A piezoelectric thin film pressure sensor and its self-assembly preparation method

Abstract

The application provides a piezoelectric thin film pressure sensor and a self-assembly preparation method thereof. The self-assembly preparation method comprises the following steps: S1, depositing a sacrificial layer on a substrate; S2, depositing a bottom electrode layer on the sacrificial layer; S3, depositing a piezoelectric layer formed by inorganic piezoelectric single crystals on the bottom electrode layer; S4, depositing a top electrode layer on the electrode area of the piezoelectric layer; S5, etching part of the non-electrode area of the piezoelectric layer to the sacrificial layer, so as to expose the end surface of the substrate and the bottom electrode layer, and obtain a piezoelectric thin film; S6, depositing a fixing layer on the piezoelectric thin film to fix one edge of the piezoelectric thin film on the substrate; S7, depositing an electrode pin layer of the bottom electrode layer and the top electrode layer; and S8, wet etching the sacrificial layer, washing, drying, and then performing polarization treatment, so as to obtain the piezoelectric thin film pressure sensor. The piezoelectric thin film pressure sensor has good flexibility, a larger deformation range, good piezoelectric performance, and a simple preparation method.

Description

Technical Field

[0001] This invention belongs to the field of pressure sensor technology, and relates to a piezoelectric thin film pressure sensor and its self-assembly preparation method. Background Technology

[0002] Pressure sensors detect pressure signals and convert them into electrical signals according to a certain rule. Due to their high sensitivity, fast response, high piezoelectric coefficient, and good environmental adaptability, flexible piezoelectric materials play a crucial role in pressure sensors and have wide applications in stress sensing, strain sensing, sound sensing, and vibration sensing. Especially for pressure sensors that sense normal stress / strain, such as tactile sensors and pneumatic pressure monitoring sensors, the piezoelectric material must have a large deformation capacity to adapt to various pressure changes and ensure stable sensor operation. Therefore, the development of highly sensitive flexible piezoelectric sensing materials and pressure sensors has become particularly urgent.

[0003] Currently, flexible piezoelectric materials mainly include organic piezoelectric polymers and inorganic piezoelectric films. Generally speaking, although organic piezoelectric polymers have high flexibility, their piezoelectric coefficient is low, resulting in poor piezoelectric sensing performance; inorganic piezoelectric films have high piezoelectric properties, but due to their poor flexibility, they deform little under pressure, resulting in poor piezoelectric sensing performance.

[0004] (1) Organic piezoelectric polymers: Polyvinylidene fluoride (PVDF) and its copolymer P(VDF-TrFE) are the most widely used organic piezoelectric materials because they possess good flexibility and exhibit the highest piezoelectric properties among existing piezoelectric polymers. However, their piezoelectric coefficient... d 33 Typically, the piezoelectric ratio is less than 30 pC / N, far lower than that of inorganic piezoelectric materials; furthermore, its glass transition temperature is usually below 120 °C, making it difficult to use in high-temperature environments; it also has a large coefficient of thermal expansion, resulting in poor device temperature stability. To improve the piezoelectric properties of organic piezoelectric polymers, in addition to material composition modification, researchers have used structural design to create three-dimensional structures such as wrinkles, micro-domes, and curved surfaces. The stress / strain applied to these geometries can be amplified, thereby generating a large piezoelectric response. For example, Chinese invention patent CN113043582B discloses a method for improving the piezoelectric response of polymer materials by forming wrinkles in flexible polymer films to enhance the piezoelectric response. Chinese patent application CN108542377A uses an arched PVDF piezoelectric polymer as a pressure-sensitive module. However, such three-dimensional structures typically involve complex manufacturing processes; and the size of these devices is usually in the millimeter or even centimeter range, making miniaturization difficult.

[0005] (2) Inorganic piezoelectric thin films: Flexible piezoelectric thin films can be prepared by directly depositing inorganic piezoelectric thin films on high-temperature resistant flexible substrates (such as metal foils, mica, etc.). For example, Yeo et al. (Sensors and Actuators A: Physical 273, 90-97 (2018)) prepared PZT thin films on nickel foil substrates, which showed good piezoelectric energy harvesting characteristics. However, since oxide thin films are usually deposited at relatively high temperatures, these base metal foils are easily oxidized and introduce interface defects; in addition, these oxide thin films usually exist in polycrystalline form, with defects such as grain boundaries. These factors will lead to poor piezoelectric performance of the device. Mica is widely used as a flexible substrate due to its low price, high light transmittance, mechanical flexibility and high temperature resistance. For example, Wang et al. (Nano Energy, 2018, 43: 351–358) prepared PZT thin films on fluorinated mica substrates by spin coating, and then thinned the mica substrate to 20.0 μm by mechanical thinning. PZT / Mica exhibits good flexibility and mechanical stability, with a low radius of curvature as low as 2.2 mm, and remains stable even after 40,000 mechanical deformations. Chinese patent application CN117881266A proposes growing PZT on a mica substrate with a Pt bottom electrode; this flexible PZT film possesses high piezoelectric response, low relative permittivity, and low leakage current. Zhang et al. (J Mater Sci: MaterElectron (2024) 35:298) reported an adaptable bending radius of approximately 1.5 mm for PZT films (80-200 nm) deposited on thinned mica (~10 μm). However, this inorganic flexible piezoelectric film is still constrained by the mica substrate, resulting in small deformation under pressure, thus limiting its sensitivity when applied to weak mechanical sensing such as tactile perception.

