Wafer inspection system and apparatus

Abstract

To improve resolution in the scan direction in a line scan inspection system. A position detector 117 associated with a slit aperture filter 108 monitors and corrects the position of the illuminated line image relative to the slit aperture to maintain image position variations within acceptable limits. Each detector measures line position and then uses the line position signal to adjust optical, mechanical, and electronic components in the collection path in a feedback loop. The feedback loop can be utilized in a run-time calibration process or during inspection to improve stability.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE This disclosure relates generally to wafer inspection systems, and more specifically to line-scan inspection devices having confocal scanning elements. [Background technology]

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 61 / 982,754, filed April 22, 2014, which is incorporated herein by reference in its entirety.

[0003] In a confocal microscope, a pinhole filter at the confocal plane of the lens removes out-of-focus light, but the confocal pinhole filter clips the illumination line projected onto the sensor, so any shift of that line relative to the pinhole filter results in a change in image intensity.

[0004] While improving resolution, confocal pinhole filters also increase the sensitivity of the instrument to focus and boresight variations due to thermal drift, vibration, and mechanical repeatability of components. The increased sensitivity affects the instrument stability, and eliminating all of the increased instrument sensitivity factor is impractical due to the complexity of the instrument. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] US Patent Application Publication No. 2005 / 0018180 Summary of the Invention [Problem to be solved by the invention]

[0006] Thus, it would be beneficial if an apparatus existed that was suitable and increased stability for wafer inspection using a confocal slit aperture filter. [Means for solving the problem]

[0007] Therefore, the present invention is directed to a novel apparatus suitable for and increasing the stability of wafer inspection using a confocal slit aperture filter.

[0008] In one embodiment, a line-scan wafer inspection system includes a detector associated with each slit aperture filter at the confocal plane that monitors and corrects the line position relative to the slit aperture filter to maintain image intensity variations within acceptable limits. Each detector measures the line position and then uses the line position signal to adjust optical components in the collection path in a feedback loop.

[0009] In some embodiments, the feedback loop is utilized within a run-time calibration process, while in other embodiments, the feedback loop is utilized as a real-time compensation mechanism during testing to improve stability.

[0010] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain embodiments of the invention and, together with the general description, serve to explain its principles.

[0011] Many advantages of the present disclosure may be better understood by those skilled in the art by reference to the following drawings. [Brief description of the drawings]

[0012] [Figure 1A] FIG. 1 is a front view of a wafer inspection system according to one embodiment of the present disclosure. [Figure 1B] FIG. 2 is a detailed top view of one pair of an image sensor and a position detector. [Diagram 2] FIG. 2 is a close-up depiction of scattered wafer light in a moving wafer inspection system. [Diagram 3] FIG. 2 shows a close-up of a portion of the wafer inspection system of FIG. [Figure 4] FIG. 2 shows a close-up of a portion of the wafer inspection system of FIG. [Diagram 5] FIG. 1 is a block diagram of a computer system for implementing embodiments of the present disclosure. [Figure 6] 1 is a flow diagram of a method for adjusting inspection optics in a line-illumination wafer inspection system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings. The scope of the present invention is limited only by the claims. Numerous alternatives, modifications, and equivalents are encompassed. For purposes of clarity, technical material known in the art related to the embodiments has not been described in detail so as not to unnecessarily obscure the description.

[0014] An understanding of one or more embodiments of the present disclosure may be further illustrated by U.S. Patent No. 7,525,649, which is incorporated herein by reference.

[0015] Referring to FIG. 1A, a front view of a wafer inspection system according to one embodiment of the present disclosure is shown. In some embodiments, a wafer inspection system, such as a line-scanning optical inspection system 100, includes three collection channels 102, 104, 106 to collect scattered light from defects in the wafer 101 and image the light onto a corresponding array line sensor 114, 116, 118. A position detector 115, 117, 119 is associated with each array line sensor 114, 116, 118. In some embodiments, the illumination optics forms a narrow line 120 at a scan location on the wafer 101. The scattered light from the illuminated scan location on the wafer 101 is collected by the three collection channels 102, 104, 106 and focused onto each array line sensor 114, 116, 118. In an exemplary embodiment, the narrow line 120 can have a width on the wafer 101 that is narrower than 1 micrometer, but is not limited to such a width. Light scattered from features and defects on the wafer 101 is imaged onto array line sensors 114, 116, 118 in each collection channel 102, 104, 106. The array line sensors 114, 116, 118 may include any imaging technology known in the art. In some embodiments, the array line sensors 114, 116, 118 include charge-coupled devices (CCDs) or time-delay integration (TDI) devices.

