Abstract:
Disclosed are a spectroscopic optical system and a spectrometer both enabling vertical illumination by means of an optical system using only refractive lenses and enabling wide-band color correction in the DUV-UV (190 to 400 nm) range. The spectroscopic optical system and spectrometer each comprise a light source ( 100 ), a folding mirror ( 110 ), a field stop ( 120 ), an object-side focusing lens system ( 130 ) for focusing light onto a sample, an image-side focusing lens ( 140 ) disposed on the image forming plane of the object-side focusing lens system, and a spectroscope ( 150 ) for dispersing regularly reflected light from the sample. The object-side focusing lens system ( 130 ) and the image-side focusing lens system ( 140 ) are each a spectroscopic optical system corrected with respect to color in a broad band of wavelength from 190 to 400 nm and configured from only refractive lenses enabling vertical illumination. The working distance (WD) of each lens is set shorter than a predetermined distance, and the doublet interval (D) is set longer than a predetermined distance.

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Phase under 35 U.S.C. §371 of International Application No. PCT/JP2009/006963, filed on Dec. 17, 2009, which in turn claims the benefit of Japanese Application No. 2009-044231, filed on Feb. 26, 2009, the disclosures of which Applications are incorporated by reference herein. 
     TECHNICAL FIELD 
     The present invention relates to a spectroscopic optical system and a spectrometer. More particular, the present invention relates to a spectroscopic optical system and a spectrometer that are color corrected for a broad range of wavelengths from deep ultraviolet (DUV) to ultraviolet (UV) (190 to 400 nm) by an arrangement of optical systems using only refractive lenses capable of vertical illumination. 
     BACKGROUND ART 
     In the defect detection of sample surface structure, there is an increasing need for spectroscopic measurement using a broad range of wavelengths in the highly sensitive DUV-UV region (from 190 to 400 nm). At this time, lens systems must be color corrected when a broad range of wavelengths and a plurality of wavelengths are used. There are two methods of color correction, one using a reflective optical system and the other using a refractive optical system. 
     As an example of the reflective optical system, Japanese Unexamined Patent Application Publication (JP-A) No. 2005-127830 (Patent Literature 1) describes a spectroscopic optical system provided with a color corrected lens system of Schwarzschild type for ultraviolet region. 
     As an example of the refractive and diffractive optical system, JP-A No. 2008-90051 (Patent Literature 2) describes an optical system that is color corrected with respect to an ultraviolet wavelength λ and a wavelength 2λ that is twice the wavelength λ, using diffractive optical element. 
     As an example of the refractive optical system, Japanese Patent No. 3288441 (Patent Literature 3) describes near-ultraviolet objective lenses that are color corrected for wavelengths over 350 nm in the near ultraviolet to visible region to have the same focal positions, allowing high resolution observation with near ultraviolet light as well as ultraviolet-fluorescent confocal imaging. Further, JP-A No. Sho 61-90115 (Patent Literature 4) describes image-forming objective lenses using fluorite and quartz as lens materials, which are color corrected in a wide range from the ultraviolet at a wavelength of about 200 nm to infrared region. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A No. 2005-127830 
     Patent Literature 2: JP-A No. 2008-90051 
     Patent Literature 3: Japanese Patent No. 3288441 
     Patent Literature 4: JP-A No. Sho 61-90115 
     SUMMARY OF INVENTION 
     Technical Problem 
     The optical system described in Patent Literature 1 is a reflective type and capable of wide-range color correction. However, the reflective-type color corrected optical system illuminates the sample surface at an oblique angle. There is no problem with oblique illumination when the sample surface remains stationary. However, a problem arises when the entire sample surface is continuously scanned by rotating the sample surface. In  FIG. 3 , when an oblique illumination  811  is applied to continuously scan the sample surface, the sample surface moves vertically (sample surface  1 ,  1 ′) due to continuous scanning, and the scan target position is displaced (scan target position  821 ,  831 ). 
     Meanwhile, the refractive-diffractive optical system and the refractive optical system, which are described in Patent Literatures 2 to 4, are designed to be capable of vertical illumination.  FIG. 4  shows the state in which displacement due to vertical movement of the sample surface is small in vertical illumination. In  FIG. 4 , vertical illumination  812  is applied to continuously scan the sample surface. In this case, even if the sample surface moves vertically (sample surface  1 ,  1 ′) due to continuous scanning, the displacement of the scan target position is small (scan target position  822 ,  832 ). However, the refractive-diffractive optical system descried in Patent Literature 2 is capable of two-wavelength color correction only for the ultraviolet wavelength λ and the wavelength 2λ that is twice the wavelength λ, and it does not supports wide-range color correction. 
     The refractive optical system described in Patent Literature 3 does not support color correction for wavelengths including highly sensitive deep ultraviolet wavelengths below 350 nm in the defect detection of sample surface structure. 
