Wafer inspection

Abstract

The invention relates to wafer inspection. A system configured to inspect a wafer includes an illumination subsystem configured to guide a first one of a plurality of pulsed light beams to a region on a wafer by the illumination subsystem earlier in time than a second one of the plurality of pulsed light beams to the region; a scanning subsystem configured to scan the plurality of pulsed light beams across the wafer; a light collection subsystem configured to image light scattered from the region on the wafer to one or more sensors; and a computer subsystem configured to detect a defect on the wafer using the output of the one or more sensors. The system in the present invention can optimize inspection speed and / or sensitivity.

Description

[0001] Relevant information of divisional application

[0002] This application is a divisional application of the invention patent application with the application date of July 10, 2012, application number 202210968490.9, and invention title "Wafer Inspection". The invention patent application with the application number 202210968490.9 is a divisional application of the invention patent application with the application date of July 10, 2012, application number 202010984361.X. The invention patent application with the application number 202010984361.X is a divisional application of the invention patent application with the application date of July 10, 2012, application number 201710053384.7. The invention patent application with the application number 201710053384.7 is a divisional application of the invention patent application with the application date of July 10, 2012, application number 201280040556.2.

[0003] Cross-reference to related applications

[0004] This application claims priority to U.S. Provisional Application No. 61 / 506,892, filed Jul. 12, 2011, entitled "Sample Inspection System", which is incorporated herein by reference as if fully set forth herein. Technical Field

[0005] The present invention generally relates to systems configured to inspect wafers. Background Art

[0006] The following description and examples should not be regarded as prior art merely because they are included in this section.

[0007] During the semiconductor manufacturing process, inspection processes are used at multiple steps to detect defects on wafers in order to increase the yield during the manufacturing process and thus obtain higher profits. Inspection has always been an important part of manufacturing semiconductor devices. However, as the size of semiconductor devices decreases, inspection becomes even more important for successfully manufacturing acceptable semiconductor devices because smaller defects can cause the devices to malfunction.

[0008] In wafer inspection systems, there is a need for increased sensitivity to particles, anomalies, and other types of defects while maintaining the overall inspection speed (calculated as the number of wafers per hour). Dark field optical inspection systems generally use laser light to illuminate the wafer in a specific pattern (individual points, lines, or regions) and use collection optics to direct the scattered light onto a corresponding set of sensors.

[0009] One advantage of an inspection system that simultaneously irradiates a large area (about 1 mm by 1 mm) of the wafer compared to irradiating a point (dimensions in microns) or a line (width (microns) times length (mm)) of the wafer is that there are many types of two-dimensional sensors from which information can be obtained in parallel from thousands to millions of individual detectors. Additionally, due to the complexity of the illumination optics and integrating the individual sensors, point-irradiation inspection systems are effectively limited to dozens of points, thus limiting the throughput that can be achieved. Another disadvantage of point and line scan systems is that the illumination energy is concentrated in a relatively small area, increasing the power density on the surface being inspected, which can undesirably alter the sample properties.

[0010] It is well known that the XY (or serpentine) inspection sequence provides a lower inspection throughput compared to a spiral sequence; thus, in some cases, a spiral trajectory (commonly referred to as R-Theta) is desirable. Examples of spiral inspection systems include the SP1 and SP2 instruments, available from KLA-Tencor Corporation, Milpitas, California, USA.

[0011] Although area inspection systems have the advantages described above and as described in the art (e.g., U.S. Patent No. 7,286,697 to Guetta), implementing this configuration on an R-Theta platform has proven challenging because of the inherent mismatch between the spiral order of the resulting images and the rectilinear nature of most two-dimensional array sensors. Detecting defects by real-time alignment and registration of polar images is a computationally intensive activity. Additionally, compared to discrete detectors (e.g., photomultiplier tubes (PMTs)), the additional noise added to the measurement by most two-dimensional silicon-based sensors actually degrades the sensitivity performance of such systems. On an XY-based area inspection system, there is no coordinate mismatch problem, but previous embodiments of such systems have not been able to detect all defects of interest at high speed because of the lack of flexibility in the illumination subsystem and the photon collection subsystem.

[0012] Accordingly, it is advantageous to develop an inspection system and / or method that does not have one or more of the above disadvantages. SUMMARY OF THE INVENTION

[0013] The following description of various embodiments should in no way be construed as limiting the subject matter of the appended claims.

[0014] One embodiment relates to a system configured to inspect a wafer. The system includes an illumination subsystem configured to simultaneously form a plurality of illumination regions on the wafer, where there is substantially no illumination flux between each of the regions. The system further includes a scanning subsystem configured to scan the plurality of illumination regions across the wafer. Additionally, the system includes a collection photon subsystem configured to simultaneously and individually image light scattered from each of the regions onto two or more sensors. The characteristics of the two or more sensors are selected such that the scattered light is not imaged into the gaps between the two or more sensors. The two or more sensors produce outputs in response to the scattered light. The system further includes a computer subsystem configured to use the outputs of the two or more sensors to detect defects on the wafer. This system can be further configured as described herein.

[0015] Another embodiment relates to another system configured to inspect a wafer. This system includes an illumination subsystem configured to direct a plurality of light beams to substantially the same region on the wafer. The plurality of light beams have substantially the same wavelength and polarization characteristics. The system further includes a scanning subsystem configured to scan the plurality of light beams across the wafer. Additionally, the system includes a collection photon subsystem configured to image light scattered from the substantially the same region on the wafer onto a sensor. The sensor produces an output in response to the scattered light. The system further includes a computer subsystem configured to use the output of the sensor to detect defects on the wafer. This system can be further configured as described herein.

[0016] An additional embodiment relates to a system configured to inspect a wafer. The system includes an illumination subsystem configured to direct a first of a plurality of pulsed light beams onto an area of the wafer earlier in time than a second of the plurality of pulsed light beams is directed onto the area. The first and the second of the plurality of pulsed light beams have different shapes and sizes from each other on the wafer. The first and the second of the plurality of pulsed light beams have different wavelengths from each other, different polarizations from each other, or different wavelengths and polarizations from each other. The system further includes a scanning subsystem configured to scan the plurality of pulsed light beams across the wafer. Additionally, the system includes a collection photon subsystem configured to image light scattered from the area on the wafer onto one or more sensors. The one or more sensors generate an output in response to the scattered light. The system further includes a computer subsystem configured to use the output of the one or more sensors to detect defects on the wafer and to determine the power of the second of the plurality of pulsed light beams that should be directed onto the area using the output of the scattered light from the area in response to illumination by the first of the plurality of pulsed light beams. The system can be further configured as described herein.

[0017] A further embodiment relates to another system configured to inspect a wafer. The system includes an illumination subsystem configured to direct light pulses onto an area of the wafer. The system further includes a scanning subsystem configured to scan the light pulses across the wafer. Additionally, the system includes a collection photon subsystem configured to image light pulses scattered from the area on the wafer onto a sensor. The sensor is configured to integrate a certain number of scattered light pulses, and the number of scattered light pulses is less than the number of scattered light pulses that can be imaged over the entire area of the sensor. The sensor is configured to generate an output in response to the integrated scattered light pulses. The system further includes a computer subsystem configured to use the output generated by the sensor to detect defects on the wafer. The system can be further configured as described herein.

[0018] Another embodiment relates to a system configured to inspect a wafer. The system includes an illumination subsystem configured to direct light onto an area on the wafer. The system also includes a scanning subsystem configured to scan the light across the wafer. Additionally, the system includes a light collection subsystem configured to image light scattered from the area on the wafer onto a sensor. The sensor is configured to generate an output in response to the scattered light. The system further includes a computer subsystem configured to use the output generated by the sensor to detect point defects on the wafer, determine the size of the point defects in pixels, determine the focusing condition of the system based on the size of the point defects, and change one or more parameters of the system based on the focusing condition. The system can be further configured as described herein.

[0019] An additional embodiment relates to a system configured to inspect a wafer. The system includes an illumination subsystem configured to direct light onto an area on the wafer. The system also includes a scanning subsystem configured to scan the light across the wafer. Additionally, the system includes a light collection subsystem configured to image light scattered from the area on the wafer onto a sensor. The sensor is configured to generate an output in response to the scattered light. The system also includes a computer subsystem configured to use the output generated by the sensor to detect defects on the wafer. The system can be further configured as described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other objects and advantages of the present invention will become immediately apparent upon reading the following detailed description and upon reference to the accompanying drawings, in which:

[0021] Figure 1 Schematic side view illustration of an embodiment of a system configured to inspect a wafer;

[0022] Figure 2 Schematic plan view illustration of an embodiment of a plurality of rectangular illumination areas on a wafer;

[0023] Figures 3 to 6 Schematic side view illustration of various embodiments of a system configured to inspect a wafer;

[0024] Figure 7 Schematic plan view illustration of an embodiment of a central region of a wafer and a region outside the central region of the wafer;

[0025] Figure 8Schematic plan view of an embodiment illustrating different ways of scanning a central region of a wafer and regions outside the central region of the wafer as described by the embodiments herein;

[0026] Figure 9 Schematic side view of an embodiment of a system configured to inspect a wafer;

[0027] Figure 10 Schematic plan view of an embodiment illustrating a plurality of light beams directed to substantially the same region on a wafer at substantially the same polar angle and different azimuthal angles;

[0028] Figure 11 Schematic plan view of an embodiment illustrating a plurality of light beams on the wafer having different shapes and sizes from each other;

[0029] Figure 12 Schematic side view of an embodiment of a system configured to inspect a wafer;

[0030] Figure 13 Schematic side view of an embodiment of a plurality of light beams, the plurality of light beams including a light beam generated by a light source of an illumination subsystem and an additional light beam formed by collecting light reflected from substantially the same region on the wafer and directing the collected light back to substantially the same region on the wafer;

[0031] Figure 14 Schematic plan view of an embodiment of a sensor including a rectangular pixel array; and

[0032] Figure 15 Schematic plan view illustrating how the size of a point defect in pixels can vary according to the focusing conditions of a system configured to inspect a wafer.

[0033] The present invention admits of many modifications and alternative forms. Specific embodiments of the present invention are shown by way of example in each of the figures and will be described in detail herein. However, it should be understood that the drawings of the present invention and the detailed description thereof are not intended to limit the present invention to the particular forms disclosed, but on the contrary, are intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Detailed Description

[0034] Generally, embodiments described herein relate to wafer inspection methods and systems, which include the following steps: causing illumination (e.g., laser illumination) to impinge on the wafer; translating the wafer or the illumination point on the wafer in some manner; collecting scattered light through a light collector system (which may include a light collecting objective lens); in the light collecting optics, the scattered light may be split based on selectable polarization and / or scattering angle characteristics; guiding a selected portion of the scattered light to one or more sensors; and detecting defects by processing the output (e.g., image information) generated by the (one or more) sensors.

[0035] Referring now to the drawings, it should be noted that the drawings are not drawn to scale. In particular, the scale of some elements in the figures has been particularly exaggerated to emphasize the characteristics of the elements. It should be noted that the drawings are not drawn to the same scale. The same reference numerals have been used to indicate elements that may be similarly configured in more than one figure.

[0036] One embodiment relates to a system configured to inspect a wafer. To optimize inspection speed and / or sensitivity, spatially discontinuous illumination profiles may be used. For example, the system includes an illumination subsystem configured to simultaneously form a plurality of illumination regions on the wafer, where there is substantially no illumination flux between each of the regions. In this way, the system is configured to perform multi-point (“multi-patch”) area inspection.

[0037] All illumination subsystems described herein include one or more light sources that may be coupled to some illumination optics. For example, multi-patch illumination can be generated by three methods: multiple lasers, with each laser used for one patch; multiple laser beams from one laser; and diffractive optical elements that split one or more laser beams. In one such example, the illumination subsystem may include a laser source (or multiple laser sources) that illuminates the wafer with polarized light at a particular incident angle or multiple discrete incident angles. The optimal illumination incident angle for inspection depends, among other factors, on the type of wafer being inspected and the defects to be detected that are of interest. The illumination subsystem may be configured to allow sequential or simultaneous illumination at near-normal incident angles and / or at oblique angles of 45 degrees or greater. Additionally, the laser source may be a pulsed laser.

