METHOD OF PRODUCING A STRUCTURE

Abstract

According to the invention there is provided a method of producing a structure comprising the steps of: a) providing a substrate comprising one or more features that correspond to the shape of the structure to be produced, wherein the one or more features comprise a hydrophobic polydimethylsiloxane (PDMS) surface; b) exposing at least a part of the hydrophobic PDMS surface to a plasma so that the part of the hydrophobic PDMS surface that is exposed to the plasma forms a hydrophilic PDMS surface; c) depositing a seed layer onto the hydrophilic PDMS surface by electroless deposition; d) depositing one or more metallic layers onto the seed layer by electrochemical deposition to form the structure; and e) removing the structure from the substrate.

Claims

1. A method of producing a structure comprising the steps of: a) providing a substrate comprising one or more features that correspond to the shape of the structure to be produced, wherein the one or more features comprise a hydrophobic polydimethylsiloxane (PDMS) surface; b) exposing at least a part of the hydrophobic PDMS surface to a plasma so that the part of the hydrophobic PDMS surface that is exposed to the plasma forms a hydrophilic PDMS surface; c) depositing a seed layer onto the hydrophilic PDMS surface by electroless deposition; d) depositing one or more metallic layers onto the seed layer by electrochemical deposition to form the structure; and e) removing the structure from the substrate.

2. The method according to claim 1 in which the plasma is an oxidising plasma.

3. The method according to claim 1 in which the plasma is an oxygen-containing plasma.

4. The method according to claim 3 in which the oxygen-containing plasma is an O.sub.2 plasma, a clean dry air (CDA) plasma, or an O.sub.2/Ar plasma.

5. The method according to claim 1 in which step b) is performed at about atmospheric pressure.

6. The method according to claim 1 in which the part of the hydrophobic PDMS surface that is exposed to the plasma is defined by a mask.

7. The method according to claim 1 further comprising the step of removing at least a part of the seed layer prior to step d).

8. The method according to claim 1 in which the one or more metallic layers comprise a plurality of metallic layers.

9. The method according to claim 1 in which the one or more metallic layers are formed from one or more of nickel, iron, cobalt, manganese, phosphorous, gold, silver, and/or any other noble metal.

10. The method according to claim 1 in which step e) comprises removing the structure from the substrate non-destructively.

11. The method according to claim 1 in which the sequence of steps b) to e) are repeated.

12. The method according to claim 1 further comprising the steps of: aa) providing a mould formed from a fluoropolymer, wherein the mould comprises one or more features that correspond to the shape of the substrate; ab) casting PDMS in the mould to form the substrate; and ac) removing the substrate from the mould.

13. The method according to claim 12 in which the fluoropolymer is polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), or fluorinated ethylene propylene (FEP).

14. The method according to claim 1 in which the features are upstanding from the substrate.

15. The method according to claim 1 in which the features have a height of 50-2000 m, preferably 100-1500 m, and more preferably 250-1000 m.

16. The method according to claim 1 in which the features have a width of 50-700 m, preferably 75-500 m, and more preferably about 350 m.

17. The method according to claim 1 in which the one or more features are tapered.

18. The method according to claim 1 in which the structure comprises a microstructure.

19. The method according to claim 1 in which the structure comprises a plurality of microneedles.

20. The method according to claim 1 in which the structure comprises a microelectronic component, a micro-coil, an inductor, or a conductive pillar.

Description

[0047] Embodiments of substrates and methods in accordance with the invention will now be described with reference to the accompanying drawings, in which:

[0048] FIG. 1 is a schematic illustration of a solid, hollow and coated microneedle;

[0049] FIG. 2 is a schematic cross-section of the surface of the skin;

[0050] FIG. 3 is a flow chart illustrating the method according to the first embodiment of the invention;

[0051] FIG. 4 is a schematic cross-sectional illustration of a structure formed by the first embodiment of the invention;

[0052] FIG. 5 is a schematic cross-sectional illustration of the production of a PDMS substrate from a mould;

[0053] FIG. 6 is a schematic cross-sectional illustration of the formation of a seed layer on a substrate, which would lead to (A) a solid microneedle, (B) a hollow microneedle with an aperture at the tip, and (C) a hollow microneedle with an aperture at a position along the taper;

[0054] FIG. 7 is a schematic cross-sectional illustration of features after an electrochemical deposition step;

[0055] FIG. 8 is a conventional electrochemical deposition apparatus; and

[0056] FIG. 9 is a cross-sectional view of a microneedle formed using the method of the first embodiment.

