Lens penetration height measurement

Abstract

The optical inspection apparatus (10, 140, 180) includes an inspection optics (14, 314) including an illumination assembly (16, 316), which includes a reticle including a predetermined pattern and a radiation source configured to emit optical radiation that illuminates the reticle. The collection optics (18, 318) projects the emitted optical radiation including the pattern (105) of the illuminated reticle (34, 120) onto a workpiece (12, 312). A deflection element (144, 146, 350, 352) is disposed at a pupil of the collection optics. The imaging assembly (20, 319) captures an image of the workpiece including a first copy (108) of the pattern projected through a first portion of the pupil area and a second copy (110) of the pattern projected through a second portion of the pupil area. A processor (24, 324) processes the captured image to estimate a distance between the inspection optics and the workpiece.

Description

[Technical Field]

[0001] The present invention relates generally to optical devices, and more particularly to inspection and metrology systems and methods. [Background technology]

[0002] In the manufacturing process of workpieces such as printed circuit boards, semiconductor wafers, display panels, and integrated circuits, the workpieces are typically inspected by an inspection system configured to image features on the circuit board. For accurate imaging of the features, the workpiece is brought to the focal plane of the inspection system's optical system. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent Application Publication No. 2021 / 102892 [Patent Document 2] U.S. Patent Application Publication No. 2015 / 377794 Summary of the Invention [Problem to be solved by the invention]

[0004] DETAILED DESCRIPTION OF THE INVENTION The embodiments of the present invention described below provide apparatus and methods that allow for accurate assessment of the distance between an optical system in an inspection system and a workpiece under inspection. [Means for solving the problem]

[0005] According to an embodiment of the present invention, an optical inspection apparatus is provided that includes an inspection optical system including an illumination assembly including a reticle including a predetermined pattern and a radiation source configured to emit optical radiation that illuminates the reticle. A collection optical system having a pupil with a given pupil area is configured to project the emitted optical radiation, including the illuminated reticle pattern, onto a workpiece. A deflection element is disposed within the pupil of the collection optical system and extends across a first portion of the pupil area but not across a second portion of the pupil area. An imaging assembly is configured to capture images of the workpiece including a first replica of the pattern projected through the first portion of the pupil area and a second replica of the pattern projected through the second portion of the pupil area. A processor is configured to process the captured images to measure a difference between the first and second replicas of the pattern and to estimate a distance between the inspection optical system and the workpiece in response to the measured difference.

[0006] In some embodiments, the deflecting element includes a transparent wedge. In disclosed embodiments, the wedge extending across a first portion of the pupil region is a first wedge having a first wedge direction, and the inspection optics includes a second wedge extending across a second portion of the pupil region and having a second wedge direction opposite to the first wedge direction.

[0007] Alternatively, the deflection element comprises a diffractive optical element. In one embodiment, the diffractive optical element extending across a first portion of the pupil region is a first diffractive optical element having a first deflection direction, and the inspection optics includes a second diffractive optical element extending across a second portion of the pupil region and having a second deflection direction opposite to the first deflection direction.

[0008] Further alternatively, the deflecting element includes a mirror. In a disclosed embodiment, the mirror extending across a first portion of the pupil region is a first mirror having a first tilt angle about an axis, and the inspection optics includes a second mirror extending across a second portion of the pupil region and having a second tilt angle about an axis opposite to the first tilt angle.

[0009] In a disclosed embodiment, the apparatus includes a motion assembly configured to adjust the distance between the inspection optics and the workpiece, and the processor is configured to drive the motion assembly in response to the measured difference.

[0010] In some embodiments, the predetermined pattern includes multiple sub-patterns arranged at different locations on the reticle. In one embodiment, the multiple sub-patterns are arranged in a periodic array on the reticle, and the first and second replicate sub-patterns are interlaced within the image of the workpiece. Alternatively or additionally, the multiple sub-patterns include at least first and second sub-patterns having different first and second pattern characteristics.

[0011] In a disclosed embodiment, the second replica of the pattern is offset in a first direction from the first replica of the pattern, and at least one of the replicas shifts in a second direction orthogonal to the first direction in response to the measured difference.

[0012] According to an embodiment of the present invention, an optical inspection apparatus is also provided, including an inspection optical system including an illumination assembly including a reticle including a predetermined pattern and a radiation source configured to emit optical radiation that illuminates the reticle. A collection optical system is configured to project the emitted optical radiation, including the illuminated reticle pattern, onto a workpiece. An imaging assembly including an image sensor and an objective optical system having a pupil with a given pupil area is configured to image the workpiece onto the image sensor. A deflection element is disposed within the pupil of the objective optical system and extends across a first portion of the pupil area but not across a second portion of the pupil area. A processor is configured to process the image captured by the image sensor to detect a first replica of the pattern imaged through the first portion of the pupil area and a second replica of the pattern imaged through the second portion of the pupil area, and to measure a difference between the first and second replicas of the pattern. The processor then estimates the distance between the inspection optical system and the workpiece based on the measured difference. Additionally, according to an embodiment of the present invention, a method for optical inspection is provided, comprising providing inspection optics including an illumination assembly including a reticle containing a predetermined pattern and a radiation source configured to emit optical radiation that illuminates the reticle. A collection optical system having a pupil with a given pupil area and configured to project the emitted optical radiation containing the illuminated reticle pattern onto a workpiece, and an imaging assembly are also provided. A deflection element is disposed within the pupil of the collection optical system and extends across a first portion of the pupil area but not across a second portion of the pupil area. Using the imaging assembly, an image of the workpiece is captured, including a first replica of the pattern projected through the first portion of the pupil area and a second replica of the pattern projected through the second portion of the pupil area. The captured image is processed to measure a difference between the first and second replicas of the pattern. The distance between the inspection optical system and the workpiece is evaluated according to the measured difference.

