CN101419109A - SAW pressure sensor and sensor making method thereof - Google Patents

Abstract

The invention relates to a SAW pressure sensor, which is characterized in that it includes two parts: a sensor and a wireless readout circuit. The sensor includes a substrate made of silicon material, and the middle of the back of the substrate is thinned to form a film structure. The structure is the high deformation area of the substrate, and the rest is the fixed end of the substrate; the front of the substrate is covered with a piezoelectric material film, and two groups of SAW elements are symmetrically arranged on the piezoelectric material film, and the SAW element includes a radio frequency transducer A reflector and a muffler, each set of SAW components corresponds to a frequency; the RF transducer is located at the fixed end of the substrate, the reflector is located in the high deformation area of the substrate, and the muffler is located at the connection line between the RF transducer and the reflector on the extension line; the wireless readout circuit includes a radio frequency system and a processing system for wirelessly reading out the pressure of the sensor. The invention uses SAW components to realize passive wireless pressure measurement, and has the characteristics of simple manufacture, wide adaptable temperature range and strong electromagnetic interference resistance.

Description

A kind of SAW pressure sensor and its sensor part manufacturing method

technical field

The invention relates to a pressure sensor, in particular to a wireless passive automobile tire SAW (surface acoustic wave) pressure sensor and a manufacturing method of the sensor part thereof.

Background technique

Due to the particularity of car tires, the usual sensors are limited in connection and cannot measure car tire pressure. Therefore, it is required that the sensor used in automobile tire pressure measurement has the function of wirelessly reading out the measurement results. In addition, due to the airtightness of the working environment of the sensor used for automobile tire pressure measurement, it is impossible to replace the battery of the sensor frequently, which requires reducing the power consumption of the sensor as much as possible. Traditional wireless sensor systems include active devices, radio frequency circuits and other high power consumption circuits, which are not suitable for long-term work when powered by batteries. In recent years, in order to solve the above problems, some passive device sensors based on SAW components and using the principle of resonance or measuring load have emerged. However, these sensors have low measurement sensitivity and complex structures, which are not suitable for precise measurement and large-scale industrial production in a large range.

Contents of the invention

In view of the above problems, the object of the present invention is to provide a SAW pressure sensor and a method for making the sensor part thereof, which is used for measuring the pressure of automobile tires, has high sensitivity and simple manufacturing process.

To achieve the above object, the present invention adopts the following technical solutions: a SAW pressure sensor, characterized in that: it includes two parts, a sensor and a wireless readout circuit, and the sensor part includes a substrate made of silicon material, and the The film-forming structure is thinned in the middle of the back of the substrate, the film structure is the high deformation region of the substrate, and the unthinned part is the fixed end of the substrate; the front of the substrate is covered with A piezoelectric material film, two groups of SAW elements are symmetrically arranged on the piezoelectric material film, the SAW elements include a radio frequency transducer, a reflector and a muffler, and each group of the SAW elements responds to a frequency respectively; The radio frequency transducer is located at the fixed end of the substrate, and the radio frequency transducer is connected to an antenna; the reflector is located in the high deformation region of the substrate, between the reflector and the radio frequency transducer The piezoelectric material thin film forms a SAW delay line; the muffler is located on the extension line of the radio frequency transducer and the reflector connection; the wireless readout circuit part includes a radio frequency system and a processing system, so The radio frequency system includes a local oscillator, the local oscillator is respectively connected to a frequency synthesizer and a mixer, the frequency synthesis is connected to a power amplifier, and the power amplifier is respectively connected to a transmitting antenna, and the mixer Connect a low noise amplifier, and the low noise amplifier is connected to a receiving antenna; the processing system includes a digital signal processor, a high-speed sampler and an intermediate frequency filter, and the digital signal processor is respectively connected to the frequency synthesizer and the high-speed sampler, the high-speed sampler is connected to the intermediate frequency filter, and the intermediate frequency filter is connected to the mixer.

The thickness of the membrane structure is at most 1/4 of the thickness of the substrate.

