Method for preparing flexible oxide memristor array through electrochemical process

Abstract

The invention relates to a method for preparing a flexible oxide memristor array by using an electrochemical anodic oxidation technology, and belongs to the crossing field of flexible semiconductor devices and electrochemistry. Depositing a plurality of mutually independent inert bottom electrodes on the flexible substrate by using a mask plate; respectively depositing active metal layers on the obtained inert bottom electrodes by using a mask plate; a connecting rod is added to each inert bottom electrode, so that all the inert bottom electrodes are connected into a whole, and the uniformity and integrity during current conduction are ensured; the connecting rod is connected to the positive electrode of a power source, all the active metal layers are completely soaked in electrolyte for anodic oxidation, and the active metal layers are oxidized into oxide middle layers; and depositing a top electrode on the oxide intermediate layer, and then removing the connecting rod to obtain the flexible oxide memristor array. The flexible memristor array is prepared by adopting a rapid and convenient process, so that the efficiency and the uniformity of preparing the memristor array through anodic oxidation are improved.

Description

Technical Field

[0001] The present invention relates to the cross-field of flexible micro-nano devices and electrochemistry, and more specifically, to a method for preparing a flexible oxide memristor array by an electrochemical process. Background Art

[0002] With the rapid development of big data, the Internet of Things, and edge devices, external information has exploded and transmitted, which has put forward higher requirements for whether information can be quickly transmitted and processed. At present, the computer architecture still maintains the von Neumann architecture. After the central processing unit processes the data, it transmits it to the memory through the data bus. The mismatch between the processor's data processing speed and the memory leads to a waste of computing power, and the data transmission process also causes delays. The new generation of storage and computing architecture intends to integrate storage and computing into one unit to achieve storage and computing integration, which can solve the above problems. As the fourth component, the memristor is an electronic device with adjustable conductivity. Its conductivity can be easily adjusted and stored. The resistive random access memory (ReRAM) prepared by it has been initially used in storage and computing integrated chips, which greatly reduces the power consumption of the chip. At the same time, it also has another volatile performance to simulate the function of brain neurons.

[0003] In the application of flexible electronic devices, flexible ReRAM devices have many advantages such as foldability and lightweight. For example, they can be used in flexible displays, flexible wearable devices, etc., and have some special advantages over non-flexible ReRAM. In the preparation process of flexible memristors, traditional processes are generally through deposition processes such as physical vapor deposition, chemical vapor deposition, atomic layer deposition, pulsed laser deposition, etc. Although these methods have certain uniformity and good precision, they generally have problems such as high prices (more than hundreds of thousands of yuan), harsh conditions, long cycles, and even high temperatures. Anodization technology is an electrochemical process in which the metal anode loses electrons in the electrolyte under the action of current and is oxidized to oxides. This technology is simple and convenient to implement, the equipment is cheap (less than one thousand yuan), and it is within the tolerance of most scientific researchers. At the same time, the samples prepared by this technology also have a gradient oxygen vacancy distribution and a uniformly oxidized surface, which can prepare flexible memristors with stable performance.

[0004] In the traditional anodization process, the metal to be oxidized is often immersed in the solution, while the other homogeneous metals are not immersed in the solution and are used to connect to the positive electrode of the power supply to facilitate current conduction. This can avoid the oxidation of the positive electrode of the power supply and the metal connection part, which causes a sharp increase in resistance and inability to conduct current. In the process of preparing memristors using anodization, the electrolyte is easily sputtered to the positive electrode connection due to the solution fluctuation caused by the need to stir the solution and the miniaturization of the device, causing oxidation of the connection and poor contact.

