Patent Publication Number: US-2005118393-A1

Title: Sheet having microsized architecture

Description:
RELATED APPLICATION  
      This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/349,596. The entire disclosure of this earlier application is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION  
      This invention relates generally to a sheet having architecture suitable for incorporation into microfluidic, microelectronic, micromechanical, and/or microoptical devices.  
     BACKGROUND OF THE INVENTION  
      Microsized architecture refers to one or more microsized (e.g., having a dimension no greater than 1000 microns) structures arranged in a predetermined pattern on a substrate that can be, for example, a rigid or flexible sheet. Typical microsized architecture includes channels, wells, and/or recesses having depths less than the thickness of the unformed original substrate. These microsized architectures can include passages extending in the x-y directions of the substrate. Dimensions of these channels and wells range from 0.00020 to 0.008 inches (5-200 microns) depth; 0.00020 inches to 10 inches (5 microns to 25.4 cm) and the channels may have convoluted shapes.  
      Volumetric accuracy of the micropassages is very important in that in many applications a 90% or greater accuracy of the cross sectional area must be conserved through the length of channel, from channel to channel, and/or well to well. In addition to volumetric accuracy, the surface texture of the channel is extremely significant, especially, for example, in microfluidic applications. For example, the smoothness or roughness of the channel can affect friction, surface drag, diffusiveness and/or laminar vs. turbulent flow patterns. Furthermore, the level of residual stresses can be very relevant in that it is directly related to strand orientation, which can result in undesirable polarization and/or because relaxation of these stresses during subsequent processing or during the life cycle of the product result in dimensional instability.  
     SUMMARY OF THE INVENTION  
      The present invention provides microsized architecture including vias which extend in the z-direction through the thickness of the substrate. In this manner, microfluidic, microelectronic, micromechanical, and/or microoptical applications requiring through-flow, through-conductivity, through-transmission, and/or other through patterns can be accommodated. Also, the present invention is believed to provide via-defining surfaces which have closer size-exactness, enhanced pattern precision, increased angle accuracy, and/or greater control of surface properties (e.g. texture) than via-defining surfaces formed by conventional methods, such as curing, ablation, stamping, roll embossing, photolithography, UV embossing and punching techniques.  
      More particularly, the present invention provides a sheet comprising a thermoplastic layer of a thermoplastic material and micro-sized architecture including at least one micro-via extending through the thickness of the layer of thermoplastic material. The sheet can have a thickness in the range of about fifteen to about three hundred microns, of about two hundred to about three hundred microns, of about forty to about one hundred microns, and/or about fifteen to about twenty-five microns. The via can have a minimal cross-sectional area with a dominating dimension that is less than the thickness of the thermoplastic material. Additionally or alternatively, the dominating dimension of the minimal cross-sectional area can be in a range of about five to twenty microns and/or about ten to about fifteen microns.  
      The via can have an axial dimension equal to the thickness of the thermoplastic layer, a first axial end corresponding to the maximum cross-sectional area of the via and a second axial end corresponding to the minimum cross-sectional area of the via. The first and second axial ends can have a similar geometry, can have different geometries, can have a polygonal geometry (regular or irregular), and/or can have a substantially circular (e.g., circle or oval) geometry. The via-defining walls of the sheet connecting the first and second axial ends can have a constant slope, can have a continuous changing slope (e.g., an arch-shaped slope) or can have a discontinuous changing slope (e.g., stepped).  
      The microsized architecture can comprise a single via or a plurality of vias. The plurality of vias can be separated from each other by a distance in the range of about thirty to about seventy microns and/or about fifty microns. They can be positioned in an array-arrangement of rows and columns and the rows/columns can be either aligned or staggered. The microsized architecture can further comprise one or more recesses (e.g., well, channel, etc.) which do not extend through the thickness of thermoplastic layer.  
      The sheet can have flat upper and lower x-y surfaces in which the vias and, if applicable, other indentations (e.g., x-y channels, recesses, or wells which do not extend through the thickness of the sheet) are formed. Instead, the microsized architecture can include structures projecting outwardly from its upper and/or lower surfaces whereby these structures, in combination with the vias, provide the sheet with multi-level topography. The projecting structures can be of the same or different heights depending on the architectural design.  
