Patent Publication Number: US-2012038911-A1

Title: Defect detection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2010-180833 filed on Aug. 12, 2010, the disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a defect detection apparatus, and in particular to a defect detection apparatus utilizing an optical interference system. 
     2. Related Art 
     According to a semiconductor lithography roadmap (for example SEMATECH Lithography Forum 2008), the next generation circuit line width HP in 2016 is expected to be 16 to 22 nm. There are investigations into the extension of existing size-reduction projection light exposure methods, and new use of nanoimprint lithography (NIL) for the next generation of light exposure apparatuses and methods. 
     With the former optical method, due to resolution being insufficient with light exposure with currently employed ArF lasers oscillating with a wavelength of 193 nm, research and development is proceeding at a fast pace into double patterning that uses an liquid immersion method and requires an EUV light source of 13.5 nm wavelength. There are issues with such a liquid immersion method regarding the reduction in throughput and increase in cost due to light exposure being performed two times. The EUV light source has a wavelength that is shorter by at least one decimal place than an ArF laser, resulting in an extremely high degree of difficulty in research towards putting such a light source and optical system into practice. 
     In contrast, nanoimprint technology is being employed as a technique for semiconductor fabrication. Nanoimprint technology is a molding processing technique in which a nanoimprint mold formed with a pattern of recesses and projections of a nanometer scale is pressed against a substrate coated with a thin resin film, thereby imparting a pattern of recesses and projections in the thin resin film. 
     A nanoimprint technology method enables nanometer scale production more simply and at a lower cost than, for example, photolithography techniques. 
     Optically there are two significant differences between a conventional light exposure mask and a nanoimprint mold. 
     (1) Fineness of Defect Size (Influence of a Same Magnification Optical System) 
     In contrast to a conventional semiconductor light exposure incorporating a 4-fold reducing optical system, a same magnification mold is employed in nanomprinting. Consequently, in contrast to a light exposure mask with a permissible defect size of 10 nm to 100 nm that is four times the semiconductor product defect size, the defect size in nanoimprint molds needs to be suppressed to about 10 nm, equivalent to the permissible defect size of the semiconductor itself. Such a defect size is smaller by at least a decimal place than the wavelength of an illumination beam (DUV light/193 nm of deep ultraviolet ArF laser). 
     (2) Optical Properties of Measurement Sample 
     A light exposure mask is manufactured with a pattern of a metal (mainly Cr) on a transparent quartz substrate. Chromium is both non-transparent and also has metallic glossiness, accordingly light illuminated on a sample is reflected/scattered or absorbed, resulting in a large difference in light intensity between transmitted light and reflected light. The presence or absence of defects can accordingly be detected directly as brightness and darkness in the light. However, in a nanoimprint mold, due to forming a pattern by recesses and projections on a quartz substrate itself, defects amount to no more than the fine level differences of recesses and projections in a transparent body. Accordingly, with a nanoimprint mold, since only fine displacements in phase occur even when there are defects present, the intensity of transmitted light is equivalent whether or not defects are present (such an object is sometimes referred to below as a “phase object”). 
     Both of these points of difference result in making the detection of defects in a nanoimprint mold more difficult. 
     A method using an interference microscope, as described for example in Japanese Patent Application Laid-Open (JP-A) No. 8-327557, is proposed as a method for detecting recesses and projections of a transparent material (phase object). 
     An apparatus is described in JP-A No. 8-327557 that performs detection by optically extracting defect portions by optically subtracting non-defective portions of a pattern. 
     There is also a technique described in non-patent publication “Dainana Hikari no Enpitsu” Volume 25 by Tadao TSURUTA for emphasizing scattered light intensity by interfering a scattered light component and an illumination beam with a phase difference of π−Δ. 
       FIG. 12  illustrates a defect detection apparatus  100  similar to a defect detection apparatus described in JP-A No. 8-327557. Explanation follows regarding detection of a defect in a nanoimprint mold  12  with the defect detection apparatus  100 . 
