NL2028376B1 - Method of and arrangement for verifying an alignment of an infinity-corrected objective. - Google Patents
Method of and arrangement for verifying an alignment of an infinity-corrected objective. Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/26—Measuring arrangements characterised by the use of optical techniques for measuring angles or tapers; for testing the alignment of axes
- G01B11/27—Measuring arrangements characterised by the use of optical techniques for measuring angles or tapers; for testing the alignment of axes for testing the alignment of axes
- G01B11/272—Measuring arrangements characterised by the use of optical techniques for measuring angles or tapers; for testing the alignment of axes for testing the alignment of axes using photoelectric detection means
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/02—Testing optical properties
- G01M11/0221—Testing optical properties by determining the optical axis or position of lenses
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/02—Testing optical properties
- G01M11/0242—Testing optical properties by measuring geometrical properties or aberrations
- G01M11/0257—Testing optical properties by measuring geometrical properties or aberrations by analyzing the image formed by the object to be tested
- G01M11/0264—Testing optical properties by measuring geometrical properties or aberrations by analyzing the image formed by the object to be tested by using targets or reference patterns
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/02—Testing optical properties
- G01M11/0285—Testing optical properties by measuring material or chromatic transmission properties
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q30/00—Auxiliary means serving to assist or improve the scanning probe techniques or apparatus, e.g. display or data processing devices
- G01Q30/02—Non-SPM analysing devices, e.g. SEM [Scanning Electron Microscope], spectrometer or optical microscope
- G01Q30/025—Optical microscopes coupled with SPM
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q40/00—Calibration, e.g. of probes
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Abstract
Method of and arrangement for verifying an alignment of an infinity- corrected objective. Abstract The invention is directed at a method of verifying an alignment of an infinity-corrected objective with an optical axis of an optical system. The optical system comprises the infinity-corrected objective configured for receiving radiation from a field of view, an optical sensor for forming an image of the radiation on an optical sensor screen, and a tube lens for focusing the radiation from the infinity-corrected objective onto the optical sensor screen. The method comprises arranging a calibration element in the field of view of the infinity-corrected objective, illuminating the calibration element with a first radiation signal at a first wavelength, and obtaining a first output signal from the optical sensor. The method further includes illuminating the calibration element with a second radiation signal at a second wavelength, obtaining a second output signal from the optical sensor, and comparing the first output signal with the second output signal for detecting a difference between the first and the second output signal, and verifying a correctness of the alignment of the infinity-corrected objective with the optical axis of the optical system dependent on the detected difference. The invention is further directed at an arrangement, an optical system, a scanning probe microscopy device, and a method of manufacturing an optical system.
Description
Title: Method of and arrangement for verifying an alignment of an infinity-corrected objective.
Field of the invention The present invention is directed at a method of verifying an alignment of an infinity-corrected objective with respect to an optical axis of an optical system, wherein the optical system comprises the infinity-corrected objective configured for receiving radiation from a field of view, an optical sensor for forming an image of the radiation on an optical sensor screen, and a tube lens for focusing the radiation from the infinity-corrected objective onto the optical sensor screen.
The invention is further directed at an arrangement, an optical system, a scanning probe microscopy device, and a method of manufacturing an optical system.
Background Scanning probe microscopy (SPM) enables to obtain a highly accurate image of a very small part of a surface.
The image can be a surface topography image or a subsurface topography image, or even a combination thereof that visualizes multiple layers, which may be surface layers or layers at different depths below a surface.
The technology enables to image surface areas having typical cross sections in the order of 0.001 to 100 micrometers (um). Because of this scale and accuracy, the technology is a suitable candidate for enabling wafer inspection, i.e. for monitoring of the manufacturing process of semiconductor elements during fabrication thereof.
Given the scale of the image in relation to the size of a typical wafer, it is essential to have a highly accurate positioning system that enables to accurately position the probe tip of the SPM in the desired location on the surface of the wafer for performing the scanning.
In an industrial application, e.g. during a semiconductor manufacturing process, maximum yield of the production process is a further requirement in addition to the required high level of accuracy.
Hence, ideally the probe tip is accurately placed in the desired location as fast as possible to start scanning and to minimize the delay caused by the positioning process.
Various techniques for positioning a probe tip on the surface of a substrate are available.
A number of these techniques thereby make use of optical microscopes to contribute in the positioning process. Apart from positioning, optical microscopes or sensors requiring optics may also be applied during scanning for various reasons, for example to measure lateral coordinates of a target (x,y). Any information obtained from the microscope image is required to be exact enough in order to enable accurate placement in the desired location or determination of sizes and dimensions on the typical scale involved with the largest magnification factors. Hence accurate calibration of the equipment is therefore of great importance. This is a difficult process in view of the desired accuracy. The optical microscope typically may be an infinity-corrected optical system, including an infinity-corrected objective and tube lens to project an image onto a camera sensor (for example a CMOS). Knowing the sensor pixel size and magnification factor of the microscope, one can calculate the size and position of the features on the target. Typically, infinity-corrected objectives have minimal aberrations on their own optical axis. However, if the optical axis is not well aligned with the optical axis of the optical system, and hence the center of the camera sensor, the minimal aberrations will not be in the microscope’s center of the field of view. That will lead to a coordinate measurement error due to distortion of the image. The level of this error may be comparable to the image processing errors thus nullify the advantage of using the advanced image processing methods.
Summary of the invention It is an object of the present invention to overcome the abovementioned disadvantages, and to enable verification of an alignment of the infinity-correeted objective. To this end, there is provided herewith a method of verifying an alignment of an infinity-corrected objective with respect to an optical axis of an optical system. The optical system thereby comprises the infinity-corrected objective, an optical sensor and a tube lens. The infinity-corrected objective is configured for receiving radiation from a field of view. The optical sensor is configured for forming an image of the radiation on an optical sensor screen, and the tube lens is configured for focusing the radiation from the infinity-corrected objective onto the optical sensor screen. The method comprises arranging a calibration element in the field of view of the infinity- corrected objective. Then, the calibration element is illuminated with a first radiation signal at a first wavelength, and a first output signal is obtained from the optical sensor. The calibration element is further illuminated with a second radiation signal at a second wavelength, and a second output signal is obtained from the optical sensor. The first output signal is compared with the second output signal for detecting a difference therebetween, and a correctness of the alignment of the infinity-corrected objective with respect to the optical axis of the optical system is verified dependent on the detected difference.