[0038] The multiple illumination regions may exhibit different cross-sectional shapes on the wafer, e.g., substantially flat-top, Gaussian beam, non-Gaussian beam, any other structured area illumination, etc. For example, multiple flat-top illumination regions may be formed on the wafer, with no illumination flux between these regions. The multiple illumination regions may be formed on the surface of the wafer (e.g., on the uppermost (surface) of the wafer). However, the multiple illumination regions may also be formed on a wafer with a film, at a specific interface in a film stack, or even at a subsurface (e.g., within the wafer).

[0039] In one embodiment, each of the multiple illumination regions is rectangular on the wafer. For example, as Figure 2 shown, each of the multiple illumination regions 200 may be rectangular on the wafer 202, and the direction of wafer travel may be in the direction shown by arrow 204. In Figure 2 the embodiment shown, three separate illumination regions (or patches) are formed on the wafer, as will be further described herein (e.g., formed by three separate laser beams or diffractive optical elements). The staggering of the patches may be accomplished using a method similar to the method used in current multi-point inspection systems.

[0040] One advantage of this embodiment in laser area inspection is that the embodiment is a solution to the problem that relatively fast sensors tend to be substantially rectangular (i.e., one dimension of the sensor is substantially longer than the other dimension). It is difficult to fabricate substantially rectangular patches (e.g., aspect ratios of 40:1 or 100:1) on a wafer. In the embodiments described herein, three patches with aspect ratios of 13:1 or 33:1 may replace the 40:1 or 100:1 patches, respectively, and three slower sensors may replace one larger substantially elongated sensor. Generally, for area inspection mode systems, ratios between 1:1 and 100: may be considered. In one embodiment, on the wafer, the multiple illumination regions do not overlap each other. For example, since this system is "flash on the fly", the patches may be arranged on the wafer in a non-overlapping manner, and the stage (described further herein) will move only the correct amount between each flash. If more convenient, the patches may be projected in a 1×3 array rather than a 3×1 array. The term "rectangular shape on the wafer" as used herein generally refers to a substantially rectangular shape but may not be an exact rectangle due to the inherent limitations in imaging any beam.

[0041] In one embodiment, the illumination subsystem forms multiple illumination regions on the wafer using multiple beams generated from a single beam. For example, a diffractive optical element may be used to generate the multiple beams from one beam. In asFigure 1 In one such embodiment shown, the illumination subsystem includes a light source 100 and a diffractive optical element 110. The light source and the diffractive optical element are configured such that a light beam generated by the light source is directed to the diffractive optical element, and the diffractive optical element generates two or more (e.g., three) light beams 112 from the single light beam. The light source can include any light source described herein, and the diffractive optical element can include any suitable diffractive optical element known in the art. As Figure 1 shown, the multiple light beams can be directed to the wafer 114 at an oblique incident angle. However, the multiple light beams can be directed to the wafer at any other suitable incident angle, as further described herein. As Figure 1 The illumination subsystem shown can include any other suitable optical elements, such as reflective optical elements, refractive optical elements, polarizers, apertures, beam shaping elements, wavelength filters, and the like.

[0042] In another embodiment, the illumination subsystem uses multiple light beams generated by multiple light sources to form multiple illumination regions on the wafer. For example, as Figure 3 shown, the illumination subsystem can include multiple light sources 300, 302, and 304. The light sources can include any light source described herein, such as pulsed lasers. Each of the multiple light sources is configured to generate light having the same characteristics (e.g., each of the multiple light sources is a laser of the same brand and model). As Figure 3 shown, the multiple light sources can generate multiple light beams 306, and the multiple light beams can be directed to the wafer 114 at the same incident angle or approximately the same incident angle. However, the multiple light beams can be three laser beams incident at different angles. Additionally, although Figure 3 the multiple light beams are shown being directed to the wafer at an oblique incident angle, the multiple light beams can also be directed to the wafer at a normal incident angle or a near-normal incident angle. Figure 3 The illumination subsystem shown can include any other suitable optical elements, such as the optical elements described above. Figure 3 The system shown can be further configured as described herein.

[0043] In some embodiments, the illumination subsystem includes a frequency-converted laser, the illumination subsystem being configured to simultaneously form the plurality of illumination regions using optical pulses, and the optical pulses directed to the regions on the wafer being spatially invariant and having a substantially constant intensity during the duration of the optical pulses. For example, the illumination subsystem described herein may use a frequency-converted laser with spatially flat-top and temporally flat-top illumination output in area mode inspection. Area mode inspection systems typically use a continuous Gaussian or "flat-top" illumination profile on the surface of the wafer. In one such embodiment, the illumination subsystem includes beam shaping optics coupled to the laser. For example, as Figure 6 shown, the illumination subsystem may include beam shaping optics 600 coupled to a light source 100, which may be a laser in this example. The beam shaping optics may include any type of beam shaping optics known in the art. Additionally, although Figure 6 the beam shaping optics are shown coupled to only one light source such that the beam shaping optics are in the path of only one beam, the beam shaping optics may be coupled to each of the light sources included in any of the illumination subsystems described herein or may be positioned in the path of each of the illumination beams used by the systems described herein. Figure 6 The illumination subsystem and system shown in

[0044] may be further configured as described herein. The flat-top beam may be generated not only by diffractive optics external to the laser or other beam shaping optical devices, but also within the laser itself as a natural result of optimizing the nonlinear frequency conversion process. An additional option is to use a laser that provides a user-specified temporal pulse shape to further reduce the likelihood of wafer damage. For example, most commonly used pulsed lasers exhibit a roughly hyperbolic secant pulse shape in time, where the peak intensity is more than twice the average intensity. However, recent developments in laser technology have enabled the generation of so-called "flat-top" or "box car" temporal pulse shapes. The peak intensity of these pulses is substantially equal to the average intensity, and can increase the inspection throughput by approximately a factor of two. In another embodiment, the illumination subsystem includes a frequency-converted laser, the illumination subsystem being configured to simultaneously form the plurality of illumination regions using optical pulses, and the optical pulses directed to the regions on the wafer having a substantially constant intensity during the duration of the optical pulses. This embodiment may be configured as described above, except that the optical pulses are allowed to vary spatially during the duration of the optical pulses.

[0045] The system further includes a scanning subsystem configured to scan the plurality of illumination regions across the wafer. The scanning subsystem may include a chuck that holds the wafer in place during inspection. For example, as Figure 1 shown, the scanning subsystem may include a chuck 116. The chuck may be an edge-grip chuck, a vacuum chuck, or a pneumatic bearing chuck. One chuck may support multiple wafer diameters (e.g., 300 mm and 450 mm) or a single substrate diameter. The scanning subsystem may further include a shaft 118 coupled to the chuck 116 and coupled to the positioning subsystem 120. The positioning subsystem may include various elements configured to rotate and / or translate the shaft 118, such as motors, gears, platforms, and the like. The shaft 118 may be coupled to the chuck 116 in such a way that rotation and / or translation of the shaft 118 causes rotation and / or translation of the chuck and thus the wafer.

[0046] The scanning subsystem may translate the wafer in a spiral or X-Y pattern or in a combination of the two as further described herein. Specifically, in addition to the spiral scanning described above, X-Y serpentine scanning and RT-XY hybrid scanning may be employed to translate the wafer relative to the illumination optics and the light collection optics. The spiral motion inspection system described herein is similar to the SP1 and SP2 inspection systems available from KLA-Tencor Corporation of Milpitas, California, USA, and some significant differences will be described herein. For example, the illumination regions on the wafer are substantially larger, typically extending hundreds of microns up to several millimeters, the spindle rotation rate is relatively moderate, typically not exceeding 1,000 rpm to 5,000 rpm, and the light collection subsystem may have near diffraction-limited performance. In addition, substrates up to and exceeding 450 mm in diameter may be inspected by the system herein.

[0047] In a spiral inspection system, the rotation rate at the center of the wafer should be sufficient to support the generation of inspection frames with the desired overlap. In one embodiment, the illumination subsystem is configured to simultaneously form a plurality of illumination regions using light pulses, the light scattered from each of the regions includes scattered light pulses, the scanning subsystem is configured to scan the light pulses across the wafer by rotating the wafer, and when scanning the light pulses across the central region of the wafer, the illumination subsystem is configured to direct the light pulses to the plurality of illumination regions on the wafer less frequently than when scanning the light pulses across the outer side of the central region of the wafer. For example, for inspection using a pulsed laser and area sensors on a rotating platform, near the center of the wafer, as the linear rate of the wafer decreases proportionally to the radius at the center of the wafer, the trigger frequency of the laser pulses can be increasingly reduced. In this way, the scanning continues at a lower rate (in terms of area per unit time), while the sensitivity of the inspection remains constant. The full average power of the laser is not used. Alternatively, during scanning, as long as the light source does not cause damage to the wafer, the illuminated area can be continuously reduced. In one such embodiment as Figure 7 shown, the central region 700 of the wafer 114 can be a region that surrounds the center 702 of the wafer and is spaced apart from the edge 704 of the wafer. The central region can surround one-third or one-quarter of the interior of the wafer. The portion of the wafer included in the central region of the wafer can depend on, for example, the rotation speed of the wafer, the diameter of the wafer, the power of the laser, and any other parameters related to the power to which the wafer is exposed at any given time.

[0048] In one embodiment, the illumination subsystem is configured to simultaneously form the plurality of illumination regions using light pulses, the light scattered from each of the regions includes scattered light pulses, the scanning subsystem is configured to scan the light pulses across the wafer by rotating and translating the wafer, the two or more sensors include area sensors, when scanning the light pulses across the central region of the wafer, the scanning subsystem scans the light pulses across the wafer along one or more non-curved lines, and when the light pulses are scanned across the outer side of the central region of the wafer, the scanning subsystem scans the light pulses across the wafer in a spiral manner. In this way, the embodiments described herein can be configured to perform hybrid scanning on a rotating platform using a pulsed laser and area sensors during inspection. For example, in the hybrid method, most of the wafer can be scanned in a spiral manner. In as Figure 8In one such instance as shown, the region 800 of the wafer 114 outside the central region 700 of the wafer can be scanned in a spiral manner 802 (which can be defined as above). Subsequently, a series of small xy serpentine movements or a single linear movement or a combination of a linear movement after an angular rotation can be performed to scan the central region 700 of the wafer. In this way, the central region can be scanned in a linear manner 804, where the scan is performed in the x direction or the y direction and is incrementally translated in the opposite direction between scans, or in a radial manner 806, where the scan is performed along the radius of the wafer between incremental rotations of the wafer. In this way, any part of the center of the wafer that cannot be scanned (which can occur when the alignment of the wafer relative to the scanning subsystem or the optics is inaccurate) can be avoided, making it less challenging to properly align the output from the sensor, and the inspection throughput can be increased. In addition, the "smearing" effect across the circular track of the rectangular sensor can be minimized according to the repetition rate of the light source.

[0049] The system includes a light collection subsystem configured to simultaneously and individually image the light scattered from each of the regions onto two or more sensors. Generally, the light collection subsystem described herein may include some kind of scattered light collector (e.g., the light collection objective lens of the scattered light collector 122 as shown, for example Figure 1 in) and some possible additional optical elements coupled to the scattered light collector (e.g., a small hole, a separator, a polarization element, one or more reflective optical elements, and one or more refractive optical elements, such as Figure 1 the refractive optical element 124 as shown in). The same light collection lens can image the scattered light from each of the regions onto multiple sensors. For example, as Figure 1 shown in, the scattered light collector 122 can collect the scattered light 126 from the multiple illumination regions on the wafer and the scattered light 128 from another of the multiple illumination regions on the wafer.

[0050] The light collection subsystem may include one or more objective lenses for collecting the light scattered from the wafer. In addition to relatively high numerical aperture (NA) objective lenses, other sets of lower NA or even non-imaging light collection optical devices can also be placed near the horizontal line in the light collection hemisphere. This will collect the light scattering information from these angles, thereby allowing further capture of defects and features of interest that cannot be detected by the main objective lens.

[0051] The light collection subsystem may also include a variety of elements to selectively filter the scattered light to increase the defect capture rate of interest and reduce the false alarm rate. The variety of elements may include, for example, optical elements and microelectromechanical systems (MEMS)-based devices as described herein. Additionally, the variety of elements may include polarizers, beam splitters, small apertures, spatial filters, and the like.

