RU2680855C1 - High-frequency thin-film capacitive pressure sensor - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: use to create a capacitive pressure sensor. Invention consists in the fact that a high-frequency thin-film capacitive pressure sensor consists of four dielectric films, the main screen is formed on the surface of the first dielectric film, first side screen is formed on the upper surface of the second dielectric film; the first dielectric film is the insulator between the main screen and the object of study, combined facing (membrane) is formed on the bottom surface of the fourth dielectric film; at least five through support holes pass through the first, second dielectric films and response plates to connect the sensor cavity under the membrane with atmospheric pressure; sensor is fixed on the surface of the object of study through the first dielectric film with grooves to maintain communication of the cell with the atmosphere through the support holes, while on the surface of the second dielectric film formed response plates of metal foil of rectangular or round shape; diameter of the perforation cell is chosen equal to 2a, where a – is the radius of the perforation cell, the height l of the perforation cell is chosen equal to the height of the total membrane thickness l₁, to maintain a linear relationship between the output and input parameters, the membrane deflection is chosen much less than the total thickness l/2; after forming the sensor membrane from a dielectric film, the thickness of the metallized layer t is chosen to be no more than one micron, otherwise, when the membrane is a metal film, its thickness t is chosen in the range from 2 microns to 40 microns, at that the same time ensuring the minimum level of the measured pressure from 1 Pa to 20 kPa; maximum – from 200 Pa to 1,100 kPa, moreover, if the membrane is formed from a polyimide film, frequency of its own oscillations is 4.5 times lower than the magnitude of the natural frequency of a nickel film; moreover, the polyimide film selected as insulation and for metal coating in vacuum should be not less than 10 microns thick.

EFFECT: providing the possibility of expanding the frequency range of thin-film capacitive sensors more audio frequency (from 20 Hz to 20 kHz).

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Description

The invention relates to measuring equipment and can be used in aviation, space technology, utilities, acoustics and in experiments for measuring rapidly varying pressure.

Known capacitive pressure sensor, the assembly of which is carried out in vacuum using the technology of manufacturing thin epitaxial films in vacuum. The sensor membrane has a flat cylindrical shape. The rational dimensions of the design of the sensitive element (CE) of the capacitive sensor are given, which make it possible to measure pressure pulsations from 10 ² μPa to 2⋅10 ⁵ Pa (15-200 dB), static pressure from 0 to 5⋅10 ⁵ Pa and their combination, i.e. . sound pressure (statodynamic pressure) in these intervals.

The SE of a capacitive sensor allows you to measure the pulsations and static pressure on the surface of the object studied (OI) by drainage and drainage methods (RF patent No. 2179308 G01L 9/08, 9/12. 2002 "Capacitive pressure sensor" by A. A. Ghazaryan).

The sensor has the following disadvantages. The assembly of the sensor takes place through a complex technological process, because of which its cost increases. The sensor does not allow measuring sound pressure (pressure pulsations) in a frequency range wider than from 20 Hz to 20 kHz.

Closest to the invention, the technical solution is a capacitive pressure sensor based on the principle of dependence of the parameters determining its electric capacity. A capacitive pressure sensor consists of a double-sided foil dielectric film, which is the base of the sensor. Capacitor plates with leads are formed on the upper surface of the sensor base. The foil on the bottom surface of the base is the sensor screen. The sensor membrane is rigidly fixed to the surface of the second dielectric film. The third and fourth dielectric films are deposited on both surfaces of the membrane. To protect the terminals, a screen is installed, covered with the sixth and seventh dielectric films. The screen is made of the same material as the membrane. The second film is perforated with a gaseous dielectric inside the cell and impregnated with glue. Additionally introduced output in the form of a wire with a diameter of 1-2 microns, coated with insulation. Based on the sensor, the plates are perforated and have at least five holes for connecting the sensor cavity under the membrane with atmospheric pressure.

Such a solution in the indicated design provides measurement by the drainless method of sound pressure (pressure pulsation), sound pressure (statodynamic pressure), static pressure, explosive, wind wave pressures, etc. (RF patent No. 2485464 G01L 9/12. 2013 by A.A. Ghazaryan).

The disadvantage of the sensor is the inability to measure sound pressure in the frequency range wider than 20 Hz-20 kHz.

The present invention is the implementation of the method of drainless measurement of sound pressure fields by expanding the measuring range of the operating frequency of thin-film capacitive sensors, more than the range of sound frequency (20 Hz to 20 kHz). At the same time, preserving the linearity of the dependence of the change in the output signal (membrane movement) on the input set pressure (load) and the required conversion coefficient (sensitivity) of the sensor.

