BACKGROUND

[0001] The present disclosure relates generally to nano-structured surfaces, and more particularly, to the formation of capped nano-pillars on surfaces. Such capped nano-pillars are suitable for use in a wide variety of applications, including applications for shock absorption, promoting adhesion, tuning surface wettability, and micro- or nano-fluidic filtration, among other applications.

[0002] Nano-scale surface structures may be formed using a template formed on the surface by an anodic oxidation process that involves immersing a workpiece in an acidic solution, and applying a voltage and/or current with the workpiece serving as an anode to cause oxidation of the workpiece surface so as to form pores (on a nanometer scale) in the workpiece surface. The porous surface structure then may be used as a template, pores in the surface being filled with a material to define nano-structures in the form of nano-pillars. Once the pores are filled, the template may be removed to expose one or more arrays of nano-pillars formed on and supported by the workpiece surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Features and advantages of embodiments of the present disclosure will become apparent with reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.

[0004] Fig. 1 is a simplified perspective view of an article including an array of capped nano-pillars formed in accordance with an embodiment of the present invention. [0005] Fig. 2 is a cross-sectional view of the article shown in Fig. 1 , taken generally along line 2-2 of Fig. 1.

[0006] Figs. 3A through 3E schematically depict a method of fabricating an article having capped nano-pillars in accordance with an embodiment of the present invention.

[0007] Fig. 4 is a flowchart showing a method of forming capped nano- pillars on a substrate in accordance with an embodiment of the present invention.

[0008] Fig. 5 is a flowchart showing a method of forming capped nano- pillars on a substrate in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0009] Referring initially to Fig. 1 , an article 10 is shown, the depicted article including a substrate 20 having a surface 22 with a nano-structure array 30 formed thereon. Nano-structure array 30 includes a plurality of nano-pillars 40, each carrying a nano-cap 45. As will be described further below, each nano- cap 45 (also referred to as a nano-dot) may substantially cover a distal end of a nano-pillar, and may be formed of a material different from the material forming such nano-pillar.

[0010] The methods disclosed herein may be used to control various properties of the nano-pillars and nano-structure array. For example, placement of nano-pillars in the nano-structure array may be selectively controlled via the methods disclosed herein. Similarly, the size of gap formed between adjacent nano-pillars may be selectively controlled, and the geometry and/or dimensions of the nano-pillars (such as their height, diameter, shape, etc.) may be controlled.

[0011] In one example, the nano-pillars 40 are elongate structures that extend orthogonal to the substrate. The geometry of the nano-pillars may be controlled so that the nano-pillars have substantially uniform columnar shape. Similarly, as shown in Fig. 2, the nano-pillars may be substantially uniform in height (H), and the pitch of the nano-pillars (the center-to-center distance between nano-pillars (D)) may be substantially uniform. Nano-pillars 40 thus may be substantially uniformly spaced across at least a portion of the substrate surface, providing a substantially uniform nano-structured surface. Dimensions of nano-pillars generally will vary by less than 10% to 20% (for nanometer scale dimensions), and in some examples, may vary by as little as 1 % or 2%.

[0012] Referring now to Figs. 1 and 2, it will be appreciated that nano- pillars 40 each include an elongate stem portion 42 extending from the substrate. The example stem portions take the form of cylindrical columns, each generally characterized as having a pillar thickness (represented by stem diameter (d1 )) and a pillar height (represented by stem height (hi )). As indicated, stem portions 42 have substantially uniform stem heights (hi ), and may have substantially uniform stem thicknesses along such stem heights. The stem portions thus terminate in distal ends 44, which collectively define a plane that is substantially uniformly spaced from substrate surface 22.

[0013] As noted above, each nano-pillar is configured to carry a nano-cap 45, which may be formed from any of a variety of materials, including materials different from the material forming nano-pillars 40. In the present example, nano- caps 45 are formed on distal ends 44 of stem portions 42. Accordingly, nano- caps 45 lie in a plane corresponding to the plane defined by the nano-pillar distal ends, and thus are substantially uniformly spaced from substrate surface 22.

