SE523969C2 - Acoustic gas sensor - Google Patents

Abstract

The invention concerns an acoustic sensor for determining the composition or concentration relations of a gas mixture within at least one measuring cell (1), including means for transport (6, 7) of said gas mixture to and from said measuring cell (1). The measuring cell is defined by at least two essentially planar and parallel plates (2, 3) at least one of which is conditioned to be subjected to mechanical vibrations by means of electrical activation. The thereby appearing interaction from acoustic waves within the gas mixture is electrically readable. Activation and readout is performed e g by electrostatic, piezoelectric, piezoresistive or electrothermal action. The readability concerns e g one in the frequency domain measurable extreme value, alternatively a phase or time difference, constituting a measure of the propagation velocity within the gas mixture, which is being the frequency controlling element of at least one oscillator circuit being built up from active and passive electronic components integrated into at least one silicon die (41). The wavelength of the acoustic waves is determined by at least one spacer (4) defining the distance between said plates (2, 3). Means (10, 11) for temperature measurement and electrical heating of said plates (2, 3) also prevail in a preferred embodiment, along with filter (51) for separation, adsorption or trapping of certain in said mixture included compounds. The geometric design of the measuring cell (1) and the plates (2, 3) is defined by a multistage process of selective or anisotropic pattern deposition and etching of single crystalline silicon or other semiconductor material.

Description

The use of gases is widespread in various sectors of society. This can apply to propellant for combustion processes or shielding gas to prevent such processes. In other contexts, specific properties of a particular gas can be used, such as anesthetic gases.

The brewing industry is an example of industrial use of CO2, both as a flavoring in beverages and as a propellant for pumping. In many use situations, it is essential to avoid leakage, and there is a need to provide warning for leaking gas in order for appropriate measures to be taken and damage to life and property to be avoided. Gas alarms for flammable gases are commercially available from a number of suppliers. The measuring principle is to allow the test gas to come into contact with a surface that catalyzes its combustion locally and in a controlled manner. A measured temperature increase on this surface corresponds to combustion taking place and thus indicates the presence of combustible gas. The method is simple, inexpensive and comparatively reliable, but is unusable for non-combustible gases. In summary, it can be stated that there is a great need for mass-produced gas sensors with more general usability, for measuring purposes, monitoring, control and alarms. So far, this need has not been met.

Acoustic gas sensors are based on the detection of variations in the lj velocity according to the following equation: c = mary / M) ............. .. (1) where c is the sound velocity, R general gas constant = 8.314 J / mol K, T absolute temperature, y the ratio of specific heat at constant pressure and volume, respectively, and M is the average molecular weight. The connection presupposes, among other things, that the gas mixture can be regarded as an ideal gas according to known physical principles.

Equation (1) has been used in measuring sensors and instruments to determine concentration ratios in binary gas mixtures, ie mixtures with two components with different molecular weights. If the temperature is kept constant, the concentration ratio between the two components can be determined by measuring the speed of sound and using equation (1). Correspondingly, air containing an 'impurity' can be considered as a two-component system between the initial air mixture and the impurity whose molecular weight and volume concentration are called MX and Xx respectively. For small concentrations of the pollutant, equation (1) can be given a linear approximation, describing small variations Ac of the speed of sound around a normal value co: AC / COZAT / zpl-'O- (Mx-Mlu fi yzMlu fi * XX....... ...

Equation (2) provides an illustration that there are both desired and unwanted functional relationships.

An observed variation in the speed of sound does not have to be caused by a variation in the carbon dioxide concentration but may be due to a variation in temperature. Equation (2) also shows that these undesired relationships have a level-shifting character, ie in the linear approximation the level of the output signal is affected. On the other hand, the influence on the calibration factor, ie the multiplicative coefficient in front of XX, is negligible according to this approximation.

Devices that measure variations in the speed of sound in order to detect the presence or concentration conditions in simple gas mixtures are known both through the literature and in the form of products available on the market. In these devices, the travel time of a sound wave between a transmitter and a receiver is measured in a gas-filled measuring cell. Compensation for the effect of temperature can Acoustic gas sensor 523 969. 3. . . . .:. .:. : preferably by actively keeping the temperature of the measuring cell constant in a control system.

The functionality and usability of these devices is undeniable, however, there are a number of problems that have not been satisfactorily solved to date. One such problem is the physical dimensions of the measuring cell, which at the state of the art so far are too large. The dimensions can be set in relation to the frequency range, and thus the resulting wavelength, which is used. A normal operating frequency is 40 kHz, corresponding to 8.5 mm wavelength, which means that the extent of the measuring cell can be calculated in centimeters. Maintaining temperature and avoiding temperature gradients are non-trivial design problems for such a measuring cell, which also leads to relatively high power consumption. Furthermore, manufacturing must be done with serial. partly manual assembly steps, which leads to relatively high manufacturing costs.

