FIELD OF THE INVENTION

The present invention relates generally to optical devices, and particularly to inspection and metrology systems and methods.

BACKGROUND

In the production process of workpieces such as printed circuit boards, semiconductor wafers, display panels and integrated circuits, the workpieces are commonly inspected by inspection systems configured to image the features of the circuit board. For accurate imaging of the features, the workpiece is brought to the focal plane of the optics of the inspection system.

SUMMARY

Embodiments of the present invention that are described hereinbelow provide apparatus and methods that enable accurate assessment of the distance between the optics in an inspection system and a workpiece under inspection.

There is therefore provided, in accordance with an embodiment of the invention, an optical inspection apparatus, including inspection optics, which include an illumination assembly, including a reticle containing a predefined pattern and a radiation source configured to emit optical radiation, which illuminates the reticle. Condensing optics, having a pupil with a given pupil area, are configured to project the emitted optical radiation including the pattern of the illuminated reticle onto a workpiece. A deflecting element is positioned in the pupil of the condensing optics and extending over a first part of the pupil area but not over a second part of the pupil area. An imaging assembly is configured to capture an image of the workpiece including a first replica of the pattern projected through the first part of the pupil area and a second replica of the pattern projected through the second part of the pupil area. A processor is configured to process the captured image so as to measure a discrepancy between the first and second replicas of the pattern and to assess a distance between the inspection optics and the workpiece responsively to the measured discrepancy.

In some embodiments, the deflecting element includes a transparent wedge. In a disclosed embodiment, the wedge extending over the first part of the pupil area is a first wedge having a first wedge direction, and the inspection optics include a second wedge extending over the second part of the pupil area and having a second wedge direction opposite the first wedge direction.

Alternatively, the deflecting element includes a diffractive optical element. In one embodiment, the diffractive optical element extending over the first part of the pupil area is a first diffractive optical element having a first deflection direction, and the inspection optics include a second diffractive optical element extending over the second part of the pupil area and having a second deflection direction opposite the first deflection direction.

Further alternatively, the deflecting element includes a mirror. In a disclosed embodiment, the mirror extending over the first part of the pupil area is a first mirror having a first tilt angle about an axis, and the inspection optics include a second mirror extending over the second part of the pupil area and having a second tilt angle about the axis, opposite the first tilt angle.

In a disclosed embodiment, the apparatus includes a motion assembly, which is configured to adjust the distance between the inspection optics and the workpiece, wherein the processor is configured to drive the motion assembly responsively to the measured discrepancy.

In some embodiments, the predefined pattern includes multiple sub-patterns disposed in different locations on the reticle. In one embodiment, the multiple sub-patterns are positioned in a periodic arrangement on the reticle, and wherein the respective sub -patterns of the first and second replicas are interlaced in the image of the workpiece. Alternatively or additionally, the multiple sub-patterns include at least first and second sub-patterns with different, first and second pattern characteristics.

In a disclosed embodiment, the second replica of the pattern is offset from the first replica of the pattern in a first direction, and at least one of the replicas shifts in a second direction that is orthogonal to the first direction responsively to the measured discrepancy.

There is also provided, in accordance with an embodiment of the invention, an optical inspection apparatus, including inspection optics, which include an illumination assembly, including a reticle containing a predefined pattern and a radiation source configured to emit optical radiation, which illuminates the reticle. Condensing optics are configured to project the emitted optical radiation including the pattern of the illuminated reticle onto a workpiece. An imaging assembly, including an image sensor and objective optics, having a pupil with a given pupil area, is configured to image the workpiece onto to image sensor. A deflecting element is positioned in the pupil of the objective optics and extends over a first part of the pupil area but not over a second part of the pupil area. A processor is configured to process an image captured by the image sensor so as to detect a first replica of the pattern imaged through the first part of the pupil area and a second replica of the pattern imaged through the second part of the pupil area, to measure a discrepancy between the first and second replicas of the pattern, and to assess a distance between the inspection optics and the workpiece responsively to the measured discrepancy. There is additionally provided, in accordance with an embodiment of the invention, a method for optical inspection, which includes providing inspection optics, including an illumination assembly, including a reticle containing a predefined pattern and a radiation source configured to emit optical radiation, which illuminates the reticle. Condensing optics, having a pupil with a given pupil area, configured to project the emitted optical radiation including the pattern of the illuminated reticle onto a workpiece, and an imaging assembly are also provided. A deflecting element is positioned in the pupil of the condensing optics and extending over a first part of the pupil area but not over a second part of the pupil area. Using the imaging assembly, an image is captured of the workpiece including a first replica of the pattern projected through the first part of the pupil area and a second replica of the pattern projected through the second part of the pupil area. The captured image is processed so as to measure a discrepancy between the first and second replicas of the pattern. A distance between the inspection optics and the workpiece is assessed responsively to the measured discrepancy.

