Abstract:
In a near-field exposure system: a light source emits exposure light having a predetermined wavelength and being unpolarized; a polarizer plate linearly polarizes the exposure light; an exposure mask which has a pattern of openings each having a dimension smaller than the wavelength of the exposure light is placed at such a position that the exposure light is applied to the exposure mask; an exposure table holds a photosensitive material sensitive to the exposure light, at a position which near-field light emerging from the openings reaches; and a polarizer-plate holding means holds the polarizer plate in such a manner that the polarizer plate can be moved between first and second positions, where the first position is in an optical path of the exposure light from the light source to the exposure mask, and the second position is outside the optical path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The subject matters disclosed in this specification are related to the subject matters disclosed in the copending, commonly-assigned U.S. patent application Ser. No. 09/562,076 filed by Masayuki Naya (the inventor of the present application) and Shinji Sakaguchi on May 1, 2000 and entitled “FINE PATTERN FORMING METHOD,” corresponding to Japanese patent application No. 2000-119670, which is disclosed in Japanese Unexamined Patent Publication No. 2001-15427.  
         [0002]    The contents of the above copending, commonly-assigned U.S. patent application and the corresponding Japanese patent application are incorporated in this specification by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention relates to a near-field exposure system which exposes a fine pattern on a photosensitive material such as a photoresist.  
           [0005]    2. Description of the Related Art  
           [0006]    The development in the photolithographic technology has been supported by the development in the reduction-projection exposure technology and the resist technology. In the reduction-projection exposure technology, performance is mainly determined by two fundamental quantities, the resolution RP and the focal depth DOP, which are respectively expressed as RP=k 1 λ/NA and DOP=k 2 λ/NA 2 , where λ is an exposure wavelength of a projection optical system, NA is a numerical aperture of a projection lens, and k 1  and k 2  are coefficients. In order to increase the resolution in lithography, it is important to reduce the wavelength λ and increase the numerical aperture NA of the projection lens. Although the resolution is improved with increase in the numerical aperture NA, the focal depth DOP is reduced in inverse proportion to the square of the numerical aperture NA. Therefore, currently, the reduction in the wavelength λ is required for realizing fine lithography. Thus, the exposure light used in lithography has been changed from the g line having the wavelength of 436 nm to the i line having the wavelength of 365 nm, and currently use of the excimer laser light having the wavelength of 248 or 193 nm is becoming mainstream.  
           [0007]    However, in the lithography using light, the resolution limit is the diffraction limit of the exposure light. Therefore, it is said that the finest line width obtained by use of the excimer laser light having the wavelength of 248 or 193 nm and a lens-series optical system is 100 nm. Further, in order to achieve a resolution of the nanometer order, it is necessary to use an electron-beam or X-ray lithography technique. In particular, the synchrotron orbital radiations (SORs) are used in the X-ray lithography.  
           [0008]    The electron-beam lithography enables highly precise control for forming a pattern including nanometer-scale structures. In addition, the electron-beam lithography can achieve a considerably greater focal depth than the lithography using an optical system. Further, the electron-beam lithography enables direct drawing on a wafer without use of a mask. However, because of low throughput and high cost, the current electron-beam lithographic techniques are far from the level which enables mass production.  
           [0009]    On the other hand, the X-ray lithography can achieve about one order of magnitude higher resolution and precision than the excimer laser exposure either when 1:1 projection exposure through a 1:1 mask is used, or when a reflective imaging X-ray optical system is used. However, it is not easy to realize the X-ray lithography since production of masks is not easy. In addition, equipment cost is high.  
           [0010]    In order to solve the above problems, a method for exposing a photosensitive material such as a photoresist to near-field light has been proposed. In this method, exposure light is applied to an exposure mask in which a pattern of openings having dimensions smaller than the wavelength of the exposure light is formed, so that near-field light emerges through the openings of the exposure mask, and the photosensitive material is exposed to the near-field light. According to the above method, a fine pattern of the same order of magnitude as the openings in the exposure mask can be formed in the photosensitive material regardless of the wavelength of the exposure light. An example of an exposure system executing the above method is disclosed in Japanese Unexamined Patent Publication No. 2000-112116.  