The present invention is suitable for solving these and related problems.

The invention utilizes a combination of known physical phenomena and engineering methodologies developed for other purposes. The combination gives rise to dramatic and partly unexpected improvements.

The characteristics of the invention are set forth in the appended claims and claims. Figure 1 shows a preferred embodiment of the gas sensor according to the invention, figure 2 frequency-dependent properties of the sensor according to the invention, and figure 3 an electrical equivalent diagram of an embodiment of the sensor according to the invention, and figure 4 an enclosure for the sensor according to the invention.

Figure 1 shows a cross-sectional view of a preferred embodiment of the sensor according to the invention. A measuring cell 1 is delimited by two substantially plane-parallel plates 2, 3 by a spacer element 4 which defines the vertical length of the measuring cell 1, called L. The function of the sensor according to the invention can be easily described by standing acoustic waves in the measuring cell 1 air mass between the plates 2, 3. The criterion for standing waves can be described with the equation L = n *> t / 2 = n * c / 2f n = 1,2,3, ... (3) where n is an integer, c is lj the velocity and f lj is exaggerated frequency. The current mode of use of eq (3) is to, for given values of n and L, determine the frequency at which standing waves to maximum amplitude occur. From this, the sound speed c can then be calculated. According to this view, the measuring cell 1 can be regarded as an acoustic resonator with multiple resonant frequencies determined by the relation (3). The distance L defined by the spacer element 4 is typically 0.3-1.0 mm, which corresponds to fundamental tones (n = 1) of the resonator in the range 200-600 kHz.

Standing sound waves in the measuring cell 1 are generated by the plate 3 which can be set in vertical vibrations by an applied alternating fold between said plate 3 and a third plate 5. By electrostatic force action a deflection of the plate 3 is obtained, since its bendability is considerably greater than that thicker plate 5. The cavity between plates 3 and 5 is closed and preferably evacuated. The alternating field between the plates 3 and 5 is generated by connecting a voltage source to two connecting wires 9, 10, which have electrically conductive connection to each of the plates 5 and 3, respectively. These are either electrically conductive in themselves, or provided with electrically conductive layers. The distance between the plates 3 and 5 is defined by a second spacer element 12 and can typically be a few thousandths of a millimeter. Larger distances mean a weakening of the electrostatic force.

The measuring cell 1 has a gas connection with the surroundings through channels 6, 7 which may for example be unetched grooves or recesses in the interface of the spacer element 4 towards the plate 2. The plate 3, the spacer elements 4 and 12 and the recesses 6, 7 are preferably designed as integrated subcrystalline silicon substructures.

From semiconductor technology for integrated circuits are additive and subtractive processing methods Acoustic gas sensor 523 969 f:. "If L '..:.:.....: Known to manufacture in a controlled manner complex bearing structures of this kind. For example, the plate 3 and the spacer element 4 can be thickness defined by so-called doping-selective etching, where the etching rate is determined by local doping of the silicon material with n- or β-dopants, eg phosphorus and boron The spacer element 12 can be defined by an additive method, eg epitaxial growth, which enables simultaneous and selective doping of this particular layer.It is also possible to make the peripheral parts of the plate considerably softer As a result of electrically actuation according to the above, the plate 3 has a more piston-like movement, which means that even the acoustic waves which propagate vertically are favored at the expense of those which are spread to the sides.This is a desirable property of the sensor according to the invention.

The lateral geometry of the silicon wafer is determined by photolithographic patterns in each of the additive or subtractive process steps.

The spacer element 4 has, as indicated in oblique walls towards the measuring cell 1. This has the advantage that multiple reactions in the lateral joint, and thus undesired standing wave phenomena, are avoided. Slanted walls with controlled geometry can according to the prior art be achieved by anisotropic etching of single crystalline silicon. Furthermore, it is desirable that the utilized resonant frequency according to equation (3) is not disturbed in an uncontrollable manner by the natural frequencies of the plate 3 for mechanical resonant oscillations.

This can be achieved by controlling the thickness, length and width of the plate 3. Typical values of these are 10-30 μm (thickness), 05-2 mm (length, width).

