

FIG. 4 illustrates an exemplary ray diagram of an imaging lens forming an image on a camera in the Z0 position;

FIG. 5 illustrates an exemplary ray diagram of the imaging lens of FIG. 4 forming an image on a camera at the Z1 position;

FIG. 6 illustrates an exemplary ray diagram of the imaging lens of FIG. 4 forming an image on a camera at the Z2 position;

FIG. 7 illustrates an exemplary ray diagram of the imaging lens of FIG. 4 forming an image on a camera at the Z3 position;

FIG. 8A illustrates an image I' formed on an imaging region of the camera of FIG. 6;

FIG. 8B illustrates an image I' formed on an imaging region of the camera of FIG. 7;

Fig. 8C illustrates an image I formed on an imaging region of the camera of fig. 5;

FIG. 9 shows an exemplary ray diagram of the photodetector at position Z0'; and

Fig. 10 shows an exemplary ray diagram of the light detector of fig. 9 repositioned to a new position Z1'.

Detailed Description

Embodiments of the present technology provide methods for calibrating optical systems (e.g., optical systems implemented in microscopes, defect detection apparatuses, specimen inspection apparatuses, etc., such as those used in semiconductor wafers and/or mask inspection equipment) to correct magnification variations in optical train devices (such as caused by production processes and other factors) by a simple operation of adjusting the position of an imaging lens along its optical axis (z-axis) relative to the position of a target plane on which the imaging lens forms a magnified image. In particular, the imaging lens is repositioned to a position in which the resulting magnified image formed by the imaging lens is constrained within a target zone defined on the target plane. The target area or the area covered by the target area may be considered as the desired image magnification specified for the instrument. For example, the target area may represent or correspond to an area covered by a photodetector for detecting the magnified image and/or an effective imaging area of the photodetector. By simply adjusting the position of the imaging lens without moving other elements of the instrument and positioning the imaging lens in a position that constrains the resulting magnified image within the target area on the target plane, the optical system can be calibrated to produce an image with the desired magnification. Thus, embodiments of the present technology enable the magnification to be calibrated or adjusted for variations caused by many different factors.

Fig. 1 schematically illustrates an exemplary inspection system 100 for inspecting specimens, such as (but not limited to) for inspecting defects, particles, and/or patterns on the surface of semiconductor wafers and/or masks as part of a quality assurance process in a semiconductor manufacturing process, according to an embodiment.

The inspection system 100 includes a set of optics or optical systems, which in this embodiment includes an objective lens or objective lens arrangement 120 and an imaging lens or imaging lens arrangement 140. A photodetector or photodetector array 170 is disposed behind the imaging lens or imaging lens arrangement 140 for detecting an image formed by the imaging lens or imaging lens arrangement 140. The detector or detector array 170 may be a camera, a photomultiplier array, or any other suitable light detector. The inspection system 100 also includes a platform 110 for receiving or securing an object or specimen for inspection. The stage 110 may be stationary or may be movable in a longitudinal direction (along the optical axis, z-axis, of the objective lens or objective lens arrangement 120) and/or in a lateral direction (x-and/or y-axis) in the same plane as the stage 110. It will be appreciated that the relative movement between the objects or samples may also be achieved by holding the platform 110 stationary while providing for moving the objective or the objective arrangement 120 to change the relative position between the objective or the objective arrangement 120 and the platform 110, or for moving all remaining elements of the inspection system 100 as a whole. For example, the table mechanism may be configured to move in coordination with the scanning sequence to enable an object placed on the platform 110 to be scanned by the light source 180. The light source 180 may be an aerial illumination, a single laser (acting as a point source focused as a point on the object), or an array of lasers.

