Patent Application
20200395453
One embodiment of the present invention relates to electrodes having nanoscale gap length and methods of manufacturing the same, and nano devices having nanogap electrodes.
Semiconductor integrated circuit has increased the degree of integration exponentially according to Moore's law. However, it is said that the miniaturization technique for semi-conductor integrated circuit is gradually approaching its limits. Faced with the limitations of such technological advances, research has been underway to realize new electronic devices by using bottom-up techniques for constructing devices from molecules in which atoms or structures, which are the smallest units of materials, are defined, rather than top-down techniques for processing and miniaturizing materials. For example, research is underway on nanogap electrodes that utilize the self-terminating function of electroless plating, and nano-devices that place metal nanoparticles between nanogap electrodes.
A nanogap electrode in an embodiment according to the present invention includes a first electrode including a first electrode layer and a first metal particle arranged at one end of the first electrode layer, and a second electrode including a second electrode layer and a second metal particle arranged at one end of the second electrode layer. The first metal particle and the second metal particle are arranged opposite to each other with a gap therebetween, and a width from one end to the other end of the first metal particle and the second metal particle is 20 nm or less. The gap between the first metal particle and the second metal particle is 10 nm or less.
A method for manufacturing nanogap electrode in an embodiment according to the present invention, the method includes forming a first electrode layer and a second electrode layer on a substrate having an insulating surface so that one ends of the first electrode layer and the second electrode layer are opposed to each other with a gap therebetween, dipping the substrate on which the first electrode layer and the second electrode layer are formed in an electroless plating solution in which a reducing agent is mixed into an electrolyte containing metal ions, forming metal particles one end of each of the first electrode layer and the second electrode layer, and forming a metallic bond between a metal forming the first electrode layer and the second electrode layer and a metal contained in the electroless plating solution, growing the metal particles to a size in which the width from one end to the other end of the metal particles is not more than 10 nm, and forming a gap of 10 nm or less between the metal particles formed at the one end of the first electrode layer and the one end of the second electrode layer.
A nanodevice in an embodiment according to the present invention includes a first electrode including a first electrode layer and a first metal particle arranged at one end of the first electrode layer, a second electrode including a second electrode layer and a second metal particle arranged at one end of the second electrode layer, and a metal nanoparticle or a functional molecule. The first metal particle and the second metal particle are arranged opposite to each other with a gap therebetween, the metal nanoparticle or the functional molecule are arranged in the gap between the first metal particle and the second metal particle, and a width from one end to the other end of the first metal particle and the second metal particle is 10 nm or less. The gap between the first metal particle and the second metal particle is 10 nm or less.
Embodiments of the present invention will be described below with reference to the drawings and the like. However, the present invention may be practiced in many ways and is not to be construed as being limited to the description of the embodiments illustrated below. In order to make the explanation clearer, the drawing may schematically show the width, thickness, shape, etc. of each part in comparison with the actual embodiment. However, it is an example and is not intended to limit the interpretation of the present invention. In this specification and each figure, elements similar to those described above with respect to the previously described figures are denoted by the same reference numerals (or a number followed by a, b, etc.) and detailed description thereof may be omitted as appropriate. In addition, the letters “First” and “Second” appended to each element are expedient signs used to distinguish between the elements and have no further meaning unless otherwise stated.
In the present specification, the nanogap electrode has a gap portion (gap) between a pair of electrodes, and the length of the gap portion (gap length) is 10 nm or less, for example, 1 nm to 10 nm, unless otherwise specified.
In the present specification, a nano-device refers to a device including a configuration of a nanogap electrode.
A structure and a manufacturing method of a nanogap electrode according to an embodiment of the present invention will be described with reference to the drawings.
1-1 Structure of the Nanogap Electrode
In the nanogap electrode 100, one end portions of a first electrode 102a and a second electrode 102b facing each other and are arranged with a gap therebetween.
In the
The gap length L2 of the nanogap electrode 100 is not more than 10 nm, but in applications to nano-devices, it is appropriately adjusted depending on the application. For example, when constructing a nano-device tunnel current flows using the nanogap electrode 100, it is preferred that the length of the gap (gap length) L2 to 10 nm or less, when applied to a nano-device that expresses Coulomb blockade, it is preferred that the length of the gap (gap length) L2 to 5 nm or less.
The length of the gap of the nanogap electrode 100, that is, the distance at which the first metal particle 106a and the second metal particle 106b are separated from each other, is controlled by the arrangement of the first electrode layer 104a and a second metal layer 114b. In this sense, the spacing L1 of end portion (tip portion) of each of the first electrode layer 104a and the second electrode layer 104b is preferably arranged at a spacing of 20 nm or less, preferably 15 nm or less.
The length of the gap of the nanogap electrode 100 can be controlled by the position at which the first metal particle 106a and the second metal particle 106b are disposed. The first metal particle 106a and the second metal particle 106b are formed by an electroless plating method. At this time, by setting the widths W1 of the first electrode layer 104a and the second electrode layer 104b to 20 nm or less, preferably 15 nm or less, metal particles can be grown preferentially each of the end portion.
The thickness T1 of the first electrode layer 104a and the second electrode layer 104b may be set as appropriate, but may be set to 20 nm or less, preferably 15 nm or less. Thus, the number of metal particles disposed at one end portion of the first electrode layer 104a and the second electrode layer 104b can be controlled. When gate electrode is disposed on the lower layer side and the upper layer side of the nanogap electrode 100, the thickness T1 of the first electrode layer 104a and the second electrode layer 104b is set to 20 nm or less, preferably 15 nm or less, whereby an electric field generated by the gate voltage can be reliably applied to the gap portion.
Even if the gap length of the nanogap electrode 100 is about 10 nm, if the widths of the first electrode layer 104a and the second electrode layer 104b are wide, operation characteristics of the nano-device are affected. For example, in a single-electron transistor having a nanogap electrode, single-electron islands disposed in the gap portion is electrically shielded by a wide electrode layer, a problem that is less susceptible to the action of the electric field caused by the gate voltage can occur.