The calibration element for example may be a pinhole, flat object, reticle, or some other structure. The shape of the calibration element may arbitrarily be selected, for example a circular type pinhole would be suitable but this may as well be a square or other shape. The calibration element may for example also be a different structure, such as a thin film coated substrate surface having a transparent part of predetermined shape. The calibration element preferable is configured for being illuminated from a back side thereof (with respect to the infinity-corrected objective) such as to provide a backlit calibration element. Preferably, the calibration element is flat, although this is not a requirement.
The abovementioned invention is based on the insight that a wavelength dependency of various kinds of lens distortions can be used to verify a correct alignment of the lens. The wavelength dependency of dispersion is encountered in different types of distortions. The amount of distortion increases if the alignment or placement of the objective is not ideal. Therefore, making use of the wavelength dependency of distortions, by comparing the first and the second output signal which are obtained at the first and the second wavelength (being different wavelengths), and determining a difference between the two signals as received on the optieal sensor screen, the amount of deviation between the first and second output signal increases if the alignment is not correct.
For example, lateral chromatic aberration causes the refraction at the first wavelength to be different from that at the second wavelength. Therefore, an image obtained from the first output signal will be different from that at a second output signal. Infinity-corrected lenses have minimal aberrations on their own optical axis, however if a beam is obliquely incident on the infinity-corrected lens (even in the center of the lens) this causes refraction which gives rise to wavelength dependent dispersion (lateral chromatic aberration). With the highly accurate optical sensor of a microscope, small deviations from alignment caused by a slight tilting of the objective can be detected in this way. This may be used in a production process of infinity-
corrected optical systems to inspect the quality of a device, to determine whether it meets the requirements, or to enable correction of a misalignment.
Furthermore, in production of high-quality objectives, the effort is to reduce lens distortions as much as possible. This is best achieved in a center area of the objective, while reducing distortions in a peripheral area of the objective is more challenging. As a result, the amount of distortion typically increases from the center towards the periphery of the objective and is never completely absent. Given the accuracy of the optical sensor applied in an infinity-corrected microscope and the magnification achievable with the tube lens, a slight off-axis displacement of an objective from the optical axis of the optical system may similarly be detected using the method of the invention.
In some embodiments, the method further comprises focusing of the optical system on the calibration element. This may be achieved by adjusting a location of the calibration element or by adjusting the focusing of the infinity-corrected objective. As may be understood, to focus the optical system on the calibration element, either the calibration element itself may be displaced along the optical axis, or — within the optical system — the infinity-corrected objective may be displaced in order to get the calibration element in focus. Preferably, the infinity-corrected objective is focused on the calibration element such as to obtain a sharp image thereof. In particular, the focus on the calibration element may be obtained in at least one of the first or second output signals, such as to be in focus for either the first or the second wavelength or both. As may be appreciated, if focus of the calibration element in the on-screen image is obtainable for both the first and the second wavelength, a difference or deviation between the two images of the first and second output signal can be accurately determined, e.g. in terms of a lateral shift, a magnification, or a deformation of the image of the calibration element. In some cases, misalignment of the infinity-corrected objective may cause the image of one of the first or second output signal to be out of focus while the other one of the first or second output signal may be in focus. In these cases, the skilled person may select which one of the first or second output signal may be in focus. From the fact that one of the images is out of focus, it may already be determined that the infinity-corrected objective is not aligned with the optical axis of the optical system.
In some embodiments, the step of detecting a difference between the first and the second output signal comprises, determining whether a focus of an image of the calibration element illuminated by the first radiation signal is different from a focus of an image of the calibration element illuminated by the second radiation signal. In some of these or further embodiments, alternatively or additionally, the step of detecting a difference between the first and the second output signal comprises, 5 determining whether on the optical sensor screen, a location of an image of the calibration element illuminated by the first radiation signal is different from a location of an image of the calibration element illuminated by the second radiation signal. In many cases a difference between an image of the first and second output signal manifests itself as a shift, an enlargement or reduction, or a blurring or sharpening.
Thus, from the first and second output signals, detecting these effects may advantageously be applied to detect a misalignment.
In some embodiments, the method further comprises: obtaining, by the optical sensor, a first image of the calibration element illuminated by the first radiation signal and a second image of the calibration element illuminated by the second radiation signal; wherein the step of comparing is performed by comparing the first and second image. It is not essential to capture and store images of the first and second output signal because the analysis of these signals may be performed directly from the signals received from the optical sensor. However, the capturing of the images in accordance with these embodiments enables to perform a more elaborate analysis in order, e.g. to obtain correction data indicative of how (e.g. direction and magnitude of correction) to correct a potential incorrect alignment.
In some embodiments, the step of comparing for example comprises: calculating, from the first output signal, at least one first reference location, such as a first centroid, of an image of the calibration element formed on the optical sensor screen; calculating, from the second output signal, at least one second reference location, such as a second centroid, of an image of the calibration element formed on the optical sensor screen; and determining, from the first and the second reference locations, a lateral shift between the on-screen locations of the calibration element as imaged using the first and the second radiation signal, for use as difference data indicative of the difference. In principle, any two well enough determined reference points in the first and second output signals may be applied, and this may be dependent on the shape of the calibration element applied. However, for example with a circular calibration element the centroid of the calibration element on the optical sensor screen (i.e. in the image captured) may be used as reference point for the image.
Alternatively, with a calibration element having a characteristic recognizable shape or having therein characteristic recognizable points (e.g. a square having corner points), these recognizable points may be applied as reference. This provides the advantage that the points may readily be recognized from the image, instead of having to be calculated (such as a centroid).
