PRIORITY DOCUMENTS

[0001] The present application claims priority from Australian Provisional Patent Application No. 2012902702 titled "APPARATUS FOR FABRICATING MICROSTRUCTURED DEVICES" and filed on 26 June 2012, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

[0002] The present invention relates to apparatus and methods for bonding substrates or embossing microstructured features onto substrates for microfluidic and other applications.

[0003] Throughout this specification reference may be made to microfluidic devices to describe the present invention. However, any such reference is for the purpose of illustrating the present invention only and it will be appreciated that the apparatus and methods described are equally applicable to microstructured substrates or devices for use in applications other than microfluidics.

BACKGROUND

[0004] Substrates having microstructured features have applications in electronics, chemical processing, diagnostics, analysis, and micromechanics. The microstructured features can include portions of electronic circuits, optical circuits, fluid circuits, and the like. Microelectromechanical systems (MEMS) and microfluidic devices are examples of microstructure devices.

[0005] Microfluidic devices are fabricated in silicon, hard polymers, elastomeric polymers as well as glass or quartz. There is a trend toward the use of polymer substrates for the fabrication of

microstructured devices. Reduced cost and favourable optical properties are often cited as reasons for choosing a polymer over an inorganic material like silicon, glass or quartz.

[0006] Typically, microstructured features are fabricated on substrates using laser ablation, injection moulding or silicone rubber embossing to form microstructured devices. Of these, hot embossing is advantageous because it is a relatively low cost process that is easily reproducible and provides high structural accuracy. For these reasons, hot embossing is suited for a wide range of applications from rapid prototyping to high volume mass fabrication.

[0007] Hot embossing uses elevated temperature and high pressure to transfer patterned structures from a template (commonly referred to as a "stamp") to a substrate. This technique has the potential to economically mass produce devices with micron-scale features. [0008] Hot embossing is generally performed between layers of different polymer materials supported by a silicon wafer substrate, between such a substrate and an etched silicon wafer hard stamp, or upon polymer or glass wafer substrates with a large micromachined metal stamp. For example, a commercially available EVG520HE Semi-automated Hot Embossing System (EV Group) is designed for embossing on silicon wafers and for the bonding of silicon or glass wafers. Using this machine, a six inch round silicon wafer is compressed between two plates with heating. The top plate of the two plates (the master plate) has a pattern formed on it. However, when one attempts to emboss substrates of a different shape, or smaller size in an apparatus of this type there is a tendency for the substrate and/or the master plate to be damaged due to the broad distribution of compressive force over the six inch round area. Furthermore, the high cost associated with the production of a six inch round stamp is restrictive and such machined stamps are difficult to produce. When smaller substrates are used, it is difficult to retain the embossed substrate in place during the disengagement process and it can also be difficult to align substrates for embossing and bonding. The failure of a substrate to disengage with a stamp after embossing and before cooling leads to damage of the stamp, fracture or disintegration of glass substrates or bending or damage to the microstructure of polymer substrates.

[0009] Despite the apparent difficulties associated with fabrication of microstructured devices on glass, glass microfluidic substrates are still preferred in many applications due to the inertness of the substrate. However glass substrates are relatively expensive because photolithography and wet chemical etching methods are required to fabricate each microstructured device.

[0010] The prior art discloses various attempts to provide apparatus and processes that can be used to hot emboss and/or bond substrates. United States patent 6,416,311 Bl (Springer et al.) discloses an apparatus for stamping a substrate that is designed to facilitate removal of a shaped substrate from the stamping tool. However, the substrate is larger than the stamping tool and, therefore, the disclosed arrangement may-not be suitable for stamping a range of substrates, and especially glass substrates, because the compressive force applied to the substrate is localised in an area that is smaller than the substrate. United States patent 7,927,976 B2 (Menard) discloses a reinforced composite stamp for dry printing

semiconductor elements. The reinforced composite stamp disclosed in Menard may not be suitable for hot embossing a range of substrates, including glass substrates. United States patent 7,611,348 B2 (Van Santen et al.) discloses an apparatus for UV imprint lithography comprising an interchangeable imprint template (stamp). There is no indication in Van Santen et al. that the apparatus could be used to hot emboss or bond a range of substrate materials and sizes and, indeed, the substrate template disclosed is considerably smaller than the substrate and this would likely lead to damage of substrates formed from some materials such as glass.

[0011] There is a need for apparatus and processes that can be reliably used to hot emboss and/or bond a range of substrates, particularly glass substrates. Alternatively, there is a need for apparatus and processes for hot embossing substrates in which the stamp can be easily interchanged. Alternatively, there is a need for apparatus and processes that can be used to emboss or bond substrates of different sizes and/or different shapes.

SUMMARY

[0012] In a first aspect, the present invention provides a platen arrangement for use in fabricating embossed or bonded substrates, the platen arrangement comprising first and second pressure plates configured to be pressed together to emboss a substrate or bond two substrates positioned therebetween, each pressure plate having a working surface comprising at least one recess configured to removably receive an insert comprising a planar substrate contact surface having a size and shape that is substantially the same as the size and shape of the substrate and an insert retainer for releasably retaining the insert in the recess, wherein when the two pressure plates are brought together the inserts are mutually aligned and the substrate contact surface of each insert contacts the substrate to apply pressure thereto.

