IL298509B2 - Optical systems and method of calibrating the same - Google Patents

Description

OPTICAL SYSTEMS AND METHOD OF CALIBRATING THE SAME TECHNOLOGICAL 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 optical inspection systems comprising an optical microscope system disclosed herein, for example for inspecting a specimen or an object such as, but not limited to, inspection of semiconductor wafers and/or masks.

BACKGROUND ART There exist a variety of systems for use in inspections of specimens. These systems may include optical systems such as various microscopes (conventional and digital) and microscope arrangements, and the specimens may include a range of objects such as semiconductor wafers and masks, food products, as well as organic specimens. US patent No. 6,407,373, incorporated herein by reference, discloses for example a system for inspecting defects on an object including both an optical microscope and a scanning electron microscope (SEM). A conventional optical inspection system generally comprises an objective lens for collecting light from a specimen under inspection. The objective lens may form a part of an objective lens arrangement that further comprises one or more additional lenses. The specimen may be illuminated by a light source, which reflects, transmits and/or scatters the light from the light source. Imaging the light collected from the specimen enables analyses of surface structure of the specimen. The specimen may be received and secured on a stationary platform or moved on a stage mechanism that allows the specimen to be moved in one dimension (e.g. varying a distance between the specimen and the objective lens or objective lens arrangement along a z-axis), two dimensions (e.g. along the z-axis and in a scan direction, x-axis or y-axis, orthogonal to the z-axis), or in three dimensions, as desired. The light source may be external to the optical inspection system 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, varying in wavelengths and intensities, as desired. The objective lens arrangement onwardly transmits the light (reflected, transmitted and/or scattered) collected from the specimen, then an imaging lens, disposed along the optical axis of the objective lens arrangement, forms with the light from the objective lens arrangement an image of the specimen (or a part of the specimen) on an image plane or back focal plane of the imaging lens. The magnified image of the specimen (or part of the specimen) may then be detected using one of a variety of optical detector apparatus, including conventional cameras, optical detectors arrays e.g. of CCD detectors, photo diodes, or photomultipliers, etc. Herein, a “magnified image” can include both an enlarged image of a (part of a) specimen, a reduced image or an image of the same scale as the specimen; in other words, a magnified image may have a magnification of >1, <1 or equal to 1. In general, the optics, e.g. the objective lens or objective lens arrangement and the imaging lens, can have a few percent of variation in image magnification at the optical detectors amongst instruments. In cases where image magnification is lower than specified for that instrument, the image does not fill the entire area of the optical detector apparatus in use and the detector apparatus is therefore not being utilized efficiently. In cases where image magnification is higher than specified for that instrument, a portion of the light signal may fall outside of the area of the detector apparatus and therefore may not being detected. In general, if image magnification deviates from the desired specification, vignetting may occur and the resulting non-uniformity along the field of view of the detector apparatus may alter detection sensitivity. Moreover, there may also be an increase in ghost images (faint second images caused by reflections within an optical component) and back reflection as light from an image lands on parts of the detector apparatus that are not intended to receive light. A main cause for such a variation in magnification is the production process of the lenses used in the instruments such as the polishing process. In some cases, there may also be minor variations due to environmental factors such as temperature. One approach to address the problem of magnification variation is to provide the instrument with zooming capability using a conventional optical zoom system, which typically comprises two or more lenses or optical modules, such that variations in magnification can be adjusted by operating the zoom system to the correct (specified) magnification for the optical detectors. However, this approach is expensive due to the additional optics involved, and potentially introduce additional uncertainties in optical performance, such as introducing field distortions, causing ghost images and back reflections, boresight, reducing transmission as a result of introducing additional optics, birefringence, etc.

It is therefore desirable to provide improved method of calibrating optical systems to address variations in magnification in microscope optics, in particular for use in semiconductor inspection and metrology equipment.

