Abstract:
Apparatus, such a embossing apparatus, for applying a controlled amount of force to an article comprises a head ( 26 ) for engaging such an article to apply said force. The head ( 26 ) is moveable along substantially linear operating or return strokes, and is mounted on a rigid support ( 1, 2 ) through flexure means such a plate ( 32, 40 ). The flexure means constrains the head ( 26 ) so that latter moves only in the direction of said strokes.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates apparatus for applying a controlled force to an article and to embossing apparatus. 
       BACKGROUND TO THE INVENTION 
       [0002]    The demand for MNT (Micro Nano Technology) and MEMS (Micro Electro Mechanical Systems) micro devices is on the increase. In particular, microfluidic devices such as biochips (or microfluidic chips) are increasingly being adopted by the biochemical analysis industry. Such chips generally comprise a polymer substrate which carries a network of channels and various other microfluidic elements, for example peristaltic pumps, valves and chambers. Accordingly, there is a growing need to be able to fabricate such devices cheaply and quickly. 
         [0003]    A number of fabrication methods are employed in the manufacture of these devices, including hot embossing, imprinting and injection moulding. 
         [0004]    Although embossing and imprinting techniques do offer the potential for rapid and cheap mass production of microfluidic devices, they do require that the magnitude of an embossing/imprinting force applied through a suitable tool to an article is precisely controlled, and that the mechanism that applies the embossing/imprinting force to the article does not allow any lateral movement of the tool relative to the article. Generally, such mechanisms include surfaces which slide relative to each other, and this can create particles (undesirable in clean room environments), produce stick-slip and cause wear. 
       SUMMARY OF THE INVENTION 
       [0005]    According to a first aspect of the invention, there is provided apparatus for applying a controlled amount of force to an article, the apparatus comprising a head for engaging such an article to apply said force, the head being moveable along substantially linear operating or return strokes; flexure means through which the head is mounted on a substantially rigid support means, and an actuator for moving the head, wherein the flexure means is so arranged as to constrain the head so that the latter moves only in the direction of said strokes. 
         [0006]    It is known to provide flexure hinges in micro/nano positioning stages, although such hinges generally allow either hinging or two axis movement. In the present invention, the use of flexure means precisely to constrain the head is particularly suitable since flexures can exhibit a very high stiffness and load capacity, are less sensitive to shock and vibration (than a constraining mechanism which has sliding contacts) do not generate particles and theoretically can have an infinite life (if operated within their elastic limits). Preferably, the apparatus is adapted for use in a process of creating surface features on an article, for example an embossing or imprinting process. 
         [0007]    Preferably, the apparatus is embossing apparatus. 
         [0008]    To that end, the apparatus may to advantage include an embossing or imprinting tool which is urged against the article by the action of the head. 
         [0009]    In one example, the tool is carried by the head. Alternatively, the tool may be provided on a separate support and the article attached to the head, so that the latter drives the articles onto the tool. 
         [0010]    Preferably, the actuator is operable to drive the head along its operating stroke. 
         [0011]    The actuator preferably comprises a piezoactuator. 
         [0012]    Such actuators provide very precise, controllable movement and force application with repeatability which is an order of magnitude better than that achievable with current stepper motor drives. 
         [0013]    The flexure means is preferably resilient so as to exert on the head a biasing force in the direction of the return stroke. 
         [0014]    This can provide a pre-loading force on the piezoactuator which can help to prevent damage to the latter. 
         [0015]    The flexure means may to advantage comprise a plate. 
         [0016]    In such a case, the direction of each stroke is preferably substantially perpendicular to the plate. 
         [0017]    A disadvantage of known types of flexure is that they only provide a relatively limited range of allowable movement. However, the above mentioned feature permits a greater range of allowable movement, particularly if the head is situated centrally relative to the plate. 
         [0018]    The movement the plate prevents will be in the plane of the plate. If the plate is non-compressible, the resistance offered to such movement will be considerable, so that the plate is also particularly suitable for providing the desired constraining effect. 
         [0019]    Preferably, the plate is symmetrically arranged relative to the head. To that end, the plate may to advantage be circular and concentric with the head. Preferably, the head is also circular. 
         [0020]    Preferably, the apparatus includes one or more conduits for enabling gas pressure on either side of the plate to be equalised. Preferably, the or each conduit is provided in the plate. 
         [0021]    The conduits enable the apparatus to be used in conjunction with a vacuum chamber. Preferably, the plate is one of two such plates which are parallel to each other and are spaced from each other in the direction of the strokes. 
         [0022]    This feature increases the biasing force exerted by the flexure means on the head, and also provide greatly increased resistance to any rocking movement of the head. Furthermore, since one plate can overlie the other, these effects are achieved without substantially increasing the lateral extent (i.e. dimensions lateral to the strokes) of the apparatus. 
