Patent Publication Number: US-6665112-B2

Title: Optical reticle substrate inspection apparatus and beam scanning method of the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This a divisional of U.S. patent application Ser. No. 09/998,521, filed Nov. 29, 2001, in the name of Motonari Tateno now U.S. Pat. No. 6,538,795. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical reticle substrate inspection apparatus that inspects a defect and a line width of a reticle substrate and a beam scanning method thereof, particularly, to an optical reticle substrate inspection apparatus for improving stability when a laser light source having a short wavelength is used and a beam scanning method thereof. 
     2. Description of the Related Art 
     In the optical reticle substrate inspection apparatus that inspects a defect and a line width of a reticle substrate, there exists an apparatus in which detection light is converged onto a substrate to detect the defect and the line width by transmission light or reflection light. In such a apparatus, a spot of a laser beam (a beam spot) is scanned in one direction parallel with a surface of the reticle substrate (hereinafter, this direction is referred to as a Y-axis direction), and a stage on which the reticle substrate is mounted is moved in an isokinetic manner in a direction parallel with the surface of the reticle substrate and a perpendicular direction to the Y-axis direction (hereinafter, this direction is referred to as an X-axis direction), and thus inspecting the defect and the like of the reticle substrate. As a scanning method of the beam spot, a method in which the beam spot is optically scanned is generally used in order to process inspection in a short time, and the beam scanning method using an acoustooptical element has been conventionally adopted. 
     In such a beam scanning method, aberration occurs to an angle of the scanning beam when the beam is converged by a normal cylindrical lens, and defocus occurs on the reticle substrate surface. For this reason, a method is adopted where the beam is converged using a lens effect by the acoustooptical element to reduce the defocus. A method converging the beam using the two acoustooptical elements is described in The U.S. Pat. No. 3,851,951, for example. And a substrate inspection apparatus that adopted the method is described in Japanese Patent Laid-Open (unexamined) No. Hei 6-294750. FIG. 1 is a schematic view showing the conventional optical reticle substrate inspection apparatus. 
     In the optical reticle substrate inspection apparatus, a laser light source  1 , an acoustooptical element  2  and a group of cylindrical lenses  3  are arranged in this order in a rectilinear direction of a laser beam output from the laser light source  1 . The acoustooptical element  2  is arranged so as to scan the laser beam output from the laser light source  1  in a perpendicular direction to the straight line by frequency modulation, and the group of cylindrical lenses  3  is arranged so as to magnify the laser beam output from the acoustooptical element  2  only in the scanning direction by the acoustooptical element  2 . 
     An acoustooptical element  4   a  which gives a cylindrical lens effect to the laser beam output from the group of cylindrical lenses  3  is further provided to the optical reticle substrate inspection apparatus. A transducer  19  which oscillates ultrasonic to the acoustooptical element  4   a  is attached to one end portion of the acoustooptical element  4   a . The acoustooptical element  4   a  is arranged such that a side where the transducer  19  is attached, that is, a side where the ultrasonic is input, is closer to the group of cylindrical lenses  3 . Furthermore, a group of the cylindrical convex lenses  18  that converges the laser beam output from the acoustooptical element  4   a  is arranged in the optical reticle substrate inspection apparatus. Moreover, a relay lens  24  which propagates the laser beam passed through the convergence spot to an optical system (not shown) in a post-step is arranged on a position apart from the convergence spot formed by the group of cylindrical convex lenses  18 . In addition, an objective lens (not shown) is arranged between the optical system and the reticle substrate being an object to be inspected, and a detector (not shown) that detects intensity and the like of the laser beam passed through the reticle substrate is further provided. 
     In the optical reticle substrate inspection apparatus constituted in this manner, scanning is performed in an angle made by an incident direction and an output direction to the laser beam output from the laser light source  1  by utilizing the frequency modulation by the first acoustooptical element  2 . An optical path of the laser beam transits from a path  12  to a path  13  due to this process. When the laser beam has its optical path on the path  12 , the laser beam is magnified by the group of cylindrical lenses  3  and output as a laser beam  30  with a width in the scanning direction. 
     Further, transducer  19  outputs a series of ultrasonic which is swept such that a wavelength lengthens in linear state as the passage of time, that is, the wavelength in a forefront portion  21  becomes shorter than that of an aftermost portion  20 . A plurality of parallel lines between an aftermost portion  20  and a forefront portion  21  in FIG. 1 show that the wider the distance between the lines the longer the wavelength. The second acoustooptical element  4   a , when the laser beam  30  is made incident thereto, outputs the laser beam  30  while converging it with functioning as a cylindrical convex lens by the ultrasonic oscillated from the transducer  19 . The laser beam  30  output from the acoustooptical element  4   a  is further converged by the group of cylindrical convex lenses  18  to form a convergence spot  22 . When the laser beam has its optical path on the path  13 , a convergence spot  23  is formed on a position off from the convergence spot  22  in the scanning direction. The laser beam  30  passed through the convergence spot  22  is made incident to the relay lens  24  while magnifying its width again. Then, the convergence spot  22  is imaged on the reticle substrate via the optical system and the objective lens, and the detector detects the intensity and the like of the transmission light. 
