Patent ID: 12209016

DETAILED DESCRIPTION

The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

According to an embodiment, there is a fabrication process that allows the formation of conductive electrodes, either symmetric, i.e., using the same material, or asymmetric, i.e., using dissimilar materials, which are spaced apart by a gap of nanoscale size, i.e., in the range of 1-100 nm. These coplanar nanogap electrodes can be used to form a soft mask, which can then be used to form various devices over large-area substrates, having features in the nanoscale range, at a low cost. The conductive electrodes may be made of different materials, such as metals, transparent conductive oxides (e.g., indium tin oxide, ITO), conductive polymers, graphene, to name a few. The conductive electrodes may be formed as an individual layer or a multilayer structure. For the sake of simplicity, this application uses the term ‘metallic’ electrodes herein to include all of the above mentioned material options and also uses the term “metallization” to include the application of any such layers to a substrate.

According to an embodiment, the gap formation occurs at a boundary of an initially deposited, patterned, surface-treated metallic layer M1and a second conductive layer M2. This is achieved by inserting a SAM layer and a thin “interlayer” of titanium (Ti) or other metals/metal oxides with suitable surface chemistry, in-between the first and second conductive layers M1and M2. In one application, the thin interlayer is considered to be part of the second metallic layer M2. However, the thin interlayer can also be treated as an independent layer from the second metallic layer M2. Examples for alternative interlayers may include Al or Cr. The method can be implemented by depositing/growing the interlayer onto the SAM layer, which is deposited on a surface of the layer M1, followed by the deposition of the layer M2. The SAM layer creates a hydrophobic, non-sticky surface on top of the first conductive layer M1, with low-surface energy, which would not make a strong bond with the second conductive layer M2. In addition, the SAM layer would be easily removable from the first conductive layer M1. The SAM layer that is formed on the M1layer can be removed later, for example, with a fast UV/ozone or plasma treatment. However, the SAM layer still offers a good adherence to the first conductive layer M1and thus, after the second conductive layer M2is formed on the SAM layer, those parts of the second conductive layer M2, which are formed on the SAM layer on top of the first conductive layer M1, would be easily removable when agitated in a liquid, or exposed to a high flow of a fluid, which may be a fluid or gas (e.g., air), as discussed later in more detail. The Ti interlayer thickness can be tuned in the range of 1 to 500 nm, and its deposition is often followed by the deposition of the M2layer, typically platinum (Pt), also of arbitrary thickness that may be in the range of 10 to 500 nm.

The inventors have found that the incorporation of the Ti interlayer and SAM layer on the first conductive layer M1decreases the adhesive forces between the two conductive layers; M1and M2, allowing the easy removal of overlapping regions (regions of layer M2overlapping the layer M1) upon a gentle agitation of the substrate (by various means including gas pressure jet, sonication in a liquid bath) or by directing a fluid jet over the second conductive layer M2, without further need of complicated or expensive equipment or processing steps, hence the name ‘self-forming’ nanogap electrodes. The process discussed herein can be extended to multiple metallization layers depending on the target application, a few examples of which will be discussed later.

By filling or covering the empty nanogaps with semiconducting, dielectric, conducting, piezoelectric, ferroelectric, piezo-resistive, electrolytic, and/or an electrolyte material, electronic devices of various kinds can be fabricated with minimum complexity and high-yield, due to the self-forming nature of the process. Post-deposition processing steps can then be carried out via traditional methods, i.e., thermal annealing or other conventional means, in order to chemically convert or modify the chemistry and microstructure of the material in the nanogap. To this end, the inventors have realized that the nanometer dimensions of the formed gap between the first and second layers, M1and M2, enables novel ways of processing the active materials deposited inside the nanogap, i.e., in-between the layers M1and M2. An extremely fast (microseconds to seconds) and efficient method for achieving this processing is via photonic annealing and/or photochemical treatment of the material deposited in-between the layers M1and M2, as will be discussed later in more detail.

In this process, the metal nanogap containing a suitable precursor material deposited in-between layers of M1and M2, is exposed to an intense pulse of light (generated, for example, by a broad or narrow spectra lamp, e.g., xenon, or other light sources such as a laser diode) of different duration and intensity, depending on (i) the electrode material, (ii) the active material in the nanogap, (iii) the nanogap architecture, (iv) the overall device geometry, and (v) the material combinations.

The high intensity of the light pulse(s) is partially absorbed by the conductive layers and/or the active material itself, which is deposited in the gap or beneath/above the nanogap electrodes. If the light is absorbed by the metal electrodes, it subsequently raises the temperature of the layers M1and M2, from room temperature to over 1000° C., within a short timeframe (from microseconds to seconds) because of their small size. Because of the nanometer dimensions separating the two electrodes, heat can propagate almost instantaneously and momentarily across the nanogap, leading to the subsequent conversion of a precursor compound pre-deposited into the nanogap, to a functional material (semiconductor, dielectric, conductor or other functional material, etc.). Because of the short duration of the photonic curing step, the process can be implemented on arbitrary substrate materials, a few examples of which include glass, plastic, paper using additive methods such as roll-to-roll (R2R), sheet-to-sheet (S2S), to name but a few, because the short duration of the process does not produce enough heat to damage the substrate material, even if the substrate material is heat sensitive. This is very advantageous when the metal or other material that is deposited inside the nanogap needs a high temperature for processing, but the substrate cannot withstand that high temperature.

Also, the nanogap electrodes can be integrated to form either single, discrete, electronic devices, such as diodes, capacitors, transistors, photodiodes, light emitting diodes, etc., or integrated circuits for logic or analog optoelectronic applications. Both the self-forming nanogap features as well as the application of the photonic curing allow for the fabrication of these devices on arbitrary substrates and surfaces, at any scale, and with minimum operator involvement as the entire process may be automatized and performed under the supervision of an electronic controller.

A self-forming nanogap based solid-state device100is now discussed with regard toFIG.1. The device100includes self-forming coplanar nanogap electrodes110and120that are separated by a gap130having a width G, which is 100 nm or smaller. The width G of the nanogap is measured along the substrate, between the first electrode110and the second electrode120. In one embodiment, the width G is 100 nm or smaller. In another embodiment, the width G is 20 nm or smaller. The electrodes110and120are formed on a substrate140, and the electrodes have heights H1and H2, respectively. The heights may be or not the same. The substrate140may be glass, the first and second electrodes/layers may be made of the same or different materials. For example, in one application, the first and second layers are made of Al. In another application, the first layer is made of Al, and the second layer is made of a combination of Ti and Pt. Other materials that may be used are discussed later. The length of the first and second electrodes may vary between mm to cm. WhileFIG.1shows the gap130being empty, as discussed later, it is possible to fill the gap with one or more materials to form other devices, such as diodes or transistors.

A method for forming the self-forming nanogap based device100ofFIG.1is now discussed with regard toFIG.2. The method includes a step200of providing the substrate140. There is a large variety of substrate materials that can be chosen for this step. While glass and Si wafers with a thermally grown oxide layer are the standard materials for the semiconductor devices, flexible plastic substrates can be used as well. Generally, any material that (1) offers good adhesion to the chosen layers M1and M2, and (2) does prevent the molecules of a self-assembling monolayer (SAM) form forming a monolayer is suitable for the substrate. When these requirements are not met, an interfacial buffer layer, exhibiting appropriate surface chemistry, may be formed on top of the substrate to render the substrate material valid for the self-forming process of nanogaps. It is additionally possible that an active material (semiconductor or dielectric) is already present on the substrate140, before the start of the nanogap fabrication, (i.e., before the deposition of the layers M1and M2and/or other functional materials), which would create the electrodes on top of the active material.

In step202, the substrate140is metallized to form the first layer M1, which in this specific embodiment is chosen to be an aluminum layer of a given thickness H1. The term metallization in this application is understood to mean the deposition of a conductive material, which may be a metal but also a non-metal. In one application, the thickness H1is selected to be 100 nm or less. The first metallic layer M1may be deposited using thermal or e-beam evaporation deposition techniques (familiar to those skilled in the art), and then it can be patterned in step204, via standard photolithography, to obtain the first electrode110, as shown inFIG.3A. Note that plural first electrodes110may be formed on the substrate140. In one embodiment, the first electrodes110are formed directly on the substrate. The first electrodes110may be patterned to have any desired shape.

However, the deposition of the first metallic layer M1is not limited to thermal or e-beam evaporation, but may include other common vacuum deposition methods, such as sputtering or pulsed laser deposition. Additionally, solution-based printing or growth methods may also be used. The patterning of the first metallic layer M1to obtain the electrode110can be achieved via standard lithography and lift-off or etching protocols. Other options include, but are not limited to: shadow-masking during evaporation, laser ablation patterning method, soft lithography, micro-molding, or other printing techniques.

For this specific embodiment, because the first electrode110is formed of aluminum, a native aluminum oxide (alumina) layer is formed upon exposure to air. Then, the first electrode110is chemically functionalized in step206with a SAM layer150, as shown inFIG.3B. The SAM layer150is made in this embodiment of small organic molecule, such as octadecylphosphonic acid (ODPA). In one application, the SAM layer150may be achieved by creating a solution of ODPA in IPA (Iso-Propyl Alcohol) with a given concentration and submersing the metalized substrate140in the solution for a fixed time. Next, the substrate is rinsed with excess IPA and dried on a hotplate at an elevated temperature of 70° C. for several minutes. The precise method of SAM functionalization may vary in accordance with established protocols.

However, a SAM layer includes molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases, molecules that form the monolayer do not interact strongly with the substrate (note that the substrate in this paragraph refers to the material to which the SAM layer is formed upon, i.e., the first electrode110inFIG.1, and not the substrate140). This is the case, for instance, of the two-dimensional supramolecular networks of e.g., perylenetetracarboxylic dianhydride (PTCDA) on gold or of e.g., porphyrins on highly oriented pyrolitic graphite (HOPG). In other cases, the molecules possess a head group that has a strong affinity to the substrate and anchors the molecule to it. Such a SAM consisting of a head group, tail, and functional end group may include head groups such as thiols, silanes, phosphonates, etc. The SAM layers are created by the chemisorption of the “head groups” onto the substrate from either the vapor or liquid phase followed by a slow organization of “tail groups”. The “head groups” assemble on the substrate, while the tail groups assemble far from the substrate. Areas of close-packed molecules nucleate and grow until the surface of the substrate is covered in a single monolayer.

The molecules that form the SAM layer150on top of the first electrode110are selected with two goals in mind. The first goal is to change the surface properties of the first electrode110. The molecule chain and tail group of the SAM layer110should be chosen to create a hydrophobic (low-surface energy), a non-sticky surface on top of the first electrode. The pure alkyl chain of ODPA is a good example of this. Meanwhile, other possible candidates include, for example, alkyl/aromatic chains with different numbers of carbons or fluorinated chains.

