Oblique uniform illumination for imaging systems

Abstract

A method and system for generating an image of a sample are provided. One system includes an axial diffractive optical element (DOE) positioned within a light beam generated by a light source. The light beam is offset relative to the center of the axial DOE. The axial DOE has concentric grating rings configured to split the light beam into multiple light beams. Additionally, the system includes focusing optics configured to focus the multiple light beams onto corresponding ones of multiple spots on the sample, the common focal plane of the multiple spots being at an oblique angle relative to the optical axis of the focusing optics. The system also includes an imaging detector configured to detect light from the multiple spots to generate an image of the sample.

Description

[Technical Field]

[0001] The present invention relates generally to oblique uniform illumination methods and systems for imaging systems, and in particular embodiments to providing a substantially uniform flat-top illumination profile with an oblique illumination spot. [Background technology]

[0002] The following descriptions and examples are not admitted to be prior art by virtue of their incorporation into this section.

[0003] In the fabrication of semiconductor devices, such as logic and memory devices, a substrate, such as a semiconductor wafer, is typically processed using a number of manufacturing processes to form the various features and multiple layers of the semiconductor devices. For example, in a semiconductor manufacturing process known as lithography, a pattern is transferred from a photomask onto a resist array on the semiconductor wafer. Additional examples of semiconductor manufacturing processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an array on a single semiconductor wafer and then separated into individual semiconductor devices.

[0004] Inspection processes are used at various steps in the semiconductor manufacturing process to detect defects on specimens, which can help increase yield and therefore profits in the manufacturing process. Inspection has always been an important part of semiconductor device manufacturing. However, as semiconductor device dimensions shrink, even smaller defects can cause device failures, making inspection more important than ever for the successful manufacture of acceptable semiconductor devices.

[0005] The configuration of an inspection system can dramatically affect the capabilities of the tool, especially as the structures on the specimen being inspected become significantly smaller and more complex, and / or as particles or defects on the specimen become significantly smaller. Although nearly every configurable parameter of an inspection system can affect the inspection capabilities of that system, the primary concern addressed in this application is the illumination configuration and how its various parameters can affect inspection performance.

[0006] Some inspection systems currently in use are configured to generate a flat-top illumination profile to provide uniformity of illumination intensity across the illuminated area on the specimen. In some of these systems, wavefront phase transformation is used to convert a Gaussian beam shape to a top-hat beam profile shape. This method is quite old, and many commercially available components exist for it. Two methods are generally considered to fall into this category: the optical wavefront transforming element can be either a refractive element or a diffractive element. The refractive element is typically a commercially available aspheric lens.

[0007] Other systems currently in use generate flat-tops by combining multiple Gaussian beams. These beams must originate from a single laser to be coherent. To avoid inter-spot interference (and hence uniformity degradation), the spots can be separated in either the spatial or temporal domain. If the inter-spot optical path length difference is larger than the coherence length, the spots separated in the time domain can be overlapped to create a flat-top without interference fringes. Typical coherence lengths for mode-locked (ML) lasers are several millimeters. Spatial spot separation relies on the integration of time-delay-integration (TDI) scans to merge the spots along one dimension and create a uniform flat-top along the orthogonal direction.

[0008] The main challenge of time-domain separation is how to recombine the beams. For wafer inspection, the beams must be nearly collinear throughout their illumination paths and focused to nearly the same location on the wafer, due to constraints on the illumination numerical aperture (NA), which must be small enough, and on the illumination field size, which must also be small enough compared to the incident beam diameter. In theory, this is a very straightforward concept, but implementation is extremely difficult due to tight requirements on stability and alignment tolerances.

[0009] Direct conversion of a Gaussian beam into a flat-top illumination profile, whether refractive or diffractive, has several important drawbacks. The main drawback is that the output flatness is sensitive to the quality and alignment of the input beam. 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 to achieve optimal defect detection sensitivity. The oblique illumination angle is defined by two parameters: polar angle and azimuth angle. The polar and azimuth angles are defined in terms of the coordinate system of the scan direction (x-axis) and the wafer surface (xy plane). The wafer surface normal is the z-axis. The best particle detection sensitivity is achieved when the illumination polar angle is between 60 and 80 degrees. At such large polar angles, the projected beam size is elongated by a factor of 1 / cos(polar angle), e.g., 2x to 6x. When the elongation is along the scan direction (90-degree azimuth angle), a larger TDI sensor is required to cover the elongated illumination field. Multiple illumination azimuth angles can reduce speckle noise and ultimately improve defect sensitivity. In dual azimuth illumination, ±45° oblique illumination angles offer advantages over the currently used 0 / 90° azimuth illumination in terms of a good compromise between steep oblique factors and symmetry of the optical system. Also, there are no proven alternatives to 45° azimuth oblique illumination. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent No. 9,945,792 Summary of the Invention [Problem to be solved by the invention]

[0012] It would therefore be advantageous to develop an imaging system and / or method that does not have one or more of the above-mentioned disadvantages. [Means for solving the problem]

[0013] The following description of various embodiments is not to be construed in any way as limiting the subject matter of the appended claims.

[0014] One embodiment relates to a system configured to generate an image of a sample. The system includes a light source configured to generate a light beam. The system also includes an axial diffractive optical element (DOE) disposed within the light beam. The light beam is offset relative to a center of the axial DOE. The axial DOE includes concentric grating rings configured to split the light beam into multiple light beams. Additionally, the system includes focusing optics configured to focus the multiple light beams onto corresponding ones of multiple spots on the sample. A common focal plane of the multiple spots is at an oblique angle relative to an optical axis of the focusing optics. The system further includes an imaging detector configured to detect light from the multiple spots to generate an image of the sample. The system may be further configured as described herein.

