A method for fabricating a nanometer or atomic scale spin valve

Abstract

The application discloses a preparation method of a nano or atomic scale spin valve, which comprises the following steps: preparing a buffer layer and a metal electrode on a substrate surface, preparing a magnetic metal point contact device based on a proximity effect photolithography method, placing the device on a vacuum probe station, preparing an atomic scale point contact structure by using a current feedback control method, and preparing a magnetic tunnel junction with a magnetic resistance of up to 40% by natural oxidation, so as to obtain the nano or atomic scale spin valve. The preparation method can effectively control the contact width of the atomic contact point, and compared with the prior art, the complexity of the spin valve structure is reduced, and the device power consumption is reduced to a certain extent. The preparation method is simple, the magnetic tunnel junction has a large magnetic resistance, and is compatible with a traditional CMOS process, so that an advantage is provided for the integration of subsequent devices.

Description

Technical Field

[0001] This invention belongs to the field of spintronics and is a method for preparing an atomic-scale spin valve. Background Technology

[0002] Electrons possess two important intrinsic properties: charge and spin. Traditional devices primarily utilize charge properties without considering spin. However, electron spin does influence electron transport. In magnetic materials, the scattering effect on electrons is related to the direction of the magnetic moment of the conducting medium. Even at the atomic scale, the direction of the magnetic moment of atomic junctions can significantly affect electron transport. The exploration of spin valves began with the study of magnetic moment phenomena. As the magnetoresistive effect has gradually developed in practical applications, researchers have recently begun to focus on the magnetoresistive effect of new materials. While multilayer film devices can achieve relatively high magnetoresistive changes, continuous miniaturization is difficult. With the continuous advancement of integrated circuit technology nodes, their application in the integrated circuit field will be limited. Therefore, there is an urgent need to study spin devices based on nanoscale / atomic scale materials and structures.

[0003] From the 1960s and 70s, the feature size of transistors shrank continuously for fifty years, as predicted by Moore's Law. However, the rate of shrinkage in transistor feature size has slowed significantly in recent years. This is because, with the shrinking of device physical size, the short-channel effect has become more prominent, leading to increased power consumption and manufacturing difficulties. Since the 1990s, many scientists have been dedicated to finding the ultimate size limit for devices while simultaneously seeking new technologies to ensure the continuation of Moore's Law. These studies can be broadly categorized into two technical fields: one falls under the category of "continuing Moore's Law," which involves continuously expanding and developing the traditional functions of CMOS through new technologies such as heterogeneous integration; the other goes beyond traditional device and process research, seeking new information processing paradigms and developing novel devices and structures, hoping to bridge the gap between the traditional CMOS field and other electronic fields outside of CMOS, such as nanoelectronics, spintronics, and molecular electronics.

[0004] Even the most advanced micro / nano fabrication techniques cannot currently fabricate atomic-scale devices. To achieve atomic-scale structure and device fabrication, special instruments and methods are often required. The main methods for fabricating atomic contact spin valves are: (1) electrochemical method; (2) mechanically controlled junction splitting method; and (3) scanning electron microscopy (STM) method. However, the devices fabricated by these methods require complex testing equipment and strict testing environment requirements, and are not compatible with traditional integrated circuit processes. For practical reasons, it is necessary to further explore the properties of nano / atomic-scale point contact structures fabricated using electrically controlled electromigration methods. Summary of the Invention

[0005] This invention proposes a method for fabricating nanoscale or atomic-scale spin valves. Magnetic metal point contact devices are fabricated by controlling the width of the deposited nanoscale gaps using proximity effect photolithography. Atomic-scale point contact structures are fabricated by electrically controlling the migration of metal atoms. Magnetic oxides are then formed through natural oxidation. The antiferromagnetic insulating properties of the oxides can be used to form atomic-scale magnetic tunnel junctions, thereby fabricating atomic-scale spin valves.

