System and method for determining size and location of minimum beam spot

The disclosure is directed to a system and method for determining at least one characteristic of an illumination beam emanating from an illumination source. A substrate having a plurality of apertures may be actuated through an illumination beam so that apertures at different spatial offsets are scanned through the illumination beam at one or more levels of focus. Portions of illumination directed through scanned apertures may be received by at least one detector. At least one characteristic of the illumination beam may be extracted from data points associated with intensity levels associated with detected portions of illumination. Furthermore, multiple determinations of a beam characteristic made over a period of time may be utilized to calibrate the illumination source.

TECHNICAL FIELD

The present disclosure generally relates to the field of illumination beams and more particularly to a system and method for determining at least one characteristic related to a minimum beam spot of an illumination beam.

BACKGROUND

Many modern metrology systems rely on optical analysis of interactions between illumination emanating from at least one illumination source and the surface of a sample being analyzed. Characteristics of an illumination beam provided by an illumination source may change over time as a result of deterioration or movement of illumination source components. Accordingly, the illumination source may need to be recalibrated from time to time to compensate for changes of illumination beam characteristics, such as minimum beam spot size and/or location, beam caustic, and the like.

Current methods for measuring illumination beam characteristics are time consuming and often impractical for on-site calibration of an illumination source. For example, one current method of measuring illumination beam characteristics entails illumination scattering from particles deposited on a wafer to analyze defects with properly aligned scattering collection optics. As such, it would be advantageous to provide a method for illumination beam characterization that cure the defects of the prior.

SUMMARY

One embodiment of the present disclosure is directed to a system for determining at least one characteristic of an illumination beam emanating from an illumination source including a substrate configured to receive illumination from an illumination source. The substrate may include a plurality of apertures being spatially distributed along a selected scanning path. The illumination source may be configured to illuminate a selected number of apertures over a selected time interval. The plurality of apertures may further be spatially offset from one another in a direction substantially perpendicular to the scanning path. An actuator may be configured to actuate the substrate to move the plurality of apertures through an illumination beam emanating from the illumination source. The distribution of the plurality of apertures may allow a first portion of illumination having a first intensity level to pass through a first aperture and at least one additional portion of illumination having at least one additional intensity level to pass through at least one additional aperture. A detector may be configured to receive at least a portion of illumination passing through at least one aperture of the plurality of apertures. A computing system communicatively coupled to the detector may be configured to: receive a first intensity level of a first portion of illumination received by the detector from a first aperture; receive at least one additional intensity level of at least one additional portion of illumination received by the detector from at least one additional aperture; and determine at least one characteristic of the illumination beam emanating from the illumination source by comparing the first intensity level of the first portion of illumination and the at least one additional intensity level of the at least one additional portion of illumination.

Another embodiment of the present disclosure is directed to a method of determining at least one characteristic of an illumination beam emanating from an illumination source, including the steps of: providing a substrate including a plurality of apertures being spatially distributed along a selected scanning path to allow the illumination source to illuminate a selected number of apertures over a selected time interval, the plurality of apertures further being spatially offset from one another in a direction substantially perpendicular to the scanning path allowing a first portion of illumination having a first intensity level to pass through a first aperture and at least one additional portion of illumination having at least one additional intensity level to pass through at least one additional aperture; actuating the substrate to move the plurality of apertures through an illumination beam emanating from the illumination source; receiving at least a portion of illumination passing through at least one aperture of the plurality of apertures utilizing a detector; receiving a first intensity level of a first portion of illumination received by the detector from a first aperture; receiving at least one additional intensity level of at least one additional portion of illumination received by the detector from at least one additional aperture; and determining at least one characteristic of the illumination beam emanating from the illumination source by comparing the first intensity level of the first portion of illumination and the at least one additional intensity level of the at least one additional portion of illumination.

Another embodiment of the present disclosure is directed to a method of calibrating the illumination source, including the steps of: actuating the substrate to move the plurality of apertures through the illumination beam emanating from the illumination source for at least one additional time; receiving at least a portion of illumination passing through at least one aperture of the plurality of apertures utilizing a detector for the at least one additional time; receiving a first intensity level of a first portion of illumination received by the detector from a first aperture for the at least one additional time; receiving at least one additional intensity level of at least one additional portion of illumination received by the detector from at least one additional aperture for the at least one additional time; determining at least one characteristic of the illumination beam emanating from the illumination source for the at least one additional time by comparing the first intensity level of the first portion of illumination and the at least one additional intensity level of the at least one additional portion of illumination for the at least one additional time; determining a rate of change of the at least one characteristic as a function of time by comparing the at least one characteristic of the illumination beam emanating from the illumination source determined for a first time and the at least one characteristic of the illumination beam emanating from the illumination source determined for the at least one additional time; and compensating for an expected change of the at least one characteristic of the illumination beam emanating from the illumination source utilizing the determined rate of change of the at least one characteristic as a function of time.

