Abstract:
In one embodiment, a surface inspection system comprises a radiation directing assembly to target radiation onto a surface of an object, the radiation directing assembly comprising a radiation source that emits a broadband radiation beam, a polarization control assembly comprising at least one of a linear polarizer and an apochromatic retarder, an aperture control mechanism, and a beam splitter, a radiation collection assembly to collect radiation reflected from the surface of the object, the radiation collection assembly comprising, a polarization control assembly comprising at least one of a linear polarizer and an apochromatic retarder, an aperture control mechanism, and at least one radiation sensing device.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/969,661, filed Sep. 3, 2007. 

   BACKGROUND 
   The subject matter described herein relates to surface inspection techniques, and more particularly to systems and methods for wafer inspection that utilize polarized broadband radiation. 
   Semiconductor materials may be inspected for defects such as, e.g., surface imperfections, particles, irregularities in the thickness of thin film coatings, and the like, which may hamper the performance of the semiconductor material. Some existing inspection systems direct a beam of radiation on the surface of the semiconductor material, then collect and analyze light reflected and/or scattered from the surface to quantify characteristics of the surface. Additional inspection techniques are desirable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a schematic illustration of an inspection system, according to embodiments. 
       FIG. 1B  is a schematic illustration of a wire grid polarizer which may be used in an inspection system, according to embodiments. 
       FIGS. 2-6  are schematic illustrations of polarization apertures which may be used in an inspection system, according to embodiments. 
       FIG. 7  is a schematic illustration of polarization orientations of radiation incident on the surface of a wafer, according to embodiments. 
       FIG. 8  is a schematic illustration of a computing system which may implement polarization inspection systems, according to embodiments. 
       FIG. 9  is a graph illustrating defect capture rates using polarized radiation, according to embodiments. 
       FIG. 10  is a graph illustrating signal to noise ratios using polarized radiation, according to embodiments. 
       FIG. 11  is a graph illustrating defect captures using polarized radiation, according to embodiments. 
       FIG. 12  is graphs illustrating cap improvements in inspection systems, according to embodiments. 
   

   DETAILED DESCRIPTION 
   Described herein are exemplary systems and methods for polarized broadband wafer inspection. Throughout this document, terms like light, optical, optics, rays and beams with reference to electromagnetic radiation may be used with no implication that the radiation is or is not in the visible portion of the spectrum. In the following description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments. However, it will be understood by those skilled in the art that the various embodiments may be practiced without the specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the particular embodiments. 
   In some embodiments, the systems and methods described herein provide polarization control of a broadband ultra-violet, including deep ultra-violet, flexible brightfield and darkfield imaging system for wafer inspection with high sensitivity and high throughput. Examples of such systems and methods are described in U.S. Pat. No. 7,304,310 to Schortt, et al., entitled Methods and Systems for Inspecting a Specimin Using Light Scattered in Different Wavelength Ranges, U.S. Pat. No. 7,259,844 to Fairley, et al. entitled High Throughput Darkfield/Brightfield Wafer Inspection Systems Using Advanced Optical Techniques, and U.S. Pat. No. 7,002,677 to Bevis, et al., entitled Darkfield Inspection System Having a Programmable Light Selection Array, the disclosures of which are incorporated herein by reference in their entireties. 
   In some embodiments, an inspection system utilizes a wire-grid polarizer with fine pitch, relative to the wavelength of the radiation used by the inspection system. For example, in some embodiments the wire grid polarizer may have a pitch (i.e., distance between wires) that is less than half the shortest wavelength of radiation used by the inspection system to inspect surfaces.  FIG. 1B  is a schematic illustration of a wire grid polarizer which may be used in an inspection system, according to embodiments. In such embodiments, the wire grids with fine pitch  102  may be manufactured on a fused substrate  104 . Wire-grid polarizers with fine pitch polarize light with sufficient transmission and extinction over a broad range of wavelengths to be useful in a broadband wafer inspection system. In addition, wire-grid polarizers are robust under DUV exposure and can be fabricated to satisfy requirements of high sensitivity optical systems (e.g., thin substrate to minimize aberrations). Further, some methods to fabricate grid polarizers lend to fabrication of polarizers with a transmitted polarization vector orientation that varies across the part, which could be useful for wafer inspection systems that incorporate large numerical aperture (NA) for either illumination or imaging. 
