FR3037652A1 - ATMOSPHERIC PRESSURE SENSOR BY PIRANI EFFECT, AND METHOD OF DESIGN AND MANUFACTURE - Google Patents

Abstract

The invention provides a sensor based Pirani gauge for measuring the gas pressure. A gas-tight enclosure, whose outer surface is exposed to the test gas, encloses a gaseous interior environment surrounding the sensing element 31 of a Pirani gauge. This enclosure has an elastically deformable structure by the pressure difference between internal and external pressure. The behavior of the gauge depends on the internal pressure, which depends on the external pressure according to a transfer function, thus providing a measure of the external pressure. This transfer function depends on the stiffness of the housing, the volume and the internal pressure. This enclosure comprises for example a deformable bellows-shaped housing 21 or a flexible membrane housing. It may include an additional element providing a reduction of the internal volume to modify this transfer function. The invention further provides a method of making such a sensor.

Description

The invention provides a Pirani gauge-based sensor for measuring the gas pressure. A gastight enclosure, whose outer surface is exposed to the test gas, encloses a gaseous interior environment surrounding the sensitive element of a Pirani gauge. This enclosure has an elastically deformable structure by the pressure difference between internal and external pressure to the enclosure. The behavior of the gauge depends on the internal pressure, which depends on the external pressure according to a transfer function, thus providing a measure of the external pressure. This transfer function depends on the stiffness of the housing, the volume and the initial encapsulation pressure and the internal pressure.

This enclosure comprises for example a deformable bellows-shaped housing or a flexible membrane housing. It may include an additional element providing a reduction of the internal volume to modify this transfer function. The invention furthermore proposes a method of producing such a sensor, called "composite" because associating a measurement of pressure by mechanical deformation and a pressure measurement by Pirani effect. State of the art The principle of the Pirani gauge is well known to those skilled in the art. This principle is based on the variation of the thermal conductivity of a gas as a function of the pressure and thus on the average free path of the molecules of this gas. The sensors implementing the principle of the Pirani gauge comprise a part, also called "sensitive element", which is immersed in the medium whose temperature and / or pressure is measured.

This sensitive element may take for example the form of a wire or a bar or a thin ribbon. It consists of an electrical conductor whose resistivity changes according to the temperature. The measurement of the resistance of the sensitive element makes it possible to determine the temperature and / or the pressure of the medium.

Passively used, the Pirani gauge measures the 3037652 -2 temperature. By heating the sensitive element, for example directly by Joule effect, it makes it possible to measure the pressure of the surrounding medium.

5 Pirani gauges are highly valued in the industry for their economical manufacture, reliability, wide range of pressure response, ease of manufacture and integration at a reduced cost. Their principle is simple, their implementation requires very little electronics since based on a simple measure of resistance.