     The refractive optical system described in Patent Literature 4 does not support doublet attachment, which poses a problem in the ultraviolet region. In general, UV curing agent (adhesive) is used for the attachment of doublets. The UV curing agent is cured by irradiating it with ultraviolet light. Thus, when the refractive optical system formed by doublets using UV curing agent is continuously irradiated with ultraviolet light, the portion of the UV curing agent is degraded. At this time, the emitted gas is attached again to the lens surface, and thus the transmittance is reduced. There is another method of attaching doublets without using the UV curing agent to prevent the degradation of quality. However, in the case of attachment, the air space is reduced and uneven brightness (the irregularity in the amount of light) occurs due to interference. 
     It is desirable to provide a spectroscopic optical system and a spectrometer using the spectroscopic optical system, which are capable of vertical illumination with little influence of sample surface movement, achieving color correction in a wide range of DUV-UV wavelengths (190 to 400 nm). 
     Other objects, advantages and novel features of the present invention will be apparent from the following detailed description and the drawings attached hereto. 
     Solution to Problem 
     Representative ones of the inventions disclosed in the present application will be explained in brief as follows. 
     A spectroscopic optical system according to the present invention includes: an illumination optical system including a light source, a folding mirror, a field stop, and an object-side objective lens system for illuminating a sample; a detection optical system including the object-side objective lens system, the field stop, the folding mirror, and an image-side focusing lens system disposed on an image forming plane on the object side; and a spectroscope for separating specularly reflected light from the sample. The object-side objective lens system and the image-side focusing lens system are color corrected in a broad range of wavelengths from 190 to 400 nm, and are formed by only refractive lenses. The working distance (WD) of each lens is set so as to satisfy WD≦10.0 mm. In this configuration, the color correction in the DUV-UV region can be achieved. 
     In the spectroscopic optical system according to the present invention, a distance D of each doublet is set to (λ 1 ·λ 2 )/(4nγ)≦D taking into account one reflection. 
     Here, n is the refractive index of air and γ is the spectroscopic resolution. It is also assumed that λ 2  is the wavelength to be studied, which is determined by selecting the longest wavelength of all the wavelengths in the range to be studied, and λ 1  is obtained by adding the spectroscopic resolution γ to the target wavelength λ 2 . 
     This makes it possible to prevent the occurrence of uneven brightness (irregularity in the amount of light) due to interference. It is to be noted that when taking into account the case of multiple reflection, the doublet distance D is made 1.5 times the value of one reflection. 
     Further, in the spectroscopic optical system according to the present invention, the illumination optical system is designed to vertically illuminate the sample. Thus, it is possible to reduce the displacement due to defocusing in high speed and continuous detection. 
     A spectrometer according to the present invention includes: the spectroscopic optical system; a stage part on which a sample placed, capable of moving a position of the sample relative to the spectroscopic optical system; a control unit for controlling the operation of a spectroscope and the stage part; and a data processing unit for detecting a shape or abnormal shape of patterns formed on the sample, based on the spectral intensity distribution detected by the spectroscope. 
     Further in the spectrometer according to the present invention, the data processing unit includes a database for storing graphs of the wavelength dependence of the spectral reflectance that is calculated in advance for a different pattern shape in the sample. The data processing unit obtains a graph of the wavelength dependency of the spectral reflectance that is measured for the sample, based on the spectral intensity distribution detected by the spectrometer. Then, the data processing unit selects the one that matches the graph of the wavelength dependence of the spectral reflectance that is measured for the sample, from the graphs of the wavelength dependence of the spectral reflectance that are stored in the database, by means of comparison of waveforms of the spectral reflectance. In this way, it is designed to identify the pattern shape formed on the sample. Note that the pattern shape also includes the film thickness. 
     Advantageous Effects of Invention 
     The effect obtained by a typical one of the inventions disclosed in the present application will be described as follows. 
     According to the present invention, it is possible to perform color correction in a broad range of DUV-UV wavelengths (190 to 400 nm). Further, it is possible to prevent the occurrence of uneven brightness (irregularity in the amount of light) due to interference. In addition, it is capable of vertical illumination, thereby reducing the displacement due to defocusing in high speed and continuous detection, which has been a problem in the oblique illumination of the existing reflection optical system. These features enable high-speed and highly accurate spectroscopic measurement (measurement of the structure, the film thickness, and the like) in the highly sensitive DUV-UV (190 to 400 nm) region. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing an example of the configuration of a spectroscopic optical system according to an embodiment of the present invention. 
         FIG. 2A  is a schematic perspective view of a patterned media used as a sample in an embodiment of the present invention. 
         FIG. 2B  and  FIG. 2C  are enlarged plan views of examples of the patterns of data part and servo part in the patterned media. 
         FIG. 3  shows the state in which displacement occurs due to vertical movement of the sample surface in the case of oblique illumination. 
