Thin Piezoelectric Layer Research Articles

Metal piezoelectric semiconductor field effect transistors for piezoelectric strain sensors

In this letter, we examine the potential of a functional device that can have good transistor and stress sensor properties. The device examined is based on the use of a thin oxide with high piezoelectric coefficients under the gate region. Channel charge and current are controlled by gate voltage or by stress. We examine the performance of two classes of heterostructures that are important semiconductor technologies: (i) Si∕SiO2∕BaTiO3 heterostructure junctions that would be an important breakthrough for silicon sensor technology and (ii) GaN∕AlN∕BaTiO3 heterostructure field effect transistors. The calculations show that with a very thin piezoelectric layer we can have a highly sensitive stress sensor and transistor. For optimum performance, the piezoelectric layer thickness should be ∼30–60Å.

Approximate dynamic boundary conditions for a thin piezoelectric layer

Approximate dynamic boundary conditions of different orders are derived for the case of a thin piezoelectric coating layer bonded to an elastic material. The approximate boundary conditions are derived using series expansions of the elastic displacements and the electric potential in the thickness coordinate of the layer. All the expansion functions are then eliminated with the aid of the equations of motion and boundary/interface conditions of the layer. This results in boundary conditions on the elastic material that may be truncated to different orders in the thickness of the layer to obtain approximate boundary conditions. The approximate boundary conditions may be used as a replacement for the piezoelectric layer and thus simplify the analysis significantly. Numerical examples show that the approximate boundary conditions give good results for low frequencies and/or thin piezoelectric layers.

Axi-symmetric wave propagation in a cylinder coated with a piezoelectric layer

This paper presents the wave propagation in a cylinder coated with a thin piezoelectric layer. The piezoelectric coupling effects are fully modeled in the mechanics model for this piezoelectric coupled cylindrical shell with bending resistance. The decoupled torsional wave velocity and the dispersion curves for the two- mode shell model are obtained theoretically. The cut-off frequency and phase velocities at limit wave number are also derived. The numerical simulations are conducted to present the results of wave propagation in this cylindrical shell and as well as to compare the results by the current bending theory and the membrane shell theory. From the comparisons, the results display obvious deference of wave propagations in terms of dispersion characteristics by different shell theories when thicker piezoelectric layer are used and when higher wave number is considered. The results of this paper can serve as a reference for future study on wave propagation in coupled structures as well as in the design of smart structures incorporating piezoelectric materials.

Free Vibration of Laminated Composite Shallow Shells Containing Piezoelectric Layers

The free vibration of laminated shallow shells containing piezoelectric layers is investigated in this article. A finite-element formulation based on the transverse shear deformation theory has been used to determine the natural frequencies, mode shapes, and modal voltages of laminated composite shell structures containing piezoelectric layers. Comparisons of degenerate cases with published results show excellent agreement. The effect of electromechanical coupling on the predicted natural frequencies is discussed. Results are presented for different geometries, laminate configurations, and boundary conditions. The effects of shell shallowness and piezoelectric layer thickness are also studied. Higher natural frequencies are obtained when the full electromechanical coupling is considered. It is also found that the natural frequencies increase considerably with the shell curvature, particularly for thin piezoelectric layers.

Free Vibration of Laminated Composite Shallow Shells Containing Piezoelectric Layers

The free vibration of laminated shallow shells containing piezoelectric layers is investigated in this article. A finite-element formulation based on the transverse shear deformation theory has been used to determine the natural frequencies, mode shapes, and modal voltages of laminated composite shell structures containing piezoelectric layers. Comparisons of degenerate cases with published results show excellent agreement. The effect of electromechanical coupling on the predicted natural frequencies is discussed. Results are presented for different geometries, laminate configurations, and boundary conditions. The effects of shell shallowness and piezoelectric layer thickness are also studied. Higher natural frequencies are obtained when the full electromechanical coupling is considered. It is also found that the natural frequencies increase considerably with the shell curvature, particularly for thin piezoelectric layers.

