Abstract:
An object is to provide a blazed diffraction grating having a smaller blaze angle than an existing blaze angle, and a method for producing the same. A method for producing a blazed diffraction grating includes the steps of forming a resin layer on a support having a saw-tooth sectional shape and having a surface on which a basic blaze surface and a basic riser surface are arranged alternately and repeatedly in a direction, such that the thickness of the resin layer contacting with the surface monotonically changes in the direction, and forming a metal coating film covering the resin layer surface. The method for monotonically changes the thickness comes in a formation method by difference in volatilization volume after applying a solvent resin on the support, and a formation method by a centrifugal force.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a blazed diffraction grating which is a wavelength separation/selection element to be used in a monochromator, a branching filter and the like, and relates to a method for producing the blazed diffraction grating. 
       BACKGROUND ART 
       [0002]    There are various kinds of diffraction gratings. A diffraction grating with grooves having a saw-tooth sectional shape, which is called a blazed diffraction grating, exhibits a high diffraction efficiency for a light of specific wavelength in a range from ultraviolet to visible light, and is often used in a visible light/ultraviolet monochromator and the like (see Patent Literature 1). 
         [0003]    Known methods for producing the blazed diffraction grating include an ion beam etching for engraving a substrate by casting ion beams at a predetermined incident angle while using a photoresist as a mask and a machine cutting for forming grooves one by one with a ruling engine. The machine cutting method has a low accuracy in groove interval, and may sometimes leave an uncut chip at a processed edge part, which causes stray light. Therefore, the ion beam etching method is often used in the making of a diffraction grating that requires accuracy. 
         [0004]    The method for producing the blazed diffraction grating by the ion beam etching method is described with reference to  FIG. 1A  to  FIG. 1F . First, a photoresist is applied on a surface of a flat substrate  1  of quartz, glass or the like to form a photoresist layer  2  ( FIG. 1A ). The photoresist layer  2  is exposed under two-beam interference, and interference stripes develop on it, thereby forming a parallel-line shaped resist pattern  3  (holographic exposure) as shown in  FIG. 1B . Then, using the resist pattern  3  as a mask, ion beam etching is performed with an oblique beam angle so as to achieve an intended blaze angle qB on the substrate  1  until the resist pattern  3  disappears, whereby a grating groove  4  having a saw-tooth sectional shape is formed on the substrate  1  ( FIG. 1C  to  FIG. 1E ). Thereafter, as shown in  FIG. 1F , the surface of the grating groove  4  is coated with a metal film  5  of aluminum, gold or the like, and the blazed diffraction grating is completed. 
         [0005]    In general, in the above-described etching step, the ion beam etching is performed using an etching gas that gives the etching rate for the substrate  1  higher than the etching rate for the resist pattern  3 , that is, yields the selection ratio (=the etching rate for the material (for example, glass) of the substrate/the etching rate for the photoresist) larger than 1. 
         [0006]    In this way, a master blazed diffraction grating is produced. On the grating surface of the master diffraction grating, a parting agent layer is formed, and a metal thin film is formed on the parting agent layer. Subsequently, an adhesive on a glass substrate is placed on the metal thin film. After the adhesive is hardened, the glass substrate is separated from the master diffraction grating. In this way, the metal thin film having a grating groove formed is transferred, in a reversed manner, to the glass substrate side, so that a replica diffraction grating is obtained. By using the replica diffraction grating as a master die, diffraction gratings as products are produced (see Patent Literature 1). 
         [0007]    As the field of analysis objects of a spectrometer grows, the wavelength of the light to be diffracted by the diffraction grating has become shorter. In order to cope with the shorter diffraction wavelength, it is required to shorten the grating interval and to decrease the blaze angle. 