     As shown in  FIG. 12 , the defect detection apparatus  100  is a field separation interference microscope including: a light source  14  that illuminates a parallel light illumination beam onto the nanoimprint mold  12 ; a focusing lens  17  that converges light that has passed through the nanoimprint mold  12 ; a half-mirror  18  that light from the focusing lens  17  into two directions; a deflector  20 A that deflects light that has passed through the half-mirror  18 ; a mirror  22  that reflects the light deflected by the deflector  20 A towards a specific direction; a phase compensation plate  24  for performing phase compensation; a deflector  20 B for deflecting light reflected by the half-mirror  18 ; a mirror  26  that reflects the light deflected by the deflector  20 B towards a specific direction; a phase shifter  28  that shifts the phase of light from the mirror  26 ; a half-mirror  30  for letting light pass through from the phase shifter  28  and wave combining by reflecting light from the phase compensation plate  24 ; and an imaging element  34  for capturing an optical image of light wave combined by the half-mirror  30 . 
     Out of the light passing through the nanoimprint mold  12 , scattered light L 1  scattered by a pattern formed on the nanoimprint mold  12  passes through the focusing lens  17  and is incident on the half-mirror  18 . The scattered light L 1  light incident on the half-mirror  18  is split into scattered light L 11  that passes through the half-mirror  18 , and scattered light L 12  that is reflected by the half-mirror  18 . In  FIG. 12 , only the scattered light L 1  out of the light that has passed through the nanoimprint mold  12  is shown. 
     The scattered light L 11  that has passed through the half-mirror  18  goes on to pass through the deflector  20 A, and is then reflected towards the phase compensation plate  24  by the mirror  22 . 
     The phase compensation plate  24  functions to adjust the relative phase difference between the scattered light L 11  and the scattered light L 12 , namely functions to adjust such that the optical path lengths of the scattered light L 11  and the scattered light L 12  are the same as each other. The scattered light L 11  that has passed through the phase compensation plate  24  is then incident on the half-mirror  30 . 
     The scattered light L 12  reflected by the half-mirror  18  passes through the deflector  20 B and is reflected towards the phase shifter  28  by the mirror  26 . 
     The phase shifter  28  is configured by wedge shaped prisms  28 A,  28 B. By shifting the prism  28 A in the arrow P direction in the drawing, the optical path difference between the scattered light L 11  and the scattered light L 12  can be adjusted according to the shift amount, namely the phase shift amount can be adjusted. 
     The light from the phase shifter  28  and the light from the phase compensation plate  24  are wave combined by the half-mirror  30 . The wave combined light is imaged on the imaging element  34  by a focusing lens. 
     Due to the defect detection apparatus  10  configured as described having a field separation function, a single image point and a conjugate object point can both be formed on the imaging element  34 . More specifically, parallel shifting can be performed so that the scattered light L 11 , L 12  move apart from each other in a direction parallel to the image plane of the imaging element  34  (an arrow P direction) when the deflectors  20 A,  20 B are inclined with respect to the optical axes by respective specific angles θ in opposite directions. Accordingly, by tilting the deflectors  20 A,  20 B by the specific angle θ, out of the two object points P 1 , P 2  separated by the separation distance D in the arrow P direction on the nanoimprint mold  12 , an interference image resulting from interference between a field separation image of the scattered light L 11  that is light from the object point P 1  and a field separation image of the scattered light L 12  that is light from the object point P 2  can be formed as an image on the imaging element  34 . 
     Consequently, light from the two object points separated on the nanoimprint mold  12  can be caused to interfere and an image can be formed on the imaging element  34  by inclining the deflectors  20 A,  20 B by a specific angle θ that depends on the separation distance D. 
     As shown in  FIG. 13 , out of light that has passed through the nanoimprint mold  12 , the illumination beam L 2 , similarly to the scattered light L 1 , passes through the focusing lens  17  and is the incident on the half-mirror  18 . The illumination beam L 2  that is incident on the half-mirror  18  is split into an illumination beam L 21  that passes through the half-mirror  18  and an illumination beam L 22  that is reflected by the half-mirror  18 . Note that in  FIG. 13  only the illumination beam L 2  is shown from the light that has passed through the nanoimprint mold  12 . 
     With respect to the illumination beam L 2 , similarly to the scattered light L 1 , the illumination beams L 21 , L 22  are parallel shifted by the deflectors  20 A,  20 B and incident to the imaging element  34 , however, as shown in  FIG. 13 , due to the wave face of the illumination beams L 21 , L 22  being the spherical wave faces L 21 A, L 22 B, interference fringes are generated in the image captured by the imaging element  34  when the illumination beams L 21 , L 22  are parallel shifted. These interference fringes are changes in the signal intensity SB of the illumination light, and due to light intensity change noise and the like it is difficult to detect the scattered light L 1 , namely to detect defects. 
