Wafer inspection

Abstract

The present invention relates to wafer inspection. The present invention discloses a system configured to inspect a wafer, the system comprising an illumination subsystem configured to direct a first of a plurality of pulsed light beams by the illumination subsystem to an area on the wafer earlier in time than directing a second of the plurality of pulsed light beams to the area; 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 area on the wafer to one or more sensors; and a computer subsystem configured to use the output of the one or more sensors to detect defects on the wafer. The system of the present invention can optimize inspection speed and / or sensitivity.

Description

[0001] Information about divisional applications

[0002] This application is a divisional application of a divisional application with an application date of July 10, 2012, application number “202010984361.X”, and invention name “Wafer Inspection”.

[0003] CROSS REFERENCE TO RELATED APPLICATIONS

[0004] This application claims priority to US Provisional Application No. 61 / 506,892, filed on July 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 are not admitted to be prior art by virtue of their inclusion in this section.

[0007] During the semiconductor manufacturing process, inspection processes are used at multiple steps to detect defects on the wafer to increase the yield and therefore higher profits in the manufacturing process. Inspection has always been an important part of manufacturing semiconductor devices. However, as the size of semiconductor devices decreases, inspection becomes more important to successfully manufacture acceptable semiconductor devices, because smaller defects may cause the device to fail.

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

[0009] One advantage of inspection systems that simultaneously illuminate a large area (on the order of 1 mm by 1 mm) of the wafer, as opposed to illuminating a point (measured in microns) or a line (width (microns) by length (mm)) of the wafer, is that there are many types of two-dimensional sensors that can acquire information from thousands to millions of individual detectors in parallel. Furthermore, because of the complexity of the illumination optics and integrating the individual sensors, point illumination inspection systems are practically limited to tens of points, thus limiting the achievable throughput. Another disadvantage of point and line scanning systems is that the illumination energy is concentrated in a relatively small area, thereby increasing the power density on the inspected surface, which can undesirably change sample properties.

[0010] It is well known that an XY (or serpentine) inspection sequence provides lower inspection throughput than a spiral sequence; therefore, 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, Milpitas, California.

[0011] Although the area inspection system has advantages as described above and in the art (e.g., U.S. Pat. No. 7,286,697 to Guetta), implementing this configuration on the R-Theta platform has proven to be challenging due to the inherent mismatch between the spiral order of the generated images and the rectilinear nature of most two-dimensional array sensors. Detecting defects by aligning and registering polar images in real time is a computationally intensive activity. Furthermore, the additional noise added to the measurement by most two-dimensional silicon-based sensors actually reduces the sensitivity performance of such systems compared to discrete detectors (e.g., photomultiplier tubes (PMTs)). On XY-based area inspection systems, there is no coordinate mismatch issue, but previous embodiments of such systems have been unable to detect all defects of concern at high speed due to the lack of flexibility of the illumination subsystem and light collection subsystem.

[0012] It would therefore be advantageous to develop inspection systems and / or methods that do not suffer from one or more of the above-mentioned 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 multiple illumination areas on the wafer, wherein there is substantially no illumination flux between each of the areas. The system also includes a scanning subsystem configured to scan multiple illumination areas across the wafer. In addition, the system includes a light collection subsystem configured to simultaneously and individually image light scattered from each of the areas onto two or more sensors. The characteristics of the two or more sensors are selected so that the scattered light is not imaged into the gaps between the two or more sensors. The two or more sensors generate 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 may 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 multiple light beams to substantially the same area on a wafer. The multiple light beams have substantially the same wavelength and polarization characteristics. The system also includes a scanning subsystem configured to scan the multiple light beams across the wafer. In addition, the system includes a light collection subsystem configured to image light scattered from the substantially the same area on the wafer to a sensor. The sensor generates 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. This system includes an illumination subsystem configured so that a first of a plurality of pulsed light beams is directed by the illumination subsystem to an area on the wafer earlier in time than a second of the plurality of pulsed light beams is directed to the area. The first and second of the plurality of pulsed light beams have different shapes and sizes on the wafer. The first and second of the plurality of pulsed light beams have different wavelengths, different polarizations, or different wavelengths and polarizations. The system also includes a scanning subsystem configured to scan the plurality of pulsed light beams across the wafer. In addition, the system includes a light collection subsystem configured to image light scattered from the area on the wafer to one or more sensors. The one or more sensors generate outputs 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 use the output of the scattered light from the area in response to illumination by the first of the plurality of pulsed light beams to determine a power of the second of the plurality of pulsed light beams that should be directed to the area. This system may be further configured as described herein.

[0017] A further embodiment relates to another system configured to inspect a wafer. This system includes an illumination subsystem configured to direct light pulses to an area on a wafer. The system also includes a scanning subsystem configured to scan the light pulses across the wafer. In addition, the system includes a light collection subsystem configured to image light pulses scattered from the area on the wafer to a sensor. The sensor is configured to integrate a 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 on 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. This system can be further configured as described herein.

[0018] Another embodiment relates to a system configured to inspect a wafer. This system includes an illumination subsystem configured to direct light to an area on a wafer. The system also includes a scanning subsystem configured to scan the light across the wafer. In addition, the system includes a light collection subsystem configured to image light scattered from the area on the wafer to 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 a point defect on the wafer, 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. This system may be further configured as described herein.

[0019] Additional embodiments relate to a system configured to inspect a wafer. The system includes an illumination subsystem configured to direct light to an area on a wafer. The system also includes a scanning subsystem configured to scan the light across the wafer. In addition, the system includes a light collection subsystem configured to image light scattered from the area on the wafer to 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 detect defects on the wafer using the output generated by the sensor. This system may be further configured as described herein. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0021] Figure 1 is a schematic diagram illustrating a side view of one embodiment of a system configured to inspect a wafer;

[0022] Figure 2 A schematic diagram of a plan view of an embodiment of a plurality of rectangular illumination areas on a wafer;

[0023] Figures 3 to 6 Schematic diagrams illustrating side views of various embodiments of a system configured to inspect a wafer;

[0024] Figure 7 A schematic diagram of a plan view of one embodiment illustrating a central region of a wafer and regions outside the central region of the wafer;

[0025] Figure 8 A schematic diagram of a plan view of one embodiment illustrating different ways in which a central region of a wafer and regions outside of the central region of the wafer may be scanned by embodiments described herein;

[0026] Fig. 9 is a schematic diagram illustrating a side view of one embodiment of a system configured to inspect a wafer;

[0027] Fig.10 A schematic diagram of a plan view illustrating one embodiment of multiple light beams directed to substantially the same area on a wafer at substantially the same polar angle and different azimuthal angles;

[0028] Fig.11 A schematic diagram of a plan view of an embodiment for illustrating a plurality of light beams having different shapes and sizes from each other on the wafer;

[0029] Fig.12 is a schematic diagram illustrating a side view of one embodiment of a system configured to inspect a wafer;

[0030] Fig.13 a schematic diagram illustrating a side view of one embodiment of a 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 the substantially same area on the wafer and directing the collected light back to the substantially same area on the wafer;

[0031] Fig.14 is a schematic diagram illustrating a plan view of one embodiment of a sensor including a rectangular array of pixels; and

[0032] Fig.15 Schematic diagram of a plan view illustrating how the size of a point defect in pixels may vary depending on the focusing conditions of a system configured to inspect a wafer.

[0033] The present invention is susceptible to many modifications and alternative forms, and specific embodiments of the present invention are shown in the various figures by way of example and will be described in detail herein. However, it should be understood that the drawings and detailed descriptions of the drawings of the present invention are not intended to limit the present invention to the specific forms disclosed, but rather, are intended to cover all modifications, equivalents and alternatives within the spirit and scope of the present invention defined by the appended claims. DETAILED DESCRIPTION

[0034] Generally speaking, the embodiments described herein relate to wafer inspection methods and systems, which include the following steps: causing illumination (e.g., laser illumination) to be incident on the wafer; translating the wafer or the illumination point on the wafer in some manner; collecting scattered light through a collection subsystem (which may include a collection objective); in the collection optical device, the scattered light can be split based on selectable polarization and / or scattering angle characteristics; directing selected portions of the scattered light to one or more sensors; and detecting defects by processing the output (e.g., image information) generated by the (multiple) sensors.

[0035] Referring now to the drawings, it should be noted that the drawings are not drawn to scale. In particular, the proportions of some elements in the drawings are 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 shown in more than one figure that may be similarly configured.

[0036] One embodiment relates to a system configured to inspect a wafer. To optimize inspection speed and / or sensitivity, a spatially discontinuous illumination profile may be used. For example, such a system includes an illumination subsystem configured to simultaneously form multiple illumination areas on the wafer with substantially no illumination flux between each of the areas. In this manner, 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-tile illumination can be produced by three methods: multiple lasers, one for each tile; multiple laser beams from one laser; and a diffractive optical element that separates one or more laser beams. In one such example, the illumination subsystem may include a laser source (or multiple) that illuminates the wafer with polarized light at a specific angle of incidence or multiple discrete angles of incidence. The optimal illumination angle of incidence for inspection depends on, among other factors, the type of wafer being inspected and the defects of interest to be detected. The illumination subsystem can be configured to allow illumination at nearly normal angles of incidence and / or oblique angles of 45 degrees or more, either sequentially or simultaneously. In addition, the laser source can be a pulsed laser.