[0052] The light collection subsystem may further include one or more optical elements configured to image the filtered light onto two or more sensors (e.g., two or more area sensors). For example, Figure 1 the refractive optical element 124 shown in Figure 1 may be configured to image the filtered light onto sensors 130 and 132 as shown in

[0053] Additionally, the light collection subsystem is preferably configured such that the light from the plurality of illumination regions on the wafer is individually imaged onto only the corresponding sensors. For example, as shown in Figure 1 the scattered light 126 from the first of the plurality of illumination regions is imaged only onto sensor 130, and the scattered light 128 from the second of the plurality of illumination regions is imaged only onto sensor 132. In this way, light from more than one of the plurality of illumination regions will not be imaged onto the same sensor.

[0054] The characteristics of the two or more sensors are selected such that the scattered light is not imaged into the gap between the two or more sensors. For example, sensors 130 and 132 may be selected and configured such that the scattered light 126 and 128 are not imaged into the gap 134 between the two sensors. In one such example, two smaller and less costly sensors may be used without experiencing an unwanted sensitivity loss from scattered laser light that would otherwise image into the "gap" between the sensors. Due to factors such as packaging limitations, support electronics, etc., a gap between discrete sensors is often inevitable. Additionally, for two-dimensional sensors and their inherent limitations (e.g., limitations on their data rate, column rate, etc.), the light source and the sensor may not be well coupled together. Some currently used systems include sensor focal plane arrays to overcome these limitations. However, in the embodiments described herein, the light source and the two-dimensional sensing characteristics are matched to overcome the limitations.

[0055] The two or more sensors generate outputs in response to the scattered light. The two or more sensors can include sensors with relatively low point or resolution. The two or more sensors can also include, for example, discrete photomultiplier tubes (PMTs), charge-coupled devices (CCDs), time delay integration (TDI), complementary metal oxide semiconductor (CMOS) sensors, scientific CMOS (sCMOS), PMT arrays, electron bombardment CCDs (EB-CCDs), electron multiplying CCDs (EM-CCDs), intensified photodiodes, or avalanche photodiode (APD) arrays. Each channel and / or sensor can be configured to respond to illumination wavelengths or additional wavelengths or a combination of both generated by the wafer interaction by using wavelength filtering techniques. This allows for more selective detection of certain types of defects of interest. Additionally, the sensors used in the systems described herein can vary depending on the type of scan used for inspection and / or the light sources included in the illumination subsystem. For example, in an XY scan configuration, a mode-locked laser with a higher repetition rate can be used to illuminate the wafer to minimize laser-induced wafer damage to sensors configured to obtain data in TDI mode.

[0056] In some cases, the elements of the light collector subsystem can be selected based on one or more characteristics of the two or more sensors. For example, in some cases, the light collector subsystem can include one or more tube lenses, and the distortion ratio of the one or more tube lenses can be selected based on the aspect ratio of the two or more sensors. Additionally, if different types of sensors are used for different channels of the system, different tube lenses can have different distortion magnifications to ensure that each sensor measures the same area on the wafer.

[0057] The objective lens included in the light collector subsystem can be a relatively high NA lens that is diffraction limited within the field of view. Alternatively, a non-diffraction limited objective lens can be employed. For example, in one embodiment, the light collector subsystem includes a scattered light collector with a resolution that is not fully diffraction limited. Specifically, to reduce costs, the collector design and manufacture can be appropriately matched to the distorted point spread function generated by the associated commonly used small apertures and polarizers in the collection channel. Given a substrate type, the resolution specifications of the objective lens can be calculated by pre-knowing the target defect geometry and material (e.g., silicon oxide spheres) and optimizing the pupil filter or Fourier plane filter for the capture rate of the defect. Reducing the requirement for fully diffraction limited resolution can save the system user a significant amount of cost.

[0058] The system described herein may further include an autofocus subsystem (not shown in the figures). The autofocus subsystem can ensure that the surface of the wafer remains at the focus of the sensor regardless of the movement of the wafer, light source, light collecting optics, and illumination optics. The autofocus subsystem can include a light source (which may or may not be the light source for inspection), a sensor, circuitry, and logic for determining the position of the wafer image relative to the sensor (e.g., two-dimensional sensing) and a feedback system for correcting any deviations noted during inspection. The autofocus subsystem can be further configured as described herein.

[0059] The system further includes a computer subsystem configured to detect defects on the wafer using the outputs from the two or more sensors. In this way, the computer subsystem provides means for detecting defects in the signals or other outputs generated by the sensors. For example, as Figure 1 shown, the system includes a computer subsystem 136 coupled to the two or more sensors such that the computer subsystem can receive the outputs generated by the two or more sensors. The computer subsystem can be configured to detect defects on the wafer using the outputs and any suitable defect detection algorithms and / or methods. For example, the computer subsystem can apply a defect detection threshold to the outputs and thus can identify any output found to exceed the defect detection threshold as a defect or a potential defect.

[0060] The computer subsystem can include any suitable computer system known in the art. For example, the computer subsystem 136 can take many forms, including a personal computer system, a mainframe computer system, a workstation, an image computer, a parallel processor, or any other device known in the art. In general, the term "computer subsystem" can be broadly defined to cover any device having one or more processors that execute instructions from a memory medium.

[0061] Regarding the collection of scattered light, an improvement of the embodiments described herein over currently used systems lies in the selective and configurable collection of surface scattering to enhance the detection of particles and defects. Some previously used systems include a rotating spatial filter system in the light collection optics to eliminate pattern scattering effects and enhance scattering from point particles and defects. In the embodiments described herein, during wafer scanning at a specific orientation relative to the illumination angle, the filter can be fixed while the wafer rotates below. The filter rejects certain solid collection angles (including unwanted scattered light from the background rather than from the defects of interest) through a combination of multiple polarizers arranged at selected angles and movable material segments opaque to the wavelengths of the scattered light. Filtering is performed in the rear Fourier plane of the objective lens such that unwanted background at each point in the illuminated field can be eliminated simultaneously.

[0062] Multiple region type sensors can be used in combination with the spatial filtering techniques described herein. For example, the system can include a flexible light collection system where multiple sensors are selectively configured to detect scattered light having multiple polarization states and / or solid scattering angles. Each sensor can be arranged to collect scattered light that cannot be collected by other sensors (if any). Additionally, each sensor can be a multi-element sensor and can have different characteristics. For example, a sensor can be an enhanced EB-CCD sensor. Another sensor can include an independent magnetic focusing image intensifier that is coupled to a CCD or a CMOS chip with a delay lens. A third sensor can be a lower resolution independent CCD chip. There can also be additional sensors. The sensor type and size can be selected for each channel based on the characteristics of the scattered background expected to be present in each channel and the sensitivity requirements for the defects of interest in that channel. A lower resolution is preferred when the point spread function projected onto a particular sensor is likely to be large due to spatial filtering. In this way, the system can be optimized to use lower cost sensors in channels dominated by other noise sources to reduce operating costs.

[0063] The above system configuration can be implemented in many different embodiments to be described in the present invention. For example, in one embodiment, the system includes optical elements configured to simultaneously and separately divide the light scattered from each of the regions collected in different segments of the light collection NA of the light collection subsystem, two or more sensors are configured to detect one of the different segments, and the system includes two or more additional sensors configured to detect another of the different segments. Figure 4 One such embodiment is shown, where the optical element 400 is positioned in the path of the light collected by the scattered light collector 122. For clarity, Figure 4Only scattered light 126 from one of the multiple illuminated regions on the wafer is shown. Preferably, the optical element is positioned in the Fourier plane or at the conjugate of the Fourier plane of the photon collection system. "At a Fourier plane" or "at a conjugate of the Fourier plane" is defined herein to mean not exactly at the Fourier plane or exactly at the conjugate of the Fourier plane. Instead, these terms are intended to mean "at or near a Fourier plane" or "at or near a conjugate of the Fourier plane", respectively. If the optical element is positioned exactly at the Fourier plane or at a position with an error within 5% of the exact position of the Fourier plane (due to any source of error in the system and / or any physical constraints of the system), then the optical element is considered herein to be "at or near the Fourier plane". "At or near the conjugate of the Fourier plane" can be described in a similar manner.

[0064] The optical element can include a variety of optical elements, such as a small hole, a mask, a mirror with a small hole, a liquid crystal display (LCD) element, or a micro-mirror array. In one such example, a suitable small hole can be formed by cutting out a portion of a folding mirror such that a portion of the mirror transmits light while another portion of the folding mirror reflects light. In another such example, a mirror with a small hole can be fabricated by forming a mask coating of a metal film and / or a dielectric film on a transparent substrate. The segmentation of the collection NA can also be achieved by using other beam-splitting optical elements, such as a prism with a plurality of facets oriented to refract light in different directions. Other ways can also be used to segment the collection NA, including digital micro-mirror devices, such as those commonly used in digital projectors.

[0065] The optical element (and other optical elements described herein) is used to separate the collection NA into different segments such that scattered light in different segments can be directed to different sensors or channels of the system. For example, as described above, the optical element can have a portion that reflects light and another portion that transmits light. Thus, the optical element can separate the collection NA into two segments, one of the two segments being directed to one channel by reflection and the other of the two segments being directed to another channel by transmission.

[0066] In one embodiment, as Figure 4As shown in the cross-section, the optical element may include transmission portions 402 and 404 corresponding to one segment of the light-collecting NA, and a reflection portion 406 corresponding to another different and mutually exclusive segment of the light-collecting NA. The reflection portion 406 may reflect substantially all of the light in the segment of the light-collecting NA corresponding to portion 406 (i.e., the transmittance of portion 406 for the scattered light is approximately 0%), while portions 402 and 404 may transmit substantially all of the light in the segments of the light-collecting NA corresponding to portions 402 and 404 (i.e., the transmittance of portions 402 and 404 for the scattered light is substantially 100%). In this way, the entire light-collecting NA can be separated into two mutually exclusive parts.

[0067] As described above, different parts of the optical element correspond to different segments of the light-collecting NA into which the scattered light is separated as it passes through the optical element. Additionally, as Figure 4 shown, portions 402 and 404 are mirror-symmetrical to each other about the incident plane of the illumination subsystem. Additionally, portions 402 and 404 may correspond to one of the different segments of the light-collecting NA. In this way, one of the different segments may include two individual segments (corresponding to portions 402 and 404) that are mirror-symmetrical about the incident plane of the illumination subsystem. Additionally, as Figure 4 shown, each of portions 402 and 404 is spaced apart from the incident plane. Additionally, each of the portions may be bounded by a first side, a second side, and a third side, and the three sides will be described below with respect to portion 402. Specifically, portion 402 includes a first side 402a, a second side 402b, and a third side 402c. The first side 402a is linear and arranged at an angle with respect to the incident plane. The second side 402b is linear and substantially parallel to the incident plane, and is substantially shorter than the first side. Additionally, the third side 402c is curved. As Figure 4 shown. The three sides also bound portion 404.

[0068] As Figure 4As further shown, two or more sensors (represented by sensor 130) are configured to detect one of the different segments, and the system includes two or more additional sensors (represented by sensor 408) configured to detect another of the different segments. Sensor 408 and the other two or more sensors can be further configured as described herein. Additionally, the two or more sensors and the two or more additional sensors can be the same type of sensor or different types of sensors. For example, the sensors can be selected based on the amount of light expected to be directed to each of the two or more sensors and the two or more additional sensors. Additionally, for example, optical elements further described herein can be coupled to the other two or more sensors. For example, as Figure 4 shown, refractive optical element 410 can be configured to image the light reflected by optical element 400 onto sensor 408 and any one of the two or more additional sensors included in the system. Figure 4 The system shown in Figure 4 can be further configured as described herein.

[0069] In these and any other embodiments described herein, each channel can ultimately have a point spread function with a different shape and different breadth (in pixels) on the corresponding sensor. Thus, to maximize sensitivity to anomalies, different analog and / or filtering techniques can be applied to each individual sensor output. Specifically, the shape of the expected point spread function can be calculated in advance based on the Fourier plane aperture before inspection, and then appropriate filter coefficients can be applied during inspection.

[0070] In one such embodiment, the system is configured to change or replace the optical element based on one of the different segments to be detected by the two or more sensors and another of the different segments to be detected by the two or more additional sensors. For example, in an area mode inspection system having multiple configurable channels, the system can include a flexible pinhole light collection space. One improvement of this area inspection system over other inspection systems is the selectivity of surface scattering and configurable collection to enhance the detection of particles and defects. The system can be configured to change or replace the optical element in any suitable manner.

