Nonvolatile strain sensor based on magnetic tunnel junction
By designing a non-volatile strain sensor based on a magnetic tunnel junction, the problems of large size, high power consumption, and volatile storage of existing sensors are solved, realizing high-sensitivity and non-volatile storage of strain event recording, which is suitable for industrial monitoring and structural health diagnosis.
Patent Information
- Application Number
- CN202511970666.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-25
- Publication Date
- 2026-02-10
AI Technical Summary
Existing stress-strain sensors suffer from problems such as large size, high power consumption, insufficient sensitivity and signal-to-noise ratio, and loss of historical data after power failure, making it difficult to achieve high sensitivity, non-volatile storage and high integrability.
A non-volatile strain sensor based on a magnetic tunnel junction (MTJ) is designed. By stacking a bottom electrode, an antiferromagnetic layer, a reference layer, a barrier layer, a free layer, and a top electrode on a substrate, strain is sensed and key strain events are recorded by utilizing the change in the magnetization direction of the free layer. The sensor achieves non-volatile storage by combining a bias magnetic field and spin torque technology.
It achieves highly sensitive strain sensing and non-volatile storage, with low power consumption and high integrability, and can quickly respond to and record data in critical stress and strain events.
Smart Images
Figure CN121498527A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of sensor technology and spintronic devices, and in particular to a method for designing and manufacturing a non-volatile strain sensor based on a magnetic tunnel junction. Technical Background
[0002] In fields such as industrial monitoring and structural health diagnostics, there is a need for continuous monitoring of mechanical stress and recording of critical events such as overload. Currently, mainstream stress-strain sensors are mostly resistive, capacitive, piezoelectric, and triboelectric. Although sensitivity has been significantly improved based on material and structural design, their large size and complex signal processing circuitry lead to a trade-off between power consumption, size, and sensitivity. Furthermore, they cease operation and lose historical data upon power loss. An ideal sensor needs high sensitivity and high signal-to-noise ratio, high durability, high integrability, small size, and extremely low static power consumption to be embedded in various systems, such as embedded bearings, to form sensor arrays.
[0003] In recent years, advancements in spintronics, particularly magnetic tunnel junction (MTJ) technology, have driven the development of magnetoresistive random access memory (MRAM). This unique physical characteristic provides significant inspiration for developing sensors that integrate sensing, storage, and decision-making. This MRAM-based structure benefits from high tunneling magnetoresistance (TMR), achieving an extremely high gauge factor (GF) and exhibiting non-volatile storage, along with excellent integrability. Summary of the Invention
[0004] The purpose of this invention is to realize a multifunctional integrated sensor that integrates "sensing-storage-judgment" of strain sensors, which has high sensitivity and provides non-volatile storage of key stress and strain events.
[0005] To achieve the above objectives, one aspect of the present invention provides a non-volatile strain sensor based on MTJ, comprising functional layers stacked sequentially from a substrate; wherein the magnetoresistance of the tunnel junction and the magnetization direction of the free layer are used to sense strain and record key strain events, respectively.
[0006] Furthermore, starting from the substrate, the functional layers are the bottom electrode (BE), antiferromagnetic layer (AFM), reference layer (PL), barrier layer (tunnel barrier layer), free layer (FL), capping layer, and top electrode (TE); wherein, the magnetization direction (i.e. magnetic moment direction) of the reference layer and the free layer is perpendicular to the thin film (parallel to the normal of the thin film), and the magnetization direction of the reference layer is fixed.
[0007] Furthermore, when the magnetization direction of the free layer is consistent with that of the reference layer, the tunneling probability is maximized, resulting in the minimum tunnel junction resistance of R. P When the magnetization directions of the two are antiparallel, the maximum resistance R is achieved. APWhen the magnetization direction of the free layer is in-plane and the included angle is 90°, the resistance is moderate. When the magnetization direction is deflected and not pulled to the plane, the tunnel junction resistance is greater than the resistance in the in-plane magnetization direction but less than R. AP .
[0008] Furthermore, when the magnetostriction coefficient of the free layer applies tensile (compressive) strain to the sensor, the stress induced in the free layer does not exceed the critical stress σ. c At this point, the magnetization direction is inclined towards the tensile strain direction (perpendicular to the compressive strain direction). When the corresponding stress exceeds the critical stress (to overcome the coercivity Hc), the magnetization direction jumps to in-plane.
[0009] Furthermore, by applying a small bias magnetic field to the sensor or by applying a spin-shifting magnetic moment through a reserved STT channel, a 180° jump in the magnetization direction can be achieved.
[0010] Furthermore, achieving a 180° jump can improve sensor sensitivity and enable non-volatile storage of stress events.