[0013] According to an embodiment of the present invention, there is further provided a method for optical inspection, comprising providing inspection optics including an illumination assembly, a reticle containing a predetermined pattern, and a radiation source configured to emit optical radiation that illuminates the reticle. A collection optical system is configured to project the emitted optical radiation, including the illuminated reticle pattern, onto a workpiece. An imaging assembly including an image sensor and an objective optical system has a pupil with a given pupil area. A deflection element is disposed within the pupil of the objective optical system and extends across a first portion of the pupil area but not across a second portion of the pupil area. Using the imaging assembly, an image of the workpiece is captured, including a first replica of the pattern imaged through the first portion of the pupil area and a second replica of the pattern imaged through the second portion of the pupil area. The captured image is processed to measure a difference between the first and second replicas of the pattern. The distance between the inspection optical system and the workpiece is evaluated according to the measured difference.

[0014] The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken in conjunction with the drawings. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic side view of an optical inspection apparatus according to an embodiment of the present invention; [Figure 2a] 2 is a partial schematic side view of the optical inspection apparatus of FIG. 1 in accordance with one embodiment of the present invention. [Figure 2b] 2 is a partial schematic side view of the optical inspection apparatus of FIG. 1 in accordance with one embodiment of the present invention. [Figure 3a] 2 is a schematic front view of a pattern of a reticle on a workpiece, in accordance with one embodiment of the present invention; [Figure 3b] 2 is a schematic front view of a pattern of a replica of a reticle on a workpiece, in accordance with one embodiment of the present invention; [Figure 4a] FIG. 10 is a schematic front view of the replication of another reticle subpattern on a workpiece in accordance with an alternative embodiment of the present invention. [Figure 4b]FIG. 10 is a schematic front view of the replication of multiple subpatterns of another reticle on a workpiece in accordance with an alternative embodiment of the present invention. [Figure 4c] FIG. 10 is a schematic front view of a reticle including multiple sub-patterns, according to another alternative embodiment of the present invention. [Figure 5] 1 is a schematic side view of an optical inspection apparatus according to an alternative embodiment of the present invention. [Figure 6] 1 is a schematic side view of an optical inspection apparatus according to an alternative embodiment of the present invention. [Figure 7] 1 is a schematic side view of an optical inspection apparatus according to an alternative embodiment of the present invention. [Figure 8] 10 is a schematic side view of an optical inspection apparatus according to a further embodiment of the present invention; [Figure 9a] 10 is a schematic side view of an optical inspection apparatus according to a further embodiment of the present invention; [Figure 9b] 10 is a schematic side view of an optical inspection apparatus according to a further embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0016] Overview In applications of optical imaging systems, such as for optical inspection of workpieces during a manufacturing process, an illuminator is used to illuminate a field on the workpiece with optical radiation. (As used herein and in the claims, the terms "optical radiation," "radiation," and "light" generally refer to any of visible, infrared, and ultraviolet radiation.) The illuminated field on the workpiece is imaged by imaging optics and detected by an image sensor.

[0017] In some applications, such as the inspection of flat panel displays, semiconductor wafers and printed circuit boards, high quality inspection requires that the field of view of the workpiece imaged onto the image sensor be precisely focused by the imaging optics.

[0018] The embodiments of the invention described herein provide a focus sensor that accurately senses the deviation of a workpiece from the focus of an imaging optical system. The focus sensor can be used, for example, to send a signal to a motion assembly, which adjusts the distance between the workpiece and the imaging optical system to focus the workpiece relative to the optical system. The focus sensor utilizes optical radiation transmitted through the imaging optical system, thus ensuring that the focus position of the workpiece is measured at the same field of view as that imaged onto the image sensor. To accomplish this task, the focus sensor illuminates a patterned reticle and projects the pattern through a collection optical system. The pupil of the inspection optical system is relayed to the collection optical system. In some embodiments, the pupil region of the collection optical system is split into two apertures that extend across different portions of the pupil region, thus splitting the beam projected from the reticle into two separate beams. Due to the lateral offset of the apertures, these two beams impinge on the workpiece at different angles, and each beam produces a replica of the reticle pattern on the workpiece. A deflection element disposed in at least one aperture laterally separates the two replicas on the workpiece. When the workpiece is in precise focus relative to the imaging assembly, the two replicas are aligned with each other, but for a defocused workpiece, the replicas are laterally shifted relative to each other. The deflection element is oriented so that the lateral separation of the two replicas is orthogonal to the replica shift due to defocus. In some embodiments, for greater lateral separation, two deflection elements with opposing deflection directions are disposed in the two apertures, respectively.

[0019] Two copies of the reticle pattern on the workpiece are imaged onto an image sensor by the imaging optics. A processor compares the two parallel images of the copies captured by the image sensor, detects workpiece defocus, and commands a motion system to move the workpiece to the correct focus position. By positioning the two pupil apertures symmetrically with respect to the optical axis of the collection optics, the two copies of the reticle pattern shift by equal but opposite amounts in response to defocus, thus enabling highly sensitive differential detection of defocus.

[0020] In these embodiments, the optical inspection apparatus includes an inspection optical system comprising an illumination assembly, a collection optical system, a deflection element, and an imaging assembly. The illumination assembly includes a reticle including a predetermined pattern and a radiation source that emits optical radiation, thereby illuminating the reticle. The collection optical system projects the emitted optical radiation, including the illuminated reticle pattern, onto a workpiece. The deflection element is disposed within the pupil of the collection optical system and extends across a first portion of the pupil region but not across a second portion of the pupil region. The imaging assembly captures an image of the workpiece, including a first replica of the pattern projected through the first portion of the pupil region and a second replica of the pattern projected through the second portion of the pupil region. A processor processes the captured image to measure a difference between the first and second replicas of the pattern and evaluate the distance between the inspection optical system and the workpiece in response to the measured difference. In an alternative embodiment, the split-pupil region and the deflection element are disposed within the objective optical system of the imaging assembly rather than within the illumination assembly. In these embodiments, the focusing optics projects a single reticle pattern onto the workpiece, and the deflection elements create two distinct copies of the reticle pattern on the image sensor in the imaging assembly. Defocus is measured and compensated for in a manner similar to that described above.

[0021] In some embodiments, the deflecting element comprises a transparent optical wedge. Alternatively, other types of deflecting elements may be used, such as diffractive optical elements or tilted mirrors. Suitable combinations of such deflecting elements, as well as other deflection modes, will be apparent to those skilled in the art and are considered to be within the scope of the present invention.

