Method for manufacturing metal-doped amorphous carbon thin film for flexible piezoresistive sensor

Abstract

A preparation method of metal-doped amorphous carbon film for flexible piezoresistive sensing, comprising the following steps: 1) installing and fixing a pure graphite target and a metal-graphite spliced target on two plane rectangular targets in a vacuum chamber of an octahedral structure thin film deposition device respectively, connecting the pure graphite target and the metal-graphite spliced target to the same positive and negative poles of a power supply, fixing a plurality of cleaned flexible substrates on corresponding positions arranged from top to bottom on a sample holder, and installing the sample holder on a revolution support of the vacuum chamber; 2) vacuumizing the vacuum chamber until the vacuum degree reaches 5*10 ‑4 Pa; 3) rotating the sample holder to the front of an ion source through the revolution support, and performing oxygen ion etching on the flexible substrate by adjusting oxygen ion etching parameters; 4) after the etching is completed, rotating the sample holder to the middle of the two plane rectangular targets through the revolution support, and depositing carbon ions and metal ions according to deposition parameters to form amorphous carbon films with different metal doping contents on each flexible substrate respectively.

Description

Technical Field

[0001] This invention relates to the field of flexible electronics and sensing materials technology, and in particular to a method for preparing metal-doped amorphous carbon thin films for flexible piezoresistive sensors. Background Technology

[0002] Flexible piezoresistive sensors are key components in wearable electronics, medical health monitoring, and artificial electronic skin. With technological advancements, higher performance requirements are being placed on flexible piezoresistive sensors, including high sensitivity, excellent flexibility, and long-term stability. Amorphous carbon thin films, due to their high mechanical strength, excellent chemical stability, and adjustable piezoresistive properties, have become one of the mainstream materials for fabricating the sensitive layer of flexible piezoresistive sensors. Amorphous carbon is mainly composed of sp... 2 and sp 3 Amorphous materials composed of hybrid carbons, in amorphous carbon thin films, sp 2 The region forms a conductive channel, sp 3 Then, acting as an insulating framework, under the action of external forces, sp 2 Changes in cluster spacing or orientation alter the overlap of π-π bonds, which in turn changes the resistivity of the amorphous carbon thin film. This is the origin of the piezoresistive effect in amorphous carbon thin films. When amorphous carbon thin films are used as the sensing material in flexible piezoresistive sensors, adjusting the resistivity of the film is the key to achieving this effect.

[0003] Existing technologies typically employ carbon ion energy modulation or metal doping to alter the resistivity of amorphous carbon thin films. The former utilizes the coordinated control of multiple parameters during magnetron sputtering, such as carbon ion energy, bias electric field strength, or cavity gas pressure, to control the resistivity of the amorphous carbon thin film. 2 / sp 3 The increased proportion of sp, which has conductive properties, makes the sp... 2 The increase of hybrid carbon clusters leads to an overall increase in thin film resistivity; while the latter involves doping metal nanoparticles into amorphous carbon films to form metal-carbon interfaces in the network of amorphous carbon films. Under stress, the interface barrier height or tunneling distance changes, thereby amplifying the piezoresistive sensitivity (GF increases by 1 to 2 orders of magnitude) and changing the resistivity of amorphous carbon films.

[0004] Compared to SP 2 / sp 3 Methods to control resistivity include metal doping, which can reduce the internal stress of amorphous carbon films and improve their adhesion to flexible substrates. However, existing methods for preparing metal-doped amorphous carbon films have the following shortcomings:

[0005] 1) Existing metal-doped amorphous carbon thin films are usually prepared using conventional DC pulsed magnetron sputtering (DCMS), with a pure metal target (such as copper) and a carbon target (such as graphite). Each target is connected to a magnetron sputtering power source, which is costly.