However, by setting the thicknesses and widths of the first electrode layer 104a and the second electrode layer 104b within the ranges of the present embodiment, the electric field generated by the gate voltage can be reliably applied to the gap portion in the nano-device including the nanogap electrode 100 and gate electrode. In addition, the number of metal particles disposed at one end portion of the first electrode layer 104a and the second electrode layer 104b can also be controlled.
The gap length of the nanogap electrode 100 can be further controlled by the sizes of the first metal particle 106a and the second metal particle 106b. By forming the first metal particle 106a and the second metal particle 106b to be large, the length (gap length) of the gap can be reduced, and by forming them to be small, the length (gap length) of the gap can be increased. As will be described later, the first metal particle 106a and the second metal particle 106b exhibit a self-terminating function in electroless plating, thereby preventing contact with each other and enabling control of the gap length.
The first metal particle 106a and the second metal particle 106b are provided as a single mass (or island-shaped region) on the respective surfaces of the first electrode layer 104a and the second electrode layer 104b. The first metal particle 106a and the second metal particle 106b have a hemispherical external view like a water droplet dripped on a hydrophobic surface. Here, the hemispherical shape refers to a spherical surface in which curved surface continues and is not limited to a true spherical surface. In the nanogap electrode 100, it is preferable that the first metal particle 106a and the second metal particle 106b do not increase in diameter. In addition, it is desired that the first metal particle 106a on the first electrode layer 104a and the second metal particle 106b on the second electrode layer 104b have widths from one end to the other end of 20 nm or less, preferably 15 nm or less, more preferably 10 nm or less in planar view. The widths of the first metal particle 106a and the second metal particle 106b mean the maximum widths of isolated metal particles observed on the surfaces of the first electrode layer 104a and the second electrode layer 104b, respectively.
In the nanogap electrode 100, the first metal layer 114a and the second metal layer 114b are formed of a first metal, and the first metal particle 106a and the second metal particle 106b are formed of a second metal. The combination of the first metal and the second metal may be appropriately selected, but it is preferable that the first metal and the second metal form a metal bond and/or an alloy. With such a combination, the first metal particle 106a and the second metal particle 106b can be provided on the surfaces of the first electrode layer 104a and the second electrode layer 104b, respectively, in a state isolated from other metal particles.
The first metal particle 106a and the second metal particle 106b may be solid solutions formed of a first metal and a second metal. Since the first metal particle 106a and the second metal particle 106b form a solid solution, the mechanical stability of the nanogap electrode 100 can be enhanced.
As a metal material for forming the nanogap electrode, gold (Au) is considered to be suitable from the viewpoints of conductivity, chemical stability, and ability to form a self-assembled monolayer on the surface. However, it is known that gold (Au) decreases its melting point as it becomes nanoscale, becomes unstable due to Rayleigh instability, and changes its shape. For example, it is known that gold (Au) cannot maintain its shape as individual particles when it becomes a nanoparticle having a diameter of 10 nm or less. On the other hand, thermal stability is required to apply nano-devices with nanogap electrodes to industry. For example, the nanogap electrode is required to have a heat resistance of about 400° C. in the manufacturing process of the semiconductor integrated circuit. Therefore, the nanogap electrode is required not only to precisely control the length of the gap (gap length), but also to have thermal stability.
Here, the surface energy of a metal surface with a nanoscale radius of curvature is proportional to the inverse of the radius of curvature. In the presence of shapes with different radii of curvature, the metal atoms tend to be spherical with a large radius of curvature, which diffuses to the surface and has a stable energy, due to the Rayleigh instability. The migration rate of surface self-diffusion is proportional to surface self-diffusion coefficient and inversely proportional to the inverse of temperature. The surface tension is proportional to the inverse of the radius of curvature. The smaller the radius of curvature, the more likely the surface self-diffusion of metal atoms occurs.
For example, on the surface of the titanium (Ti) film formed on the substrate, to deposit gold (Au) by electron beam evaporation, when trying to produce an electrode having a line width of 20 nm or less, the electrode shape is changed at room temperature by Rayleigh instability. The reason for this is considered to be that the surface self-diffusion coefficient of gold (Au) at room temperature is as high as about 10−13 cm2/sec (C. Alonso, C. Salvarezzo, J. M. Vara, and A. J. Arvia, “The Evaluation of Surface Diffusion Coefficients of Gold and Platinum Atoms at Electrochemical Interfaces from Combined STM-SEM Imaging and Electrochemical Techniques”, J. Electrochem. Soc. Vol. 137, No. 7, 2161 (1990)).
Therefore, the nanogap electrode 100 employs combinations in which surface self-diffusion coefficient of the first metal forming the first electrode layer 104a and the second electrode layer 104b is smaller than surface self-diffusion coefficient of the second metal forming the first metal particle 106a and the second metal particle 106b. In other words, when the first electrode layer 104a and the second electrode layer 104b are formed of the first metal and the first metal particle 106a and the second metal particle 106b are formed of the second metal, a combination is applied in which surface self-diffusion coefficient of the second metal on the surfaces where the first metal and the second metal are bonded to each other is smaller than surface self-diffusion coefficient of the second metal. By such a combination, surface self-diffusion of the second metal is suppressed, and the first metal particle 106a and the second metal particle 106b can be formed as independent particles having a hemispherical shape.
An example of a combination of the first metal and the second metal is to use platinum (Pt) as the first metal and gold (Au) as the second metal. Specifically, the first electrode layer 104a and the second electrode layer 104b are preferably formed of platinum (Pt), and the first metal particle 106a and the second metal particle 106b are preferably formed of gold (Au), as one embodiment.
That is, by combining gold (Au) having a surface self-diffusion coefficient of 10−13 cm2/sec and platinum (Pt) having a surface self-diffusion coefficient of about 10−18 cm2/sec at normal temperature, it is possible to eliminate the effect of the Rayleigh instability and obtain a structurally stable nanogap element 100. That is, by combining platinum (Pt) having a low surface self-diffusion coefficient with gold (Au) while using gold (Au) suitable as the electrode material, the surface self-diffusion of gold (Au) in the growing process can be suppressed, and the shape-stability of the gold nanoparticles can be greatly improved. Platinum (Pt) has a high melting point of 1768° C., is excellent in heat resistance, is hard, is also chemically stable, and has a property of high durability. In addition, since platinum (Pt) forms a metal bond with gold (Au), surface diffusion of gold (Au) is suppressed in the process of growing gold (Au) particles on the platinum (Pt) surface, and gold (Au) particles having a hemispherical surface can stably exist.