[0022] Replication separation using a light-collecting optical system Figure 1 is a schematic side view of an optical inspection apparatus 10 for inspecting a workpiece 12 according to one embodiment of the present invention. Apparatus 10 includes inspection optics 14, which includes an illumination assembly 16, collection optics 18, and an imaging assembly 20. Apparatus 10 further includes a motion assembly 22 and a processor 24. In this figure, as well as in Figures 2a-2b, 3a-3b, 4a-4b, and 5, Cartesian coordinates 26 are used to define the orientation of the depicted items.

[0023] The motion assembly 22 comprises, for example, a linear mechanical stage capable of moving the workpiece 12 in the x, y, and z directions of Cartesian coordinates 26, as well as a rotational stage capable of rotating the workpiece about the z-axis, under the control of the processor 24. In the illustrated embodiment, the motion assembly 22 moves the workpiece 12 relative to the stationary inspection optics 14. Alternatively, the motion assembly 22 may move the inspection optics 14 (along with other optical assemblies) relative to the stationary workpiece 12. Adjusting the separation between the workpiece 12 and the inspection optics 14 in the z-direction is applied for height measurement and focus adjustment purposes, while movement in the x- and y-directions and rotation about the z-axis is used to bring a desired region of the workpiece 12 into the field of view of the inspection optics 14.

[0024] Illumination assembly 16 comprises a reticle radiation source 28, a field radiation source 30, a reticle collimator lens 32, a reticle 34 containing a predetermined pattern illuminated by reticle radiation source 28, a field illumination lens 35, and a beam splitter 36. Alternatively, instead of two separate radiation sources, a single radiation source combined with appropriate movable optics may be used to illuminate workpiece 12 with or without a contribution from reticle 34. In the illustrated embodiment, beam splitter 36 (together with field radiation source 30 and field illumination lens 35) is located immediately to the right of aperture assembly 48. Alternatively, beam splitter 36 may be located, for example, between workpiece 12 and inspection optics 14, or in some other suitable location.

[0025] The collection optics 18 includes a collection lens 38, a pupil relay 40, a beam splitter 42, and an objective lens 44 having a focal plane 45. The collection optics 18 further includes an aperture assembly 48 and first and second transparent optical wedges 50 and 52 positioned in respective portions of the pupil region of the collection optics 18 within a plane 46. The plane 46 is relayed by the pupil relay 40 to a pupil plane 47 of the objective lens 44, such that the planes 46 and 47 are conjugate to one another. The collection optics 18 projects optical radiation emitted by the reticle radiation source 28, which includes the pattern of the illuminated reticle 34, onto the workpiece 12. A subset of the collection optics 18 projects optical radiation emitted by the field radiation source 30 onto the workpiece 12. The aperture assembly 48 and the optical wedges 50 and 52 are described in further detail below in FIGS. 2a-2b.

[0026] Imaging assembly 20 includes, as elements shared with collection optics 18, beam splitter 42 and objective lens 44. Imaging assembly 20 further includes a tube lens 54 and an image sensor 56. Image sensor 56 includes a two-dimensional pixelated image sensor, such as a CMOS (complementary metal-oxide semiconductor) image sensor. Thus, imaging assembly 20 captures an image of workpiece 12, including respective replicas of the reticle pattern projected through wedges 50 and 52.

[0027] Processor 24 is coupled to motion assembly 22, radiation sources 28 and 30, and image sensor 56. Processor 24 typically comprises a general-purpose programmable computer processor with suitable interfaces to the other components of apparatus 10 and is programmed with software and / or firmware to perform the functions described herein. Processor 24 is shown in the figures as a single monolithic functional block for simplicity, although in practice the processor may comprise a single chip or a set of two or more chips with suitable interfaces for receiving and outputting the signals shown in the figures and described herein.

[0028] The following describes two functions of the inspection system 10: bringing the workpiece into precise focus with respect to the objective lens 44 and imaging the workpiece 12 onto the image sensor 56.

[0029] To bring workpiece 12 into precise focus, processor 24 turns on reticle radiation source 28 while ensuring that field radiation source 30 is turned off. As described in more detail below with reference to FIGS. 2a-2b and 3a-3b, pattern 105 of reticle 34 is projected to generate two side-by-side replicas on workpiece 12, such that point 59 on the reticle is imaged to points 60 and 62 within a field of view 58 under inspection on workpiece 12. These two replicas are imaged by imaging assembly 20 onto image sensor 56 around images 64 and 66 of points 60 and 62, respectively, and captured by processor 24. Processor 24 calculates the deviation of workpiece 12 from the focus of objective lens 44 from the two captured images and drives motion assembly 22 to move the workpiece in the z-direction of Cartesian coordinates 26 to correct the deviation.

[0030] To capture an image of workpiece 12 for inspection, processor 24 turns on field radiation source 30 while ensuring that reticle radiation source 28 is turned off, thus time-multiplexing the inspection and focusing functions. Optical radiation emitted by radiation source 30 is projected by field illumination lens 35, reflected by beam splitter 36, and projected through a subset of collection optics 18 to illuminate field of view 58 on workpiece 12. The combination of field illumination lens 35 and the subset of collection optics 18 is configured to project uniform illumination onto field of view 58. Field of view 58 is then imaged onto image sensor 56 by objective lens 44, beam splitter 42, and tube lens 54. For clarity, the light rays emitted by field radiation source 30 have been omitted.

[0031] 2a-2b are partial schematic side views of an optical inspection apparatus 10 according to one embodiment of the present invention. The view in FIG. 2b is rotated 90° about optical axis 68 relative to FIG. 2a. Because the purpose of these figures is to illustrate the focusing function of inspection apparatus 10, components that are not essential for focusing have been omitted for clarity. The remaining components are labeled as in FIG. 1.