[0005] When preparing a flexible memristor array, when the bottom electrodes are arranged in multiple separate shapes, it is easy to cause the bottom electrode and the positive electrode of the power supply to not be in good contact (the positive electrode clip and the bottom electrode are not in full contact), and the current cannot be conducted to all the middle layers. Too dispersed electrode distribution will cause the positive electrode of the power supply and the bottom electrode to be connected multiple times to oxidize all the middle layers, reducing the success rate and efficiency of oxidation. Summary of the invention

[0006] In view of the above defects or improvement needs of the prior art, the present invention provides a method for preparing a flexible oxide memristor and its array by an electrochemical process. The purpose is to prepare a flexible memristor by combining an electrochemical process, improve the preparation efficiency of the memristor, and reduce its preparation cost and complexity. First, since the photolithography technology usually requires heating and drying the photoresist, which will cause the flexible substrate to deform, the technology prepares a flexible memristor array by designing a metal mask. In view of the problem that the electrolyte of small-sized devices is easily sputtered to the bottom electrode of the sample and the positive electrode of the power supply during oxidation to cause oxidation, the present invention uses an inert electrode as the bottom electrode and connects the bottom electrode to the positive electrode of the power supply. Even if all are immersed in the electrolyte, they will not be oxidized, ensuring good conduction of the current. At the same time, in view of the poor contact caused by the scattered distribution of the bottom electrode and the uneven oxidation of all the intermediate layers, by using a connecting rod, all the arrays of the bottom electrode are connected into a whole, ensuring that the oxidation current of all the intermediate layers is uniform. The present invention prepares a flexible memristor and its array with stable performance, and is applied to the fields of neuromorphic simulation, flexible wearable electronic devices and flexible electronic screens.

[0007] According to a first aspect of the present invention, a method for preparing a flexible oxide memristor array by an electrochemical process is provided, comprising the following steps:

[0008] (1) using a mask to deposit multiple independent inert bottom electrodes on a flexible substrate;

[0009] (2) using a mask to deposit active metal layers on the inert bottom electrodes obtained in step (1); the material of the active metal layers is a metal that can be oxidized among transition metals;

[0010] (3) adding a connecting rod to each inert bottom electrode to connect all the inert bottom electrodes into a whole to ensure uniformity and integrity during current conduction; connecting the connecting rod to the positive electrode of the power supply to completely immerse all the active metal layers in the electrolyte; using the active metal layer as the anode and platinum, gold, graphite or stainless steel as the cathode to perform anodic oxidation, and conducting current through the inert bottom electrode to the active metal layer, so that the active metal layer is oxidized into an oxide intermediate layer;

[0011] (4) A top electrode is deposited on the oxide intermediate layer, and then the connecting rod is removed to obtain a flexible oxide memristor array.

[0012] Preferably, the flexible substrate is a polyethylene naphthalate film, a polyethylene terephthalate film, a polyimide film, a polymethyl methacrylate film or a cellulose film.

[0013] Preferably, the material of the inert bottom electrode is Pt, Au or Pd, so that the flexible substrate is completely immersed in the electrolyte and the part connected to the positive electrode of the power source is not oxidized, thereby continuously transmitting the current to the active metal layer.

[0014] Preferably, the material of the active metal layer is Ta, Hf, Zn, Al, Mg, Ti, Cu, Ni, Nb or Zr.

[0015] Preferably, the active metal layer has a thickness of 10-500 nm.

[0016] Preferably, the voltage range of the anodization is 5-24V.

[0017] Preferably, the anodization time is 5 s to 5 min.

[0018] Preferably, the cathode is a sheet-like or mesh-like structure, and has an area larger than that of the flexible substrate.

[0019] According to another aspect of the present invention, a flexible oxide memristor array prepared by any one of the methods is provided.

[0020] According to another aspect of the present invention, there is provided application of the flexible oxide memristor array in neuromorphic computing, brain function simulation or non-volatile storage.

[0021] In general, the above technical solution conceived by the present invention has the following technical advantages compared with the prior art:

[0022] (1) The present invention allows the entire flexible substrate to be immersed in an electrolyte by clamping an inert bottom electrode, making oxidation more convenient; at the same time, the inert electrode will not be oxidized, so that the current can be continuously conducted to the middle layer, thereby preparing a cross-array memristor with different oxidation degrees. The mask shape design in the present invention can ensure that all the bottom electrodes are connected as a whole through the connecting rod, so that the current is evenly conducted to the middle layer; avoid the uneven oxidation phenomenon caused by poor contact between some bottom electrodes and the power anode, thereby increasing the oxidation uniformity and oxidation efficiency.