         FIG. 4  shows the state in which displacement due to vertical movement of the sample surface is reduced in the case of vertical epi-illumination. 
         FIG. 5  is a diagram showing the principle of chromatic aberration correction and the method of calculating illumination width. 
         FIG. 6  is a diagram showing the method of calculating illumination width. 
         FIG. 7A  is a diagram showing the working distance (WD) in which a lens as well as a lens barrel for accommodating the lens are taken into account. 
         FIG. 7B  is a diagram showing the relationship between the working distance (WD) and the color shift (Δx). 
         FIG. 8  is a diagram showing the principle of uneven brightness (irregularity in the amount of light) due to interference caused by the air space D between two lenses. 
         FIG. 9A  is a schematic diagram in which the air space D between two lenses is set from 0 to 0.5 μm. 
         FIG. 9B  is a graph of simulation results of the transmittance at wavelengths from 199.50 to 200.50 nm when the air space D is changed from 0 to 0.5 μm. 
         FIG. 10A  is a schematic diagram of the case in which the air space D between two lenses is set from 30 to 30.1 
         FIG. 10B  is a graph of simulation results of the transmittance at wavelengths from 199.50 to 200.50 nm when the air space D is changed from 30 to 30.1 μm. 
         FIG. 11A  is a graph plotting Peak to Valley values of the transmittance with a spectrometer resolution γ=0.3 nm at a wavelength of 400 nm, for each air space D. 
         FIG. 11B  is a graph plotting Peak to Valley values of the transmittance with a spectrometer resolution γ=1.0 nm at a wavelength of 400 nm, for each air space D. 
         FIG. 12  is a diagram showing an example of the configuration of a hard disk detection device using the spectroscopic optical system which is an embodiment of the present invention. 
         FIG. 13  is a schematic diagram showing the process of the data processing unit in an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof is omitted. 
     First Embodiment 
     First, a spectroscopic optical system which is an embodiment of the present invention will be described with reference to  FIGS. 1 and 2 . 
       FIG. 1  is a diagram showing an example of the configuration of a spectroscopic optical system which is an embodiment of the present invention. The spectroscopic optical system includes an illumination optical system and a detection optical system. 
     The illumination optical system is configured to vertically illuminate a sample  1  on a stage, through a light source  100  for emitting illumination light, a folding member  110 , a field stop  120 , and an object-side objective lens system  130 . 
     Similarly, the detection optical system is configured to vertically illuminate a spectroscope  150  for separating the specularly reflected light from the surface of the sample  1 , through the object-side objective lens system  130 , the field stop  120 , the folding mirror  110 , and an image-side focusing lens system  140  disposed on the image forming plane on the object side. It is to be noted that the surface of the sample  1  and the incident surface of the spectroscope  150  are conjugate to each other. 
       FIG. 2A  is a schematic perspective view of a patterned media used as the sample  1 . A patterned media  2000  is a recording medium in which magnetic particles are arranged artificially and regularly on a disk. An example of the patterned media  2000  is a magnetic recording medium used for hard disk devices. For example, on the surface of the patterned media  2000 , there are a data part  2100  for writing user data as well as a servo part  2200  for tracking control and data access control including a burst signal, address, and preamble. In  FIG. 2A , the arrangement of the data part  2100  and the servo part  2200  on the disk surface is schematically shown by the lines. 
       FIG. 2B  is an enlarged plan view showing examples of patterns of the data part  2100  and the servo part  2200  in the patterned media  2000  shown in  FIG. 2A . In a servo part  2210  of  FIG. 2B , magnetic thin film patterns on the convex portions of a substrate with a concave-convex surface, correspond to servo patterns of the patterned media  2000 . The servo part  2210  includes a burst signal  2220  for tracking control. In a data part  2110 , a magnetic thin film is divided by the concave portions to form continuous tracks in the circumferential direction. The patterned media  2000  of this type is called discrete track media. 
     Similarly,  FIG. 2C  is an enlarged plan view showing examples of patterns of the data part  2100  and the servo part  2200  in the patterned media  2000  shown in  FIG. 2A . In a data part  2120  of  FIG. 2C , a magnetic thin film is divided by the concave portions to form data bits. The patterned media  2000  of this type is called bit patterned media. 
     The data part  2100  and the servo part  2200  must be separated from each other. The reason is that when the sample surface is inspected by illuminating the data part  2100  with light to detect a spectral waveform, an accurate spectral waveform may not be detected when the light spot enters the servo part  2200 . In particular, in the case of oblique illumination shown in  FIG. 3 , when the entire surface of the data part  2100  of the patterned media  2000  is continuously scanned, the surface of the sample  1  moves vertically. As a result, scan target positions  821 ,  831  are displaced and are very likely to enter the servo part  2200 . 
     Next, the principle of chromatic aberration correction of the spectroscopic optical system will be described with reference to  FIGS. 5 to 7 . 
     First, the principle of the optical system for correcting chromatic aberration will be described with reference to  FIG. 5 . Achromatization of object points to converge different color lights into one point by a lens system can be expressed by the following equation (1). 
     
