WO2013043122A1

WO2013043122A1 - A reinforced filter with a metallic filtering layer

Info

Publication number: WO2013043122A1
Application number: PCT/SG2012/000341
Authority: WO
Inventors: Haiqing Gong; Majid EBRAHIMI WARKIANI
Original assignee: Nanyang Technological University
Priority date: 2011-09-19
Filing date: 2012-09-19
Publication date: 2013-03-28

Abstract

The present invention relates to a reinforced filter with a metallic filtering layer and a method for fabricating the same. The filter comprises a plurality of openings, having at least a portion of at least one surface of the metallic filtering layer bonded to a support permeable to fluid or gas. The filter can be used for isolation of waterborne pathogens, blood filtration, cell harvesting, and removal of unwanted particles from other food and industrial fluid made of liquid, gas, or a mixture thereof. There is also provided a method for fabricating a reinforced filter with a metallic filtering layer.

Description

A reinforced filter with a metallic filtering layer

Field of the invention

The present invention relates to a filter comprising at least one metallic filtering layer bonded to a support, and a method for fabricating the same. In particular, the filter device is for separation of particles from either fluid or gas. For example, the filter can be used for, but is not limited to applications like isolation of waterborne, airborne or foodborne pathogens, blood filtration to harvest cells of interest or separating different types of cells by size discrimination or cell affinity to the filter surface, chemotaxis, bioassays and cytology. Background of the invention

Many problems associated with the physical, chemical, and microbiological studies of the human environment are that they require rapid concentration of small particles suspended in minute concentration within the two foremost environmental vehicles in which humans are involved continuously in large quantities - namely, air and water [1]. In the laboratory and on industrial-scale, it is often needed to separate and concentrate small particles according to the particle size. Traditional depth filters such as cellulose ester, which are normally made of a thick bed of fibre or other materials, capture small particles with other bigger particles because their pore diameter is an average value in a certain scale range. Therefore, it is often difficult to realize an absolute separation according to the small particle dimension and a full collection of separated particles using these filters. In addition, it is normally required to apply a high transmembrane pressure (TMP) to facilitate the flow through the depth filters due to their large thickness and tortuous pore path [2]. Screen type filters like track-etched membrane are another well-known type of filters that employs relatively thin filter membranes in contrast to the depth filters. Although they have nominal pore sizes and have been used in a wide variety of industrial and medical applications, they tend to have a limited porosity [3] (= 5-10%) which normally leads to a small throughput due to the blockage on the surface of membrane. Moreover, track-etched membrane can only be made with circular pores; therefore, they are unsuitable for discrimination based on non-circular particle shape [4].

Recent developments in MEMS technology have provided novel techniques for controlling the detailed microstructure of membrane materials, allowing the fabrication of membranes with precise pore size (i.e. down to hundreds of nanometer) and shape. In recent years, different methods have been proposed to create membranes with cylindrical pores like conventional lithography and silicon etching technology [5,6], nanoimprinting using alumina template [7], phase separation micromolding [8] and dissolving mold techniques [9]. It has been reported that some major impediments in fabrication of membrane with the aforementioned methods such as inefficient (and/or costly) processes, limitation in materials in use, and low-yield techniques make these methods unreliable for mass production purposes [10]. A highlight of those problems is the lack of ability of handling the filter membrane during and after the filter manufacturing process, due the thin thickness and fragile structure of the membrane, which typically cause damages of the membranes which are not structurally supported or reinforced properly during and after the membrane manufacturing.

Hence, when developing a particle filter, desirable and important properties include a smooth surface, identical pore-diameter, and high porosity for rapid separation and concentration of small particles according to their dimensions from fluid (liquid or gas) that includes many kinds of particles with different dimensions but the particle filter should not be readily damaged during fabrication or use. Summary of the invention

In general terms, the invention relates to a metallic filter bonded to or with an integrated support and its fabrication method.

According to a first aspect, the invention provides a method for fabricating a filter comprising the steps of:

(i) providing a first layer comprising a first plurality of pillars on a substrate;

(ii) depositing a metallic material to fill the space between the pillars to form a metallic filtering layer comprising a plurality of openings; and (iii) fabricating and/or bonding a support to at least a portion of at least one surface of the metallic filtering layer.

The invention also provides a filter comprising a metallic filtering layer comprising a plurality of openings and with at least a portion of at least one surface of the metallic filtering layer bonded to a support permeable to fluid or gas.

A more complete understanding of the present invention, as well as further features and advantages of the present invention, will become apparent from the detailed description and figures below.

Brief description of the figures Reference numerals indicated in the drawings and referred to in the detailed description are intended for illustrative purposes only and should not be construed as limited to the particular structure indicated in the drawings.

Figure 1 shows a cut-view of the metallic micro-fabricated filtering layer with an integrated back-support. Figure 2 depicts a micro-fabricated metallic filtering layer with an integrated back-support on top of a silicon wafer before release. Close-ups show the SEM images of the integrated support and filtering layers.

Figure 3 (a.1 )-(i.1 ) depicts cross-sectional (left column) and (a.2)-(i.2) shows 3- D (right column) schematic diagram of the entire fabrication process.

Figure 4 shows a schematics of pore size reduction using conformal deposition of metals or polymers.

Figure 5 shows an SEM image of a high aspect ratio nanofilter of rectangular pores of 424.28nm in pore width, made by conformal electroless deposition of Ni onto the pore walls of a microfilter.

Figure 6 shows the contact angle and topographic AFM images of a filtering layer, (a) untreated surface, (b) treated surface with 02 plasma for 8 min. The scale was set to the maximum peak to valley distance in each image as shown. Figure 7 (a) shows various shapes of pore openings having low aspect-ratio and (b) shows various shapes of pore openings having high aspect- ratio.

Figure 8 (a) shows various shapes of particles having low aspect-ratio and (b) shows various shapes of particles having high aspect-ratio. Figure 9 shows a schematic of blocking particles of various shapes on pore openings of various shapes.

Figure 10 shows SEM images of the photo-resist pattern (micro-pillars) used to electroplate the microfilter, (b) SEM image of a thin Ni layer electroplated between photo-resist pillars, (c) SEM image of a through-hole microfilter with slotted openings, and (d) cross-section view of the obtained microfilter.

Figure 1 1 illustrates a cut-away view of a filter holder made using laser cutting.

Figure 12 shows reciprocal value of the rupture pressure for the metallic micro- fabricated filtering layers with different thickness in comparison to the theoretical values calculated using Rijn's model.

Figure 13 shows SEM images of the micro-fabricated filter (a) after filtration of latex particle, (b) after back-washing with buffer solution.

Figure 14 depicts fluorescence microscopic image of C. parvum oocysts captured by a micro-fabricated filter.

Figure 15 represents the flow rate of two commercial filters and micro-fabricated filter for filtration of tap water at a pressure of 1 bar and turbidity of 0.5 NTU.

