Flexible strain sensor and method of making the same

Abstract

The present disclosure provides a flexible strain sensor and a preparation method thereof. The flexible strain sensor of the present disclosure comprises a substrate, a conductive layer and a packaging layer. The conductive layer is stacked by two or more dissimilar metal layers. Each dissimilar metal layer comprises a first metal layer and a second metal layer. The second metal layer is formed on the first metal layer and the thickness of the second metal layer is greater than that of the first metal layer. The first metal layer is formed by a first metal material and the second metal layer is formed by a second metal material. The ductility of the first metal material is lower than that of the second metal material. The flexible strain sensor of the present disclosure can greatly adjust the sensitivity and working range.

Description

Technical Field

[0001] This disclosure relates to a flexible strain sensor and a method for fabricating the same. Background Technology

[0002] Flexible strain sensors can be used to monitor the mechanical deformation of complex surfaces, such as the human body surface and the surface of soft robot arms. They also hold promise for collecting human motion and health information and constructing machine-human interfaces in intelligent scenarios, making them a highly anticipated type of flexible mechanical sensor. To adapt to scenarios with high deformation, such as the 60% strain range required for human elbow joint movement, the sensor needs to operate stably within a wide response range. Simultaneously, the sensor also needs high sensitivity to monitor detailed movements. Therefore, improving the sensor's operating range and sensitivity remains a key challenge for flexible strain sensors.

[0003] Flexible strain sensors mostly use resistance as the response quantity, and their conductive layer is the key response element. Depending on the differences in the mechanical properties of the conductive material or its structural differences, they will exhibit different resistance response characteristics during the strain process.

[0004] Crack-type strain sensors are a type of sensor that utilizes the increase in resistance caused by the formation of cracks in a conductive layer during strain. The conductive layer typically consists of a conductive thin film with a thickness ranging from nanometers to micrometers. This conductive film can include a nanometer-thick metal layer, nano-conductive materials (such as conductive carbon nanotubes, silver nanowires, and gold nanoparticles), etc. In the optimization of crack-type sensors, it has been found that it is difficult to simultaneously improve their operating range and sensitivity. When the crack exhibits a parallel through-type structure, the sensor displays high sensitivity (>10). 3 However, the strain range is low (<2%), while when the crack has a network structure, the sensor has a high response range (>100%), but the sensitivity is usually low (<10). To improve the sensor response characteristics, the mechanical properties of the conductive layer need to be controlled to induce controllable cracks.

[0005] Therefore, how to control the mechanical properties of the conductive layer to induce controllable cracks in order to improve sensitivity and slow down the decline of the sensor's operating range is a technical problem that urgently needs to be solved in the flexible strain sensor and its fabrication process. Summary of the Invention

[0006] To address at least one of the aforementioned technical problems, this disclosure provides a flexible strain sensor and a method for its fabrication.

[0007] According to a first aspect of this disclosure, a flexible strain sensor is provided, comprising: a substrate, a conductive layer, and an encapsulation layer, wherein the conductive layer is formed by stacking two or more dissimilar metal stacks, each of the dissimilar metal stacks comprising a first metal layer and a second metal layer, the second metal layer being formed on the first metal layer and having a thickness greater than that of the first metal layer, the first metal layer being formed of a first metal material, the second metal layer being formed of a second metal material, and the ductility of the first metal material being lower than that of the second metal material.

[0008] In one implementation of the first aspect of this disclosure, the first metal layer in different dissimilar metal stacks uses the same first metal material; and / or, the second metal layer in different dissimilar metal stacks uses the same second metal material.

[0009] In one implementation of the first aspect of this disclosure, the first metal layer in different dissimilar metal stacks uses a different first metal material; and / or, the second metal layer in different dissimilar metal stacks uses a different second metal material.

[0010] In one implementation of the first aspect of this disclosure, the thickness of the first metal layer is different in different dissimilar metal stacks; and / or, the thickness of the second metal layer is different in different dissimilar metal stacks.

[0011] In one implementation of the first aspect of this disclosure, the thickness of the first metal layer is the same in different dissimilar metal stacks; and / or, the thickness of the second metal layer is the same in different dissimilar metal stacks.

[0012] In one implementation of the first aspect of this disclosure, the total thickness of the conductive layer is between 80 and 200 nm; and / or, the single-layer thickness of the first metal layer in the dissimilar metal stack is between 5 and 20 nm, and the total thickness of the second metal layer is 2 to 10 times the total thickness of the first metal layer.

[0013] In one implementation of the first aspect of this disclosure, the thickness difference between the total thickness of the second metal layer and the total thickness of the first metal layer in the dissimilar metal stack is greater than or equal to a first predetermined value; and / or, the sum of the thicknesses of the first metal layers of all dissimilar metal stacks in the conductive layer is less than or equal to a second predetermined value; and / or, the sum of the thicknesses of the second metal layers of all dissimilar metal stacks in the conductive layer is less than or equal to a third predetermined value.

[0014] In one implementation of the first aspect of this disclosure, the reduction of area RA% of the second metallic material is higher than or equal to a fourth predetermined value; and / or, the RA% of the first metallic material is lower than or equal to a fifth predetermined value.