The second goal of the SAM material is the selective binding of the head group, i.e., the SAM material is desired to bind to the first layer M1but not to the substrate140. The phosphonic acid-based ODPA used in this embodiment is known to bind to certain metal oxide surfaces such as the native alumina, but not to, for example, non-oxide surfaces such as plastic substrate, or substrates covered by an interlayer that can be processed atop prior to metal and SAM deposition. This way, the adhesion of the second layer M2with the substrate140's surface (or the surface of the interlayer) stays unaltered while the adhesion of the second layer M2towards the first layer M1is diminished. The head group can be changed depending on the nature of the first layer M1and needs to be matched/selected accordingly amongst several existing options.

For example, thiol-based molecules are known to attach preferentially onto noble metals such as gold, platinum, or silver, whereas phosphonic acids do not. The SAM layer deposition step is mostly carried out via self-assembly from the liquid phase (SAM molecules are dissolved in a solvent), but a gas phase deposition is also possible.

A few examples of possible SAM forming molecules as well as a conceptual visualization of the SAM selection criteria are illustrated inFIGS.4A to4C.FIG.4Ashows 7 different SAM materials having a head group (bottom region of the figure), a chain part (middle region of the figure), and an end group (top region of the figure). Note that examples 1-3, 6, and 7 have the end group very similar to the chain part, while examples 4 and 5 have the three groups distinct from each other. The head group may include the phosphonic acid400, or the carboxylic acid402, or the thiol404, while the chain/end groups may include an alkyl410or a fluorinated element412.

The first example 1 of SAM material is Octadecylphosphonic acid, the second example 2 is: Tetradecylphosphonic acid, the third example 3 is Decylphosphonic acid, the fourth example 4 is: 12,12,13,13,14,14,15,15,16,16,17,17-Tridecafluoroseptadecylphosphonic acid, the fifth example 5 is: 3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluorooctylphosphonic acid, the sixth example 6 is: Decanoic acid, and the seventh example 7 is: Octadecanethiol.

FIGS.4B and4Cschematically illustrate the selective binding of different SAM molecules to the first electrode110or an oxide layer112formed on the first electrode110, but the lack of binding to the substrate140. For example,FIG.4Bshows a first electrode110made of Al on the substrate140, and the oxide layer112formed at the surface of the first electrode110. A phosphonic acid (group400inFIG.4A) based SAM layer150is binding to a native or grown alumina layer112on the first Al electrode110.FIG.4Cshows a thiol (group404inFIG.4A) based SAM layer150binding to the surface of a first Au electrode110. In either case, the SAM layer150does not form a strong bond with the chosen substrate.

Returning toFIG.2, the method then advances to step208, in which the second metallization layer M2is formed over the SAM layer150and over the exposed substrate140, as shown inFIG.3C. The second layer M2may have a thickness of about 50 to 200 nm, but preferably 100 nm. In one embodiment, the second layer M2is formed to include a first thin layer122(e.g., less than 20 nm, preferably 5 nm) of titanium (which may be considered to be the interlayer discussed above) and a second thick layer124(e.g., less than 200 nm, but preferably 100 nm) of platinum. The two layers are schematically illustrated inFIG.3C, but not at scale. The two layers122and124may be deposited by e-beam evaporation, or other methods, over the pre-patterned first electrodes110on the substrate140.

The kind of metal that can be employed for the first layer M1is related to the availability of a suitable SAM forming molecule. Some examples of these metals are Al, Au, Ti, and ITO, but a much wider selection is possible. The second layer M2for the self-forming nanogap procedure may include a combination of the thin layer122(thickness in the range 1-100 nm) of titanium (or others mentioned above) followed by the second layer124, e.g., a platinum layer. Other materials instead of Pt may be used.

With regard toFIG.3C, in all areas where the second layer M2overlaps with the first electrode110, the SAM treated surface of the first electrode110drastically reduces the adhesion between the material of the first electrode110and the material of the second layer M2. Thus, when immersing in step210the substrate140in a liquid160(e.g., acetone or other liquids) contained in a container162, as shown inFIG.3D, while gently shaking/agitating in step212the container162, either through manual agitation or ultrasonication for a few minutes, or as long as it is required, makes the second layer M2to be removed (spontaneously being lift-off) from all the areas where it overlaps the first electrode110, but not from the areas where the second layer M2directly contacts the substrate140. Note that the step of agitation may also be implemented by directing a fluid jet over the second layer M2to remove it from the areas where the SAM layer is present. The fluid may be a liquid or a gas, e.g., air. The jet needs to have enough speed to accomplish this task. However, if the fluid jet is directed over the second layer M2, it may not be necessary to move/shake/agitate the substrate for removing the second layer M2from the first M1layer, and there is no need for the liquid160, i.e., the fluid jet may be directed to remove the second layer M2from the first layer M1. At the same time the entire device is kept in air.

As a result of the agitation step212(which can be achieved with a motorized device164), the second layer M2is fractured along the boundary of the first electrode110, and because of the low-energy adherence between the SAM layer150and the parts126of the second layer M2that are directly formed over the first electrodes110, these parts126are falling off the first electrodes110. However, the other parts of the second layer M2, which are directly formed on the substrate140, remain adhered to the substrate, and thus they form the second electrode120. As the parts126of the second layer M2are falling off the device100, the nanogap130appears between the first electrode110and the second electrode120, as shown inFIG.3E. The nanogap130has a gap width G, as previously shown inFIG.1. The gap width G is 20 nm or less. In one embodiment, the gap width G is between 1 and 100 nm. This gap130is self-forming and can be used for depositing other materials to make a diode, transistor, sensor, integrated circuit, solar cell, Wireless Energy Harvesting (WEH) devices, etc.

To remove the second layer M2from the overlapping areas of the first electrode110, several techniques are possible. These include the submersion of the substrate into a solvent bath (water, IPA, acetone amongst many other options) either with intermittent manual shaking for a duration of <1 h or without additional manual agitation over a prolonged period of time, as shown inFIG.3D. Instead of manual agitation, in another embodiment, it is possible that the second layer M2removal is initiated by ultrasonication for a shorter time frame (tenths of seconds to minutes). Another possibility without the usage of solvents is to expose the substrate to a pressurized gas stream (using compressed air, nitrogen, or other gases) that can remove the overlapping areas between the first electrode and the second metallic layer. Alternatively, an adhesive layer (glue, tape, etc.) is placed over the second layer M2. Then this layer is peeled-off, thereby taking away all weakly adhered M1/M2overlapping regions, similar to a process described in [1]. However, this step described in [1] needs to be carried out manually, with great care. Variations in the peel-off rate and the pulling force can lead to process variations, depending on the operator. Eliminating this peel-off step by using self-forming nanogaps, as described in the method illustrated inFIG.2, especially step212, increase the uniformity of the gap, removes operator-to-operator fluctuations, as well as saves material costs for specialized glues.

The nanogap130may be used, as discussed next, to receive one or more materials (semiconductor material or dielectric material). For this process, the SAM layer can be removed, for example, using a plasma treatment of less than 20 min, or an equivalent process, and a different SAM layer with a particular functionality (i.e., ability to change the electronic properties or chemistry of both or one of the electrodes) may be post-deposited inside the nanogap, for making a desired solid state device.

Various factors that may affect the nanogap fabrication process are now discussed. One such factor is the edge profile of the first electrode110. Because the nanogap formation takes place around the lateral boundary line of the first electrode110, this area appears to affect the process discussed concerningFIG.2. The height profile500, from the substrate140, of the first electrode110at this region can vary between a steep and sharp transition, as shown inFIG.5A, to a more gradual one or even an undercut, as shown inFIG.5B, depending on the technique employed to pattern the first layer M1to obtain the first electrode110. A sharp transition, as shown inFIG.5Ais generally preferred to create a nanogap of smallest dimensions, but it may not be necessary, depending on the application envisioned.

The deposition rate of the materials making the first layer M1and the second layer M2also play a role. It is known that the deposition rate during a gas phase deposition process influences specific properties of the final film. This applies to the average grain size within the metal film and to the surface roughness (root-mean-square roughness, RMS). In the case of aluminum, for example, the formed films tend to show a reduced grain size and roughness values at higher deposition rates. The grain size in both the first and second layers influence the nanogap formation process. For the first metallic layer M1, the grains define the lateral boundary, meaning that smaller grain sizes lead to a generally preferred more uniform outline, whereas larger grains result in more irregularity. Because it is the second metallic layer that fractures during the gap formation process, its average grain size also plays a role, as fractures tend to occur along grain boundaries, which constitute structurally weaker parts of the layer. Additionally, the adhesion of a deposited layer to the substrate material can change in magnitude, depending on the deposition rate and the nature of materials employed.

The thicknesses of the first layer M1and the second layer M2also influence the gap formation process. The dimensions of the self-forming nanogap may be influenced by the layer thickness of both metals involved. This applies to cases where both metals in the first and second layers M1and M2are of equal thickness, which can be varied from thin to thick, within the range of approximately 10 to 500 nm. Similarly, variations of the relative thicknesses between the first layer M1and the possible interlayer122(formed between the substrate140and the second metallic layer M2) and the second layer M2can affect the final gap size. In this regard,FIG.6Ashows possible implementations of the device100shown inFIG.1, in which the thickness of the second metallic layer M2is constant, while a thickness of the first metallic layer M1can vary from less than a thickness of the second metallic layer M2to more than the thickness of the second layer M2.FIG.6Bshows the thicknesses of both layers M1and M2being equal, but having different values, andFIG.6Cshows possible variations of the thickness of the second metallic layer M2relative to the first metallic layer M1.FIG.6Calso illustrates various materials that can be used for these layers.

A self-forming nanogap solid-state device is now discussed. This device may be a TFT transistor700, as shown inFIG.7Aor a Schottky gated TFT transistor710, as shown inFIG.7B. Other devices may be made with this novel technique, as for example, a solar cell. A method for manufacturing such solid-state devices is discussed with regard toFIGS.8A to8F.FIG.8Ashows a substrate140on which first and second electrodes110and120have been formed with a nanogap130in between them, based on the method discussed above with regard toFIG.2. The two electrodes,110and120are SAM functionalized with a SAM layer800, which may be the same or different from the SAM layer150. Note that for the embodiment illustrated inFIG.8B, the SAM layer800is formed after the formation of the gap130, so that the SAM layer800extends inside the gap, along with the side parts of the first and second electrodes110and120. Note that the SAM layer is not formed directly on the substrate140due to the properties of the substrate, as discussed above. Also, the SAM layer does not fill the nanogap. Then, as shown inFIG.8B, an interlayer810is formed over the SAM layer800and the substrate140, followed by the formation of another metallic layer M3, to cover an entirety of the interlayer810. The interlayer810may be made of similar materials as the interlayer122previously discussed.

Then, the device shown inFIG.8Bis placed in a liquid (for example, step210inFIG.2) and agitated (see step212inFIG.2) so that the interlayer810and the third metallic layer M3that are in direct contact with the SAM layer800fall out, while a portion of the third metallic layer M3that is directly or indirectly formed on the substrate140remains attached to the substrate140, within the gap130, to form the third electrode820, as shown inFIG.8C. In other words, the method discussed above with regard toFIG.2is applied to form additional first and second nanogaps832and834, between the third electrode820and the adjacent first and second electrodes110and120, as illustrated inFIG.8C.