In this embodiment, a reflector 190 may be placed along the optical axis of the objective lens or objective lens arrangement 120 to direct the light beam from the light source 180 (through the objective lens or objective lens arrangement 120) towards the platform 110. This enables the light source 180 to be placed outside the optical axis of the objective lens or objective lens arrangement 120, for example to achieve a more compact instrument. Similarly, the imaging lens or imaging lens arrangement 140 and the detector or detector array 170 may also be arranged outside the optical axis of the objective lens or objective lens arrangement 120, for example to achieve compactness by using a partially reflective element 130. The element 130 may also act as a beam splitter arranged to allow a portion of the light from the object to pass through to an imaging lens or imaging lens arrangement 140 and to direct another portion of the light in a different direction, e.g. towards a set of separate imaging lens arrangements and detectors. It should be appreciated that positioning the imaging lens or imaging lens arrangement 140 and the detector or detector array 170 and/or the light source 180 at an angle with respect to the optical axis of the objective lens or objective lens arrangement 120 is entirely optional and not necessary to the present technology.

The objective lens or objective lens arrangement 120 is arranged to collect light from the object (e.g. light from the light source 180 that is reflected and/or scattered by a part of the object, or light that is transmitted through a part of the object as in the case of a transmission microscope). In some embodiments, the objective lens or objective lens arrangement 120 may optionally be configured to telecentric image on the object side such that light leaves the objective lens or objective lens arrangement 120 and passes through an exit pupil (not shown) of the objective lens or objective lens arrangement 120 as parallel rays. The embodiment of the objective lens or the objective lens arrangement 120 being telecentric on the object side advantageously enables the axial position of the object (i.e. the stage 110) to be adjusted to focus the image without affecting the final image magnification. Optionally, if desired, the objective lens or objective lens arrangement 120 may be configured such that the exit pupil is located at a position outside (at its back focal plane) the objective lens or objective lens arrangement 120. In such an embodiment, partially reflective element 130 may optionally be disposed at the external exit pupil, although this is not required. In this embodiment, the imaging lens 140 receives light from the objective lens or objective lens arrangement 120 and forms an image onto a detector or detector array 170 disposed on the target plane 150, and the detector or detector array 170 detects the image formed thereon. In this embodiment, any image desired to be formed by imaging lens 140 is constrained within the effective imaging area of photodetector 170; in this way, the effective imaging area of photodetector 170 defines a target zone 160 within which the image is preferably constrained.

It should be appreciated that the present technique may equally be implemented in inspection systems that utilize a transmissive optical system in which light transmitted through the sample is collected and analyzed, or in inspection systems that utilize infrared radiation.

Fig. 2 schematically illustrates an optical system used in the inspection system 100 of fig. 1. For ease of understanding, the objective lens arrangement 120 and the imaging lens 140 are arranged along the same (z) axis in fig. 2.

In this embodiment, the objective arrangement 120 is arranged to collect light reflected (or transmitted) and/or scattered from a portion h of the object 200, which object 200 is for example placed on the platform 110 of fig. 1 at an axial position at a distance dl from the objective arrangement 120. Although not necessary, in the present embodiment, the objective lens arrangement 120 is arranged and aligned such that the exit pupil of the objective lens arrangement 120 is outside the objective lens arrangement 120 and is located on the back focal plane (exit pupil plane) 135 of the objective lens arrangement 120. For illustration purposes, the objective lens arrangement 120 in the present embodiment is shown as being arranged for telecentricity on the object side, although this is not necessary. The arrangement and configuration of the optical elements within the objective lens arrangement 120 for achieving external exit pupils or telecentricity is not within the scope of the present technology and is therefore not discussed herein.

As shown in fig. 2, the objective lens arrangement 120 gathers light reflected or scattered from different points of the portion h of the object 200 (shown as a cone of light having a half angle θ) and transmits the light to the exit pupil 135. The imaging lens 140 receives the light from the objective lens arrangement 120 and forms an image of the portion h of the object 200 on the target plane 150 at a distance d2 from the imaging lens 140. The image of portion h of object 200 may then be imaged or detected using a suitable light detector device (e.g., camera) 170. Since the light detector device may be considered to have a specific effective imaging area (the area on the detector where light detection is desired), it is desirable to ensure that the image is constrained within a target area on the target plane 150 that corresponds directly or indirectly to the effective area of the detector used.

