Abstract:
A fine-structure transfer apparatus has a base plate, a first post and a second post erected on the upper surface of the base plate, an elongated stamper that is fixed at one end to the upper end face of the first post. The stamper is supported at the other end in a vertically movable manner by means of an ascending/descending unit provided on the second post. A transfer element holding stage is provided on the upper surface of the base plate between the first and second posts in a position that corresponds to the position of the lower surface of the elongated stamper where a fine pattern is formed. A pressure-applying unit is provided to reciprocate on the upper surface of the elongated stamper along a longitudinal direction thereof. The transfer apparatus is characterized by the ease with which the stamper can be detached from the transfer element.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a fine-structure transfer or nanoimprint apparatus. More particularly, the present invention relates to a fine-structure transfer or nanoimprint apparatus that not only allows a stamper to be easily attached to and detached from a transfer element but also effectively prevents the occurrence of air bubbles as a transfer defect. 
       BACKGROUND ART 
       [0002]    With remarkable advances in the performance of computers and other information equipment, the volume of information that is handled by users has been constantly increasing and the unit of measurement is now in terabytes rather than gigabytes. Under these circumstances, there exists an ever-growing demand for semiconductor devices such as information storage/reproduce equipment and memories that are capable of recording at even higher densities. 
         [0003]    To achieve higher recording densities, technologies for even finer microfabrication are required. Conventional photolithography which uses the exposure process is capable of microfabrication over a large area in one step; however, since its resolution is not finer than the wavelength of light, conventional photolithography is inevitably unsuitable for creating fine structures smaller than the wavelength of light (say, 100 nm and less). Technologies currently available for processing finer structures than the wavelength of light include exposure using electron beams, exposure using X-rays, and exposure using ion beams. However, pattern formation with an electron beam lithographic apparatus differs from patterning by one-shot exposure using such light sources as i-line and an excimer laser in that the more patterns that need be written with electron beams, the longer the time that is required for writing (exposure). Therefore, as the recording density increases, the time it takes to form a fine-featured pattern is prolonged to cause a marked drop in throughput. With a view to forming patterns at a faster speed by the e-beam lithographic equipment, the development of a method for one-shot irradiation of geometric figures is underway in which combinations of variously shaped masks are subjected to one-shot exposure to electron beams; however, the e-beam lithographic apparatus that uses the method for one-shot irradiation of geometric figures is not only bulky but it also needs an additional mechanism for controlling the positions of masks to an even higher precision; this increases the cost of the lithographic apparatus, eventually leading to a higher cost for manufacturing the media. 
         [0004]    Printing-based approaches have been proposed as an alternative to the conventional exposure technologies for creating fine structures smaller than the wavelength of light. See, for example, US 005772905A which describes an invention relating to the technology of nanoimprint lithography (NIL). The technology of nanoimprint lithography (NIL) is a technique in which a pattern of predetermined fine structures is formed on a mold by exposure to electron beams or using some other methods of creating finer structures than the wavelength of light and the mold is urged under pressure against a resist-coated transfer substrate so that the fine-structured pattern is transferred to the resist coating on the transfer substrate. As long as the mold is available, there is no particular need to employ an expensive exposure unit but an apparatus in the class of ordinary printing presses will suffice to produce replicas in large quantities; hence, in comparison with the conventional methods such as exposure to electron beams, there is achieved a marked improvement in throughput whereas the manufacturing cost is significantly reduced. 
         [0005]    As descried in US 005772905A, when a thermoplastic resin (say, PMMA) is used as a resist material in the technology of nanoimprint lithography (NIL), transfer is performed with the thermoplastic resin being heated under pressure to a temperature near its glass transition temperature (Tg) or higher. This approach is called a thermal transfer process. The thermal transfer process has the advantage of permitting the use of general-purpose, thermoplastic resins. If a photosensitive resin is used as a resist in the NIL technology, a photocurable resin that hardens upon exposure to light such as UV radiation is chosen as the resin to which the original fine-featured pattern is transferred. This approach is called an optical transfer process. 
         [0006]    In the nanoimprint processing technology using the optical transfer process, a special photocurable resin must be used but, on the other hand, compared to the thermal transfer process, the optical transfer process has the advantage of reducing the dimensional errors in finished products due to the thermal expansion of transfer printing plates or printing media. Other advantages that are related to the apparatus include elimination of the need for equipping it with a heating mechanism and providing accessories such as for performing temperature elevation, temperature control, and cooling. There is a further advantage concerning the nanoimprint apparatus taken as a whole and that is elimination of the need for design considerations against thermal distortions, such as heat insulation. 
