CN115694405B - A structure and method for generating flexoelectricity - Google Patents

Abstract

The invention discloses a structure and a method for generating flexoelectric power, which are used for carrying out on-chip integration on an interdigital transducer and a thin film device, and simultaneously applying sine waves with the same frequency, phase and amplitude to the interdigital transducers at two sides. By arranging the thin film device at the antinode of the surface acoustic wave standing wave generated when the pair of interdigital transducers are applied with sine waves, the thin film device at the antinode is strongly stretched under the action of the surface acoustic wave, and the generated flexoelectric effect is stronger. The structure of the invention can generate the flexoelectric effect without damaging the surface of the thin film device, and has important significance for the application and development of the aspects of flexoelectric energy collection, flexoelectric sensing and the like.

Description

A structure and method for generating flexoelectricity

Technical Field

The present invention belongs to the field of electromechanical coupling, and more specifically, relates to a structure and method for generating flexoelectricity.

Background Art

With the rapid development of microelectronics technology, the use of electromechanical coupling effects to regulate the performance of devices at the micro-nano scale has attracted widespread attention from researchers. Electromechanical coupling effects include traditional piezoelectric coupling effects and flexoelectric coupling effects. The traditional electromechanical coupling effect refers to the piezoelectric effect, which is a linear coupling between electric polarization and uniform strain. The flexoelectric effect is a high-order electromechanical coupling effect in dielectric materials. It usually refers to the electric polarization caused by non-uniform deformation of dielectric materials, that is, the electric polarization caused by strain gradient. The advantages of the flexoelectric effect over the piezoelectric effect are: first, the dielectric materials that produce the flexoelectric effect are no longer limited to non-centrosymmetric crystals, and the flexoelectric effect exists in all dielectrics; second, the flexoelectric effect is independent of temperature; third, since the strain gradient is inversely proportional to the characteristic size of the material, the flexoelectric effect will be more significant when the device is miniaturized to the nanoscale. Based on these advantages, the flexoelectric effect has received more and more attention.

At present, there are methods such as bending cantilever beams, bending films, using pyramid samples, trapezoidal samples, etc. to produce flexoelectric effect, and it is pointed out that flexoelectric effect has obvious size dependence, but the above methods are all methods of generating flexoelectricity in quasi-static state. It can be seen that it is urgent to develop a dynamic way to generate flexoelectric effect, so as to lay a foundation for the future flexoelectric effect to control the performance of electronic devices in dynamic and medium-high frequency directions.

Summary of the invention

In response to the above defects or improvement needs of the prior art, the present invention provides a structure and method for generating flexoelectricity, the purpose of which is to lay a foundation for regulating the performance of electronic devices in dynamic and medium and high frequency directions by the flexoelectric effect.

To achieve the above object, according to a first aspect of the present invention, there is provided a structure for generating flexoelectricity, comprising: an IDT I and an IDT II integrated on the upper surface of a piezoelectric substrate and at least one thin film device located between the IDT I and the IDT II; the IDT I and the IDT II are of the same material, size and frequency;

When sinusoidal voltage signals with the same frequency, amplitude and phase are applied to the IDT I and IDT II simultaneously, standing surface acoustic waves vibrating in the XY plane are generated, causing the at least one thin film device to be periodically stretched, thereby generating a flexoelectric effect.

Preferably, the at least one thin film device is located at the antinode of the surface acoustic wave standing wave, and the position between the IDT I and IDT II satisfies the following relationship:

Δx＝n(a+b)

Wherein, a represents the width of the fingers of the IDT; b represents the spacing between the fingers of the IDT; Δx represents the spacing between the center point of the last finger electrode of the IDT I and the center point of the nth thin film device, and n is an integer.

Preferably, the thickness of the dielectric layer of the thin film device is 10 nm to 20 nm.

Preferably, the size of the thin film device is smaller than the wavelength of the surface acoustic wave.

Preferably, the electrode materials of the IDT I and IDT II are Cr and Au.

Preferably, the upper surface of the piezoelectric substrate is a polished surface, and the lower surface is a rough surface.

Preferably, the material of the piezoelectric substrate is a LiNbO ₃ single crystal crystal cut at 64° in the Y direction and propagated in the X direction.

According to a second aspect of the present invention, there is provided a method for generating flexoelectricity, which is applied to the structure as described in the first aspect, comprising:

Sinusoidal voltage signals with the same frequency, amplitude and phase are applied to the IDT I and IDT II simultaneously to generate standing surface acoustic wave waves vibrating in the XY plane, so that at least one thin film device is periodically stretched, thereby generating a flexoelectric effect.