There is further provided, in accordance with an embodiment of the invention, a method for optical inspection, which includes providing inspection optics, including an illumination assembly, and including a reticle containing a predefined pattern and a radiation source configured to emit optical radiation, which illuminates the reticle. Condensing optics are configured to project the emitted optical radiation including the pattern of the illuminated reticle onto a workpiece. An imaging assembly, including an image sensor and objective optics, has a pupil with a given pupil area. A deflecting element is positioned in the pupil of the objective optics and extending over a first part of the pupil area but not over a second part of the pupil area. Using the imaging assembly, an image is captured of the workpiece including a first replica of the pattern imaged through the first part of the pupil area and a second replica of the pattern imaged through the second part of the pupil area. The captured image is processed so as to measure a discrepancy between the first and second replicas of the pattern. A distance between the inspection optics and the workpiece is assessed responsively to the measured discrepancy.

The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic side view of an optical inspection apparatus, in accordance with an embodiment of the invention;

Figs. 2a and 2b are partial schematic side views of the optical inspection apparatus of Fig. 1, in accordance with an embodiment of the invention; Figs. 3a and 3b are schematic frontal views of a pattern of a reticle and its replicas on a workpiece, respectively, in accordance with an embodiment of the invention;

Figs. 4a and 4b are schematic frontal views of another reticle and the replicas of its multiple sub-patterns on a workpiece, respectively, in accordance with an alternative embodiment of the invention;

Fig. 4c is a schematic frontal view of a reticle containing multiple sub-patterns, in accordance with another alternative embodiment of the invention;

Figs. 5 and 6 are a schematic side views of two optical inspection apparatuses, in accordance with alternative embodiments of the invention;

Fig. 7 is a schematic side view of an optical inspection apparatus, in accordance with an alternative embodiment of the invention; and

Figs. 8 and 9a-9b are schematic side views of an optical inspection apparatus, in accordance with a further embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

OVERVIEW

In applications of optical imaging systems, for example for optical inspection of workpieces during a manufacturing process, an illuminator is used to illuminate a field on the workpiece with optical radiation. (The terms “optical radiation,” “radiation,” and “light” as used in the present description and in the claims refer generally to any and ah of visible, infrared, and ultraviolet radiation.) The illuminated field of the workpiece is imaged by imaging optics and detected by an image sensor.

In some applications, such as inspection of flat panel displays, semiconductor wafers and printed circuit boards, high-quality inspection requires that the field of the workpiece that is imaged onto the image sensor be brought to an accurate focus of the imaging optics.

The embodiments of the present invention that are described herein provide a focus sensor that accurately senses a deviation of the workpiece from the focus of the imaging optics. 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 optics so as to focus the workpiece with respect to the optics. The focus sensor utilizes optical radiation transmitted through the imaging optics, thus ensuring that the focal position of the workpiece is measured at the same field position as is imaged onto the image sensor. In order to accomplish this task, the focus sensor illuminates a patterned reticle and projects the pattern through condensing optics. The pupil of the inspection optics is relayed to the condensing optics. In some embodiments, the pupil area of the condensing optics is divided into two apertures, extending over different parts of the pupil area, thus dividing 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, with each beam generating a replica of the reticle pattern on the workpiece. A deflecting element, disposed in at least one of the apertures, causes the two replicas to separate laterally on the workpiece. When the workpiece is at an exact focus with respect to the imaging assembly, the two replicas are in mutual alignment, whereas for a defocused workpiece the replicas are shifted laterally with respect to each other. The deflecting 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 deflecting elements with opposing deflection directions are disposed respectively in the two apertures.