Since the surface self-diffusion coefficient of gold (Au) is 10−13 cm2/sec and that of platinum (Pt) is about 10−18 cm2/sec, the surface self-diffusion coefficient of gold (Au) is five orders of magnitude smaller than that of platinum, and due to the existence of an alloy of gold (Au) and platinum (Pt), the surface self-diffusion coefficient of gold (Au) atoms on a platinum (Pt) surface is smaller than that of gold (Au) atoms when platinum is replaced with gold. Therefore, it is expected that metal particles 106 transverse direction (in-plane) formed of gold (Au) are prevented from diffusing on the surfaces of the electrode layers 104 formed of platinum (Pt).
If transverse direction diffusivity of the second metal on the surfaces of the first electrode layer 104a and the second electrode layer 104b formed of the first metal is large, the metal particles formed of the second metal have a large particle diameter, and the particles are connected to each other. Such a situation results in a problem that the shape of the nanogap electrode affects the properties of the nano-device and the desired properties cannot be obtained.
On the other hand, as exemplified in the present embodiment, since the metal particles 106 formed of the second metal (gold (Au)) are prevented from diffusing transverse direction on the surfaces of the first electrode layer 104a and the second electrode layer 104b formed of the first metal (platinum (Pt)), the large particle diameter is suppressed and the particles become small hemispherical particles. For example, the first metal particles 106a and the second metal particles 106b formed of gold (Au) have widths of 20 nm or less, preferably 15 nm or less, more preferably 10 nm or less, from one end to the other end in planar view on the surfaces of the first electrode layer 104a and the second electrode layer 104b formed of platinum (Pt), and can stably retain their shapes. The first metal particles 106a and the second metal particles 106b preferably have a curvature radius of 12 nm or less.
In the present embodiment, platinum (Pt) is used as the first metal for forming the first electrode layer 104a and the second electrode layer 104b, and gold (Au) is used as the second metal for forming the first metal particles 106a and the second metal particles 106b, but the present invention is not limited thereto. Other metal materials may be used as long as the first metal and the second metal form alloys and satisfy surface self-diffusion coefficient relation as described above.
A platinum (Pt) layer which forming the first electrode layer 104a and the second electrode layer 104b is provided on insulating surface. In the first electrode layer 104a and the second electrode layer 104b, another metal layer may be provided between the platinum layer and the base surface. As shown in the
In the nanogap electrode 100, it is preferable that the first metal particles 106a and the second metal particles 106b having a width of 20 nm or less from one end to the other end are arranged in pairs in the gap portion. If a plurality of metal particles are disposed at one end of each of the first electrode layer 104a and the second electrode layer 104b, the metal nanoparticles or the functional molecules disposed in the gap portion of the nanogap electrode 100 cannot be controlled properly. In addition, when one or both of the third electrode 102c and the fourth electrode 102d used as gate electrode are disposed, it becomes difficult to apply gate-bias to the metal nanoparticles or the functional molecules disposed in the gap portion of the nanogap electrode 100.
The
As shown in the
On the other hand, in the nano-device, when the presence of a plurality of single electron islands in the gap portion (in the gap) is allowed, a plurality of sets of metal particles paired with the gap portion of the nanogap electrode may be arranged.
As shown in the
In the nanogap electrode, when the arrangement of a plurality of metal particles on end portion of each of the first electrode layer 104a and the second electrode layer 104b is permitted, the widths and the film thicknesses of the first electrode layer 104a and the second electrode layer 104b may be appropriately set. For example, the width of the first electrode layer 104a and the second electrode layer 104b may be W1 and the film thickness may be T2, the width may be W2 and the film thickness may be T1, the width may be W2 and the film thickness may be T2.
The shapes of the first electrode layer 104a and the second electrode layer 104b are not limited to rectangular shapes. For example, as shown in the
The first metal particles 106a and the second metal particles 106b as shown in the
According to the present embodiment, the width from one end to the other end of the first metal particle 106a and the second metal particle 106b which spaced apart from each other can be set to 20 nm or less respectively, and the distance between them can be arrange to 10 nm or less in the gap portion of the nanogap electrode 100.
As shown in the
1-2 Method of Manufacturing a Nanogap Electrode
1-2-1 Manufacturing Process
A method of manufacturing the nanogap electrode 100 will be described with reference to the drawings.
On the upper surface of the first insulating layer 112, a metal layer 114 is formed.
When the electroless plating is performed, the metal particles 106 grow on the surfaces of the first electrode layer 104a and the second electrode layer 104b. The first metal particle 106a and the second metal particle 106b may grow at any position on the surfaces of the first electrode layer 104a and the second electrode layer 104b. However, since one end of each of the first electrode layer 104a and the second electrode layer 104b is formed to have a width of 20 nm or less, nucleation is preferentially performed by end portion, and the metal particles 106 are reliably generated.
In the process of electroless plating, monovalent positive ions of ascorbic acid and gold exist on the surfaces of the first electrode layer 104a and the second electrode layer 104b, and ascorbic acid acts as a reducing agent, so that a state of electrons is formed. At this time, on the surfaces of the first electrode layer 104a and the second electrode layer 104b, gold ions are reduced to gold by the surface autocatalytic reaction, and are plated. As a result, as shown in the
The first metal particles 106a and the second metal particles 106b are formed in a hemispherical shape on the surfaces of the first electrode layer 104a and the second electrode layer 104b. The width from one end to the other end of the first metal particle 106a and the second metal particle 106b having a hemispherical surface is preferably 20 nm or less. The radius of curvature of the first metal particle 106a and the second metal particle 106b is preferably 12 nm or less. The width and the radius of curvature from one end to the other end of the first metal particle 106a and the second metal particle 106b can be controlled by the processing time of the electroless plating.