[0023] (2) The flexible memristor prepared by the present invention has good stability and uniformity, and can be used to simulate brain synaptic functions, neuron functions or realize non-volatile storage and other functions.

[0024] (3) The present invention combines the electrochemical anodization process to prepare the flexible memristor, which has a fast preparation rate and only takes a few seconds to a few minutes to prepare the functional layer required for the memristor. The preparation cost is greatly reduced compared to the traditional process.

[0025] (4) The present invention has universal applicability. Using the method of the present invention, various types of flexible memristors such as conductive volatile type and non-volatile type can be prepared. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The present invention is a schematic diagram of a method for preparing a flexible oxide memristor and an array thereof by an electrochemical process according to an embodiment of the present invention.

[0027] Figure 2 A top view of the structure of 2*2 and 4*4 cross arrays of flexible memristors prepared by electrochemical oxidation provided by the present invention.

[0028] Figure 3 A top view of the structure of a flexible memristor cross array prepared by electrochemical oxidation provided by the present invention.

[0029] Figure 4 This is a physical picture of the flexible memristor prepared in Example 1 of the present invention.

[0030] Figure 5 This is an IV curve diagram of the memristor device after bending according to Example 1 of the present invention.

[0031] Figure 6 This is a 100-cycle IV cycle curve diagram of the volatile memristor provided in Example 2 of the present invention.

[0032] Figure 7 The conductivity retention characteristics of the volatile memristor under the read voltage provided in the second embodiment of the present invention.

[0033] Figure 8 The current response of the volatile memristor under a pulse voltage provided in Example 2 of the present invention.

[0034] Fig. 9 This is an IV cycle curve diagram of the non-volatile memristor provided in Example 3 of the present invention.

[0035] Fig.10 The conductance of the non-volatile memristor provided in Example 3 of the present invention is in high and low resistance states under a pulse voltage.

[0036] Fig.11 This is a 128-cycle IV cycle curve diagram of the volatile memristor provided in Example 4 of the present invention. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0038] A method for preparing a flexible oxide memristor array by an electrochemical process comprises the following steps:

[0039] (1) using a mask to deposit multiple independent inert bottom electrodes on a flexible substrate;

[0040] (2) using a mask to deposit active metal layers on the inert bottom electrodes obtained in step (1); the material of the active metal layers is a metal that can be oxidized among transition metals;

[0041] (3) adding a connecting rod to each inert bottom electrode to connect all the inert bottom electrodes into a whole to ensure uniformity and integrity during current conduction; connecting the connecting rod to the positive electrode of the power supply to completely immerse all the active metal layers in the electrolyte; using the active metal layer as the anode and platinum, gold, graphite or stainless steel as the cathode to perform anodic oxidation, and conducting current through the inert bottom electrode to the active metal layer, so that the active metal layer is oxidized into an oxide intermediate layer;

[0042] (4) A top electrode is deposited on the oxide intermediate layer, and then the connecting rod is removed to obtain a flexible oxide memristor array.

[0043] In some embodiments, the flexible substrate material is various non-conductive organic, inorganic or composite flexible films such as polyethylene naphthalate (PEN) film, polyethylene terephthalate (PET) film, polyimide thin film (PDMS) film, polymethyl methacrylate (PMMA), cellulose film or flexible films of other materials.

[0044] In some embodiments, the bottom electrode in step (1) needs to be an inert metal, such as Pt, Au, Pd and other non-oxidizable metals, to ensure that the part of the flexible substrate connected to the positive electrode of the power supply is not oxidized when it is completely immersed in the electrolyte, and the current is continuously transmitted to the active metal layer.

[0045] In some embodiments, the active metal layer in step (2) is made of Ta, Hf, Zn, Al, Mg, Ti, Cu, Ni, Nb, Zr, etc. The active metal layer is to cover the bottom electrode, which is generally square or circular.