[0032] The positions and orientations of the first optical wedge 50 and the second optical wedge 52 are viewed from two orthogonal directions (as indicated by the respective oriented Cartesian coordinates 26) in FIGS. 2a and 2b, as well as in a cross-sectional view of the insert 74. Wedge directions 70 and 72 are defined for the wedges 50 and 52, respectively, as vectors directed from the base of the respective wedges to their edges. For further clarity, the wedge direction 70 of the first wedge 50 is shown in a perspective view of the first wedge within the insert 76. The wedge direction 70 is perpendicular to the base 78 of the first wedge 50 and points toward the edge 80 of the first wedge. The first and second optical wedges 50 and 52 are positioned over respective first and second apertures 90 and 92 of the aperture assembly 48 and extend over different respective portions of the pupil region of the collection optics 18.

[0033] For the focusing function, light rays are emitted by the reticle radiation source 28 toward the reticle collimator lens 32. The reticle collimator lens 32 projects the optical radiation onto the reticle 34 and from there to the beam splitter 36, the collector lens 38, and the optical wedges 50 and 52. Referring to FIG. 2a, these rays from point 59 striking the first wedge 50 and first aperture 90 are refracted by the first wedge in the yz plane toward the positive y-axis as ray 94 and imaged to point 60 by the pupil relay 40 and objective lens 44. The light rays from point 59 striking the second wedge 52 and second aperture 92 are refracted by the second wedge in the yz plane as ray 96, but now toward the negative y-axis, and imaged to point 62.

[0034] These opposing actions of the first and second wedges 50 and 52 split the replica of the pattern 105 of the reticle 34 on the workpiece 12 into two replicas (hereafter labeled as first and second replicas 108 and 110, respectively, in FIG. 3b). In the xz plane (FIG. 2b), the wedges 50 and 52 behave as plane-parallel plates of glass with the same refractive properties. However, the opposing lateral offsets of the apertures 90 and 92 from the optical axis 68 cause the ray bundles passing through each aperture to impinge on the workpiece 12 at equal but opposite oblique angles, as indicated by the respective rays 98 and 100 and angles α and β. Thus, the replica of the pattern 105 formed at point 60 is formed by the oblique rays impinging on the workpiece 12 at angle α, and the replica formed at point 62 is formed by the oblique rays impinging on the workpiece at angle β, which has the opposite sign to the sign of angle β. The opposite signs of angles α and β cause the two replicas at points 60 and 62 to shift in opposite x directions, respectively, as workpiece 12 is moved in the z direction (into or out of focal plane 45 of objective lens 44). An example of image shift for workpiece 12 moving a distance Δz relative to plane 102 is shown in Figure 3b below.

[0035] 3a and 3b are schematic front views (viewed in the z-direction) of a pattern 105 of reticle 34 and its replicas 108 and 110 formed on workpiece 12 through wedges 50 and 52, respectively, in accordance with an embodiment of the present invention. Reticle 34 includes a pattern 105 of opaque lines 104 on a transparent substrate 106. (Alternatively, transparent lines on an opaque substrate may be used.)

[0036] In Figure 3b, replicas 108 and 110 are separated from each other in the y-direction. Replica 108 is formed around point 60, and replica 110 is formed around point 62, as shown in Figure 2a. With respect to workpiece 12 at focal plane 45 of objective lens 44, line 104 is imaged onto line 112 in first replica 108 and onto line 114 in second replica 110, with lines 112 and 114 aligned with each other in the x-direction. When workpiece 12 is defocused by Δz relative to plane 102 (see Figures 2a-2b), line 112 shifts in the positive x-direction, shown as dotted line 112', and line 114 shifts in the negative x-direction, shown as dotted line 114', with the relative shift Δx between lines 112' and 114' being proportional to Δz. Thus, the relative shift of the two replicas 108 and 110 occurs in a direction perpendicular to their direction of separation. The described embodiment refers to a system that is perfectly aligned in the sense that lines 112 and 114 are aligned with one another when workpiece 12 is at focal plane 45. However, even in a misaligned system in which lines 112 and 114 are not aligned with one another when workpiece 12 is at focal plane 45, the defocus Δz can be inferred from the relative shift of the lines.

[0037] 1, replicas 108 and 110 are imaged onto image sensor 56 and processed by processor 24 to estimate the distance between inspection optics 14 and workpiece 12. To this end, processor 24 measures the relative difference Δx between replicas 108 and 110 from the captured images, calculates from Δx a deviation Δz of workpiece 12 from the focus of objective lens 44, and drives motion assembly 22 to move the workpiece in the z direction of Cartesian coordinates 26 to correct the deviation.

[0038] In the illustrated embodiment, the apertures 90 and 92 and associated wedges 50 and 52 are laterally offset symmetrically with respect to the optical axis 68. Alternatively, they may be arranged asymmetrically. For example, the first aperture 90 and first wedge 50 may be centered on the optical axis 68, while the second aperture 92 and second wedge 52 are laterally offset from the axis. In such an arrangement, the first replica 108 is stationary with respect to defocus, while the second replica 110 shifts with defocus. As another alternative, wedges may be used in part of the pupil region to shift one of the reticle replicas, with another portion of the pupil region accompanied by a flat, transparent optical element (or no optical element). In the depicted embodiment, the pattern 105 is depicted as comprising a plurality of parallel lines 104. Alternatively, the pattern may include a set of parallel lines with a fixed or varying spatial period and varying orientation, as well as patterns with different pattern characteristics, as described in further detail below.

[0039] 4a and 4b are schematic front views of a reticle 120 including multiple subpatterns 122 and replication of the subpatterns 122 on a workpiece 12, according to an alternative embodiment of the present invention. Each subpattern 122 is similar to pattern 105 of FIG. 3a.

[0040] Reticle 120 is positioned in optical inspection system 10 in the same position as reticle 34 of Figures 1 and 2a-2b. Subpattern 122 is projected to form first and second replicas 124 and 126 on workpiece 12, similar to Figure 3b above. By designing subpatterns 122 to be the same width W and separating them by the same width W, and by appropriate selection of the optical properties of first wedge 50 and second wedge 52, replicas 124 and 126 are interlaced on workpiece 12. As in Figure 3b, the relative shift Δx between replicas 124 and 126 is proportional to the deviation Δz of workpiece 12 from the focus of objective lens 44.