         [0023]    The piezoactuator may conveniently be connected to the head through a linkage for providing mechanical amplification of the actuator stroke. 
         [0024]    This helps to compensate for the relatively limited stroke length provided by piezoactuators. In addition, the characteristics of the linkage may be chosen to provide any selected force/length characteristics from a range of possible characteristics for a given type of piezoactuator. 
         [0025]    Preferably, the linkage comprises a rod attached to the head and a lever attaching the rod to the actuator. 
         [0026]    In such a case, the magnitude of mechanical amplification will be related to the position of the pivot for the lever relative to the piezoactuator and the rod. 
         [0027]    Preferably, the rigid support means comprises a frame on which the flexure means is mounted. 
         [0028]    Because the piezoactuator is separate from the head, it can be replaced without interfering with the head and the flexure means. 
         [0029]    According to a second aspect of the invention, there is provided embossing apparatus comprising a head for urging an embossing tool and an article together to cause the article to be embossed, a piezoactuator for moving the head relative to a support frame and flexure means mounting the head on the frame. 
         [0030]    Preferably, the piezoactuator and rod are disposed substantially parallel and adjacent to each other, the piezoactuator having an operating stroke in the opposite direction to that of the head, and hence the rod. 
         [0031]    This position of the rod and piezoactuator enable the apparatus to be of a relatively compact construction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0033]      FIG. 1  is an isometric view of an embodiment embossing apparatus in accordance with the invention; 
           [0034]      FIG. 2  is a sectional side view of the apparatus; 
           [0035]      FIG. 3  is an isometric view of the apparatus from a different angle from  FIG. 1 ; 
           [0036]      FIG. 4  is an exploded isometric view of part of the apparatus, the view being taken from one side and slightly above; 
           [0037]      FIG. 5  is a corresponding view to  FIG. 4  but taken from one side and below; 
           [0038]      FIG. 6  is a sectional side view of the parts shown in  FIGS. 4 and 5  when assembled; 
           [0039]      FIG. 6A  is a more detailed view of part of the arrangement as shown in  FIG. 6 ; 
           [0040]      FIGS. 7 and 8  are isometric and sectional side views respectively of the apparatus when attached to a vacuum chamber; 
           [0041]      FIG. 8A  is a more detailed view of part of the arrangement as shown in  FIG. 8 ; and 
           [0042]      FIGS. 9 and 9A  are isometric views, from different angles, of parts of a second embodiment of the embossing apparatus, having a pair of parallel piezoactuators operating through a common linkage. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    With reference to  FIGS. 1-3 , an embodiment of embossing apparatus in accordance with the invention has a frame comprising a circular base plate  1  and a vertical support post  2  attached to the plate  1  at a position offset from the centre of the latter. The post  2  is substantially U-shaped in horizontal cross-section so as to define two side portions  4  and  6  and a back portion  8 . The post  2  thus also defines a vertical gap, and this accommodates a preloaded high-load piezoactuator  10  which is also vertically disposed and attached at its base to the base plate  1 . 
         [0044]    Piezoactuators of this type are known and this element has not therefore been described in detail (the cross-sectional view of the element in  FIG. 2  omitting all of the internal structures of the actuator for the sake of clarity). The piezoactuator  10  has a cylindrical body  12  from the top of which a rod  14  extends. The rod  14  is slideable relative to the body  12  and coupled to the piezoelectric materials (typically polycrystalline ceramics) that expand and contract when a voltage is applied. Thus increasing decreasing voltage applied to the piezoelectric materials within the body  12  causes the rod  14  to move up or down. 
         [0045]    On top of the post  2  there is provided a bracket  16  in the form of a parallelepipidal block provided with a slot that is aligned with the gap defined by the post  2 , and into which the top of the rod  14  extends to act on a lever  18  which projects forwardly from the front of the bracket  16 , and which is pivotally mounted on that bracket by a pin  20 . As can be seen from  FIG. 2 , the line of action of the piezoactuator  10  is disposed behind the pin  20 , i.e. at the inboard end of the lever  18 . The outboard end of the lever  18  extends over the central axis of the plate  1  and acts on a vertical push rod  22  which is coaxial with the plate  1 . The rod  22  terminates at its lower end in a flange  29  attached to a head assembly  26 , underneath the plate  1 , through an adapter plate  31 . 
         [0046]    As can be seen from  FIG. 1 , the flange  29  is attached to the adapter plate  31  by six screws (e.g. screw  33 ). The adapter plate  31  is, in turn, attached to a central cylindrical spigot  35  in the head assembly  26 , the spigot extending through a central aperture  24  ( FIGS. 4 and 5 ) in the base plate  1 . In this example the adapter plate  31  is attached to the spit  35  by means of vertical screws (not shown). 