[0046] In some embodiments, the active metal layer in step (2) has a thickness of 10-500 nm, and the metal layer is a composite of one or more layers.

[0047] In some embodiments, the electrolyte in step (3) is an acid solution, an alkaline solution, a salt solution or an alcohol solution, the concentration of the solute in the electrolyte is 0.05M / L to 1M / L, the temperature of the electrolyte is 10 to 20°C, too high a temperature will cause excessive heat generation on the membrane surface and damage the surface morphology; and the electrolyte cannot react with the substrate material, thereby affecting the oxidation process. For example, a relatively concentrated strong oxidizing acid (concentrated sulfuric acid, concentrated hydrochloric acid, concentrated nitric acid, etc.).

[0048] In some embodiments, in step (3), the oxidation voltage generally has to meet two conditions: first, the oxidation voltage cannot exceed the limit value of film shedding. If the voltage is too high, the surface current will be too large to ablate the surface of the film, causing film shedding or pores; second, the voltage cannot be too low. When the voltage is too low, the degree of metal oxidation is too low, which will cause ohmic contact between the electrodes and thus fail to produce the resistive switching effect of the memristor. The voltage range is 5-24V; the oxidation time is generally between 5s and 5min. While ensuring that the flexible film will not be over-oxidized and produce bubbles, the flexible device can also be quickly prepared.

[0049] In some embodiments, in step (4), the top electrode material can be an active metal, an inert metal, an oxide, a nitride, etc., and the thickness is 10 to 200 nm; the top electrode position should cover the middle layer and should not contact the bottom electrode to avoid direct short circuit.

[0050] In some embodiments, the active metal layer is one or more combinations of copper, tantalum, titanium, nickel, tungsten, hafnium, niobium, yttrium, zinc, cobalt, aluminum, silicon and zirconium.

[0051] The flexible memristor prepared according to the method of the present invention and its application in neuromorphic computing, brain function simulation or non-volatile storage.

[0052] Figure 1 The present invention is a schematic diagram of a method for preparing a flexible oxide memristor and an array thereof by an electrochemical process according to an embodiment of the present invention.

[0053] When preparing a flexible memristor array, when the bottom electrode is a plurality of separately arranged shapes, such as Figure 2 and Figure 3 When the electrodes are too dispersed, the positive electrode of the power source and the bottom electrode are connected multiple times to oxidize all the middle layers, which reduces the success rate and efficiency of oxidation.

[0054] The shapes of the bottom and top electrodes form a cross array, and the overlapping area is a square, such as Figure 2 As shown. Figure 2 and Figure 3 As described above, the independent bottom electrodes are connected as a whole through connecting rods to ensure the uniformity of current conduction when the intermediate layer is oxidized.

[0055] The following are specific embodiments

[0056] Example 1

[0057] This embodiment provides a method for preparing a flexible oxide memristor and an array thereof by an electrochemical process, comprising the following steps:

[0058] (1) Preparation of bottom electrode: The cleaned flexible PDMS substrate is placed in a magnetron sputtering device. The target materials are Ti and Pt, with Pt as the bottom electrode. The vacuum degree of the chamber is pumped to 2*10 -6 Torr or less, high-purity argon was introduced to sputter 10nmTi and 100nmPt. The sputtering power of Ti was 25W and that of Pt was 20W.

[0059] (2) Preparation of composite metal active layer: Place a square mask on the flexible substrate in step (1), install the Nb target, introduce high-purity argon gas, and sputter 18nm of metal Nb at a power of 20W.

[0060] (3) Preparation of anodizing electrolyte: Prepare 0.5 M / L dilute phosphoric acid, and then stir it for 2 hours using a stirring device to mix it evenly.

[0061] (4) Oxidation of metal layer: clamp the bottom electrode of step (1) with the electrode clamp of the positive electrode of the power supply, immerse the entire flexible substrate in the electrolyte, and then oxidize the Nb metal deposited in step (2) under the oxidation condition of oxidizing at a constant voltage of 23.5 V for 1 minute. Then wash with deionized water and blow dry for use.