The two replicas of the reticle pattern on the workpiece are imaged by the imaging optics onto the image sensor. A processor compares the two side-by-side images of the replicas captured by the image sensor, detects the defocus of the workpiece, and commands a motion system to move the workpiece to the correct focal position. By locating the two pupil apertures symmetrically with respect to the optical axis of the condensing optics, the two replicas of the reticle pattern shift by equal amounts but in opposite directions in response to defocus, thus enabling a high-sensitivity differential detection of the defocus.

In these embodiments, an optical inspection apparatus includes inspection optics, comprising an illumination assembly, condensing optics, a deflecting element, and an imaging assembly. The illumination assembly comprises a reticle containing a predefined pattern and a radiation source, which emits optical radiation and thus illuminates the reticle. The condensing optics project the emitted optical radiation including the pattern of the illuminated reticle onto a workpiece. The deflecting element is positioned in the pupil of the condensing optics and extends over a first part of the pupil area but not over a second part of the pupil area. The imaging assembly captures an image of the workpiece including a first replica of the pattern projected through the first part of the pupil area and a second replica of the pattern projected through the second part of the pupil area. A processor processes the captured image so as to measure the discrepancy between the first and second replicas of the pattern and to assess the distance between the inspection optics and the workpiece responsively to the measured discrepancy. In alternative embodiments, the split pupil area and deflecting element are located in the objective optics of an imaging assembly, rather than in the illumination assembly. In these embodiments, the condensing optics project a single reticle pattern onto the workpiece, and the deflecting element creates two different replicas 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.

In some embodiments, the deflecting element or elements comprise transparent optical wedges. Alternatively, other sorts of deflecting elements may be used, such as diffractive optical elements or tilted mirrors. Suitable combinations of such deflecting elements, as well as other modes of deflection, will be apparent to those skilled in the art after reading the present description and are considered to be within the scope of the present invention.

REPLICA SEPARATION BY CONDENSING OPTICS

Fig. 1 is a schematic side view of an optical inspection apparatus 10 for inspection of a workpiece 12, in accordance with an embodiment of the invention. Apparatus 10 comprises inspection optics 14, which comprise an illumination assembly 16, condensing optics 18, and an imaging assembly 20. Apparatus 10 further comprises a motion assembly 22 and a processor 24. Cartesian coordinates 26 are used in this figure, as well as in Figs. 2a-2b, 3a-3b, 4a-4b, and 5, for defining the orientations of the pictured items.

Motion assembly 22 comprises, for example, linear mechanical stages that can, under the control of processor 24, move workpiece 12 in the x-, y-, and z-directions of Cartesian coordinates 26, as well as a rotational stage that can rotate the workpiece around the z-axis. In the pictured embodiments, motion assembly 22 moves workpiece 12 with respect to a stationary inspection optics 14. Alternatively, motion assembly 22 may move inspection optics 14 (together with the other optical assemblies) with respect to a stationary workpiece 12. Adjusting the separation of workpiece 12 and inspection optics 14 in the z-direction is applied for the purpose of height measurement and focus adjustment, whereas movement in the x- and y-directions and rotation around the z-axis are used for bringing a desired area of workpiece 12 into the field-of-view of inspection optics 14.

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

Condensing optics 18 comprise a collector lens 38, a pupil relay 40, a beamsplitter 42, and an objective lens 44 with a focal plane 45. Condensing optics 18 further comprise an aperture assembly 48 and first and second transparent optical wedges 50 and 52, respectively, located in respective parts of the area of the pupil of condensing optics 18, in a plane 46. Plane 46 is relayed by pupil relay 40 to a pupil plane 47 of objective lens 44, and planes 46 and 47 are thus conjugate to one another. Condensing optics 18 project the optical radiation emitted by reticle radiation source 28, including the pattern of the illuminated reticle 34 onto workpiece 12. A subset of condensing optics 18 projects the optical radiation emitted by field radiation source 30 onto workpiece 12. Aperture assembly 48 and optical wedges 50 and 52 will be described in further detail in Figs. 2a-2b hereinbelow.