     Note that the same detection can be performed even if the acoustooptical element  4   a  is arranged such that the side where the ultrasonic is input is made far from the group of cylindrical lenses  3  and the ultrasonic is swept such that the wavelength becomes shorter in linear state as the passage of time. 
     However, in recent years, although higher resolving power has been demanded for an apparatus for inspecting the reticle substrate with demand for finer pattern, there is a problem that the conventional optical reticle substrate inspection apparatus cannot provide sufficient high resolving power. 
     As a method of obtaining the high resolving power, a method is considered where the convergence spot size on the reticle substrate surface is made small by shortening the wavelength of the detection light. Progress of shorter wavelength has been made also in the field of exposure due to advancement in technology. It is important that the wavelength of the detection light is shortened to make it close to that of an exposure light because the way how the defect appears changes depending on a detection wavelength. However, in the case of shortening the wavelength of the detection light, a usable material of the acoustooptical element is limited due to a problem of material absorption regarding the scanning method where convergence is performed by the conventional acoustooptical element. For example, since tellurium dioxide, which has been generally used as the acoustooptical element, does not transmit the detection light having the wavelength of 300 nm or less, the acoustooptical element made of quartz, which has a sonic speed of 5960 m/sec being about ten times faster than comparing to tellurium dioxide, needs to be used. However, there is a problem that a focal length lengthens when the acoustooptical element made of quartz is used, because a focal length of a cylindrical lens effect by the acoustooptical element is inversely proportional to the wavelength of light output from the light source and proportional to a square of the sonic speed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide the optical reticle substrate inspection apparatus that can improve scanning stability when the laser light source having a short wavelength is used and the beam scanning method thereof. 
     An optical reticle substrate inspection apparatus according to the present invention comprises a laser, a first acoustooptical element which scans a laser beam output from said laser, a second acoustooptical element which generates a virtual image with a concave lens effect to said laser beam output from said first acoustooptical element, a concave lens arranged on the output side of said laser beam of said second acoustooptical element, and an optical system which images said virtual image on a reticle substrate being an object to be inspected. The concave lens magnifies said laser beam in a perpendicular direction to the scanning direction by said first acoustooptical element. 
     In the present invention, the first acoustooptical element performs laser beam scanning. Further, the second acoustooptical element generates a virtual image whose aberration is reduced to the laser beam, and the concave lens adjusts the shape of the virtual image in a circle, for example. Then, the optical system images the virtual image on the reticle substrate. As described, in the present invention, since not the convergence spot but the virtual image is imaged on the reticle substrate and its scanning is performed, stable scanning can be performed even if the laser beam having a short wavelength is used. 
     A beam scanning method of an optical reticle substrate inspection apparatus according to the present invention comprises the steps of generating a virtual image with an acoustooptical element to a laser beam, and imaging said virtual image on a reticle substrate being an object to be inspected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing a conventional optical reticle substrate inspection apparatus. 
     FIG. 2 is a schematic view showing a structure of an optical reticle substrate inspection apparatus according to a first embodiment of the present invention. 
     FIG. 3 is a schematic view showing a beam scanning method of the optical reticle substrate inspection apparatus according to the first embodiment of the present invention. 
     FIG. 4 is a schematic view showing a structure of an optical reticle substrate inspection apparatus according to a second embodiment of the present invention and its beam scanning method. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be specifically described with reference to the accompanying drawings as follows. FIG. 2 is a schematic view showing a structure of an optical reticle substrate inspection apparatus according to a first embodiment of the present invention. 
     In the optical reticle substrate inspection apparatus according to the first embodiment, similarly to the conventional apparatus, the laser light source  1 , the acoustooptical element  2  and the group of cylindrical lenses  3  are arranged in this order in the rectilinear direction of the laser beam output from the laser light source  1 . A laser wavelength of the laser light source  1  is 300 nm or less, for example. The acoustooptical element  2  is made of quartz, for example, and is arranged so as to scan the laser beam output from the laser light source  1  in a vertical direction to the straight line by the frequency modulation. The group of cylindrical lenses  3  is arranged so as to magnify the laser beam output form the acoustooptical element  2  only in the scanning direction by the acoustooptical element  2 . Note that the scanning direction by the acoustooptical element  2  corresponds to the Y-axis direction. 