However, this type of sensor also has a number of disadvantages and limitations. An important limitation relates to the pressure ranges that can be measured by this type of sensor. Mathematical models of behavior of such a gauge have been proposed for example by Mastrangelo et al., Microfabricated thermal absolute-pressure sensor with on-chip digital front-end processor. Solid-State Circuits, IEEE Journal of, 1991. 26 (12): p. 1998-2007, then by Santagata et al., An analytical model and verification for MEMS Pirani gauges. Journal of Micromechanics and Microengineering, 2011. 21 (11): p. 115007. When the wire is heated, some of its energy is transferred to the nearest substrate. This work tells us that a drop in the distance (or "gap") separating the bridge or hot wire from the thermalized substrate (or "thermal bath") makes it possible to shift the range of sensitive pressures to higher values. Research has been conducted since the 1980s to reduce the gap to micro- or nanometric dimensions by techniques from the fields of electronics. These attempts are apparent from US6860153 or the following publications: Kourosh, K. and M.L. Albert, The nanogap Pirani-a pressure sensor with superior linearity in an atmospheric pressure range. Journal of Micromechanics and Microengineering, 2009. 19 (4): p. 045007. - Brun, T., et al., Silicon nanowire based Pirani sensor for vacuum measurements. Applied Physics Letters, 2012. 101 (18): p.183506-1-4. Masanori, K., M. Yoshio, and S. Masakazu, Silicon sub-micron-gap deep trench Pirani vacuum gauge for operation at atmospheric pressure. Journal of Micromechanics and Microengineering, 2011. 21 (4): p. However, the materials constituting the son or film used in these sensors having low temperature coefficients, between 9.10-4 K-1 and 3.10-3 K-1, the sensitivity of these sensors is thus very limited. because 5 proportional to this coefficient. These sensors often use suspended sensitive elements based on metal films, such as gold or platinum films, or even aluminum, nickel or chromium, or possibly semiconductor films. In addition, the range of pressures measurable by these sensors is determined by the geometry of the sensitive element, and most often limited only to low pressures since it is technologically and / or economically complicated to industrially guarantee a nano-sized gap. . This range is thus usually limited to pressures of less than 100 mbar with good sensitivity, or at most up to 1000 mbar with low accuracy. Another limitation is inherent in the principle of the Pirani gauge, since it must be immersed in the gas to be tested. This results in a fragility to mechanical shocks and dust, sensitivity to environmental disturbances such as anemometric flows or variations in the composition of the gas tested. They also have a fragility to the aggressiveness of the gaseous environment, for example moisture and corrosion. Certain types of "packaging" have been proposed, which partially surround the sensing element to protect it from certain influences, for example to limit anemometric influences or to provide mechanical protection or to limit the nonlinear effects of thermal convection, such as for example the package described in Dozoretz, P., Stone C., and O. Wenzel, "Shrinking the Pirani Vacuum Gauge", Sensors (Magazine), 2005. Thus, FIGURE 1, taken from this reference, illustrates a micro-gauge Pirani on a silicon support 18 carrying resistors 12 for measuring the temperature around its periphery. The sensitive element 11 is made by a nickel filament coiled into several successive "S". The sensitive element is disposed under a protective shell 19 of silicon open on the side, which limits the mechanical contacts and reduce disturbances 35 due to gas flows for example by convection effect. It has also been proposed to use a micro-filtering grid to protect the sensitive element from surrounding dust while maintaining the communication of the gauge with the gas to be tested. These protections, however, remain incomplete, and generally do not allow use in any environment, for example outdoors or in aggressive environments within within chemical reactors. This is why Pirani gauges are used almost exclusively in controlled environments such as laboratory or industrial vacuum structures where the measured gas is often a neutral gas such as nitrogen or argon, or even in a few cases. rare case of air. An object of the invention is to propose a temperature and / or pressure sensor implementing the principle of the Pirani gauge which is improved in its performances, its ranges of use, its robustness, its reliability, and or the environments and situations in which it is usable. Another aim is to allow a better adaptation of the sensor and its performance to its conditions of use, to optimize its manufacture for example in terms of simplicity, cost, technical constraints and flexibility of industrialization.

SUMMARY OF THE INVENTION The invention proposes a heat loss type sensor implementing the principle of the Pirani gauge for measuring the pressure, and / or the temperature, as a result, of a gas in an environment. tested.

According to the invention, this sensor comprises a gastight enclosure, or aneroid capsule, here also called housing, which is intended to be exposed to said environment tested by its outer surface, and which encloses a gaseous interior environment in which is disposed a resistive sensing element to be exposed to said gaseous interior environment. This sealed enclosure has a deformable structure under the effect of a pressure difference between said indoor environment and said environment tested. The resistive behavior of said resistive detection element thus depends on the so-called external gas pressure prevailing within said tested environment according to a transfer function, and thus makes it possible to provide a measurement of said external pressure. Thus, compared with the prior art illustrated in FIGURE 1, the invention provides an overall solution to many of these difficulties or limitations, typically by sealing the gauge in a capsule under a given pressure of inert gas. The capsule used as a seal, which can be called a package or "package", is deformable to reproduce all or part of the pressure variations that it undergoes externally. The internal Pirani gauge then remains sensitive to external pressure while operating in a controlled environment since it is completely protected even when the entire sensor is used in harsher environments. Although protecting the Pirani gauge in a leakproof way may appear a simple principle, this type of package has never been proposed. In fact, unlike partial protections such as microgrids or open protections such as that of FIG. 1, it clearly appears that sealing the existing enclosures would prevent the internal gauge from detecting the external pressure, in any case with significant sensitivity. In contrast to this natural notion, the inventors propose to use a sealed enclosure anyway, by proposing a different type and having deformability characteristics which allow the internal gauge to reflect the external pressure in a usable manner. , even improved on some points. Even if the "raw" sensitivity of the gauge is reduced by the housing, as would be expected, it appears that this decrease can be limited to remain exploitable. In addition, the invention proposes to use the influence of this housing in a positive manner, for example to obtain a higher sensitivity in a specific range or even restricted, and / or to control the positioning of the range of better sensitivity .