[0015] Another embodiment relates to a method for generating an image of a sample, comprising splitting a light beam into multiple light beams with an axial DOE positioned within the light beam. The light beam is offset relative to the center of the axial DOE. The axial DOE has concentric grating rings configured to split the light beam into the multiple light beams. The method also focuses the multiple light beams onto corresponding ones of multiple spots on the sample, where a common focal plane of the multiple spots is at an oblique angle relative to the optical axes of the multiple light beams. Additionally, the method generates an image of the sample by detecting light from the multiple spots.

[0016] The steps of this method can be performed as detailed herein. In addition, any other steps of any other method(s) described herein can be incorporated into this method. This method can be performed by any of the systems described herein.

[0017] An additional embodiment relates to a system configured to inspect a specimen, the system including 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 specimen based on the image. The system may be configured as described herein.

[0018] 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: [Brief explanation of the drawings]

[0019] [Figure 1] This is a schematic diagram depicting the side view of an example of a conventionally used diffractive optical element (DOE) and an embodiment of an axial DOE, illustrating the principle of generating a spot array at a tilt angle using an axial grating beam splitter. [Figure 2]FIG. 1 is a schematic diagram illustrating a plan view of one embodiment of multiple spots and time delay integration (TDI) design parameters used to generate a uniform high-hat illumination profile. [Figure 3] FIG. 1 is a schematic diagram illustrating a plan view of one embodiment of a spot array layout consisting of nine spots. [Figure 4] FIG. 4 is a plot depicting the effective intensity profile obtained by TDI integration of nine spots in the layout shown in FIG. 3. [Figure 5] FIG. 1 is a schematic diagram depicting a side view of the design parameters of an embodiment that produces a tilted spot array with an axial DOE and a focusing lens. [Figure 6a] FIG. 1 is a schematic diagram depicting a top view (relative to the sample surface) of one embodiment of an illumination beam path. [Figure 6b] 6b is a schematic diagram illustrating a side view of the illumination beam path of FIG. 6a. [Figure 7] FIG. 1 is a schematic diagram illustrating one embodiment of an incident plane defined angle, a spot array angle, and an illumination incident angle. [Figure 8] FIG. 1 is a schematic diagram depicting a side view of one embodiment of elements that convert and distribute a single laser beam into an array of tilted spots on a sample surface. [Figure 9] FIG. 1 is a schematic diagram illustrating a planar appearance of an example of a definition of an illumination azimuth angle. [Figure 10] FIG. 1 is a schematic diagram depicting a side view of one embodiment of a system configured to generate an image of and / or inspect a sample. [Figure 11] 1 is a block diagram illustrating one embodiment of a non-transitory computer-readable medium having stored thereon program instructions executable on a computer system for performing one or more of the computer-implemented methods described herein. DETAILED DESCRIPTION OF THE INVENTION

[0020] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are described in detail herein. However, it is not intended that the drawings and detailed description thereof limit the invention to the specific forms disclosed; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

[0021] Turning now to the drawings, please note that the figures are not drawn to scale. In particular, the scale of some of the elements in the figures has been greatly exaggerated to emphasize the characteristics of those elements. Also, please note that the figures are not drawn to the same scale. Elements that appear in multiple figures and may be similarly configured are indicated using the same reference numerals. Unless otherwise noted herein, any of the elements described and shown may include any suitable commercially available elements.

[0022] The embodiments described herein generally relate to methods and systems for generating images of a specimen. The embodiments described herein are particularly advantageous for oblique uniform illumination for imaging systems. Additionally, the embodiments described herein provide oblique uniform illumination for laser scanning time delay and integration (TDI) imaging systems. For example, the embodiments described herein advantageously provide methods and systems for generating flat-top illumination for oblique laser dark-field (DF) wafer inspection (the terms "flat-top," "flat-top," "high hat," and "top hat" are used interchangeably herein to refer to an intensity profile having a generally step-function shape, as opposed to other intensity profile shapes or functions, such as Gaussian). However, the illumination configurations described herein can be incorporated into any suitable imaging system known in the art.

[0023] Figure 1 illustrates the basic operating principles of the embodiments described herein. By comparing and contrasting two types of beam-splitting diffractive optical elements (DOEs), it details how to generate a one-dimensional (1D) spot array with a focal plane at an oblique angle (i.e., a non-perpendicular angle) relative to its optical axis. More specifically, the figure illustrates the principle of using an off-axis grating beamsplitter to generate a spot array at an oblique angle. The terms "off-axis DOE," "off-axis grating," "off-axis grating beamsplitter," and "off-axis DOE" are used interchangeably herein.

[0024] A currently used grating beamsplitter 104 splits a collimated incident beam 100 into multiple collimated beams 110 (three beams in this illustration), which are focused by a focusing lens 108 into three spots whose common focal plane 112 is perpendicular to the optical axis 102 of the collimated incident beam 100 and the focusing lens 108. As shown in cross-sectional view 106 of the grating beamsplitter 104, the beamsplitter may have parallel grating lines that split the light beam into multiple light beams.

[0025] In the embodiments described herein, the beam is split by an axial DOE. The system includes an axial DOE 118 positioned within the light beam 114. In one embodiment, the axial DOE is configured to function as a combination diffractive beam splitter and Fresnel lens, exhibiting weak diffraction order focusing power. In other words, the axial DOE functions similarly to a Fresnel lens, but with significantly weaker diffraction order focusing power. For example, an axial grating 118 (configured to perform the combined functions of a beam splitter grating and a Fresnel lens) splits the collimated beam 114 into multiple beams 124 (three beams are shown, but the light beam can be split into any suitable number of beams).