When platinum (Pt) is used as the first electrode layer 104a and the second electrode layer 104b, gold (Au) deposited by reduction on the platinum (Pt) surface is metallurgically bonded to platinum (Pt). As a result, gold (Au) is grown on the platinum (Pt) surface such that transverse direction is suppressed from diffusing and a spherical surface is formed on the platinum (Pt) surface.
As described above, by performing electroless gold plating on platinum (Pt) surfaces, which are not frequently used in the related art, as shown in the
1-2-2 Principle of Electroless Plating
As the electroless plating solution used in the present embodiment, an iodine tincture solution (a solution in which 12 and KI2− are dissolved in ethanol solvents) in which a gold foil is dissolved is used. When such an electroless plating solution is used, it is possible to perform autocatalytic type electroless gold plating using a chemical reaction by the saturation state of gold.
The principle of this electroless plating is as follows. Gold dissolved in iodine tincture becomes saturated and the following equilibrium occurs.
2Au+I3−I−↔2[AuI2]− (1)
[AuI2]−+I3−↔[AuI4]−+I− (2)
The following equilibrium states exist in tincture of iodine solution.
2KI+I2↔2K++I3−+I− (3)
Equation (3) is an endothermic reaction, and the equilibrium tilts to the right by heating the solution. Then, iodine ions (I−, I3−) are generated, and a tri-valued gold ion (Au3+) is generated from the responses of Equations (1) and (2). In this condition, by introducing L(+)-ascorbic acid (C6H8O) as a reducing agent, the ratio of I− ions is increased by the reduction of Equation (3).
C6H8O6+I3−→C6H6O6+3I−+2H+ (4)
When the electrode is immersed in the solution in this reaction, the reaction of Equation (1) and Equation (2) of chemical equilibrium is directed toward the reaction on the left side where gold is electroless plated.
Monovalent gold ions (Au+) are reduced to nuclei on the platinum-electrode surfaces. In addition, electroless gold plating of the autocatalytic type progresses on the gold surface as a nucleus. Since L(+)-ascorbic acid is supersaturated in this plate, I3− continues to be reduced to I− and the process is suppressed.
As noted above, in plating baths, the two reactions of nucleation electroless gold plating by reduction of monovalent gold ions (Au+) on the platinum surface and electroless gold plating on gold (Au) nuclei occur competitively.
1-2-3 Molecular Ruler Electroless Plating
In the step of manufacturing the first metal particles 106a and the second metal particles 106b shown in the
In the molecular ruler electroless plating method, an electroless plating solution containing a surfactant which serves a function of a molecular ruler is used in addition to an iodine tincture solution containing gold (Au) and a reducing agent. As the surfactant, for example, alkyltrimethylammonium bromide, alkyltrimethylammonium halide, alkyltrimethylammonium chloride, alkyltrimethylammonium iodide, dialkyldimethylammonium bromide, dialkyldimethylammonium chloride, dialkyldimethylammonium iodide, alkylbenzyldimethylammonium bromide, alkylbenzyldimethylammonium iodide, alkylbenzyl dimethylammonium iodide, alkylamine iodide, N-methyl amine, N-methyl-1-dialkylamine, N-methyl-1-dialkylamine, alkylphosphine, alkyl phosphine, and the like can be used.
The surfactant chemisorbs to the metal particles deposited during the process of electroless plating. The surfactant has an alkyl chain, and the alkyl chain fills the gaps between the first metal particles 106a and the second metal particles 106b with an interleaved fit, thereby self-terminating the electroless plating. In this electroless plating method, it is possible to control the length of the gap (gap length) by changing the length of the alkyl chain of the surfactant. That is, when the alkyl chain length is increased, the gap length of the nanogap electrode can be increased.
As described above, the nanogap electrode having at least a pair of metal particles in the gap portion can also be manufactured by a molecular ruler electroless plating method. By using the molecular ruler electroless plating method, the length of the gap of the nanogap electrode can be controlled by the alkyl chain length of the surfactant.
According to this embodiment, by using the electroless plating method, it is possible to precisely control the electrode spacing (gap) of the nanogap electrode. More specifically, by performing electroless gold plating on platinum (Pt) surfaces, a nanogap electrode having an electrode spacing (gap) of 10 nm or less can be manufactured. In addition, by dissolving non-toxic iodine tincture and gold foil as the electroless plating solution and using L(+)-ascorbic acid (C6H8O) as the reducing agent, nanogap electrodes can be produced in large quantities at one time at room temperature.
This embodiment shows an example of a nano-device using the nanogap electrode shown in the first embodiment. A nano-device 200a shown in the present embodiment has an operation configuration as a single-current transistor.
2-1 First Structure of the Nano-Device
The nano-device 200a further includes a self-assembled monolayer (SAM) 118. The self-assembled monolayer 118 is provided so as to cover at least the first electrode 102a and the second electrode 102b. In other words, the self-assembled monolayer 118 is provided so as to cover at least the surface of the first metal particle 106a and the second metal particle 106b.
The self-assembled monolayer 118 includes a first functional group that chemically adsorbs to a metal atom forming the first metal particle 106a and the second metal particle 106b, and a second functional group that is bonded to the first functional group. The first functional group is either a thiol group, a dithiocarbamate group, or a xanthate group. The second functional group is one in which some or all of the hydrogen molecules of an alkane, an alkene, an alkane or an alkene are substituted with fluorine, an amino group, a nitro group or an amide group.
For example, the self-assembled monolayer 118 is formed of a monomolecular film in which an alkanethiol is self-assembled. The self-assembled monolayer 118 is water-repellent and acts to keep the surface stable. A small number of alkane dithiols are mixed in the alkanethiol of the self-assembled monolayer 118. Alkane dithiol is formed by placing a bonding group thiol containing sulfur (S) at both ends of an alkane chain and has a shape in which sulfur (S) is present at each position of an alkanethiol monomolecular film. In order to incorporate an alkane dithiol into an alkanethiol, an electrode coated with an alkanethiol self-assembled monolayer 118 is immersed in a solution of an alkane dithiol, and a part of the alkanethiol is replaced with an alkane dithiol.