         [0007]    An example of nanoimprint apparatuses based on the optical transfer process is described in JP 2008-12844A. This apparatus is so designed that a stamper capable of transmitting UV light is urged against a photocurable resin coated transfer substrate and irradiated with UV light from above. The stamper has a predetermined pattern of fine structures formed in the surface that is to be pressed against the transfer substrate. 
         [0008]      FIG. 9  in the accompanying drawings is a schematic diagram showing major steps in a fine-structure transfer method involving the nanoimprint technology based on the optical transfer process. In step (a), a transfer element  100  comprising a substrate  102  coated with a resist  104  on its topside is placed in a face-to-face relationship with a stamper  108  having a fine-featured pattern  106  formed on the side that is to be brought into contact with the resist  104 . In step (b), the stamper  108  is pressed against the resist-coated surface of the transfer element  100 . In step (c), ultraviolet (UV) light is applied to the stamper  108  from above, whereby the resist  104  is hardened. Then, in step (d), the stamper  108  is detached from the transfer element  100 , leaving a patterned layer  110  on a surface of the substrate  102  of the transfer element  100 . The patterned layer  110  is the obverse image of the fine-featured pattern  106 . 
         [0009]    To press the stamper  108  against the transfer element  100 , a press mechanism is used, as indicated by  111  in  FIG. 10 . The conventional press mechanism  111  generally comprises a uniaxial moving stage  112  which is a combination of a linear guide and a servo motor. With the transfer element  100  being fixed on a base  114 , the stamper  108  mounted on the moving stage  112  is moved down so that it is urged against the transfer element  100 . The moving stage  112  is controlled by an XYZ control circuit  116 . Since the positions of the transfer element  100  and the stamper  108  are important for precision in transfer, the moving stage  112  requires precision in both motion and position. Consequently, the press mechanism  111  has assumed a bulky and complicated structure. 
         [0010]    Another problem with the conventional press mechanism  111  is the occurrence of transfer defects due to air bubbles. If the pressing of the stamper  108  in the position shown in step (a) of  FIG. 9  into intimate contact with the resist  104  as shown in step (b) is performed in the atmosphere, air bubbles are trapped in the space between the recesses in the fine-featured pattern  106  on the stamper  108  and the resist  104  and even after the pressing action is over, those air bubbles will stay unremoved, causing the external shape of the resist  104  to be fixed as irregularly deformed. This makes it difficult or even impossible to ensure that the shape of the fine-featured pattern  106  is transferred correctly. 
         [0011]    Still another problem with the conventional press mechanism  111  is the difficulty involved in detaching the stamper  108  from the transfer element  100 . After the stamper  108  has been pressed into contact with the transfer element  100  and pattern transfer effected as shown in step (c) of  FIG. 9 , the stamper  108  has to be detached from the transfer element  100  in subsequent step (d). If the stamper  108  and the transfer element  100  are securely adhered to each other via the hardened resist  104 , it is extremely difficult to detach the stamper  108  from the transfer element  100 . A wedge or the like may be inserted from the outside to form a gap between the transfer element  100  and the stamper  108  so that the stamper  108  can be gradually detached from the transfer element  100 ; however, this method takes so much time and labor that the operating efficiency drops considerably. In addition, when a wedge or the like is being driven into the interface between the transfer element  100  and the stamper  108  that are in intimate contact with each other, the transfer element  100  and/or the stamper  108  might be damaged. 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0012]    An object, therefore, of the present invention is to provide a fine-structure transfer apparatus that does not use anything like the conventional press mechanism and which is not only free from the occurrence of transfer defects due to air bubbles but also characterized by ease with which the stamper can be detached from the transfer element. 
         [0013]    Another object of the present invention is to provide a two-side fine-structure transfer apparatus that does not use anything like the conventional press mechanism and which is not only free from the occurrence of transfer defects due to air bubbles but also characterized by ease with which the stamper can be detached from the transfer element. 
       Solution to the Problem 
       [0014]    The first object of the present invention can be attained by a fine-structure transfer apparatus comprising a base plate, a first post and a second post erected on the upper surface of the base plate, an elongated stamper that is fixed at one end to the upper end face of the first post and which is supported at the other end in a vertically movable fashion by means of an ascending/descending unit provided on the second post, a transfer element holding stage provided on the upper surface of the base plate between the first and second posts in a position that corresponds to that position of the lower surface of the elongated stamper where a fine pattern is formed, and a pressure-applying unit provided to reciprocate on the upper surface of the elongated stamper along a longitudinal direction of the elongated stamper. 
         [0015]    In this fine-structure transfer apparatus, the ascending/descending unit may be a pivoting arm that pivots through a predetermined angle about a mounting shaft provided on the second post, the free end of the pivoting arm being in engagement with the other end of the elongated stamper, whereby the elongated stamper is supported in a vertically movable fashion. 
