Strain sensor based on vertically aligned carbon nanotubes

Abstract

A method of manufacturing a strain sensor is provided. The method includes growing a patterned thin catalytic layer of iron (Fe) on a top surface of a silicon isolation layer formed on a top surface of a silicon wafer; synthesizing a plurality of vertically aligned carbon nanotubes (VACNTs) on a top surface of the thin catalytic layer of iron (Fe) to form electrodes of the strain sensor; forming a first polydimethylsiloxane (PDMS) layer disposed on and between adjacent VACNTs of the plurality of VACNTs; peeling the first PDMS layer and the plurality of VACNTs embedded in the first PDMS layer from the top surface of the silicon isolation layer; and forming a second PDMS layer on a bottom surface of the plurality of VACNTs embedded in the first PDMS layer.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of U.S. Provisional Application Serial No. 63 / 257,128, filed October 19, 2021, the entire contents of which (including any forms, figures or drawings) are incorporated herein by reference. Background Technology

[0003] In recent years, the rapid development of wearable electronic devices has attracted considerable interest for various potential applications, including human motion detection, soft robotics, electronic skin, and human-machine interfaces. Stretchable and flexible strain sensors can convert mechanical deformation into electrical signals, enabling the detection of strain caused by human activity. To meet the requirements of these potential applications, there is a need to develop strain sensors with fast response, low hysteresis, rapid response, high tensile strength, and robust long-term reliability.

[0004] Furthermore, there is a desire to integrate transparent strain sensors that do not obstruct light transmission with multi-sensor components requiring clear optical paths. Different types of materials can be used to manufacture flexible substrates and stretchable electrodes for strain sensors. For example, polydimethylsiloxane (PDMS), which is soft, deformable, and has good light transmittance, can be used to manufacture flexible substrates. Additionally, aliphatic aromatic random copolyesters (ecoflex) and hydrogels with excellent stretchability can also be used to manufacture large stretchable strain sensors (ε>100%). Furthermore, stretchable materials, including stretchable elastomers such as epoxy aliphatic acrylate and aliphatic urethane diacrylate, polyvinylidene fluoride (PVDF), thermoplastic polyurethane (TPU), silicone rubber, styrene-(ethylene-butylene)-styrene (SEBS), and polyimide, can also be used to manufacture stretchable substrates. For fabricating electrodes, carbon materials (such as carbon black, graphene nanosheets, carbon nanotubes, and graphene) are preferred due to their good conductivity, low cost, and simple manufacturing process. Silver nanoparticles, liquid metals, Au, and Cu wires have also been investigated for fabricating conductive electrodes.

[0005] Based on different sensing mechanisms, strain sensors can be classified into several categories, including optical strain sensors, capacitive strain sensors, piezoelectric strain sensors, and resistive strain sensors. Optical strain sensors rely on changes in the transmittance of the sensing material under different strain conditions to measure strain, thus requiring integration with an additional light intensity detection component. Piezoelectric strain sensors generate signals due to the normal force between two electrodes under bending or tensile deformation. Therefore, their application over a wide measurement range is limited. Resistive strain sensors are also widely used due to their simple structure and manufacturing process. However, this sensor can experience resistance changes not only due to geometric changes but also due to changes in the material's conductivity under applied varying strain, resulting in poor linearity and significant hysteresis. In contrast, the capacitance change of capacitive strain sensors primarily depends on changes in the dielectric material and the geometry of the sensing electrodes, thus exhibiting good linearity and low hysteresis. Summary of the Invention

[0006] There is still a need in this field for improved designs and techniques for manufacturing strain sensors.

[0007] According to a first embodiment of the present invention, a method for manufacturing a strain sensor is provided. The method includes: growing a patterned thin iron (Fe) catalyst layer on the top surface of a silicon oxide insulating layer, the silicon oxide insulating layer being formed on the top surface of a silicon wafer; synthesizing a plurality of vertically aligned carbon nanotubes (VACNTs) on the top surface of the thin iron (Fe) catalyst layer to form electrodes of the strain sensor; forming a first polydimethylsiloxane (PDMS) layer disposed on adjacent VACNTs and between the adjacent VACNTs; peeling the first PDMS layer and the plurality of VACNTs embedded in the first PDMS layer from the top surface of the silicon oxide insulating layer; and forming a second PDMS layer on the bottom surface of the plurality of VACNTs embedded in the first PDMS layer. Furthermore, the thin iron (Fe) catalyst layer has a thickness of approximately 2 nm, and the silicon oxide insulating layer has a thickness of approximately 1 μm. The synthesis of the plurality of VACNTs is performed by microwave plasma-enhanced chemical vapor deposition (PECVD). The formation of the first polydimethylsiloxane (PDMS) layer is performed by spin-coating a first degassed PDMS precursor mixer onto the top and side surfaces of the plurality of VACNTs to cover the top and side surfaces of the plurality of VACNTs. Furthermore, the monomer to curing agent ratio of the PDMS precursor mixer is in the range of approximately 10:1. The spin-coating is performed using a spin coater at a speed of approximately 150 rpm for approximately 40 seconds. The formation of the second PDMS layer is performed by coating a second degassed PDMS precursor mixer at a speed of approximately 2000 rpm for approximately 40 seconds and curing at a temperature of approximately 70°C for approximately two hours.

