WO2005034348A1

WO2005034348A1 - Piezoacoustic resonator and use of said piezoacoustic resonator

Info

Publication number: WO2005034348A1
Application number: PCT/EP2004/051909
Authority: WO
Inventors: Reinhard Gabl; Evan Green
Original assignee: Siemens Aktiengesellschaft
Priority date: 2003-09-30
Filing date: 2004-08-26
Publication date: 2005-04-14

Abstract

The invention relates to a piezoacoustic resonator (2) comprising a lower stratiform electrode (6) located on a substrate (3), an upper stratiform electrode (5) and a piezoelectric layer (4) interposed between the electrodes (5, 6). The resonator (2) is characterized in that the lower electrode (6) has a layer thickness that is selected from the range ranging from a tenth to nine tenth of a wavelength (μ) of the oscillation of the piezoelectric resonator (2). Preferably, the layer thickness of the lower electrode (6) substantially amounts to a fourth (μ/4) or three quarters (3μ/4) of the wavelength of the oscillation of the resonator (2). The inventive resonator is characterized by a very high mass sensitivity. The invention also relates to a method for detecting at least one substance of a fluid using the piezoacoustic resonator (2).

Description

Fiezoa ustischen resonator and use of the piezoacoustic resonator

The invention relates to a piezoacoustic resonator with a lower layered electrode arranged on a substrate, an upper layered electrode and a piezoelectric layer arranged between the electrodes, the piezoelectric layer and the electrodes being arranged in such a way that the electrodes are electrically driven to oscillate of the resonator with a certain resonance frequency. In addition, a method for the detection of at least one substance of a fluid using the piezoacoustic resonator is specified.

Biosensors are increasingly used in modern biological analysis technology and in medical diagnostics. A biosensor consists of a biological detection system for a biological substance and a so-called physical transducer. The substance is "recognized" via the biological recognition system. This "recognition" is converted into an electronic signal using the physical transducer. Antibodies, enzymes and nucleic acids are frequently used biological detection systems. The biological detection systems are usually immobilized (fixed) in approximately two-dimensional layers on the transducer. Immobilization (fixing) can take place through covalent bonds, through affinity interactions and through hydrophilic or hydrophobic interactions. I. Willner and E. Katz in Angew give an overview of a structure of almost two-dimensional biological recognition layers. Chem. 112 (2000), 1230 A device and a method for the detection of a substance of a fluid using a piezoacoustic resonator as a physical transducer is known from C. Koßlinger et al., Biosensors & Bioelectronics, 7 (1992), pp. 397-404. The surface section of the resonator represents a detection system for a substance. The piezoelectric layer of the known resonator consists of a quartz crystal. Gold electrodes are attached to the quartz crystal. Electrical control of the electrodes turns the quartz crystal into bulk acoustic waves (bulk

Akoustic aves) in the form of thickness shear vibrations. The resonance frequency is about 20 MHz. One of the electrodes forms the surface section for sorption of the substance of the fluid. The substance is a macromolecular protein that is in a liquid and is physically adsorbed on the electrode. Adsorption of the protein changes the mass and thus the resonance frequency of the resonator. The following general relationship applies to the change in the resonance frequency (Δf) as a function of the change in the amount of substance adsorbed per unit area (Δm) (see G. Sauerbrey, Zeitschrift für Physik, 155 (1959), pp. 206-222) :

_S = jL _{= c ∞f} > ( ₁ ) Am m

Here, S is the mass sensitivity of the resonator, fo is the Re ^¬ sonanzfrequenz of the resonator without adsorbed substance, c is a material-specific constant and the mass m of the resonator per unit area. The mass sensitivity is proportional to the square of the resonance frequency of the resonator. At a relatively low resonance frequency f ₀ of about 20 MHz, the mass sensitivity of the known device can be estimated at about 1 Hz-ng ^' ^ cm ² .

A physical transducer in the form of a piezoacoustic resonator is also known from V. Ferrari et al. , Sensors and Actua- tors, B 68 (2000), pp. 81-87. The piezoelectric layer of the piezoacoustic resonator is a lead zirconate titanate (PZT) layer. Layered electrodes (electrode layers) made of a silver-palladium alloy are attached to opposite sides of the PZT layer. The resonator can be excited to form a longitudinal oscillation (longitudinal oscillation) along the layer thickness of the PZT layer by means of electrical control of the electrodes.