The technical result is achieved by the fact that a high-frequency thin-film capacitive pressure sensor, consisting of four dielectric films, a main screen is formed on the surface of the first dielectric film, a first side screen is formed on the upper surface of the second dielectric film, the first dielectric film is an insulator between the main screen and the object of study on the lower surface of the fourth dielectric film a united lining (membrane) is formed, through the first, second dielectric an insulating film and counter electrode are at least five through holes for connection reference sensor cavity under the membrane at atmospheric pressure; the sensor is mounted on the surface of the object of study through the first dielectric film with grooves to maintain communication between the cell and the atmosphere through the support holes; additionally, response plates of rectangular or round metal foil are formed on the surface of the second dielectric film; the diameter of the perforation cell is chosen equal to 2 a , where a is the radius of the perforation cell, the height ℓ of the perforation cell is chosen equal to the height of the total thickness of the membrane ℓ ₁ ; to maintain a linear relationship between the output and input parameters, the deflection of the membrane is chosen much less than the total thickness ℓ / 2; after the sensor membrane is formed from a dielectric film (for example, from a polyimide film), the thickness of the metallized layer t is chosen to be no more than one μm, otherwise, when the membrane is a metal film, its thickness t is selected in the range from 2 μm to 40 μm, while respectively providing a minimum level of measured pressure from 1 Pa to 20 kPa; the maximum is from 200 Pa to 1100 kPa, and if the membrane is formed from a polyimide film, the frequency of its natural vibrations is 4.5 times less than the frequency of natural vibrations of the nickel film; moreover, the polyimide film selected as insulation and for coating with metal in vacuum should be at least 10 microns thick.

The expansion of the frequency range of a capacitive sensor in aeronautical acoustics is necessary to measure such an important parameter as the pressure behind the shock wave reflected from the closed end of the shock tube. For example, from reviews of open foreign literature it is known that piezoelectric sensors with a resonant frequency of ≥400 kHz and a signal rise time of 10 ^-6 s are used to measure the braking pressure of a gas medium.

The extension of the frequency range of the capacitive sensor is also important for the study of two-phase liquid media. For example, in an aqueous medium in air bubbles with a diameter of one mm, the characteristic values of the natural frequency are ω ₀ / 2π≈3 kHz; but with a diameter of 0.1 mm, the natural frequency of the bubble reaches ω ₀ / 2π≈30 kHz, where ω ₀ is the angular frequency of the bubble.

In FIG. 1 shows the design of the sensor from several SE and its individual nodes. In FIG. 2 shows the dependence of the change in frequency f on the membrane thickness t. In FIG. 3 and FIG. 4 - dependence of the change in frequency f on the diameter 2 a of the hole under the membrane, where a is the radius of the holes of the cell perforation cell.

In FIG. 1, a capacitive pressure sensor consisting of four dielectric films 2, 4, 7, and 8 is glued on the surface of the OI 1, the main screen 3 being applied to the upper surface of the first insulating dielectric film 2; on the second dielectric film 4, a first side screen 5 and a response plate 6 are formed. The third dielectric film 7 is perforated. On the fourth dielectric film 8, a united plate (membrane) 9 and a second side screen 10 are formed (Fig. 1 sec. BB). The third dielectric film 7 is simultaneously an insulator between the membrane 9 and the mating plates 6 (Fig. 1 sec. A-A, sec. B-B). Through the dielectric film 2, 4 of the reciprocal plates 6 and the main screen 3 pass the support holes 11 with a diameter of d. The groove 12 extends along the lower surface of the dielectric film 2 (FIG. 1, section BB). For signal pick-up, conclusions are provided with pads 13 (Fig. 1 sec. A-A, sec. B-B).

In FIG. 2 shows the dependence of the change in the frequency ƒ of the natural vibrations of the nickel membrane 9 on the thickness t of the membrane 9 at different values of the radii a of the perforation cell 7. FIG. 3 - FIG. Figure 4 shows the dependence of the change in the frequency ƒ of the natural vibrations of the membrane on the change in the radius a of the cell 7 for given values of the thickness t of the membrane 9.

The first and second dielectric films 2, 4 and the main screen 3 are made of a polyimide film with a thickness of 5 μm to 10 μm, foil copper or nickel with a thickness of 5 μm. This design is well protected from external electromagnetic interference.