Figure 16 shows the total collect volume for the micro-fabricated filter with 90 mm diameter for filtration of water at different turbidities.

Figure 17 illustrates (a) SEM image of a metallic pre-filter (12x9 m) made from

Ni. (b) Total collected volume of a 90mm filtering layer during filtration of tap-water with and without pre-filtration unit.

Definitions

Porosity refers to the proportion of pore area per unit surface area.

A slotted pore refers to a narrow opening (or an opening in the form of a slot or a groove). The slotted pore may be of any suitable narrow shape, for example: quadrilateral (such as a rectangle) or oval. Detailed description of the invention

A method for fabricating a filter with a support layer according to any aspect of the invention is described herein.

The method comprises the steps of

(ii) depositing a metallic material to fill the space between the pillars to form a metallic filtering layer comprising a plurality of openings; and

(iii) fabricating and/or bonding a support to at least a portion of at least one surface of the metallic filtering layer.

It can be understood that at this stage, the metallic filtering layer comprises a plurality of openings although these openings are occupied by the pillars of the first layer.

According to a further aspect, step (i) comprises fabricating the first layer on the substrate. Any suitable technique for fabricating the pillars in the first layer may be used. In particular, step (i) comprises depositing a first layer comprising a resist layer on the substrate or on the seed layer and performing lithography to fabricate the first plurality of pillars in the first layer. Any suitable lithography technique and/or resist may be used. For example, the resist used would depend on the lithography technique used.

The first layer may either be fabricated directly on the substrate or on a seed layer on the substrate. If a seed layer is required, the seed layer may first be deposited on the substrate by any suitable method before the first layer. For example, the seed layer may be a metallic layer or an electrically conductive polymer or monomer. The seed layer may be deposited by any suitable method, for example by sputtering, thermal evaporation or electroless plating. The seed layer may comprise aluminium, gold, chromium, copper, titanium, silver or a combination thereof or an electrically conductive polymer or monomer.

The metallic material of the metallic filtering layer may be made of any suitable metal. For depositing the metallic material, techniques such as electroplating, sputtering, thermal evaporation or electroless plating may be used. In particular, the metallic material may be deposited by an electroplating process. Examples of the metallic material include but are not limited to nickel, aluminium, gold, chromium, copper, titanium, silver or a combination thereof.

According to one exemplary embodiment, step (i) comprises depositing a seed layer comprising an electrically conductive layer on the substrate and fabricating the first layer comprising a first plurality of pillars on the seed layer; and step (ii) comprises electroplating to deposit the metallic material to fill the spaces between the pillars of the first layer.

The support may be of any suitable material. For example, the support may be a metallic or a polymeric support. In particular, the metallic support comprises nickel, aluminium, gold, chromium, copper, titanium, or silver or a combination thereof. The support may also be a silicon support.

There are a number of options for obtaining the bonded support of the filter. In a very direct option, this may involve bonding (or adhering) the support permeable to gas or fluid to the metallic filtering layer. The support to be bonded may be of any suitable material, for example, metallic or polymeric. Bonding a polymeric support would require using adhesives. Techniques for bonding of a metallic support include but are not limited to electroplating, sintering, adhesive bonding and/or welding. As another option, the support may be fabricated onto the metallic filtering layer. A first fabrication method for the support is very similar to the method of fabricating the metallic filtering layer itself. This may involve: (a) depositing a second layer on the metallic and first layers;

(b) fabricating the second layer into a layer comprising a second plurality of pillars; and

(c) depositing a second material into the space between the second plurality of pillars to form a permeable support comprising a plurality of apertures.

At this stage, the permeable support comprises a plurality of apertures even though these apertures are occupied by the pillars of the second layer.

The step of fabricating the second layer into a layer comprising a second plurality of pillars may be by any suitable method. For example, lithography methods may be used.

The above first fabrication method may be polymeric or metallic material as the second material. With a polymeric material, this may involve curing a suitable resin to form the support. With a metallic material, this may involve an electroplating process to deposit the metallic material to form a metallic support. A second fabrication method for the support comprises depositing a second layer on the first metallic and first layers and fabricating the second layer into a permeable support comprising a plurality of apertures. In particular, lithography may be performed to fabricate the second layer into the permeable support.

A third fabrication method comprises forming the support comprising a plurality of apertures and bonded to the metallic filter layer from the substrate. For example, the substrate may be etched to form a permeable support comprising a plurality of apertures. As mentioned above, the openings of the first metallic layer are occupied by the pillars of the first layer. In order to reveal the openings of the metallic filtering layer, the pillars of the first layer may be removed. Unless the support is formed from the substrate, the filter comprising the metallic filtering layer bonded to the support must be released from the substrate. Accordingly, the step of removing the pillars of the first layer may serve to release the metallic filtering layer bonded to the support from the substrate.

The filter comprising the metallic filtering layer bonded to the support may also be released from the substrate by other means. For example, if a seed layer was used, the seed layer may be removed to release the filter.

Any suitable method may be utilised to remove the pillars of the first layer and/or the seed layer; including but not limited to etching, using a chemical solvent, water or gas to remove the pillars, or mechanical removing.

Optionally, the size of the openings of the metallic filtering layer could be reduced. In particular, this may be achieved by depositing a material to reduce the size of the openings in the metallic layer. Possible materials include but are not limited to a metal or a polymer.

Surface treatment may be performed to the metallic filtering layer, for example to improve performance of the metallic filtering layer. Examples of surface treatment include but are not limited to plasma treatment and/or wet chemical etching.

The invention also provides a filter comprising a metallic filtering layer comprising a plurality of openings and with at least a portion of at least one surface of the metallic filtering layer bonded to a support permeable to fluid or gas. In one exemplary embodiment, at least a portion of each surface of the metallic filtering layer is bonded to the support.

Other exemplary features of the metallic filtering layer and/or support are further described below. The metallic filtering layer is of a defined thickness and the thickness of the metallic filtering layer is substantially even. For example, the thickness of the fabricated layer is from 10 nm to 50 μιτι. The metallic filtering layer also has a substantially smooth and/or flat surface. The openings of the metallic filtering layer may be of substantially the same size and/or arranged in a substantially regular array. The openings of the metallic filter may be of any suitable shape and/or size. The shape and/or size of the openings of the metallic filtering layer may be fabricated according to the specific applications, for example, depending on the type of particles, cells and/or organisms to be trapped. The openings may be quadrilateral, oval shaped or circular. In particular, the openings may be rectangles or squares. The size of the openings may be from 10 nm to 1000 pm in width and/or length. In particular, the size of the openings may be from 100 nm to 1000 μηη in width and/or length. For example, the support may comprise a plurality of apertures for passage of fluid or gas. The apertures of the support are larger than the openings of the metallic filtering layer. The support may also be a grid structure. The support may be substantially rigid. The thickness of the support may be thicker than the metallic filtering layer. Further exemplary aspects and/or advantages of the invention are described below.