[0062] (5) Top electrode preparation: Place a circular metal mask on a flexible substrate, and then deposit 100nm of Pt and 100nm of Au top electrodes in a magnetron device. Finally, a PDMS / Ti / Pt / NbOx / Pt / Au flexible memristor is prepared. Figure 4 This is a physical picture of the flexible memristor prepared in this embodiment.

[0063] Figure 5This is the IV curve of the device after bending. When voltage is applied to the bottom electrode, oxygen vacancy filaments are formed inside, which turns into low resistance and the Set process occurs. On the contrary, when voltage is applied to the top electrode, due to the combined effect of internal heat accumulation and voltage field strength, the oxygen vacancy filaments break and the device turns into high resistance. It can be seen that the flexible device can maintain the resistive switching performance of the memristor after bending.

[0064] Example 2

[0065] This embodiment provides a method for preparing a flexible oxide memristor and an array thereof by an electrochemical process, comprising the following steps:

[0066] (1) Preparation of bottom electrode: The cleaned flexible PET substrate is placed in a magnetron sputtering device. The target materials are Ti and Pt, with Pt as the bottom electrode. The vacuum degree of the chamber is pumped to 2*10 -6 Torr or less, high-purity argon was introduced to sputter 10nmTi and 100nmPt. The sputtering power of Ti was 35W and that of Pt was 20W.

[0067] (2) Preparation of composite metal active layer: Place a square mask on the flexible substrate in step (1), install Ta target and Ti target, introduce high-purity argon gas, and sputter 60nm of metal Ta at a power of 30W. Then continue to sputter 1nm of Ti at a power of 20W to prepare a composite metal layer.

[0068] (3) Same as step (3) of Example 1.

[0069] (4) Oxidation of metal layer: clamp the bottom electrode of step (1) with the electrode clamp of the positive electrode of the power supply, immerse the entire flexible substrate in the electrolyte, and then oxidize the composite metal layer deposited in step (2) under the oxidation condition of oxidizing for 20 seconds at a constant voltage of 7V. Then wash with deionized water and blow dry for use.

[0070] (5) Same as step (5) of Example 1.

[0071] The 4200 semiconductor characteristics analysis system is used to test its electrical performance. Figure 6 The IV curve under cyclic scanning shows that the device has good stability under 100 cycles and there is no large variability under 100 cycles. Figure 7 As shown in the figure, after testing the conductivity retention performance of the device, it can be found that after reverse scanning, the device resistance becomes low resistance, and then the device resistance will spontaneously return to high resistance after 200s. It can be seen that the performance of this device belongs to a volatile memristor, which can be used for reserve pool calculation neural network, neuron function simulation, etc. Figure 8 As shown, when pulse voltage stimulation is performed, it can be seen that its conductance can be adjusted and the device has multiple conductance states.

[0072] Example 3

[0073] (1) Preparation of bottom electrode: Place a layer of the following on the upper surface of the cleaned flexible PET substrate: Figure 2 The metal mask with the bottom electrode shape shown in the figure is then placed in the magnetron sputtering equipment. The target materials are Ti and Pt, where Pt is used as the bottom electrode. The vacuum degree of the chamber is pumped to 2*10 -6 Torr or less, high-purity argon was introduced to sputter 10nmTi and 100nmPt. The sputtering power of Ti was 35W and that of Pt was 20W.

[0074] (2) Preparation of metal active layer: Place a metal active layer such as Figure 2 The square mask shown is installed with a Ta target, high-purity argon gas is introduced, and a 60nm metal Ta layer is sputtered at a power of 30W.

[0075] (3) Preparation of anodizing electrolyte: Prepare 0.4 M / L of dilute phosphoric acid and 0.1 M / L of ethylene glycol, and then stir with a stirring device for 2 h to mix them evenly.