[0016] In some embodiments, each array line sensor 114, 116, 118 is associated with a slit aperture filter 108, 110, 112. In at least one embodiment, the line-scanning optical inspection system 100 includes a line illumination source, and the array line sensors 114, 116, 118 may be configured to have a large field of view in the x-direction (orthogonal to the illumination line 120) to collect all scattered light. Position detectors 115, 117, 119 are positioned just outside of the corresponding array line sensors 114, 116, 118 in the y-direction (along the long axis of the illumination line 120). These position detectors 115, 117, 119 track the light scattered from the wafer 101 and compare the position of the scattered light to a calibrated position that corresponds to the center of the corresponding slit aperture filter 108, 110, 112. The error signals generated by the position detectors 115, 117, 119 are used to move the optical elements in the corresponding collection channels 102, 104, 106 to ensure that the scattered light from the wafer 101 is centered on the slit aperture filters 108, 110, 112. Although the position detectors 115, 117, 119 in Figure 1 are shown offset from the array line sensors 114, 116, 118 in a direction perpendicular to the illumination line 120, those skilled in the art will appreciate that such a depiction is merely a function of the constraints of a two-dimensional medium.

[0017] In some embodiments, the slit aperture filters 108, 110, 112 are substantially abutted against the corresponding array line sensors 114, 116, 118. In other embodiments, the slit aperture filters 108, 110, 112 are a distance away from the corresponding array line sensors 114, 116, 118, and a cylindrical lens refocuses the light onto the slit aperture filters 108, 110, 112.

[0018] For a line-scanning inspection tool, the resolution in the scan direction of the wafer 101 may be determined by the profile of the illuminated line. The resolution in the x-direction may be determined by the illuminated line width. This line width may be limited by the numerical aperture (NA) of the line-forming cylinder (LFC), which has a theoretical limit of 1, and by the Gaussian beam at the entrance pupil. Smaller line widths can be achieved with more aggressive Gaussian fill factors, but note that side lobes from diffraction rings create performance limits. A confocal slit aperture filter at the detector improves the resolution in the scan direction beyond this limit. In the line-scanning optical inspection system 100, the confocal slit aperture filters 108, 110, 112 at each array line sensor 114, 116, 118 improve the resolution in the scan direction and reduce sensitivity to the illuminated line side lobes.

[0019] When point scanning the wafer 101 in a direction orthogonal to the illumination line (the x-direction), the image produced on the array line sensors 114, 116, 118 can be described using a point spread function defined for each collection channel 102, 104, 106. F PS_Channel (X,Y,Z) where X, Y are the local coordinates for each array line sensor 114, 116, 118, and Z is the defocus value when the illumination point does not conjugate with the array line sensors 114, 116, 118. The illumination line spread function is given in wafer coordinates xw by F PS_Illuminationl (xw) It is described as follows.

[0020] Given a magnification factor M for the collection channels 102, 104, 106, the total point spread function including the illumination profile is given by: F PS (xw,yw,X)=F PS_Illuminationl (xw)*F PS_Channel (XM*xw,M*yw,Z) x is integrated over the length of the array line sensors 114, 116, 118.

[0021] For the central collection channel 104, Z is constant and it has a narrow slit aperture filter 110 centered at x=0. F PS (xw,yw)=F PS_Illuminationl (xw)*F PS_Channel (-M*xw,M*yw,Z) In one embodiment, both point spread functions have a Gaussian shape exp(-X 2 / W 2 ), the point spread function of the system also has a width W (W of the illumination point spread function) IL and the channel point spread function W C ) and 1 / W 2 =1 / W IL 2 +1 / W C 2 Such a point spread function may have a smaller line width and higher resolution. For large slit aperture filter 108, 110, 112 widths, after integration of the channel point spread function, the line width in the x-direction is determined only by the illuminated line width. Note that in at least some embodiments, narrow slit aperture filters 108, 110, 112 do not collect side lobe energy, while wide slit aperture filters 108, 110, 112 do collect side lobe energy.

[0022] It is further noted that a similar effect is observed in the case of the side collection channels 114, 118. In this case, the overall line width is determined by the illumination line width, relative to the width of the large slit aperture filters 108, 110, 112 and detectors 114, 116, 118. Furthermore, for the slit aperture filters 108, 110, 112, the line width is reduced due to the multiplication factor.

[0023] When narrow slit aperture filters 108, 110, 112 are implemented, either mechanically or electronically in the sensor, the side lobes can be significantly suppressed. Side lobe suppression allows for higher resolution with smaller line widths. Proper side lobe suppression requires that the focused light is correctly centered on the slits of the slit aperture filters 108, 110, 112, and therefore a position detector 115, 117, 119 that allows the line illumination system 100 to change the position of the optical elements in one or more of the collection channels 102, 104, 106 is desirable. However, because the slit aperture filters 108, 110, 112 will necessarily cause some light loss, the inspection sensitivity may be affected. Therefore, it is important that the slit aperture filters 108, 110, 112 are replaceable when more light is required. Systems that include mechanical slit aperture filters 108, 110, 112 may include a mechanism for replacing such slit aperture filters. Alternatively, the system including the electronic slit aperture filters 108, 110, 112 may be configured such that the electronic slit aperture filters may be adjusted to have different slit aperture widths for different applications.