         [0016]    In the fine-structure transfer apparatus, the ascending/descending unit may be a horizontal arm supported by a drive mechanism provided on the second post to move up and down in a vertical direction, the free end of the horizontal arm being in engagement with the other end of the elongated stamper, whereby the elongated stamper is supported in a vertically movable fashion. 
         [0017]    In the fine-structure transfer apparatus, the pressure-applying unit may consist of one or more pressure-applying rollers that rotate to move in contact with the upper surface of the elongated stamper. 
         [0018]    In the fine-structure transfer apparatus, the pressure-applying unit may be an air pad that moves along, while making no contact with, the upper surface of the elongated stamper as it applies high-pressure air to the upper surface of the elongated stamper. 
         [0019]    In the fine-structure transfer apparatus, an engagement finger for engaging the free end of the pivoting arm may be fastened to the upper surface of the other end of the elongated stamper and provided in an end portion with an engagement hook for preventing the free end of the pivoting arm from dropping out. 
         [0020]    The second object of the present invention can be attained by a two-side fine-structure transfer apparatus for transferring a fine structure to both sides of a transfer element or an annular disk of substrate having a through-hole of a predetermined inner diameter in the center, which apparatus comprises a lower fine-structure transfer device and an upper fine-structure transfer device, wherein: 
         [0021]    the lower fine-structure transfer device comprises a base plate, a first post and a second post erected to face up on the upper surface of the base plate, a lower elongated stamper that is fixed at one end to the upper end face of the first post and which is supported at the other end in a vertically movable fashion by means of an ascending and descending unit provided on the second post, a transfer element holding cylindrical stage provided on the upper surface of the base plate between the first and second posts in a position that corresponds to that position of the upper surface of the lower elongated stamper where a fine-featured pattern is formed, a lower pressure-applying unit provided on the upper surface of the base plate for pressing the lower elongated stamper against the lower surface of the transfer element from below the lower elongated stamper, and a lower UV light irradiating unit provided on the lower surface of the base plate, the transfer element holding cylindrical stage having a cylindrical projection of a predetermined outer diameter provided on the upper end face, a through-hole of a predetermined inner diameter being opened in the center of the fine-featured pattern on the upper side of the lower elongated stamper, and the transfer element holding cylindrical stage being passed through the through-hole; and 
         [0022]    the upper fine-structure transfer device comprises a base plate, a first post and a second post erected to face down on the lower surface of the base plate, an upper elongated stamper that is fixed at one end to the upper end face of the first post and which is supported at the other end in a vertically movable fashion by means of an ascending/descending unit provided on the second post, an upper pressure-applying unit provided on the lower surface of the base plate for pressing the upper elongated stamper against the upper surface of the transfer element from above the upper elongated stamper, and an upper UV light irradiating unit provided on the upper surface of the base plate, a through-hole of a predetermined inner diameter being opened in the center of the fine-featured pattern on the lower side of the upper elongated stamper such that the cylindrical projection on the upper end face of the transfer element holding cylindrical stage can be passed through the through-hole; and wherein the lower fine-structure transfer device and the upper fine-structure transfer device are positioned in symmetry with respect to the center point of the transfer element holding cylindrical stage. 
         [0023]    In this two-side fine-structure transfer apparatus, each of the ascending/descending units may be a pivoting arm that pivots through a predetermined angle about a mounting shaft provided on the associated second post, the free end of the pivoting arm being in engagement with the other end of the associated elongated stamper, whereby the associated elongated stamper is supported in a vertically movable fashion. 
         [0024]    In the two-side fine-structure transfer apparatus, each of the ascending/descending units may be a horizontal arm supported by a drive mechanism provided on the associated second post to move up and down in a vertical direction, the free end of the horizontal arm being in engagement with the other end of the associated elongated stamper, whereby the associated elongated stamper is supported in a vertically movable fashion. 
         [0025]    In the fine-structure transfer apparatuses described above, each of the lower and upper elongated stampers may have flexibility. 
       ADVANTAGEOUS EFFECTS OF THE INVENTION 
       [0026]    In the fine-structure transfer apparatus according to the first aspect of the present invention, a flexible stamper is rendered to have an elongated structure and by fixing one of its end while designing the other end to be vertically movable, the stamper can be deformed into a curved shape so that the entire surface of the transfer element is gradually covered with the curved stamper, starting from its outer periphery and directed radially towards its center. As a result, the air present at the interface between the stamper and the transfer element will be pushed to the outside of the interface as the transfer element is being covered with the stamper, ensuring that no air will be left at the interface. This substantially eliminates transfer defects that would otherwise occur on account of air bubbles in the conventional fine-structure transfer apparatus. When the transfer step is over, the stamper is gradually detached from the transfer element, starting from its outer periphery and directed radially towards its center, thus eliminating the heretofore required step of driving a wedge or the like into the interface between the stamper and the transfer element. As a result, the detaching operation will proceed rapidly enough to achieve a marked improvement in efficiency; what is more, little damage will be caused to the transferred pattern, as well as to the stamper and the transfer element. 