Another embodiment of the present disclosure is directed to a method of determining at least one characteristic of an illumination beam emanating from an illumination source, including the steps of: providing a substrate including a plurality of apertures being spatially distributed along a selected scanning path to allow the illumination source to illuminate a selected number of apertures over a selected time interval, the plurality of apertures further being spatially offset from one another in a direction substantially perpendicular to the scanning path allowing a first portion of illumination having a first intensity level to pass through a first aperture and at least one additional portion of illumination having at least one additional intensity level to pass through at least one additional aperture; actuating the substrate to move the plurality of apertures through an illumination beam emanating from the illumination source; receiving at least a portion of illumination passing through at least one aperture of the plurality of apertures utilizing a detector; receiving a first plurality of intensity levels of a first plurality of portions of illumination received by the detector from the plurality of apertures at a first level of focus; receiving at least one additional plurality of intensity levels of at least one additional plurality of portions of illumination received by the detector from the plurality of apertures at at least one additional level of focus; and determining at least one characteristic of the illumination beam emanating from the illumination source by comparing the first plurality of intensity levels of the first plurality of portions of illumination received by the detector at the first level of focus and the at least one additional plurality of intensity levels of the at least one additional plurality of portions of illumination received by the detector at the at least one additional level of focus.

Another embodiment of the present disclosure is directed to a method of calibrating the illumination source, including the steps of: actuating the substrate to move the plurality of apertures through the illumination beam emanating from the illumination source for at least one additional time; receiving at least a portion of illumination passing through at least one aperture of the plurality of apertures utilizing a detector for the at least one additional time; receiving a first plurality of intensity levels of a first plurality of portions of illumination received by the detector from the plurality of apertures at a first level of focus for the at least one additional time; receiving at least one additional plurality of intensity levels of at least one additional plurality of portions of illumination received by the detector from the plurality of apertures at at least one additional level of focus for the at least one additional time; determining the at least one characteristic of the illumination beam emanating from the illumination source for the at least one additional time by comparing the first plurality of intensity levels of the first plurality of portions of illumination received by the detector at the first level of focus and the at least one additional plurality of intensity levels of the at least one additional plurality of portions of illumination received by the detector at the at least one additional level of focus for at least one additional time; determining a rate of change of the at least one characteristic as a function of time by comparing the at least one characteristic of the illumination beam emanating from the illumination source determined for a first time and the at least one characteristic of the illumination beam emanating from the illumination source determined for the at least one additional time; and compensating for an expected change of the at least one characteristic of the illumination beam emanating from the illumination source utilizing the determined rate of change of the at least one characteristic as a function of time.

DETAILED DESCRIPTION

FIGS. 1A through 5generally illustrate a system and method for determining at least one characteristic of an illumination beam emanating from an illumination source. Illumination sources often experience negative changes in performance over time resulting from defocusing caused by deterioration or movement of illumination source components and/or optical elements when the illumination source is utilized. An illumination source may be refocused utilizing information associated with one or more beam characteristics such as minimum beam spot size, minimum beam spot location, and/or beam caustic. Accordingly, the present disclosure is directed to a system and associated method for monitoring at least one characteristic of an illumination beam and further directed to a method of calibrating an illumination source by monitoring a change in at least one beam characteristic over time.

FIGS. 1A through 1Cillustrate a system100for determining at least one characteristic of an illumination beam emanating from an illumination source106in accordance with an embodiment of this disclosure. The system100may include a substrate102configured to receive a beam of illumination from the illumination source106. The substrate102may include a rigid or semi-rigid opaque material. The substrate102may further include a plurality of apertures configured for permitting a portion of illumination received by at least one of the apertures from the illumination source106to traverse the substrate102through the aperture.