The parameters of the fourth dielectric film 8 are selected based on the frequency and amplitude characteristics of the measured pressure pulsations. For example, if it is necessary to measure low levels of pressure pulsations from 50 dB to 140 dB with a frequency of more than 20 kHz, then a polyimide film with a thickness of 10 μm to 20 μm with one-sided nickel plating with a thickness of up to t = 1 μm is chosen. When choosing the thickness of the fourth dielectric film less than 10 μm, technological difficulties arise - the film is twisted, which complicates both the metallization of the film itself and the assembly of the sensor package.

When designing thin-film capacitive sensors, methodological difficulties arise. Namely, what is a membrane - the fourth dielectric film 8 with a thickness of 10-20 microns or an integrated lining (membrane) 9 with a thickness of t up to one micron of metal, for example nickel. It is proposed in the calculations of capacitive film sensors to make the assumption that the membrane material is homogeneous and its elastic properties are the same in all three directions; a metallized layer, up to one micron thick, playing the role of capacitor plates, does not affect the film elasticity. Moreover, in the theoretical model of the sensor, a single-layer case is considered. If the thickness of the foil (metallized) is significantly less than the thickness of the dielectric film, then the elastic modulus E, Poisson's ratio μ and dielectric thickness are used in the calculations, for example, for a polyimide film E = 3 =10 ⁹ Pa, μ = 0.22. With the opposite ratio, these are the same foil parameters. Such a simplified approach allows us to describe the main implementations of the developed sensors and to obtain a good agreement between the calculated and experimental data. In FIG. 2 - FIG. 4 shows the dependences of the natural vibration frequency on the radius a of the perforation cell and the thickness t of the integrated plate 9, i.e. the thickness of the membrane, in this case, which is a nickel film of different thicknesses. If a polyimide film is selected as the membrane material, then the frequency of natural vibrations decreases by 4.5 times (see Fig. 2 - Fig. 4). In the case of a membrane made of copper foil, the value of the natural vibration frequency decreases 1.64 times (see Fig. 2 - Fig. 4).

The fourth dielectric film 8 is formed by applying to the lower surface of the metal membrane (film) 9 a polyimide film or polyamide-acid varnish according to known technology. The thickness t of the metal membrane 9 can be selected in the range from 1.0 μm to 40 μm, (Fig. 2-Fig. 4),

The height ℓ of the perforation cell must not exceed the total thickness of the membrane ℓ ₁ . This ensures the linearity of the characteristics of the dependence of the output signal of the sensor from the input, i.e. set pressure. In this case, the deflection of the membrane y should be much smaller than the total thickness of the membrane (ℓ / 2 times).

The thickness of the metal film of the combined plate is selected depending on the range of the measured pressure. For example, in the case of a metal film of nickel and with a diameter of the perforation cell 2 a = 0.8 mm or less:

Binders between the membrane 9, the response plates 6 and the conclusions 13 are layers of glue K, deposited on the surface of the perforated film prior to perforation (Fig. 1 section. BB). The dimensions of the rectangular plate can be 4 × 6 mm or 6 × 9 mm; or with a diameter of 3-6 mm in the case of a round shape.

In the manufacture on one substrate, from one SE to several tens, with or without leads, can be formed. The materials used are well known in the industry. The support hole under the membrane is chosen with a diameter of d = 0.25-0.3 mm. Communication with atmospheric pressure P _{0 is} provided by a groove 12 on the first dielectric film 2, with a width equal to the height of the plates 6 glued on the surface of OI 1 (Fig. 1 sec. A-A, sec. BB).

The main screen 3 and the second dielectric film 4 are fastened together with glue on an epoxy-rubber basis. The sensor on the surface of the IO is fastened with any type of cold curing adhesive.

The support holes on the mating plate 6 are located symmetrically with respect to the central axis. The support holes pass through the second, first dielectric films 4, 2 and the main screen 3. The equalization of the static pressure behind the membrane is necessary to achieve the most accurate pressure measurement and to reduce the lower limit of the working frequency range.

The presence of an air layer, depending on the operating conditions of the membrane, allows additional attenuation or additional elasticity to be introduced into the membrane operating mode. The presence of support holes when changing the sound frequency in a wide range, the influence of inertia of the air, the influence of internal friction of the membrane, etc., equally and slightly change the acoustic resistance of a capacitive pressure sensor.

The deflection of the membrane inside the perforation cell with a diameter of 2 a (Fig. 1 sec. BB) is determined with the assumption that at any pressure level (from minimum to maximum value) on the surface of the membrane, the center moves at _poppy = ℓ of the membrane and its stress state σ _pop possesses fairly linear functions of pressure.