         [0027]    In the two-side fine-structure transfer apparatus according to the second aspect of the present invention, by providing an upper and a lower transfer device each using a flexible stamper rendered to have an elongated structure and having one end of the associated flexible stamper fixed while designing the other end to be vertically movable, a fine-featured pattern can be transferred to both surfaces of the transfer element in a very easy and efficient manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1(   a ) is a front view showing an outline of an embodiment of the fine-structure transfer apparatus according to the first aspect of the present invention, before pressure is applied to a stamper. 
           [0029]      FIG. 1(   b ) is a front view showing the same, with pressure applied to the stamper. 
           [0030]      FIG. 2  is a plan view of the fine-structure transfer apparatus shown in  FIG. 1 . 
           [0031]      FIG. 3  is a front view showing an outline of another embodiment of the fine-structure transfer apparatus according to the first aspect of the present invention. 
           [0032]      FIG. 4  is a front view showing an outline of yet another embodiment of the fine-structure transfer apparatus according to the first aspect of the present invention. 
           [0033]      FIG. 5(   a ) is a front view showing an outline of still another embodiment of the fine-structure transfer apparatus according to the first aspect of the present invention, before pressure is applied to a stamper. 
           [0034]      FIG. 5(   b ) is a front view showing the same, with pressure applied to the stamper. 
           [0035]      FIG. 6  is a front view showing an outline of an embodiment of the two-side fine-structure transfer apparatus according to the second aspect of the present invention. 
           [0036]      FIG. 7  is a perspective view showing an outline of a lower fine-structure transfer device indicated by  30  in the two-side fine-structure transfer apparatus indicated by  1 D in  FIG. 6 . 
           [0037]      FIGS. 8  ( a )-(f) show a sequence of essential steps in the process of transferring a fine-featured structure to both surfaces of a doughnut-shaped transfer element indicated by  100 ′ in  FIG. 6  by means of the two-side fine-structure transfer apparatus  1 D. 
           [0038]      FIGS. 9  ( a )-(d) is a schematic diagram showing major steps in a fine-structure transfer method involving the nanoimprint technology based on the optical transfer process. 
           [0039]      FIG. 10  shows an outline of a press mechanism conventionally used to press the stamper of  FIG. 9  against a transfer element. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0040]      FIG. 1(   a ) is a front view showing an outline of an embodiment of the fine-structure transfer apparatus according to the first aspect of the present invention, before pressure is applied to a stamper.  FIG. 1(   b ) is a front view showing the same, with pressure applied to the stamper.  FIG. 2  is a plan view of the fine-structure transfer apparatus shown in  FIG. 1 . The fine-structure transfer apparatus of the present invention which is generally indicated by  1  has the following basic components: a base plate  3 , a stage  5  placed on top of the base plate  3  for holding a transfer element  100  in position, and a stamper  7  provided over the stage  5 . The stamper  7  is elongated in shape; one of its ends is fixed to a first post  9  and the other end can be lifted upward like a cantilevered spring by means of a pivoting arm  11  that reciprocates in a vertical direction. The pivoting arm  11  is mounted on a second post  17  in such a way that by means of a suitable cam mechanism not shown, it can pivot through a predetermined angle about the mounting shaft. The other end of the stamper  7  has an engagement finger  19  fastened thereto for establishing engagement with the pivoting arm  11  and the free end  21  of the pivoting arm  11  is capable of sliding on the bottom surface of the engagement finger  19 . An end portion of the engagement finger  19  is preferably provided with an engagement hook  22  for preventing the free end  21  of the pivoting arm  11  from dropping out. A pressure-applying roller  13  is provided above the stamper  7 . The pressure-applying roller  13  is axially supported by a drive arm  15 , which creates a depressing force for the roller  13  as it reciprocates on the upper surface of the stamper  7 . As shown in  FIG. 1(   a ), the pressure-applying roller  13  is retracted to the farthest end close to the first post  9  and the pivoting arm  11  is caused to pivot to the higher end of its pivoting action so that the stamper  7  is curved upward to provide a wide enough space to allow the transfer element  100  to be attached to or detached from the upper surface of the stage  5 . When the transfer element  100  has been placed on top of the stage  5 , the pressure-applying roller  13  starts to move towards the second post  17 . With the progress of the pressure-applying roller  13 , the pivoting arm  11  is caused to descend slowly, whereupon in proportion as the scanning with the pressure-applying roller  13  progresses, the stamper  7  descends as it is gradually pressed against the transfer element  100 , starting from its outer periphery and directed radially towards its center. As shown in  FIG. 1(   b ), at the point in time when the scanning with the pressure-applying roller  13  ends, the pivoting arm  11  reaches the lower end of its pivoting action. The lower end of the pivoting action of the arm  11  is preferably equal to slightly lower than the height of the stage  5 . The pressure-applying roller  13  is stopped by the engagement finger  19 . When the transfer step is over, the pressure-applying roller  13  is moved backwards to the first post  9  as the pivoting arm  11  is caused to pivot to the higher end of its pivoting action until it resumes the position depicted in  FIG. 1(   a ); then, the transfer element  100  to which the fine-featured pattern has been transferred is removed from the stage  5  and a new transfer element  100  is placed on top of the stage  5 . In the case of a thermal transfer process, sufficient heat is applied from the pressure-applying roller  13  or the stage  5  to effect transfer. In the case of an optical transfer process, the transfer element  100  is irradiated with UV light from above the stamper  7  to effect transfer. Note that scanning with the pressure-applying roller  13  may be performed independently of the pivoting action of the arm  11 . For instance, scanning under pressure with the pressure-applying roller  13  may be started after the arm  11  has descended to the lower end of its pivoting action to bring the stamper  7  into intimate contact with the upper surface of the transfer element  100 . 
