SYSTEM AND METHOD FOR IMPROVED HOLOGRAPHIC IMAGING

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Patent Application Serial No. 60/709,328, filed August 18, 2005 by Arkady Feldman, and entitled "System and Method for Detection of Defects on Patterned Semiconductor Wafers . ^' " TECHNICAL FIELD OF THE INVENTION

This invention relates in general to the field of image processing and, more particularly, to a system and method for detecting differences on substrates. BACKGROUND OF THE INVENTION

The use of direct-to-digital holography techniques have become prevalent in imaging systems, and have become prevalent in a number of applications. For example, in a direct-to-digital holography system, holograms from highly similar objects can be obtained. Consecutive processing of the holograms allows comparison of actual image waves of the objects. These image waves contain significantly more information for both small and large details of the objects than conventional non-holographic images, because image phase information is retained in the holograms, but lost in conventional images. In some applications, such as defect inspection for a semiconductor wafer, the holography system may be used to acquire multiple images from different locations on objects that were meant to be identical .

Holographic images differ from real images because holographic images contain intensity and phase information while real images only contain intensity

information. The additional phase information in holographic imaging adds a new dimension of complexity, as well as new possibilities beyond standard image processing tools and capabilities. For example, wave front matching capabilities would have little merit for intensity images (e.g., real images), whereas they are important for image waves (e.g., complex images), as they address the phase in the image wave that does not exist in the intensity image. Such holographic imaging systems may be used to optically scan or capture images of the surface of a target such as a semiconductor wafer to measure the topography of a target surface. This topography may then be analyzed to identify manufacturing or material defects existing on the target. Such analysis is critical in diagnosing manufacturing problems to maintain a desired manufacturing throughput .

Some holography systems may include a reference arm for generating a reference beam and a target arm for generating a target beam. In existing holography systems, the reference beam and target beam follow a different optical path. Typically, the target beam will interact with a target, e.g. a semiconductor substrate, before being combined with the reference beam. The reference and target beams may be combined in order to produce a complex image that is captured by a digital recorder, such as a high resolution charge coupled device (CCD) camera. In order to produce the image, the zero order waves associated with the reference and target beams should be matched. Conventional systems match each of the beams by providing identical optics in each of the arms. The optics used in these systems, however, may be

expensive and may increase the overall cost of the systems .

In addition, because the reference beam and the target beam follow different paths, noise caused by environmental factors (such as temperature and air turbulence) , mechanical disturbances (such as vibrations) , and other factors may affect each beam differently and introduce phase variations between the reference arm and target arm that are not induced by the target. These phase variations may affect the effectiveness of the imaging system. For example, in applications in which the imaging system is used to detect defects on a semiconductor wafer, noise may cause false positives or false negatives in the detection of defects on a substrate, and a reduced defect capture rate. Because noise represents an erroneous signal, noise significantly reduces the effectiveness of imaging and inspection systems. Accordingly noise can severely hamper the ability to identify and remedy manufacturing and material defects, negatively effecting manufacturing throughput and yield. SUMMARY OF THE DISCLOSURE

In accordance with the teachings of the present invention, disadvantages and problems associated with detecting differences between complex images have been substantially reduced or eliminated.

In one embodiment of the disclosure, an interferometric imaging system may comprise a common path section and an equal path section. The common path section may include a source operable to produce a first beam of electromagnetic energy and may be configured to cause at least a portion of the first beam to interact

with a target. The equal path section may include a beamsplitter, a spatial filter and a beam combiner. The beamsplitter may be operable to separate the portion of the first beam interacting with the target into a reference beam and a target beam after the first beam has interacted with the target. The spatial filter may be operable to extract zero order image information from the reference beam and transmit the zero order image information as a zero order beam. The beam combiner may be operable to combine the zero order beam with the target beam to create a holographic image.

In another embodiment of the disclosure, an imaging system may comprise a source, one or more imaging system components, a beamsplitter, a spatial filter and a beam combiner. The source may be operable to produce a first beam of electromagnetic energy, and the one or more imaging system components may be operable to cause at least a portion of the first beam to interact with a target. The beamsplitter may be operable to separate the portion of the first beam interacting with the target into a reference beam and a target beam after the first beam has interacted with the target . The spatial filter may be operable to extract zero order image information from the reference beam and transmit the zero order image information as a zero order beam. The beam combiner may be operable to combine the zero order beam with the target beam to create a holographic image .

In yet another embodiment of the disclosure a method of creating a holographic image is provided. A first beam of electromagnetic energy is produces and a portion of the first beam interacts with a target. After the first beam has interacted with the target, the portion of

the first beam interacting with the target is separated into a reference beam and a target beam. Zero order image information is extracted from the reference beam and transmitted as a zero order beam. The zero order beam is combined with the target beam to create a holographic image.