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

[0039] In one embodiment, each of the plurality of illumination areas is rectangular on the wafer. Figure 2 As shown in , each of the plurality of illumination areas 200 may be rectangular on the wafer 202, and the direction of wafer travel may be in the direction shown by arrow 204. Figure 2 In the illustrated embodiment, three separate illumination areas (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). Interleaving of the patches can be accomplished in a manner similar to that used in current multi-point inspection systems.

[0040] One advantage of this implementation in laser area inspection is that it 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 another dimension). It is difficult to make substantially rectangular patches on a wafer (e.g., 40:1 or 100:1 aspect ratios). In the implementation described herein, three patches with aspect ratios of 13:1 or 33:1 can replace the 40:1 or 100:1 patches, respectively, and three slower sensors can replace one larger substantially elongated sensor. In general, ratios between 1:1 and 100: can be considered for area inspection mode systems. In one embodiment, the multiple illumination areas do not overlap each other on the wafer. For example, because this system is "flash on the fly," the patches can be arranged in a way that they do not overlap each other on the wafer, and the platform (described further herein) will only move the correct amount between each flash. If more convenient, the patches can be projected in a 1×3 array instead of a 3×1 array. As used herein, the term "rectangular shape on the wafer" generally refers to being substantially rectangular but may not be a precise rectangle due to inherent limitations in imaging any light beam.

[0041] In one embodiment, the illumination subsystem uses multiple light beams generated from a single light beam to form multiple illumination areas on the wafer. For example, a diffractive optical element may be used to generate the multiple light beams from a single light beam. Figure 1 In one such embodiment shown in , 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 may include any light source described herein, and the diffractive optical element may include any suitable diffractive optical element known in the art. Figure 1 As shown in , the multiple light beams can be directed to the wafer 114 at oblique incident angles. However, the multiple light beams can be directed to the wafer at any other suitable incident angles, as further described herein. Figure 1 The illumination subsystem shown in may 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 areas on the wafer. Figure 3 As shown in , the illumination subsystem may include multiple light sources 300, 302, and 304. The light sources may include any light source described herein, such as a pulsed laser. Each of the multiple light sources is configured to produce light having the same characteristics (e.g., each of the multiple light sources is the same brand and model of laser). Figure 3 As shown in , 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. In addition, although Figure 3 The multiple light beams are shown as being directed to the wafer at oblique incident angles. The multiple light beams can also be directed to the wafer at normal incident angles or nearly normal incident angles. Figure 3 The illumination subsystem shown in may include any other suitable optical elements, such as the optical elements described above. Figure 3 The system shown in 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 multiple illumination areas using light pulses, and the light pulses directed to the areas on the wafer are spatially non-varying over the duration of the light pulses and have a substantially constant intensity over the duration of the light pulses. For example, the illumination subsystem described herein may use a frequency converted laser having a spatially flat top illumination and a temporally flat top illumination output in area mode inspection. Area mode inspection systems typically use a Gaussian or "flat top" illumination profile that is continuous over the surface of the wafer. In one such embodiment, the illumination subsystem includes a beam shaping optical element coupled to the laser. For example, as Figure 6 As shown in FIG. 1 , the illumination subsystem may include a beam shaping optical element 600 coupled to a light source 100, which in this example may be a laser. The beam shaping optical element may include any type of beam shaping optical element known in the art. In addition, although Figure 6 The beam shaping optical element is shown coupled to only one light source such that the beam shaping optical element is in the path of only one light beam, but the beam shaping optical element 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 can be further configured as described herein. The flat top beam can be generated not only by a diffractive optical element or other beam shaping optical device external to the laser, 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 possibility 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 made it possible to produce so-called "flat top" or "box car" temporal pulse shapes. The peak intensity of these pulses is essentially equal to the average intensity and can increase inspection throughput by about 2 times.

[0044] In another embodiment, the illumination subsystem includes a frequency converted laser, the illumination subsystem is configured to simultaneously form the plurality of illumination areas using light pulses, and the light pulses directed to the areas on the wafer have substantially constant intensity over the duration of the light pulses. This embodiment may be configured as described above, but differs in that the light pulses are allowed to vary spatially over the duration of the light pulses.

[0045] The system also includes a scanning subsystem configured to scan the plurality of illumination areas across the wafer. The scanning subsystem may include a chuck that holds the wafer in place during inspection. For example, Figure 1 As 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 also include a shaft 118 coupled to the chuck 116 and to a positioning subsystem 120. The positioning subsystem may include a variety of elements configured to rotate and / or translate the shaft 118, such as motors, gears, platforms, etc. 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, therefore, the wafer.

[0046] The scanning subsystem may translate the wafer in a spiral or XY manner, or some combination of the two as further described herein. Specifically, in addition to the spiral scanning described above, XY serpentine scanning and RT-XY hybrid scanning may also be used to translate the wafer relative to the illumination optics and 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, with some significant differences described herein. For example, the illumination area on the wafer is substantially larger, generally extending from hundreds of microns to several millimeters, the spindle rotation rate is relatively modest, generally not exceeding 1,000 rpm to 5,000 rpm, and the collection subsystem may have nearly 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 multiple illumination areas using light pulses, light scattered from each of the areas comprising scattered light pulses, the scanning subsystem is configured to scan the light pulses across the wafer by rotating the wafer, and when the light pulses are scanned across the central region of the wafer, the illumination subsystem is configured to simultaneously direct the light pulses to the multiple illumination areas on the wafer less frequently than when the light pulses are scanned across the outside of the central region of the wafer. For example, for inspection using a pulsed laser and an area sensor on a rotating platform, the triggering frequency of the laser pulses can be increasingly reduced as the linear velocity of the wafer decreases in proportion to the radius at the center of the wafer near the center of the wafer. In this way, scanning continues at a smaller 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 yet used. Alternatively, during scanning, the illuminated area may be continuously reduced as long as the light source does not cause damage to the wafer. Figure 7 In one such embodiment shown, the central region 700 of the wafer 114 may be a region surrounding the center 702 of the wafer and spaced from the edge 704 of the wafer. The central region may surround one-third of the interior of the wafer or one-quarter of the interior of the wafer. The portion of the wafer included in the central region of the wafer may 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 multiple illumination areas using light pulses, light scattered from each of the areas comprises 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 comprise area sensors, the scanning subsystem causes the light pulses to scan across the wafer in one or more non-bent lines when scanning the light pulses across a central region of the wafer, and the scanning subsystem causes the light pulses to scan across the wafer in a spiral manner when scanning across the outside of the central region of the wafer. In this manner, the embodiments described herein may be configured to perform hybrid scanning with pulsed lasers and area sensors on a rotating platform during inspection. For example, in the hybrid approach, a large portion of the wafer may be scanned in a spiral manner. In an embodiment, the scanning subsystem causes the light pulses to scan across the wafer in a spiral manner. Figure 8In one such example shown in , a region 800 of the wafer 114 outside of the central region 700 of the wafer (which may be defined as above) may be scanned in a spiral fashion 802. The central region 700 of the wafer may then be scanned in a series of small xy serpentine movements or a single linear motion or a combination of linear motions followed by angular rotations. In this manner, the central region may be scanned in a linear fashion 804, where scanning is performed in either the x-direction or the y-direction with stepwise translations in opposite directions between scans, or in a radial fashion 806, where scanning is performed along a radius of the wafer between stepwise rotations of the wafer. In this manner, failure to scan any portion of the center of the wafer (which may occur if the alignment of the wafer relative to the scanning subsystem or optics is not precise) may be avoided, thereby making it less challenging to properly align the output from the sensor and inspection throughput may be increased. Additionally, the effect of "smearing" across the circular track of the rectangular sensor may be minimized depending on the repetition rate of the light source.

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

[0050] The light collection subsystem may include one or more objective lenses for collecting 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 optics may be placed in the light collection hemisphere close to the horizon. Light scattering information from these angles will thereby be collected, allowing for 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 various elements to selectively filter the scattered light to increase the capture rate of defects of interest and reduce false alarm rates. The various elements may include, for example, optical elements and micro-electromechanical system (MEMS)-based devices as described herein. In addition, the various elements may include polarizers, beam splitters, pinholes, 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). Figure 1 The refractive optical element 124 shown in FIG. 1 may be configured to image the filtered light onto a Figure 1 On sensors 130 and 132 shown in .

[0053] Furthermore, the light collection subsystem is preferably configured so that light from the plurality of illumination areas on the wafer is imaged individually onto only corresponding sensors. Figure 1 , scattered light 126 from a first of the multiple illumination regions is imaged only onto sensor 130, while scattered light 128 from a second of the multiple illumination regions is imaged only onto sensor 132. In this way, light from more than one of the multiple illumination regions will not be imaged onto the same sensor.