[0015] In one implementation of the first aspect of this disclosure, the first metallic material is a brittle metallic material with strong adhesion.

[0016] In one implementation of the first aspect of this disclosure, the first metallic material is a brittle metallic material, which includes one or more of the following: Ti, Ni, Cr, W; and / or, the second metallic material is a ductile metallic material, which includes one or more of the following: Au, Pd, Pt, Ag, Al, Cu.

[0017] In one implementation of the first aspect of this disclosure, the first metal material in the dissimilar metal stack is Ti and the second metal material is Au.

[0018] In one implementation of the first aspect of this disclosure, the first metal material in the dissimilar metal stack is Cr and the second metal material is Cu.

[0019] According to a second aspect of this disclosure, a method for fabricating a flexible strain sensor is provided, comprising:

[0020] Substrate preparation;

[0021] After surface treatment of the substrate to enrich the substrate surface with hydroxyl groups, two or more metal thin film depositions are performed on the substrate to prepare a conductive layer. The conductive layer is a stacked structure containing two or more dissimilar metal stacks. Each dissimilar metal stack includes a first metal layer and a second metal layer. The second metal layer is located on the first metal layer and the thickness of the second metal layer is greater than the thickness of the first metal layer. The first metal layer is formed of a first metal material, and the second metal layer is formed of a second metal material. The ductility of the first metal material is lower than that of the second metal material.

[0022] An encapsulation layer is formed on the surface of the conductive layer.

[0023] In one implementation of the second aspect of this disclosure, an oxygen plasma cleaning process is used to treat the substrate surface to enrich the substrate surface with hydroxyl groups.

[0024] In one implementation of the second aspect of this disclosure, a conductive layer is prepared by performing two or more metal thin film depositions on a substrate using physical vapor deposition.

[0025] In one implementation of the second aspect of this disclosure, a conductive layer is prepared by performing two or more metal thin film evaporations on a substrate using one of the following methods: electron beam evaporation deposition, magnetron sputtering, or thermal evaporation deposition.

[0026] The flexible strain sensor disclosed herein uses a structure of alternating stacks of brittle and ductile metal materials as the conductive layer. By selecting the number of dissimilar metal stacks, different metal materials, and adjusting the thickness of the brittle and ductile metals, the sensor's sensitivity and operating range can be significantly adjusted to meet the different needs of various application scenarios. Attached Figure Description

[0027] The accompanying drawings illustrate exemplary embodiments of the present disclosure and, together with the description thereof, serve to explain the principles of the present disclosure. These drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification.

[0028] Figure 1 This is a structural schematic diagram of a flexible strain sensor according to some embodiments of the present disclosure.

[0029] Figure 2 This is a schematic diagram of the structure of the conductive layer in a flexible strain sensor according to some embodiments of the present disclosure.

[0030] Figure 3 This is a schematic flowchart of a method for fabricating a flexible strain sensor according to some embodiments of the present disclosure.

[0031] Explanation of reference numerals in the attached figures

[0032] 100 Flexible Strain Sensor

[0033] 110 substrate

[0034] 120 conductive layer

[0035] 121 Dissimilar metal stacks

[0036] 1211 First Metal Layer

[0037] 1212 Second Metal Layer

[0038] 130 encapsulation layer Detailed Implementation

[0039] The present disclosure will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the disclosure. Furthermore, it should be noted that, for ease of description, only the parts relevant to the present disclosure are shown in the accompanying drawings.

[0040] It should be noted that, where there is no conflict, the embodiments and features described in this disclosure can be combined with each other. The technical solutions of this disclosure will now be described in detail with reference to the accompanying drawings and embodiments.

[0041] Unless otherwise stated, the exemplary implementations / embodiments shown are to be understood as providing exemplary features of various details that provide ways in which the technical concepts of this disclosure can be implemented in practice. Therefore, unless otherwise stated, the features of various implementations / embodiments may be additionally combined, separated, interchanged and / or rearranged without departing from the technical concepts of this disclosure.

[0042] The use of crosshairs and / or shading in the accompanying drawings is generally used to clarify the boundaries between adjacent components. Thus, unless otherwise stated, the presence or absence of crosshairs or shading does not convey or indicate any preference or requirement for the specific material, material properties, dimensions, proportions, commonalities between the illustrated components, or any other characteristics, properties, etc., of the components. Furthermore, in the accompanying drawings, the dimensions and relative dimensions of components may be exaggerated for clarity and / or descriptive purposes. When exemplary embodiments can be implemented differently, a specific process sequence may be performed in a different order than that described. For example, two consecutively described processes may be performed substantially simultaneously or in the reverse order of their description. Furthermore, the same reference numerals denote the same components.

[0043] When a component is referred to as being "on" or "above" another component, "connected to," or "joined to" another component, the component may be directly on, directly connected to, or directly joined to the other component, or there may be intermediate components. However, when a component is referred to as being "directly on" another component, "directly connected to," or "directly joined to" another component, there are no intermediate components. Therefore, the term "connection" can refer to a physical connection, an electrical connection, etc., and may or may not have intermediate components.