Accordingly, in the present invention, a high-yield process for the fabrication of metallic micro/nano-filters with a structural supporting layer using, for example, UV lithography and electroplating is disclosed. Other patterning techniques may also be applied, including but not limited to hot embossing, micro- and nano- molding and casting, electron beam lithography, nano imprinting, pattern transfer by stamping, interferometry lithography, x-ray or proton lithography, inkjet pattern deposition, micelle and other self-assembly of particles and molecules, amplification of electrohydrodynamic instabilities [18], micro or nano templates such as anodized porous alumina sheets, etc. Such a supporting layer bonded to or integrated to the filtration (or filtering) layer significantly strengthens the filtration (or filtering) layer during the manufacturing process and use of the filter without degrading the filtration flux and particle recovery after the filtration process. The manufacturing process also solves the issue of handling a delicate un-supported filter layer, since the filter layer is typically thin and has no rigidity. The invention substantially circumvents existing limitations in current micro- and nano-fabrication techniques described above.

The filter may comprise several layers which one (or two) of them forms the filtering layer and one (or two) of them forms the support structure to reinforce the filtering layer. In the present invention, the support layer plays several key roles. Firstly, it keeps the filtering layer flat upon release from a substrate such as a silicon wafer and subsequent handling of the filtering layer, and secondly, it improves the filtering layer strength during the filtration process (i.e. especially during the back-washing step). Moreover, the support layer safeguards the filtering layer from tearing or breaking during use and fabrication (or manufacturing).

The filtering layer could be made of any metal. In particular, the metal may be deposited by electroplating process, including but not limited to nickel, aluminium, gold, chromium, copper, titanium, silver or a combination thereof. A wide variety of support structures may be employed in the present invention to support the metallic micro-fabricated filtering layer. For example, the support can be made from a second layer of metal, photoresist such as SU-8 or Polyimide, other plastic sheets, silicon wafer, or even a glass plate with through- holes.

In the present invention, the support layer plays several key roles. Firstly, it keeps the filtering layer flat upon release from a substrate such as a silicon wafer and subsequent handling of the filtering layer, and secondly, it improves the filtering layer strength during the filtration process (i.e. especially during the back-washing step). Moreover, the support layer safeguards the filtering layer from tearing or breaking during use and fabrication (or manufacturing). For example, the support layer is made of a structure (or macrostructure) of considerably greater thickness and with apertures which do not cause any significant increase in hydraulic resistance during filtration. The integrated support layer with large apertures attached to the filtering layer improves the strength during filtration. A wide variety of support structures may be employed in the present invention to support the metallic micro-fabricated filtering layer. For example, the support can be made from a second layer of metal, photoresist such as SU-8 or Polyimide, other plastic sheets, silicon wafer. The support can also be a glass plate with through-holes.

More specifically, the present invention describes the fabrication of a micro- or nano-porous filtering layer with regularly-arranged array of openings (pores) and smooth pore and filtering layer surfaces, which has small flow resistance during filtration of fluid (liquid, including water, and gas, including air). This novel methodology fabricates a high-flux micro/nano filter with an integrated support, which is mechanically strong and poses precise pores with circular or rectangular shape. The openings of the filtering layer may be of any suitable shape. The openings may be quadrilateral, oval shaped or circular. For example, the openings may be squares, rectangles, oval or slotted pores. Openings in the form of quadrilateral, oval or slotted pores in the filtering layer described in the present invention with different width and length (i.e. 100 nm to 1000 μσι) have some advantages over circular pore filtering layer with the same pore size. For example, initial rate of flux decline is slower for the filtering layer with slotted pores compared to the filtering layer with circular pores since the initial particle deposition only covers a small fraction of the pores [1 1]. Furthermore, the filtering layer resistance during filtration is also much lower for the slotted pore filtering layer compared to the circular pore filtering layer. It should be noted that exact pore size of the filtering layer can be tailored according to the desired application. For instance, a filtering layer with pore size of around 0.4-0.7 μιη can be ideal for removing most cells and cell fragments from blood, leaving essentially cell-free plasma, or can even be used for filtration of wine. Pore size of around 2 μιτι can be used for isolation of C. parvum oocysts and Giardia from water in order to secure the hygiene of drinking water. The standard deviation of obtained filtering layers is less than 5 %.

The filtering layers of the present invention generally have extremely smooth surfaces, uniform pore size and high porosity. The porosity of the filtering layer may be selected according to the intended application. In accordance with the present invention, the porosity of the micro-fabricated filtering layer can be substantially higher than commercially available filtering layers (i.e. up to 60 %). The uniform pore distribution makes the filters suitable for many applications like isolation of waterborne, airborne or foodborne pathogens, blood filtration to harvest cells of interest or separating different types of cells by size discrimination.

Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting of the present invention.

Example 1 : Structure of the filter

Figure 1 shows a cut-view of a filter schematically, generally at 8, illustrating the present invention. In accordance with the present invention, the filter includes at least a metallic filtering layer that comprises an array of mono-sized openings (pores), and a support layer that includes a precision-shaped support structure (Figure 1 ) with larger apertures (openings). For purpose of illustration, the filtering layer 8 as shown in Figure 1 is not to scale. Although, theoretically, the support layer 6 could have a thickness in the order of the thickness of the filtering layer, more typically the filtering layer 4 will be substantially thinner than, the support layer. In the present invention, the thickness of filtering layer 4 is between 10nm-50 pm while support structure 6 has the thickness of around 100nm-5mm. The thickness of both filtering layer 4 and support structure 6 can be varied, depending on the desired pore size, pore shape, porosity and filter strength. Depending on the fabrication procedure, the support layer 6 can either be on top or at the bottom of the filtering layer 4.

Example 2: Fabrication of metallic filtering layer

Schematic fabrication process flow diagram of metallic microfilter 8 is illustrated in Figure 3. On the left column (Figure 3(a.1 to i.1 ), cross-sectional view of the fabrication process is illustrated. For the purpose of illustration and better understating the three dimensional, hereinafter 3-D, view of the entire fabrication is illustrated in the right column (Figure 3(a.2 to i.2)).

First, a (1 ,0,0)-oriented silicon wafer 1 , was cleaned carefully in piranha solution (96% H₂S0₄ and 30% H₂0₂) for 20 minutes at 120 °C to remove any organic contaminations on the wafer surface 1 . After rinsing with Dl water and drying with N₂ gas, the substrate 1 was submerged in the buffered oxide etchant (BOE) for 2 minutes to clean the natural oxide layer. This step has a significant impact on adhesion of seed layer 2 to the substrate 1. Then, electrical contact for the subsequent electroplating step was provided through a 300 nm thick Cr/Cu seed layer 2 (Figure 3(a.1 and a.2) that was deposited on the contact wafer by sputtering process, using a magnetron sputtering machine. In another aspect of the present invention, electrical contact can be deposited (i.e. with any thickness range from 10 nm to 10 pm) by using other metals such as Aluminium, Gold, Chromium, Copper, Titanium, Silver and, etc. alone or in combination. Also, electrically conductive polymer or monomer can be spin- coated on a flat substrate such as silicon wafer 1 and cured to form the seed layer 2.

