Abstract:
A method for detecting an anomaly on a first surface of a transparent substrate starts with providing a transparent substrate that has a reflective second surface. The method then comprises directing a radiation beam at the first surface of the substrate so that at least a portion of the radiation penetrates the substrate and strikes the reflective second surface. This radiation is reflected back as a reflected radiation beam through the first surface of the substrate. The method then comprises detecting radiation from the reflected radiation beam. This method can further comprise causing relative motion between the radiation beam and the first surface of the substrate. This method can also further comprise documenting the presence of an anomaly if the detected radiation shows that the reflected radiation beam was scattered upon traversing the first surface.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to the field of wafer or photomask surface inspection, and more particularly, to illumination and light collection optics for inspecting transparent glass substrates.  
           [0003]    2. Background Information  
           [0004]    Monitoring anomalies, such as pattern defects and particulate contamination, during the manufacture of semiconductor wafers is an important factor in increasing production yields. Numerous types of defects and contamination, especially particles, can occur on a wafer&#39;s surface. Determining the presence, location and type of an anomaly on the wafer surface can aid in both locating process steps at which the anomaly occurred and determining whether a wafer should be discarded.  
           [0005]    Originally, particles were monitored manually by visual inspection of wafer surfaces. These particles, usually dust or microscopic silicon particles, caused many of the wafer pattern defects. However, manual inspection proved time-consuming and unreliable due to operator errors or an operator&#39;s inability to observe certain defects.  
           [0006]    To decrease the time required to inspect wafer surfaces, many automatic inspection systems were introduced. A substantial majority of these automatic inspection systems detect particles and other anomalies based on the scattering of light. These systems include two major components: illumination optics and collection-detection optics. Illumination optics generally consists of scanning a wafer surface with a source of radiation, e.g., a laser or white light. Particles present on the wafer&#39;s surface scatter incident radiation. The collection optics detect increases in the amount of scattered radiation received, and these increases generally correspond to particles encountered by the illumination optics. This data is reconciled with reference to the known beam position at those moments when the increases in scattered radiation were detected. The data is then converted to electrical signals which can be measured, counted and displayed on a monitor.  
           [0007]    Known systems for inspecting wafers that utilize scattered radiation suffer from severe limitations when they are used to inspect transparent articles such as glass mask substrates. One important limitation is that anomalies on transparent substrates generate substantially less scattered radiation than anomalies on non-transparent substrates. There are at least two factors that contribute to this low scattered radiation output. The first is the presence of destructive interference generated between air-side incident and air-side reflected radiation at the surface of the substrate. The second is a substantial reduction in forward scattered radiation that reaches the collection-detection optics.  
           [0008]    Forward scattered radiation is radiation that scatters in the same general direction as the radiation from which it originates. For instance, incident radiation that strikes the substrate can generate forward scattered radiation that travels into the substrate. Incident radiation that strikes an anomaly can generate forward scattered radiation that travels past the anomaly and strikes the substrate surface. And radiation that reflects off the substrate surface (reflected radiation) and then strikes an anomaly from below it can generate forward scattered radiation that tends to travel away from the substrate and into the collection-detection optics. Since this last form of forward scattered radiation tends to travel directly into the collection-detection optics, it generally makes up a sizeable portion of the scattered radiation that is collected during a wafer inspection process. Accordingly, the term “forward scattered radiation” as used herein refers primarily to forward scattered radiation generated by reflected radiation striking an anomaly from below it.  
           [0009]    When a radiation source is directed at the surface of a transparent substrate, very little of the incident radiation reflects off the surface as reflected radiation. This is because a substantial portion of the incident light penetrates into the transparent substrate. In fact, only around 0% to 10% of the incident radiation reflects off the surface. This substantial reduction in reflected radiation off transparent substrates (as compared to silicon wafers) results in a correspondingly substantial reduction in forward scattered radiation off anomalies that is directed at the collection-detection optics.  
           [0010]    In addition to these problems, background noise increases on a transparent substrate because incident light penetrates the substrate and then scatters as it hits the chuck used to hold the substrate in position. So this and all of the above factors significantly reduce the signal-to-noise ratio when known systems inspect transparent substrates, resulting in poor detection of particles. Accordingly, there is a need for an inspection system that can produce stronger scattered light signals with higher signal-to-noise ratios when encountering anomalies present on transparent substrates.  