[0008] In a second embodiment of the present invention, another method for manufacturing a strain sensor is provided. The method includes: growing a thin iron (Fe) catalyst layer on the top surface of a silicon oxide insulating layer, the silicon oxide insulating layer being formed on the top surface of a silicon wafer; synthesizing a plurality of vertically aligned carbon nanotubes (VACNTs) on the top surface of the iron (Fe) thin catalyst layer to form electrodes of the strain sensor; forming a first polydimethylsiloxane (PDMS) layer disposed on adjacent VACNTs and between the adjacent VACNTs; peeling the first PDMS layer and the plurality of VACNTs embedded in the first PDMS layer from the top surface of the silicon oxide insulating layer; inverting the peeled first PDMS layer having the plurality of VACNTs; attaching the silicon wafer to the bottom surface of the peeled first PDMS layer having the plurality of VACNTs; covering the contact areas of the plurality of VACNTs with protective tape; forming a second PDMS layer on the top surface of the plurality of VACNTs embedded in the first PDMS layer; and removing the protective tape from the silicon wafer. Furthermore, the thin iron (Fe) catalyst layer has a thickness of approximately 2 nm, and the silicon oxide isolation layer has a thickness of approximately 1 μm. Multiple VACNTs are synthesized using microwave plasma-enhanced chemical vapor deposition (PECVD). The formation of the first PDMS layer is performed by spin-coating a first degassed PDMS precursor mixer onto the top and side surfaces of the multiple VACNTs to cover them. Furthermore, the monomer to curing agent ratio of the first PDMS precursor mixer is in the range of approximately 10:1. The spin-coating is performed using a spin coater at a speed of approximately 150 rpm for approximately 40 seconds. The formation of the second PDMS layer is performed by coating a second degassed PDMS precursor mixer at a speed of approximately 2000 rpm for approximately 40 seconds; and curing at a temperature of approximately 70°C for approximately two hours.

[0009] In some embodiments of the invention, the strain sensor includes a flexible substrate made of polydimethylsiloxane (PDMS) and a plurality of vertically aligned carbon nanotubes (VACNTs) embedded in the flexible substrate. The flexible substrate and the plurality of VACNTs are manufactured according to a first embodiment of a method for manufacturing a strain sensor.

[0010] In some embodiments of the invention, the strain sensor includes a flexible substrate made of polydimethylsiloxane (PDMS) and a plurality of vertically aligned carbon nanotubes (VACNTs) embedded in the flexible substrate. The flexible substrate and the plurality of VACNTs are manufactured according to a second embodiment of a method for manufacturing a strain sensor. Attached Figure Description

[0011] Figure 1A flowchart illustrating a process for manufacturing a flexible transparent strain sensor with 3D electrodes according to an embodiment of the present invention is shown.

[0012] Figure 2 (i) to Figure 2 (viii) is a schematic representation of the steps of a manufacturing process for a flexible transparent strain sensor having 3D electrodes based on vertically aligned carbon nanotubes according to an embodiment of the present invention. Detailed Implementation

[0013] Embodiments of the present invention relate to methods and systems for manufacturing strain sensors based on vertically aligned carbon nanotubes.

[0014] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. As used herein, the singular forms "a" and "the" are intended to include both the plural and singular forms unless the context clearly indicates otherwise. It will also be understood that the terms "comprises and / or comprising," when used in this specification, specify the presence of the stated features, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof.

[0015] Unless otherwise defined, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It will also be understood that terms such as those defined in common dictionaries shall be interpreted as having the same meaning as they have in the relevant field and in the context of this disclosure, and shall not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0016] When the term “about” is used in conjunction with a numerical value in this document, it should be understood that the value may be in the range of 90% to 110% of the value, that is, the value may be + / - 10% of the value. For example, “about 1 kg” means 0.90 kg to 1.1 kg.