BRIEF DESCRIPTION OF FIGURES

A more complete understanding of the present invention and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:

FIGURE 1 illustrates a schematic view of a prior art direct-to-digital holography system; FIGURE 2 illustrates a schematic view of a common path direct-to-digital holography system in accordance with teachings of the present disclosure;

FIGURE 3 illustrates a schematic view of another common path direct-to-digital holography system in accordance with teachings of the present disclosure;

FIGURE 4 illustrates a schematic view of a common path direct-to-digital holography system utilizing dark- field illumination in accordance with teachings of the present disclosure; FIGURE 5 illustrates a schematic view of a direct- to-digital holography system using dark-field illumination wherein the target illumination beam is coaxial with the optical axis of an objective lens in accordance with teachings of the present disclosure; and FIGURE 6 illustrates a schematic view of direct-to- digital holography system using dark-field illumination wherein the target illumination beam is coaxial with the

optical axis of an objective lens in accordance with teachings of the present disclosure. DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention and its advantages are best understood by reference to FIGURES 1- 6 wherein like numbers refer to like and corresponding parts and like element names to like and corresponding elements. A listing of the reference numbers and their corresponding elements is included below. The following invention generally relates to digital holographic imaging systems and applications as described in U.S. Patent No. 6,078,392 entitled Direct-to-Digital Holography and Holovision; U.S. Patent No. 6,525,821 entitled, Acquisition and Replay Systems for Direct to Digital Holography and Holovision; U.S. Patent No.

6,763,142 entitled System and Method for Correlated Noise Removal in Complex Imaging Systems; U.S. Patent No. 6,873,354 entitled, System and Method for Registering Complex Images; U.S. Patent Application Serial No. 10/661,187 entitled System and Method for Acquiring and Processing Complex Images; U.S. Patent Application Serial No. 10/661,873 entitled, System and Method for Detecting Differences Between Complex Images; and U.S. Patent Application Serial No. 10/661,174 entitled, Optical Acquisition Systems for Direct-to Digital Holography and Holovision; all of which are incorporated herein by reference.

FIGURE 1 illustrates a schematic view of a prior art direct-to-digital holography system 100. System 100 includes laser 110, modulator 112, beam expander 114, beam splitter 116, target beamsplitter 130, target objective 142, quarter wave plate 144, target 146, half

wave plate 132, reference arm mirror 122, target arm mirror 134, target tube lens 136, reference tube lens 124, beam combiner 128 and digital recorder 140. In operation, laser 110 directs a beam of light toward modulator 112 and the modulated light travels to beam expander 114. The expanded light travels to beamsplitter 116 which splits the beam of light into target beam 120 and reference beam 118.

Still referring to FIGURE 1, target beam 120 is directed through target beamsplitter 130 which reflects all of the target beam 120 light toward target objective 142 and quarter wave plate (QWP) 144. Target beam 120 then interacts with target 146 and passes back through QWP 144 and target objective 142. During each pass through QWP 144, QWP 144 creates a one-quarter wavelength shift of target beam 120. Thus, if target beamsplitter 130 is configured to transmit light of a certain polarization and reflect light of opposite polarization, beamsplitter 130 may direct target beam 120 through objective 142 and quarter wave plate 144 and interact with target 146 on its first pass through beamsplitter 130. Light reflected from target 146 may again pass through quarter wave plate 144 and objective 142, and through beamsplitter 130 on its second pass. Target beam 120 may then become incident onto mirror 134 where it is reflected through target tube lens 136 and onto mirror 138 where it is reflected to beam combiner 128.

In the reference arm, reference beam 118 from beamsplitter 116 is reflected by mirror 122 and transmitted through reference tube lens 124. Reference tube lens 124 includes a spatial filter 126 which filters or conditions reference beam 118 to provide desired

optical characteristics of the reference beam. Reference beam 118 then travels toward beam combiner 128, which combines light from the target arm and the reference arm and directs the combined beams to digital recorder 140, such as a charge coupled device (CCD) camera. Together, target beam 120 and reference beam 118 combine at beam combiner 128 to form an interference pattern or hologram at digital recorder 140. Interference infringes of the hologram are created by phase differences of the light between reference beam 118 and target beam 120. The phase differences are induced in the target beam 120 by- features appearing on target 146. Thus, system 100 may be used to compare complex images obtained from the same object or target after a physical change occurs to the target or to compare complex images from different targets. In addition, system 100 may be used to acquire an image from different locations on target 146. In some applications, system 100 may be used to detect defects on target 146, for example a semiconductor wafer. If a defect exists on a target, such a defect will cause a different hologram than that of a defect-free target. Using image processing, one can detect hologram differences, and thus detect target defects.

As used in this disclosure, a "beamsplitter" may be any optical element that allows a portion of the beam to be transmitted and a portion of the beam to be reflected. In some embodiments, a beamsplitter may be a 50/50 beamsplitter where approximately fifty percent (50%) of a beam is reflected and approximately fifty percent (50%) of the beam is transmitted. In the same or alternative embodiments, a beamsplitter may reflect and/or transmit any suitable percentage of light. References to a

beamsplitter may include, but are not limited to, a cube beamsplitter, a plate beamsplitter, a polarizing beamsplitter, and a non-polarizing beam splitter.

As used in this disclosure, a "beam combiner" may be any optical element that allows two or more separate electromagnetic beams to be combined into one beam of electromagnetic energy.

As used in this disclosure, a "beam expander" may be any system or apparatus configured to increase the diameter or width of a radiation beam.