The plates 3 and 5 can, for example, be made of glass, whereby so-called anodic bonding can be used for joining to the intermediate silicon structure. A metal layer 8 is then applied to the underside of the plate 5, thereby forming an electrically conductive plane with connection to the connecting wire 9. Correspondingly, the wire 10 is connected to the plate 3. Insulation over the spacer element 12 can be supported by a so-called depletion layer, which according to known physical principles are formed in silicon that carries boundary layers between doping substances of different polarity, so-called pn-diode effect.

The manufacturing technique for the bearing structure formed by the measuring cell 1 delimited by the silicon plate 3 with integrated spacers 4 and 12 and the glass plates 2 and 5 is parallel in the sense that thousands or tens of thousands of components are manufactured in one and the same process, each component occupying a small area, 2 -4 mmz, on circular plates of silicon and glass with a diameter of 100-200 mm, which are processed simultaneously. Although process technology is advanced and each process step resource-intensive, the cost of each component is very low, both in terms of resource utilization and time consumption.

However, this presupposes very careful process control, and that the design meets the rules set by the individual process steps.

A third wire connection 11 is drawn in the figure to indicate that the silicon structure also carries elements for temperature measurement and resistive heating, which according to known principles can be connected in a control circuit for keeping the temperature constant. In fact, a single element, such as a silicon resistor made by diffusion of the dopant and integrated into the silicon structure described above with connections 10 and 11 to an external control circuit. Namely, it is known that silicon resistors have temperature-dependent resistance, which can thus be used for temperature measurement while at the same time the identical resistor is used for resistive heating from an external current source.

The device described in Figure 1 can be modified in many ways. For example, instead of electrostatic driving of the vibrations of the plate 3, the piezoelectric effect can be used, provided that the plate 3 constitutes or is provided with a coating of a piezoelectric material, eg zinc oxide, aluminum nitride, polyvinylidene chloride or lead zirconate titanate. A third possibility is thermal excitation by means of superficially located resistive layers in the plate 3. In this case, a separate reading mechanism is required, eg utilization of the piezoresistive effect in silicon.

Acoustic gas sensor 523 969 5 u. . . . . . -. The air or gas in the measuring cell 1 communicates with the environment via recesses in the spacer element 4 which constitute means for passive gas transport 6, 7 to and from the measuring cell 1. The gas transport can be effected by a pressure gradient in the environment or by diffusion.

The electric alternating field gives rise to vibrations in the plate 3 which in turn gives rise to an acoustic wave which propagates vertically in the measuring cell 1. Since the acoustic impedance in air differs considerably from that in solid bodies such as the plates 2 and 3, a large part of the acoustic energy to be reflected and give rise to standing waves, the maximum amplitude of which occurs at frequencies when the distance L corresponds to multiples of half the wavelength according to eq (3).

Figure 2 schematically shows how the impedance Z of the sensor element between the wire connections 9 and 10 in figure 1 varies with the frequency. The diagram which has logarithmic scales shows three extreme values, 21, 22, 23 of which two maxima 21, 23 and a minimum value 22. Maxima correspond to the occurrence of standing waves according to eq (3), where at the lowest frequency 21 it corresponds to n = 1 eq ( 3), the next maximum 22 corresponds to n = 2 etc. It should be noted in this context that said extreme values according to elementary theory coincide with certain phase relations between current and voltage for said impedance Z.

Measurement of phase or time difference can thus, as well as sensing of extreme values, form the basis for the above-mentioned reading principle.

Figure 3 shows an electrical equivalent diagram for the corresponding sensor element. The diagram comprises a capacitance 31 which corresponds to the plate capacitor formed between the plates 3 and 5 (fi g 1) according to known elementary theory. Furthermore, the diagram comprises a transformer 32 which describes the connection between the electrical and acoustic domains. Ideally, this connection is determined by a simple, frequency-independent constant, which presupposes, among other things, that the natural frequencies of the plates 3, 5 for resonant oscillations do not occur in the current frequency range according to Figure 2. The acoustic part of the sensor element is described in the equivalence diagram as a transmission line 33. for such lines possess a certain characteristic impedance which is related to the wave propagation speed, and a certain attenuation factor. Finally, the equivalent scheme comprises an acoustic load 34, corresponding to the plate 2.

The electrical equivalent scheme in Figure 3 makes it possible to explain how different structural elements affect the impedance characteristics of Figure 2. Standing waves in the transmission line 33 occur as soon as there is a mismatch between its characteristic impedance, load 34 and the acoustically coupled impedance of the capacitor 31 and vibration. impedance. Such maladaptation is always present to a greater or lesser extent. With small resistive losses, a large so-called standing wave ratio is obtained, ie a large value of the ratio between the maximum and minimum values 21, 22. Furthermore, it should be clear that the position of the extreme values 21, 22, 23 on the frequency scale is related to the propagation speed, ie sound speed, if L is assumed to be constant.