. Это обеспечивает линейность при условии, что у_мак≤0,5t [м], напряжение по окружности

и наибольшая частота собственных колебаний мембраны

, где Р - давление н/м²; E - модуль Юнга для никеля E=20,5⋅10¹⁰ н/м², μ - коэффициент Пуассона μ=0,31; плотность j=8,8 кг/м² и t - толщина мембраны [м].These parameters can be calculated according to well-known formulas:

. This ensures linearity, provided that at _poppy ≤0.5t [m], the circumferential stress

and the highest natural vibration frequency of the membrane

where P is the pressure n / m ² ; E - Young's modulus for nickel E = 20.5⋅10 ¹⁰ n / m ² , μ - Poisson's ratio μ = 0.31; density j = 8.8 kg / m ² and t is the membrane thickness [m].

For a polyimide film, these parameters are equal: E = 3⋅10 ⁹ n / m ² , μ = 0.22; j = 1.42 g / cm ³ . In this case, the dependence of the change in the lowest frequency of the membrane natural oscillations at given values by different radii a and the membrane thickness t of the nickel foil is shown in FIG. 2 - FIG. four

Based on the frequency and amplitude characteristics of the measured pressure pulsations, determine the type of sensor for single-piece manufacturing or in the matrix manufacturing option (several SEs on one substrate). According to FIG. 1 SE of the sensor is formed from a membrane i.e. the combined plate 9, the second dielectric film 4 metallized on the upper surface, the mating plate 6. Around the CE is formed the first side screen 5. The mating plate 6 (or plates - in the case of the matrix manufacturing option) of the sensor is formed on the second dielectric film 4. Metallization of the membrane, the response plates with leads, screens from a dielectric film (for example, from polyimide) are formed in a vacuum chamber through masks.

From the first and second dielectric films collect the package. Further, according to the markings of the support holes 11, through the specified dielectric films 2, 4 and the counter plates 6 drill support holes with a diameter of 0.25-0.3 mm

The complete capacitive sensor package according to FIG. 1 sec Bb. The formed package is held between the punch and the matrix under a pressure of (3.0-3.5) ⋅10 ⁵ Pa / cm with a cyclic temperature effect of 80-110 ° C for a duration of 5-10 minutes. Then increase the temperature from 110 ° C to 170 ° C for a duration of 30-40 minutes The assembled bag is cooled to a temperature of 50-60 ° C, disassembled, the operability of each SE of the pressure sensor is checked, if necessary, a passport is made.

The assembled ready-made sensor on the same substrate with several CEs on the surface of OI 1 is fastened with any type of curing adhesive. The area of the support holes 11 (Fig. 1 section. DD) should be free of glue.

It is known that the sensitivity of a sensor with a small air gap ℓ between the membrane and the counter plate does not depend on ℓ if the external shunt capacitance is negligible compared to the capacitance of the sensor. It is also known that this leads to the cancellation of the effect of increasing stiffness of the air cushion, when the gap decreases and vice versa. Support holes with perforation cells under the membrane increase the sensitivity of the sensor by twenty times.

The proposed sensor design makes it possible to implement the method of drainless determination of the fields of pressure pulsations of aircraft acoustics in the frequency range from 5 Hz to 100 kHz. High-frequency thin-film sensors of this design can be successfully used in the following tasks:

- conducting strength and aerodynamic research without violating the integrity of the product;

- combination and simultaneous measurement of distribution with weight, temperature and other types of measurements.

To provide the operating principles of the proposed capacitive sensor, we neglect the cushioning effect of the air gap and edge effects.

и напряжению поляризации датчика U, т.е. на выходе датчика имеем напряжение

.The principle of operation of the sensor. When the pressure changes by a value of P, the membrane 9 bends into the inside of the perforation cells 7, the distance 2 and between the counter plates 6 and the combined membrane 9 changes. A change in this distance leads to a change in the capacitance C and the increment of the capacitance ΔC. The sensor is polarized with a DC voltage from 40 V to 200 V. This voltage is applied to the integrated plate 9 through the contact pad with leads 13. The output voltage taken from the outputs of the CE of the combined cover 9 and the contact pad with leads 13 (Fig. 1, section A- A, section B-B), in proportion to the increment coefficient of the capacitive sensor

and the polarization voltage of the sensor U, i.e. at the output of the sensor we have voltage

.