[0006] Furthermore, thin-film self-assembly technology holds great potential in the fabrication of flexible piezoelectric thin films with large deformation capabilities. For example, Chinese patent application CN116471917A discloses a controllable self-assembly method and device for elongated thin films, proposing a bilayer self-rolling thin film based on nickel ferrite / lead zirconate titanate, which can naturally form a three-dimensional roll after detaching from the substrate. However, due to the significant difference in lattice constant between nickel ferrite and the sacrificial layer of strontium aluminate and lead zirconate titanate, lattice defects are easily introduced during thin film growth, limiting device performance. Therefore, there is an urgent need to propose a novel piezoelectric thin-film pressure sensor and its self-assembly manufacturing method. Summary of the Invention

[0007] To address the problems of the prior art, this invention provides a piezoelectric thin film pressure sensor and its self-assembly preparation method. The piezoelectric thin film pressure sensor has good flexibility, a large deformation range, and good piezoelectric properties, and the preparation method is simple.

[0008] This invention is achieved through the following technical solution:

[0009] In a first aspect, the present invention provides a self-assembly method for fabricating a piezoelectric thin-film pressure sensor, comprising the following steps:

[0010] S1, deposit a sacrificial layer on the substrate;

[0011] S2, deposit a bottom electrode layer on the sacrificial layer;

[0012] S3, depositing a piezoelectric layer formed by an inorganic piezoelectric single crystal on the bottom electrode layer; wherein, the crystal structure of the bottom electrode layer matches that of the piezoelectric layer, and the lattice constant of the bottom electrode layer is smaller than that of the piezoelectric layer;

[0013] S4, deposit a top electrode layer in the electrode region of the piezoelectric layer;

[0014] S5, etching a portion of the non-electrode region of the piezoelectric layer down to the sacrificial layer using an etching method, exposing the end faces of the substrate and the bottom electrode layer to obtain a piezoelectric thin film;

[0015] S6, Deposit a fixing layer on the piezoelectric thin film to fix one edge of the piezoelectric thin film to the substrate;

[0016] S7, deposit an electrode lead layer of bottom electrode layer and top electrode layer to bring out the bottom electrode layer and top electrode layer;

[0017] S8, wet etching removes the sacrificial layer, followed by washing, drying, and then polarization treatment to obtain a piezoelectric thin film pressure sensor.

[0018] Preferably, the bottom electrode layer is Nb-doped SrTiO3 or SrRuO3, and the piezoelectric layer is Pb(Zr,Ti)O3 or PMN-PT.

[0019] Preferably, the substrate is an oxide single crystal substrate.

[0020] Furthermore, the substrate is SrTiO3.

[0021] Preferably, the sacrificial layer is CaSr2Al2O6.

[0022] Preferably, the fixing layer is a negative photoresist.

[0023] Preferably, S6 specifically involves: spin-coating negative photoresist onto a piezoelectric thin film, heating and baking to obtain a negative photoresist coating, ultraviolet exposure, heating and baking again, developing in a developing solution, and then baking to form a patterned fixing layer, fixing one edge of the piezoelectric layer onto the substrate.

[0024] Preferably, S8 specifically involves immersing the sample obtained in S7 in water to dissolve the sacrificial layer, thereby achieving the self-assembly of the arched piezoelectric film as the sacrificial layer dissolves.

[0025] Furthermore, in S8, the drying is freeze drying, and the polarization treatment is polarization for 20-30 minutes at 80℃-100℃ and a DC electric field of 150kV / cm-200 kV / cm.