     FIG. 1A  shows a diagram of a wafer inspection system with polarization control. Light emitted from a broadband source  101  is illuminated onto a test object  115  and imaged onto a time delay integration (TDI) detector  127 . The polarization of the light source is controlled in both illumination and imaging paths by manipulating a linear polarizer  105 ,  119  (e.g., wire grid) and apochromatic retarder  107 ,  121  pair. In some embodiments, different apertures  109 ,  123  may be placed in the illumination pupils  101  and imaging pupils  125  to dampen the polarization aberrations caused by the specimen and the inspection system to enhance the purity of the desired polarization both at the specimen and the TDI detector  127 . 
   The broadband wafer inspection system controls a polarization vector of the source and the polarization vector near the imaging plane to enhance the system&#39;s sensitivity to specific objects located on the inspected specimen. Polarization control is made possible through the appropriate orientation of a linear polarizer which operates over a broad range of wavelengths. The ellipticity of the polarization vector may be modified with an apochromatic retarder  107 ,  121 . Sensitivity can be further enhanced by masking the polarized beam with an appropriate aperture set  109 ,  123  (see  FIGS. 2 ,  3 ,  4 ,  5 , and  6 ). The apparatus allows for polarization and aperture control in both illumination and imaging paths. The physical location of the linear polarizer  105 ,  119  and apochromatic retarder  107 ,  121  is flexible: They can be located either close to pupil plane, close to field plane or any other favorable location to enhance the object of interest which may have variable amplitude and phase properties. 
   With the correct polarization and aperture configuration  109 ,  123 , a larger concentration of the electric field may be localized at a specific layer of the specimen, making the inspection system more sensitive to targeted objects of interest while suppressing noise from previous or overlaying layer (see  FIGS. 9 ,  10 ,  11  and  12 ). The advantage of the fine pitch wire-grid polarizer is its performance across a broad spectrum to DUV wavelengths. 
   Multiple fabrication methods may be used to construct wire-grid polarizers as well as other optical components capable of polarizing deep ultraviolet (DUV) light for inspection systems and is not meant to limit the scope of the invention. In some embodiments, fine pitch wire grid polarizers may be produced using 266 nm, 193 and other DUV interference lithography systems, immersion lithography, dual patterning lithography, combination of interference, immersion or dual patterning lithography, EUV lithography, e-Beam writing and nanoimprint technologies. 
   Some of these methods permit fabrication of polarizers with a transmitted polarization vector orientation that varies across the polarizer. This could be used to generate or analyze linear polarization vectors that vary as a function of incident angle or reflected/scattered angle at the wafer specimen. 
   Dielectric fine-pitch grids and diffraction gratings are capable of polarizing light, but they generally polarize for a narrow range of wavelengths. For broadband applications, use of several grids or gratings designed for varying wavelengths stacked in series may be used to polarize broadband DUV radiation. 
   A multi-layer coating on a plane parallel plate is capable of obtaining both high extinction and high transmission, however, such a design is limited to both narrowband and narrow angle applications. A combination in series of any narrow band polarizers may be used to produce a broadband polarizer. 
   A series of plane parallel plates oriented at Brewster&#39;s angle is able to polarize a broadband source, however, the design is limited to a narrow scope of angles. In addition, the large amount of glass needed for the design leads to increased wavefront distortion. Further, the large number of air to glass interfaces would increase the number of ghost reflections in the system. 
   Another approach to polarizing a broadband ultraviolet source is to use a polarizing prism. It is difficult to manufacture a large one, however, due to the difficulty in obtaining a large crystal with adequate homogeneity. In addition, polarizing prisms have a much smaller acceptance angle than the polarizing element described. Last, the large amount of glass required for the part leads to a larger amount wavefront distortion. 
   Various processing techniques may be used produce these polarizers: 266 nm,  193  and other DUV interference lithography systems, immersion lithography, dual patterning lithography, combination of interference, immersion or dual patterning lithography, EUV lithography, e-Beam writing and nanoimprint technologies. 