To aid good adhesion of the photo-resist 3 to the seed layer 2, the dehydration bake-step was typically performed under vacuum in Suss machine (Delta 150 VPO) for 2 minutes. Afterwards, A thick layer of an appropriate photo-resist 3 such as AZ9260 (Microchemicals GmbH) was spin-coated on the wafer to create the microstructure patterns in later step, as shown in Figure 3(b.1 and b.2). In order to avoid bubbles, the photo-resist 3 was poured onto the substrate 1 directly from a bottle with a large aperture. Soft baking process was performed on a carefully levelled hot-plate at 1 1 ^{n q}C for 4 min and followed by 2 min relaxation at 25 °C. After soft baking the AZ film 3 on the hot plate, a chrome coated glass mask with desired features was used to transfer the patterns into the AZ photoresist 3. UV-Lithography was processed by Karl Suss MA6 mask aligner (Karl Suss Inc.) in the vacuum contact mode between the silicon wafer 1 and the mask with a 350W mercury lamp with a high pass UV filter in order to cut off undesired short wavelength. Finally, the exposed film was developed at room temperature in AZ 400k (Microchemicals GmbH) developer, which was diluted with Dl water (1 :2) for 2 minutes with manual agitation, as shown in Figure 3(c.1 and c.2). In the above steps, a dry photoresist film can be used to replace the liquid photo-resist 3,

A variety of techniques can be employed to transfer the pattern in micrometer or nanometer scale into the photo-resist film such as laser interference lithography, X-ray lithography, Electron-beam lithography, extreme ultraviolet lithography, Ion-beam lithography, stamping, Nano imprint lithography (NILT) and, etc. the photo-resist film can be formed from either a wet resist material or a dry film resist material. Instead of a photo resist material, other pattern forming material can be used, including electron-beam resist, thermoset, thermoplastics, as long as they can be removed after the formation of the metallic filter layer such as by dissolving in a chemical solvent, water or corrosive gas, or mechanical removal such as peeling, etc. The present invention in its broadest respects is not limited to any particular techniques or combination of them.

Figure 3(d.1 and d.2) show that, after rinsing the wafer 1 with Dl water and drying with N₂ gas, the Si wafer 1 was mounted in a holder that provides a homogeneous electric contact with the conducting layer coated on the substrate. Then, the holder was immersed in the electroplating bath to electroplate the nickel 4 between photo-resist pillars 3. The bath solution has a volume of about 5 litres, which operates at 50 °C, and its composition is shown in Table 1. In another aspect of the present invention, the filtering layer material 4 can be deposited (i.e. with any thickness range from 10 nm to 100 μητι) with nickel or by using other metals such as Aluminium, Gold, Chromium, Copper, Titanium, Silver and, etc. alone or in combination of them. The deposition methods include sputtering, thermal evaporation, electroless plating, etc.

Nickel sulphamate is the main source of nickel ions for electroforming that will be deposited in the cathode while nickel chloride promotes nickel anode dissolution and prevents anode passivation (i.e. causing pH increase and sulphamate hydrolysis, which leads to increase internal stress) [12].

Table 1 Chemical composition of the Ni electrolyte bath.

The boric acid acts as a pH buffer that prevents basic nickel compound formation at the cathode and thereby minimizing the internal stress of the nickel deposits. Appropriate additives have been added to the bath solution in order to shift the stress from tensile to compressive, prevent pitting formation on the cathode surface and also adjust the pH value during operation [13].

The filter pore may have various shapes which can be either a pore opening of low aspect-ratio, as shown in Figure 7(a), in which the dimension of the opening in all directions are nearly the same, or a pore opening of high aspect-ratio, as shown in Figure 7(b), in which the dimension of the opening in all directions are significantly different. The particles may have various shapes having either low- aspect-ratio, as shown in Figure 8(a), in which the dimension of the particles in all directions are nearly the same, or shapes of high-aspect-ratio, as shown in Figure 8(b), in which the dimension of the particles in all directions are significantly different. During the particle filtration or separation process, the filter pore shape should be designed to suit the shape of the particle of interest. Figure 9 shows a few examples of choosing suitable shapes of pore openings for given particles of certain shapes. The opening size which is designed to suit the size and shape of the particles of interest can be in the range of 10nm-5mm which can be the smallest size of the opening or the largest size of the opening.

Example 3: Fabrication of the support structure

(3. 1 ) Metallic support structure The micro-fabricated filtering layer 4 is too thin and may easily warp, fold or break during filtration or handling. Hence, it is required to strengthen the filtering layer with a back-support with large apertures. For this purpose, a thick layer of AZ 9260 photoresist 5 was again spin-coated on the filtering layer surface (Figure 3(e.1 and e.2)) and lithography was performed using aplastic mask with square shape features to transfer the pattern into the photo-resist 5 (Figure 3(f.1 and f.2)). The back support structure can be made in a variety of sizes and shapes such as the array of hexagonal, rectangular or circular features. In the present invention, we used array of 600x600 μιτι features to make the support layer. The back support openings should be large enough so as to not contribute any resistance during filtration [6].

Then, the exposed film was developed at room temperature in AZ 400k developer for 3 minutes followed by rinsing with Dl water and N₂ gas drying. Figure 3(g.1 and g.2) show that the second electroplating was performed 6 in the same bath for around 15 minutes to form the support structure between the photo-resist gaps 5. Afterwards, photo-resists 3, 5, which remain from the previous steps, were dissolved in an acetone bath, as shown in Figure 3(h.1 and h.2). After washing the wafer with Isopropanol and also Dl water, the seed layer 2 was removed using Cu etchant (Sigma Aldrich) in order to release the micro-fabricated filtering layer 8 from the substrate 1 , as shown in Figure 3(i.1 and i.2). Ultrasonic agitation can be useful in this step because it expedites the releasing process and also prevents from adhesion of the filtering layer 8 to the substrate 1. In the above processing of forming the support layer 6, the liquid photo-resist 5 can be replaced by a dry film photo-resist.

(3.2) Polymeric support structure

In another aspect of the present invention, as shown in Figure 3, the support structure 6 can be also made using a thick layer of negative photoresist like SU- 8 or Polyamide which have good mechanical properties. For this purpose, a thick layer of SU-8 photoresist 5 was deposited on the wafer during (i.e. step e in Figure 3) and soft-baked on the hotplate for 10 minutes at 95 °C. Then the same plastic mask with square shape features was used to transfer the pattern to the SU-8 photoresist. After exposing the film with an appropriate dosage, the wafer was kept again on the hotplate for post-exposure purpose and also good adhesion to the metallic layer for around 15 minutes. Development of the SU-8 film was performed on a SU-8 developer (MicroChem Corp) solution for 5 minutes with manual agitation. The areas which received UV light were cross linked to each other and form a rigid and robust structure which can be used as a back-support for the filtering layer 6. After washing the wafer with Isopropanol and also Dl water, the seed layer 2 was removed using Cu etchant, similar to the previous section, in order to release the micro-fabricated filtering layer 8 from the substrate 1 .