[0026] As shown in cross-sectional view 120 of axial grating 118, this axial DOE has concentric grating rings configured to split a light beam into multiple light beams. The layout of these concentric rings can be the same as that of a conventional Fresnel zone plate (FZP). However, the grating profile is designed to generate multiple uniform diffraction orders, as opposed to the single diffraction order of a conventional FZP. This DOE thus combines the functions of an FZP and a beam splitter. This axial DOE has the same grating groove layout as an FZP and the same grating groove profile as a diffractive beam splitter. Similar to the focusing force of an FZP, this axial DOE generates a focusing force for multiple diffraction orders. The focusing force is proportional to the diffraction order, so positive diffracted orders converge and negative diffracted orders diverge. The axial DOE is designed so that the focusing power of any order is fairly small for focal lengths of many meters, which is how the "fairly weak focusing power" is defined in the embodiments described herein. In some embodiments, at least two of the optical beams have different powers. For example, if each branch beam has a slightly different power, when focused by a focusing lens as described herein, the beams form an array of spots with a common focal plane at an oblique angle to the incident beam.

[0027] As shown in FIG. 1 , the light beam 114 is offset from the center of the axial DOE. For example, the axis 116 passing through the center of the axial DOE is offset from the light beam 114. This offset is further illustrated in cross-sectional view 120, in which the position of the light beam cross-section 120a is offset from the center of the axial DOE (the DOE center is defined by the center of curvature of the DOE grating grooves). This results in the axial DOE being used in an off-axis position, meaning that only the portion of the DOE that is away from the optical axis is used by the laser beam. In one embodiment, the pitch of the grating rings varies with the radius of the axial DOE. This results in the DOE having concentric grating rings, with the pitch of the grating varying 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 along the radius of the axial DOE. The axial DOE can be further configured as described in U.S. Patent Application Publication No. 2013 / 0129998, issued April 17, 2018 to Zhao, which is incorporated by reference as if fully set forth herein.

[0028] The focusing optics (focusing lens 122 shown in FIG. 1) are configured to focus the multiple light beams 124 generated by the axial DOE onto corresponding ones of the multiple spots on the sample (not shown in FIG. 1). As shown in FIG. 1 and other figures herein, the focusing lens can be positioned on-axis, i.e., on the same optical axis as the incoming light beams and any other illumination optics. The focusing lens, as well as any other focusing optics described herein, can have any suitable configuration known in the art.

[0029] The common focal plane of the multiple spots is at an inclined angle relative to the optical axis of the focusing optical system. In addition, the embodiments described herein can generate spot arrays at any angle relative to the illumination beam. In other words, the spot array line can be at an angle relative to the incident direction, and the angle of the spot line can be controlled by the DOE's decentering relative to the optical axis of the illumination optical system and the light beam. For example, as shown in FIG. 1 , when the incident beam is offset from the axial center of the DOE and the beams are focused by a focusing lens, the focal point of each beam (the longitudinal position of each spot) will be located at a different distance from the focusing lens. In particular, as shown in FIG. 1 , the focal position of each beam 124 will be located at a different distance from the focusing lens 122. Therefore, the common focal plane of all spots will be at an inclined angle relative to the optical axis of the focusing lens. 1, if we were to draw a plane on which all of the focal points of the beams 124 are located, the common focal plane would be at an inclined angle to the optical axis of the focusing lens 122. In other words, the common focal plane is not perpendicular to the optical axis of the focusing lens.

[0030] Therefore, in principle, it is possible to generate a linear spot array that is tilted relative to the off-axis DOE and the focusing lens, which is the first step in generating a flat-top profile from the spot array. In certain embodiments, the spots have an elliptical shape with their major axes at an angle relative to the scanning direction of the imaging detector. For example, as shown in FIG. 2, each illumination spot 200 on the sample (not shown in FIG. 2) has an elliptical shape. To effectively create a uniform flat-top illumination profile, the spot shape is elongated and tilted relative to the TDI integration (scan) direction 204, as shown in FIG. 2. In other words, the major axis of each elliptical spot is neither parallel nor perpendicular to the scanning direction of the imaging detector. Instead, the spots have an elliptical shape, with the major axis of the elliptical spots at an angle relative to the TDI scanning direction.

[0031] FIG. 2 also illustrates design parameters for generating a uniform high-hat illumination profile using multiple spots and TDI integration. In some embodiments, the imaging detector is configured as a TDI camera (also referred to herein as a TDI or TDI sensor). In some such embodiments, the projection of each of the multiple spots onto the TDI camera's field of view, which is perpendicular to the TDI camera's scanning direction, is overlapped with the projection of one or more spots adjacent to each of the multiple spots onto the field of view. In other words, the projection of each elliptical spot onto the TDI field of view (perpendicular to the TDI scanning direction) is overlapped with the projection of its adjacent spot onto the TDI field of view. For example, in FIG. 2, the TDI camera's field of view is perpendicular to the TDI camera's scanning direction 204. The projection of each of the spots 200 (here five spots, although any other number of spots could equally well be used) onto the field of view of the TDI camera (not shown in FIG. 2), perpendicular to the scan direction 204, will overlap with the projection of light from its neighboring spot(s). In other words, when the spots are projected from the sample onto the TDI field of view 202 and further onto line H shown in FIG. 2 (i.e., flattened), the light from neighboring spots will overlap along that line.

[0032] In one embodiment, the spots are separated from one another on the sample. This separates the spots in space. In other words, the spots are illuminated and formed on the sample (and therefore within the common focal plane of the spot array) and are spatially separated from one another. Specifically, no part of any spot overlaps any other spot on the sample. By spatially separating the spots, interference between neighboring spots can be avoided. On the other hand, in one embodiment, the projection distance between each of the spots and one or more spots adjacent to each of the spots and included in the plurality of spots is approximately half the size of the spots. In other words, the projection distance between neighboring spots is approximately half the spot size. This allows the spots on the sample to be separated in space, while allowing approximately half of the projection of a first spot to overlap with approximately half of the projection of an adjacent spot. In other words, the distance between the projected centers of two adjacent spots along the TDI's viewing direction can be set to about half the size of the spots along that direction.