The radio frequency transducer is a cross unit formed of aluminum metal, and the reflector is a closed ladder structure.

A leakage signal is drawn out from the output terminal of the power amplifier, and is input to the mixer after a phase shift of 180 degrees.

The wireless readout circuit uses direct frequency synthesis technology to form a frequency synthesis system. One wireless readout circuit supports the measurement of multiple sensor parts. Different sensor parts are distinguished by different frequency groups, and frequency division multiple address access method.

A method for manufacturing the sensor part in the above-mentioned SAW pressure sensor, which includes the following steps: 1. Using a piece of <100> crystalline double-thrown silicon wafer as the substrate, the thickness of the silicon wafer is about 200 microns; 2. Silicon nitride is deposited on the upper and lower surfaces of the silicon wafer; 3. Photoetching the silicon nitride window, using anisotropic silicon to etch the silicon wafer to form a silicon cup structure. The thickness of the etched circular film is about 50 microns, and the nitrogen is removed after the etching is completed. 4. Deposit the piezoelectric material zinc oxide on the silicon wafer to form a piezoelectric material film; 5. Coat the photoresist on the zinc oxide, and cover the photoresist with a mask plate with SAW element pattern, to light The resist is exposed, developed, and finally aluminum is sputtered and the glue is removed, so that the SAW element of aluminum material is formed by photolithography on the zinc oxide.

The present invention has the following advantages due to the adoption of the above technical solutions: 1. The present invention receives and emits electromagnetic waves through the radio frequency transducer in the sensor part and the radio frequency system in the wireless readout circuit, thereby realizing passive wireless pressure measurement. 2. The present invention adopts piezoelectric material film and dual-frequency processing method, so that the sensitivity of the pressure sensor is high and the measurement range is large. 3. Since the sensor part of the present invention uses silicon material as the substrate, and the sensor part does not amplify and filter electromagnetic signals, the present invention has the characteristics of high temperature resistance and electromagnetic radiation resistance, and can be applied in harsh environments. 4. Since the present invention does not have the complicated technological process in the traditional integrated circuit, and the structure and manufacturing process of the sensor part are simple, the production cost is low.

Description of drawings

Fig. 1 is a partial top view of the sensor of the present invention

Fig. 2 wireless readout circuit part principle block diagram of the present invention

Figure 3 is a cross-sectional view of Figure 1

Fig. 4 is the structural representation of radio frequency transducer of the present invention

Fig. 5 is a schematic structural view of the reflector of the present invention

Fig. 6 schematic diagram of the working principle of the sensor part of the present invention

Fig. 7 a～e is the flow chart of the fabrication process of the sensor part of the present invention

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

As shown in Fig. 1 and Fig. 2, the present invention includes two parts: a sensor and a wireless readout circuit.

As shown in Fig. 1 and Fig. 3, the sensor part of the present invention includes a substrate 1 made of silicon material, and a circular cavity, namely a silicon cup, is etched out in the middle of the back of the substrate 1. The thinned part on the substrate 1 forms a circular membrane 2 like this, and the thickness of the circular membrane 2 is at most 1/4 of the substrate 1 thickness, and this circular membrane 2 is the high deformation region of the substrate 1, and the substrate 1 and the rest are fixed ends. The front side of the substrate 1 is covered with a piezoelectric material film 3, and two groups of SAW elements are symmetrically arranged on the piezoelectric material film 3, and each group of SAW elements includes a radio frequency transducer 4, a reflector 5 and a muffler 6, each group The SAW elements respond to a frequency f ₁ , f _{2 ,} respectively. The radio frequency transducer 4 can convert radio frequency radio waves of a specific frequency into surface acoustic waves or convert surface acoustic waves of the frequency into radio frequency electric waves. The radio frequency transducer 4 is located at the fixed end of the substrate 1, and the radio frequency transducer 4 is connected to an antenna (not shown in the figure) for receiving and emitting electromagnetic waves, so as to realize communication with the wireless readout circuit. The reflector 5 is located on the circular film 2 of the substrate 1, and the piezoelectric material film 3 between the reflector 5 and the radio frequency transducer 4 forms a SAW delay line. The muffler 6 is located on the extension line of the connection between the radio frequency transducer 4 and the reflector 5, it can absorb surface acoustic waves, and its function is to eliminate the interference of multiple reflected waves. As shown in FIG. 4 , the radio frequency transducer 4 is a cross unit formed of aluminum metal. As shown in FIG. 5 , the reflector 5 is a closed ladder structure.