(3.3) Silicon back support In an alternative embodiment of the filter according to the invention, as shown in Figure 3, the support 6 can be made by anisotropic etching of Si wafer using KOH like the process in reference [5]. For this purpose, we have used a double sided polished silicon wafer in which we have done the filtering layer electroplating 4 for one side and back-side etching of the Si wafer 1 from other side using KOH. In this case, it is not required to release the filtering layer 4 from the Si substrate 1. With this technique, we can make a metallic micro/nano filter 4 on top of a Si wafer 1 with different thickness (i.e. ^"l OOnm -1000 μιη) which is etched anisotropically or isotropic ally using KOH or deep reactive ion etching (DRIE), respectively. A great explanation, of the back-side etching can be found in references [4, 5].

Example 4: Pore size reduction using conformal deposition of metals or polymers

With many lower cost UV lithographical techniques, it is technically difficult to project a clear image of a small feature (< 2 μιη) inside a thick photoresist layer due to the limitation in wavelength of the light and also reduction lens system. In view of this, a variety of techniques including electro/electroless-plating, atomic layer deposition (ALD) as well as polymer deposition (i.e. Parylene C, Teflon, Heparin and, etc.) may be employed to reduce the pore size of the micro- fabricated microfilter evenly in order to achieve a high aspect ratio micro or nanofilter which is robust during filtration. The pore size of the final micro or nanofilter can be controlled via the amount of the metal (Ni, Cu, Pt, Ag and, etc.) or the polymer deposited. Figure 4 shows schematically the mechanism of pore reduction. Figure 4(a) shows the filter structure 22 and the pore 10 which yields an initial pore size before pore reduction. Figure 4(b) shows the reduced pore 1 1 having a smaller final pore size after deposition of material 24 over the initial pore structure 22. Figure 5 also depicts a micro or nanofilter (i.e. ^« 400 nm pore size) which has been obtained by conformal deposition of nickel onto the pore walls of a microfilter with initial pore size of 2 μιη. Example 5: Surface treatment for reducing the fouling

The controlled adjustment of wettability, surface roughness, and surface chemistry of the filtering layer are important tasks, because, these factors have a great impact on the rate of protein and particulate fouling during filtration. The change of surface composition can cause loss in performance and adsorption of undesired components as well as low reproducibility of the separation. Plasma treatment is widely used to tune the surface properties of many metals and polymers to promote adhesion or to enhance wetting properties. Plasma processes can introduce specific binding sites or modify the filtering layer surface by cross-linking, chain scission or incorporation of functionalities. The stability of the hydrophilization depends on the intensity of the plasma treatment and also subsequent functionalization with polymer grafting such as PEG, PEG- PEI and, etc. Wet chemical processing can be also employed for surface hydrophilization like etching the polymer/metal surface with Ceric Ammonium Nitrate (CAN) followed by coating with ethanolamine. In another aspect of present invention, we employed both plasma treatment and wet chemical etching process using CAN to change the surface properties of our micro- fabricated filtering layer in order to increase the surface hydrophilicity and reduce the fouling during micro-filtration. To verify the hydrophilization and the integrity of the surfaces, these were analyzed by contact angle measurements and topographic imaging with an atomic force microscope (AFM). The filtration properties of the modified and unmodified layer were examined using latex particles and BSA. A significant increase in total collected volume of filtrate was observed for the treated layer during filtration of BSA and latex particles. Figure 6 shows the AFM images of a sample before and after plasma treatment. The contact angle of the samples was also measured with a goniometer after each surface treatment. It can be seen that wettability increased significantly with this method.

Example 6: Filtering layer morphology There is provided a quick method for fabrication of high-flux metallic micro- fabricated filters 8 with perfectly ordered pores and integrated back-support 6 by using lithographic and electroplating techniques. This method can be scaled up for mass production of micro-fabricated filtering layers. Figure 2 shows an optical microscopic image of a 100 mm diameter metallic layer 8 on a silicon substrate 1 . The close-up views show the SEM images of the integrated back-support 6 and through-hole microsieve 4, respectively.

Figure 10(a) and 10(b) show also the SEM photographs of the photo-resist pillars 3 with the nickel thin film deposited between the gaps 4, respectively. The nickel film cannot be thicker than about 85-90% of the photo-resist structure's height; otherwise the nickel film will fill the microfilter pores. The obtained metallic layer 4 with slotted pores 7 is also illustrated in Figures 10(c) and 10(d). It can be seen that array of rectangular pores (2.5 χ8 μιτη) is perfectly formed. The width of photo-resist pillars 3 was reduced from 3 to 2.5 μηη precisely by controlling the exposure and development time during lithography. It should be noted that a slotted pore design provides a significantly lower pressure drop than a circular pore filtering layer because the resistance is much smaller [1 1]. In addition, it has been shown by other researchers that slotted pore filtering layer is less vulnerable to the particle bridging and therefore, fouling than the circular one [14].

As discussed before, integrated back-support 6 safeguards the filtering layer 4 from tearing and failing during micro-filtration besides keeping it flat. The pore- size distribution of the fabricated filtering layers was measured using digitalized images from an image analysis program (Semicaps 2200, Semicaps Pte Ltd). The average pore width was around 2.5 μιη and the standard deviation of the pore was 50 nm. Therefore, the calculated coefficient of variation (CV) was 2 %, which is much lower than commercially available filtering layers [3].

Example 7: Filtration testing (7.1 ) Filter holder for filtration and back-flush

In order to test and compare the result of our micro-fabricated filter with commercial filters, a filter holder was designed and fabricated from PMMA (Polymethyl methacrylate) by laser cutting and thermal bonding technique. It comprises a base for sample flow in and eluent flow out, a cap for flow out of the filtered sample and for intake of eluent, a gasket for leakage prevention and also an upper and a lower perforated support for keeping commercial filters during the test firmly. The major parts of this holder are depicted schematically in Figure 1 1 . 7.2 Filtering layer strength

Experimental and numerical investigations were carried out to calculate the mechanical stability of the metallic micro-fabricated filtering layers 8. Based on the following correlation which was introduced by van Rijm et al. [15], the mechanical strength of a perforated filtering layer depends on the thickness of filtering layer, the Young's modulus of filtering layer, the intrinsic tensile stress, and also distance between the bars of integrated back-support [15].