In some embodiments, the method further comprises determining, based on the difference detected in the step of comparing, a correction for correcting the alignment of the infinity-corrected objective. In addition to verifying whether or not the alignment of an infinity-corrected objective with the optical axis of the optical system is correct, the present invention further allows to calculate correction data in order to correct the alignment. For example, the tilting of the infinity-corrected objective may be changed in order to correctly align the objective with the optical axis. Additionally or alternatively, the infinity-corrected objective may be laterally displaced in order to shift the center of the objective onto the optical axis.
In these, other or further embodiments, the step of determining the correction comprises: determining one or more of a correction amount or correction direction for correcting a tilt angle of the infinity-corrected objective, for example a tilt angle around a point of rotation coinciding with a location of the calibration element. It has been found that good results may be achieved if the tilt angle of the infinity- corrected objective is rotated around a point of rotation that coincides with the location of the calibration element on the optical axis. However, this is not the only manner in which the infinity-corrected objective may be moved in order to correct the alignment, as discussed above.
In some embodiments, the method further comprises a step of correcting an orientation or position of the infinity-corrected objective such as to align the objective with the optical axis of the optical system. Using the method of the present invention, in these embodiments it is possible (e.g. by using piezo type actuators) to perform meticulous corrections to the alignment of the infinity-corrected objective in order to align it with the optical axis accordingly.
In some embodiments, the steps of illuminating the calibration element with the first radiation signal and the second radiation signal are performed using a first and a second radiation source respectively. Two (or more) radiation sources may well be tuned to each provide an optical signal at a specifically desired wavelength.
For example, a first and a second laser source, or other monochrome optical sources, may be applied to illuminate the calibration element.
In some of the above embodiments, a beam splitter may be placed or located between the calibration element and the first and the second radiation source, such as to combine the first and second radiation signals from the first and the second radiation source respectively for illuminating the calibration element. The two radiation sources may be at different locations, while their ineident beams can be combined by the beam splitter. In some of these embodiments, the beam splitter is at least one of a plan parallel plate, a semi-transparent mirror or a cube, although other beam splitters may likewise be applied.
In yet other embodiments that include multiple radiation sources, the first and the second radiation source illuminate the calibration element from different angles. No beam splitter is in that case applied, but the incident beams illuminate the calibration element (from the backside with respect to the infinity-corrected objective) from different angles. Furthermore, in some embodiments, a diffuser may be located between the calibration element and the first and the second radiation source, for diminishing a coherence effect between the first and the second radiation signal. In case coherent radiation sources are applied, the coherence effect may give rise to disturbing fringes or interference patterns in the images, which can be reduced or prevented by using a diffuser.
In some embodiments, the steps of illuminating the calibration element with the first radiation signal and the second radiation signal are performed using a broadband radiation source for emitting radiation at a plurality of wavelengths including the first and the second wavelength, wherein a filter arrangement is located between the calibration element and the broadband radiation source, wherein the filter arrangement is configured for selectively filtering radiation from the broadband radiation source such as to selectively transmit the first or the second radiation signal or both, for illuminating the calibration element. In these innovative embodiments, a single radiation source may be used to provide the radiation signals at any desired wavelength by applying changeable or selectable filters. By selecting a different filter, a different wavelength will be transmitted to illuminate the calibration element. This allows to quickly perform the method of the invention without requiring beam splitters or diffusers.
In some embodiments, the first and the second wavelengths are wavelengths within a range between 100 nanometer and 1000 nanometer.
For example, in some embodiments, the first wavelength is within a range of 400 to 550 nanometer and wherein the second wavelength is within a range of 750 nanometer to 1000 nanometer.
These latter embodiments are based on the insight that the difference in dispersive effect is largest between light of a blue color and infra-red radiation.
Thus, by applying wavelengths from these two ranges, these differences on the basis of which misalignment can be detected are maximized and thus optimal.
In some embodiments, the tube lens is adjusted such that an on-screen size of the calibration element on the optical sensor screen is larger than one pixel of the screen, preferably larger than 5 pixels, more preferably larger than 10 pixels.
Having a larger image of the calibration element enables more accurate determination of the difference and hence more accurate correction of a perceived incorrect alignment.
In accordance with a second aspect, there is provided an arrangement for use in a method according to any one or more of the preceding claims, for verifying an alignment of an infinity-corrected objective with an optical axis of an optical system, wherein the arrangement comprises: a calibration element configured for being mounted on front of the infinity-corrected objective; and an illumination arrangement including at least one radiation source, for illuminating the calibration element with a first radiation signal at a first wavelength and for illuminating the calibration element with a second radiation signal at a second wavelength.
This arrangement may be mounted in front of an infinity-corrected optical system to perform a method as described above.
In some embodiments, the arrangement further comprises, or is configured for cooperating with, a controller for enabling to perform a steps of: obtaining, from an optical sensor of the optical system, a first output signal received in response to said illumination with the first radiation signal, and a second output signal received in response to said illumination with the second radiation signal; comparing the first output signal with the second output signal for detecting a difference between the first and the second output signal, and at least one of: verifying a correctness of the alignment of the infinity-corrected objective with the optical axis of the optical system dependent on the detected difference; or determining, based on the difference detected in the step of comparing, a correction for correcting the alignment of the infinity- corrected objective.
For example, in some of these embodiments, the step of determining the correction comprises: determining one or more of a correction amount or correction direction for correcting a tilt angle of the infinity-corrected objective, for example a tilt angle around a point of rotation coinciding with a location of the calibration element. Advantageously, the ability to comprise or cooperate with a controller in order to perform the abovementioned steps allows the arrangement to be useable as an add-on or auxiliary tool that can be used in combination with a system comprising the infinity-corrected optical system, such as a scanning probe microscopy device, a wafer inspection tool or any other microscopic instrument including a controller. The arrangement may also be used in a system that is used during manufacturing of an infinity-corrected microscope or similar tool, to be used during manufacturing.