GENERAL DESCRIPTION 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 lens arrangement for receiving reflected light from at least a portion of an object, and an imaging lens for projecting light received from the objective lens arrangement to form an image of at least the portion of the object on a target plane, the method comprising: defining a target region on the target plane based on a predetermined dimension of the image of at least the portion of the object to be formed on the target plane; moving the imaging lens along an optical axis thereof to a plurality of imaging lens positions, the plurality of imaging lens positions varying in distances in relation to the target plane, to vary a magnification of the image on the target plane; determining a first imaging lens position from the plurality of imaging lens position when the image of at least the portion of the object fits within the target region on the target plane; and positioning the imaging lens at an operation position based on the first imaging lens position such that the image of at least the portion of the object is confined within the target region on the target plane. According to embodiments of the present technology, a method is provided in which an optical system is calibrated for magnification variance in the optics by a simple operation of adjusting the position of the imaging lens along its optical axis (z-axis) with respect to the position of a target plane on which the magnified image is formed. In particular, a target region is defined on the target plane within which the magnified image is to be confined. The target region or the size of the target region may be regarded as the intended magnification as specified during production. For example, the target region may represent the FOV of the optical detector apparatus used (e.g. a camera), a predetermined area within an imaging region of the optical detector apparatus, and/or a predetermined number of imaging regions (e.g. pixels) of the optical detector apparatus. By simply adjusting the axial position of the imaging lens and positioning the imaging lens where the resulting magnified image is confined within the target region on the target plane, the optical system may be calibrated to produce an image with the specified magnification. Embodiments of the present technology therefore enables calibration of magnification, instead of adapting for the variations in magnification, without the need for expensive additional optics. There may be various suitable ways of detecting or otherwise determining when the image fits within the target region on the target plane. In some embodiments, the method may further comprise detecting the image of at least the portion of the object using one or more optical sensor at a position substantially on the target plane. In some embodiments, the light detector apparatus may comprise an imaging area formed of a plurality of imaging regions, e.g. pixels, and the target region may be defined as a predetermined number of imaging regions on the light detector apparatus. For example, a calibration target may be used as the object and the first imaging lens position may be determined as when an image of the calibration target fits within the imaging area comprising all the pixels or within a given number of (one or more) pixels on a camera. In some embodiments, the object may be a calibration object of a known dimension, such that defining a target region on the target plane based on a predetermined dimension of the image of at least the portion of the object to be formed on the target plane defines a magnification of the object. Using a calibration object that has a known dimension or size means that the dimension of the target region may be defined so as to achieve a desired magnification. Following the adjustment of the imaging lens position, the image formed on the target plane may become out of focus. Thus, in some embodiments, the method may further comprise: adjusting an axial position 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 setting the object at the object axial position. In doing so, the image formed on the target plane may be straightforwardly refocused. In embodiments where the objective lens arrangement is telecentric on the object side, the image formed on the target plane may be refocused through adjustment of the axial position of the object along the optical axis without impacting the magnification of the image. There may be instances when the image covers only a portion of the target region while 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 determining a second imaging lens position from the plurality of imaging lens position when the image extends beyond the target region on the target plane, and setting the operation position as a position between the first imaging lens position and the second imaging lens position. In some embodiments, the operation position may be set at substantially halfway 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 an object; and an imaging lens disposed at an operation position spaced apart from the objective lens arrangement, the imaging lens being configured to receive light from the objective lens arrangement and to 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 region on the target plane based on a predetermined dimension of the image of at least the portion of the object to be formed on the target plane; moving the imaging lens along an optical axis thereof to a plurality of imaging lens positions, the plurality of imaging lens positions varying in distances in relation to the target plane, to vary a magnification of the image on the target plane; determining a first imaging lens position from the plurality of imaging lens position when the image of at least the portion of the object fits within the target region on the target plane; and setting the operation position based on the first imaging lens position such that the image of at least the portion of the object is confined within the target region on the target plane. In some embodiments, the objective lens arrangement may be configured such that an exit pupil of the objective lens arrangement is positioned external to the objective lens arrangement on a back focal plane of the objective lens arrangement. A yet 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 arranged to transmit reflected light from the object onto the target plane; and at least one light detector apparatus arranged to detect light transmitted through the optical system. In some embodiments, the at least one light detector apparatus comprises an array of light detectors. In some embodiments, the at least one light detector apparatus may comprise 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 with respect to a distance from the objective lens arrangement of the optical system. Implementations of the present technology each have at least one of the above-mentioned objects and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present technology that have resulted from attempting to attain the above-mentioned object may not satisfy this object and/or may satisfy other objects not specifically recited herein. Additional and/or alternative features, aspects and advantages of implementations of the present technology will become apparent from the following description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: FIG. 1 shows schematically an exemplary inspection system for inspecting a specimen; FIG. 2 shows schematically an exemplary optical microscope system used in the inspection system of FIG. 1; FIG. 3 shows a representation of a target region; FIG. 4 shows an exemplary ray diagram of an imaging lens at a position Z0 forming an image on a camera; FIG. 5 shows an exemplary ray diagram of the imaging lens of FIG. 4 at