In an embodiment, the object 200 may be a calibration target for a calibration process. Light detector 170 (e.g., a camera) may be placed on target plane 150. The light detector (or light sensitive portion of the light detector) may be considered to include an imaging region formed of a plurality of image areas (e.g., pixels), and the target area within which the image should be confined may be defined as an area 160, the area 160 covering a predetermined number of image areas on the imaging region or light detector.

Fig. 3 shows a representation of a target zone 160, the target zone 160 being an imaging zone of the camera 170 formed by a plurality of pixels 301, 302, 303.

As described above, it is desirable to calibrate the optical system such that an image formed by an imaging lens (e.g., imaging lens 140) of the optical system is constrained within a target zone. In the optical system of fig. 2, it is desirable to constrain the image formed by the imaging lens 140 within a predefined target area, for example, defined by a plurality of image areas (pixels) of a light detector (camera). Shown in fig. 4 is a ray diagram of imaging lens 140 forming an image at position Z0 (zero position), for example, on an imaging region of a light detector device (e.g., camera). As shown in fig. 4, the image formed by the imaging lens 140 at the position Z0 is larger than the area covered by the imaging area of the light detection device 160 (target area), and thus the detected image pattern extends beyond the FOV of the light detector device.

In accordance with the present technique, the optical system of fig. 2 may be calibrated (image magnification adjusted) by repositioning the imaging lens 140, as shown in fig. 5. In fig. 5, imaging lens 140 is repositioned to a position Z1 a distance dZ relative to position Z0 of imaging lens 140 in fig. 4. As the imaging lens 140 is moved to Z1, the imaging lens 140 is closer to the target plane 150, and the image formed thereby falls within the imaging region of the light detection device 160. As shown in fig. 4 and 5, since the angle at which the light enters the imaging lens 140 remains the same, the light is incident on different points (heights) on the imaging lens 140, so that the light is refracted by the imaging lens 140 to different degrees. As shown in fig. 5, the light rays exiting the imaging lens 140 form an exit angle different from that of the light rays exiting the imaging lens 140 in fig. 4. Accordingly, the magnification of the image is changed by the axial movement of the imaging lens 140 that changes the relative distance between the imaging lens 140 and the target plane 150, and by the change of the exit angle.

In practice, the imaging lens 140 may be moved through a number of different positions along the Z-axis, for example using an automated mechanism controlled by software executing on a processor, until the optimal position, e.g., position Z1, is found.

In some embodiments, it may be desirable to fine tune the position of the imaging lens 140 so that the magnified image is not only constrained within the target region 160 (e.g., the imaging region of the photodetector device) but is not so small that the photodetector arrangement is underutilized. Thus, according to some embodiments, imaging lens 140 may be moved along the Z-axis toward target plane 150 through a plurality of positions until it reaches position Z2, at which the magnified image formed on the imaging area of light detector device 160 becomes too small, as shown in fig. 6.

For this purpose, a point (i.e., position Z2) at which the image formed on the imaging region of the light detector device 160 is considered too small may be selected, for example, when the image I' is formed on less than a predetermined number of imaging regions (e.g., pixels) on the light detection device 170, as shown in fig. 8A. In this scenario, light from the image formed by imaging lens 140 is constrained within image area I' such that some of imaging regions 301, 302, 303, 304 … … do not detect any light or only partially receive light and thus utilize less than the area of the entire imaging region of the photodetector device used to detect the image.

When establishing position Z2 for imaging lens 140, it may be assumed that a better position of imaging lens 140 is located farther from photodetector device 170 (or object plane 150). In some embodiments, imaging lens 140 may then be moved through a plurality of positions along the Z-axis in the opposite direction (away from target plane 150) until it reaches position Z3 where the image formed on the photodetector device becomes too large, as shown in fig. 7.

For this purpose, a point (i.e. position Z3) may be selected at which the image formed on the light detector device is considered too large, for example when the image i″ formed on the light detector device falls partly outside the imaging area (or a predetermined number of imaging areas, e.g. pixels) of the light detector device, as shown in fig. 8B clock. In this case, the light from the image formed by the imaging lens 140 covers the image area i″ such that the light from the image extends beyond the area covered by the imaging area of the light detection device 160.