         [0041]    In the fine-structure transfer apparatus according to the first aspect of the present invention, the entire surface of the transfer element  100  is gradually covered with the stamper  7 , starting from its outer periphery and directed radially towards its center. As a result, the air present at the interface between the stamper  7  and the transfer element  100  will be pushed to the outside of the interface as the transfer element  100  is being covered with the stamper  7 , ensuring that no air will be left at the interface. This substantially eliminates transfer defects that would otherwise occur on account of air bubbles in the conventional fine-structure transfer apparatus. When the transfer step is over, the stamper  7  is gradually detached from the transfer element  100 , starting from its outer periphery and directed radially towards its center, thus eliminating the heretofore required step of driving a wedge or the like into the interface between the stamper  7  and the transfer element  100 . As a result, the detaching operation will proceed rapidly enough to achieve a marked improvement in efficiency; what is more, little damage will be caused to the transferred pattern, as well as to the stamper  7  and the transfer element  100 . 
         [0042]    The stamper  7  preferably has sufficient flexibility or elasticity that it can be not only be deformed into a curved shape but also restored to the initial flat state by means of the pivoting arm  11 . If the stamper  7  is unduly rigid, it might either bend to break or deform permanently under stress while it is curved by means of the pivoting arm  11 , with the result that it can no longer be restored to the initial flat shape. Hence, the stamper  7  is preferably formed of plastics (e.g. polyvinyl chloride, polyethylene, polypropylene, polystyrene, polyethylene terephthalate, and polylactic acid), metals (e.g. aluminum, stainless steel, and copper), or ceramics (e.g. silicon- or glass-based). In the case of a thermal transfer process, the stamper  7  need not be transparent to light but in the case of an optical transfer process, the stamper  7  must be formed of a light-transmitting material. 
         [0043]    A fine-featured pattern  8  to be transferred to the transfer element  100  is present on the underside of the stamper  7  in the area that is to be brought into contact with the transfer element  100 . The fine-featured pattern  8  may be directly formed in the specified area of the stamper  7 ; alternatively, a sheet having a separately formed fine-structured pattern may be glued to the specified area of the stamper  7 . 
         [0044]      FIG. 3(   a ) and  FIG. 3(   b ) are front views showing an outline of another embodiment of the fine-structure transfer apparatus according to the first aspect of the present invention. In the apparatus shown, the means of vertically moving the stamper  7  is not the pivoting arm but a horizontal arm  23  that vertically moves up and down. The free end  21  of the horizontal arm  23  is capable of sliding on the bottom surface of the engagement finger  19 . The horizontal arm  23  may be driven by a known, commonly used drive means or mechanism such as a chain, water pressure, oil pressure, a ball screw, or a linear motor. 
         [0045]    The fine-structure transfer apparatus shown in  FIGS. 1 and 3  uses only one unit of pressure-applying roller  13  but the number of pressure-applying rollers to be used is by no means limited to one. For instance, two or more units of pressure-applying roller  13  may be arranged in parallel as shown in  FIG. 4 . If more than one unit of pressure-applying roller  13  is used in this way, a sufficiently uniform depressing force can be obtained during transfer to produce a transferred pattern of enhanced quality. 