a position Zforming an image on the camera; FIG. 6 shows an exemplary ray diagram of the imaging lens of FIG. 4 at a position Zforming an image on the camera; FIG. 7 shows an exemplary ray diagram of the imaging lens of FIG. 4 at a position Zforming an image on the camera; FIG. 8A illustrates an image I’ formed on an imaging area of the camera of FIG. 6; FIG. 8B illustrates an image I’’ formed on an imaging area of the camera of FIG. 7; FIG. 8C illustrates an image I formed on an imaging area of the camera of FIG. 5; FIG. 9 shows an exemplary ray diagram of a light detector at a 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 OF EMBODIMENTS Embodiments of the present technology provides methods for calibrating optical systems, for example optical systems implemented in microscopes, defect detection apparatuses, specimen inspection apparatuses, etc. such as those used in semiconductor wafer and/or mask inspection equipment, to correct for magnification variance in the optics, such as caused by production processes and other factors, through a simple operation of adjusting the position of an imaging lens along its optical axis (z-axis) with respect 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 at which the resulting magnified image formed by the imaging lens is confined within a target region defined on the target plane. The target region or the area covered by the target region may be regarded as the intended image magnification specified for the instrument. For example, the target region may represent or correspond to the area covered by the light detectors used for detecting the magnified image, and/or an effective imaging area of the light detectors. By simply adjusting the position of the imaging lens without moving other elements of the instrument, and positioning the imaging lens where the resulting magnified image is confined within the target region on the target plane, the optical system may be calibrated to produce an image with the intended magnification. Embodiments of the present technology therefore enables calibration or adjustment of magnification for variations caused by many different factors. FIG. 1 shows schematically an exemplary inspection system 100 for inspecting a specimen according to an embodiment, for example (but not limited to) for inspecting defects, particles and/or patterns on the surface of a semiconductor wafer and/or mask as part of a quality assurance process in semiconductor manufacturing processes. The inspection system 100 comprises a set of optics or an optical system, which, in the present embodiment, includes an objective lens or objective lens arrangement 120 and an imaging lens or imaging lens arrangement 140. A light detector or light detectors 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 detectors array 170 may be a camera, a photomultiplier array or any other suitable light detectors. The inspection system 100 also comprises a platform 110 for receiving or securing an object or specimen for inspection. The platform 110 may be stationary or it may be a stage mechanism movable in a longitudinal direction (along the optical axis of the objective lens or objective lens arrangement 120, z-axis) and/or in a transverse direction (x- and/or y-axis) in the same plane as the platform 110. It should be understood that relative movements between the object or specimen may also be achieved by maintaining the platform 110 stationary while providing mechanism for moving the objective lens or objective lens arrangement 120 to change the relative position between the objective lens or objective lens arrangement 120 and the platform 110, or for moving all remaining elements of the inspection system 100 as a whole. For example, a stage mechanism may be configured to move in coordination with a scanning sequence to enable an object placed on the platform 110 to be scanned by a light source 180. The light source 180 may be aerial illumination, a single laser that functions as a point light source focused as a spot onto the object or an array of lasers. In the present 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 off 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 detectors array 170 may also be arranged off the optical axis of the objective lens or objective lens arrangement 120 to e.g. achieve compactness, through the use of a partially reflective element 130. The element 130 may also function as a beam splitter arranged to allow a portion of the light from the object to pass through to the imaging lens or imaging lens arrangement 140 and direct another portion of the light in a different direction, e.g. towards a separate set of imaging lens arrangement and detectors. It should be understood that positioning the imaging lens or imaging lens arrangement 140 and the detector or detectors 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 essential 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 reflected and/or scattered off a portion of the object, or transmitted through a portion 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 for telecentric imaging at the object side such that light exits 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. Embodiments in which the objective lens or objective lens arrangement 120 is telecentric at the object side advantageously enable the axial position of the object (i.e. the platform 110) to be adjusted to focus an image without impacting the resulting magnification of the image. Optionally, the objective lens or objective lens arrangement 120 may be configured such that the exit pupil is located at a position external to (at the back focal plane of) the objective lens or objective lens arrangement 120, if desired. In such embodiments, the partially reflective element 130 may optionally be disposed at the external exit pupil though it is not essential. In the present embodiment, the imaging lens 140 receives the light from the objective lens or objective lens arrangement 120 and forms an image onto the detector or detectors array 170 disposed on a target plane 150, and the detector or detectors array 170 detects an image formed thereon. In the present embodiment, it is desirable for any image formed by the imaging lens 140 to be confined within an effective imaging area of the light detector 170; as such, the effective imaging area of the light detector 170 defines a target region 160 within which the image is preferably confined. It should be understood that the present technology may equally be implemented in inspection systems that utilizes transmission optical systems in which light transmitted through a specimen is collected and analyzed, or inspection systems that utilizes infrared radiation. FIG. 2 shows schematically 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 the present embodiment, the objective lens arrangement 120 is arranged to collect light reflected (or transmitted) and/or scattered off a portion h of an object 200, placed, e.g. on the platform 110 of FIG. 1, at an axial position at a distance d1 from the objective lens arrangement 120. While not essential, the objective lens arrangement 120 is arranged and aligned, in the present embodiment, such that the exit pupil of the objective lens arrangement 120 is external to the objective lens arrangement 120 and lies on the back focal plane (exit pupil plane) 135 of the objective lens arrangement 120. For the purpose of illustration, though not essential, the objective lens arrangement 120 in the present embodiment is shown as being arranged for telecentricity at the object side. Arrangements and configurations of optical elements within the objective lens arrangement 120 to achieve an external exit pupil or telecentricity are not within the scope of the present technology and as such are not discussed herein.