[0044] For descriptive purposes, this disclosure may use spatial relative terms such as “below,” “under,” “below,” “down,” “above,” “above,” “higher,” and “side (e.g., in a “sidewall”)” to describe the relationship between one component and another component as shown in the accompanying drawings. In addition to the orientations depicted in the drawings, the spatial relative terms are also intended to encompass different orientations of the device during use, operation, and / or manufacture. For example, if the device in the drawings is flipped, a component described as “below” or “under” another component or feature would subsequently be positioned “above” said other component or feature. Thus, the exemplary term “below” can encompass both “above” and “below” orientations. Furthermore, the device may be otherwise positioned (e.g., rotated 90 degrees or in other orientations), thus interpreting the spatial relative descriptive terms used herein accordingly.

[0045] The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used herein, unless the context clearly indicates otherwise, the singular forms “a” and “the” are intended to include the plural forms as well. Furthermore, when the terms “comprising” and / or “including” and variations thereof are used in this specification, it indicates the presence of the stated features, integrals, steps, operations, parts, components, and / or groups thereof, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, parts, components, and / or groups thereof. It should also be noted that, as used herein, the terms “substantially,” “about,” and other similar terms are used as approximate terms rather than as terms of degree, thus explaining the inherent biases in measurements, calculated values, and / or provided values ​​that would be recognized by one of ordinary skill in the art.

[0046] As mentioned earlier, to improve the sensor's response characteristics, it is necessary to control the mechanical properties of the conductive layer to induce controllable cracks. Currently, how to control the mechanical properties of the conductive layer to induce controllable cracks, thereby improving sensitivity and slowing down the degradation of the sensor's operating range, is a pressing technical problem to be solved in flexible strain sensors and their fabrication.

[0047] Typically, the choice of conductive layer material has a decisive influence on the fracture characteristics of the conductive layer in flexible strain sensors. However, the fabrication process of nanomaterials is complex, and the molding process on the surface of flexible substrates is limited by the low surface energy of polymers, resulting in low interfacial strength between the polymer and the substrate. This increases the difficulty of film formation, leading to complex processes, and also makes the conductive layer prone to detachment from the substrate during large strains or cyclic stretching, causing device failure. In addition, most nanomaterials need to be cured after contacting the substrate in solution form, but organic solvents can easily cause swelling of the flexible substrate, and it is difficult to achieve electrode patterning through photolithography, often requiring additional transfer processes. In contrast, physical vapor deposition (PVD) for preparing metal thin films is a stable process, and the film material and thickness are controllable. The control of electrode patterns and linewidths can be achieved by covering the substrate with a perforated mask, thereby reducing the limitations when choosing a flexible substrate. However, current related technologies and research mainly focus on improving the adhesion between the substrate and the conductive layer interface, and the impact of different metal material thicknesses on sensor sensitivity and operating range, while research on multilayer metal thin film stacking is lacking.

[0048] In view of this, the present disclosure provides the following flexible strain sensor and its fabrication method. The conductive layer of the flexible strain sensor adopts a multilayer metal thin film stack structure combining brittle metal and ductile metal. By selecting the number of layers of dissimilar metal stacks, selecting the materials of brittle metal and ductile metal, and controlling the thickness of each metal thin film, the mechanical properties of the conductive layer can be effectively controlled, inducing controllable cracks, thereby improving sensitivity and slowing down the degradation of the sensor's working range.

[0049] Figure 1 This is a schematic diagram of the structure of a flexible strain sensor 100 according to one embodiment of this disclosure. Figure 1 As shown, the flexible strain sensor 100 of this embodiment may include: a substrate 110, a conductive layer 120 and an encapsulation layer 130, wherein the conductive layer 120 is formed on the substrate 110 and the encapsulation layer 130 is formed on the conductive layer 120.

[0050] Figure 2 A schematic diagram of the conductive layer 120 in a flexible strain sensor 100 according to some embodiments of this disclosure is shown. For example... Figure 2 As shown, the conductive layer 120 is formed by stacking two or more dissimilar metal stacks 121. That is, the conductive layer 120 may contain two or more dissimilar metal stacks 121, which are stacked in layers.

[0051] like Figure 2 As shown, each dissimilar metal stack 121 includes a first metal layer 1211 and a second metal layer 1212. The second metal layer 1212 is formed on the first metal layer 1211, and the thickness of the second metal layer 1212 is greater than the thickness of the first metal layer 1211. The first metal layer 1211 is formed of a first metal material, and the second metal layer 1212 is formed of a second metal material. The ductility of the first metal material is lower than that of the second metal material. In other words, in each dissimilar metal stack 121, the first metal material used in the first metal layer 1211 can be a brittle metal material, and the second metal material used in the second metal layer 1212 can be a ductile metal material. Thus, brittle metal material A and ductile metal material B can be coupled in the order "A1-B1-…-A n -B n The conductive layer 120 is formed by stacking a “-shaped structure. The second metal layer 1212, made of ductile metal material B, is used as the main conductive layer, which has high conductivity and high ductility. The first metal layer 1211, made of brittle metal material A, has high brittleness and can induce crack initiation and propagation. By controlling the selection and thickness of these two materials layer by layer, the mechanical properties of the conductive layer 120 can be adjusted, thereby controlling crack growth within a large response range and obtaining a flexible strain sensor 100 with high sensitivity and a large working range.