         SUMMARY OF THE INVENTION  
         [0011]    The disadvantages and problems associated with inspecting transparent articles such as glass mask substrates have been improved using the present invention.  
           [0012]    In accordance with an embodiment of the invention, a method for detecting an anomaly on a first surface of a transparent substrate starts with providing a transparent substrate that has a reflective second surface. The method then comprises directing a radiation beam at the first surface of the substrate so that at least a portion of the radiation penetrates the substrate and strikes the reflective second surface. This radiation is reflected back as a reflected radiation beam through the first surface of the substrate. The method then comprises detecting radiation from the reflected radiation beam. This method can further comprise causing relative motion between the radiation beam and the first surface of the substrate. This method can also further comprise documenting the presence of an anomaly if the detected radiation shows that the reflected radiation beam was scattered upon traversing the first surface.  
           [0013]    In accordance with another embodiment, the above method can further comprise directing a second radiation beam at a location on the first surface of the substrate that corresponds to where the reflected radiation beam traverses the first surface, and detecting radiation from the second radiation beam.  
           [0014]    In accordance with another embodiment, a method for detecting an anomaly on a first surface of a transparent substrate comprises directing a radiation beam at a second surface of the substrate so that at least a portion of the radiation beam penetrates the substrate and traverses the first surface, and detecting radiation from the radiation beam as it traverses the first surface.  
           [0015]    In accordance with another embodiment of the invention, a system for detecting an anomaly on a first surface of a transparent substrate comprises a radiation source operable to emit radiation, an objective operable to focus the radiation into a radiation beam, and a detector mounted to detect radiation. The objective is mounted to direct the radiation beam onto a first location on the first surface of the substrate so that at least a portion of the radiation beam penetrates the substrate and strikes a reflective second surface of the substrate, thereby reflecting the radiation beam back through a second location on the first surface of the substrate.  
           [0016]    In accordance other embodiments, the above system can further comprise any one or all of a compensatory plate operable to correct any aberration introduced by the substrate, a collector operable to collect radiation and focus the radiation onto the detector, and/or an optical element operable to redirect the radiation beam to the second location on the first surface of the substrate.  
           [0017]    An important technical advantage of the present invention includes reflecting the radiation beam off the reflective second surface of the substrate so that the radiation beam strikes anomalies from the substrate side, rather than from the air side. The use of substrate side radiation increases the sensitivity of the system by reducing radiation loss, reducing interference between scattered and reflected radiation by eliminating collection of the reflected radiation component, reducing background noise, and greatly increasing the amount of forward scattered radiation generated by the system. Another advantage of the invention is that the methods disclosed herein can be performed without significant design changes to current wafer inspection systems and wafer mounting systems.  
           [0018]    Other important technical advantages of the present invention are readily apparent to one skilled in the art from the following figures, descriptions, and claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:  
         [0020]    [0020]FIGS. 1 and 2 demonstrate a method for inspecting a surface of a substrate where a radiation beam is directed at the surface of the substrate, and a particle is detected when the radiation beam is scattered by the particle.  
         [0021]    [0021]FIGS. 3 and 4 demonstrate a method for inspecting the surface of a substrate in accordance with an embodiment of the invention using substrate-side radiation produced by reflecting radiation off a reflective bottom surface of the substrate.  
         [0022]    [0022]FIGS. 5A and 5B illustrate a system and method in accordance with an alternative embodiment of the invention where a first radiation beam is operable to perform substrate-side inspection of the surface of a substrate and a second radiation beam is operable to perform air-side inspection of the surface of a substrate.  
         [0023]    [0023]FIG. 6 demonstrates a method for performing substrate-side inspection of a surface of a substrate without the use of a reflective bottom surface in accordance with an alternative embodiment of the invention.  
         [0024]    [0024]FIG. 7 is a chart that illustrates a difference between substrate-side illumination and air-side illumination for different angles of incidence of the radiation beam.  
         [0025]    [0025]FIG. 8 illustrates a substrate inspection system constructed in accordance with an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    The preferred embodiments of the present invention and their advantages are best understood by referring to FIGS. 1 through 8 of the drawings. Like numerals are used for like and corresponding parts of the various drawings.  