As shown in FIG. 2 , the wireless readout circuit part includes a radio frequency system 7 and a processing system 8 . The radio frequency system 7 includes a local oscillator 71, the signals of two measurement frequencies are generated by the local oscillator 71, after being processed by the frequency synthesizer 72 and amplified by the power amplifier 73, they are transmitted by the transmitting antenna 74, and then transmitted by the radio frequency transducer 4. An electromagnetic echo is generated after the antenna picks up. The wireless receiving antenna 75 receives the reflected electromagnetic echo signal, which is amplified by the low noise amplifier 76 and then input to the mixer 77. The mixer 77 mixes the amplified echo to an intermediate frequency signal. The processing system 8 includes a digital signal processor 81 (DSP), a high-speed sampler 82 and an IF filter 83 . The intermediate frequency signal is sent back to the DSP after passing through the intermediate frequency filter 83 and the high-speed sampler 82, and the DSP uses the method of fast Fourier transform (FFT) to filter and process, and measure the phases of the signals of the two measurement frequencies. The phase of the signal on a frequency, the deformation is calculated by DSP, and finally the pressure is calculated. In the wireless readout circuit, the signal transmitted from the transmitting antenna 74 will directly enter the receiving antenna 75. In addition, the transmitted electromagnetic wave may be reflected by other obstacles to form clutter echo, which will cause interference to the measurement. Therefore, a leaked signal is drawn from the output terminal of the power amplifier 73, and after a phase shift of 180 degrees, it is fed into the mixer 77 to compensate for the interference.

The wireless readout circuit of the present invention adopts direct frequency synthesis (DDS) technology to form a frequency synthesis system. One wireless readout circuit supports partial measurement of multiple sensors. Different sensors are distinguished by different frequency groups, and frequency division multiple access is used. In this way, the difference between two frequencies in different frequency groups is a constant constant. At the same time, in order to avoid mutual interference of multiple wireless readout circuits, an intermittent working mode is adopted to minimize the occupation of radio wave frequency resources.

The principle of the invention is to measure the elastic deformation of the material by using the propagation reflection theory of the surface acoustic wave and the piezoelectric effect, thereby indirectly calculating the pressure on the sensor part. In the present invention, the electromagnetic wave is emitted by the wireless readout circuit part, and the radio frequency transducer 4 of the sensor part receives the electromagnetic wave, and converts the electromagnetic wave of a specific frequency into a surface acoustic wave, and the surface acoustic wave propagates on the substrate 1 of the SAW element and encounters reflection The radio frequency transducer 5 is reflected back to the radio frequency transducer 4, and the radio frequency transducer 4 converts it into an electromagnetic echo, and the receiving antenna 75 receives the electromagnetic echo, and the wireless readout circuit calculates the surface acoustic wave by measuring the phase difference of the electromagnetic echo propagation distance. When there is pressure, the SAW element deforms, causing the propagation distance of the surface acoustic wave to change, thereby causing the phase of the electromagnetic echo to change. The wireless readout circuit calculates the deformation of the SAW element by calculating the change of the phase, and then calculates the pressure on the sensor part. The principle of the present invention will be described in detail below through an embodiment.