In preferred embodiments, such as those presented herein, the enclosure is resiliently deformable, even though its stiffness is low. However, the invention also proposes to use a plastically deformable enclosure or without significant stiffness, for example a very flexible elastomer or plastomer. The sensor is then designed to operate in equipressure between its indoor environment and the tested external environment. Although the sensitivity then undergoes no modification other than degradation, it becomes possible to use the sensor in more varied environments, for example unknown gases, possibly reactive and / or aggressive.

In another aspect, the invention provides a method of designing or adjusting such a sensor. This method comprises the following steps. One step is to select at least two pressure (and / or temperature) values forming a target measurement range for the tested environment for which said sensor is intended. From this target range, a virtual gap value, representing the dummy distance between a mechanically released sensitive element and the nearest thermalized surface, which is adapted to perform at least one measurement within this measurement range is determined. aimed with a standard Pirani gauge, that is to say without waterproof enclosure and bathing directly in the environment that one wishes to test. This value can be determined or optimized according to one or more determined criteria, for example according to a known model of operation of a Pirani gauge in a free environment. Moreover, a determined value is chosen representing a real gap, that is to say the actual value of the gap that is to be achieved physically for the gauge that will be made inside the housing. Typically, this real gap value will be chosen to allow a performance determined according to at least one criterion other than the target measurement range, for example ease of realization, resistance, reliability, cost, etc. From the value of the virtual gap that one wishes to simulate, and the actual gap value that is expected to be achieved, a transfer function representing a behavioral transformation from a gauge made with said real gap to a gauge made with said virtual gap. That is to say the function which represents the influence of the encapsulation on a gauge with the real gap, and which makes the whole behaves like a gauge which would present the virtual gap when arranged directly in the environment tested. This transfer function thus makes it possible to manufacture and use a real gauge, having said real gap, to achieve at least one measurement within said target range of measurement. This transfer function is for example determined by inversion of a model of mechanical behavior of encapsulation, or chosen from a list of known or tested transfer functions for different structures. It is then possible to determine or choose, for example from a list of structures and / or models calculated and / or tested, the characteristics of a sealed enclosure whose structure, or the internal pressure, or a torque 10 of these two characteristics, provides said transfer function. We then have implementation characteristics for manufacturing a "composite" sensor operating in an optimized manner for said target range of measurements, while having a gauge having said actual gap.

According to one feature, the sealed enclosure is determined to provide a maximum sensitivity to a pressure value of the tested environment that is shifted to high pressures relative to the pressure value that would provide the maximum sensitivity if the same element of detection was exposed directly to said tested environment (ie without a sealed enclosure). Thus, the virtual gap is always less than or equal to the real gap since it is inversely proportional, according to the models mentioned, to the position of the maximum sensitivity. Thanks to the invention, a wider and easier choice and control of the sensitivity range, in position, width and / or sensitivity, is thus obtained by acting on the main parameters of this composite sensor: choice of stiffness of the chamber, internal pressure, encapsulation pressure (that is to say the internal pressure at the time the enclosure is sealed, also called packaging pressure) and / or actual gap value. The presence of this deformable sealed enclosure makes it possible in particular to move the sensitivity range of the sensor relative to the gauge alone, even in pressure levels where the gauge alone is little or not sensitive, and thus to improve the performances. absolute in certain pressure ranges. It also allows a local modification of the sensitivity, for example an amplification in a restricted range, or a stabilization or linearization of its values. These possibilities make it possible to relax the constraints imposed by the pressure range on the dimensions of the gauge, in particular that of the real gap, and thus make it possible to optimize the manufacturing and to make it simpler, more reliable and more economical. They allow for example a translation of the sensitivity range, thus allowing the use for high pressures of a micrometric gap instead of a nanometric gap, which is much simpler to achieve. In addition, this architecture makes it possible to use such a Pirani gauge-based sensor in an environment that would not allow the use of a bare gauge, for example in an aggressive or reactive environment, such as an atmosphere containing oxygen or different flammable gases or substances, inter alia because of the filament temperature and / or the risks of corrosion and / or other unwanted chemical reactions. Such resistance to the external environment under atmospheric pressure conditions is thus likely to allow new applications, in particular onboard applications such as for barometric measurement or altitude or air speed. The manufacturing methods make it possible to obtain sensors of good miniaturization, possibly integrable into miniaturized or integrated electronic circuits or micrometric mechanisms. According to an optional feature, the sealed enclosure also contains at least one volume element called reducing element occupying part of the interior space of said sealed enclosure, thereby reducing the volume occupied by the internal gaseous environment.