where P_max is the maximum load (i.e. bursting pressure), h is the filtering layer thickness, / is the distance between the back-support bars, E_eff and σ_/β# are the effective Young's modulus and yield strength, respectively. The filtering layer porosity in this model is considered by a factor (1 - ) for calculation of E_{eff a}nd Oyeff, where K is the porosity. Based on this theory, a perforated filtering layer can be modelled as a non-perforated filtering layer with adjusted Young's modulus and yield strength [15]. For evaluation of the obtained results from the analytical correlation with experimental ones, a small test device similar to the reference [15] has been made in which burst pressure of micro-fabricated filtering layers with different thickness could be measured using a pressure sensor. Figure 12 shows the reciprocal values of bursting pressures for metallic micro-fabricated filtering layers with square-shaped integrated back-support (600x600 Mm openings) in comparison to the reciprocal values of theoretical bursting pressures calculated using Rijn's model [13]. The following typical values were used for calculations: h= 2,4,6,8 and 10 μηι, /= 600 μηι, E = 210 GPa [14], o_yei|_d = 400 MPa [14] and = 36%. It can be seen that the experimental results for mechanical stability of the micro- fabricated filtering layers with different thickness are larger than the stability calculated with Rijn's model. The under-estimation from this theory is due to the fact that for ductile metals like Ni, stainless steel, and Ti, there is a linear relationship between the strain and the applied stress up to the yield stress (Oyieid)- After this point, the filtering layer may not break and the stress in the middle of the edge (i.e. where the maximum stress is happening) may increase up to the ultimate stress (animate)- Between the Oyieid and animate, the filtering layer strain can increase significantly and E cannot be considered as a constant value [18,19]. Therefore, only an under-estimate of the maximum load can be given with above correlation for the ductile materials.

(7.3) Microfiltration with latex particles

Microfiltration with latex particles with specific size can reveal how a pore is blocked and how the performance of a filtering layer will be with feeds of different particle concentrations. In addition, direct integrity test of the micro- filters before any biological test can be done using appropriate surrogates (like latex particles) by directly assessing the removal of the surrogates [17]. Therefore, we evaluate the performance of metallic micro-fabricated filters with slotted pores (2.5χ8 μι^η pore size and 6 pm thickness) using challenge suspensions containing latex particles with 3 μιη size. For this purpose, two test solutions with different concentrations (1 g/L and 0.1 g/L) were prepared and filtered through the filtering layers by a dead-end filtration set up under a constant pressure (0.2 bar). Then, the permeate solutions filtered for the second time through an Anopore™ aluminium membrane (Cat No: 6809-5022) with nominal pore size of 0.2 μιη to capture any microbeads that may have passed from the metallic micro-fabricated filter. Subsequently, the surface of aluminium membrane was fully observed under the microscope, and it was realized that no latex particle have been passed through the micro-fabricated membrane (i.e. 100 % capturing). Figure 13 shows SEM photos of the metallic micro-fabricated filter after filtration of latex solution (0.1 g/L) and after back-flushing. Unique features of metallic micro-fabricated filter such as smooth surface and uniform pore-size greatly reduce the latex adhesion to the filter surface and enable us to achieve a very high recovery rate. (7.4) Microorganism isolation

The capturing and recovering performance of the metallic filtering layers was also verified by a two-step C. parvum oocysts filtration method. For this purpose, 10 litres of a challenge sample, which was tap water-spiked with 2x 10³ viable C. parvum oocysts with fluorescent tagged antibodies, was filtered using a metallic micro-fabricated filter. Filtering layers were wetted by immersion in ethanol (70%) for 15 min, and then flushed with sterile distilled water before mounting on the filtering layer support. The filtration process was performed in dead-end mode with a peristaltic pump under 0.5 bar pressure. After filtration, C. parvum oocysts were recovered from the filter either by lateral shacking or by back-washing using a 10 ml back-flush buffer solution containing 1 % sodium polyphosphate (NaPP) and 0.1 % Tween 80. C. parvum oocysts attached to the filter before and after the recovery were visualized by staining the filtering layers with FITC technique followed by observation under the fluorescence microscope. Then the filtered water goes to a subsequent filtration step using Anodisc filter to capture any oocyst that may pass through the micro-fabricated filter. C. parvum oocysts attached to both filters were observed under a fluorescence microscope by FITC (fluorescence iso-thiocyanate) technique (Waterborne. Inc., New Orleans, LA, Cat no. A400FLK). It was observed that all oocysts were captured by the micro-fabricated filter and no oocyst was found on the Anodisc filter (see Figure 14).

Unique features of the metallic micro-fabricated filter like the smooth surface, straight pore path and uniform pore-size greatly reduce the oocyst adhesion to the filter surface and enable us to achieve a very high recovery rate (90%-95%) of C. parvum oocysts when applying back-flush or lateral shaking.

(ix) Filtration throughput

Figure 15 shows the flow rate of two commercial micro-filters and the metallic micro-fabricated filter under a constant pressure condition using tap water. The operating pressure was 1 bar and the turbidity was around 0.5 NTU. The results indicate that the micro-fabricated filter has much higher flow rate in comparison to other filters. This can be attributed to the higher porosity, slit shape openings of the filtering layer and its lower thickness. Experiments were performed in triplicate and presented results are the average of the measurements. The results of the tests for higher turbidities also confirm that metallic micro- fabricated filter is superior to the commercial filtering layer.

By increasing the porosity, pressure of filtration (2bar) and decreasing the thickness of micro-fabricated filtering layer, flow rate of about 2000 ml/min/cm² has been achieved from our filtration testes. The following bar chart (Figure 16) shows the total collected volume of a 90mm micro-fabricated filter for filtration of water at different turbidities. It can be seen that just a 90 mm diameter filtering layer with 10 Mm thickness can pass around 90 litre of tap-water before complete plugging in less than 20 minutes. (7.5) Pre-filtration

During microfiltration of C. parvum oocysts, presence of large particles (i.e. 8-15 μιπ) bigger than the desired ones to be filtered and recovered, can cause cake formation on the filtering layer surface and compromise the efficiency of the micro-filter during operation. For this purpose, we also fabricate a metallic pre- filter with 12x9 Mm openings to capture the unwanted particles and enhance the flow rate and recovery during microfiltration of oocysts. Figure 17(a) shows SEM image of a metallic pre-filter made with the same technique as described earlier. The obtained results from our experiments confirmed that employing a pre-filter during filtration process can boost the throughput (see Figure 17(b)) and also enhance the recovery rate.

(7.6) Turbidity of eluent infiltration of tap water

The efficiency of genetic testing during downstream analysis may be compromised if the back-flushed eluent contains a large volume of dirt. For this purpose, the turbidity of eluent of metallic micro-fabricated filter was evaluated by comparing with three different types of commercially available micro-filters. In all experiments, 10 L of tap water was passed through the filtering layers with a diameter of 47 mm. Then 250 ml of buffer containing 1 % NAPP and 0.1% Tween 20 was back-flushing the filtering layers with an air pressure of 1 bar. Table 2 shows the summary of experiments.

Table 2 Measured turbidity of eiuent for different types of micro-filters.