         [0027]    [0027]FIGS. 1 and 2 demonstrate a method for detecting particles on the surface of a substrate using a wafer inspection system. For purposes of this description, any reference to “particles” is intended to include other types of anomalies as well, including crystal-originated particles (COPs), which are surface breaking defects in a semiconductor wafer that have in the past been classified as “particles” due to the inability of earlier inspection systems to distinguish them from real particles. FIG. 1 illustrates an instance where an incident radiation beam  100  is directed at a transparent or glass substrate  102  to inspect its surface and does not encounter any particles. Incident radiation beam  100  can be in the form of light, and in particular, a laser beam. Common types of laser beams used for detecting particles include Argon-Ion lasers which can emit radiation at around 488 nm and 514 nm, and solid-state YAG lasers which can emit radiation at around 266 nm, 355 nm, or 1064 nm. Incident radiation beam  100  is generally scanned across the surface of a substrate to look for particles.  
         [0028]    Incident radiation beam  100  strikes a first surface  106  at an angle of incidence θ. In an embodiment of the invention, angle θ is equal to around 70 degrees, which is the same angle of incidence used by known semiconductor wafer inspection systems such as the Surfscan® SP1 TBI  Wafer Inspection Tool by KLA-Tencor of San Jose, Calif.  
         [0029]    [0029]FIG. 1 includes a particle  104  resting on first surface  106  of substrate  102  and away from incident radiation beam  100 . First surface  106  of substrate  102  is also referred to herein as top surface  106 . Particles typically found on substrate surfaces, and on semiconductor wafer surfaces in particular, include microscopic particles such as microscopic silicon particles (e.g. Si, SiO 2 , or Si 3 N 4 ), or dust particles. In FIG. 1, particle  104  can also be a polystyrene latex sphere (PSL). This is one type of man-made particle generally used to calibrate tools such as particle deposition systems and wafer scanners.  
         [0030]    When incident radiation beam  100  strikes top surface  106 , a component of incident radiation beam  100  is reflected off top surface  106  as a reflected radiation beam  108 , and another component is transmitted into substrate  102  as a refracted radiation beam  110 . Reflected radiation beam  108  has an angle of incidence θ′ measured from normal and this incidence angle is equal to the incidence angle θ for incident radiation beam  100 . Because substrate  102  is transparent, much of the energy in incident radiation beam  100  is transmitted into substrate  102 . Thus, reflected radiation beam  108  generally contains only a small portion of the energy from incident radiation beam  100 , typically only between 0% to 10% of the energy.  
         [0031]    Refracted radiation beam  110  is refracted as it enters substrate  102 . Refracted radiation beam  110  then travels through substrate  102  and exits out a second surface  112  of substrate  102  where it is refracted once again, at an angle of incidence θ′ as measured from normal. This angle θ″ is equal to angle θ for incident radiation beam  100  and θ′ of reflected radiation beam  108 . Unlike reflected radiation beam  108 , refracted radiation beam  110  generally carries a substantially large portion (between 90% to 100%) of the energy from incident radiation beam  100 . Second surface  112  of substrate  102  is also referred to herein as bottom surface  112 .  
         [0032]    Most of the remaining energy from incident radiation beam  100  is exhausted in different ways, for example, as scattered radiation, by absorption into substrate  102 , or as dissipated heat energy. Scattered radiation can be generated at several locations, including where incident radiation beam  100  strikes top surface  106  and where refracted radiation beam  110  strikes bottom surface  112 .  
         [0033]    [0033]FIG. 2 illustrates an instance where particle  104  is detected. Here, portions of incident radiation beam  100  and reflected radiation beam  108  are now scattered by particle  104 . The resulting scattered radiation  200  is then collected by the collection-detection optics of the wafer inspection system (shown in FIG. 8). A substantial portion of the scattered radiation  200  that is collected by the system originates from reflected radiation beam  108 . This is because reflected radiation beam  108  strikes particle  104  from below, and the resulting forward scattered radiation  200  tends to travel directly into the collection-detection optics of the system.  
         [0034]    The collection-detection optics of the system consist of a collector  812  and a detector  814  (both shown in FIG. 8), and are used to collect and detect scattered radiation  200 . It is detector  814  that records increases in the level of scattered radiation  200  collected that correspond to particles  104  being found by radiation beams  100  and  108 . Normally, detector  814  continually receives a low threshold level of scattered radiation due to scattered radiation being generated by incident beam  100  striking top surface  106  and refracted beam  110  striking bottom surface  112 . Therefore, it is typically the increases in the level of scattered radiation collected that indicate a particle has been found.  