     In this embodiment, an acoustooptical element  4  showing the lens effect is further provided for the laser beam output from the group of cylindrical lenses  3 . The transducer  19  is attached to one end portion of the acoustooptical element  4 . The acoustooptical element  4  is arranged such that a side where the transducer  19  is attached, that is, a side where the ultrasonic is input, is closer to the group of cylindrical lenses  3 . Furthermore, in this embodiment, a group of cylindrical concave lenses  5 , which magnifies the laser beam output form the acoustooptical element  4  only in a perpendicular direction to the scanning direction and outputs it, is provided. A numerical aperture of the group of cylindrical concave lenses  5  equals a numerical aperture of the acoustooptical element  4  when it functions as the cylindrical concave lens. Therefore, the shape of the virtual image formed by the acoustooptical element  4  and the group of cylindrical concave lenses  5  is the circle. 
     Moreover, an optical system  8  is provided which images the laser beam output from the group of cylindrical concave lenses  5 . A relay lens  7  which propagates the laser beam output from the group of cylindrical concave lenses  5  to the optical system  8  in the post-step is arranged between the group of cylindrical concave lenses  5  and the optical system  8 . The optical system  8  has a structure same as that of a conventional system. For example, the optical system is provided with a group of lenses necessary for imaging, a measure for detecting fluctuation of the intensity and the like of the laser beam, a splitter necessary in a multi-beam inspection method, a detector for detecting a reticle substrate reflection light and the like. Further, an objective lens  9  that performs convergence close to a theoretical spot limit is arranged between the optical system  8  and a reticle substrate  10  being an object to be inspected. Furthermore, a detector  11  that detects the intensity and the like of the laser beam passed through the reticle substrate  10  is provided to the optical reticle substrate inspection apparatus. The acoustooptical element  2 , the group of cylindrical lenses  3 , the acoustooptical element  4  and the group of cylindrical concave lenses  5  may constitute a concave lens type scanning portion  6 . 
     Note that a magnification of the group of cylindrical lenses  3  may be set so as to equal a distance between the forefront portion and the aftermost portion when a series of ultrasonic oscillated from the transducer  19  propagates the acoustooptical element  4 , for example. 
     Next, description will be made for the beam scanning method of the optical reticle substrate inspection apparatus according to the first embodiment constituted as above. FIG. 3 is a schematic view showing a beam scanning method of the optical reticle substrate inspection apparatus according to the first embodiment of the present invention. FIG. 3 shows the laser light source  1  to the relay lens  7 . 
     In this beam scanning method, scanning is performed in the angle made by the incident direction and the output direction to the laser beam output from the laser light source  1  by utilizing the frequency modulation by the acoustooptical element  2 . At this point, a frequency band modulated by the acoustooptical element  2  for scanning in the angle can be arbitrarily decided based on a size of the virtual image formed by the acoustooptical element  4 , a moving distance of the virtual image with scanning, and a spot size imaged on a surface of the reticle substrate  10 . The optical path of the laser beam transits from the path  12  to the path  13  due to this scanning. When the laser beam has its optical path on the path  12 , the laser beam is magnified by the group of cylindrical lenses  3  and output as the laser beam  30  with the width in the scanning direction. 
     Further, the transducer  19  outputs a series of ultrasonic which is swept such that the wavelength becomes shorter in linear state as the passage of time. A plurality of the parallel lines between an aftermost portion  25  and a forefront portion  26  in FIG. 3 show that the wider the distance between the lines the longer the wavelength. A distance between the forefront portion  26  and the aftermost portion  25  of a series of the ultrasonic equals the width of the laser beam  30 . When the laser beam  30  is made incident to the acoustooptical element  4 , diffraction of the laser beam  30  occurs by an ultrasonic pulse, in which the frequency is swept such that the wavelength becomes shorter in linear state from the forefront portion  26  to the aftermost portion  25  as described above. At this time, since the wavelength of the ultrasonic is different in accordance with a spatial position of the laser beam  30 , Bragg angle is also different, and thus the concave lens effect occurs in this embodiment. Accordingly, the acoustooptical element  4  functions as the concave lens, and a circular virtual image  14  is formed on a side where the laser beam  30  is made incident by a combinational function with the group of cylindrical concave lenses  5 . Further, when the laser beam has its optical path on the path  13 , a circular virtual image  15  is formed at a position off from the virtual image  14  in the scanning direction. 