         [0008]    For producing a blazed diffraction grating with a small blaze angle using the above-described ion beam etching method, the incident angle a of the ion beam needs to be large. In this case, as shown in  FIG. 2 , the riser surface  12  between a blaze surface  11  and an adjacent blaze surface  11  is deeply etched, and the riser surface  12  recedes under the blaze surface  11  (that is, the angle b between the blaze surface  11  and the riser surface is acute), resulting in an eaves-like shape in which the edge of the blaze surface  11  hangs over the adjacent blaze surface  11  (this is called an overhang). A replica diffraction grating cannot be produced with a master diffraction grating having such an overhang. 
         [0009]    Further, in the case of a concave diffraction grating, if the incident angle a of the ion beam is too small, a part on the periphery remains unetched. For example, in the case of a concave diffraction grating having the radius R of curvature with the diameter D of the substrate, the edge part of the substrate is curved at an angle of q=sin −1 (D/2R) from the horizontal direction, as shown in  FIG. 3 . Therefore, when the incident angle a of the ion beam is (90−q) degrees or larger, a part (shadow part) at which the ion beam does not reach the substrate surface remains, where the etching cannot be performed. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [Patent Literature 1] JP 2009-92687 A 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0011]    An object of the present invention is to provide a blazed diffraction grating having a smaller blaze angle than an existing blaze angle, and a method for producing the same. 
       Solution to Problem 
       [0012]    A blazed diffraction grating according to the present invention, which has been made for achieving the above object, includes:
       a) a support having a saw-tooth sectional shape and having a basic blaze surface and a basic riser surface arranged alternately and repeatedly in a direction;   b) a resin layer covering the basic blaze surface and the basic riser surface of the support and having monotonically changing thickness on the basic blaze surface in the direction; and   c) a metal coating film covering a surface of the resin layer.       
 
         [0016]    In the blazed diffraction grating according to the present invention, as shown in  FIG. 4 , on a “resin layer blaze surface” of the resin layer covering the basic blaze surface of the support, the thickness of the resin layer monotonically changes in the direction. Therefore, regardless of the blaze angle qB 1  of the basic blaze surface of the support, the blaze angle qB 2  of the resin layer blaze surface can be arbitrarily determined. For example, as shown in  FIG. 4 , when a thickness t 1  of the resin layer is larger at a trough portion of the support (i.e. the deeper portion of the basic blaze surface) and a thickness t 2  of the resin layer is smaller at a crest portion of the support (i.e. the shallower portion of the basic blaze surface), the blaze angle qB 2  of the resin layer blaze surface can be made to be smaller than the blaze angle qB 1  of the basic blaze surface. Conversely, in the case of t 1 &lt;t 2 , it is possible to obtain a blazed diffraction grating having a larger blaze angle than the blaze angle qB 1  of the basic blaze surface. 
         [0017]    The metal coating film is a film that is thin enough to have no influence on the thickness of the resin layer. 
         [0018]    A method for producing a blazed diffraction grating according to the present invention, which has been made for achieving the above object, includes the steps of:
       a) preparing a support having a saw-tooth sectional shape and having a basic blaze surface and a basic riser surface arranged alternately and repeatedly in a direction;   b) forming a resin layer covering the basic blaze surface and the basic riser surface, on the support, such that a thickness of the resin layer monotonically changes on the basic blaze surface in the direction; and   c) forming a metal coating film covering a surface of the resin layer.       
 
         [0022]    The method for forming the resin layer such that the thickness monotonically changes on the basic blaze surface in the direction comes in two methods: a method of utilizing the volatilization of a volatile solvent and a method of utilizing an external force. 
         [0023]    The method of utilizing the volatilization of a volatile solvent includes making a solvent resin by dissolving a resin for forming the resin layer in a volatile solvent, applying the solvent resin on the support such that the surface of the solvent resin after the application is flat, and forming the resin layer through a step of volatilizing the volatile solvent and solidifying the resin after the application. In the state in which the solvent resin is applied on the support so that the outermost surface is a flat surface, the depth from the flat surface to the basic blaze surface varies for each part of the basic blaze surface. Therefore, the volume of the volatilization of the solvent from the solvent resin also varies, and after the solidification of the resin, the resin layer blaze surface having different blaze angle from the basic blaze surface is obtained. 