[0026] Secondly, the present invention provides a piezoelectric thin film pressure sensor obtained by the self-assembly preparation method described above.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The fabrication method of the piezoelectric thin-film pressure sensor of this invention employs a bottom electrode layer with a lattice constant smaller than that of the piezoelectric layer. This involves a strategy of gradually increasing the lattice constant from the bottom to the top layer, ensuring strict epitaxial growth of the thin film and avoiding defects introduced during growth due to a large lattice constant in the bottom layer, thereby improving device performance. Because the lattice constant of the bottom electrode layer is smaller than that of the piezoelectric layer, the bottom electrode layer also acts as a stress layer. During wet etching of the sacrificial layer, due to the lattice mismatch strain gradient between the bottom electrode layer and the piezoelectric layer, the upper piezoelectric layer experiences in-plane compressive stress. This stress / strain is released during the peeling of the piezoelectric thin film. Therefore, after wet etching removes the sacrificial layer, the planar piezoelectric thin film curls downwards to self-assemble into an arched piezoelectric thin film. This arched piezoelectric thin film can detach from the substrate and will not break under repeated pressure, exhibiting better elasticity. Compared to planar piezoelectric thin films, the arched piezoelectric thin film structure design can produce greater deformation under the same pressure, thus significantly improving the piezoelectric response. Therefore, the arched piezoelectric thin-film pressure sensor has higher sensitivity when applied to flexible pressure sensing. Compared to traditional methods that improve piezoelectric device performance through structural design, this invention features a simpler process that can be achieved in a single step. Individual devices are only a few hundred micrometers in length and can be further miniaturized, enabling the micro-sizing of piezoelectric device units. Furthermore, this invention uses inorganic piezoelectric single crystals as the piezoelectric layer. Compared to traditional flexible piezoelectric polymers, the piezoelectric layer formed by inorganic piezoelectric single crystals has a higher piezoelectric coefficient, resulting in a piezoelectric thin-film pressure sensor with a strong piezoelectric sensing response. Simultaneously, compared to traditional flexible pressure sensors based on piezoelectric polymers, the pressure sensor based on inorganic piezoelectric single crystals in this invention has a wider operating temperature range, reaching up to ~300℃, and exhibits better temperature stability. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a flowchart of the self-assembly fabrication method for the piezoelectric thin film pressure sensor of the present invention.

[0031] Figure 2 These are actual images of the piezoelectric film before (a) and after (b) curling in Example 1.

[0032] Figure 3 This is a comparison diagram of the piezoelectric responses of the planar piezoelectric film and the arched piezoelectric film under pressure in Example 1.

[0033] Figure 4 The results show the sensitivity test results of the arched piezoelectric thin film sensing unit in Example 1. Detailed Implementation

[0034] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0035] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0036] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products, or apparatuses. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not intended to limit the order of the method steps or define the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0037] The self-assembly fabrication method of the piezoelectric thin film pressure sensor of the present invention includes the following steps:

[0038] S1, deposit a sacrificial layer on the substrate;

[0039] S2, deposit a bottom electrode layer on the sacrificial layer;

[0040] S3, depositing a piezoelectric layer on the bottom electrode layer; wherein the piezoelectric layer is an inorganic piezoelectric single crystal, the crystal structure of the bottom electrode layer matches that of the piezoelectric layer, and the lattice constant of the bottom electrode layer is smaller than that of the piezoelectric layer;

[0041] S4, deposit a top electrode layer in the electrode region of the piezoelectric layer;

[0042] S5, etching a portion of the non-electrode region of the piezoelectric layer to the sacrificial layer by etching, exposing the end faces of the substrate and the bottom electrode layer, and forming a piezoelectric thin film on the substrate;

[0043] S6, Deposit a fixing layer on the piezoelectric thin film to fix one edge of the piezoelectric thin film to the substrate;

[0044] S7, deposit electrode lead layers for the bottom electrode layer and the top electrode layer to bring out the bottom electrode layer and the top electrode layer;

[0045] S8, wet etching removes the sacrificial layer to obtain a piezoelectric thin film pressure sensor.

[0046] This invention employs a bottom electrode layer with a crystal structure matched to the piezoelectric layer, having a smaller lattice constant than the piezoelectric layer. This strategy involves a gradual increase in the lattice constant from the bottom to the top layer, ensuring strict epitaxial growth and avoiding defects introduced during film growth due to a large lattice constant in the bottom layer, thus improving device performance. Because the bottom electrode layer has a smaller lattice constant than the piezoelectric layer, it also acts as a stress layer. During wet etching of the sacrificial layer, the upper piezoelectric layer experiences in-plane compressive stress due to the lattice mismatch strain gradient between the bottom electrode layer and the piezoelectric layer. This stress / strain is released during etching and peeling, causing the planar piezoelectric film to curl downwards and self-assemble into an arched shape. Compared to planar thin-film devices, the arched structure design allows for greater deformation under the same stress, resulting in a significant improvement in piezoelectric response. Therefore, the arched piezoelectric film pressure sensor exhibits higher sensitivity when applied to flexible pressure sensing. Compared to traditional methods of improving piezoelectric device performance through structural design, this invention features a simpler process and can be manufactured in one step. Individual devices are only a few hundred micrometers long and can be further reduced in size, enabling the miniaturization of piezoelectric device units.

[0047] This invention employs inorganic piezoelectric single crystals as the piezoelectric layer. Compared to traditional flexible piezoelectric polymers, the piezoelectric layer formed by inorganic piezoelectric single crystals has a higher piezoelectric coefficient, thus enabling the fabricated piezoelectric thin-film pressure sensor to exhibit a strong piezoelectric sensing response. Furthermore, compared to traditional flexible pressure sensors based on piezoelectric polymers, the pressure sensor based on inorganic piezoelectric single crystals in this invention has a wider operating temperature range, reaching up to ~300℃, and exhibits good temperature stability.