[0014] In some examples, the nano-caps 45 take the form of cylindrical columns, each generally characterized as having a cap diameter (d2) and a cap height (h2). Nano-caps 45 also may have substantially uniform cap heights (h2), and substantially uniform cap thicknesses (represented by cap diameter (d2)) along such cap heights.

[0015] Nano-caps 45 may substantially cover stem portion distal ends 44. Cap diameter (d2) thus may be substantially the same as stem diameter (d1 ). It will be noted, however, that the example nano-pillars have stem heights (hi ) that are substantially greater than cap heights (h2), generally on the order of 3 to 6 or more. The nano-caps thus also may be referred to as nano-dots.

[0016] Although columnar nano-pillars and nano-caps are shown for illustrative purposes, the nano-pillars can have other geometries, which may be determined at least in part by parameters of the fabrication process (e.g., anodization voltage, current density, nature of electrolyte, etc.). For example, it is possible to control height, diameter, shape, and spacing between nano-pillars. It thus will be appreciated that the fabrication process may be manipulated to tune nano-pillar geometry and spacing to accommodate a variety of particular nano-cap geometries and spacing.

[0017] Figs. 3A-3E depict an article 10 through various stages of fabrication. As shown, a substrate 20 thus may be adapted, through the present method, to include a nano-structured surface that includes an array of capped nano-pillars. Although a particular nano-pillar geometry is shown, it will be understood that the fabrication process parameters may be altered to achieve different nano-pillar geometries.

[0018] Referring initially to Fig. 3A, fabrication begins with a substrate 20 having a surface 22. Substrate 20 may be selected based, at least in part, on whether or not the material will provide a suitably a planar surface 22. In some examples, substrate 20 may be formed from a substantially planar silicon wafer. Substrate 20 similarly may be formed from other materials, e.g., glass, quartz, alumina, stainless steel, plastic, and/or the like, and may take any of a variety of forms, including a multilayer structure.

[0019] As shown, a first oxidizable material is deposited on substrate 20 to form a layer of first oxidizable material 50. The first oxidizable material layer 50 may be formed using any suitable deposition technique known in the art. Some non-limiting examples of suitable deposition techniques include physical vapor deposition (PVD) (such as sputtering, thermal evaporation and pulsed laser deposition), atomic layer deposition (ALD), or, in some instances, chemical vapor deposition (CVD).

[0020] In some examples, the first oxidizable material layer 50 may be formed of a metal or metal alloy that forms a dense metal oxide after electrochemical oxidation. Suitable oxidizable materials include oxidizable refractory metals such as tantalum (Ta), niobium (Nb), titanium (Ti), tungsten (W), or their alloys. Such oxidizable materials can be electrochemically and/or thermally oxidized, and have expansion coefficients (the ratio between thickness of the grown oxide and thickness of the consumed material) that are greater than 1.

[0021] In the present example, first oxidizable material layer 50 is formed of tantalum (Ta), which has been found suitable for use in the method described herein. The example first oxidizable material layer also may be referred to herein as the "Ta layer". The Ta layer may have any suitable thickness that will produce (during electrochemical oxidation) enough oxide to form the nano-structures (which will be described in further detail below). In some examples, the thickness of the Ta layer may be approximately 100 to 1000 nanometers.

[0022] Referring still to Fig. 3A, it will be noted that a second oxidizable material is deposited on the Ta layer to form a layer of second oxidizable material 60. The second oxidizable material layer may have a thickness selected to produce a porous oxide (as described below), which corresponds to the desired nano-structures to be formed. The second oxidizable material may be aluminum (Al), or may be an aluminum alloy such as an alloy having aluminum as the main component. Second oxidizable material layer 60 also may be referred to herein as the "Al layer". The Al layer may have any suitable thickness that will produce (during electrochemical oxidation) enough oxide to form a template sufficient to produce the intended nano-structures. In some examples, the thickness of the Al layer may be approximately 100 to 1000 nanometers.

[0023] Deposition of the second oxidizable material layer on the first oxidizable material layer may be accomplished using any suitable deposition technique known in the art. Some non-limiting examples of suitable deposition techniques include physical vapor deposition (PVD) (such as sputtering, thermal evaporation and pulsed laser deposition.