[0006] The technical solution provided by this invention is as follows:

[0007] A method for preparing nanoscale or atomic-scale spin valves, comprising the following steps:

[0008] (1) Fabrication of a buffer layer and metal electrodes on the substrate surface. Depending on the experimental or application requirements, suitable substrate types can be selected, including but not limited to: silicon, silicon oxide, silicon oxide / silicon, silicon nitride, mica, sapphire, etc. A rigid substrate is required. Flexible substrates should not be used because multilayer film structures are prone to cracking under bending stress, leading to a sharp decrease in device lifespan. The buffer layer serves to: a) increase the adhesion between the metal and the substrate; b) provide a smoother surface, improving the quality of the subsequently fabricated metal film; c) improve the contact between the metal and the substrate. A good contact surface can reduce the entry of impurities or contaminants, thereby increasing device lifespan. Buffer layers are generally made of metals, such as titanium (Ti) or chromium (Cr), with a thickness of 5 nm to 20 nm. Electrode materials include, but are not limited to, metals such as gold, silver, palladium, platinum, and aluminum. The fabrication method for the buffer layer and metal electrodes is not specified; electron beam deposition, thermal evaporation deposition, magnetron sputtering, molecular beam epitaxy, etc., can be used.

[0009] Fabrication of metal electrodes: An adhesion layer and metal electrodes are fabricated on a substrate. The fabrication process can either use mask patterning followed by metal deposition-lifting, or it can directly pattern the metal layer without a mask. If mask patterning is used, the patterning methods include, but are not limited to, ultraviolet lithography, electron beam lithography, imprinting, or pattern transfer processes; the metal layer fabrication methods include, but are not limited to, electron beam evaporation deposition, thermal evaporation deposition, magnetron sputtering, electroplating, atomic layer deposition, and epitaxial growth. If a mask is not used and the metal layer is directly patterned, methods such as focused ion beam (FIB) etching, helium ion etching, and plasma etching can be used to directly etch the electrode structure onto the metal layer.

[0010] (2) Fabrication of magnetic metal point contact devices based on proximity effect photolithography: First, a magnetic point contact structure tens of nanometers thick is fabricated as a conductive channel through photolithography, deposition, and lift-off processes. The two sides of this magnetic metal film are connected to metal electrodes, and the middle portion of the magnetic metal film is used to fabricate the magnetic metal point contact structure. The location where atomic-scale point contacts are formed is called an atomic junction. The magnetic metal film is a 3d transition metal that does not oxidize drastically, such as iron, cobalt, or nickel. The fabrication of the magnetic metal film can be selected according to experimental requirements, and can include electron beam deposition, sputtering, thermal evaporation, epitaxial growth, etc.

[0011] This invention utilizes proximity effect photolithography to fabricate magnetic metal point contact devices. The proximity effect refers to the interconnection of closely spaced exposure locations due to lateral diffusion caused by electron scattering. This non-ideal effect is inherent and unavoidable in electron beam lithography. The device fabrication strategy based on the proximity effect of the electron beam is as follows: Due to lateral electron diffusion, the electron beam dose at each exposure location eventually decreases with increasing distance from the exposure center. There is an overlap of electron diffusion between two closely spaced exposure locations. Although this overlap is not the designed exposure area, it is still affected by a certain dose of electron beam. Photoresist is generally sensitive to electron beams; although these laterally diffused electrons have low energy, they are sufficient to denature the photoresist in the overlap area. By adjusting the pattern spacing, the exposure dose in the overlap area can be controlled. There exists a critical distance where the middle position achieves the optimal exposure effect. At this point, the contact should be at the minimum value under the current exposure state, thus obtaining a magnetic metal point contact device. To prevent oxidation, the magnetic metal thin film is kept isolated from air in acetone after fabrication.