[0006] 2) The existing preparation methods can only prepare metal-doped amorphous carbon films with a specific content in a single deposition. To achieve rapid preparation of metal-doped amorphous carbon films with the optimal content, a large number of experiments are required for preparation and testing. Obviously, the above preparation-testing-screening process is time-consuming, and the metal doping amount in the preparation of amorphous carbon films is always a discrete specific value. The whole process is time-consuming and affects efficiency. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a method for preparing metal-doped amorphous carbon thin films for flexible piezoresistive sensors. Using a flexible substrate as a carrier, a high-power pulsed magnetron sputtering (HiPIMS) method is employed to prepare amorphous carbon thin films with varying metal doping content in a single deposition at low temperatures (<70℃), saving preparation time. Furthermore, the prepared amorphous carbon thin films exhibit dense structure, strong adhesion, low internal stress, and excellent flexibility and piezoresistive response characteristics.

[0008] The technical solution of this invention is: a method for preparing a metal-doped amorphous carbon thin film for a flexible piezoresistive sensor, comprising the following steps:

[0009] 1) A pure graphite target is installed and fixed on the first planar rectangular target in the vacuum chamber of the octahedral thin film deposition equipment, and a metal-graphite splicing target is installed and fixed on the second planar rectangular target. The pure graphite target and the metal-graphite splicing target are respectively connected to the positive and negative terminals of the same power supply. Multiple cleaned flexible substrates are fixed from top to bottom in the sample grids arranged vertically on the sample holder. The sample holder with the fixed flexible substrates is installed on the orbital support in the vacuum chamber.

[0010] 2) Evacuate the vacuum chamber until the vacuum level reaches 5×10⁻⁶. -4 Pa;

[0011] 3) Rotate the sample holder to the front of the ion source using the orbital support, and perform oxygen ion etching on the flexible substrate by adjusting the etching parameters of the ion source;

[0012] 4) After etching, the sample holder is rotated to the center of the first and second planar rectangular targets using a revolution support. Carbon ions and metal ions are deposited according to the deposition parameters, so that amorphous carbon films with different metal doping contents are formed on each flexible substrate fixed at different heights of the sample holder.

[0013] Preferably, in step 1), the metal of the metal-graphite splicing target is copper.

[0014] Preferably, in step 1), the metal block in the metal-graphite splicing target is a triangular block located at one corner of the planar rectangular target, occupying 1 / 4 to 1 / 3 of the entire area of ​​the planar rectangular target.

[0015] Preferably, multiple flexible substrates are arranged at equal intervals along the diagonal direction on the sample holder surface, with the tilting direction being the same as that of the inclined side of the metal block.

[0016] Preferably, in step 1), the flexible substrate is made of one or more of polydimethylsiloxane, polyvinylidene fluoride, polyethylene terephthalate, and polypropylene.

[0017] Preferably, in step 1), the flexible substrate is ultrasonically cleaned in deionized water and anhydrous ethanol for 15 minutes in sequence.

[0018] Preferably, in step 3), the oxygen ion etching parameters of the octahedral thin film deposition equipment are set as follows: ion source current 1A, ion source voltage 1200V, oxygen flow rate 30-50sccm, vacuum chamber vacuum degree 0.5Pa, substrate bias voltage -500V. During the etching process, the sample holder first rotates clockwise at 30rpm for 10-15 minutes, and then rotates counterclockwise at 30rpm for 10-15 minutes.

[0019] Preferably, in step 4), the deposition parameters are: target voltage: -1500V, sputtering current: 2A~3A, output power: 3000W, pulse period: 2000μs, duty cycle: 5%, substrate negative bias voltage: -50V, cavity gas pressure: 0.01Pa, deposition time: 30 minutes, and the sample holder rotates at 30rpm during the deposition process.

[0020] Preferably, in step 4), the temperature of the flexible substrate does not exceed 70°C during the deposition of carbon ions and metal ions.

[0021] Preferably, the metal element content of the flexible substrate decreases or increases sequentially from top to bottom.

[0022] The advantages of this invention are:

[0023] 1. This invention, by setting up a metal-graphite splicing target and placing multiple flexible substrates at different heights on a sample holder, allows for varying distances between each flexible substrate and the metal portion of the metal-graphite splicing target. This enables the formation of amorphous carbon films with different metal doping contents on each flexible substrate during high-power pulsed magnetron co-sputtering deposition. In other words, this invention can prepare multiple amorphous carbon film samples with different metal doping contents in a single deposition, greatly improving performance screening efficiency and facilitating subsequent experimental selection of the amorphous carbon film with the optimal metal doping content.

