WO2018096215A1

WO2018096215A1 - Sensor

Info

Publication number: WO2018096215A1
Application number: PCT/FI2017/050815
Authority: WO
Inventors: Panu KOPPINEN; Teuvo SILLANPÄÄ
Original assignee: Teknologian Tutkimuskeskus Vtt Oy
Priority date: 2016-11-24
Filing date: 2017-11-23
Publication date: 2018-05-31

Abstract

A sensor (1) comprising an ultrasonic transducer (2) and a resonance cavity (6) arranged in connection with the transducer (2). The cavity (6) has a constant cavity length. The sensor (1) further comprises at least one of a DC voltage tuning arrangement for adjusting a resonance frequency of the transducer (2) and a temperature compensation arrangement for adjusting a temperature of the cavity (6) for adjusting a resonance condition of the sensor (1). Also a method for adjusting a resonance condition of the sensor (1).

Description

SENSOR

FIELD OF THE INVENTION

The present invention relates to a sensor to be used in microelectro- mechanical systems or devices. BACKGROUND OF THE INVENTION

A sensor to be used in microelectromechanical systems or devices comprises an ultrasonic transducer to put out or emit as well as to receive ultrasound or ultrasonic waves. The ultrasound or the ultrasonic waves are applied in an identification of a substance or an agent to be measured, in a measurement of a property of a substance or an agent to be measured, or in a measurement of a physical magnitude of a phenomenon, such as sound or voice, to be measured by the sensor. The sensor like that may be used for example for measuring pressure, variation in acoustic pressure, magnetic field, acceleration, vibration, or a composition of gas or liquid.

The sensor comprises a resonance cavity arranged in connection with the transducer. For a proper operation of the sensor a resonance condition between the transducer and the cavity should be met. At the resonance condition a cavity length of the cavity matches typically to a half of the wavelength or a quarter of the wavelength of the ultrasound or the ultrasonic waves or any integer multiple of the half or the quarter of the wavelength of the ultrasound or the ultrasonic waves. This has been tried to achieve with very high accuracy of manufacture for the cavity during the fabrication of the sensor. The tight manufacturing tolerance requirements increase costs in the manufacturing of sensors but still do not necessarily provide sufficiently accurate resonance conditions because of possible variations of properties between individual sensors and the transducers therein.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a novel sensor applicable to be used also in microelectromechanical systems or devices and a method for using the sensor.

The invention is characterized by the features of the independent claim.

The invention is based on the idea of having a constant cavity length and using at least one of a DC voltage tuning arrangement for adjusting a reso- nance frequency of the transducer and a temperature compensation arrangement for adjusting a temperature of the cavity for adjusting a resonance condition of the sensor.

An advantage of the invention is that it may be affected on a resonance condition of the sensor so that a resonance frequency of the transducer can be matched to the fabricated cavity length of the cavity of the sensor, whereby the requirement for the accuracy of manufacture of the cavity need not be so tight as earlier.

Some embodiments of the invention are disclosed in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figure 1 shows schematically an axonometric view of a sensor;

Figure 2 shows schematically an axonometric view of a part of the sensor of Figure 1;

Figure 3 shows schematically an axonometric view of a part of the sensor of Figure 1;

Figure 4 shows schematically an axonometric view of a transducer of the sensor of Figure 1;

Figure 5 shows schematically a cross-sectional side view of a sensor according to Figure 1;

Figure 6 shows schematically an example of a DC bias tuning of a reso- nance frequency of capacitive micromachined ultrasonic transducer; and

Figure 7 shows schematically an example of a temperature tuning of a cavity of the sensor.

For the sake of clarity, the figures show some embodiments of the invention in a simplified manner. Like reference numerals identify like elements in the figures.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows schematically an axonometric view of a sensor 1. Figure 2 shows schematically an axonometric view of a part of the sensor 1 of Figure 1 and Figure 3 also shows schematically an axonometric view of a part of the sen- sor 1 of Figure 1. The sensor 1 comprises an ultrasonic transducer 2 being con- nected to or being in connection with an acoustic resonance cavity 6. Some examples of sensors like that are disclosed in the following description.