[0054] The characteristics of the two or more sensors are selected so 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 so that scattered light 126 and 128 are not imaged into the gap 134 between the two sensors. In one such example, two smaller and less expensive sensors may be used without experiencing the undesired sensitivity loss from the scattered laser light that would otherwise be imaged into the "gap" between the sensors. Gaps between discrete sensors are generally unavoidable due to packaging limitations, supporting electronics, and the like. In addition, for two-dimensional sensors and their inherent limitations (e.g., limitations on their data rates, column rates, and the like), 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 may include point or relatively low resolution sensors. The two or more sensors may also include, for example, discrete photomultiplier tubes (PMTs), charge coupled devices (CCDs), time delay integrators (TDIs), complementary metal oxide semiconductor (CMOS) sensors, scientific CMOS (sCMOS), PMT arrays, electron bombardment CCDs (EB-CCDs), electron multiplication CCDs (EM-CCDs), enhanced photodiodes, or avalanche photodiode (APD) arrays. Each channel and / or sensor may be configured to respond to an illumination wavelength or additional wavelengths or some combination of both generated by wafer interaction using wavelength filtering techniques. This allows for more selective detection of certain types of defects of interest. In addition, the sensors used in the systems described herein may vary depending on the type of scan used for inspection and / or the light source included in the illumination subsystem. For example, in an XY scan configuration, a mode locked laser with a higher repetition rate may be used to illuminate the wafer to minimize laser induced wafer damage for sensors configured to acquire data in TDI mode.

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

[0057] The objective lens included in the light collection 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 used. For example, in one embodiment, the light collection subsystem includes a scattered light collector with a resolution that is not completely diffraction limited. In particular, to reduce costs, the collector design and manufacture can be appropriately matched to the distorted point spread function produced by the associated commonly used pinholes and polarizers in the light collection channel. Given a substrate type, the specification of the resolution of the objective lens can be calculated by knowing in advance the target defect geometry and material (e.g., silicon oxide spheres) and a pupil filter or Fourier plane filter that optimizes the capture rate of the defect. Reducing the resolution requirement to completely diffraction limited can save a lot of cost for the system user.

[0058] The systems described herein may also include an autofocus subsystem (not shown). The autofocus subsystem may ensure that the surface of the wafer is always in focus of the sensor regardless of movement of the wafer, light source, light collection optics, and illumination optics. The autofocus subsystem may include a light source (which may or may not be the light source used 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 may be further configured as described herein.

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

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

[0061] With respect to the collection of scattered light, one improvement of the embodiments described herein over currently used systems is 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 collection optics to eliminate pattern scattering effects and enhance scattering from point particles and defects. In the embodiments described herein, the filtering can be fixed while the wafer is rotated underneath during a wafer scan at a specific orientation relative to the illumination angle. The filter rejects certain stereo collection angles (including unwanted scattered light from the background rather than from the defects of interest) by using a combination of multiple polarizers arranged at selected angles and movable material sections that are opaque to the scattered light wavelengths. The filtering is performed in the post-Fourier plane of the objective lens so that unwanted background at every point in the illuminated field can be eliminated simultaneously.

[0062] A variety of area type sensors can be used in combination with the spatial filtering techniques described herein. For example, the system may include a flexible light collection system in which multiple sensors are selectively configured to detect scattered light with multiple polarization states and / or stereo scattering angles. Each sensor can be arranged to collect scattered light that other sensors (if present) cannot collect. In addition, 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, which is a CMOS chip coupled to a CCD or with a delay lens. The third sensor can be an independent CCD chip with lower resolution. Additional sensors may also exist. The sensor type and size can be selected for the channel based on the scattering background characteristics expected to exist in each channel and the sensitivity requirements for the channel to the defects of interest. When the point spread function expected to be projected onto a specific sensor may be larger due to spatial filtering, a lower resolution is preferred. 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 system configuration described above can be implemented in many different embodiments to be described in the present invention. For example, in one embodiment, the system includes an optical element configured to simultaneously and individually divide 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 are configured to detect one of the different segments, and the system includes another two or more sensors configured to detect another of the different segments. Figure 4 , where the optical element 400 is positioned in the path of light collected by the scattered light collector 122. For clarity, Figure 4Only scattered light 126 from one of the multiple illuminated areas on the wafer is shown in FIG. The optical element is preferably positioned in a Fourier plane or at a conjugate of the Fourier planes of the light collection subsystem. "At a Fourier plane" or "at a conjugate of the Fourier plane" are defined herein to mean not exactly at the Fourier plane or exactly at the conjugate of the Fourier planes. Rather, these terms are intended to mean "at or near a Fourier plane" or "at or near a conjugate of the Fourier plane," respectively. An optical element is considered herein to be "at or near a Fourier plane" if it is positioned at the exact location of the Fourier plane or at a location that is within 5% of the exact location of the Fourier plane (due to any error sources in the system and / or any physical constraints in the system). "Located at or near the junction of the Fourier planes" may be described in a similar manner.

[0064] The optical element may include a variety of optical elements, such as an aperture, a mask, a mirror with an aperture, a liquid crystal display (LCD) element, or a micromirror array. In one such example, a suitable aperture may be formed by cutting out a portion of a fold mirror so that a portion of the mirror transmits light and another portion of the fold mirror reflects light. In another such example, a mirror with an aperture may be fabricated by forming a mask coating of a metal film and / or a dielectric film on a transparent substrate. The segmentation of the light collection NA may also be achieved by using other beam splitting optical elements, such as a prism with multiple facet orientations for refracting light in different directions. Other means may also be used to segment the light collection NA, including a digital micromirror device, such as a digital micromirror device 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 so 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 may have a portion of reflected light and another portion of transmitted light. Thus, the optical element can separate the collection NA into two segments, one of which is directed into one channel by reflection and the other of which is directed into another channel by transmission.

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

[0067] As described above, different portions of the optical element correspond to different sections of the collection NA into which the scattered light is separated by the optical element. Figure 4 As shown in , portions 402 and 404 are mirror-symmetric to each other about the incident plane of the illumination subsystem. In addition, portions 402 and 404 may correspond to one of the different sections of the light collection NA. In this way, one of the different sections may include two individual sections (corresponding to portions 402 and 404) that are mirror-symmetric about the incident plane of the illumination subsystem. In addition, as Figure 4 As shown in , each of portions 402 and 404 is spaced from the incident plane. In addition, each of the portions can be defined by a first side, a second side, and a third side, which will be described below with respect to portion 402. In particular, portion 402 includes a first side 402a, a second side 402b, and a third side 402c. First side 402a is linear and arranged at an angle relative to the incident plane. Second side 402b is linear and substantially parallel to the incident plane, and substantially shorter than the first side. In addition, third side 402c is curved. As shown in FIG. Figure 4 These three sides also define portion 404.

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

[0069] In these and any other embodiments described herein, each channel may ultimately have a point spread function of a different shape and different extent (in pixels) on the corresponding sensor. Thus, to maximize sensitivity to anomalies, different simulation and / or filtering techniques may be applied to each individual sensor output. In particular, the shape of the expected point spread function may be calculated in advance based on the Fourier plane aperture prior to inspection, and then the appropriate filter coefficients may 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 sections to be detected by the two or more sensors and another of the different sections to be detected by the other two or more sensors. For example, in an area mode inspection system with multiple configurable channels, the system may include a flexible pinhole light collection space. One improvement of this area inspection system over other inspection systems is the selective and configurable collection of surface scattering to enhance the detection of particles and defects. The system may 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 divide 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 are configured to detect one of the different segments using a portion of the two or more sensors and detect the other of the different segments using a different portion of the two or more sensors, and the portion and the other portion of the two or more sensors do not overlap and are not adjacent on the two or more sensors. For example, the system can be configured to separate the scattered light in the light collection space by an angle and image the light again on a single sensor in two independent small pieces. In particular, the number of effective pixels on the sensor can be controlled during or before scanning in conjunction with or independently of the illumination shape and illumination breadth. 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 solid angle range, and another portion of the sensor can receive scattered light from another solid angle range. For example, if a sensor includes 1000×1000 individual elements, 1000×500 of the elements may receive images of scattered light generated between 40 and 60 degree frontal azimuth angles. The other fifty percent of the sensor (1000×500) may receive images of scattered light from the surface generated between 120 and 160 degree azimuth angles. In some cases, the portion of the scattered light imaged onto the sensor surface may be inverted, and the other portion may remain uninverted. An additional configuration is to read sensor data from both ends of each column (e.g., row 1 and row N) simultaneously, which in some sensors can effectively double the sensor data rate.