As shown in FIG. 2, the objective lens arrangement 120 gathers light reflected or scattered off different points of the portion h of the object 200, shown as light cones with half angle , and transmits the light towards 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 a target plane 150 at a distance d2 from the imaging lens 140. The image of the portion h of the object 200 may then be imaged or detected using a suitable light detector apparatus, e.g. a camera, 170. Since a light detector apparatus may be regarded as having a specific effective imaging area (an area on the detector where light detection is desired), it is desirable to ensure that the image is confined within a target region 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 used for the calibration process. A light detector 170, e.g. a camera, may be placed on the target plane 150. The light detector (or light sensitive part of the light detector) may be regarded as comprising an imaging area formed of a plurality of image regions, e.g. pixels, and a target region within which the image should be confined may be defined as an area 160 covering the imaging area or a predetermined number of image regions on the light detector. FIG. 3 shows a representation of a target region 160 that is an imaging area of a camera 170 formed of a plurality of pixels 301, 302, 303, …, according to an embodiment. As discussed above, it is desirable to calibrate an optical system such that an image formed by an imaging lens of the optical system (e.g. imaging lens 140) is confined within a target region. In the optical system of FIG. 2, it is desirable to confine an image formed by the imaging lens 140 within a predefined target region, e.g. defined by a number of image regions (pixels) of a light detector (camera). Shown in FIG. 4 is a ray diagram of the imaging lens 1at a position Z0 (zero position) forming an image on e.g. an imaging area of a light detector apparatus (e.g. a camera). As can be seen in FIG. 4, the image formed by the imaging lens 1at the position Z0 is larger than an area covered by the imaging area of the light detector apparatus 160 (target region), and as a result, the detected image pattern extends beyond the FOV of the light detector apparatus. According to the present technology, the optical system of FIG. 2 can be calibrated (image magnification adjusted) by repositioning the imaging lens 140, as illustrated in FIG. 5. In FIG. 5, the imaging lens 140 is repositioned to a position Z1 that is a distance dZ with respect to the position Z0 of the imaging lens 140 in FIG. 4. As a result of moving the imaging lens 140 to Z1, the imaging lens 140 is closer to the target plane 150 and the image thus formed falls within the imaging area of the light detector apparatus 160. As can be seen in FIGs. 4 and 5, since the angle made by the rays entering the imaging lens 140 remains the same, the rays incident at different points (height) on the imaging lens 140, such that the rays are refracted differently by the imaging lens 140. As can be seen in FIG. 5, the exit angle made by the rays exiting the imaging lens 140 is different from the exit angle made by the rays exiting the imaging lens 140 in FIG. 4. The magnification of the image is therefore changed both by the axial movement of the imaging lens 140 that changes the relative distance between the imaging lens 140 and the target plane 150 as well as by changes in the exit angle. In practice, the imaging lens 140 may move through a plurality of different positions along the z-axis, for example using an automated mechanism controlled by software executed on a processor, until an 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 such that the magnified image not only is confined within the target region 160 (e.g. the imaging area of a light detector apparatus), but not so small that the light detector arrangement is not fully utilized. Thus, according to some embodiments, the imaging lens 140 may be moved through a plurality of positions along the z-axis towards the target plane 150 until it reaches a position Z2 at which the magnified image formed on the imaging area of the light detector apparatus 160 becomes too small, as shown in FIG. 6. To this purpose, the point at which the image formed on the imaging area of the light detector apparatus 160 is deemed too small (i.e. the position Z2) may be chosen as when an image I’ is formed on less than a predetermined number of imaging regions (e.g. pixels) on the light detector apparatus 170, as shown in FIG. 8A. In this scenario, light from the image formed by the imaging lens 140 is confined within an image area I’ such that some imaging regions 301, 302, 303, 304, … do not detect any light or receive light only partially, and so less than the whole imaging area of the light detector apparatus used for detecting the image is utilized. When the position Z2 is established for the imaging lens 140, it may be assumed that a more optimal position for the imaging lens 140 lies further away from the light detector apparatus 170 (or target plane 150). In some embodiments, the imaging lens 140 may then be moved through a plurality of positions along the z-axis in the opposite direction (away from the target plane 150) until it reaches a position Z3 at which the image formed on the light detector apparatus becomes too large, as shown in FIG. 7. To this purpose, the point at which the image formed on the light detector apparatus is deemed too large (i.e. the position Z3) may be chosen as when an image I’’ that is formed on the light detector apparatus falls partially outside the imaging area (or a predetermined number of imaging regions, e.g. pixels) of the light detector apparatus, as shown in FIG. 8B. In this scenario, light from the image formed by the imaging lens 140 covers an image area I’’ such that light from the image extends beyond the area covered by the imaging area of the light detector apparatus 160. When the position Z3 is established for the imaging lens 140, it may be assumed that an optimal position for the imaging lens 140 lies somewhere between the position Z2 and the position Z3. The positions Z0, Z1, Z2 and Z3 are shown for comparison in FIGs. 6 and 7. The final operation position, Z2, for the imaging lens 140 may be set at a position between Z2 and Z3. In some embodiments, Z1 may be set at halfway between Z2 and Z3, but other options with different ratio is also possible. For illustrative purposes, FIG. 8C shows the position of an image area I covered by an image formed on the imaging area of the light detector apparatus 160 when the imaging lens 140 is at the position Z1. When the imaging lens 140 is positioned at the operation position Z2, at which an image formed by the imaging lens 140 on the target plane 150 is confined within the target region 160, the image formed 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, e.g. by controlling the height or axial position of the platform 110, either by increasing or reducing the axial position d1, 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 reduce 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 preferable as it may affect the magnification of the image formed on the light detector. However, a person skilled in the art would be able to implement additional steps of adjustment based on the discussion above. In an alternative embodiment, since the size/dimension of an image formed behind the imaging lens 140 changes dependent on the distance from the imaging lens 140, the target plane 150, or the light detector (e.g. camera) that defines the target region 160, may be repositioned along the optical axis until a desired magnification (or image size) is reached for an image formed on the repositioned target plane. As shown in FIGs. 9 and 10, where the light detector 170 is repositioned, from Z0’ to Z1’ by a distance dZ’, along the optical (z-) axis towards the imaging lens 140 to reduce the magnification of an image formed thereon. Again, following the repositioning of the light detector 170, the height or axial position d1 of the object 200 may be adjusted, either by increasing or reducing the axial position d1, to refocus the image formed on the light detector 170.