[0013] The platen arrangement can be used for any purpose requiring embossing of microstructured features or bonding of substrates. For example, the platen arrangement can be used to emboss any microscale three dimensional structures such as scaffolds to be used for three-dimensionally

microstructured free thin films, for the manufacture of micro lens arrays, fabrication of micro and nanostructured surface textures to effect different wetting phenomena or for optical sensor surfaces.

[0014] In embodiments, when the insert is fitted in the recess of a pressure plate the substrate contact surface of the insert is co-planar with the working surface of the pressure plate.

[0015] In embodiments, when the insert is fitted in the recess of a pressure plate the substrate contact surface of the insert is plane-parallel with and is spaced away from the working surface of the pressure plate.

[0016] In embodiments, the insert comprises a retaining flange. The retaining flange engages with the insert retainer on the pressure plate to retain the insert in the recess. In embodiments, the retaining flange extends outwardly from a side of the insert in a plane that is parallel with the plane of the substrate contact surface. The peripherally extending flange may extend from the insert adjacent a back surface of the insert.

[0017] In embodiments, the insert is square or rectangular in plan view. In these embodiments, the insert may have a retaining flange extending from two opposing sides of the insert. The retaining flange may extend along the length of each of the sides. In other embodiments, the insert is round in plan view. In these embodiments, the insert may have a retaining flange extending diametrically from all or part of the circumference of the insert.

[0018] In embodiments, at least one of the pressure plates or inserts comprises a substrate retainer for retaining a substrate on the insert. In embodiments, the substrate retainer comprises: a fixed retaining lug that is fixed to the pressure plate or insert in a position whereby it abuts one side of the substrate when it is correctly positioned on the insert; and a movable retaining lug that is fixed to the pressure plate or insert whereby it abuts an opposing side of the substrate when it is correctly positioned on the insert, the moveable retaining lug moveable between an engaged position in which it abuts the opposing side of the substrate, and a disengaged position whereby it is spaced from the opposing side of the substrate to allow for insertion or removal of the substrate from the insert. In embodiments, the movable retaining lug is biased to the engaged position. At embossing temperature, the bias towards the engaged position may impress into the side walls of the substrate a small groove that serves to increase the strength of the retaining force imposed by the retaining lugs in the direction perpendicular to the plane of the substrate in contact with the stamp.

[0019] In embodiments, at least one of the inserts comprises positioning means for aligning and/or locating at least two substrates in a stacked configuration on the substrate contact surface. The platen arrangement of these embodiments is suitable for hot bonding applications in which the two substrates are to be bonded together. In embodiments, the platen arrangement comprising positioning means also comprises a retainer for retaining at least one of the substrates in the stacked configuration on the substrate contact surface of the insert.

[0020] In embodiments, one of the inserts is a stamp having a pattern to be transferred to the substrate. The platen arrangement of these embodiments is suitable for hot embossing substrates.

[0021] In embodiments, at least one of the pressure plates comprises a compliant layer on a back surface that is opposite the working surface, wherein the compliant layer is of a size and shape substantially the same as the substrate and insert. The compliant layer in these embodiments compensates for any lack of planarity and/or parallel in the platen arrangement when it is under pressure and/or distributes force solely into those areas of the platen arrangement that engage with the insert.

[0022] In embodiments, the platen arrangement further comprises means of aligning the first pressure plate with the second pressure plate prior to contact between both inserts and the substrate.

[0023] In embodiments, the platen arrangement further comprises means for aligning each insert within each pressure plate. [0024] In embodiments, the platen arrangement further comprises means of aligning two or more substrates with respect to datum points on their periphery to allow alignment and structural agreement between communicating microstructures on each substrate layer during bonding.

[0025] The aforementioned alignment features are particularly useful for aligned bonding wherein the microstructured features all have to be correctly positioned when embossing and if there are

microstructured features in both (or subsequent) substrates, these have to be accurately aligned.

[0026] In a second aspect, the present invention provides a pressure plate for use in a platen arrangement as described herein, the pressure plate having a working surface comprising a recess configured to removably receive an insert comprising a planar substrate contact surface having a size and shape that is substantially the same as the size and shape of the substrate and an insert retainer for releasably retaining the insert in the recess.

[0027] In a third aspect, the present invention provides an apparatus for fabricating embossed or bonded substrates, the apparatus comprising:

- a platen arrangement of the first aspect of the invention described herein;

a press for applying pressure to at least one of the pressure plates in the platen arrangement; and

a heater for optionally heating a substrate positioned on an insert on one of the pressure plates.

[0028] In a fourth aspect, the present invention provides a process for embossing a microstructured device, the process comprising:

- providing a substrate to be embossed;

- providing a stamp insert having a pattern to be embossed on the substrate formed thereon;

- inserting the stamp insert into a recess in a first pressure plate of the platen arrangement of the first aspect of the invention described herein;

- positioning a substrate on an insert on the second pressure plate of the platen arrangement;

- optionally heating the substrate;

- bringing the pressure plates together so that the pattern on the stamp insert contacts the substrate; and

- applying pressure onto at least one of the pressure plates to transfer the pattern from the stamp insert onto the substrate. [0029] In a fifth aspect, the present invention provides a process for bonding two substrates, the process comprising:

- providing a first substrate and a second substrate to be bonded together;