[0024] 2. This invention reduces power supply costs by connecting the metal-graphite spliced ​​target and the pure graphite target to the positive and negative terminals of the same power supply, respectively. Furthermore, it achieves coordinated control of the deposition process of the two targets through a single power supply for dual-target co-sputtering. By precisely controlling the metal doping content and adjusting the sp(s) content in the amorphous carbon thin film according to the set deposition parameters, this invention allows for precise control of the metal doping content. 2 / sp 3 The structural ratio is adjusted to overcome the problem of decoupling that is difficult to solve with traditional single-target doping.

[0025] 3. This invention employs a high-power pulsed magnetron dual-target co-sputtering process to deposit amorphous carbon thin films. Throughout the deposition process, the temperature of the flexible substrate is kept below 70°C. This process enables the preparation of dense, low-stress, high-adhesion, and piezoresistive sensitive metal-doped amorphous carbon thin films with varying ranges of piezoresistive sensitivity. These films are suitable for flexible pressure detection applications such as wearable devices, medical bandage pressure monitoring, wound care pressure measurement, and implantable pressure sensors. They possess significant technical advantages and practical application value. Attached Figure Description

[0026] Figure 1 In Figure a, a is a schematic diagram of the target composition and sample distribution in the planar structure thin film deposition equipment, and b is a top view of the target layout and distribution of the equipment.

[0027] Figure 2 This is a schematic diagram showing the layout of the sample holder and target material in the vacuum chamber of this invention;

[0028] Figure 3 This is a cross-sectional structural diagram of the amorphous carbon thin film of sample S5 in an embodiment of the present invention;

[0029] Figure 4 This is an XPS result image of a copper-doped amorphous carbon thin film in an embodiment of the present invention;

[0030] Figure 5 The figure shows the internal stress test results of amorphous carbon films with different copper contents in the embodiments of the present invention.

[0031] Figure 6 The figure shows the initial resistance test results of amorphous carbon films with different copper contents in the embodiments of the present invention. Detailed Implementation

[0032] A method for preparing a metal-doped amorphous carbon thin film for a flexible piezoresistive sensor includes the following steps:

[0033] 1) A pure graphite target is fixed on a first planar rectangular target within the vacuum chamber of an octahedral thin film deposition apparatus, and a metal-graphite spliced ​​target is fixed on a second planar rectangular target. The pure graphite target and the metal-graphite spliced ​​target are respectively connected to the positive and negative terminals of the same power supply, reducing power supply costs. Furthermore, the co-sputtering of the two targets through a single power supply enables coordinated control of the deposition process of the two target materials, allowing for precise control of the metal doping content and adjustment of sp in the amorphous carbon thin film. 2 / sp 3 Regarding the structural proportions, the metal in the metal-graphite splicing target is preferably copper, but other metals such as silver, nickel, and chromium can also be used. The metal block in the metal-graphite splicing target is a triangular block located at one corner of the planar rectangular target, occupying 1 / 4 to 1 / 3 of the entire planar rectangular target area. This arrangement ensures different metal contents at different heights, allowing for deposition of multiple amorphous carbon films with varying metal doping levels in a single deposition, while avoiding excessive metal doping. Multiple flexible substrates, after being ultrasonically cleaned sequentially in deionized water and anhydrous ethanol for 15 minutes, are sequentially fixed from top to bottom in sample holders arranged vertically. The flexible substrates are arranged at an angle within the sample holders, with the angle matching the angle of the metal block's inclined side. The sample holder with the fixed flexible substrates is mounted on a rotating support within the vacuum chamber. Through the coordination of revolution and rotation, the sample holder can sequentially pass through different positions within the vacuum chamber, achieving cleaning, etching, and compositional gradient deposition. The flexible substrate is made of one or more of the following: polydimethylsiloxane, polyvinylidene fluoride, polyethylene terephthalate, and polypropylene.