[0052] With a fixed total thickness of the conductive layer 120, increasing the thickness of the second metal layer 1212 can suppress crack propagation, giving the flexible strain sensor 100 a wider operating range. Conversely, increasing the thickness of the first metal layer 1211 can accelerate crack propagation, resulting in higher sensitivity for the flexible strain sensor 100. However, it has been verified that further increases in the thickness of the first metal layer 1211 beyond a critical thickness can rapidly reduce the operating range of the flexible strain sensor 100. Furthermore, different brittle metal materials have different critical thicknesses. In specific applications, the crack morphology of the conductive layer 120 under strain can be controlled by adjusting the number of dissimilar metal stacks 121 in the conductive layer 120, the material selection of the first metal layer 1211 and the second metal layer 1212 in each dissimilar metal stack 121, and their film thickness, thereby adjusting the sensitivity and operating range of the flexible strain sensor 100.

[0053] Verification revealed that when the thickness of the first metal layer 1211 and the thickness of the second metal layer 1212 are fixed, compared to a conductive layer 120 containing only one dissimilar metal stack 121, a conductive layer 120 with a stacked structure of x (x = 2, 3, 4, ..., n, where n is a predetermined value) dissimilar metal stack 121 can transmit the crack in the first metal layer 1211 to the second metal layer 1212 more quickly, thereby significantly improving the sensitivity of the flexible strain sensor 100.

[0054] In practical applications, the mechanical properties of the conductive layer 120 can be controlled by adjusting the number of dissimilar metal stacks 121, the material selection of the first metal layer 1211 and the second metal layer 1212, the relative thickness ratio of the first metal layer 1211 and the second metal layer 1212, and the total thickness of each dissimilar metal stack 121, thereby obtaining a flexible strain sensor 100 that meets actual needs. For example, when higher stretchability is required, the ratio of the second metal layer 1212 to the first metal layer 1211 can be increased. That is, the stretchability of the conductive layer 120 can be improved by increasing the thickness difference between the second metal layer 1212 and the first metal layer 1211, thereby improving the sensitivity of the flexible strain sensor 100.

[0055] In some implementations, in order to keep the flexible strain sensor 100 in a high operating range, the thickness of the second metal layer 1212 can be much greater than the thickness of the first metal layer 1211.

[0056] In some embodiments, the total thickness of the conductive layer 120 is preferably between 80 and 200 nm. For example, the thickness of the conductive layer 120 can preferably be 80 nm, 90 nm, 100 nm, 200 nm or other values. In this case, better sensitivity can be obtained and a higher working range can be maintained. At the same time, it is easier to form a uniform film during preparation.

[0057] Considering that a metal layer that is too thin may not be easy to form a uniform film, while a metal layer that is too thick may reduce the operating range of the device, in some embodiments of this disclosure, the total thickness and / or single-layer thickness of the first metal layer 1211 is preferably between 5 nm and 20 nm, and the total thickness and / or single-layer thickness of the second metal layer 1212 is preferably between 2 and 10 times the total thickness of the first metal layer 1211. This not only achieves better sensitivity and maintains a higher operating range, but also facilitates uniform film formation during fabrication.

[0058] In some embodiments, the thickness difference between the total thickness of the second metal layer 1212 and the total thickness of the first metal layer 1211 in the dissimilar metal stack 121 can be greater than or equal to a first predetermined value. The first predetermined value can be an empirical value. For example, the first predetermined value can be, but is not limited to, 70nm, 80nm, 100nm or other values, and the specific value of the first predetermined value can be different in different scenarios. In this way, by controlling the thickness difference between the second metal layer and the first metal layer in the dissimilar metal stack, the mechanical properties of the conductive layer can also be adjusted, thereby obtaining a flexible strain sensor 100 with high sensitivity and maintaining a wide operating range.

[0059] In some embodiments, the thickness of the first metal layer 1211 of all dissimilar metal stacks 121 in the conductive layer 120 can be less than or equal to a second predetermined value. For example, based on experience, when the conductive layer 120 includes two or more dissimilar metal stacks 121, the thickness of the first metal layer 1211 of all dissimilar metal stacks 121 can be less than or equal to 15 nm or 20 nm. That is, the second predetermined value can be, but is not limited to, 15 nm, 20 nm or other values. In this way, the flexible strain sensor 100 can have a relatively large control range while ensuring easy film formation.

[0060] In some embodiments, the sum of the thicknesses of the second metal layers 1212 of all dissimilar metal stacks 121 in the conductive layer 120 may be less than or equal to a third predetermined value. For example, based on experience, when the conductive layer 120 includes two or more dissimilar metal stacks 121, the sum of the thicknesses of the first metal layers 1211 of all dissimilar metal stacks 121 may be less than or equal to 180nm, 100nm, 80nm, or 50nm. That is, the third predetermined value may be, but is not limited to, 180nm, 100nm, 80nm, 50nm, or other values. In this way, the flexible strain sensor 100 can have a relatively large control range while ensuring easy film formation.