[0057] FIG. 3 shows a flow chart summarising the method of manufacturing a structure according to a first embodiment of the invention. Firstly, a 3D structure is designed to be suitable for the desired end use (step 31). By way of example only, the structure may be in the form of an array of microneedles, or a microelectronic device, such as a micro-coil, an inductor or a series of conductive pillars.

[0058] In the first embodiment, the structure 40 to be formed is a 55 array of hollow microneedles. FIG. 4 shows a simplified cross-section of a free-standing structure 40 of the first embodiment. Each microneedle has a height of 500 m, an outer diameter (OD) of 300 m, and a pitch of 1100 m. Each microneedle has an internal bore with an inner diameter (ID) of 150 m, and a wall thickness of 75 m. The 55 array has a total size of 6.7 mm6.7 mm. It will be apparent to the skilled reader that the size and dimension of each microneedle in the array can individually be chosen to be any convenient size depending on the device design, including different sizes.

[0059] When the shape and arrangement of the structure 40 has been defined, a master mould 50 is produced (step 32). The master mould 50 has features 52 that correspond to the shape of the structure 40. The master mould may conveniently be a negative of the shape of the structure 40. Alternatively, the master mould may be a positive of the shape of the structure 40.

[0060] In the first embodiment the master mould 50 is formed from a fluoropolymer, such as polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), or fluorinated ethylene propylene (FEP). Fluoropolymers are preferred due to their non-stick properties and due to their low chemical reactivity.

[0061] The master mould 50 is typically produced using conventional direct write laser methods. Direct write laser methods are convenient to produce a master mould including micro-scale features (i.e. a few hundred micrometres and below) with good resolution and accuracy. Despite the relatively high cost of the direct write laser process, the master mould 50 can be reused many times as required. Therefore, the direct write laser step is a single time cost.

[0062] At step 33, a polydimethylsiloxane (PDMS) substrate is formed based on the master mould 50. PDMS was sourced from The Dow Chemical Company. A mixed PDMS is poured into the features 52 of the master mould 50. The PDMS is degassed and thermally cured at a temperature of 85-100 C. The cured PDMS is subsequently removed from the master mould 50 to provide a solid, hydrophobic PDMS substrate 60, 60a (FIG. 5). Since the PDMS substrate 60 is flexible and has low adhesion to the fluoropolymer mould 50, the substrate 60 can be conveniently separated from the mould 50, for example by peeling, without damaging either the substrate 60 or master mould 50. The PDMS substrate has features 62, 62a that correspond to the shape of the desired structure 40. For simplicity, only one feature 62, 62a is labelled on FIG. 5. PDMS can resolve features that are as small as about 10 nm in diameter. The features 62, 62a are formed during the curing process simultaneously to the formation of the base 63 of the substrate 60. The cured PDMS substrate 60, including the base 63 and features 62, is monolithic. In the first embodiment, the PDMS substrate 60 is a positive of the shape of the structure 40. Alternatively, the PDMS substrate 60a is a negative of the shape of the structure 40.

[0063] The cured PDMS substrate 60 is a flexible and soft material having a Young's modulus of about 2 MPa. The highly flexible nature of the cured PDMS aids in the removal of the PDMS from the mast mould 50. PDMS is preferably used due to its low adhesion to the fluoropolymer master mould 50. Since PDMS does not adhere well to the fluoropolymer master mould 50, it is possible to create high aspect ratio features 62, 62a with excellent reproducibility from the master mould 50. In the first embodiment, the features 62 correspond to an array of microneedles. The microneedles have a slightly tapered tip, which aids removal of the PDMS substrate from the master mould 50, and aids penetration of the microneedles into tissue.

[0064] From a single master mould 50 it is possible to produce many PDMS substrates at a low cost and on a quick timescale. The PDMS substrate 60 is used as a substrate in subsequent processing steps.

[0065] Parts of the hydrophobic PDMS substrate 60 are subsequently subjected to a surface treatment step (step 34) to selectively reduce the hydrophobicity of these parts. In some instances, the surface treatment step 34 selectively converts the hydrophobic surface of the PDMS substrate 60 to a hydrophilic surface. Here, hydrophobic is used to mean a surface having a contact angle for water of greater than 90. Hydrophilic is used to mean a surface that is wetted by water.