[0048] When this invention is implemented, it specifically includes the following steps (such as...). Figure 1 (as shown)

[0049] (1) Clean the substrate, such as oxide single crystal strontium titanate (SrTiO3), and epitaxially deposit a sacrificial layer on the substrate by pulsed laser deposition or magnetron sputtering, such as calcium-doped strontium aluminate (CaSr2Al2O6).

[0050] (2) An epitaxially deposited bottom electrode layer, i.e., an oxide single crystal thin film, such as strontium ruthenate (SrRuO3) or Nb-doped SrTiO3, etc., is deposited. The crystal structure of the bottom electrode layer matches that of the piezoelectric layer, and the lattice constant is smaller than that of the piezoelectric layer. The bottom electrode layer also serves as a stress layer.

[0051] (3) Epitaxially deposit a piezoelectric layer formed by an inorganic piezoelectric single crystal, such as lead zirconate titanate (Pb(Zr,Ti)O3) or lead magnesium niobate-lead titanate (PMN-PT).

[0052] (4) A patterned metal thin film, such as Au or Pt, is deposited in the electrode region of the piezoelectric layer as the top electrode layer by combining photolithography and magnetron sputtering.

[0053] (5) By combining photolithography, dry etching and other methods, a portion of the non-electrode region of the piezoelectric layer is etched to the sacrificial layer, exposing the end face of the substrate and the bottom electrode layer, thus obtaining a piezoelectric thin film and realizing the patterning of planar devices.

[0054] (6) A patterned negative photoresist (such as SU8) is photolithographically applied to form a fixing layer, so that the end of the piezoelectric film is fixed on the substrate.

[0055] (7) A patterned electrode lead layer, such as Au or Pt, is deposited on the fixed layer by combining photolithography and magnetron sputtering to bring out the bottom electrode layer and the top electrode layer.

[0056] (8) Wet etching is performed on the sacrificial layer. Due to the lattice mismatch strain gradient between the bottom electrode layer and the piezoelectric layer, the lattice constant of the lower bottom electrode layer is smaller and the lattice constant of the upper piezoelectric layer is larger. Therefore, the upper piezoelectric layer will be subjected to in-plane compressive stress. When the stress / strain is released during the etching and peeling process, the planar thin film will spontaneously curl into an arched piezoelectric thin film after the wet etching removes the sacrificial layer.

[0057] (9) Rinse the substrate and piezoelectric film in deionized water 2-3 times, remove the deionized water using freeze-drying technology, and then further polarize at a higher temperature to obtain a self-assembled arched piezoelectric film pressure sensor.

[0058] Example 1: Self-assembled lead zirconate titanate-based flexible piezoelectric thin film pressure sensor

[0059] The high-sensitivity pressure sensor based on lead zirconate titanate (Pb(Zr,Ti)O3) arched piezoelectric thin film provided in this embodiment consists of, from bottom to top, a SrTiO3 substrate, a SrRuO3 bottom electrode layer, a Pb(Zr,Ti)O3 piezoelectric layer, a Pt top electrode layer, an SU8 fixing layer, and a Pt electrode pin layer.

[0060] The specific implementation steps of this embodiment are as follows:

[0061] Step 1: A CaSr2Al2O6 sacrificial layer is epitaxially deposited on a SrTiO3 substrate using pulsed laser deposition.

[0062] (1-1) Pretreatment of substrate: The (001) oriented SrTiO3 substrate was placed in acetone, alcohol and deionized water in sequence, ultrasonically cleaned for 10 minutes in sequence, and dried with dry nitrogen. Then the substrate was placed in an ultraviolet ozone cleaner for 15 minutes to ensure that the substrate surface was clean and dust-free.

[0063] (1-2) A CaSr2Al2O6 sacrificial layer was deposited on a SrTiO3 substrate using pulsed laser deposition. The deposition conditions included: SrTiO3 substrate heating temperature of 760 ℃, laser energy of 0.8 W, laser frequency of 3 Hz, working gas of O2 with oxygen pressure of 15 Pa, and deposition time of 10 min, resulting in a CaSr2Al2O6 sacrificial layer with a thickness of about 20 nm.

[0064] Step 2: An epitaxial SrRuO3 bottom electrode layer is deposited on the CaSr2Al2O6 sacrificial layer using pulsed laser deposition.

[0065] (2-1) A SrRuO3 bottom electrode layer was deposited on a CaSr2Al2O6 sacrificial layer using pulsed laser deposition. The deposition conditions included: substrate heating temperature of 650 °C, laser energy of 0.8 W, laser frequency of 5 Hz, working gas of O2 with oxygen pressure of 10 Pa, and deposition time of 60 min, resulting in a SrRuO3 bottom electrode layer with a thickness of about 200 nm.