The thin film device (3) at the antinode of the standing wave is connected to a measuring instrument by a probe station needle piercing method, and the voltage signal generated under the flexoelectric effect is measured by the measuring instrument.

Preferably, the measuring instrument is an oscilloscope.

In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

1. The structure and method for generating flexoelectricity provided by the present invention integrates the interdigital transducer and the thin film device on a chip, and simultaneously applies a sine wave with the same frequency, phase, and amplitude to the interdigital transducers on both sides. Since no load is applied to the upper electrode of the thin film device, the dielectric layer of the thin film device will produce a strain gradient under the periodic stretching of the surface acoustic wave. The periodically changing strain gradient will produce polarization charge, that is, the flexoelectric effect is generated, which provides a new idea for the flexoelectric effect to control the performance of electronic devices in the dynamic and medium and high frequency directions. In addition, the method provided by the present invention can generate the flexoelectric effect through this structure without damaging the surface of the thin film device, which is of great significance to the application development of flexoelectric energy collection, flexoelectric sensing, etc.

2. The structure and method for generating flexoelectricity provided by the present invention arranges at least one thin film device at the antinode of the standing surface acoustic wave generated by the interdigital transducer I and the interdigital transducer II when a sinusoidal wave with the same frequency, phase and amplitude is applied. The thin film device located at the antinode is strongly stretched under the action of the surface acoustic wave, and the flexoelectric effect generated is stronger.

3. The structure and method for generating flexoelectricity provided by the present invention have a unit size of the thin film device that is smaller than the wavelength of the surface acoustic wave, and no load is applied to the upper electrode of the thin film device. Under the action of the periodic stretching of the surface acoustic wave, the dielectric layer of the thin film device will produce a strain gradient, and the strain gradient will cause electric polarization. When the unit size of the thin film device is smaller than the wavelength of the surface acoustic wave, the generated electric polarization will not cancel each other out.

4. The structure and method for generating flexoelectricity provided by the present invention can be used to design interdigital transducers of different frequencies. Since the wavelength determines the size of the strain gradient, the flexoelectric effect generated by a high-frequency surface acoustic wave standing wave with a smaller wavelength is stronger. Therefore, the wavelength of the surface acoustic wave standing wave can be changed by changing the frequency of the interdigital transducer, thereby changing the intensity of the flexoelectric effect. Therefore, the performance of electronic devices can be regulated in the dynamic and medium and high frequency directions through the flexoelectric effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of a structure for generating flexoelectricity provided in an embodiment of the present invention.

DETAILED DESCRIPTION

In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

An embodiment of the present invention provides a structure for generating flexoelectricity, as shown in FIG1 , comprising: an IDT I2, an IDT II4, and at least one thin film device 3 located between the IDT I2 and the IDT II4, which are integrated on the upper surface of a piezoelectric substrate 1; the IDT I2 and the IDT II4 have the same material and size and the same frequency;

When sinusoidal voltage signals with the same frequency, amplitude and phase are applied simultaneously to the IDT I2 and IDT II4, a surface acoustic wave standing wave 5 vibrating in the XY plane is generated, so that the at least one thin film device 3 is periodically stretched, thereby generating a flexoelectric effect.

It can be understood that in this embodiment, the thin film device 3 is composed of three parts from bottom to top: a lower electrode (Pt), a functional layer (dielectric layer, such as SiO ₂ ; HfO ₂ ) and an upper electrode (W). The thickness of the lower electrode is 100nm; the thickness of the functional layer is 10nm; and the width of the upper electrode is 100μm.

The thin film device 3 is located between the IDT I2 and the IDT II4, in the propagation direction of the surface acoustic waves generated by the IDT I2 and the IDT II4.

Preferably, there is a conductive adhesive layer between the thin film device 3 and the piezoelectric substrate 1 , the conductive adhesive layer forms a covalent bond with the piezoelectric substrate 1 , and the conductive adhesive layer forms a metal bond with the lower electrode of the thin film device 3 .

Preferably, the size of the thin film device 3 is smaller than the wavelength of the surface acoustic wave.

The finger width and finger spacing of the interdigital transducer are both 100μm. The wavelength of the surface acoustic wave generated by the two interdigital transducers is λ=2×(finger width+finger spacing)=400μm, and the surface acoustic wave frequency is f=v/T, where v is the surface acoustic wave velocity of about 4000m/s, so the surface acoustic wave frequency is 10MHz. The interdigital transducers on both sides generate surface acoustic waves, and the surface acoustic waves propagating in opposite directions are superimposed in the area between the interdigital transducers on both sides to form a surface acoustic standing wave. Since no load is applied to the upper electrode of the thin film device 3, and the size of the thin film device 3 is smaller than the wavelength of the surface acoustic wave, the dielectric layer of the thin film device 3 will produce a strain gradient under the periodic stretching of the surface acoustic wave, and the periodically changing strain gradient will produce polarization charge, that is, the flexoelectric effect is generated.