Such a reducing element is for example rigid, or is compressible with a stiffness combining with the stiffness of the enclosure. The characteristics of this reducing element make it possible to adjust the characteristics of the overall transfer function, leaving more freedom in the definition of the housing and the gauge, and therefore in the constraints and technical conditions for producing the complete sensor. Such an element makes it possible for example to modify the effects of an existing box, and thus to adjust the characteristics of a sensor from an existing box and an existing gauge without the need to design and manufacture a gauge and / or a different housing. It can thus provide additional design flexibility and / or adjustment fineness. Thus, it allows a better adaptation of the sensor and its performance to its conditions of use, for example by giving more flexibility in the choice of the optimum sensitivity range. Type of Gauge and Sensitive Element According to another aspect of the invention, advantageously combinable with other aspects and features, it is proposed a particular type of pressure sensor and / or temperature on the principle of the gauge Pirani, in particular through its sensitive element (s) and the materials that compose it. It should be noted that this aspect can also be implemented independently of the other aspects and features described and described herein, for example without including packaging or active packaging. According to this aspect, the sensitive element of the Pirani gauge is made at least partially of a perovskite oxide, preferably a crystalline oxide, corresponding to the formulation: (M1) ix (M2) xM303 with x ranging from 0 to 1 including case x = 0 and case x = 1, and where: M1 is a lanthanide, M2 is strontium or barium, M3 is selected from titanium (Ti), manganese (Mn), iron (Fe), chromium ( Cr) and Cobalt (Co); Except for barium titanate and strontium titanate. Optionally, we exclude the SrMnO3, even the case where x = 1. According to a feature of this aspect, the element M3 is selected from titanium (Ti) or manganese (Mn). Particularly interesting examples of perovskite oxides suitable for the sensing element are the following: manganite of the Lai_xSrxMnO3 (or LSMO) type, manganite of the Lai_xBaxMnO3 (or LBMO) type. Optionally, such a perovskite oxide is (La, Sr) TiO 3 - (LSTO), or SrTiO 3 (STO) with doping, for example with Nobium in the form of Nb: SrTiO 3. These materials allow better performance, particularly in terms of sensitivity and / or stability, which are particularly advantageous in the context of higher pressure ranges or optimized and adapted by the encapsulation proposed by the invention.

In particular, this aspect of the invention proposes to provide the sensitive member with a film of such a perovskite oxide forming a bridge connected at both ends and released over most or all of its periphery. (viewed along a cross-section to the measuring current), so that it can be immersed in the medium to be measured.

This sensitive element may for example take the form of a plate or a ribbon or a wire extending between two electrical connection terminals. It can be connected to terminals made of different materials. It can also itself constitute a narrow part between two wider regions of the same layer of perovskite oxide, these wider parts then forming said electrical connection terminals or forming part of an integrated circuit. The gauge comprises one or more sensitive elements according to this aspect, which can each be released from their support or not, possibly by combining the two within the same sensor. Each has for example a length between 100 nm and 2 cm, or even between 10 pm and 100 pm, and / or a width between 50 nm and 1 mm, even between 2 pm and 15 pm, and / or a thickness between 1 nm and 10 pm, or even between 10 nm and 200nm. For example, for a laboratory sensor, the sensing element has a 100nm x 50nm ribbon section, or a 3cm x 2mm wire section for a general purpose commercial sensor, or a ribbon section. 5pm x 2pm x 160nm for other types of intermediate uses. Such a sensitive element is produced for example by deposition on a support of a layer of perovskite oxide including the sensitive element, followed by a release of the sensitive element of this support, in whole or in part, by 25 -gravure of the support by at least one chemical attack, for example by XeF2. The support is advantageously a monocrystalline silicon substrate, for example treated by epitaxial growth of at least one intermediate layer, chosen to make it possible to obtain good quality for the perovskite oxide layer, and if possible chosen not to be attacked by the release burn step. Preferably, the perovskite oxide layer is deposited as a crystalline coating on the entire sensing element. Examples of advantages Perovskite oxides have been known for a very long time, but some of them under certain conditions have a temperature coefficient of between 20 × 10 -3 K -1 and 500 × 10 -3 K -1. that is to say 10 to 1000 times greater than the thermal coefficients of the metal films used to produce the sensitive elements of the current sensors.