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

[0073] In a further embodiment, the system includes an additional two or more sensors including an image intensifier, the light collection subsystem is configured to simultaneously and individually image light scattered from each of the regions to the additional two or more sensors, the additional two or more sensors generate additional outputs in response to the scattered light, and the computer subsystem is configured to use the additional outputs to replace the outputs when sensor electronic noise dominates the total channel noise in the two or more sensors to detect defects on the wafer. For example, this embodiment may include one or more flexible pinholes 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 not shown) 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 in , the light collection subsystem can include a refractive optical element 506 configured to image scattered light from the optical element 500 to two or more additional sensors. The refractive optical element 506 can be further configured as described herein. Figure 5 The system shown in can be further configured as described herein.

[0074] This embodiment may also or alternatively include one or more flexible pinholes as described above to direct the 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 enhancement sensor) may be impaired by the large amount of scattering and may not be necessary to achieve optimal sensitivity even against a relatively large background. Therefore, 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 another 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 an additional two or more sensors configured to perform photon counting, the light collection subsystem is configured to simultaneously and individually direct light scattered from each of the regions to the additional two or more sensors, the additional two or more 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 multiple 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. Figure 4 5, but replacing the additional two or more sensors (represented by the combination of sensor 408 or sensor 502 and image intensifier 504, respectively) with sensors configured for photon counting. The sensors configured for photon counting may be any suitable such sensors known in this field, such as avalanche light emitting diodes.

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

[0077] In one such embodiment, the system includes two or more additional sensors, the illumination subsystem is configured to simultaneously form a plurality of illumination areas using light pulses, light scattered from each of the areas includes scattered light pulses, and the optical switching device is configured to direct a first group of scattered light pulses generated by a first group of light pulses to the two or more sensors and direct a second group of scattered light pulses generated by a second group of light pulses subsequent to the first group of light pulses to the additional two or more sensors. For example, if the two sensors are respectively Figure 4 and 5 The optical elements 400 and 500 shown in FIG. 4 are replaced by the optical switching device described above. Figure 4 The additional two or more sensors represented by detector 408 in and detector 502 in 5 can be used for additional two or more sensors in this embodiment. In this way, the optical switching device in the light collecting optical device can direct alternating frames to alternating sensors to save costs. In this way, if a relatively high repetition rate laser 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, scattered light generated by a pulsed laser at a frequency of f can be directed to an optoelectronic beam splitter operating at a frequency of f. The optoelectronic 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 multiply the data rate (for example, double, triple, etc.) by directing the light to different sensors over time. Limitations of individual sensor components may thus be overcome.Such embodiments are particularly suitable for Q-switched lasers having a repetition rate of about 2 kHz to about 40 kHz (eg, low enough to permit the use of optical switches).

[0078] In another such embodiment, the illumination subsystem is configured to simultaneously form a plurality of illumination areas using light pulses, light scattered from each of the areas comprising scattered light pulses, the optical switching device is configured to simultaneously and individually segment scattered light from each of the areas 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 following the first set of light pulses to the two or more sensors. For example, one sensor may receive and process different segments of a hemisphere of scattered light on subsequent laser impacts. The MEMS device will be configured to spatially select a specific scattered light beam and direct the specific scattered light beam to the sensor. In particular, the MEMS device may be configured to function similarly to the optical element 400 described above. Using this configuration, the field of view size of the imaging objective may be reduced by at least a factor of two, which may provide substantial cost savings, even after the additional cost of the switching device is taken into account.

[0079] In some embodiments, the illumination subsystem is configured to simultaneously form multiple illumination areas using light pulses, light scattered from each of the areas comprises scattered light pulses, and the two or more sensors are synchronized in time with respect 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, etc. In particular, each channel and / or sensor may be synchronized in time with respect to the laser beam to capture only photons having a specific arrival time. In this way, time-dependent effects, such as fluorescence or photoluminescence, may be observed independently of scattered light produced by conventional means, thereby providing additional information about the surface and / or defects of interest.

[0080] In another embodiment where light pulses are used to form the multiple illumination areas, the sensor acquisition and scanning subsystem rotation and / or translation rate may be synchronized with this pulse frequency (or vice versa) according to a "flash on the fly" technique. As described above, some spatial overlap between subsequent laser pulses may be desirable.

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

[0082] Each of the above-described 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 multiple light beams to the substantially same area on the wafer. The multiple 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 Fig. 9 In some embodiments, the multiple beams are laser beams. In this way, the illumination subsystem may have a multi-laser beam configuration. In another embodiment, the multiple beams are generated by only a single laser of the illumination system. For example, the multiple illumination beams for an area pattern may be generated within a single laser. Some lasers have frequency conversion crystals whose lifetime may be limited by the intensity of the spot incident on the crystal. With multiple simultaneous incident spots, the lifetime of the crystal may be extended. Fig. 9 In one such embodiment shown in , the illumination subsystem may include only one single light source 900, which may be a laser. The laser may include any of the lasers described herein or any other suitable laser known in the art.

[0084] Light from the light source may 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 also 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. Beam splitters 902 and 906 may include any suitable beam splitters known in the art. The illumination subsystem may also 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 also 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. Reflective optical elements 914 and 916 are configured to direct the first light beam and the second light beam, respectively, to the refractive optical element 912, 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 element 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. Reflective optical elements 910, 914, and 916 can include any suitable reflective optical element known in the art, and refractive optical element 912 can include any suitable refractive optical element known in the art. Fig. 9 The system shown in can be further configured as described herein.

[0085] In another embodiment, light source 900, beam splitters 902 and 906, and reflective optical elements 910, 914, and 916 may 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 may be directed at substantially the same angle to refractive optical element 912 and then directed to the wafer through the refractive optical element. This embodiment may be further configured as described herein.

[0086] If two or more (e.g., three) beams are arranged parallel to each other, the beams can be focused by a lens (e.g., refractive optical element 912) to the same location on the wafer. This configuration allows spot sizes on the wafer to be 100 μm or larger. For example, in some such embodiments, the illumination subsystem may include a lens (e.g., refractive optical element 912) configured to direct the multiple beams onto the wafer, and the lens may have an NA of about 0.1 or higher, so that the lens can focus several relatively low NA input beams simultaneously. In one such example, for an illumination subsystem including a 266 nm laser, a spot size of about 100 μm to about 1 mm on the wafer only requires an NA of less than 0.01 to form the spot. Due to the relatively low NA of the lens, all beams can illuminate the same small patch of substantially equal size on the wafer. In contrast, for the case of a spot of approximately 1 μm, the lens NA would need to be 0.5 or greater, making it impossible for a single lens to inject multiple beams of the same wavelength. In general, the illumination subsystems described herein may use any number of light beams or light sources.

[0087] In one embodiment, the multiple beams may be directed to substantially the same area on the wafer at substantially the same polar angle and different azimuthal angles. In this way, the multiple beams (e.g., laser beams) may illuminate the wafer at approximately the same angle of incidence. For example, one laser beam may be incident at a polar angle of 55 degrees and an azimuthal angle of 0 degrees, while a second laser beam may be incident at a polar angle of 55 degrees and an azimuthal angle of 2 degrees. Approximately identical angles of incidence and polarization vectors may be used so that the scattered light produced by each beam has the same characteristics and polarization state, thereby effectively filtering in the light collection subsystem. Beams of the same wavelength cannot be combined and incident at the exact same angle, but may be incident at angles that differ from each other by a range of 5 degrees and will result in approximately the same surface scattering characteristics, so that sensitivity will not be compromised. In another example, if the central beam is incident on the wafer at a polar angle of approximately X degrees, then the other two beams may be incident at polar angles of approximately X-2 degrees and polar angles of approximately X+2, and thus the surface scattering resulting from each of the beams will be substantially less different.

[0088] In another embodiment, the multiple light beams can be directed to the substantially same area on the wafer simultaneously. For example, even though the multiple light beams have substantially the same wavelength and polarization characteristics, as described above, by splitting light from a single light source into multiple light beams directed to the wafer at slightly different azimuthal and / or polar angles and by using a light beam that produces multiple light beams directed to the wafer at slightly different azimuthal and / or polar angles, as described above, the multiple light beams can be directed to the substantially same area on the wafer simultaneously. Directing multiple light beams having the same wavelength and polarization characteristics to the wafer simultaneously has several advantages, which are further described herein.

[0089] In another embodiment, the plurality of light beams illuminate the substantially identical area on the wafer in an area illumination mode. For example, the illumination subsystem may have a multiple illumination beam configuration for an area mode. In some embodiments, the substantially identical area on the wafer has a lateral dimension greater than 50 microns. For example, Fig.10 As shown in FIG. 1 , multiple light beams 904, 908, and 911 can be directed to substantially the same area 1000 on the wafer 114 by a refractive optical element 912. The lateral dimension 1002, which is the smallest dimension of the substantially same area 1000, can be greater than 50 microns. Fig.10 As shown in , the substantially identical regions 1000 may be elliptical on the wafer, and the substantially identical regions may have any other shape (eg, rectangular) on the wafer, as will be further described herein.