The ultrasonic transducer 2 is configured to put out or emit ultrasound or ultrasonic waves, or to receive or absorb ultrasound or ultrasonic waves or both to put out and receive ultrasound or ultrasonic waves. The ultrasound or ultrasonic waves may for example be applied in an identification of a substance or an agent, such as liquid or gas, in a measurement of a property of a substance or an agent, or in a measurement of a physical magnitude of a phenomenon, such as sound or voice, to be measured by the sensor 1. Figure 4 shows schematically an axonometric view of a transducer 2 which may be applied in the sensor 1.

The sensor 1 comprises a base plate 3. The base plate 3 provides a body of the sensor 1. The base plate 3 comprises a space 4 or a room 4 for accommodating the transducer 2 in the sensor 1. The base plate 3 thus provides or forms a frame or a holder for the transducer 2. According to an embodiment the base plate 3 is formed of a silicon wafer but the base plate 3 may be made of any other material applicable to be used for providing the base or the frame for the transducer 2.

In the embodiment of the base plate 3 disclosed in Figures 1 to 3 the space 4 is a hole arranged through the base plate 3 in the thickness direction thereof. The space 4 is thus arranged to extend from a top surface 3' or a front surface 3' of the base plate 3 up to a bottom surface 3" or a backside surface 3" of the base plate 3.

The sensor 1 further comprises a silicon-on-insulator plate 5, i.e. a SOI plate 5, made of a silicon wafer and arranged on top of the base plate 3. The sili- con-on-insulator plate 5 at least partly defines the resonance cavity 6 that is a free space extending horizontally and vertically in the sensor structure level provided by the silicon-on-insulator plate 5. The cavity 6 is located on top of the space 4 where the transducer 2 is arranged to remain, the cavity 6 being arranged to be open to the space 4 so that the transducer 2 is arranged to be connected to or to be in open connection with the cavity 6 when the transducer 2 is assembled in the sensor 1.

The ultrasound or the ultrasonic waves are generated into the cavity 6 by the transducer 2. The cavity 6 is also arranged to receive the substance or the agent to be identified or the property of which is to be measured, or arranged to be in connection with a phenomenon, such as sound or voice, the physical magnitude of which is to be measured. The cavity 6 may be formed of the silicon-on- insulator plate 5 by removing material from the silicon-on-insulator plate 5 for example by etching either before it is stacked on top of the base plate 3 or after it has been stacked on top of the base plate 3.

In the embodiment of the sensor 1 disclosed above the cavity 6 was at least partly defined by the silicon-on-insulator plate 5. The cavity 6 in the sensor 1 may, however, have a numerous number of different implementations so that a combination of an ultrasonic transducer 2 connected to or being in connection with an acoustic resonance cavity 6 is provided in the sensor 1.

The sensor 1 of Figures 1 to 3 further comprises a flow channel 8 which is arranged in connection with the cavity 6 and which is at least partly defined by the silicon-on-insulator layer 5. The flow channel 8 is arranged to extend substantially horizontally through the silicon-on-insulator plate 5 via the cavity 6, whereby the transducer 2 forms a bottom of the flow channel 8 at the cavity 6. The flow channel 8 is intended for a fluid exchange or a gas exchange in the cavity 6 of the sensor 1 when the fluid or the gas flowing through the cavity 6 is the substance or the agent which is to be identified or the property of which is to be measured with the sensor 1. Alternatively the flow channel 8 will bring the phenomenon, the physical magnitude of which is to be measured, into the cavity 6 into contact with the transducer 2. In the embodiment of the sensor 1 disclosed in the Figures 1 to 3 both ends 8' 8" of the flow channel 8 are open out of the sensor 1 so that the fluid or the gas may flow into the flow channel 8 from the first end 8' of the flow channel 8 and out of the flow channel 8 from the second end 8" of the flow channel 8.

If the fluid is liquid, the fluid may be composed of only one liquid or it may be a mixture of two or more different liquids. Alternatively, if the fluid is gas, the fluid may be composed of only one gas or it may be a mixture of two or more different gases. Alternatively the fluid may be a mixture of at least one liquid and at least one gas. The gas may be composed of only one gas or it may be a mixture of two or more gases.