[0024] 1B, a detailed top view of one image sensor and position detector pair is shown. In an exemplary embodiment, the first collection channel of FIG. 1A has an associated image sensor 116 and position detector 117. An illuminated line image 122 from the first collection channel illuminates both the image sensor 116 and the position detector 117. The portion of the illuminated line image 122 that illuminates the image sensor is filtered by a corresponding slit aperture filter 108, while the portion that illuminates the position detector 117 is not filtered. The position detector 117 generates a signal that corresponds to a portion of the illuminated line image 122 in the scan direction (x-direction). In some embodiments, a similar image sensor 116 / position detector 117 pair is used for each collection channel.

[0025] 2, a close-up depiction of scattered wafer light in a moving wafer inspection system is shown. During the wafer inspection process, the wafer is illuminated by a line illumination source having an illumination profile 206 while the wafer moves in a moving direction 200. Primary light 210 is scattered by the wafer and is received and focused by one or more collection channels. Additionally, secondary light 212 may be scattered by unwanted structures 202.

[0026] Referring to Figure 3, a close-up of a portion of the wafer inspection system of Figure 1 is shown. In at least one embodiment of the present disclosure, a first slit aperture filter 108 is associated with the first collection channel. The first slit aperture filter 108 is positioned and oriented at a confocal plane as defined by the first collection channel optics. The first slit aperture filter 108 transmits a primary beam 300 associated with primary light scattered by the wafer from the line illumination source and filters out an incident beam 302 associated with secondary light or side lobes of the line illumination beam scattered by the wafer.

[0027] In at least one embodiment, the first slit aperture filter 108 comprises a mechanical filter. The mechanical filter may be replaceable with a mechanical filter having a larger slit aperture to allow more light if greater sensitivity is required. Alternatively, in at least one embodiment, the first slit aperture filter 108 comprises an electronic filter. The electronic filter may be adjustable to create a larger or smaller aperture as desired for system sensitivity for a particular application.

[0028] Referring to Figure 4, a close-up of a portion of the wafer inspection system of Figure 1 is shown. In at least one embodiment of the present disclosure, a second slit aperture filter 110 is associated with the second collection channel. The second slit aperture filter 110 is positioned and oriented at a confocal plane as defined by the second collection channel optics. The second slit aperture filter 110 transmits a primary beam 400 associated with primary light scattered by the wafer from the line illumination source and filters out an incident beam 402 associated with secondary light or side lobes of the line illumination beam scattered by the wafer.

[0029] In at least one embodiment, the second slit aperture filter 110 comprises a mechanical filter. The mechanical filter may be replaceable with a mechanical filter having a larger slit aperture to allow more light if greater sensitivity is required. Alternatively, in at least one embodiment, the second slit aperture filter 110 comprises an electronic filter. The electronic filter may be adjustable to create a larger or smaller aperture as desired for system sensitivity to a particular application.

[0030] 5, a block diagram of a computer system 500 for implementing embodiments of the present disclosure is shown. The computer system 500 includes a processor 502, a memory 504 coupled to the processor 502 for implementing processor executable program code, and one or more detectors 508, 510, 512 coupled to the processor 502. In some embodiments, each of the one or more detectors 508, 510, 512 is associated with one collection channel. In some embodiments, the computer system 500 includes a data storage element 506 coupled to the processor 502. In some embodiments, the data storage element 506 is configured to store one or more illumination profiles and one or more line images received from the one or more detectors 508, 510, 512.

[0031] In some embodiments, the computer system 500 may further include an electronic aperture 514, 516, 518 associated with each detector 508, 510, 512 and connected to the processor 502. In some embodiments, the electronic apertures 514, 516, 518 may be adjustable to create larger or smaller slit apertures as needed.

[0032] 6, a flow diagram illustrating a method for adjusting inspection optics in a line-illuminated wafer inspection system according to one embodiment of the present disclosure is shown. In a first step 600, a wafer illumination line image from one or more collection channels in the inspection system is filtered with a corresponding slit aperture. In a second step 602, one or more detectors receive the illumination line image.

[0033] In a third step 604, a processor coupled to the one or more detectors analyzes the illumination line image to determine whether the collection channel is configured to focus the illumination line image at a predetermined confocal plane location. In a fourth step 606, the processor adjusts the confocal plane of the collection channel. The confocal plane can be adjusted by changing the relative or absolute position of one or more optical elements in the collection channel, by changing mechanical elements in the line illumination wafer inspection system that performs the wafer inspection process, by changing electronic or optical elements in the illumination device that generates the thin line illumination, or by changing electronic components in the line illumination wafer inspection system that performs the wafer inspection process.