           [0011]    It is known that when a fine pattern in an exposure mask is transferred to a photosensitive material such as a photoresist by exposure to near-field light as above, and the fine pattern is constituted by straight lines extending in an identical direction as in the case of a diffraction grating, and the exposure light is linearly polarized in the same direction as the direction of the straight lines, thickening of straight lines formed on the photosensitive material can be prevented so that a finer pattern can be formed on the photosensitive material.  
           [0012]    Based on the above knowledge, JUPP No. 2000-112116 discloses a near-field exposure mask in which an opening pattern and a grid polarizer are stacked on a mask substrate, where the opening pattern has dimensions smaller than the wavelength of exposure light, and the grid polarizer polarizes light in a direction parallel to the direction of the openings constituting the opening pattern.  
           [0013]    However, when a polarizer is produced for each exposure mask as above, the exposure mask becomes expensive, and thus the cost of fine pattern exposure increases.  
         SUMMARY OF THE INVENTION  
         [0014]    An object of the present invention is to provide a near-field exposure system which can expose a photosensitive material such as a photoresist to near-field light so as to form a fine pattern including thin linear portions, and suppress the cost of the exposure.  
           [0015]    (1) According to the first aspect of the present invention, there is provided a near-field exposure system comprising: a light source which emits exposure light having a predetermined wavelength and being unpolarized; a polarizer plate which linearly polarizes the exposure light; an exposure mask which has a pattern of openings each having a dimension smaller than the wavelength of the exposure light, and is placed at such a position that the exposure light is applied to the exposure mask; an exposure table which holds a photosensitive material sensitive to the exposure light, at a position which near-field light emerging from the openings reaches; and a polarizer-plate holding means which holds the polarizer plate in such a manner that the polarizer plate can be moved between first and second positions, where the first position is in an optical path of the exposure light from the light source to the exposure mask, and the second position is outside the optical path.  
           [0016]    The near-field exposure system according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv).  
           [0017]    (i) The polarizer-plate holding means may hold the polarizer plate in such a manner that the polarizer plate can be rotated and directed to a direction in which polarizer plate linearly polarizes the exposure light.  
           [0018]    (ii) The near-field exposure system the first aspect of the present invention may further comprise an indication means for indicating a direction in which the polarizer plate linearly polarizes the exposure light. For example, the indication means is an arrow indication provided on the polarizer plate.  
           [0019]    (iii) The exposure light which is applied to the exposure mask may have a first component which is linearly polarized in a desired direction and a second component which is linearly polarized in a direction perpendicular to the desired direction, and a ratio of the second component to the first component may be 25% or smaller.  
           [0020]    (iv) In the near-field exposure system having the above feature (iii), the ratio may be 15% or smaller.  
           [0021]    (2) The advantages of the near-field exposure system according to the first aspect of the present invention are explained below.  
           [0022]    The near-field exposure system according to the first aspect of the present invention comprises the polarizer-plate holding means, so that the polarizer plate which linearly polarizes the exposure light can be moved between the first position in the optical path of the exposure light from the light source to the exposure mask and the second position outside the optical path. That is, the polarizer plate can be selectively inserted in the optical path to the exposure mask.  
           [0023]    In the case where the exposure mask has an opening pattern constituted by only lines extending in an identical direction, thickening of lines formed on the photosensitive material can be prevented when the exposure mask is placed in an optical path of the linearly polarized exposure light in such a manner that the direction of the lines constituting the opening pattern is identical to the direction of the linear polarization of the linearly polarized exposure light. That is, a fine pattern constituted by lines having small widths can be formed on the photosensitive material by the exposure when the exposure light is linearly polarized by insertion of the polarizer plate in the optical path, and coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization is achieved.  
           [0024]    The coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization can be achieved, for example, by changing the direction of the exposure table on which the photosensitive material is held, or adjusting the direction of the photosensitive material held on the exposure table. Alternatively, in the case where the polarizer-plate holding means holds the polarizer plate in such a manner that the polarizer plate can be rotated, the direction of the linear polarization can be easily adjusted to the direction of the lines constituting the opening pattern in the exposure mask by rotating the polarizer plate.  
           [0025]    In addition, in the case where the indication means for indicating the direction in which the polarizer plate linearly polarizes the exposure light is provided, the user can visually confirm the direction of the linear polarization in the operation for achieving the coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization. Therefore, the operation for achieving the coincidence between the above directions becomes further easier.  