The flow channel 8 is formed of the silicon-on-insulator plate 5 by removing material from the silicon-on-insulator plate 5 either after it has been stacked on top of the base plate 3 or before it is stacked on top of the base plate 3. The material removal may be implemented for example by etching. The bottom 8"' of the flow channel 8 is thereby formed for example by an insulation layer of the silicon-on-insulator plate 5 at other portions of the flow channel 8 but not at the cavity 6 at which the material of the silicon-on-insulator plate 5 is totally re- moved so that at the cavity 6 the bottom 8"' of the flow channel 8 is formed by the top surface of the transducer 2. Alternatively the bottom 8"' of the flow channel 8 at other portions of the flow channel 8 but not at the cavity 6 is provided by the top surface 3' of the base plate 3, which may be implemented by etching the sili- con-on-insulator plate 5 up to the top surface 3' of the base plate 3 or by forming the silicon-on-insulator plate 5 of two separate pieces that together form the sili- con-on-insulator plate 5.

The sensor 1 disclosed above is arranged to comprise the flow channel 8. Depending on the intended application of the sensor 1, the sensor 1 may, how- ever, be implemented without any flow channel 8.

The sensor 1 further comprises a top element 7 on top of the silicon- on-insulator plate 5 for terminating the cavity 6. According to an embodiment of the sensor 1 the top element 7 is formed of a silicon wafer. The distance between the transducer 2 and the top element 7, or in other words a thickness of the sili- con-on-insulator plate 5 determines a cavity length of the cavity 6, i.e. a vertical dimension of the cavity 6. For a proper operation of the sensor 1 the cavity length should be dimensioned such that a resonance condition between the cavity 6 and the transducer 2 is met. Generally in resonance condition of the sensor the cavity length is a half or a quarter of the wavelength of the ultrasound or the ultrasonic waves or any integer multiple of the half or the quarter of the wavelength of the ultrasound or the ultrasonic waves put out by the transducer 2.

When the sensor 1 is assembled, the silicon-on-insulator plate 5 is stacked onto the base plate 3 and the cavity 6 is formed as disclosed above unless the cavity 6 has been manufactured earlier in the silicon-on-insulator plate 5. Af- ter that the transducer 2 may be inserted into the space 4 in the base plate 3 through the hole in the bottom surface 3" of the base plate 3. A horizontal dimensioning of the cavity 6 is arranged to be smaller than a horizontal dimensioning of the space 4, whereby, when the transducer 2 is moved towards the front surface 3' of the base plate 3, the transducer 2 will stop at its final location at the bottom of the cavity 6 when the transducer 2 meets the silicon-on-insulator plate 5 that at least partly defines the cavity 6.

After that the top element 7 is stacked onto the silicon-on-insulator plate 5 for providing a sensor 1 having a three-layer structure. The different layers of the sensor 1, i.e. the base plate 3, the silicon-on-insulator plate 5 and the top element 7 as well as the transducer 2 are glued together with adhesive that does not deform when drying. According to an embodiment of the sensor 1, the transducer 2 is a ca- pacitive micromachined ultrasonic transducer, i.e. a CMUT. In CMUTs, an energy transduction is due to a change in capacitance in the transducer 2. The transducer 2 has a silicon substrate 9 formed of a silicon wafer and provides a base 9 of the transducer 2. The transducer 2 comprises a vacuum space 10 that is schematically shown later in Figure 5. The vacuum space 10 of the transducer 2 is formed in the silicon substrate 9. On top of the vacuum space 10 of the transducer 2 there is a thin vibrating member 11, such as a thin membrane. The vibrating member 11 comprises a metallized layer that acts as an electrode, together with the silicon substrate 9 which serves as a bottom electrode.

In the embodiment of Figure 4 the transducer 2 comprises a number of transducer elements 12 that are separate from each other, each element 12 having the vibrating member 11 of its own, meaning that the transducer 2 is formed as a composition of several transducer elements 12 wherein each element 12 provides an operable transducer unit. Some of the transducer elements 12 may put out the ultrasound and the rest of the transducer elements 12 may receive the ultrasound. In the embodiment of Figure 5 there is a single transducer 2 that is arranged to both put out and receive the ultrasound. Electrical contacts of the transducer 2 are shown only very schematically with boxes denoted with refer- ence signs 13, 14 and 15.

When an AC signal is applied across the contact elements 13, 14 with an AC voltage source 17 or oscillator 17, as shown schematically in Figure 5, the vibrating membrane 11 will produce the ultrasound or the ultrasonic waves in the medium or the substance or the agent flowing in the flow channel 8 or being in another way in connection with the cavity 6 of the sensor 1 and the transducer 2 at the bottom of the cavity 6. In that case the transducer 2 works as a transmitter. On the other hand, when the ultrasound or the ultrasonic waves are received onto the membrane 11 of the CMUT, it will generate alternating signal as the capacitance of the CMUT is varied, whereby the transducer 2 works as a receiver.