This resonator has a surface section on which a substance can be sorbed. For this purpose, the resonator has a chemically sensitive coating forming the surface section. The chemically sensitive coating is a polymer film that is applied to one of the electrodes. The polymer film is, for example, polystyrene or polymethyl acrylate. Various substances, for example hydrocarbons, can be adsorbed on these polymer films. The mass of the resonator changes as a result of the adsorption. As a result, the resonance frequency of the resonator changes. An extent of the change in the resonance frequency depends on the adsorbed amount of the substance. The more substance is adsorbed, the greater the change in the resonance frequency.

The layer thickness of the PZT layer of the resonator is approximately 100 μm. The electrodes are about 10 μm thick. The polymer film is applied, for example, with a thickness of approximately 3 μm. A lateral extension of the resonator is approximately 6 mm. The resonance frequency of the resonator is approximately 7 MHz. The known device with the piezoacoustic resonator is suitable for the detection of a substance of a fluid. The fluid is either a liquid or a gas or gas mixture.

The resonator is applied to a substrate made of aluminum oxide. To manufacture the resonator or to apply the resonator to the substrate, the so-called thick film technology (Thick Film Technology, TFT). Due to the relatively low resonance frequency, the resonator is characterized by a relatively low mass sensitivity (see equation (1)). A low concentration of the substance of the fluid or a slight change in the concentration of the substance in the fluid can only be determined with a relatively large error.

From H. Baltes, Proceedings of the IEEE, Vol. 86, No. 8, August 1998, pages 1660-1678, a so-called flexural plate wave (FPW) sensor is known. The sensor has a piezoacoustic resonator, which is applied to a semiconductor substrate made of silicon. Vapor deposition processes, CMOS (Complementary Metal Oxide Semiconductor) technology and front or rear side etching of the semiconductor substrate (bulk micromachining) are used to manufacture the resonator. The electrodes and the piezoelectric layer are arranged on the semiconductor substrate in the form of a so-called cantilever in such a way that electrical control of the electrodes leads to a transverse oscillation of the resonator with a resonance frequency of approximately 140 kHz. The resonator has a chemically sensitive coating made of polyurethane or polysiloxane. These polymers are suitable for the adsorption and thus the detection of holographic hydrocarbons. The fluid is gaseous, for example. When the fluid is directed past the surface portion formed by one of the polymers, the hydrocarbons are adsorbed on the surface portion. Depending on the concentration of the hydrocarbons, the mass of the resonator changes and thus also the resonance frequency of the resonator. The lateral dimension of the resonator is relatively small. For example, it is 300 μm. However, due to the relatively low resonance frequency of approximately 140 kHz, the resonator is distinguished by a relatively low mass sensitivity. The object of the present invention is to show how a substance with an increased mass sensitivity compared to the prior art can be detected.

To achieve the object, a piezoacoustic resonator with a lower layer-shaped electrode arranged on a substrate, an upper layer-shaped electrode and a piezoelectric layer arranged between the electrodes is specified, the piezoelectric layer and the electrodes being arranged in such a way that an electrical control of the electrodes leads to an oscillation of the resonator with a certain resonance frequency. The resonator is characterized in that the lower electrode has a layer thickness that ranges from one tenth to nine tenths of a wavelength (λ)

Vibration of the piezoacoustic resonator is selected. The layer thickness of the lower electrode preferably corresponds essentially to a quarter (λ / 4) or three quarters (3λ / 4) of the wavelength of the oscillation of the resonator.

To achieve the object, a method for detecting at least one substance of a fluid using the piezoacoustic resonator is also specified with the following method steps: a) bringing the fluid and the piezoacoustic resonator together such that the substance can be sorbed on a surface section of the resonator and b) Determining a resonance frequency of the resonator, the amount of the substance sorbed at the surface section being deduced from the resonance frequency. The resonator is part of a device for detecting the substance of a fluid. For this purpose, a surface section for sorbing a substance of the fluid is arranged on the resonator such that the resonance frequency of the resonator is dependent on an amount of the substance sorbed on the surface section.

A layer thickness of the piezoelectric layer is preferably in the range from 0.1 μm to finally 20 μm and the resonance frequency of the vibration is selected from the range from 500 MHz to 10 GHz inclusive. A piezoacoustic resonator with such a thin piezoelectric layer is referred to as a thin-film or thin-film resonator.