[0066] (2-2) The pseudocubic lattice constant of SrRuO3 is a = 3.93 Å, and the lattice constant of the cubic CaSr2Al2O6 sacrificial layer is a = 3.907 Å. Therefore, the SrRuO3 layer can be epitaxially grown on the CaSr2Al2O6 layer.

[0067] Step 3: Pb(Zr,Ti)O3 piezoelectric layer is epitaxially deposited on the SrRuO3 bottom electrode layer using pulsed laser deposition.

[0068] (3-1) A Pb(Zr,Ti)O3 piezoelectric layer was deposited on the SrRuO3 bottom electrode layer by pulsed laser deposition. The deposition conditions included: substrate heating temperature of 625 ℃, laser energy of 0.8 W, laser frequency of 5 Hz, working gas of O2 with oxygen pressure of 26 Pa, and deposition time of 60 min, resulting in a Pb(Zr,Ti)O3 piezoelectric layer with a thickness of about 200 nm.

[0069] (3-2) The Pb(Zr,Ti)O3 piezoelectric layer has a tetragonal phase structure with lattice constants a=b=4.017 Å and c=4.139 Å, while the pseudocubic lattice constant of SrRuO3 is a=3.93 Å. Since 3.93 Å < 4.017 Å, the Pb(Zr,Ti)O3 piezoelectric layer will be subjected to in-plane compressive stress when grown on the SrRuO3 bottom electrode layer, which provides a strain gradient for the self-assembly of the arched piezoelectric film.

[0070] Step 4: A Pt top electrode layer is deposited on the surface of the Pb(Zr,Ti)O3 piezoelectric layer by a combination of photolithography and magnetron sputtering to construct a surface electrode pattern.

[0071] (4-1) Spin-coat AR-P 3510T positive photoresist. Drop the photoresist onto the sample surface, spin-coat at 500 rpm for 10 s, spin-coat at 4000 rpm for 50 s, place the sample on a heating stage at 120℃ to dry for 5 min, expose to ultraviolet light for 8 s, and develop in developer for 60 s to pattern the photoresist.

[0072] (4-2) Deposition of noble metal electrode: A noble metal electrode Pt was deposited in the electrode region of the piezoelectric layer using magnetron sputtering. The working gas was Ar, the working pressure was 3 mTorr, and the DC sputtering power was 70 W for 400 s, resulting in a 20 nm thick Pt top electrode layer. The positive photoresist was then removed in acetone to obtain a Pt top electrode layer with a pattern size of 500 μm × 600 μm. Finally, the layer was annealed at 400 °C for 5 minutes to ensure a strong bond between the Pt top electrode layer and the Pb(Zr,Ti)O3 piezoelectric layer.

[0073] Step 5: Spin-coat a thicker protective photoresist layer and expose it with ultraviolet light to pattern it. Then, etch a portion of the non-electrode area of ​​the piezoelectric layer down to the sacrificial layer using ion beam etching, fully exposing the bottom electrode layer end face. Remove the photoresist to achieve planar device patterning and obtain the piezoelectric thin film.

[0074] (5-1) Spin-coating AR-P 3220 positive photoresist: drop the photoresist onto the thin film surface, spin-coat at 500 rpm for 10 s, spin-coat at 2000 rpm for 50 s, place the sample on a heating stage at 100℃ and dry for 2 min. Repeat the operation 3 times to obtain a photoresist coating of about 30 μm thickness, and expose it with ultraviolet light to form a patterned protective layer.

[0075] (5-2) Part of the non-electrode region of the piezoelectric layer was etched using ion beam etching technology until the sacrificial layer was completely removed, and the end face of the bottom electrode layer was fully exposed. The etching conditions were: vacuum degree 8×10 -4 The etching process was carried out at Pa, with an anolyte current of 1 A for 50 min. The photoresist was then removed in acetone.

[0076] Step 6: A fixing layer is formed on the patterned piezoelectric film surface using photolithography negative photoresist (SU8) to fix one edge of the piezoelectric film onto the substrate.

[0077] (6-1) Spin-coating SU8 2002 negative photoresist: Drop the negative photoresist onto the surface of the piezoelectric film, spin-coat at 500 rpm for 10 s, spin-coat at 4000 rpm for 50 s, place the sample on a heating stage at 100℃ and dry for 5 min to obtain a negative photoresist coating of about 2 μm thickness, expose to ultraviolet light for 15 s, place the sample on a heating stage at 100℃ again and bake for 5 min, develop in developer for 60 s, and then place the sample on a heating stage at 200℃ and bake for 15 min to form a patterned fixing layer, fixing one edge of the piezoelectric film to the substrate.

[0078] Step 7: Pt electrode lead layers are deposited using a combination of photolithography and magnetron sputtering to bring out the top / bottom electrode layers for easy subsequent testing.