[0024] As shown generally in Fig. 3B, the multi-layer structure of Fig. 3A may be further processed to form a nano-structure template 80 on substrate 20. The nano-structure template defines a plurality of nano-pores 82, each having a first width (indicated as nano-pore diameter (d _p i), in the present example). Such nano-pores are suitable for use in forming nano-structures on the substrate, as will be described herein. [0025] In some examples, further processing includes a first anodization process whereby second oxidizable material layer 60 (Fig. 3A) is anodized to define a plurality of substantially uniform, cylindrical nano-pores 82. Such nano- pores may be formed by completely anodizing the second oxidizable material layer 60 (e.g., the Al layer) so as to produce a nano-structure template 80 in the form of a layer of porous oxide (e.g., anodic porous alumina, AI ₂ O3) with nano- pores 82. Complete anodization refers to the oxidation sufficiently through the thickness of the layer being anodized to allow anodization of underlying first oxidizable material layer 50, as will be described below.

[0026] Anodization (i.e., electrochemical oxidation) is a process of forming an oxide layer on a material by making the material the anode in an electrolytic cell and passing an electric current through the cell. For anodization of aluminum, as in the present example, applied voltage may be kept constant at voltage within a range of about 10 V to 200 V. In some examples, the first anodization process may occur at a voltage of about 30 V

[0027] As indicated generally above, it is possible to adjust geometry by adjusting parameters of the fabrication process. For example, geometry of the nano-structure template 80 may be adjusted by varying one or more of anodization voltage, current density and electrolyte. Such adjustments to the first anodization process may alter nano-pore pitch (D _p ) and/or nano-pore diameter (d _p i), which characteristics are illustrated in Fig. 3B. For example, nano-pore pitch may be related to anodization voltage, where nanometer pitch (D _p ) is 2.8 nanometres per volt of anodization voltage. Nano-pore pitch (D _p ) generally may be adjusted within a range of from about 30 nanometers to about 500 nanometers. Nano-pore diameter (d _p i) generally may be adjusted within a range of from about 10 nanometers to about 350 nanometers.

[0028] Anodization can be performed at constant current (galvanostatic regime), at constant voltage (potentiostatic regime) or at some combination of these regimes. Nano-pore diameter (d _p i) is proportional to anodization voltage. Accordingly, a potentiostatic regime may be employed to produce a porous substrate with nano-pores having substantially uniform nano-pore diameter (d _p i). Substantially uniform nano-pores 82, in turn, will yield substantially uniform nano- pillars 40, as will be described below.

[0029] The first anodization process may be carried out by exposing Al layer 60 to an electrolytic bath containing an oxidizing acid such as sulfuric acid (H ₂ S0 ), phosphoric acid (H ₃ P0 ) oxalic acid (C ₂ H ₂ 0 ) and/or chromic acid (H ₂ Cr0 ₄ ). The electrolyte may be present, for example, in a water-based solution. The voltage applied during the first anodization process may be selected based on the electrolyte composition. For example, the voltage may range from 5-25V for an electrolyte based on sulfuric acid, 10-80V for an electrolyte based on oxalic acid, and 50-150V for an electrolyte based on phosphoric acid. The particular voltage used will depend on the desired pore diameter (and the suitability of such voltage for the electrolyte).

[0030] Nano-pore diameter (d _p i) also is related to the nature of the electrolyte used. Accordingly, an electrolyte may be selected to achieve a particular desired nano-pore diameter (d _p i). As non-limiting examples, nano- pores 82 of the following sizes may be obtained using the following electrolytes: nano-pore diameters (d _p i) of about 20 nanometers may be obtained using H ₂ S0 ₄ (in a water-based solution) as the electrolyte; nano-pores diameters (d _p i) of about 40 nanometers may be obtained using C ₂ H ₂ 0 ₄ (in a water-based solution) as the electrolyte; and nano-pores diameters (d _p i) of about 120 nanometers may be obtained using H ₃ P0 ₄ (in a water-based solution) as the electrolyte.