Physically, the transformer 32 corresponds to the plate 3, which can be subjected to mechanical vibrations with electrical activation in the manner described above. These vibrations are partly converted into acoustic waves in the adjacent gas medium according to known physical principles. The coupling factor for this transition between the electric and the acoustic domain is determined by a number of factors, for example the stiffness and mass of the plate 3. If this switching factor is high, the extreme values 21, 22, 23 become very noticeable, while a low switching factor results in a smaller difference between maximum and minimum values.

In the sensor according to the invention, measurement of one of the extreme values 21, 22, 23 takes place, which thus constitutes an indirect measurement of the variations of the speed of sound according to equations (1), (2) and (3). The measurement takes place by connecting the connections 9, 10 of the sensor element in an oscillator circuit, the oscillation frequency of which Acoustic gas sensor 523 959 a "z" I =. . "If b. :: '. °.:..:. É is determined by any of these extreme values. The oscillator circuit may consist of a linear amplifier coupling, dimensioned so that the self-oscillation conditions are met for any of the extreme values 21, 22, or 23, or ex. The latter solution entails, among other things, the advantage that the frequency of oscillation frequency due to undesirable factors is minimal.

A verbal and shorter description of the operation of the sensor according to the invention is that the standing acoustic waves react to the mechanism which initiates the mechanical oscillations of the plate 3, and that this reaction is electrically readable by measuring the electrical impedance of the sensor element.

As previously pointed out, other alternative embodiments of sensor elements can be selected, for example using piezoelectric, piezoresistive electrothermal elements. This choice of course affects the exact form of frequency characteristic and equivalence scheme according to Figures 2 and 3. However, the fundamentally important property that the frequency characteristic includes extreme values, whose positions on the frequency scale are determined by the speed of sound, remains.

Figure 4 shows a cross section of a preferred enclosure shape for the sensor according to the invention. The previously described measuring cell 1 surrounded by a silicon plate 3 and the glass plates 2, 5, with electrical connections 9, 10, 11 is housed in a primary capsule 47, 48 with electrical bushings 44, 45.

The capsule lid 48 is provided with at least one hole 49 which allows passage of gas to and from the measuring cell 1 via channels 6, 7 as previously described. The capsule 47, 48 is preferably made of a polymeric material and with a standardized design which allows automatic mounting and soldering according to so-called surface mounting technology on a circuit board substrate 46, which includes printed conduits for electrical connection of the sensor according to the invention with other electronic components and subsystems. The capsule 47, 48 also houses a silicon wafer 41, containing certain integrated active and passive electronic circuit elements needed for the function of the sensor. It can be, for example, the oscillator circuit described above and a control circuit for keeping the temperature of the measuring cell constant. The electrical connections 9, 10, 11, 43 between the structure supporting the measuring cell 1, the silicon wafer 41 and the bushings 44, 45 consist of thin wires, preferably aluminum wire with a diameter of 0.025-0.05 mm, with connection points made by means of so-called wire bonding technology according to either a procedure based on hay effect ultrasound or terrno compression. This technique can be performed automatically under a microscope, but requires that the connection points of the wires 9, 10, 11, 43 be substantially in one and the same plane, which is achieved with support elements 42, 52 of suitable thickness.

The measuring cell 1 with surrounding plates 2, 3, 5 is, as previously described, preferably provided with a control function for keeping its temperature constant. For optimal control function it is required that the measuring cell 1 and its supporting material 2, 3, 5 are thermally insulated from the rest of the capsule 47, 48. The supporting element 52 is therefore preferably a porous material with very low thermal conductivity.

The encapsulation process described above and its construction constitute the standard method with very low manufacturing cost, provided that the manufacturing volume is high. The component contained in the capsule 47, 48 has a very large general utility. The capsule shape is referred to as "dual in line" according to the double-sided geometry of the connections 44, 45. The capsule 47, 48 has typical dimensions 5 x 5 x 10 mm, and the number of connections can be 8-16, depending on how many active or passive circuit elements which are integrated on the silicon wafer 41 or alternatively need to be connected from the outside to the capsule 47, 48.