Imaging assembly 20 comprises beamsplitter 42 and objective lens 44 as shared elements with condensing optics 18. Imaging assembly 20 further comprises a tube lens 54 and an image sensor 56. Image sensor 56 comprises a two-dimensional pixelated image sensor, for example a CMOS (Complementary Metal-Oxide Semiconductor) image sensor. Imaging assembly 20 thus captures images of workpiece 12, including respective replicas of the reticle pattern that are projected through wedges 50 and 52.

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 in software and/or firmware to carry out the functions that are described herein. Although processor 24 is shown in the figures, for the sake of simplicity, as a single, monolithic functional block, 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 that are illustrated in the figures and are described in the text.

Two functions of inspection apparatus 10 are described hereinbelow: bringing the workpiece to an accurate focus with respect to objective lens 44, and imaging workpiece 12 onto image sensor 56.

For bringing workpiece 12 to an accurate focus, processor 24 turns on reticle radiation source 28, while ascertaining that field radiation source 30 is turned off. As will be described in further detail with reference to Figs. 2a-2b and 3a-3b hereinbelow, a pattern 105 of reticle 34 is projected to create two side-by-side replicas on workpiece 12, so that a point 59 on the reticle is imaged to points 60 and 62 within a field 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 computes from the two captured images a deviation 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 so as to correct for the deviation.

To capture an image of workpiece 12 for the purpose of inspection, processor 24 turns on field radiation source 30, while ascertaining that reticle radiation source 28 is turned off, thus multiplexing in time the inspection and focusing functions. The optical radiation emitted by radiation source 30 is projected by field illumination lens 35, reflected by beamsplitter 36, and projected through a subset of condensing optics 18 to illuminate field 58 on workpiece 12. The combination of field illumination lens 35 and a subset of condensing optics 18 is configured to project uniform illumination onto field 58. Field 58 is then imaged by objective lens 44, beamsplitter 42, and tube lens 54 onto image sensor 56. For the sake of clarity, the optical rays emitted by field radiation source 30 have been omitted.

Figs. 2a-2b are partial schematic side views of optical inspection apparatus 10, in accordance with an embodiment of the invention. The view of Fig. 2b has been rotated by 90° around an optical axis 68 with respect to Fig. 2a. As the purpose of these figures is to describe the focusing function of inspection apparatus 10, components not essential for focusing have been omitted for the sake of clarity. The remaining components have been labelled as in Fig. 1.

The position and orientation of first and second optical wedges 50 and 52 are seen from two orthogonal directions (as shown by respectively oriented Cartesian coordinates 26) in Figs. 2a and 2b, as well as in a cross-sectional view in an insert 74. Wedge directions 70 and 72, respectively, are defined for wedges 50 and 52 as vectors oriented from the base of the respective wedge to its edge. For further clarification, wedge direction 70 for first wedge 50 is shown in a perspective view of the first wedge in an insert 76: Wedge direction 70 is perpendicular to a base 78 of first wedge 50, and points toward an edge 80 of the first wedge. First and second optical wedges 50 and 52 are positioned over respective first and second apertures 90 and 92 of aperture assembly 48, extending over different, respective parts of the pupil area of condensing optics 18.

For purposes of the focusing function, optical rays are emitted by reticle radiation source 28 toward reticle collimator lens 32. Reticle collimator lens 32 projects the optical radiation onto reticle 34, and from there to beamsplitter 36, collector lens 38, and to optical wedges 50 and 52. Referring to Fig. 2a, those optical rays from point 59 that impinge on first wedge 50 and first aperture 90 are refracted by the first wedge in the yz -plane towards the positive y-axis as rays 94, and are imaged by pupil relay 40 and objective lens 44 to point 60. Those rays from point 59 that impinge on second wedge 52 and second aperture 92 are refracted by the second wedge similarly in the yz-plane, but now toward the negative y-axis as rays 96, and are imaged to point 62.