       
         
           
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     s 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             s 
                             N 
                           
                           
                             h 
                             N 
                           
                         
                         ) 
                       
                       2 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         N 
                       
                       ⁢ 
                       
                         · 
                         
                           
                             
                               h 
                               i 
                               2 
                             
                             ⁢ 
                             
                               ϕ 
                               i 
                             
                           
                           
                             v 
                             i 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Here, as shown in  FIG. 5 , it is assumed that the number of lenses is N, the marginal ray height of each lens is hi, the marginal ray height of the last lens is hN, the distance from the last lens surface to the object surface is SN, and the refractive power and Abbe number of each lens are φi and νi, respectively. Further, the refractive power φi of each lens surface can be calculated from the lens surface curvature radius r and from the difference between the refractive indices n(λ) on the two sides thereof. The Abbe number νi can be calculated from the refractive indices of the center wavelength λ 0 , the short wavelength λ 1 , and the long wavelength λ 2 . In this way, the chromatic aberration ΔS is derived. 
     Further, the marginal ray height hi of each lens is substantially equal to the marginal ray height hN of the last lens. In other words, these rays are substantially parallel to each other. Thus, the equation (1) can be expressed by the following equation (2). 
     
       
         
           
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     s 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           s 
                           N 
                         
                         ) 
                       
                       2 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           1 
                         
                         N 
                       
                       ⁢ 
                       
                         
                           · 
                           
                             
                               ϕ 
                               i 
                             
                             
                               v 
                               i 
                             
                           
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           ( 
                           
                             
                               h 
                               i 
                             
                             ≈ 
                             
                               h 
                               N 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The chromatic aberration can be reduced by reducing ΔS. In other words, the chromatic aberration can be reduced by reducing Σ(φi/νi), or reducing the distance SN from the last lens surface to the object surface. In the former case, the reduction can be achieved by adjusting the curvature radius, thickness, and space of the individual lenses. In the latter case, the reduction can be achieved by reducing the working distance (WD) of each lens. 
       FIG. 7  shows the color shift when the surface distance (WD) between the sample  1  and a lens  910  is reduced. 
     The color shift is the difference in an illumination width  854  of each wavelength in a surface S 0  that is perpendicular to the optical axis. As shown in  FIGS. 5 and 6 , the calculation method of the difference in the illumination width  854  of each wavelength in the surface S 0  is as follows. 
     First, a position  852  and an RMS value 853 are calculated by ray tracing with respect to the outermost image forming spot  851  in the surface S 0  that is perpendicular to the optical axis. 
     Next, the illumination width  854  is given by the following equation (3) based on the position  852  and the RMS value  853  with respect to the image forming spot  851 . 
     Equation (3)
 
 Xλ= 2 ·x+Pλ   (3)
 
     Here, Xλ is the illumination width, x is the position of the image forming spot, and Pλ is the RMS value. 
     The above calculation is performed for each wavelength. 
     Finally, the color shift is defined by the following equation (4) based on the illumination width  854  of each wavelength. 
     
       
         
           
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     4 
                     ) 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     x 
                   
                   = 
                   
                     
                       
                         
                           Max 
                           . 
                           
                             ( 
                             
                               X 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               λ 
                             
                             ) 
                           
                         
                         - 
                         
                           Min 
                           . 
                           
                             ( 
                             
                               X 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               λ 
                             
                             ) 
                           
                         
                       
                       
                         2 
                         × 
                         
                           Ave 
                           . 
                           
                             ( 
                             
                               X 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
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                     × 
                     100 
                     ⁢ 
                     
                       ( 
                       % 
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Here, Δx is the color shift (%) of each wavelength, Max. (Xλ) is the largest illumination width of the wavelengths λ, Min. (Xλ) is the smallest illumination width of the wavelengths λ, Ave. (Xλ) is the average illumination width of the wavelengths λ. At this time, it is divided by 2 because the color shift of the wavelength is estimated for one side. 
       FIG. 7B  shows the simulation result of plotting the color shifts of the individual wavelengths calculated as described above, with the surface distance (WD) being reduced. As a result, the color shift can be 10% or less by setting WD≦10.0 mm. At this time, the lower limit of WD≦10.0 mm is defined by the limit value for the implementation such as the lens barrel  920 . The color shift is significantly increased with WD≧10.0 mm. 
     Next to be discussed using  FIGS. 8 to 11  is uneven brightness (irregularity in the amount of light) due to interference, caused by lens attachment or thin spacer insertion when the UV curing agent (adhesive) is not used. 
     As shown in  FIG. 8 , a ray of light  960  is incident at an angle θ to the surface of a first lens  930 , which includes a transmitted light  961  through a second lens  950 , and a reflected light  970 . In particular, the reflected light  970  after being transmitted through the first lens  930  is reflected on the surfaces of the second lens  950  and the first lens  930 . Then, the reflected light  970  is transmitted through the second lens  950 . Thus, there is a difference in the optical path length between the transmitted light  961  and the reflected light  970  depending on the air space  980 . At this time, uneven brightness (irregularity in the amount of light) occurs due to interference, which is expressed by the following equation (5) in the case of multiple reflections. 
     Equation 5
 