[0090] In another embodiment, the multiple light beams are pulsed light beams, and the illumination subsystem is configured so that one of the multiple light beams is directed by the illumination subsystem to the substantially same area on the wafer later than another of the multiple light beams is directed to the substantially same area, such that the pulsed light beam illuminates the substantially same area as one continuous light pulse of a duration longer than the duration of each of the pulsed light beams. In another embodiment, the multiple light beams are pulsed light beams, and the illumination subsystem is configured so that one of the multiple light beams is directed by the illumination subsystem to the substantially same area on the wafer later than another of the multiple light beams is directed to the substantially same area, such that the peak pulse power incident on the wafer due to the multiple light beams is less than the peak pulse power if the multiple light beams are directed simultaneously to the substantially same area on the wafer. In this way, these embodiments have the advantage over single beams in that pulse duration can actually be extended. In the multiple illumination beam configuration for area mode, the pulse duration can be extended to reduce the peak pulse power incident on the wafer, thereby reducing the likelihood of damaging the wafer. In one specific example, assuming that each of the lasers or light sources is a pulsed light source with a repetition rate between about 2 kHz and 50 kHz and a pulse duration between about 10 ns and 200 ns, if all pulses are incident on the wafer simultaneously, the energy intensity will be quite high. However, because the wafer moves quite slowly compared to the pulse duration, the pulses can be dispersed in time and still expose substantially the same area. For example, a first pulse can be incident on the wafer at time t0, a second pulse can be incident on the wafer at time t0+t1, and a third pulse can be incident on the wafer at time t0+2*t1. Therefore, as long as the wafer (referred to as the sensor) does not move more than one sensor pixel in 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 roughly the same as if the pulses were 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 an area mode may be generated by a plurality of lasers. Fig.12In one such embodiment shown, the illumination subsystem may include lasers 1200, 1202, and 1204 configured to generate light beams 1206, 1208, and 1210, respectively. Lasers 1200, 1202, and 1204 may be identical 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 generate light beams having the same wavelength and polarization characteristics as each of the other light beams. Fig.12 As shown in , 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 further described herein, Fig.12 The light beams shown in can be directed to the wafer (e.g., simultaneously or sequentially). Fig.12 The system shown in can be further configured as described herein.

[0092] In some embodiments, the multiple light beams include one light beam generated by the light source of the illumination subsystem and another light beam formed by collecting light reflected from the substantially same area on the wafer and directing the collected reflected light back to the substantially same area on the wafer. This embodiment may be functionally similar to using multiple light sources to generate the multiple light beams. For example, the multiple illumination beams for the area pattern may be generated by recycling multiple orders of the same light beam (by collecting reflected light beams from the wafer and redirecting the light beams back onto the wafer). In this way, reflected light from the first order may be collected and reshaped into a second light beam that is incident on the substantially same area of ​​the wafer. In the option of multiple order beams, the power available for each subsequent illumination order is reduced due to surface reflectivity and the efficiency of the recycling optics (so, most likely two additional beams are more practical, but more recycling beams are certainly possible), but the effective illumination power will be enhanced by a factor of 50% or more. It should be noted that, in general, these multi-beam techniques are difficult to implement in an online or spot inspection system compared to the system described herein. An alternative illumination option is to use multiple laser beams with different wavelengths to achieve more efficient defect detection.

[0093] In one such embodiment, Fig.13 As shown in , the multiple light beams may include a light source ( Fig.13 The incident light beam 1300 is generated by any light source described herein. Fig.13As shown in , light beam 1300 is directed to wafer 114 by refractive optical element 1302, which may be configured as further described herein (e.g., relative to refractive optical element 912). Light 1304 specularly reflected from the substantially same area on the wafer is collected by reflective optical element 1306 of the illumination subsystem, which directs the collected reflected light beam to beam reshaping optics 1308. Reflective optical element 1306 may include any suitable reflective optical element, and beam reshaping optics 1308 may include any suitable beam shaping element (e.g., an anamorphic optical element, a field stop, a spatial filter, a polarization filter, etc.). Beam reshaping optics 1308 directs the collected reflected light beam to reflective optical element 1310, which reflects the collected reflected light back to substantially the same area on the wafer as light beam 1312. For example, as Fig.13 As shown in FIG. 1 , light 1314 specularly reflected from wafer 114 may be collected by reflective optical element 1306, which directs the collected reflected light beam to beam reshaping optics 1308. Beam reshaping optics 1308 directs this collected reflected light as beam 1316 back to substantially the same area on the wafer. Fig.13 The portions of the lighting subsystem shown in may be included in any system embodiments described and shown herein.

[0094] In one embodiment, the illumination subsystem comprises a frequency converted laser, the multiple light beams comprise light pulses, and the light pulses directed to the substantially same area on the wafer are spatially non-varying over the duration of the light pulses and have a substantially constant intensity over the duration of the light pulses. In one such embodiment, the illumination subsystem comprises a beam shaping optical element coupled to the laser. In another embodiment, the illumination subsystem comprises a frequency converted laser, the multiple light beams comprise light pulses, and the light pulses directed to the substantially same area on the wafer have a substantially constant intensity over the duration of the light pulses. Such embodiments may be further configured as described herein.

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

[0096] The zoom lens group within the collection optics allows different sized areas on the wafer to be imaged onto the same sensor depending on the inspection speed and / or inspection sensitivity required. When relatively fast inspection is required (more wafers per hour), a larger area of ​​the wafer (e.g., 2 mm x 2 mm) is imaged onto a sensor of fixed size. When higher sensitivity (typically lower speed) inspection is required, a smaller area is imaged onto the sensor after a magnifying element is inserted into or moved in the collection optics path. This change in speed or sensitivity can generally be performed during the inspection as well as prior to the inspection. The area of ​​the illumination spot is simultaneously increased to expose the appropriate area. When the illumination spot area is changed, the intensity of the illumination spot preferably remains the same, but the intensity can be increased to improve inspection sensitivity to a point where laser induced damage can be avoided (the multiple beam techniques described previously can reduce the possibility of laser induced damage). Alternatively, a smaller zoom factor may be employed in the collection optics, and a larger illuminated area used, and additional sensors may be used to image this larger portion of the wafer, thereby increasing inspection speed while maintaining inspection sensitivity.

[0097] In one embodiment, the light collection subsystem comprises a scattered light collector having a resolution that is not completely diffraction limited. This embodiment may be further configured as described herein.

[0098] In some embodiments, the illumination subsystem is configured to vary the multiple light beams directed to the substantially same area on the wafer over time, the light collection subsystem is configured to image scattered light from the multiple areas on the wafer onto the sensor, and the sensor and the light source of the illumination subsystem are gated to be synchronized with each other. In this way, the system can be configured to perform time domain multi-point inspection in regional mode. For example, the illumination profile does not only vary with position as described above, but also varies with time. Scattered light from different parts of the wafer can be received by the same sensor, and it may be advantageous to gate the illumination and the sensor together (which will be referred to as time domain multi-point) to improve throughput, defect capture, or reduce the likelihood of surface damage. Temporally different illumination profiles 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 inspecting the same wafer twice with different optical configurations is that the fixed time overhead associated with inspecting each wafer, for example, loading, unloading, alignment, acceleration, and deceleration, is only applied once, thereby increasing overall throughput.

[0099] In one such embodiment, the plurality of light beams are directed to the substantially same area on the wafer at different azimuth angles, different polar angles, or different azimuth angles and different polar angles. For example, although as described above, the plurality of light beams may be directed to the substantially same area on the wafer at different azimuth angles and the same polar angle at the same time, both the azimuth angles and the polar angles at which the plurality of light beams are directed to the wafer may be varied over time (e.g., by varying the azimuth angles). Fig. 9 The reflective optical element shown in Fig.12 ).

[0100] In another such embodiment, the illumination subsystem is configured to vary the wavelength and polarization characteristics of the plurality of light beams, and the plurality of light beams directed to the substantially same area on the wafer over time have different wavelength characteristics, different polarization characteristics, or different wavelength and polarization characteristics from one another. For example, while the plurality of light beams may have the same wavelength and polarization characteristics as described above, both the wavelength and polarization characteristics of the plurality of light beams may be varied over time (e.g., using one or more polarizers having time-dependent polarization characteristics (e.g., due to rotation of the polarizers) 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 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 includes a computer subsystem configured to detect defects on the wafer using the output of the sensor. The computer subsystem may be configured as further described herein.