[0071] In another embodiment, the system includes an optical element configured to simultaneously and separately split the light scattered from each of the regions collected in different segments of the light collection NA of the light collection subsystem, the two or more sensors being configured to detect one of the different segments using a portion of the two or more sensors and to detect another of the different segments using a different portion of the two or more sensors, and the portion and the different portion of the two or more sensors not overlapping and not adjacent on the two or more sensors. For example, the system can be configured to angularly separate the scattered light in the light collection space and re-image the light as two separate patches onto a single sensor. Specifically, the number of effective pixels on the sensor can be controlled during or prior to scanning, either in combination with or independent of the illumination shape and illumination extent. All or some of the elements located on a particular sensor can be used. A portion of a sensor (including several elements) can receive scattered light from one range of solid angles, and another portion of the sensor can receive scattered light from another range of solid angles. For example, if a sensor includes 1000×1000 individual elements, then 1000×500 of the elements can receive an image of the scattered light generated between 40 degrees and 60 degrees of azimuth. The other fifty percent (1000×500) of the sensor can receive an image of the scattered light from the surface generated between 120 degrees and 160 degrees of azimuth. In some cases, the portion of the scattered light imaged onto the sensor surface can be inverted, and other portions can remain non-inverted. An additional configuration is to read sensor data simultaneously from both ends of each column (e.g., row 1 and row N), which in some sensors can effectively double the sensor data rate.

[0072] Each of the system embodiments described herein can be further configured according to the International Application Publication No. 2012 / 082501 filed on Dec. 7, 2011 by Zhao et al., which is incorporated herein by reference in its entirety.

[0073] In a further embodiment, the system includes an additional two or more sensors that include image intensifiers, the light collection subsystem being configured to simultaneously and separately image the light scattered from each of the regions onto the additional two or more sensors, the additional two or more sensors generating additional outputs in response to the scattered light, and the computer subsystem being configured to use the additional outputs to detect defects on the wafer in place of the outputs when sensor electronic noise dominates the total channel noise in the two or more sensors. For example, this embodiment can include one or more flexible apertures as described above to optimize the performance, cost, and reliability of the sensors. In one such embodiment,Figure 5 The system shown in FIG. 5 includes an optical element 500 configured to simultaneously and individually image light scattered from each of the regions to an additional two or more sensors ( Figure 5 , represented by sensor 502). The optical element 500 may be further configured as described herein. However, the optical element 500 may also include a beam splitter configured to transmit a portion of the scattered light across the entire collection NA of the collection subsystem and to reflect a portion of the scattered light across the entire collection NA of the collection subsystem. For example, the optical element 500 may be a simple 70 / 30 beam splitter. In addition, as described above, the sensor type and size may be selected for each channel based on the expected scattering background characteristics in the channel and the sensitivity requirements to the defects of interest in the channel. In some such cases, when the sensor electronic noise dominates the total channel noise, an enhanced sensor may be desirable. However, when another noise source other than the sensor read noise dominates, a non-enhanced sensor may be preferred. For example, the additional two or more sensors (in Figure 5 ) each including an image intensifier (in Figure 5 ), and the two or more sensors (in Figure 5 ) may not include any image intensifier. This configuration may also be reversed so that the two or more sensors ( Figure 5 ) each including an image intensifier ( Figure 5 ) and making the additional two or more sensors ( Figure 5 ) does not include any image intensifier. In this way, various optical elements described herein (e.g., flexible pinholes and mirror arrangements) can be used to direct light to enhanced sensors when the light is low, and to other non-enhanced sensors when the light is high. According to sampling theory, when the point spread function projected onto a particular sensor is expected to be large due to spatial filtering, a lower overall sensor resolution can be allowed to be used. For example, in some channels, the illuminated patch on the chip may be approximately 2000 point spread functions in breadth when imaged through the collection optics and spatial filters. In other channels, due to different spatial filters limiting the collection NA, the image of the illuminated patch on the chip may be 1000 point spread functions in breadth. In this way, lower cost sensors can be used in channels dominated by other noise sources to optimize the system in order to reduce operating costs. In addition, enhanced sensors generally have a shorter lifespan than non-enhanced sensors, so this particular configuration also allows for improved system reliability. Figure 5 The system shown in can be further configured as described herein. For example,Figure 5 As shown, the photon collection subsystem may include a refractive optical element 506 configured to image scattered light from the optical element 500 onto two or more additional sensors. The refractive optical element 506 may be further configured as described herein. Figure 5 The system shown may be further configured as described herein.

[0074] This embodiment may also or alternatively include one or more flexible apertures as described above to direct scattered light to the most appropriate sensor. For example, one sensor may be optimized for a large amount of light scattering, while another sensor may be optimized for very weak light scattering. In this configuration, a sensor optimized for relatively low light scattering (e.g., an image intensifier sensor) may be impaired by a large amount of scattering and may not be necessary for achieving optimal sensitivity even in a relatively large background. Thus, during some portions of the scan, the optical element may be configured to direct a portion of the scattered light to a sensor optimized for a large amount of light scattering and a different portion of the scattered light to a different sensor optimized for low light scattering. In different instances, the optical element may be configured to direct all of the scattered light to only one of the multiple sensors included in the system, and the sensor to which the scattered light is directed may be changed during the scan.

[0075] In another embodiment, the system includes two or more additional sensors configured to perform photon counting, the photon collection subsystem is configured to simultaneously and separately direct light scattered from each of the regions to the two or more additional sensors, the two or more additional sensors generate additional outputs in response to the scattered light, and the computer subsystem is configured to use the additional outputs to detect defects on the wafer. For example, this embodiment may include a plurality of flexible apertures as described above to optimize the performance, cost, and reliability of the sensors. In some such cases, so-called photon counting techniques may be employed for one or more of the sensors included in the system. This embodiment may be configured as shown in Figure 4 or 5, but with the two or more additional sensors (represented by the combination of sensor 408 or sensor 502 and image intensifier 504, respectively) replaced with sensors configured for photon counting. The sensors configured for photon counting may be any suitable such sensors known in the art, such as avalanche photodiodes.

[0076] In one embodiment, the system includes a MEMS-based optical switching device positioned between the photon collection subsystem and the two or more sensors. For example, there are relatively fast MEMS-based optical switching devices that can be reconfigured between each laser pulse. One or more of these optical switching devices can be placed at a suitable location in the light collection optics. For example, the optical elements 400 and 500 as shown in Figure 4 and 5 can be replaced by MEMS-based optical switching devices, respectively. The MEMS-based optical switching device can include any suitable such elements known in the art.

[0077] In one such embodiment, the system includes an additional two or more sensors, the illumination subsystem is configured to simultaneously form a plurality of illumination regions using light pulses, the light scattered from each of the regions includes scattered light pulses, and the optical switching device is configured to direct a first set of scattered light pulses generated by a first set of light pulses to the two or more sensors and direct a second set of scattered light pulses generated by a second set of light pulses after the first set of light pulses to the additional two or more sensors. For example, if the optical elements 400 and 500 as shown in Figure 4 and 5 are replaced by the optical switching devices as described above, then the additional two or more sensors represented by the detectors 408 in Figure 4 and the detectors 502 in 5 can be used as the additional two or more sensors in this embodiment. In this way, the optical switching device in the light collection optics can direct alternating frames to alternating sensors to save cost. In this way, if a laser with a relatively high repetition rate can be used, but the data rate and / or frame rate of a particular type of sensor is limited, then the scattered light generated by subsequent laser pulses can be directed to alternating sensors by reconfiguring the MEMS device between laser pulses. For example, the scattered light generated by a pulsed laser at a frequency f can be directed to an optical beam splitter operating at the frequency f. The optical beam splitter can be used to alternately switch the scattered light relatively quickly between two sensors, each sensor having an effective frame rate of f / 2. Therefore, if a sensor has a limited readout rate and the system does not allow two or more sensors to be placed side by side due to cost, packaging, or other reasons, then some types of optical switching elements can double (e.g., increase to double, triple, etc.) the data rate by directing the light to different sensors over time. Thus, the limitations of individual sensor components can be overcome. Such embodiments are particularly suitable for Q-switched lasers having a repetition rate of about 2 kHz to about 40 kHz (e.g., low enough to permit the use of an optical switch).

[0078] In another such embodiment, the illumination subsystem is configured to simultaneously form a plurality of illumination regions using light pulses, the light scattered from each of the regions comprising scattered light pulses, the optical switching device is configured to simultaneously and separately split the scattered light from each of the regions collected in different segments of the light collection NA of the light collection subsystem, and the optical switching device is configured to direct only one of the different segments of a first set of scattered light pulses generated by a first set of light pulses to the two or more sensors and direct only another of the different segments of a second set of scattered light pulses generated by a second set of light pulses subsequent to the first set of light pulses to the two or more sensors. For example, one sensor may receive and process different segments of the scattered light hemisphere on a subsequent laser strike. The MEMS device will be configured to spatially select a particular scattered light beam and direct the particular scattered light beam to the sensor. Specifically, the MEMS device may be configured to function similar to the optical element 400 described above. Using this configuration, the field of view size of the imaging objective can be reduced by at least a factor of two, which can provide significant cost savings, even when taking into account the additional cost of the switching device.

[0079] In some embodiments, the illumination subsystem is configured to simultaneously form a plurality of illumination regions using light pulses, the light scattered from each of the regions comprising scattered light pulses, and the two or more sensors are synchronized in time relative to the light pulses to detect only scattered light pulses having a predetermined arrival time. In one such embodiment, the scattered light pulses having a predetermined arrival time comprise fluorescence or photoluminescence. For example, the embodiments described herein may use camera shutter synchronization to look for fluorescence and the like. Specifically, each channel and / or sensor may be synchronized in time relative to the laser beam to capture only photons having a particular arrival time. In this way, time-dependent effects such as fluorescence or photoluminescence can be observed independently of the scattered light generated by conventional means, thereby providing additional information about the surface and / or defects of interest.

[0080] In another embodiment in which light pulses are used to form the plurality of illumination regions, the acquisition and scanning subsystem of the sensor may be synchronized with this pulse frequency (or vice versa) according to the "flash on the fly" technique. As described above, some spatial overlap between subsequent laser pulses may be desirable.

[0081] In any embodiment in which scattered light is split between two or more channels of the inspection system, each channel may include a separate deformable optical element positioned between the Fourier plane and the sensor of the channel. The separate deformable optical element in each channel may be different and may depend on the characteristics of the scattered light (e.g., the segment of the scattered light) that the channel is used to detect.

[0082] Each of the above embodiments may be further configured as described herein.

[0083] Another embodiment relates to another system configured for inspecting a wafer. This system includes an illumination subsystem configured to direct a plurality of light beams to substantially the same region on the wafer. The plurality of light beams have substantially the same wavelength and polarization characteristics. For example, two or more light beams having the same wavelength and polarization cannot be combined without loss, but the two or more light beams can be made parallel to each other (e.g., using Figure 9 the folding mirrors shown in). In some embodiments, the plurality of light beams are laser beams. In this way, the illumination subsystem can have a multi-laser beam configuration. In another embodiment, the plurality of light beams are generated by only a single laser of the illumination system. For example, the plurality of illumination beams for the area mode can be generated within a single laser. The lifetime of the frequency conversion crystal of certain lasers can be limited by the intensity of the light spot incident on the crystal. In the case of having multiple simultaneous incident spots, the lifetime of the crystal can be extended. In Figure 9 one such embodiment shown in, the illumination subsystem may include only a single light source 900 that can be a laser. The laser can include any of the lasers described herein or any other suitable laser known in the art.

[0084] Light from the light source can be directed to a beam splitter 902 of the illumination subsystem, which is configured to split the light beam from the light source into a first light beam 904 and another light beam. The illumination subsystem may further include a beam splitter 906, which is configured to split the light from the beam splitter 902 into a second light beam 908 and another light beam. The beam splitters 902 and 906 may include any suitable beam splitters known in the art. The illumination subsystem may further include a reflective optical element 910, which is configured to reflect the light beam from the beam splitter 906 as a third light beam 911 to a refractive optical element 912 of the illumination subsystem. The illumination subsystem may further include a reflective optical element 914 positioned in the path of the first light beam and a second optical element 916 positioned in the path of the second light beam. The reflective optical elements 914 and 916 are configured to direct the first light beam and the second light beam to the refractive optical element 912, respectively, such that when the first light beam, the second light beam, and the third light beam are incident on the refractive optical element 912, the three are substantially parallel to each other. In this way, the reflective optical elements positioned in the path of each of the light beams can control the angle at which each of the light beams is directed to the refractive optical element and the refractive optical element controls the angle at which each of the light beams is directed to the wafer. The reflective optical elements 910, 914, and 916 may include any suitable reflective optical elements known in the art, and the refractive optical element 912 may include any suitable refractive optical elements known in the art. Figure 9 The system shown in Figure 9 can be further configured as described herein.