[0011] Furthermore, the barrier layer, as the main component in tunneling, controls the barrier width by adjusting its thickness, thereby controlling the tunneling probability and TMR. During tensile stress, the tunnel barrier is modulated, which, together with a slight deflection of the magnetization direction, alters the overall resistance.
[0012] Furthermore, by applying a bias voltage to regulate the vertical magnetic anisotropy at the CoFeB / MgO interface, the threshold and sensitivity of the critical strain can be controlled.
[0013] Furthermore, during the measurement process, by comparing the sensor resistances of two reference layers with opposite magnetization directions, the magnetization direction and strain state of the free layer can be quickly obtained.
[0014] Furthermore, the sensor shape should be processed into a regular pattern, such as a circle or a rectangle.
[0015] Furthermore, the barrier layer material should be one of MgO(001) or h-BN. The thickness of the MgO barrier layer is approximately 1-1.6 nm, and not higher than 2.6 nm.
[0016] Furthermore, the reference layer and the free layer are Co. 40 Fe 40 B 20 Its thickness affects the perpendicular magnetic anisotropy constant and coercivity.
[0017] Furthermore, the thicknesses of the reference layer and the free layer are approximately 0.8-1.1 nm and approximately 1-2 nm, respectively.
[0018] Furthermore, the antiferromagnetic layer is either IrMn or PtMn, wherein the thickness of IrMn is 8-15 nm, and must be greater than 6 nm; the thickness of PtMn is 15-30 nm, and must be greater than 10 nm. If it is necessary to reduce the influence of stray fields, an antiferromagnetic layer (SAF) can be deposited on the AFM, and the common forms are CoFeB or CoFe(2.2) / Ru(0.9) / CoFeB(1.1).
[0019] Furthermore, the free layer can be processed into a wedge shape to guide magnetization reversal, and it is necessary to ensure that there is a gradient of the perpendicular magnetic anisotropy constant, with the thickness gradient range being 1.2-1.8 nm.
[0020] Furthermore, the capping layer should be one of Ta, IrMn, and PtMn, serving a protective function or utilizing a small exchange bias field H. eb It forms an exchange-biased free layer with the free layer, guiding the free layer to achieve a 180° flip from metastable to steady state, and adjusting the critical stress, which provides H eb It is approximately 0.1-0.2 times that of the AFM layer.
[0021] Another aspect of the present invention provides a method for manufacturing the sensor, the method comprising:
[0022] A bottom electrode is formed on a Si / SiO2 or flexible substrate, and the bottom electrode is one of Cu, Ru, or Ta.
[0023] Each layer of the sensor film is sequentially fabricated on the bottom electrode. An antiferromagnetic layer and a reference layer are fabricated using magnetron sputtering, an MgO barrier layer is fabricated using radio frequency sputtering, and then a free layer capping layer and a top electrode are grown using magnetron sputtering.
[0024] Furthermore, magnetron sputtering is combined with substrate tilting and rotation to process wedge-shaped free layers.
[0025] After the sensor completes the growth of the material layer, it undergoes post-annealing treatment in the temperature range of 250-400℃ for half an hour to several hours. During the annealing process, an external magnetic field is applied along the expected magnetization direction.
[0026] The annealed sensor is patterned using MEMS techniques such as ion beam etching to complete the strain sensor.
[0027] Compared with the prior art, the present invention has at least the following beneficial effects: the present invention is based on the non-volatile recording of key strain events by magnetic tunnel junctions, has low power consumption, high integrability, less susceptibility to environmental interference, and the tunnel junction has a fairly high gauge factor. Attached Figure Description
[0028] Figure 1This is a schematic diagram of the main structure of the strain sensor of the present invention.
[0029] Figure 2 The sensor in this embodiment of the invention consists of thin films of various layers.
[0030] Figure 3 The vertical magnetic anisotropy constant of the wedge-shaped free layer of the strain sensor of the present invention is given when there is no stress.
[0031] Figure 4 This is a flowchart illustrating the operation of the strain sensor of the present invention.
[0032] Figure 5 This is a resistance-stress response trend diagram of the strain sensor of the present invention.
[0033] Figure 6 This is a schematic diagram illustrating the principle of magnetization direction reversal during the operation of the strain sensor of the present invention.
[0034] Figure 7 This is a flowchart illustrating the manufacturing process of the strain sensor of the present invention. Detailed Implementation
[0035] To make the design scheme and features of the present invention clearer, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The embodiments of the present invention described in the accompanying drawings are merely exemplary, and the present invention can be embodied in many different forms and should not be construed as being limited to the specific embodiments set forth herein.