[0017] In describing this invention, it should be understood that numerous techniques and steps are disclosed. Each of these techniques and steps has its own advantages and can be used in combination with one or more (or in some cases all) of the other disclosed techniques. Therefore, for clarity, this specification will avoid unnecessarily repeating every possible combination of the steps. However, the specification and claims should be understood as meaning that such combinations are fully within the scope of this invention and the claims.

[0018] Reference Figure 1 The diagram shows a flowchart of the manufacturing process for a flexible capacitive strain sensor. First, in step 110, a thin iron (Fe) catalyst layer with a thickness of, for example, about 2 nm is grown on a silicon wafer, and a silicon oxide isolation layer with a thickness of, for example, 1 μm is provided on the top surface of the silicon wafer.

[0019] Then, in step 120, vertically aligned carbon nanotube (VACNT) arrays are synthesized by microwave plasma-enhanced chemical vapor deposition (PECVD) to form sensor electrodes.

[0020] Next, in step 130, a degassed PDMS precursor mixer / mixture having a monomer / curing agent ratio of approximately 10:1 is spin-coated onto the wafer surface for approximately 40 seconds using a spin coater at a speed of approximately 150 rpm for example, to cover VACNT.

[0021] Then, in step 140, the VACNT electrodes are stripped from the silicon wafer using PDMS.

[0022] Next, in step 150, an additional PDMS layer is applied at a rotation speed of, for example, about 2000 rpm for about 40 seconds; and the additional PDMS layer is cured on a hot plate at a temperature of about 70°C for about two hours to protect the other side of the CNT electrode.

[0023] Reference Figure 2 (i) to Figure 2 (viii) shows in more detail the steps of the fabrication process of a flexible transparent strain sensor with 3D electrodes based on vertically aligned carbon nanotubes.

[0024] First, such as Figure 2 As shown in (i), a silicon oxide isolation layer with a thickness of, for example, about 1 μm is formed on the top surface of a silicon wafer.

[0025] Then, a thin iron (Fe) catalyst layer with a thickness of, for example, about 2 nm, is grown on the top surface of the formed silicon oxide isolation layer, and an interdigitated pattern is formed on the iron (Fe) catalyst layer using an electron beam evaporator via a lift-off process, such as... Figure 2 As shown in (ii).

[0026] Next, a VACNT array was synthesized using microwave plasma-enhanced chemical vapor deposition (PECVD) to form sensor electrodes on a silicon wafer, such as... Figure 2 As shown in (iii).

[0027] Then, as Figure 2As shown in (iv), a degassed PDMS precursor mixer having a monomer / curing agent ratio of approximately 10:1 is spin-coated onto the top surface of the wafer for approximately 40 seconds using a spin coater at a speed of approximately 150 rpm for example, to cover VACNT.

[0028] Next, to reduce air bubbles in the PDMS substrate and improve the filling performance of PDMS in the gaps between carbon nanotubes, the silicon wafer was placed in a vacuum chamber and evacuated for, for example, approximately 20 minutes. Then, the PDMS was cured on a hot plate at a temperature of approximately 70°C for approximately two hours.

[0029] PDMS has a thickness characteristic of, for example, about 400 μm, and is used to remove VACNT electrodes from silicon wafers. PDMS is available from Dowsil.

[0030] like Figure 2 As shown in (v), PDMS with 3D electrodes is peeled off from a silicon wafer.

[0031] Then, the PDMS flexible substrate with 3D electrodes is inverted, and the peeled silicon wafer is attached to the opposite side of the flexible substrate. The contact area is then protected with tape, such as... Figure 2 As shown in (vi).

[0032] To protect the other side of the CNT electrode, such as Figure 2 As shown in (vii), an additional PDMS layer is applied to the flexible substrate at a rotation speed of, for example, about 2000 rpm for about 40 seconds, and then cured on a hot plate at a temperature of about 70°C for about two hours.

[0033] Then, as Figure 2 As shown in (viii), the tape is removed and the flexible strain sensor is peeled off from the silicon wafer.

[0034] The capacitive strain sensor obtained by the above manufacturing method is designed with interdigitated electrodes including a flexible PDMS substrate to improve sensitivity, transparency, linearity, and reduce hysteresis. Furthermore, compared to conventional capacitive strain sensors with parallel electrodes, the interdigitated electrodes of this invention can reduce the sensor's thickness. Additionally, the flexible sensor exhibits excellent dynamic response, such as fast response, stability, and robustness, making the strain sensor suitable for use in wearable devices to monitor various human activities, including large-scale movements such as finger bending, knee bending, neck bending, wrist bending, and elbow bending, as well as small-scale movements such as speaking, and real-time identification of robot movements. As a result, patients with throat and speech problems can signal an alarm to a doctor by opening their mouths. In another example, the strain sensor can be used to train deaf people to practice pronunciation.