- providing a first insert comprising a planar substrate contact surface having a size and shape that is substantially the same as the size and shape of the first substrate;

- providing a second insert comprising a planar substrate contact surface having a size and shape that is substantially the same as the size and shape of the second substrate;

- fitting the first insert into a recess in a first pressure plate of the platen arrangement of the first aspect of the invention described herein;

- fitting the second insert into a recess in a second pressure plate of the platen arrangement of the first aspect of the invention described herein;

- positioning the first and second substrates in a stacked arrangement on the second insert on the second pressure plate of the platen arrangement;

- optionally heating one or both of the substrates;

- bringing the pressure plates together so that the inserts contact the substrates; and

- applying pressure onto at least one of the pressure plates under conditions to bond the substrates.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

[0030] Embodiments of the present invention will be discussed with reference to the accompanying drawings wherein:

[0031] Figure 1 shows an exploded perspective view from the side of an embodiment of a platen arrangement of the invention for embossing;

[0032] Figure 2 shows a side view of an embodiment of a platen arrangement of the invention for embossing;

[0033] Figure 3 shows a perspective view from below of an embodiment of a platen arrangement of the invention for embossing;

[0034] Figure 4 shows a perspective view from above of an embodiment of a platen arrangement of the invention for embossing; [0035] Figure 5 shows a partial exploded perspective view from above of an embodiment of a platen arrangement of the invention for bonding;

[0036] Figure 6 shows a side view of an embodiment of a platen arrangement of the invention for bonding;

[0037] Figure 7 shows a side line view of an embodiment of a platen arrangement of the invention for bonding;

[0038] Figure 8 shows a perspective view from below of an embodiment of a platen arrangement of the invention for bonding;

[0039] Figure 9 shows a perspective view of an embodiment of a pressure plate of the invention for bonding;

[0040] Figure 10 shows a close up of the perspective view of Figure 9;

[0041] Figure 11 shows a top view of an embodiment of a pressure plate of the invention for bonding;

[0042] Figure 12 shows an exploded perspective of an embodiment of a pressure plate of the invention - for bonding showing a range of inserts and compliant layers that can be fitted to the pressure plate;

[0043] Figure 13 shows an exploded perspective of an embodiment of a pressure plate of the invention for bonding;

[0044] Figure 14 shows a perspective view of an embodiment of a pressure plate of the invention having a round insert for bonding; and

[0045] Figure 15 shows a perspective view of an embodiment of a pressure plate of the invention having a round insert for bonding.

[0046] In the following description, like reference characters designate like or corresponding parts throughout the figures.

DESCRIPTION OF EMBODIMENTS

[0047] The following description is for the purpose of describing particular embodiments only and is not intended to be limiting with respect to the above description and/or the claims. [0048] As used in this specification, the singular forms "a", "an" and "the" include plural aspects unless the context clearly dictates otherwise.

[0049] Where a range of values is expressed, it will be understood that this range encompasses the upper and lower limits of the range, and all values in between these limits.

[0050] The term "microfluidic", and variants thereof, as used throughout the specification means that the device, apparatus or substrate accommodates picolitre, nanolitre or microlitre fluid volumes.

[0051] The present invention arises from research into embossing and bonding systems that can be used to hot emboss or bond glass substrates, or other materials. In particular, the commercially available EVG520 hot embossing system has been designed primarily to emboss or bond on a platform of six inch diameter silicon or glass wafers. However, the system is not suitable for embossing or bonding glass or polymer substrates of a different shape or size to six inch diameter wafers. In particular, the present inventor has developed a platen arrangement having inserts that are interchangeable and can, therefore, be changed to accommodate substrates of different sizes and/or different shapes.

[0052] The present invention provides a platen arrangement 10 for use in fabricating embossed or bonded substrates for use in microfluidic applications. The platen arrangement 10 comprises first 12 and second 14 pressure plates configured to be pressed together to emboss a substrate 16 or bond two substrates 16a, 16b positioned therebetween. Each pressure plate 12, 14 has a working surface 18, 20 comprising a recess 22, 24 configured to removably receive an insert 26, 28 comprising a planar substrate contact surface 30, 32 having a size and shape that is substantially the same as the size and shape of the substrate 16 and an insert retainer 34 for releasably retaining the insert 26, 28 in the recess 22, 24. When the two pressure plates 12, 14 are brought together the inserts 26, 28 are mutually aligned and the substrate contact surface 30, 32 of each insert 26, 28 contacts the substrate to apply pressure thereto.

[0053] The platen arrangement 10 is suitable for use used in an embossing or bonding apparatus 36. Embossing machines are available commercially. Whilst not limited to only this specific use, the platen arrangement 10 is particularly suitable for use in a commercially available EVG520HE Semi-automated Hot Embossing System (EV Group).

[0054] For the purposes of further description, the illustrated embodiments of the invention will described with reference to a platen arrangement 10 in which the pressure plates 12, 14 are orientated horizontally in an embossing or bonding apparatus 36. However, the skilled person will appreciate that other orientations (e.g. vertical) of the pressure plates 12, 14 are also possible. Furthermore, the second pressure plate 14 is the lower of the two plates in the platen arrangement 10 in use and it is typically connected to the force frame of the apparatus 36 such that it is a stationary or dead plate. This lower plate 14 is commonly referred to as the "chuck". In contrast, the first pressure plate 12 is the upper plate in the platen arrangement 10 and it is connected to the force frame or press of the apparatus 36 and is moveable between a retracted position in which it is spaced from the chuck, and an engaged position in which it is in close proximity to, and face to face arrangement with, the chuck such that pressure is applied to the substrate 16.

