Tilted uniform illumination for imaging systems

Abstract

Methods and systems are provided for generating an image of a sample. A system includes an axially diffractive optical element (DOE) positioned in a light beam generated by a light source. The light beam is offset to the center of the axial DOE. The axial DOE includes a concentric grating ring configured to split the light beam into a plurality of light beams. In addition, the system includes focusing optics configured to focus the plurality of light beams to a plurality of spots on a sample, respectively. A common focal plane of the plurality of light spots is at an inclined angle with respect to an optical axis of the focusing optics. The system also includes an imaging detector configured to generate an image of the sample by detecting light from the plurality of light spots.

Description

Technical Field

[0001] The present invention generally relates to methods and systems for oblique uniform illumination of imaging systems. Certain embodiments are directed to providing a substantially uniform flat-top illumination profile via an oblique illumination spot. Background Art

[0002] The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.

[0003] The fabrication of semiconductor devices, such as logic and memory devices, typically involves processing a substrate, such as a semiconductor wafer, using a number of semiconductor fabrication processes to form the various features and multiple layers of the semiconductor device. For example, photolithography is a semiconductor fabrication process that involves transferring a pattern from a mask to a photoresist disposed on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. Multiple semiconductor devices can be fabricated as an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.

[0004] Inspection processes are used at various steps during the semiconductor manufacturing process to detect defects on samples to drive higher yields and, therefore, higher profits in the manufacturing process. Inspection has always been an important part of manufacturing semiconductor devices. However, as the size of semiconductor devices decreases, inspection becomes even more important to successfully manufacturing acceptable semiconductor devices, as even small defects can lead to device failure.

[0005] Especially as the structures on the inspected samples become much smaller and more complex and / or the particles or defects on the samples become significantly smaller, the configuration of the inspection system can significantly affect the capabilities of the tool. Although almost every configurable parameter of the inspection system can have an impact on the inspection capabilities of the system, the main focus addressed herein is on how the illumination configuration and its various parameters can affect inspection performance.

[0006] Some currently used inspection systems are configured to produce a flat-top illumination profile to provide uniform illumination intensity across the illuminated area on the sample. Some of these systems use a wavefront phase shift to transform a Gaussian beam shape into a top-hat beam profile. These methods are quite old, with many commercially available components. There are generally two approaches in this category. The optical element for the wavefront transformation can be either a refractive element or a diffractive element. Refractive elements are typically commercially available aspheric lenses.

[0007] Other currently used systems combine multiple Gaussian beams to produce a flat top. The multiple Gaussian beams must come from a single laser so that they are coherent. To avoid interference between the spots (which reduces uniformity), the spots can be separated in the spatial domain or in the temporal domain. If the difference in optical path length between the spots is greater than the coherence length, then the spots separated in the temporal domain can partially overlap to form a flat top without interference fringes. The typical coherence length of a mode-locked (ML) laser is a few millimeters. Spots separated in the spatial domain rely on the integration of a time delay integral (TDI) scan to merge the spots in one dimension to produce a uniform flat top in an orthogonal direction.

[0008] The primary challenge with time-domain separation methods is how to reassemble the beams. For wafer inspection, due to limitations on the numerical aperture (NA) of the illumination (e.g., which is substantially small) and the size of the illumination field (which is also substantially small compared to the input beam diameter), each of the multiple beams must be nearly collinear through the illumination path and focused at nearly the same location on the wafer. While this concept is simple in theory, implementation is extremely challenging due to stringent stability and alignment tolerance requirements.

[0009] Directly converting a Gaussian beam into a flat-top illumination profile, regardless of the method (refractive or diffractive), has several significant drawbacks. The primary drawback is that the output flat-top is sensitive to input beam quality and alignment. Relatively small changes in the input beam wavefront, beam size, or misalignment can significantly degrade the output uniformity.

[0010] Wafer inspection systems employ oblique illumination for optimal defect detection sensitivity. The angle of oblique illumination is defined by two parameters: polar angle and azimuth angle. These are defined by the coordinate system of the wafer surface (xy plane) and the scan direction (x-axis). The wafer surface normal is the z-axis. For optimal particle detection sensitivity, the polar angle of illumination is between 60 and 80 degrees. At these large polar angles, the projected beam size stretches by a factor of 1 / cos(polar angle), which can be between 2x and 6x. When the stretch is in the scan direction (90 degrees azimuth), a larger TDI sensor size is required to cover the extended illumination field. Multiple azimuth illumination angles reduce speckle noise, thereby improving defect sensitivity. For dual azimuth illumination, + / -45-degree oblique illumination angles are more advantageous than the currently used 0 / 90 azimuth illumination because of the good compromise between symmetrical optics and a steep tilt factor. There is also no proven solution for 45-degree azimuth oblique illumination.

[0011] Therefore, it would be advantageous to develop imaging methods and systems that do not suffer from one or more of the disadvantages described above. Summary of the Invention

[0012] The following description of various embodiments should not be construed in any way as limiting the subject matter of the appended claims.

[0013] One embodiment relates to a system configured for producing an image of a sample. The system includes a light source configured to produce a light beam. The system also includes an axial diffractive optical element (DOE) positioned in the light beam. The light beam is offset to the center of the axial DOE. The axial DOE includes concentric grating rings configured to separate the light beam into a plurality of light beams. In addition, the system includes focusing optics configured to focus the plurality of light beams onto a plurality of light spots on the sample, respectively. A common focal plane of the plurality of light spots is at an oblique angle relative to an optical axis of the focusing optics. The system further includes an imaging detector configured to produce an image of the sample by detecting light from the plurality of light spots. The system can be further configured as described herein.

[0014] Another embodiment relates to a method for generating an image of a sample. The method includes splitting a light beam into a plurality of light beams, wherein an axial direction of focus (DOE) is positioned in the light beams. The light beams are offset to the center of the axial direction of focus (DOE). The axial direction of focus (DOE) includes concentric grating rings configured to split the light beam into the plurality of light beams. The method also includes focusing the plurality of light beams onto a plurality of light spots on the sample. A common focal plane of the plurality of light spots is at an oblique angle relative to the optical axes of the plurality of light beams. The method also includes generating an image of the sample by detecting light from the plurality of light spots.