[0103] For substrates where the scattering intensity across the surface exceeds a predetermined value in one or more of the scattering channels of the collected light, the light attenuation or optical or electronic gain of the specific sensors associated with those channels can 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, the illumination subsystem is configured to first direct a first of the plurality of light beams to the substantially same area on the wafer, and thereafter direct a second of the plurality of light beams to the substantially same area by the illumination subsystem, the first and second of the plurality of light beams having different shapes and sizes on the wafer from each other, and the computer subsystem is configured to use the output of scattered light from the substantially same area in response to illumination due to the first of the plurality of light beams to determine whether the second of the plurality of light beams should be directed to the substantially same area. This embodiment can be advantageous because the guided 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). In addition, the guided light beam can be used to detect whether haze on the wafer has become too high, thereby preventing damage to the sensor that may be caused by light scattering due to haze exceeding a threshold of the sensor or a usable dynamic range of the sensor. Fig.11 A secondary beam illumination spot 1100 is shown on wafer 1102, which is located in front of the spot 1104 illuminated by the main inspection beam. The advantage of the relatively thin spot of the secondary beam is that it does not significantly increase the optical field of view. Fig.11 , the two beams may have very different profiles on the wafer, both in time and in space. Arrow 1106 indicates the direction of travel of the points on the wafer. As further described herein, light scattered from the illumination area of ​​the inspection beam may be imaged on multiple sensors, and the illumination for the inspection beam may be pulsed illumination. Depending on the scattered light from the auxiliary beam, the computer subsystem may generate a trigger, which is a signal to the light source indicating not to emit a pulse to be used for the inspection beam.

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

[0106] In this way, elements of the system for inspection (or additional optical subsystems included in the system) can be configured to detect relatively large defects or to detect scattered light from other light scattering events before these events are incident on the area inspection sensor. For example, a large amount of scattered light can saturate or damage an image sensor or exceed the ability of the sensor to quantitatively measure the scattering. It is preferred to reduce the incident intensity before inspecting the scattered area. The system (or additional optical subsystem) scans the wafer first, then scans the main inspection point and corresponding sensor area, and if a large amount of scattered light is detected, a control signal reduces (e.g., eliminates) the power incident on the area of ​​the surface. Alternatively, scattered light attenuation can be added in the light collection subsystem, or the optical gain or electronic gain of the sensor or enhancement element can be temporarily adjusted (e.g., using an optoelectronic 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 light beams should not be directed to the substantially same area, the computer subsystem prevents the second of the plurality of light beams from irradiating the substantially same area. For example, as described above, depending on scattered light from the auxiliary beam, the computer subsystem may generate a trigger that is a signal to the Q-switched laser indicating not to emit pulses to be used for the inspection beam. However, the computer subsystem may prevent the second of the plurality of light beams from irradiating the substantially same area without controlling the Q-switched laser. For example, the computer subsystem may control an optical element, such as a relatively fast optoelectronic shutter coupled to the Q-switched laser, such that the optical element prevents pulses generated by the Q-switched laser from irradiating the substantially same area. Furthermore, the above-described embodiments may be implemented in combination with other pulsed light sources, such as CW lasers or mode-locked lasers.

[0108] In another embodiment, the plurality of light beams are pulsed light beams, the illumination subsystem is configured to direct a first one of the plurality of light beams to the substantially same area on the wafer before directing a second one of the plurality of light beams to the substantially same area by the illumination subsystem, the first one of the plurality of light beams and the second one of the plurality of light beams having different shapes and sizes on the wafer, and the computer subsystem is configured to use the output of scattered light from the substantially same area in response to 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 some fraction of the power of the second light beam. For example, if the scattered light generated by the first light beam indicates a relatively large particle on the wafer, the computer subsystem may determine that the second light beam should be directed to the wafer at zero power or fractional power to prevent the particle from being fragmented due to heating by the full power of the second light beam. Alternatively, if the scattered light produced by the first beam indicates that relatively large particles are absent from the wafer, the computer subsystem may determine that the second beam should be directed at the wafer at full power to enable detection of relatively small particles. This embodiment may be further configured as described herein.

[0109] In one such embodiment, the illumination subsystem includes a Q-switched laser, and the computer subsystem reduces the power of the Q-switched laser based on the determined power. For example, the computer subsystem may 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 may be configured to monitor the power of the second one of the plurality of beams that is actually directed to the substantially same area to normalize the power of the second one of the plurality of beams that is actually directed to the substantially same area. In this way, software and hardware may be used to normalize pulse-to-pulse laser energy variations. An advantage of this embodiment is that pulse-to-pulse energy variations of Q-switched lasers may not be minimal.

[0111] Although the system described herein may normalize variations in the laser pulse energy of the system by attenuating the power of the light beam directed to the substantially same area, the system described herein may also or alternatively normalize variations in the laser pulse energy of the system by detecting the energy of the laser pulse and normalizing the gain of the sensor based on the detected energy and / or using the computer subsystem to normalize the output produced by the sensor.

[0112] In one embodiment, the system includes an optical element configured to separate scattered light collected in different segments of the collection NA of the collection subsystem, the sensor is configured to detect one of the different segments, and the system includes another sensor configured to detect the other 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. 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 collection NA of the collection subsystem, the sensor is configured to detect one of the different segments using a portion of the sensor and to detect another of the different segments using a different portion of the sensor, and the portion of the sensor and the other portion do not overlap and are not 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 including an image intensifier, the light collection subsystem is configured to image light scattered from the substantially same area on the wafer to 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 in place of the output when sensor electronic noise dominates the total channel noise in the sensor. This embodiment may be configured as further described and shown herein.

[0115] In another embodiment, the system includes an additional sensor configured to perform photon counting, the light collection subsystem is configured to image light scattered from the substantially same area on the wafer to 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 may be configured as further described and shown herein.

[0116] In one embodiment, the system includes a MEMS-based optical switching device positioned between the light collection subsystem and the sensor. In one such embodiment, the system includes at least one additional sensor, the multiple light beams include light pulses, the scattered light includes scattered light pulses, and the optical switching device is configured to direct a first of the scattered light pulses generated by a first group of light pulses to the sensor and a second of the scattered light pulses generated by a second group of light pulses following the first group of light pulses to the at least another sensor. In another such embodiment, the multiple light beams include light pulses, the scattered light includes scattered light pulses, the optical switching device is configured to separate scattered light pulses collected in different sections of the light collection NA of the light collection subsystem, and the optical switching device is configured to direct only one of the different sections of the scattered light pulses generated by the first group of light pulses to the sensor and then direct only another of the different sections of the scattered light pulses generated by the second group of light pulses following the first group 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 include 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 a central region of the wafer, the illumination subsystem is configured to direct the light pulses to the substantially same area on the wafer less frequently than when scanning the light pulses across the wafer outside of the central region. This embodiment may be configured as further described herein.

[0118] In another embodiment, the plurality of light beams include 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, the scanning subsystem scans the light pulses across the wafer in one or more non-bent lines when scanning the light pulses across a central region of the wafer, and the scanning subsystem scans the light pulses across the wafer in a spiral manner when scanning the light pulses across the wafer outside of the central region. This embodiment may be configured as further described herein.

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

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

[0121] The system also includes a scanning subsystem configured to scan the multiple pulsed light beams across the wafer. This scanning subsystem can be configured as further described and shown herein. In addition, the system includes a light collection subsystem configured to image light scattered from the area on the wafer to one or more sensors. The one or more sensors generate an output in response to the scattered light. The light collection subsystem and the one or more sensors can be configured as further described and shown herein.

[0122] 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 use the output of scattered light from the area in response to illumination due to the first one of the plurality of pulsed light beams to determine a power of the second one of the plurality of pulsed light beams that should be directed to the area. 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 reduces 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 generating the second one of the plurality of pulsed beams that will illuminate the area. In additional embodiments, the computer subsystem is configured to monitor the power of the second one of the plurality of pulsed beams actually directed to the area and to alter one or more parameters of the system based on the power directed to the area to normalize the power of the second one of the plurality of pulsed beams actually directed to the area. Such embodiments may be configured as further described herein.

[0124] Each of the above-described 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 light pulses to an area on a wafer. The illumination subsystem may be configured as further described and shown herein.

[0126] In one embodiment, the illumination subsystem includes a frequency converted laser, the light pulses directed to the area on the wafer are spatially non-varying over the duration of the light pulses and have a substantially constant intensity over the duration of the light pulses, and the light pulses illuminate the area on the wafer in an area illumination pattern. In one such embodiment, the illumination subsystem includes a beam shaping optical element coupled to the laser. In another embodiment, the illumination subsystem includes a frequency converted laser, the light pulses directed to the area on the wafer have a substantially constant intensity over the duration of the light pulses, and the light pulses illuminate the area on the wafer in an area illumination pattern. These embodiments may be configured as further described herein.

[0127] The system also includes a scanning subsystem configured to scan the light pulses across the wafer. The scanning subsystem may be configured as further described and shown herein.