           [0026]    Further, investigations made by the present inventor have revealed that a pattern formed on a photosensitive material by exposure to linearly polarized exposure light is likely to be deformed to an elliptic shape when the linearly polarized exposure light is applied to the photosensitive material through a photomask having a circular opening pattern. Therefore, in the case where a photomask having a circular opening pattern or the like is used, a circular pattern which is similar to the circular opening pattern in the exposure mask can be accurately formed on a photoresist material by placing the polarizer plate at the aforementioned second position outside the optical path and applying the unpolarized exposure light to the exposure mask.  
           [0027]    (3) According to the second aspect of the present invention, there is provided a near-field exposure system comprising: a light source which emits exposure light having a predetermined wavelength and being linearly polarized; a phase plate which circularly or elliptically polarizes the exposure light; an exposure mask which has a pattern of openings each having a dimension smaller than the wavelength of the exposure light, and is placed at such a position that the exposure light is applied to the exposure mask; an exposure table which holds a photosensitive material sensitive to the exposure light, at a position which near-field light emerging from the openings reaches; and a phase-plate holding means which holds the phase plate in such a manner that the phase plate can be moved between first and second positions, where the first position is in an optical path of the exposure light from the light source to the exposure mask, and the second position is outside the optical path.  
           [0028]    The near-field exposure system according to the second aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii).  
           [0029]    (i) The near-field exposure system according to the second aspect of the present invention may further comprise an indication means for indicating a direction in which the exposure light emitted by the light source is linearly polarized.  
           [0030]    (ii) The exposure light which is applied to the exposure mask may have a first component which is linearly polarized in a desired direction and a second component which is linearly polarized in a direction perpendicular to the desired direction, and the ratio of the second component to the first component may be 25% or smaller.  
           [0031]    (iii) In the near-field exposure system having the above feature (ii), the ratio may be 15% or smaller.  
           [0032]    (4) The advantages of the near-field exposure system according to the second aspect of the present invention are explained below.  
           [0033]    The near-field exposure system as the second aspect of the present invention comprises a phase-plate holding means, so that the phase plate which circularly or elliptically polarizes the linearly polarized exposure light can be moved between the first position in the optical path of the exposure light from the light source to the exposure mask and the second position outside the optical path. That is, the phase plate can be selectively inserted in the optical path of the exposure light to the exposure mask.  
           [0034]    In the case where the exposure mask has an opening pattern constituted by only lines extending in an identical direction, a fine pattern constituted by lines having small widths can be formed on a photosensitive material by placing the phase plate outside the optical path, achieving coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization, and applying the linearly polarized exposure light to the exposure mask.  
           [0035]    The coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization can be achieved, for example, by changing the direction of the exposure table on which the photosensitive material is held, or adjusting the direction of the photosensitive material held on the exposure table.  
           [0036]    In addition, in the case where the indication means for indicating a direction in which the exposure light emitted by the light source is linearly polarized is provided, the user can visually confirm the direction of the linear polarization in the operation for achieving the coincidence between the direction of the lines constituting the opening pattern in the exposure mask and the direction of the linear polarization. Therefore, the operation for achieving the coincidence between the above directions becomes further easier.  
           [0037]    Further, in the case where a photomask having a circular opening pattern is used, a circular pattern which is similar to the circular opening pattern in the exposure mask can be accurately formed on a photoresist material by inserting the phase plate in the optical path of the exposure light to the exposure mask and applying the circularly or elliptically polarized exposure light to the exposure mask.  
           [0038]    (5) The advantages of the near-field exposure systems according to the first and second aspects of the present invention having the additional features indicated in paragraphs (1) (iii), (1) (iv), (3) (ii), and (3)(iii) are explained below.  
           [0039]    When the ratio of the component of the exposure light which reaches the exposure mask and is linearly polarized in a desired direction (i.e., the component linearly polarized in the direction identical to the direction of the lines constituting the exposure mask) to the component of the exposure light which reaches the exposure mask and is linearly polarized in a direction perpendicular to the desired direction is 25% or smaller, thickening of lines formed by exposure (i.e., the ratio of the increase over the mask pattern line width to the mask pattern line width) can be suppressed to 50% or less. When the thickening of lines formed on a photosensitive material based on a mask pattern having a line-and-space ratio of 1:1 is 50% or less, adjacent lines formed on the photosensitive material do not overlap.  