In some embodiments, the arrangement further comprising at least one of: a first radiation source configured for providing the first radiation signal, a second radiation source configured for providing the second radiation signal, and a beam splitter configured for combining the first and the second radiation signal for illuminating the calibration element; or a broadband radiation source for emitting radiation at a plurality of wavelengths including the first and the second wavelength, a filter arrangement located between the calibration element and the broadband radiation source, wherein the filter arrangement is configured for selectively filtering radiation from the broadband radiation source such as to selectively transmit the first or the second radiation signal or both, for illuminating the calibration element. These various implementations have been discussed above. The embodiments comprising multiple radiation sources may further comprise a diffuser configured for being located between the calibration element and the first and the second radiation source, for diminishing a coherence effect between the first and the second radiation signal.
In other or further embodiments, the arrangement further comprises one or more actuators for correcting an orientation or position of the infinity-corrected objective such as to align the objective with the optical axis of the optical system. For example, these actuators may be controlled by the abovementioned controller in order to correct the alignment. Alternatively or additionally, the actuators may be manually controlled while the first and second output signals are displayed on a display screen to an operator, without departing from the invention.
In accordance with a third aspect of the invention, there is provided an optical system comprising an infinity-corrected objective configured for receiving radiation from a field of view, an optical sensor for forming an image of the radiation on an optical sensor screen, and a tube lens for focusing the radiation from the infinity-corrected objective onto the optical sensor screen, wherein the optical system comprises an arrangement according to the second aspect, for use in a method according to the first aspect, for verifying an alignment of the infinity-corrected objective with an optical axis of the optical system.
In accordance with a fourth aspect of the invention, there is provided a scanning probe microscopy device comprising a substrate carrier for supporting a substrate, a scan head including a probe holder for receiving a probe comprising a cantilever and a probe tip, and actuator means for moving the probe tip relative to a surface of the substrate in use, while the probe tip at least intermittently is brought in contact with the surface for performing topography or subsurface measurements, wherein the scan head further comprises an optical system including an infinity- corrected objective, and wherein the system includes an arrangement according to the second aspect, for use in a method according to the first aspect, for verifying an alignment of the infinity-corrected objective with an optical axis of the optical system.
In accordance with a fifth aspect of the invention, there is provided a method of manufacturing an optical system, comprising the steps of: providing an infinity-corrected objective configured for receiving radiation from a field of view; providing an optical sensor for forming an image of the radiation on an optical sensor screen; and providing a tube lens for focusing the radiation from the infinity-corrected objective onto the optical sensor screen; wherein the method further comprises: using an arrangement according to the second aspect or a method according to the first aspect, for verifying an alignment of the infinity-corrected objective with an optical axis of the optical system.
Brief description of the drawings The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falling under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings:
Figure 1 schematically illustrates a scanning probe microscopy system including an infinity-corrected optical sensor; Figure 2 schematically illustrates an infinity-corrected optical sensor for use in the system figure 1; Figure 3 schematically illustrates a method in accordance with an embodiment of the present invention; Figure 4A schematically illustrates an arrangement in accordance with an embodiment; Figure 4B illustrates pictures obtained with the arrangement of figure 4A; Figures 4C and 4D illustrate graphs indicative of a deviation obtained using the arrangement of figure 4A; Figure 5 schematically illustrates an arrangement in accordance with an embodiment; Figure 6 schematically illustrates an arrangement in accordance with an embodiment; Figure 7 schematically illustrates an arrangement in accordance with an embodiment. Detailed description In figure 1, the scanning probe microscopy (SPM) system 1 comprises a base 5 and a substrate carrier 3. The substrate carrier 3 includes a bearer surface 7 onto which a substrate 4 may be placed. The substrate 4 may be placed such that a surface 8 thereof, which is to be examined using the SPM system 1, faces the base 5. The base 5 comprises a coordinate reference grid plate 6. The coordinate reference grid plate 6 is part of a grid encoder, which consists of the plate 6 and at least one encoder
15. Typically, in the system 1 in accordance with the present invention, a plurality of encoders cooperates with the grid plate 6. For example, each element that is moving within the working space 2 between the sample carrier 3 and the grid plate 6 may comprise an encoder 15 that cooperates with the grid plate 6 in order to determine its position on the grid plate 6. The encoder 15, and each other encoder cooperating with the coordinate reference grid plate 6, reads the reference grid in order to obtain the coordinate data of its current location on the grid 6. In figure 1, the encoder 15 is mounted on a support 13 providing the scan head 10, which is connected to an arm 12 of a positioning unit module. The support 13 comprises an optical mieroscope 14, the encoder 15 and a probe 26 including a cantilever 27 and a probe tip 28 for scanning the surface 8 of a substrate 4. To examine the substrate 4, the probe tip 28 is brought in contact with the substrate surface 8 at a desired location, and an area of the substrate surface 8 is scanned using the probe tip 28. The probe tip 28 thereby encounters the various nanometer or tens of nanometer sized features on the surface 8, which changed the deflection of the cantilever 27. This can be measured using sensing arrangement, which typically includes an optical beam deflection (OBD) arrangement (not shown) wherein the position of the probe tip 28 is monitored by a laser beam impinging on the back side of the probe tip 28 and reflected back towards an photodetector (e.g. four quadrant photodetector). As may be appreciated, other suitable deflection detection methods may be applied as an alternative to the above, or additionally thereto — for example, piezoelectric, piezoresistive or capacitive sensing methods. The probe 26 may be scanned with the probe tip 28 in contact mode, non-contact mode, tapping mode, or any other mode. Furthermore, the SPM system 1 may perform an acoustic or ultrasonic measurement technique to investigate structures below the surface 8. The optical microscope 14 may be applied to support correct positioning of the probe tip on the surface in a fast and reliable manner. The optical microscope 14 enables to aid in navigation across the surface 8, in the approach method to place the probe tip 28 onto the surface, and in calibrating the system, e.g. by observing fiducial markers 9 or determination of the exact location of the probe tip 28 relative to the system (e.g. after each replacement thereof). Preferably, for all these purposes, the optical microscope 14 needs to be as accurate as possible, and also its location within the system 1 (e.g. relative to fixed points on the arm 12 or in relation to the base 5 or substrate carrier 3) needs to be known. In the embodiments discussed herein, the optical microscope 14 is an infinity-corrected microscope, but the invention is not limited to a specific design. The optical microscope 14 comprises an optical system 16, comprising an optical sensor 20, for example a CMOS camera to obtain images of the substrate surface. Alternatively, a CCD camera may be applied or a different type of optical sensor unit. The optical system 16 further comprises an aperture 21 and a tube lens
22. The tube lens 22 connects to an actuator that enables to adjust the distance between the camera 20 and the tube lens 22 in order to focus the image of the substrate surface 8 or the surface to be imaged onto a sensor screen 30 (see figure 2) of optical sensor 20 to obtain a sharp image.