[0041] Additionally or alternatively, the relative shift between replicas 124 and 126 may be sensed locally by processor 24, as indicated by shift Δx′ in region 128. Processor 24 can measure the local topography of the workpiece by comparing the relative image shifts in multiple regions. Figure 4c is a schematic front view of reticle 130 including multiple subpatterns 131, 132, 133, 134, 135, and 136, in accordance with another alternative embodiment of the present invention.

[0042] While a full-field image (including the entire field of view (FOV) of the inspection optics 14 of the apparatus 10) is captured to inspect the workpiece 12, an image of a region of interest (ROI), i.e., a partial FOV, may be captured for the purpose of assessing the workpiece's deviation Dz from the focus of the objective lens 44. Thus, by confining the ROI to the vicinity of one of the subpatterns 131, 132, 133, 134, 135, or 136, a particular quality of that subpattern can be beneficially utilized, either separately or in combination with other subpatterns. For example, the large period of subpattern 132 allows a coarse measurement range of Δz to be used in a "search and converge" mode, while the small period of subpattern 131 allows a fine measurement range for topography tracking, with a combination of large and small periods enabling both functions. Additionally, various orientations or shapes of reticle features such as subpatterns 133, 134, and 135 can be used to avoid projecting a reticle pattern onto a similar pattern on the workpiece, thus preventing pattern aliasing. A combination of two subpatterns, such as 131 and 136, can be utilized while scanning workpiece 12 back and forth, such that subpattern 131 is utilized for one scan direction and subpattern 136 is utilized for the opposite scan direction.

[0043] Similar to subpatterns 131, 132, 133, 134, 135, and 136 in reticle 130, other subpatterns having various characteristics may alternatively be used. These characteristics may include different resolutions, different shapes, and different orientations of the pattern features. Additionally or alternatively, the pattern features may include regular, random, and pseudo-random patterns, as well as combinations thereof. In general, different patterns in each reticle may be located in different regions of the FOV of inspection optics 14. This allows for measuring and correcting defocus of workpiece 12 with different patterns by having processor 24 dynamically change the ROI within the field of view based on operational criteria such as scan direction and speed, measurement results, and material properties of the workpiece.

[0044] 5 and 6 are schematic side views of optical inspection apparatuses 140 and 180, respectively, according to alternative embodiments of the present invention. Apparatuses 140 and 180 are similar to apparatus 10 (FIG. 1), except for their respective different implementations for splitting the reticle image. Only those parts of apparatuses 140 and 180 that differ from their counterparts in apparatus 10 are labeled differently from those in FIG. 1. Referring to FIG. 5, in apparatus 140, wedges 50 and 52 of apparatus 10 are replaced with a diffractive optical element (DOE) 142, while all other components of apparatus 140 are the same as apparatus 10. DOE 142 comprises two separate subgratings. Subgratings 144 and 146 have deflection directions indicated by arrows 145 and 147, respectively. DOE 142 is shown within insert 148 in a cross-sectional view taken along the positive z-axis relative to aperture assembly 48. Subgratings 144 and 146 act as deflecting elements due to their particular diffractive properties, deflecting the light rays towards points 60 and 62 respectively, in the deflection directions indicated by arrows 145 and 147 .

[0045] Referring to FIG. 6, in apparatus 180, wedges 50 and 52 of apparatus 10 are replaced by mirrors 182 and 184 positioned in plane 46 at or near the pupil region of collection optics 18. Mirrors 182 and 184 are tilted in opposite directions about the x-axis to have respective deflection directions indicated by arrows 183 and 185. Furthermore, unlike apparatus 10, apparatus 180 does not include aperture assembly 48, as mirrors 182 and 184 also serve as first and second apertures 90 and 92, respectively. Mirrors 182 and 184 are shown in a cross-sectional view of insert 188 as viewed from plane 46 along the positive z-axis. Based on their respective tilt angles, mirrors 182 and 184 function as deflection elements, deflecting light rays toward points 60 and 62, respectively, as indicated by arrows 183 and 185.

[0046] FIG. 7 is a schematic side view of an optical inspection apparatus 200 for inspecting a workpiece 12 in accordance with an alternative embodiment of the present invention.

[0047] Optical inspection apparatus 200 is similar to apparatus 10 of Figure 1, with like components labeled the same. The difference between the two apparatus is that collection assembly 218 in apparatus 200 has been modified compared to collection assembly 18 by removing pupil relay 40. In this case, aperture assembly 48 and wedges 50 and 52 are positioned directly at or near pupil plane 47 of objective lens 44.

[0048] First and second optical wedges 50 and 52, and aperture assembly 48 are similar to those of apparatus 10. As a result, like apparatus 10, apparatus 200 images point 59 of reticle 34 onto points 260 and 262 on workpiece 12, which are further imaged onto respective points 264 and 266 on image sensor 56. As in FIG. 3b, first and second replicas 108 and 110 within region 258 are laterally offset from one another and shifted due to the focus shift of workpiece 12.

[0049] Replicate separation by imaging assembly 8 and 9a-9b are schematic side views of an optical inspection apparatus 310 for inspecting a workpiece 312 in accordance with a further embodiment of the present invention. The apparatus 310 includes an inspection optics 314 comprising an illumination assembly 316, collection optics 318, and an imaging assembly 319. The imaging assembly 319 includes a reticle imaging subassembly 320 and a field of view imaging subassembly 321. The apparatus 310 further includes a motion assembly 322 and a processor 324. In this figure and in FIGS. 9a-9b, Cartesian coordinates 326 are used to define the orientation of the depicted items. The motion assembly 322, similar to the motion assembly 22 of FIG. 1, includes, for example, a linear mechanical stage capable of moving the workpiece 312 in the x, y, and z directions of the Cartesian coordinates 326, as well as a rotational stage capable of rotating the workpiece about the z-axis under the control of the processor 324.