       FIG. 14  illustrates a defect detection apparatus  101  equipped with a different optical interference system to that of the defect detection apparatus  100 . As shown in  FIG. 14 , the defect detection apparatus  101  employs the half-mirror  18  also as a deflector, and employs the mirror  22  also as a phase shifter. Other parts of the configuration are similar to those of the defect detection apparatus  100 . 
     The half-mirror  18  has a similar function to that of the deflectors  20 A,  20 B described above, and is capable of laterally shifting the two split beams in the arrow P direction in the drawing by pivoting about the deflection direction center point C. 
     The mirror  22  also has a similar function to that of the phase shifter  28  described above, and is capable of adjusting the phase difference between the two separated beams by movement of the mirror  22  in a direction orthogonal to the arrow P direction in the drawing. 
     In the defect detection apparatus  101  too, similarly to in the defect detection apparatus  100 , an interference image from interference between the scattered light L 11  and the scattered light L 12  can be formed as an image on the imaging element  34 . 
     As shown in  FIG. 14 , the illumination beam L 2 , similarly to the scattered light L 1 , is split into the two illumination beams L 21 , L 22  and is also shifted laterally in the arrow P direction in the drawing by the half-mirror  18 . 
     However, as shown in  FIG. 14 , after having passed through the focusing lens  17 , the illumination beams L 21 , L 22  are not parallel to each other due to deflection by the half-mirror  18 . As a result, when the illumination beams L 21 , L 22  interfere with each other, the signal intensity SB of the illumination light changes similarly to in the defect detection apparatus  100  due to the wave faces L 21 A, L 22 A being tilted, and interference fringes are generated in the image captured by the imaging element  34 , making it difficult to detect defects. 
     SUMMARY 
     The present invention addresses the above issues and is directed towards provision of a defect detection apparatus capable of detecting defects with high precision when detecting defects of a detection subject using an optical interference system. 
     To address the above issues, a first aspect of the present invention provides a defect detection apparatus including: 
     a light illumination section that illuminates an illumination beam onto a detection subject that transmits light and is formed with a predetermined pattern; 
     a group of lenses including an object lens and a focusing lens for focusing the illumination beam illuminated on and passing through the detection subject; 
     a light splitter section that splits the light passing through the lens group into two beams; 
     a deflecting section that deflects at least one of the two beams from the two split beams so as to be laterally shifted along a predetermined direction; 
     a phase shifting section that shifts the phase of the at least one of the beams from the two beams deflected by the deflecting section; 
     a wave combining section that wave combines the two beams phase shifted by the phase shifting section; and 
     an imaging section that captures an optical image of light wave combined by the wave combining section, wherein the object lens and the focusing lens are disposed such that two beams that have passed through the focusing lens are parallel to each other and the main axes of the two beams that have passed through the focusing lens are parallel to each other. 
     According to the present invention, due to the object lens and the focusing lens being disposed such that the two beams that have passed through the focusing lens and the main axes of the two beams that have passed through the focusing lens are parallel to each other, no interference fringes are generated in the captured image even though the two beams interfere with each other, and so defects can be detected with high precision. 
     A second aspect of the present invention provides the defect detection apparatus of the first aspect, wherein the object lens and the focusing lens are disposed such that the back focal point position of the object lens and the front focal point position of the focusing lens coincide with each other. 
     A third aspect of the present invention provides the defect detection apparatus of the second aspect, wherein: 
     the focusing lens is provided between the wave combining section and the imaging section; and 
     the light splitter section comprises a half-mirror that causes a portion of the light passed through the object lens to pass through and reflect another portion of the light passed through the object lens; 
     the light splitter section also functions as the deflecting section; and the object lens, the focusing lens and the half-mirror are disposed such that the deflection direction central point of the half-mirror is at the back focal point position of the object lens and at the front focal point position of the focusing lens. 
     A fourth aspect of the present invention provides the defect detection apparatus of the second aspect, wherein: 
     a relay lens is provided on an optical path between the object lens and the focusing lens; and 
     the deflecting section is also employed as a mirror that reflects the light that has passed through the half-mirror towards the wave combining section, and the object lens, the focusing lens and the half-mirror are disposed such that the deflection direction central point of the mirror is at the back focal point position of the object lens and at the front focal point position of the focusing lens. 