[0036] Furthermore, to avoid obscuring the invention with unnecessary details, only the main structure and method of the invention are described in the accompanying drawings, while some irrelevant details are omitted. In addition, the terms "top," "bottom," "upper," and "lower" used in the description of this invention are only for the convenience of describing the invention and do not indicate the necessary orientation of the elements.
[0037] To address the technical problems mentioned in the background section, this invention provides a non-volatile strain sensor based on MTJ. For example... Figure 1 As shown, the main structure of the strain sensor in this embodiment includes, in sequence, a substrate 1, a bottom electrode (BE), a bottom electrode (BE), an antiferromagnetic layer (AFM), a reference layer (PL), a barrier layer, a free layer (FL), a capping layer, and a top electrode (TE). The sensor described in this embodiment should be either circular or rectangular. The resistance of the tunnel junction can be detected between the top and bottom electrodes to determine the magnetization direction of the free layer and to read the magnitude of the applied strain.
[0038] like Figure 2The diagram shows the composition of the thin films in each layer of the sensor according to an embodiment of the present invention. In the specific fabrication of this embodiment, to ensure uniform growth of each material layer, the bottom electrode 2 is divided into three layers: bottom electrode 21, buffer layer 22, and lattice template layer 23. The buffer layer is a Ta layer approximately 5 nm thick, and the lattice template is a Ru layer 10 nm thick; the latter ensures the lattice growth quality of the antiferromagnetic layer. The antiferromagnetic layer 3 is either IrMn or PtMn, where IrMn has a thickness of 8-15 nm; and PtMn has a thickness of 15-30 nm, to provide a sufficient exchange bias field H. eb To fix the magnetization direction of reference layer 4.
[0039] The reference layer 4 is made of CoFeB, and its thickness in the sensor is approximately 0.8-1.1 nm or slightly thicker. Considering stray field interference, the reference layer 4 is fabricated as a synthetic antiferromagnetic layer as shown in the figure. Specifically, it is configured as a CoFeB or CoFe layer 41 with a thickness of approximately 2.2 nm; a Ru layer 42 with a thickness of approximately 0.9 nm; and a CoFeB reference layer 43 with a thickness of approximately 1.1 nm.
[0040] The free layer 6 is made of CoFeB with a thickness of approximately 1-2 nm. Its thickness has a significant impact on the perpendicular magnetic anisotropy constant (PMA). K eff It has a significant impact. CoFeB is a magnetostrictive material with a magnetostriction coefficient greater than 0. λ s When free layer 6 is subjected to tensile stress or strain, it increases its magnetoelastic coupling energy. E σ This causes its easy magnetization axis (easy axis) to turn in the direction of tensile stress.
[0041] The magnetization directions (i.e. magnetic moment directions) of the reference layer 4 and the free layer 6 are both perpendicular to the thin film (parallel to the normal of the thin film). The magnetization direction of the reference layer 4 is fixed downward (pointing towards the substrate), while the magnetization direction of the free layer 6 deflects with the initial bias magnetic field and the strain it experiences during operation, thereby causing a change in the tunneling probability and a jump in resistance.
[0042] In this embodiment, the sensor barrier layer 5 should be one of MgO(001) or h-BN. The thickness of the MgO barrier layer 5 is approximately 1-1.6 nm, and not more than 2.6 nm.
[0043] Barrier layer 5, as a key component of the sensor described in this embodiment, is the main part where tunneling occurs. When the magnetization directions of free layer 6 and reference layer 4 are parallel, it has the minimum resistance RP; when their magnetization directions are antiparallel, it has the maximum resistance R. AP When the magnetization direction of free layer 6 is in-plane with an included angle of 90°, the resistance is moderate. When the magnetization direction is deflected under stress and not pulled to the plane, the resistance is greater than the in-plane magnetization resistance but less than R.AP .