[0066] The surface treatment step 34 typically comprises exposing parts of the PDMS substrate to an oxidising plasma. The surface treatment step 34 may be performed at a reduced pressure. Alternatively, an atmospheric plasma may be used. The plasma typically comprises oxygen. The plasma may be a clean dry air (CDA) plasma, an O.sub.2 plasma, an O.sub.2/Ar plasma, or any other suitable plasma. The plasma typically uses RF or DC excitation means. Without wishing to be bound by any theory or conjecture, it is believed that an oxidising plasma reacts with the surface of the PDMS. More specifically, the oxidising plasma removes methyl groups (CH.sub.3) from the PDMS surface to form a hydrophilic surface.

[0067] The hydrophilic surface formed by the surface treatment step 34 is suitable for use as a substrate to deposit a metallic seed layer via electroless deposition (step 35). Electroless deposition is used to deposit a metallic seed layer 42 of copper (Cu), nickel (Ni) or the like. Electroless deposition is performed using conventional methods. Typically, nanoparticle initiators, such as Pd and Sn nanoparticles, are used to initiate the electroless deposition. Other metal nanoparticle catalysts may also be used. The electroless deposition typically terminates after 1 m of metal has been deposited. Electroless deposition is generally a less expensive processing technique compared to sputtering methods, such as PVD methods.

[0068] Electroless deposition only occurs on parts of the PDMS substrate that have been subjected to the surface treatment step. That is, the seed layer only forms where the PDMS is hydrophilic.

[0069] FIG. 6A shows a seed layer 42 formed across the entirety of the PDMS feature 62. In this instance, the entire feature 62 was subjected to the surface treatment step 34. This configuration would lead to a solid microneedle.

[0070] In the first embodiment, the structure 40 is an array of hollow microneedles. In order to form a hollow microneedle having an aperture 44, it is desirable to avoid depositing the seed layer either at the tip of the microneedle (FIG. 6B) or at a specific location along the taper (FIG. 6C).

[0071] It is possible to selectively control where the electroless deposition of the seed layer occurs by selectively controlling which areas of the PDMS substrate are hydrophilic.

[0072] In one embodiment a mask layer is applied to the PDMS substrate prior to the surface treatment step 34. In this instance, only the areas that are exposed to the oxidising plasma undergo a surface change. The areas below the mask are not exposed to the plasma, and remain hydrophobic. Therefore, electroless deposition only occurs at areas that have been exposed to the surface treatment step.

[0073] In another embodiment, the PDMS substrate 60 is subjected to the surface treatment step 34 so that all of the PDMS surface becomes hydrophilic. Hydrophilic regions can be selectively converted back to their original hydrophobic state by removal of small amounts of material (1 m) from the PDMS surface by, for example, laser ablation. It is noted that the change in hydrophobicity is a surface effect, and such changes occur at a depth of <<1 m. Therefore, the selective change in hydrophobicity has a minimal influence on the shape and dimension of the PDMS substrate 60 and subsequent structure 40.

[0074] In another embodiment, the entire PDMS substrate 60 is subjected to the surface treatment step 34 so that the entire PDMS surface becomes hydrophilic. The electroless deposition step 35 forms a seed layer on the hydrophilic PDMS surface. Regions of the seed layer can subsequently be removed by wet etching or laser ablation to form a patterned seed layer 42.

[0075] After the seed layer 42 has been deposited in the desired locations, one or more additional metal layers 46 are subsequently deposited on the seed layer 42 by electrochemical deposition (step 36). The seed layer 42 acts as a cathode and deposition surface for the electrochemical deposition process. Electrochemical deposition is performed using conventional methods. FIG. 8 shows a schematic of apparatus suitable for performing the electrochemical deposition step 36. The electrochemical cell comprises an anode 70, a cathode 72 and a plating bath 74. The potential of the electrodes is controlled by a controller or power supply 76.

[0076] Electrochemical deposition provides a method to controllably increase the thickness of the additional metallic layers 46. The additional metallic layers 46 provide the bulk material of the structure 40. In the first embodiment, the additional metallic layers 46 correspond to the walls of the microneedle. Electrochemical deposition allows high aspect ratio features, and features having a size of several hundred micrometres and below to be produced with high resolution and good reproducibility. Electrochemical deposition does not occur in regions where the seed layer has not been formed. Electrochemical deposition therefore provides a convenient method to produce an aperture 44 in the structure, such as apertures in the tip of a microneedle (as shown in FIG. 7).