         [0024]    The method of utilizing an external force includes giving the external force in the direction when coating the support with a liquid resin for forming the resin layer, and thereby, monotonically changing the thickness of the resin layer coating the basic blaze surface. The external force to be used includes a centrifugal force, a gravitational force by the inclination of the support, a wind force to be applied by an injection of air, or others, to the surface, and the like. The blaze angle of the blazed diffraction grating to be produced can be controlled not only by the control of the intensity of the external force but also by the adjustment of the viscosity of the solvent resin. 
       Advantageous Effects of Invention 
       [0025]    By using the method for producing the blazed diffraction grating according to the present invention, a blazed diffraction grating having any blaze angle can be produced, by a simple production method of forming a resin layer covering a basic blaze surface and a basic riser surface, on a support having a saw-tooth sectional shape and having the basic blaze surface and the basic riser surface arranged alternately and repeatedly in a direction, such that the thickness of the resin layer monotonically changes on the basic blaze surface in the direction, and forming a metal coating film covering a surface of the resin layer. Therefore, it is possible to produce a blazed diffraction grating having a smaller blaze angle than an existing blazed diffraction grating. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]      FIG. 1A ,  FIG. 1B ,  FIG. 1C ,  FIG. 1D ,  FIG. 1E  and  FIG. 1F  are schematic sectional views for describing a procedure of a method for producing a blazed diffraction grating. 
           [0027]      FIG. 2  is a diagram for describing an overhang. 
           [0028]      FIG. 3  is a diagram for describing a curve of an edge part of a substrate in the case of a concave diffraction grating. 
           [0029]      FIG. 4  is a diagram for describing a change in blaze angle. 
           [0030]      FIG. 5  is a schematic sectional view after solvent resin application in the case where a solvent resin mixed with a volatile solvent is applied on a blazed diffraction grating by dip coating. 
           [0031]      FIG. 6  is a schematic sectional view after the volatilization of the volatile solvent in the case where the solvent resin mixed with the volatile solvent is applied on the blazed diffraction grating by dip coating. 
           [0032]      FIG. 7  is a simplified schematic view of a unit that gives a centrifugal force. 
           [0033]      FIG. 8  is a schematic sectional view of a liquid resin on a blazed diffraction grating when the centrifugal force is applied. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    Hereinafter, modes for carrying out the present invention will be described using embodiments. 
       Embodiment 1 
       [0035]    A first embodiment of the present invention will be described with reference to  FIG. 5  and  FIG. 6 .  FIG. 5  is a schematic sectional view after solvent resin application in the case where a solvent resin mixed with a volatile solvent is applied on a blazed diffraction grating by dip coating.  FIG. 6  is a schematic sectional view after the volatilization of the volatile solvent in the case where the solvent resin mixed with the volatile solvent is applied on the blazed diffraction grating by dip coating. 
         [0036]    The first embodiment is an example of the case where a method of utilizing the volatilization of a volatile solvent is used for forming a resin layer such that the thickness monotonically changes on a basic blaze surface in a direction. 
         [0037]    In the first embodiment, a blazed diffraction grating  20  having a first blaze angle is used as a support. As an example, in the blazed diffraction grating  20 , the number of grooves is 900/mm, the depth of the groove is 0.30 mm, the first blaze angle is 14°, and a master blazed diffraction grating produced by an ion beam etching method is used. 
         [0038]    A resin (solvent resin) dissolved in a volatile solvent is applied on a surface of the blazed diffraction grating  20  having multiple grooves each of which is composed of a basic blaze surface  21  and a basic riser surface  22 . A dip coating method is used to apply the solvent resin such that the resin surface after the application is flat as shown in  FIG. 5 . Here, the thickness of the resin layer  26  is, at the thinnest spot (at the part of the crest of the groove), approximately one-half to three times of the groove depth of the blazed diffraction grating  20 . The thickness of the resin layer  26  depends also on the proportion of the solvent in the solvent resin. In the case of the blazed diffraction grating  20 , the thickness of the solvent resin after the application is about 1 mm at the thickest spot. 