     A fifth aspect of the present invention provides the defect detection apparatus of the fourth aspect, further comprising a mask section disposed at a Fourier transform plane where an optical image of a Fourier transform pattern corresponding to the pattern is formed, the mask section configured to cut out an optical image of the Fourier transform pattern. 
     A sixth aspect of the present invention provides the defect detection apparatus of the first aspect, wherein the focusing lens is provided between the object lens and the light splitter section. 
     A seventh aspect of the present invention provides the defect detection apparatus of the first aspect, wherein: 
     the light illumination section illuminates an illumination beam of wavelength greater than nanometer size onto a nanoimprint mold that transmits light and is formed with a predetermined pattern of nanometer size; and 
     the phase shifting section shifts the phase of at least one of the beams from the two beams such that the phase difference between the two beams deflected by the deflecting section is π−Δ(−90°&lt;Δ&lt;90°). 
     According to the present invention, an effect is exhibited by which defects can be detected at high precision when detecting for defects in a detection subject using an optical interference system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a configuration diagram of a defect detection apparatus according to a first exemplary embodiment; 
         FIG. 2  is a bloc diagram of a control system of a defect detection apparatus according to the first exemplary embodiment; 
         FIGS. 3A and 3B  are explanatory diagrams regarding detection of an isolated defect; 
         FIG. 4  is a diagram illustrating an image captured of an isolated defect; 
         FIG. 5  is a graph illustrating measurement results of the contrast of a simulated defect and the contrast of background light; 
         FIG. 6  is a diagram illustrating an image captured of an isolated defect; 
         FIG. 7  is an explanatory diagram regarding detecting a defect in a cell by interfering an image of an adjacent cell; 
         FIG. 8  is an explanatory diagram regarding detecting defects in a die by interfering an image of an adjacent die; 
         FIG. 9  is a configuration diagram of a defect detection apparatus according to a second exemplary embodiment; 
         FIG. 10  is a configuration diagram of a defect detection apparatus according to the second exemplary embodiment; 
         FIG. 11  is a configuration diagram of a defect detection apparatus according to a third exemplary embodiment; 
         FIG. 12  is a configuration diagram of a defect detection apparatus according to a related example; 
         FIG. 13  is a configuration diagram of a defect detection apparatus according to a related example; and 
         FIG. 14  is a configuration diagram of a defect detection apparatus according to a related example. 
     
    
    
     DETAILED DESCRIPTION 
     Explanation follows regarding an exemplary embodiment of the present invention, with reference to the drawings. 
     First Exemplary Embodiment 
       FIG. 1  illustrates a defect detection apparatus  10  according to the first exemplary embodiment. The defect detection apparatus  10  is an apparatus for detecting defects in a nanoimprint mold  12  formed with a predetermined pattern of nanometer size. Portions of the defect detection apparatus  10  similar to the above defect detection apparatus  100  are allocated the same reference numerals. 
     The nanoimprint mold  12  is manufactured by nanoimprint lithography (NIL) using a light transmitting material, such as quartz for example. A predetermined pattern is formed on one face  12 A of the nanoimprint mold  12 , with a pattern width and pattern pitch of several nm to several tens of nm. 
     As shown in  FIG. 1 , the defect detection apparatus  10  is a field separation interference microscope, configured including: a light source  14  that illuminates a parallel light illumination beam onto the nanoimprint mold  12 ; an object lens  15  that converges light that has passed through the nanoimprint mold  12 ; a focusing lens  17  that converges light that has passed through the object lens  15 ; a half-mirror  18  that splits scattered light L 1  that has passed though the focusing lens  17  into scattered light L 11 , L 12  and splits an illumination beam L 2  into illumination beams L 21 , L 22 ; a deflector  20 A that deflects light that has passed through the half-mirror  18 ; a mirror  22  that reflects the light deflected by the deflector  20 A towards a specific direction; a phase compensation plate  24  for performing phase compensation; a deflector  20 B for deflecting light reflected by the half-mirror  18 ; a mirror  26  that reflects the light deflected by the deflector  20 B towards a specific direction; a phase shifter  28  that shifts the phase of light from the mirror  26 ; a half-mirror  30  for letting light pass through from the phase shifter  28  and wave combining by reflecting light from the phase compensation plate  24 ; and an imaging element  34  for capturing an image of light from the half-mirror  30 . 