Epitaxy of AIN and GaN thin films on silicon or sapphire for the development of high frequency saw devices

The increase in operating frequencies of telecommunication systems requires to use Surface Acoustic Wave (SAW) devices for filtering applications that propagate on “high velocity” subtrates like sapphire or even <100 > silicon. The excitation of SAW on such materials requires using thin piezoelectric layers deposited on their top surface, like AIN or GaN. This paper is devoted to the theoretical and experimental study of such material combinations. The theoretical models for multimaterial substrates are first presented. Experimental results are then reported, allowing to define the application field of AIN or GaN thin-film based devices. Epitaxie de films minces de AIN et GaN sur silicium et saphir pour le d e ́ veloppement de dispositifs d'ondes de surface a ̀ haute fr e ́ quence. La montée en fréquence des systémes de télécommunication impose l'utilisation de composants à ondes de surface pour les besoins de filtrage de fréquence fabriqués sur des substrats pour lesquels celles-ci présentent des vitesses de phase élevées, tels que le saphir ou encore le silicium d'orientation <100 >. L'excitation d'ondes élastiques sur de tels matériaux réclame la présence d'une fine couche piézoélectrique en surface de travail, composée par exemple d'AIN ou de GaN. Cet article est dédié l'étude théorique et expérimentale de telles combinaisons de matéraiaux. Les modéles théoriques développés pour la simulation de propagation d'ondes sur substrats multimatériaux sont tout d'abord briévement évoqués. Les résultats expérimentaux sont alors interprétés à la lumiére des prévisions théoriques pour bien définir la gamme d'application des substrats à base de couches d'AIN ou de GaN.

Effect of initial stress on the propagation behavior of Love waves in a layered piezoelectric structure

The propagation behavior of Love waves in a layered piezoelectric structure with an initial stress is investigated in this article. It involves a thin piezoelectric layer bonded perfectly to an elastic substrate. Solutions of the mechanical displacement and electrical potential function are obtained for the piezoelectric layer and elastic substrate by solving the coupled electromechanical field equations. The phase velocity equations of the Love wave propagation and the stress fields in the layered piezoelectric structure are obtained for electrical open and short cases on the free surface, respectively. The effect of the initial stress on the phase velocity, the stress fields and the coupled electromechanical factor are discussed, respectively. Three sets of piezoelectric layer–elastic substrate systems are considered, i.e. BaTiO 3 ceramic layer–borosilicate glass substrate, PZT-5H ceramic layer–borosilicate glass substrate, and PZT-5H ceramic layer–SiO 2 glass substrate. It is seen that the phase velocity of the Love wave propagation decreases with the increase of the magnitude of the initial stress. The coupled electromechanical factor increases remarkably, as the magnitude of the initial the stress is greater than 100 MPa. This is useful for the design of acoustic surface wave devices.

Fabrication of piezoelectric thin film of zinc oxide in composite membrane of ultrasonic microsensors

The fabrication of the thin piezoelectric layers on the silicon substrate by means of the pulse dc and the dc magnetron sputtering methods is discussed. The influences of the different kind of the surface, the conditions of deposition as well as the process of the thermal stabilising on the properties of the piezoelectric layer were investigated. The results of the investigations of the diffraction spectrum for the thin ZnO films are presented and discussed. The selection of the technological process parameters for the production of the thin ZnO layers in the sensor membranes with the ultrasonic wave generated was analysed.

Method of forming integral transducer and impedance matching layers

A method of forming a transducer device having integral transducer and impedance matching portions includes forming grooves partially through a thickness of a piezoelectric member. A groove volume fraction at the impedance matching portion controls the electrical impedance. The impedance matching portion may be at either or both of the front and rear surfaces of the transducer portion, which generates acoustic wave energy in response to application of a drive signal. The drive signal is introduced by electrodes. In one embodiment, the electrode at the impedance matching portion extends into the grooves, but preferably a filler material is selected and deposited to allow use of a planar electrode. An alternative embodiment to fabricating the transducer device is to assemble piezoelectric material. For example, an integral transducer and impedance matching portions may be formed by using molding techniques or by stacking dimensionally different thin piezoelectric layers. The acoustic impedance of the matching layer can be varied spatially to provide apodization of a radiating aperture. Moreover, a graded impedance matching layer can be formed, resulting in a tapered variation in the acoustic impedance of the matching layer.