      The sheet can comprise a single layer of thermoplastic material. Alternatively, the sheet can comprise multiple layers of the same or different thermoplastic materials. With particular reference to multi-layer sheets made of different materials, co-extruded films can be used to provide a gradient of surface properties along the z-axis of the via(s).  
      According to a method of the present invention, the sheet can be made with a tool having a projection that is sized, shaped, and arranged to correspond to each via. Accordingly, if the microsized architecture includes a plurality of vias, the tool will include a plurality of projections. Also, if the desired architecture includes other indentations (e.g., channels, recesses, wells, etc.) and/or outwardly projecting structures, the tool can include reverse features of these architectural items so that they can be made simultaneously with the via(s).  
      In this method, the thermoplastic layer is heated so that the thermoplastic material is sufficiently flowable so that, when the tool and the thermoplastic layer are appropriately positioned relative to each other, the projections extend through the sufficiently flowable thermoplastic layer. The thermoplastic layer is then cooled so that the thermoplastic material solidifies around the projection(s). The tool and the thermoplastic layer are thereafter stripped from each other (e.g., the tool is stripped from the thermoplastic layer or the thermoplastic layer is stripped from the tool).  
      A carrier layer can be superimposed on the thermoplastic layer to provide the adjacent side of the thermoplastic layer with a desired surface morphology (e.g., a flat and highly finished surface) and/or to support the layer during certain method steps. To this end, the plastic carrier layer, if thermoplastic, can have a glass transition temperature substantially greater than the glass transition temperature of the target thermoplastic layer. During the manufacture of the sheet, the projections can extend partially or completely through the carrier sheet whereby recesses, aligned with the vias in the thermoplastic material, will be formed in the carrier sheet.  
      These and other features of the invention are fully described and particularly pointed out in the claims. The following description and drawings set forth in detail certain illustrative embodiments of the invention which are indicative of but a few of the various ways in which the principles of the invention may be employed. 
    
    
     DRAWINGS  
       FIG. 1  is a top view of a sheet according to the present invention, the sheet having microsized architecture including an array of vias extending through the thickness (i.e., the z-direction) of the sheet.  
       FIG. 2  is side cross-sectional view of the sheet.  
       FIG. 2A  is a schematic view showing the geometry of one of the vias in the sheet shown in  FIGS. 1 and 2 .  
       FIGS. 2B-2M  are schematic views showing other possible geometries of the via according to the present invention.  
       FIGS. 3A-3C  are side cross-sectional views of multi-layer sheets.  
       FIGS. 4A-4C  are side schematic views of sheets incorporating other non-via architectural features.  
       FIGS. 5A-5I  are schematic views of steps of a method of making the resinous sheet according to the present invention.  
       FIGS. 6A-6C  are schematic views of the sheet wherein the vias are made electrically conductive according to the present invention.  
       FIGS. 7A-7C  are schematic views of a plurality of sheets stacked according to the present invention and tools for making such sheets.  
       FIGS. 8A-8C  are schematic views of covered sheets according to the present invention.  
       FIGS. 9A-9C  are schematic views of a via having a microstructure block contained therein and assembly steps for positioning the microstructure blocks in the vias. 
    
    
     DETAILED DESCRIPTION  
      Referring now to the drawings in detail, and initially to  FIGS. 1 and 2 , a sheet  20  according to the present invention is shown. The sheet  20  includes microstructure architecture including an array of vias  22  extending completely through the sheet  20 . In this manner, applications requiring through-flow, through-conductivity, or other through patterns can be accommodated by the sheet  20 .  
      The sheet  20  can be a single layer of a thermoplastic material or a plurality of thermoplastic layers compatible with its intended application. For example, the thermoplastic material may comprise polyolefins, both linear and branched, polyamides, polystyrenes, polyurethanes, polysulfones, polyvinyl chloride, polycarbonates, and acrylic polymer and copolymer. If the sheet  20  is to be incorporated into a chemical, biochemical, or pharmaceutical assay, then a polymer/copolymer can be chosen that is chemically inert to the samples and reagents used in the assay or has other innate features that may enhance overall performance of the device, such as surface hydrophilicity/hydrophobicity. If the sheet  20  is to be incorporated into an instrument that relies on emissive or reflective characteristics for detection of an event of interest (e.g., fluorimetry, colormetry or spectroscopy), then a polymer/copolymer can be selected that does not interfere with the absorption or emission of the signals to or from the sample. If the product sheet  20  is to be incorporated into electrical circuitry, then the electrical/dielectric qualities of the polymer/copolymer can be considered.  