[0034] 2) Close the vacuum chamber door, and sequentially turn on the mechanical pump and molecular pump to evacuate the vacuum chamber until the vacuum level reaches 5×10⁻⁶. -4 Pa;

[0035] 3) Rotate the sample holder to the front of the ion source using the orbital support, and perform oxygen ion etching on the flexible substrate by adjusting the oxygen ion etching parameters of the octahedral thin film deposition equipment; oxygen ion etching is used to improve the adhesion of subsequent amorphous carbon thin films.

[0036] The oxygen ion etching parameters are set as follows: ion source current 1A, ion source voltage 1200V, oxygen flow rate 30-50sccm, vacuum chamber vacuum degree 0.5Pa, substrate bias voltage -500V. During the etching process, the sample holder first rotates clockwise at 30rpm for 10-15 minutes, and then rotates counterclockwise at 30rpm for 10-15 minutes. By rotating in different directions, uniform etching of the flexible substrate can be achieved.

[0037] 4) After etching, the sample holder is rotated to the center of the first and second planar rectangular targets using a rotating support. Carbon and metal ion deposition is controlled according to the deposition parameters, resulting in the formation of amorphous carbon films with thicknesses ranging from 100 to 500 nm on various flexible substrates fixed at different heights of the sample holder. The metal doping content decreases or increases sequentially from top to bottom. The deposition parameters are: target voltage: -1500V, sputtering current: 2A–3A, output power: 3000W, pulse period: 2000μs, duty cycle: 5%, substrate negative bias: -50V, chamber pressure: 0.01Pa, deposition time: 30 minutes, with the sample holder rotating at 30 rpm during deposition. The temperature of the flexible substrate does not exceed 70°C during the carbon and metal ion deposition process.

[0038] Example: Amorphous carbon thin films with different copper doping contents were prepared using copper-graphite spliced ​​targets and pure graphite targets.

[0039] This invention adopts Figure 1 The octahedral thin film deposition apparatus shown is equipped with a high-power pulsed power supply, two planar rectangular targets, and an ion source. The first planar rectangular target is a pure graphite target, and the second planar rectangular target is a copper-graphite spliced ​​target. A copper block is placed in the lower left corner of the second planar rectangular target in a triangular shape. Since the secondary electron emission coefficient of copper is approximately 1.6, which is greater than that of graphite (approximately 0.8), the area of ​​the copper block is smaller than that of the graphite block. That is, compared to graphite, the higher the secondary electron emission coefficient of the metal, the smaller the area it occupies on the second planar rectangular target. Therefore, in this embodiment, the copper block occupies 1 / 4 of the area of ​​the second planar rectangular target. The pure graphite target and the copper-graphite spliced ​​target are respectively connected to the positive and negative terminals of the high-power pulsed power supply, and metal-doped amorphous carbon thin film deposition is performed using a high-power pulsed magnetron co-sputtering method. In this embodiment, the flexible substrate is made of polydimethylsiloxane (PDMS), with dimensions of 20mm × 20mm × 1mm, and a total of 5 substrates are arranged, labeled S1 to S5. Before deposition, the 5 flexible substrates are ultrasonically cleaned for 15 minutes in sequence with deionized water and anhydrous ethanol to remove oil and other re-adsorbed impurities from their surfaces. After cleaning, the residual liquid on the surface is dried with a blower. The 5 cleaned flexible substrates are bonded and fixed at equal intervals along the diagonal direction of a rotatable cuboid sample holder using conductive high-temperature adhesive. This diagonal direction is the same as the hypotenuse of the triangular copper block. From the upper right to the lower left, they are numbered S1, S2, S3, S4, and S5, as shown in the schematic diagram. Figure 2 As shown. Finally, the sample holder with the five flexible substrates S1 to S5 fixed is then fixed in the positioning hole of the orbital support that can rotate in the vacuum chamber.