In some embodiments, transmissive optical systems may be used, for example (but not limited to) in mask inspection tools, where light transmits through a specimen (e.g. a semiconductor mask) and exits the other side of the specimen instead of being reflected. In these embodiments, an imaging lens and/or the light detector for detecting the image formed by the imaging lens may similarly be repositioned axially along the optical axis (with respect to the objective lens arrangement) so as to adjust the magnification of the image on the light detector, as described above. In such embodiments, the specimen can be repositioned axially (changing the distance between the specimen and the objective lens arrangement) to correct the resulting image focus. It will be understood by a person skilled in the art that the illumination system may require adjustment in some manner to maintain the illumination field stop focus at the specimen. Since the degree of telecentricity (or non-telecentricity) is dependent 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 potentially impacts telecentricity as well as magnification. In particular, repositioning the imaging lens 140 in relation to the target plane 150 not only has a technical effect of adjusting the magnification (or image size) of an image formed on the target plane 150, but can have an additional technical effect of changing the angle of the centroid rays, which facilitates the image thus formed in converging more effectively towards the desired size/magnification compared to repositioning the target plane 150 (or light detector). It should be noted that in embodiments where the imaging lens 140 is repositioned axially along the optical axis with respect to the objective lens arrangement 120 and/or the light detector 170, it is possible to significantly separate the adjustment of image magnification from image focusing to enable simple straightforward alignment (or realignment) of the optical elements of the instrument. A person skilled in the art would be able to optimize magnification and focus on a case-by-case basis (e.g. through iterative alignment, etc.). While there may still be some degree of dependency between magnification and focus, in comparison, addition of a zoom system to the instrument leads to more significant non-trivial dependency between magnification and focus, thus introducing uncertainties and complications to alignment. Techniques describe herein enable calibration of image magnification in optical systems through adjusting the position of an imaging lens in an optical system and positioning the imaging lens where the resulting magnified image is confined within a target region on a target plane (on which the image is produced), where the magnified image is detected. As such, techniques described herein enable optical systems to be calibrated for variations in magnification whether as a result of production processes or otherwise, without resorting to additional instrumentations such as conventional zoom systems. Embodiments of the present techniques may be implemented in the manufacturing of semiconductor inspection and metrology equipment; in particular, embodiments of the present techniques may be implemented in the manufacturing or treatment process for testing, measuring and/or calibrating an optical system to address variations in magnification without requiring further processing to modify elements of the optical system. As will be appreciated by one skilled in the art, the present techniques may be embodied as a system, method or computer program product. Accordingly, the present techniques may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware. Furthermore, the present techniques 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. A computer readable medium may be, for example, but is 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 techniques may be written in any combination of one or more programming languages, including object-oriented programming languages and conventional procedural programming languages. For example, program code for carrying out operations of the present techniques may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up 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 procedures, methods or the like, and may comprise sub-components which may take the form of instructions or sequences of instructions at any of the levels of abstraction, from the direct machine instructions of a native instruction set to high-level compiled or interpreted language constructs.