As used in this disclosure, a "target" may be any object, assembly or component from which a complex image may be generated. In some embodiments, a target may be an electronic device fabricated from silicon, germanium or any compound containing group III and/or group V elements. In the same or alternative embodiments, a target may be a photomask or reticle that includes a pattern formed on a substrate. In the same or other embodiments, a target may be any other object, including without limitation a reflective target, assembly or component from which a complex image may be generated.

As used in this disclosure, "polarization rotator" may refer to any apparatus or system that transforms a beam of electromagnetic radiation of any polarization angle and produces a new beam, substantially coaxial with the incident beam, and having a specified new polarization angle.

As used in this disclosure, "digital recorder" may refer to any system or apparatus operable to record and/or playback an interference pattern or hologram acquired from a target in an imaging system. In some embodiments, a digital recorder may be a high resolution

charge coupled device (CCD) camera that may record and playback a hologram acquired from a target in an imaging system. A digital recorder may further be interfaced with a computer (not expressly shown) that includes processing resources. In the same or alternative embodiments, the processing resources may be one or a combination of a microprocessor, a microcontroller, a digital signal processor (DSP) or any other digital circuitry configured to process information. As used in this disclosure, "spatial filter" may refer to any system or apparatus operable to extract lower order or zero order image information from an electromagnetic beam and/or mask higher order image information. As used in this disclosure, "mirror" may refer to any system or apparatus operable to reflect an electromagnetic beam. In some embodiments, a mirror may comprise a smooth, highly polished surface that may be planar or curved. In the same or alternative embodiments, the actual reflecting surface may be a thin coating of silver or aluminum on glass. In the same or alternative embodiments, a mirror may comprise a right angle prism and/or other type of prism.

As used in this disclosure, "ghost" may refer to an unwanted or undesired image caused by reflection that is sometimes seen when observing through an optical instrument .

As used in this disclosure, "modulator" may refer to any apparatus or system that turns on and off a beam of electromagnetic radiation of any polarization angle at controlled intervals. In the same or alternative embodiments, the actual modulator may be any mechanical

or electronic shuttering or gating device, or laser pulse generating apparatus suitable for creating timed ans synchronized illumination exposures.

FIGURE 2 illustrates a schematic view of a common path direct-to-digital holography system 200 in accordance with teachings of the present disclosure. System 200 may include common path section 211 which may comprise laser 210, modulator 212, beam expander 214, polarization rotator 216, beamsplitter 230, objective 242, quarter wave plate (QWP) 244, target 246; and equal, path section 221, which may comprise half wave plate 232, beamsplitter 223, reference arm mirror 222, target arm mirror 234, target tube lens 236, reference tube lens 224 and beam combiner 228. System 200 may also include half wave plate 218 and digital recorder 240.

In operation, laser 210 may direct a beam of light toward modulator 212 and the modulated light may travel to beam expander 214. The expanded light may then travel through polarization rotator 216 to create a desired polarization of the light, then to beamsplitter 230 which may reflect all of a portion of the light towards objective 242. Light reflected from beamsplitter 230 may travel through objective 242 and through QWP 244 and then interact with target 246. The beam of light may then reflect from target 246, pass back through QWP 244 and objective 242, and travel to beamsplitter 230 which may transmit all or a portion of the light. In some embodiments, beamsplitter 230 may comprise a polarizing beam splitter cube, which may permit light of a certain polarization to be reflected while permitting light of an opposite polarization to be transmitted. In the same or alternative embodiments, polarization rotator 216 may be

configured to polarize the light emitted from laser 210 such that a desired amount of light is reflected and/or transmitted by beamsplitter 230.

Still referring to FIGURE 2, the light transmitted by beamsplitter 230 may then exit common path section 211 and travel through half wave plate 219, then to beamsplitter 223 which may split the beam of light into target beam 220 and reference beam 218. In some embodiments, beamsplitter 223 may comprise a polarizing beam splitter cube, which may split light of one polarization into target beam 220 and light of the opposite polarization into reference beam 218. Target beam 220 may be directed through target tube lens 236 and onto mirror 238 where it may be reflected towards beam combiner 228. In the reference arm, reference beam 218 from beamsplitter 223 may pass through half wave plate 232 to match polarity with target beam 220 and may be reflected by mirror 222. Reference beam 218 may then be transmitted through reference tube lens 224. Reference tube lens 224 may include a spatial filter 226 which may extract zero order image information from reference beam 218, thus allowing reference beam 218 to have characteristics similar to a reference beam that has not interacted with a target . The zero order beam extracted from reference beam 218 may then travel toward beam combiner 228, which combines light from the target arm and the zero order light from reference arm and directs the combined beams to digital recorder 240, e.g. a CCD camera. Together target beam 220 and the zero order portion of reference beam 218 may combine at beam combiner 228 to form an interference pattern or hologram at digital recorder 240. In system 200, common mode

phase changes in the reference arm and target arm induced by noise, e.g. noise induced by vibration and temperature, may cancel out, making system 200 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating reducing the effects of noise in the interference pattern created at digital recorder 240. Testing of an imaging system consistent with system 200 has shown decreased sensitivity to noise compared to that observed in prior art imaging systems. Thus, in application in which system 200 is used to detect differences between complex images, e.g. detection of defects on a semiconductor wafer, thus use of system 200 may substantially reduce or eliminate the occurrence of false positives or false negatives, and may improve defect capture rate as compared to prior art imaging systems .