[0055] The first and second pressure plates 12, 14 can be fitted to the embossing or bonding apparatus 36 following procedures required for the particular apparatus. In the illustrated embodiments fastening points 37 are located at the periphery of the bottom side of the second pressure plate 14 for attachment to a frame device provided with the EVG520 machine. The bottom face of the pressure plate 14 also includes a centrally-placed location bore 39 and a location slot 41 each of which locates onto pins on the bottom heating plate of the EVG520. The first pressure plate 12 attaches to the top heater plate of the EVG520 machine by way of spring-steel clips which locate in six slots 43 around the periphery of the pressure plate 12. These clips retain the pressure plate 12 on the top heater plate whilst still allowing sliding along the plane of the heater face in a manner common to the EVG-produced top plates. The skilled person understands that other arrangements for attaching the pressure plates 12 and 14 to other forms of bonding or embossing machine can be incorporated into the pressure plates 12, 14.

[0056] Advantageously, the insert 26, 28 may be any suitable shape such as rectangular, square, circular, etc. The shape and size of the insert 26, 28 will be dictated by the shape and size of the substrate 16 and, therefore, will typically be about the same shape and size as the substrate to be embossed or bonded. This means that a variety of shapes and sizes of substrates 16 can be embossed or bonded in a single apparatus 36 and without having to remove pressure plates 12, 14. In the embodiments illustrated in Figures 1 to 11 the insert 26, 28 is rectangular in plan view. The inserts 26, 28 in these illustrated embodiments are 72mm by 32mrn and these inserts are suitable for embossing or bonding substrates 16 that are 70mm by 30mm. The substrates may range from about 0.5 mm to 4 mm in thickness.

[0057]> A back surface 36, 38 of the insert 26, 28 is shaped to fit in the recess 22, 24. In the embodiments illustrated in Figures 1 to 11 the back surface 36, 38 is rectangular. As discussed, the substrate contact surface 30, 32 is of a size and shape that approximates the size and shape of the substrate 16. Thus, the substrate contact surface 30, 32 may be a different shape from that of the pressure plates 12, 14.

[0058] In embodiments illustrated in Figures 12 to 15 the recess 22 of the pressure plate 12 is configured to receive round or rectangular inserts 26. These embodiments are advantageous because the inserts 26 of different shapes (in this case rectangular or circular) can be fitted into a single pressure plate 12, thereby eliminating the need to change pressure plates 12 when changing between rectangular or circular inserts 26. The recess 22 in these embodiments comprises rectangular recesses of two common sizes and a contiguous circular recess. [0059] When the insert 26, 28 is fitted in the recess 22, 24 of the pressure plate 12, 14 the substrate contact surface 30, 32 is co-planar with the working surface 18, 20. The substrate contact surface 30, 32 is spaced away from the working surface 18, 20. In these embodiments, the depth of the recess 22, 24 in each pressure plate 12, 14 is less than the thickness of the corresponding insert 26, 28.

[0060] The inserts 26, 28 can be manufactured from any material that is capable of withstanding the pressures and temperatures used in the bonding or embossing applications contemplated. Examples of suitable materials for use in manufacturing inserts 26, 28 include metals such as brass, aluminium, stainless steel, tool steels, nickel and its alloys, and titanium and its alloys. Stainless steel and nickel inserts 26, 28 may be particularly suitable for embossing or bonding glass substrates 16, whilst brass inserts 26, 28 may be particularly suitable for embossing or bonding plastic substrates 16. A number of hard polymers such as polyetheretherketone (PEEK), polymethyl methacrylates (PMMA), polyester, polyethylene and its derivatives could also be used to manufacture the inserts 26, 28. It is also contemplated that soft elastomers, such as polydimethylsiloxane (PDMS), could be used. The materials used to manufacture the inserts 26, 28 may be chosen on the basis of fabrication route and glass transition temperatures, melting points and thermal expansion coefficient with respect to the substrate to be embossed. It is also envisaged that the inserts 26, 28 may incorporate a thin micro- or nanostructured layer of silicon, metal or polymer adhered to a contact surface 30, 32.

[0061] In embodiments that are suitable for bonding two substrates 16a, 16b, the substrate contact surface 30, 32 of each insert 26, 28 is a planar, polished surface. This causes pressure to be distributed evenly over the substrate/s to affect complete bonding of the two substrates across the entire area. The polished fmish prevents the transfer of rough surface features from the substrate contact surface of the insert 30, 32 during bonding. In other embodiments, the substrate contact surface 30, 32 on at least one of the inserts 26, 28 may be slightly curved in a convex manner with respect to the substrates 16a, 16b so that the centre of the substrates 16a, 16b are compressed together first in a bonding procedure thus minimising the likelihood of gas becoming entrapped between the substrate layers 16a, 16b to be bonded. The substrate contact surface 30 or 32 can be made slightly convex by leaving mechanical stress in the insert 26 or 28 as it is machined parallel, thereby making the front and back faces of the insert concentric on a very large radius. The convex substrate contact surface 30 or 32 could also be formed by lapping procedures.