     Out of the light passing through the nanoimprint mold  12 , the scattered light L 1  is made into parallel light by the object lens  15 , and the illumination beam L 2  first converges before diverging and being incident on the focusing lens  17 . 
     The light that has passed through the focusing lens  17  is incident on the half-mirror  18 . Out of the light incident on the half-mirror  18 , the scattered light L 1  is split into scattered light L 11  that passes through the half-mirror  18 , and scattered light L 12  that is reflected by the half-mirror  18 , and the illumination beam L 2  is split into the illumination beam L 21  that passes through the half-mirror  18  and the illumination beam L 22  that is reflected by the half-mirror  18 . 
     The scattered light L 11  and the illumination beam L 21  passing through the half-mirror  18  go on to pass through the deflector  20 A, and are then reflected towards the phase compensation plate  24  by the mirror  22 . 
     The phase compensation plate  24  functions to adjust the relative phase difference between the scattered light L 11  and the scattered light L 12 , and the illumination beam L 21  and the illumination beam L 22 , namely function to adjust such that the optical path lengths of the scattered light L 11  and the scattered light L 12 , and the optical path lengths of the illumination beam L 21  and the illumination beam L 22 , are respectively the same as each other. The scattered light L 11  and the illumination beam L 21  that have passed through the phase compensation plate  24  are then incident on the half-mirror  30 . 
     The scattered light L 12  and the illumination beam L 22  reflected by the half-mirror  18  pass through the deflector  20 B and are reflected towards the phase shifter  28  by the mirror  26 . 
     The phase shifter  28  is configured by wedge shaped prisms  28 A,  28 B. By shifting the prism  28 A in the arrow P direction in the drawing, the optical path difference between the scattered light L 11  and the scattered light L 12 , and the optical path difference between the illumination beam L 21  and the illumination beam L 22 , can be adjusted according to the shift amount, namely the respective phase shift amounts can be adjusted. 
     The light from the phase shifter  28  and the light from the phase compensation plate  24  are wave combined by the half-mirror  30 . The wave combined light makes an image on the imaging element  34 . 
     The object lens  15  and the focusing lens  17  are disposed so such that the back focal point position of the object lens  15  and the front focal point position of the focusing lens  17  coincide with each other at the point Q in  FIG. 1 . The defect detection apparatus  10  accordingly configures a two sided telecentric optical system, hence out of the illumination beam L 2  that has passed through the focusing lens  17 , the illumination beam L 21  and the main axis L 21 B of the illumination beam L 21  that have passed through the half-mirror  18  are parallel to each other, and the illumination beam L 22  and the main axis L 22 B of the illumination beam L 22  that have been reflected by the half-mirror  18  are parallel to each other, and the illumination beams L 21 , L 22  and the respective main axes of the illumination beams L 21 , L 22  after either passing through or being reflected by the half-mirror  30  are all parallel to each other. 
     Accordingly, even though the illumination beams L 21 , L 22  interfere with each other as shown in  FIG. 1  there are no interference fringes formed in the captured image, and since the signal strength SB or the illumination beam is constant detection can be made at high precision when detecting for defects as described below. 
     Due to the defect detection apparatus  10  configured as described having a field separation function, a single image point and a conjugates object point can both be formed on the imaging element  34 . More specifically, parallel shifting can be performed so that the two separated light beams move apart from each other in a direction parallel to the image plane of the imaging element  34  (an arrow P direction) when the deflectors  20 A,  20 B are inclined with respect to the optical axes by respective specific angles θ in opposite directions. Accordingly, by tilting the deflectors  20 A,  20 B by the specific angle θ, out of the two object points P 1 , P 2  separated by the separation distance D in the arrow P direction on the nanoimprint mold  12 , an interference image resulting from interference between a field separation image of light from the object point P 1  and a field separation image of light from the object point P 2  can be formed as an image on the imaging element  34 . 
     Consequently, light from the two object points separated on the nanoimprint mold  12  can be caused to interfere and an image can be formed on the imaging element  34  by inclining the deflectors  20 A,  20 B by a specific angle θ that depends on the separation distance D. 
       FIG. 2  is a block diagram illustrating a control system of the defect detection apparatus  10 . As shown in  FIG. 2 , the defect detection apparatus  10  is equipped with a controller  40 . The controller  40  is connected to a drive section  42 A for driving the deflector  20 A, a drive section  42 B for driving the deflector  20 B, a drive section  44  for driving the prism  28 A of the phase shifter  28 , the imaging element  34  and a memory  46 . 