System 200 may be used in any suitable application. For example, system 200 may be used to compare complex images obtained from the same object or target after a physical change occurs to the target or to compare complex images from different targets. In addition, system 200 may be used to acquire an image from different locations on target 246. In some applications, system 200 may be used to detect defects on target 246, for example a semiconductor wafer. If a defect exists on a target, such a defect will cause a difference hologram than that of a defect -free target. Using image processing, one can detect hologram differences, and thus detect target defects. In the same or alternative embodiments, system 200 may be used to determine the sizes and/or shapes of features or devices appearing on

target 246, for example sizes devices manufactured on a semiconductor wafer. Furthermore, in the same or other embodiments, system 200 may be used to add holographic imaging capability to existing non-holographic imaging systems. For example, system 200 may comprise a standalone modular component that may be integrated into an existing imaging system.

FIGURE 3 illustrates a schematic view of another common path direct-to-digital holography system 300 in accordance with teachings of the present disclosure.

System 300 may include common path section 311 which may comprise laser 310, modulator 312, beam expander 314, polarization rotator 316, beamsplitter 330, objective 342, QWP 344, target 346; and equal path section 321, which may comprise, beamsplitter 323, reference arm mirror 322, reference arm lenses 343 and 345, spatial filter 326, blocking screen 346, dark field filter 347 and beam combiner lens 328. System 300 may also include digital recorder 340. In operation, laser 310 may direct a beam of light toward modulator 312 and the modulated light may travel to beam expander 314. The expanded light may then travel through polarization rotator 316 to create a desired polarization of the light, then to beamsplitter 330 which may reflect all of a portion of the light towards objective 342. Light reflected from beamsplitter 330 may travel through objective 342 and through QWP 344 and then interact with target 346. The beam of light may then reflect from target 346, pass back through QWP 344 and objective 342, and travel to beamsplitter 330 which may transmit all or a portion of the light. In some embodiments, beamsplitter 330 may comprise a polarizing

beam splitter cube, which may permit light of a certain polarization to be reflected while permitting light of an opposite polarization to be transmitted. In the same or alternative embodiments, polarization rotator 316 may be configured to polarize the light emitted from laser 310 such that a desired amount of light is reflected and/or transmitted by beamsplitter 330.

Still referring to FIGURE 3, the light transmitted by beamsplitter 330 may then exit common path section 311 and travel to beamsplitter 323 of equal path section 321 which may split the beam of light into target beam 320 and reference beam 318. Beamsplitter may direct target beam 320 towards beam combiner lens 328. In the reference arm, reference beam 318 from beamsplitter 323 may pass through lens 343 and spatial filter 326. Spatial filter 326 may extract zero order image information from reference beam 318, thus allowing reference beam 318 to have characteristics similar to a reference beam that has not interacted with a target . The zero order beam extracted from reference beam 318 may then travel through lens 345 and toward beam combiner 328, which combines light from the target arm and the zero order light from reference arm and directs the combined beams to digital recorder 340, e.g. a CCD camera. Together target beam 320 and the zero order portion of reference beam 318 may combine at beam combiner 328 to form an interference pattern or hologram at digital recorder 340. System 300 may also include a blocking screen 349 to block or prevent noise, e.g. stray light or air currents, from being introduced into system 300.

In system 300, common mode phase changes in the reference arm and target arm induced by noise, e.g. noise induced by vibration and temperature, may cancel out, making system 300 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating the effects of noise in the interference pattern created at digital recorder 340.

In some embodiments of system 300, equal path section 321 may be built as a dual-axis, monolithic optical system housed within a single cover. As described above, a single beam may enter equal path section 321 prior to separation into target beam 320 and reference beam 318, and the suitably combined target beam 320 and reference beam 318 exit equal path section 321 before forming a pattern on digital recorder 340. In such a configuration, noise e.g. noise induced by vibration and temperature, may cancel out, further making system 300 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating the effects of noise in the interference pattern created at digital recorder 340.

Testing of an imaging system consistent with system 300 has shown decreased sensitivity to noise compared to that observed in prior art imaging systems. Thus, in application in which system 300 is used to detect differences between complex images, e.g. detection of defects on a semiconductor wafer, thus use of system 300 may substantially reduce or eliminate the occurrence of false positives or false negatives, and may improve defect capture rate as compared to prior art imaging

systems. In addition, testing of an imaging system consistent with system 300 has shown a reduction in ghosts and scattered light levels, as well as increased transmission as compared to that observed in prior art imaging systems, and as such, shows improved signal to noise ratio and sensitivity. Such improved transmission may be related to, at least in part, the fewer number of optical components present in system 300 as compared to prior art imaging systems as well as the fact that system 300 may permit: (i) an optical system in which the optical paths of the reference arm and target arm have a significant common path in common path section 311 and (ii) an equal path section built as a dual-axis, monolithic optical system housed within a single cover. Because of improved transmission, power of the optical beam directed into an imaging system can be reduced, potentially increasing the lifetime of optical components and reducing laser damage to the target .

