Abstract:
A device for transferring a pattern, especially micro- or nanostructures, from a stamp ( 1 ) to an object ( 2 ) comprises a first contacting means ( 11 ) with a receiving surface ( 13 ) for the stamp ( 1 ) and a second contacting means ( 20 ) with a receiving surface ( 23 ) for the object ( 2 ). A pressing means ( 10 ) is adapted to operate at least one of the contacting means ( 11, 20 ) for contacting the stamp ( 1 ) with the object ( 2 ). One contacting means ( 11, 20 ) comprises a base ( 21 ) and a holder ( 22 ). The holder ( 22 ) has a first end which defines one of the receiving surfaces ( 23 ) and a second opposite end which is pivotally connected to the base ( 21 ) in such a manner that the stamp ( 1 ) and the object ( 2 ) are automatically placed in a mutually parallel position when contacting each other.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to transferring a pattern from a stamp to an object. The invention relates especially to production of micro- and nanostructures.  
         BACKGROUND ART  
         [0002]    A promising technique of producing nanostructures, i.e. structures in the order of 100 nm and smaller, is so-called nanoimprint lithography. This technique is described in the document U.S. Pat. No. 5,772,905, which is incorporated herewith by reference. In such lithography, the main steps of which are schematically shown in FIGS. 1 a - d , a pattern of nanostructures is transferred from a stamp  1  to an object  2 . The object  2  consists of a substrate  2   a  and, applied thereto, a film  2   b  of a polymer material (resist). After heating of the film  2   b  to a suitable temperature, the stamp  1  is pressed into the same (FIG. 1 b ). The stamp  1  is then released from the object  2  when recesses  3  of a desired depth have been formed in the layer  2   b  (FIG. 1 c ). Subsequently any remaining film in the recesses  3  is removed, for instance by etching, thereby exposing the substrate  2   a . In subsequent process steps (not shown), the pattern in the film is reproduced in the substrate or in some other material which is applied to the substrate.  
           [0003]    A device according to the above-mentioned U.S. patent for carrying out the above lithographic process comprises a first contacting means with a receiving surface for the stamp, a second contacting means with a receiving surface for the object, and a pressing means for contacting or joining the first and second receiving surfaces with each other.  
           [0004]    The film applied to the substrate is very thin, typically 50-200 nm. For even structuring of the object, the stamp and the object must thus be mutually parallel with an accuracy of a few nanometres. In industrial applications, the object can have a diameter of about 15-30 cm, which means that the surfaces contacting each other can have a maximum angle of inclination of about 10 −7  rad. A greater inclination between the stamp and the object can, in addition to uneven structuring of the object, also result in the latter being smashed. In fact the substrate is usually made of a brittle material, e.g. Si/SiO 2 , GaAs or InP, and the pressure exerted upon the substrate during-contacting is high, typically 4-10 MPa.  
           [0005]    A conceivable solution to the above problems would be to fix the receiving surfaces of the device in a mutually fully parallel relationship once and for all. This requires, however, that all objects have perfect plane-parallel flat sides. For reasons of manufacture, this is not possible, and therefore the device must be adjusted for each individual object.  
           [0006]    According to a previously suggested solution to this problem a plurality of power or pressure sensors are mounted in the receiving surface of the stamp or the object. A control unit is adapted to actively control the mutual angular position of the contacting means based on the thus measured pressure distribution. However, this is an expensive and complicated solution, which is also difficult to scale up for structuring of large objects.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the invention is to wholly or partly overcome the above problems of prior art. More specifically, it is an object to provide a simple device for transferring a pattern from a stamp to an object with a high degree of mutual parallelism between the stamp and the object.  
           [0008]    It is also an object of the invention to provide such a device that allows contacting of the stamp with the object under high pressure.  
           [0009]    It is a specific object of the invention to provide a device which is suited for transferring micro- or nano-structures to the object.  
           [0010]    These and other objects that will appear from the following description are now achieved by means of a device according to claim 1. Preferred embodiments are defined in the subclaims.  
           [0011]    In the inventive device, the receiving surfaces, and the stamp and the object which are received thereon, can be put together in a non-parallel position. When the stamp is contacted with the object, they will automatically be brought into a mutually parallel position by the force acting between them pivoting one receiving surface relative to the other receiving surface. The device is of a simple design and can be formed to allow contacting of the stamp with the object under high pressure. The device can also easily be scaled up for structuring of objects having a greater diameter or thickness.  