[0077] The electrochemical deposition step 36 is conveniently continued or repeated until the desired metal thickness is achieved. Typically, the desired total thickness of the additional metallic layers 46 is less than about 75 m. In some embodiments, a single metal layer 46 is formed from a single metal. In other embodiments, a plurality of metal layers 46 is formed from a plurality of metals. The additional metal layers 46 may be formed from iron, nickel, copper cobalt, manganese, phosphorous, or any other metal.

[0078] In some embodiments, it is desirable for the final external layer (i.e. the outermost layer) to be a thin layer of a noble metal, such as gold or silver. An outermost noble metal layer helps to avoid reaction if the structure 40 is used to penetrate living tissue, for example, if the structure is an array of microneedles. Such a thin layer of noble metal is conveniently deposited using conventional displacement plating techniques.

[0079] The metal (or metals) deposited via the electrochemical deposition step 36 provides the structural integrity of the resultant structure 40. Typically, depositing a plurality of metal layers formed from a plurality of metals provides a stronger structure 40.

[0080] When the desired thickness has been achieved, the resultant structure 40 is removed from the PDMS template 60. Preferably, the structure 40 is removed from the PDMS template 60 in a non-destructive manner, for example, by peeling the structure away from the PDMS template 60. Since the PDMS substrate 60 is flexible, it can be separated from the structure 40 easily without damaging any features of the structure. In this way, the PDMS template 60 can be used again to make further structures. Reusing the PDMS template 60 significantly reduces the material cost and material waste during the manufacturing process.

[0081] After removal from the template, the structure 40 may be used directly or be subjected to further processing steps 38 as desired. For example, it may be convenient to coat the surface of the structure 40 that was previously in contact with the PDMS template 60. In the instance that the structure 40 is an array of hollow microneedles, the internal bore of the microneedles may be coated with a noble metal 48, such as silver. Other processing steps may also include depositing a low-friction polymer coating or laminate 49 on the outer surface of the structure 40 (FIG. 9). The coatings 48, 49 act as a protective layer for the structure 40. The polymer coating 49 may prevent leaching of metal from the additional metal layers 46. The single microstructure illustrated in FIG. 9 may be part of an array, such as an array of microneedles.

[0082] The resultant structure 40 is typically a microstructure, such as an array of microneedles, or a microelectronic device, such as a micro-coil, an inductor or a series of conductive pillars.

[0083] The additive process of the present invention provides an economical method to manufacture structures 40, in particular microstructures. By way of example only, a 55 array of microneedles was produced using the additive method of the present invention. For comparison, a 55 array of microneedles was produced from a substrate using a known subtractive photolithographic and etch method. In each case, the microneedles had an outer diameter (OD) of 300 m, an inner diameter (ID) of 150 m, and a wall thickness of 75 m. Table 1 illustrates the normalised volumes of materials used to form the 55 array.

TABLE-US-00001 TABLE 1 Normalised Normalised Microneedle subtractive vol. additive vol. length (comparative) (invention) 300 m (0.3 mm) 1 0.27 900 m (0.9 mm) 3 0.3

[0084] The (comparative) subtractive photolithographic process requires at least two deep Bosch style long etch steps (e.g. DRIE etch steps) and at least two masking steps to form the microneedle array. Further etch and masking steps are required to form any additional features on the microneedles, such as a bevel. Photolithographic masking steps and plasma etch steps require expensive equipment, and are expensive processes to operate, which increases the cost of the subtractive process. The method of the present invention does not require the use of such expensive photolithographic or plasma etch apparatus. In the comparative example, 96% of the starting silicon substrate was etched to form the microneedle array. This leads to a significant amount of waste material. Further, when using the subtractive photolithographic process, a greater amount of the substrate must be etched if a longer microneedle is desired.

[0085] In contrast, the present invention uses an additive process to form the microneedle array. The additive approach of the present invention requires a significantly lower volume of material to form the microneedle array compared to the conventional subtractive approach. As the microneedle length increases, the variance between the subtractive approach and the present invention becomes more pronounced. The present invention is consequently particularly suited to structures comprising micro-sized features (i.e. features having a dimension of several hundred micrometres or less). For example, the present invention is particularly well-suited to produce features, such as microneedles, having a length between 100-1500 m, an OD between 100-300 m, and a pitch between 100-1500 m. The method of the present invention is suitable for producing an array of features, each feature having different shapes and dimensions. Since no photolithographic processes are used in the present invention, the cost of each processing step is kept to a minimum. Additionally, after the PDMS template has been formed it can be reused multiple times. This further helps make the process of the present invention more economical.