 I =( E   t ) 2 =( E   t1   +E   t2   +E   t3 + . . . ) 2   (5)
 
     Here, I is the intensity, Et is the transmitted light amplitude, Et 1  is the amplitude of the transmitted light  961  through the second lens  950 , and Et 2  is the amplitude of the transmitted light  970  through the second lens  950  after reflection (one reflection) on the surface of the first lens  930 . In the case of multiple reflections, it continues with transmitted light amplitudes Et 3 , Et 4 , and so on. 
     When considering two-beam interference, the transmitted light  961  through the second lens  950  and the reflected light  970  have an optical path length difference Δ given by the following equation (6). 
     Equation (6)
 
Δ=2nD cos θ  (6)
 
     Here, it is assumed that n is the refractive index of air, and θ is the incident angle. 
     Further, a phase difference δ of π occurs between the transmitted light  961  through the second lens  950  and the reflected light  970 , which can be expressed by the following equation (7). 
     
       
         
           
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     7 
                     ) 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   δ 
                   = 
                   
                     
                       
                         
                           2 
                           ⁢ 
                           π 
                         
                         λ 
                       
                       ⁢ 
                       Δ 
                     
                     + 
                     π 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Here, Δ is the optical path length difference, and λ is the wavelength. 
     By using the equations (5) and (7), the following equation (8) of two-beam interference can be obtained. 
     Equation (8)
 
 I =( E   t ) 2 =( E   t1   +E   t2 ) 2   =E   t1   2   +E   t2   2 +2E t1   E   t2  cos δ  (8)
 
     Here, I is the transmitted light intensity of two-beam interference, Et is the transmitted light amplitude, Et 1  is the amplitude of the transmitted light  961  through the second lens  950 , and Et 2  is the amplitude of the transmitted light  970  through the second lens  950  after reflections (two reflections) on the surfaces of the second lens  950  and the first lens  930 . The degree of uneven brightness (irregularity in the amount of light) due to interference is dependent on the phase difference  6 . 
     The optical path length difference Δ given by the equation (6) has the phase difference of π. Thus, the brightness variation due to interference can be expressed as the following equation (9). 
     
       
         
           
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     9 
                     ) 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   Δ 
                   = 
                   
                     
                       2 
                       ⁢ 
                       nD 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                     = 
                     
                       
                         2 
                         ⁢ 
                         
                           m 
                           · 
                           
                             
                               λ 
                               1 
                             
                             2 
                           
                         
                       
                       = 
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               m 
                             
                             + 
                             1 
                           
                           ) 
                         
                         · 
                         
                           
                             λ 
                             2 
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Here, the wavelength λ 2  is the target wavelength, and λ 1  is the result of adding the spectroscopic resolution γ to the target wavelength λ 2 . At this time, the longest wavelength of all the wavelengths in the range to be studied is selected as the wavelength λ 2 . This is because the value D increases as the wavelength becomes longer. 
     The following equations (10), (11), and (12) can be derived from the equation (9). 
     
       
         
           
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     10 
                     ) 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     2 
                     ⁢ 
                     
                       m 
                       · 
                       
                         
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                           1 
                         
                         2 
                       
                     
                   
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                         1 
                       
                       ) 
                     
                     · 
                     
                       
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                         2 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   10 
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                   Equation 
                   ⁢ 
                   
                       
                   
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                     ( 
                     11 
                     ) 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     2 
                     ⁢ 
                     
                       m 
                       · 
                       
                         ( 
                         
                           
                             λ 
                             1 
                           
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                             2 
                           
                         
                         ) 
                       
                     
                   
                   = 
                   
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                     2 
                   
                 
               
               
                 
                   ( 
                   11 
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     12 
                     ) 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   γ 
                   = 
                   
                     
                       
                         λ 
                         1 
                       
                       - 
                       
                         λ 
                         2 
                       
                     
                     = 
                     
                       
                         λ 
                         2 
                       
                       
                         2 
                         ⁢ 
                         m 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     Here, the difference between the wavelengths λ 1  and λ 2  can be defined as the spectroscopic resolution γ. The spectroscopic resolution γ and the integer value m are inversely related. The following equation (13) for calculating the air space D is derived by substituting the equation (12) into the equation (9). 
     