[0062] In embodiments that are suitable for embossing a substrate 16, the insert 28 on the second (lower) pressure plate 14 has a planar, milled and polished substrate contact surface 32 on which the substrate 16 to be embossed sits. The insert 26 on the first (upper) pressure plate 12 is a stamp that is used to define the pattern in the substrate 16. The pattern on the stamp 26 may be formed on the substrate contact surface 30 using any of the methods known in the art for that purpose, such as micromachining, LIGA, machining using a CNC tool, electroplating, electroforming, photolithography, laser ablation, focussed ion-beam (FB) ablation, deep reactive ion etching, wet etching, soft lithography, etc.

[0063] There are a number of advantages associated with the use of inserts 26, 28. Firstly, the insert 26, 28 transfers the force from the pressure plate 12, 14 onto the substrate 16. However, because the substrate contact surface 30, 32 of the insert 26, 28 is about the same shape and size as the substrate 16, the force applied through the stack is resisted by the substrate 16 over the entire area of the insert 26, 28, which avoids force being applied to areas of the insert 26, 28 which may be unsupported by the resistance of the substrate 16, allowing it to deform. Similarly, the use of compliant layers (discussed in more detail later) that are substantially the same size and shape as the substrate 16 ensures that force is only applied to those areas of the pressure plate 12 that are supported by the stacked substrate 16, compliant layer 112 and insert 26, 28.

[0064] Secondly, each insert 26, 28 is removable and interchangeable which means that different shaped and sized substrates 16 can be embossed or bonded using a single platen arrangement 10. Furthermore, in embodiments in which the arrangement is used for embossing a substrate the insert 26 on the first pressure plate 12 is patterned with a pattern to be transferred onto the substrate 16. The interchangability of the inserts 26, 28 means that inserts or stamps having different patterns can be used in the same embossing apparatus and can be easily interchanged without having to completely remove one of the pressure plates 12, 14 from the apparatus. Furthermore, in embodiments the pressure plates 12, 14 may be configured in such a manner to receive inserts 26, 28 of different shapes and sizes with corresponding compliant layers of shapes and sizes that are substantially the same to allow high-pressure embossing or bonding of substrates with corresponding shapes and sizes.

[0065] The inserts 26, 28 have a retaining flange 40. The retaining flange 40 extends peripherally from the body and on a side of the insert 26, 28 in a plane that is parallel with the plane of the substrate contact surface 30, 32. The retaining flange 40 extends from the insert 26, 28 adjacent a back surface 36, 38 of the insert 26, 28.

[0066] In the embodiments illustrated in Figures 1 to 11 the inserts 26, 28 are rectangular in plan view and they have a retaining flange 40 extending from opposing sides 42a, 42b of the insert 26, 28. Square inserts 26, 28 could also be used.

[0067] In the embodiments illustrated in Figures 12 to 15 the inserts 26, 28 are circular and the retaining flange 40 extends diametrically from the circumference of the insert. The retaining flange 40 need not extend from the entire circumference of the insert 26, 28. For example, the retaining flange 40 may extend diametrically from diametrically opposed parts of the circumference of the insert. [0068] The retaining flange 40 engages with the insert retainer 34 when the insert 26, 28 is located in the recess 22, 24. Specifically, the insert retainer 34 is in the form of a tab 44 that is fitted over the retaining flange 40 and fastened or fixed to the working surface 18, 20 of the pressure plate 12, 14. The tab 44 is housed in an indent 46 formed on the working surface 18, 20 adjacent one side of the recess 22, 24. The depth of the indent 46 is substantially the same as the thickness of the tab 44 so that, when the tab 44 is fitted in the indent 46, a top surface of the tab is substantially flush with the working surface 18, 20. The tab 44 has one or more holes through which a fastener, such as a bolt 48, can pass. The bolt 48 is threaded into corresponding threaded holes 50 in the top surface of the indent 46. To fit an insert 26, 28 to a pressure plate 12, 14, the insert 26, 28 is placed in the recess 22, 24 and a tab 44 is positioned on each side of the insert 26, 28 and in the indent 46. When correctly fitted, one edge of each tab 44 is positioned on top of a respective retaining flange 40. Bolts 48 are then placed in the holes of the tab and tightened. The insert 26, 28 is then securely fastened to the pressure plate 12, 14.

[0069] Advantageously, the retaining flange 40 is capable of bending if a force above a predetermined force is applied to the insert so as to indicate if the insert has been overloaded. When force is placed on the pressure plates 12, 14, some of the resultant load is borne by the retaining flanges 40. Above a predetermined force, the flanges 40 will bend in the direction that the force is applied due to the deformation of a compliant layer 112 behind the insert. Thus, when the platen arrangement 10 is disassembled after use any deformation in the flange 40 provides an indication that the insert 26, 28 was overloaded. Importantly, this occurs at a load that is significantly smaller than that which is required to cause damage to other parts of the assembly or to the machine. The thickness and depth of the flange 40 may be varied so that the flange 40 will only bend above the required predetermined forces.

[0070] The insert retainer 34 may be in other forms and, for example, the tabs 44 may be in the form of relatively small lugs that are positioned over the retaining flange 40. Furthermore, the tabs 44 may be relatively small tabs that are rotatably connected to the working surface 18, 20 such that they can be rotated into a position over the retaining flange 40. These embodiments may accommodate inserts 26, 28 of several shapes and sizes.