The interdigital transducers of different frequencies and at least one thin film device 3 are integrated on the same piezoelectric substrate 1 (i.e., the two interdigital transducers and at least one thin film device 3 are both located on the upper surface of the piezoelectric substrate 1), and a pair of interdigital transducers on the same piezoelectric substrate 1 are simultaneously applied with a sine wave signal with the same frequency, amplitude, and phase. Due to the inverse piezoelectric effect, the interdigital transducers located on both sides of the at least one thin film device 3 both generate surface acoustic waves, and the surface acoustic waves propagating in opposite directions are superimposed in the area between the interdigital transducers on both sides, thereby forming a surface acoustic standing wave in the area between the interdigital transducers on both sides. Under the action of the surface acoustic standing wave, the at least one thin film device 3 will be periodically stretched. Since no load is applied to the upper electrode of the thin film device 3, under the action of the periodic stretching of the surface acoustic wave, the dielectric layer of the thin film device 3 will be non-uniformly strained, that is, the dielectric layer of the thin film device 3 will produce a strain gradient, and the periodically changing strain gradient will produce polarization charge, that is, the flexoelectric effect is produced.

And the size of the thin film device 3 is smaller than the wavelength of the surface acoustic wave. The interdigital transducer on the piezoelectric substrate 1 generates surface acoustic waves, and the upper electrode of the thin film device 3 does not apply any load. Under the periodic stretching of the surface acoustic wave, the dielectric layer of the thin film device 3 will produce non-uniform strain, and the periodically changing strain gradient will produce polarization charge, that is, the flexoelectric effect. The present invention integrates the surface acoustic wave and the thin film device 3 on a chip to produce the flexoelectric effect, which provides a new idea for the flexoelectric effect to control the performance of electronic devices in the dynamic and medium and high frequency directions.

Preferably, the amplitude of the sinusoidal voltage signal does not exceed the maximum withstand voltage of the piezoelectric substrate 1. In this embodiment, the non-polished surface of the piezoelectric substrate 1 is pasted on a circuit board, and the printed circuit channels on the circuit board are connected to the IDTs, and sinusoidal voltage signals with an amplitude of 1V and a frequency of 10MHz are applied to the two pairs of IDTs simultaneously.

Preferably, at least one thin film device 3 is located at the antinode of the surface acoustic wave standing wave 5 (that is, the at least one thin film device 3 is arranged in the direction of propagation of the surface acoustic wave generated by the interdigital transducer on the piezoelectric substrate 1), and the position between the interdigital transducer I2 and the interdigital transducer II4 satisfies the following relationship:

Δx＝n(a+b)

Wherein, a represents the width of the fingers of the IDT; b represents the spacing between the fingers of the IDT; Δx represents the spacing between the center point of the last finger electrode of the IDT I and the center point of the nth thin film device 3, and n is an integer.

Specifically, the thin film device 3 is integrated on a chip with the interdigital transducer so that the thin film device 3 is located at the antinode of the surface acoustic wave standing wave 5. The thin film device 3 located at the antinode is strongly stretched under the action of the surface acoustic wave, and the flexoelectric effect generated is stronger.

Preferably, the thickness of the dielectric layer of the thin film device 3 is 10 nm to 20 nm.

Specifically, the thickness of the thin film device 3 is in the nanometer range. Since the strain gradient is inversely proportional to the characteristic size of the material, when the device is miniaturized to the nanometer scale, the flexoelectric effect will be more significant.

Preferably, the electrode material of the IDT I2 and IDT II4 is Cr/Au.

Specifically, the electrode material of the IDT is Cr/Au, and the thickness is generally 50nm-100nm.

The metal Cr serves as an adhesion layer between Au and the piezoelectric substrate 1 , that is, the metal Cr can increase the adhesion between the metal film Au and the substrate.

Preferably, the contact surfaces of the IDT I2 and IDT II4 with the piezoelectric substrate 1 (ie, the upper surface of the piezoelectric substrate 1) are polished surfaces, and the lower surface of the piezoelectric substrate 1 is a rough surface.

Specifically, the upper surface of the piezoelectric substrate 1 is a polished surface, which is beneficial to the propagation of surface acoustic waves and reduces losses. The lower surface of the piezoelectric substrate 1 is a rough surface, which is beneficial to reduce the reflection of surface acoustic waves.

Preferably, the material of the piezoelectric substrate 1 is a LiNbO ₃ single crystal crystal cut in the Y direction at 64° and propagated in the X direction, that is, 64°YX-LiNbO ₃ .