[0015] The steps of the method may be implemented as further described herein. In addition, the method may include any other steps of any other method described herein. The method may be performed by any of the systems described herein.

[0016] Additional embodiments relate to a system configured for inspecting a sample. The system includes the light source, axial DOE, focusing optics, and imaging detector described above. The system also includes a computer subsystem configured to detect defects on the sample based on the image. The system can be configured as further described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings, in which:

[0018] Figure 1 is a schematic diagram illustrating an example of a previously used diffractive optical element (DOE) and a side view of an embodiment of an axial DOE illustrating the principle of using an axial grating beam splitter to generate an array of light spots at oblique angles;

[0019] Figure 2 is a schematic diagram illustrating a plan view of an embodiment of design parameters for producing a uniform top-hat lighting profile using multiple light points and time delay integration (TDI);

[0020] Figure 3 is a schematic diagram illustrating a plan view of an embodiment of a light spot array layout of 9 light spots;

[0021] Figure 4 It is explained from Figure 3 Plot of the effective intensity profile of the TDI integration for the 9 spots in the layout shown;

[0022] Figure 5 is a schematic diagram illustrating a side view of design parameters of an embodiment of an axial DOE and a focusing lens for generating an array of tilted light spots;

[0023] Figure 6a is a schematic diagram illustrating a top view (from the sample surface) of an embodiment of an illumination beam path;

[0024] Figure 6b It is an explanation Figure 6a A schematic diagram of a side view of an illumination beam path;

[0025] Figure 7 is a schematic diagram illustrating an embodiment of defining the angle of the incident plane, the angle of the light spot array, and the illumination incident angle;

[0026] Figure 8 is a schematic diagram illustrating a side view of one embodiment of elements for transforming and delivering a single laser beam to an array of tilted spots on a sample surface;

[0027] Figure 9 is a schematic diagram of a plan view illustrating an example of the definition of an illumination azimuth angle;

[0028] Figure 10 is a schematic diagram illustrating a side view of one embodiment of a system configured for generating an image of a sample and / or for inspecting a sample; and

[0029] Figure 11 is a block diagram illustrating one embodiment of a non-transitory computer-readable medium storing program instructions executable on a computer system for performing one or more of the computer-implemented methods described herein.

[0030] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the drawings and detailed description thereof are not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. DETAILED DESCRIPTION

[0031] Turning now to the drawings, it should be noted that the figures are not drawn to scale. In particular, the proportions of some elements of the figures are greatly exaggerated to emphasize the characteristics of the elements. It should also be noted that the figures are not drawn to the same scale. The same reference numerals have been used to indicate elements shown in more than one figure that may be similarly configured. Unless otherwise specified herein, any elements described and shown may include any suitable commercially available components.

[0032] Embodiments described herein generally relate to methods and systems for generating images of samples. The embodiments described herein are particularly advantageous for providing tilted, uniform illumination for imaging systems. Additionally, the embodiments described herein provide tilted, uniform illumination for laser scanning, time-delay integration (TDI) imaging systems. For example, the embodiments described herein advantageously provide methods and systems for generating flat-top illumination for tilted laser dark-field (DF) wafer inspection. (The terms "flat top" and "top-hat" are used interchangeably herein to collectively refer to an intensity profile having a generally step-function-type shape, as opposed to other intensity profile shapes or functions, such as Gaussian.) However, the illumination configurations described herein may be incorporated into any suitable imaging system known in the art.

[0033] Figure 1 The basic operating principles of the embodiments described herein are shown. Two types of beam-splitting diffractive optical elements (DOEs) are compared to demonstrate how to generate a one-dimensional (1D) array of light spots with a focal plane at an oblique (i.e., non-perpendicular) angle relative to the optical axis. More specifically, this figure shows the principle of using an axial grating beam splitter to generate an array of light spots at an oblique angle. The terms "axial DOE," "axial grating," "axial grating beam splitter," and "off-axis DOE" are used interchangeably herein.

[0034] The currently used grating beam splitter 104 splits the collimated input beam 100 into multiple collimated beams 110 (three beams are shown here), and the focusing lens 108 focuses the three beams into three spots. The common focal plane 112 of the focused spots is perpendicular to the collimated input beam 100 and the optical axis 102 of the focusing lens 108. As shown in the cross-sectional view 106 of the grating beam splitter 104, the beam splitter may include parallel grating lines that separate the light beam into multiple beams.

[0035] In the embodiments described herein, the beam is split by an axial DOE. The system includes an axial DOE 118 positioned in the beam 114. In one embodiment, the axial DOE is configured to function as a combination of a diffraction beam splitter and a Fresnel lens, with weakly focused power for the diffraction orders. In other words, the axial DOE functions similarly to a Fresnel lens, but with substantially weakly focused power for the diffraction orders. For example, the axial grating 118 (which is configured to perform a combination of the functions of a beam splitter grating and a Fresnel lens) splits the collimated beam 114 into a plurality of beams 124 (three beams are shown, but the beam can be split into any suitable number of beams).

[0036] As shown in cross-sectional view 120 of axial grating 118, the axial DOE comprises concentric grating rings configured to split a beam into multiple beams. The layout of the concentric rings can be identical to a conventional Fresnel zone plate (FZP). However, the grating profile is designed to produce multiple uniform diffraction orders, rather than just one diffraction order as in a conventional FZP. Thus, this DOE combines the functionality of an FZP and a beam splitter. The axial DOE has the same grating groove layout of an FZP and the same grating groove profile of a diffraction beam splitter. Like the focused power of an FZP, the axial DOE also produces focused power for multiple diffraction orders. The focused power is proportional to the diffraction order, so positive orders converge and negative orders diverge. The axial DOE is designed so that the focused power for all orders is substantially small, with focal lengths of many meters, which is how "substantially weak focused power" can be defined in the embodiments described herein. In one embodiment, at least two of the multiple beams have different powers. For example, each split beam has a slightly different power, and when focused by a focusing lens as described herein, the beams form an array of spots having a common focal plane at an oblique angle to the incident beam.