           [0040]    Further, when the ratio of the component of the exposure light which reaches the exposure mask and is linearly polarized in a desired direction to the component of the exposure light which reaches the exposure mask and is linearly polarized in a direction perpendicular to the desired direction is 15% or smaller, the thickening of lines formed by exposure can be suppressed to 30% or less. It is said that the thickening of 30% or less is preferable for obtaining a very fine pattern constituted by lines formed by exposure. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0041]    [0041]FIG. 1 is a side view, partly in cross section, of a near-field exposure system as a first embodiment of the present invention in a first state.  
         [0042]    [0042]FIG. 2 is a side view, partly in cross section, of a near-field exposure system as the first embodiment of the present invention in a second state.  
         [0043]    [0043]FIG. 3 is a plan view of an example of a photomask used in the near-field exposure system of FIG. 1.  
         [0044]    [0044]FIG. 4 is a side view, partly in cross section, of a near-field exposure system as a second embodiment of the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0045]    Embodiments of the present invention are explained in detail below with reference to drawings.  
       Construction of First Embodiment  
       [0046]    [0046]FIG. 1 is a side view, partly in cross section, of a near-field exposure system as the first embodiment of the present invention in a first state. As illustrated in FIG. 1, the near-field exposure system as the first embodiment comprises a substrate  10 , an exposure table  12 , a photomask holding member  13 , a photomask (exposure mask)  14 , and an exposure light source  16 .  
         [0047]    A photoresist  11  is applied to a surface  10   a  of the substrate  10 . The exposure table  12  has a nearly cylindrical shape, and the substrate  10  is held by the exposure table  12  so that the back surface  10   b  of the substrate  10  is in contact with the exposure table  12 . The photomask holding member  13  is arranged to contain the exposure table  12 . The photomask  14  is placed on the photomask holding member  13 . The exposure light source emits exposure light  15 , which is applied to photoresist  11  through the photomask  14 .  
         [0048]    The exposure light  15  emitted by the exposure light source  16  is unpolarized and has a wavelength of about 400 nm or longer. For example, the exposure light is g-line light (having the wavelength of 436 nm).  
         [0049]    The photomask  14  is designed for near-field exposure. That is, the photomask  14  has a pattern of openings  14   a  having widths smaller than the wavelength of the exposure light  15  so that near-field light emerges from the openings  14   a  when the exposure light  15  is applied to the photomask  14 . In particular, the photomask  14  in the first embodiment is designed for forming a plurality of diffraction grating patterns on the photoresist  11 , as illustrated in FIG. 3, which is a plan view of an example of a photomask used in the near-field exposure system of FIG. 1. Specifically, all of the openings  14   a  have a linear shape and extend in the direction of the arrow A indicated in FIG. 3, and the plurality of diffraction grating patterns are each formed with a group of openings. In FIG. 1, the photomask  14  is placed on the photomask holding member so that the arrow A is perpendicular to the plane of the figure.  
         [0050]    The upper surface of the exposure table  12  is a substrate holding surface  12   a , and the substrate  10  is placed in contact with the substrate holding surface  12   a. In addition, a plurality of ventilation paths  12     b  are provided in the center and a plurality of near-edge positions of the exposure table  12  so that each of the plurality of ventilation paths  12   b  passes through the thickness of the exposure table  12  in the vertical direction.  
         [0051]    Further, a sealing member  17  is arranged around the exposure table  12  so as to be interposed between the exposure table  12  and the photomask holding member  13 . That is, the space between the exposure table  12  and the photomask holding member  13  is separated by the sealing member  17  into upper and lower spaces. In addition, the photomask holding member  13  has a ventilation path  13   b  at an elevation above the sealing member  17 , and the ventilation path  13   b  passes through the photomask holding member  13  in the horizontal direction.  
         [0052]    The ventilation paths  12   b  and  13   b  are connected through pipes  18  to an air suction means  19  such as a vacuum pump. The air suction means  19 , the pipes  18 , and the photomask holding member  13  realize a means for bringing the photomask  14  close to the photoresist  11  and pressing the photomask  14  on the photoresist  11 .  
         [0053]    A polarizer disk  20  can be inserted in the optical path of the exposure light  15  from the exposure light source  16  to the photomask  14 . Specifically, the polarizer disk  20  is held by a polarizer-disk holding means  22 , which can move the polarizer disk  20  around a rotation axis  21  and fix the polarizer disk  20  at a position in the optical path of the exposure light  15  as illustrated with solid lines in FIG. 1 or another position outside the optical path as illustrated with dashed lines in FIG. 1.  