[0031] In one example, nano-structure template 80 is formed by anodization of the second oxidizable material layer 60 in a 4% solution of oxalic acid (C ₂ H ₂ 0 ₄ ), at a voltage of 30 Volts until substantially the entire Al layer is consumed. For a suitably thick Al layer, the resulting nano-structure template 80 will define nano-pores 82 that are approximately 30 nanometers wide, and that will allow oxidation of underlying first oxidizable material layer 50. The nano- structure template should have a template height (h _T ) sufficient to allow deposit of nano-caps 45 following complete growth of a nano-pillars 40 within the nano- pores. [0032] After the first anodization process, the nano-pore diameter (d _p i) may be further tuned to a target nano-pore diameter by anisotropic etching, or other suitable process. Anisotropic etching may be performed using diluted phosphoric acid (5 vol.%). The time for etching may vary, depending, at least in part, upon the desirable average diameter for the final pores. The temperature for etching may also depend upon the process, the etching rate, and the etchant used.

[0033] In some examples, prior to performing the first anodization process, the first oxidizable material layer may be patterned to precisely define locations of nano-pores 82 in the resulting nano-structure template 80. Patterning may be accomplished via any suitable technique. The patterned layer (not shown) is then anodized, for example, by employing the patterned layer as the anode of an electrolytic cell. A suitable amount of voltage and current is then applied to the electrolytic cell for an amount of time to completely anodize the patterned layer in accordance with the first anodization process described above. This can result in substantially uniformly spaced nano-structures where the variance in spacing between nano-structures differs by less than 1 % (for nanometer scale dimensions).

[0034] Referring now to Fig. 3C, nano-pores 82 may be partially filled to define nano-pillar stem portions 42. Nano-pillar stem portions may be formed via a second anodization process selected to partially anodize the underlying first oxidizable material layer 50 (e.g., the Ta layer). Such second anodization process will grow an oxide from the first oxidizable material, the oxide forming in the nano-pores 82 of the nano-structure template 80 from the bottom up. Where the first oxidizable material layer 50 is formed of a metal such as tantalum (Ta), the resulting oxide may take the form of a dense oxide such as anodic tantalum pentoxide (Ta ₂ 0 ₅ ).

[0035] The second anodization process may be accomplished, for example, using a process similar to the first anodization process described above. More specifically, the first oxidizable material layer 50 is anodized by employing the first oxidizable material layer as the anode of an electrolytic cell to achieve a desired oxidation of the first oxidizable material.

[0036] For oxidation of tantalum, non-limiting examples of electrolyte may include solutions containing citric acid (C ₆ H ₈ 07), oxalic acid (C ₂ H ₂ 0 ), boric acid (H3BO3), ammonium pentaborate ((NH ₄ ) ₂ BioOi ₆ x 8H ₂ 0), and/or ammonium tartrate (H ₄ N0 ₂ CCH(OH)CH(OH)C0 ₂ NH ₄ ). It is to be understood that this type of anodization forms a dense oxide, where both the interface between the remaining first oxidizable material and the formed oxide, and the interface between the formed oxide and the electrolyte are planarized.

[0037] During anodization of the first oxidizable material layer 50 (in this example, a tantalum layer), the formed oxide (in this example, tantalum pentoxide (Ta ₂ Os)) grows through the individual nano-pores 82 defined in nano- structure template 80 to form a nano-pillar stem portion 42 in each nano-pore. The orientation of nano-pillar stem portions 42 is generally controlled by the orientation of the nano-pores 82. In the present example, the nano-pillar stem portions 42 are substantially orthogonal to the surface 22 of substrate 20.

[0038] The expansion coefficient of a material to be oxidized is defined as the ratio of oxide volume to consumed material volume. The expansion coefficient for oxidation of tantalum (Ta) is approximately 2.3. Accordingly, in the present example, due to the significant expansion of tantalum pentoxide (Ta ₂ Os), and the fact that the resulting oxide (Ta ₂ Os) is dense, the nano-pores 82 are filled from the bottom up. It will be understood that although the first oxidizable material is tantalum (Ta) in the present example, other materials with an expansion coefficient greater than 1 would similarly allow the oxidizable material to squeeze into the nano-pores 82 of template 80.

[0039] As indicated, the grown oxide will partially fill nano-pores 82 of nano-structure template 80 to define nano-pillar stem portions 42. The geometries of the nano-pillar stem portions 42 will substantially conform to the geometries of corresponding nano-pores 82, within which the nano-pillar stem portions are growing. Nano-pillar stem portions 42 thus may take the form of substantially uniform cylindrical columns, substantially uniformly spaced across surface 22 of substrate 20.