      The sheet  20  can have a generally planar geometry having, for example, a width W, a length L, and a thickness T. The width W can be constant across the sheet&#39;s length and can be of a dimension compatible with the equipment used to incorporate the sheet  20  into the desired final product. The length L can be a predetermined distance in the same general range as the width W or can be substantially longer so that the sheet  20  resembles a continuous web. The thickness T is generally in the range of about fifteen to about three hundred microns, of about two hundred to about three hundred microns, of about forty to about one hundred microns, and/or about fifteen to about twenty-five microns. The thickness T can be constant across the sheet&#39;s length and/or width.  
      The array-arrangement of the vias  22  can be in aligned rows/columns, staggered rows/columns, and/or changing rows/columns. Additionally or alternatively, the spacing between the vias  22  can be the same, can change proportionally, and/or can simply be different. Also, the vias  22  can be randomly arranged so that an array pattern or spacing sequence is not apparent. In any case, the minimum spacing between adjacent vias  22  (center-to-center) can be in the range of about thirty to seventy microns, about forty to sixty microns, and/or about fifty microns.  
      Referring now to  FIG. 2A , the geometry of one of the vias  22  is schematically shown. The illustrated via  22  has a frustoconical shape having a z-axial dimension A equal to the thickness T of the sheet  20 , a first (top) circular axial end and second (bottom) circular axial end. The area of the top end is greater than the area of the bottom end so that the via  22  tapers downwardly. (It may be appreciated, however, that the sheet  22  could simply be turned over to provide a via that tapers upwardly.)  
      The tapering shape of the via  22  is preferred as the geometry accommodates certain methods for making the sheet  20  as an appropriate “release angle” is necessary. In certain situations, a small release angle in the range of about 3° to about 5° might be desired so that cross-sectional areas along the axis of the via do not differ significantly. In other situations, however, large taper angles, in the range of about 30° to 60° might be more appropriate.  
      The tapering shape of the via  22  is preferred as the geometry accommodates certain methods and/or apparatus for making the sheet  20 . In other words, one axial end will define the maximum cross-sectional area of the via  22  and the other axial end will define the minimum cross-sectional area of the via  22 . In many cases, the dominating dimension (e.g., the diameter of a circular end, the length of a rectangular end, the height/base of a triangular end, etc.) defining the maximum cross-sectional axial end will be less than the thickness T of the sheet  20  and thus less than the axial dimension of the via  22 . Such a dominating dimension in the range of about 0.10 microns to about 3.0 microns is contemplated by the present invention.  
      Additionally or alternatively, the dominating dimension of the larger axial end will be in the range of about five to twenty microns and/or about ten to about fifteen microns. If the dominating dimension of the larger axial end is in the range of five to twenty microns, the dominating dimension of the smaller axial end can be in the range of about two to about ten microns and/or about three to about five microns. For example, in the frustoconical shape shown in  FIGS. 1-2 , the top axial end could have a diameter of about thirteen microns and/or the bottom axial end could have a diameter of about three microns.  
      Other via geometries are certainly possible with and contemplated by the present invention. For example, as shown in  FIGS. 2B-2J , the axial ends instead can be triangular ( FIG. 2B ), square ( FIG. 2C ), rectangular ( FIG. 2D ), oval ( FIG. 2E ), or an irregular polygon ( FIG. 2K ) or any other irregular shape ( FIG. 2L ). The walls connecting the axial ends can have a constant slope ( FIGS. 2A-2E ,  2 K,  2 L), can have a continuous changing slope ( FIG. 2H ), or can have a discontinuous changing slope ( FIG. 2G ). The geometry of the cross-sectional shape can remain the same ( FIGS. 2A-2H  and  2 J) or can change at a predetermined depth in the via ( FIG. 2I ). Also, the centers of the axial ends can be aligned ( FIGS. 2A-2L ) or can be offset relative to one another to provide a “non-symmetrical” via ( FIG. 2M ). It should be noted, however, that regardless of the via geometry, an appropriate angle of release may be required across any continuous “vertical” wall segment.  
      As was indicated above, the sheet  20  can be a single thermoplastic layer or a plurality of thermoplastic layers. If the sheet  20  is multi-layered as shown in  FIGS. 3A-3C , it can comprise co-extruded and/or laminated layers of the same thermoplastic material ( FIGS. 3A and 3B ). Additionally or alternatively, the sheet  20  can comprise co-extruded and/or laminated layers of different thermoplastic materials ( FIGS. 3B and 3C ). The layers may be of the same or different thicknesses.  