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

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

[0130] Laser repetition rates of 1 kHz and higher are generally acceptable. An obvious disadvantage of relatively high repetition rates or CW lasers on rotating area inspection systems is that an undesirably high data rate is required to avoid image smearing during a single sensor acquisition cycle. 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 nonlinear heating effects are ignored, a lower repetition rate will more easily damage the wafer than a higher repetition rate. For relatively low repetition rates, the intensity of the laser illumination can be diffused over a larger area, thereby reducing the likelihood of damage, but the field of view of the optics (and possible sensor size) requirements will increase and will significantly increase the cost of the system. 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 be potentially 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 on the wafer. The sample still moves 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 inspection throughput. The scattered signal 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 collection subsystem 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 a 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 for 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 from the sensor. For a small number of pulses, the "smear" 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 may be only 2 or 3 pixels, which can be achieved by limiting the number of pixels integrated, which is different from the way integration is typically performed using a sensor (e.g., pixels that are typically integrated over the entire sensor). The sensor is configured to generate an output in response to the integrated scattered light pulse. The light collection subsystem and the sensor may be further configured as described and shown herein. This embodiment may be configured as described in U.S. Patent Application Serial No. 61 / 569,611 filed by Chaung et al. on December 12, 2011, which is incorporated herein by reference as if fully set forth herein.

[0133] In one embodiment, the collection subsystem comprises a scattered light collector having a resolution that is not fully diffraction limited.This embodiment may 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 comprises a rectangular array of pixels. Fig.14 As shown in . Sensor 1400 may include a rectangular array of pixels 1402.

[0135] In another embodiment, the light collection subsystem includes one or more anamorphic optical elements configured to image all scattered light in one of the scattered light pulses to only one pixel of the sensor. For example, the anamorphic ratio of the optical device included in the light collection subsystem can be changed to collect all light onto one pixel. Another way to solve the problem of laser pulses extending across several pixels due to the duration of the pulse is to have a magnification optical device that magnifies the smear image of the point so that the image is finally circular instead of 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 with limited duration, these anamorphic optical configurations can also be used to match channels that employ optical sensors with different aspect ratios, resolutions and / or sizes.

[0136] In another embodiment, the sensor integrates unidirectionally for the duration of a scattered light pulse and then bidirectionally transfers any charge away from the sensor in response to the scattered light pulse. Fig.14 As shown in , the sensor can integrate unidirectionally in one direction shown by arrow 1404 and then transfer any charge bidirectionally as shown by arrow 1406. Fig.14 As further shown in , 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. Furthermore, the sensor can be a CCD, and many CCDs allow charge to be transferred off both sides of the CCD, thus effectively doubling the data rate. However, the laser spot only smears to one of the sides of the sensor. Therefore, if one performs unidirectional integration, stops integrating once the laser pulse is complete, and then moves charge bidirectionally away, the maximum data rate / throughput advantage can be obtained while still optically integrating all the light.

[0137] In some embodiments, the sensor includes an image intensifier and an area sensor, and the sensor integrates for the duration of the scattered pulse 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 may be a TDI sensor, a CCD, or a CMOS sensor. For example, if the sensor is detecting the output of an image intensifier, then the image intensifier includes a phosphor that takes a relatively long time to decay (e.g., a TV). The pixels of the sensor may be integrated for the time required to collect all of this phosphor energy, and then begin transferring charge (in the case of a CCD) or reading out the pixel voltage (in the case of a CMOS). Obviously, this will cause a loss in throughput due to the need to wait for the phosphor to decay, but at least all of the energy will be collected in a small number of pixels. This embodiment may be further configured as described and shown herein.

[0138] In one embodiment, the sensor is synchronized in time with respect 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 include fluorescence or photoluminescence. Such embodiments may be further configured as described and shown herein.

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

[0140] In one embodiment, the system includes an optical element configured for separating the scattered light pulses collected in different segments of the collection NA of the 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 another of the different segments to be detected by the other sensor. Such embodiments may be further configured as described and shown herein.

[0141] In one embodiment, the system includes an optical element configured for separating the scattered light pulses collected in different segments of the collection NA of the collection subsystem, the sensor is configured to detect one of the different segments using a portion of the sensor and to detect another of the different segments using a different portion of the sensor, and the portion of the sensor and the other portion do not overlap and are not adjacent to each other on the sensor. This embodiment may be further configured as described and shown herein.

[0142] In some embodiments, the system includes an additional sensor including an image intensifier, the light collection subsystem is configured to image light pulses scattered from the area on the wafer to 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 may be further configured as described and shown herein.

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

[0144] In some embodiments, the system includes a MEMS-based optical switching device positioned between the light collection subsystem 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 of the scattered light pulses generated by the first of the light pulses to the sensor and a second of the scattered light pulses generated by the second of the light pulses following the first 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 light collection NA of the light collection subsystem, and the optical switching device is configured to direct only one of the different segments of the scattered light generated by the first of the light pulses to the sensor and direct only another of the different segments of the scattered light generated by the second of the light pulses following the first of the light pulses to the sensor. Such embodiments may be configured as further described and shown herein.

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

[0146] Another embodiment relates to a system configured to inspect a wafer. This system includes an illumination subsystem configured to direct light to an area on a wafer. This 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. This scanning subsystem may be configured as further described and shown herein. In addition, the system includes a light collection subsystem configured to direct light scattered from the area on the wafer to a sensor. The sensor is configured to generate an output in response to the scattered light. The light collection subsystem and the sensor may be configured as described and shown herein.

[0147] The system further includes a computer subsystem configured to: detect point defects on the wafer using the output generated 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, one 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. Therefore, the embodiments described herein may be configured for measuring focus conditions using an inspection algorithm. Specifically, since many defects detected on a non-patterned inspection system will be point defects on a substantially small background, the algorithm that detects these defects (imaged onto a 2D sensor) may also characterize the size of these defects. If the defect is larger than the size specified by the system calibration, then this may correspond to a defocus condition. For example, if Fig.15 As shown in , a defect 1500 imaged onto a two-dimensional sensor 1502 has one size when the system is in focus, and a 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 eliminate the need for a separate auto-focus sensing system or may make an existing auto-focus sensing system simpler.

[0148] The one or more parameters of the system altered by the computer subsystem may include: the position of the inspection illumination; the position of the illumination optics, the 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 altered using feed-forward techniques. The depth of focus of the system may depend on the presence of an aperture and / or polarizer in the 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 to optimize inspection of different types of wafers.

[0149] Determining the height of the wafer based on the size of point defects is most advantageously done in non-patterned inspection applications. In patterned wafer inspection applications, there are many different structures on the wafer that scatter light onto the sensor. Each of these structures may have a size that is smaller or larger than the imaging lens point spread function. It will be difficult to determine which light pattern or patterns in any particular sensor frame will provide a suitable autofocus error signal. On the other hand, in non-patterned inspection applications, many defects are point defects and are much smaller in size than the imaging system point spread function (which may be approximately 250nm to 300nm), and therefore, 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 alter the one or more parameters during an inspection process performed 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 manner to perform in situ control.

[0151] In another embodiment, the system includes an additional subsystem configured to direct additional light to an additional area on the wafer before the light is directed to the area by the illumination subsystem, the additional subsystem includes an additional sensor configured to detect light scattered from the additional area, and the computer subsystem is configured to alter the power of the light directed to the area by the illumination subsystem based on the detected light scattered from the additional area. Fig.11 The area and the additional area are configured as shown, but in this embodiment, the area and the additional area do not necessarily have different sizes and shapes. Fig.12 , wherein one of light sources 1200, 1202, and 1204 is used as the light source of the additional subsystem and one of 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 refractive optical element 912 and scattered light collector 122. The additional subsystem may include any other suitable optical elements. The computer subsystem may be configured in this embodiment as further described herein to change the power of light directed to the area by the illumination subsystem.

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

[0153] In another embodiment, the light collection subsystem is configured to image the light scattered from different point defects on the wafer onto different portions of the sensor, and the computer subsystem is configured to determine if and how the wafer is tilted based on the relationship between the size of the different point defects and the size of the different portions of the sensor onto which the light scattered from the different point defects is imaged. For example, the tilt of the wafer may be corrected in real time based on 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 may indicate that the wafer is not level and therefore the wafer should be tilted).

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

[0155] Additional embodiments are directed to a system configured to inspect a wafer. The system includes an illumination subsystem configured to direct light to an area on a wafer. The light illuminates the area on the wafer in an area illumination pattern. This illumination subsystem may be further configured as described and shown herein.

[0156] In one embodiment, the illumination subsystem comprises a frequency converted laser, the light comprises light pulses, and the light pulses directed to the area on the wafer are spatially non-varying over the duration of the light pulses and have a substantially constant intensity over the duration of the light pulses. In one such embodiment, the illumination subsystem comprises a beam shaping optical element coupled to the laser. Such embodiments may be further configured as described and shown herein. In some embodiments, the illumination subsystem comprises a frequency converted laser, the light comprises light pulses, and the light pulses directed to the area on the wafer have a substantially constant intensity over the duration of the light pulses. This embodiment may be configured as further described herein.