[0040] In the present example, each nano-pillar stem portion has a substantially uniform stem thickness (indicated as stem diameter (d1 )) that corresponds to the nano-pore diameter (d _p i). Nano-pillar stem portions 42 are grown to a stem height (hi ) that is less than template height (h _T ) so as to allow subsequent deposit of nano-caps 45 on distal ends 44 of stem portions 42. As will be explained further below, nano-caps 45 may be electrochemically deposited onto distal ends 44 of stem portions 42 before removal of nano- structure template 80. Nano-structure template 80 thus may be used to define the geometry of both nano-pillars 40 and nano-caps 45.

[0041] The geometry and/or dimensions of the nano-pillar stem portions 42 may further be controlled by adjusting one or more parameters of the anodization process. For example, the stem height (hi ) will depend on the anodization voltage applied to the first oxidizable material layer 50 during its anodization. In some examples, nano-pillar stem portions are formed by anodizing the first oxidizable material at a first voltage corresponding to a target nano-pillar stem portion height.

[0042] In one example, nano-pillar stem portions having a stem height (hi ) of 90 nanometers (at a stem diameter of approximately 30 nanometers) may be formed by anodization of Ta layer 50 in a 0.1 % solution of citric acid (C ₆ H ₈ 0 ₇ ), at a current density of 2 mA/cm ² until voltage reaches 55 Volts, and for 5 minutes more at 55V. It will be appreciated that stem height (hi ) may be tuned to a target stem height by selecting a corresponding anodization voltage. For example, nano-pillar stem portions having a stem height of 155 nanometers may be formed by anodization of Ta layer 50 in a 0.1 % solution of citric acid (C ₆ H ₈ 0 ₇ ) at a current density of 2 mA/cm ² until voltage reaches 100 Volts, and for 5 minutes more at 100V.

[0043] As indicated in Fig. 3D, once stem portions 42 are grown to the target stem height (hi ), nano-caps 45 may be formed on the distal ends 44 of the stem portions. In some examples, nano-caps 45 are formed of a cap material that is different from the stem material. The cap material may be a metal, a polymer, or some other material suitable for electrochemical deposit on the distal ends of stem portions 42. Nano-caps 45 thus may be formed from conductors, semiconductors, dielectric materials, magnetic materials, piezoelectric materials, and other suitable materials.

[0044] As noted above, nano-caps 45 may be electrochemically deposited on the distal ends 44 of nano-stems 42. Such electrochemical deposit may be achieved by using the nano-pillars as a cathode in a solution of cap material, before removal of nano-structure template 80. The resulting nano-caps 45 will be deposited on stem portion distal ends 44, within nano-pores 82, and thus will substantially conform to the geometries of nano-pores 82. Nano-caps 45 may take the form of substantially uniform cylindrical columns having diameters corresponding to the diameters of stem portions 42. Furthermore, the nano-caps may be substantially uniformly spaced across surface 22 of substrate 20.

[0045] Although not particularly shown, nano-pores 82 may be re-shaped prior to deposit of nano-caps 45, thereby providing for formation of nano-caps shaped differently than stem portions 42. The nano-pores may remain substantially unchanged in stem-forming sections 82a, but may be changed in cap-forming sections 82b. In the depiced example, stem-forming sections and cap-forming sections are unchanged, and thus both have a same width (indicated as original nano-pore diameter (d _p i)).

[0046] In some examples, nano-pores 82 are re-shaped by broadening unfilled sections of the nano-pores 82 (the sections of the nano-pores above the formed stem portions 42). Such broadening may be achieved by selective etching of the nano-structure template 80. Selective etching may be accomplished by employing an etchant solution configured to etch the exposed areas of porous oxide forming the nano-structure template 80 (e.g., anodic porous alumina, AI ₂ O ₃ ) at a rate that is substantially higher than the etch rate for the oxide of the first oxidizable material (e.g., anodic tantalum pentoxide (Ta ₂ 0s)).