Next, the SAM layer800may be removed, as shown inFIG.8D, using, for example, UV/Ozone or plasma treatment or any other known method. A dielectric layer840may then be formed around the third electrode820, inside the additional gaps832and834, using any traditional method, as shown inFIG.8E. For example, the dielectric deposition may include either the formation of a native oxide on the electrode820via oxidization and/or the deposition of a SAM using self-assembly (which does not require any manual alignment step). The formation of the dielectric material840inside the additional nanogaps832and/or834does not mean that these gaps are filled with the dielectric material. Next, a semiconductor material is deposited in the remaining parts of the additional gaps832and834and over the dielectric layer840to form a semiconductor layer860, as illustrated inFIG.8F. Note that both the dielectric layer840and the semiconductor layer860are formed inside the gaps830.

In this way, the TFT transistor700is obtained, where the first and second electrodes110and120play the roles of the source and drain of the transistor, and the third electrode820plays the role of the gate of the transistor. The semiconductor layer860acts as the channel between the source and the drain, and the electric charge flow through the channel is controlled by the gate820, through the dielectric material840.

The novel technology discussed above may also be used to prepare large-area, soft stamps containing features with sizes ranging from 1 nm to potentially any scale. This technology relies on the use of a nanoscale mould formed between two or more pre-patterned electrodes (made of similar or dissimilar materials and of predefined thickness ranging between 1 nm to 1 micrometer—or more) separated laterally by a distance that can range between 1-500 nm. The lateral size of the electrodes, on the other hand, can vary from hundreds of nanometres to meters, in length, depending on the method of patterning employed and the particular needs of the targeted application(s). This specific nanogap mould would be referred herein as the master mould.

The nanogaps formed between sequentially patterned electrodes in the nanogap mould could be prepared, as discussed above, by the method illustrated inFIG.2. In another embodiment, the nanogap mould may be formed using the method known as adhesion lithography (a-Lith) discussed in [1]. In another embodiment, the nanogap mould can be formed by any other large-area compatible nanogap patterning technology [2], [3]. The formed nanogap between the two or more electrodes act as a mould when a UV curable polymer is casted over and used to prepare the soft stamp by first curing it and then peeling it off.

In this regard, a method for making a soft stamp900is now discussed with regard toFIGS.9A-9D. The self-forming nanogap based device100discussed inFIGS.1and3Eis shown inFIG.9Aas being the starting point, i.e., the master mould. The device100has aluminum-aluminum (Al—Al) as the electrode material for electrodes110and120. In this example, the electrodes110and120are made of the same material, but in principle, they can be realized using numerous/arbitrary material combinations, if needed. The electrodes may be made to be transparent to light or not.

The master mould100, which can be fabricated on known and suitable substrate materials (of any size), includes 100 nm-thick (a range of thicknesses can be considered here depending on the particular features one wants to create) Al—Al electrodes110and120separated by a nanogap Ng smaller than 20 nm (other values can be considered, e.g., smaller than 100 nm), which was prepared via a-Lith method or self-forming method previously discussed. The master mould100may be cleaned using different processes (e.g., with oxygen plasma) and/or treated with different surfactants (SAMs, etc.), before starting the soft stamp preparation process. Other cleaning steps, such as rinsing or heating, may also be incorporated depending on the materials used and their interaction with the stamp material formulation.

After cleaning the master mould, an Anti-Sticking Layer (ASL)910, which can be EVG ASL 1 (from EVG group GmbH), is spin-coated on top of the exposed substrate140and the electrodes110and120and thermally annealed in air at 120° C. for a couple of minutes, for example, 10 minutes. Different ASL materials can be used and are known to those experts in the art of NIL or other fields.

A soft polymeric/UV curable solution902for forming the soft stamp900was prepared by mixing the EVGNIL AF1 (from EVG group) with the Photo-Initiator (PI) in a 98:2 ratio (volume of 10 ml:0.2 ml) and stirred vigorously for 30 minutes. The solution902was then poured on top of the master mould100containing the nanogaps, as shown inFIG.9B.

Next, as shown inFIG.9C, a portion of a movable substrate920is brought into physical contact with the solution902. The movable substrate920includes two parts: the top922and the bottom part of924. The bottom part924is pressed against the master mould100, sandwiching the solution902in-between and the top portion922is a transparent substrate which allows for UV light930to pass and cure the soft polymer solution902sandwiched between the master mould100and the bottom portion924of the movable substrate920.

The top portion922of the movable substrate920may be made of a glass substrate, which is chosen to have minimum absorption in the UV range, spin-coated with an adhesion promoter layer, known as EVG PRIM K (from the EVG group), followed by annealing at 120° C. for 2 minutes to obtain the bottom portion924. The adhesion promoter layer924enables the adhesion between the soft stamp900and the glass substrate922during the delamination of the soft stamp from the master mould.

UV light930, having a wavelength 375 nm, may be used to cross-link the polymer inside the solution902, which is placed in the nanogap Ng, and solidify. A UV light intensity of 18 mJ/cm2for 30 minutes exposure time was used in this embodiment and the photo-initiator enhanced the cross-linking of this soft polymer.

The soft stamp900(the solidified polymer solution902) is then delaminated from the master mould100, as shown inFIG.9Dand cleaned with oxygen plasma and HFE 7100 cleaner (from EVG group GmbH) for future use. Other known cleaning methods may be used. In this embodiment, the soft stamp900was cleaned with isopropyl alcohol (IPA) or HFE 7100 cleaner, but other solvents can be used.

FIG.10illustrates a method for using the soft stamp900to form various devices. WhileFIG.10the formation of a semiconductor device, those skilled in the art will understand that the soft stamp900can be used for forming other devices. In step S1000, a substrate140on which plural electrodes110and120have been formed is provided. The substrate with the plural electrodes is called herein the master mould. The gap Ng between the electrodes is in the order of nm, and these electrodes and the corresponding gaps have been formed as described in the method ofFIG.2. The electrodes could be made of the same or different material and they could have the same or different shapes and/or sizes.

The master mould can be made of different materials, such as metals, metal oxides, polymers, 2D materials, dielectrics, insulators, and nitrides, etc., as an individual layer or a combination of two or more layers. For the sake of simplicity, the electrodes in the following examples are considered to include metals and/or transparent conductive oxides (e.g., indium tin oxide, ITO), and for this reason, the discussion herein refers to these electrodes as metal electrodes, although non-metal electrodes may also be used. The electrodes may be made to have a predefined thickness, ranging between 1 nm to 1 mm—or more. The electrodes may be separated laterally by a distance that can range between 1-500 nm. The lateral size of the electrodes can vary from hundreds of nanometres to meters, depending on the method of patterning employed and the particular needs of the targeted application(s).

The nano-gaps Ng between the electrodes may be patterned to have any desired shape and/or size. The nano-size features (pillars, trenches, etc.) cast and/or replicated using the nanogaps Ng of the mould100, can be extended to any shape and/or size depending on the target application(s). Nanogap substrates can be made of rigid materials (Si, Si/SiO2, glass etc.) or flexible ones (e.g., plastics), or other suitable substrate materials known to those skilled in the art. The non-/conductive nanogap electrodes can be made on any choice of substrate materials and are not limited to the above mentioned materials/combinations. If a-Lith or self-forming nanogap method (as discussed inFIG.2) are used to create the nanogaps, then one possible limiting factor on the choice of the electrode material will be the self-assembled monolayer (SAM) used to perform the a-Lith or self-forming of nanogap. For instance, octadecyl phosphonic acid (ODPA) can be used for various metals and metal oxides whereas octadecane thiol (ODT) can be used for the noble metals like Au, Ag and Pt. In principle, however, SAMs with different functionalities could be synthesised and/or other methods for tuning the surface energy of the electrodes could be used.

In step S1002, the UV curable substance902, for example, a UV curable resin, is poured over the electrodes110and120. A UV curable resin is a class of materials that is polymerized and cured in a short time by the energy radiated from ultraviolet irradiation devices. In one application, the curable substance902may include a magnetic material (e.g., small particles, or nano-particles, or nano-tubes, or nano-wires). The UV curable substance is fluid enough to enter inside and fill the nano-gaps Ng, between the various electrodes110and120. A UV light904is irradiated in step S1004over the UV curable substance902to cure the resin. In step S1004, the cured resin, which is now the soft stamp900, is peeled off from the substrate140. The soft stamp900includes a planar-type base900A on which nanogaps901are formed, and the nanogaps901correspond to the electrodes110and120, i.e., the nanogaps901has the size of the electrodes110/120. The soft stamp900also includes nano-features903, which are also formed on the base900A and extend away from the base, and the nano-features903correspond to the nano-gap Ng from the master mould100. If a magnetic material has been added to the curable substance902, then the formed nano-features903are magnetic and would respond when a magnetic field is applied to them.

The height of the replicated nano-features903on the base900A of the soft stamp900depends on the depth of the nano-gap Ng, which in turn can be tuned by varying the thickness of the electrodes110and120employed in step S1000to form the nano-gaps Ng. In one embodiment, the height is 50 nm or smaller. Possible shapes of the nano-gaps901and/or nano-features903formed on the base900A of the soft mould900may include cylinders, triangles, squares, pillars, circles, or others arbitrary shapes desirable for the targeted applications. The targeted application depends on the application field, which may include any of optoelectronics, electronics, memory devices, solar cells, and bio-electronics/sensor applications, to name but a few. The technologies discussed with regard toFIG.10could also be adopted in the development of nanoimprint lithography (NIL) tools and their applications.

The soft stamp900can now be used in various processes for replicating it into a number of copies for making various devices. The replicating process may use any known soft lithography route, such as nano-contact printing, moulding/embossing (NIL), phase shifting edge lithography, and nanoskiving/mechanical sectioning, etc. The nano-contact printing method, as further illustrated inFIG.10, involves a step B1000of dipping the soft stamp900into a solution1010, e.g., SAM or ink so that traces of the solution1010are attached to the tips of the nano-features903. In step B1002, a substrate1012is brought next to the soft stamp900, and solution1010is transferred from the tips of the nano-features903to the substrate1012, as shown inFIG.10. The size and shape and distances between the solution traces1010on the substrate1012mirrors the size and shape and distances between the nano-features903of the soft stamp900. The substrate1012and the solution traces1010transferred onto the substrate1012form a new mask1014, which may be used, for example, to manufacture an optoelectronic device, like a grating. Many other devices may be manufactured with the nano-contact printing process discussed above.

The molding/embossing process uses the soft stamp900to imprint in step C1000the nano-features903into a curable polymer1020, which are hardened in step C1002by curing the polymer1020. In the same step, the soft stamp900is removed, thus obtaining the nano-gaps1022formed into the polymer1020, which also acts as the new mask1014.