[0033] In another embodiment, the imaging detector is configured to integrate the light intensity from the multiple spots along the scan direction. In an additional embodiment, the projected overlap of the multiple spots produces a uniform flat-top illumination profile. As used herein, the term "uniform flat-top illumination profile" is generally defined as an illumination profile that exhibits substantially the same intensity value (i.e., statistically the same intensity value) across the entire dimension (or nearly all of the entire dimensions) of its illuminated area. For example, TDI integration can sum energy along the scan direction 204 to produce a substantially flat-top intensity profile. Thus, although the spots are separated in space, integration of the TDI scan can merge the spots along one dimension to produce a substantially uniform flat-top along the orthogonal direction. Thus, the TDI scan integrates the illumination intensity along the scan direction, and the projected overlap of neighboring spots produces a substantially uniform flat-top illumination profile.

[0034] Each spot has a spot size (D a ) and spot size along the short direction (D b ) and the orientation angle α. The flat-top efficiency is defined by the available power in the flat portion of the illumination profile, η~(N-1) / N The condition for forming a uniform flat-top area is given by D a <(1 / 2)·(F / (N-1))sinα D b >(2F / (N-1))·1 / cosα where F is the field size.

[0035] One embodiment of a spot array layout consisting of nine spots is depicted in Figure 3. Its integrated intensity profile, i.e., the sum of the electric field amplitudes with random phase offsets between the spots, is shown in Figure 4. More specifically, Figure 4 depicts the effective intensity profile obtained by TDI integration of the nine-spot layout shown in Figure 3. As shown in Figure 4, both the sum amplitude plot 400 and the sum intensity plot 402 exhibit a nearly flat-topped intensity profile.

[0036] FIG. 5 illustrates key design parameters and how they are determined. More specifically, FIG. 5 depicts the design parameters of an axial DOE and a focusing lens for generating a tilted spot array. As shown in this figure, a light beam 500 is directed toward an axial DOE 502, which is shown in cross-sectional view 502a. The light beam is offset relative to the center of the axial DOE. For example, as shown in FIG. 5, the light beam is centered on the optical axis 504 of the illumination optics, which is offset from the optical axis 506 of the DOE. Thus, the center of the light beam and the center of the axial DOE are offset relative to each other. This offset is detailed in cross-sectional view 502a of axial DOE 502, overlaid with cross-sectional view 500a of the light beam, which illustrates the offset of the light beam from the center of the axial DOE. The axial DOE can be further configured as described herein to split the light beam into multiple light beams and direct them to focusing lenses 508 in the focusing optics. The focusing lens focuses the multiple light beams onto corresponding ones of multiple spots on the sample (not shown in FIG. 5). The common focal plane 510 of the multiple spots is at an oblique angle (i.e., not perpendicular) to the optical axis of the focusing optics (e.g., axis 504 shown in FIG. 5). The embodiment shown in FIG. 5 can be further configured as described herein.

[0037] The spot array layout is defined by two parameters: the spacing between neighboring 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. The spot array tilt angle β depends on the illumination angle. cosβ=sinθsinφ In the formula, θ and φ are the polar angle and azimuthal angle, respectively. The deviation of the axial lattice is y=f tanβ where f is the focal length of the focusing lens.

[0038] Axial gratings have concentric circular grating grooves whose pitch varies linearly with radius. The grating layout can be determined by two parameters: wavelength (λ) and the focal point of the first-order diffracted light (z1). The focal point of the first-order diffracted light is z1=fy / (d·sinβ) The grating pitch is a function of the offset distance, P(y)=λf / (d·sinβ) The grating profile is defined by a number of diffraction orders, and the profile is generally optimized for maximum total diffraction efficiency and maximum uniformity among the diffraction orders.

[0039] In one embodiment, each of the plurality of spots is focused onto the surface of the specimen. In another embodiment, a focusing optical system is configured to direct the plurality of light beams onto corresponding ones of the plurality of spots at a 75-degree polar angle. A geometric lens design embodiment for oblique illumination at a 75-degree polar angle and a 45-degree azimuth angle is shown in FIG. 6a (a top view (relative to the wafer surface) of the illumination beam path showing the 45-degree azimuth angle) and FIG. 6b (a side view of the illumination beam path showing the 75-degree polar angle). More specifically, as shown in FIG. 6a, the plurality of light beams generated by the off-axis DOE 600 can be focused onto the wafer plane 604 by a paraxial lens 602 at a 45-degree azimuth angle 606. Additionally, as shown in FIG. 6b, the plurality of light beams generated by the off-axis DOE 600 can be focused onto the wafer plane 604 by a paraxial lens 602 at a 75-degree polar angle 608. A close-up 610 of the focal plane portion 612 shows that all spots are perfectly focused on the wafer even when the wafer surface is at a 75 degree angle to the illumination beam.

[0040] As shown in Figure 7, the direction of the incident optical axis in an optical design is usually defined as the angle (β) between the illumination beam (incident beam 700) and the illumination line (line 702 on the wafer), as well as the roll angle (ρ) of the incident plane. The relationship of these two angles to the illumination polar angle (θ) and illumination azimuthal angle (φ) is shown in Figure 7 and can be determined by the following equations: TDI scan direction 704 is also shown in this figure. cosβ=sinθsinφ tanρ=sinθcosφ / cosθ

[0041] One embodiment of the overall illumination optics design is shown in FIG. 8. The beam size of the output beam from a laser 800 is asymmetrically expanded by a beam expansion optics 802, which in this embodiment includes 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 along the short dimension is amplified by a factor of the first cylindrical lens to the focusing lens ratio, and the beam size along the short dimension is amplified by a factor of the second cylindrical lens to the focusing lens ratio. 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, whose parameters are determined by the illumination conditions as discussed above. A focusing lens 810 then forms a linear spot array on the wafer (sample 812) with the desired illumination angle and layout dimensions. Thus, the elements shown in Figure 8 are configured to convert and distribute a single laser beam into an array of tilted spots on the wafer surface. Other optical elements that can be incorporated into the illumination path include waveplate(s), polarizer(s), etc. to control the illumination characteristics, and folding mirrors to reduce the footprint and optimize the location of other optical elements.