      With particular reference to multi-layer sheets made of different materials, co-extruded films can be used to provide a gradient of surface properties along the z-axis of the via(s). By way of an example, a hydrophilic upper layer of a co-extruded film might hold a fluid sample while a lower layer having a more hydrophobic property might prevent flow out of the via(s). By way of another example, a gradient of hydrophilic layers could be provided that might promote or alter the energy required for flow through the via(s) due to the gradient of surface hydrophilicity differences. By way of a further example, different layers could have different resistances to etching.  
      The vias  22  can be the only formed working feature on the sheet  20  or can be part of an architectural scheme including other elements, as shown in  FIGS. 4A-4C . For example, the microsized architecture can include other indentations  24  not extending through the thickness of the sheet  20 , such as recesses, wells, and/or channels ( FIGS. 4A and 4C ). Additionally or alternatively, projecting structures  26  of the same or different heights can be provided ( FIGS. 4B and 4C ). If the microsized architecture includes only indentations ( FIG. 2  and  FIG. 4A ), the sheet  20  can have flat upper and lower x-y surfaces. If the microsized architecture includes projecting structures  26  ( FIGS. 4B and 4C ), the sheet  20  will have a multi-height topology.  
      Referring now to  FIGS. 5A-5I , the steps of a method for making the embossed sheet  20  are schematically shown. In this method, a web  30  is provided, having at least a thermoplastic layer  32 , and the web  30  can also include a plastic carrier layer  34  ( FIG. 5A ). As was explained above, the thermoplastic layer  32  can comprise a polymer or copolymer having properties compatible with the assembly steps and with the eventual intended use of the sheet  22 .  
      The carrier layer  34  can provide several functions. First, it can serve to maintain the thermoplastic layer  32  under pressure against a belt while traveling around heating and cooling stations and/or while traversing the distance between them, thus assuring conformity of the thermoplastic layer  32  with the precision pattern of the tool  56  during the change in temperature gradient as the web (now embossed sheet) drops below the glass transition temperature of the material. Second, the film can act as a carrier for the web in its weak “molten” state and prevents the web from adhering to the pressure rollers  58  as the web is heated above the glass transition temperature. Thirdly, the carrier layer can receive an impression, or at least act as an “anvil,” during the process of embossing through holes in the thermoplastic layer  32  and thereby facilitate the embossing of through holes in accordance with the present invention.  
      Accordingly, the plastic carrier layer  34  can be selected based upon its having a glass transition temperature substantially greater than the glass transition temperature of the thermoplastic layer  32 . Additionally or alternatively, the carrier layer  34  can be chosen to provide the adjacent surface of the layer  32  with a flat and highly finished profile suitable for other processing. The ability of the carrier layer  34  to support the thermoplastic layer  32  during certain method steps can also be taken into consideration when picking a carrier material. Possible material candidates for the carrier layer  34  include, but are not limited to, polyester, such as a Mylar film. That being said, any carrier material, thermoplastic, thermosetting or otherwise, compatible with the manufacturing method, is contemplated by the present invention.  
      A tool  36  is provided, having a series of projections  38  sized, shaped and arranged to correspond to the desired array of vias  22  on the sheet  22 . ( FIGS. 5B and 5C ). Thus, to make the sheet  20  illustrated in  FIGS. 1 and 2 , the projections  38  would have a frustoconical shape and would be arranged in aligned rows/columns. It may be noted, however, that the distal end portions of the projections might need to represent an extension of the smaller axial end of the via  22 , as it may extend past the distance defined bottom surface of the sheet  22 .  
      The tool  36  can be made of a suitable material, such as nickel, which will withstand the subsequent method steps. For example, the method includes steps which can involve heating and cooling of the tool  36 . Accordingly, the dimensions of the tool  36  may affect the heating/cooling energy necessary to reach the required temperature gradients. A thin tool (about 0.010 inches [0.254 mm] to about 0.030 inches [0.768 mm]) will facilitate rapid heating and cooling while a thicker tool will retain heat.  