[0079] (7-1) Spin-coat AR-P 3510T positive photoresist. Drop the photoresist onto the sample surface, spin-coat at 500 rpm for 10 s, spin-coat at 4000 rpm for 50 s, place the sample on a heating stage at 120℃ to dry for 5 min, expose to ultraviolet light for 8 s, and develop in developer for 60 s to pattern the photoresist.

[0080] (7-2) Deposition of noble metal electrode leads: A noble metal Pt electrode lead layer was deposited on the surface of the fixed layer by magnetron sputtering. The working gas was Ar, the working pressure was 3 mTorr, the DC sputtering power was 70 W, and the deposition time was 4000 s to obtain a 200 nm thick Pt film. Then, the positive photoresist was removed in acetone to obtain a patterned Pt electrode lead layer.

[0081] Step 8: Perform wet etching on the sacrificial layer to completely remove it.

[0082] (8-1) The substrate and piezoelectric film are immersed in deionized water, allowing the CaSr2Al2O6 sacrificial layer to dissolve at room temperature, thereby releasing the upper piezoelectric film. Due to the lattice mismatch strain gradient between the bottom electrode layer and the piezoelectric layer, the self-assembly of the arched piezoelectric film is ultimately achieved as the sacrificial layer dissolves, as shown in Figure 8. Figure 2 As shown.

[0083] Step nine involves using freeze-drying technology to remove deionized water while simultaneously polarizing the piezoelectric device.

[0084] The piezoelectric thin film was polarized at 100℃ and a DC electric field of 200 kV / cm for 20 minutes, then cooled to room temperature under pressure before testing. Compared to planar thin films, the arched piezoelectric thin film can withstand greater stress / strain, thus producing a larger piezoelectric response, such as... Figure 3 As shown. The piezoelectric coefficient of this piezoelectric film. d 33 It can reach 120 pC / N, and the sensitivity can reach 0.004 Pa (e.g. Figure 4 As shown in the figure, it can work stably from -50 to 300℃.

[0085] Example 2: Self-assembled PMN-PT based flexible piezoelectric thin film pressure sensor

[0086] The high-sensitivity pressure sensor based on the PMN-PT arched piezoelectric thin film provided in this embodiment consists of, from bottom to top, a SrTiO3 substrate, a 0.5% Nb-doped SrTiO3 bottom electrode layer, a PMN-PT piezoelectric layer, a Pt top electrode layer, an SU8 fixing layer, and a Pt electrode pin layer.

[0087] The specific implementation steps of this embodiment are as follows:

[0088] Step 1: A CaSr2Al2O6 sacrificial layer is epitaxially deposited on a SrTiO3 substrate using pulsed laser deposition.

[0089] (1-1) Pretreatment of substrate: The (001) oriented SrTiO3 substrate was placed in acetone, alcohol and deionized water in sequence, ultrasonically cleaned for 10 minutes in sequence, and dried with dry nitrogen. Then the substrate was placed in an ultraviolet ozone cleaner for 15 minutes to ensure that the substrate surface was clean and dust-free.

[0090] (1-2) A CaSr2Al2O6 sacrificial layer was deposited on a SrTiO3 substrate using pulsed laser deposition. The deposition conditions included: SrTiO3 substrate heating temperature of 760 ℃, laser energy of 0.8 W, laser frequency of 3 Hz, working gas of O2 with oxygen pressure of 15 Pa, and deposition time of 10 min, resulting in a CaSr2Al2O6 sacrificial layer with a thickness of about 20 nm.

[0091] Step 2: A 0.5% Nb-doped SrTiO3 bottom electrode layer is epitaxially deposited on the CaSr2Al2O6 sacrificial layer using pulsed laser deposition.

[0092] (2-1) A 0.5% Nb-doped SrTiO3 bottom electrode layer was deposited on a CaSr2Al2O6 sacrificial layer using pulsed laser deposition. The deposition conditions included: substrate heating temperature of 650 °C, laser energy of 0.8 W, laser frequency of 5 Hz, working gas of O2 with oxygen pressure of 20 Pa, and deposition time of 60 min, resulting in a 0.5% Nb-doped SrTiO3 bottom electrode layer with a thickness of about 200 nm.

[0093] (2-2) The lattice constant of cubic 0.5% Nb-doped SrTiO3 is a = 3.905 Å, and the lattice constant of the cubic CaSr2Al2O6 sacrificial layer is a = 3.907 Å. Therefore, the 0.5% Nb-doped SrTiO3 layer can be epitaxially grown on the CaSr2Al2O6 layer.

[0094] Step 3: A PMN-PT piezoelectric layer is epitaxially deposited on the 0.5% Nb-doped SrTiO3 bottom electrode layer using pulsed laser deposition.

[0095] (3-1) A PMN-PT piezoelectric layer was deposited on a 0.5% Nb-doped SrTiO3 bottom electrode layer using pulsed laser deposition. The deposition conditions included: substrate heating temperature of 625 °C, laser energy of 0.8 W, laser frequency of 5 Hz, working gas of O2 with oxygen pressure of 25 Pa, and deposition time of 60 min, resulting in a PMN-PT piezoelectric layer with a thickness of about 200 nm.