[0061] For example, when Ti is selected as the first metal material and Au is selected as the second metal material, the single-layer thickness of the first metal layer 1211 in all dissimilar metal stacks 121 can be controlled between 5nm and 20nm, and the total thickness of the second metal layer 1212 in all dissimilar metal stacks 121 can be controlled between 70nm and 180nm. In this case, the flexible strain sensor 100 adopts the "A1-B1-...-A" configuration. n -B n The sensitivity will be significantly increased when using a "-shaped" stacked structure.

[0062] The ductility of metallic materials can be characterized by their reduction of area (RA%). A higher RA% indicates better ductility, while a lower RA% indicates poorer ductility. In other words, RA% can be used to select suitable metallic materials as the primary and secondary metallic materials.

[0063] In some embodiments, the second metallic material is a ductile metallic material, which may be, but is not limited to, a metallic material whose RA% is higher than or equal to a fourth predetermined value; and / or, the first metallic material is a brittle metallic material, which may be, but is not limited to, a metallic material whose RA% is lower than or equal to a fifth predetermined value. The fourth and fifth predetermined values ​​are empirical values ​​and can be flexibly determined according to the specific application scenario's requirements for the sensor's sensitivity and operating range. For example, the fourth predetermined value may be the RA% of Cu in Table 1 below, i.e., 0.62. The fifth predetermined value may be the RA% of Ti in Table 1 below, i.e., 0.54. In this case, the brittle metallic material may include one or more of the following: Ti, Ni, Cr, W; the ductile metallic material may include one or more of the following: Au, Pd, Pt, Ag, Al, Cu.

[0064] Since the second metal layer 1212 needs to be formed on the first metal layer 1211, the first metal material in this embodiment is preferably a brittle metal material with strong adhesion, such as Cr, Ti, Ni, or W. In this way, the adhesion between the second metal layer 1212 and the substrate can be enhanced by the first metal layer 1211, thereby effectively preventing phenomena such as detachment.

[0065] Table 1 shows the RA% of various metallic materials listed from ductile to brittle. The first and second metallic materials can be selected from these metallic materials. Among them, metals with higher RA% such as Au, Pd, Pt, Ag, Al, and Cu can be considered ductile metallic materials and can be used as second metallic materials; metals with lower RA% such as Ti, Ni, Cr, and W can be considered brittle metallic materials and can be used as first metallic materials.

[0066] Gold (Au) 0.93 Palladium (Pd) 0.80 Platinum (Pt) 0.76 Silver (Ag) 0.73 Aluminum (Al) 0.65 Copper (Cu) 0.62 Titanium (Ti) 0.54 Nickel (Ni) 0.50 Tungsten (W) 0.40 Chromium (Cr) 0.18

[0067] Table 1

[0068] It should be noted that the ductility of the first metal material and the second metal material in this embodiment is relative. As long as the ductility (e.g., RA%) of the second metal material forming the second metal layer is higher than that of the first metal material forming the first metal layer, "A1-B1-...-A" can be formed. n -B n Conductive layers with a stacked structure.

[0069] In some embodiments, when the conductive layer 120 includes two or more dissimilar metal stacks 121, the first metal material used in the first metal layer 1211 of the different dissimilar metal stacks 121 may be the same.

[0070] In some embodiments, the second metal material used in the second metal layer 1212 of different dissimilar metal stacks 121 can be the same.

[0071] In some embodiments, when the conductive layer 120 includes two or more dissimilar metal stacks 121, the first metal material used in the first metal layer 1211 of the different dissimilar metal stacks 121 may be different.

[0072] In some embodiments, the second metal material used in the second metal layer 1212 of different dissimilar metal stacks 121 may be different.

[0073] In some embodiments, when the conductive layer 120 includes two or more dissimilar metal stacks 121, the thickness of the first metal layer 1211 in the different dissimilar metal stacks 121 may be different.

[0074] In some embodiments, the thickness of the second metal layer 1212 in different dissimilar metal stacks 121 may be different.

[0075] In some embodiments, when the conductive layer 120 includes two or more dissimilar metal stacks 121, the thickness of the first metal layer 1211 in different dissimilar metal stacks 121 may be the same.

[0076] In some embodiments, the thickness of the second metal layer 1212 in different dissimilar metal stacks 121 can be the same.

[0077] For example, when the conductive layer 120 includes two dissimilar metal stacks 121, the first metal layer 1211 in the bottom layer of the first dissimilar metal stack 121 uses Ti as the first metal material, and the first metal layer 1211 in the top layer of the second dissimilar metal stack 121 can also use Ti as the first metal material. The second metal layer 1212 in the first dissimilar metal stack 121 can use Au as the second metal material, and the second metal layer 1212 in the second dissimilar metal stack 121 can also use Au as the second metal material.