         [0035]    The amount of scattered radiation  200  generated in FIG. 2 during the inspection of transparent substrate  102  is relatively small compared to the amount generated during the inspection of non-transparent silicon wafers. Again, this is because reflected radiation beam  108 , which generates a substantial portion of the detected scattered radiation, contains only 0% to 10% of the energy of incident radiation beam  100  when a transparent surface is being inspected. In addition, deconstructive interference between incident radiation beam  100  and reflected radiation beam  108  further decreases the energy level of reflected radiation beam  108 . Accordingly, only a relatively small amount of forward scattered radiation can be generated. This decrease in the level of scattered radiation  200  unfortunately results in decreased system sensitivity.  
         [0036]    [0036]FIGS. 3 and 4 demonstrate an exemplary method for detecting particles on the surface of a substrate in accordance with an embodiment of the invention. Here, particle detection is performed using substrate-side radiation, as opposed to air-side radiation. In particular, the radiation beam used to detect particles in this embodiment travels through substrate  102  prior to striking the particle, as opposed to traveling just through the air as in FIG. 1. In the embodiment of FIGS. 3 and 4, this is accomplished using a modified substrate  300  having a first surface  302  that is being inspected and a second surface  304  that has been made reflective. First surface  302  is also referred to herein as top surface  302 , and second surface  304  is also referred to herein as bottom surface  304 . The use of reflective bottom surface  304  causes refracted radiation beam  110  to reflect back up towards top surface  302  as a reflected radiation beam  306 . It is reflected radiation beam  306  that is now used to detect particle  104  on top surface  302 .  
         [0037]    Modified substrate  300  is different from substrate  102  only in that bottom surface  304  has been made reflective. In one embodiment, this modification can be done by taking substrate  102  and coating bottom surface  112  with a reflective layer, for instance, by using an aluminization process as is used in extreme ultraviolet (EUV) lithography to enable electrostatic chucking. The use of reflective bottom surface  304  enables the radiation source to remain above top surface  302 . This allows the methods of the invention to be implemented on existing wafer inspection systems that use top side, obliquely directed radiation, without the need for having substantial modifications made to them. The primary modification is displacing the optical axis of incident radiation beam  100  so that reflected radiation beam  306  is now focused onto the location of interest.  
         [0038]    The use of substrate-side radiation provides several advantages that result in a considerably large increase in the amount of scattered radiation generated off particle  104 . One advantage is that substrate-side radiation strikes particle  104  from below at angles that generate forward scattered radiation  200  directed at the collection-detection optics, similar to reflected radiation beam  108 . Unlike reflected radiation beam  108 , however, substrate-side radiation retains a majority of the energy from incident radiation beam  100 , and therefore generates substantially more forward scattering radiation.  
         [0039]    Another advantage of substrate-side radiation is that as the substrate-side radiation traverses the top surface of a substrate, constructive interference occurs at the point where the radiation crosses from substrate to air. This constructive interference tends to intensify the radiation leaving the substrate, sometimes intensifying it by 40-60% at certain angles of incidence. This increase in the intensity of the radiation also aids in generating more scattered radiation.  
         [0040]    [0040]FIG. 3 illustrates a moment where incident radiation beam  100  is directed at substrate  300  to inspect top surface  302  and does not encounter particle  104 . Incident radiation beam  100  strikes top surface  302  at a first location  308  where a component of incident radiation beam  100  is refracted into substrate  300  as refracted radiation beam  110 . In an embodiment, incident radiation beam  100  has an angle of incidence that is around 70 degrees, as it can have when existing systems are used. Refracted radiation beam  110  then travels through substrate  300  and strikes reflective bottom surface  304  where it is reflected back as reflected radiation beam  306 . Reflected radiation beam  306  then travels through substrate  300  and traverses top surface  302  at a second location  310  where it is refracted once again.  
         [0041]    It should be noted that when incident radiation beam  100  strikes top surface  302 , reflected radiation beam  108  and scattered radiation (not shown) are still produced, as is scattered radiation (not shown) from refracted radiation beam  110  striking bottom surface  304 . Detection of these components by the detector (shown in FIG. 8), which tends to interfere with the results obtained, can be minimized or eliminated, as is discussed below with reference to FIG. 8. Thus, these components are not discussed here, or illustrated in FIG. 3, for clarity.  