     Explanation follows regarding detecting an isolated defect with the defect detection apparatus  10 , and regarding a simulated result from electromagnetic field analysis optics simulation. 
     In the simulation, as shown in  FIGS. 3A and 3B , a case is simulated in which, as an example, a rectangular box-shaped isolated defect of length and width 300 nm and height 200 nm present on a flat planar region  50  of the nanoimprint mold  12  is detected as a projecting portion  52 . 
     The object point P 1  is a projecting portion  52  on the flat planar region  50 , and the object point P 2  is a point on the flat planar region  50  separated from the projecting portion  52  by the separation distance D. Explanation follows regarding a simulation in which an interference image from interference between an image of light from the projecting portion  52  and an image of light from the flat planar region  50  is achieved by tilting the deflectors  20 A,  20 B by the angle θ corresponding to the separation distance D in opposite directions. The simulation investigates the state of the interference image as the phase difference φ=π−Δ between the two separated beams is changed using the phase shifter  28 . A is the phase shift amount (bias phase). The wavelength of the illuminated light is, for example, 638 nm. 
       FIG. 4  illustrates the simulation results of the interference image when there is no background light and when there is background light for each of a phase difference φ=0° (Δ=π, wherein Δ is the bias phase), φ=π−60° (Δ=60°), φ=π−30° (Δ=30°), φ=π(Δ=0°. The background light is scattered light caused by defects in optical members or by dirt/scratches/dust, for example, escaping light such as from a mirror tube or holder, or light generated by dark current, noise or the like. 
     When there is no background light then this represents an ideal case envisaging no background light from defects or dirt on the nanoimprint mold  12  or other optical members. 
     The light intensity of background light when present is set at 0.05. This value is based on a value of light intensity of 1 when the phase difference φ=0°, namely a bright field image without interference between the two separated light beams. 
     The projecting portion  52  cannot be detected for a bright field image at phase difference φ=0°, whether or not there is background light present. When the phase difference φ=π, due to the images interfering of the two separated beams with phases misaligned with each other by π, in an ideal state with no background light, the images of portions the same in the two images cancel each other out, but the portions that are different in the two images, namely only the portions of the projecting portion  52 , appear bright. This results in an extremely high contrast of 1, however the signal light intensity is extremely small due to the light intensity being proportional to the sixth power of the size. 
     However, in practice there is normally background light present caused by defects in optical members or by dirt/scratches/dust. Consequently, as shown in  FIG. 4 , when there is background light present and φ=π, the contrast is reduced to 0.065 due to being affected by the background light, and it is difficult to detect the projecting portion  52 . 
     In contrast thereto, as shown in  FIG. 4 , even when there is background light present the contrast is higher for φ=π−30° and φ=π−60° than when φ=0° or π, enabling detection of the projecting portion  52 . The signal light intensity from the projecting portion  52  can be increased in amplitude by making the phase difference φ of the two separated beams π−Δ. Accordingly, even in actual measurement systems in which there is background light present, detection with good precision is possible for defects of nanometer size, smaller than the wavelength of the illumination beam, by controlling the phase difference φ. 
       FIG. 5  illustrates simulation results for the contrast of a simulated defect  54  in a measurement image when the simulated defect  54  like that of  FIG. 6  is imaged by the defect detection apparatus  10 , and the contrast of background light. The size of the simulated defect  54  is length and width of 500 nm and height of 200 nm, the NA of the aperture of the optical system of the defect detection apparatus  10  is 0.45. Line  56  of  FIG. 6  shows the brightness on a line including the simulated defect  54 . The horizontal axis in  FIG. 5  is the bias phase, namely Δ, and the vertical axis is the contrast. As shown in  FIG. 5 , in the bright field image when Δ=0°, namely when φ=π, the contrast of the simulated defect is substantially the same as the contrast of the background light, making it difficult to detect the defect. In the bright field image when Δ=π, namely when φ=2π(0°), the simulated device contrast is also low, and it is also difficult to detect the defect. The contrast reaches a maximum close to Δ=−30°. 
     Accordingly, even when there is a defect of size less than the wavelength of the illuminated light, the defect can be detected by setting the phase difference between the two separated beams to π− 66  , rather than to π. Note that Δ is set according to the light intensity of background light, for example, the greater the light intensity of background light the larger the value set for Δ. Δ is set such that the contrast of the defect portion and the contrast of the background light are contrasts sufficient to enable detection of the defect portion. 