         [0039]    Any photo-curing resin and thermosetting resin can be used as the resin in the present invention. As long as the reaction with these resins does not occur, any volatile organic solvent such as ethanol, benzene or acetone can be used as the volatile solvent in the present invention. In the embodiment, ethanol is used as the volatile organic solvent. 
         [0040]    Next, the ethanol in the solvent resin is volatilized. The volatilization may be accomplished by heating the whole of the blazed diffraction grating  20  having the solvent resin applied for accelerating the volatilization, or by waiting until the volatile solvent naturally volatilizes with time elapse. As the ethanol in the solvent resin volatilizes as shown by arrows in  FIG. 6 , the thickness of the solvent resin becomes thinner, and the surface of the solvent resin tends to have a concavo-convex shape which reflects the shape of the grooves of the blazed diffraction grating  20 . 
         [0041]    Thereafter, the resin is solidified by a predetermined method, so that a resin layer  36  having the concavo-convex shape which reflects the shape of the grooves of the blazed diffraction grating  20  is formed on the surface of the blazed diffraction grating  20  as shown in  FIG. 6 . Here, the thickness of the resin layer  36  is not uniform, and monotonically changes on the basic blaze surface  21  in a direction, and the thickness (t 2  in  FIG. 4 ) on the crest (the convex part of the concavo-convex shape) of the blazed diffraction grating  20  is smaller than the thickness (t 1  in  FIG. 4 ) on the trough (the concave part of the concavo-convex shape) of the blazed diffraction grating  20 . As a result, the groove of the resin layer  36  is shallower than the groove of the blazed diffraction grating  20 . In the above-described example, the thickness of the resin layer  36  is approximately 100 nm to 200 nm at the thinnest spot. 
         [0042]    The reason why the thickness of the resin layer  36  is not uniform and the thickness on the crest of the blazed diffraction grating  20  is smaller than the thickness on the trough of the blazed diffraction grating  20  is thought to be as follows. As shown in  FIG. 5 , in the dip coating method of performing the resin application such that the resin surface after the application is flat, the thickness of the solvent resin after the application is thinner on the crest of the blazed diffraction grating  20 , and is thicker on the trough of the blazed diffraction grating  20 . The proportion of the volatile solvent contained in the solvent resin is even, and therefore, assuming that the solvent resin volatilizes by a constant proportion (for example, half) of the thickness immediately after the application of the solvent resin, film thickness is reduced by the constant proportion while the relation of the thickness of the solvent resin after the above application is maintained. As a result, the thickness of the resin layer  36  is not uniform, and the thickness on the crest of the blazed diffraction grating  20  is smaller than the thickness on the trough of the blazed diffraction grating  20 . 
         [0043]    Thereafter, for protecting the surface of the resin layer  36 , a metal coating film of aluminum or the like, which is not illustrated, is formed on the surface using a vacuum evaporation method, a sputtering method or the like, and the production of the blazed diffraction grating is completed. The thickness of the metal coating film is approximately several tens nanometers, which has no influence on the shape of the surface of the resin layer  36 . 
         [0044]    The resin layer  36  has a blaze surface  31  on the basic blaze surface  21  and a riser surface  32  on the basic riser surface  22 , and the slopes of the blaze surface  31  and riser surface  32  are more gradual than the slopes of the basic blaze surface  21  and basic riser surface  22 , respectively. The angle g 1  between the blaze surface  31  and the riser surface  32  is larger than the angle b′ 1  between the basic blaze surface  21  and the basic riser surface  22 . The blaze angle (second blaze angle) of the blazed diffraction grating produced in this way is smaller than the blaze angle (first blaze angle) of the blazed diffraction grating  20 . The above-described example gives a blazed diffraction grating in which the number of grooves is 900/mm, the depth of the groove is 0.15 mm and the second blaze angle is 7°. 