[0037] like Figure 1 As shown, beam 114 is offset from the center of the axial DOE. For example, axis 116, which passes through the center of the axial DOE, is offset from beam 114. This offset is further illustrated in cross-sectional view 120, where the position of beam cross section 120a is offset from the center of the axial DOE (where the center of the DOE is defined by the center of curvature of the DOE grating grooves). In this way, the axial DOE is used in an off-axis position, i.e., the laser beam only uses the portion of the DOE away from its optical axis. In one embodiment, the pitch of the grating rings varies with the radius of the axial DOE. In this way, the DOE includes concentric grating rings, and the pitch of the gratings varies with the radius of the DOE. For example, as shown in cross-sectional view 120 of axial DOE 118, the pitch of the grating rings varies across the radius of the axial DOE. The axial DOE can be further configured as described in U.S. Patent No. 9,945,792 to Zhao, issued on April 17, 2018, which is incorporated herein by reference as if set forth in its entirety.

[0038] Focusing optics ( Figure 1 The focusing lens 122 is shown configured to focus the multiple beams 124 generated by the axial DOE onto the sample ( Figure 1 (not shown in the figure). Figure 1 As shown in Figures 1 and 2 and other figures described herein, the focusing lens can be positioned on-axis, meaning on the same optical axis of the incident light beam and any other illumination optics. The focusing lens and any other focusing optics described herein can have any suitable configuration known in the art.

[0039] The common focal plane of the plurality of light spots is at an oblique angle relative to the optical axis of the focusing optics. In addition, the embodiments described herein can generate an array of light spots at any angle relative to the illumination beam. In other words, the light spot array line can be angled relative to the incident direction, and the light spot line angle can be controlled by decentering the DOE relative to the optical axis of the beam and the illumination optics. For example, when the input beam is as Figure 1 The shown offset is to the center of the axial DOE, and when the beams are focused by a focusing lens, the focus of each beam (the longitudinal position of each spot) is located at a different distance from the focusing lens. Figure 1 As shown, the focus of each beam 124 is positioned at a different distance from the focusing lens 122. Therefore, the common focal plane of all the light points is at an oblique angle relative to the optical axis of the focusing lens. Figure 1 , if a plane is drawn in which all the focal points of the plurality of light beams 124 are located, the common focal plane is at an oblique angle relative to the optical axis of the focusing lens 122. In other words, the common focal plane is non-perpendicular to the optical axis of the focusing lens.

[0040] Thus, in principle, an off-axis axial DOE and a focusing lens can be used to generate a linear array of spots at an oblique angle, which is a first step in generating a flat-top profile from the array of spots. In some embodiments, the plurality of spots have an elliptical shape with the major axis angled relative to the scanning direction of the imaging detector. For example, Figure 2 As shown, the sample ( Figure 2 Each illumination spot 200 on the image sensor (not shown) has an elliptical shape. To form an effectively uniform flat-top illumination profile, the spot shape is elongated and tilted relative to the TDI integration (scanning) direction 204, as shown in FIG. Figure 2 In other words, the major axis of each elliptical spot is not parallel or perpendicular to the scanning direction of the imaging detector. Instead, the spot has an elliptical shape, and the major axis of the elliptical spot has an angle relative to the TDI scanning direction.

[0041] Figure 2Design parameters for producing a uniform top-hat illumination profile using multiple light spots and TDI integration are also shown. In one embodiment, the imaging detector is configured as a TDI camera (also referred to herein simply as TDI and TDI sensor). In one such embodiment, the projection of each of the multiple light spots into the field direction (perpendicular to the scanning direction of the TDI camera) of the TDI camera overlaps with the projection into the field direction of one or more of the multiple light spots adjacent to each of the multiple light spots. In other words, the projection of an elliptical light spot into the TDI field direction (perpendicular to the TDI scanning direction) overlaps with the projection of its respective adjacent light spot into the TDI field direction. For example, in Figure 2 In FIG, the field direction of the TDI camera is perpendicular to the scanning direction 204 of the TDI camera. Each of the plurality of light spots 200 (in this case 5 light spots, but any other number of light spots can be used) is directed to the TDI camera ( FIG. Figure 2 In other words, if multiple light spots are projected from the sample onto the field 202 of the TDI and then projected (i.e., flattened) onto the field of view of the sample, the projection of the light from the adjacent light spots will overlap with the projection of the light from its adjacent light spots. Figure 2 On the shown line H, the light from adjacent light points will then overlap along that line.

[0042] In an embodiment, the multiple light spots on the sample are separated from one another. In this way, each light spot is spatially separated. In other words, the light spots as illuminated on the sample (and therefore in the common focal plane of the array of light spots) are spatially separated from one another. Specifically, no portion of any one light spot on the sample overlaps with any other light spot on the sample. The light spots may be spatially separated to avoid interference between adjacent light spots. However, in one embodiment, the projection distance between each of the multiple light spots and each of one or more of the multiple light spots adjacent to each of the multiple light spots is approximately half the size of the multiple light spots. In other words, the projection distance between adjacent light spots is approximately half the size of the light spots. In this way, although the light spots are spatially separated on the sample, approximately half of the projection of a first light spot may overlap approximately half of the projection of its adjacent light spot. In other words, the distance between the projection centers of two adjacent light spots along the field direction of TDI may be approximately half the size of the multiple light spots in that direction.