       
         
           
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     13 
                     ) 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   D 
                   = 
                   
                     
                       
                         λ 
                         1 
                       
                       ⁢ 
                       
                         λ 
                         2 
                       
                     
                     
                       4 
                       ⁢ 
                       n 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       γ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     The spectroscopic resolution γ and the air space D are inversely related. Thus, when the spectroscopic resolution γ is large, the air space D becomes small. On the other hand, when the spectroscopic resolution γ is small, the air space D becomes large. 
     For example, the air space D with a spectroscopic resolution γ of 1.0 nm will be discussed. At this time it is assumed that the wavelength range is 200 to 400 nm. Further, as the spectroscopic resolution γ is 1.0 nm, the target wavelengths are set to (1) 200±0.25, 0.50 nm, (2) 300±0.25, 0.50 nm, and (3) 400±0.25, 0.50 nm, respectively. The transmittance in the air space D for each wavelength will be discussed below. 
     It is assumed a structure having the first lens  930 , the second lens  950 , and an air layer  940  as shown in  FIG. 9A . The light  960 , which is incident at an angle θ to the surface of the first lens  930 , includes the transmitted light  961  through the second lens  950 , and the transmitted light  970  after reflection on the surface of the second lens  950 . A discussion will be made on the transmittance with respect to these two lights when the air space D  980  is changed from 0 to 0.5 μm. Here, the wavelengths are set to (1) 200 nm, (2) 300 nm, and (3) 400 nm, respectively. 
       FIG. 9B  shows the simulation result of the transmittance at the wavelength (1) 200 nm when the air space D  980  is changed from 0 to 0.5 μm. The wavelengths of transmittance waveforms shown in  FIG. 9B  are 199.50 nm, 199.75 nm, 200.00 nm, 200.25 nm, and 200.50 nm. It is also shown an average  990  of the transmittance waveforms of the respective wavelengths. As a result, it is found that when the air space D  980  is small in the range of 0 to 0.5 μm, the waveform phases of all the wavelengths are the same and uneven brightness (irregularity in the amount of light) occurs at the average  990 . This is because the air space D  980  is so small that the optical path length difference hardly occurs between the transmitted light  961  and the transmitted light  970  after reflection, so that mutual cancelling effect is not applied. 
     A discussion will be made on the transmittance with respect to the two lights shown in  FIG. 10A  when an air space D  980 ′ is changed from 30 to 30.1 μm. Here, the wavelengths are set to (1) 200 nm, (2) 300 nm, and (3) 400 nm, respectively. 
       FIG. 10B  shows the simulation result of the transmittance at the wavelength (1) 200 nm when the air space D  980 ′ is increased to 30 μm and changed from 30 to 30.1 μm. The wavelengths of transmittance waveforms shown in  FIG. 10B  are 199.50 nm, 199.75 nm, 200.00 nm, 200.25 nm, and 200.50 nm, respectively. It is also shown an average  990 ′ of the transmittance waveforms of the respective wavelengths. As a result, it is found that when the air space D  980 ′ is large in the range of 30 to 30.1 μm, the waveform phases of the individual wavelengths are shifted and uneven brightness (irregularity in the amount of light) is reduced and equalized at the average  990 ′. This is because the air space D  980 ′ is so large that the optical path difference occurs between a transmitted light  961 ′ and a transmitted light  970 ′ after reflection, so that mutual cancellation is applied. 
     When assuming that the wavelength λ 1 =401 nm, the wavelength λ 2 =400 nm, the spectroscopic resolution γ=1.0 nm, the incident angle θ=0° (vertical incidence), and the air refraction index n is substantially equal to 1, the air space D must satisfy D≧40.1 μm from the equation (13). When the air space D does not satisfy D≧40.1 μm, there is little difference in the optical path length between the transmitted light  961  and the transmitted light  970  after reflection. The mutual cancellation is not applied. As a result, uneven brightness (irregularity in the amount of light) occurs. 
       FIG. 11A  is a graph plotting a Peak to Valley value  999  of the transmittance waveform with the spectroscopic resolution γ=0.3 nm at the wavelength=400 nm, for each air space D. 
       FIG. 11B  is a graph plotting a Peak to Valley value  999 ′ of the transmittance waveform with the spectroscopic resolution γ=1.0 nm at the wavelength=400 nm, for each air space D. 
     When the spectroscopic resolution γ is 0.3 nm, the air space D must satisfy D≧133.4 μm from the equation (13). When the air space D does not satisfy D≧133.4 μm, there is little difference in the optical path length between the transmitted light  961  and the transmitted light  970  after reflection. The mutual cancellation is not applied. As a result, uneven brightness (irregularity in the amount of light) occurs. 
     There is a mismatch between the simulation results  1040 ,  1040 ′ shown in  FIG. 11 , and the results  1050 ,  1050 ′ of the equation (13). This is because only one reflection is taken into account in the equation (13), although actually with multiple reflections. Thus, it is necessary to multiply 1.5 times the air space D calculated by the equation (13) to obtain 1060, 1060′. 