[0085] In another embodiment, the light source 900, the beam splitters 902 and 906, and the reflective optical elements 910, 914, and 916 can be replaced with a single light source (such as a laser from which multiple (e.g., three) light beams are emitted). The multiple light beams emitted from the light source can be directed to the refractive optical element 912 at substantially the same angle and then directed to the wafer through the refractive optical element. This embodiment can be further configured as described herein.

[0086] If two or more (e.g., three) light beams are arranged parallel to each other, then the light beams can be focused by a lens (e.g., refractive optical element 912) at the same location on the wafer. This configuration allows a spot size on the wafer of 100 μm or greater. For example, in some such embodiments, the illumination subsystem can include a lens (e.g., refractive optical element 912) configured to direct the plurality of light beams onto the wafer, and the lens can have a NA of about 0.1 or higher such that the lens can simultaneously focus several relatively low-NA input light beams. In one such example, for an illumination subsystem that includes a single 266 nm laser, a spot size on the wafer of from about 100 μm to about 1 mm only requires a NA of less than 0.01 to form the spot. Due to the relatively low NA of the lens, all the light beams can illuminate the same small patch on the wafer having substantially equal size. In contrast, for a spot of approximately 1 μm, the lens NA would need to be 0.5 or greater such that a single lens cannot inject multiple light beams having the same wavelength. Generally, the illumination subsystems described herein can use any number of light beams or light sources.

[0087] In one embodiment, the plurality of light beams can be directed to substantially the same region on the wafer at substantially the same polar angle and different azimuthal angles. In this way, the plurality of light beams (e.g., laser beams) can illuminate the wafer at nearly the same angle of incidence. For example, one laser beam can be incident at a polar angle of 55 degrees and an azimuthal angle of 0 degrees, while a second laser beam can be incident at a polar angle of 55 degrees and an azimuthal angle of 2 degrees. Nearly the same angle of incidence and polarization vectors can be used such that the scattered light generated by each light beam has the same characteristics and polarization state, thereby enabling effective filtering in the photon collection subsystem. Light beams having the same wavelength cannot be combined and incident at exactly the same angle, but can be incident at angles within a range of 5 degrees of each other and will result in nearly the same surface scattering characteristics such that sensitivity will not be impaired. In another example, if the central light beam is incident on the wafer at a polar angle of approximately X degrees, then the other two light beams can be incident at polar angles of approximately X - 2 degrees and approximately X + 2 degrees, and thus there will be substantially little difference in the surface scattering obtained from each of the light beams.

[0088] In another embodiment, the multiple light beams may be simultaneously directed to the substantially same region on the wafer. For example, although the multiple light beams have substantially the same wavelength and polarization characteristics, as described above, the multiple light beams may be simultaneously directed to the substantially same region on the wafer by splitting the light from a single light source into multiple light beams directed to the wafer at slightly different azimuthal angles and / or polar angles and by using light beams that produce multiple light beams directed to the wafer at slightly different azimuthal angles and / or polar angles, as described above. Simultaneously directing multiple light beams having the same wavelength and polarization characteristics to the wafer has several advantages, which will be further described herein.

[0089] In another embodiment, the multiple light beams illuminate the substantially same region on the wafer in a zonal illumination mode. For example, the illumination subsystem may have a multi-illumination beam configuration for the zonal mode. In some embodiments, the substantially same region on the wafer has a lateral dimension greater than 50 microns. For example, as Figure 10 shown, multiple light beams 904, 908, and 911 may be directed to substantially the same region 1000 on wafer 114 by refractive optical element 912. The lateral dimension 1002, which is the minimum dimension of substantially the same region 1000, may be greater than 50 microns. Additionally, although as Figure 10 shown, substantially the same region 1000 may be elliptical on the wafer, substantially the same region may have any other shape (e.g., rectangular) on the wafer, which will be further described herein.

[0090] In another embodiment, the plurality of light beams are pulsed light beams, and the illumination subsystem is configured such that guiding one of the plurality of light beams to the substantially same area on the wafer by the illumination subsystem is later than guiding another one of the plurality of light beams to the substantially same area, so that the pulsed light beams illuminate the substantially same area as a continuous light pulse having a duration longer than the duration of each of the pulsed light beams. In another embodiment, the plurality of light beams are pulsed light beams, and the illumination subsystem is configured such that guiding one of the plurality of light beams to the substantially same area of the wafer by the illumination subsystem is later than guiding another one of the plurality of light beams to the substantially same area, so that the peak pulse power incident on the wafer due to the plurality of light beams is less than the peak pulse power in the case where the plurality of light beams are simultaneously guided to the substantially same area on the wafer. In this way, the advantage of these embodiments compared to a single light beam is that the pulse duration can actually be extended. In the configuration of the plurality of illumination beams for the area mode, the pulse duration can be extended to reduce the peak pulse power incident on the wafer, thereby reducing the possibility of damaging the wafer. In a specific example, assuming that each of the lasers or light sources is a pulsed light source having a repetition rate between about 2 kHz and 50 kHz and a pulse duration between about 10 ns and 200 ns, if all the pulses are incident on the wafer simultaneously, the energy intensity will be quite high. However, since the wafer moves quite slowly compared to the pulse duration, the pulses can be dispersed in time and still substantially expose the same area. For example, the first pulse can be incident on the wafer at time t0, the second pulse can be incident on the wafer at time t0 + t1, and the third pulse can be incident on the wafer at time t0 + 2 * t1. Therefore, as long as the wafer (referred to as a sensor) does not move more than one sensor pixel during the time interval between the first pulse and the last pulse (e.g., 2 * t1 in the above example), the overall signal-to-noise ratio will be approximately the same as the signal-to-noise ratio when the pulses are incident on the wafer simultaneously, but the peak power density incident on the wafer will be reduced to avoid damaging the wafer.

[0091] In another embodiment, the plurality of light beams are generated by a plurality of lasers of the illumination subsystem. For example, the plurality of illumination beams for the area mode can be generated by a plurality of lasers. In Figure 12In one such embodiment, the illumination subsystem may include lasers 1200, 1202, and 1204, which are configured to generate light beams 1206, 1208, and 1210, respectively. Lasers 1200, 1202, and 1204 may be the same lasers (i.e., lasers of the same brand and model). Alternatively, lasers 1200, 1202, and 1204 may be different lasers (i.e., lasers of different brands and / or models) that produce light beams having the same wavelength and polarization characteristics as each of the other light beams. As Figure 12 shown, each of the light beams may be directed to the wafer 114 by a single refractive optical element (e.g., refractive optical element 912), which may be configured as described above. As will be described further herein, Figure 12 the light beams shown may be directed to the wafer (e.g., simultaneously or sequentially). Additionally, Figure 12 the system shown may be further configured as described herein.

[0092] In some embodiments, the plurality of light beams includes one light beam generated by a light source of the illumination subsystem and another light beam formed by collecting light reflected from substantially the same region on the wafer and directing the collected reflected light back to substantially the same region on the wafer. This embodiment may be functionally similar to using multiple light sources to generate the plurality of light beams. For example, the plurality of illumination beams for the area pattern may be generated by cycling multiple sequenced identical light beams (by collecting the reflected light beam from the wafer and redirecting the light beam back to the wafer). In this manner, the reflected light from the first sequence may be collected and reformed into a second light beam that is incident on substantially the same region of the wafer. In the option of multiple sequenced light beams, the power available for each subsequent illumination sequence is reduced due to surface reflectivity and the efficiency of the recycling optics (thus, most likely an additional two beams are more practical, but more recycled beams are of course possible), but the effective illumination power will be enhanced by a factor of 50% or more. It should be noted that, generally speaking, these multi-beam techniques are difficult to implement in in-line or point inspection systems as compared to the systems described herein. An alternative illumination option is to use multiple laser beams having different wavelengths to achieve more effective defect detection.

[0093] In one such embodiment, as Figure 13 shown, the plurality of light beams may include an incident light beam 1300 generated by a light source of the illumination subsystem ( Figure 13 not shown in). Light beam 1300 may be generated by any light source described herein. As Figure 13As shown, the light beam 1300 is directed by a refractive optical element 1302 to the wafer 114, and the refractive optical element 1302 can be configured as further described herein (e.g., relative to the refractive optical element 912). The light 1304 specularly reflected from the substantially same area on the wafer is collected by a reflective optical element 1306 of the illumination subsystem, and the reflective optical element 1306 directs the collected reflected light beam to a beam reshaping optical device 1308. The reflective optical element 1306 can include any suitable reflective optical element, and the beam reshaping optical device 1308 can include any suitable beam shaping element (e.g., anamorphic optics, field stop, spatial filter, polarization filter, etc.). The beam reshaping optical device 1308 directs the collected reflected light beam to a reflective optical element 1310, and the reflective optical element 1310 reflects the collected reflected light as a beam 1312 back to the substantially same area on the wafer. For example, as Figure 13 shown, the light 1314 specularly reflected from the wafer 114 can be collected by the reflective optical element 1306, and the reflective optical element 1306 directs the collected reflected light beam to the beam reshaping optical device 1308. The beam reshaping optical device 1308 directs this collected reflected light as a beam 1316 back to the substantially same area on the wafer. Figure 13 The portion of the illumination subsystem shown in Figure 13 can be included in any system embodiment described and shown herein.

[0094] In one embodiment, the illumination subsystem includes a frequency-converted laser, the plurality of light beams include light pulses, and the light pulses directed to the substantially same area on the wafer are spatially invariant and have a substantially constant intensity during the duration of the light pulses. In one such embodiment, the illumination subsystem includes beam shaping optics coupled to the laser. In another embodiment, the illumination subsystem includes a frequency-converted laser, the plurality of light beams include light pulses, and the light pulses directed to the substantially same area on the wafer have a substantially constant intensity during the duration of the light pulses. Such embodiments can be further configured as described herein.

[0095] The system further includes a scanning subsystem configured to scan the plurality of light beams across the wafer. The scanning subsystem can be further configured as described herein. Additionally, the system includes a collection photon subsystem configured to image light scattered from the substantially same area on the wafer onto a sensor. The sensor generates an output in response to the scattered light. The collection photon subsystem and the sensor can be further configured as described herein.

[0096] The zoom lens group within the light collection optical device allows different sized areas on the wafer to be imaged onto the same sensor according to the desired inspection speed and / or inspection sensitivity. When relatively fast inspection is required (more wafers inspected per hour), a larger area of the wafer (e.g., 2 mm × 2 mm) is imaged onto a sensor of a fixed size. When higher sensitivity (usually lower speed) inspection is required, after an amplification element is inserted into or moved within the light collection optical device path, a smaller area is imaged onto the sensor. This change in speed or sensitivity can generally be performed both during and prior to inspection. The area of the illumination spot is increased simultaneously to expose the appropriate area. When the illumination spot area changes, the intensity of the illumination spot preferably remains the same, but the intensity can be increased to improve the inspection sensitivity to a level that can avoid laser-induced damage (the multi-beam technique described previously can reduce the likelihood of laser-induced damage). Alternatively, a smaller zoom factor can be employed in the light collection optical device, and a larger illuminated area can be used, and additional sensors can be used to image this larger portion of the wafer, thereby increasing the inspection speed while maintaining the inspection sensitivity.

[0097] In one embodiment, the light collection subsystem includes a scattered light collector with a resolution that is not fully diffraction-limited. This embodiment can be further configured as described herein.

[0098] In some embodiments, the illumination subsystem is configured to vary over time the plurality of beams directed to the substantially same area on the wafer, the light collection subsystem is configured to image light scattered from the plurality of areas on the wafer onto the sensor, and the sensor and the light source of the illumination subsystem are gated to be synchronous with each other. In this way, the system can be configured to perform time-domain multi-point inspection in area mode. For example, the illumination profile varies not only as a function of position as described above, but also as a function of time. Scattered light from different parts of the wafer can be received by the same sensor, and it can be advantageous to gate the illumination and the sensor together (referred to as time-domain multi-point) to improve throughput, defect capture, or reduce the likelihood of surface damage. Different illumination profiles over time can be generated by lasers or laser beams incident on the wafer at different azimuthal and / or polar angles. A significant advantage of time-domain multi-point inspection over performing two inspections of the same wafer with different optical configurations is that the fixed time overhead associated with inspecting each wafer, e.g., only one load, unload, alignment, acceleration, and deceleration are applied, thereby increasing the total throughput.