The phase-shifting edge lithography process also uses the soft stamp900to form nano-features1034on a substrate1030. In this method, the soft stamp900is placed in step D1000above a photo-curable layer1032, which is formed over the substrate1030. In one embodiment, the soft stamp900is placed in direct contact with the photo-curable layer1032. Then, in the same step, light1036having a desired wavelength is directed to the soft stamp900. The light passes through the soft stamp900and interacts with the curable layer1032. Because of the nano-features903of the soft stamp900, some regions of the curable layer1032receive less light while the regions correspond to the nano-gaps901of the soft stamp receive more light. Depending on the type of material used for the soft stamp, it is possible that the regions of the layer1032corresponding to the nano-gaps901receive more light than the regions corresponding to the nano-features903. Either way, the regions1034that receive more light are removed while the other regions remain and thus, in step D1002, the remaining regions, corresponding either to the nano-gaps901or the nano-features903, form the nano-features1034on the substrate1030. The substrate1030with the nano-features1034forms the mask1014.

Another process that can use the soft stamp900is the nanoskiving/mechanical sectioning process. Illustrated also inFIG.10, this process starts in step E1000in which an epoxy substrate1040is used to make a replica of the soft stamp900. After the epoxy substrate1040is cured in step E1002(by light or other means), top and bottom layers900B and1040A are identified and removed in step E1004, by mechanical sectioning, to obtain a slab1050. The slab1050includes the nano-features903of the soft stamp900and the epoxy material corresponding to the nano-gaps901, which form the epoxy nano-gaps1052. The slab1050thus has nano-elements903and1052formed with two different materials. The slab1050can be used as the mask1014for making new devices.

In the process discussed above with regard toFIGS.9A to9D, the soft stamp900's size is limited by the master mould's size, which in this embodiment has been chosen to be a 4″ wafer. Various kinds of nanogaps and nano-features may be formed from simple geometric shapes, i.e., square, circle, stars, etc., to complex structures such as interdigitated electrodes, which are successfully formed on the soft stamp.FIGS.11A to11Fillustrate such shapes formed on the soft stamp900.FIG.12shows the profile1200of the soft stamp900, indicating the thickness (in nm) of the nano-gaps901and of the nano-features903. It is noted that a difference (d) of about 27 nm is present between the nanogap and the nano-feature. This advantage indicates that a-Lith/self-forming lithography method can serve as a powerful alternative method for producing the master mould(s), replicating nanoscale features (<100 nm), and that this provides a truly facile route for producing soft stamps where the nanogap substrates are made without the need of costly production processes such as electron-beam lithography, or other known specialized, but highly precise, techniques that are known to suffer by low-throughput, high-cost, and slow writing speeds.

The processes discussed above with regard toFIGS.9to10Dused specific substances and numbers for describing the various conditions applied to make the soft stamp. However, these substances and specific conditions are not intended to limit the invention in any way. For example, the electrode materials (conductive, semiconducting, insulating, etc.) that can be used to make the soft stamp and the nano-features on the additional substrate should be compatible with the a-Lith and/or self-forming nanogap lithography process discussed with regard toFIG.2, or any other large-area compatible nanogap forming/patterning methods. For convenience, conductive electrodes such as Al, Cr, Au, Ti, Pt, and ITO were used by the inventors, but other materials with different physical properties can also be used. Any choice material such as metals, metal oxides, nitrides, insulators, dielectrics, and polymers that are compatible with the a-Lith and/or self-forming nanogap process can be utilized for creating the master mould100.

In the particular example illustrated inFIGS.10A to10D, the inventors verified the applicability of this novel approach by using Al—Al (i.e., electrodes110and120inFIG.10Aare made of the same material) electrodes of 100 nm in thickness patterned on Borofloat glass substrate. Other substrates such as plastics have also been used to create the nanogap moulds without any difficulty, or fundamental limitation.

In addition, the choice of the nanogap/master substrate material is also a possibility for this novel approach. There is a large variety of substrate materials that can, in principle, be used depending on the particular application. While glass and Si wafers with a thermally grown oxide layers are preferred, flexible and plastic substrates can be used as well. Generally, any substrate that is compatible with the a-Lith and the self-forming nanogap process could be considered as a substrate material for creating the master mould100.

Regarding the choice of the solidification route for the polymeric solution902, after being poured into the master mould100as shown inFIG.9B, the cross-linking of the polymer and its solidification can be achieved via thermal annealing, UV light, laser, and flashlight methods, among many others known to the experts in the field. The solidification of the soft polymers is typically achieved by photo-chemical (UV), or thermal (heating). In the embodiment ofFIGS.9A to9D, the inventors employed the UV photochemical cross-linking route to fabricate the soft stamp900from the master mould100. However, other polymer cross-linking methods mentioned above can also be used.

Regarding the polymeric stamp material, a desired function of the ASL layer910is to mitigate issues associated with strong adhesion of the soft polymer stamp900(following cross-linking) onto the master mould100. Conversely, the adhesion promoter portion924enhances the adhesion between the soft stamp900and the transparent glass substrate922, so that the peel-off step can be easily performed, without damaging the soft stamp900. Thus, the ASL layer facilitates smooth and easy delamination of the nanoscale features from the master mould once the fabrication and treatment processes have been completed. There are many material choices available for the soft stamp, ASL, and adhesion promoters. For the embodiment illustrated inFIGS.9A to9D, commercially available materials purchased from the EV Group GmbH were used. However, a broad range of suitable material choices exists, and could in principle, be used during the soft stamp manufacturing process.

For instance, materials of choice for the soft polymer stamp may include elastomers known as Poly Di-Methyl Siloxane PDMS, polyurethane (PU), polyimide (PI), cross-linked Novolac resins (a phenol formaldehyde polymer), fluoro carbon modified siloxanes, Poly-Methyl-Metha-Acrylyte (PMMA), Poly ([3-Mercaptopropyl] Methyl-Siloxane), known as PMMS, poly Styrene (PS), Per-Fluoro-Poly Ether (PFPE) and epoxy resins. Moreover, inherently present/adopted SAMs on the master mould (such as ODPA on Al/Ti/Cr/ITO/metal oxides and ODT on Au/Pt/Ag) in the a-Lith and self-forming nanogap lithography methods can be an alternative choice for the ASL layer910. The end groups of ODPA/ODT SAMs having methyl (—CH3) group pose non-reactive/hydrophobic nature, which facilitates weak adhesion between the soft polymer902and the master mould100. Hence, the SAM layer used during a-Lith or self-forming method can act as the ASL layer for ease of stamp release process, as well. Similarly, for adhesion promoter glues, various types of epoxy resins can also be used, as long as they are promoting a good adhesion to the polymer surface.

The various materials and elements of the master mould and the soft stamp impact the processes discussed above. For example, the electrode quality impacts the quality of the soft stamp900. The precise dimensions of the master mould100depend on the edge profile of the electrodes110and120used to form the nanogaps Ng. The parameters that influence the edge profile of the master mould and subsequently the soft stamp are the deposition rate, grain size, thickness, and etching technique (wet vs dry etching) involved in the deposition of the conductive electrodes110and120on the substrate140. The edge profile from the nanogap substrate to the110/120electrodes' top surface can vary between steep and sharp transition (seeFIGS.5A and5Band associated discussion) to a more gradual one or even an undercut, depending on the above mentioned factors. A sharp transition is desired for stamp preparation, but it is not necessary. Potentially, the shape of the nanogap Ng may offer advantages in terms of the sturdiness of the replicated nano-features due to the often wider dimension of the structure at the point of contact with the main body of the soft stamp.

The surface roughness of the electrodes110/120also plays a role in the quality of the soft stamp preparation and nanoscale features replication. Because the features of interest have a size below 100 nm, the surface roughness of the conductive electrodes should be kept as low as possible with an ultra-smooth surface being preferred. Since the electrodes deposited via different deposition techniques are down to achieve ultra-smooth films, it is easy to control the surface morphology of these electrodes. However, the process should be carried out precisely for solution-based electrode formation routes.

The dimension of the nanogap Ng also plays a role with regard to the soft stamp fabrication and its nano-features. The patterned electrodes can be made of similar/dissimilar material with varying thicknesses from 10-500 nm (or more), depending on the feature of interest and target application. The thickness of the electrodes110and120may also differ, giving the ability to produce stamps with tuneable mechanical properties and nano-features with improved mechanical properties/sturdiness (e.g.,FIGS.11A to11F). The aspect ratio of the final nano-feature of interest can be manipulated by altering the thickness of the conductive electrodes110/120. In this regard, as shown inFIGS.11A to11F, the nanogap dimensions and the soft stamp dimensions can also be varied allowing step-like profile features to be formed on the soft stamp.

The process parameters are known as the processing route of the soft polymer902(spin coating, drop-casting, etc.) and the curing route of the soft stamp (UV, thermal, LASER, and flashlight) mainly affect the soft stamp preparation. In the embodiments discussed herein, the inventors adopted the drop-casting and UV curing route for processing the soft stamp, and the factors associated with this approach, such as UV light intensity, exposure time, and annealing temperature, have an impact on the cross-linking of soft polymers. In this process, the thickness of the soft stamp900is controlled by adjusting the contact distance between the bottom portion924of the movable substrate920and the master mould100. However, for a spin coating approach, the thickness of the soft stamp is controlled through parameters like spin speed, spin duration, and subsequent annealing temperature and time. The LASER and flash lamp irradiation parameters such as energy, intensity, pulse duration, and frequency of pulses influence the stamp's size and shape for LASER and flash lamp annealing routes. The precise control of these parameters achieves a soft stamp having a high aspect ratio of the nanoscale features over a large area.

While the electrode nanogaps have been formed for the master mould100using the a-Lith or the self-forming nanogap methods previously discussed, where the metal electrodes were patterned using standard lithography and etching/lift-off processes, other methods for forming the electrode nanogaps can be used. For example, it is possible to form the electrodes and the associated nanogaps by using other techniques, which are described in the literature as the Atomic Layer Lithography (ALL), nanomasking, electro lithography, and nanosphere-lithography. In atomic layer lithography, directional evaporation of second metal dictate the discontinuity between the first and second metal layers, which helps to selectively peel off the second layer to make sub 10 nm nanogap electrodes. In the nanomasking method, the oxidation of chromium may be utilized for the spontaneous formation of the shadow mask to fabricate the nanogap electrodes. In electro-lithography, conductive scanning probes may be used to pattern the metal films and the underneath polymer films to create features having a size down to 9 nm. However, the resolution and mass production of such devices is limited by the need and fabrication of the conducting scanning probe. In nanosphere lithography, critical features and shapes are not achievable. All these traditional techniques are either not viable for large area implementation and fast prototyping or limited to the choice of materials and substrates.

In all the methods, including the a-Lith and self-forming nanogap methods, the density of the nanogap features rely on the first layer patterning via standard lithography such as photolithography, E-beam Lithography (EBL) and Focused Ion beam lithography (FIBL). The cost and time associated with patterning the first metal remain high. However, in the embodiments discussed herein, the patterning of the first metal has been simplified by using the LASER scribing/patterning technique. LASER scribing falls under digital and additive manufacturing that allows the rapid and low-cost patterning of the first layer on any substrate.