[0040] After closing the vacuum chamber door, sequentially turn on the mechanical pump and then the molecular pump to evacuate the chamber until the vacuum level reaches 5×5×10⁻⁶.-4 After Pa, the sample holder is rotated to be directly in front of the ion source using a rotating support, and oxygen ion etching is performed on the flexible substrate. The oxygen ion etching parameters are as follows:

[0041] The ion source current was 1A, the ion source voltage was 1200V, the oxygen flow rate was 40sccm, the vacuum degree of the control chamber was 0.5Pa, and the substrate bias voltage was -500V. During the etching process, the sample holder first rotated clockwise at 30rpm for 15 minutes, and then rotated counterclockwise at 30rpm for 15 minutes, for a total etching time of 30 minutes.

[0042] After the etching process, the sample holder is positioned precisely between the two planar targets by rotating the orbital support. A copper-doped amorphous carbon thin film is then prepared using high-power pulsed magnetron co-sputtering. The positive and negative power lines are sequentially output with negative voltages by setting the output voltage to negative, thus achieving the purpose of dual-target co-sputtering. The deposition parameters are set as follows during the sputtering process:

[0043] Target voltage: -1500V, sputtering current: 2A~3A, output power: 3000W, pulse period: 2000μs, duty cycle: 5%, substrate negative bias: -50V, cavity gas pressure: 0.01Pa, deposition time: 30 minutes, the sample holder rotates at 30rpm throughout the deposition process.

[0044] In this invention, the deposition parameters are not only set for copper, but also for other metals such as silver, nickel, and chromium to deposit and prepare metal-doped amorphous carbon thin films.

[0045] In this embodiment, since the copper region of the copper-graphite splicing target occupies a fixed portion of the runway area, copper atoms exhibit a gradient distribution along the diagonal of the second-plane rectangular target during splicing. The flexible substrate S5 at the lower left of the sample holder is initially aligned with the copper-rich region, resulting in the highest copper content. The other flexible substrates S4 to S1 are positioned further and further away from the copper block region, thus the copper content in the amorphous carbon films deposited on the flexible substrates S4 to S1 decreases progressively. This results in a doped amorphous carbon film with a copper content that gradually decreases from the lower left to the upper right, and the thickness of the amorphous carbon film is 220 nm for all of them.

[0046] In this embodiment, a series of samples with copper content ranging from 0.5 at% to 10 at% can be obtained through a single deposition, and the temperature of the flexible substrate during the deposition process does not exceed 70°C, which greatly improves the quality and preparation efficiency of amorphous carbon thin films and shortens the material development cycle.

[0047] Structural and performance characterization of copper-doped amorphous carbon thin films

[0048] See Figure 3Scanning electron microscopy was used to characterize the amorphous carbon film on the flexible substrate S5 with the highest copper content. After high copper doping, the amorphous carbon film showed a columnar structure, the film was relatively dense, and there was no peeling or cracking. It also showed good bonding stability with the flexible substrate.

[0049] XPS analysis of the composition of copper-doped amorphous carbon thin films deposited on flexible substrates S1–S5

[0050] See Figure 4 Figure 1 shows the XPS spectra of the outermost C1s and Cu 2p orbitals of a copper-doped amorphous carbon thin film. The figure shows that the closer the flexible substrate is to the copper target, the higher the copper content in the amorphous carbon thin film, and the higher the sp2p orbital density. 2 The higher the content, the more copper exists in the amorphous carbon film in the form of nanoclusters and the more it gradually increases. That is, in this embodiment, the amorphous carbon film deposited on the flexible substrate S5 contains more sp... 2 The copper content reached its highest value. This indicates that metal doping can achieve the desired effect on sp2+ content in amorphous carbon thin films. 2 / sp 3 The controllable adjustment of carbon bond composition enables the regulation of stress and resistance in amorphous carbon thin films.

[0051] The copper content in the amorphous carbon films deposited on flexible substrates S1–S5 was calculated using XPS fitting, as shown in Table 1. The closer the flexible substrate is to the Cu target, the higher the Cu content in the amorphous carbon films, reaching a maximum of 5.62 at%.