[0078] For example, when the conductive layer 120 includes two dissimilar metal stacks 121, the first metal layer 1211 in the bottom layer of the first dissimilar metal stack 121 may use Ti as the first metal material, the first metal layer 1211 in the top layer of the second dissimilar metal stack 121 may use Cr as the first metal material, the second metal layer 1212 in the first dissimilar metal stack 121 may use Au as the second metal material, and the second metal layer 1212 in the second dissimilar metal stack 121 may use Cu as the second metal material.

[0079] For example, when the conductive layer 120 includes three dissimilar metal stacks 121, the first metal layer 1211 in the bottom layer of the first dissimilar metal stack 121 may use Ti as the first metal material, the first metal layer 1211 in the middle layer of the second dissimilar metal stack 121 may also use Ti as the first metal material, the first metal layer 1211 in the top layer of the third dissimilar metal stack 121 may use Cr as the first metal material, and the second metal layer 1212 in the first dissimilar metal stack 121, the second dissimilar metal stack 121 and the third dissimilar metal stack 121 may use Au, Au and Cu respectively as the second metal materials.

[0080] Table 2 shows the stacking structure, metal material, and thickness values ​​of the conductive layer 120 in several preferred sample examples of the flexible strain sensor 100 according to embodiments of the present disclosure.

[0081]

[0082] Table 2

[0083] Those skilled in the art should understand that the number of dissimilar metal stacks 121 in the conductive layer 120, and the materials and thicknesses of the first and second metal layers in each dissimilar metal stack 121, can be flexibly adjusted according to the requirements of the flexible strain sensor 100 for sensitivity, operating range, etc. in the actual application scenario. This disclosure does not impose any limitations on this.

[0084] The substrate 110 is a flexible material. For example, the substrate 110 may be made of, but is not limited to, one of the following materials: polydimethylsiloxane (PDMS), Ecoflex, Dragon skin, hydrogenated styrene-butadiene-styrene block copolymer (SEBS), polyurethane (PU), or other flexible substrates.

[0085] The encapsulation layer 130 can also be a flexible material. For example, the encapsulation layer 130 can be made of the same flexible material as the substrate, that is, the encapsulation layer 130 can be made of, but is not limited to, one of the following materials: polydimethylsiloxane (PDMS), Ecoflex, Dragon skin, hydrogenated styrene-butadiene-styrene block copolymer (SEBS), polyurethane (PU), or other flexible materials.

[0086] The flexible strain sensor 100 of this disclosure uses a stacked structure of brittle metal and ductile metal as the conductive layer 120. By selecting the number of dissimilar metal stacks, different metal materials, and adjusting the thickness of the brittle metal and the ductile metal, the sensor sensitivity and working range can be greatly adjusted, thereby meeting the different needs of various application scenarios.

[0087] Figure 3 A schematic flowchart illustrating a method for fabricating a flexible strain sensor according to an embodiment of this disclosure is shown. Figure 3 As shown, the method for fabricating a flexible strain sensor according to an embodiment of this disclosure may include the following steps S302 to S306:

[0088] Step S302: Prepare the substrate;

[0089] Specifically, flexible thin-film substrates can be prepared using, but are not limited to, one of the following materials: polydimethylsiloxane (PDMS), Ecoflex, Dragon Skin, styrene-butylene-styrene block copolymer (SEBS), polyurethane (PU), or other flexible substrates.

[0090] Step S304: After surface treatment of the substrate to enrich the substrate surface with hydroxyl groups (-OH), two or more metal thin film depositions are performed on the substrate to prepare a conductive layer. The conductive layer is a stacked structure containing two or more dissimilar metal stacks. Each dissimilar metal stack includes a first metal layer and a second metal layer. The second metal layer is located on the first metal layer and the thickness of the second metal layer is greater than the thickness of the first metal layer. The first metal layer is formed of a first metal material, and the second metal layer is formed of a second metal material. The ductility of the first metal material is lower than that of the second metal material.

[0091] In some embodiments, the surface treatment of the substrate can be, but is not limited to, oxygen plasma cleaning (O2 Plasma) or other surface treatment methods. A suitable cleaning method can be selected based on the flexible material used in the substrate. For example, oxygen plasma cleaning can be used for silicone substrates such as PDMS and Ecoflex. Other flexible substrates can have their surface energy increased through other suitable surface treatment methods.

[0092] During the surface treatment of a substrate, the degree of hydroxyl enrichment on the substrate surface can be determined by a contact angle test. The higher the degree of hydroxyl enrichment, the stronger the hydrophilicity, and the smaller the contact angle. When the degree of hydroxyl enrichment on the substrate surface meets the requirements determined by the contact angle test, the surface treatment step is considered complete.

[0093] For example, when the substrate is a PDMS thin film, the substrate surface can be cleaned for 2 minutes at 250W under Faraday cage protection using oxygen plasma cleaning (O2Plasma) to enrich the substrate surface with hydroxyl groups to increase its surface energy. Subsequently, a metal thin film can be deposited on the substrate surface immediately (e.g., within 10 minutes) to prepare a conductive layer.