The nano-device 200a includes the metal nanoparticle 116 in the gap between the first electrode 102a and the second electrode 102b. The metal nanoparticle 116 are particles having a diameter of several nanometers, and gold (Au), silver (Ag), copper (Cu), nickel (Ni), iron (Fe), cobalt (Co), ruthenium (Ru), rhodium (Rh), palladium (Pd), iridium (Ir), platinum (Pt), or the like is used. The metal nanoparticle 116 are adsorbed on a self-assembled monomolecular 118 mixed film formed by the reaction of a self-assembled monomolecular with an organic molecule. Molecules such as alkanethiols that bind to the straight chain portion of the molecules constituting the self-assembled monolayer 118 are bound to the periphery. The metal nanoparticle 116 introduced into the gap portion between the first electrode 102a and the second electrode 102b are chemically bonded to the sulfur (S) contained in the alkane dithiol of the self-assembled monolayer 118 and become a stable state.
The nano-device 200a is covered with a second insulating layer 120 provided to bury the self-assembled monolayer 118 and the metal nanoparticle 116. The second insulating layer 120 is used as a protective film of the nano-device 200a.
As the substrate 110, a silicon wafer, silica substrate, alumina substrate, zirconia substrate, alkali-free glass substrate, or the like is used. As the substrate 110, when a silicon wafer is used, in order to ensure the insulating properties of the surface forming the electrode 102, it is preferable that the first insulating layer 112 is provided. The first insulating layer 112 is formed of an inorganic insulating film such as a silicon oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxide film, or a magnesium oxide film.
The first electrode 102a, the second electrode 102b, the third electrode 102c, and the fourth electrode 102d have the same structure as that shown in the first embodiment and are manufactured in the same manner.
The nano-device 200a operate as a single-electron transistor. That is, the first electrode 102a is the source electrode, the second electrode 102b is drain electrode, and the third electrode 102c and the fourth electrode 102d are gate electrode. In the nano-device 200a of the present embodiment, the same voltage is applied to the third electrode 102c and the fourth electrode 102d. One of the third electrode 102c and the fourth electrode 102d used as gate electrode may be omitted.
The metal nanoparticle 116 disposed in the gap between the first electrode 102a and the second electrode 102b function as single-electron islands (also referred to as “Coulomb islands”). The nano-device 200a develops electron flow between the first electrode 102a and the second electrode 102b due to a tunnel effect with Coulomb blockade phenomenon.
A second insulating layer 120 is provided between the third electrode 102c and the fourth electrode 102d functioning as a gate electrode and the metal nanoparticle 116. In other words, the third electrode 102c and the fourth electrode 102d are insulated from the metal nanoparticle 116. The third electrode 102c and the fourth electrode 102d function as a gate electrode, and can modulate a current flowing between the first electrode 102a and the second electrode 102b. The nano-device 200a, i.e., the current (drain current) due to the tunnel effect with Coulomb blockade phenomenon between the source and the drain, allows the drain current to be modulated by the voltage applied to the gate.
The nano-device 200a can replace the metal nanoparticle 116 with functional molecules. That is, functional molecules can be disposed in the gap between the first electrode 102a and the second electrode 102b. Examples of the functional molecule include a molecule having a π-conjugated system skeleton and an oligomer. Even if the metal nanoparticle 116 are replaced with functional molecules, operation of the nano-device 200a can be similarly performed.
2-2 Second Structure of the Nano-Device
As shown in the
In the nano-device 200a shown in
In the nano-device 200a shown in
Note that although both the third electrode 102c and the fourth electrode 102d are shown in
As described in this embodiment, by using the nanogap electrode shown in the first embodiment, as one of the nano-devices, it is possible to realize a single-electron transistor. Since the length of gap of the nanogap electrode (gap length) is precisely controlled by the self-terminating function of the electroless plating, it is possible to suppress the characteristic variation of the single-electron transistor. Furthermore, since the nanogap electrode is thermally stable, it is possible to increase the reliability of the single-electron element.
This embodiment shows an example of a nano-device using the nanogap electrode shown in the first embodiment. A nano-device 200b shown in this embodiment has an operation configuration as a logical operation device.
As in the second embodiment, the self-assembled monolayer 118 may be provided on the surfaces of the first metal particle 106a and the 2 metal particle 106b, and the metal nanoparticle 116 may be chemically bonded to the sulfur (S) contained in the alkane dithiol of the self-assembled monolayer 18 The metal nanoparticle 116 may be replaced with functional molecules as in the second embodiment.
Similar to the second embodiment, the self-assembled monolayer 118 may be provided on the surfaces of the first metal particle 106a and the second metal particle 106b, and the metal nanoparticles 116 may be chemically bonded to sulfur (S) contained in the alkanedithiol of the self-assembled monolayer 118. As shown in
The nano-device 200b in the present embodiment has the same structure as the single-electron transistor. The nano-device 200b can modulate charges to single-electron islands formed with metal nanoparticle 116 with a gate-voltage applied to gate electrode. Thus, between the source-drain (nanogap electrode 100), a state in which a current flow, that two states of a state in which no current flows appear periodically, so-called Coulomb oscillation phenomenon is observed.
The nano-device 200b having three gate electrodes can be used as a logical operation element for operation of exclusive OR (XORs), exclusive not OR (XNOR) by utilizing such phenomena. That is, by applying a voltage corresponding to the logical values “0” and “1” to the three gate electrodes of the nano-device 200b, it is possible to obtain a logic output corresponding to the logic of the XOR or XNOR. The detail of operation of the nano-device 200b capable of performing such a logical operation is the same as that of the logical operation device disclosed in WO2014/142039.
The nano-device 200b according to the present embodiment, by using the nanogap electrode shown in the first embodiment, even when operation as a logical operation element, it is possible to improve the stability and reliability of operation. That is, the length of the gap of the nanogap electrode (gap length), since it is precisely controlled by the self-terminating function of the electroless plating, it is possible to suppress the characteristic variation of the logic operation element. Furthermore, since the nanogap electrode is thermally stable, it is possible to increase the reliability of the logic operation element.