System 300 may be used in any suitable application. For example, system 300 may be used to compare complex images obtained from the same object or target after a physical change occurs to the target or to compare complex images from different targets. In addition, system 300 may be used to acquire an image from different locations on target 346. In some applications, system 300 may be used to detect defects on target 346, for example a semiconductor wafer. If a defect exists on a target, such a defect will cause a different hologram than that of a defect-free target. Using image processing, one can detect hologram differences, and thus detect target defects. In the same or alternative embodiments, system 300 may be used to determine the

sizes and/or shapes of features or devices appearing on target 346, for example sizes devices manufactured on a semiconductor wafer. Furthermore, in the same or other embodiments, system 300 may be used to add holographic imaging capability to existing non-holographic imaging systems. For example, system 300 may comprise a standalone modular component that may be integrated into an existing imaging system.

FIGURE 4 illustrates a schematic view of a common path direct-to-digital holography system 400 utilizing dark-field illumination in accordance with teachings of the present disclosure. System 400 may include common path section 411 which may comprise laser 41O ₇ modulator 412, beam expander 414, polarization rotator 416, mirror 417, mirror 428, beamsplitter 430, objective 442, and target 446; and equal path section 421, which may comprise mirror 423, reference arm lenses 443 and 445, spatial filter 426, blocking screen 449, and beam combiner lens 428. System 400 may also include diffraction grating or mirror 434 and digital recorder 440.

In operation, laser 410 may direct a beam of light toward modulator 412 and the modulated light may travel to beam expander 414. The expanded light may then travel through polarization rotator 416 to create a desired polarization of the light, then to beamsplitter 430 which may reflect all of a portion of the light towards mirror 417. Light reflected from mirror 417 may be reflected by mirror 428 onto target 446. In some embodiments of system 400, mirror 428 may be located in a position that is not coaxial with objective 442, and may be positioned and/or configured such that the specular reflection of

the beam reflected by mirror 428 is directed out of the optical path of objective 442 and becomes the target beam 420. In such an embodiment, the portion of the target beam 420 reflected from target 446 and within the optical path of objective 442 may pass through objective 442.

The specular component of the light reflected from mirror 428 may become reference beam 418, as described in greater detail below.

Still referring to FIGURE 4, the light transmitted through objective 442 may then exit common path section 411 and travel to mirror 423 of equal path section 421. Mirror 423 may direct target beam 420 towards beam combiner lens 428. In the reference arm, the specular component light reflected from mirror 417 may be reflected towards mirror 417 and become the reference beam 418. Such a reflection may be created by any suitable means, including without limitation, another mirror (not shown) which is configured to direct the specular component of the light reflected from mirror 428 onto mirror 417. After reflection from mirror 417, reference beam 418 may pass through beamsplitter 430, exit common path section 411, and become incident upon diffraction grating 434. Zero order light diffracted by diffraction grating 434 may enter equal path section 421 while higher order light diffracted by diffraction grating 434 may be blocked by blocking screen 449 from entering equal path section 421. In addition to blocking higher order light, blocking screen 449 may also block or prevent noise, e.g. stray light or air currents, from being introduced into system 400.

Such filtering may allow reference beam 418 to have characteristics similar to a reference beam that has not

interacted with a target. In other embodiments of system 400, diffraction grating 434 may be replaced by a mirror, in which case higher order light may not filtered prior to reference beam 418 entering equal path section 421. The zero order light diffracted from diffraction grating 434 may pass through lens 443 and spatial filter 426. Spatial filter 426 may extract zero order image information from reference beam 418, thus providing additional filtering to the reference arm to further condition the light of the reference arm in a desired manner. The zero order beam extracted from reference beam 418 may then travel through lens 445 and toward beam combiner 428, which combines light from the target arm and the zero order light from reference arm and directs the combined beams to digital recorder 440, e.g. a CCD camera. Together target beam 420 and the zero order portion of reference beam 418 may combine at beam combiner 428 to form an interference pattern or hologram at digital recorder 440. In system 400, common mode phase changes in the reference arm and target arm induced by noise, e.g. noise induced by vibration and temperature, may cancel out, making system 400 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating the effects of noise in the interference pattern created at digital recorder 440.

In some embodiments of system 400, equal path section 421 may be built as a dual-axis, monolithic optical system housed within a single cover. As described above, target beam 420 and a reference beam 418 may enter equal path section 421, and the suitably

combined target beam 420 and reference beam 418 exit equal path section 421 before forming a pattern on digital recorder 440. In such a configuration, noise e.g. noise induced by vibration and temperature, may cancel out, further making system 400 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating reducing the effects of noise in the interference pattern created at digital recorder 440. Testing of an imaging system consistent with system 400 has shown decreased sensitivity to noise compared to that observed in prior art imaging systems. Thus, in application in which system 400 is used to detect differences between complex images, e.g. detection of defects on a semiconductor wafer, thus use of system 400 may substantially reduce or eliminate the occurrence of false positives or false negatives, and may improve defect capture rate as compared to prior art imaging systems. In addition, testing of an imaging system consistent with system 400 has shown a reduction in ghosts and scattered light levels, as well as increased transmission as compared to that observed in prior art imaging systems, and as such, shows improved signal to noise ratio and sensitivity. Such improved transmission may be related to, at least in part, the fewer number of optical components present in system 400 as compared to prior art imaging systems as well as the fact that system 400 may permit: (i) an optical system in which the optical paths of the reference arm and target arm have a significant common path in common path section 411; (ii) an equal path section built as a dual -axis, monolithic optical system housed within a single cover; (iii) the