     Second Embodiment 
     Hereinafter a hard disk inspection device will be described as an example of the spectrometer using the spectroscopic optical system according to this embodiment.  FIG. 12  is a diagram showing an example of the configuration of a hard disk inspection device using the spectroscopic optical system according to this embodiment. 
     The hard disk inspection device includes: a spectroscopic optical system  200  for illuminating the sample  1  with light to separate and detect specularly reflected light from the sample  1 ; a stage part  300  on which the sample  1  is placed, capable of moving the position of the sample  1  relative to the spectroscopic optical system  200 ; a control unit  400  for controlling the operation of the spectroscope  150  and the stage part  300 ; and a data processing unit  500  for detecting the shape or abnormal shape of patterns formed on the sample  1  based on the spectral waveform data detected by the spectroscope. 
     The spectroscopic optical system  200  has the same configuration as the spectroscopic optical system shown in  FIG. 1 . At this time, when the entrance position of the spectroscope  150  is defined as the image forming position, it is possible to control the size of the area to be spectroscopically surveyed in the sample  1  by the size of the entrance of the spectroscope  150 . For example, the size of the entrance is set to φ400 μm and the magnification on the image forming plane is set to ×8. In this case, the size of the spectroscopically surveyed area is φ50 μm on the target disk (sample  1 ). 
     As described above, when the wavelength around 400 nm is used, the types of optical devices and the like that can be applied are limited. Examples of the light source  100  are xenon and deuterium lamps that emit light at a wavelength of about 190 nm or more. However, depending on the sample  1 , it is also possible to achieve sufficient performance with a wavelength of about 400 nm or more. In such a case, a halogen lamp or other light source that emits light from visible to infrared can be used as the light source  100 . 
     Finally, a description will be given of the stage part  300 , the control unit  400 , and the data processing unit  500  in the hard disk inspection device according to this embodiment. In  FIG. 12 , the stage part  300  includes an X stage  301  for moving in the direction perpendicular to the surface of the sample  1 , a Z stage  302  for moving in the direction perpendicular to the surface of the sample  1 , and a θ stage  303  for rotating the disk (patterned media  2000 ) of the sample  1 . The Z stage  302  causes the sample  1  to move to the focal position of the spectroscopic optical system  200 . The X stage  301  and the θ stage  303  cause the spectroscopic optical system  200  to move to any position on the sample  1 . 
     Further, an XY stage can be used as a method of moving the spectroscopic optical system to any position on the sample  1 . An Xθ stage is more appropriate than the XY stage when the sample  1  is a disk and the pattern on the sample surface is formed concentrically. Also, the Xθ stage is more appropriate than the XY stage when it is desired to inspect the entire surface of the disk at high speed, because the operation is simpler with the Xθ stage than with XY stage. For this reason, the hard disk inspection device according to this embodiment has an Xθ stage structure using the X stage  301  and the θ stage  303 . 
       FIG. 13  is a schematic diagram of the process in the data processing unit  500 . The process of the data processing unit  500  is roughly divided into the following two processes. One is the spectral reflectance calculation, and the other is the pattern shape and defect detection process. As described above, in the hard disk inspection device according to this embodiment, the pattern shape and defects of the sample  1  are detected based on the spectral reflectance of the surface of the sample  1 . 
     The spectroscopic optical system  200  can detect the spectral intensity distribution of the surface of the sample  1 . Thus, optical simulation is applied to the sample  1  having a different pattern shape  510  in advance. Then, a graph  511  of the wavelength dependence of the calculated spectral reflectance is stored in a database  513 . Next, the sample  1  on which the pattern is repeatedly formed is illuminated with light from the light source  100  through the spectroscopic optical system  200 . In this way, the specularly reflected light from the sample surface is received by the spectroscope  150 . 
     The data processing unit  500  obtains a graph  512  of the wavelength dependence of the spectral reflectance, based on the spectral intensity distribution detected by the spectroscope  150 . Finally, an approximation to the graph  512  of the wavelength dependence of the spectral reflectance that is obtained by measurement, is selected from the graph  511  of the wavelength dependence of the spectral reflectance that is calculated by optical simulation and is stored in the database  513 , by means of a comparison  514  of the waveforms of the spectral reflectance. In this way, the shape of the sample  1  can be identified. 