[0043] In another embodiment, the imaging detector is configured to integrate the intensity of light from multiple light spots in a scan direction. In additional embodiments, the projections of the multiple light spots overlap to produce a uniform flat-top illumination profile. As used herein, the term "uniform flat-top illumination profile" is generally defined as an illumination profile having substantially the same intensity value (i.e., statistically the same intensity value) across the entire size (or substantially all of the entire size) of the illumination area. For example, TDI integration can sum the energy in the scan direction 204 to effectively form a flat-top intensity profile. Thus, although the light spots are separated in the spatial domain, the integration of the TDI scan can combine the light spots in one dimension to produce a substantially uniform flat-top in an orthogonal direction. In this way, the TDI scan integrates the illumination intensity in the scan direction, and the overlap of the projections of adjacent light spots produces a substantially uniform flat-top illumination profile.

[0044] Each light point is from the long direction (D a ) and short direction (D b ) and the orientation angle α. Flat-top efficiency is defined by the available power within the flat portion of the illumination profile and is roughly given by:

[0045]

[0046] The conditions for forming a uniform flat-top area are as follows:

[0047]

[0048] where F is the field size.

[0049] Figure 3 An embodiment of a light spot array layout of 9 light spots is described. Figure 4 The integrated intensity profile is shown in , which is the sum of the electric field amplitudes of the random phase shifts between the spots. More specifically, Figure 4 Show from Figure 3 The effective intensity profile of the TDI integration for the 9-spot layout shown. Figure 4 As shown, both the amplitude sum plot 400 and the intensity sum plot 402 have approximately flat-top intensity profiles.

[0050] Figure 5 Demonstrate key design parameters and how to determine them. More specifically, Figure 5 The design parameters of the axial DOE and focusing lens for generating an array of tilted light spots are shown. As shown in this figure, the light beam 500 is directed to the axial DOE 502, which is shown in cross-sectional view 502a. The light beam is offset to the center of the axial DOE. For example, Figure 5As shown, the optical axis of the illumination optics 504, on which the light beam is centered, is offset from the optical axis of the DOE 506. In this way, the center of the light beam and the center of the axial DOE are offset from each other. This offset is further shown in the cross-sectional view 502a of the axial DOE 502, on which the cross-sectional view 500a of the light beam is superimposed to show the offset of the light beam from the center of the axial DOE. The axial DOE can be further configured as described herein to separate the light beam into multiple light beams, which are directed to focusing optics including a focusing lens 508. The focusing lens focuses the multiple light beams onto the sample ( Figure 5 The common focal plane 510 of the plurality of light points is relative to (ie, not perpendicular to) the optical axis of the focusing optics (eg, Figure 5 The axis 504 is shown at an inclined angle. Figure 5 The embodiments shown in can be further configured as described herein.

[0051] The spot array layout is defined by two parameters: the spacing between adjacent spots (d) and the tilt angle (β) of the linear spot array relative to the optical axis. These two parameters determine the grating parameters and optical parameters.

[0052] The tilt angle β of the light spot array is determined by the illumination angle,

[0053] cosβ=sinθsinφ

[0054] where θ and φ are the polar and azimuthal angles, respectively. The offset of the axial grating is given by:

[0055] y=f tanβ

[0056] Where f is the focal length of the focusing lens.

[0057] Axial gratings have concentric grating grooves with a pitch that varies linearly with radius. The grating layout can be defined by two parameters: the wavelength (λ) and the focal point (z1) of the first diffraction order. The focal point of the first diffraction order is given by:

[0058]

[0059] The grating pitch as a function of the offset distance is given by:

[0060]

[0061] The grating profile is defined by the number of diffraction orders, and the profile is typically optimized for maximum overall diffraction efficiency and maximum uniformity between the diffraction orders.

[0062] In one embodiment, the plurality of light spots are each focused on the surface of the sample.In another embodiment, the focusing optics are configured to direct the plurality of light beams to the plurality of light spots, respectively, at a polar angle of 75 degrees. Figure 6a (Top view (from the wafer surface) showing the illumination beam path at a 45 degree azimuth angle) and Figure 6b An embodiment of a geometric lens design for oblique illumination at a polar angle of 75 degrees and an azimuth angle of 45 degrees is shown in FIG. Figure 6a As shown, multiple beams generated by the off-axis DOE 600 can be focused by the paraxial lens 602 onto the wafer plane 604 at an azimuth angle 606 of 45 degrees. Figure 6b As shown, multiple light beams generated by off-axis DOE 600 can be focused by paraxial lens 602 onto wafer plane 604 at a 75-degree polar angle 608. A magnified view 610 of focal plane portion 612 shows that all light spots are perfectly focused on the wafer even though the wafer surface is at a 75-degree angle relative to the illumination beam.

[0063] like Figure 7 As shown, in optical design, the direction of the input optical axis is usually defined as the angle (β) between the illumination beam (incident beam 700) and the illumination line (line 702 on the wafer) and the roll angle (ρ) of the incident plane. The relationship between these two angles and the illumination polar angle (θ) and azimuthal angle (φ) is shown in Figure 7 is shown and can be determined by the following equation: Also shown in this figure is the TDI scan direction 704.

[0064] cosβ=sinθsinφ

[0065]

[0066] exist Figure 8 One embodiment of the overall illumination optical design is shown in . The output beam from the laser 800 is asymmetrically expanded in beam size by a beam expansion optic 802, which in this embodiment comprises a focusing lens 804 and two cylindrical lenses 806. The beam size is determined by the laser output beam size and the focal lengths of the three lenses. The beam size in the short direction is magnified by a multiple of the ratio of the first cylindrical lens to the focusing lens, and the beam size in the short direction is magnified by a multiple of the ratio of the second cylindrical lens to the focusing lens. The orientation of the elliptical beam can be controlled by the orientation of the two cylindrical lenses. The expanded beam then passes through an off-axis DOE 808, which has parameters determined by the illumination requirements as discussed previously. Next, a linear array of light spots is formed on the wafer (sample 812) at the desired illumination angle and layout size by a focusing lens 810. In this way, Figure 8The elements shown are configured for transforming and delivering a single laser beam to an array of tilted spots on the wafer surface. Other optical elements can be included in the illumination path to control the illumination properties, such as wave plates and polarizers, and to reduce the footprint and optimize the position of other optical elements, such as folding mirrors.