     In the defect detection apparatus  10 , the controller  40  instructs the drive section  44  to drive the prism  28 A such that the phase difference φ between the two separated beams satisfies φ=π−Δ, and the nanoimprint mold  12  is imaged by the imaging element  34 . An interference image in which the defect portion is emphasized can thereby be obtained, and isolated defects smaller in size than the wavelength of illuminated light can be detected with good precision. 
     Explanation follows regarding a nanoimprint mold  12  employed, for example, in fabrication of a semiconductor circuit board, regarding detection of a defect in periodic circuit pattern formed on the nanoimprint mold  12 . 
     As shown in  FIG. 7 , when detecting defects on a nanoimprint mold  12  when forming plural dies  64  including plural repetitions of cells  62  of the same circuit pattern, the circuit patterns can be made to cancel each other out and a defect can be detected by interfering a reference beam from a nearby, preferably adjacent, cell  62  with the measurement beam. Namely, by setting separation distance D as the interval to the reference cell by inclining the deflectors  20 A,  20 B by an angle θ corresponding to the separation distance D, and then driving the prism  28 A of the phase shifter  28  such that the phase difference between the two separated beams from adjacent cells is φ=π−Δ an interference image of interference between two beams from comparison cells is captured by the imaging element  34 . Δ is determined according to the light intensity of background light as described above. 
     When, for example, there is a defect  66  present in the cell  62 C and the reference cell  62 B is a normal cell with no defect present, then in an interference image  68 B from the two images, as shown in  FIG. 7 , this results in an image emphasizing the differences between the two cells, namely emphasizing only the defect  66  (the white round portion in the drawing), with the other portions cancelling each other out. A similar phenomenon is observed in the interference image formed with the other reference cell  62 D. Accordingly, a defect in a cell of a circuit pattern of periodic structure can be detected. 
     Similarly, when there are plural adjacent dies  64  each of the same pattern, a defect can be detected by imaging an interference image from interfering a reference beam from a nearby, preferably adjacent, die  64  with the measurement beam. For example, as shown in  FIG. 8 , when there are the defects  66 A,  66 B present in the die  64 B, the interference image  68 A with the normal die  64 A results in an image in which the defects  66 A,  66 B are emphasized, as shown in  FIG. 8 . Similar applies to an interference image with the normal die  64 C. 
     In the first exemplary embodiment, as stated above, optical members are disposed from the object lens  15  to the focusing lens  17  such that the back focal position of the object lens  15  and the front focal position of the focusing lens  17  coincide with each other at the position of point Q in  FIG. 1 , so as to configure a two sided telecentric optical system. Out of the illumination beam L 2  that has passed through the focusing lens  17 , the illumination beam L 21  and the main axis L 21 B of the illumination beam L 21  that has passed through the half-mirror  18 , and the illumination beam L 22  and the main axis L 22 B of the illumination beam L 22  reflected by the half-mirror  18 , are respectively parallel to each other, and the main axes of the illumination beams L 21 , L 22  passing through or reflected by the half-mirror  30  are all respectively parallel to their main beams. Accordingly, even though the illumination beams L 21 , L 22  interfere with each other, no interference fringe is formed in the captured image, and since the signal strength SB is constant. Defect detection can consequently be achieved at high precision. 
     Second Exemplary Embodiment 
     Explanation follows regarding a second exemplary embodiment of the present invention. Similar portions to those of the defect detection apparatus  101  are allocated the same reference number and detailed explanation is omitted. 
       FIG. 9  illustrates a defect detection apparatus  10 A according to the present exemplary embodiment. As shown in  FIG. 9 , the defect detection apparatus  10 A has similar configuration members to those of the defect detection apparatus  101 , however the defect detection apparatus  10 A differs from the defect detection apparatus  101  in configuration as a two sided telecentric optical system. Namely, as shown in  FIG. 9 , the object lens  15  and the focusing lens  17  are disposed such that the back focal point position of the object lens  15  and the front focal point position of the focusing lens  17  coincide with each other at the deflection direction central point C of the half-mirror  18 . 