The plurality of apertures of the substrate102may be disposed along a scanning path configured to receive illumination from the illumination source106. The apertures of the substrate102may be uniformly distributed along the scanning path. For example,FIG. 1Billustrates the substrate102having uniform separation X between apertures of the substrate102in a first direction along a linear scanning path. The apertures may be sufficiently spaced apart so that a desired number of apertures are illuminated by the illumination beam emanating from the illumination source106at one time. For example, the apertures may be uniformly distributed along the scanning path with sufficient separation X so that the illumination source106is configured to illuminate only one aperture at a time.

The apertures of the substrate102may be further disposed with an offset from one aperture to the next in a second direction substantially perpendicular to the scanning path. For example,FIG. 1Billustrates the substrate102having uniform perpendicular offsets Y between the apertures. The perpendicular offsets Y may allow a first aperture of the substrate102to pass a portion of illumination having a different intensity level from another aperture of the substrate depending on a spatial relationship between an illuminated aperture and the illumination beam emanating from the illumination source106. Accordingly, translating the plurality of apertures of the substrate102along the scanning path through the illumination beam may allow detection of different intensity levels for portions of illumination received by apertures having different Y offsets.

An alternative embodiment of the substrate102is illustrated inFIG. 1Cwhere the substrate102is substantially circular or at least partially curved with the plurality of apertures disposed along a curved scanning path. The apertures may be further disposed with tangential offsets to the curved scanning path. For example, a circular substrate102may have apertures disposed uniformly along a circular scanning path with uniform offsets extending from the center of the circular substrate102. Accordingly, the actuator104may be configured to rotate the substrate102through the curved scanning path so that the apertures are translated through the illumination beam at a plurality of tangential offsets.

The system100may further include an actuator104configured to actuate the substrate102to move the plurality of apertures along the scanning path through the illumination beam of the illumination source102. The actuator104may include a stage, clamp, or other holder (hereinafter “stage”) configured for receiving the substrate102and one or more motors configured for translating and/or rotating the stage to actuate the substrate102. In one embodiment, the actuator104may be configured to linearly translate the stage to move the substrate along a linear scanning path through the illumination beam at a desired speed, such as 0.1 to 5 mm/s. The foregoing range is included for illustrative purposes only and is not intended to limit the present disclosure in any way.

The system100may further include at least one detector108configured to receive at least a portion of illumination passing through an aperture of the plurality of apertures of the substrate102. In one embodiment, the detector108may include a photodiode, photoresistor, camera, or any other photodetector. The detector108may be configured to receive a first portion of illumination having a first intensity level through a first aperture. The detector108may be further configured to receive at least one additional portion of illumination having at least one additional intensity level through at least one additional aperture.

In a further embodiment, the detector108may be configured to receive a first plurality of portions of illumination having a first plurality of intensity levels from the plurality of apertures translated along the scanning path through the illumination beam emanating from the illumination source106at a first focus level or Z-height. The detector108may be further configured to receive at least one additional plurality of portions of illumination having as at least one additional plurality of intensity levels from the plurality of apertures translated along the scanning path through the illumination beam at at least one additional focus level. The focus level may be adjusted by changing the spatial relationship between the substrate and the illumination source106or optical elements defining an illumination beam delivery path of the illumination source106.

The system100may further include at least one computing system110communicatively coupled to the detector108. The computing system110may include one or more processors configured to execute program instructions114from carrier media112to complete one or more steps to determine at least one characteristic of the illumination beam emanating from the illumination source106. The computing system110may be further configured to execute one or more steps to calibrate the illumination source106. It is further noted that several analysis algorithms are known to the art for conditioning a plurality of data points to extract relevant information, such as interpolation, extrapolation, best fit analysis, Gaussian beam analysis, and the like. The system and methods described herein are intended to encompass all analysis algorithms, techniques, and/or mathematical models known to the art.

In one embodiment, the computing system110may be configured to receive a first intensity level corresponding to a first detected portion of illumination from the detector108. The computing system110may be further configured to receive at least one additional intensity level corresponding to at least one additional portion of illumination corresponding to at least one additional detected portion of illumination from the detector108. The computing system110may be further configured to compare the plurality of intensity levels corresponding to portions of illumination detected from the plurality of apertures translated through the illumination beam to determine at least one characteristic, such as minimum beam spot size, minimum beam spot location, and/or beam caustic of the illumination beam emanating from the illumination source106.

In a further embodiment, the computing system110may be configured to receive a first plurality of intensity levels corresponding to a first plurality of detected portions of illumination from the detector108at a first focus level. The computing system110may be further configured to receive at least one additional plurality of intensity levels corresponding to at least one additional plurality of intensity levels corresponding to at least one additional plurality of detected portions of illumination from the detector108at at least one additional focus level. The computing system110may be further configured to compare the plurality of intensity levels obtained by translating the plurality of apertures through the illumination beam at a plurality of focus levels to determine at least one characteristic of the illumination beam emanating from the illumination source106.