[0034] Alternatively, a processor connected to one or more detectors may analyze 608 the illuminated line image to determine whether the illuminated line image on the detector is at a desired location. The processor may then adjust 610 the position of the slit aperture, the position of one or more optical elements in the corresponding collection channel, elements in the line illumination wafer inspection system that perform the wafer inspection process, electronic or optical elements in the illumination device that generate the thin line, or the position of electronic components in the line illumination wafer inspection system that perform the wafer inspection process to move the illuminated line image to the desired location.

[0035] In either embodiment, the inspection system may filter 600 and receive 602 subsequent illumination line images in a feedback loop adjustment process.

[0036] It is believed that the present invention and many of its attendant advantages will be understood from the foregoing description of the embodiments of the invention. It will be apparent that various changes may be made in the form, construction and arrangement of components thereof without departing from the scope and spirit of the invention and without sacrificing all of its material advantages. The forms hereinbefore described are merely exemplary embodiments thereof, and it is the intent of the following claims to embrace and include such changes.

Claims

1. 1. A wafer inspection system comprising: one or more collection channels, at least one of the one or more collection channels comprising: a plurality of optical elements configured to collect light from a wafer being inspected and provide a line image of the wafer to at least one confocal plane of the one or more collection channels in response to illumination from a line illumination system; a position detector configured to receive at least a portion of the line image of the wafer; a slit aperture filter disposed at the confocal plane and configured to filter out side lobes from the wafer line image; an image sensor configured to receive a line image filtered by the slit aperture filter, wherein the line image of the wafer illuminates both the position detector and the image sensor, a portion of the line image that illuminates the image sensor is filtered by the slit aperture filter and a portion of the line image that illuminates the position detector is not filtered by the slit aperture filter; a processor coupled to the position detector; receiving a line position signal from the position detector indicative of a position of at least a portion of a line image of the wafer on the position detector; analyzing the line position signal to determine a position of a confocal plane beam associated with a corresponding collection channel among the one or more collection channels; adjusting at least one of the positions of the optical elements in the collection channel, mechanical elements in the wafer inspection system, or electronic elements in the wafer inspection system to produce a desired confocal plane beam position corresponding to the slit aperture filter; analyzing the line position signal to determine a position of the slit aperture filtered line image projected onto the image sensor; Adjusting the position of the slit aperture filter so that the slit of the slit aperture filter is centered on the line image in an x-direction perpendicular to the long axis direction of the line image. a processor configured to Equipped with The wafer inspection system, wherein the slit aperture filter is an electronic slit aperture filter whose slit aperture width is adjustable by the processor.

2. A processor; a memory coupled to said processor and embodying processor-executable code; one or more position detectors coupled to the processor for providing positions of the line images of the wafer; one or more image sensors coupled to the processor and configured to receive one or more line images during a wafer inspection process; one or more slit aperture filters disposed at one or more confocal planes of a collection channel including the one or more image sensors, at least one of the slit aperture filters configured to filter out side lobes from a line image received at an image sensor of the one or more image sensors; Equipped with a line image of the wafer illuminates both the position detector and the image sensor, a portion of the line image that illuminates the image sensor being filtered by the slit aperture filter and a portion of the line image that illuminates the position detector being unfiltered by the slit aperture filter; The processor executable code causes the processor to: receiving one or more line images from the one or more image sensors, where in the case of the one image sensor, receiving the one line image, and in the case of the more image sensors, receiving the more line images; Analyzing the one or more line images; determining a position of the one or more confocal planes of the collection channels; adjusting the position of an optical element in the collection channel to produce a desired confocal plane location corresponding to the one or more slit aperture filters; determining a position of the one or more line images filtered by the one or more slit aperture filters projected onto the one or more image sensors; adjusting a position of the one or more slit aperture filters such that the slits of the one or more slit aperture filters are centered on the one or more line images in an x-direction perpendicular to a major axis direction of the one or more line images; and wherein the slit aperture filter is an electronic slit aperture filter having a slit aperture width adjustable by the processor.

3. a data storage element coupled to the processor; The processor executable code further causes the processor to: comparing positions of the one or more line images on the one or more image sensors to one or more calibrated positions associated with center positions of the one or more slit aperture filters stored in the data storage element; adjusting a position of at least one of the one or more line images on at least one of the one or more image sensors to a corresponding calibrated position of the one or more calibrated positions; The apparatus of claim 2 , configured to:

4. a data storage element coupled to the processor; The processor executable code further causes the processor to: comparing the position of the confocal plane to a calibrated confocal plane position associated with a position of a corresponding one of the one or more slit aperture filters stored in the data storage element; Adjusting the position of the confocal plane to the position of a calibrated confocal plane; The apparatus of claim 2 , configured to:

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