     Accordingly, similarly to the first exemplary embodiment, since out of the illumination beam L 2  that has passed through the focusing lens  17 , the illumination beam L 21  and main axis L 21 B of the illumination beam L 21 , and the illumination beam L 22  and main axis L 22 B of the illumination beam L 22  are respectively parallel to each other, the wave faces L 21 A, L 22 A of the illumination beams L 21 , L 22  are also parallel. Consequently, even though the illumination beams L 21 , L 22  interfere with each other interference fringes do not occur in the captured image, and the signal intensity SB of the illumination light is constant. Defect detection can hence be achieved with high precision. 
     Third Exemplary Embodiment 
     Explanation follows regarding a third exemplary embodiment of the present invention. Portions similar to those of the defect detection apparatus  10 A are allocated the same reference numerals and detailed explanation thereof is omitted. 
       FIG. 10  illustrates a defect detection apparatus  10 B according to the third exemplary embodiment. As shown in  FIG. 10 , the defect detection apparatus  10 B differs from the defect detection apparatus  10 A in that: relay lenses  72 A,  72 B are provided between the object lens  15  and the half-mirror  18 ; the mirror  22  is also employed as a deflector; and the mirror  26  is also employed as a phase shifter. 
     The optical system components from the object lens  15  to the focusing lens  17  are disposed such that the back focal point position of the object lens  15  and the front focal point position of the focusing lens  17  coincide at the deflection direction central point C of the mirror  22 . 
     In this exemplary embodiment, similarly to the second exemplary embodiment, out of the illumination beam L 2  when it has passed through the focusing lens  17 , the illumination beam L 21  and the main axis L 21 B of the illumination beam L 21 , and the illumination beam L 22  and the main axis L 22 B of the illumination beam L 22  are respectively parallel to each other. Consequently, even though the illumination beams L 21 , L 22  interfere with each other interference fringes do not occur in the captured image, and the signal intensity SB of the illumination light is constant. Defect detection can hence be achieved with high precision. 
     Due to provision of the relay lenses  72 A,  72 B between the object lens  15  and the half-mirror  18 , the degrees of freedom in the interferometer position from the half-mirror  18  to the half-mirror  30  can be raised. 
     Fourth Exemplary Embodiment 
     Explanation follows regarding a fourth exemplary embodiment of the present invention. Portions similar to those of the defect detection apparatus  10 B are allocated the same reference numerals and detailed explanation is omitted. 
     In the fourth exemplary embodiment explanation is of a defect detection apparatus employing a periodic pattern cut mask for cutting diffracted light due to periodic patterns formed on the nanoimprint mold  12 . 
     For example, in a memory device such as SRAM, there is generally a regular periodic circuit pattern maintained within a single chip. In such a case, a nanoimprint mold  12  for forming such a circuit pattern also has a repeating pattern with a periodicity larger than the wavelength of the illumination light. Due to such a repeating pattern acting as a diffraction grid on the illumination light to generate diffracted light in characteristic angles. Accordingly, when the diffracted light from the periodic circuit pattern is brighter than the signal light intensity from a defect of a few nm to a few tens of nm then sometimes the ability to detect a defect is reduced. 
     As shown in  FIG. 11 , the defect detection apparatus  10 C according to the present exemplary embodiment is provided between the object lens  15  and the relay lens  72 A with a periodic pattern cut mask  90  formed with a Fourier transform pattern on a Fourier transformation plane, where an optical image of a Fourier transform pattern is formed corresponding to the pattern formed on the nanoimprint mold  12 . 
     The periodic pattern cut mask  90  is configured, for example, by birefringent elements, liquid crystals or the like. The controller  40  in such a case controls the periodic pattern cut mask  90  such that a Fourier transform pattern corresponding to the periodic pattern formed on the nanoimprint mold  12  is formed (displayed). 
     Disposing the periodic pattern cut mask  90  at the Fourier transform plane enables diffracted light due to the periodic circuit pattern to be cut, and hence the defect portions can be detected with high precision. 
     The present invention is not limited to the above exemplary embodiment and obviously various modifications and improvements are possible within a scope not departing from the technical intention as recited in the scope of patent claims. For example, as a method to obtain an interference image of a defect detection apparatus according to the present exemplary embodiment, while there are the examples in the present exemplary embodiments of a Mach-Zehnder method (see  FIG. 1 ), another method may also be applied as appropriate, such as, for example, a Jamin method, a Michelson method, a Fizeau method, or a Twyman-Green method.