These opposite actions of first and second wedges 50 and 52 split the replica of pattern 105 of reticle 34 on workpiece 12 into two replicas (labelled as respective first and second replicas 108 and 110 in Fig. 3b hereinbelow). In the xz-plane (Fig. 2b), wedges 50 and 52 behave as plane- parallel plates of glass, with identical refractive properties. Flowever, the opposite lateral offsets of apertures 90 and 92 from optical axis 68 cause the optical ray bundles through the respective apertures to impinge on workpiece 12 at equal but opposite oblique angles, as shown by respective rays 98 and 100 and angles a and b. Thus, the replica of pattern 105 formed at point 60 is formed by oblique rays impinging on workpiece 12 at the angle a, and the replica formed at point 62 is formed by oblique rays impinging on the workpiece at the angle b, which has an opposite sign to that of angle a. The opposite signs of angles a and b cause the two replicas at points 60 and 62, respectively, to shift in opposite x-directions when workpiece 12 is moved in the z-direction (in or out of focal plane 45 of objective lens 44). An example of the image shift for workpiece 12 moving by a distance Az to a plane 102 is shown in Fig. 3b hereinbelow.

Figs. 3a and 3b are schematic frontal views (viewed in the z-direction) of 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 invention. Reticle 34 comprises pattern 105 of opaque lines 104 on a transparent substrate 106. (Alternatively, transparent lines on an opaque substrate may be used.)

In Fig. 3b, replicas 108 and 110 are separated from each other in the y-direction. Replica 108 is formed, with reference to Fig. 2a, around point 60, and replica 110 is formed around point 62. For workpiece 12 at focal plane 45 of objective lens 44, lines 104 image to lines 112 in first replica 108 and to lines 114 in second replica 110, with lines 112 and 114 aligned with each other in the x-direction. When workpiece 12 is defocused by Az to plane 102 (with reference to Figs. 2a-2b), lines 112 shift in the positive x-direction, shown as dotted lines 112’, and lines 114 shift in the negative x-direction, shown as dotted lines 114’, with the relative shift Ax between respective lines 112’ and 114’ proportional to Az. Thus, the relative shift of the two replicas 108 and 110 takes place in a direction orthogonal to the direction of their separation. The described embodiment refers to a system that is aligned perfectly in the sense that when workpiece 12 is at focal plane 45, lines 112 and 114 are aligned with each other. However, even in a misaligned system, in which lines 112 and 114 are not aligned with each other when workpiece 12 is at focal plane 45, the defocus Dz may be inferred from the relative shift of the lines.

As shown in Fig. 1, replicas 108 and 110 are imaged onto image sensor 56 and processed by processor 24 to assess the distance between inspection optics 14 and workpiece 12. For this purpose, processor 24 measures from the captured images a relative discrepancy Dc between replicas 108 and 110, computes from Ax the deviation Az 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 so as to correct for the deviation.

In the pictured embodiment, apertures 90 and 92 and associated wedges 50 and 52 are laterally offset in a symmetrical fashion with respect to optical axis 68. Alternatively, they may be positioned in an asymmetrical fashion. For example, first aperture 90 and first wedge 50 may be centered on optical axis 68, while second aperture 92 and second wedge 52 are laterally offset from the axis. In such an arrangement, first replica 108 is stationary with respect to defocus, while second replica 110 shifts with defocus. As another alternative, a wedge may be used in one part of the pupil area to cause one of the replicas of the reticle to shift, with a flat, transparent optical element (or no optical element) in the other part of the pupil area. In the pictured embodiment, pattern 105 is depicted as comprising multiple parallel lines 104. Alternatively, the pattern may comprise sets of parallel lines with fixed or varying spatial periods and in varying orientations, as well as patterns with different pattern characteristics, as will be further detailed hereinbelow.

Figs. 4a and 4b are schematic frontal views of a reticle 120 containing multiple sub-patterns 122 and of the replicas of sub-patterns 122 on workpiece 12, respectively, in accordance with an alternative embodiment of the invention. Each sub-pattern 122 is similar to pattern 105 in Fig. 3a.

Reticle 120 is positioned in optical inspection apparatus 10 in the same location as reticle 34 in Figs. 1 and 2a-2b. Sub-patterns 122 are projected, similarly to Fig. 3b hereinabove, to form first and second replicas 124 and 126 on workpiece 12. By designing sub-patterns 122 to be of equal width W and to be separated by the same width W, and by a suitable choice of the optical properties of first and second wedges 50 and 52, replicas 124 and 126 are interlaced on workpiece 12. Similarly to Fig. 3b, the relative shift Dc between replicas 124 and 126 is proportional to the deviation Dz of workpiece 12 from the focus of objective lens 44.