   In some embodiments, the tailored polarizers having a transmitted polarization vector orientation that varies across the part may be used. For example, in some embodiments multiple wire-grid patterns may be printed, oriented at different but certain directions across polarizer part substrate. For example, if the wire grids are oriented along the radial direction of the polarizer part, P-polarized light could be utilized for wafer inspection. Contrarily, if the wire grids are oriented along a circumference, S-polarized light could be utilized for wafer inspection. 
     FIG. 7  shows how X and Y polarization at the illumination pupil are found to have different S and P projections at the wafer which vary as a function of the pupil position. Aside from having the ability to target specific layers on an inspected specimen,  FIGS. 2 and 3  show apertures that have the benefit of sharpening the image by removing birefringent effects present in the system. 
   By placing the illumination and collection polarizers in orthogonal orientations, the apparatus can be used as a broadband, cross-polarized imaging system. A special aperture may be inserted to improve the extinction ratio of this mode by blocking the leakage created in the four quadrants of a high NA cross polarization imaging system (see  FIGS. 4 and 6 ). 
   In addition, a quarter wave plate may be inserted into the illumination or collection path to alter the generated/analyzed linear state to a circular state. By rotating the quarter wave plate, the ellipticity of the polarization may be changed to vary the penetration depth of the incident electric field. 
     FIG. 8  is a schematic illustration of one embodiment of a computing system which may be used to process data received from the imaging optics  125  of  FIG. 1A . The computer system  800  includes a computer  808  and one or more accompanying input/output devices  806  including a display  802  having a screen  804 , a keyboard  810 , other I/O device(s)  812 , and a mouse  814 . The other device(s)  812  can include a touch screen, a voice-activated input device, a track ball, and any other device that allows the system  800  to receive input from a developer and/or a user. The computer  808  includes system hardware  820  and random access memory and/or read-only memory  830 . A file store  880  is communicatively connected to computer  808 . File store  880  may be internal such as, e.g., one or more hard drives, or external such as, e.g., one or more external hard drives, network attached storage, or a separate storage network. 
   Memory  830  includes an operating system  840  for managing operations of computer  808 . In one embodiment, operating system  840  includes a hardware interface module  854  that provides an interface to system hardware  820 . In addition, operating system  840  includes one or more file systems  850  that manage files used in the operation of computer  808  and a process control subsystem  852  that manages processes executing on computer  808 . Operating system  840  further includes a system call interface module  842  that provides an interface between the operating system  840  and one or more application modules  862 . 
   In operation, one or more application modules and/or libraries executing on computer  808  make calls to the system call interface module  842  to execute one or more commands on the computer&#39;s processor. The system call interface module  842  invokes the services of the file system(s)  850  to manage the files required by the command(s) and the process control subsystem  852  to manage the process required by the command(s). The file system(s)  850  and the process control subsystem  852 , in turn, invoke the services of the hardware interface module  854  to interface with the system hardware  820 . 
   The particular embodiment of operating system  840  is not critical to the subject matter described herein. Operating system  840  may be embodied as a UNIX operating system or any derivative thereof (e.g., Linux, Solaris, etc.) or as a Windows® brand operating system. 
   In some embodiments, computer system  800  includes one or more modules to implement a surface inspection system. In the embodiment depicted in  FIG. 8 , computer system  800  includes a surface analysis module  862  which implements the system described with reference to  FIG. 1A . 
   Using broadband, linear polarized light for inspection has the benefit of targeting specific layers of a specimen while controlling defect contrast with wavelength. Such a configuration has the benefit of maximizing sensitivity to the area of interest while minimizing noise. By way of example and not limitation, consider  FIGS. 9 ,  10  and  11  which show that orienting the polarization vector such that it is perpendicular to a line space array will penetrate the electric field deeper into the specimen, increasing the capture rate of objects of interest embedded within the dense structure. Conversely, orienting the polarization parallel to the line space array will concentrate the electric field at the surface of structure, making the inspection system sensitive to the objects such as particles on the surface of the wafer specimen. These examples make use of a single polarizer in either the illumination or imaging paths of the inspection system. 
   Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an implementation. The appearances of the phrase “in one embodiment” in various places in the specification may or may not be all referring to the same embodiment. 
   Thus, although embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that claimed subject matter may not be limited to the specific features or acts described. Rather, the specific features and acts are disclosed as sample forms of implementing the claimed subject matter.