The technical and economic effect of the proposed invention is achieved by increasing the frequency of natural vibrations of the membrane.

In order to confirm the technical and economic effect, a single-film thin-film capacitive sensor was manufactured. A perforated polyimide film with a thickness of ~ 20 μm with openings of perforation cells with a diameter of 2 a = 0.8 mm was located between the membrane of a metallized polyimide film and the counter plates of copper-foil polyimide film. The output signal of the capacitive sensor was matched with the input of the charge amplifier and amplified by a normalizing voltage amplifier. The polarization voltage of the sensor is 100 V. The bandwidth range of the operating frequency of the measuring equipment corresponds to the sound frequency from 20 Hz to 20 kHz.

When measuring the frequency response of the capacitive sensor behind the passband, it was recorded that the upper limit of the operating frequency range far exceeds 50 kHz. The experiment was conducted at the shock tube stand UT-4. As reference equipment, equipment with a microphone of model 4136 from Bruhl & Kj фирмыr (Denmark) was used.

In ["Receivers of pressure pulsations used for aerodynamic research (based on foreign press)." Department of Scientific and Technical Information TsAGI No. 619 1983 p. 19] describes a receiver from which it was possible to obtain an operating frequency range of up to 500 kHz by providing a very small gap between the plates (0.6-0.7 μm). In this work, a capacitor receiver (sensor) of pressure with a rough surface of the fixed plate is also of interest. The roughness is irregular in nature, and the membrane is not stretched at a certain distance from the plate, but lies directly on a stationary fourth dielectric film.

Thus, the design of the sensor is simplified and the mass of air between the plates of the capacitor is reduced. In a certain approximation, the entire sensor can be considered as a set of independent sensors formed by the deflections of individual sections of the membrane on the microroughnesses of the stationary response plate connected in parallel.

The frequency response of the capacitive sensor depends on the average pressure levels at which it is used, which is explained by the low self-weight of the membrane, the influence of the connecting masses of air, and the practical implementation of the damping of the resonance peak, which depends on the ambient pressure and worsens when deviated from the design conditions.

Both domestic and foreign studies have shown that capacitive sensors have very wide dynamic ranges for measured pressures. The upper limit of the measurement of pressure pulsation is usually determined by permissible distortions and can reach 170 dB or more. The lower levels of the measured pressures are 45-50 dB. Studies have shown that when using low-noise amplification equipment, the total noise of the pre-amplifier with a sensor is 3-5 μV. In the low-frequency region, the main role is played by the noise of the preliminary charge amplifier, and in the higher-frequency region, the noise from the Brownian motion of air molecules near the membrane plays a large role.

If it is necessary to use a capacitive sensor at very low frequencies from 0.01 Hz to 5.0 Hz, the support holes must be closed and a capacitive sensor with equipment at the carrier frequency used.

In domestic and foreign measuring equipment, usually the membrane of microphones is formed thin and strongly stretched. Nickel films with a thickness of 5-20 μm are used as the membrane material.

Claims (1)

A high-frequency thin-film capacitive pressure transducer consisting of four dielectric films, a main screen is formed on the surface of the first dielectric film, a first side screen is formed on the upper surface of the second dielectric film, the first dielectric film is an insulator between the main screen and the object of study, on the lower surface of the fourth dielectric film a combined lining (membrane) is formed, through the first, second dielectric films and the mating passages at least five through support holes for connecting the sensor cavity under the membrane with atmospheric pressure; the sensor is fixed on the surface of the object of study through the first dielectric film with grooves to maintain communication between the cell and the atmosphere through the support holes, characterized in that on the surface of the second dielectric film mating plates of rectangular or round metal foil are formed; the diameter of the perforation cell is chosen equal to 2a, where a is the radius of the perforation cell, the height l of the perforation cell is chosen equal to the height of the total thickness of the membrane l ₁ , to maintain a linear relationship between the output and input parameters, the deflection of the membrane is chosen much less than the total thickness l / 2; after the sensor membrane is formed from a dielectric film, the thickness of the metallized layer t is selected no more than one μm, otherwise, when the membrane is a metal film, its thickness t is selected in the range from 2 μm to 40 μm, while providing a minimum measured pressure level of 1 Pa to 20 kPa; the maximum is from 200 Pa to 1100 kPa, and if the membrane is formed from a polyimide film, the frequency of its natural vibrations is 4.5 times less than the frequency of natural vibrations of the nickel film; moreover, the polyimide film selected as insulation and for coating with metal in vacuum should be at least 10 microns thick.

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