elimination of certain imaging system components, such as quarter wave plates, that may introduce noise or other deleterious effects into an imaging system; (iv) in some embodiments of system 400, two levels of filtering of reference beam, thus allowing a higher quality hologram to be captured by digital recorder 440; and (v) off-axis dark-field illumination of target 446 without necessitating re-alignment of the objective and may eliminate "through-the-objective illumination" of the target, thus reducing backreflections and ghosts.

Because of improved transmission and other improved qualities, power of the optical beam directed into an imaging system can be reduced, potentially increasing the lifetime of optical components and reducing laser damage to the target.

System 400 may be used in any suitable application. For example, system 400 may be used to compare complex images obtained from the same object or target after a physical change occurs to the target or to compare complex images from different targets. In addition, system 400 may be used to acquire an image from different locations on target 446. In some applications, system 400 may be used to detect defects on target 446, for example a semiconductor wafer. If a defect exists on a target, such a defect will cause a difference hologram than that of a defect-free target. Using image processing, one can detect hologram differences, and thus detect target defects. In the same or alternative embodiments, system 400 may be used to determine the sizes and/or shapes of features or devices appearing on target 446, for example sizes devices manufactured on a semiconductor wafer. Furthermore, in the same or other

embodiments, system 400 may be used to add holographic imaging capability to existing non-holographic imaging systems. For example, system 400 may comprise a standalone modular component that may be integrated into an existing imaging system.

FIGURE 5 illustrates a schematic view of a direct- to-digital holography system 500 using dark-field illumination wherein the target illumination beam is coaxial with the optical axis of an objective lens 542. System 500 may include common path section 511 which may comprise laser 510, modulator 512, beam expander 514, polarization rotator 516, mirror 517, mirror 524, mirror 528, beamsplitter 530, objective 542, and target 546; and equal path section 521, which may comprise mirror 523, reference arm lenses 543 and 545, spatial filter 526, blocking screen 549, and beam combiner lens 528. System 500 may also include diffraction grating or mirror 534 and digital recorder 540.

In operation, laser 510 may direct a beam of light toward modulator 512 and the modulated light may travel to beam expander 514. The expanded light may then travel through polarization rotator 516 to create a desired polarization of the light, then to beamsplitter 530 which may split the beam of light into target beam 520 and reference beam 518. In some embodiments, beamsplitter 530 may comprise a polarizing beam splitter cube, which may split light of one polarization into target beam 520 and light of the opposite polarization into reference beam 518. In such an embodiment, a half wave plate (not shown) may be inserted in the reference arm between beamsplitter 530 and diffraction grating 534 in the reference arm, or between mirrors 524 and 517 in the

target arm, in order to match the polarization of reference beam 518 to reference beam 520. In other embodiments, beamsplitter 530 may comprise a non- polarizing beam splitter cube. In such an embodiment, polarization rotator 516 may be excluded from system 500.

Target beam 520 from beamsplitter 530 may be reflected towards mirror 524, towards mirror 517, towards mirror 528 and interact with target 546. In some embodiments of system 500, mirror 528 may be located in a position that is coaxial with the focal axis of objective 542, and thus may be positioned and/or configured such that the specular reflection of the target beam reflected from target 546 is directed out of the optical path of objective 542 (although such specular reflection is not necessarily reflected back towards mirror 517 by mirror 528. The non-specular components of the light reflected from target 526 may pass through objective 542 reflect from mirror 523 of equal path section 521 towards beam combiner lens 538. In the reference arm, the light transmitted by beamsplitter 530 may exit common path section 511, and become incident upon diffraction grating 534. Zero order light diffracted by diffraction grating 534 may enter equal path section 521 while higher order light diffracted by diffraction grating 534 may be blocked by blocking screen 549 from entering equal path section 521. In addition to blocking higher order light, blocking screen 549 may also block or prevent noise, e.g. stray light or air currents, from being introduced into system 500.

Such filtering may allow reference beam 518 to have characteristics similar to a reference beam that has not

interacted with a target. In other embodiments of system 500, diffraction grating 534 may be replaced by a mirror, in which case higher order light may not filtered prior to reference beam 518 entering equal path section 521. The zero order light diffracted from diffraction grating 534 may pass through lens 543 and spatial filter 526. Spatial filter 526 may extract zero order image information from reference beam 518, thus providing additional filtering to the reference arm to further condition the light of the reference arm in a desired manner. The zero order beam extracted from reference beam 518 may then travel through lens 545 and toward beam combiner 528, which combines light from the target arm and the zero order light from reference arm and directs the combined beams to digital recorder 540, e.g. a CCD camera. Together target beam 520 and the zero order portion of reference beam 518 may combine at beam combiner 528 to form an interference pattern or hologram at digital recorder 540. In system 500, common mode phase changes in the reference arm and target arm induced by noise, e.g. noise induced by vibration and temperature, may cancel out, making system 500 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating reducing the effects of noise in the interference pattern created at digital recorder 540.