           [0012]    According to a preferred embodiment, the holder is pivotally connected to the body via coacting male and female portions, which between them define a compartment for receiving a fluid, preferably a hydraulic fluid. Such a device allows soft and gradual contacting of the stamp with the object owing to the fact that the fluid is able to impart a certain amount of inertia to the pivoting motion of the holder. The fluid in the compartment between the male portion and the female portion can also absorb sudden changes in pressure that may arise during contacting, which also promotes soft and gradual contacting. A further advantage of this embodiment resides in the possibility of detecting via a pressure sensor the pressure established in the chamber, which can be converted into an almost exact value of the actual contact pressure between the stamp and the object. Based on the pressure detected by the pressure sensor, the pressing means can thus be caused to produce a well-defined pressure between the stamp and the object.  
           [0013]    According to a further preferred embodiment, the male portion is inserted into the female portion in such manner that the compartment encompasses the male portion arranged in the female portion. As a result, the mechanical contact between the male and female portions is minimised.  
           [0014]    When the stamp and the object, during contacting, are pivoted to a mutually parallel position, the largest amount of forces is exerted upon the peripheral edges of the object. These forces are minimised in a preferred embodiment by the female portion defining a mouth portion, which is in sealing contact with a circumferential portion of the male portion, and by the circumferential portion having a diameter which is equal to or smaller than the diameter of the receiving surface of the holder.  
           [0015]    To achieve an optimally even contacting of the stamp with the object, it is preferable for the female or male portion arranged on the holder to be coaxial with the receiving surface of the holder. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The invention and its advantages will be described in more detail below with reference to the accompanying schematic drawings which by way of example illustrate currently preferred embodiments of the invention.  
         [0017]    [0017]FIGS. 1 a - 1   d  illustrate transfer of a pattern from a stamp to a substrate by nanoimprint lithography according to a known process.  
         [0018]    [0018]FIG. 2 is a basic sketch of a device according to the first embodiment of the invention.  
         [0019]    [0019]FIG. 3 a  is a side view on a larger scale of the abutment illustrated in FIG. 2, some inner parts being indicated by dashed lines, and FIG. 3 b  is a sectional view taken in the centre plane of the abutment in FIG. 3 a.    
         [0020]    FIGS.  4 - 7  are sectional views taken in the centre plane of abutments according to alternative embodiments of the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 2 is a schematic view of a device according to a first embodiment of the invention. The device, which is intended to transfer a pattern from a stamp  1  to an object  2 , comprises a press  10  and an abutment  20  coacting with the press. The press  10 , which can be of a prior-art type, such as a hydraulically operated press, has a piston element  11  which is movable back and forth in a well-defined direction relative to a fixedly arranged body  12  of the press  10 . The piston element  11  has on its side facing away from the body  12  a surface  13  for receiving the stamp  1 .  
         [0022]    The device further comprises an abutment  20  which comprises a fixedly arranged base  21  and a holder  22 , which on its side facing away from the base  21  has a surface  23  for receiving the object  2 . The surface  23  is of essentially the same extent as the object  2  to be received thereon. A locking means (not shown) for securing the object  2  is arranged at the surface  23 . This locking means can of an arbitrary kind, but for automatic production it is preferable to use sub-atmospheric pressure in this securing of the object. The holder  22  is pivotable relative to the base  21 , as will be described in more detail with reference to FIGS. 3 a - 3   b.    
         [0023]    In the embodiment according to FIGS. 3 a - 3   b , the base  21  comprises a vertical supporting plate  24  and a female portion in the form of a projection  25 , which is sleeve-shaped and, thus, defines a cavity  26 . The holder  22  comprises a base plate  27 , which on one side defines the surface  23  and on its other side has a male portion in the form of a projecting shaft  28  which is arranged coaxially with the receiving surface  23 . The distal end of the shaft  28  is inserted into the cavity  26  and forms together therewith a closed compartment  29 . Both the shaft  28  and the cavity  26  are circular in cross-section. A seal in the form of an O ring  30  is arranged in a circumferential groove  31  at the mouth area  32  of the cavity  26 . The O ring  30  rests against the circumferential surface of the shaft  28  to seal the compartment  29  against the ambient air. The compartment  29  is filled with a suitable hydraulic fluid of low compressibility, such as a brake fluid for vehicles.  