[0052]

[0053] Table 1

[0054] Internal stress test results of copper-doped amorphous carbon films deposited on flexible substrates S1–S5

[0055] See Figure 5 The internal stress of five amorphous carbon films was tested using a substrate bending method. With increasing Cu doping concentration, the internal stress of the amorphous carbon films generally decreased, with the lowest internal stress (1.1 GPa) observed on the flexible substrate S5. Low internal stress is crucial for improving the stability of amorphous carbon films and can prevent them from peeling off under bending or other conditions.

[0056] Initial resistance of copper-doped amorphous carbon thin films deposited on flexible substrates S1–S5

[0057] See Figure 6The initial resistance of copper-doped amorphous carbon thin films deposited on flexible substrates S1–S5 was tested using a four-probe method. The highest initial resistance of 2.31 MΩ was observed in the amorphous carbon thin film on flexible substrate S1, which had the lowest copper content, while the lowest initial resistance of 0.51 MΩ was observed in the amorphous carbon thin film on flexible substrate S5, which had the highest copper content. Therefore, high-power pulsed magnetron co-sputtering technology combined with target splicing can be used to adjust the overall initial resistance of the amorphous carbon thin film on demand by controlling the doping metal content, thus achieving on-demand material design.

[0058] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications made to the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a metal-doped amorphous carbon thin film for a flexible piezoresistive sensor, characterized in that, The method comprises the following steps: 1) fixing a pure graphite target on a first planar rectangular target and fixing a metal-graphite spliced target on a second planar rectangular target in a vacuum chamber of an octahedral structure thin film deposition device, the metal of the metal-graphite spliced target is copper, the metal block in the metal-graphite spliced target is a triangular block, located at a corner of the planar rectangular target, and accounts for 1 / 4 to 1 / 3 of the area of the planar rectangular target, the pure graphite target and the metal-graphite spliced target are connected to the positive and negative poles of the same power supply respectively, a plurality of cleaned flexible substrates are sequentially fixed in sample slots arranged in an up-down manner on a sample holder, the plurality of flexible substrates are arranged in an inclined manner in the sample slots arranged in an up-down manner on the sample holder, the inclined direction is the same as the inclined direction of the hypotenuse of the metal block, and the sample holder with the flexible substrates fixed thereon is installed on a revolution support in the vacuum chamber; 2) The vacuum chamber is evacuated until the vacuum reaches Pa; 3) rotating the sample holder to the front of the ion source through the revolution support, and performing oxygen ion etching on the flexible substrates by adjusting oxygen ion etching parameters of the octahedral structure thin film deposition device; 4) after the etching is completed, rotating the sample holder to the middle of the first and second planar rectangular targets through the revolution support, and controlling carbon ion and metal ion deposition according to deposition parameters, so that amorphous carbon films with different metal doping contents are respectively formed on the flexible substrates at different heights of the sample holder.

2. The method of claim 1, wherein: In step 1), the flexible substrate is made of one or more of polydimethylsiloxane, polyvinylidene fluoride, polyethylene terephthalate and polypropylene.

3. The method of claim 1, wherein: In step 1), the flexible substrate is sequentially ultrasonically cleaned in deionized water and anhydrous ethanol for 15 minutes.

4. The method of claim 1, wherein: In step 3), the oxygen ion etching parameters of the octahedral structure thin film deposition device are set as follows: ion source current 1A, ion source voltage 1200V, oxygen flow rate 30-50sccm, vacuum degree of the vacuum chamber 0.5Pa, substrate bias-500V, during the etching process, the sample holder is first self-rotated clockwise at 30 rpm for 10-15 minutes, and then self-rotated counterclockwise at 30 rpm for 10-15 minutes.

5. The method of claim 1, wherein: In step 4), the deposition parameters are as follows: target material voltage-1500V, sputtering current 2A-3A, output power 3000W, pulse period 2000μs, duty cycle 5%, substrate negative bias-50V, cavity gas pressure 0.01Pa, deposition time 30 minutes, and the sample holder is self-rotated at 30 rpm during the deposition process.

6. The method of claim 1, wherein: In step 4), during the deposition of carbon ions and metal ions, the temperature of the flexible substrate is not higher than 70℃.

7. The method of claim 1, wherein: The metal element content of the flexible substrate decreases or increases from top to bottom.

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