[0035] Therefore, a flexible capacitive sensor structure with three-dimensional (3D) interdigitated electrodes fabricated from vertically aligned carbon nanotubes (VACNTs) is ideal for developing highly stable, low-hysteresis, transparent, and pressure-insensitive strain sensors. Compared to the traditional "sandwich" structure with thick dielectric material, the parallel interdigitated electrodes of this invention can significantly reduce the overall thickness of the device. Furthermore, the strain sensor fabricated by the method of this invention exhibits ultra-low hysteresis, excellent pressure insensitivity, fast response, good long-term stability, and durability.

[0036] Enhanced flexible strain sensors have been successfully demonstrated as wearable devices for accurately monitoring different types of human movement, including fingers, knees, elbows, wrists, and necks, using large strain signals. Due to their enhanced performance, flexible strain sensors can be applied to a wide range of fields, including human motion detection, soft robotics, and healthcare.

[0037] All patents, patent applications, provisional applications and publications referenced or cited herein are incorporated herein by reference in their entirety (including all figures and tables) to the extent that they are consistent with the explicit teachings of this specification.

[0038] It should be understood that the examples and embodiments described herein are for illustrative purposes only, and various modifications or variations thereof will occur to those skilled in the art, and such modifications or variations will be included within the spirit and scope of this application and the scope of the appended claims. Furthermore, any element or limitation of any invention or embodiment disclosed herein may be combined with any and / or all other elements or limitations disclosed herein (alone or in any combination) or any other invention or embodiment thereof, and all such combinations are contemplated within the scope of the invention but are not limited thereto.

Claims

1. A method for manufacturing a strain sensor, the method comprising: A thin iron (Fe) catalytic layer is grown on the top surface of a silicon oxide isolation layer, which is formed on the top surface of a silicon wafer; Multiple vertically aligned carbon nanotubes are synthesized on the top surface of the thin iron (Fe) catalyst layer to form the electrodes of the strain sensor; A first polydimethylsiloxane layer is formed on adjacent vertically arranged carbon nanotubes and between the adjacent vertically arranged carbon nanotubes. Peel the first polydimethylsiloxane layer and the plurality of vertically arranged carbon nanotubes embedded in the first polydimethylsiloxane layer from the top surface of the silicon oxide isolation layer; The first polydimethylsiloxane layer stripped from the plurality of vertically arranged carbon nanotubes is inverted; The silicon wafer is attached to the bottom surface of the stripped first polydimethylsiloxane layer having the plurality of vertically arranged carbon nanotubes; Cover the contact areas of the plurality of vertically arranged carbon nanotubes with protective tape; A second polydimethylsiloxane layer is formed on the top surface of the plurality of vertically aligned carbon nanotubes embedded in the first polydimethylsiloxane layer; and Remove the protective tape from the silicon wafer.

2. The method according to claim 1, wherein, The iron (Fe) thin catalyst layer has a thickness of 2 nm, and the silicon oxide isolation layer has a thickness of 1 µm.

3. The method according to claim 1, wherein, The synthesis of multiple vertically aligned carbon nanotubes was performed using microwave plasma-enhanced chemical vapor deposition (PECVD).

4. The method according to claim 1, wherein, The formation of the first polydimethylsiloxane layer is carried out by spin-coating a first degassed polydimethylsiloxane precursor mixer onto the top and side surfaces of the plurality of vertically arranged carbon nanotubes to cover the top and side surfaces of the plurality of vertically arranged carbon nanotubes.

5. The method according to claim 4, wherein, The ratio of monomer to curing agent in the first polydimethylsiloxane precursor mixer is in the range of 10:

1.

6. The method according to claim 4, wherein, The spin coating is performed using a rotary coater at a speed of 150 rpm for 40 seconds.

7. The method according to claim 1, wherein, The formation of the second polydimethylsiloxane layer is carried out by coating the second degassed polydimethylsiloxane precursor mixer at a rotation speed of 2000 rpm for 40 seconds. It is then cured at 70°C for two hours.

8. A strain sensor, comprising: Flexible substrate made of polydimethylsiloxane; as well as Multiple vertically aligned carbon nanotubes are embedded in the flexible substrate. The flexible substrate and the plurality of vertically arranged carbon nanotubes are manufactured according to the method of claim 1.

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