     As described above, the spectroscopic optical system  200  according to this embodiment includes: the illumination optical system including the light source  100 , the folding mirror  110 , the field stop  120 , and the object-side objective lens system  130  for illuminating the sample  1 ; the detection optical system including the object-side objective lens system  130 , the field stop  120 , the folding mirror  110 , and the image-side focusing lens system  140  disposed on the image forming plane of the sample  1 ; and the spectroscope  150  for separating the specularly reflected light from the sample  1 . In this configuration, the object-side objective lens system  130  and the image-side focusing lens system  140  are color corrected in a wide range of deep ultraviolet to ultraviolet light at wavelengths of 190 to 400 nm. The object-side objective lens system  130  and the image-side focusing lens system  140  are formed only by refractive lenses. 
     At this time, when the working distance (WD) of each lens is set to WD≦10.0 mm, it is possible to reduce the color shift. 
     Further, when the distance D of each doublet is set to (λ 1 ·λ 2 )/(4nγ)≦D, it is possible to prevent the occurrence of uneven brightness (irregularity in the amount of light). 
     Still further, because the vertical epi-illumination is used, it is possible to reduce the displacement due to defocusing, which has been a problem in oblique illumination of the reflection optical system. In addition, the illumination positions of the individual wavelengths coincide by wide range color correction. Thus, highly accurate spectroscopic measurement (structure and film thickness measurement, and the like) can be achieved. 
     While the invention made by the present inventors has been described specifically based on its embodiments hereinbefore, it will be appreciated that the present invention is not limited to the embodiments and various modifications may be made without departing from the gist of the invention. 
     For example, the spectroscopic optical system  200  and the spectrometer according to this embodiment are designed to detect the pattern shape and defects on the surface of the patterned media  2000  by spectroscopic measurement. However, the sample  1  is not limited to the patterned media  2000 . As long as the sample  1  has a structure and pattern on the surface, the data processing unit  500  can detect the structure by matching the spectral reflectance. Further, in addition to the detection of the surface structure of the sample  1 , it is also possible to apply the measurement of the thin film thickness, and the like, by spectroscopic measurement. 
     Industrial Applicability 
     The spectroscopic optical system and the spectrometer according to the present invention can be applied to spectroscopic optical systems and spectrometers for performing wide range color correction by optical systems using refractive lenses, such as semiconductor and patterned media inspection devices, as well as thin film thickness measuring devices by spectroscopic measurement. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  1 ′: sample 
           100 : light source 
           110 : folding member 
           120 : field stop 
           130 : object-side objective lens system 
           140 : image-side focusing lens system 
           150 : spectroscope 
           200 : spectroscopic optical system 
           300 : stage part 
           301 : X stage 
           302 : Z stage 
           303 : θ stage 
           400 : control unit 
           500 : data processing unit 
           510 : pattern shape 
           511 : graph of wavelength dependence of spectral reflectance calculated by simulation 
           512 : graph of wavelength dependence of spectral reflectance based on the detected spectral intensity distribution 
           513 : database 
           514 : comparison of waveforms of spectral reflectance 
           811 : oblique illumination 
           812 : vertical illumination 
           821 ,  822 : position to be scanned before defocusing 
           831 ,  832 : position to be scanned after defocusing 
           850 : marginal ray 
           851 : outermost image forming spot 
           852 : position x of the outermost image forming spot 
           853 : RMS value Pλ of the outermost image forming spot 
           854 : illumination width xλ 
           910 : lens 
           920 : lens barrel 
           930 ,  930 ′: first lens 
           940 ,  940 ′: air 
           950 ,  950 ′: second lens 
           960 ,  960 ′: incident light 
           861 ,  961 ′: transmitted light through the second lens 
           970 ,  970 ′: reflected light on the first and second lens surfaces 
           980 ,  980 ′: air space D 
           990 : transmittance waveforms at wavelengths of 199.50 nm, 199.75 nm, 200.00 nm, 200.25 nm, and 200.50 nm, and average of the transmittance waveforms from 199.50 to 200.50 nm 
           990 ′: average of the transmittance waveforms from 199.50 to 200.50 nm 
           991 ′: transmittance waveform of 199.50 nm 
           992 : transmittance waveform of 199.75 nm 
           993 ′: transmittance waveform of 200.00 nm 
           994 ′: transmittance waveform of 200.25 nm 
           995 ′: transmittance waveform of 200.50 nm 
           1040 : plot of Peak to Valley values for each air space with respect to the average of the transmittance waveforms from 399.85 to 400.1 nm 
           1040 ′: plot of Peak to Valley values for each air space with respect to the average of the transmittance waveforms from 399.50 to 400.50 nm 
           2000 : patterned media 
           2100 - 2120 : data part 
           2200 - 2210 : servo part 
           2220 : burst signal pattern