         [0054]    In addition, the polarizer disk  20  is held by a polarizer-disk holding means  22  in such a manner that the polarizer disk  20  per se can be rotated around the center axis  20   a  of the polarizer disk  20 . Further, an indication  23  of the direction of the linear polarization of the exposure light which passes through the polarizer disk  20  is provided on a near-edge area of the upper surface of the polarizer disk  20 . For example, the indication  23  is an arrow indication.  
       Operations of First Embodiment  
       [0055]    The operations of the near-field exposure system as the first embodiment of the present invention are explained below.  
         [0056]    The substrate  10  is placed on the exposure table so that the back surface  10   b  of the substrate  10  is in contact with the substrate holding surface  12   a , where the photoresist  11  is applied on the upper surface  10   a  of the substrate  10 . In addition, the photomask  14  which has the aforementioned pattern is placed on the photomask holding member  13 . At this time, the polarizer disk  20  is inserted in the optical path of the exposure light  15 . This situation is illustrated in FIG. 1.  
         [0057]    When the air suction means  19  is activated, air is sucked by the air suction means  19  from the plurality of ventilation paths  12   b  so that vacuum evacuation is achieved in the plurality of ventilation paths  12   b , and the substrate  10  comes in contact with the substrate holding surface  12   a  of the exposure table  12  and is held by the exposure table  12 . At the same time, air is also sucked from the ventilation path  13   b  of the photomask holding member  13  so that vacuum evacuation is also achieved in the space formed inside the photomask holding member  13  between the photomask  14  and the sealing member  17 , and the photomask  14  is warped onto the photoresist  11  applied on the substrate  10 , i.e., the photomask  14  comes in contact with the photoresist  11 , as illustrated in FIG. 2.  
         [0058]    When the exposure light source  16  is activated in the above situation, the exposure light  15  is applied to the photoresist  11  through the photomask  14 , and the photoresist  11  is exposed to near-field light. That is, near-field light emerges from the openings  14   a  having the dimensions smaller than the wavelength of the exposure light  15  (as illustrated in FIG. 3), and fine diffraction grating patterns corresponding to the openings  14   a  are formed on the photoresist  11  by the exposure, i.e., the pattern constituted by the openings  14   a  is transferred to the photoresist  11  by the exposure.  
         [0059]    The photoresist  11  is a positive or negative photoresist. Only portions of the positive photoresist which are exposed to the near-field light become soluble in a developer solution, and only portions of the negative photoresist which are not exposed to the near-field light become soluble in a developer solution. Therefore, when the photoresist  11  undergoes a developing process after the exposure, and etching processing is performed on the substrate  10  by using the developed photoresist  11 , a plurality of diffraction gratings are formed on the substrate  10 .  
         [0060]    At the time of the exposure of the photoresist  11 , the user can manually set the polarizer-disk holding means  22  so that the polarizer disk  20  is inserted in the optical path of the exposure light  15 . Thus, the exposure light  15  applied to the photomask  14  is linearly polarized by the polarizer disk  20 . In addition, the user can set the rotational position of the polarizer disk  20  so that the direction of the linear polarization of the exposure light  15  coincides with the direction of the linearly shaped openings  14   a , for example, by referring to the indication  23 . In this case, the exposure light  15  which is applied to the photomask  14  is linearly polarized in the same direction as the linearly shaped openings  14   a . Therefore, it is possible to prevent thickening of the lines of the patterns formed in the photoresist  11  by the exposure, and form fine diffraction grating patterns having small line widths by the exposure.  
         [0061]    For the reason explained before, the ratio of the component of the exposure light  15  which is applied to the photomask  14  and linearly polarized in the direction perpendicular to the linearly shaped openings  14   a  to the component of the exposure light  15  which is applied to the photomask  14  and linearly polarized in the same direction as the linearly shaped openings  14   a  is preferably 25% or smaller, and more preferably 15% or smaller.  
         [0062]    The above photoresist  11  may be a conventional photoresist comprised of a single layer. Alternatively, the photoresist  11  may be the two-layer photoresist disclosed in the copending, commonly-assigned U.S. patent application Ser. No. 09/562,076 and the corresponding Japanese patent application No. 2000-119670 (which is laid open in Japanese Unexamined Patent Publication No. 2001-15427).  