         [0047]    The rod  14  is connected to the lever  18  via a flexible stainless steel connector  25  whilst the connection between the lever  18  and rod  22  is via another connector (referenced  27 ) of this type. The connectors enable the linear piezoelectric actuator motion to be converted into a tilt motion at the lever  18  and vice versa at the rod  22 . 
         [0048]    As can be seen from  FIG. 1 , the flange  29  is attached to the adapter plate  31  by six screws (e.g. screw  33 ). The adapter plate  31  is, in turn, attached to a central, cylindrical spigot  35  in the head assembly  26 , the spigot extending through a central aperture  24  ( FIGS. 4 and 5 ) in the base plate  1 . In this example the adapter plate  31  is attached to the spigot  35  by means of vertical screws (not shown). 
         [0049]    The frame further comprises two rings  28  and  30  attached to the underside of the plate  1  and defining (with the plate) a circular recess in which the head assembly  26  is housed. All of the components of the frame and head assembly are stainless steel, BSEN10088-3:1995, 1.4301 (formerly grade 304), with manufacturing tolerances applied to ensure suitable alignment of components. The head assembly  26  is mounted on the frame through a pair of parallel circular flexure plates  32  and  40  which are also stainless steel (17-4PH heat treated to condition A), and have a thickness of approximately 0.8 mm and a diameter of 350 mm. The desired dimensional characteristics of the plates are determined from such parameters as the required operational stroke and the mechanical properties of the plates (yield stress, for example). 
         [0050]    The construction and interrelationship of the head assembly  26 , the rings  28  and  30  and the flexure plates  32  and  40  can be more clearly seen in  FIGS. 4 and 5 . 
         [0051]    The head assembly  26  comprises an upper disk  42  attached to middle and lower disks, referenced  44  and  46  respectively by 12 equi-angularly arranged stainless steel fixing bolts, which pass through corresponding holes in the three disks of the head assembly. One of those fixing bolts is denoted by the reference numeral  48  in  FIG. 4 . Since the other fixing bolts are apparent and identical to the bolt  48 , they have not been denoted by reference numerals. 
         [0052]    Those fixing bolts also extend through corresponding holes in the flexure plates  32  and  40  so that the plate  32  is held firmly between disks  42  and  44 , and the plate  40  between disks  44  and  46 . The disk  46  has four equi-angularly arranged axial passages  50 - 53  (i.e. passages extending parallel to the axis of the rod  22 ) these are aligned with corresponding apertures  54 - 57  in the plate  40 , similar channels  58 - 61  in the middle disk  44 , channels  62 - 65  in the upper disk  42  and apertures  66 - 69  in the plate  32 . Similarly, the plates  32  and  40  have further holes  70 - 73  which are in register with the remaining holes in the upper, middle and lower disks of the head assembly. 
         [0053]    The head assembly therefore provides four passages which extend axially from the underside of the lower disk  46  to just beneath the top surface of the upper disk  42 , said passages also extending through the plates  32  and  40 . 
         [0054]    In  FIG. 2  two of these passages are generally referenced  74  and  76 . Each of the middle and upper disks of the head assembly also has four radial passages, each communicating with a respective axial passage. In  FIG. 2 , two of the four radial passages in the middle disk  44  are shown at  78  and  80 , whilst the corresponding passages in the upper disk  42  are shown at  82  and  84 . 
         [0055]    Thus, when the apparatus is attached to a vacuum chamber, the passages permit the pressures on either sides of the two flexure disks to equalise (as a result of gas being exhausted through the axial and radial passages). 
         [0056]    The frame rings  28  and  30  have annular grooves  86  and  88  in their upper surfaces, each of which accommodates a respective O ring seal. 
         [0057]    The base plate  1  and frame ring  28  also have respective annular grooves  90  and  92  in their lower surfaces, also for accommodating O-ring seals. Accordingly, flexure plate  32  is sealed within the underside of the base plate  1  and the top surface of the ring  28 , whilst the plate  40  is sealed against the underside of the ring  28  and the top of the ring  30 . The components of the head assembly  26  also have annular grooves for O ring seals ( 100 - 111  in  FIG. 6A ). 
         [0058]    Consequently, when the apparatus is mounted on a vacuum chamber (referenced  100  in  FIGS. 7 and 8 ) the vacuum in the chamber can be communicated in and maintained in the spaces between the flexure plates and the frame of the apparatus. In this arrangement, there is also, of course, a seal between the underside of the ring  30  and the top of the vacuum chamber  100 , but this has been omitted for the sake of clarity. 