Starting from a digital design, which can easily be altered and adapted to specific layouts and purposes, a common laser cutter/scriber1302, as shown inFIG.13, can be used to selectively remove parts110A of a blanket first metal layer110via an ablation process. The resulting patterns1304can then be used to form nanogap electrodes120via the a-Lith or self-forming process. After the successful nanogap fabrication, further laser ablation steps can be applied to remove and shape parts of the second metal120additionally, thus enabling more intricate device layouts. In this regard,FIG.13shows the addition of the second electrode120by adhesion lithography or self-forming, between the first electrodes110. Nanogaps Ng are formed between the first and second electrodes during this process.

The a-Lith and self-forming nanogap methods may be used to control a thickness of the deposited electrodes110. For example, as shown inFIG.14A, the thickness t (or height) of all the electrodes110(indicated as M in the figure) may be made to be the same.FIG.14Aalso shows a width w1formed between adjacent electrodes M. The soft stamp900manufactured with these electrodes is further shown inFIG.14A, including the nano-features903having a width w2and a height h. The replicated nano-gaps901are shown formed on the base900A, between adjacent nano-features903, having the height h. In this embodiment, w1=w2<100 nm and t=h<50 nm. The resulting device1402that is formed with the soft stamp900is a replica of the master mould100.

FIG.14Bshows another embodiment in which the master mould100′ has the electrodes M made with to have a higher thickness t′, i.e., t′>t. In this case, the soft stamp900′ has the nano-features903longer than for the case shown inFIG.14A, and the resulting replica1404of the master mould100′ has thicker electrodes M than the replica1402inFIG.14A.

The embodiment ofFIG.14Cshows that the electrodes M being made to have different thicknesses, which results in the soft stamp900″ having various height nano-features903and nano-gaps901. For example, the same soft stamp900″ has gaps901and901′ having different depths. The replica1404of the master mould100″ has electrodes M having different thicknesses. The examples presented in embodiments14A to14C are only a couple of possible variations that can be achieved with the processes discussed above. More sophisticated electrode shapes and gaps may be formed with these processes.

The master mould100could be used, as now discussed with regard toFIG.15, to manufacture a soft stamp to be used, for example, in the manufacturing of a semiconductor device. The method includes a step1500of providing a substrate140having a first electrode110and a second electrode120, the second electrode120being formed at a distance less than 20 nm (or less than 100 nm) from the first electrode110so that a nanogap Ng is formed between the first and second electrodes110,120. A step1502of pouring a curable substance902over the first and second electrodes110,120and into the nanogap Ng, a step1504of curing the curable substance902to form a soft stamp900, and a step1506of removing the soft stamp900from the first and second electrodes110,120. The soft stamp900has a nano-feature903and a nanogap901, having sizes less than 100 nm. In one application, the soft stamp has a base900A from which extends the nano-feature903. The nano-feature has a height less than 50 nm. In another application, a width of the nanogap, formed between two adjacent nano-features, is 20 nm or less.

In one embodiment, the step1500includes a step of patterning a first metallic layer M1to form the first electrode110on the substrate140, a step of depositing a self-assembling monolayer, SAM, layer150over and around the first electrode110, a step of forming a second metallic layer M2in contact with the SAM layer150and the substrate140, and a step of touchless removing parts of the second metallic layer M2that are formed directly above the SAM layer150, to form the second electrode120, and the nanogap Ng between the first electrode110and the second electrode120. The method may further include a step of placing the substrate, the first electrode, the SAM layer, and the second metallic layer in a liquid and agitating either the substrate or the liquid, and a step of directing a fluid flow over the second metallic layer. In still another embodiment, the curing step may include irradiating the curable substance with ultra-violet light.

In still another embodiment, the method discussed above with regard toFIG.15may also include a step1508of forming a mask1014based on the soft stamp900, where the mask1014is used for manufacturing the semiconductor device. The step of forming the mask may include a sub-step of dipping the soft stamp in ink and transferring the ink on an additional substrate. In another embodiment, the step of forming the mask includes dipping the nano-feature of the soft stamp into a deformable polymer, curing the polymer to form the mask, and removing the soft stamp. In yet another embodiment, the step of forming the mask includes placing the mask over a photo-curable layer, irradiating the mask with light to cure portions of the photo-curable layer, and removing the soft stamp. In another embodiment, the step of forming the mask includes dipping the nano-feature of the soft stamp into an epoxy substrate, curing the epoxy substrate to form a gap that corresponds to the nano-feature, removing a layer of the soft stamp, and removing a layer of the epoxy substrate to form the mask.

In yet another embodiment, which is illustrated inFIG.16, the method for manufacturing a soft stamp to be used in the manufacturing of a semiconductor device. The method includes a step1600of providing a substrate140having a first electrode110and a second electrode120, the second electrode120being formed at a distance less than 20 nm from the first electrode110so that a nanogap Ng is formed between the first and second electrodes110,120, a step1602of depositing an anti-sticking layer910over the first and second electrodes110,120and the substrate140, a step1604of pouring a curable substance902over the first and second electrodes110,120and into the nanogap Ng, a step1606of pressing the curable substance902with a movable substrate920, toward the substrate140, a step1608of curing the curable substance902to form a soft stamp900, and a step1610of removing the soft stamp900from the first and second electrodes110,120, with the movable substrate920.

The master mould100obtained with the a-Lith or self-forming nanogaps methods discussed herein may also be used as a mask in a dry or wet-etching process. As indicated inFIGS.17A to17C, the nanogaps Ng formed between the electrodes M1and M2(or110and120) in the master mould100may be used to allow an etchant1710for performing dry or wet etching into the substrate140. Note that the etchant1710enters through the nanogap Ng and reaches the underneath substrate140(e.g., Si/SiO2or glass, or an interlayer) while the electrodes M1and M2prevent the etchant from reaching the portions of the substrate beneath them. Thus, only the etchants1710going through the nanogap (seeFIG.17A), which acts as the nm-size aperture, or mask, can react with the substrate underneath and etch it to form trenches1712, as illustrated in the cross-section view ofFIG.17Band the side view ofFIG.17C. In other words, the electrodes M1and M2act as the mask for etching the substrate (or material, such as an interlayer) beneath. Once the etching is completed, the M1and M2electrodes can be removed (etched) using an orthogonal (to the substrate) chemical route. This process enables the formation of nanoscale trenches1712, holes, and vias directly on the substrate140. This approach could be used to replicate higher aspect ratio nanoscale features directly into the substrate via the aforementioned soft stamp method. The interlayers, deposited between the substrate and the metal electrodes, can be used to control the depth of the formed nanogaps/etched features, hence further expanding the possible uses of the approaches discussed above.

In the embodiment illustrated inFIG.17D, a second substrate1720may be added on top of the nanoscale trenches1712, to seal them from the ambient. In this case, the nanoscale trenches become nanoscale tunnels or channels. A tunnel or channel is understood herein to mean that only its opposite end are open to the ambient and all other sides are closed. A tunnel or channel may also have inlets or outlets1713, that extend through one of the two substrates, as also shown in the figure. For this embodiment, the M1and M2electrodes may be removed from the first substrate140, after the trenches1712were formed, and then the second substrate1720is placed directly on top the first substrate140, directly facing the trenches1712. The second substrate may be made of glass or Si substrate, or other materials. External pressure1730and/or heating1732may be applied to the device to bond the second substrate to the first substrate. If a power source1740is applied between the two substrates, an anodic bonding may be achieved between them. In one application, a glue type material may be placed between the two substrates to achieve the bonding.

In a different embodiment, as illustrated inFIG.17E, the second substrate1720is formed on top of the M1, M2electrodes, without making any trenches1712in the top surface of the first substrate140. In this case, the nanogaps Ng become the nano-fluidic channels1750. The same bonding methods as for the embodiment illustrated inFIG.17Dmay be used herein for attaching the second substrate to the metal electrodes, i.e., anodic bonding (for example, if the second substrate is made of borosilicate glass) using pressure and/or heat, or any equivalent method.

In still a further embodiment, as illustrated inFIG.17F, a bonding promoter layer1760can be placed between the metal electrodes and the second substrate. In one application, the bonding promoter layer1760is applied to one or more of the metal electrodes M1and M2, and then the second substrate is added on top of the metal electrodes. For each of the embodiments ofFIGS.17D to17F, the nanogaps Ng can be made with any of the methods discussed herein. Any number of nano-fluidic channels1750may be formed in any given device1700. The length of the nano-fluidic channels1750may be larger than 1 mm or 1 cm, up to 10 m, as the method for forming the nanogaps can be applied to any length of the metal electrodes. Because the nanogaps Ng have a size between 1 and 100 nm, with a preferred size of 20 nm, the closed nano-fluidic channels1750have a width between 1 and 100 nm and a length from 1 mm to 10 m. The term “closed” is used herein to indicate that the nano-fluidic channel is completed bounded by the first and second substrates140and1720, and the metal electrodes M1and M2, fora given cross-section of the channel1750. The term “closed” does not exclude that the ends of the channel1750are open, or that one or more inlets or outlets are formed along the channel, and they can fluidly communicate with the environment or any desired device.

The methods discussed above could be applied for the fabrication of ultra-high aspect ratio nano-fluidic (nf) channels, as illustrated in the embodiment shown inFIGS.18A and18B.FIG.18Ashows a cross-section of a nano-fluidic device1800, whileFIG.18Bshows a top view of the same device. The channel1814has been formed in the substrate1810with the dry or wet etching method discussed with regard toFIGS.17A to17C, or with any of the methods discussed with regard toFIGS.17D to17F, where the electrodes in1812have been deposited using the a-Lith or the self-forming nanogap methods.FIG.18Bshows that it is possible to have plural pairs of electrodes1812formed along the channel1814. Such channels could be used for numerous applications in optics, electronics, materials science, chemistry, biology, biochemistry, genetics, and many other fields. In the area of biology/genetics, such as nanoscale trenches/channels1814could emulate nanopore DNA sequencing technologies and their application in biosciences and genetics. In this regard, note that the channel1814may have a length L between 50 nm and up to 1 m or more, while the width W of the trench is less than 100 nm, or less than 20 nm, or less than 10 nm. In one embodiment, a depth of the channel1814is less than 100 nm, or less than 20 nm or less than 10 nm.