[0044] Specifically, tunneling magnetoresistance (TMR) reflects the performance of the tunnel junction; a higher TMR is advantageous, thereby improving the sensor's sensitivity, and is described by the normalization factor (GF). The following are the mathematical expressions for the TMR and GF parameters of the sensor described in this embodiment:
[0045] Among them, the easy axis direction of free layer 6 determines the tendency of magnetization to deflect easily. K eff Determine the direction of the easy axis. K eff When the magnetization direction is greater than 0, the easy axis is perpendicular to the thin film surface, and the magnetization direction tends to remain perpendicular. Under no stress... K eff Record K u From a mechanical perspective, magnetization reversal requires overcoming the coercivity Hc; the larger Hc is, the more difficult it is to change the magnetization direction. Overcoming Hc when subjected to stress σ or corresponding strain will... K eff The change from positive to negative causes the magnetization direction to deflect towards the tensile stress direction. K eff The relationship between stress and the critical stress σ at which deflection occurs. c as follows:
[0046] Therefore, the critical stress of the sensor can be designed by combining the Young's modulus of the material. Furthermore, a bias voltage is applied between the top electrode 8 and the bottom electrode 21 of the MTJ device. V It can be adjusted K eff This allows for adjustment of sensitivity, and also provides a VCMA coefficient. ξ (Negative numbers, related to the thickness of free layer 6, etc.), the corresponding relationships are as follows:
[0047] Under stress or strain, the magnetization direction of free layer 6 flips to the tensile stress direction, while the magnetization direction of reference layer 4 remains unchanged, resulting in a jump in resistance. Based on the original embodiment, the cross-sectional shape of free layer 6 is optimized into a wedge-shaped cross-section with a thickness gradient ranging from approximately 1.2 to 1.8 nm. t right K eff The regulatory effect forms a free layer with gradient vertical magnetic anisotropy, such as Figure 3 The figure shows the shape of the wedge-shaped free layer and K u Distribution. This further promotes deterministic magnetization reversal of free layer 6, while simultaneously modulating sensor sensitivity and widening the dynamic range. Within a given range, when the thickness is small, K eff Mainly affected by the interface anisotropy constant K i The effect is that when the thickness is large, it is affected by the volume anisotropy constant. K v The following are the relationships between various parameters, and the thickness gradient is established accordingly:
[0048] To ensure non-volatile storage, an external bias magnetic field is required to induce the magnetization direction of the free layer 6 to align with that of the reference layer 4. Furthermore, in this embodiment, the sensor can inject STT current through an MTJ structure to assist in flipping the magnetization direction of the free layer 6 towards the magnetization direction of the reference layer 4.
[0049] Furthermore, in addition to the bias voltage, the capping layer 7 can be designed as an antiferromagnetic material, either IrMn or PtMn, with the exchange bias field Heb being approximately 0.1 to 0.2 times that of the antiferromagnetic layer 3. The antiferromagnetic magnetization direction in the capping layer 7 is downward, consistent with the reference layer 4, making it easier for the free layer 6 to flip from the metastable state (upper) to the stable state (lower), thus achieving non-volatile storage.
[0050] The material of the top electrode 8 is the same as that of the bottom electrode 21.
[0051] like Figure 4 The diagram shown is a flowchart of the sensor's workflow. Figure 5 The sensor resistance-strain response trend graph, such as... Figure 6 The diagram shows the principle of magnetization direction reversal. The sensor described in this embodiment is initialized before operation by applying a strong magnetic field or writing an STT current to make the magnetization direction of the free layer 6 opposite to that of the reference layer 4 (bottom) (top), resulting in maximum resistance. R AP Tensile strain is then applied to the monitored surface.
[0052] In this embodiment, the stress induced in the free layer 6 does not exceed the critical stress σ. c When the magnetization direction tilts towards the tensile strain direction, deviating from the antiparallel direction, the tunneling probability increases, and the resistance continuously decreases. Secondly, the MgO barrier layer 5, as a crucial component for tunneling, controls the barrier width through its thickness, thus affecting the tunneling probability. Under tensile strain, the thickness of barrier layer 5 slightly decreases, reducing the tunnel junction resistance. At this point, the resistance value undergoes a continuous change.
[0053] When the tensile stress on free layer 6 exceeds the critical value σ c At this point, its magnetization direction overcomes the coercivity and is pulled into the plane, parallel to the stress direction. At this time, the tunnel junction resistance undergoes a sudden decrease, and this significantly contributes to the high GF value of the sensor.
[0054] When a bias magnetic field is applied to the sensor or a small STT current is written to promote the downward deflection of the magnetization direction of the free layer 6, after reaching the critical stress, the magnetization direction of the free layer 6 of the sensor will remain downward without strain, so as to achieve non-volatile storage function.
[0055] In particular, in this embodiment, sensors with opposite magnetization directions reference layers 4 are arranged in pairs, which can more conveniently and accurately read the magnetization direction and stress value of the free layer 6.
[0056] In addition, such as Figure 7 As shown, the sensor in this embodiment can be manufactured in the following way:
[0057] S1. A bottom electrode 21 is formed on a substrate 1 of Si / SiO2 or flexible material, wherein the bottom electrode is one of Cu, Ru, or Ta.
[0058] S2. In this embodiment, the various thin films of the sensor are sequentially fabricated on the bottom electrode 21. The buffer layer 22, the lattice template layer 23, the antiferromagnetic layer 3, and the reference layer 4 are fabricated using a magnetron sputtering process.