Specific applications often require a certain degree of specificity in properties, such as the ability to sense certain gases and suppress sensitivity to others. To achieve such properties, the sensor according to the invention can be provided with a secondary capsule 50 outside the primary 47, 48. The capsule 50 is Acoustic gas sensor 523 969 »- a. -. . . , ,, partially perforated to allow gas passage and includes at least one 51lter 51 with selective permeability for the gas (s) desired to be detected. For example, passage of gases with polar molecular structure, eg water vapor, can be prevented or delayed with a porous filter material capable of binding polar molecular groups. The capsule 50 and the filter 51, like other sub-components included in the sensor, can be manufactured at a very low cost. Dividing generally desired properties at the primary encapsulation level with specific properties at the secondary, means unique, and hitherto unattainable opportunities to achieve very good measurement performance at very low manufacturing cost. For measuring CO 2 concentration, resolution and linearity in the ppm range have been experimentally verified, with minimal simultaneous sensitivity to water vapor.

The invention can be varied in many ways within the scope of the following claims.

Acoustic gas sensor

Claims (1)

523 969 8 PATENT CLAIMS Acoustic sensor for determining the composition or concentration ratio of a gas mixture in at least one measuring cell (1), including means (6, 7) for transporting said gas mixture to and from said measuring cell (1), characterized in that said measuring cell (1) is limited by at least two substantially plane-parallel plates (2, 3), at least one of which is arranged to be subjected to mechanical oscillations by means of electrical activation, which give rise to acoustic waves in said gas mixture, that the reaction of said waves to said oscillations is electrically readable. , that at least one of said plates (2, 3) includes integrated substructures in single crystalline silicon, and that the geometric design of said plates (2, 3) and measuring cells (1) is defined by a step-by-step process of selective or anisotropic pattern deposition and pattern etching. Acoustic sensor according to claim 1 characterizes! by said electrical activation taking place by electrothermal action on a resistive layer integrated in said plate, by piezoelectric action on a piezoelectric layer integrated in said plate, or by electrostatic action between said plates or to a third plate (S) included in said sensor, with at least one electrically conductive surface, and that said electrical reading takes place using piezoresistive, piezoelectric or capacitive elements, which are identical or integrated with said activating elements. Acoustic sensor according to claim 1, characterized in that said feedback and readability refer to at least one extreme value (21, 22, 23) present in the frequency domain, alternatively a phase or time difference, for at least one electrically measurable quantity, and that said extreme value (21, 22, 23), alternatively phase or time difference, in at least one interval is a monotonic function of said wave propagation speed in said gas mixture and constitutes frequency determining elements in at least one oscillator coupling which is built up of active and passive electronic components in integrated form on at least one silicon wafer (41 ). Acoustic sensor according to claim 1, characterized in that the wavelength of said acoustic waves is determined by at least one distance element (4) defining the distance between said plates (2, 3) whose natural frequencies substantially exceed the frequency of said oscillations. Acoustic sensor according to claim 1, characterized by at least one device (10, 11) for temperature measurement and electric heating of said plates (2, 3) and that these are made of materials with significantly higher thermal conductivity than materials (52) used to support said plates (2, 3), and that said devices (10, 11) can be connected in at least one temperature regulator through which said measuring cell (1) is kept at a substantially constant temperature. Acoustic sensor according to claim 1, characterized by at least one filter (51) for separation, adsorption or capture of certain substances present in said gas mixture in solid, surface or gas form, said filter (51) being arranged to cooperate with said device (6, 7). ) for transport so that said substances are substantially separated or delayed before they reach said measuring cell (1). Acoustic sensor according to claim 1, characterized in that at least that of said plates (2, 3) which is arranged to be subjected to mechanical oscillations by means of electrical activation can be divided into at least one rigid part and an elastically flexible part. Acoustic sensor according to claim 1, characterized by at least one silicon wafer (41) with integrated active and passive components electrically connected by wire connection (9, 10) by wire bonding to contact surfaces on at least one of said plates (2, 3) and said silicon wafer (41) for said electrical activation and reading, and that said contact surfaces are located on a substantially common plane. en: u. 523 969,. -. . An acoustic sensor according to claim 1, characterized by at least one capsule (47, 48) which 10. contains a number of metallic conductors (44, 45) for electrical passage between the interior of said capsule (47, 48) and outer, the design of which is suitable partly for wire connection by means of so-called Wire bonding between contact surfaces on at least one of said plates (2, 3) or at least one silicon washer (41) present in said capsule and partly for surface mounting and soldering on a circuit board laminate. Acoustic sensor according to claim 1, characterized by at least two measuring cells (61, 62) placed in mutual thermal contact, with substantially identical construction and function as said measuring cell (1) except that their respective associated devices (63, 64) for transporting said gas mixture to said measuring cells (61, 62) possess substantially deviating fl resistance resistors, said transport to at least one of said measuring cells (61, 62) being substantially delayed, whereby the composition of said gas mixture in said measuring cell (62) represents a time average value for a certain time.