If the nanofluidic device1800is used in the field of biopolymer (e.g., DNA) sequencing, with the above-discussed methods would be easy to apply an industrial fabrication process of a large number of lateral nm-size channels (trenches) and the incorporation of planar electrodes by using the soft stamps produced by the nanogap method. The nanofluidic device1800would enable the measurement of the tunnelling currents during the passage of a single DNA strand1830through the channel1814, a process dictated by the low dimensionality of the nanofluidic channel and the electrolyte1832and the biasing conditions applied at the electrodes1812. The nano-fluidic system resembles a nanopore system, which contains an electrolytic solution and which applies a constant electric field when the DNA strand is passing through the nanopore. The magnitude of the electric current density across the nanopore surface S depends on the nanopore's dimensions and the composition of DNA or RNA that is crossing the nanopore. Thus, by having a sensing device1840electrically connected to the electrodes1812, as shown inFIG.18B, it is possible to sequence the DNA1832by identifying the changes in electric current density across the nanopore surface S. As the strand1830moves through the nanofluidic channel1814, one base at a time, the current recorded by the sensing device1840is associated with a particular base, which a computing device1842associated with the sensing device can identify based on an existing library of bases and their corresponding currents when passing between the electrodes1812, through the channel1814.

In the case of the coplanar nano-channel pores shown inFIG.18B, the electrical current (other electrical quantities, for example, voltage, may also be used) between different pairs of electrodes1812,1812′, and1812″, which are separated by a few nm from each other by the nano-trench1814, due to the flowing of the DNA along the channel, can be sensed. In this way, the current (or other electrical quantity) is measured at multiple places along the nanofluidic channel making such measurements more accurate than the traditional single nanopore technologies. In one application, the plural currents are averaged. In one application, the plural currents are compared to each other, and/or are correlated with each other. Various processing methods may be applied for sequencing the biomaterial that is passing between the electrodes.

Further, the nanofluidic device1800may be inclined so that the DNA strands1830flow from one end to the other end of the channel1814due to gravity and/or potential difference. In an alternative embodiment, a supply system1850may be located at one end of the channel1814and in fluid communication with the channel so that the DNA1830is continuously supplied to the channel. A pump1852may be fluidly connected to the supply system1850to force the DNA along the channel. In this case, a top of the channel may be sealed with a layer of material1854, which is illustrated inFIGS.18A and18Bwith a dashed line to indicate that this layer is optional. The cover layer1854may be made from the same material as the substrate1810, to prevent the DNA from1830to spill out of the channel1814. The cover layer1854may be added to close the channel1814either after the channel has been formed with the dry/wet etching process discussed above with regard toFIGS.17A to17Cor from a method illustrated inFIG.17D to17F. In one variation of this embodiment, it is possible to make plural parallel channels1814into the same substrate1810, and thus, to supply the DNA material simultaneously in all these channels, and also to measure the electrical currents between the electrodes, through the DNA, simultaneously in all the parallel channels1814, to increase the sequencing speed.

A method for making a nanofluidic device1800for biological material sequencing is now discussed with regard toFIG.19. The method includes a step1900of patterning a first metallic layer (M1) to form a first electrode110on a substrate140, a step1902of depositing a self-assembling monolayer, SAM, layer150over and around the first electrode110, a step1904of forming a second metallic layer (M2) in contact with the SAM layer150and the substrate140, a step1906of touchless removing parts of the second metallic layer (M2) that are formed directly above the SAM layer150and the first electrode110, to form the second electrode120, and the nanogap Ng between the first electrode110and the second electrode120, and a step1908of removing material from the substrate140, in the nanogap Ng, to form a nano-channel1814into the substrate140, where the nano-channel1814is configured to receive a biological material1830for sequencing.

The method may further include supplying the biological material to the nano-channel, and/or sensing a first electrical quantity (e.g., voltage or current) associated with a base of the biological material when passing the first and second electrodes, and/or identifying the base based on the measured first electrical quantity. In one application, the nano-channel has a length larger than 1 m and a depth and a width less than 20 nm. The method may further include forming third and fourth electrodes across the nano-channel, which are configured to sense a second electrical quantity associated with the base of the biological material sequencing, and/or averaging or comparing or cross-correlating the first and second electrical quantities, and/or forming an additional nano-channel, extending in parallel to the channel on the surface of the substrate. In one application, the step of removing includes dry etching the substrate inside the nanogap.

The embodiments discussed with regard toFIGS.17A to19may be summarized as follows using dedicated numbered paragraphs:1. A nano-fluidic device (1800) for biological material sequencing, and the nano-fluidic device includes a substrate (1810); a nano-channel (1814) extending along a surface of the substrate (1810); and a first pair of electrodes (1812) formed to sandwich the nano-channel (1814), wherein the nano-channel (1814) has a depth and width less than 100 nm, and wherein the first pair of electrodes (1812) are configured to sense a first electrical quantity, which is related to a base of a biological material (1830) when passing between the first pair of electrodes (1812).2. The device of paragraph 1, wherein the channel has a length up to 1 m.3. The device of paragraph 1, further comprising:a second pair of electrodes that are configured to sense a second electrical quantity associated with the base of the biological material.4. The device of paragraph 3, further comprising:a sensing device configured to receive the first and second electrical quantities.5. The device of paragraph 1, further comprising:a supply system that releases the biological material at a first end of the nano-channel.6. The device of paragraph 5, further comprising:an additional nano-channel, extending in parallel to the channel on the surface of the substrate.7. The device of paragraph 1, further comprising:another substrate (1720) formed either directly on the first substrate (140), or directly on the first pair of electrodes (M1, M2), such that the nano-channel (1750/1814) is sandwiched between the first and second substrates.8. The device of paragraph 7, wherein the nano-channel is formed above the first substrate.9. A method for making a nano-fluidic device (1800) for biological material sequencing, the method comprising:patterning (1900) a first metallic layer (M1) to form a first electrode (110) on a substrate (140);depositing (1902) a self-assembling monolayer, SAM, layer (150) over and around the first electrode (110);forming (1904) a second metallic layer (M2) in contact with the SAM layer (150) and the substrate (140);touchless removing (1906) parts of the second metallic layer (M2) that are formed directly above the SAM layer (150) and the first electrode (110), to form the second electrode (120), and the nanogap Ng between the first electrode (110) and the second electrode (120); andremoving (1908) material from the substrate (140), in the nanogap Ng, to form a nano-channel (1814) into the substrate (140),wherein the nano-channel (1814) is configured to receive a biological material (1830) for sequencing.10. The method of paragraph 9, further comprising:supplying the biological material to the nano-channel.11. The method of paragraph 10, further comprising:sensing a first voltage or current associated with a base of the biological material when passing the first and second electrodes.12. The method of paragraph 11, further comprising:identifying the base based on the measured first voltage or current.13. The method of paragraph 9, wherein the nano-channel has a length larger than 1 m.14. The method of paragraph 13, wherein the nano-channel has a depth and a width less than 20 nm.15. The method of paragraph 11, further comprising:forming third and fourth electrodes across the nano-channel, which are configured to sense a second voltage associated with the base of the biological material sequencing.16. The method of paragraph 9, further comprising:forming a second substrate directly over the first substrate, or directly over the first pair of electrodes, to sandwich the nano-channel between the first and second substrate.17. The method of paragraph 9, further comprising:forming an additional nano-channel, extending in parallel to the channel on the surface of the substrate.18. The method of paragraph 9, wherein the step of removing comprises:dry or wet etching the substrate corresponding to the nanogap.19. A method for making a nano-channel in a substrate, the method comprising:patterning (1900) a first metallic layer (M1) to form a first electrode (110) on a substrate (140);depositing (1902) a self-assembling monolayer, SAM, layer (150) over and around the first electrode (110);forming (1904) a second metallic layer (M2) in contact with the SAM layer (150) and the substrate (140);touchless removing (1906) parts of the second metallic layer (M2) that are formed directly above the SAM layer (150) and the first electrode (110), to form the second electrode (120), and the nanogap Ng between the first electrode (110) and the second electrode (120); andremoving (1908) material from the substrate (140), in the nanogap Ng, to form the nano-channel (1814) into a surface of the substrate (140).20. The method of paragraph 19, further comprising:removing the first and second electrodes.21. The method of paragraph 20, further comprising:forming a second substrate over the first substrate to sandwich the nano-channel between the first and second substrates.22. The method of paragraph 19, wherein the nano-channel is over 1 m long and a depth and width is less than 20 nm.23. The method of paragraph 17, further comprising:forming a second substrate over the first and second metallic layers to sandwich the nano-channel between the first and second substrates.

The various technologies discussed above are now contrasted to the existing technologies. In the last decade, nano-electronics, optoelectronics, and bio-electronics received significant attention based on their enhanced properties and improved fabrication tools. Considering the fabrication of devices in these fields requires functional structures with arbitrary patterns of maximum dimensions ≤100 nm, commercial methods such as photolithography, E-Beam Lithography (EBL), and Ion-Beam Lithography (FIBL) have the potential to fabricate these nanoscale patterns. However, many factors limit the usage of these techniques for the mass manufacturing process, in addition to the compatibility issues for patterning on non-planar surfaces and arbitrary substrates.

Advancing the feature size below 100 nm needs to overcome a few technological barriers. For instance, the diffraction of light limits the minimum achievable resolution or feature size in conventional photolithography. Several approaches such as Deep/Extreme UV light source and immersion lithography allow the industry to mitigate these issues in reaching the feature size beyond 50 nm. Yet, they still need added high-resolution lens/optics systems and a way to integrate the water/other liquid as a medium to increase the numerical aperture respectively. On the other hand, EBL and FIBL offer excellent downscaling. Nevertheless, capital cost and production time ultimately increases for these methods and thus, limit their capability for large-area manufacturing.

There is a necessity to identify alternative, viable and low cost techniques, some of which include non-conventional lithography techniques that are grouped into soft lithography known as micro-contact printing, moulding/embossing, scanning probe lithography (SPL), nano-skiving and edge lithography developed by several researchers in past two decades. Some of these techniques (such as SPL and nano-skiving) are not suitable for large area manufacturing while the rest of them rely on the master fabrication, which is typically made by EBL or FIBL, which are in turn slow and very expensive. All the soft lithography techniques use a soft stamp, which replicates the features of the stamp/master mould. Master moulds are typically made by EBL/FIBL, which dictate the minimum attainable resolution in all these non-conventional lithography methods. In addition to that, master fabrication via EBL and FIBL has many technological drawbacks, such as limited materials/substrate that are suitable for that method, they are time consuming, are slow processes, require costly equipment, and require skilled human resources in a clean room facility.

To address these challenges, the a-lithography and self-forming nanogap lithography methods based approach (also those relying on other large-area compatible and known nanogap formation techniques) facilitate and radically pave a new way for fabricating the master mould and soft stamp on a large scale with a low capital cost. The a-Lith and the self-forming nanogap lithography methods discussed herein both use SAMs to modify the surface functionality (hydro/philic or phobic) of one metal electrode specifically by selective deposition on top of a patterned first metal (M1) while leaving the substrate intact, which is followed by deposition of a second metal (M2, can be similar/dissimilar metal as M1).