[0099] In one such embodiment, the plurality of light beams are directed to substantially the same region on the wafer at different azimuthal angles, different polar angles, or different azimuthal angles and different polar angles. For example, although as described above, the plurality of light beams may be simultaneously directed to substantially the same region on the wafer at different azimuthal angles and the same polar angle, both the azimuthal angle and the polar angle at which the plurality of light beams are directed to the wafer may vary with time (e.g., by changing Figure 9 the position of the reflective optical element shown in Figure 12 or the positions of the plurality of light sources shown in

[0100] In another such embodiment, the illumination subsystem is configured to change the wavelength and polarization characteristics of the plurality of light beams, and the plurality of light beams directed to substantially the same region on the wafer over time have different wavelength characteristics from each other, different polarization characteristics from each other, or different wavelength and polarization characteristics from each other. For example, although as described above, the plurality of light beams may have the same wavelength and polarization characteristics, both the wavelength and polarization characteristics of the plurality of light beams may be changed over time (e.g., using one or more polarizers having time-dependent polarization characteristics (e.g., due to rotation of the polarizer)) and / or using one or more wavelength filters having time-dependent wavelength characteristics).

[0101] In one embodiment, the plurality of light beams comprise light pulses, and the scattered light comprises scattered light pulses, and the sensor is synchronized in time with respect to the light pulses to detect only the scattered light pulses having a predetermined arrival time. In one such embodiment, the scattered light pulses having a predetermined arrival time comprise fluorescence or photoluminescence. Such embodiments may be further configured as described herein.

[0102] The system further comprises a computer subsystem configured to use the output of the sensor to detect defects on the wafer. The computer subsystem may be configured as further described herein.

[0103] For a substrate in which the scattering intensity of the entire surface exceeds a predetermined value in one or more of the scattering channels of the collected light, the optical attenuation or optical gain or electronic gain of a particular sensor associated with those channels may be adjusted prior to inspection to maximize the inspection sensitivity or dynamic range.

[0104] In one embodiment, the plurality of light beams are pulsed light beams, and the illumination subsystem is configured to first direct a first one of the plurality of light beams to the substantially same area on the wafer and then direct a second one of the plurality of light beams to the substantially same area through the illumination subsystem. The first and the second of the plurality of light beams have different shapes and sizes from each other on the wafer, and the computer subsystem is configured to use the output of the scattered light from the substantially same area in response to the illumination by the first one of the plurality of light beams to determine whether the second one of the plurality of light beams should be directed to the substantially same area. This embodiment can be advantageous because the guiding light beam (e.g., the first multi-beam) can be used to sense relatively large particles on the wafer, thereby preventing damage to the wafer that may be caused by irradiating the relatively large particles with the main inspection beam (e.g., the second multi-beam). Additionally, the guiding light beam can be used to detect whether the haze on the wafer has become too high, thereby preventing damage to the sensor caused by light scattering that may occur when the haze exceeds the threshold of the sensor or the usable dynamic range of the sensor. Figure 11 An auxiliary beam illumination point 1100 on the wafer 1102 is shown, which is located in front of the point 1104 irradiated by the main inspection beam. The advantage of the relatively thin point of the auxiliary beam is that it does not significantly increase the optical field of view. As Figure 11 shown, the two light beams can have distinct profiles on the wafer, both temporally and spatially. Arrow 1106 indicates the direction of travel of the point on the wafer. As further described herein, the light scattered from the illumination area of the inspection beam can be imaged on a plurality of sensors, and the illumination for the inspection beam can be pulsed illumination. Depending on the scattered light from the auxiliary beam, the computer subsystem can generate a trigger, which is a signal to the light source indicating not to emit the pulse that will be used for the inspection beam.

[0105] The scattered light from the substantially same area due to the illumination by the first and the second of the plurality of light beams can be detected by the same sensor. However, the scattered light from the substantially same area due to the illumination by the first and the second of the plurality of light beams can be detected using different sensors. In this case, the above-mentioned sensor will be used to detect the scattered light from the substantially same area due to the illumination by the second one of the plurality of light beams, and another sensor will be used to detect the scattered light from the substantially same area due to the illumination by the first one of the plurality of light beams. The another sensor can be further configured as described herein.

[0106] In this manner, the elements of the system for inspection (or additional optical subsystems included in the system) can be configured to detect relatively large defects or detect these events before scattered light from other light scattering events is incident on the area inspection sensor. For example, a large amount of scattered light can saturate the image sensor or damage the image sensor or exceed the ability of the sensor to quantitatively measure the scattering. Preferably, the incident intensity is first reduced, and then the scattered area is inspected. The system (or additional optical subsystem) first scans the wafer, and then scans the main inspection points and the corresponding sensor areas, and if a large amount of scattered light is detected, then the control signal reduces (e.g., eliminates) the power incident on the area of the surface. Alternatively, scattered light attenuation can be added in the photon collection subsystem, or the optical gain or electronic gain of the sensor or enhanced element can be temporarily adjusted (e.g., using a photoelectric shutter).

[0107] In one such embodiment, the illumination subsystem includes a Q-switched laser, and if the computer subsystem determines that the second of the plurality of beams should not be directed to the substantially same area, then the computer subsystem prevents the second of the plurality of beams from illuminating the substantially same area. For example, as described above, depending on the scattered light from the auxiliary beam, the computer subsystem can generate a trigger, which is a signal to the Q-switched laser indicating not to emit the pulse that will be used for the inspection beam. However, the computer subsystem can prevent the second of the plurality of beams from illuminating the substantially same area without controlling the Q-switched laser. For example, the computer subsystem can control an optical element, such as a relatively fast photoelectric shutter coupled to the Q-switched laser, such that the optical element prevents the pulse generated by the Q-switched laser from illuminating the substantially same area. Additionally, the above embodiments can be implemented in combination with other pulsed light sources (e.g., CW lasers or mode-locked lasers).

[0108] In another embodiment, the plurality of light beams are pulsed light beams, and the illumination subsystem is configured to first direct a first one of the plurality of light beams to the substantially same area on the wafer and then direct a second one of the plurality of light beams to the substantially same area by the illumination subsystem. The first and the second of the plurality of light beams have different shapes and sizes from each other on the wafer, and the computer subsystem is configured to use the output of the scattered light from the substantially same area in response to the illumination by the first one of the plurality of light beams to determine the power of the second one of the plurality of light beams that should be directed to the substantially same area. The power of the second one of the plurality of light beams determined by the computer subsystem may be zero power, full power, or a partial power of the second light beam. For example, if the scattered light generated by the first light beam indicates relatively large particles on the wafer, then the computer subsystem may determine that the second light beam should be directed to the wafer with zero power or partial power to prevent the particles from breaking due to the heating by the full power of the second light beam. Or, if the scattered light generated by the first light beam indicates no relatively large particles on the wafer, then the computer subsystem may determine that the second light beam should be directed to the wafer with full power to enable the detection of relatively small particles. This embodiment can be further configured as described herein.

[0109] In one such embodiment, the illumination subsystem includes a Q-switched laser, and the computer subsystem attenuates the power of the Q-switched laser based on the determined power. For example, the computer subsystem can be coupled to the Q-switched laser in any suitable manner such that the computer subsystem can control the power of the laser to match the power determined by the computer subsystem.

[0110] In one such embodiment, the computer subsystem can be configured to monitor the power of the second one of the plurality of light beams that is actually directed to the substantially same area to normalize the power of the second one of the plurality of light beams that is actually directed to the substantially same area. In this way, software and hardware can be used to normalize the pulse-to-pulse laser energy variation. The advantage of this embodiment is that the pulse-to-pulse energy variation of the Q-switched laser may not be negligible.

[0111] Although the system described herein can regularize the laser pulse energy variation of the system by attenuating the power of the beam directed to the substantially same region, the system described herein can also or alternatively regularize the laser pulse energy variation of the system by detecting the energy of the laser pulse and regularizing the gain of the sensor based on the detected energy and / or using the computer subsystem to regularize the output generated by the sensor.

[0112] In one embodiment, the system includes an optical element configured to separate scattered light collected in different segments of the light collection NA of the photon collection system, the sensor is configured to detect one of the different segments, and the system includes another sensor configured to detect another of the different segments. In one such embodiment, the system is configured to change or replace the optical element depending on one of the different segments to be detected by the sensor and another of the different segments to be detected by the other sensor. These embodiments can be configured as further described and shown herein.

[0113] In another embodiment, the system includes an optical element configured to separate scattered light collected in different segments of the light collection NA of the photon collection system, the sensor is configured to detect one of the different segments using a portion of the sensor and another of the different segments using a different portion of the sensor, and the portion of the sensor and the different portion are non-overlapping and non-adjacent to each other on the sensor. This embodiment can be configured as further described and shown herein.

[0114] In some embodiments, the system includes an additional sensor that includes an image intensifier, the photon collection system is configured to image light scattered from the substantially same region on the wafer onto the additional sensor, the additional sensor generates an additional output in response to the scattered light, and the computer subsystem is configured to use the additional output to detect defects on the wafer instead of the output when sensor electronic noise dominates the total channel noise in the sensor. This embodiment can be configured as further described and shown herein.

[0115] In another embodiment, the system includes an additional sensor configured to perform photon counting, the photon collection system is configured to image light scattered from the substantially same region on the wafer onto the additional sensor, the additional sensor generates an additional output in response to the scattered light, and the computer subsystem is configured to use the additional output to detect defects on the wafer. This embodiment can be configured as further described and shown herein.

[0116] In one embodiment, the system includes a MEMS-based optical switching device positioned between the photon collection system and the sensor. In one such embodiment, the system includes at least one additional sensor, the plurality of light beams includes light pulses, the scattered light includes scattered light pulses, and the optical switching device is configured to direct a first one of the scattered light pulses generated by a first set of light pulses to the sensor and direct a second one of the scattered light pulses generated by a second set of light pulses following the first set of light pulses to the at least one additional sensor. In another such embodiment, the plurality of light beams includes light pulses, the scattered light includes scattered light pulses, the optical switching device is configured to separate scattered light pulses collected in different segments of the collection NA of the photon collection system, and the optical switching device is configured to direct only one of the different segments of the scattered light pulses generated by the first set of light pulses to the sensor and then direct only another one of the different segments of the scattered light pulses generated by a second set of light pulses following the first set of light pulses to the sensor. Such embodiments may be configured as further described and shown herein.

[0117] In one embodiment, the plurality of light beams includes light pulses, the scattered light includes scattered light pulses, the scanning subsystem is configured to scan the light pulses across the wafer by rotating the wafer, and when scanning the light pulses across the central region of the wafer, the illumination subsystem is configured to direct the light pulses to substantially the same region of the wafer less frequently than when scanning the light pulses across the wafer outside the central region. This embodiment may be configured as further described herein.

[0118] In another embodiment, the plurality of light beams includes light pulses, the scattered light includes scattered light pulses, the scanning subsystem is configured to scan the light pulses across the wafer by rotating and translating the wafer, the sensor includes an area sensor, when scanning the light pulses across the central region of the wafer, the scanning subsystem scans the light pulses across the wafer along one or more non-curved lines, and when scanning the light pulses across the wafer outside the central region, the scanning subsystem scans the light pulses across the wafer in a spiral manner. This embodiment may be configured as further described herein.

[0119] Each of the above-described embodiments of the system may be further configured as described herein.

[0120] Additional embodiments relate to a system configured to inspect a wafer. The system includes an illumination subsystem configured to direct a first of a plurality of pulsed light beams to an area on the wafer by the illumination subsystem earlier in time than a second of the plurality of pulsed light beams is directed to the area. The first and the second of the plurality of pulsed light beams have different shapes and sizes from each other on the wafer. The first and the second of the plurality of pulsed light beams have different wavelengths from each other, different polarizations from each other, or different wavelengths and polarizations from each other. The illumination subsystem may be configured as further described and shown herein.