Additionally or alternatively, the relative shift between replicas 124 and 126 may be sensed locally by processor 24, as shown by shift Dc’ in an area 128. Processor 24 may measure the local topography of workpiece by comparing the relative image shifts in multiple areas. Fig. 4c is a schematic frontal view of a reticle 130 containing multiple sub-patterns 131, 132, 133, 134, 135, and 136, in accordance with another alternative embodiment of the invention.

While a full-field image (an image comprising the full field of view (FOV) of inspection optics 14 of apparatus 10) is captured for inspecting workpiece 12, an image of a region of interest (ROI), i.e., a partial FOV, may be captured for the purpose of assessing the deviation Dz of the workpiece from the focus of objective lens 44. Thus limiting the ROI to the vicinity of one of sub patterns 131, 132, 133, 134, 135, or 136, the specific qualities of that sub-pattern may be beneficially utilized separately or in combination with other sub-patterns. For example, the large period of sub-pattern 132 enables a large measurement range of Dz with a coarse measurement accuracy, to be used in a “search and converge” mode, and the small period of sub-pattern 131 enables a small measurement range with a fine measurement accuracy for tracking of the topography, with a combination of large and small periods enabling both functionalities. Furthermore, varying orientations or shapes of the reticle features, such as in sub-patterns 133, 134, and 135, may be used for avoiding projecting a reticle pattern on a similar pattern of the workpiece, and thus preventing pattern aliasing. A combination of two sub-patterns, such as 131 and 136, may be utilized while scanning workpiece 12 in a back-and-forth fashion, so that sub pattern 131 is utilized for one scan direction, and sub-pattern 136 is utilized for the opposite scan direction.

Similarly to sub-patterns 131, 132, 133, 134, 135, and 136 within reticle 130, other sub patterns with various characteristics may alternatively be used. These characteristics may include different resolutions, different shapes, and different orientations of pattern features. Additionally or alternatively, the pattern features may include regular, random, and pseudo-random patterns, as well as their combinations. In general, different patterns within the respective reticle may be placed in different areas of the FOV of inspection optics 14. This allows measuring and correcting the 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.

Figs. 5 and 6 are a schematic side views of optical inspection apparatuses 140 and 180, respectively, in accordance with alternative embodiments of the invention. Apparatuses 140 and 180 are similar to apparatus 10 (Fig. 1), except for different respective implementations for the splitting of reticle images. In apparatuses 140 and 180, only those parts that are different from the corresponding parts in apparatus 10 are labeled differently than in Fig. 1. With reference to Fig. 5, in apparatus 140 wedges 50 and 52 of apparatus 10 have been replaced by a diffractive optical element (DOE) 142, with all other components of apparatus 140 being the same as in apparatus 10. DOE 142 comprises two separate sub-gratings: a sub-grating 144 and a sub-grating 146 having respective deflection directions shown by arrows 145 and 147. DOE 142 is shown in an insert 148 in a cross-sectional view, as viewed from the positive z-axis against aperture assembly 48. Sub-gratings 144 and 146 function as deflecting elements due to their specific diffractive properties, deflecting the optical rays toward respective points 60 and 62, with the directions of deflection indicated by arrows 145 and 147.

With reference to Fig. 6, in apparatus 180 wedges 50 and 52 of apparatus 10 have been replaced by mirrors 182 and 184, positioned in plane 46 at or close to pupil area of condensing optics 18. Mirrors 182 and 184 have been tilted in opposite directions around the x-axis so as to have respective deflection directions shown by arrows 183 and 185. Further, differing from apparatus 10, apparatus 180 does not comprise aperture assembly 48, as mirrors 182 and 184 also serve the respective functions of first and second apertures 90 and 92. Mirrors 182 and 184 are shown in a cross-sectional view in an insert 188, as viewed from the positive z-axis from plane 46. Mirrors 182 and 184 function as deflective elements based on their respective tilt angles, deflecting the optical rays toward respective points 60 and 62, as indicated by arrows 183 and 185.