The magnification of the tube lens 22 for example may be three times to eight times, although any different magnification factor may be applied within or outside this range.
The optical system 16 is infinity-corrected.
At the front side, the optical microscope 14 comprises a sensor opening 17, and the optical system 16 includes a redirection mirror 25 which makes an angle of o/4 radians with the longitudinal axis through the optical microscope 14 in order to redirect the view of the imaging plane of the surface 8 of the substrate 4 to the optical system.
Furthermore, the optical system 16 comprises an infinity-corrected microscopy objective 29 with a long working distance, which is used to obtain a correct focus on the Z-level perpendicular to the sample surface 8. The numeric aperture of this objective 29 for example may be 0,28. The objective 29 may likewise be moved, using a precision actuator 24 suspending with flexures from a structure of the optical microscope 14, along the optical axis 23 of the optical system 16 in order to obtain focus at an exact Z-level.
The actuator 24 may be a piezo actuator and the flexures may be provided by bending elements or leaf springs or a system of leaf springs to allow very accurate focusing adjustment and stability.
The optical system 16 of infinity-corrected microscope 14 is schematically illustrated in figure 2, consisting of the infinity-corrected lens system explained in relation to figure 3 above.
In figure 2, the objective 29 can be precisely moved in the direction along optical axis 23 in order to adjust the focus of the system onto the surface 8. In the situation illustrated, the surface 8 in the area 35 at the location imaged includes a recognizable feature 9, which for example may be a fiducial marker on a wafer surface or a semiconductor device structure manufactured on the wafer 4. This image is obtained by focusing lens 29 at the correct Z-level onto the surface 8, after which it is focused and magnified by tube lens 22 on the optical sensor screen 30 of optical sensor (or camera) 20. The image 36 is obtained with optical sensor 20, from which the location (X,Y) on the surface 8 as well as the size of feature 9 can be obtained.
In metrology applications, the optical microscope 14 comprising the optical system 16 is used to measure lateral coordinates (X,Y) of a target 9 on the surface 8. The features on the target are projected by the infinity-corrected objective 29 and tube lens 22 onto the optical sensor 20 (typically CMOS). Figure 2 visualizes the image 36 on screen 30 of the optical sensor 20. Knowing the pixel size of the sensor 20 and the magnification factor of the microscope 14, one can calculate the size and position of the features on the target 9. Figure 2 also illustrates the optical axis 23 of optical system 16. In order to enable the precise measurement of lateral positions in the X and Y directions, the corrected aligning of infinity-corrected objective 29 with the optical axis 23 is of importance. If the optical axis 23 and the infinity-corrected objective 29 are not aligned with the center of the optical sensor 20, the area of minimal aberration of the infinity-corrected objective 29 will not be in the center of the microscope. This will lead to a coordinate measurement error due to distortion of the image caused by the aberrations. Because the level of error may well be comparable to the image processing errors, such misalignment would nullify the advantages of the high quality optics and advanced imaging processing methods used.
In accordance with the present invention, in order to determine whether the alignment of infinity-corrected objective 29 and the optical axis 23 through the optical system 16 is correct, the present invention makes use of the wavelength dependent properties of the aberrations of the lens 29. Most aberrations find their nature in refraction of optical radiation through an interface between two media. This refraction is dependent on the angle of incidence of the optical radiation on the interface, and is wavelength dependent. Typically, only in the exact center of a lens, provided that an incoming optical beam is aligned with the optical axis (and thus incident in a straight angle), refraction will not occur. In reality, the position and orientation of a lens is not perfectly aligned with the optical axis of an optical system, and therefore some refraction will always occur. At some distance from the optical axis, towards the periphery of the lens 29, the amount of refraction will increase due to the curvature of the lens surface. Therefore, lens distortions such as lateral chromatic aberration will be of larger magnitude towards the periphery of a lens. Although these aberrations can be minimized in high quality lenses, it is impossible to completely get rid of them.
In the present invention, if an infinity-corrected objective 29 is not perfectly aligned with the optical axis 23 of optical system 16, the amount of distortion is increased due to the (slightly) oblique incidence of the optical beam on the optical axis falling onto the objective 29. The chromatic property of the aberrations, will cause light of different wavelengths to be refracted differently by the objective 29, and therefore a misalignment of the infinity-corrected objective 29 can be detected by comparing images taking at a first wavelength with images taken at a second wavelength.
For example, the differences between such two images may be detected as a lateral shift, an enlargement or reduction of size or a deformation of the expected image for one or hoth of the images obtained.
Therefore, by comparing the output signal at a first wavelength with the output signal at a second wavelength, it can be readily detected whether the infinity-corrected objective 29 is correctly aligned with the optical axis 23. Figure 3 schematically illustrates a method in accordance with an embodiment of the present invention.
Many of the steps discussed below are optional, and this embodiment is merely discussed as an example of a method in accordance with the present invention.
While discussing the method illustrated in figure 3, reference is made to figures 1, 2 and 4 in respect of certain elements of the scanning probe microscopy system 1, the arrangement 38 for verifying the alignment, and other elements relevant to the invention.