[0096] (3-2) The PMN-PT piezoelectric layer has a tetragonal phase structure with a lattice constant of a=b=4.024 Å, while the cubic phase 0.5% Nb-doped SrTiO3 has a lattice constant of a=3.905 Å. Since 3.905 Å < 4.024 Å, the PMN-PT piezoelectric layer will be subjected to in-plane compressive stress when grown on the 0.5% Nb-doped SrTiO3 bottom electrode layer, which provides a strain gradient for the self-assembly of the arched piezoelectric film.

[0097] Step 4: A Pt top electrode layer is deposited on the surface of the PMN-PT piezoelectric layer using a combination of photolithography and magnetron sputtering to construct a surface electrode pattern.

[0098] (4-1) Spin-coat AR-P 3510T positive photoresist. Drop the photoresist onto the sample surface, spin-coat at 500 rpm for 10 s, spin-coat at 4000 rpm for 50 s, place the sample on a heating stage at 120℃ to dry for 5 min, expose to ultraviolet light for 8 s, and develop in developer for 60 s to pattern the photoresist.

[0099] (4-2) Deposition of noble metal electrode: A noble metal electrode Pt was deposited in the electrode region of the piezoelectric layer using magnetron sputtering. The working gas was Ar, the working pressure was 3 mTorr, and the DC sputtering power was 70 W for 400 s, resulting in a 20 nm thick Pt top electrode layer. The positive photoresist was then removed in acetone to obtain a Pt top electrode layer with a pattern size of 500 μm × 600 μm. Finally, the layer was annealed at 400 °C for 5 minutes to ensure a strong bond between the Pt top electrode layer and the PMN-PT piezoelectric layer.

[0100] Step 5: Spin-coat a thicker protective photoresist layer and expose it with ultraviolet light to pattern it. Then, etch a portion of the non-electrode area of ​​the piezoelectric layer down to the sacrificial layer using ion beam etching, fully exposing the bottom electrode layer end face. Remove the photoresist to achieve planar device patterning and obtain the piezoelectric thin film.

[0101] (5-1) Spin-coating AR-P 3220 positive photoresist: drop the photoresist onto the thin film surface, spin-coat at 500 rpm for 10 s, spin-coat at 2000 rpm for 50 s, place the sample on a heating stage at 100℃ and dry for 2 min. Repeat the operation 3 times to obtain a photoresist coating of about 30 μm thickness, and expose it with ultraviolet light to form a patterned protective layer.

[0102] (5-2) Part of the non-electrode region of the piezoelectric layer was etched using ion beam etching technology until the sacrificial layer was completely removed, and the end face of the bottom electrode layer was fully exposed. The etching conditions were: vacuum degree 8×10 -4 The etching process was carried out at 1 Pa, with an anolyte current of 1 A, for approximately 40 minutes. The photoresist was then removed in acetone.

[0103] Step 6: A fixing layer is formed on the patterned piezoelectric film surface using photolithography negative photoresist (SU8) to fix one edge of the piezoelectric film onto the substrate.

[0104] (6-1) Spin-coating SU8 2002 negative photoresist: Drop the negative photoresist onto the surface of the piezoelectric film, spin-coat at 500 rpm for 10 s, spin-coat at 4000 rpm for 50 s, place the sample on a heating stage at 100℃ and dry for 5 min to obtain a negative photoresist coating of about 2 μm thickness, expose to ultraviolet light for 15 s, place the sample on a heating stage at 100℃ again and bake for 5 min, develop in developer for 60 s, and then place the sample on a heating stage at 200℃ and bake for 15 min to form a patterned fixing layer, fixing one edge of the piezoelectric film to the substrate.

[0105] Step 7: Pt electrode lead layers are deposited using a combination of photolithography and magnetron sputtering to bring out the top / bottom electrode layers for easy subsequent testing.

[0106] (7-1) Spin-coat AR-P 3510T positive photoresist. Drop the photoresist onto the sample surface, spin-coat at 500 rpm for 10 s, spin-coat at 4000 rpm for 50 s, place the sample on a heating stage at 120℃ to dry for 5 min, expose to ultraviolet light for 8 s, and develop in developer for 60 s to pattern the photoresist.

[0107] (7-2) Deposition of noble metal electrode leads: A noble metal Pt electrode lead layer was deposited on the surface of the fixed layer by magnetron sputtering. The working gas was Ar, the working pressure was 3 mTorr, the DC sputtering power was 70 W, and the deposition time was 4000 s to obtain a 200 nm thick Pt film. Then, the positive photoresist was removed in acetone to obtain a patterned Pt electrode lead layer.

[0108] Step 8: Perform wet etching on the sacrificial layer to completely remove it.