According to an embodiment the transducer 2 may be a piezoelectric micromachined ultrasonic transducer, i.e. a PMUT. PMUTs are based on the flex- ural motion of a thin membrane which is coupled with a thin piezoelectric film. The transducer 2 implemented as a PMUT can also function as a transmitter and a receiver depending on the intended use of the sensor 1.

General structures and operation principles of capacitive micromachined ultrasonic transducers and piezoelectric micromachined ultrasonic transducers are known for a person skilled in the art and therefore they are not considered here in more detail.

Other types of transducers, wherein an impedance of the transducer is modulated, are also applicable.

According to an embodiment of the sensor 1, the top element 7 is an

Application Specific Integrated Circuit, an ASIC. When the top element 7 of the sensor 1 is the ASIC, the sensor 1 may form an independently operable unit, i.e. all the electronics needed for the operation of the sensor 1 may be contained by the sensor 1 itself, or in other words, all the necessary electronics needed for the op- eration of the sensor 1 may be embedded into the ASIC. The sensor 1 may comprise electrical feed-through connections 16 arranged through the base plate 3, whereby the sensor 1 may be assembled in connection to a circuit board of the actual device where the sensor 1 is utilized via the electrical feed-through connections 16 extending through the base plate 3. A cross-sectional side view of a sen- sor 1 of this type is shown schematically in Figure 5.

According to an embodiment of the sensor 1, the top element 7 is a micro hotplate. When the top element 7 is the micro hotplate, the cavity 6 and/or the flow channel 8 of the sensor 1 as well as the fluid or the gas flowing in the flow channel 8 may be heated to a temperature suitable for the intended meas- urement operation or other intended application of the sensor 1.

According to an embodiment the sensor 1, and especially the transducer 2, may be tuned to operate at a resonance frequency by adjusting a DC bias of the transducer 2. This may be implemented by an adjustable or a regulable DC voltage source 18, which is coupled in parallel with the AC voltage source 17, as shown schematically in Figure 5. Alternatively the DC voltage source 18 could be coupled in series with the AC voltage source 17. With the DC voltage source 18 the bias of the vibrating member 11 of the transducer 2, or in other words a bias tension of the vibrating member 11, is tuned so that the transducer 2 will have a desired resonance frequency for an intended use of the sensor 1. Figure 5 disclos- es schematically a block 19, which contains necessary equipment or electronics for obtaining the actual information or variable intended to be measured. The block 19 may also contain necessary equipment for controlling the operation of the DC voltage source 18 so that the DC bias voltage providing the transducer 2 to operate at the resonance frequency is achieved. The control connection from the block 19 to the DC voltage source 18 is shown schematically with a line CL18 in Figure 5. If the top element 7 is implemented with the ASIC, the block 19, or es- pecially the functionalities provided by it may be implemented with the ASIC.

In Figure 6 it is shown an example of the DC bias tuning of a transducer 2 for adjusting a resonance condition of the sensor 1. In Figure 6 a transfer function of the transducer 2 in decibels is shown on y-axis and a frequency is shown on x-axis in megaherzes. In the example of Figure 6 three different DC biasing voltages is used, i.e.

20 V, Vwas = 15 V and 10 V. From Figure 6 it can be seen that with the DC biasing voltage

20 V a resonance of the transducer 2 is achieved at the frequency of about 2.69 MHz. The DC biasing will change the resonance frequency of the transducer 2 membrane 11 so that the resonance frequency can be matched to the fabricated cavity length, whereby the requirement for the accuracy of manufacture of the cavity 6 need not be so tight as in prior art.

According to an embodiment the sensor 1, and especially a cooperation of the transducer 2 and the cavity 6, may be tuned to operate at a reso- nance frequency by adjusting or tuning a temperature of the cavity 6 of the sensor 1, preferably so that the cavity 6 is in a constant temperature. The temperature of the cavity 6 of the sensor 1 may be adjusted by an adjustable or a regulable heating element arranged at the sensor 1. Figure 5 discloses a micro hotplate 20, which provides a kind of a heating element, arranged at the top element 7 above the cavity 6 for heating the cavity 6. Alternatively the micro hotplate 20 may form the entire top element 7. Alternatively the heating element, such as a micro hotplate 20, may be arranged at some other location in the sensor 1 as long as the heating element is able to heat the cavity 6 of the sensor 1.