         [0042]    [0042]FIG. 4 illustrates a moment where incident radiation beam  100  is directed at substrate  300  to inspect its top surface  302  and reflected radiation beam  306  encounters particle  104 . As in FIG. 3, incident radiation beam  100  is directed at top surface  302  at first location  308 , and a component enters substrate  300  as refracted radiation beam  110 . Refracted radiation beam  110  then travels through substrate  300  and reflects of reflective bottom surface  304  as reflected radiation beam  306 . Reflected radiation beam  306  then travels through substrate  300  and traverses top surface  302  where it strikes particle  104 . Constructive interference can occur as reflected radiation beam  306  traverses top surface  302 , thereby amplifying the intensity of reflected radiation beam  306 . As shown in FIG. 4, scattered radiation  200  is then generated off particle  104 , and this scattered radiation  200  can be collected and detected by collector  812  and detector  814  (shown in FIG. 8).  
         [0043]    [0043]FIGS. 5A and 5B illustrate a system and method designed in accordance with an alternative embodiment of the invention where a first radiation beam is operable to perform substrate-side inspection of the surface of a substrate and a second radiation beam is operable to perform air-side inspection of the surface of a substrate. FIG. 5A illustrates a first step of this alternative embodiment where particle inspection is performed in accordance with the invention, in particular, using reflected radiation beam  306  to detect particle  104 . FIG. 5A includes a radiation source  500  that emits incident radiation beam  100  at first location  308  of top surface  302 . Then, as described above in FIGS. 3 and 4, a component of incident radiation beam  100  enters substrate  300  as refracted radiation beam  110 , refracted radiation beam  110  reflects off reflective bottom surface  304  as reflected radiation beam  306 , and reflected radiation beam  306  traverses top surface  302  where it strikes particle  104 .  
         [0044]    [0044]FIG. 5B illustrates a second step of this alternative embodiment where an optical element  502 , which can be provided by an objective or a prism (as shown in FIG. 5B), is introduced into the path of incident radiation beam  100 . The presence of optical element  502  alters the path of incident radiation beam  100  so that is now strikes top surface  302  at second location  310 . Therefore, second location  310  is undergoing substrate-side inspection by reflected radiation beam  306  in FIG. 5A, and air-side inspection by incident radiation beam  100  in FIG. 5B. Through inspection of second location  310  using both air-side and substrate-side inspection techniques, one can discern what type of defect particle  104  is by analyzing the differences in scattered radiation  200  received during the air-side and substrate-side inspections.  
         [0045]    [0045]FIG. 6 demonstrates a method for performing substrate-side inspection of a surface of a substrate without the use of a reflective bottom surface in accordance with an alternative embodiment of the invention. Here, rather than providing a reflective bottom surface  304  for refracted radiation beam  110  to reflect off, incident radiation beam  100  is directed from below and directly strikes the substrate on its bottom surface  112 . In this embodiment, substrate  102  from FIGS. 1 and 2 is utilized because a reflective bottom surface is no longer necessary. A component of incident radiation beam  100  enters substrate  102  as refracted radiation beam  110 , and refracted radiation beam  110  travels through substrate  102  and traverses top surface  106  where it strikes particle  104 . Then as before, scattered radiation  200  is collected by collector  812  (shown in FIG. 8) that is still positioned above top surface  106 . Thus, substrate-side inspection is performed here without the use of a reflective bottom surface.  
         [0046]    [0046]FIG. 7 is a chart that illustrates the difference between substrate-side illumination and air-side illumination for different angles of incidence of the radiation beam. As shown in FIG. 7, at angles of incidence below 30 degrees and above 60 degrees, much more scattered radiation is detected using substrate-side radiation than using air-side radiation. Most importantly, at an angle of incidence of 70 degrees, which is the angle of incidence used by most known wafer inspection systems, there is substantially more scattered radiation detected when using substrate-side radiation.  
         [0047]    [0047]FIG. 8 is a schematic view of a sample inspection system  800  according to an embodiment of the invention. Sample inspection system  800  includes a radiation source  802  that operates to emit incident radiation beam  100  at one or more wavelengths. As described above, one device that can be used as radiation source  802  is a solid-state laser. Solid-state lasers tend to be more stable, reliable, and compact than other types of lasers, making them attractive for use in sample inspection systems.  