Specifically, the electromechanical coupling coefficient of the _LiNbO3 single crystal is large and the insertion loss is small. Fabricating an interdigital transducer on the polished surface of the LiNbO3 _single crystal can reduce the propagation loss of the surface acoustic wave.

In this embodiment, the piezoelectric substrate 1 is made of 64° YX-LiNbO ₃ , has a size of 30×15×0.5 mm, and is polished on one side.

An embodiment of the present invention provides a method for generating flexoelectricity, which is applied to the structure described in any of the above embodiments, including:

Sinusoidal voltage signals with the same frequency, amplitude and phase are applied to the IDT I2 and IDT II4 simultaneously to generate a surface acoustic wave standing wave 5 vibrating in the XY plane, so that the at least one thin film device 3 is periodically stretched, thereby generating a flexoelectric effect.

Specifically, a sinusoidal voltage signal with the same frequency and amplitude is applied to the IDTs on both sides. Due to the inverse piezoelectric effect, the IDTs on both sides will generate surface acoustic waves. The surface acoustic waves propagating in opposite directions are superimposed in the area between the IDTs on both sides, thereby forming a surface acoustic standing wave.

Preferably, the method further comprises: measuring a voltage signal generated by the thin film device 3 under the flexoelectric effect by means of a measuring instrument; preferably, the measuring instrument is an oscilloscope.

Specifically, for example, when the at least one thin film device 3 is located at the antinode of the standing wave, the thin film device 3 at the antinode of the standing wave is connected to an oscilloscope by a probe station needle method. Since no load is applied to the upper electrode of the thin film device 3, the dielectric layer of the thin film device 3 will produce a strain gradient under the action of the periodic stretching of the surface acoustic wave. The strain gradient will cause electric polarization, and the sinusoidal voltage signal generated by the thin film device 3 can be measured by the oscilloscope.

It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (4)

1. A structure for generating flexoelectric power, comprising: the interdigital transducer I (2) and the interdigital transducer II (4) are integrated on the upper surface of the piezoelectric substrate (1), and at least one thin film device (3) is positioned between the interdigital transducer I (2) and the interdigital transducer II (4); the materials, the sizes and the frequencies of the interdigital transducer I (2) and the interdigital transducer II (4) are the same;

the interdigital transducer I (2) and the interdigital transducer II (4) generate a surface acoustic wave standing wave (5) vibrating in an XY plane when sinusoidal voltage signals with the same frequency, amplitude and phase are applied simultaneously, so that the thin film device (3) is periodically stretched, and a flexoelectric effect is generated;

the piezoelectric substrate (1) is made of a LiNbO ₃ single crystal plate which is cut in the Y direction at 64 degrees and propagates in the X direction; the upper surface of the piezoelectric substrate (1) is a polished surface, and the lower surface is a rough surface;

The adopted thin film device (3) consists of a lower electrode, a functional layer and an upper electrode from bottom to top in sequence; the lower electrode material is Pt, the functional layer, namely the dielectric layer material is SiO ₂; HfO₂, and the upper electrode material is W; the thickness of the lower electrode is 100 nm; the thickness of the functional layer is 10 nm; the width of the upper electrode is 100 μm;

the size of the thin-film device (3) is smaller than the wavelength of the surface acoustic wave;

The thin film device (3) is positioned at the antinode of the surface acoustic wave standing wave (5), and the positions of the thin film device, the interdigital transducer I (2) and the interdigital transducer II (4) meet the following relational expression:

Δx =n(a+b)

Wherein a represents the width of the interdigital transducer finger; b represents the spacing between the interdigital transducer fingers; Δx represents the distance between the center point of the last interdigital electrode of the interdigital transducer I and the center point of the nth thin film device (3), and n is an integer;

the electrode materials of the interdigital transducer I (2) and the interdigital transducer II (4) are Cr and Au.

2. A method of generating flex electricity applied to a flex electricity generating structure according to claim 1, comprising:

Sinusoidal voltage signals with the same frequency, amplitude and phase are simultaneously applied to the interdigital transducer I (2) and the interdigital transducer II (4) to generate a surface acoustic wave standing wave (5) vibrating in an XY plane, so that the thin film device (3) is periodically stretched, and a flexoelectric effect is generated.

3. A method of generating electricity as defined in claim 2, further comprising: the thin film device (3) at the antinode of the standing wave is connected with the measuring instrument by a probe station needle insertion method, and the voltage signal generated under the action of flex electricity is measured by the measuring instrument.

4. A method of generating electricity according to claim 3, wherein the measuring instrument is an oscilloscope.