         [0045]    In the embodiment, by utilizing the unevenness of the thickness of the residual resin layer due to the even volatilization of the solvent from the solvent resin whose thickness is uneven, the resin layer is formed such that the resin layer monotonically changes on the basic blaze surface of the blazed diffraction grating  20 , which is the support, in the direction and the thickness of the resin layer on the crest of diffraction grating  20  is smaller than the thickness on the trough of the blazed diffraction grating  20 , whereby the blaze angle of the produced blazed diffraction grating is made to be smaller than that of the blazed diffraction grating  20  used as the support. Therefore, it is possible to produce a blazed diffraction grating having a small blaze angle, which is difficult to achieve by the conventional methods for producing the diffraction grating. 
         [0046]    In the case where the blaze angle of the produced blazed diffraction grating is not an intended angle, it is only necessary to remove the formed resin layer and redo the formation of the resin layer. Therefore, even if the processing is unsuccessful, the processing object material does not need to be discarded, leading to the reduction in production cost. 
       Embodiment 2 
       [0047]    A second embodiment of the present invention will be described with reference to  FIG. 7  and  FIG. 8 .  FIG. 7  is a simplified schematic view of a unit for giving a centrifugal force.  FIG. 8  is a schematic sectional view of a liquid resin on a blazed diffraction grating  40  when the centrifugal force is applied. In the following description of the second embodiment, a liquid resin is used. A resin liquefied by the same volatile solvent as that in the first embodiment may be used. Hereinafter, descriptions that are the repetition of the first embodiment is omitted, and differences from the first embodiment is mainly described. 
         [0048]    The second embodiment is an example of the case where a method of utilizing an external force is used for forming a resin layer such that the thickness monotonically changes on a basic blaze surface in a direction, and a centrifugal force is used as the external force. 
         [0049]    In the second embodiment, a liquid resin is applied on a surface of a blazed diffraction grating  40  that is a support and that has multiple grooves each of which is composed of a basic blaze surface  41  and a basic riser surface  42 . The application of the liquid resin may be done by any method, such as a dip method and a centrifugation method (spin coating). In the case of the spin coating, the spin speed is not to be very high, and the coating thickness needs to be sufficiently large. 
         [0050]    The blazed diffraction grating  40  is set on a centrifugal apparatus such that a centrifugal force is applied, in the blaze direction (the direction perpendicular to the groove), to the blazed diffraction grating  40  having the liquid resin applied on the surface. Specifically, as shown in  FIG. 7 , the blazed diffraction grating  40  is fixed at a peripheral part that is as far away from the center of a discoid rotating plate  60  as possible. At this time, the blazed diffraction grating  40  is fixed such that the blaze direction coincides with the radial direction of the rotating plate  60 . If a resin layer having a smaller blaze angle than the blaze angle of the basic blaze surface  41  is desired, the blazed diffraction grating  40  is fixed such that the basic blaze surface  41  is oriented to the outer side of the rotating plate  60 . Conversely, if a resin layer having a larger angle than the blaze angle of the basic blaze surface  41  is desired, the basic blaze surface  41  is oriented to the center of the rotating plate  60 . For the fixation, preferably, a conductive adhesive or tape (for example, a copper tape or the like) should be used, in order to release static electricity toward the rotating plate  60  when the blazed diffraction grating  40  is charged. 
         [0051]    Next, the rotating plate  60  is rotated at a high speed of several thousand rpms to apply a force (centrifugal force), in the blaze direction, to the liquid resin on the surface of the blazed diffraction grating  40  (an arrow in  FIG. 8 ). In this way, the liquid resin flows on the basic blaze surface  41 . As described above, in the case where the basic blaze surface  41  is oriented to the outer side, the blaze angle of a resin layer  56  becomes smaller than that of the basic blaze surface  41 , and in the case where the basic blaze surface  41  is oriented to the center side, the blaze angle of the resin layer  56  becomes larger than that of the basic blaze surface  41 . 