When establishing the position Z3 for the imaging lens 140, it can be assumed that the optimal position of the imaging lens 140 is somewhere between the positions Z2 and Z3. Positions Z0, Z1, Z2 and Z3 are shown in fig. 6 and 7 for comparison. The final operating position Z2 of the imaging lens 140 may be disposed at a position between Z2 and Z3. In some embodiments, Z1 may be disposed midway between Z2 and Z3, although other choices with different ratios are possible.

For illustration purposes, fig. 8C shows the position of the image area I covered by the image formed on the imaging area of the photodetector device 160 when the imaging lens 140 is located at position Z1.

When the imaging lens 140 is positioned in the operative position Z2 (the image formed by the imaging lens 140 on the target plane 150 is constrained within the target zone 160 at the operative position Z2), the formed image may become out of focus. Thus, in some embodiments, the height or axial position d1 of the object 200 (see fig. 2) may be adjusted, for example, by controlling the height or axial position of the platform 110 (by increasing or decreasing the axial position dl) until the image formed on the target plane 150 is in focus. Alternatively, the light detector (e.g., camera) may be repositioned along the z-axis to decrease or increase its distance from the imaging lens 140 until the image formed on the light detector is in focus. This option may be less preferred because it may affect the magnification of the image formed on the light detector. However, a person skilled in the art will be able to implement additional adjustment steps based on the above discussion. In an alternative embodiment, the target plane 150 or a light detector (e.g., camera) defining the target area 160 may be repositioned along the optical axis as the size/dimension of the image formed behind the imaging lens 140 varies depending on the distance from the imaging lens 140 until a desired magnification (or image size) is achieved for the image formed on the repositioned target plane. As shown in fig. 9 and 10, wherein light detector 170 is repositioned along the light (Z) axis from Z0' to Z1' toward imaging lens 140 by a distance dZ ' to reduce the magnification of the image formed thereon. Again, after repositioning light detector 170, the height or axial position dl of subject 200 may be adjusted by increasing or decreasing axial position dl to refocus the image formed on light detector 170.

In some embodiments, the transmissive optical system may be used in, for example, but not limited to, a mask inspection tool, wherein light is transmitted through a specimen (e.g., a semiconductor mask) and out the other side of the specimen rather than being reflected. In these embodiments, the imaging lens and/or the light detector for detecting the image formed by the imaging lens may similarly be axially repositioned along the optical axis (relative to the objective lens arrangement) in order to adjust the magnification of the image on the light detector, as described above. In such an embodiment, the specimen may be axially repositioned (changing the distance between the specimen and the objective lens arrangement) to correct the resulting image focus. Those skilled in the art will appreciate that the illumination system may need to be adjusted in some way to maintain the illumination field stop focus at the sample.

Since telecentricity (or non-telecentricity) depends on the distance between the imaging lens 140 and the back focal plane of the objective lens arrangement 120, repositioning the imaging lens 140 along the optical axis may affect telecentricity as well as magnification. In particular, repositioning the imaging lens 140 relative to the target plane 150 not only has the technical effect of adjusting the magnification (or image size) of the image formed on the target plane 150, but may also have the additional technical effect of changing the angle of the centroid ray, which facilitates more efficient convergence of the image formed thereby to a desired size/magnification than repositioning the target plane 150 (or photodetector).

It should be noted that in embodiments in which the imaging lens 140 is axially repositioned along the optical axis relative to the objective lens arrangement 120 and/or the light detector 170, the adjustment of the image magnification may be significantly separated from the image focusing to achieve simple direct alignment (or realignment) of the instrument optics. Those skilled in the art will be able to optimize the magnification and focus (e.g., by iterative alignment, etc.) as the case may be. While there may still be some degree of dependence between magnification and focus, adding a zoom system to the instrument in contrast results in a more significant non-trivial dependence between magnification and focus, thereby introducing uncertainty and complexity to alignment.