[0042] Dual illumination azimuthal angles provide a 1.4x improvement in defect signal-to-noise ratio for film wafers. For example, oblique illumination at multiple azimuthal angles can reduce speckle noise from wafer surface scattering. A 90-degree difference in azimuthal angles provides the greatest speckle noise reduction. Current systems use illumination angles of 0 and 90 degrees, which necessitate two different sets of optics and non-uniform performance. In contrast, the illumination optics embodiments described herein can be used with illumination at both +45 and -45 degrees azimuthal angles. Using ±45 degrees illumination simplifies the illumination optics (the optics are identical at both azimuthal angles) and provides uniform performance. For example, a spot array formed at a 75-degree polar angle and ±45 degrees azimuthal angles is perfectly focused on the wafer surface, resulting in the most uniform illumination profile and maximum depth of focus.

[0043] The advantages and disadvantages of the two illumination methods are listed in Table 1 and illustrated in Figure 9. In particular, Figure 9 shows an example 900 of 0 and 90 degree illumination angles used to simultaneously illuminate a field of view 904 on a wafer 902, and an example 906 of ±45 degree illumination angles used to simultaneously illuminate the same field of view on the wafer. [Table 1]

[0044] In some embodiments, the system includes beam splitting optics configured to split each of the plurality of light beams into first and second portions of the plurality of light beams to generate first and second sets of light beams, respectively, and the focusing optics configured to focus the first and second sets of light beams onto corresponding ones of a plurality of spots on the sample at different azimuthal angles and the same polar angle. For example, in the embodiment shown in Figure 10, the beam shaping optics 1002 and / or element 1004 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 the plurality of light beams to generate the first and second sets of light beams. The focusing optics 1006 and 1010 (and potentially elements 1004 and 1008) described herein can be configured to focus the first and second sets of light beams onto corresponding ones of a plurality of spots on the sample 1012 at different azimuthal angles and the same polar angle. Each of these elements can be configured as described herein.

[0045] In some embodiments, the difference between the different azimuthal angles is 90 degrees. For example, as noted above, a particularly advantageous set of azimuthal angles is ±45 degrees. However, other azimuthal angles may be employed in the described embodiments, such as the aforementioned 0-degree and 90-degree azimuthal angles, or other azimuthal angles that differ by more or less than 90 degrees. Additionally, while it may be most useful to have the azimuthal angles symmetric about the x- or y-axis of the sample, this is not required for the described embodiments. In general, the azimuthal angles used for inspection may be selected based on the characteristics of the sample, the characteristics of structures formed on the sample, such as material or composition, size, aspect ratio, orientation, shape, etc., the defects or particles of interest thereon, and various other characteristics thereof, such as those described above.

[0046] In one embodiment of the multi-azimuthal configuration (as well as the single-azimuthal configuration), the system includes a set of cylindrical lenses positioned within the light beam between the light source and the axial DOE, configured to asymmetrically expand the size of the light beam. For example, as shown in FIG. 8 and described in detail above, the beam size of the output beam from laser 800 can be asymmetrically expanded by beam expansion optics 802, which in this embodiment includes a focusing lens 804 and two cylindrical lenses 806. As described in detail above, the beam size and beam orientation are determined by the laser output beam size and the configuration of these three lenses. The expanded and / or shaped beam then passes through off-axis DOE 808, configured as described in detail herein. Thus, by configuring the system to include a set of cylindrical lenses configured to asymmetrically expand the beam size prior to the beam-splitting DOE, the illumination optics for multi-azimuthal illumination can be simplified.

[0047] One embodiment of an implementation of a system configured to generate an image of a sample and / or inspect a sample is shown in FIG. 10. The system includes a light source 1000 configured to generate a light beam, which may include a laser or any other suitable light source known in the art. The laser may include any suitable laser known in the art. Additionally, one advantage of the described embodiments is that, because a 1D spot array is used to generate a flat-top illumination intensity profile, there is no time delay in the system, and the laser can be a cw laser.

[0048] The light beam from the light source can be directed to beam shaping optics 1002, which can include any of the elements described herein (not shown in FIG. 10). For example, the beam shaping optics 1002 can be configured as shown in FIG. 8. In this way, the beam shaping optics can include at least an axial DOE 808, shown in FIG. 8, positioned within the light beam, which is offset relative to the axial DOE. The axial DOE includes concentric grating rings (e.g., those shown in cross section 120 in FIG. 1) configured to split the light beam into multiple light beams. Additionally, the beam shaping optics 1002 can be configured to split each of the multiple light beams into different portions of the light beam, each focused at a different azimuthal angle on the sample.

[0049] The system also includes focusing optics configured to focus the multiple light beams onto corresponding ones of multiple spots on the sample, with a common focal plane of the multiple spots at an oblique angle relative to the optical axis of the focusing optics. For example, in a system configured for multi-azimuthal illumination, the focusing optics in FIG. 10 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 beams onto elements 1008 and 1010, such as focusing lenses, which focus the different portions of the multiple beams onto multiple spots on the sample 1012. For example, a first portion of each of the plurality of 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 plurality of light beams may be directed to element 1008, which directs the second portion to focusing optics 1010. In another optical configuration, element 1004 may be configured to perform a beam splitting function. For example, element 1004 may be configured as a beam splitter that passes a portion of each of the plurality of beams into element 1008 and refracts another portion of each of the plurality of beams toward focusing lens 1006. In this case, the two illumination angles can simultaneously illuminate the same area on the sample, as long as the beam path length is longer than the coherence length. Element 1004 may also be a switchable element (e.g., a flip-in mirror) that directs a beam to either element 1008 or 1006. In this case, the two illumination angles are sequential, and the wafer is scanned twice at two different azimuthal angles.