The invention is based on the knowledge that, in the case of a thin-film resonator, the electrodes cannot be neglected with regard to the resonance properties. The upper and lower electrodes of the resonator are in the acoustic

Path of vibration. This is not taken into account in equation (1). Equation (1) relates to piezoacoustic resonators with a thick piezoelectric layer. According to the prior art on which the equation (1) is based, the piezoelectric layer is, for example, one

0.12 mm thick quartz crystal plate. In this case, the mass sensitivity essentially depends only on the properties of the quartz crystal. The influences of the electrodes, which are thin compared to the thick piezoelectric layer, can be neglected. In contrast, in a thin-film resonator there is a significant part of an acoustic wave in the electrodes. This means that the mass sensitivity depends not only on the material properties of the piezoelectric material, but also on the material properties of the electrode materials used and also on the dimensions of the electrodes. The dimensions are in particular the layer thicknesses of the electrodes.

An increased mass sensitivity of a piezoacoustic thin-film resonator can be observed when the layer thickness of the lower electrode is λ / 1 to λ / 9. A broad maximum occurs in each of the areas in which the layer thickness of the lower electrode corresponds to approximately a quarter and approximately three quarters of the wavelength of the oscillation of the resonator. For a lower electrode made of platinum, this means, for example, that when the resonator is driven with a fundamental frequency of approximately 2 GHz, the mass sensitivity is greatest when the layer thickness of the lower electrode is approximately 500 nm. Another maximum of the mass sensitivity occurs with a layer thickness of approximately 1500 nm. In contrast, a minimum occurs at a layer thickness of approximately 1000 nm.

Any electrode material is conceivable as the electrode material of the lower electrode. The electrode material is gold, for example. However, the lower electrode preferably has an electrode material selected from the group consisting of tungsten and / or platinum.

The resonator is preferably configured such that bulk acoustic waves are generated in the piezoelectric layer of the resonator by the electrical control. Longitudinal vibration and / or thickness shear vibration occur. Which type of vibration is preferably excited depends, among other things, on a symmetry group of the piezoelectric material of the piezoelectric layer, the orientation of the piezoelectric layer to the surface and the arrangement of the electrodes and the piezoelectric layer. For example, the piezoelectric layer consists of a <111> oriented lead zirconate titanate. If an electric field is only applied in the z direction along the layer thickness of the piezoelectric layer, there is primarily a longitudinal vibration along the layer thickness. In contrast, a thickness shear oscillation occurs in the arrangement described along a lateral extent of the piezoelectric layer, that is to say transversely to the z direction. For this purpose, the thickness shear oscillation requires a lateral component of the exciting electric field.

Bulk waves in the form of longitudinal vibrations are used in particular to examine a gaseous fluid. In the case of a liquid fluid, longitudinal vibrations are damped relatively strongly, which greatly reduces the mass sensitivity of the resonator. For examining a liquid fluid using the longitudinal vibrations of the resonators, the fluid is therefore removed from the chemically sensitive coatings of the resonators after sorption. The measurement of the resonance frequency of the resonator takes place after sorption in the absence of the fluid. In contrast, the measurement of the thickness shear vibrations of the resonator is suitable for the direct investigation of a liquid fluid. A thickness shear vibration is only imperceptibly dampened in a liquid. The measurement can take place when the resonators come into contact with the liquid fluid.

In a special embodiment, the piezoelectric layer has a piezoelectric material which is selected from the group of lead zirconate titanate, zinc oxide and / or aluminum nitride. These materials are suitable for depositing the materials from the gas phase on a substrate. The separation takes place, for example, in a chemical vapor deposition process (Chemical Vapor Deposition, CVD) or a physical vapor deposition process (Physical Vapor Deposition, PVD). The physical vapor deposition process is sputtering, for example. The small layer thickness of the piezoelectric layer is accessible with the aid of the vapor deposition process.

The resonator can be arranged on any substrate that has a low loss for high-frequency signals. This substrate has, for example, a sapphire as the dielectric. A high-frequency substrate is particularly conceivable. The high frequency substrate is characterized in that a high frequency signal with a high quality and thus with a low loss is passed on. In particular, an LTCC substrate (Low Temperature Cofired Ceramics) is used as the high-frequency substrate. Due to the use of glass ceramic sintering at low temperature, electrically highly conductive materials such as metallic copper or silver can be integrated in the LTCC substrate. In a special embodiment, the resonator is arranged on a semiconductor substrate. The semiconductor substrate has in particular a semiconductor material which is selected from the group consisting of silicon and / or gallium arsenide. These semiconductor materials are suitable for the application of bipolar and CMOS technology. With the help of these technologies, a circuit, for example an evaluation circuit for determining the resonance frequency of the resonator, can be integrated in the semiconductor substrate. The result is a high integration density.