[0012] (3) The device is placed on a vacuum probe stage, and an atomic-scale point contact structure is prepared using the current feedback control method: The current feedback control method is a technique for electrically controlling the migration of metal atoms. The preparation techniques for atomic-scale point contact structures using electrically controlled metal atom migration can be divided into two categories: one is to pre-prepare a metal tunnel junction, with the ultimate goal of reconnecting the metals at both ends; this preparation technique is called the tunneling distance control method. The other is to use electrical control to allow metal atoms to migrate slowly, gradually reducing the contact width at the metal point contact position, ultimately obtaining an atomic-scale point contact structure; this preparation technique is called the current feedback control method. The electrical conductivity characteristics during the atomic junction fracture process are an important experimental basis for controlling atomic-scale point contacts. Since the electromigration process and effect are different for each feedback operation, simply using the feedback control method cannot observe the continuous changes in conductivity or electrical conductivity processes during the formation of the atomic junction, thus failing to effectively control its scale. To observe the continuous conductance changes during the atomic junction fracture process, this invention adopts a combination of feedback control and direct burn-out methods. First, the conductance of the device is adjusted to a low level, such as 10G0 (G0 = 2e2 / h ≈ 77.5uS), through feedback control. Then, the voltage is continuously and slowly increased to cause the metal atoms at the point contact to continuously undergo electromigration, thus obtaining an atomic-scale point contact structure.

[0013] (4) Device oxidation to form magnetic tunnel junction: After the atomic-scale point contact structure is prepared, it is allowed to come into full contact with the air, which causes the magnetic atoms exposed in the air to be fully oxidized to form magnetic oxides. The atomic-scale point contact structure is a sandwich structure of magnetic metal-metal oxide-magnetic metal. The antiferromagnetic insulation properties of the oxide can be used to form an atomic-scale magnetic tunnel junction, thereby obtaining a nano or atomic-scale spin valve.

[0014] This invention fabricates magnetic point contact devices using proximity effect photolithography and further reduces the device scale to construct atomic-scale point contact structures using current feedback control. Through natural oxidation, a sandwich structure of magnetic metal-metal oxide-magnetic metal is formed, thereby constructing an atomic-scale spin valve.

[0015] The beneficial effects of this invention are as follows:

[0016] This invention utilizes a current feedback control method to fabricate magnetic metal point contact structures and forms magnetic oxides through natural oxidation, thereby creating a magnetic tunnel junction with a magnetoresistance effect as high as 40%. The fabrication method proposed in this invention can effectively control the contact width of atomic contact points, offering greater precision compared to previous methods. Furthermore, the use of natural oxidation to prepare antiferromagnetic oxides reduces the complexity of the spin valve structure and, to some extent, lowers device power consumption. This atomic-scale spin valve fabrication method is simple, produces a magnetic tunnel junction with high magnetoresistance, and is compatible with traditional CMOS processes, providing favorable conditions for subsequent device integration. Attached Figure Description

[0017] Figure 1 The following are flowcharts and characterization diagrams of the device fabrication process in specific embodiments of the present invention: (a) Flowchart of device fabrication process; (b) Schematic diagram of device fabrication; (c) Optical microscope photograph of a single device; (d) AFM scan image of the point contact area.

[0018] Figure 2 The figure shows the preparation process and characterization of the CoO thin layer in a specific embodiment of the present invention. In the figure: (a) the IV curves of the Co atomic-scale point contact structure prepared using the feedback control method during successive feedback operations; (b) the change in device conductivity with the number of feedback control operations, affecting the quantum conductivity (G0 = 2e⁻¹). 2 (h≈77.5μS) was normalized, and the dashed line represents the position of 1G0; (c)~(e) Changes in the corresponding point contact structure during the feedback control operation; (c) is the initial state; (d) is after partial atomic migration; (e) is the formation of an atomic-scale point contact structure; (f) after the formation of atomic-scale Co contact, it is fully oxidized to form an oxide; (g) XPS full spectrum analysis curve of the Co film surface (0~1100eV), with the element corresponding to each peak position marked in the figure; (h) Co2p spectrum peak scanning curve of the Co film (770~810eV), where the gray curve is the original data, and the other color curves are the results of peak fitting, with the corresponding Co valence state marked near each peak;

[0019] Figure 3 The figure shows the magnetoresistive test results of the prepared spin valve according to a specific embodiment of the present invention. In the figure: (a) IV characteristic curve of device #1 in the initial state; (b) magnetoresistive test curve of device #1 under a constant voltage of 200mV; (c) IV characteristic curve of device #2 in the initial state; (d) magnetoresistive test curve of device #2 under a constant voltage of 200mV, with a test temperature of 12K. Detailed Implementation

[0020] The present invention will be further illustrated below through examples of the preparation of Co-CoO-Co magnetic tunnel junctions. It should be noted that the purpose of disclosing the embodiments is to aid in further understanding of the invention; however, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the invention is defined by the claims.