[0071] At least one of the pressure plates 12, 14 comprises a substrate retainer 52 for holding the substrate 16 on the insert 26, 28. In embodiments illustrated in Figures 1 to 4 that are suitable for hot embossing applications, the substrate retainer 52 comprises at least one fixed retaining lug 54 that is fixed to the pressure plate 12, 14 in a position whereby it abuts one side of the substrate 16 when it is correctly positioned on the insert 26, 28. At least one movable retaining lug 56 is fixed to the pressure plate 12, 14 whereby it abuts an opposing side of the substrate 16 when it is correctly positioned on the insert 26, 28. The moveable retaining lug 56 is slideable between an engaged position in which it abuts the opposing side of the substrate 16, and a disengaged position whereby it is spaced from the opposing side of the substrate to allow for insertion or removal of the substrate 16 from the insert 26, 28. Advantageously, the movable retaining lug 52 is biased to the engaged position. In embodiments illustrated in Figures 1 to 4 there are two fixed retaining lugs 54 and two moveable retaining lugs 56. Each of the retaining lugs 54, 56 has a finger 58 extending from one end thereof. The finger 58 abuts a side of the substrate 16 when it is placed on the insert 26, 28 and prevents lateral movement of the substrate 16 on the insert 26, 28. The fixed retaining lugs 54 are fastened to the pressure plate 12, 14 on the working surface 18, 20 using a suitable fastener such as a bolt 60 which passes through a hole (not shown) in the retaining lug 54 and is screwed into a corresponding threaded hole 62 in the pressure plate 12, 14. The retaining lugs 54, 46 are positioned in a lug recess 64 formed in the working surface 20.

[0072] The movable retaining lug 56 comprises a slot 66 extending through the lug 56. A bolt 68 having a shoulder 70 passes through the slot 66 and is threaded into a corresponding threaded hole 72 in the working surface 18, 20 of the pressure plate 12, 14. The movable retaining lug 56 is free to slide along the bolt 68 between the engaged position and the disengaged position. The moveable retaining lug 56 is biased to the engaged position by a biasing means such as a coil spring 74 that is positioned between a wall 76 of the retainer recess 64 and the retaining lug 56. A rear surface 78 of the moveable retaining lug 56 has a bore 80 into which the spring 74 fits.

[0073] The illustrated substrate retainer 52 is particularly advantageous as it allows for easy fitting and removal of a substrate 16 on the insert 26, 28. The moveable retaining lugs 56 can be slid back, the substrate 16 lowered onto the substrate contact surface 30, 32 and the biased moveable retaining lugs 56 will then push the substrate 16 into engagement with the fixed retaining lugs 54. As best seen in Figures 2 and 4, the finger 58 on each retaining lug 54, 56 abuts a side of the substrate 16 and exerts pressure thereon to hold the substrate 16 in place. In hot embossing operations, it is important to disengage the working surface 30 of the stamp insert 26 from the embossed substrate 16 prior to significant cooling of the assembly. This is because differences in the thermal expansion coefficients of the substrate 16 and insert 26, 28 material can lead to damage to the microstructure of both the contact surface 30, 32 of the insert 26, 28 and of the embossed substrate 16, deformation of the substrate 16, or cracking or disintegration of the embossed substrate 16. This is particularly the case when embossing glass substrates 16. The fingers 58 on the retaining lugs 54, 56 impose sufficient force on the substrate 16 during embossing to withstand the attachment force of the substrate contact surface 30 of the insert 26 to the substrate 16 during disengagement thus preventing the substrate from lifting away from the bottom insert 28 and remaining attached to the contact surface 30 of the insert 26. To remove the substrate 16, the moveable retaining lugs 56 are slid back and the substrate 16 lifted from the insert 26, 28. At embossing temperature, the magnitude of the bias of the retaining lugs 52, 56 may be great enough to impress a small groove in the side of the substrate 16 thus increasing the retaining force in a direction perpendicular to the plane of the contact surface 30 that may be withstood during the disengagement process. [0074] In embodiments illustrated in Figures 5 to 10 that are suitable for bonding applications, the substrate retainer 52 comprises moveable retaining lugs 56 and positioning means 82 for locating one or more substrates on the insert 28. The positioning means 82 comprises one or more pins 84 that extend from the substrate contact surface 32 adjacent one or more sides thereof. As best seen in Figure 10, the positioning means 52 in the illustrated embodiments comprises four pins 84 with two adjacent one long side of the substrate 16 when it is correctly positioned on the substrate contact surface 32 and one each at either end of the substrate 16 when it is correctly positioned on the substrate contact surface 32. Moveable retaining lugs 56 are fitted into a retainer recess 64 and are biased to an engaged position, as described previously. The height of the pins 84 is such that the pin abuts a side of two substrates 16a, 16b when they are fitted in a stacked arrangement on the insert 28. In this way the pins 84 assist in maintaining both substrates 16a, 16b in position throughout the bonding process. This allows alignment of the layered substrates 16a, 16b to tight tolerances. The height of the pins 84 is also less than the combined thickness of the substrates 16a, 16b so that the pins 84 do not make contact with the insert 26 of the first pressure plate 12 during the bonding process.