In one embodiment, the computing system110may be further configured to make multiple determinations of at least one characteristic of the illumination beam at different times to determine a rate of change of the at least one characteristic. The computing system110may be further configured to determine compensation necessary to substantially neutralize an expected change of the illumination beam emanating from the illumination source106over time. The computing system110may be further configured to calibrate a compensator, such as an astigmatism compensator, of the illumination source106utilizing the determined compensation necessary for substantially neutralizing the expected rate of beam characteristic change.

FIGS. 2 through 5illustrate methods of determining at least one characteristic and/or a rate of change of at least one characteristic of an illumination beam emanating from an illumination source106in association with system100. The computing system110or a plurality of computing systems may be configured to execute one or more steps of methods200,300,400, and500described in further detail herein. However, it is noted that methods200,300,400, and500are not limited to embodiments of system100described herein. Rather, the present disclosure is intended to encompass any and all systems or apparatuses known to the art for carrying out any of the steps described herein.

FIG. 2is a flow diagram illustrating method200for determining at least one characteristic of an illumination beam emanating from an illumination source106. In step202, a substrate102having a plurality of apertures distributed along a linear or nonlinear scanning path may be placed between the illumination source106and a detector104configured to receive at least a portion of illumination directed through an aperture of the substrate102. In step204, the substrate102may be actuated so that the apertures are translated or rotated through the illumination beam along the scanning path. In step206, the detector104may receive portions of illumination passing through the apertures as they are actuated through the illumination beam. Portions of illumination received by the detector104may have different intensity levels due to different perpendicular or tangential offsets of the apertures disposed along the scanning path of the substrate102.

In steps208and210, a computing system110may receive a first intensity level corresponding to a first aperture of the substrate102and at least one additional intensity level corresponding to at least one additional aperture of the substrate102. The data received may be mapped to X and Y (Cartesian) offsets from a reference point, such as a location on the substrate102, or to offsets defined by any alternative coordinate system (e.g. polar, cylindrical, etc.). In steps212and214, the two or more received intensity levels may be compared to determine at least one characteristic of the illumination beam, such as minimum beam spot size, minimum beam spot location, and/or beam caustic. In one embodiment, the received intensity levels may be compared by analyzing data points collected at different X and Y offsets utilizing a data conditioning algorithm or analysis model known to the art. At least one characteristic of the illumination beam emanating from the illumination source106may be determined utilizing a spatial relationship between the offsets associated with apertures of the substrate102corresponding to peak intensity levels and the illumination source106.

FIG. 3illustrates a method300of determining a rate of change for at least one characteristic of the illumination beam emanating from the illumination source106and utilizing the determined rate of change to calibrate the illumination source106. The method300may include steps202through214of method200. The method300may further include step302of executing one or more of steps204through214of method200at least one additional time so that at least one characteristic of the illumination beam is determined for a first time and for at least one additional time. In steps304and306, values obtained for the at least one characteristic at different times may be compared to determine a rate of change for the at least one characteristic over time. In step308, the determined rate of change may be utilized to predict an expected change for the at least one characteristic of the illumination beam with time. Accordingly, the illumination source106may be calibrated by periodically adjusting components or by manually or automatically configuring a compensator, such as an astigmatism compensator, to neutralize negative performance effects resulting from a change of the at least one characteristic of the illumination beam emanating from the illumination source106over time.

FIG. 4is a flow diagram illustrating method400for determining at least one characteristic of an illumination beam emanating from an illumination source106by comparing a plurality of intensity levels corresponding to portions of illumination detected through the apertures of the substrate102scanned through the illumination beam at a plurality of focus levels. In step402, the substrate102having a plurality of apertures distributed along a linear or nonlinear scanning path may be placed between the illumination source106and a detector104configured to receive at least a portion of illumination directed through an aperture of the substrate102. In step404, the substrate102may be actuated so that the apertures are translated or rotated through the illumination beam along the scanning path. The substrate102may be actuated through the illumination beam with the illumination source106resolved at a first level of focus and actuated through the illumination beam at least one additional time with the illumination source106resolved at at least one additional level of focus. The focus levels may be mapped to a Z offset in a Cartesian coordinate system or to a spatial offset defined by any other coordinate system. In step406, the detector104may receive portions of illumination passing through the apertures as they are actuated through the illumination beam at the plurality of focus levels. Portions of illumination received by the detector104may have different intensity levels due to different perpendicular or tangential offsets of the apertures disposed along the scanning path of the substrate102. In addition, the focus level of the illumination source104may affect intensity levels of portions of illumination received by the detector104.