[0067] Dual illumination azimuth angles provide a 1.4x improvement in defect signal-to-noise ratio for thin film wafers. For example, multiple azimuth angles of oblique illumination can reduce speckle noise from scattering on the wafer surface. A 90-degree difference in azimuth angles provides the greatest speckle noise reduction. Current implementations use 0-degree and 90-degree illumination angles, which require two different sets of optics and uneven performance. However, the illumination optics embodiments described herein can be used for both +45-degree and -45-degree azimuth angle illumination. + / -45-degree illumination simplifies the illumination optics (the optics are the same for both azimuth angles) and provides consistent performance. For example, an array of light spots formed at a 75-degree polar angle and + / -45-degree azimuth angles is perfectly focused on the wafer surface, providing maximum uniformity of the illumination profile and maximum depth of focus.

[0068] Table 1 lists and Figure 9 Explain the advantages and disadvantages of the two lighting methods. Specifically, Figure 9 Examples 900 of 0 and 90 degree illumination angles for simultaneously illuminating an imaging field 904 on a wafer 902 and examples 906 of + / - 45 degree illumination angles for simultaneously illuminating the same imaging field on the wafer are shown.

[0069] Table 1

[0070]

[0071] In one embodiment, the system includes a beam splitting optic configured to split each of the plurality of light beams into first and second portions of each of the plurality of light beams, thereby generating first and second pluralities of light beams, respectively, and a focusing optic configured to focus the first and second pluralities of light beams, respectively, to a plurality of light spots on the sample at different azimuthal angles and the same polar angle. Figure 10 In the illustrated embodiment, the beam shaping optics 1002 and / or element 1004, further described herein, can be configured to split each of the plurality of light beams generated by the beam shaping optics 1002 into first and second portions of each of the plurality of light beams, thereby generating first and second pluralities of light beams. The focusing optics 1006 and 1010 (and possibly elements 1004 and 1008), further described herein, can be configured to focus the first and second pluralities of light beams, respectively, onto a plurality of spots on a sample 1012 at different azimuthal angles and the same polar angle. Each of these elements can be configured as further described herein.

[0072] In one embodiment, the different azimuth angles differ by 90 degrees. For example, as described above, a particular set of favorable azimuth angles is + / - 45 degrees. However, in the embodiments described herein, other azimuth angles are possible, such as the 0 and 90 azimuth angles described above, as well as other azimuth angles that differ by more or less than 90 degrees. Additionally, while it may make the most sense to make the azimuth angles symmetrical about the x or y axis of the sample, this is not required in the embodiments described herein. In general, the azimuth angles used for inspection can be selected based on the characteristics of the sample, the characteristics of the structures formed on the sample (e.g., material or composition, size, aspect ratio, orientation, shape, and the like), the defects or particles of interest thereon and their various characteristics (e.g., the characteristics described above), etc.

[0073] In one embodiment of a multi-azimuth configuration (as well as a single-azimuth configuration), the system includes a set of cylindrical lenses positioned in the beam between the light source and the axial DOE and configured to asymmetrically expand the size of the beam. Figure 8 As shown and described further above, the output beam from laser 800 can be asymmetrically expanded in beam size by beam expansion optics 802, which in this embodiment includes a focusing lens 804 and two cylindrical lenses 806. As described further above, the beam size and orientation of the beam are determined by the laser output beam size and the configuration of the three lenses. The expanded and / or shaped beam then passes through an off-axis DOE 808, which is configured as described further herein. In this way, the system can include a set of cylindrical lenses configured to asymmetrically expand the beam size prior to the beam-splitting DOE, thereby simplifying the illumination optics for multi-azimuth illumination.

[0074] Figure 10 One example of an implementation of a system configured for generating an image of a sample and / or inspecting a sample is shown. This system includes a light source 1000 configured to generate a light beam, which can include a laser or any other suitable light source known in the art. The laser can include any suitable laser known in the art. Furthermore, an advantage of the embodiments described herein is that by using a 1D array of light spots to generate a flat-top illumination intensity profile, there is no time delay in the system, and thus the laser can be a cw laser.

[0075] The light beam from the light source may be directed to the beam shaping optics 1002, which may include any of the elements described herein ( Figure 10 (not shown). For example, the beam shaping optics 1002 may be as Figure 8 In this way, the beam shaping optics may include at least a Figure 8The axial DOE 808 is shown, and the beam is offset to the center of the axial DOE. The axial DOE includes concentric grating rings (e.g., Figure 1 120 ), the concentric grating rings are configured to separate the light beam into multiple light beams. Furthermore, the beam shaping optics 1002 can be configured to separate each of the multiple light beams into different portions of the light beam, each portion being focused onto the sample at a different azimuth angle.

[0076] The system further includes a focusing optical device configured to focus the plurality of light beams onto a plurality of light spots on the sample, respectively, wherein a common focal plane of the plurality of light spots is at an oblique angle relative to an optical axis of the focusing optical device. Figure 10 In the case of a system configured for multi-azimuth illumination, the focusing optics may include elements 1004, 1006, 1008, and 1010. Elements 1004 and 1008 may or may not be part of the focusing optics. These elements may simply be reflective and / or refractive optical elements that direct different portions of the multiple light beams to elements 1008 and 1010, which may be focusing lenses that focus different portions of the multiple light beams to multiple spots on sample 1012. For example, a first portion of each of the multiple light beams may be directed to element 1004, which directs the first portion to focusing optics 1006, and a second portion of each of the multiple light beams may be directed to element 1008, which directs the second portion to focusing optics 1010. In another optional configuration, element 1004 may be configured to perform a beam splitting function. For example, element 1004 can be configured as a beam splitter that allows one portion of each of the multiple light beams to pass through the beam splitter to element 1008 and refracts another portion of each of the multiple light beams to focus lens 1006. In this case, as long as the beam path length is longer than the coherence length, two illumination angles can simultaneously illuminate the same area on the sample. Element 1004 can also be a switchable element (e.g., a flip mirror) that directs the light beams to either element 1008 or 1006. In this case, the two illumination angles are sequential, and the wafer is scanned twice with two different azimuth angles.

