Porous metallic membrane

Abstract

The present disclosure relates to a method of forming a metallic layer having pores extending therethrough, the method comprising the steps of: (a) contacting a cathode substrate with an electrolyte solution comprising at least one cation; reducing the cation to deposit the metallic layer on a surface of the cathode substrate; and (c) generating a plurality of non-conductive regions on the cathode substrate surface during reducing step (b); wherein the deposition of the metallic layer is substantially prevented on the non-conductive regions on the cathode substrate surface to thereby form pores extending through the deposited metallic layer. The present disclosure further provides a metallic porous membrane fabricated by the disclosed process.

Claims

1. A method of forming a metallic layer having pores extending therethrough, the method comprising the steps of: (a) contacting a surface of a cathode substrate with an electrolyte solution comprising at least one cation; (b) reducing said cation to deposit the metallic layer on the surface of said cathode substrate; and (c) generating a plurality of non-conductive regions on the cathode substrate surface during reducing step (b); wherein: the deposition of the metallic layer is substantially prevented on said non-conductive regions on the cathode substrate surface to thereby form pores extending through the deposited metallic layer; and step (c) comprises a step of reducing an electron acceptor species contained within said electrolyte solution to dispose a non-conductive material on the cathode substrate surface to thereby generate said non-conductive regions.

2. The method of claim 1, further comprising maintaining said electrolyte at a pH of between 3.0 to 3.5.

3. The method of claim 1, further comprising passing a constant current flow through said electrolyte, and optionally providing a localized current density at said cathode of between 1 to 60 A/dm.sup.2.

4. The method of claim 1, further comprising maintaining a constant concentration of said cations in the electrolyte solution.

5. The method of claim 1, wherein said electron acceptor is a hydrogen-generating species or wherein said electron acceptor is a hydrogen ion.

6. The method of claim 1, wherein said cation is selected from monovalent, divalent, trivalent or tetravalent ions of a metallic material selected from the group consisting of gold, palladium, platinum, silver, molybdenum, titanium, cobalt, copper, nickel, zinc, brass, solder and alloys thereof; or wherein said cation is divalent nickel.

7. The method of claim 1, further comprising a step of disposing at least one patternable material on said cathode substrate surface, said material being patterned to cooperate with the non-conductive material to define a desired pore geometry.

8. The method of claim 7, further comprising a step of removing the patterned material after deposition of the metallic layer.

9. The method of claim 1, wherein said cathode substrate surface is an uneven surface, said uneven surface comprising needle-like projections having a diameter of less than 100 nm.

10. The method according to claim 1, wherein said generating step (c) comprises a step of providing a patterned conductive layer on said cathode substrate surface, wherein the patterned conductive layer comprises a plurality of trench features having an aspect ratio of at least 3, and wherein the patterned conductive layer optionally comprises a resist layer substantially coated with a conductive seed layer.

11. The method of claim 10, further comprising a step of removing the patterned conductive layer after deposition of the metallic layer.

12. The method of claim 1, wherein said generating step (c) comprises a step of providing a patterned, non-conductive layer on said cathode substrate surface, said non-conductive layer being a resist layer having an aspect ratio of less than 1.

13. The method of claim 1, wherein the non-conductive material is a hydrogen bubble.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

(2) FIG. 1 is a schematic diagram depicting the formation of a porous nickel membrane.

(3) FIG. 2a is an optical micrograph depicting the porous nickel membrane generated in Example 1 on a plated Ni disk with 4 wafer scale. FIG. 2b is a SEM image showing the side wall of a pore channel of the porous nickel membrane generated in Example 1.

(4) FIG. 3 is a schematic diagram depicting a pore channel.

(5) FIG. 4 shows SEM images of the porous nickel membrane fabricated in Example 1 with rectangular pores 22 m.sup.2 in 100 m thick nickel membranes.

(6) FIG. 5 is a schematic diagram depicting the formation of a porous nickel membrane.

(7) FIG. 6 shows SEM images and optical micrographs depicting the porous nickel membrane generated in Example 2 having pores with circular, hexagonal and triangular cross sections.

(8) FIG. 7 is a schematic diagram depicting the formation of a porous nickel membrane by using a patterned conductive layer with high aspect ratio.

(9) FIG. 8 is a simulation conducted in Example 3 of the distribution of an electric field when a metal template comprising patterned trenches is placed in a uniform external electric field.

(10) FIG. 9 shows SEM images of the porous nickel membrane fabricated in Example 3 with rectangular pores 22 m.sup.2 in 100 m thick Ni membranes.

(11) FIG. 10 is a schematic diagram depicting the formation of a porous nickel membrane by using a patterned conductive layer with low aspect ratio.