Due to the poor adhesion between SAM's tail group and the M2electrode, M1/SAM/M2overlapping regions can easily be peeled off via applying a glue/tape (as in the a-Lith method) or by blowing with air/N2stream, rinsing with liquids or sonication (as in the self-forming lithography method). The interface formed between the M1/M2materials has a gap <20 nm, and the SAM molecules on M1can be cleaned via UVO/Oxygen plasma route. Consequently, the scalable, low cost and solution-based a-Lith or self-forming nanogap methods can be used in many functional device applications such as RF diodes, nano-LEDs, memory devices, photo-detectors, Full Wave Rectifiers (FWR) and nano-trenches (made with the underlying substrate etching) discussed with regard toFIGS.15A to16B. The nano-trenches may be used for nano-bio fluidics applications. The novel self-forming nanogap method eliminates the additional peeling off step used in the a-Lith process, which further simplifies the processing, reduces the costs, increases uniformity, reliability, and allows more precise control of the nanogap size.

Thus, the embodiments discussed herein can achieve one or more of the following advantages:

(1) The use of the a-Lith and/or self-forming nanogap methods serve as an alternative route to fabricate soft/hard master and stamps on large-area substrates using existing scalable patterning techniques such as conventional photolithography or laser scribing. The replicated stamps can contain features with a minimum size down to 1 nm and maximum size of >>1m if needed.

(2) The methods discussed herein provide low cost and rapid prototyping compared to current technologies such as e-beam lithography (EBL) and Focused Ion Beam Lithography (FIBL) for master fabrication, but without being limited in size, unlike the nanogap patterning which is a parallel process—

(3) A soft stamp prepared from a nanogap master mould can be used to replicate high-aspect-ratio nanoscale features over large areas via any soft lithography techniques such as nano-contact printing, embossing/NIL, phase shift lithography and nanoskiving.

(4) The control on the master/soft stamp feature dimensions, size, and shape can be manipulated via a-Lith/self-forming nanogap lithography process parameters.

(5) The conductive electrodes can act as a shadow mask for dry/wet etching the underneath substrate or other interlayers and/or substrate materials. This approach allows for the patterning of high/low aspect ratio nanofluidic channels on the substrate. Such nm-deep, nm-wide, but mm- or m-length features may find an application on biological sciences, among other fields.

(6) The ability to prepare such soft stamps for any soft lithography (micro/nano contact printing, NIL, phase shift lithography and mechanical sectioning) using additive manufacturing techniques such as an individual or combination of laser scribing, a-Lith, self-forming nanogap method, and others techniques (known from the literature) can find applications in different fields including, optoelectronics, electronics, bio-electronics, solar cells, and sensors field, to name but a few.

The methods discussed above may be used for other applications, as now discussed. In one embodiment, it is possible to form nanogaps or nano-channels based on the above discussed methods, and to use these nanogaps or nano-channels for electro, photo or photo-electro chemical conversion of earth abundant natural chemicals such as water, sea-water, O2, N2, CO2and natural gas, like methane, into a value added chemical commodity like H2fuel, NH3, H2O2, Cl2, CH3OH and HCHO to name but a few. Any of the methods discussed above may be used to manufacture conductive and/or catalytic electrodes (symmetric, i.e., the same material, or asymmetric i.e., dissimilar material) M1and M2, which are spaced apart by a gap of nanoscale size in the range of 1-100 nm. The conductive and/or catalytic electrodes M1and M2may include one or more different materials, such as metals, transparent conductive oxides (e.g., indium tin oxide, ITO), conductive polymers, 2D, 1D and 0D materials, to name but a few, as an individual layer or in multilayer combinations. The co-planar nanogap electrodes M1and M2can be made by either of the above mentioned methods, or by other methods known in the literature, for example, mechanical break junction, metal oxidation, scanning probe lithography, nano masking methods, to name but a few.

A novel nanogap electrochemical cell2000is shown inFIG.20Abeing implemented as an electrochemical conversion cell, inFIG.20Bas a photo-chemical conversion cell, and inFIG.20Cas a photo-electrochemical conversion cell.FIG.20Ashows the cell2000having the substrate140, the first and second metal electrodes M1and M2, which define the nanogap Ng, and a power source2010that supplies electrical power to two contact pads2012and2014, which are connected to the two electrodes M1and M2, respectively. In one application, the minimum voltage applied to the two electrodes M1and M2is 1.3 V. Larger voltages may be used. An electrolyte2020(e.g., water) is placed inside the nanogap Ng. The electrical field E generated through electrolyte2020in the nanogap Ng, is due to the voltage applied by the power source2010in this embodiment.

However, for the cell2000shown inFIG.20B, the electrical field E is generated by the interaction between the solar energy2016received from the sun or other illumination source, and a photo-active material2030that is deposited on one of the electrodes M1and M2, or both of them, as shown inFIG.21A. The photo-active material2030may be TiO2. However, other photo-active materials may be used. The embodiment illustrated inFIG.21Ahas a first photo-active material (n-type)2030, on the first electrode M1and a different, second photo-active material (p-type)2032, on the second electrode M2.

The cell2000shown inFIG.20Cuses both the power source2010and the photo-active material2030to generate the electrical field E. The cell shown inFIG.21Bhas the two different photo-materials2030and2032formed on the two electrodes M1and M2, respectively, similar to the configuration shown inFIG.21A. The photo-material2030and/or2032may be added to one or more of the electrodes M1and M2by using UVO/O2plasma to promote native oxide growth (TiO2, CuO, NiO2, etc.) and/or selective deposition of n-type/p-type SAMs on the metal electrodes. These steps have been discussed above with regard toFIG.2, for example, step206, and various types of SAM materials have also been discussed above, with regard toFIGS.4A to4C. P-type SAM materials may include BTBT-PA, C13-BTBT, C2-4T-C12-PA, and 2PACZ, see the table inFIG.22, and n-type SAM materials may include PTCDI-PA, PTCDI-Ph, and Glycol-C60-C6-Pa, as also shown in the table inFIG.22.

The implementation of the cell2000shown inFIG.21Bmay be further modified, as shown inFIG.21C, so that the solar energy2016is provided from the back of the cell, i.e., through the substrate140. For this specific implementation, the solar energy2016first passes the substrate140and then arrives at the photo-active material2030. In one application, one or both of the metal electrodes M1and M2are transparent to the solar energy2016, so that the solar light passes the substrate and the metal electrodes before arriving at the photo-active material2030and/or2032. If the substrate140is made of a flexible material, then the cell2000can be bent as illustrated inFIG.21D. In one application, the substrate140may be bent to form a semicircle.

For the above implementations of the cell2000, the empty nanogap Ng is filled with an electrolyte such as DI water, H2SO4or Na2SO4dissolved in water at various concentrations. The voltage applied to the electrodes M1and M2leads to a strong electric field across the nanoscale channel while extending over larger widths, thus causing a robust electro-chemical reaction in the electrolyte2020added between the electrodes M1and M2. The nanogap electrodes M1and M2can be integrated to form either single discrete cells2000, for electrochemical sensing, or they can be used as a wafer scale electro-chemical apparatus and/or integrated with photo-active materials for photo or photo-electro-chemical conversion. The planar architecture illustrated inFIGS.20A to21D(i.e., all the electrodes are formed in a given horizontal plane, next to each other, and not on top of each other), the transparent substrates140and1720, and the photo catalysts can enable both front and back illuminations. The planar device configuration shown in these figures not only mimic the conventional electro chemical cells, but also enhances the kinetic of the reactions due to the large field produced within the nanogap. In this regard, it is noted that the cell2000inFIGS.20A to21Dshow at least one of an Oxygen Evolution Reaction (OER) and a Hydrogen Evolution Reaction (HER).

These reactions are now discussed with regard toFIG.23A, which corresponds to cell2000shown inFIG.21B. These reactions are similar for the other cell implementations. The electrical power source2010(seeFIGS.20A and21B) is connected to the contact pads2012and2014of the M1and M2electrodes, respectively, so that no direct contact between the contact pads and the water is taking place. The water only contacts the M1and M2electrodes. Under the external bias (solar power2016or electrical current from the power source2010, or both), electrons e−at the M2electrode (cathode) are involved in the HER, while at the M1electrode, holes h+initiate the OER. As the HER and OER reactions are known in the art, details of these reactions are omitted herein. An advantage of a planar nanogap architecture over the conventional electrochemical is the large electric field E generated in the nanogap Ng during this process. Thus, the kinetics of both HER and OER, which are subjected to such a large electric field, will be positively influenced.

A method for electro-catalysis (EC) of water using any of the cells2000discussed above is now discussed with regard toFIG.23B. In step2300, the cell2000is fabricated either via the self-forming nanogap method discussed with regard toFIG.2or according to the a-lith method. In one application, the prepared nanogap substrate holds Al (M1)-Ti/Pt (M2) planar inter-digitated electrodes fabricated by the self-forming nanogap method. The nanogap electrodes were cleaned with UV-Ozone (UVO) or oxygen plasma for 2 minutes in order to remove the SAM from the previous fabrication method. The inter-digitated electrodes can be formed on a die (e.g., on a wafer having a length of about 10 cm) to have a circular shape, as shown inFIG.24A, or linear shape, as shown inFIG.24B. A diameter of the M1and M2electrodes inFIG.24Amay be between 100 to 900 μm, while a length of the same electrodes inFIG.24Bmay be between 1 mm or 1 cm up to 10 cm. One skilled in the art would understand that these dimensions may be scaled up or down to fit the substrate.

In step2302, an electrolyte (e.g., pure DI water) is applied only on the areas between the IDE fingers, leaving the contact pads2012and2014on both metal electrodes M1and M2untouched. This ensures that the probe does not have any direct contact with the DI water and the reaction is solely initiated due to the induced large electric field E and electrochemical reaction in the nanoscale channel Ng. In step2304, both metal electrodes' contact pads are connected to an external source2010via needle probes and an external bias is applied, which induces the electrochemical reaction that splits the DI water into its constituents such as H2and O2molecules. Under certain bias conditions (mostly >1.3 V), fine bubbles are emerging only at the interface between the Al/Ti—Pt metal electrodes, where the nanogap exists.

In step2306, the by-products of the electrochemical reaction, e.g., H2and O2molecules, are processed to generate another chemical component, as now discussed. In one embodiment, as illustrated inFIG.25, a reactor2500may include a closed housing2502that hosts one or more cells2000. The power source2010applies a voltage V to the metal electrodes M1and M2to generate an electrical field E in the nanogap Ng. The electrolyte2020present in the nanogap Ng experiences the OER and HER reactions. At the same time, a nozzle or gun2510, placed partially or totally inside the housing2502, is configured to inject a desired chemical component2512, for example, CO2or N2. Other gases or combination of gases may be injected by the gun2510. The chemical component2512is stored outside the housing2502, into a storage tank2514. A pressure regulator2516controls the amount of the chemical component2512that is supplied by the storage tank2514to the gun2510. A computing system2520may be associated with the reactor2500and may be linked, in a wired or wireless manner, to the power source2010and the pressure regulator2516to control them, and the chemical reactions occurring inside the housing2502.

If the chemical component2512is CO2, and the gun2510is located and oriented inside the housing2502to expel the gas onto the metal electrodes M1and M2, a CO2reduction reaction (CO2RR) may take place inside the reactor2500. The CO2RR is described by the following chemical reaction:
CO2+nH++ne−→CnHn.