As shown in FIG. 6, when a pressure P acts on the piezoelectric material film 3 of the substrate 1, the piezoelectric material film 3 deforms. On a circular membrane 2 with thickness h and radius R, the radial strain ε _r at a point r away from the center of the circle is:

${\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 8</span>}{\mathrm{<span\; class="notranslate">\; Eh</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; R</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 3</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span>$

Among them, P is the pressure on the sensor part, E is the Young's modulus of the material, v is Poisson's ratio, and r is the distance from any point in the circular membrane to the center of the circle. Therefore, the total change in length between the reflector 5 and the radio frequency transducer 4 on the circular film 2 is:

$\mathrm{<span\; class="notranslate">\; \&Delta;R</span>}<span\; class="notranslate">\; =</span>\frac{\sqrt{\mathrm{<span\; class="notranslate">\; 3</span>}}\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; twenty\; four</span>}\mathrm{<span\; class="notranslate">\; E.</span>}{\mathrm{<span\; class="notranslate">\; h</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; R</span>}}^{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; .</span>$

When the radio frequency transducer 4 receives the electromagnetic wave, it converts it into a surface acoustic wave, propagates to the reflector 5 along a straight line, is reflected back to the radio frequency transducer 4, and is converted into an electromagnetic wave, which is received by the wireless readout circuit. In this process, the total stroke of the surface acoustic wave is 2(R+ΔR). If the velocity of the surface acoustic wave is v and the frequency is f, then the reflected wave of the RF transducer 4 has a phase change of 2πf( R+ΔR)/v. By measuring the phase change of the electromagnetic wave, calculate ΔR, and substitute it into the above formula to calculate the pressure P on the sensor part.

There is a serious problem in the above measurement process. Since the cosine function is a multi-valued function, in the above measurement process, one phase change corresponds to the deformation of multiple membranes, and the difference between these deformation values is an integer multiple of half a wavelength. That is to say, when the possible deformation range is larger than the half-wavelength of the used frequency raw surface wave, it will make the measurement result indistinguishable and cause the measurement to be blurred. In order to solve the above problems, the sensor part uses signals of two frequencies to measure the deformation simultaneously. Assuming that the phase change measured by the electromagnetic wave with frequency f ₁ is φ ₁ , then

${\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;R</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; ,</span>$

Similarly, the phase change measured by the electromagnetic wave with frequency f ₂ is

${\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;R</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; .</span>$

Assuming that the difference between two frequencies is Δf=f ₁ -f ₂ , and measuring the phase difference of two different frequencies, we can get

${\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\frac{\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;R</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; v</span>}}<span\; class="notranslate">\; .</span>$

It can be seen that in this case the maximum resolvable deformation measured becomes v/2Δf. If the difference between the two frequencies is not very large, that is, Δf is small, the resolvable deformation is greatly increased, which solves the problem of too small measurement range. In the actual process, the propagation of electromagnetic waves will also cause a certain phase change, but due to the high speed of electromagnetic waves, this phase change is negligible compared to the phase change of surface acoustic waves.

The manufacturing method of the sensor part of the present invention is:

1. As shown in FIG. 7a, a <100> double-thrown silicon wafer 7 is used as the substrate 1, and the thickness of the silicon wafer 7 is about 200 microns.

2. As shown in FIG. 7 b , deposit silicon nitride 8 on the upper and lower surfaces of the silicon wafer 7 .

3. As shown in Figure 7c, photoetching the silicon nitride 8 window, using anisotropic silicon to etch the silicon wafer 7 to form a silicon cup structure, the thickness of the etched circular film 2 is about 50 microns, and the silicon nitride is removed after the etching is completed 8.

4. As shown in FIG. 7 d , deposit the piezoelectric material zinc oxide 9 on the silicon wafer 7 to form a piezoelectric material film 3 .

5. As shown in Figure 7e, apply photoresist on the zinc oxide 9, cover the photoresist with a mask plate with SAW element graphics, expose and develop the photoresist, and finally sputter aluminum and remove the glue. A SAW element of aluminum material is formed by photolithography on the zinc oxide 9 .

In the above embodiments, the silicon cup can be in other shapes such as circle, ellipse, square or regular polygon.

Although the preferred embodiments and drawings of the present invention are disclosed for the purpose of illustration, the purpose is to help understand the content of the present invention and implement it accordingly, those skilled in the art will not depart from the present invention and the appended claims Various substitutions, changes and modifications may be made within the spirit and scope of . Therefore, the present invention should not be limited to the content disclosed in the preferred embodiment and the accompanying drawings, and the scope of protection of the present invention should be defined by the appended claims.