Furthermore, although not illustrated in the figures, either one or both of the arrangement for verifying the alignment of the infinity-corrected objective 29 or the scanning probe microscopy system 1 may comprise a controller in order to perform one or more steps of the method.
The term controller may refer to a single processing unit or a plurality of controllers, or processing units that cooperate together to perform a certain task.
Such controllers or processing units may be a part of the scanning probe microscopy system 1 or the arrangement 38 for verifying the alignment of the infinity-corrected objective 29, or of an external system or computer connected or cooperating with the arrangement 38 or system 1 for performing a method of the present invention.
In accordance with the method of the present invention, a calibration element 48 is placed or arranged on the optical axis 23 of an optical system 16, such that it is within the field of view of the infinity-corrected objective 29. The calibration element 48 in the present embodiment is a pinhole, but may as well be formed by a different structure, such as a reticle or a thin film coated substrate surface having a transparent part of predetermined shape.
The calibration element 48 is configured for being illuminated from the back side (i.e. the opposite side of the calibration element with respect to where the objective 29 resides) such as to provide a backlit calibration element 48. The calibration element is typically flat, but the shape of the calibration element 48 in a plane transverse to the optical axis 23 is not essential and may be selected freely by the skilled person.
For example, the pinhole 48 may be of circular shape, but may as well be of different shape (e.g. square, an arbitrary shape, star- shaped, oval, the shape of one or more characters, a logo, etc). In embodiments wherein a structure, such as a reticle, is used this structure is preferably flat (although again this is not essential) and may likewise be of various shapes in a plane transverse to the optical axis 23. With the term ‘field of view’ it is meant that the calibration element 48 is placed in an area that is visible to the infinity-corrected objective 29 and within the focusing range of the infinity-corrected objective 29 such that the calibration element 48 may be brought in focus. In accordance with the invention, the calibration element 48 thereafter is illuminated with a first radiation signal at a first wavelength and with a second radiation signal at a second wavelength. This happens in steps 110 and 120, and an output signal is obtained in steps 115 and 125 from the optical sensor 20 of the optical system 16. In accordance with the method, by comparing in step 130, the first output signal with the second output signal, a difference may be detected in step 140 between the first and the second output signal, and based on this detected difference, it can be verified whether or not the alignment of the infinity-corrected objective 29 with the optical axis 23 of the optical system 16 is correct. In some embodiments also a magnitude and direction for correcting an alignment can be determined.
In more detail, in figure 3, the method starts with step 100 wherein the calibration element 48 is placed within the field of view of the infinity-corrected objective 29. Thereafter, in step 102 the location of the calibration element 48 may be adjusted, or alternatively the focusing of objective 29 may be adjusted such that the calibration element 48 is in focus to the infinity-corrected objective 29. To adjust the focusing of the objective 29, the objective 29 may be slightly displaced in the direction along the optical axis 23 towards or away from the calibration element 48. Furthermore, dependent on whether or not the output signal from optical sensor 20, obtained via the optical sensor screen 30, provides sufficient detail for analysis of output signals, the magnification and/or focus of the tube lens 22 onto the optical sensor screen 30 may be adjusted by displacing the tube lens 22 along the optical axis 23 in a similar manner. The magnification, preferably, should be such that an image of the calibration element 48 formed on the optical sensor screen 30 covers several pixels (in cross section at least two pixels, preferably at least five or ten pixels) in order to determine whether or not the image of calibration element 48 is displaced, deformed, or out of focus.
Once the setup and adjustment of elements is established as described above, in accordance with the method of the present invention the calibration element 48 will be illuminated in step 110 with a first radiation signal at a first wavelength. This may be achieved by using a first radiation source 40 to illuminate the calibration element 48. The first radiation source 40 may be a monochromatic light source, e.g. a laser beam, or a different light source suitable for providing a radiation signal at the first wavelength. In step 115, from the optical sensor 20 as using the optical sensor screen 30, a first output signal is obtained from the image of the calibration element 48 formed on the screen 30 of the optical sensor 20. Optionally, in step 116, the obtaining of the first output signal may comprise a step of obtaining a first image of the first output signal on the screen 30 of the optical sensor 20. Such an image may be obtained from the sensor screen 30 and stored in a memory 34. The memory 34 may be a memory of the scanning probe microscopy system 1, the arrangement 38, or an external memory of an external system. The image at least must be made available to analyze, and it is in principle not necessary (although possible) to store the image in the memory 34 for a long time. The memory 34 may thus for example be a working memory of a computer or system.
Simultaneously or subsequently in step 120, the calibration element 48 is illuminated with a second radiation signal at a second wavelength. The second radiation signal may for example be obtained using a radiation source 42, which may be a monochromatic radiation source such as a laser or a different radiation source. Regarding the first and second wavelength, it is important to the invention that the wavelengths selected are different from each other, such that the first wavelength is not the same as the second wavelength. Although any two different wavelengths may be selected for providing the first and second radiation signal, good results have been achieved by using blue light and infrared radiation for the first and the second wavelength. It has been noticed that the difference between the effects of an aberration is relatively large between these two wavelengths, and thereby makes these wavelengths suitable for a method in accordance with the present invention.
Blue light in air roughly has a wavelength between 400 nanometer and 550 nanometer, whereas infrared radiation in air roughly has a wavelength within a range of 750 nanometer to 1000 nanometer. As may be appreciated, any two wavelengths may be selected, such as any two wavelengths between 1 nanometer and 1000 nanometer. Practically, wavelengths within a range of 100 nanometer to 1000 nanometer may be applied in a method of the present invention. However, none of these wavelengths or wavelength ranges must be considered as limiting on the invention. The wavelength mentioned above are all measured in air as a transport medium, and may be different in a different transmission medium. To this end, we note that the method of the present invention may be carried out in any desired ambient medium, although air or vacuum may typically be applied as suitable ambient media. However, the effects of aberrations on optical radiation of different wavelengths will not be absent if a different ambient medium is used, and therefore the skilled person will appreciate that any desired ambient medium can be applied to carry out a method of the invention.
In step 130, the first and the second output signals will be analyzed.