[0050] Illumination assembly 316, similar to illumination assembly 16 of Figure 1, includes a reticle radiation source 328, a field radiation source 330, a reticle collimator lens 332, a reticle 334, a field illumination lens 335, and a beam splitter 336. Reticle 334, similar to reticles 34 and 120, includes either a single pattern or multiple subpatterns (e.g., as shown in Figure 4a) that are illuminated by reticle radiation source 328. As in the previous embodiments, the reticle pattern may be periodic, quasi-periodic, or random.

[0051] Collection optics 318 includes collimator lens 338, beam splitters 341 and 342, and objective lens 344 having focal plane 343. Collection optics 318 projects optical radiation emitted by reticle radiation source 328, including the pattern of illuminated reticle 334, onto workpiece 312.

[0052] Reticle imaging subassembly 320 includes objective lens 344 and beam splitters 341 and 342 as elements shared with collection optics 318. Additionally, reticle imaging subassembly 320 includes pupil relay 340, reticle imaging lens 345, and reticle image sensor 347. Additionally, reticle imaging subassembly 320 includes aperture assembly 348 and first and second optical wedges 350 and 352 located at plane 346 conjugate to the pupil plane of objective lens 344 (pupil plane not shown for clarity). Thus, wedges 350 and 352 are positioned within the relay pupil of objective lens 344 and extend across distinct respective portions of the pupil region. Further details of aperture assembly 348 and optical wedges 350 and 352 are shown in FIGS. 9a-9b.

[0053] Field imaging subassembly 321 includes objective lens 344 and beam splitter 342 as elements shared with collection optics 318 and reticle imaging assembly 320. In addition, field imaging assembly 321 includes tube lens 354 and field image sensor 356.

[0054] Image sensors 356 and 347 include two-dimensional pixelated image sensors, such as CMOS (complementary metal-oxide semiconductor) image sensors. Processor 324 is coupled to motion assembly 322, light sources 328 and 330, and image sensors 356 and 347, similar to processor 24 described above.

[0055] To bring workpiece 312 into precise focus, processor 324 turns on reticle radiation source 328 (ensuring that field radiation source 330 is turned off). The pattern of reticle 334 is projected by collection optics 318 to form a single replica on field of view 358 on workpiece 312, with point 359 on the reticle projected onto point 361 on the workpiece. This single replica of the pattern of reticle 334 on workpiece 312 is imaged along first optical axis 368 through wedges 350 and 352 by reticle imaging optics 321 to form two parallel replicas of the reticle pattern on reticle image sensor 347, resulting in point 361 on the workpiece being imaged to points 360 and 362. Processor 324 processes the image captured by image sensor 347 and measures the difference between the two replicas of the pattern, thus assessing the distance between the inspection optics and workpiece 312. Based on this, processor 324 drives motion assembly 322 to move the workpiece in the z-direction of Cartesian coordinates 326 to correct the deviation.

[0056] To capture images of structures on workpiece 312 for inspection purposes, processor 324 turns on field radiation source 330 (and ensures that reticle radiation source 328 is turned off). Optical radiation emitted by light source 330 is projected by field illumination lens 335, reflected by beam splitter 336, and projected through collection optics 318 to illuminate field of view 358 on workpiece 312. The combination of field illumination lens 335 and collection optics 318 is configured to project uniform illumination onto field of view 358. Field of view 358 is then imaged along second optical axis 369 by objective lens 344, beam splitter 342, and tube lens 354 onto field of view image sensor 356. For clarity, the light rays emitted by field radiation source 330 have been omitted.

[0057] Similar to the time multiplexing of focusing and imaging in optical inspection apparatus 10 (FIG. 1), alternatingly turning on reticle radiation source 328 and field radiation source 330 time multiplexes the focusing and imaging of workpiece 312 for apparatus 310. Alternatively, reticle radiation source 328 and field radiation source 330 may each be spectrally multiplexed by having the two radiation sources emit at distinct wavelengths and by using appropriately configured dichroic beam splitters and / or mirrors.

[0058] 9a-9b are partial schematic side views of optical inspection apparatus 310. The view in FIG. 9b is rotated 90° about first optical axis 368 relative to FIG. 9a. Because the purpose of these figures is to illustrate the focusing function of inspection apparatus 310, components that are not essential for focusing have been omitted for clarity. The remaining components are labeled as in FIG. 8.

[0059] Aperture assembly 348 includes first and second apertures 390 and 392 that extend across different respective portions of the pupil region of objective optics 344. First and second wedges 350 and 352 are positioned over apertures 390 and 392, respectively, as shown in the cross-sectional view of insert 374. Wedges 350 and 352 are oriented in opposite wedge directions 351 and 353, respectively. Referring to FIG. 9 a, a ray from point 361 passing through first wedge 350 is refracted in the negative y-direction and projected to point 360, while a ray passing through second wedge 352 is refracted in the positive y-direction and projected to point 362, thus splitting the replica of the pattern of reticle 334 on field of view 358 into two adjacent replicas 408 and 410, respectively, as shown in insert 420.

[0060] 9b, a ray from point 361 passing through first aperture 390 strikes reticle image sensor 347 at angle α', and a ray passing through second aperture 392 strikes reticle image sensor 347 at angle β', which is the same magnitude as angle α' but has the opposite sign to angle α'. As in the previous embodiment, the opposite signs of angles α' and β' cause replicas 408 and 410 to shift in opposite x-directions in response to deviation of workpiece 312 by Δz from focal plane 343 of objective lens 344 to plane 402.

[0061] Processor 324 processes the images of replicas 408 and 410 to measure the difference Δx between the replicas, calculates the deviation Δz from Δx, and drives motion assembly 322 to move the workpiece in the z direction of Cartesian coordinates 326 to correct the deviation.

[0062] One advantage of having separate pattern replicas within imaging assembly 319 (rather than within collection optics 18 as in FIG. 1 ) is that replicas 408 and 410 are derived from a single replica on workpiece 312. This in turn provides defocus information from a single, smaller area, rather than comparing two replicas located side-by-side on the workpiece.

[0063] In the embodiment shown in Figures 8 and 9a-9b, wedges 350 and 352 may be replaced by other types of deflecting elements, such as DOEs or mirrors, similar to those shown in Figures 5 and 6 and described above.