Claims (8)

1, a kind of SAW pressure transducer, it is characterized in that: it comprises sensor and two parts of wireless sensing circuit, described Sensor section comprises a substrate that adopts silicon materials to make, attenuate in the middle of the back side of described substrate is become membrane structure, described membrane structure is the high deformation district of described substrate, and the part that is not thinned is the stiff end of described substrate; The front of described substrate is coated with piezoelectric material film, is symmetrically arranged with two groups of SAW elements on the described piezoelectric material film, and described SAW element comprises a radio frequency transducer, a reverberator and a sound suppressor, and every group of described SAW element responds a frequency respectively; Described radio frequency transducer is positioned at the stiff end of described substrate, and described radio frequency transducer connects an antenna; Described reverberator is positioned at the high deformation district of described substrate, and the described piezoelectric material film between described reverberator and the radio frequency transducer forms the SAW delay line; Described sound suppressor is positioned on the extended line of described radio frequency transducer and reverberator line;

Described wireless sensing circuit partly comprises a radio system and a disposal system, described radio system comprises a local oscillator, described local oscillator connects a frequency synthesizer and a frequency mixer respectively, described frequency synthesis connects a power amplifier, described power amplifier connects an emitting antenna respectively, described frequency mixer connects a low noise amplifier, and described low noise amplifier connects a receiving antenna; Described disposal system comprises a digital signal processor, a high-speed sampler and an intermediate-frequency filter, described digital signal processor connects described frequency synthesizer and described high-speed sampler respectively, described high-speed sampler connects described intermediate-frequency filter, and described intermediate-frequency filter connects described frequency mixer.

2, a kind of SAW pressure transducer as claimed in claim 1, it is characterized in that: the thickness of described membrane structure is at most 1/4 of described substrate thickness.

3, a kind of SAW pressure transducer as claimed in claim 1 is characterized in that: described radio frequency transducer is the cross unit that aluminum metal forms, and described reverberator is the sealing ladder-shaper structure.

4, a kind of SAW pressure transducer as claimed in claim 2 is characterized in that: described radio frequency transducer is the cross unit that aluminum metal forms, and described reverberator is the sealing ladder-shaper structure.

5, as claim 1 or 2 or 3 or 4 described a kind of SAW pressure transducers, it is characterized in that: draw a leakage signal from described power amplifier output terminal,, deficiently go into to described frequency mixer through 180 degree phase shifts.

6, as claim 1 or 2 or 3 or 4 described a kind of SAW pressure transducers, it is characterized in that: described wireless sensing circuit adopts direct frequency synthesis technique to constitute the frequency synthesis system, a described wireless sensing circuit supports a plurality of described Sensor sections to measure, different described Sensor sections adopts different group of frequencies to distinguish, and adopts the frequency division multiple access access way.

7, a kind of SAW pressure transducer as claimed in claim 5, it is characterized in that: described wireless sensing circuit adopts direct frequency synthesis technique to constitute the frequency synthesis system, a described wireless sensing circuit is supported the measurement of a plurality of described Sensor sections, different described Sensor sections adopts different group of frequencies to distinguish, and adopts the frequency division multiple access access way.

8, a kind of method for making as Sensor section in the described SAW pressure transducer of claim 1～7, it may further comprise the steps:

1) adopting a slice＜100〉the two silicon chips of throwing in crystal orientation are as substrate, and the thickness of silicon chip is about 200 microns;

2) in the upper and lower surface deposit silicon nitride of silicon chip;

3) photoetching silicon nitride window utilizes the anisotropic silicon corrosion of silicon to form structure of silicon cup, and the circular membrane thickness after the corrosion is about 50 microns, removes silicon nitride after etching;

4) deposit piezoelectric zinc paste on silicon chip forms piezoelectric material film;

5) resist coating on zinc paste covers the mask plate do SAW element figure on photoresist, to resist exposure, development, last sputtered aluminum, remove photoresist, so just the SAW element of photoetching formation aluminum on zinc paste.

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