[0050] In any case, the beam shaping optics 1002, the combination of elements 1004 and 1008, and the focusing lenses 1006 and 1010 can be configured to direct first and second portions of a first of the plurality of light beams to the same or substantially the same first area on the sample at corresponding ones of the first and second azimuthal angles, direct first and second portions of a second of the plurality of light beams to the same or substantially the same second area on the sample (spatially separated from the first area) at corresponding ones of the first and second azimuthal angles, etc. In this way, elliptical spots on the sample, such as those shown in Figures 2 and 3, can be simultaneously illuminated with different portions of one of the plurality of light beams at different azimuthal angles (and the same polar angle).

[0051] Alternatively, in another configuration, the beam shaping optics 1002, the combination of elements 1004 and 1008, and the focusing lenses 1006 and 1010 shown in FIG. 10 can be configured such that first and second portions of a first of the plurality of light beams are directed at corresponding ones of first and second azimuthal angles to separate first areas on the sample that are spatially separated from one another, and that first and second portions of a second of the plurality of light beams are directed at corresponding ones of first and second azimuthal angles to separate second areas on the sample that are spatially separated from one another and from the separate first areas on the sample, and so on. In this way, a first portion of each of the plurality of light beams can be directed at a first azimuthal angle to a first set of elliptical spots on the sample, which may be configured, for example, as shown in Figures 2 and 3, and a second portion of each of the plurality of light beams can be directed at a second azimuthal angle to a second set of elliptical spots on the sample that are spatially separated from the first set, which may also be configured, for example, as shown in Figures 2 and 3. Thus, two spatially separated arrays, each having spatially separated elliptical spots, can be simultaneously illuminated and formed on the sample at different azimuthal angles (and the same polar angle).

[0052] As described further herein, the different azimuth angles listed above may differ by 90 degrees from one another. For example, the beam shaping optics, in cooperation with the focusing optics, may be configured for oblique illumination at ±45° azimuth angles to successfully generate two substantially uniform, flat-topped linear illuminations. Additionally, while the ±45° azimuth angles shown in FIG. 9 may be particularly advantageous in the described embodiments, the embodiments may be configured with any other suitable azimuth angle. For example, instead of the generally more advantageous ±45° azimuth angle, the 0° and 90° azimuth angles may be selected for detecting certain types of defects. In other words, while the ±45° azimuth angle is expected to be most advantageous for the majority of inspections performed using the described systems, the embodiments are not limited to such azimuth angles.

[0053] The system also includes an imaging detector configured to generate an image of the sample by detecting light from the multiple illumination spots. Light from each illumination spot on the sample (e.g., scattered light in the case of DF) can be imaged via an objective lens 1016, as shown in FIG. 10 . Light collected by the objective lens 1016 and imaged can be relayed by a pupil relay lens 1018 and passed through one or more polarization elements, such as a polarization-converting waveplate 1020 and a polarizing beam splitter (PBS) 1022. Light emerging from the polarization element(s) can be directed by a tube lens 1024 to one or more TDI sensors (shown generally as TDI 1026 in FIG. 10 ). Each of these elements can have any suitable configuration known in the art. Together, these elements form what are commonly referred to in the art as detection channels. Any suitable optical element known in the art may be incorporated into the detection channel, such as a spectral filter, a spatial filter, etc.

[0054] The system can be configured to have multiple detection channels (not shown). The multiple detection channels can be configured to detect light from the same illumination spot on the sample (e.g., an array of spots on the sample that can be illuminated at one or more azimuthal angles) and can be configured with detector characteristics that cause them to "appear" to the detector as a substantially uniform, flat-topped line of illumination. Also, when there are multiple illumination spot arrays on the sample (e.g., separate arrays illuminated at different azimuthal angles), one detection channel can be configured to detect light from a first illumination spot array and another detection channel can be configured to detect light from a second illumination spot array. Each of these spot arrays can also be configured with detector characteristics that cause them to "appear" as differently illuminated lines on the sample, each with a substantially uniform, flat-topped illumination intensity profile.

[0055] The multiple detection channels can have a variety of other configurations as well. For example, the detection channels can be configured to detect light scattered from the sample (from the same set of spots or different sets of spots) at different scattering angles and / or polarizations. In this way, different detection channels can be used in different modes of the system; a "mode" is generally defined as a set of parameters of the system used to generate images of the sample (other than the locations at which those images are generated).

[0056] The sample can be scanned along a spiral path (i.e., as in an RT-type scan) to detect defects on the sample. For example, the system can also include a scanning subsystem (shown generally in FIG. 10 by shaft 1014) configured to scan the light across the sample. The rotary stage (spindle) 1014 can be coupled to a linear stage (not shown) on which the sample 1012 is positioned and coupled to any suitable mechanical and / or robotic assembly (not shown), the combination of which can be configured to move the sample and scan the light across the sample along a spiral trajectory. Additionally or alternatively, the system can be configured such that some of the light scanning across the sample is performed by one or more optical elements. The light scanning across the sample can be performed in any suitable manner.

[0057] The system also includes a computer subsystem 1028 configured to detect defects on the specimen based on the images. The computer subsystem 1028 may be coupled to the TDI 1026 in any suitable manner (e.g., via one or more transmission media, including, for example, "wired" and / or "wireless" transmission media) to receive the images generated by the TDI 1026. The computer subsystem 1028 may be configured to detect defects on the specimen 1012 by applying a defect detection method to the images generated by the TDI 1026. Detection of defects on the specimen may be performed using any suitable defect detection method and / or algorithm in any manner known and suitable in the art (e.g., by applying a defect detection threshold to the outputs and classifying all outputs having values ​​above the threshold as representing defects (or potential defects)).