In steps408and410, a computing system110may receive a first plurality of intensity levels corresponding to the plurality of apertures of the substrate102scanned at a first focus level of the illumination source106and at least one additional plurality of intensity levels corresponding to the plurality of apertures scanned at at least one additional focus level. The data received may be mapped to X, Y, and Z (Cartesian) offsets from a reference point, such as a location on the substrate102, or to offsets defined by any alternative coordinate system (e.g. polar, cylindrical, etc.). In steps412and414, the two or more pluralities of received intensity levels corresponding to different focus levels may be compared to determine at least one characteristic of the illumination beam, such as minimum beam spot size, minimum beam spot location, and/or beam caustic. In one embodiment, the pluralities of received intensity levels may be compared by analyzing data points collected at different X, Y, and Z offsets utilizing a data conditioning algorithm or analysis model known to the art. At least one characteristic of the illumination beam emanating from the illumination source106may be determined utilizing a spatial relationship between the offsets associated with apertures of the substrate102corresponding to peak intensity levels and the illumination source106.

FIG. 5illustrates a method500of determining a rate of change for at least one characteristic of the illumination beam emanating from the illumination source106and utilizing the determined rate of change to calibrate the illumination source106. The method500may include steps402through414of method400. The method500may further include step502of executing one or more of steps404through414of method400at least one additional time so that at least one characteristic of the illumination beam is determined for a first time and for at least one additional time. In steps504and506, values obtained for the at least one characteristic at different times may be compared to determine a rate of change for the at least one characteristic over time. In step508, the determined rate of change may be utilized to predict an expected change for the at least one characteristic of the illumination beam with time. Accordingly, the illumination source106may be calibrated by adjusting components of the illumination source and/or by manually or automatically configuring a compensator, such as an astigmatism compensator, to neutralize negative performance effects resulting from a change of the at least one characteristic of the illumination beam emanating from the illumination source106over time.

Any of the systems or methods described above may be further extended to diagnostic testing, maintenance, or calibration of an inspection system or an optical metrology system including the illumination source106. Several inspection and metrology systems are known to the art and the foregoing systems and methods apply to any such system including an illumination source configured to emanate an illumination beam. For example, several inspection systems are described or referenced by U.S. Pat. No. 7,548,308, U.S. Pat. No. 6,271,916, and U.S. Pat. No. 6,201,601, all incorporation herein by reference.

It should be recognized that the various steps described throughout the present disclosure may be carried out by a single computing system or, alternatively, a multiple computing system. Moreover, different subsystems of the system may include a computing system suitable for carrying out at least a portion of the steps described above. Therefore, the above description should not be interpreted as a limitation on the present invention but merely an illustration. Further, the one or more computing systems may be configured to perform any other step(s) of any of the method embodiments described herein.

The computing system may include, but is not limited to, a personal computing system, mainframe computing system, workstation, image computer, parallel processor, or any other device known in the art. In general, the term “computing system” may be broadly defined to encompass any device having one or more processors, which execute instructions from a memory medium.

Program instructions implementing methods such as those described herein may be transmitted over or stored on carrier medium. The carrier medium may be a transmission medium such as a wire, cable, or wireless transmission link. The carrier medium may also include a storage medium such as a read-only memory, a random access memory, a magnetic or optical disk, or a magnetic tape.

All of the methods described herein may include storing results of one or more steps of the method embodiments in a storage medium. The results may include any of the results described herein and may be stored in any manner known in the art. The storage medium may include any storage medium described herein or any other suitable storage medium known in the art. After the results have been stored, the results can be accessed in the storage medium and used by any of the method or system embodiments described herein, formatted for display to a user, used by another software module, method, or system, etc. Furthermore, the results may be stored “permanently,” “semi-permanently,” temporarily, or for some period of time. For example, the storage medium may be random access memory (RAM), and the results may not necessarily persist indefinitely in the storage medium.

Although particular embodiments of this invention have been illustrated, it is apparent that various modifications and embodiments of the invention may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure. Accordingly, the scope of the invention should be limited only by the claims appended hereto.