      The tool  36  can be manufactured by known techniques to create micropatterns in rigid substrates such as ruling, diamond turning, photolithography, deep reaction ion etching, plasma etching, reactive ion etching, deep x-ray lithography, electron beam lithography, ion milling or combinations thereof. For example, a female master can be electroformed and used to create several male patterns that are assembled together to form the tool  36 . Further details of making the tool  36  can be found in U.S. Pat. Nos. 4,478,769 and 5,156,863. (These patents are now assigned to the assignee of the present invention and their entire disclosures are hereby incorporated by reference.)  
      In the method of the present invention, the thermoplastic layer  32  is heated until it is sufficiently flowable. ( FIG. 5D .) In many cases, this will require that the layer  32  is heated to at least the glass transition temperature T g —that is, the temperature at which the material changes from the glassy state to the rubbery state. The term “glass transition temperature” is a well known term of art and is applied to thermoplastic materials as well as glass. It is the temperature at which the material begins to flow when heated. For various extendable types of acrylic, the glass transition temperatures begin at about 200° F. and, for polyester (Mylar), it begins at about 480° F. to 490° F.  
      Glass transition temperatures in the range of about 325° F. to about 410° F. (about 160° C. to about 215° C.) are typical for materials used to make the thermoplastic layer  32 . In some cases, the temperature will have to be increased to a flow temperature T e  in excess of the glass transition temperature T g  for the material to go from the rubbery state to a flowable state. For example, Polysulfone has a beginning glass transition temperature T g  of about 190° C., changing into a rubbery state at about 210° C. and beginning to flow at about 230° C.  
      Accordingly, two temperature reference points are significant in the present invention: T g  and T e . T g  is defined as the glass transition temperature, at which plastic material will change from the glassy state to the rubbery state. It may comprise a range before the material may actually flow. T e  is defined as the embossing or flow temperature where the material flows enough to be permanently deformed by the embossing process, and will, upon cooling, retain form and shape that matches, or has a controlled variation (e.g. with shrinkage) of, the embossed shape. Because T e  will vary from material to material, and also will depend on the thickness of the film material and the nature of the dynamics of the embossing apparatus, the exact T e  temperature is related to conditions including the embossing pressure(s), the temperature input of apparatus and the speed of apparatus, as well as the extent of both the heating and cooling sections in the reaction zone.  
      The embossing temperature T e  must be high enough to exceed the glass transition temperature T g , so that adequate flow of the material can be achieved to provide highly accurate embossing of the film by the apparatus. Numerous thermoplastic materials may be considered as polymeric materials to provide the layer  32 . (However, not all can be embossed on a continuous basis.) These materials include thermoplastics of a relatively low glass transition temperature (up to 302° F./150° C.), as well as materials of a higher glass transition temperature (above 302° F./150° C.). Typical lower glass transition temperatures (i.e. up to 302° F./150° C.) include materials used, for example, to emboss cube corner sheeting, such as vinyl, polymethyl methylacrylate, low T g  polycarbonate, polyurethane, and acrylonitrile butadiene styrene (ABS). The glass transition T g  temperatures for such materials are 158° F., 212° F., 302° F., and 140° to 212° F. (272° C., 100° C., 150° C., and 60° to 100° C.). Higher glass transition temperature thermoplastic materials (i.e. with glass transition temperatures above 302° F./150° C.) which applicants&#39; assignee has found suitable for embossing precision microvias, are disclosed in U.S. patent application Ser. No. 09/596,240 filed on Jun. 16, 2000, U.S. patent application Ser. No. 09/781,756 filed on Feb. 12, 2001, and/or U.S. patent application Ser. No. 10/015,319 filed on Dec. 12, 2001. These polymers include polysulfone, polyarylate, cyclo-olefinic copolymer, high T g  polycarbonate, and polyether imide. These earlier applications are owned by the assignee of the present invention and their entire disclosures are hereby incorporated by reference.  
      A table of exemplary thermoplastic materials, and their glass transition temperatures, appears below as Table I:  
                           TABLE I                       Symbol   Polymer Chemical Name   Tg ° C.   Tg ° F.                                                PVC   Polyvinyl Chloride   70   158       Phenoxy   Phenoxy PKHH   95   203       PMMA   Polymethyl methacrylate   100   212       BPA-PC   Bisphenol-A Polycarbonate   150   302       COC   Cyclo-olefinic copolymer   163   325       Polysulfone   Polysulfone   190   374       Polyacrylate   Polyacrylate   210   410       PC   High T g  polycarbonate   260   500       PEIPI   Polyether imide   260   500       Polyurethane   Polyurethane   varies   varies       ABS   Acrylonitrile Butadiene Styrene   60-100   140-212                  
 
      The thermoplastic material also may comprise a filled polymeric material, or composite, such as a microfiber filled polymer, and may comprise a multilayer material, such as a coextrudate of PMMA and BPA-PC.  