It will also be clear to one of skill in the art that all or part of a logical method according to the preferred embodiments of the present techniques may suitably be embodied in a logic apparatus comprising logic elements to perform the steps of the method, and that such logic elements may comprise components such as logic gates in, for example a programmable logic array or application-specific integrated circuit. Such a logic arrangement may further be embodied in enabling elements for temporarily or permanently establishing logic structures 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 present technology and not to limit its scope 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, nonetheless embody the principles of the present technology and are included within its scope as defined by the appended claims. Furthermore, as an aid to understanding, the above description may describe relatively simplified implementations of the present technology. As persons skilled in the art would understand, various implementations of the present technology may be of a greater complexity. In some cases, what are believed to be helpful examples of modifications to the present technology may also be set forth. This is done merely as an aid to understanding, and, again, not to limit the scope or set forth the bounds of the present technology. These modifications are not an exhaustive list, and a person skilled in the art may make other modifications while nonetheless remaining within the scope of the present technology. Further, where no examples of modifications have been set forth, it should not be interpreted that no modifications are possible and/or that what is described is the sole manner of implementing that element of the present technology. Moreover, all statements herein reciting principles, aspects, and implementations of the technology, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof, whether they are currently known or developed in the future. 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 present technology. Similarly, it will be appreciated that any flowcharts, flow diagrams, state transition diagrams, pseudo-code, 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 block labeled as a "processor", 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. Moreover, 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 hardware, conventional and/or custom, may also be included. Software modules, or simply modules which are implied to be software, may be represented herein as any combination of flowchart elements or other elements indicating performance of process steps and/or textual description. Such modules may be executed by hardware that is expressly or implicitly shown. It will be clear to one 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 techniques.