         [0046]      FIG. 5(   a ) and  FIG. 5(   b ) are front views showing an outline of still another embodiment of the fine-structure transfer apparatus according to the first aspect of the present invention. The fine-structure transfer apparatus shown in  FIG. 1  uses a single unit of pressure-applying roller  13  and the depressing force that is exerted on a given area of the stamper  7  becomes virtually zero after the roller has passed that area during scanning. The fine-structure transfer apparatus shown in  FIG. 4  uses more than one unit of pressure-applying roller  13  but the depressing force that is exerted on the valley between adjacent rollers is again zero. To solve the problem of nonuniformity in the depressing force that is exerted by roller scanning, the apparatus shown in  FIG. 5  uses an air pad  25  instead of the rollers. The air pad  25  is supported in such a way that it is capable of moving above the stamper  7  keeping a certain distance from the upper surface of the transfer element  100 . The air pad  25  is also capable of forcing high-pressure air against the upper surface of the stamper  7 . High-pressure air is supplied through a pipe  27 . By holding the pipe  27  with an advancing/retracting mechanism not shown, the air pad  25  can be rendered to be capable of reciprocating between the first post  9  and the second post  17 . 
         [0047]    As shown in  FIG. 5(   a ), the air pad  25  is retracted to the farthest end close to the first post  9  and the pivoting arm  11  is caused to pivot to the higher end of its pivoting action so that the stamper  7  is curved upward to provide a wide enough space to allow the transfer element  100  to be attached to or detached from the upper surface of the stage  5 . When the transfer element  100  has been placed on top of the stage  5 , the air pad  25  starts to move towards the second post  17 . In synchronism with the movement of the air pad  25 , the pivoting arm  11  is caused to descend slowly, whereupon in proportion as the air pad  25  advances, the stamper  7  descends as it is gradually pressed against the transfer element  100 , starting from its outer periphery and directed radially towards its center. 
         [0048]    As shown in  FIG. 5(   b ), at the point in time when the air pad  25  stops advancing, the pivoting arm  11  reaches the lower end of its pivoting action. The air pad  25  supplies high-pressure air to ensure that a sufficiently uniform pressure to produce a depressing force is applied to the entire surface of the transfer element  100  from above the stamper  7 . Another advantage of the air pad  25  is that with it being maintained immediately above the transfer element  100 , the depressing force can be applied from above the stamper  7  and retained on the entire surface of the transfer element  100  for any length of time. This ensures that the fine structure on the stamper  7  can be transferred to the transfer element  100  without suffering any defects. 
         [0049]    When the transfer step is over, the air pad  25  is moved backwards to the first post  9  as the pivoting arm  11  is caused to pivot to the higher end of its pivoting action. In the case of a thermal transfer process, air heated to a predetermined temperature is forced from the air pad  25  or, alternatively, sufficient heat is applied from the stage  5  to effect transfer. In the case of an optical transfer process, the transfer element  100  is irradiated with UV light from above the air pad  25  that is made of a transparent material (e.g. quartz) and the stamper  7 , also made of a transparent material, to effect transfer. Note that scanning with the air pad  25  may be performed independently of the pivoting action of the arm  11 . For instance, scanning under pressure with the air pad  25  may be started after the arm  11  has descended to the lower end of its pivoting action to bring the stamper  7  into intimate contact with the upper surface of the transfer element  100 . 
         [0050]    The transfer element that may be used in a transfer process by means of the fine-structure transfer apparatus according to the first aspect of the present invention is not limited to a completely round substrate but encompasses an annular (doughnut-shaped) disk of substrate having a circular through-hole of a predetermined inner diameter at the center. Needless to say, the upper surface of the stage  5  and the stamper  7  may be changed in shape according to the shape of the transfer element to be used. 
         [0051]    Each of the fine-structure transfer apparatuses depicted in  FIGS. 1 to 5  is only capable of performing a transfer process on one side of the transfer element  100 . If two-side transfer is to be performed, the conventional parallel-plate press mechanism involves difficulty achieving simultaneous transfer to both surfaces of the transfer element. If two-side simultaneous transfer need be performed with this mechanism, the backside of the transfer element must be kept out of contact with any extraneous matter before the actual transfer step is started. However, if, in order to ensure this, the transfer element is gripped only by the outer periphery, it might deflect during the application of a depressing force, which might then fail to be transmitted properly, causing poor transfer. 
         [0052]    To solve this problem, the present invention provides a two-side fine-structure transfer apparatus that performs a transfer process simultaneously on both sides of the transfer element  100 .  FIG. 6  is a front view showing an outline of an embodiment of the two-side fine-structure transfer apparatus according to the second aspect of the present invention, which is indicated by  1 D and intended for simultaneous transfer of a fine structure to both sides of a transfer element  100 ′ which is a doughnut-shaped disk of substrate. The two-side fine-structure transfer apparatus  1 D comprises a lower fine-structure transfer device  30  and an upper fine-structure transfer device  32 . Although not shown, a suitable retainer is used to hold the two fine-structure transfer devices  30  and  32  in such a way that they are spaced apart by a predetermined distance to face each other. 