[0075] In both bonding and embossing operations, alignment of the microstructure between the stamp insert 26 and the substrate 16 in embossing applications, or between the substrates 16a and 16b in bonding applications is important. In embossing operations, the imprinted features must be aligned and positioned in a well-defined position with respect to some datum points, ideally placed on the edge of the substrate 16. In bonding operations, alignment between the features of the top 16a and bottom 16b or subsequent substrates must also be accurate to allow the fabrication of devices with communicating features on each substrate, for example. The platen arrangement 10 of the present invention comprises several alignment features that allow such alignment without the need for elaborate optical preparation.

[0076] The platen arrangement 10 comprises alignment means 100 for aligning the first 12 and second 14 pressure plates as they are brought together. In the illustrated embodiments, the alignment means 100 comprises at least two posts 102 extending upwardly from the working surface 20 of the bottom plate 14 and at least two corresponding slots or bores 104 in corresponding positions on the upper pressure plate 12. It will be appreciated that the post and bore arrangement of the alignment means 100 could be also be the other way around, with the posts 102 extending from the working surface 18 of the top pressure plate 12 and the bores in the bottom pressure plate 14. It will also be appreciated that the posts 102 may take forms other than predominantly cylindrical, such as rectangular or oblong lugs which may provide a better service life through diminished wear. Such lugs may also be positioned to act on the periphery of the pressure plates 12, 14 to effect the alignment in at least one axis. In each arrangement, as the two pressure plates 12, 14 are brought together the posts 102 enter a corresponding slot or bore 104 and, in doing so, align the two pressure plates 12, 14 with respect to one another. To assist in the alignment, the slots or bores 104 are chamfered so that if the plates 12, 14 are not properly aligned when they are brought together, the angle of the chamfer will direct the post 102 into the corresponding bore or slot 104. The pressure plates 12, 14 may be arranged in a manner prior to an operation that ensures that the operation of the alignment pins 104 as the pressure plates 12, 14 approach each other is through interference on only one side of each of the alignment slots 104. Thus, the alignment occurs in one direction only for each axis, allowing the small geometric tolerance between the dimension of the posts 102 and slots 104 to have no bearing on the alignment accuracy. In other embodiments, it is envisaged that this mode of operation for alignment may be affected by some form of mechanical bias, such as compression springs, for example.

[0077] As best seen in Figure 3, the first pressure plate 12 comprises a compliant layer 1 0 on a back surface 108 that is opposite the working surface 18. The compliant layer 110 is of a size and shape substantially the same as the substrate 16 and/or the inserts 26, 28. The compliant layer 110 is located in a back face recess 106 on the back surface 108. As seen in Figure 13, the back face recess 106 may be shaped to accommodate a round, rectangular or square compliant layer 110, with the size and shape of the compliant layer 110 being determined by the size and shape of the substrate 16 and inserts 26, 28. The compliant layer 110 may be a layer of any suitable material such as graphite, polymethdisiloxane (PDMS), latex, etc. The compliant layer 110 may have a thickness of approximately 0.5 mm-1 mm and is of a shape and size to fit snugly in the back face recess 106 whilst remaining proud of the back surface of the pressure plate 12. The compliant layer 110 distributes force solely into those areas of the platen arrangement that engage with the insert.

[0078] As illustrated in Figures 1 and 12, a second compliant layer 112 may also be inserted in the recess 22 of the first pressure plate 12 between the working surface 18 and a back of the insert 26. Like the first compliant layer 110, the second compliant layer 112 is of a size and shape substantially the same as the substrate 16 and/or the inserts 26, 28. The second compliant layer 112 may be the same of different to the compliant layer 110 and accommodates any subtle errors in planar alignment and flatness of the inserts 26, 28.

[0079] The present invention also provides an apparatus for fabricating embossed or bonded substrates for use in microfluidic applications. The apparatus comprises a platen arrangement 10 as described herein, a press for applying pressure to at least one of the pressure plates 12, 14 in the platen arrangement 10, and a heater for optionally heating a substrate 16 positioned on an insert 26, 28 on one of the pressure plates 12, 14.

[0080] A typical apparatus comprises a force frame which delivers the embossing force to the platen arrangement 10. The planar substrate 16 is mounted on the lower pressure plate 14. The pressure plate 14 is heated using a heater. At the beginning of the embossing cycle, the apparatus is either evacuated or filled with inert gas and the substrate 16 is heated. The substrate 16 is heated to a temperature just above the glass transition temperature of the substrate material. In the case of glass, the substrate is heated to a temperature of about 630°C. Prior to the application of any force, the chamber is generally evacuated. The insert 26 on the first pressure plate 12 is then brought into contact with the substrate 16 and an embossing or bonding force, which is typically of the order of 1-60 kN, is applied. The embossing or bonding force is then removed and the pressure plates 12, 14 are drawn apart. This effects the insert- substrate disengagement in embossing processes. The insert-substrate sandwich is then cooled and the embossed or bonded substrate 16 removed.

[0081] In one specific application, the platen arrangement 10 may be used in a hot embossing process for forming a micro fluidic network on a substrate 16. The substrate may be glass, quartz, a polymeric material such as polymethyl methacrylate (PMMA), polycarbonate (PC), cyclic olefin copolymer (COC), etc. or a film or thin layer of hard or soft polymer materials supported by a silicon or glass wafer substrate. In embodiments, the substrate is glass.