The techniques described herein are capable of calibrating image magnification in an optical system by adjusting the position of an imaging lens in the optical system and positioning the imaging lens within a target region (where the magnified image is detected) that constrains the resulting magnified image on a target plane (on which the image is generated). Thus, the techniques described herein enable an optical system to be calibrated for magnification variations (whether due to a production process or other reasons) without the need for additional instruments such as conventional zoom systems. Embodiments of the present technology may be implemented in the manufacture of semiconductor inspection and metrology equipment; in particular, embodiments of the present technology may be implemented in manufacturing or processing processes for testing, measuring, and/or calibrating optical systems to account for variations in magnification without requiring further processing to modify elements of the optical systems.

As will be appreciated by one skilled in the art, the present technology may be embodied as a system, method or computer program product. Accordingly, the present technology may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects.

Furthermore, the present technology may take the form of a computer program product embodied in a computer-readable medium having computer-readable program code embodied thereon. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. The computer readable medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing.

Computer program code for carrying out operations of the present technology may be written in any combination of one or more programming languages, including an object oriented programming language and conventional procedural programming languages.

For example, program code for performing operations of the present technology may include source code, object code, or executable code (such as C) in a conventional programming language (interpreted or compiled), or assembly code, code for setting or controlling an ASIC (application specific integrated circuit) or FPGA (field programmable gate array), or code for a hardware description language, such as VerilogTM or VHDL (very high speed integrated circuit hardware description language).

The program code may execute entirely on the user's computer, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network. Code components may be embodied as processes, methods, and the like, and may include subcomponents that may take the form of instructions or sequences of instructions at any level of abstraction (from direct machine instructions of a native instruction set to a high-level compiled or interpreted language structure).

It will also be apparent to those skilled in the art that all or part of the logic methods according to the preferred embodiments of the present technology may be suitably embodied in a logic device that includes logic elements for performing the steps of the methods, and that such logic elements may include components such as, for example, logic gates in a programmable logic array or application specific integrated circuit. Such a logic arrangement may further be embodied as enabling elements that temporarily or permanently establish a logic structure in such an array or circuit using, for example, a virtual hardware descriptor language, which may be stored and transmitted using fixed or transmittable carrier media.

The examples and conditional language recited herein are intended to aid the reader in understanding the principles of the technology and are not intended to limit the scope of the same to such specifically recited examples and conditions. It will be appreciated that those skilled in the art may devise various arrangements which, although not explicitly described or shown herein, embody the principles of the technology and are included within its scope as defined by the following claims.

Furthermore, to facilitate understanding, the foregoing description may describe relatively simplified embodiments of the present technology. Those skilled in the art will appreciate that various embodiments of the present technology may have greater complexity.

In some cases, useful examples that are considered modifications to the present technology may also be set forth. This is done merely to aid in understanding and is again emphasized that it is not intended to limit the scope of the technology or set forth the limits of the technology. Such modifications are not an exhaustive list and other modifications may be made by those skilled in the art while remaining within the scope of the present technology. Furthermore, without setting forth a modified example, it should not be construed that modifications are possible and/or that the only way to implement the element of the present technology is described.

Moreover, all statements herein reciting principles, aspects, and embodiments of the technology, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof, whether presently known or later developed. Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the technology. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudocode, and the like represent various processes which may be substantially represented in computer readable media and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

The functions of the various elements shown in the figures, including any functional blocks labeled as "processors", may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Furthermore, explicit use of the term "processor" or "controller" should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital Signal Processor (DSP) hardware, network processor, application Specific Integrated Circuit (ASIC), field Programmable Gate Array (FPGA), read Only Memory (ROM) for storing software, random Access Memory (RAM), and non volatile storage. Other conventional and/or custom hardware may also be included.

A software module or simple module implied as software may be represented herein as any combination of flow chart elements or other elements that indicate execution of process steps and/or textual descriptions. Such modules may be performed by hardware explicitly or implicitly shown.