         [0063]    The patterns which can be formed by the near-field exposure system as the first embodiment of the present invention are not limited to diffraction grating patterns. When an opening pattern is not constituted by only lines extending in an identical direction, e.g., when the opening pattern is a circular pattern or a pattern constituted by lines extending in more than one direction, the polarizer disk  20  is set in the position outside the optical path of the exposure light  15 . That is, the exposure light  15  which is applied to the photomask  14  is unpolarized.  
         [0064]    If linearly polarized exposure light is applied to a photomask having a circular pattern, a pattern formed on the photoresist  11  by the exposure may have an elliptic shape. On the other hand, if linearly polarized exposure light is applied to a photomask having a pattern constituted by lines extending in more than one direction, lines formed on the photoresist  11  may be differently thickened depending on the directions of the lines. Therefore, when the exposure light  15  which is applied to the photoresist  11  is unpolarized, it is possible to prevent occurrence of the above problems, and form a fine pattern by exposure.  
         [0065]    Alternatively, it is possible to move the polarizer-disk holding means  22  by using an actuator, instead of manually moving the polarizer-disk holding means  22 . The actuator may be any type. In addition, it is also possible to rotate the polarizer disk  20  by using an actuator.  
       Second Embodiment  
       [0066]    [0066]FIG. 4 is a side view, partly in cross section, of a near-field exposure system as the second embodiment of the present invention. In FIG. 4, elements having the same functions as the elements in the near-field exposure system illustrated in FIG. 1 bear the same reference numerals as FIG. 1, respectively, and only the differences from the construction of FIG. 1 are explained below.  
         [0067]    The near-field exposure system as the second embodiment is essentially different from the first embodiment in that the exposure light source  36  emits linearly polarized exposure light  35 , and a quarter-wavelength plate  30  (as a phase plate) is provided instead of the polarizer disk  20 . For example, the exposure light  35  may be laser light.  
         [0068]    The quarter-wavelength plate  30  can be inserted in the optical path of the exposure light  35  from the exposure light source  36  to the photomask  14 . Specifically, the quarter-wavelength plate  30  is held by a polarizer-disk holding means  32 , which can move the quarter-wavelength plate  30  around a rotation axis  31  and fix the quarter-wavelength plate  30  at a position in the optical path of the exposure light  35  as illustrated with solid lines in FIG. 4 or another position outside the optical path as illustrated with dashed lines in FIG. 4.  
         [0069]    When the photomask  14  has a pattern constituted by linear openings  14   a  as illustrated in FIG. 3, the user manually sets the polarizer-disk holding means  32  so that the quarter-wavelength plate  30  is outside the optical path of the exposure light  35 . Therefore, the linearly polarized exposure light  35  is applied to the photomask  14 . At this time, the user can adjust the direction of the linearly shaped openings  14   a  in the photomask  14  to the direction of the linear polarization of the exposure light  35 , for example, by adjusting the rotational position of the photomask  14 . Thus, it is possible to prevent thickening of the lines of the patterns formed in the photoresist  11  by the exposure, and form fine diffraction grating patterns having small line widths by the exposure.  
         [0070]    The patterns which can be formed by the near-field exposure system as the second embodiment of the present invention are also not limited to diffraction grating patterns. When an opening pattern is not constituted by only lines extending in an identical direction, e.g., when the opening pattern is a circular pattern or a pattern constituted by lines extending in more than one direction, the quarter-wavelength plate  30  is set in the optical path of the exposure light  35  from the exposure light source  36  to the photomask  14 . That is, the exposure light  35  which is applied to the photomask  14  is circularly polarized.  
         [0071]    As explained before, if linearly polarized exposure light is applied to a photomask having a circular pattern or a pattern constituted by lines extending in more than one direction, a pattern formed on the photoresist  11  by the exposure may have an elliptic shape, or lines formed on the photoresist  11  may be differently thickened depending on the directions of the lines. On the other hand, when the exposure light  35  which is applied to the above photomask is circularly polarized, it is possible to prevent occurrence of the above problems, and form a fine pattern by exposure.  
         [0072]    Alternatively, a phase plate which converts the linearly polarized exposure light  35  into elliptically polarized exposure light can be used instead of the quarter-wavelength plate  30 . The near-field exposure system having such a phase plate has similar advantages to the near-field exposure system having the quarter-wavelength plate  30 .