All experiments were performed in triplicate and presented results are the average of the measurements. The obtained results indicate that metallic micro- fabricated filtering layer presents much lower turbidity of eiuent during the back- flushing step. This is attributed to the smooth surface of the micro-filter and also passage of small particles (i.e. unwanted) through the pores during the filtration process. Inspection of micro-filters using SEM images also revealed that majority of the pores (more than 95%) in the commercial micro-filters clogged during the filtration process while surface of micro-fabricated filter was quite clean and just some pores were closed.

Since 250 ml of eiuent is a large volume for genetic analysis, we tried to condense this amount by a second stage filtration and lateral shaking via 1 ml of buffer using a 13mm micro-fabricated filter. The initial experiments and microscopic inspections revealed that trapped particles (i.e. latex or oocysts) can be recovered from the filtering layer surface efficiently by employing lateral shaking using a high frequency lab shaker for around 10 minutes. The corresponding turbidity of eiuent at this stage was around 7 NTU. In this invention, a new process for fabrication of high-flux metallic microfilter which can be high-yield and low cost is disclosed herein. The obtained filtering layers may have smooth surface, high porosity and are highly reusable. The slotted pore design provides a significantly lower pressure drop than a circular pore and has a much lower tendency to foul. The significant properties of the micro-fabricated filter for isolation of C. parvum oocysts are as follows: a) High sample throughput: According to the tests and measurements, the flow rate and throughput of the micro-fabricated filter is much higher than the commercial filters like track-etched filtering layer filter for the same purpose of C. parvum oocyst capturing. The higher throughput of tap water leads to higher concentration ratio of the oocysts. b) High recovery rate: Micro-fabricated filter shows a high recovery rate (i.e. 90% to 95%) when it was employed for filtration of C. parvum oocysts. This valuable property is attributed to its properties like smooth surface, low coefficient of variation and straight pores. c) Reusability: Commercial filters are not designed to be reusable, and they can hardly retain their initial status after the filtration and back-flushing process while the micro-fabricated filter can retain its original condition with a back-flush or shaking process. This example highlighted the potential application of high-flux metallic microfilters in rapid filtration and recovery of C. parvum oocysts for downstream detection. Additionally, this filter can be used in other applications such as air monitoring, blood filtration, cell analysis, and protein fractionation.

(7.8) Reusability Commercial membranes are not designed to be reusable, and they can hardly retain their initial status after the filtration and backflushing process while the polymeric micro-fabricated filter can easily retain its original condition with a simple backflushing or lateral shaking process. Hence, a long lifetime and the ability to be cleaned easily make the micro-fabricated multi-layer filters a good choice for large scale applications where conventional filters must be replaced regularly, like in the water purification industry or in wineries. Due to the metallic filter's high strength and chemical resistance too many cleaning agents, the metallic filter can be washed and cleaned for repeated use. This opens a new application area of field deployment of the filtration device and autonomous monitoring and filtration of various types of particles such as pathogens from environmental samples, and the automated or on-line monitoring of patients' blood samples.

Example 8 Applications of the filter I. Healthcare Applications

1 ) Biosensors: as a barrier offering controlled diffusion for biological reagents and electrochemical detectors.

2) Diagnostic assays: for flow control, sample preparation, blood separation (i.e. separation of CTC's from other blood cells or separation of WBC from RBC), and capture of latex microparticles. They can be used also for in vitro applications including diagnosis and protein separation. 3) Cell biology: for cell culture, chemotaxis, and cytological analyses (i.e. direct staining, isotopic and fluorescence based assays).

4) Transdermal drug delivery: as an inert matrix for retention of therapeutics.

5) Hemodialysis: removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. 6) Single Molecular Analysis: a promising tool in probing biomacromolecules (DNA, RNA, and proteins) one by one for single-molecule analysis. 7) Nucleic acid studies: Alkaline elution and DNA fragment fractionation. II. Additional Applications:

1 ) Water analysis: Absorbable organic halides (AOX), direct count of microorganisms, marine biology and dissolved phosphates, nitrates, and ammonia analysis.

2) Air monitoring: Trace elements (chemicals, radioactivity) and particulate analysis (dust, pollens, and airborne particles).

4) General filtration: Particulate and bacteria removal, cross flow filtration, HPLC sample preparation, and solution filtration (i.e. milk filtration, oil filtration and etc).

5) Microscopy: Electron microscopy, epifluorescence microscopy, and direct optical microscopy

6) Microorganism analysis: Direct total microbial count, harvesting, concentration, fractionation, yeast, molds, Giardia, Legionella, coliform, and canine microfilaria.

8) Oceanographic studies: Transparent micro-fabricated or track etched filtering layer filters provide a new tool for studying planktonic organisms. These ultra-thin transparent filtering layers are strong yet flexible, allowing for planktonic samples to be filtered and the filtering layers to be mounted directly onto microscope slides.

9) Fouling investigation: Transparent micro-fabricated filtering layers can be employed as a great tool for investigation of mechanism of fouling on the surface of micro/nano filters. References:

[1] R.W. Baker, Membrane Technology and Applications, Wiley, 2nd Edition, (2004).

[2] R.D. Noble, Membrane separations technology: principles and applications, Elsevier science, 1995.

[3] Ramachandran V and Fogler H S 1999 Plugging by hydrodynamic bridging during flow of stable colloidal particles within cylindrical pores Journal of Fluid Mechanics 385 129-56

[4] C. Van Rijn. Nano and micro engineered membrane technology, Elsevier Science, 2004.

[5] Kuiper S, Van Rijn C J M, Nijdam W and Elwenspoek M C 1998 Development and applications of very high flux microfiltration membranes Journal of Membrane Science 150 1 -8

[6] M.E. Warkiani, L. Chen, CP. Lou, H.B. Liu, Z. Rui, H.Q. Gong, Capturing and recovering of Cryptosporidium parvum oocysts with polymeric micro- fabricated filter, Journal of membrane science, 369 (201 1 ) 560-568.

[7] T. Yanagishita, K. Nishio, H. Masuda, Polymer through-hole membrane fabricated by nanoimprinting using metal molds with high aspect ratios, Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures, 25 (2007) L35-L38.

[8] M. Girones, I.J. Akbarsyah, W. Nijdam, C.J.M. van Rijn, H.V. Jansen, R.G.H. Lammertink, M. Wessling, Polymeric microsieves produced by phase separation micromolding, Journal of Membrane Science, 283 (2006) 41 1 -424. [9] L. Chen, M.E. Warkiani, H.B. Liu, H.Q. Gong, Polymeric micro-filter manufactured by a dissolving mold technique, Journal of Micromechanics and Microengineering, 20 (2010) 075005.

[10] M.E. Warkiani, L. Chen, CP. Lou, H.B. Liu, Z. Rui, H.Q. Gong, Capturing and recovering of Cryptosporidium parvum oocysts with polymeric micro- fabricated filter, Journal of membrane science, 369 (201 1 ) 560-568.