[0094] The substrate surface prepared in step S302 has a submicron-scale microstructure. In step S304, a conductive layer can preferably be prepared by conformal deposition (i.e., evaporation deposition) of a metal thin film. Conformal deposition of the metal thin film on the microstructure surface enables the metal thin film to have a special three-dimensional structure, which disperses the stress at the crack tip and prevents the rapid propagation of cracks that could lead to electrode failure under low strain.

[0095] Conformal deposition involves depositing a metal thin film approximately perpendicular to the substrate. When the substrate has uneven surfaces, the deposited metal film follows these undulations rather than flattening the structure. Thin films deposited via physical vapor deposition typically exhibit conformal deposition characteristics. Therefore, in some embodiments, step S304 may involve two or more metal thin film depositions on the substrate using physical vapor deposition to prepare a conductive layer.

[0096] The embodiments disclosed herein employ metal thin film evaporation to efficiently prepare the conductive layer of the flexible strain sensor, avoiding the difficulty of film formation on low-energy surfaces of flexible substrates in liquid phase deposition processes. At the same time, it fully utilizes the advantages of this technology, such as stable environment, replaceable materials, and adjustable thickness, and can achieve significant adjustments to the device sensitivity and operating range without the need for additional equipment or intermediate processes, thereby meeting the needs of different application scenarios.

[0097] Specifically, the preparation process of the conductive layer in step S304 can be as follows: after cleaning the substrate, firstly, a first metal material is deposited on the substrate surface as the first metal layer of the bottom dissimilar metal stack, and then a second metal material is deposited on the first metal layer of the bottom dissimilar metal stack to form the second metal layer of the bottom dissimilar metal stack. This process is repeated layer by layer until the second metal layer of the top dissimilar metal stack is deposited.

[0098] For example, if the conductive layer to be prepared contains two dissimilar metal stacks, the first metal layer of the bottom dissimilar metal stack is Ti and the second metal layer is Au, and the first metal layer of the top dissimilar metal stack is Cr and the second metal layer is Cu, the formation process of the conductive layer can be as follows: first, a Ti thin film is deposited on the substrate surface as the first metal layer of the first dissimilar metal stack; then, an Au thin film is deposited on the first metal layer of the first dissimilar metal stack as the second metal layer of the first dissimilar metal stack; then, a Cr thin film is deposited on the second metal layer of the first dissimilar metal stack as the first metal layer of the second dissimilar metal stack; finally, a Cu thin film is deposited on the Cr layer of the first metal layer of the second dissimilar metal stack as the second metal layer of the second dissimilar metal stack.

[0099] In some embodiments, when preparing the conductive layer, various metal layers can be formed by vapor deposition using, but not limited to, electron beam evaporation, magnetron sputtering, thermal evaporation, or other physical vapor deposition techniques.

[0100] In the process of preparing conductive layers using electron beam evaporation deposition, an electron beam thermal evaporation apparatus can be used to deposit various metal layers. For example, if the structure of the conductive layer is Ti / Au, the evaporation parameters can be set as follows: evaporation rate of the Ti layer... The deposition rate of the Au layer is In this structure, the bottom Ti layer also serves as an adhesion layer, improving the adhesion between the Au layer and the substrate. The Au layer, as the main conductive layer, has high conductivity and high ductility, while the Ti layer is highly brittle and can induce crack initiation and propagation.

[0101] Step S306: Form an encapsulation layer on the surface of the conductive layer.

[0102] Specifically, a flexible material such as PDMS can be spin-coated onto the conductive layer surface and cured as an encapsulation layer. The encapsulation layer prevents the flexible strain sensor from failing due to surface abrasion of the conductive layer, while also slowing down the process of the conductive layer detaching from the substrate, thus improving the robustness of the flexible strain sensor. Table 3 shows the flexible strain sensor samples prepared by the above method and their parameters such as sensitivity and operating range.

[0103]

[0104] Table 3

[0105] The sensitivity of the strain sensor is characterized by GF (Gauge Factor), where GF = ΔR / (R0ε), ΔR is the difference between the current resistance and the initial resistance R0 of the flexible strain sensor, and ε is the current strain of the flexible strain sensor.

[0106] In Table 3, the conductive layers in samples 1 and 2 each contain only one dissimilar metal stack, with Ti as the first metal layer and Au as the second metal layer. As can be seen from Table 3, sample 1 has lower sensitivity due to its thinner conductive layer (less than 80 nm). In contrast, samples 2, 3, and 4, with conductive layers of 90 nm, show significantly improved sensitivity compared to sample 2 (using a single dissimilar metal film) and existing flexible strain sensors using related technologies.

[0107] In Table 3, the conductive layers of samples 3 and 4 each contain only two dissimilar metal stacks. In the lower dissimilar metal stack, the first metal layer is Ti and the second metal layer is Au, and in the upper dissimilar metal stack, the first metal layer is also Ti and the second metal layer is also Au. As can be seen from Table 3, the sensitivity of samples 3 and 4 is much higher than that of samples 1, 2, and existing flexible strain sensors in related technologies. Although their operating range is slightly reduced, the operating range of sample 3 can still meet the needs of most health monitoring (body surface monitoring generally requires 0-60%).