[0077] In any case, the combination of the beam shaping optics 1002, elements 1004 and 1008, and focusing lenses 1006 and 1010 can be configured so that the first and second portions of a first beam of the plurality of beams are directed to the same or substantially the same first area on the sample at the first and second azimuth angles, respectively, the first and second portions of a second beam of the plurality of beams are directed to the same or substantially the same second area (spaced from the first area) on the sample at the first and second azimuth angles, respectively, and so on. In this way, an elliptical light spot (e.g., Figure 2 and3 The light spots shown) can each be illuminated simultaneously with different portions of one of the multiple light beams at different azimuth angles (and the same polar angle).

[0078] However, in another configuration, Figure 10 The illustrated combination of beam shaping optics 1002, elements 1004 and 1008, and focusing lenses 1006 and 1010 can be configured such that a first portion and a second portion of a first beam of the plurality of beams are directed at first and second azimuth angles, respectively, to different first areas spaced apart from one another on the sample, a first portion and a second portion of a second beam of the plurality of beams are directed at first and second azimuth angles, respectively, to different second areas spaced apart from one another on the sample and to different first areas on the sample, and so on. In this way, a first portion of each of the plurality of beams can be directed at a first azimuth angle onto the sample, for example, as Figure 2 and 3 A first set of elliptical light spots configured as shown, and a second portion of each of the plurality of light beams can be directed at a second azimuth angle to a second set of elliptical light spots on the sample, the second set of elliptical light spots being spaced apart from the first set on the sample and also being able to be directed, for example, as Figure 2 and 3 Thus, two spatially separated arrays (each comprising spaced elliptical spots) can illuminate a sample simultaneously at different azimuthal angles (and the same polar angle).

[0079] As further described herein, the different azimuth angles can differ from each other by 90 degrees. For example, the beam shaping optics in combination with the focusing optics can be configured for oblique illumination at + / - 45 degrees azimuth, thereby advantageously producing two effectively uniform top-hat illuminations. Additionally, although Figure 9 The illustrated + / - 45 degree azimuth angles may be particularly advantageous for the embodiments described herein, but the embodiments may be configured for any other suitable azimuth angles. For example, to detect certain defect types, azimuth angles of 0 and 90 degrees may be selected rather than the more generally advantageous + / - 45 degree azimuth angle. In other words, while it is expected that + / - 45 degree azimuth angles are most advantageous for most inspections performed using the systems described herein, the embodiments are not limited to such azimuth angles.

[0080] The system also includes an imaging detector configured to generate an image of the sample by detecting light from the plurality of light points. Figure 10 As shown, light from each illumination point on the sample (e.g., scattered light in the case of DF) can be imaged by objective lens 1016. The light collected and imaged by objective lens 1016 can be relayed by pupil relay lens 1018 through one or more polarization components (e.g., polarization conversion wave plate 1020 and polarization beam splitter (PBS) 1022). Light exiting the polarization components can be guided to one or more TDI sensors (at Figure 10 1026). Each of these elements can have any suitable configuration known in the art. In combination, these elements form what is generally referred to in the art as a detection channel. The detection channel can include any other suitable optical elements known in the art, such as spectral filters, spatial filters, etc.

[0081] The system can be configured to have multiple detection channels (not shown). Multiple detection channels can be configured to detect light from the same illumination light point on the sample (e.g., an array of light points on the sample that can illuminate at a single azimuth angle or at multiple azimuth angles), which, in combination with the detector characteristics, is configured so that it is "seen" by the detector as an effectively uniform, flat-top line illumination. Alternatively, when there is more than one array of illumination light points on the sample (e.g., different arrays illuminated at different azimuth angles), one detection channel can be configured to detect light from a first array of illumination light points, and another detection channel can be configured to detect light from a second array of illumination light points. Each of these arrays of light points can also be configured in combination with the detector characteristics so that it is "seen" as a different illumination line on the sample, each having an effectively uniform, flat-top illumination intensity profile.

[0082] The multiple detection channels can also have various other configurations. For example, the detection channels can be configured to detect light scattered from the sample (whether from the same set of light points or different sets of light points) at different scattering angles and / or polarizations. In this way, different detection channels can be used for different modes of the system, where a "mode" is generally defined as a set of parameters of the system used to generate an image of the sample (except for the location at which the image is generated).

[0083] The sample can be scanned in a spiral path (ie, as in RT-type scanning) to detect defects on the sample. For example, the system also includes a scanning subsystem (in Figure 10 1014), the scanning subsystem is configured to scan light across the sample. A rotating stage (spindle) 1014 can be connected to a linear stage (not shown) on which the sample 1012 is mounted, and coupled to any suitable mechanical and / or robotic assembly (not shown), which in combination can be configured to move the sample so that light can be scanned across the sample in a spiral trajectory. Additionally or alternatively, the system can be configured so that one or more optical elements perform some form of scanning of light across the sample. The light can be scanned across the sample in any suitable manner.

[0084] The system also includes a computer subsystem 1028 configured to detect defects on the sample based on the image. The computer subsystem 1028 can be coupled to the TDI 1026 in any suitable manner (e.g., via one or more transmission media, which may include "wired" and / or "wireless" transmission media) so that the computer subsystem can receive the image generated by the TDI 1026. The computer subsystem 1028 can be configured to detect defects on the sample 1012 by applying a defect detection method to the image generated by the TDI 1026. Defect detection on the sample can be performed using any suitable defect detection method and / or algorithm known in the art (e.g., applying a defect detection threshold to the output and determining that any output having a value above the threshold corresponds to a defect (or potential defect)).

[0085] A computer subsystem may also be referred to herein as a computer system. A computer subsystem or system may take various forms, including a personal computer system, a graphics computer, a mainframe computer system, a workstation, a network appliance, an Internet appliance, or other devices. In general, the term "computer system" may be broadly defined to encompass any device having one or more processors that execute instructions from a memory medium. A computer subsystem or system may also include any suitable processor known in the art, such as a parallel processor. In addition, a computer subsystem or system may include a computer platform with high-speed processing and software (either as a stand-alone or network-linked appliance).