[0076] (4) Oxidation of metal layer: clamp the bottom electrode of step (1) with the electrode clamp of the positive electrode of the power supply, immerse the entire flexible substrate in the electrolyte, and then oxidize the composite metal layer deposited in step (2) under the oxidation condition of 15V constant voltage for 15s. Then wash with deionized water and blow dry for use.

[0077] (5) Top electrode preparation: Place the device prepared in step (4) on top of the Figure 2 The top electrode shape is shown, and then 100nm Pt and 100nm Au top electrodes are deposited in the magnetron device.

[0078] (6) The connecting rod of the bottom electrode is removed to prepare the final cross-array flexible memristor.

[0079] Fig. 9 This is the IV cycle curve of the flexible memristor. It can be seen that the device has good stability and the Set voltage is within 1V. Fig.10 Figure 3 is the conductivity change of the device under positive and negative pulse voltages (+1V and -2.5V). It can be seen that the high and low resistance states of the device are still stable after 200 cycles.

[0080] Example 4

[0081] (1) Same as step (1) of Example 2;

[0082] (2) Same as step (2) of Example 2;

[0083] (3) Same as step (3) of Example 2;

[0084] (4) Oxidation of metal layer: Clamp the bottom electrode of step (1) with the electrode clamp of the positive electrode of the power supply, immerse the entire flexible substrate in the electrolyte, and then oxidize the composite metal layer deposited in step (2) under the oxidation condition of 7V constant voltage for 30s. Then wash with deionized water and blow dry for use;

[0085] (5) Same as step (5) of Example 2.

[0086] The performance of the device prepared using the process of this embodiment is similar to that of Example 2. Fig.11 The IV cycle curve of 128 cycles for this example shows very good cycle consistency.

[0087] Comparative Example 1

[0088] The difference between Comparative Example 1 and Example 2 is that the oxidation voltage in step (4) is 25 V, and the surface of the film obtained by oxidation has multiple holes and detached films. This is because the large oxidation voltage leads to local overheating, and holes are formed under the combined action of surface bubbles. Therefore, the memristor under this process cannot be used normally.

[0089] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a flexible oxide memristor array by electrochemical process, characterized in that: The following steps are involved: (1) using a mask to deposit multiple independent inert bottom electrodes on a flexible substrate; (2) using a mask to deposit active metal layers on the inert bottom electrodes obtained in step (1); the material of the active metal layers is a metal that can be oxidized among transition metals; (3) adding a connecting rod to each inert bottom electrode to connect all the inert bottom electrodes into a whole to ensure uniformity and integrity during current conduction; connecting the connecting rod to the positive electrode of the power supply to completely immerse all the active metal layers in the electrolyte; using the active metal layer as the anode and platinum, gold, graphite or stainless steel as the cathode to perform anodic oxidation, and conducting current through the inert bottom electrode to the active metal layer, so that the active metal layer is oxidized into an oxide intermediate layer; (4) A top electrode is deposited on the oxide intermediate layer, and then the connecting rod is removed to obtain a flexible oxide memristor array.

2. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The flexible substrate is a polyethylene naphthalate film, a polyethylene terephthalate film, a polyimide film, a polymethyl methacrylate film or a cellulose film.

3. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The material of the inert bottom electrode is Pt, Au or Pd, so that the flexible substrate is completely immersed in the electrolyte and the part connected to the positive electrode of the power source is not oxidized, thereby continuously transmitting the current to the active metal layer.

4. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The material of the active metal layer is Ta, Hf, Zn, Al, Mg, Ti, Cu, Ni, Nb or Zr.

5. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The thickness of the active metal layer is 10-500 nm.

6. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The voltage range of the anodization is 5-24V.

7. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The anodizing time is 5s to 5min.

8. The method for preparing a flexible oxide memristor array by electrochemical process according to claim 1, characterized in that: The cathode is in a sheet or mesh structure, and has an area larger than that of the flexible substrate.

9. A flexible oxide memristor array prepared by the method according to any one of claims 1 to 8.

10. Application of the flexible oxide memristor array as claimed in claim 9 in neuromorphic computing, brain function simulation or non-volatile storage.

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