      The tool  36  and the thermoplastic layer  32  are brought into contact with each other so that, when thermoplastic material is sufficiently flowable, the projections  38  extend through the thermoplastic layer  32  to the carrier layer  34 . ( FIGS. 5E and 5F .) The resinous material of the layer  32  is sufficiently flowable to mold around the projections  38 . ( FIG. 5G .) Thus, the projections  38  do not puncture or pierce the thermoplastic layer  32  as occurs when a nail is hammered through a block of wood. Instead, the interaction between the thermoplastic layer  32  and the projections  38  more accurately duplicates what would occur if this nail was dipped in a bucket of water. Applicants have observed as a rule of thumb that for good fluidity of the molten thermoplastic material, the embossing temperature T e  should be at least 50° F. (10° F. C), and more advantageously between 100° F. to 150° F. (38° C. to 66° C.), above the glass transition temperature of the thermoplastic layer  32 .  
      The distal end portions of the projections  38  can extend partially into the carrier layer  34  ( FIG. 5E ) or can extend entirely therethrough ( FIG. 5F ). It is noted that since the size and shape of the via  20  can change depending upon the penetration of the projection  38 , some type of depth registration may be required. This registration can be accomplished by measuring the vertical position of the tool  36  ( FIGS. 5E and 5F ) and/or by sensing the penetration of the projections  38  through the carrier layer  34  ( FIG. 5F ). It may be noted that the carrier layer  34  acts as anvil, in effect, as the via  22  is embossed through the thermoplastic layer  32 . While it is desirable to control the form of the via, the carrier layer does not have to be cleanly embossed, since this is not part of the final product. Accordingly, the carrier layer  32  can be “punched” while it is below its glass transition temperature.  
      With the projections  38  still extending to or through the carrier layer  34 , the web  30  is cooled so that the thermoplastic material solidifies around the projections. ( FIG. 5H .) After sufficient solidification, the material surrounding the projections  38  will no longer depend upon the tool  10  for shape-defining purposes. The tool  36  is then stripped from the web  30 , leaving behind the vias  22 . ( FIG. 5I .)  
      The forming steps of the present invention are believed to provide essentially exact-sized surfaces and very precise inter-via patterns. The molded via-defining surfaces are formed without distortion, thereby allowing the enhanced smoothness of flat and curved regions of the via geometry. Also, with via shapes incorporating polygonal geometries (see e.g.,  FIGS. 2B-2D ,  2 G and/or  2 I), the via-defining surfaces have increased angular accuracy, and sharp corners can be incisively obtained.  
      The via-defining surfaces of the present invention are believed to be structurally superior (and in any event structurally different) than vias formed by conventional methods, such as curing, injection molding, ablation, stamping, and punching techniques. In a curing process, for example, the molded material must undergo a significant chemical change, thereby making final geometries (dimensions and surface profiles) difficult to predict in a micro-tolerance situation, especially via-to-via. Also, since a curing process by definition changes the chemistry of the starting polymer, the properties of the post-cure structure can differ from those of the pre-cure structure. Accordingly, while testing local properties of the starting polymer may help estimate the characteristics of the cured material, these characteristics usually must be re-tested in the final product. Moreover, even the same starting polymer can yield different final-product properties (depending upon the exact nature of the curing process), whereby testing of each batch of products is often necessary.  
      In an injection molding process, pressure is required to push the material into the appropriate cavities. This almost always results in some degree of orientation twist and/or relaxation stress. Also, certain parts of the mold often tend to cool faster than other parts of the mold, whereby uniform films are difficult to achieve.  
      An ablation process (such as laser ablation) involves the vaporization of a via-shaped piece of material, a stamping process requires the compaction of a via-shaped piece of material into surrounding regions, and a punching process requires the removal of a via-shaped piece of material. To the extent that sizing-specification and/or pattern-precision could be obtained with an ablation, stamping, and/or punching process, the profile of the surfaces would be difficult, if not impossible, to maintain, and the thrust of the tooling would have to be very precisely controlled.  