Potential distribution in a bounded piezoelectric in the presence of the photothermoacoustic effect

The photothermoacoustic effect in a bounded piezoelectric is studied theoretically and experimentally. Calculations are made of the potential distribution over the thickness and the potential difference in a thin piezoelectric layer as a function of the coordinate of this layer. The amplitude distribution of the potential oscillations over the thickness of the piezoelectric reveals two maxima. The phase of the oscillations at these maxima is shifted by approximately 180°. It is shown that the potential difference on the opposite surfaces of the plate is zero regardless of the propagation constant of the thermal wave. An experiment was carried out using a layered disk formed by seven identical piezoelements. The amplitude and phase shift of the potential difference were investigated for each element at different light modulation frequencies. The experimental results show good agreement with the theory.

Mode selection for a piezoelectric layer in a bulk-acoustic-wave composite acoustic resonator

An analysis is made of the mode selection for a thin piezoelectric layer in a bulk-acoustic-wave composite layered acoustic resonator by transformation of the impedance using a set of quarter-wave layers. Depending on the number of layers, the impedance of the substrate on which the piezoelectric layer is deposited may be abruptly increased or sharply reduced. This serves to simulate a “fixed” or “free” surface. As a result, a mechanically strong resonator structure is produced, operating at the natural frequencies of a thin piezoelectric layer. A numerical simulation is made of multilayer structures formed by alternating quarter-wave layers of LiNbO3 and LiTaO3. It is shown that if these layers are of micron thickness and there is a sufficiently large number of them, the acoustic properties of the substrate do not influence the frequency characteristics and Q factor of the thin piezoelectric vibrating layer in the microwave range.

The application of a finite shell element for composites containing piezo-electric polymers in vibration control

Advanced composite structures with integrated sensors and actuators are becoming increasingly important due to the demand for very large, low-mass space structures. This paper focuses on the finite element analysis of shell structures with thin piezo-electric layers bonded to the surfaces. A finite element formulation taking the piezo-electric effect into account is given and a finite shell element is presented, allowing for the computation of these advanced composite structures. The dynamic matrix equation accounting for the piezo-electric material response is formulated and found to have the same form as the well-known equation of structural dynamics. By using the second or direct method of Lyapunov, a control law is derived, which is used in vibration control and ensures asymptotic stability. A numerical example is given to show the accuracy of the finite element, as well as the efficiency of the algorithm.

Circular interdigital transducers

The purpose of this article is to establish the focusing properties of a new kind of transducer consisting of circular electrodes deposited on an isotropic substrate and covered by a thin piezoelectric layer. We first express the electromechanical equations of the device and determine its boundary conditions. Then, using the effective dielectric function and an appropriate approximation of the charge density, we obtain various functions (electric potential, displacements) giving a physical description of each medium. Last, we present the numerical results of our study, namely those concerning the displacements and proving the focusing capabilities of our device.

A planar process for microfabrication of a scanning tunneling microscope

With decreasing size, a scanning tunneling microscope (STM) becomes less sensitive to vibrations and thermal drift and easier to manufacture by batch fabrication techniques. An integrated-circuit-compatible planar process has been developed to fabricate arrays of STMs on Si wafers. Using planar technology, it is possible to integrate the tunneling tip, the actuation mechanism and (if desired) the counter electrode on a Si ‘chip’. The actuator mechanism is a cantilevered bimorph consisting of alternating layers of metal and piezoelectric zinc oxide. Each metal layer is divided into two electrodes that can be individually addressed, thereby enabling the cantilever to move in three orthogonal directions. The thin (3 μm) piezoelectric layers allow low voltage (< 30 V) operation. The theoretical response of the actuator is 2500 A/V in z, 104 A/V in y and 14 A/V in x. The device has been successfully fabricated and its operation has been demonstrated by imaging the surface of graphite with atomic resolution. A planar process for microfabrication of an integrated STM has been developed and operation of this ‘STM on a chip’ has been demonstrated by imaging the surface of graphite with atomic resolution. The devices are small (typically 8 × 200 × 1000 μm) and are fabricated in large batches of over 200 cantilevers per wafer. By utilizing the bimorph action of a cantilever made of metal and piezoelectric ZnO, it is possible to achieve three-dimensional scanning motions using low-voltage feedback circuits. Initial results show a z-sensitivity of up to 2500 A/V for the cantilever, which is sufficient to use the device as an STM and obtain atomic resolution images. Processes for growing a sharp tunneling tip on the cantilever are being prepared and we envision integrating onboard signal-processing circuitry with the device. The piezoelectric bimorph cantilever with three-dimensional motion is likely to have numerous and diverse applications in the field of microfabrication.