By heating the cavity 6 of the sensor 1 it is changed a temperature of the medium or the substance or the agent flowing in the flow channel 8 or being in another way in connection with the cavity 6 of the sensor 1 and the transducer 2 at the bottom of the cavity 6. When the temperature of the medium or the substance or the agent changes, the speed or velocity of the ultrasound in the cavity 6 changes. The energy for heating the micro hotplate 20 may be provided for ex- ample by an adjustable or a regulable second DC voltage source 21 being electrically coupled to the micro hotplate 20, as also shown schematically in Figure 5. The block 19 may also contain necessary equipment for controlling the operation of the DC voltage source 21 so that the necessary heating power for achieving the cavity temperature providing the transducer 2 to operate at the resonance fre- quency is achieved. The control connection from the block 19 to the second DC voltage source 21 is shown schematically with a line CL21 in Figure 5. Instead of the micro hotplate 20 some other heating element may be used. Alternatively the DC voltage source 18 could be used as a source of electrical energy to heat the micro hotplate 20 or some other heating element. In the heating element the electrical energy is transformed to heat for heating the cavity 6 and the parts and materials at least partly defining the cavity 6.

In Figure 7 it is shown an example of a temperature tuning of the cavity 6 of a sensor 1 for adjusting a resonance condition of the sensor 1. In Figure 7 a transfer function of the transducer 2 in decibels is shown on y-axis and a frequency is shown on x-axis in megaherzes. In the example of Figure 7 two different cav- ity temperatures are used, i.e. T = 111 °C and T = 115 °C. From Figure 7 it can be seen that with the cavity temperature of T = 115°C a resonance condition of the transducer 2 is met at the frequency of about 2.69 MHz. The temperature tuning of the cavity 6 will affect on the temperature of the cavity 6 and the temperature of the medium or the substance or the agent therein so that the resonance condi- tion of the transducer 2 can be achieved although the actual cavity length varies from the designed cavity length, meaning that the requirement for the accuracy of manufacture of the cavity 6 need not be so tight as in prior art.

In the embodiments disclosed above only one of the DC bias tuning of the transducer 2 and the tuning of the temperature of the cavity 6 were applied for adjusting the resonance condition of the sensor 1. It is, however, also possible to use both tuning methods in the one and same sensor for adjusting the resonance condition of the sensor 1.

The sensor 1 as presented may be used for various applications.

According to an embodiment, the sensor 1 may be used as a gas sen- sor. The sensor can for example be used to measure both a damping and either a speed or a velocity of the ultrasound in the gas, whereby the gas can be determined or identified based on these measurements. Because the damping and the speed and velocity of the ultrasound depend on temperature and humidity of the gas, an accurate measurement may also require the measurement of the tempera- ture and humidity. The humidity of the gas may also be determined only from the damping of the ultrasound if measured in a broad frequency range. If the top element is implemented as a micro hotplate or if the top element comprises a micro hotplate, the temperature and/or humidity of the gas to be measured may be arranged to be a specific predetermined constant. In that case temperature and/or humidity measurement are not needed. This may be achieved for example by arranging the cavity to have a temperature that is substantially high relative to the temperature of ambient of the sensor.

The sensor 1 may be used correspondingly to determine or identify other fluids, such as liquids.

The sensor 1 having the top element 7 being formed of the micro hot- plate or comprising a micro hotplate may also be used as a combo gas sensor. The sensor 1 of this type may be arranged to measure the properties of inert gases by using the ultrasound or ultrasonic waves provided by the transducer 2 as well as the properties of volatile organic compounds by using a micro pellistor technique utilizing the micro hotplate to heat the gas flowing in the cavity 6 and in the flow channel 8 of the sensor 1. In the micro pellistor technique the sensor 1 comprises also a detecting element consisting of small pellets or thin film of catalyst loaded ceramic whose resistance changes in the presence of the gas. Some of the pellets or thin films of catalyst loaded ceramic require a gentle heating in use what may be provided by the micro hotplate.

According to an embodiment, the sensor 1 may be used as a pressure sensor. The pressure of the fluid or gas can be measured by determining a deflection of the vibrating membrane of the transducer 2 because of the fluid or gas affecting through the flow channel 8 and the cavity 6 to the vibrating membrane 11 of the transducer 2. This causes a change in the impedance of the transducer 2 which indicates the pressure of the fluid or gas.