         [0059]    The embossing tool is carried on the underside of the lower disk  46  of the head assembly. This head will be specific to the type of embossing operation to be carried out. 
         [0060]    In use, a suitable voltage fed to the piezoactuator  10  will cause an extending force to exerted by the actuator on its rod  14 . This force will create a moment on the lever  18  about the pivot  20  which, in turn, results in a downward force being exerted on the rod  22 . The head assembly  26  is constrained by the flexure plates  32  and  40 . However, those plates can flex to a limited degree in a direction normal to their faces, so as to allow a controlled downward movement of the head assembly  26 . As the plates are deformed in this way, they will exert an upward biasing force on the head assembly  26 . However, the plates  32  and  40  have a high tensile strength and will therefore strongly resist movement of the head assembly  26  in directions parallel to the planes of the plates. The plates  32  and  40  thus will substantially prevent horizontal movement of the head assembly  26 . In addition since the plates are vertically spaced from each other, rocking movement of the head assembly  26  (about a horizontal axis) is also effectively prevented. 
         [0061]    The permitted vertical movement of the head  26  in response to extension of the actuator  10  drives an embossing tool into a substrate to be embossed. The constraining effects of the flexure plates ensure that the movement of the embossing tool is normal to the surface of the substrate so that any errors or inaccuracies caused by lateral movements of the tool are avoided. 
         [0062]    The actuator  10  can be removed and replaced without affecting the mounting of the head assembly  26  on the plates  32  and  40 . In addition the mechanical reduction or amplification of the stroke of the actuator  10  as transmitted to the rod  22  can be changed by selecting a post  2  having a different geometry, more particularly a post in which the pivot point of the lever  18  is in a different position. 
         [0063]    It has been found that the apparatus in accordance with the invention is able to exert typical embossing forces (several kN) on a substrate whilst constraining the head assembly  26  to move only in a vertical downwards direction (i.e. along the axis of rod  22 ), constituting the operating stroke of the device. When the voltage applied to piezoactuator  10  is reduced the rod  14  retracts, causing the head assembly  26  to move vertically upwards (in the reverse stroke). The resilience of the plates  32  and  40  assists in this upward movement, but the additional retracting force exerted by the piezoactuator  10  may be needed to separate the embossing tool from the substrate. 
         [0064]    The flexibility of the design of the apparatus enables a range of processes to be conducted by the same mechanism. Accurate linear motion is provided with sub-micron position capability should embossing with precise depth and feature control be required. Emboss depths can be accommodated up to several hundred microns, to satisfy most precision embossing requirements. Alternatively, embossing by force control is also possible with applied forces available up to several kN. 
         [0065]    To provide additional flexibility within the MEMS market, the apparatus is designed to interface with typical vacuum chambers, such as those used in wafer bonding processes. This is illustrated in  FIGS. 7 ,  8  and  8 A. 
         [0066]    The vacuum chamber comprises a cylindrical housing  150  having radial inlet/outlet ports  151 - 154  and upper flange  156  to which the lower ring  30  of the frame of the apparatus is attached by, for example, screws extending from the underside of the flange  156  into correspondingly threaded blind bores in the underside of the ring  30 . Reference numeral  158  denotes an annular groove in the in the top of the flange  156  for accommodating an O ring seal so that the apparatus is sealed against the vacuum chamber. 
         [0067]    As can be seen from  FIG. 8 , the vacuum chamber includes a table  160  for carrying a substrate  162  to be embossed by an embossing tool  164  attached to the head assembly  26 . 
         [0068]    The ability to perform wafer bonding is not impaired by the addition of the apparatus, meaning that multiple processes can be conducted by the same assembly. 
         [0069]    For example, the embossing apparatus (with the embossing tool removed) may be used to hold together the faces of wafers (typically of silicone and glass) with forces which are much lower than those used in embossing processes, but which are suitable for wafer bonding processes. Thus the apparatus could be used, for example, to create an embossed substrate and/or to bond that substrate to another wafer to create, for example, a sealed biochip. 
         [0070]      FIGS. 9 and 9A  show components of a modified version of the apparatus which are similar to the corresponding components on the first described embodiment, and are therefore denoted by the reference numerals used in relation to the first embodiment, raised by  200 . The components include a post  202  which is wider than the post  2 , so as to be able to accommodate a pair of parallel piezoactuators  210  and  210 ′, which are respectively connected to a common overhead lever  218  by means of connectors  227  and  227 ′. It can be seen from the figures that the lever  218  is, in plan, in the shape of a truncated triangle so as to be wide enough to be connected to both piezoactuators, whilst acting on the rod  222  which is in a central position relative to the piezoactuators.