[0086] This article provides Figure 10 To generally illustrate some configurations of inspection systems, which may include an axial DOE and various other optical elements. Obviously, as is typically done when designing commercial systems, the inspection system configurations described herein may be modified to optimize the performance of the system. In addition, the systems described herein may be implemented using existing optical systems (e.g., by adding the axial DOE and other functionality described herein to the existing inspection system) (e.g., inspection systems available from KLA Corp., Milpitas, Calif.). For some such systems, the embodiments described herein may be provided as optional functionality for the existing system (e.g., to supplement the other functionality of the system). Alternatively, the systems described herein may be designed "from the ground up" to provide an entirely new system.

[0087] In one embodiment, the sample is a wafer. The wafer may include any wafer known in the semiconductor art. Furthermore, although some embodiments may be described herein with respect to wafers, the embodiments are not limited to the samples to which they may be applied. For example, the embodiments described herein may be applied to samples such as photomasks, flat panels, personal computer (PC) boards, and other semiconductor samples.

[0088] In another embodiment, the system is configured as a metrology system. In a further embodiment, the system is configured as a defect inspection system. For example, Figure 10 The embodiments of the system shown in can be modified in one or more parameters to provide different imaging capabilities depending on the application it is to be used for. In one such example, if the system is to be used for metrology rather than for inspection, it can be configured to have a higher resolution. In other words, Figure 10 The embodiments of the system shown in describe some general and various configurations for an imaging system that can be customized in a number of ways that will be apparent to those skilled in the art to produce a system with different imaging capabilities that are more or less suitable for different applications.

[0089] Figure 10 The computer subsystem 1028 shown can be configured to generate results that include at least information determined for the sample based on the image generated by the imaging detector and possibly any other output generated by the computer subsystem. The results can be in any suitable format (e.g., a KLARF file, which is a proprietary file format used by tools purchased from Klarity; a results file generated by Klarity, which is a tool purchased from Klarity; batch results, etc.). In addition, all embodiments described herein can be configured to store the results of one or more steps of the embodiments in a computer-readable storage medium. The results can include any results described herein and can be stored in any manner known in the art. The storage medium can include any storage medium described herein or any other suitable storage medium known in the art. After the results have been stored, the results can be accessed in the storage medium and used by any of the method or system embodiments described herein, formatted for display to a user, used by another software module, method or system, etc. to perform one or more functions on the sample or another sample.

[0090] Such functionality includes, but is not limited to, modifying a process, such as a manufacturing process or step, that has been or will be performed on a sample in a feedback, feedforward, in-situ, or other manner. For example, the computer subsystem may be configured to determine one or more changes to a process that has been or will be performed on the sample based on the detected defect and / or other determined information. The changes to the process may include any suitable changes to one or more parameters of the process. For example, if the determined information is for a defect detected on the sample, the computer subsystem preferably determines those changes so that the defect can be reduced or prevented on other samples that perform the modified process, the defect on the sample can be corrected or eliminated in another process performed on the sample, the defect can be compensated in another process performed on the sample, and so on. The computer subsystem may determine such changes in any suitable manner known in the art.

[0091] The changes may then be sent to a semiconductor manufacturing system (not shown) or to a storage medium accessible to both the computer subsystem and the semiconductor manufacturing system ( Figure 10 (not shown). A semiconductor manufacturing system may or may not be part of the system embodiments described herein. For example, the systems described herein may be coupled to a semiconductor manufacturing system, for example, via one or more common elements (e.g., a housing, a power supply, a sample handling device or mechanism, etc.). The semiconductor manufacturing system may include any semiconductor manufacturing system known in the art, such as a lithography tool, an etching tool, a chemical mechanical polishing (CMP) tool, a deposition tool, and the like.

[0092] The embodiments described herein provide several important advantages over currently used methods for oblique multi-spot illumination imaging. For example, the embodiments described herein provide a method and apparatus for producing a substantially uniform linear array of spots at oblique illumination angles, particularly for large polar angles and + / - 45 degree azimuthal angles. In addition, the number of spots can be relatively large to achieve substantially high illumination efficiency and uniformity. The embodiments described herein also have better tolerance for incident beam quality and mechanical alignment. Therefore, the implementation of the embodiments is more cost-effective and long-term stability can be maintained. A further advantage of the embodiments described herein is that due to the substantially small separation angle between the beams, the DOE grating pitch can be relatively large. Therefore, DOE manufacturing can be easily obtained. An additional advantage of the embodiments described herein is that the illumination path from the laser to the wafer is much simpler and more compact, with improved illumination efficiency and footprint. In addition to the simpler design of the embodiments described herein, the embodiments described herein also provide a more stable design.

[0093] Each of the embodiments of the system described above may be further configured according to any other embodiment described herein.

[0094] Another embodiment relates to a method for generating an image of a sample. The method comprises directing a light beam (e.g., Figure 1 The method further includes separating the light beam (shown as light beam 114) into a plurality of light beams (124), wherein an axial DOE (118) is positioned in the light beam. The light beam is offset to the center of the axial DOE (e.g., as shown in cross-sectional view 120 of the axial DOE and cross-sectional view 120a of the light beam 114 showing the relative position of the light beam with respect to the center of the axial DOE). The axial DOE includes concentric grating rings (shown in cross-sectional view 120) configured to separate the light beam into the plurality of light beams. The method further includes focusing the plurality of light beams onto a plurality of light spots (e.g., Figure 2 The light point 200 is shown. A common focal plane of multiple light points (e.g., Figure 5510) is tilted (ie, non-perpendicular) at an angle (β) relative to the optical axis 504 of the plurality of light beams. Additionally, the method includes detecting light from a plurality of light points (eg, with Figure 10 The TDI sensor 1026 is shown to produce an image of the sample.