According to an embodiment, the sensor 1 may be used as a magnetometer. In this application a coil, in which either direct current or alternating current travels, is arranged on top of the membrane of the transducer, whereby the membrane will either move or oscillate as a function of the external magnetic field. In the case of direct current the change in the impedance is determined, whereas in the case of alternating current impedance modulation taking place in the sensor 1 is inspected.

According to an embodiment, the sensor 1 may be used as a microphone. In this application the movement of the membrane 11 of the transducer 2 is measured, the movement of the membrane being directly proportional to the pressure and the effective surface area of the membrane 11 of the transducer 2.

In addition to the application areas listed above, the sensor 1 may also be utilized for location, velocity, acceleration, surface roughness and vibration measurement applications.

The measurement principle using the ultrasound or the ultrasonic waves for the applications mentioned above, or for other applications not specifi- cally listed above, are generally known for a person skilled in the art and therefore they are not described herein in more detail. For example WO-publication 2009/071746 Al discloses some possible applications listed above in more detail.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. A sensor (1) comprising

an ultrasonic transducer (2),

a resonance cavity (6) arranged in connection with the transducer (2), the cavity (6) having a constant cavity length, and

at least one of a DC voltage tuning arrangement for adjusting a resonance frequency of the transducer (2) and a temperature compensation arrangement for adjusting a temperature of the cavity (6) for adjusting a resonance condition of the sensor (1).

2. A sensor as claimed in claim 1, characterized in that the transducer (2) is one of a capacitive micromachined ultrasonic transducer and a DC biased tuneable piezoelectric transducer.

3. A sensor as claimed in claim 1 or 2, characterized in that the DC voltage tuning arrangement comprises an adjustable DC voltage source (18) for adjusting the resonance frequency of the transducer (2).

4. A sensor as claimed in any one of the preceding claims, charac- t e r i z e d in that

the temperature compensation arrangement comprises an adjustable heating element for heating the cavity (6).

5. A sensor as claimed in any one of the preceding claims, charac- t e r i z e d in that

the temperature compensation arrangement is arranged to adjust the temperature of the cavity (6) to a constant temperature.

6. A sensor as claimed in any one of the preceding claims, charac- t e r i z e d in that the sensor (1) further comprises

a base plate (3) comprising a space (4) for accommodating the transducer (2), the base plate (3) forming a frame for the transducer (2),

a silicon-on-insulator plate (5) on top of the base plate (3), the silicon- on-insulator plate (5) comprising the cavity (6) arranged in connection with the transducer (2), and a top element (7) on top of the silicon-on-insulator plate (5) for terminating the cavity (6).

7. A sensor as claimed in claim 6, characterized in that the top element (7) is the adjustable heating element for heating the resonance cavity (6).

8. A sensor as claimed in any one of claims 4 to 7, characterized in that the heating element is a microhotplate (20).

9. A sensor as claimed in claim 6, characterized in that the top element (7) is an Application Specific Integrated Circuit (ASIC).

10. A sensor as claimed in any one of claims 6 to 9, characterize d in that the sensor (1) is a gas sensor, wherein the silicon-on-insulator plate (5) at least partly defines a flow channel (8) arranged in connection with the cavity (6) for a gas exchange in the sensor (1), the flow channel (8) extending substantially horizontally through the silicon-on-insulator plate (5) via the cavity (6), and the transducer (2) forming a bottom of the flow channel (8) at the cavity (6).

11. A sensor as claimed in any one of claims 6 to 10, characterize d in that electrical feed-through connections (16) are arranged through the base plate (3) for electrical signals.

12. A method for adjusting a resonance condition of a sensor (1) comprising an ultrasonic transducer (2) and a resonance cavity (6) arranged in connection with the transducer (2) and having a constant cavity length, the method comprising

at least one of adjusting a resonance frequency of the transducer (2) and adjusting a temperature of the cavity (6) for adjusting a resonance condition of the sensor (1).

13. A method as claimed in claim 12, characterized by adjusting a resonance frequency of the transducer (2) by adjusting a DC biasing of the transducer (2).

14. A method as claimed in claim 12 or 13, c h a r a c t e r i z e d by adjusting the temperature of the cavity (6) by heating the cavity (6).

15. A method as claimed in any one of claims 12 to 14, c h a r a c - terized by

adjusting the temperature of the cavity (6) to a constant temperature.