         [0053]    The lower fine-structure transfer device  30  has a lower base plate  3   a  with a stage  5 ′ provided on its upper surface. The stage  5 ′ has a circular projection  33  of a predetermined height on its upper surface; the projection  33  is passed through a circular through-hole of a predetermined inner diameter bored at the center of the transfer element  100 ′ which is a doughnut-shaped disk of substrate. The lower base plate  3   a  is also provided with a first post  9   a  and a second post  17   a ; to the first post  9   a , one end of a lower stamper  7   a  is fixed, and the second post  17   a  is provided with a lower pivoting arm  11   a . The other end of the lower stamper  7   a  has an engagement finger  19   a  fastened thereto for establishing engagement with the lower pivoting arm  11   a  and the free end  21   a  of the lower pivoting arm  11   a  is capable of sliding on the surface of the engagement finger  19   a . An end portion of the engaging finger  19   a  is preferably provided with an engagement hook  22   a  for preventing the free end  21   a  of the lower pivoting arm  11   a  from dropping out. A first fine-featured pattern  8   a  is formed on the transfer surface of the lower stamper  7   a . The apparatus  1 D does not use the pressure-applying roller  13  or aid pad  25  but uses a lower depressing unit  34   a  and an upper depressing unit  34   b , each being capable of advancing or retracting vertically. The lower depressing unit  34   a  is provided on the topside of the lower base plate  3   a . It has two lower shafts  36   a  passing through the lower base plate  3   a , a lower depressing plate  38   a  to which the lower shafts  36   a  are fixed, and a lower resin cushion layer  40   a  provided on the lower depressing plate  38   a . The lower resin cushion layer  49   a  works like a cushion and serves to ensure that pressure application from the lower depressing plate  38   a  to the lower stamper  7   a  will be uniform or have a pressure gradient. The bottom side of the lower base plate  3   a  is provided with a lower UV light irradiating unit  42   a.    
         [0054]    The upper fine-structure transfer device  32  has generally the same configuration as the lower fine-structure transfer unit  30 , except that it does not have the stage  5 ′. To be more specific, it has an upper base plate  3   b  that is provided with a first post  9   b  and a second post  17   b ; to the first post  9   b , one end of an upper stamper  7   b  is fixed, and the second post  17   b  is provided with an upper pivoting arm  11   b . The other end of the upper stamper  7   b  has an engagement finger  19   b  fastened thereto for establishing engagement with the upper pivoting arm  11   b  and the free end  21   b  of the upper pivoting arm  11   b  is capable of sliding on the surface of the engagement finger  19   b . A second fine-featured pattern  8   b  is formed on the transfer surface of the upper stamper  7   b . The upper depressing unit  34   b  is provided on the bottom side of the upper base plate  3   b . It has two upper shafts  36   b  passing through the upper base plate  3   b , an upper depressing plate  38   b  to which the upper shafts  36   b  are fixed, and an upper resin cushion layer  40   b  provided on the upper depressing plate  38   b . The topside of the upper base plate  3   b  is provided with an upper UV light irradiating unit  42   b . The major difference between the upper fine-structure transfer device  32  and the lower fine-structure transfer device  30  is that the first post  9   b  and the second post  17   b  in the former are positioned on the side away from the first post  9   a  and the second post  17   b  in the latter. The first fine-featured pattern  8   a  on the lower stamper  7   a  may be the same as or different from the second fine-featured pattern  8   b  on the upper stamper  7   b.    
         [0055]    To ensure simultaneous transfer of a fine structure to both surfaces of a transfer element, the two-side fine-structure transfer apparatus according to the second aspect of the present invention is preferably adapted to be such that the lower fine-structure transfer device  30  and the upper fine-structure transfer device  32  are positioned in symmetry with respect to the center point of the stage  5 ′. The advantage of this positional relationship is that when the upper stamper  7   b  and the lower stamper  7   a  are curved with respect to the transfer element  100 ′ by means of the respective associated pivoting arms, they will be curved in opposite directions with respect to the transfer element  100 ′, securing a sufficient space for accommodating a handling mechanism  50  (see  FIG. 8 ) that may be used to attach the transfer element  100 ′ to the stage  5 ′ or detach it from the latter. 
         [0056]      FIG. 7  is a perspective view showing an outline of the lower fine-structure transfer device  30 . A through-hole of a predetermined inner diameter that is sufficient to allow passage of the stage  5 ′ fixed to the lower base plate  3   a  is formed in generally the center of the lower depressing plate  38   a  and the lower resin cushion layer  40   a . A hole through which the stage  5 ′ can pass is also opened in the lower stamper  7   a , and the first fine-featured pattern  8   a  is provided around this hole. The upper fine-structure transfer device  32  has generally the same configuration as the lower fine-structure transfer device  30  depicted in  FIG. 7 . 