Considering figure 3, multiple different comparison steps may be performed in order to detect the difference between the first and the second output signal. For example, from the images obtained in step 116 and 126, in step 132 and 134 in accordance with the present invention the method may calculate for each of the first output signal and the second output signal a reference location to be associated with the image of the calibration element 48 on the screen 30. For example in step 132, from the first output signal in the image obtained in step 116, a first centroid may be calculated for the calibration element 48 on the screen 30. In step 134, similarly, for the second output signal captured in the image obtained in step 126, a second centroid will be calculated associated with the location of the image of the calibration element 48 of the screen 30 at the second wavelength. Then in step 130”, the two calculated centroids (the first and the second centroid) are compared with each other such as to determine in step 136, a lateral shift between the on screen locations of the calibration element 48 as imaged on the screen 30 using the first and second radiation signal. This calculated lateral shift may be provided as different data obtained during the comparison step
130.
Alternatively or additionally, it is also possible to compare the output signals obtained in steps 115 and 125 or the images thereof obtained in steps 116 and 126, directly with each other in step 130’ in order to determine whether either one or both of these output signals is out of focus. For example if one of the first or second output signal is in focus, the other one of the first and second output signal may be out of focus. In step 131, it is then determined whether or not there is a difference in the focusing of the first and second output signal. Further additionally or alternatively, in step 130”, the first and second output signals obtained from steps 115 and 125 or the images thereof obtained in steps 116 and 126 respectively, may be compared in order to determine whether one of the images of the calibration element 48 on the screen 30 in either the first or the second wavelength is deformed. In step 138, it can then be determined whether there is a difference in shape between the first output signal and a second output signal. Any of the findings from steps 131, 136 and 138 may be provided as output of the step 130 as difference data. It is to be noted that, as we will see, a determination of the lateral shift in step 136 is one of the preferred embodiments, any of these comparison methods discussed above associated with either step 130’, step 130” or step 1307”, can be performed individually or in combination with any or both of the other steps. In the most basic embodiment, the output of the method of the present invention will be a determination as to whether or not the infinity- corrected objective 29 is aligned with the optical axis 23. Such a determination may already be made by for example comparing the focusing of the first and second output signal as in step 130’, even in absence of the other steps. To gain more information, the lateral shift in step 130” may be determined additionally or alternatively. Furthermore, also the detection of a deformation of the image of the calibration element 48 on the screen 30 may provide additional information or insufficient information to detect an incorrect alignment of the infinity-corrected objective 29 from the optical axis 23 of the optical system 16.
Thereafter, in step 140 the difference data gathered in step 130 is assessed in order to determine whether or not there is a difference between the first and second output signal. Based on this, in step 150, the correctness of the alignment of the infinity-corrected objective 29 with the optical axis 23 is verified. If the infinity- corrected objective 29 is correctly aligned with the optical axis 23, then in step 180 the method ends. However, if the result of step 150 is that it is determined that the objective 29 is misaligned with the optical axis 23, a correction may be advised, and in step 160 such a correction may be determined based on the difference detected in step
140. For example, it may be determined in step 160 in which direction a tilt angle of the objective 29 needs to be corrected in order to align it with the optical axis 23. Furthermore, also the amount of correction may be determined in step 160, based on the detected difference. Best results have been achieved by correcting a tilt angle of the objective 29 with respect to a point of rotation that coincides with the location of the calibration element 48 on the optical axis 23. Thus, by establishing a point of rotation on the axis 23 in the mid of the calibration element 48, the tilt angle of the objective 29 may be corrected by rotating the objective around this point of rotation, for example in the direction indicated by step 160 and with an amount as indicated by step 160. Then, in step 170, if the correction data has been established in step 160, the correction of objective 29 may for example be corrected using piezo type actuators or other precision actuators to correct the location or tilt angle of the objective 29. After this correction step, the method may continue again in steps 110 and 120 as illustrated in figure 3. This loop may be continued until a correct alignment is determined in step 150, such that the method can end in step 180.
The arrangement 38 illustrated in figure 4A, which can be placed in front of the optical system 16, further comprises a beam splitter 45. This beam splitter, in the embodiment illustrated in figure 4A is added in order to enable illumination by both radiation sources 40 and 42. The beam splitter 45 reflects at least a part of the radiation coming from optical source 40 along the optical axis 23 to the calibration element 48 from the backside thereof (with respect to the optical system 16). Similarly, light or radiation from radiation source 42 encounters the beam splitter 45 and part of it will be transmitted along the optical axis 23 towards the calibration element 48.
Figure 4B schematically illustrates images taken (e.g. in steps 116 and 126) from the first and second output signal. The images 36-1, 36-2 and 36-3 are images taken from the first output signal e.g. provided by radiation source 40. The images 36-1’, 36-2" and 36-3 are images from the second output signal obtained using the radiation source 42.
Figures 4C and 4D illustrate graphs that are indicative of the difference (in micrometer) between the location of the centroids of the images 36 and 36 in X and Y.
For example, in figure 4C it can be seen that if no correction is performed (spot X position = Om), the difference in x location of the spot formed by the calibration element 48 on the screen 30 between the infrared light and the blue light, is approximately 4 micrometer. Thus, by changing the radiation wavelength from the infrared source 42 to the blue radiation source 40, the spot formed on screen 30 will shift 4 micrometer in the positive x direction. The line 58 can be followed if a certain correction is performed. By displacing the arrangement 38 along an X axis, the position of the spot 37 and 37 in the figures 36 and 36’ will shift to either the positive x or the negative x direction. From the graph in figure 4C, it can be derived that the difference in X location of the spot 37 between the blue light and the infrared radiation, will become zero in point 60 indicated by arrow 59. From figure 4C, it can be estimated that point 60 is approximately located -800 micrometer from the starting location. From this, the exact corrections to be performed to the objective 29 in tilt angle and displacement can be calculated in step 160.