This embodiment shows an example of a nano-device using the nanogap electrode shown in the first embodiment. A nano-device 200c shown in this embodiment mode has hysteresis in current-voltage characteristics and functions as a memory element.
As the halogen ion 124, a bromine ion, a chlorine ion, an Iodine ion, or the like is applied. Halogen ion 124 are present in the gap of the nanogap electrode 100 and affect electrical conduction. The halogen ions 124 are not arranged in equal numbers on both the first electrode 102a and the second electrode 102b, but are arranged biased to one of the metal particles.
Halogen ions 124 change in valence when a voltage is applied to the nanogap electrode 100. As a result, a redox reaction occurs, or the number of halogen ions present in the gap changes. The number of halogen ions contributing to conduction changes, and the conductivity between the first electrode 102a and the second electrode 102b changes. As another interpretation, it is believed that the application of a voltage to the nanogap electrode 100 altered the conductivity due to the migration of the halogen ions 124. By such a phenomenon, the current-voltage characteristics of the nanogap electrode 100 will have a hysteresis.
Therefore, the nano-device 200c sets the writing voltage (Vwrite), reading voltage (Vread), and erasing voltage (Verase) as voltages to apply to the first electrode 102a, which is then operation as a memory element. The relationship between these three types of voltages is set so that the following relationship is satisfied.
By setting operation voltages as described above, the nano-device 200c can realize three functions of writing, reading, and erasing as memory elements. Since the nano-device 200c can generate a high electric field in the gap even when the voltage applied to the nano gap electrode 100 is low, the valence of the halogen ion 124 can be easily changed. The nano-device 200c does not require a high voltage and can reduce power consumption.
Halogen ions 124, the electroless plating solution shown in the first embodiment, by performing electroless plating by mixing a surfactant containing halogen ions, it is possible to arrange the halogen ions 124 in the nanogap electrode 100.
In the present embodiment, by using the nanogap electrodes for realizing the memory element by the nano-device 200c, it is possible to improve the stability of operation, the low-voltage driving, and the reliability of the memory element. That is, the length of the gap of the nanogap electrode (gap length) is precisely controlled by the self-terminating function of the electroless plating, it is possible to suppress the characteristic variation of the memory element. Furthermore, since the nanogap electrode is thermally stable, the reliability of the memory element can be enhanced.
This embodiment shows an example of a nano-device using the nanogap electrode shown in the first embodiment. A nano-device 200d shown in this embodiment mode has a floating gate and can be used as a memory element.
The nano-device 200d is configured such that the fourth electrodes 102d are used as a gate electrode and a gate voltage Vg is applied to the gate voltage Vg. The third electrode 102c is used as a floating gate electrode, and is configured to be applied with a floating voltage Vf via the switch 126. In the nanogap electrode 100, a first electrode 102a is used as a source electrode, and an ammeter is connected thereto. The second electrode 102b is used as a drain electrode, and is configured so that the drain voltage Vd is applied.
The nano-device 200d can store the states of charges of the metal nanoparticle 116 with charges stored in the third electrode 102c (corresponding to the floating gate electrode) even when the switch 126 is turned off after a current is passed between the first electrode 102a (corresponding to the source electrode) and the second electrode 102b (corresponding to drain electrode), and the floating voltage Vf is applied to the third electrode 102c (corresponding to the floating gate electrode). In addition, the charge state of the metal nanoparticle 116 can be changed stepwise by a voltage applied to the third electrode 102c, which corresponds to the floating gate electrode. As a result, the current flowing between the nanogap electrodes 100 can be stepwise varied. Therefore, by changing the floating gate voltage Vf in multiple stages, the charge state of the metal nanoparticle 116 are stepwise different, it is possible to use as a multi-valued memory.
Such an operation is similar to the nano-devices disclosed in WO2016/031836. However, since the nano-device 200d according to the present embodiment has the nano gap electrode 100 shown in the first embodiment, variations in element characteristics can be suppressed, heat resistance can be excellent, and reliability can be enhanced.
The present embodiment shows an integrated circuit in which the nano-device exemplified in the second to fifth embodiments and an electronic device such as a MOS transistor are formed.
The MOS transistor 130 is buried in interlayer insulating film 132. Between the nano-device 200 and the MOS transistor 130, several layers of interlayer insulating film may be stacked to form a multilayer wiring.
The nano-device 200 is provided on the second interlayer insulating film 132b. The nano-device 200 is electrically connected to the MOS transistor 130, for example, by wiring 134 passing through the second interlayer insulating film 132b.
The type of the nano-device 200 is appropriately selected according to the application. That is, the nano-device 200 can be applied to integrated circuit 202 with various structures depending on the application, such as the single-electron transistor shown in the second embodiment, the logical operation element shown in the third embodiment, the memory element shown in the fourth embodiment, and the memory element provided with the floating gate shown in the fifth embodiment. For example, by using the nano-device 200a according to the second embodiment, it is possible to realize an integrated circuit for operation with low power dissipation. In addition, a memory cell can be formed using the nano-device 200c of the fourth embodiment and the nano-device 200d of the fifth embodiment.
The nano-device 200 is further buried in a second insulating layer 120. The upper layer of the second insulating layer 120, further multilayered wiring, bumps or the like may be formed. As described in the first embodiment, the nanogap electrodes 100 that make up the nano-devices 200 can be incorporated into the process of the semiconductor integrated circuit because they are highly heat resistant. For example, the fabrication of the nanogap electrode as described in the first embodiment can be performed in a metallization process.
As shown in this embodiment, the nano-device can be used as one of the elements constituting the semiconductor integrated circuit.
Example 1 shows an example of fabrication of a nanogap electrode. Fabrication step of the nanogap electrode has a step of producing a platinum electrode serving as a base of the electrode, and a step of applying an electroless gold plating on the surface of the platinum electrode.
1 Fabrication of Platinum (Pt) Electrodes
This example 1 shows an example in which the first electrode 102a and the second electrode 102b are formed using platinum. In this example 1, the first to fourth electrodes are referred to as platinum electrodes.