[0064] It is to be understood that the above-described embodiments are given by way of example, and that the present invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described above, as well as variations and modifications thereof not disclosed in the prior art that will occur to those skilled in the art upon reading the foregoing description.

Claims

1. An optical inspection device, An inspection optical system, an illumination assembly comprising a reticle including a predetermined pattern and a radiation source configured to emit optical radiation that illuminates the reticle; a collection optic having a pupil with a given pupil area and configured to project the emitted optical radiation containing the pattern of the illuminated reticle onto a workpiece; a deflection element disposed in a pupil of the collection optics, the deflection element extending across a first portion of the pupil area but not across a second portion of the pupil area; an imaging assembly configured to capture an image of a workpiece including a first replica of a pattern projected through a first portion of the pupil area and a second replica of the pattern projected through a second portion of the pupil area; an inspection optical system comprising: a processor configured to process the captured images to measure a difference between the first and second copies of the pattern and to estimate a distance between the inspection optics and the workpiece in response to the measured difference; An optical inspection device comprising:

2. 10. The apparatus of claim 1, wherein the deflecting element comprises a transparent wedge.

3. 3. The apparatus of claim 2, wherein the wedge extending across a first portion of the pupil region is a first wedge having a first wedge direction, and the inspection optics comprises a second wedge extending across a second portion of the pupil region and having a second wedge direction opposite the first wedge direction.

4. The apparatus of claim 1 , wherein the deflecting element comprises a diffractive optical element.

5. 5. The apparatus of claim 4, wherein the diffractive optical element extending across a first portion of the pupil region is a first diffractive optical element having a first polarization direction, and the inspection optics comprises a second diffractive optical element extending across a second portion of the pupil region and having a second polarization direction opposite to the first polarization direction.

6. The apparatus of claim 1 , wherein the deflecting element comprises a mirror.

7. 7. The apparatus of claim 6, wherein the mirror extending across a first portion of the pupil region is a first mirror having a first tilt angle about an axis, and the inspection optics comprises a second mirror extending across a second portion of the pupil region and having a second tilt angle about an axis opposite to the first tilt angle.

8. a motion assembly configured to adjust the distance between the inspection optics and the workpiece; 2. The apparatus of claim 1, wherein the processor is configured to actuate the motion assembly in response to the measured difference.

9. 9. The apparatus of claim 1, wherein the predetermined pattern comprises a plurality of sub-patterns located at different positions on a reticle.

10. 10. The apparatus of claim 9, wherein the plurality of sub-patterns are arranged in a periodic arrangement on the reticle, and the sub-patterns of each of the first and second replicas are interlaced within the image of the workpiece.

11. 10. The apparatus of claim 9, wherein the plurality of sub-patterns includes at least first and second sub-patterns having different first and second pattern characteristics.

12. 9. The apparatus of claim 1, wherein the second replica of the pattern is offset in a first direction from the first replica of the pattern, and at least one of the replicas is shifted in a second direction orthogonal to the first direction in response to the measured difference.

13. An optical inspection device, An inspection optical system, an illumination assembly comprising a reticle including a predetermined pattern and a radiation source configured to emit optical radiation that illuminates the reticle; a collection optical system configured to project the emitted optical radiation containing the illuminated reticle pattern onto a workpiece; an imaging assembly comprising an image sensor and an objective having a pupil with a given pupil area and configured to image the workpiece onto the image sensor; a deflection element disposed in a pupil of the objective optical system, the deflection element extending across a first portion of the pupil area but not across a second portion of the pupil area; an inspection optical system comprising: a processor configured to process images captured by the image sensor to detect a first replica of a pattern imaged through the first portion of the pupil region and a second replica of the pattern imaged through the second portion of the pupil region, measure a difference between the first replica and the second replica of the pattern, and estimate a distance between the inspection optics and the workpiece in response to the measured difference; An optical inspection device comprising:

14. the imaging assembly having the deflection element disposed at a pupil of the objective optical system is a first imaging assembly having a first optical axis; 14. The apparatus of claim 13, wherein the inspection optics comprises: a second imaging assembly configured to capture an image of a structure on the workpiece along a second optical axis; and a beam splitter configured to split optical radiation reflected from the workpiece between the first optical axis and the second optical axis.

15. 14. The apparatus of claim 13, wherein the deflecting element comprises a transparent wedge.

16. 16. The apparatus of claim 15, wherein the wedge extending across the first portion of the pupil region is a first wedge having a first wedge direction, and the inspection optics comprises a second wedge extending across the second portion of the pupil region and having a second wedge direction opposite the first wedge direction.

17. The apparatus of claim 13 , wherein the deflecting element comprises a diffractive optical element.

18. 18. The apparatus of claim 17, wherein the diffractive optical element extending across the first portion of the pupil region is a first diffractive optical element having a first polarization direction, and the inspection optics comprises a second diffractive optical element extending across the second portion of the pupil region and having a second polarization direction opposite to the first polarization direction.

19. 14. The apparatus of claim 13, wherein the deflecting element comprises a mirror.

20. 20. The apparatus of claim 19, wherein the mirror extending across a first portion of the pupil region is a first mirror having a first tilt angle about an axis, and the inspection optics comprises a second mirror extending across a second portion of the pupil region and having a second tilt angle about an axis opposite to the first tilt angle.

21. a motion assembly configured to adjust the distance between the inspection optics and the workpiece; 14. The apparatus of claim 13, wherein the processor is configured to actuate the motion assembly in response to the measured difference.

22. 22. The apparatus of claim 13, wherein the predetermined pattern comprises a plurality of sub-patterns located at different positions on the reticle.

23. 23. The apparatus of claim 22, wherein the plurality of sub-patterns are arranged in a periodic arrangement on the reticle, and the sub-patterns of each of the first and second replicas are interlaced within the image of the workpiece.

24. 23. The apparatus of claim 22, wherein the plurality of sub-patterns includes at least first and second sub-patterns having different first and second pattern characteristics.

25. 22. The apparatus of claim 13, wherein the second replica of the pattern is offset in a first direction from the first replica of the pattern, and at least one of the replicas is shifted in a second direction orthogonal to the first direction in response to the measured difference.