         [0024]    When the stamp  1  is pressed against the object  2 , an increased pressure is generated in the compartment  29  and strives to press the O ring  30  out of the groove  31 . In order to counteract this, the mouth area  32  has a circumferential guide lug  33  connecting to the groove  31 . In order to minimise fluid leakage from the compartment  29  via the O ring  30 , a flexible cloth  38 , for instance of rubber material, can according to an alternative embodiment as shown in FIG. 7 be sealingly fixed to the inner walls of the compartment, thereby forming a fluid-limiting membrane  38  in the compartment  29 . The fluid is caught in the space between the membrane  38  and the inner walls of the compartment  29  and absorbs forces from the distal end of the shaft  28 , which distal end abuts against the membrane when the stamp  1  is pressed against the object  2 . If there is an O ring as shown in FIGS. 3 a - b , this will affect the distal end of the shaft in case of great inclinations of the same. By not having an O ring in this alternative embodiment, the distribution of forces will be more uniform across the entire surface of the stamp while pressing against the object.  
         [0025]    A pressure sensor  34  extends through a bore  35  in the supporting plate  24  into contact with the fluid in the compartment  29 , either direct or via a partition (not shown). According to FIG. 1, the pressure sensor  34  is connected to a control unit  36 , such as a computer. The control unit  36  is connected to the press  10  and controls its contacting of the stamp  1  with the object  2  via a pressure control means  37 , e.g. a pump/valve which controls a hydraulic pressure acting on the piston element  11 .  
         [0026]    In a preferred embodiment, the holder  22  comprises a heating means (not shown) for heating the object  2 . Moreover the holder  22  suitably has a cooling means in the form of a cooling loop K which is passed by a cooling medium (FIG. 3 a ). This allows, in addition to cooling of the object  2 , a temperature stabilisation of the shaft  28  to prevent changes in the shape thereof.  
         [0027]    The operating mode of the device will be briefly described below with reference to FIG. 1. The control unit  36  actuates the pressure control means  37  to move the piston element  11  towards the immovable abutment  20  for contacting with the same. Normally, the stamp  1  mounted on the piston element  11  is not quite parallel with the object  2  mounted on the abutment  20 . In the contacting with the object  2 , any inclination between the stamp  1  and the object  2  will be automatically eliminated by the action of forces between them causing the holder  22  to pivot relative to the base  21  about a pivot point located in the centre of the mouth area  32 . The fluid in the compartment  29  damps the motion of the holder  22  during the paralleling, thereby allowing the contacting to take place softly. Subsequently the parallel receiving surfaces  13 ,  23  are pressed against each other until the pressure sensor  34  registers a desired contact pressure, after which the control unit  36  causes the press  10  to lock the piston element  11  for a given period of time. After that the control unit  36  actuates the pressure control means  37  to move the piston element  11  away from the abutment  20 , so that the structured object  2  can be removed from the receiving surface  23 .  
         [0028]    Most satisfactory results have been achieved with the device illustrated in FIGS.  2 - 3  in the manufacture of micro- and nanostructures. A substrate of Si/SiO 2  was provided with a 300 nm thick layer of 950K PMMA. This object was baked at 180° C. for 24 h. Then the object was mounted in the device, after which the PMMA layer was heated to about 170° C. At this temperature, the heating was terminated, and a stamp with a pattern of 300 nm deep structures was pressed against the object at a pressure of about 60 bar for about 1 min. At the same time the object was cooled to about 80° C. The thus structured object had a depth of pattern of about 200 nm with good evenness across the entire substrate, whose diameter in this case was about 5 cm.  
         [0029]    FIGS.  4 - 6  are schematic views of different alternative embodiments of the abutment  20 , parts equivalent to those in FIGS.  2 - 3  having the same reference numerals.  
         [0030]    In FIG. 4, the coacting female and male portions  25 ,  28  of the base  21  and the holder  22  are of a complementary shape, more specifically a part-spherical shape. This embodiment will probably result in a somewhat more flexible contacting of the stamp  1  with the object  2 .  
         [0031]    In FIG. 5, the receiving surface  23  of the holder  22  is of essentially the same diameter as the circumferential surface of the male portion  28  in the mouth area.  
         [0032]    In FIG. 6, the base  21  is provided with a projecting male portion  28 ′ which coacts with a female portion  25  formed in the holder  22 .  
         [0033]    However, it should be emphasised that the invention is not restricted to the above embodiments and that several modifications are feasible within the scope of the appended claims. For instance, the stamp  1  and the object  2  can change places in the accompanying drawings. In another alternative, the holder  22  is pivotally arranged on the piston element  11  instead of on the base  21  of the abutment  20 . According to one more alternative, the abutment  20  is arranged to move towards the piston element  11 . It should also be appreciated that the holder  22  in a conceivable embodiment is pivotally connected to the base  21  via a mechanical coupling, instead of the hydromechanical coupling described above.