As a substrate for manufacturing platinum electrodes, a silicon wafer having a silicon oxide film formed on its surfaces was used. The substrate was cleaned by ultrasonic cleaning using acetone, ethanol, ultraviolet (UV) ozonation, or the like to form a clean surface.
An electron-beam resist solution (a resist solution obtained by mixing ZEP-520A (Nippon Zeon Corporation) and ZEP-A (Nippon Zeon Corporation)) was applied to the surface of substrate (the surface of the silicon oxide film) by a spinner to form a resist film, and then a prebake was performed. The substrate on which the resist film was formed was set in an electron-beam lithography device (ELS-7500EX manufactured by ELIONIX), and electron-beam lithography was performed on the resist film to form a resist film on which patterns for forming electrodes were formed. After that, development treatment was performed to form a resist pattern in which the drawn portions (portions corresponding to the electrode patterns) were in opening with each other.
Next, a titanium (Ti) film was formed on the patterned resist film using an electron-beam evaporation device (E-400EBS manufactured by Shimadzu Corporation), and a platinum (Pt) film was further formed on the patterned resist film. The titanium (Ti) film was formed to improve the adhesion of the platinum (Pt) film. A thickness of the titanium (Ti) film was 3 nm, and the thickness of the platinum (Pt) film was 10 nm.
The patterned resist film was peeled off by bubbling substrate on which the titanium (Ti) film and the platinum (Pt) film were laminated, immersed in a peeling solution (ZDMAC (manufactured by Zeon Corporation)) and allowed to stand. The metal layer in which the titanium (Ti) film and the platinum (Pt) film were laminated was lifted off together with peeling of the resist film. As a result, metallic layers remained in portions of opening patterns of the resist film, and other portions were peeled off and removed together with the resist film. In this way, a platinum electrode (more precisely, an electrode with a stack of titanium/platinum) was fabricated on the substrate.
Then, the fabrication of contact pads for electrical characteristics measurement was carried out. After the substrate on which the platinum electrodes were formed was cleaned, a positive resist was applied and prebaked to form a resist film. The resist film was exposed by a mask aligner (MA-20 manufactured by Mikasa Corporation) and developed to form a resist film having opening patterns corresponding to the pads for probe contacts.
Using an electron-beam evaporation apparatus (Shimadzu Corporation E-400EBS), titanium (Ti) film and platinum (Pt) film is laminated to form a metallic layer. Thereafter, the resist film was peeled off and the metal layer was lifted off to form a pad for probe contact.
The platinum (Pt) electrode thus prepared was observed with a scanning electron microscope (SEM), and the results are shown in
2 Formation of Metal Particles
Metal particles were formed on a platinum (Pt) electrode. Gold (Au) was used as the material of the metal particles. The gold (Au) particles were formed on the platinum (Pt) electrode by an electroless plating method. Details of the manufacturing procedure of the nanogap electrode by the iodine electroless gold plating method on the platinum (Pt) electrode are shown below.
2-1 Preparation of Electroless Plating Solution
An electroless plating solution was produced. A 99.99% pure gold (Au) foil was placed in a container, and iodine tincture was added and stirred, followed by standing. In addition, L(+)-ascorbic acid (C6H8O) was added, and the mixture was allowed to stand after being heated. The solution allowed to stand was separated in a centrifuge. The supernatant of the solution after centrifugation was collected, heated in addition to another container containing L(+)-ascorbic acid (C6H8O), and agitated. Thereafter, an iodine tincture solution containing gold (Au) used for electroless plating was prepared by standing.
2-2 Electroless Plating
Before the electroless gold plating was performed, the platinum electrode was cleaned. Washing was carried out by acetone and ethanol. After cleaning, the surface was dried with nitrogen blow, and the organic matter on the surface was removed by UV-ozone treatment.
A pretreatment of the electroless gold plating was carried out. As the pretreatment of platinum (Pt) electrodes, the surface was treated with acid.
An iodine tincture solution containing ultrapure water and gold (Au) was placed in the plating bath to adjust the density of the electroless plating solution. To the plating bath, 8 mL of ultrapure water was added to 8 μL of an iodine tincture solution containing gold (Au). The platinum-electrode-formed substrate was immersed for 10 seconds. Substrate removed from the plating bath was rinsed with ultrapure water followed by sequential boiling with ethanol and acetone. Substrate was then dried by blowing.
The SEM image of the sample thus produced is shown in
Table 1 shows the results of evaluating the dimensions of the platinum electrode before and after the electroless plating by length measurement SEM. The length of the gap (gap length) of the platinum electrode was measured to be 17.8 nm, while the length of the gap (gap length) after electroless plating was measured to be 2 nm. In addition, the width of the platinum electrode changed from 17 nm to 20 nm. Further, the radius of curvature of the gold particles in the gap portion was observed to be 10 nm or less.
Further, from the SEM image shown in
According to the results of the example 1, the platinum electrode, by applying an electroless gold plating, it was confirmed that it is possible to produce a nanogap electrode nanogap is formed with gold particles.
This example 2 shows the treatment condition dependence of the electroless plating. As the conditions of the electroless plating, the concentration of the electroless plating solution and the treatment time were compared and evaluated.
Evaluation was carried out using an iodine tincture solution containing gold (Au) prepared in the first example and varying the concentration diluted with ultrapure water. The prepared electroless plating solution was evaluated at two levels: a condition in which 8 μL of the stock solution was diluted with 8 mL of ultrapure water (hereinafter referred to as “condition 1”) and a condition in which 10 μL of the stock solution was diluted with 8 mL of ultrapure water (hereinafter referred to as “condition 2”).
According to the SEM images shown in
Next,
Compared to the case where the electroless plating treatment time shown in
Furthermore, from the comparison of
When the electroless plating solution of the condition 1 is used, when one gold atom is reduced on the platinum surface and the nucleus grows, if the electroless plating time is set to 20 seconds, neighboring nuclei are connected and particle diameter of hemispherical gold particles becomes large. This suggests that at the platinum surface, the reduction of monovalent gold ions continues to proceed, and hemispherical gold particles are formed.