In a special embodiment, the lower electrode is part of an acoustic mirror for acoustically isolating the resonator and the substrate from one another. The acoustic isolation of the resonator and the substrate ensures that the resonance frequency of the resonator is independent of the substrate. The result is a relatively high mass sensitivity. For this purpose, for example, the acoustic mirror is integrated in the substrate. The acoustic mirror is, for example, an acoustic Bragg reflector, which consists of several λ / 4 thick layers of different acoustic impedance. The lower electrode of the resonator can be one of the λ / 4 thick layers.

To further increase the mass sensitivity, it is ensured that the layer thickness of the upper electrode is as small as possible. In a special embodiment, a layer thickness of the upper electrode is therefore smaller than the layer thickness of the lower electrode. The layer thickness of the upper electrode is preferably less than a quarter of the layer thickness of the lower electrode.

According to a further embodiment, the upper electrode has at least one electrode material selected from the group aluminum and / or gold. Aluminum is characterized by a low density (about 2.7 g / cm ³ ). This leads to a further increase in the mass sensitivity of the resonator. gold is particularly suitable for the binding of thiol-containing molecules via the formation of sulfur-gold bonds. The surface portion for sorbing the substance is formed on the upper electrode of the resonator.

In particular, the upper electrode can be multi-layered. The upper electrode can consist of a relatively thick sub-layer made of aluminum, which ensures increased mass sensitivity. The aluminum partial layer has, for example, a partial layer thickness of 80 nm. On the aluminum sublayer, another gold sublayer is applied to the sorption of the substance. The gold partial layer has, for example, a partial layer thickness of 20 nm.

In a special embodiment, the surface section for sorption of the substance of the fluid is formed by a chemically sensitive coating of the resonator. A coating is applied to the resonator, which can sorb a certain substance or a certain substance class. According to the examples described above, the chemically sensitive coating can be formed by the upper electrode of the resonator or a partial layer of the upper electrode. The chemically sensitive coating can also be applied to the upper electrode or another area of the resonator. A coating applied in this way can be, for example, a polymer film described at the beginning. It is also conceivable that the coating has a specific DNA sequence. The corresponding DNA sequence can dock onto this DNA sequence according to the key-lock principle. The corresponding DNA sequence is chemisorbed to form hydrogen bonds. In this form, the piezoacoustic resonator is suitable for the detection of a specific DNA sequence. Finally, a chemically sensitive coating for various chemical and biological substances is also conceivable. Such a coating is, for example, a layer of silicon dioxide. Any conceivable chemical or biological compound can be considered as a substance. The prerequisite is that the substance is sorbed. Sorption means the formation of a chemical or physical bond between the substance and the surface section. Sorption encompasses both absorption and adsorption. During absorption, the substance is absorbed, for example, by a coating of the resonator, which forms the surface section, without forming a phase boundary. The substance is built into the coating. In contrast, a phase boundary is formed during adsorption. Adsorption in the form of a physisorption is particularly conceivable. The substance attaches to the surface section of the resonator through Van der Waals or dipole-dipole interactions. Alternatively, an adsorption in

Form of chemisorption. During chemisorption, the substance attaches to the surface section to form a chemical bond.

Because of the sorption, the device can be used as a gas sensor for detecting a gas. Any gases, for example carbon monoxide, carbon dioxide, nitrogen oxides or sulfur oxides, can be detected. Other examples are low molecular weight hydrocarbons such as methane, ethane, benzene and alcohols. In particular, a complex gas mixture such as air is also conceivable. The device can be used to monitor air quality. A gaseous substance can also be an explosive or a component, an intermediate or a decomposition product of an explosive. The device can be used as an explosives detector. It is also conceivable that the device is designed as a biosensor for the detection of any biomolecule. The biomolecule is, for example, a DNA (deoxyribonucleic acid) sequence or a macromolecular protein.