Figure 4D illustrates the situation wherein in the starting position, the objective 29 is correctly aligned with the optical axis 23. Here, in the starting position (where spot X position = Opm), the difference between the infrared radiation and the blue light with respect to the location of spot 37 and 37’, is 0 micrometer. As can be seen, if the X position of the arrangement 38 is changed, such as to shift the spot 37 in the positive or negative x direction, the difference between these two spots 37 and 37 will grow as indicated by line 58’. In both figures 4C and 4D, it can be seen that the objective 29 in the Y direction is already aligned, the line 55 and 55 is approximately 0 micrometer and does not change if the X location of the spot 37 on the screen 30 is changed.
Figures 5, 6 and 7 illustrate different embodiments of the arrangement 38 of the present invention. In figure 5, the arrangement 38 is largely the same as the arrangement 38 of figure 4A, however an additional diffuser 46 is added directly contiguous (and at the backside) of the calibration element 48. The diffuser 46 prevents a coherence effect between light from the two light sources 40 and 42, which are both monochromatic. This, as a result, improves the accuracy of determining a correction for the alignment of objective 29. As will be appreciated by the skilled person, due to the coherence effect, fringes around the spot 37 may be created caused by diffraction of monochromatic light.
In figure 6, arrangement 38’ is a different embodiment of an arrangement for verifying an alignment of an infinity-corrected objective 29 with the optical axis 23. In the arrangement of figure 6, the radiation sources 40 and 42 illuminate the calibration element 48 from two different angles obliquely. Radiation from the two radiation sources 40 and 42 is incident on the diffuser 46 and thereby provides the desired first radiation signal and second radiation signal from pinpoint 48 similarly.
Advantageously, a beam splitter 45 is absent in the embodiment 38’. As a result, because a beam splitter 45 only reflects/transmits part of the optical signal, the complete optical signals from radiation sources 40 and 42 is incident on the diffuser
46. As a result, the required optical power of radiation sources 40 and 42 can be half the optical power of radiation sources 40 and 42 of arrangement 38 in figure 5 and figure 4A.
A further alternative arrangement 38” for verifying the correct alignment of an infinity-corrected objective 29 in respect of the optical axis 23 is illustrated in figure 7. The arrangement 38 comprises a broad band radiation source 70 that is configured for transmitting radiation in a broad wavelength range. In between the calibration element 48 and the broad band radiation source 70, a controllable optical filter 72 is installed that enables to selectively apply a desired filter 75 from a plurality of filters to the radiation emitted by broadband radiation source 70.
Therefore, at the calibration element 48, only the wavelength transmitted by the filter 75 will be received. As a result, by changing the filter 75 to transmit a different wavelength, the calibration element 48 can be illuminated at a plurality of different wavelengths such as to obtain corresponding signals of each wavelength at optical sensor screen 30. The advantages of the arrangement 38” is that with a single radiation source 70, a large amount of different wavelengths can be transmitted onto the calibration element 48, such that it is not necessary to install a separate radiation source for each wavelength. Furthermore, due to the arrangement 38” comprising a combination of broadband radiation source 70 at a filter arrangement 72 in combination with the calibration element 48, the hole arrangement 38’ can be kept relatively compact. As may be appreciated though, in order to transmit only a small range of wavelengths by the filter 75, most of the radiation from broadband radiation source 70 is to be filtered out by the filter arrangement 72. Therefore, the arrangement 38 may require some additional means for heat dissipation in order to prevent the ambient elements to rise in temperature. As may be appreciated, at the level of accuracy desired, the generation of heat is to be avoided as much as possible or dealt with.
In most embodiments shown and described, the calibration element 48 applied is a pinhole. However, it is to be understood that the invention is not limited to the application of a pinhole as calibration element, and a different structure may likewise be applied. Preferably, the calibration element 48 is flat (although the thickness is not critical), because in backlighting this enables to fix the optical distance to the objective 29. However, any edges on the calibration structure may likewise provide this advantage, and therefore it is not essential but merely advantageous to apply a flat structure providing the calibration element.
Furthermore, and structures that are not visible in the field of view of the objective 29 or which are not in focus, do not affect the measurement and may thus be ignored.
In a plane transverse to the optical axis 23, the shape of the calibration structure 48 as it will be imaged by the objective 29 may arbitrarily be selected.
Typically, pinholes with circular cross sections are widely available and frequently used in optical structures, and may thus form a calibration element 48 of preference.
However, the invention is not limited thereto.
The present invention has been described in terms of some specific embodiments thereof.
It will be appreciated that the embodiments shown in the drawings and described herein are intended for illustrated purposes only and are not by any manner or means intended to be restrictive on the invention.
The context of the invention discussed here is merely restricted by the scope of the appended claims.
Claims (24)
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| US5453606A (en) * | 1993-03-11 | 1995-09-26 | Minolta Co. Ltd. | Apparatus for adjusting the optical axis of an optical system |
| US5479252A (en) * | 1993-06-17 | 1995-12-26 | Ultrapointe Corporation | Laser imaging system for inspection and analysis of sub-micron particles |
| US20180180642A1 (en) * | 2016-11-29 | 2018-06-28 | Anaysis Instruments Corp. | Method and apparatus for rapid sub-diffraction infrared imaging and spectroscopy and complementary techniques |
| WO2021026515A1 (en) * | 2019-08-07 | 2021-02-11 | Agilent Technologies, Inc. | Optical imaging performance test system and method |
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|---|---|---|---|---|
| US5453606A (en) * | 1993-03-11 | 1995-09-26 | Minolta Co. Ltd. | Apparatus for adjusting the optical axis of an optical system |
| US5479252A (en) * | 1993-06-17 | 1995-12-26 | Ultrapointe Corporation | Laser imaging system for inspection and analysis of sub-micron particles |
| US20180180642A1 (en) * | 2016-11-29 | 2018-06-28 | Anaysis Instruments Corp. | Method and apparatus for rapid sub-diffraction infrared imaging and spectroscopy and complementary techniques |
| WO2021026515A1 (en) * | 2019-08-07 | 2021-02-11 | Agilent Technologies, Inc. | Optical imaging performance test system and method |
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