[0082] For the purpose of embossing, the first insert 26 is a stamp having a pattern to be transferred to the substrate 16. In the illustrated embodiments the stamp insert 26 is fitted to the first pressure plate 12. Insert 28 having a flat substrate contact surface 32 is fitted to the second pressure plate 14 as described previously. The first pressure plate 12 is then lowered toward the second pressure plate 14 until the stamp contacts the upper surface of the substrate 16. Pressure and heat are then applied under appropriate conditions to impress the pattern onto the substrate surface 16. The heat used is generally marginally in excess of the glass transition temperature of the substrate material.

[0083] As will be appreciated by the skilled person, a range of microfluidic architectures can be formed on the substrate 16. Typical components of microfluidic architectures include wells, channels, chambers, valves, etc.

[0084] Alternatively, or in addition, the platen arrangement 10 may be used in a hot bonding process for bonding two substrates 16 a, 16b together. The substrates 16a, 16b may be glass, silicon or a polymeric material. For bonding applications a second or top substrate 16b is placed on top of the first or lower substrate 16a and the two substrates are bonded by applying heat and pressure. In these embodiments, both inserts 26, 28 have a lapped, polished and parallel planar substrate contact surface. Substrates of a variety of materials, such as glass, polymethyl methacrylate (PMMA), polycarbonate (PC), cyclic olefin copolymer (COC), etc. may be bonded to lid and seal microfluidic devices. This is performed by heating the parts to be bonded to their glass transition temperature and forcing them together in a vacuum, causing intimate contact and irreversible bonding.

[0085] The present invention also provides a process for embossing a microstructured device. The process comprises providing a substrate 16 to be embossed and a stamp insert 26 having a pattern to be embossed on the substrate formed thereon. The stamp insert 26 is inserted into a recess 22 in a first pressure plate 12 of the platen arrangement 10 of the invention described herein. The substrate 16 is placed on an insert 28 on the second pressure plate 14 of the platen arrangement 10. Optionally, the substrate 16 is heated. The pressure plates 12 and 14 are then brought together so that the pattern on the stamp insert 26 contacts the substrate 16. Pressure is applied onto at least one of the pressure plates 12 and 14 to transfer the pattern from the stamp insert 26 onto the substrate 16.

[0086] The present invention also provides a process for bonding two substrates 16a, 16b wherein at least one of the substrates has microstructured features. The process comprises providing a first substrate 16a, a second substrate 16b and, optionally subsequent substrates to be bonded together; providing a first insert 26 comprising a planar substrate contact surface 30 having a size and shape that is substantially the same as the size and shape of the first substrate 16b; providing a second insert 28 comprising a planar substrate contact surface 32 having a size and shape that is substantially the same as the size and shape of the second substrate 16a. The first insert 26 is fitted into a recess 22 in a first pressure plate 12 of the platen arrangement 10 and the second insert 28 is fitted into a recess 24 in a second pressure plate 14 of the platen arrangement 10. The first and second substrates 16a, 16b and optionally any subsequent substrates are positioned in a stacked arrangement on the second insert 28 on the second pressure plate 14 of the platen arrangement 10. Optionally one or more of the substrates is heated. The pressure plates 12 and 14 are then brought together so that the inserts 26, 28 contact the substrates 16a, 16b. Pressure is applied onto at least one of the pressure plates under conditions to bond the substrates.

The bonding process may be used to bond two or more substrates 16a, 16b in an aligned manner. For bonding, the platen arrangement 10 further comprises substrate alignment means for aligning two or more substrates 16a, 16b with respect to datum points on their periphery to allow alignment and structural agreement between communicating microstructures on each substrate 16a, 16b during bonding. The substrate alignment means may take any form but in the illustrated embodiments it comprises alignment pins 84 that are positioned on the insert 28 so that they correspond relatively closely to the positioning on the substrates 16a, 16b of any alignment features such as fiducials used in previous fabrication procedures, such as photolithography. For example, if fiducials have been aligned to the edge of the substrate 16a, 16b, then the alignment pins 84 are best placed in corresponding locations on the insert 28 so that their physical engagement with the edge of the substrate 16 corresponds to the points of alignment in earlier steps. In this manner, any subtle inaccuracies or inconsistencies in the shape or straightness of the alignment edge will not translate as inaccuracy in alignment between the two substrates 16a, 16b.

[0087] The alignment pins 84 may be of variable length to accommodate substrate stacks of different thicknesses. The alignment pins 84 or other form of alignment means may be sprung to allow some movement along their axis, perpendicular to the plane of the substrate contact surface 32 thus accommodating a degree of interference between the top of these structures and the opposing substrate contact surface should they protrude beyond the height of the substrate stack 16a, 16b thus avoiding damage.

[0088] Some substrates such as hydrophilic clean glass are inclined to bond on contact to some extent due to electrostatic and molecular forces. In practice, the alignment of such substrates may be facilitated by the use of a separate alignment tool having alignment pins placed so that they correspond to the locations of the alignment pins 84 in the insert 28 and to the positions of fiducials used in

photolithographic steps.

[0089] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to, or indicated in this specification, individually or collectively, and any and all combinations of any two or more of the steps or features.

[0090] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0091] All publications mentioned in this specification are herein incorporated by reference. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia or elsewhere before the priority date of each claim of this application.