The surface section is preferably designed such that a specific substance or class of substances is selectively selected sorbed according to the lock and key principle and thus recognized. It is thus possible to selectively detect a specific substance from a mixture of a large number of substances with the aid of the device. The detection includes both a qualitative and a quantitative determination of the substance. The absence or presence of the substance in the fluid can be detected. The concentration of the substance in the fluid can also be determined. A differential change in the concentration of the substance can also be determined by differential detection of the substance. The resonator is therefore also suitable, for example, for controlling the reaction of a chemical reaction in which the substance is involved.

According to a special embodiment, the chemically sensitive coating has molecules for recognizing the substance. To recognize a specific DNA sequence, such molecules are corresponding oligo nucleotides (DNA oligos) made up of several nucleotide units.

The molecules for recognizing the substance can be directly connected to a transducer surface. For example, the transducer surface is a gold electrode of the resonator. Molecules that have a thiol group are bound directly to the transducer surface by forming a gold-sulfur bond.

In a special embodiment, the chemically sensitive coating has an immobilization layer for connecting the resonator and the molecules for recognizing the substance. For example, a transducer surface has NH or OH groups. The molecules for recognizing the substance can be immobilized via alkoxysilanes, cyanuric chloride or carbodiimide. These connections form the immobilization layer. Several such piezoacoustic resonators can be present for the simultaneous detection of different substances. The resonators are preferably designed such that a specific substance or a specific class of substance is detected in each case. It is also conceivable that several substances are detected with each of the resonators. In order to be able to draw conclusions about the concentrations of the individual substances, the mass sensitivities of the resonators with respect to the substances are different.

Each of the resonators can be arranged on a separate substrate. However, the resonators can also be combined on a common substrate to form a resonator matrix or a resonator row (resonator array). Each of the resonators forms a row element of the resonator row or a matrix element of the resonator matrix.

In order to be able to infer the amount of the substance sorbed on the surface section of a resonator, the resonance frequency of the resonator without the sorbed substance is generally determined before the fluid and the resonator are brought together. The resonance frequency is then determined with the substance sorbed on the surface section. The resonance frequency is preferably determined after the fluid and the resonator have been brought together in the presence of the fluid. For example, the fluid is a gas or gas mixture. The gas mixture is conducted past the surface section of one or more resonators. The substance is sorbed on the surface section. Sorption changes the mass of the resonator and thus the resonance frequency of the resonator. The more substance is sorbed, the greater the change in the resonance frequency compared to the resonator, on the surface section of which no substance is adsorbed.

According to a further embodiment, the resonance frequency is determined in the absence of the fluid. For example, in a first step, the fluid is passed by sonators. Sorption takes place. The fluid is then removed such that the substance remains sorbed on the surface section of the resonator. The resonance frequency of the resonator is then determined. This method is used in particular when a liquid is used as the fluid and the longitudinal vibration of the resonator is used as the vibration. Incidentally, it is also conceivable that the resonance frequency is determined both in the presence and in the absence of the fluid.

The piezoacoustic resonator and its application for the detection of a substance of a fluid are presented on the basis of several exemplary embodiments and the associated figures. The figures are schematic and do not represent true-to-scale illustrations.

Figure 1 shows a piezoacoustic resonator in a lateral cross section.

FIG. 2 shows a profile of the mass sensitivity of a piezoacoustic resonator from the layer thickness of the lower electrode.

FIG. 3 shows the course of the mass sensitivity of the piezoacoustic resonator from the layer thickness of the lower electrode, depending on the electrode material of the upper electrode.

Figure 4 shows a multi-layer electrode.

FIG. 5 shows a method for the detection of a substance of a fluid.

The piezoacoustic resonator 2 is a component of a device 1 for detecting a substance. The resonator 2 consists of a layer structure composed of a lower electrode 6, a piezoelectric layer 4 and an upper electrode 5. The lower one Electrode 6 of resonator 2 is applied to a semiconductor substrate 3.

The piezoelectric layer 4 of the resonator 2 is made of lead zirconate titanate. The lead zirconate titanate has a <111> orientation with respect to the semiconductor substrate 3. The layer thickness 7 of the piezoelectric layer 4 is approximately 0.8 μm. The lateral dimension 11 of the resonator 2 is approximately 100 μm. In two alternative designs, the piezoelectric layer 4 consists of aluminum nitride or zinc oxide.