         [0057]      FIG. 8  shows a sequence of essential steps in the process of transferring a fine structure to both surfaces of the doughnut-shaped transfer element  100 ′ by means of the two-side fine-structure transfer apparatus  1 D depicted in  FIG. 6 . 
         [0058]    In step (a), a pair of handling arms  50  are operated such that the projection  33  on top of the stage  5 ′ is passed through the through-hole in the transfer element  100 ′ which is a doughnut-shaped disk of substrate having a resist coat on both the upper and lower surfaces. 
         [0059]    In step (b), the upper pivoting arm  11   b  in the upper fine-structure transfer device  32  is allowed to pivot downward until the upper stamper  7   b  is brought into intimate contact with the upper surface of the transfer element  100 ′; at the same time, the lower pivoting arm  11   a  in the lower fine-structure transfer device  30  is allowed to pivot upward until the lower stamper  7   a  is brought into intimate contact with the lower surface of the transfer element  100 ′. The process of making intimate contact between the stamper and the transfer element proceeds in two opposite directions, first starting from the outer periphery of the transfer element and directed radially toward the center, and then vice versa. As a result, any residual air that may be present at the interface between the stamper and the transfer element can be pushed outside to establish intimate contact between the two parts. 
         [0060]    In step (c), the lower shafts  36   a  in the lower depressing unit  34   a  of the lower fine-structure transfer device  30  are lifted so that the lower resin cushion layer  40   a  provided on the lower depressing plate  38   a  is depressed towards the bottom of the lower stamper  7   a ; at the same time, the upper shafts  36   b  in the upper depressing unit  34   b  of the upper fine-structure transfer device  32  are lowered so that the upper resin cushion layer  40   b  provided on the upper depressing plate  38   b  is depressed towards the top of the upper stamper  7   b . In conjunction with these actions, the upper UV light irradiating unit  42   a  and the lower UV light irradiating unit  42   b  apply UV light to harden the resist coat. Since the lower depressing unit  34   a  and the upper depressing unit  34   b  push each other, the first fine-featured pattern  8   a  on the lower stamper  7   a  is transferred to the lower surface of the transfer element  100 ′ and the second fine-featured pattern  8   b  on the upper stamper  7   b  to its upper surface. 
         [0061]    When the transfer step is over, the process goes to step (d), in which the upper pivoting arm  11   b  in the upper fine-structure transfer device  32  is caused to pivot upward so that the upper stamper  7   b  is detached from the upper surface of the transfer element  100 ′. Since the lower stamper  7   a  is in intimate contact with the lower surface of the transfer element  100 ′, the upper stamper  7   b  can be detached from the upper surface of the transfer element  100 ′ with no problem. 
         [0062]    Then, in step (e), with the outer periphery of the transfer element  100 ′ being held in position by one of the two handling arms  50 , the lower pivoting arm  11   a  in the lower fine-structure transfer device  30  is caused to pivot downward so that the lower stamper  7   a  is detached from the lower surface of the transfer element  100 ′. The process of detaching the stampers proceeds in two opposite directions, first starting from the outer periphery of the transfer element and directed radially toward the center, and then vice versa. As a result, the stampers can be easily detached from the transfer element without damaging the transferred patterns. 
         [0063]    Lastly, in t step (f), the transfer element  100 ′ to which the fine patterns have been transferred is removed from the stage  5 ′ and a new transfer element  100 ′ mounted on the stage  5 ′ by manipulation with the pair of handling arms  50 . 
         [0064]    Since the two-side fine-structure transfer apparatus  1 D depicted in  FIG. 6  is operated by the optical transfer process, the base plate  3   a , depressing plate  38   a , resin cushion layer  40   a  and stamper  7   a  in the lower fine-structure transfer device  30 , as well as the base plate  3   b , depressing plate  38   b , resin cushion layer  40   b  and stamper  7   b  in the upper fine-structure transfer device  32  must all be formed of a light-transmitting material. 
       INDUSTRIAL APPLICABILITY 
       [0065]    While the fine-structure transfer apparatus of the present invention has been described on the foregoing pages with reference to the preferred embodiments, it should be understood that the present invention is by no means limited to those embodiments but may be modified in various ways. For example, the longitudinal dimension of the first post  9  may be sufficiently increased to ensure that even when more than one roller  13  or an air pad  25  of an increased longitudinal dimension is used, the stamper  7  will not curve irregularly in areas near the point where it is fixed. 
         [0066]    In another modified version, the stages  5  and  5 ′ may be provided with an XY movement control mechanism in order to ensure positional alignment between the fine-featured pattern on the stamper and the transfer element.