[0059] S3. MgO barrier layer 5 is processed by radio frequency sputtering to form a surface of sufficiently high quality.
[0060] S4. Next, magnetron sputtering is used to grow a free layer 6, a capping layer 7, and a top electrode 8. Specifically, magnetron sputtering is combined with substrate tilting and rotation to process the wedge-shaped free layer 6.
[0061] S5. After the sensor completes the growth of the material layer, it undergoes post-annealing treatment in the temperature range of 250-400℃ for half an hour to several hours. During the annealing process, an external magnetic field is strengthened along the expected magnetization direction.
[0062] S6. The annealed sensor is patterned using MEMS techniques such as ion beam etching to complete the strain sensor.
[0063] The above embodiments merely illustrate the implementation method of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. For those skilled in the art, various changes, modifications, and substitutions can be made without departing from the principles of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.
Claims
1. A non-volatile strain sensor based on a magnetic tunnel junction, characterized in that, The sensor comprises, starting from the substrate, a bottom electrode, an antiferromagnetic layer, a reference layer, a barrier layer, a free layer, a capping layer, and a top electrode, stacked sequentially. The sensor is characterized by its circular or rectangular shape. The layers from the antiferromagnetic layer to the free layer are used to measure strain and store critical stress events. The magnetization directions of both the reference layer and the free layer are perpendicular to the thin film. The magnetization direction of the reference layer is fixed, while the magnetization direction of the free layer affects the sensor resistance and stores critical stress events. Specifically, the sensor resistance is minimum when the magnetization direction of the free layer is aligned with that of the reference layer; maximum when their magnetization directions are antiparallel; moderate resistance when the free layer's magnetization direction is in-plane and perpendicular to the reference layer's magnetization direction; and moderate resistance when the magnetization direction is deflected and not pulled to the plane, resulting in a tunnel junction resistance greater than the in-plane resistance but less than the maximum resistance.
2. The non-volatile strain sensor based on a magnetic tunnel junction according to claim 1, characterized in that, When a tensile force is applied to the sensor, and the strain on the free layer increases but does not exceed the critical stress, the magnetization direction of the free layer tilts towards the tensile strain direction, and the resistance decreases. When the stress corresponding to the free layer exceeds the critical stress, the magnetization direction jumps to in-plane, and the resistance decreases and jumps. Specifically, applying a small bias magnetic field or introducing an STT current into the sensor guides the magnetization direction to jump 180°, improving sensitivity and enabling non-volatile storage.
3. A non-volatile strain sensor based on a magnetic tunnel junction according to claim 1, characterized in that, The barrier layer should be one of MgO(001) or h-BN. The sensor sensitivity and critical stress are adjusted by changing the thickness of the barrier layer or by applying a bias voltage.
4. A non-volatile strain sensor based on a magnetic tunnel junction according to claim 1, characterized in that, The reference layer and the free layer are Co 40 Fe 40 B 20 The cross-sectional shape of the free layer can be designed as a wedge to induce a deterministic reversal of the magnetization direction.
5. A non-volatile strain sensor based on a magnetic tunnel junction according to claim 1, characterized in that, Its antiferromagnetic layer is either IrMn or PtMn. Specifically, a synthetic antiferromagnetic layer is deposited on top of the antiferromagnetic layer to reduce the influence of stray fields.
6. A non-volatile strain sensor based on a magnetic tunnel junction according to claim 1, characterized in that, Its capping layer should be one of Ta, IrMn, and PtMn, and its bottom electrode and top electrode should be one of Cu, Ru, and Ta.
7. A non-volatile strain sensor based on a magnetic tunnel junction according to claim 1, characterized in that, By setting up a pair of sensors with reference layers having opposite magnetization directions, the magnetization direction and strain of the free layer can be quickly read.
8. A method for manufacturing a non-volatile strain sensor based on a magnetic tunnel junction, characterized in that, The method for manufacturing the strain sensor according to claim 1 is as follows: S1. Fabricate the bottom electrode on the substrate; S2. An antiferromagnetic layer and a reference layer are sequentially fabricated on the bottom electrode using a magnetron sputtering process; S3. The barrier layer is fabricated using radio frequency sputtering technology to form a surface of sufficiently high quality; S4. The free layer, capping layer and top electrode are processed sequentially using magnetron sputtering. S5. After the material layer growth is completed, post-annealing is carried out in the temperature range of 250-400℃ for half an hour to several hours. During the annealing process, an external strong magnetic field is applied along the expected magnetization direction. S6. The annealed sensor is patterned using MEMS techniques such as ion beam etching to complete the strain sensor.