[0058] This computer subsystem may also be referred to herein as a computer system. This computer subsystem or system may take various forms, including a personal computer system, an image computer, a mainframe computer system, a workstation, a network appliance, an Internet appliance, or other device. In general, the term "computer system" may be broadly defined to encompass any device having one or more processors that execute instructions obtained from a storage medium. This computer subsystem or system may include any suitable processor known in the art, such as a parallel processor. Additionally, this computer subsystem or system may include a computer platform with high-speed processing and software, whether a stand-alone or networked tool.

[0059] FIG. 10 is provided herein to generally illustrate several configurations of inspection systems that can incorporate axial DOEs and various other optical elements. Obviously, the configurations of the inspection systems described herein can be modified to optimize the performance of the systems, as is commonly done when designing commercial systems. Additionally, the systems described herein can be implemented with existing optical systems (e.g., by adding the axial DOEs described herein and other features to the existing inspection system), such as inspection systems commercially available from KLA Corp., Milpitas, California. In some such systems, the embodiments described herein can be provided as optional features of the existing system (e.g., in addition to other features of the system). Alternatively, the systems described herein can be designed "from scratch" to provide an entirely new system.

[0060] In some embodiments, the specimen is a wafer, which may include any wafer known in the semiconductor art. Additionally, although some embodiments may be described herein in the context of a wafer, the embodiments are not limited by the specimen with which they may be used. For example, the embodiments described herein may be used with specimens such as reticles, flat panels, personal computer (PC) boards, other semiconductor specimens, and the like.

[0061] In another embodiment, the system is configured as a metrology system. In a further embodiment, the system is configured as a defect review system. For example, one or more parameters of the system embodiment shown in FIG. 10 can be modified to provide different imaging capabilities depending on the application for which the system is to be used. In some such examples, the system can be configured to provide higher resolution when used for metrology rather than inspection. In other words, the system embodiment shown in FIG. 10 illustrates several common and diverse configurations of imaging systems that can be tailored in a variety of ways that would be obvious to one skilled in the art, thereby creating systems with different imaging capabilities that are more or less suitable for different applications.

[0062] The computer subsystem 1028 shown in FIG. 10 can be configured to generate results including at least information determined about the sample based on the image generated by the imaging detector and potentially any other output generated by the computer subsystem. The results may have any suitable format (e.g., a KLARF™ file, a proprietary file format used by tools commercially available from KLA; a results file generated by Klarity™, a tool commercially available from KLA; lot results, etc.). Additionally, any of the embodiments described herein can be configured to store results of one or more steps of the embodiment in a computer-readable storage medium. The results may include any of the results described herein and may be stored in any manner known in the art. The storage medium may include any of the storage media described herein, as well as any other suitable storage media known in the art. After the results are stored, they can be accessed in the storage medium and used in any of the method or system embodiments described herein, formatted for display to a user, or used by other software modules, methods, systems, etc. to perform one or more functions on the sample or other samples.

[0063] Such functions include, but are not limited to, modifying a process, such as modifying a manufacturing process or operation performed or to be performed on a specimen in a feedback, feedforward, in situ, or other manner. For example, the computer subsystem may be configured to determine one or more modifications to a process performed or to be performed on a specimen based on the detected defect(s) and / or other determined information. The process modifications may include any suitable modification to one or more parameters of the process. For example, if the determined information relates to a defect detected on the specimen, the computer subsystem may determine the appropriate modifications to reduce or prevent the defect on another specimen that is subjected to a revised process, correct or eliminate the defect on the specimen in another process performed on the specimen, compensate for the defect in another process performed on the specimen, etc. The computer subsystem may determine such modifications in any suitable manner known in the art.

[0064] The changes can then be sent to a semiconductor manufacturing system (not shown) or to a storage medium (not shown in FIG. 10 ) accessible to both the computer subsystem and the semiconductor manufacturing system. The 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 the semiconductor manufacturing system through one or more common elements, such as 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 etch tool, a chemical-mechanical polishing (CMP) tool, a deposition tool, etc.

[0065] The described embodiments offer several important advantages over currently used oblique, multi-spot illumination imaging methods. For example, the described embodiments provide methods and apparatus for generating substantially uniform linear spot arrays at oblique illumination angles, particularly at large polar and ±45° azimuthal angles. Additionally, the relatively large size of the multiple spots allows for significantly higher illumination efficiency and uniformity. The described embodiments also exhibit better tolerance to incident beam quality and mechanical alignment. Therefore, the described embodiments are more cost-effective to implement and offer longer-term stability. A further advantage of the described embodiments is that the relatively small inter-beam separation angle allows for a relatively large DOE grating pitch, thereby facilitating DOE fabrication. An additional advantage of the described embodiments is that the illumination path from the laser to the wafer is significantly simpler and more compact, improving illumination efficiency and footprint. In addition to the simplified design of the described embodiments, the described embodiments also provide a more stable design.

[0066] Each of the system embodiments described above may also be configured according to any other embodiment(s) described herein.

[0067] Another embodiment relates to a method for generating an image of a sample. The method splits a light beam (e.g., light beam 114 shown in FIG. 1 ) into multiple light beams (124) using an axial DOE (118) positioned within the light beam. The light beam is offset from 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 light beam 114, which illustrate the relative positions of the light beams with respect to the center of the axial DOE). The axial DOE includes concentric grating rings (shown in cross-sectional view 120) configured to split the light beam into multiple light beams. The method also focuses the multiple light beams onto corresponding ones of multiple spots (e.g., spot 200 shown in FIG. 2 ) on the sample. A common focal plane (e.g., 510 shown in FIG. 5 ) of the multiple spots forms an oblique (i.e., non-perpendicular) angle (β) with respect to the optical axis 504 of the multiple light beams. Additionally, the method generates an image of the sample by detecting light from the multiple spots (eg, with TDI sensor 1026 shown in FIG. 10).