[0109] (8-1) The substrate and piezoelectric film are immersed in deionized water, allowing the CaSr2Al2O6 sacrificial layer to dissolve at room temperature, thereby releasing the upper piezoelectric film. Due to the lattice mismatch strain gradient between the bottom electrode layer and the piezoelectric layer, the self-assembly of the arched piezoelectric film is ultimately achieved as the sacrificial layer dissolves, as shown in Figure 8. Figure 2 As shown.

[0110] Step nine involves using freeze-drying technology to remove deionized water while simultaneously polarizing the piezoelectric thin film.

[0111] The piezoelectric thin film was polarized at 90℃ and a DC electric field of 200 kV / cm for 20 minutes, then cooled to room temperature under pressure before testing. Compared to planar thin films, the arched piezoelectric thin film can withstand greater stress / strain, thus producing a larger piezoelectric response. The piezoelectric coefficient of this thin film is... d 33 It can reach 200 pC / N, with a sensitivity of 0.002 Pa, and can operate stably from -50 to 120℃.

[0112] The advantages of the piezoelectric thin film pressure sensor of the present invention include: (1) Strong piezoelectric sensing response: Compared with traditional flexible piezoelectric polymers, the inorganic piezoelectric single crystal thin film of the present invention has a higher piezoelectric coefficient. Compared with planar thin film devices, the arched structure design can produce greater deformation under the same stress, thereby achieving a significant improvement in piezoelectric response. Therefore, the self-curving arched piezoelectric device has higher sensitivity when applied to flexible pressure sensing. (2) Wide operating temperature range and good temperature stability: The operating temperature of traditional polymer-based flexible pressure sensors is usually below 120°C. The operating temperature range of the inorganic piezoelectric single crystal thin film of the present invention is wider, up to ~300°C, and has good temperature stability. (3) Compared with the traditional method of improving the performance of piezoelectric devices through structural design, the present invention has a simple process and can be formed in one step. The length of a single device is only a few hundred micrometers, and the size can be further reduced, realizing the miniaturization of piezoelectric device units.

Claims

1. A self-assembly method for preparing a piezoelectric thin film pressure sensor, characterized by, The method comprises the following steps: S1, depositing a sacrificial layer on a substrate; S2, depositing a bottom electrode layer on the sacrificial layer; S3, depositing a piezoelectric layer formed by an inorganic piezoelectric single crystal on the bottom electrode layer; wherein the crystal structure of the bottom electrode layer matches the piezoelectric layer, and the lattice constant of the bottom electrode layer is smaller than that of the piezoelectric layer; S4, depositing a top electrode layer on the electrode area of the piezoelectric layer; S5, etching part of the non-electrode area of the piezoelectric layer to the sacrificial layer by etching method to expose the end surface of the substrate and the bottom electrode layer, and obtaining a piezoelectric thin film; S6, depositing a fixing layer on the piezoelectric thin film to fix one edge of the piezoelectric thin film on the substrate; S7, depositing an electrode pin layer of the bottom electrode layer and the top electrode layer to lead out the bottom electrode layer and the top electrode layer; S8, immersing the sample obtained in S7 in water, so that the sacrificial layer is dissolved by water, and the self-assembly of the arched piezoelectric thin film is realized with the dissolution of the sacrificial layer, then washing, drying, and then polarization treatment, to obtain a piezoelectric thin film pressure sensor.

2. The self-assembly method for the preparation of piezoelectric thin film pressure sensors according to claim 1, characterized in that, The bottom electrode layer is Nb-doped SrTiO3 or SrRuO3, and the piezoelectric layer is Pb(Zr,Ti)O3 or PMN-PT.

3. The self-assembly method for the preparation of piezoelectric thin film pressure sensor according to claim 1, wherein The substrate is an oxide single crystal substrate.

4. The self-assembly method for the preparation of piezoelectric thin film pressure sensor according to claim 2, wherein The substrate is SrTiO3.

5. The self-assembly method for the preparation of piezoelectric thin film pressure sensor according to claim 1, wherein The sacrificial layer is CaSr2Al2O6.

6. The self-assembly method for the preparation of piezoelectric thin film pressure sensors according to claim 1, characterized in that, The fixing layer is a negative photoresist.

7. The self-assembly method for the preparation of piezoelectric thin film pressure sensors according to claim 1, characterized in that, S6 specifically comprises: spin-coating a negative photoresist on the piezoelectric thin film, heating and baking, obtaining a negative photoresist coating, ultraviolet exposure, again heating and baking, developing in a developing solution, then baking, to form a patterned fixing layer, fixing one edge of the piezoelectric layer on the substrate.

8. The self-assembly method for the preparation of piezoelectric thin film pressure sensors according to claim 7, characterized in that, In S8, the drying is freeze-drying, and the polarization treatment is polarization under a direct current electric field of 80-100℃ and 150-200 kV / cm for 20-30 minutes.

9. A piezoelectric thin film pressure sensor obtained by the self-assembly preparation method according to any one of claims 1-8.

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