[0021] (1) Fabrication of metal electrodes: An n-type doped low-resistivity silicon wafer (0.005Ωcm~0.2Ωcm) of size 1.1cm×1.1cm was selected as the substrate. A layer of silicon dioxide with a thickness of 300nm formed by thermal oxidation was formed on the surface of the substrate. Titanium / gold bilayer film was prepared as the electrode by electron beam lithography, electron beam evaporation coating and lift-off process. Ti was used as the adhesion layer with a thickness of 5nm and Au was used as the thickness of 50nm. The metal electrode was designed as a four-terminal structure, which can increase the number of times the Pad is used and the device utilization rate.

[0022] (2) Fabrication of Co point contact devices: A 15nm thick Co point contact was fabricated as a conductive channel using photolithography, deposition, and lift-off processes. The Co films on both sides served as metal electrodes connecting to the Ti / Au electrodes, and the Co film in the middle was used to fabricate the magnetic metal point contact device. The point contact width was approximately 100nm. The device was annealed at 300℃ in a protective gas atmosphere. The resulting device structure is shown below. Figure 1 As shown in (c), the structure of the atomic junction is as follows: Figure 1 As shown in (d).

[0023] (3) Preparation of Co atomic-scale point contact structures using feedback control: The electrodynamic characteristics during Co fracture are an important experimental basis for controlling atomic-scale Co contacts. (Refer to...) Figure 2 As shown in (a) to (b), the device conductivity is first adjusted to a low level using feedback control. Then, the voltage is continuously and slowly increased to cause continuous electromigration of Co at the point contact. The contact scale is determined by the conductivity value. When the voltage is adjusted to near the atomic scale, a Co point contact structure is considered to have been fabricated.

[0024] (4) The Co point contact structure is subjected to full oxidation in an atmospheric environment, causing the Co atoms in contact with the air to be oxidized to form atomic-scale CoO, thereby constructing an atomic-scale spin valve of Co / CoO / Co. The preparation mechanism is shown in the diagram below. Figure 2As shown in (c) to (f). Analysis of the Co surface oxide layer composition: X-ray photoelectron spectroscopy (XPS) can analyze elemental composition, valence states, and bonding, and infer the material composition accordingly. To avoid oxidation of the point contact device during XPS experiments, a 50 nm thick Co film was used to replace the oxide layer on the Co surface. The changes in the oxide layer before and after annealing were studied. Annealing was performed using hydrogen (3%) and argon (97%) at atmospheric pressure at a temperature of 300 °C for 1 hour. XPS used Al-Kα monochromatic rays. The test sample was a 50 nm thick Co film prepared on a SiO2 / Si substrate. The preparation process was exactly the same as that used for the Co film in the point contact device. Figure 2 (g) shows the full spectrum (0–1100 eV) scan results of the Co film after annealing. Full spectrum scanning allows for the identification of sample elements; the element corresponding to each peak is labeled in the figure. To identify the composition of the oxide layer, the Co 2p peak was scanned, and the results are as follows. Figure 2 As shown in (h), a strong metallic Co peak, i.e., Co2p(0), can be observed in the figure. 3 / 2 With Co2p(0) 1 / 2 This indicates that the oxide layer is relatively thin, allowing X-rays to penetrate directly and detect unoxidized Co atoms inside. The image also clearly shows the spectral peak formed by divalent Co ions, i.e., Co2p(П). 3 / 2 With Co2p(П) 1 / 2 There are also two corresponding satellite peaks. The appearance of satellite peaks is a significant indicator of the presence of divalent Co ions, and also indicates that trivalent Co ions are almost non-existent. This proves that the oxide layer formed by the Co film is CoO rather than Co3O4.