[0121] The system further includes a scanning subsystem configured to scan the plurality of pulsed light beams across the wafer. The scanning subsystem may be configured in a manner as further described and shown herein. Additionally, the system includes a light collector subsystem configured to image light scattered from the area on the wafer onto one or more sensors. The one or more sensors produce outputs in response to the scattered light. The light collector subsystem and the one or more sensors may be configured as further described and shown herein.

[0122] The system further includes a computer subsystem configured to detect defects on the wafer using the outputs of the one or more sensors and to determine the power of the second of the plurality of pulsed light beams that should be directed to the area using the output of the scattered light from the area in response to illumination by the first of the plurality of pulsed light beams. The computer subsystem may be configured as further described and shown herein.

[0123] In one embodiment, the illumination subsystem includes a Q-switched laser, and the computer subsystem attenuates the power of the Q-switched laser based on the determined power. In another embodiment, the illumination subsystem includes a Q-switched laser, and if the determined power is zero, the computer subsystem prevents the Q-switched laser from producing the second of the plurality of pulsed light beams that will illuminate the area. In additional embodiments, the computer subsystem is configured to monitor the power of the second of the plurality of pulsed light beams that is actually directed to the area and to change one or more parameters of the system based on the power directed to the area to normalize the power of the second of the plurality of pulsed light beams that is actually directed to the area. Such embodiments may be configured as further described herein.

[0124] Each of the above embodiments may be further configured as described herein.

[0125] A further embodiment relates to another system configured to inspect a wafer. This system includes an illumination subsystem configured to direct an optical pulse to an area on the wafer. The illumination subsystem can be configured as further described and shown herein.

[0126] In one embodiment, the illumination subsystem includes a frequency-converted laser, the optical pulse directed to the area on the wafer is spatially invariant during the duration of the optical pulse and has a substantially constant intensity during the duration of the optical pulse, and the optical pulse illuminates the area on the wafer in an area illumination mode. In one such embodiment, the illumination subsystem includes beam shaping optics coupled to the laser. In another embodiment, the illumination subsystem includes a frequency-converted laser, the optical pulse directed to the area on the wafer has a substantially constant intensity during the duration of the optical pulse and the optical pulse illuminates the area on the wafer in an area illumination mode. These embodiments can be configured as further described herein.

[0127] The system further includes a scanning subsystem configured to scan the optical pulse across the wafer. The scanning subsystem can be configured as further described and shown herein.

[0128] In one embodiment, the scanning subsystem is configured to scan the optical pulse across the wafer by rotating the wafer, and when scanning the optical pulse across the central region of the wafer, the illumination subsystem is configured to direct the optical pulse to the area on the wafer less frequently than when scanning the optical pulse across the wafer outside the central region. This embodiment can be configured as further described herein.

[0129] In another embodiment, the scanning subsystem is configured to scan the optical pulse across the wafer by rotating and translating the wafer, the sensor includes an area sensor, when scanning the optical pulse across the central region of the wafer, the scanning subsystem scans the optical pulse across the wafer along one or more non-curved lines, and when scanning the optical pulse across the wafer outside the central region, the scanning subsystem scans the optical pulse across the wafer in a spiral manner. This embodiment can be configured as further described herein.

[0130] Laser repetition rates of 1 KHz and higher are generally acceptable. A significant drawback of relatively high repetition rates or CW lasers on a rotating area inspection system is that, during a single sensor acquisition cycle, an undesirably high data rate is required to avoid image smear. However, lower repetition rates can increase the likelihood of damaging the wafer. Some wafers (e.g., wafers containing organic films) are more susceptible to damage. For the same average laser intensity incident on the wafer, if non-linear heating effects are ignored, lower repetition rates will be more likely to damage the wafer than higher repetition rates. For relatively low repetition rates, the intensity of the laser illumination can be spread over a larger area, thus reducing the likelihood of damage, but the optical device field of view (and possibly the sensor size) requirements will increase and the cost of the system will increase significantly. For one laser pulse per frame, the maximum laser repetition rate can also be limited by the maximum sensor frame rate. However, the sensor frame rate can potentially be reduced by reducing the number of effective pixels or elements on the sensor.

[0131] To avoid surface damage while improving sensitivity, multiple laser pulses can be used on each area of the wafer. The sample is still moved continuously, but there will be a relatively large overlap in the exposed area between subsequent light source pulses. In this case, the sensor speed (frame rate) can be increased to maintain the inspection throughput. The scattered signals generated by each individual laser pulse can be read from the sensor and aligned, overlapped, and processed in post-sensor hardware or software.

[0132] Alternatively, the system includes a light collector system configured to image light pulses scattered from the area on the wafer onto a sensor. The sensor is configured to integrate a number of scattered light pulses that is less than the number of scattered light pulses that can be imaged over the entire area of the sensor. In this way, the sensor can operate in a partial TDI mode / partial CCD mode. For example, the sensor can operate in TDI mode to effectively optically integrate one or two (or another suitable number) of pulses. In some embodiments, the number of pulses integrated by the sensor is one pulse of scattered light, and the sensor integrates during the duration of the one pulse of scattered light and then transfers any charge away from the sensor in response to the one pulse of scattered light of the sensor. For a small number of pulses, the "smearing" effect of a rectangular sensor operating on an r-theta inspection system may be limited. For example, the number of pixels that can be integrated can be only 2 or 3 pixels, which can be achieved by limiting the number of pixels integrated, which is different from the way of integrating using a sensor (e.g., integrating pixels over the entire sensor) typically. The sensor is configured to produce an output in response to the integrated scattered light pulses. The light collector system and the sensor can be further configured as described and shown herein. This embodiment can be configured as described in U.S. Patent Application Serial No. 61 / 569,611, filed December 12, 2011, by Chang et al., which is incorporated herein by reference as if fully set forth herein.

[0133] In one embodiment, the light collector system includes a scattered light collector having a resolution that is not fully diffraction-limited. This embodiment can be further configured as described herein.

[0134] In one embodiment, the scanning subsystem is configured to scan the light pulses across the wafer by simultaneously rotating and translating the wafer, and the sensor includes a rectangular array of pixels. For example, as Figure 14 shown. Sensor 1400 can include a rectangular array of pixels 1402.

[0135] In another embodiment, the photon collection system includes one or more deformable optical elements configured to image all of the scattered light in one of the scattered light pulses to only one pixel of the sensor. For example, the aspect ratio of the optics included in the photon collection system can be varied to collect all of the light onto one pixel. Another way to address the problem of a laser pulse extending across several pixels due to the duration of the pulse is to have magnification optics that magnify the trailing image of the spot such that ultimately the image is circular rather than elliptical on the sensor. One of the optical axes will have a different magnification than the other optical axis. In addition to the problem of laser pulses having a limited duration, these deformable optical configurations can also be used to match channels that employ optical sensors having different aspect ratios, resolutions, and / or sizes.

[0136] In another embodiment, the sensor performs unidirectional integration during the duration of the scattered light pulse and then transfers any charge away from the sensor bidirectionally in response to the scattered light pulse. For example, as Figure 14 shown, the sensor can perform unidirectional integration in one direction as shown by arrow 1404 and then transfer any charge bidirectionally as shown by arrow 1406. As Figure 14 further shown, the integration direction can be perpendicular to the direction of charge transfer. In this way, the sensor can integrate during the pulse and then reverse the direction of charge transfer on the sensor. Additionally, the sensor can be a CCD, and many CCDs allow charge to be transferred away from both sides of the CCD, thus effectively doubling the data rate. However, the laser spot trails only to one of the sides of the sensor. Therefore, if unidirectional integration is performed and once the laser pulse is complete, the integration is stopped and then the charge is moved away bidirectionally, the maximum data rate / processing throughput advantage can be obtained while still being able to optically integrate all of the light.

[0137] In some embodiments, the sensor includes an image intensifier and an area sensor, and the sensor integrates during the duration of the scattered pulsed light and until all of the phosphor energy of the image intensifier corresponding to the scattered light pulse has completely decayed. In such embodiments, the sensor can be a TDI sensor, a CCD, or a CMOS sensor. For example, if the sensor is detecting the output of an image intensifier, the image intensifier includes phosphors that take a relatively long time to decay (e.g., a television). The pixels of the sensor can be integrated for the time required to collect all of this phosphor energy and then charge transfer can begin (in the case of a CCD) or the pixel voltages can be read out (in the case of a CMOS). Obviously, this will result in a throughput penalty due to the need to wait for the phosphors to decay, but at least all of the energy will be collected in a small number of pixels. Such embodiments can be further configured as described and shown herein.

[0138] In one embodiment, the sensor is synchronized in time with respect to the light pulse to detect only scattered light pulses having a predetermined arrival time. In one such embodiment, the scattered light pulses having a predetermined arrival time include fluorescence or photoluminescence. Such embodiments can be further configured as described and shown herein.

[0139] The system further includes a computer subsystem configured to detect defects on the wafer using the output generated by the sensor. The computer subsystem can be further configured as described and shown herein.

[0140] In one embodiment, the system includes an optical element configured to separate the scattered light pulses collected in different segments of the light collection NA of the light collection subsystem, the sensor is configured to detect one of the different segments, and the system includes another sensor configured to detect another of the different segments. In one such embodiment, the system is configured to change or replace the optical element depending on one of the different segments to be detected by the sensor and the other of the different segments to be detected by the other sensor. Such embodiments can be further configured as described and shown herein.

[0141] In one embodiment, the system includes an optical element configured to separate the scattered light pulses collected in different segments of the light collection NA of the light collection subsystem, the sensor is configured to detect one of the different segments using a portion of the sensor and detect another of the different segments using a different portion of the sensor, and the portion of the sensor and the different portion are non-overlapping and non-adjacent to each other on the sensor. Such embodiments can be further configured as described and shown herein.

[0142] In some embodiments, the system includes an additional sensor that includes an image intensifier, the photon collection system being configured to image light pulses scattered from the region on the wafer onto the additional sensor, the additional sensor generating an additional output in response to the scattered light pulses, and the computer subsystem being configured to use the additional output to detect defects on the wafer instead of the output when sensor electronic noise dominates the total channel noise in the sensor. This embodiment can be further configured as described and shown herein.

[0143] In one embodiment, the system includes an additional sensor configured to perform photon counting, the photon collection system being configured to image the light pulses scattered from the region on the wafer onto the additional sensor, the additional sensor generating an additional output in response to the scattered light pulses, and the computer subsystem being configured to use the additional output to detect defects on the wafer. This embodiment can be further configured as described and shown herein.

[0144] In some embodiments, the system includes a MEMS-based optical switching device positioned between the photon collection system and the sensor. In one such embodiment, the system includes at least one additional sensor, the optical switching device being configured to direct a first one of the scattered light pulses generated by a first one of the light pulses to the sensor and a second one of the scattered light pulses generated by a second one of the light pulses after the first one of the light pulses to the at least one additional sensor. In another such embodiment, the optical switching device is configured to separate scattered light pulses collected in different segments of a collection NA of the photon collection system, and the optical switching device is configured to direct only one of the different segments of the scattered light generated by a first one of the light pulses to the sensor and only another one of the different segments of the scattered light generated by a second one of the light pulses after the first one of the light pulses to the sensor. Such embodiments can be configured as further described and shown herein.

[0145] Each of the above embodiments can be further configured as described herein.

[0146] Another embodiment relates to a system configured to inspect a wafer. The system includes an illumination subsystem configured to direct light onto an area of the wafer. The illumination subsystem may be configured as further described and shown herein. The system also includes a scanning subsystem configured to scan the light across the wafer. The scanning subsystem may be configured as further described and shown herein. Additionally, the system includes a light collector subsystem configured to direct light scattered from the area on the wafer to a sensor. The sensor is configured to produce an output in response to the scattered light. The light collector subsystem and the sensor may be configured as described and shown herein.

[0147] The system further includes a computer subsystem configured to: detect a point defect on the wafer using the output produced by the sensor; determine the size of the point defect in pixels; determine a focus condition of the system based on the size of the point defect; and change one or more parameters of the system based on the focus condition. In this way, the system can perform autofocus by looking at the point spread function of the defect. In other words, a potential means for determining the height of the wafer is to look at the size in pixels of the point defects detected by the actual inspection process. Thus, the embodiments described herein may be configured for measuring a focus condition with an inspection algorithm. Specifically, since many defects detected on a non-patterned inspection system will be point defects on a substantially small background, an algorithm for detecting these defects (imaging onto a 2D sensor) can also characterize the size of these defects. If the defect is larger than a size specified by the system calibration, then this may correspond to a defocus condition. For example, as Figure 15 shown in, defect 1500 imaged onto two-dimensional sensor 1502 has one size when the system is in focus, and defect 1504 imaged onto the same two-dimensional sensor 1502 will have a different (e.g., larger) size when the system is out of focus. Such embodiments may obviate the need for a separate autofocus sensing system or may make an existing autofocus sensing system simpler.