      Accordingly, the present invention is believed to provide via-defining surfaces which have closer size-exactness, enhanced pattern precision, increased angle accuracy, and/or greater surface texture control than via-defining surfaces formed by prior art methods. Additionally, residual stresses are avoided with the present invention, thereby providing essentially stress-free microstructures. Moreover, the local properties of the sheet material will not change during the via-forming process (since there is no change in chemistry), whereby post-forming testing of these properties is not necessary.  
      Once the web  30  and the tool  36  have been stripped from each other, the carrier layer  34  can be removed (e.g., peeled) from the thermoplastic layer  32  ( FIG. 5J ). If the web  30  reflected the desired size of the sheet  20 , then the production of the sheet  20  is complete and it is ready for further processing, assembly, and/or finishing. If the web  30  was of a continuous length, the product can be wound onto a roll ( FIG. 5K ) for later sectioning into desired lengths. Alternatively, the web  30  can be cut into sections of the desired sheet dimensions ( FIG. 5L ). It should be noted that the peeling step can be performed before, during or after the winding and/or cutting steps.  
      The method of the present invention can be performed with the machines and apparatus disclosed in U.S. patent application Ser. No. 09/596,240 filed on Jun. 16, 2000, U.S. patent application Ser. No. 09/781,756 filed on Feb. 12, 2001, and/or U.S. patent application Ser. No. 10/015,319 filed on Dec. 12, 2001. These applications are owned by the assignee of the present invention and their entire disclosures are hereby incorporated by reference.  
      As was indicated above, the sheet  20  can be incorporated into a variety of applications, each of which may require further processing and/or assembly. By way of example, in electrical circuitry constructions, the via-defining surfaces can be coated with an electrical conductive coating  90  ( FIG. 6A ), electrically conductive particles  90 ′ can be placed in the via  22  ( FIG. 6B ), and/or an electrically conductive object  90 ″ (e.g. a sphere having a diameter less than that of the circular top end and greater than that of the circular bottom end of a frustoconical shaped via) can be dropped into the via  22  ( FIG. 6C ). Further details of possible conductive vias are set forth in co-pending U.S. application Ser. No. 60/349,907 filed concurrently with the present application. This application is assigned to the assignee of the present invention and its entire disclosure is hereby incorporated by reference.  
      A plurality of sheets  20  can be stacked to provide a three-dimensional network of passageways with the vias  22  providing inter-level communication ( FIG. 7A ). Multi-level sheet assemblies might be especially helpful in fluid applications where the sheet  20  contains other microsized architecture, forming passageways  92  to and from the vias  22  ( FIG. 7B ). The passageways  92  can be formed simultaneously with the vias  22  by modifying the tool  36  to include “shorter” projections  94  which do not extend through the thermoplastic layer  32 . ( FIGS. 7C-7E ). Also, in filtering situations, vias  22  between stacked sheets  20  could be used to distribute and equalize flow downstream of the filter entrance.  
      A lid or cover  96  can be provided for the sheet  22  which results in the top of each or some of the vias  22  being covered ( FIGS. 8A-8C ). Details of possible lidded and/or covered constructions are set forth in co-pending U.S. application Ser. No. 60/349,909, filed on Jan. 18, 2002. This application is assigned to the assignee of the present invention and its entire disclosure is hereby incorporated by reference.  
      The vias  22  can define recesses which receive complementary shaped microstructure blocks  98  ( FIGS. 9A and 9B ). For efficient assembly, a multitude of the blocks  98  (e.g., chips) can be provided in a slurry that is passed over the sheet  22  by, for example, a soft air stream ( FIG. 9C ). Properly positioned blocks  98  will drop into the vias  22  with the remainder being swept downstream ( FIG. 9D ).  
      These and other further processing and assembly steps can be performed to create a product suitable for incorporation into a filtering, sampling, electrical or other application. Also, such processing and assembly steps can be combined as appropriate. For example, sheets  20  containing the electrically conductive vias  22  shown in  FIGS. 6A-6C  can be stacked as shown in  FIG. 7A  and/or provided with a lid  96  as shown in  FIG. 8A-8C . Additionally or alternatively, sheets  20  containing the microstructure blocks  98  shown in  FIG. 9A  can be likewise stacked and/or covered.  
      Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent and obvious alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification. The present invention includes all such alterations and modifications and is limited only by the scope of the following claims.