According to the results of this example, it has been shown that by adjusting the concentration of the electroless plating solution and the processing time of the electroless plating, it is possible to control the length (gap length) of the gap in accordance with the size of the nanoparticles or functional molecules introduced between the gaps of the nanogap electrodes while utilizing the self-terminating function.
This example 3 shows the results of evaluating the curing of the pretreatment before the electroless gold plating is performed on the platinum electrode. The conditions for manufacturing the platinum electrode are the same as those in the first embodiment.
The pretreatment was evaluated under three conditions: (1) without pretreatment, (2) treatment with solution A (HCl diluted with ultrapure water), and (3) treatment with solution B (HClO4 diluted with ultrapure water).
As shown in
According to this example 4, it was confirmed that the growth of Au was different depending on the presence or absence of the pretreatment and the difference in the pretreatment conditions before the electroless plating was performed on the platinum electrode. The pretreatment is considered to contribute to nucleation when gold particles grow, and it was confirmed that the gold particles can be grown in a dispersed state by delaying the speed of electroless plating.
This example 4 shows the results of evaluating the heat resistance of the nanogap electrode. The nanogap electrode produced in the first embodiment 200° C., subjected to heat treatment for 2 hours, the shape changes before and after the heat treatment was observed by SEM.
On the other hand, the gold particles on the first pad 108a and the second pad 108b, which are wider than the first electrode 102a and the second electrode 102b, are in a state in which the grains cannot be confirmed after the heat treatment. The gold particles on the first pad 108a and the second pad 108b are difficult to be disposed apart from each other, and the gold atoms are diffused to change the shape of the gold particles, so that the platinum electrode surface is covered with the gold particles. From this, it is clear that the electrode width influences the formation process of the gold particles.
The gold particles on the first electrode 102a and the second electrode 102b whose particle diameter is changed are contacted with the adjoining gold particles on the surface of the platinum electrode, and the gold atoms are self-diffused on the surface due to Rayleigh instability, and tend to have a spherical shape with stable radii of curvature. At this time, since one of the adjoining gold particles is incorporated into the other gold particle, disappearance of the gold particle and the gold particle having a large particle diameter is observed at the same time.
On the other hand, gold particles that do not touch each other and are spaced apart at the platinum-electrode surfaces do not change in particle diameter and remain structural. In particular, it is important that the gold particles in the gap portion exist in the same state as before the heat treatment, which suggests that the gold particles in the gap portion have a strong tendency to be disposed apart from each other.
In addition, the fact that the shape is not changed even by the heat treatment at 200° C. promotes solid solution of the gold particles with platinum of the platinum electrode, and solid solution strengthening can form solid solution particles that are stronger than the gold particles.
On the other hand, the platinum electrode, in the nanogap electrode was subjected to electroless gold plating in place of the gold electrode, it has been reported that the electrode structure is broken by heat treatment at 200° C. (V. M. Serdio, et al., Nanoscale, 4, (2012), p. 7161). From this, it was confirmed that the nanogap electrode produced in this example was thermally stable.
A titanium (Ti)/platinum (Pt) nanogap electrode subjected to electroless gold plating (hereinafter referred to as Sample 1) and a titanium (Ti)/gold (Au) nanogap electrode (hereinafter referred to as Sample 2) were evaluated for heat resistance. Both Sample 1 and Sample 2 have a structure in which gold is uniformly formed on the electrode surface by electroless plating. The heat resistance test was carried out at 400° C., 2 hours.
Considering the above results, it is considered that the gold (Au) atom electroless plated on the platinum (Pt) forms a metal-metal bond with the platinum (Pt) atom, and the platinum (Pt)-gold (Au) bond has a higher bonding energy than the gold (Au)-gold (Au) bond, so that the shape of the nanogap electrode can be maintained.
Further, not only the gold-platinum interface is formed, but also gold and platinum are formed into an alloy, and the gold particles are solidified, whereby gold-platinum particles with solid solution strengthening are formed, and the heat resistance is higher than that of platinum upper gold particles, and a strong gap structure can be produced.
Furthermore, than the nanogap electrode gold (Au) is uniformly formed by electroless plating, the nanogap electrode gold particles are formed by dispersing, due to the presence of the platinum electrode surface, the gold-platinum bonding because the surface self-diffusion of gold is less likely to occur, the radius of curvature of the gold particles is small, it is considered that more structurally stable. That is, in order to obtain a strong gap structure, it is important that the gold particles are not in contact with each other on the adjacent gold particles and the platinum surface, but are spaced apart from each other. Therefore, in the active device such as a transistor for performing the switching operation, as in the present embodiment, it is considered that the nanogap electrode gold (Au) particles are dispersed on the platinum electrode is suitable.
As an example 5, a nanogap electrode was produced using a molecular ruler electroless plating method in the following manner.
The first electrode layer 104a and the second electrode layer 104b are formed. Next, an electroless plating solution was prepared. As a molecular ruler, 25 mmol of alkyl-trimethylammonium bromide is measured by 28 mL. To that, 50 mmol of aqueous solution gold chloride is weighed 120 μL. Acetic acid was added to 1 mL as acid, and 0.1 mol of L(+)-ascorbic acid serving as a reducing agent and 3.6 mL were added, and the mixture was stirred well to obtain a plating solution.
In the example 5, molecular C12TAB was used as alkyl-trimethylammonium bromide.
Substrate with the first electrode 102a and the second electrode 102b prepared above was immersed in an electroless plating solution for about 3 minutes, 6 minutes, and 10 minutes. Thus, an electrode having a gap was produced by the molecular ruler electroless plating method of example 5.
From the above, it has been shown that when the molecular ruler electroless gold plating method is used, it is possible to form the gap in which the gold particles face each other by hemispherical electroless gold plating, and it is possible to precisely control the gap length by the molecular ruler.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-038092 | Mar 2018 | JP | national |
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2018-038092, filed on Mar. 2, 2018, and PCT Application No. PCT/JP2019/007937 filed on Feb. 28, 2019, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2019/007937 | Feb 2019 | US |
| Child | 17006437 | US |