[0068] The steps of the method may be performed as detailed herein. The method may also include any other step(s) that may be performed by the system(s) described herein. The steps of the method may be performed by the systems described herein, which may be configured according to any of the embodiments described herein.

[0069] An additional embodiment relates to a non-transitory computer-readable medium having stored thereon program instructions executable on a computer system for performing a computer-implemented method for sample imaging and / or sample inspection. One such embodiment is shown in Figure 11. Specifically, as shown in Figure 11, non-transitory computer-readable medium 1100 includes program instructions 1102 executable on computer system 1104. The computer-implemented method may include any step(s) of any method(s) described herein.

[0070] Program instructions 1102 implementing methods, such as those described herein, can be stored on computer-readable medium 1100. The computer-readable medium can be a storage medium, such as a magnetic or optical disk, magnetic tape, or any other suitable non-transitory computer-readable medium known in the art.

[0071] The program instructions may be implemented in any of a variety of ways, including procedure-based, component-based, and / or object-oriented techniques, among others. For example, the program instructions may be implemented using ActiveX controls, C++ objects, JavaBeans, Microsoft Foundation Classes (MFC), Streaming SIMD Extensions (SSE), or any other technology or methodology, as desired.

[0072] The computer system 1104 may be configured according to any of the embodiments described herein.

[0073] Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art upon review of this specification. For example, methods and systems for generating images of a sample are presented. Accordingly, this specification is to be construed as illustrative only and is intended to teach those skilled in the art the general practice of the invention. The forms of the invention shown and described herein are to be taken as presently preferred embodiments. Elements and materials may be substituted for those shown and described herein, components and processes may be reversed, and certain features of the invention may be utilized independently, all of which would be apparent to those skilled in the art and having reference to this description of the invention. Changes may be made to the elements described herein without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

1. 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 disposed within the light beam, the light beam being offset relative to a center of the axial diffractive optical element, the axial diffractive optical element comprising concentric grating rings configured to split the light beam into a plurality of light beams; a focusing optical system configured to focus the plurality of light beams onto corresponding ones of a plurality of spots on the sample, the common focal plane of the plurality of spots being at an inclination angle with respect to an optical axis of the focusing optical system; an imaging detector configured to detect light from the plurality of spots to generate an image of the sample; A system comprising:

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

3. 10. The system of claim 1, wherein the axial diffractive optical element is further configured to function as a combination diffractive beam splitter and Fresnel lens with weak focusing power of diffraction orders.

4. 10. The system of claim 1, wherein at least two of the plurality of light beams have different powers.

5. 2. The system of claim 1, wherein the plurality of spots have an elliptical shape, the elliptical shape having a major axis at an angle to a scan direction of the imaging detector.

6. 10. The system of claim 1, wherein the imaging detector is further configured as a time delay integration camera.

7. 2. The system of claim 1, wherein the plurality of spots are separated from one another on the sample.

8. 2. The system of claim 1, wherein the imaging detector is further configured as a time delay integration camera, and wherein a projection of each of the plurality of spots onto a line of sight of the time delay integration camera, perpendicular to a scanning direction of the time delay integration camera, overlaps with a projection onto the line of sight of one or more spots adjacent to each of the plurality of spots and included in the plurality of spots.

9. 9. The system of claim 8, wherein a projected distance between each of the plurality of spots and each of the one or more spots adjacent to and included in the plurality of spots is approximately half the size of the plurality of spots.

10. 9. The system of claim 8, wherein the imaging detector is further configured to integrate the light intensity from the plurality of spots along the scan direction.

11. 9. The system of claim 8, wherein the projected overlap of the plurality of spots produces a uniform flat-topped illumination profile.

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

13. 2. The system of claim 1, further comprising beam splitting optics configured to split each of the plurality of light beams into the first and second portions of each of the plurality of light beams to generate first and second sets of light beams, respectively, and wherein the focusing optics is further configured to focus the first and second sets of light beams onto corresponding ones of the plurality of spots on the sample at different azimuthal angles and the same polar angle.

14. 14. The system of claim 13, wherein the difference between the different azimuth angles is 90 degrees.

15. 14. The system of claim 13, further comprising a set of cylindrical lenses disposed 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. 2. The system of claim 1, wherein each of the plurality of spots is focused onto the surface of the specimen.

17. 10. The system of claim 1, further configured for inspection of the specimen.

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

19. 1. A method for producing an image of a sample, comprising: an axial diffractive optical element disposed within the light beam to split the light beam into a plurality of light beams, the light beam being offset relative to a center of the axial diffractive optical element, the axial diffractive optical element comprising concentric grating rings configured to split the light beam into the plurality of light beams; focusing the plurality of light beams onto corresponding ones of a plurality of spots on the specimen, wherein a common focal plane of the plurality of spots is at an oblique angle with respect to the optical axes of the plurality of light beams; and generating an image of the sample by detecting light from the plurality of spots; method.

20. 1. A system configured to inspect a specimen, comprising: a light source configured to generate a light beam; an axial diffractive optical element disposed within the light beam, the light beam being offset relative to a center of the axial diffractive optical element, the axial diffractive optical element comprising concentric grating rings configured to split the light beam into a plurality of light beams; a focusing optical system configured to focus the plurality of light beams onto corresponding ones of a plurality of spots on the sample, the common focal plane of the plurality of spots being at an inclination angle with respect to an optical axis of the focusing optical system; an imaging detector configured to detect light from the plurality of spots to generate an image of the sample; a computer subsystem configured to detect defects on the specimen based on the image; and A system comprising:

Images