[0025] (5) Magnetoresistive testing of atomic-scale spin valves: The device was placed in a low-temperature magnetic field probe station. First, the IV curve of the device without an external magnetic field was measured to calculate its initial conductance. Then, a constant voltage of 200mV was applied, and the magnetic field strength of the test chamber was adjusted, slowly changing from -1T to 1T. The current-time curve was measured, and finally converted into the resistance and rate of change of resistance with the applied magnetic field to test its giant magnetoresistive effect. Specifically, a LakeShore CRX-VF probe station and a Keysight B1500 semiconductor parameter analyzer were used for magnetoresistive testing. First, its IV characteristic curve was measured to calculate the initial conductance of the device. Then, a constant voltage of 200mV was applied across the device, and the current was measured by changing the magnetic field strength, thereby calculating the resistance-magnetic field curve. Through testing and analysis, it was found that the spin valve prepared by this method exhibited a certain magnetoresistive response, with the maximum magnetoresistive response reaching 40% during the test. Figure 3(a) shows the IV characteristic curve of device #1 in its initial state. Based on the IV curve, its initial resistance can be calculated to be approximately 14 MΩ, and its conductance is around 10 MΩ. -3 G0 indicates the tunneling state. 3(b) shows the magnetoresistive curve measured for this device. Based on the magnetoresistive curve analysis, the maximum magnetoresistive resistance of this spin valve can reach 40%. Another device that tested and showed a relatively large magnetoresistive resistance is device #2, whose initial IV curve is as follows... Figure 3 As shown in (c), the initial resistance is calculated to be 80 MΩ based on the IV curve, and the corresponding conductance is 10. -4 G0, Figure 3 (d) shows the magnetoresistance change curve during the test. The graph shows that the maximum magnetoresistance can reach 10%.

[0026] The embodiments described above are not intended to limit the present invention. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention is defined by the scope of the claims.

Claims

1. A method for preparing nanoscale or atomic-scale spin valves, comprising the following steps: 1) A buffer layer and a metal electrode are fabricated on the substrate surface; 2) Fabrication of magnetic metal point contact devices based on proximity effect photolithography, specifically: a magnetic metal thin film is prepared by photolithography, deposition and lift-off processes, with both sides of the magnetic metal thin film connected to metal electrodes; then, point contact devices are prepared in the middle part of the magnetic metal thin film by utilizing the proximity effect of the electron beam. 3) Place the magnetic metal point contact device on a vacuum probe stage and use the current feedback control method to prepare an atomic-scale point contact structure; 4) The prepared atomic-scale point contact structure is brought into full contact with air, causing the magnetic atoms exposed in the air to be fully oxidized to form magnetic oxides. The atomic-scale point contact structure is a sandwich structure of magnetic metal-metal oxide-magnetic metal. The antiferromagnetic insulation properties of the magnetic oxide are used to form an atomic-scale magnetic tunnel junction, thereby obtaining a nanoscale or atomic-scale spin valve.

2. The method as described in claim 1, characterized in that, The substrate mentioned in step 1) is silicon, silicon oxide, silicon oxide / silicon, silicon nitride, mica, or sapphire.

3. The method as described in claim 1, characterized in that, The buffer layer mentioned in step 1) is made of titanium and chromium, with a thickness of 5nm to 20nm.

4. The method as described in claim 1, characterized in that, The metal electrode described in step 1) is prepared by electron beam evaporation deposition, thermal evaporation deposition, magnetron sputtering, electroplating, atomic layer deposition, and epitaxial growth.

5. The method as described in claim 1, characterized in that, The magnetic metal thin film material mentioned in step 2) includes cobalt, iron, and nickel.

6. The method as described in claim 1, characterized in that, The magnetic metal thin film described in step 2) is prepared by electron beam deposition, sputtering, thermal evaporation or epitaxial growth methods.

7. The method as described in claim 1, characterized in that, After the magnetic metal thin film described in step 2) is prepared, it is kept isolated from air in acetone.

8. The method as described in claim 1, characterized in that, In step 3), an atomic-scale point contact structure is prepared using a current feedback control method. Specifically, the conductivity of the device is first adjusted to a low level using a feedback control method, and then the voltage is continuously and slowly increased to allow the metal atoms at the point contact to undergo continuous electromigration. The contact scale is determined by the conductivity value. When the voltage is adjusted to be close to the atomic scale, an atomic-scale point contact structure is prepared.

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