     The laser beam  30  output from the acoustooptical element  4  is made incident to the optical system  8  via the group of cylindrical concave lenses  5  and the relay lens  7  while magnifying its width. Then, the optical system  8  controls a traveling direction, an aperture and the like of the laser beam  30 , and makes the virtual image  14  to be imaged as a spot  17  on the surface of the reticle substrate  10  via the objective lens  9 . The spot  17  imaged on the surface of the reticle substrate  10  moves in the Y-axis direction with an angle scanning by the acoustooptical element  2 , and when the laser beam output from the acoustooptical element  2  has its optical path on the path  13 , the virtual image  15  is imaged on the surface of the reticle substrate  10  as a spot  16 . Then, the intensity and the like of the transmission light are detected by the detector  11 . 
     In such a scanning method, assuming that a propagation speed of ultrasonic in the acoustooptical element  4  is V (m/sec.), the wavelength of the laser beam is (m), a sweeping time of ultrasonic (output time of a series of ultrasonic) in the transducer  19  is T (sec.), and the band of ultrasonic oscillated from the transducer  19  is f (Hz), a focal length f (m) when the acoustooptical element  4  shows the cylindrical concave lens effect is expressed by the following expression 1.              f   =              V   2        T       λ                 Δ                 f                    [     Expression                 1     ]                         
     This expression 1 also shows that the focal length is inversely proportional to the wavelength of the incident laser and proportional to a square of the ultrasonic speed. Thus it is understood that the focal length lengthens when the acoustooptical element made of quartz is used, which has a fast ultrasonic propagation speed of 5960 (m/sec.), comparing to the acoustooptical element made of tellurium dioxide having the ultrasonic propagation speed of 620 (m/sec.) as in the foregoing. 
     Since this embodiment has a constitution where the acoustooptical element  4  is used so as to generate the concave lens effect and relay the virtual image, a total optical path length can be shortened comparing to the conventional beam scanning method using a cylindrical convex lens effect. Particularly in a current state, only the acoustooptical element made of quartz having a fast sonic speed (ultrasonic propagation speed) can be used in a laser wavelength shorter than 300 nm, a remarkable effect is exerted. For example, in the case where the propagation speed V is 5960 (m/sec.), the wavelength of the laser beam is 244×10 −9  (m), the sweeping time of ultrasonic (output time of a series of ultrasonic) T is 2×10 −6  (sec.), and the band of ultrasonic f is 80×10 −6  (Hz), the focal length f is 3.64 m according to the foregoing expression 1. Therefore, when the acoustooptical element is made to exert the convex lens effect to relay the convergence spot and scanning is performed conventionally, the optical path lengthens by a length corresponding to about 2×f, which is not realistic when the stability is considered. On the other hand, according to the present embodiment, since the acoustooptical element is used so as to function as the concave lens, the virtual image is relayed, and thus the optical path of a scanning optical system can be shortened. For this reason, the stable scanning can be performed even when the acoustooptical element made of quartz having a fast ultrasonic propagation speed is used. 
     Next, description will be made for a second embodiment of the present invention. In the first embodiment, the frequency is swept such that the wavelength of the ultrasonic pulse of the acoustooptical element  4  becomes shorter from the forefront portion  26  to the aftermost portion  25 , but the sweeping direction of the frequency is reversed in the second embodiment. FIG. 4 is a schematic view showing a structure of an optical reticle substrate inspection apparatus according to the second embodiment of the present invention and its beam scanning method. Note that the same reference numerals are added to the same constituent elements as the first embodiment shown in FIG.  2  and FIG. 3, and their detail description will be omitted. 
     In the second embodiment, the acoustooptical element  4  is arranged such that a side where the transducer  19  is attached, that is, a side where ultrasonic is input is made far from the group of cylindrical lenses  3 . Other part of the structure is the same as that of the first embodiment. 
     In the beam scanning method for the second embodiment constituted in this manner, the transducer  19  outputs a series of ultrasonic such that the wavelength lengthens in linear state as the passage of time. A plurality of parallel lines between the aftermost portion  27  and the forefront portion  28  in FIG. 4 show that the wider the distance between the lines the longer the wavelength. The distance between the forefront portion  28  and the aftermost portion  27  of the series of ultrasonic equals the width of the laser beam  30  similarly to the first embodiment. When the laser beam  30  is made incident to the acoustooptical element  4 , diffraction of the laser beam  30  occurs due to the ultrasonic pulse, in which the frequency is swept such that the wavelength lengthens in linear state as described above. Thus, the concave lens effect occurs in this embodiment as well. Then, the circular virtual image  14  is formed on a side where the laser beam  30  is made incident by the combinational function with the group of cylindrical concave lenses  5 . When the laser beam has its optical path on the path  13 , the circular virtual image  15  is formed on a position off from the virtual image  14  in the scanning direction. 
     Therefore, an effect that the stable scanning can be performed is obtained by the second embodiment similarly to the first embodiment by performing the same scanning as the first embodiment.