Piezoelectric thin film ZnO on Si integrated microphone

Recent technology advances in thin film deposition and in fabrication of Si microstructures have made possible the development of a new class of “intelligent” sensors having electronics and sensing element integrated on the same Si chip. The present paper describes a fully integrated microphone which combines high sensitivity, wide frequency range, low power, and low cost of batch processed miniature silicon components. A thin piezoelectric layer of ZnO transforms the mechanical deflection of a thin Si diaphragm into a piezoelectric output voltage. This signal is directly connected to an amplifier built on the same chip. A unique annular electrode design provides cancellation of temperature induced parasitic signal due to the pyroelectric effect in ZnO. Integrated devices were fabricated and tested. Measured performances were: sensitivity of 25 μV/μbar, a signal to noise ratio of 5:1 at 2 μbars, a frequency response of 0.1 Hz to 10 kHz and a power consumption below 40 μW. Further improvements in the onchip electronics should increase the signal to noise ratio by an order of magnitude bringing its value in the range of the conventional hearing aid devices.

Electroelastic equilibrium of a thin anisotropic layer with piezoelectric effects taken into account: PMM vol. 42, no. 4, 1978, pp 731–736

The method of asymptotic integration [1] of three-dimensional equations of electroelasticity is used to construct a solution for the general cale of anisotropy of the problem of equilibrium of a thin piezoelectric layer the plane edges of which are free of electrode coating and are acted upon by specified, external mechanical forces. A fundamental iterative process is constructed, which makes possible the estimation of the validity of various simplifying assumptions made in the course of constructing the practical theories [2–4]. It is established that the absence of a plane of material symmetry within the layer parallel to its middle surface does not affect the construction of the first iterative step, but affects the derivation of the relations in the subsequent approximations.

Transverse waves in piezoelectric (622) crystal class

The transmission of transverse surface waves in piezoelectric (622) crystal class has been studied in four cases. In the first case, a thin piezoelectric layer is resting on a deep elastic layer whereas in the second case, both the layers are considered piezoelectric with different materials. In the third case, a piezoelectric stratum is bounded on both sides by deep elastic layers whereas, in the fourth case, the upper and lower elastic layers are replaced by piezoelectric layers of different materials. The wave velocity equations are obtained for all cases.

Depletion Layer High-Frequency Transducers

The depletion layer transducer is a new ultrasonic transducer made from piezoelectric semiconductors, such as gallium arsenide, and is best suited for use at microwave frequencies. The electromechanically active region is a thin, flat, high-resistance depletion layer such as a p-n junction or the surface barrier at a metal semiconductor contact. Since the transducer is a thin piezoelectric layer, its behavior is more like that of a flat piezoelectric transducer plate used at low frequencies than other microwave transducers. Advantages expected when the thickness is comparable to a half wavelength of sound are large electromechanical coupling and large bandwidth. The major difficulty is the low impedance due to the extreme thinness. It is also possible to change the thickness, hence the resonant frequency, by a dc bias. The first models utilized metal semiconductor contacts to form the depletion layers. Frequencies above 1000 Mc were generated and detected, and bandwidths of 5% were observed. The insertion loss was much higher than theoretically predicted. Current efforts are directed toward transducers formed by p-n junctions. It is anticipated that improvements in fabrication techniques and circuitry will extend the frequency range and lower the insertion loss.