DETAILED DESCRIPTION OF DRAWINGS

(12) FIG. 1 is a schematic diagram depicting the formation of a porous nickel membrane 10. A cathode substrate 2 comprising a plain metal sheet with a smooth and flat surface is used for electroplating to fabricate a porous Ni membrane 10. Evolution of hydrogen bubbles 4 during the electroplating process led to the formation of non-conductive regions on the cathode substrate surface. A nickel layer 6 is deposited on areas not covered by the non-conductive regions, thereby forming pores 8. The bubble 4, confined between the adjacent deposited Ni layer 6, may gradually enlarge as electroplating progresses. In the process, the enlarging bubble 4 may rise above the deposited Ni layer 6 to occupy a larger space. Further Ni deposition then results in the formation of a tapering pore 8. In some cases, the tapering pore 8 may exhibit a step-wise tapered profile.

(13) FIG. 3 is a schematic diagram depicting a pore channel 18 in accordance with an embodiment of the present disclosure. Pore channel 18 has tapered side walls 16, a first pore size 12 disposed at one side, and a second pore size 14 disposed at an opposite side, wherein said second pore size is greater than said first pore size.

(14) FIG. 5 is a schematic diagram depicting the formation of a porous nickel membrane 24. A cathode substrate 2 comprising a plain metal sheet with a smooth and flat surface has disposed thereon, a conductive seed layer 20 and a patterned conductive layer 22. Evolution of hydrogen bubbles 4 during the electroplating process led to the formation of non-conductive regions on the cathode substrate surface 2. Patterned layer 22 is configured to provide a shape for confining the bubble 4 therein. A nickel layer 6 is then deposited on seed layer 20. However, nickel deposition is prevented in areas covered by the patterned layer 20 and bubble 4. After a desired membrane thickness has been formed, the conductive seed layer 20 and patterned layer 22 are removed, leaving behind the membrane 24 having tapered pores 8 wherein the shape of the pore opening is based on the patterned layer 22.

(15) FIG. 7 is a schematic diagram depicting the formation of a porous nickel membrane 28 by using a patterned conductive layer 22 with a high aspect ratio (d/H is more than 3). A cathode substrate 2 comprising a plain metal sheet with a smooth and flat surface, and further comprising a disposed patterned conductive layer 22 coated with a conductive seed layer 20 on said cathode substrate 2, is used for electroplating to fabricate a porous Ni membrane 28. A nickel layer 6 is deposited on the exterior regions of the patterned conductive layer 22, thereby forming pores 8, after which the conductive seed layer 20 and patterned conductive layer 22 are removed. During electroplating, the electric field lines 24 are deflected and terminate at the top peripheral edges of the patterned conductive layer 22. The electroplating proceeds at the top surface with faster growth upward and near the edge of the patterned conductive layer 22, resulting in a region 26, where the electric field lines are low or zero.

(16) FIG. 10 is a schematic diagram depicting the formation of a porous nickel membrane 30 by using a patterned layer 22 with a low aspect ratio. A cathode substrate 2 comprising a plain metal sheet with a smooth and flat surface, and further comprising a disposed patterned conductive layer 22 and a conductive seed layer 20 on said cathode substrate 2, is used for electroplating to fabricate a porous Ni membrane 30. If the electroplating is conducted for a long period of time, the porous Ni membrane 30 has a first pore size 12 disposed at one side, and a second pore size 14 disposed at an opposite side, wherein said first pore size is greater than said second pore size.

EXAMPLES

(17) Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1

Method A

(18) The evolution of hydrogen bubbles is largely influenced by electrochemical factors, namely the type of electroplated metal, the bath composition, the pH of the solution, the current density employed, the temperature of the solution and the cathode current efficiency (CCE). Accordingly, in this example, the various factors are explored.

(19) One of the main factors influencing the evolution of hydrogen is the chemical nature of the electrode inside the bath solution. Different metals demonstrate different levels of electrocatalysis. Furthermore, electroplating at acidic conditions with increased H.sup.+ discharge reactions leads to the formation of a greater number of pores on the metallic surface.

(20) The Ni electroforming was performed in this example using a commercial plating system from Technotrans microform.200 with a Ni sulfamate bath solution of pH 3.5 and a temperature of 50 C., without the use of organic additives, as depicted in FIG. 1. Exemplary electroplating conditions are provided in Table 1 below.