26. 1. A method for optical inspection, comprising: Providing an inspection optical system, the inspection optical system comprising: an illumination assembly comprising a reticle including a predetermined pattern and a radiation source configured to emit optical radiation that illuminates the reticle; a collection optic having a pupil with a given pupil area and configured to project the emitted optical radiation containing the pattern of the illuminated reticle onto a workpiece; an imaging assembly; Equipped with disposing a deflection element in a pupil of the collection optics, the deflection element extending across a first portion of the pupil region but not across a second portion of the pupil region; capturing an image of a workpiece using an imaging assembly, the image including a first replica of a pattern projected through a first portion of the pupil region and a second replica of the pattern projected through a second portion of the pupil region; processing the captured images to measure differences between the first and second copies of the pattern; assessing the distance between the inspection optics and the workpiece in response to the measured difference; A method for providing the above.

27. 27. The method of claim 26, wherein the step of placing a deflecting element comprises placing a transparent wedge in the pupil.

28. 28. The method of claim 27, wherein disposing the transparent wedge comprises disposing a first wedge having a first wedge direction across the first portion of the pupil region, and disposing a second wedge having a second wedge direction opposite the first wedge direction across the second portion of the pupil region.

29. 27. The method of claim 26, wherein placing the deflecting element comprises placing a diffractive optical element in the pupil.

30. 30. The method of claim 29, wherein the step of arranging the diffractive optical element comprises: arranging a first diffractive optical element having a first polarization direction to extend across a first portion of the pupil region; and arranging a second diffractive optical element having a second polarization direction opposite the first polarization direction across a second portion of the pupil region.

31. 27. The method of claim 26, wherein the step of positioning a deflecting element comprises positioning a mirror in the pupil.

32. 32. The method of claim 31 , wherein positioning the mirrors comprises positioning a first mirror having a first tilt angle about an axis to extend across a first portion of the pupil region, and positioning a second mirror having a second tilt angle about an axis opposite the first tilt angle across a second portion of the pupil region.

33. adjusting the distance between the inspection optics and the workpiece in response to the measured difference.

27. The method of claim 26, comprising:

34. A method according to any one of claims 26 to 33, wherein the predetermined pattern comprises a plurality of sub-patterns located at different positions on the reticle.

35. Arranging the plurality of sub-patterns in a periodic arrangement on the reticle.

35. The method of claim 34, comprising: positioning the deflecting elements such that the sub-patterns of each of the first and second replicas are interlaced within the image of the workpiece.

36. 35. The method of claim 34, wherein the plurality of sub-patterns includes at least first and second sub-patterns having different first and second pattern characteristics.

37. 34. A method according to any one of claims 26 to 33, wherein the second replica of the pattern is offset in a first direction from the first replica of the pattern, and at least one of the replicas is shifted in a second direction orthogonal to the first direction in response to the measured difference.

38. 1. A method for optical inspection, comprising: Providing an inspection optical system, the inspection optical system comprising: an illumination assembly comprising a reticle including a predetermined pattern and a radiation source configured to emit optical radiation that illuminates the reticle; a collection optical system configured to project the emitted optical radiation containing the illuminated reticle pattern onto a workpiece; an imaging assembly including an image sensor and an objective optical system, the imaging assembly having a pupil with a given pupil area; Equipped with disposing a deflection element in a pupil of the objective optical system, the deflection element extending across a first portion of the pupil area but not across a second portion of the pupil area; capturing an image of a workpiece using the imaging assembly, the image including a first replica of a pattern imaged through the first portion of the pupil area and a second replica of the pattern imaged through the second portion of the pupil area; processing the captured images to measure differences between the first and second copies of the pattern; assessing the distance between the inspection optics and the workpiece in response to the measured difference; A method for providing the above.

39. the imaging assembly having the deflection element disposed at the pupil of the objective optical system is a first imaging assembly having a first optical axis; 39. The method of claim 38, wherein providing the inspection optics step includes providing a second imaging assembly configured to capture an image of a structure on the workpiece along a second optical axis, and a beam splitter positioned to split optical radiation reflected from the workpiece between the first and second optical axes.

40. 39. The method of claim 38, wherein the step of placing a deflecting element comprises placing a transparent wedge in the pupil.

41. 41. The method of claim 40, wherein the step of disposing the transparent wedge comprises disposing a first wedge having a first wedge direction across the first portion of the pupil region, and disposing a second wedge having a second wedge direction opposite the first wedge direction across the second portion of the pupil region.

42. 39. The method of claim 38, wherein the step of disposing a deflecting element comprises disposing a diffractive optical element in the pupil.

43. 43. The method of claim 42, wherein the step of positioning the diffractive optical element comprises: positioning a first diffractive optical element having a first polarization direction to extend across a first portion of the pupil region; and positioning a second diffractive optical element having a second polarization direction opposite the first polarization direction across a second portion of the pupil region.

44. 39. The method of claim 38, wherein the step of positioning a deflecting element comprises positioning a mirror in the pupil.

45. 45. The method of claim 44, wherein the step of positioning the mirrors comprises: positioning a first mirror having a first tilt angle about an axis to extend across the first portion of the pupil area; and positioning a second mirror having a second tilt angle about an axis opposite the first tilt angle across the second portion of the pupil area.

46. adjusting the distance between the inspection optics and the workpiece in response to the measured difference; 39. The method of claim 38, comprising:

47. A method according to any one of claims 38 to 46, wherein the predetermined pattern comprises a plurality of sub-patterns located at different positions on the reticle.

48. Arranging the plurality of sub-patterns in a periodic arrangement on the reticle.

48. The method of claim 47, comprising: positioning the deflecting elements such that the sub-patterns of each of the first and second replicas are interlaced within the image of the workpiece.

49. 48. The method of claim 47, wherein the plurality of sub-patterns includes at least first and second sub-patterns having different first and second pattern characteristics.

50. 47. A method according to any one of claims 38 to 46, wherein the second replica of the pattern is offset in a first direction from the first replica of the pattern, and at least one of the replicas is shifted in a second direction orthogonal to the first direction in response to the measured difference.

Images