Fig. 7 is a schematic side view of an optical inspection apparatus 200 inspecting workpiece 12, in accordance with an alternative embodiment of the invention.

Optical inspection apparatus 200 is similar to apparatus 10 in Fig. 1, with similar components labelled with the same labels. The difference between the two apparatuses is that a condensing assembly 218 in apparatus 200 has been modified, as compared to condensing assembly 18, by removing pupil relay 40. In this case, aperture assembly 48 and wedges 50 and 52 are located directly at or near pupil plane 47 of objective lens 44.

First and second optical wedges 50 and 52, as well as aperture assembly 48 are similar to those of apparatus 10. Consequently, apparatus 200, similarly to apparatus 10, images point 59 of reticle 34 onto points 260 and 262 on workpiece 12, and these points are further imaged onto respective points 264 and 266 on image sensor 56. As in Fig. 3b, first and second replicas 108 and 110 in area 258 are laterally offset from each other and shifted due to a focal shift of workpiece 12.

REPLICA SEPARATION BY IMAGING ASSEMBLY

Figs. 8 and 9a-9b are schematic side views of an optical inspection apparatus 310 for inspection of a workpiece 312, in accordance with a further embodiment of the invention. Apparatus 310 comprises inspection optics 314, which comprise an illumination assembly 316, condensing optics 318, and an imaging assembly 319. Imaging assembly 319 comprises a reticle imaging subassembly 320 and a field imaging subassembly 321. Apparatus 310 further comprises a motion assembly 322 and a processor 324. Cartesian coordinates 326 are used in this figure and in Figs. 9a-9b for defining the orientation of the pictured items. Motion assembly 322 comprises similarly to motion assembly 22 Of Fig. 1 , for example, linear mechanical stages that can, under the control of processor 324, move workpiece 312 in the x-, y-, and z-directions of Cartesian coordinates 326, as well as a rotational stage that can rotate the workpiece around the z- axis.

Illumination assembly 316 comprises, similarly to illumination assembly 16 of Fig. 1, 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 beamsplitter 336. Reticle 334 comprises, similarly to reticles 34 and 120, either a single pattern or multiple sub-patterns (for example as shown in Fig. 4a), which are illuminated by reticle radiation source 328. As in the preceding embodiments, the reticle pattern may be periodic, quasi-periodic, or random.

Condensing optics 318 comprise a collimator lens 338, beamsplitters 341 and 342, and an objective lens 344 with a focal plane 343. Condensing optics 318 project the optical radiation emitted by reticle radiation source 328, including the pattern of the illuminated reticle 334, onto workpiece 312.

Reticle imaging subassembly 320 comprises objective lens 344 and beamsplitters 341 and 342 as shared elements with condensing optics 318. Additionally, reticle imaging subassembly 320 comprises a pupil relay 340, a reticle imaging lens 345, and a reticle image sensor 347. Furthermore, reticle imaging subassembly 320 comprises an aperture assembly 348 and first and second optical wedges 350 and 352, located in a plane 346 conjugate to the pupil plane of objective lens 344 (pupil plane not shown for the sake of clarity). Wedges 350 and 352 are thus positioned in the relayed pupil of objective lens 344 and extend over separate, respective parts of the pupil area. Further details of aperture assembly 348 and optical wedges 350 and 352 are shown in Figs. 9a-9b.

Field imaging subassembly 321 comprises objective lens 344 and beamsplitter 342 as shared elements with condensing optics 318 and reticle imaging assembly 320. Additionally, field imaging assembly 321 comprises a tube lens 354 and a field image sensor 356.

Image sensors 356 and 347 comprise two-dimensional pixelated image sensors, for example CMOS (Complementary Metal-Oxide Semiconductor) image sensors. Processor 324, similar to processor 24 hereinabove, is coupled to motion assembly 322, to light sources 328 and 330, and to image sensors 356 and 347.