It will be apparent to those skilled in the art that many improvements and modifications can be made to the foregoing exemplary embodiments without departing from the scope of the present technology.

Claims

1. A method of calibrating an optical microscope system, the optical microscope system comprising: an objective arrangement (120) for receiving light from at least a portion of an object (200), and an imaging lens (140) for projecting the light received from the objective arrangement to form an image of at least the portion of the object on a target plane (150), the method comprising:

defining a target area (160) on the target plane based on a predetermined size of the image of at least the portion of the object to be formed on the target plane;

Moving the imaging lens along its optical axis to a plurality of imaging lens positions that vary in distance relative to the target plane to vary the magnification of the image on the target plane;

determining a first imaging lens position (Z1; Z2) from the plurality of imaging lens positions when the image of at least the portion of the object fits within the target area on the target plane; and

Positioning the imaging lens at an operating position (Z1) based on the first imaging lens position such that the image of at least the portion of the object is constrained within the target zone on the target plane.

2. The method of claim 1, wherein the optical microscope system comprises a light detector device configured to detect the image of at least the portion of the object, the method further comprising detecting the image of at least the portion of the object using the light detector device at a location substantially on the target plane.

3. The method of claim 2, wherein the light detector device comprises an imaging area formed by a plurality of imaging areas (301, 302, 303, … …), and the target area (160) is defined by a predetermined number of imaging areas on the light detector device.

4. The method of claim 1, wherein the object is a calibration object of known size such that defining a target area (160) on the target plane defines a magnification of the object based on a predetermined size of the image of at least the portion of the object to be formed on the target plane.

5. The method of claim 1, further comprising:

Adjusting an axial position (d 1) of the object along the optical axis to vary a distance between the object and the objective lens arrangement;

Determining an object axial position of the object, at which the image of at least the portion of the object is focused on the target plane; and

The object is disposed at the object axial position.

6. The method of claim 1, further comprising: when the image (I ") extends beyond the target area on the target plane, a second imaging lens position (Z3) is determined from the plurality of imaging lens positions, and the operating position (Z1) is set to a position between the first imaging lens position (Z2) and the second imaging lens position (Z3).

7. The method of claim 6, wherein the operational position is disposed substantially midway between the first imaging lens position and the second imaging lens position.

8. A non-transitory computer readable medium comprising machine readable code, which when executed by a processor, causes the processor to perform the method of claim 1.

9. An optical microscope system, comprising:

An objective lens arrangement (120) configured to receive reflected light from at least a portion of the object;

An imaging lens (140) disposed at an operative position spaced apart from the objective arrangement, the imaging lens configured to receive light from the objective arrangement and project the received light from the objective arrangement to form an image of at least the portion of the object on a target plane (150); and

A control unit configured to determine the operation position by:

Determining a first imaging lens position from the plurality of imaging lens positions when the image of at least the portion of the object fits within the target area on the target plane; and

The operating position is set based on the first imaging lens position such that the image of at least the portion of the object is constrained within the target area on the target plane.

10. The optical microscope system according to claim 13, wherein the objective arrangement is configured such that an exit pupil of the objective arrangement is positioned outside the objective arrangement on a back focal plane of the objective arrangement.

11. An inspection system (100) for inspecting an object, comprising:

a light source (180) arranged to illuminate the object;

An optical system according to any one of claims 9 to 10, the optical system being arranged to transmit reflected light from the object onto the target plane; and

At least one light detector device (170) arranged to detect light transmitted through the optical system.

12. The inspection system of claim 11, wherein the at least one light detector device comprises a light detector array.

13. The inspection system of claim 11, wherein the at least one light detector device includes an imaging region formed by a plurality of imaging regions (301, 302, 303, … …).

14. The inspection system of claim 11, further comprising a platform (110) configured to receive the object, wherein a position of the platform is adjustable relative to a distance from the objective lens arrangement of the optical system.

15. The inspection system of claim 11, wherein the inspection system is an optical semiconductor wafer and/or mask inspection system.