[0148] The one or more parameters of the system changed by the computer subsystem may include: the position of the inspection illumination; the positions of the illumination optics, the light collection optics; the wafer height; the tilt of the wafer; the tilt of the chuck; or the temperature and / or pressure within the inspection system. The one or more parameters may be changed using feedforward techniques. The depth of focus of the system may depend on the small hole and / or polarizer present in the light collection optics between the wafer and the sensor as described above, and the system operation may be configured to take into account these various inspection modes developed for optimizing the inspection of different types of wafers.

[0149] It is most advantageous to determine the height of the wafer based on the size of point defects in a non-patterned inspection application. In a patterned wafer inspection application, there are many different structures on the wafer that scatter light onto the sensor. Each of these structures can have a size that is smaller or larger than the size of the imaging lens point spread function. It will be difficult to determine which or which light patterns in any particular sensor frame will provide a suitable autofocus error signal. On the other hand, in a non-patterned inspection application, many defects are point defects and are much smaller than the size of the imaging system point spread function (which can be approximately 250 nm to 300 nm), and thus, all defects will appear on the sensor close to the exact size of the point spread function. In this case, the deviation can be easily calculated.

[0150] In one embodiment, the computer subsystem is configured to determine the focus condition and change the one or more parameters during the process of performing an inspection on the wafer. In this way, the computer subsystem can control the focus in situ and thus keep the wafer in focus during the inspection process. The computer subsystem can be configured in any suitable way to perform in situ control.

[0151] In another embodiment, the system includes an additional subsystem configured to direct other light onto an additional area of the wafer before the illumination subsystem directs the light onto the area. The additional subsystem includes an additional sensor configured to detect the light scattered from the additional area, and the computer subsystem is configured to change the power of the light directed by the illumination subsystem onto the area based on the detected light scattered from the additional area. The area and the additional area can be configured as shown in Figure 11 , but in this embodiment, the area and the additional area do not necessarily have different sizes and shapes. Additionally, the additional subsystem can be arranged in a manner similar to that shown in Figure 12 , where one of the light sources 1200, 1202, and 1204 is used as the light source of the additional subsystem and one of the sensors 130 and 502 is used as the sensor of the additional subsystem. In this way, both the additional subsystem and the main inspection subsystem can use some of the same optical elements, such as the refractive optical element 912 and the scattered light collector 122. The additional subsystem can include any other suitable optical elements. The computer subsystem can be configured as further described herein to change the power of the light directed by the illumination subsystem onto the area.

[0152] In some embodiments, the light collector system is configured to direct light scattered from regions on the wafer to one or more additional sensors, the one or more additional sensors being configured to generate an output in response to the scattered light, each of the sensor and the one or more additional sensors being configured to detect scattered light collected in different segments of the light collection NA of the light collector system, and the computer subsystem being configured to: detect point defects on the wafer using the output generated by the one or more additional sensors; determine different sizes in pixels for at least some of the point defects using different outputs (respectively generated by the one or more additional sensors for at least some of the point defects); determine a weighted size for at least some of the point defects based on the size and the different sizes; determine the focusing condition of the system based on the weighted size; and change one or more parameters of the system based on the focusing condition. For example, the point spread function can be weighted through various channels (each channel generating a slightly different point spread function because each channel collects a different part of the scattered hemisphere) to obtain a better feedback signal. This light collector system, additional sensors, and computer subsystem can be further configured as described herein.

[0153] In another embodiment, the light collector system is configured to image the light scattered from different point defects on the wafer onto different parts of the sensor, and the computer subsystem is configured to determine whether and how the wafer is tilted based on the relationship between the sizes of the different point defects and the sizes of the different parts of the sensor onto which the light scattered from the different point defects is imaged. For example, the tilt of the wafer can be corrected in real time by tilting the gripping chuck based on the response across the sensor (e.g., the point spread function of one defect at the edge of the sensor versus the point spread function of another defect at the midpoint of the sensor can indicate that the wafer is not horizontal and thus the wafer should be tilted).

[0154] Each of the above embodiments can be further configured as described herein.

[0155] Additional embodiments relate to a system configured to inspect a wafer. The system includes an illumination subsystem configured to direct light to a region on the wafer. The light illuminates the region on the wafer in a region illumination mode. This illumination subsystem can be further configured as described and shown herein.

[0156] In one embodiment, the illumination subsystem includes a frequency-converted laser, the light includes optical pulses, and the optical pulses directed onto the area on the wafer are spatially invariant during the duration of the optical pulses and have a substantially constant intensity during the optical pulse duration. In such an embodiment, the illumination subsystem includes beam shaping optics coupled to the laser. Such embodiments can be further configured as described and shown herein. In some embodiments, the illumination subsystem includes a frequency-converted laser, the light includes optical pulses, and the optical pulses directed onto the area on the wafer have a substantially constant intensity during the duration of the optical pulses. This embodiment can be configured as further described herein.

[0157] The system further includes a scanning subsystem configured to scan the light across the wafer. This scanning subsystem can be configured as further described and shown herein.

[0158] In one embodiment, the light includes optical pulses, the scattered light includes scattered optical pulses, the scanning subsystem is configured to scan the optical pulses across the wafer by rotating the wafer, and when scanning the optical pulses across the central region of the wafer, the illumination subsystem is configured to direct the optical pulses onto the area on the wafer less frequently than when scanning the optical pulses across the area outside the central region of the wafer. In another embodiment, the light includes optical pulses, the scattered light includes scattered optical pulses, the scanning subsystem is configured to scan the optical pulses across the wafer by rotating and translating the wafer, the sensor includes an area sensor, when scanning the optical pulses across the central region of the wafer, the scanning subsystem scans the optical pulses across the wafer along one or more non-curved lines, and when scanning the optical pulses across the area outside the central region of the wafer, the scanning subsystem scans the optical pulses across the wafer in a spiral manner. Such embodiments can be further configured as described and shown herein.

[0159] In addition, the system includes a light collecting subsystem configured to image the light scattered from the area on the wafer onto a sensor. The sensor is configured to generate a light output in response to the scattered light. This light collecting subsystem and the sensor can be configured as further described and shown herein.

[0160] In one embodiment, the light includes light pulses, the scattered light includes scattered light pulses, and the sensor is synchronized in time with respect to the light pulses to detect only the scattered light pulses having a predetermined arrival time. In one such embodiment, the scattered light pulses having a predetermined arrival time include fluorescence or photoluminescence. Such embodiments can be further configured as described herein.

[0161] The system further includes a computer subsystem configured to detect defects on the wafer using the output generated by the sensor. The computer subsystem can be configured as further described and shown herein.

[0162] In one embodiment, the system includes an additional sensor that includes an image intensifier, the photon collection subsystem is configured to image the light pulses scattered from the region on the wafer onto the additional sensor, the additional sensor generates an additional output in response to the scattered light pulses, and the computer subsystem is configured to detect defects on the wafer using the additional output in place of the output when sensor electronic noise dominates the total channel noise in the sensor. This embodiment can be further configured as described and shown herein.

[0163] In another embodiment, the system includes an additional sensor configured to perform photon counting, the photon collection subsystem is configured to image the light scattered from the region on the wafer onto the additional sensor, the additional sensor generates an additional output in response to the scattered light, and the computer subsystem is configured to detect defects on the wafer using the additional output. This embodiment can be configured as described herein.

[0164] In some embodiments, the system includes a MEMS-based optical switching device positioned between the photon collection system and the sensor. In one such embodiment, the system includes at least one additional sensor, the light includes light pulses, the scattered light includes scattered light pulses, and the optical switching device is configured to direct the first of the scattered light pulses generated by the first of the light pulses to the sensor and direct the second of the scattered light generated by the second of the light pulses after the first of the light pulses to the at least one additional sensor. In another such embodiment, the light includes light pulses, the scattered light includes scattered light pulses, the optical switching device is configured to separate scattered light pulses collected in different segments of the numerical aperture (NA) of the photon collection system, and the optical switching device is configured to direct only the first of the different segments of the scattered light pulses generated by the first of the light pulses to the sensor and direct only the other of the different segments of the scattered light pulses generated by the second of the light pulses after the first of the light pulses to the sensor. Such embodiments can be further configured as described and shown herein.

[0165] Each of the above embodiments can be further configured as described herein.

[0166] Any of the systems described herein can include additional channels and / or subsystems (not shown in the figures) designed to detect defects independently of or in combination with the main inspection optical channel described above. An example of such an additional channel is a Nomarski differential interference contrast (DIC) "bright field" channel.

[0167] All channels of any inspection system described herein generate information regarding the surface quality of interest and defects. The outputs from multiple channels can be combined by a variety of logic components and / or using a variety of mathematical operations, as described in U.S. Patent Application Publication No. 2010 / 0188657, issued July 29, 2010 to Chen et al. and U.S. Patent Application Publication No. 2012 / 0044486, issued February 23, 2012 to Chen et al., which are incorporated herein by reference as if fully set forth herein. This is sometimes referred to as image or channel fusion and can advantageously improve the anomaly capture rate while reducing the false count rate.

[0168] The embodiments described herein may be further configured as described in U.S. Patent No. 7,286,697 to Guetta, U.S. Patent No. 7,339,661 to Korngut et al., U.S. Patent No. 7,525,659 to Furman et al., U.S. Patent No. 7,826,049 to Furman et al., and U.S. Patent No. 7,843,558 to Furman, which patents are hereby incorporated by reference in their entirety as if fully set forth herein.

[0169] Those skilled in the art will appreciate further modifications and alternative embodiments of various aspects of the invention in view of this description. For example, a system configured to inspect wafers is provided. Accordingly, this description should be construed only as illustrative and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as presently preferred embodiments. As will be apparent to those skilled in the art in this technology who have benefited from this description of the invention, elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be used independently. Changes may be made to the elements described herein without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

1. A system configured to inspect a wafer, comprising: An illumination subsystem configured to direct light to an area on the wafer; A scanning subsystem configured to scan the light across the wafer; A light collection subsystem configured to image light scattered from the area on the wafer onto a sensor, wherein the sensor is configured to generate an output in response to the scattered light; And A computer subsystem configured to use the output generated by the sensor to detect point defects on the wafer, determine the size of the point defects in pixels, determine the focus condition of the system based on the size of the point defects, and change one or more parameters of the system based on the focus condition.

2. The system according to claim 1, wherein the computer subsystem is further configured to determine the focus condition and change the one or more parameters during the process of inspecting the wafer.

3. The system according to claim 1, further comprising an additional subsystem configured to direct other light to an additional area on the wafer before the illumination subsystem directs the light to the area, wherein the additional subsystem includes an additional sensor configured to detect light scattered from the additional area, and wherein the computer subsystem is further configured to change the power of the light directed by the illumination subsystem to the area based on the detected light scattered from the additional area.

4. The system according to claim 1, wherein the light collection subsystem is further configured to direct light scattered from an area on the wafer to one or more additional sensors, wherein the one or more additional sensors are configured to generate an output in response to the scattered light, wherein each of the sensor and the one or more additional sensors is configured to detect the scattered light collected in different segments of the numerical aperture of the light collection of the light collection subsystem, and wherein the computer subsystem is further configured to use the outputs generated by the one or more additional sensors to detect the point defects on the wafer, determine different sizes in pixels for at least one of the point defects respectively using different outputs generated by the one or more additional sensors for at least one of the point defects, determine a weighted size for at least one of the point defects based on the size and the different sizes, determine the focus condition of the system based on the weighted size, and change the one or more parameters of the system based on the focus condition.

5. The system according to claim 1, wherein the light collection subsystem is further configured to image the light scattered from different point defects on the wafer onto different parts of the sensor, and wherein the computer subsystem is further configured to determine whether and how the wafer is tilted based on the relationship between the sizes of the different point defects and the sizes of the different parts of the sensor onto which the light scattered from the different point defects is imaged.

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