(21) TABLE-US-00001 TABLE 1 Parameter Value Nickel sulfamate 380 (Ni(SO.sub.3NH.sub.2).sub.2) (g/l) Nickel (g/l) 3-89 Boric acid (H.sub.3BO.sub.3) (g/l) 5-55 pH 3.5-3.7 Temperature ( C.) 52-54 Current density (A/dm.sup.2) 4-15

(22) The experimental conditions that led to pore generation while electroplating at optimum conditions of low stress, high flatness, minimal grain size for plating, and high uniformity over large areas are: 1) a pH range of from, 3.5 to 3.0, where the pore density was observed to increase rapidly with decreasing pH; and 2) current density. It was observed that a current density of 5 A/dm.sup.2 is the lowest stress allowed for electroplating. However, for large areas and thick membrane layers of more than 100 m, this current density results in a long plating time of typically about 10 hours for a 100 m thick membrane covering an 8 area. Consequently, it would be more advantageous to increase the current density, while maintaining the thickness of the membrane below 100 m. The resultant membranes are typically no thicker than 50 m, and require a plating time of about 2 hours for a plating area of 4, and 6 hours for a plating area of 8. In general, the formation of a thicker membrane was facilitated by a lower current density. It was also observed that a low stress for electroplating was achieved at a current density of between 4 A/dm.sup.2 to 15 A/dm.sup.2.

(23) The electroplating process requires a constant plating rate to maintain a uniform growth of the electrodeposit element onto the substrate. This was realized by employing a constant current rather than a constant voltage power supply between anode and cathode. The plating growth rate is the rate at which the thickness of the electroplated material is increasing according to time, thickness/time or m/s. This is controlled by the amount of the constant current applied between anode and cathode. The speed of plating can be represented by plating growth rate as mm/s or by plating current density or total applied current/total plating area; I/A (A/dm.sup.2).

(24) For manufacturing purposes of the membranes, there are four important characteristics that define the membrane performances and commercial values: the pore density, the pore uniformity, the pore dimension and the pore shape.

(25) Statistical measurements were carried out on these parameters and the most influential parameters that control these characteristics were identified.

(26) The pore density is mainly controlled by pH. Changing the pH from 3 to 3.3 to 3.5 changed the pore density from 10% to as high as 60% to 90%.

(27) The pore size is mainly controlled by current density considering all other parameters remain constant. Changing the current density from 11.46 A/dm.sup.2 equivalent to 2.35 m/min to 56.3 A/dm.sup.2 equivalent to 11.54 m/min reduced the pore sizes from 10 m to 1 m.

(28) FIG. 2a is an optical micrograph depicting the top view of the porous nickel membrane generated in this example on a plated Ni disk with 4 wafer scale at 11.46 A/dm.sup.2 (2.35 m/min). Scanning electron microscopic (SEM) images showing the side wall of a pore channel of the generated porous nickel membrane is shown in FIG. 2b. The pore channel extends through the membrane and has an opening having a size of about 2 mm in diameter disposed on one side of the membrane and an opening having a size of about 1 m in diameter on the opposite side of the membrane. The inset shows the top view of the pore channel from the end with the larger opening. The dark spot in the middle of the inset is the 1 m opening on the other side.

(29) The pore generation relies on H.sub.2 bubbles and if the conditions for electroplating chemical reactions such as concentration of ions at the site and time of H.sub.2 generation, the temperature uniformity and stability, the electric field uniformity, the flow rate, and the template surface characteristics all remain exactly the same, the bubbles will be generated all with the same sizes. However, the current density has a limit depending on the template conductive surface. If the current passes a certain limit, the surface of the template burns and turns black. For example, for membranes having a conductive seed layer, e.g. Au (50 nm)/Cr (20 nm) conductive seed layer as will be described in method B below, the current density should not be more than limit of 65 A/dm.sup.2 equivalent to 12 m/min.

(30) It was further found that the generation of bubbles was dependent on the local electric field strength i.e., the higher the electric field strength, the more focused the field lines. That is, the footprints of the field lines become more focused and consequently, the generated bubbles become smaller. This ultimately leads to smaller pore sizes.

(31) In order to quantify this control mechanism on pore sizes, measurements were carried out at different electric field strengths. The strategy that was adopted was to decrease the area of plating rather than increasing the magnitude of applied external voltage. This is a better approach because the electric field lines remain highly parallel and uniform, especially at the center. Thus, by confining the area, the uniformity of the electric field lines is guaranteed, while the electric field strength is increased. Moreover, control of the electric field is more precise because of the high resolution of the microfabrication techniques.

(32) Accordingly, a high electric field strength associated with a plating current of 56.3 A/dm.sup.2, which is equivalent to 11.54 m/min, was used to fabricate the porous nickel membrane. FIG. 4 shows SEM images of the fabricated porous nickel membrane with rectangular pores 22 m.sup.2 in 100 m thick nickel membranes.

Example 2

Method B

(33) Electroplating was used along with the lithographic patterning technique to create pores within the patterned conductive layer comprising of different shapes, dimensions and defined geometries, as depicted in FIG. 5.

(34) The first step comprises of coating of a silicon wafer with a conductive seed layer for electroplating. A layer of Cr (20 nm) was used as an adhesion promoter for Au (50 nm) while the conductive seed layer was sputter coated on the silicon wafer. Other metals such as NiV, and Cu were also used in place of Au.