For bringing workpiece 312 to an accurate focus, processor 324 turns on reticle radiation source 328 (and ascertains that field radiation source 330 is turned off). The pattern of reticle 334 is projected by condensing optics 318 to form a single replica on a field 358 on workpiece 312, with a point 359 on the reticle projected onto a point 361 on the workpiece. This single replica of the pattern of reticle 334 on workpiece 312 is imaged along a first optical axis 368 by reticle imaging optics 321 through wedges 350 and 352 to form two side-by-side replicas of the reticle pattern on reticle image sensor 347, so that point 361 on the workpiece is imaged to points 360 and 362. Processor 324 processes the image captured by image sensor 347 to measure the discrepancy between the two replicas of the pattern, and thus to assess the distance between the inspection optics and workpiece 312. On this basis, processor 324 drives motion assembly 322 to move the workpiece in the z-direction of Cartesian coordinates 326 so as to correct for the deviation.

For capturing images of structures on workpiece 312 for the purpose of inspection, processor 324 turns on field radiation source 330 (and ascertains that reticle radiation source 328 is turned off). The optical radiation emitted by light source 330 is projected by field illumination lens 335, reflected by beamsplitter 336, and projected through condensing optics 318 to illuminate field 358 on workpiece 312. The combination of field illumination lens 335 and condensing optics 318 is configured to project a uniform illumination on field 358. Field 358 is subsequently imaged along a second optical axis 369 by objective lens 344, beamsplitter 342, and tube lens 354 to field image sensor 356. For the sake of clarity, the optical rays emitted by field radiation source 330 have been omitted.

Similarly to the time-multiplexing of focusing and imaging in optical inspection apparatus 10 (Fig. 1), turning on reticle radiation source 328 and field radiation source 330 in alternation time-multiplexes the focusing and imaging of workpiece 312 for apparatus 310. Alternatively, reticle and field radiation sources 328 and 330, respectively, may be spectrally multiplexed by having the two radiation sources radiate at separate wavelengths and by using suitably configured dichroic beamsplitters and/or mirrors.

Figs. 9a-9b are partial schematic side views of optical inspection apparatus 310. The view of Fig. 9b has been rotated by 90° around first optical axis 368 with respect to Fig. 9a. As the purpose of these figures is to describe the focusing function of inspection apparatus 310, components not essential to focusing have been omitted for the sake of clarity. The remaining components have been labelled as in Fig. 8.

Aperture assembly 348 comprises first and second apertures 390 and 392, extending over different, respective parts of the pupil area of objective optics 344. First and second wedges 350 and 352 are positioned respectively over apertures 390 and 392, as shown in a cross-sectional view in an insert 374. Wedges 350 and 352 are oriented with opposing respective wedge directions 351 and 353. With reference to Fig. 9a, optical rays from point 361 that pass through first wedge 350 are refracted into the negative y-direction and projected to point 360, whereas the rays that pass through second wedge 352 refract into the positive y-direction and to point 362, thus splitting the replica of the pattern of reticle 334 on field 358 into two side-by-side replicas 408 and 410, respectively, shown in an insert 420.

With reference to Fig. 9b, optical rays from point 361 passing through first aperture 390 impinge on reticle image sensor 347 at an angle a’ , and the rays passing through second aperture 392 impinge on the reticle image sensor at an angle b’, which is of the same magnitude as but with opposite sign to angle a’ . As in the preceding embodiments, the opposite signs of angles a’ and b’ causes replicas 408 and 410 to shift in opposite x-directions in response to a deviation by Az of workpiece 312 from focal plane 343 of objective lens 344 to a plane 402.

Processor 324 processes the images of replicas 408 and 410 in order to measure the discrepancy Ax between the replicas, computes from Ax the deviation Az, and drives motion assembly 322 to move the workpiece in the z-direction of Cartesian coordinates 326 so as to correct for the deviation.

One advantage of having the replicas of the pattern split in imaging assembly 319 (rather than in condensing optics 18 as in Fig. 1) is that replicas 408 and 410 are derived from a single replica on workpiece 312. This, in turn, yields the defocus information from a single smaller area rather than by comparing two replicas that are located side-by-side on the workpiece.

In the embodiments shown in Figs. 8 and 9a-9b, wedges 350 and 352 may be replaced by other sorts of deflecting elements, such as a DOE or mirrors, similarly to those shown in Figs. 5 and 6 and described hereinabove.

It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.