         [0052]    Through such a high-speed rotation, the liquid resin is naturally solidified. The solidification may be further accelerated by the light casting to the rotating surface or the heating of the rotating plate  60 . 
         [0053]    In such steps, by solidifying the liquid resin while giving the centrifugal force to the liquid resin, the resin layer  56  reflecting the shape of the grooves of the blazed diffraction grating  40  and having a different blaze angle from the blazed diffraction grating  40  is formed on the surface of the support. 
         [0054]    Thereafter, as a metal coating film covering the surface of the resin layer  56 , which is not illustrated, aluminum is formed (for example, the thickness is several tens nanometers) using a vacuum evaporation method, a sputtering method or the like while the blaze shape of the resin layer  56  is maintained, and the blazed diffraction grating is completed. 
         [0055]    In  FIG. 8 , the resin layer  56  has a blaze surface  51  on the basic blaze surface  41  and a riser surface  52  on the basic riser surface  42 , and the slopes of the blaze surface  51  and riser surface  52  are more gradual than the slopes of the basic blaze surface  41  and basic riser surface  42 , respectively. Therefore, the angle g 2  between the blaze surface  51  and the riser surface  52  is larger than the angle b′ 2  between the basic blaze surface  41  and the basic riser surface  42 . 
         [0056]    In the embodiment, the resin layer is formed such that the resin layer monotonically changes on the basic blaze surface of the blazed diffraction grating  40 , which is the support, in the direction and the thickness (t 2  in  FIG. 4 ) of the resin layer on the crest of the blazed diffraction grating  40  is smaller than the thickness (t 1  in  FIG. 4 ) on the trough of the blazed diffraction grating  40 , whereby the blaze angle of the produced blazed diffraction grating is made to be smaller than the blaze angle of the blazed diffraction grating  40  used as the support. Therefore, it is possible to produce a blazed diffraction grating having a small blaze angle, which is difficult to achieve by the conventional methods for producing the diffraction grating. 
         [0057]    In the case where the blaze angle of the produced blazed diffraction grating is not an intended angle, it is only necessary to remove the formed resin layer and redo the formation of the resin layer. Therefore, even if the processing is unsuccessful, the processing object material does not need to be discarded, leading to the reduction in production cost. 
         [0058]    By using the present embodiment, the blaze angle of the blazed diffraction grating to be produced can be controlled by controlling the rotation speed of the rotating plate  60  or adjusting the viscosity of the liquid resin to be applied. The adjustment of the viscosity of the liquid resin may be performed by mixing the volatile solvent mentioned in Embodiment 1. Therefore, the flexibility for adjusting the parameter in the production stage for the blazed diffraction grating is high. 
         [0059]    The embodiment shows an example in which a centrifugal force is used as the external force to be given to the liquid resin, but the present invention is not limited to this. As an example, the support having the liquid resin applied may be inclined such that a gravitational force is given in the direction shown by the arrow in  FIG. 8 . Further, a wind force may be used as the external gas, by injecting inert gas such as nitrogen gas or air in the direction shown by the arrow in  FIG. 8 . 
         [0060]    The above embodiments show an example in which the master diffraction grating is used as the blazed diffraction grating to be used for the support, but the present invention is not limited to this. As the blazed diffraction grating to be used for the support, a replica diffraction grating may be used. Further, from the blazed diffraction grating produced in the above embodiment, a replica diffraction grating can be produced. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  . . . Substrate 
           2  . . . Photoresist Layer 
           3  . . . Resist Pattern 
           4  . . . Grating Groove 
           5  . . . Metal Film 
           20 ,  40  . . . Blazed Diffraction Grating 
           21 ,  41  . . . Basic Blaze Surface 
           11 ,  31 ,  51  . . . Blaze Surface 
           22 ,  42  . . . Basic Riser Surface 
           12 ,  32 ,  52  . . . Riser Surface 
           26 ,  36 ,  56  . . . Resin Layer 
           60  . . . Rotating Plate