In some embodiments of system 500, equal path section 521 may be built as a dual-axis, monolithic optical system housed within a single cover. As described above, target beam 520 and a reference beam 518 may enter equal path section 521, and the suitably

combined target beam 520 and reference beam 518 exit equal path section 521 before forming a pattern on digital recorder 540. In such a configuration, noise e.g. noise induced by vibration and temperature, may cancel out, further making system 500 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating reducing the effects of noise in the interference pattern created at digital recorder 540. In addition, as detailed above, mirror 528 directs light towards wafer 546 and into objective 542. This permits light, incident on wafer 546, to penetrate deep into the target 546, thus allowing maximum phase modulation of the beam reflected or scattered into objective 542. Accordingly, system 500 has shown increased sensitivity and reduced signal to noise ratios with respect to deep aspect ratio patterns and/or structures on target 546.

Testing of an imaging system consistent with system 500 has shown decreased sensitivity to noise compared to ■ that observed in prior art imaging systems . Thus , in application in which system 500 is used to detect differences between complex images, e.g. detection of defects on a semiconductor wafer, thus use of system 500 may substantially reduce or eliminate the occurrence of false positives or false negatives, and may improve defect capture rate as compared to prior art imaging systems. In addition, testing of an imaging system consistent with system 500 has shown a reduction in ghosts and scattered light levels, as well as increased transmission as compared to that observed in prior art imaging systems, and as such, shows improved signal to

noise ratio and sensitivity. Such improved transmission may be related to, at least in part, the fewer number of optical components present in system 500 as compared to prior art imaging systems as well as the fact that system 500 permits: (i) an optical system in which the optical paths of the reference arm and target arm have a significant common path in common path section 511; (ii) an equal path section built as a dual-axis, monolithic optical system housed within a single cover; (iii) the elimination of certain imagine system components, such as quarter wave plates, that may introduce noise or other deleterious effects into an imaging system, and (iv) in some embodiments of system 500, two levels of filtering of reference beam, thus allowing a higher quality hologram to be captured by digital recorder 540; and (v) on-axis dark-field illumination of target 546 without necessitating re-alignment of the objective and may eliminate "through-the-objective illumination" of the target, thus reducing backreflections and ghosts while increasinging maximum phase modulation of the beam reflected or scattered from target 546. Because of improved transmission and other improved qualities, power of the optical beam directed into an imaging system can be reduced, potentially increasing the lifetime of optical components and reducing laser damage to the target .

System 500 may be used in any suitable application. For example, system 500 may be used to compare complex images obtained from the same object or target after a physical change occurs to the target or to compare complex images from different targets. In addition, system 500 may be used to acquire an image from different

locations on target 546. In some applications, system 500 may be used to detect defects on target 546, for example a semiconductor wafer. If a defect exists on a target, such a defect will cause a difference hologram than that of a defect-free target. Using image processing, one can detect hologram differences, and thus detect target defects . In the same or alternative embodiments, system 500 may be used to determine the sizes and/or shapes of features or devices appearing on target 546, for example sizes devices manufactured on a semiconductor wafer. Furthermore, in the same or other embodiments, system 500 may be used to add holographic imaging capability to existing non-holographic imaging systems. For example, system 500 may comprise a standalone modular component that may be integrated into an existing imaging system.

FIGURE 6 illustrates a schematic view of direct-to- digital holography system 600 using dark-field illumination wherein the target illumination beam is coaxial with the optical axis of an objective lens 642 in accordance with teachings of the present disclosure. System 600 may include common path section 611 which may comprise laser 610, modulator 612, beam expander 614, polarization rotator 616, mirror 617, mirror 628, beamsplitter 630, objective 642, and target 646; and equal path section 621, which may comprise mirror 623, reference arm lenses 643 and 645, spatial filter 626, blocking screen 649, and beam combiner lens 628. System 600 may also include diffraction grating or mirror 634 and digital recorder 640.

In operation, laser 610 may direct a beam of light toward modulator 612 and the modulated light may travel

to beam expander 614. The expanded light may then travel through polarization rotator 616 to create a desired polarization of the light, then to beamsplitter 630 which may direct all or a portion of the beam towards mirror 617. In some embodiments, beamsplitter 630 may comprise a non-polarizing beam splitter cube. In such an embodiment, polarization rotator 616 may be excluded from system 600. The portion of the beam transmitted by beamsplitter 630 may be directed towards mirror 617, towards mirror 628 and interact with target 646. In some embodiments of system 600, mirror 628 may be located in a position that is coaxial with with the focal axis of objective 642, and thus may be positioned and/or configured such that the specular reflection of the target beam reflected from target 646 is directed out of the optical path of objective 642 and reflected back towards mirror 617 by mirror 628. The non-specular components of the light reflected from target 626 may become the target beam 620 and pass through objective 642 reflect from mirror 623 of equal path section 621 towards beam combiner lens 638.