The electrodes 5 and 6 are arranged on two sides of the piezoelectric layer 4 facing away from one another. Electrical insulation 12 made of aluminum oxide, which is attached to the side, additionally separates the electrodes 5 and 6.

The resonator 2 is excited to oscillate by electrically actuating the electrodes 5 and 6 with a fundamental frequency of approximately 2 GHz. Depending on the arrangement of the piezoelectric layer 4 and the electrodes 5 and 6 relative to one another, the resonator 2 is excited to a thickness shear vibration along a lateral extent 11 of the piezoelectric layer 4 and / or to a longitudinal vibration along the layer thickness 7 of the piezoelectric layer 4.

The upper electrode 5 is made of gold with a layer thickness 51 of approximately 0.1 μm. In an alternative exemplary embodiment, the upper electrode 5 is multilayer (FIG. 4). The multilayer electrode 5 consists of a relatively thick partial layer 53 made of aluminum with a partial layer thickness 531 of approximately 0.08 μm and a relatively thin partial layer 52 of gold with a partial layer thickness 521 of approximately 0.02 μm applied thereon.

The lower electrode 6 is made of platinum. The layer thickness 61 of the lower electrode 6 is approximately a quarter of the wave length of the vibration of the piezoacoustic resonator 2. This means that the layer thickness is approximately 0.5 μm.

In order to increase the mass sensitivity of the resonator 2, a device 15 for acoustically isolating the resonator 2 and the semiconductor substrate 3 from one another is provided. The resonator 2 and the semiconductor substrate 3 are acoustically isolated from one another. The device 15 is a Bragg reflector with λ / 4 thick layers of different impedance. The lower electrode 6 can be regarded as one of the layers of the acoustic mirror.

FIG. 2 shows a simulation result for the resonator 2, in which the upper electrode 5 consists only of gold. The mass sensitivity 21 (in Hz-ng ^_1- cm ² ) is plotted as

Function of the layer thickness 61 (in nm) of the lower electrode 6 made of platinum. The following considerations form the basis for the simulation results: Since the mirror does not have perfect reflection, a frequency-dependent phase change (dφjdf) takes place at a boundary between the piezoelectric layer and the electrode (the lower electrode 6 is considered here as part of the acoustic mirror). If the upper electrode 5 is regarded as thin against the wavelength, then a mass coating on the upper electrode 5 causes an additional phase shift (dφ / dm Δm). Since these changes are small, the total change in phase (Δφ) is given by:

Aφ ^ Af ^ Am (2) of dm

The resonance condition for such a resonator requires that the phase is equal to π over a return movement of the acoustic wave, ie half a wavelength must be in the resonator. From this condition follows a relationship between the resonance frequency (fo), the basic phase (φo) and its derivatives: dφ (3)

Substituting equation (3) into equation (2), we get the equation modified to (1)

/ _{s =} ^ ₌ _pä A

with d the thickness of the piezoelectric layer, p the density of the resonator material and A the area of the resonator. Equation (4) shows that the sensitivity differs significantly from that

Properties of the mirror depends and thus also on the lower electrode 6. Since, strictly speaking, the thickness of the upper electrode 5 is not thin in the case of 2 GHz resonators, the sensitivity also depends essentially on the material and dimensions of the upper electrode.

In the areas in which the layer thickness is 61λ / 4 and 3λ / 4, that is 500 nm and 1500 nm (FIG. 2), the mass sensitivity 21 is in each case characterized by a broad maximum.

FIG. 3 also shows simulation results for the mass sensitivity 31 of the resonator 2. The layer thickness 61 of the lower electrode 6 is varied. The electrode material of the upper electrode 5 is also varied. The layer thickness 51 of the upper electrode 5 is 0.1 μm in each case. The lowest mass sensitivity results with the platinum 54 electrode material. A medium mass sensitivity results for the upper electrode from the electrode material gold 55. Highest mass sensitivity results for the electrode material aluminum 56. In any case, a maximum of the mass sensitivity results with a layer thickness 61 of the lower electrode 6 of approximately 500 nm. The simulation results are correct Incidentally, they are in very good agreement with measurement results 541 and 542 for the electrode material platinum 54 of the upper electrode was obtained.

So that the resonator 2 can be used as a device for detecting a substance, the resonator 2 has a surface section 8, onto which a substance of a fluid 9 can be sorbed. For this purpose, the resonator 2 has a chemically sensitive coating 10. The chemically sensitive coating 10 is applied to the electrode 5.