[0157] The system also 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 comprises light pulses, the scattered light comprises scattered light pulses, the scanning subsystem is configured to scan the light pulses across the wafer by rotating the wafer, and when the light pulses are scanned across a central region of the wafer, the illumination subsystem is configured to direct the light pulses to the region on the wafer less frequently than when the light pulses are scanned across the wafer outside of the central region. In another embodiment, the light comprises light pulses, the scattered light comprises scattered light pulses, the scanning subsystem is configured to scan the light pulses across the wafer by rotating and translating the wafer, the sensor comprises an area sensor, when the light pulses are scanned across the central region of the wafer, the scanning subsystem scans the light pulses across the wafer in one or more non-bent lines, and when the light pulses are scanned across the wafer outside of the central region, the scanning subsystem scans the light pulses across the wafer in a spiral manner. Such embodiments may be further configured as described and shown herein.

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

[0160] In one embodiment, the light comprises light pulses, 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.

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

[0162] In one embodiment, the system includes an additional sensor including an image intensifier, the light collection subsystem is configured to image light pulses scattered from the area on the wafer to 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 may be further configured as described and shown herein.

[0163] In another embodiment, the system includes an additional sensor configured to perform photon counting, the light collection subsystem is configured to image the light scattered from the area on the wafer to 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 may be configured as described herein.

[0164] In some embodiments, the system includes a MEMS-based optical switching device positioned between the light collection subsystem 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 a first of the scattered light pulses generated by a first of the light pulses to the sensor and direct a second of the scattered light generated by a 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 sections of the light collection NA of the light collection subsystem, and the optical switching device is configured to direct only a first of the different sections of the scattered light pulses generated by the first of the light pulses to the sensor and direct only another of the different sections 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 may be further configured as described and shown herein.

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

[0166] Any of the systems described herein may include additional channels and / or subsystems (not shown) designed to detect defects independently of or in conjunction with the primary inspection optical channel described above. One 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 produce information about the surface quality and defects of interest. The outputs from multiple channels may be combined by various logical components and / or with various mathematical operations, as described in U.S. Patent Application Publication No. 2010 / 0188657, issued to Chen et al. on July 29, 2010, and U.S. Patent Application Publication No. 2012 / 0044486, issued to Chen et al. on February 23, 2012, 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 anomaly capture rates while reducing false count rates.

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

[0169] Those skilled in the art will appreciate further modifications and alternative embodiments of aspects of the present invention in light of this description. For example, a system configured to inspect a wafer is provided. Therefore, this description should be interpreted only as illustrative and is intended to teach those skilled in the art the general manner of implementing the present invention. It should be understood that the forms of the present invention shown and described herein should be considered as currently preferred embodiments. As will be appreciated by those skilled in the art who benefit from this description of the present invention, elements and materials may be used to replace the elements and materials illustrated and described herein, parts and processes may be reversed, and certain features of the present invention may be used independently. Changes may be made to the elements described herein without departing from the spirit and scope of the present invention as described in the appended claims.

Claims

1. A system configured to inspect a wafer, comprising: an illumination subsystem configured to direct a first one of a plurality of pulsed light beams to an area on a wafer and then direct a second one of the plurality of pulsed light beams to the area through the illumination subsystem, wherein the first one of the plurality of pulsed light beams and the second one of the plurality of pulsed light beams have different shapes and sizes on the wafer, wherein the first one of the plurality of pulsed light beams and the second one of the plurality of pulsed light beams have different wavelengths, different polarizations, or different wavelengths and polarizations; a scanning subsystem configured to scan the plurality of pulsed beams across the wafer; a light collection subsystem configured to image light scattered from the area on the wafer to one or more sensors, wherein the one or more sensors generate an output in response to the scattered light; a computer subsystem configured to use the output of the one or more sensors to detect defects on the wafer and to use the output of the scattered light from the area in response to illumination by the first of the plurality of pulsed light beams to determine a power of the second of the plurality of pulsed light beams that should be directed to the area; An optical switching device based on a micro-electromechanical system, which is located between the light collection subsystem and the sensor; as well as at least one additional sensor, wherein the light comprises light pulses, wherein the scattered light comprises scattered light pulses, and wherein the optical switching device is configured to direct a first of the scattered light pulses generated by a first group of the light pulses to the sensor and to direct a second of the scattered light pulses generated by a second group of the light pulses following the first group of the light pulses to the at least one additional sensor.

2. The system of claim 1 , wherein the illumination subsystem comprises a Q-switched laser, and wherein the computer subsystem is configured to prevent the second one of the plurality of pulsed light beams that should be directed to the area from being generated from the Q-switched laser when the power of the second one of the plurality of pulsed light beams that should be directed to the area is determined to be zero.

3. The system of claim 1 , wherein the computer subsystem is further configured to monitor the power of the second one of the plurality of pulsed light beams actually directed to the area and to change one or more parameters of the system based on the power directed to the area so as to normalize the power of the second one of the plurality of pulsed light beams actually directed to the area.

4. A system configured to inspect a wafer, comprising: an illumination subsystem configured to direct the plurality of light beams to substantially the same area on the wafer; a scanning subsystem configured to scan the plurality of light beams across the wafer; a light collection subsystem configured to image light scattered from the substantially same area on the wafer to a sensor, wherein the sensor is configured to generate an output in response to the scattered light; as well as a computer subsystem configured to use the output generated by the sensor to detect defects on the wafer, wherein the computer subsystem is further configured to use the output of scattered light from the substantially same area in response to illumination due to the first of the plurality of light beams to determine a power of a second of the plurality of light beams that should be directed to the substantially same area; An optical switching device based on a micro-electromechanical system, which is located between the light collection subsystem and the sensor; as well as at least one additional sensor, wherein the light comprises light pulses, wherein the scattered light comprises scattered light pulses, and wherein the optical switching device is configured to direct a first of the scattered light pulses generated by a first group of the light pulses to the sensor and to direct a second of the scattered light pulses generated by a second group of the light pulses following the first group of the light pulses to the at least one additional sensor.

5. The system of claim 4, wherein the illumination subsystem comprises a Q-switched laser, and wherein the computer subsystem is configured to prevent the second one of the plurality of light beams that should be directed to the area from being generated from the Q-switched laser when the power of the second one of the plurality of light beams that should be directed to the area is determined to be zero.

6. The system of claim 4 further comprising an additional sensor including an image intensifier, wherein the light collection subsystem is further configured to image light scattered from the substantially same area on the wafer onto the additional sensor, wherein the additional sensor generates an additional output in response to the scattered light, and wherein the computer subsystem is further configured to use the additional output to detect defects on the wafer in place of the output when sensor electronic noise dominates the total channel noise in the sensor.

7. The system of claim 4, further comprising additional sensors configured to perform photon counting, wherein the light collection subsystem is further configured to image light scattered from the substantially same area on the wafer onto the additional sensors, wherein the additional sensors generate additional outputs in response to the scattered light, and wherein the computer subsystem is further configured to use the additional outputs to detect defects on the wafer.

8. The system of claim 4, wherein the illumination subsystem comprises a frequency converted laser, wherein the light comprises light pulses, and wherein the light pulses directed to the area on the wafer are spatially non-varying over a duration of the light pulses and have a substantially constant intensity over the duration of the light pulses.

9. The system of claim 8, wherein the illumination subsystem comprises beam shaping optics coupled to the laser.

10. The system of claim 4, wherein the illumination subsystem comprises a frequency converted laser, wherein the light comprises light pulses, and wherein the light pulses directed to the area on the wafer have a substantially constant intensity over a duration of the light pulses.

11. A system according to claim 4, wherein the light comprises light pulses, wherein the scattered light comprises scattered light pulses, wherein the optical switching device is configured to separate the scattered light pulses collected in different segments of the collection numerical aperture of the collection subsystem, and wherein the optical switching device is further configured to direct only one of the different segments of the scattered light pulses generated by a first group of the light pulses to the sensor and then direct only another one of the different segments of the scattered light pulses generated by a second group of the light pulses after the first group of the light pulses to the sensor.

12. The system of claim 4, wherein the light comprises light pulses, wherein the scattered light comprises scattered light pulses, and wherein the sensor is synchronized in time with respect to the light pulses to detect only the scattered light pulses having predetermined arrival times.

13. The system of claim 12, wherein the scattered light pulses having predetermined arrival times comprise fluorescence or photoluminescence.

14. The system of claim 4 , wherein the light comprises light pulses, wherein the scattered light comprises scattered light pulses, wherein the scanning subsystem is further configured to scan the light pulses across the wafer by rotating the wafer, and wherein when the light pulses are scanned across a central region of the wafer, the illumination subsystem is further configured to direct the light pulses to the area on the wafer less frequently than when the light pulses are scanned across the wafer outside of the central region.

15. The system of claim 4, wherein the light comprises light pulses, wherein the scattered light comprises scattered light pulses, wherein the scanning subsystem is further configured to scan the light pulses across the wafer by rotating and translating the wafer, wherein the sensor comprises an area sensor, wherein when scanning the light pulses across a central region of the wafer, the scanning subsystem scans the light pulses across the wafer in one or more non-bent lines, and wherein when scanning the light pulses across the wafer outside of the central region, the scanning subsystem scans the light pulses across the wafer in a spiral manner.

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