         [0048]    In particular, a YAG solid-state laser can be used as radiation source  802 . A YAG laser generally operates by generating radiation at one frequency, and then shifting that frequency to a desired frequency by passing the radiation through Yttrium Aluminum Garnet crystals that are doped with neodymium or erbium. This process is called harmonic laser light generation. The crystals can double, triple, or quadruple the frequency of the radiation. So if the radiation begins at 1064 nanometers (nm), then harmonic laser light generation can produce radiation at 532 nm radiation, 355 nm radiation, or 266 nm radiation. In alternative laser sources, materials such as gas, plasma, or other types of crystals can be used in place of the YAG crystals in the harmonic laser light generation process. In other embodiments of the invention, radiation source  802  can be provided by these alternative laser sources, including Argon Ion lasers.  
         [0049]    Sample inspection system  800  can include a lens  804  that can focus incident radiation beam  100  through a pinhole filter  806 . This lens-pinhole assembly is a spatial filter that is used to remove spatial noise from incident radiation beam  100 , which consists of random fluctuations in the intensity profile of a radiation beam caused by particles and other objects that the radiation beam encounters. These particles tend to degrade the spatial coherence of the radiation beam.  
         [0050]    After incident radiation beam  100  passes through lens  804  and filter  806  of the spatial filter, it is focused by lens  808  into oblique illumination channel  810 . Incident radiation beam  100  then strikes top surface  302  of glass substrate  300  at an oblique angle, and in an embodiment, this angle is around 70 degrees measured from normal to the substrate surface. As incident radiation beam  100  strikes surface  302 , refracted radiation beam  110  enters substrate  300  and is reflected off reflective bottom surface  304 . This creates reflected radiation beam  306  that is then used for detecting particles on top surface  302 .  
         [0051]    In another embodiment, system  800  can include a compensatory plate (not shown) that is mounted between lens  808  and top surface  302 . The use of a compensatory plate corrects significant third order aberrations that can be introduced by transparent substrate  300 , thereby producing a small illumination spot on top surface  302  for detecting particles.  
         [0052]    A portion of scattered radiation  200  generated by reflected radiation beam  306  as it traverses top surface  302  is collected by a collection system  812 , provided in this embodiment by an ellipsoidal mirror. The scattered radiation is also focused by collection system  812  onto a detector  814 . In an embodiment, detector  814  can be provided by a photomultiplier tube. Collection system  812  and detector  814  can be identical to what is used in known systems, such as the Surfscan® SP1 TBI  tool by KLA-Tencor described above.  
         [0053]    In an embodiment of the invention, collection system  812  can include a field stop to prevent any unwanted scattered radiation components from entering detector  814  and degrading the sensitivity of system  800 . This can include scattered radiation created by incident radiation beam  100  striking first location  308  on top surface  302  and/or refracted radiation beam  110  striking reflective bottom surface  304  of substrate  300 .  
         [0054]    As shown in FIG. 8, glass substrate  300  is mounted onto a chuck  816  which is rotated by a motor  818 . Mounting chuck  816  is preferably an edge support chuck, similar to what is currently used in known systems. These elements are then moved linearly by a transducer  820 . Both movements are controlled by a controller  822 , so that incident radiation beam  100  and reflected radiation beam  306  can scan surface  302  along a spiral scan to cover the entire surface.  
         [0055]    In alternate embodiments of the invention, instead of using an ellipsoidal mirror as collection system  812 , other curved mirrors or objectives can be used, including but not limited to a paraboloidal mirror. A paraboloidal mirror will collimate the scattered radiation from surface  302  into a collimated beam, and this collimated beam can then be focused by a lens to detector  814 . Curved mirrored surfaces having shapes other than ellipsoidal or paraboloidal shapes may also be used.  
         [0056]    Accordingly, systems and methods of the invention have been described for inspection of transparent glass substrates. Unlike previously developed techniques in which systems suffered from severe limitations and collected poor data when inspecting transparent substrates, the systems and methods of the present invention utilize substrate-side inspection techniques that can overcome these limitations and provide improved data and greater signal-to-noise ratios. In particular, substrate-side radiation produces greater forward scattered radiation off anomalies on transparent substrates. Also, because the incident radiation beam is offset from the reflected radiation beam as it traverses the top surface, there is no destructive interference between the two. Rather, there is constructive interference as the reflected radiation beam crosses into the air out of the substrate. In addition, the systems and methods of the invention can provide both substrate-side and air-side inspections to aid in defect discrimination. The methods of the invention can even be implemented on existing systems with only minor modifications needed.  
         [0057]    While various embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that numerous alterations may be made without departing from the inventive concepts presented herein. Thus, the invention is not to be limited except in accordance with the following claims and their equivalents.