IL298509B1

IL298509B1 - Optical systems and method of calibrating the same

Info

Publication number: IL298509B1
Application number: IL298509A
Authority: IL
Original assignee: Applied Materials Israel Ltd
Priority date: 2022-11-23
Filing date: 2022-11-23
Publication date: 2023-12-01
Also published as: IL298509A; CN118068551A; IL298509B2

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’.

Claims

- 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-