(35) The second step involves patterning a non-conductive layer on the smooth flat cathode surface. UV lithography was carried out to create the desired patterns on the surface of the silicon wafer. For high resolution nano-scale patterning, other resists and/or lithographic techniques can be used.

(36) The third step involves the Ni electroplating step, which was carried out using a DC current at low pH to create favorable conditions for the formation of non-conductive regions on the cathode substrate surface due to the generation of hydrogen.

(37) FIG. 6 shows a SEM and an optical micrograph depicting the porous nickel membrane generated in this example having pores with circular, hexagonal and triangular cross sections.

(38) Accordingly, the geometry and size of the pores was controlled by the surface pattern design in this example.

Example 3

Method C

(39) This method involves using a patterned conductive layer with a high aspect ratio, as depicted in FIG. 7. Due to the more rapid growth of electroplating at the side walls than the bottom of the pore channel, the side walls impinge at the top before the bottom can escape, thereby resulting in conical-shaped voids. If the aspect ratio is very high, the pore channels form semi-closed conductive surfaces that prevent the electric field lines from penetrating them.

(40) FIG. 8 is a simulation plot of the distribution of an electric field when a Ni porous membrane comprising of trenches with high aspect ratios was placed in a uniform external electric field. The simulation shows that the electric field inside the trenches is zero. In addition, it illustrates that if the aspect ratio of the trenches is more than 3, the conductive surface of the deep trenches repels the electric field lines towards the outer surfaces. The electric field lines would be deflected and would terminate at the top peripheral edges of the patterns. The electroplating proceeds at the top surface with faster growth upward and near the edges of the patterns. This leads to V-shaped patterns moving outwards from the interior regions where the electric field lines are low or zero.

(41) After electroplating to the desired thickness, the membrane with well-defined pores can be delaminated. Since the membrane and template have a planar interface with each other and have minimal adhesion, they are easily separated with minimal defects to the fabricated porous membrane.

(42) FIG. 9 shows SEM images of the porous nickel membrane fabricated in this example with rectangular pores 22 m.sup.2 in 100 m thick Ni membranes.

Example 4

Method D

(43) Large resist patterns with low aspect ratio can also be created on a conductive seed layer surface. Using electroplating conditions of low H.sub.2 formation, i.e. high pH conditions, Ni deposition will take place on the exposed conductible areas. In addition to the H.sub.2 non-conductive regions, the resist areas will also not support Ni deposition reactions and voids will be created.

(44) If the plating is conducted for an adequate period of time, these voids will gradually become smaller in size due to overplating. By controlling the plating time, the terminal pore size can be defined.

(45) This method is depicted in FIG. 10 which shows the fabrication of a conical shaped porous membrane by overplating on large area patterns.

Example 5

Method E

(46) To produce arrays of pores with small dimensions, a patterned template with properties that concentrate the electric field lines into particular spots can be used. Such a substrate consists of dense arrays of needle-shaped Si tips. The sharp needles serve to concentrate the electric field on the sharp tips. As a result, the electroplating current density will increase at the tips with most reactions taking place at the sharp tips. Consequently, generation of the bubbles will be directed and promoted on the sharp tips. Hence, the tips provide conditions to generate the bubbles at the sharp tips and the ability to produce high density porous membranes with dimensions in sub 100 nm scales.

Example 6

Membrane Characterization

(47) The porous metal membranes produced by Methods A, B and C above were characterized in this example. The results are summarized in table 2.

(48) TABLE-US-00002 TABLE 2 Membrane Minimum Pore Pore Pore type Size (m) Uniformity* Density** Area Method A 20 to 0.05 10% to 70% 10% to 70% 8 and larger Method B 3.0 to 0.02 90% 10% to 70% 8 and larger Method C 3.0 to 0.02 90% 10% to 70% 8 and larger *Pore size refers to the largest pore in the membrane through which a particle can pass; determined by SEM measurements. **Pore density was measured by placing a membrane sample under a scanning electron microscope (SEM) and physically counting the number of pores per unit area. The stated percentage values refer to the percentage of surface area on the membrane surface covered by pores.

APPLICATIONS

(49) The disclosed method for preparing a metallic porous membrane is particularly advantageous for preparing membranes having customized pore geometry and dimensions. In particular, it has been shown that the tapered pore channels disclosed herein allow good selectivity without compromising the flux of the membrane. Through the control of electroplating parameters, the disclosed method also advantageously prepares porous membranes of high pore uniformity, thereby reducing or eliminating the risk of contaminants entering the filtrate. These porous metal membranes can be used in reverse osmosis, ultrafiltration, or gas separation. In addition, they may be used for the removal of pathogens or particulates from water or food samples.

(50) It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.