Claims (15)

- 17 - 298509/2 02902444119- CLAIMS:

1. A method of calibrating an optical microscope system, said optical microscope system comprising an objective lens arrangement (120) for receiving light from at least a portion of an object (200), and an imaging lens (140) for projecting light received from said objective lens arrangement to form an image of at least the portion of the object on a target plane (150), the method comprising: defining a target region (160) on said target plane based on a predetermined dimension of said image of at least the portion of the object to be formed on said target plane; moving said imaging lens along an optical axis thereof to a plurality of imaging lens positions, the plurality of imaging lens positions varying in distances in relation to said target plane, to vary a magnification of the image on said target plane; determining a first imaging lens position (Z1; Z2) from said plurality of imaging lens position when said image of at least the portion of the object fits within said target region on said target plane; and positioning the imaging lens at an operation position (Z1) based on said first imaging lens position such that said image of at least the portion of the object is confined within said target region on said target plane.

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

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

4. The method of claim 1, 2 or 3, wherein said object is a calibration object of a known dimension, such that defining a target region (160) on said target plane based on a - 18 - 298509/2 02902444119- predetermined dimension of said image of at least the portion of the object to be formed on said target plane defines a magnification of said object.

5. The method of any preceding claim, further comprising: adjusting an axial position (d1) of the object along said optical axis to vary a distance between the object and said objective lens arrangement; determining an object axial position of the object at which said image of at least the portion of the object is focused on said target plane; and setting the object at said object axial position.

6. The method of any preceding claim, further comprising determining a second imaging lens position (Z3) from said plurality of imaging lens position when the image (I’’) extends beyond said target region on said target plane, and setting said operation position (Z1) as a position between said first imaging lens position (Z2) and said second imaging lens position (Z3).

7. The method of claim 6, wherein said operation position is set at halfway between said first imaging lens position and said 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 any preceding claim.

9. An optical microscope system comprising: an objective lens arrangement (120) configured to receive reflected light from at least a portion of an object; an imaging lens (140) disposed at an operation position spaced apart from said objective lens arrangement, said imaging lens being configured to receive light from said objective lens - 19 - 298509/2 02902444119- arrangement and to project said received light from said objective lens arrangement to form an image of at least the portion of the object on a target plane (150); and a control unit configured for determining said operation position by: defining a target region (160) on said target plane based on a predetermined dimension of said image of at least the portion of the object to be formed on said target plane; moving said imaging lens along an optical axis thereof to a plurality of imaging lens positions, the plurality of imaging lens positions varying in distances in relation to said target plane, to vary a magnification of the image on said target plane; determining a first imaging lens position from said plurality of imaging lens position when said image of at least the portion of the object fits within said target region on said target plane; and setting said operation position based on said first imaging lens position such that said image of at least the portion of the object is confined within said target region on said target plane.

10. The optical microscope system of claim 9, wherein said objective lens arrangement is configured such that an exit pupil of said objective lens arrangement is positioned external to said objective lens arrangement on a back focal plane of said objective lens arrangement.

11. An inspection system (100) for inspecting an object, comprising: a light source (180) arranged to illuminate the object; an optical system of any of claims 9 to 10 arranged to transmit reflected light from the object onto the target plane; and at least one light detector apparatus (170) arranged to detect light transmitted through said optical system.

12. The inspection system of claim 11, wherein said at least one light detector apparatus comprises an array of light detectors. - 20 - 298509/2 02902444119-

13. The inspection system of claim 11 or 12, wherein said at least one light detector apparatus comprises an imaging area formed of a plurality of imaging regions (301, 302, 303, …).

14. The inspection system of claim 11, 12 or 13, further comprising a platform (110) configured to receive the object, wherein a position of said platform is adjustable with respect to a distance from said objective lens arrangement of said optical system.

15. The inspection system of any one of claims 11 to 14, wherein said inspection system is an optical semiconductor wafer and/or mask inspection system. - 21 - 298509/2 02902444119-