All or a portion of the specular light reflected back towards mirror 617 may be transmitted by beamsplitter 630, may exit common path section 611, and become incident upon diffraction grating 634. Zero order light diffracted by diffraction grating 634 may enter equal path section 621 while higher order light diffracted by diffraction grating 634 may be blocked by blocking screen 649 from entering equal path section 621. In addition to blocking higher order light, blocking screen 649 may also block or prevent noise, e.g. stray

light or air currents, from being introduced into system 600.

Such filtering may allow reference beam 618 to have characteristics similar to a reference beam that has not interacted with a target. In other embodiments of system 600, diffraction grating 634 may be replaced by a mirror, in which case higher order light may not filtered prior to reference beam 618 entering equal path section 621. The zero order light diffracted from diffraction grating 634 may pass through lens 643 and spatial filter 626. Spatial filter 626 may extract zero order image information from reference beam 618, thus providing additional filtering to the reference arm to further condition the light of the reference arm in a desired manner. The zero order beam extracted from reference beam 618 may then travel through lens 645 and toward beam combiner 628, which combines light from the target arm and the zero order light from reference arm and directs the combined beams to digital recorder 640, e.g. a CCD camera. Together target beam 620 and the zero order portion of reference beam 618 may combine at beam combiner 628 to form an interference pattern or hologram at digital recorder 640.

In system 600, common mode phase changes in the reference arm and target arm induced by noise, e.g. noise induced by vibration and temperature, may cancel out, making system 600 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating the effects of noise in the interference pattern created at digital recorder 640.

In some embodiments of system 600, equal path section 621 may be built as a dual-axis, monolithic optical system housed within a single cover. As described above, target beam 620 and a reference beam 618 may enter equal path section 621, and the suitably combined target beam 620 and reference beam 618 exit equal path section 621 before forming a pattern on digital recorder 640. In such a configuration, noise e.g. noise induced by vibration and temperature, may cancel out, further making system 600 essentially insensitive to temperature variations, vibrations, and other sources of noise thus substantially reducing or eliminating reducing the effects of noise in the interference pattern created at digital recorder 640. In addition, as detailed above, mirror 628 directs light towards wafer 646 and into objective 642. This permits light, incident on wafer 646, to penetrate deep into the target 646, thus allowing maximum phase modulation of the beam reflected or scattered into objective 642. Accordingly, system 600 has shown increased sensitivity and reduced signal to noise ratios with respect to deep aspect ratio patterns and/or structures on target 646.

Testing of an imaging system consistent with system 600 has shown decreased sensitivity to noise compared to that observed in prior art imaging systems. Thus, in application in which system 600 is used to detect differences between complex images, e.g. detection of defects on a semiconductor wafer, thus use of system 600 may substantially reduce or eliminate the occurrence of false positives or false negatives, and may improve defect capture rate as compared to prior art imaging

systems. In addition, testing of an imaging system consistent with system 600 has shown a reduction in ghosts and scattered light levels, as well as increased transmission as compared to that observed in prior art imaging systems, and as such, shows improved signal to noise ratio and sensitivity. Such improved transmission may be related to, at least in part, the fewer number of optical components present in system 600 as compared to prior art imaging systems as well as the fact that system 600 permits: (i) an optical system in which the optical paths of the reference arm and target arm have a significant common path in common path section 611; (ii) an equal path section built as a dual-axis, monolithic optical system housed within a single cover; (iii) the elimination of certain imagine system components, such as quarter wave plates, that may introduce noise or other deleterious effects into an imaging system, and (iv) in some embodiments of system 600, two levels of filtering of reference beam, thus allowing a higher quality hologram to be captured by digital recorder 640; (v) on- axis dark-field illumination of target 646 without necessitating re-alignment of the objective and may eliminate "through-the-objective illumination" of the target, thus reducing backreflections and ghosts while increasing maximum phase modulation of the beam reflected or scattered from target 646; and (vi) both zero order and diffracted light incident on target 646 to interact with target 646, thus allowing any effects of noise related to target 646, e.g. vibration, to cancel out. Because of improved transmission and other improved qualities, power of the optical beam directed into an imaging system can be reduced, potentially increasing the

lifetime of optical components and reducing laser damage to the target .

System 600 may be used in any suitable application. For example, system 600 may be used to compare complex images obtained from the same object or target after a physical change occurs to the target or to compare complex images from different targets. In addition, system 600 may be used to acquire an image from different locations on target 646. In some applications, system 600 may be used to detect defects on target 646, for example a semiconductor wafer. If a defect exists on a target, such a defect will cause a difference hologram than that of a defect-free target. Using image processing, one can detect hologram differences, and thus detect target defects. In the same or alternative embodiments, system 600 may be used to determine the sizes and/or shapes of features or devices appearing on target 646, for example sizes devices manufactured on a semiconductor wafer. Furthermore, in the same or other embodiments, system 600 may be used to add holographic imaging capability to existing non-holographic imaging systems. For example, system 600 may comprise a standalone modular component that may be integrated into an existing imaging system. Although the disclosed embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments without departing from their spirit and scope.