If the chemical component2512is N2, then a Nitrogen Reduction Reaction (NRR) takes place, which is described by the following chemical reaction:
N2+6H++6e−→2NH3.

For both reactions, the N2and CO2gases are adsorbed on the metal electrode M1or M2where the HER occurs, and the N2and H2gases or the CO2and H2gases bind to each other due to their potential and dissociate as NH3or CnHnfrom the metal surface of the electrodes. The number n can be any integer. These newly formed gases2518are then collected at a port2530, and optionally pumped with a pump2532into another storage tank2534. The computing system2520is configured to also control the pump2530.

While the embodiment illustrated inFIG.25shows only the cell2000illustrated inFIG.20A, those skilled in the art would understand that any of the cells shown inFIGS.20A to21Dmay be used inside the reactor, i.e., those that use only solar energy or those that use a combination of solar and electrical energy. In one embodiment, a mixture of these cells may be used inside the reactor2500. Any number of cells2000may be placed inside the reactor2500, e.g., hundreds, or thousands or more.

The cells2000for electrochemical conversion are not limited to water splitting, as discussed above, but they can also serve as a basic foundation for other useful value-added chemical commodity conversions and chemical detections. For instance, the planar nanogap cells can be used, in addition to the ammonia conversion via NRR from naturally existing H2O and N2as reactants (N2→NH3), and CO2reduction (CO2→CnHn), also for methane partial reduction (CH4→CH3OH), for two-electron oxygen reduction (O2→H2O2), Chlorine evolution from seawater (Cl−→Cl2), etc. In addition, the planar nanogap cells can be used for electrolyte free chemical conversion and/or detection as shown inFIG.25.

In addition, the electrostatic or External Electric Field (EEF) induced catalysis of a Diels-Alder reaction is another possible candidate for reactions induced by the nanogap cell2000. For this application, the field-induced change in redox and non-redox chemical reactions can be measured by the change in the tunnelling current. The electrochemical reaction in the planar nanogap cell2000could also be used for chemical detections, such as the fake alcoholic content detection in beverages, as illustrated inFIGS.26A and26B. More specifically, a material of interest2600may be placed in the nanogap Ng of the cell2000, as shown inFIG.26A. The power source2010applies a voltage V between the first and second metal electrodes M1and M2, to create an electrical field E in the nanogap Ng. This electrical field affects the redox and non-redox chemical reactions taking place in the material of interest2600, which is reflected in the current I. The current I is measured with a device2610, for example, a multimeter, an then plotted as shown inFIG.26B, against the applied voltage V. If no material is placed in the nanogap Ng, then the current curve2620is obtained. If 1 M of Na2SO4is placed in the nanogap Ng, and the current I is measured for various voltage ranges, the curve2622is obtained for V varying between −1.5 to 1.5 V, the curve2624is obtained for V varying between −3.5 to 3.5 V, and the curve2626is obtained for V varying between −4.5 to 4.5 V. As these curves are different from each other, it means that the current can be used to identify the EEF induced chemistry of the material placed in the nanogap, and thus, to assess various redox and non-redox reactions. In one application, the computing device2520may be connected to the power source2010and the current measuring device2610, and configured in software to automatically detect the material placed in the nanogap, based on a library of current measurements priorly obtained for various substances. It is noted thatFIG.26Bshows the cell2000having a length of about 2 cm, which means that the metal electrodes M1and M2have substantially the same length. However, the metal electrodes M1and M2may have a 1 cm to 5 cm length. The current levels and/or I-V slopes can also be used to extract the amount or percentage of certain chemicals that are present in the material of interest2600, if a prior calibration of these chemicals is performed for the cell2000.

The processes described above were based on the use of a specific set of conditions and materials combinations. However, different variants of these processes can be envisioned and are now discussed. The choice of electrolyte used in the nanogap electrochemical cells2000influences the various parameters such as on-set potential (the potential required to turn on the chemical reactions), over-potential, type of chemical reaction mechanism involved, the yield of the converted chemicals, etc. Similarly, the choice of the electrode materials will also influence these parameters. However, both the a-lith and the self-forming methods of fabrication are versatile and can be adapted to any choice of substrate and electrode combination. For instance, the initial cells2000were based on Al/Ti—Pt, Al/Au, ITO/Au, ITO/Ti—Pt and Ti/Ti—Pt nanogap electrodes formed on glass and Si substrates. Different electrolytes, such as pure DI water, 0.05M H2SO4dissolved in water, 1M Na2SO4dissolved in water, tap water, and bottled drinking water, have also been employed to investigate the HER and OER. The choice of electrolyte and electrodes dictates the rate of the OER/HER.

There is a large variety of substrate materials that can be used for the cell2000, depending on the particular application. While glass and Si wafers with thermally grown oxide layers have been used herein, a flexible and plastic substrate can be used as well. Generally, any substrate that is compatible with the a-lith or self-forming nanogap processes could be considered as a substrate material for the proposed cell.

As discussed above with regard toFIGS.20A to21D, based on the external stimuli due to either an external voltage (electro catalysis, EC) and/or light, the principle of operation of the cell changes (photo catalysis (PC) or photo electro catalysis (PEC)). However, in the PC or PEC modes of operation, certain kinds of photo-active semi-conductors (photo catalyst) and electrode materials are required. As also discussed above, the process illustrated inFIG.2can be adapted to have additional steps to selectively grow native oxides, which are intrinsic photo catalysts (such as Ti into TiO2, Ni into NiO2, and Cu into CuO) via UV-Ozone or oxygen plasma treatments. Furthermore, semiconducting and photoactive SAMs (seeFIG.22) can also be selectively coated on the M1and M2electrodes. In general, any semiconducting and photoactive material (photo catalyst), which has selective binding towards one of the metal electrodes M1or M2or that can be treated to achieve such selectivity, can be used in the process ofFIG.2.

Nonetheless, the mechanism for water splitting or other possible chemical reactions varies based on the external stimuli, the types of photoactive materials, and principles of operation. Thus, the important figures of merit known as on-set potential, power consumption, yield, and cost of the devices may vary depending on the choice of principles of operation. The inventors are not aware of a planar nanogap device technology employed for water splitting or any other chemical conversion and detection. All the reported devices are mostly in vertical configurations. This is mainly due to the difficulties in fabrication and constraints imposed by the current state of the art lithography methods. Thus, based on the versatile lithography approach illustrated inFIG.2, it is possible to achieve a completely planar device architecture that can be scalable, low-cost, and a viable route for electro-chemical conversion, detection and analysis.

The embodiments discussed with regard toFIGS.20A to26Bmay be summarized as follows, using dedicated numbered paragraphs:1. A nanogap electrochemical cell (2000) includes a substrate (140), first and second metal electrodes (M1, M2) formed on the substrate (140) such that a nanogap Ng delineates the first metal electrode (M1) from the second metal electrode (M2), and a power source (2010,2030) that converts external energy into an electrical field E between the first and second metal electrodes (M1, M2), wherein the nanogap Ng is smaller than 100 nm.2. The cell of paragraph 1, wherein the power source is an electrical power source.3. The cell of paragraph 1, wherein the power source is solar energy.4. The cell of paragraph 1, wherein the power source includes a photo-active material formed on at least one of the first and second electrodes.5. The cell of paragraph 4, wherein the photo-active material includes a first photo-active material formed on the first metal electrode, and a second photo-active material formed on the second metal electrode, and the second photo-active material is different from the first photo-active material.6. The cell of paragraph 5, wherein the first photo-active material is n-type and the second photo-active material is p-type.7. The cell of paragraph 1, wherein the power source includes an electrical power source and a photo-active material formed on at least one of the first and second electrodes.8. The cell of paragraph 1, wherein the power source is a photo-active material formed on at least one of the first and second electrodes, and solar energy reaches the photo-active material through the substrate.9. The cell of paragraph 1, wherein the substrate is bendable.10. The cell of paragraph 1, further including a current measuring device for measuring a current between the first and second electrodes, and a computing device that determines a material placed in the nanogap, based on the measured current.11. A reactor (2500) for performing electrochemical reactions, the reactor (2500) including, a housing (2502), a nanogap electrochemical cell (2000) placed inside the housing (2502), a gun (2510) configured to provide a first gas (2512) to the nanogap electrochemical cell (2000), a port (2530) configured to collect a second gas (2518) from the housing, which is different from the first gas (2512), an electrolyte (2020) placed in a nanogap Ng of the nanogap electrochemical cell (2000), wherein the nanogap Ng is formed above a substrate (140) and delineates a first metal electrode (M1) from a second metal electrode (M2), and a power source (2010,2016) that converts external energy into an electrical field E between the first and second metal electrodes (M1, M2), wherein the nanogap Ng is smaller than 100 nm.12. The reactor of paragraph 11, wherein the electrolyte is water, the first gas is N2, and the second gas is NH3.13. The reactor of paragraph 11, wherein the electrolyte is water, the first gas is CO2, and the second gas is CnHn, where n is an integer.14. The reactor of paragraph 11, further including a first storage tank for storing the first gas, a second storage tank for storing the second gas, a pump for pumping the second gas from the housing to the second storing tank, and a computing system that controls the pump and an amount of the first gas entering the housing.15. The reactor of paragraph 11, wherein the power source is an electrical power source, or a solar energy, or a combination of both.16. The reactor of paragraph 11, wherein the power source includes a photo-active material formed on at least one of the first and second electrodes.17. The reactor of paragraph 16, wherein the photo-active material includes a first photo-active material formed on the first metal electrode, and a second photo-active material formed on the second metal electrode, and the second photo-active material is different from the first photo-active material.18. The reactor of paragraph 11, wherein the power source includes a photo-active material formed on at least one of the first and second electrodes, and solar energy reaches the photo-active material through the substrate.19. The reactor of paragraph 11, wherein the substrate is bendable.20. The reactor of paragraph 11, wherein the nanogap formed between the first and second electrodes is 20 nm.

The disclosed embodiments provide a new method for forming (1) a soft stamp that corresponds to a master mould, (2) a nano-fluidic device, or (3) a nanogap electrochemical cell. The soft stamp can then be used with various methods for forming various solid-state devices with a small footprint, which would make these devices appropriate components for the IoT environment, allow large scale manufacturing, and offer a low-cost solution for many applications. The embodiments are intended to cover alternatives, modifications, and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

REFERENCES

[1] Beesley et al., “Sub-15-Nm Patterning of Asymmetric Metal Electrodes and Devices by Adhesion Lithography,” Nat Commun 2014, 5.[2] Chen et al., “Atomic layer lithography of wafer-scale nanogap arrays for extreme confinement of electromagnetic waves,” Nat. Commun. 2013, 4, 2361.[3] Chen et al., “Nanogap-Enhanced Infrared Spectroscopy with Template-Stripped Wafer-Scale Arrays of Buried Plasmonic Cavities,” Nano Lett. 2014, 15, 107.