[1 1 ] Chandler, M.; Zydney, A., Effects of membrane pore geometry on fouling behavior during yeast cell microfiltration. Journal of membrane science 2006, 285 (1 -2), 334-342.

[12] J. McGeough, M. Leu, K. Rajurkar, A. De Silva, Q. Liu, Electroforming process and application to micro/macro manufacturing, CIRP Annals- Manufacturing Technology, 50 (2001 ) 499-514.

[13] P. Spiro, Electroforming: a comprehensive survey of theory, practice and commercial applications, Draper, 1971.

[14] S. Kuiper, R. Brink, W. Nijdam, G. Krijnen, M. Elwenspoek, Ceramic microsieves: influence of perforation shape and distribution on flow resistance and membrane strength, Journal of membrane science, 196 (2002) 149-157. [15] C. Van Rijn, M. van der Wekken, W. Nijdam, M. Elwenspoek, Deflection and maximum load of microfiltration membrane sieves made with silicon micromachining, Microelectromechanical Systems, Journal of, 6 (1997) 48-54.

[16] A.P. Boresi and R.J. Schmidt, Advanced mechanics of materials, Wiley, sixth edition, (2003). [17] B. Leonard, Membrane filtration guidance manual, United States, Environmental Protection Agency, office of Water, 2003.

Chou SY, et al, J Vac Sci Technology, B 17 (1999) 3197

Claims

Claims:

1. A method for fabricating a filter comprising the steps of:

(i) providing a first layer comprising a first plurality of pillars on a substrate; (ii) depositing a metallic material to fill the space between the pillars to form a metallic filtering layer comprising a plurality of openings; and

2. The method according to claim 1 , wherein step (i) comprises fabricating the first layer on the substrate.

3. The method according to claim 1 or 2, comprising fabricating the first plurality of pillars directly on the substrate or on a seed layer on the substrate.

4. The method according to any one of the preceding claims, wherein step (i) comprises depositing a first layer comprising a resist layer on the substrate or seed layer and performing lithography to fabricate the first plurality of pillars in the first layer.

5. The method according to any one of the preceding claims, wherein depositing the metallic material in step (ii) comprises using electroplating, sputtering, thermal evaporation or electroless plating.

6. The method according to any one of the preceding claims, wherein step (i) comprises depositing a seed layer comprising an electrically conductive layer on the substrate and fabricating the first layer comprising a first plurality of pillars on the seed layer; and step (ii) comprises electroplating to deposit the metallic material to fill the spaces between the pillars of the first layer.

7. The method according to any one of claims 3 to 6, wherein the seed layer comprises aluminium, gold, chromium, copper, titanium, silver or a combination thereof or an electrically conductive polymer or monomer.

8. The method according to any one of the preceding claims, wherein the metallic material comprises nickel, aluminium, gold, chromium, copper, titanium, silver or a combination thereof.

9. The method according to any one of the preceding claims, wherein step (iii) comprises

(a) depositing a second layer on the metallic and the first layers;

(b) fabricating the second layer into a layer comprising a second plurality of pillars;

10. The method according to claim 9, wherein the support comprises a metallic or a polymeric support.

11 . The method according to claim 10, wherein the metallic support comprises nickel, aluminium, gold, chromium, copper, titanium, or silver or a combination thereof.

12. The method according to any one of claims 1 to 8, wherein step (iii) comprises depositing a second layer on the first metallic and first layers and fabricating the second layer into a permeable support comprising a plurality of apertures.

13. The method according to claim 12, comprising performing lithography to fabricate the second layer into the permeable support.

14. The method according to any one of claims 1 to 8, wherein step (iii) comprises bonding a metallic support comprising a plurality of apertures to the metallic and first layers.

15. The method according to claim 14, wherein bonding the metallic support comprises electroplating, sintering, adhesive bonding and/or welding.

16. The method according to any one of the preceding claims, further comprising removing the pillars of the first layer to reveal the openings of the metallic filtering layer.

17. The method according to any one of the preceding claims, further comprising releasing the metallic filtering layer bonded to the support from the substrate.

18. The method according to claim 17, wherein releasing the metallic filtering layer bonded to the support from the substrate comprises removing the seed layer.

19. The method according to any one of claims 1 to 8, wherein step (iii) comprises forming the support comprising a plurality of apertures and bonded to the metallic filter layer from the substrate.

20. The method according to any one of the preceding claims, further comprising depositing a material to reduce the size of the openings in the metallic layer.

21 . The method according to claim 20, wherein the material comprises a metal or a polymer.

22. The method according to any one of the preceding claims, further comprising surface treating the metallic filtering layer.

23. The method according to claim 22, wherein the surface treating comprises plasma treatment and/or wet chemical etching.

24. A filter comprising: a metallic filtering layer comprising a plurality of openings and with at least a portion of at least one surface of the metallic filtering layer bonded to a support permeable to fluid or gas.

25. The filter according to claim 24, comprising a plurality of openings of substantially the same size.

26. The filter according to claims 24 or 25, wherein at least a portion of each surface of the metallic filtering layer is bonded to the support.

27. The filter according to any one of claims 24 to 26, wherein the plurality of openings is in a substantially regular array.

28. The filter according to any one of claims 24 to 27, wherein the support comprises a plurality of apertures for passage of fluid or gas.

29. The filter according to any one of claims 24 to 28, wherein the support comprises a grid structure.

30. The filter according to any one of claims 24 to 29, wherein the metallic filtering layer comprises nickel, aluminium, gold, chromium, copper, titanium, silver or a combination thereof.

31 . The filter according to any one of claims 24 to 30, wherein the support comprises a metal, photoresist, polymer, silicon or glass support.

32. The filter according to any one of claims 24 to 31 , wherein the openings are quadrilateral, oval shaped or circular.

33. The filter according to any one of claims 24 to 32, wherein the openings are rectangles or squares.

34. The filter according to any one of claims 24 to 33, comprising a plurality of openings of 10 nm to 1000 μηπ in width and/or length.

35. The filter according to any one of claims 24 to 34, comprising a plurality of openings of 100 nm to 1000 μηη in width and/or length.

36. The filter according to any one of claims 24 to 35, wherein the support is substantially rigid.

37. The filter according to any one of claims 24 to 36, wherein the metallic filtering layer is of a defined thickness.

38. The filter according to claim 37, wherein the thickness of the metallic filtering layer is substantially even.

39. The filter according to claim 37 or 38, wherein the thickness of the metallic filtering layer is from 10 nm to 50 μιη.

40. The filter according to any one of claims 24 to 39, wherein the support is thicker than the metallic filtering layer.

41 . The filter according to any one of claims 24 to 40, wherein each surface of the metallic filtering layer is substantially flat.

42. The filter according to any one of claims 24 to 41 , wherein each surface of the metallic filtering layer is substantially smooth.

43. The filter according to any one of claims 28 to 42, wherein the apertures of the support are larger than the openings of the metallic filtering layer.