[0047] In the present example, each nano-cap has a substantially uniform cap thickness (indicated as cap diameter (d2)) that corresponds to the nano-pore diameter (d _p i). Nano-caps 45 are deposited to a cap height (h2), providing structures with an overall height (h). As shown, some residual first oxidizable material may remain beneath the grown oxide after the second anodization process (Fig. 3C).

[0048] In Fig. 3E, the nano-structure template 80 is removed to expose the fully formed nano-structure array 30, including nano-pillars 40 having nano-caps 45. The nano-structure template 80 may be removed using a second selective etching process that will remove the nano-structure template 80 without deleteriously affecting the nano-pillars 40, nano-caps 45 or other features of article 10. In one example, the selective etching may be performed using a selective etchant containing H ₃ P0 ₄ (92g), Cr0 ₃ (32g) and H ₂ 0 (200g), at approximately 95°C. It has been found that the example tantalum pentoxide (Ta ₂ 0s) nano-pillars 40 can withstand this particular etching process for more than one hour, while the example anodic porous alumina (AI ₂ O3) nano-structure template 80 is etched away at a rate of about 1 micron per minute. Other selective etchants are also contemplated, dependent on the particular characteristics of the nano-pillars, nano-caps and other structures.

[0049] Fig. 4 shows a high-level flowchart 100 of a method of forming capped capped nano-pillars on a substrate, as described herein. The method generally includes: (1 ) forming a template on the substrate, the template defining nano-pores; (2) at least partially filling the nano-pores with a pillar material to define nano-pillars; and (3) electrochemically depositing a cap material on the nano-pillars to define nano-caps on the nano-pillars.

[0050] More particularly, at 1 10, a template is formed on the substrate, the template defining nano-pores. At 120, the nano-pores are at least partially filled with a pillar material to define nano-pillars. At 130, cap material is electrochemically deposited on the nano-pillars to define nano-caps on the nano- pillars.

[0051] Partially filling the nano-pores may include forming a layer of a first oxidizable material, and anodizing the layer of first oxidizable material to grow oxide from the first oxidizable material into the nano-pores. The cap material may be deposited into the nano-pores using the nano-pillars as a cathode in an electrochemical deposition process. The resulting nano-pillars may have a pillar thickness and the resulting nano-caps may have a cap thickness substantially the same as the pillar thickness.

[0052] The template may be formed by forming a layer of a second oxidizable material and anodizing the layer of second oxidizable material to define the nano-pores. Once the capped nano-pillars are formed, the template may be removed.

[0053] Fig. 5 shows a flowchart 200 of a method of forming capped capped nano-pillars on a substrate, as described herein. The method generally includes: (1 ) depositing a first oxidizable material onto a substrate; (2) depositing a second oxidizable material onto the first oxidizable material; and (3) anodizing the second oxidizable material to form a porous oxide having nano-pores that extend through the porous oxide to expose portions of the first oxidizable material; (4) anodizing the first oxidizable material so as to partially fill the nano- pores in the porous oxide with a pillar material including an oxide of the first oxidizable material, thereby forming an array of nano-pillars of substantially uniform height; (5) electrochemically depositing a cap material on the nano-pillars so as further fill the nano-pores in the porous oxide with a cap different form the pillar material; and (6) removing porous oxide by selective etching, thereby yielding an array of capped nano-pillars on the substrate.

[0054] More particularly, at 210, a first oxidizable material (which may take the form of Tantalum (Ta)) is deposited onto a substrate. At 220, a second oxidizable material is deposited onto the first oxidizable material. At 230, the second oxidizable material is anodized to form a porous oxide having nano-pores that extend through the porous oxide to expose portions of the first oxidizable material. At 240 the first oxidizable material is anodized so as to partially fill the nano-pores in the porous oxide with a pillar material including an oxide of the first oxidizable material, thereby forming an array of nano-pillars of substantially uniform height. At 250, a cap material is electrochemically deposited on the nano-pillars so as further fill the nano-pores in the porous oxide with a cap different form the pillar material. At 260, the porous oxide is removed by selective etching, thereby yielding an array of capped nano-pillars on the substrate. In some examples,

[0055] Although the present invention has been described with reference to certain representative examples, various modifications may be made to these representative examples without departing from the scope of the appended claims.