[0095] Each of the steps of the method may be performed as further described herein. The method may also include any other steps that can be performed by the system described herein. The steps of the method may be performed by the system described herein, which may be configured according to any of the embodiments described herein.

[0096] Additional embodiments relate to a non-transitory computer-readable medium storing program instructions executable on a computer system for performing a computer-implemented method for generating an image of a sample and / or examining a sample. Figure 11 In particular, as shown in Figure 11 , non-transitory computer-readable medium 1100 includes program instructions 1102 executable on a computer system 1104. The computer-implemented method may include any steps of any method described herein.

[0097] Program instructions 1102 implementing methods, such as those described herein, may be stored on a computer-readable medium 1100. The computer-readable medium may be a storage medium such as a magnetic or optical disk, tape, or any other suitable non-transitory computer-readable medium known in the art.

[0098] The program instructions may be implemented in any of a variety of ways, including procedural, component-based, and / or object-oriented techniques, etc. For example, the program instructions may be implemented using ActiveX controls, C++ objects, JavaBeans, Microsoft Foundation Classes ("MFC"), SSE (Streaming SIMD Extensions), or other techniques or methods, as desired.

[0099] Computer system 1104 may be configured according to any of the embodiments described herein.

[0100] In view of this description, further modifications and alternative embodiments of various aspects of the present invention will be apparent to those skilled in the art. For example, methods and systems for generating images of samples are provided. Therefore, this description should be interpreted as merely illustrative and for the purpose of teaching those skilled in the art the general manner of implementing the present invention. It should be understood that the forms of the invention shown and described herein should be considered to be presently preferred embodiments. As will be apparent to those skilled in the art after having the benefit of this description of the present invention, elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the present invention may be utilized separately. Changes may be made to the elements herein without departing from the spirit and scope of the present invention as described in the appended claims.

Claims

1. A system configured to generate an image of a sample, comprising: a light source configured to generate a light beam; an axial diffractive optical element positioned in the light beam, wherein the light beam is offset to a center of the axial diffractive optical element, and wherein the axial diffractive optical element comprises concentric grating rings configured to separate the light beam into a plurality of light beams; focusing optics configured to focus the plurality of light beams to a plurality of light spots on a sample, respectively, wherein a common focal plane of the plurality of light spots is at an oblique angle relative to an optical axis of the focusing optics; and An imaging detector is configured to generate an image of the sample by detecting light from the plurality of light points.

2. The system of claim 1, wherein a pitch of the grating rings varies with a radius of the axial diffractive optical element.

3. The system of claim 1, wherein the axial diffractive optical element is further configured to function as a combination of a diffractive beam splitter and a Fresnel lens, with weak focusing power of diffraction orders. The system of claim 1 , wherein at least two of the plurality of light beams have different powers.

5. The system of claim 1, wherein the plurality of light spots have an elliptical shape with a major axis angled relative to a scan direction of the imaging detector. The system of claim 1 , wherein the imaging detector is further configured as a time-delay integration camera. The system of claim 1 , wherein the plurality of light spots are separated from one another on the sample.

8. A system according to claim 1, wherein the imaging detector is further configured as a time-delay integration camera, and wherein the projection of each of the multiple light points into the field direction of the time-delay integration camera perpendicular to the scanning direction of the time-delay integration camera overlaps with the projection of one or more of the multiple light points adjacent to each of the multiple light points into the field direction.

9. The system of claim 8, wherein a projection distance between said each of said plurality of light points and each of said one or more of said plurality of light points adjacent to said each of said plurality of light points is approximately half the size of said plurality of light points.

10. The system of claim 8, wherein the imaging detector is further configured to integrate the intensity of the light from the plurality of light points in the scanning direction.

11. The system of claim 8, wherein the projections of the plurality of light points overlap to produce a uniform flat top lighting profile.

12. The system of claim 1, wherein the focusing optics are further configured to direct the plurality of light beams to the plurality of light spots, respectively, at a polar angle of 75 degrees.

13. The system of claim 1 , further comprising a beam splitting optical device configured to split each of the plurality of light beams into first and second portions of the each of the plurality of light beams, thereby generating first and second groups of plurality of light beams, respectively, wherein the focusing optical device is further configured to focus the first and second groups of plurality of light beams, respectively, to the plurality of light spots on the sample at different azimuthal angles and the same polar angle. The system of claim 13 , wherein the different azimuth angles differ by 90 degrees.

15. The system of claim 13, further comprising a set of cylindrical lenses positioned in the light beam between the light source and the axial diffractive optical element and configured to asymmetrically expand the size of the light beam.

16. The system of claim 1, wherein each of the plurality of light spots is focused on a surface of the sample.

17. The system of claim 1, wherein the system is further configured for testing the sample.

18. The system of claim 1, wherein the sample is a wafer.

19. A method for generating an image of a sample, comprising: separating a light beam into a plurality of light beams with an axial diffractive optical element positioned in the light beam, wherein the light beam is offset to a center of the axial diffractive optical element, and wherein the axial diffractive optical element comprises concentric grating rings configured to separate the light beam into the plurality of light beams; focusing the plurality of light beams onto a plurality of light spots on the sample, respectively, wherein a common focal plane of the plurality of light spots is at an oblique angle relative to the optical axes of the plurality of light beams; and An image of the sample is generated by detecting light from the plurality of light points.

20. A system configured for testing a sample, comprising: a light source configured to generate a light beam; an axial diffractive optical element positioned in the light beam, wherein the light beam is offset to a center of the axial diffractive optical element, and wherein the axial diffractive optical element comprises concentric grating rings configured to separate the light beam into a plurality of light beams; focusing optics configured to focus the plurality of light beams to a plurality of light spots on a sample, respectively, wherein a common focal plane of the plurality of light spots is at an oblique angle relative to an optical axis of the focusing optics; an imaging detector configured to generate an image of the sample by detecting light from the plurality of light points; and A computer subsystem is configured to detect defects on the sample based on the image.

Images