To detect the substance of the fluid 9, the surface section 8 of the resonator 2 and the fluid 9 are brought together in a first step (FIG. 5, step 501). Fluid 9 and resonator 2 are brought together in such a way that the substance of the fluid 9 can be sorbed on the surface section 8 of the resonator 2. The mass of the resonator 2 changes as a result of the sorption. By subsequently measuring the resonance frequency of the resonator 2 (FIG. 5, step 502), it is possible to infer the type of substance and its concentration in the fluid 9. The sorption of the substance changes the resonance frequency of the resonator 2 compared to the resonance frequency of the resonator 2, on the surface section 8 of which no substance is sorbed. In order to be able to determine the change in the resonance frequency, a resonator 2 with a known resonance frequency is used. In an alternative embodiment, the resonance frequency of the resonator 2 without the sorbed substance is determined before the fluid 9 and the resonator 2 are brought together.

Claims

claims

1. Piezoacoustic resonator (2) with a lower layered electrode (6) arranged on a substrate (3), an upper layered electrode (5) and a piezoelectric layer (4) arranged between the electrodes (5, 6), wherein the piezoelectric Layer (4) and the electrodes (5, 6) are arranged in such a way that electrical control of the electrodes (5, 6) leads to oscillation of the resonator (2) with a specific resonance frequency, characterized in that - the lower electrode (6) has a layer thickness (61) which is selected from the range from one tenth to nine tenths of a wavelength of the oscillation of the piezoacoustic resonator (2).

2. Resonator according to claim 1, wherein the layer thickness (61) corresponds substantially to a quarter or three quarters of the wavelength of the oscillation of the resonator (2).

3. Resonator according to claim 1 or 2, wherein - a layer thickness (7) of the piezoelectric layer (4) from the range of 0.1 μm up to and including 20 μm and the resonance frequency of the oscillation from the range of 500 MHz up to and including 10 GHz is selected.

4. Resonator according to one of claims 1 to 3, wherein the piezoelectric layer (4) comprises a piezoelectric material which is selected from the group aluminum nitride, lead zirconate titanate and / or zinc oxide.

5. Resonator according to one of claims 1 to 4, wherein the lower electrode (6) has an electrode material selected from the group consisting of tungsten and / or platinum.

6. Resonator according to one of claims 1 to 5, wherein the substrate (3) is a semiconductor substrate with a semiconductor material.

7. Resonator according to one of claims 1 to 6, wherein the lower electrode (6) is part of an acoustic mirror (15) for acoustic isolation of the resonator (2) and the substrate (3) from each other.

8. Resonator according to one of claims 1 to 7, wherein a layer thickness (51) of the upper electrode (5) is smaller than the layer thickness (61) of the lower electrode (6).

9. The resonator according to claim 8, wherein the layer thickness (51) of the upper electrode (5) is less than a quarter of the layer thickness (61) of the lower electrode (6).

10. Resonator according to one of claims 1 to 9, wherein the upper electrode (5) comprises an electrode material selected from the group aluminum and / or gold.

11. Resonator according to one of claims 1 to 10, wherein the upper electrode (5) is multilayer.

12. Resonator according to one of claims 1 to 11, wherein a surface section (8) for sorbing a substance of a fluid (9) is arranged on the resonator (2) such that the resonance frequency of the resonator (2) is dependent on one on the surface section ( 8) amount of substance sorbed.

13. The resonator according to claim 12, wherein the surface section (8) for sorbing the substance of the fluid (9) from a chemically sensitive coating (10) of the resonator (2) is formed.

14. A method for detecting at least one substance of a fluid using the piezoacoustic resonator according to one of claims 1 to 13 with the following method steps: a) bringing the fluid and the piezoacoustic resonator together such that the substance can be sorbed on the surface section of the resonator and b) determining a resonance frequency of the resonator, the amount of the substance sorbed at the surface section being deduced from the resonance frequency.

15. The method of claim 14, wherein the resonance frequency is determined in the presence of the fluid.

16. The method of claim 14 or 15, wherein the resonance frequency is determined in the absence of the fluid.

17. The method according to claim 16, wherein a liquid is used as the fluid and after the bringing together of the fluid and the resonator and before the determination of the resonance frequency, the fluid is removed in such a way that the substance remains sorbed on the surface section of the resonator.