[0108] Furthermore, a comparison between Sample 3 and Sample 4 shows that by adjusting the thickness of each metal layer in different dissimilar metal stacks, a suitable metal stack thickness (such as Sample 3) can significantly slow down the degradation of the sensor's operating range while improving sensitivity.

[0109] In the description of this specification, the references to terms such as "one embodiment / mode," "some embodiments / modes," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment / mode or example is included in at least one embodiment / mode or example of this disclosure. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment / mode or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments / modes or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments / modes or examples described in this specification, as well as the features of different embodiments / modes or examples.

[0110] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this disclosure, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0111] Those skilled in the art should understand that the above embodiments are merely for illustrating the present disclosure and are not intended to limit the scope of the disclosure. Those skilled in the art can make other changes or modifications based on the above disclosure, and these changes or modifications still fall within the scope of the present disclosure.

Claims

1. A flexible strain sensor, comprising: A substrate, a conductive layer, and a packaging layer, characterized in that the conductive layer is formed by stacking two or more dissimilar metal stacks, each of the dissimilar metal stacks including a first metal layer and a second metal layer, the second metal layer being formed on the first metal layer and having a thickness greater than that of the first metal layer, the first metal layer being formed of a first metal material, the second metal layer being formed of a second metal material, and the ductility of the first metal material being lower than that of the second metal material; The total thickness of the conductive layer is between 80 and 200 nm. The thickness of the first metal layer in the dissimilar metal stack is between 5 and 20 nm, and the total thickness of the second metal layer is 2 to 10 times the total thickness of the first metal layer. The first metallic material is a brittle metallic material with strong adhesion.

2. The flexible strain sensor according to claim 1, characterized in that, In different dissimilar metal stacks, the first metal layer uses the same first metal material; and / or, in different dissimilar metal stacks, the second metal layer uses the same second metal material.

3. The flexible strain sensor according to claim 1, characterized in that, In different dissimilar metal stacks, the first metal layer uses a different first metal material; and / or, in different dissimilar metal stacks, the second metal layer uses a different second metal material.

4. The flexible strain sensor according to claim 1, characterized in that, The thickness of the first metal layer varies in different dissimilar metal stacks; and / or, the thickness of the second metal layer varies in different dissimilar metal stacks.

5. The flexible strain sensor according to claim 1, characterized in that, In different dissimilar metal stacks, the thickness of the first metal layer is the same; and / or, in different dissimilar metal stacks, the thickness of the second metal layer is the same.

6. The flexible strain sensor according to claim 1, characterized in that, The thickness difference between the total thickness of the second metal layer and the total thickness of the first metal layer in the dissimilar metal stack is greater than or equal to a first predetermined value; and / or, The thickness of the first metal layer in all dissimilar metal stacks in the conductive layer is less than or equal to a second predetermined value; and / or, The sum of the thicknesses of the second metal layers of all dissimilar metal stacks in the conductive layer is less than or equal to a third predetermined value.

7. The flexible strain sensor according to claim 1, characterized in that, The reduction of area RA% of the second metal material is higher than or equal to a fourth predetermined value; and / or, the RA% of the first metal material is lower than or equal to a fifth predetermined value.

8. The flexible strain sensor according to claim 1, characterized in that, The first metallic material is a brittle metallic material, which includes one or more of the following: Ti, Ni, Cr, W; and / or, The second metallic material is a ductile metallic material, which includes one or more of the following: Au, Pd, Pt, Ag, Al, Cu.

9. The flexible strain sensor according to claim 1, characterized in that, The first metal material in the dissimilar metal stack is Ti, and the second metal material is Au.

10. The flexible strain sensor according to claim 1, characterized in that, The first metal material in the dissimilar metal stack is Cr and the second metal material is Cu.

11. A method for fabricating a flexible strain sensor, used to fabricate the flexible strain sensor according to any one of claims 1 to 10, characterized in that, The preparation method includes: Substrate preparation; After surface treatment of the substrate to enrich the substrate surface with hydroxyl groups, two or more metal thin film depositions are performed on the substrate to prepare a conductive layer. The conductive layer is a stacked structure containing two or more dissimilar metal stacks. Each dissimilar metal stack includes a first metal layer and a second metal layer. The second metal layer is located on the first metal layer and the thickness of the second metal layer is greater than the thickness of the first metal layer. The first metal layer is formed of a first metal material, and the second metal layer is formed of a second metal material. The ductility of the first metal material is lower than that of the second metal material. An encapsulation layer is formed on the surface of the conductive layer.

12. The method for fabricating the flexible strain sensor according to claim 11, characterized in that, The substrate is surface treated using oxygen plasma cleaning to enrich the substrate surface with hydroxyl groups.

13. The method for fabricating the flexible strain sensor according to claim 11, characterized in that, A conductive layer is prepared by performing two or more metal thin film depositions on a substrate using physical vapor deposition.

14. The method for fabricating the flexible strain sensor according to claim 11 or 13, characterized in that, A conductive layer is prepared by performing two or more metal thin film evaporations on a substrate using one of the following methods: electron beam evaporation deposition, magnetron sputtering, or thermal evaporation deposition.

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