With this type of material, the inventors have determined that a number of interesting and unexpected advantages are obtained. Sensitivity: In a Pirani type sensor, the evolution of the resistance of the sensing element as a function of temperature and / or pressure shows a significant variation within a certain range of temperature or pressure, determining a range of measured. This can be determined by sensor realization parameters, in particular geometric parameters, such as the gap between the sensitive element and the support. The magnitude of the variation d R of the electrical resistance R over the whole of this measurement range, relative to the value of R, determines the sensitivity that can be obtained by this sensing element. Over this measurement range, the sensor according to the invention can allow a measurement of R, and therefore directly of T negligible magnetic field, with a greater sensitivity. Due to the increase in the temperature-dependent resistivity variation with respect to prior art materials, i.e. the large value of TCR = 1 / p.dp / dT, gets an increase in sensor sensitivity. It is thus possible to obtain a dR / R value well above 10%, and for example a dR / R value of about 80% in certain configurations. Such a sensitive element thus makes it possible to obtain a sensor that is more sensitive than the current sensors. Pressure range: In addition, this better sensitivity makes the ends of the electrical resistance variation area more exploitable, which slightly extends the usable measuring range compared to current sensors. In certain configurations, the pressure measuring range can thus extend above 10mbar or even 100mbar. A pulse type measurement will also be more sensitive. Consumption: For pressure measurement, the sensitive element according to the invention also requires a lower supply current than the known sensitive elements, which makes it possible to obtain a less energy-hungry sensor. It becomes possible to obtain a consumption of less than 10 pW, a gain of about a factor of 1000. Noise in 1 / f: Moreover, it appears that the very nature of the perovskite oxides gives a noise in 1 / f less than that obtained by the metallic or semiconductor sensitive elements.

In addition, the oxides are deposited in ultrathin layers, but their high structural rigidity makes them easier to handle than other materials of the same dimensions, which facilitates the manufacture of miniature devices. It is thus possible thanks to the sensor element according to the invention to design temperature and / or pressure sensors which are more compact and smaller in size than the current sensors, which enables them to be even more sensitive to higher pressures and especially less energy hungry. These advantages can thus be obtained with a small footprint, for example by a factor of about 1000. It thus becomes possible to produce sensors whose sensitive element is of the order of microns or even a few tens of nanometers, and can be integrated into an integrated circuit of the electronic or electro-optical type. Such a sensor can then be made very economically and reproducibly by technologies used for example for electronics or MEMS or NEMS, and be arranged and positioned extremely flexibly with respect to other circuits or the medium to be measured. . In addition, it becomes possible to integrate the sensor in the same chip as some electronic circuits, including correction circuits that serve to correct its non-linearity, thus greatly limit the variations and interference between them. For example, the temperature compensation can be obtained in-situ by integrated circuits in the same chip as the sensor. The non-linearity of this type of measurement thus becomes much more acceptable and less troublesome for the final measurement, which further increases the accessible measurement range.

Moreover, such a sensor can be implemented with voltages lower than 5V, and for example with an output voltage in the range of 3.3V amplifiers, which allows great flexibility, simplicity and economy in the device. using or integrating the sensor. Environment and Temperature: In addition, the perovskite oxides 35 of the given formulation are more stable and robust than metal films and can be used in hostile environments and especially in an oxidizing atmosphere, unlike the present sensitive elements. . They are also more temperature resistant, and can withstand higher temperatures than electrical contacts of a conventional type, i.e., for example of the order of 400 ° C or higher. In addition, the sensitive element is very strong mechanically, because of the rigidity of the material but also because of its small dimensions. It has no creep and is less sensitive to aging. It also has a better structural behavior because the oxides have a Young's modulus 10 greater than that of metals and a lower coefficient of thermal expansion, which makes them less sensitive structurally to temperature variations and mechanical shocks. Measuring method According to yet another aspect, the invention furthermore proposes a method for measuring a pressure and / or temperature within a tested gaseous environment, which method comprises using a sensor as explained here, within said gaseous environment. According to a particularity, the sensor used comprises a deformable sealed enclosure which is adapted to transmit the external pressure to its internal environment in the same way. According to another particularity, the sensor used comprises a deformable sealed enclosure which is adapted to transmit the external pressure to its interior environment according to a specific transfer function. According to yet another particularity, the sensor comprises a temperature change (typically by heating, for example the substrate) of the interior environment of the sealed enclosure. First Embodiment According to a first embodiment of the invention, the sealed enclosure comprises at least one deformable bellows. Such a bellows is typically formed by a thin and pleated or corrugated wall, wholly or partly surrounding the internal volume of said sealed enclosure, and which is deformable in one direction and which is deformable in a direction parallel to the average surface of this wall. This enclosure is made for example by various known technologies, such as molding, machining, stamping, electrolytic deposition and / or numerical control additive manufacturing (for example by three-dimensional printing based on metal or polymer). It can be made of any material or combination of material that can be used in these types of process. It may be for example metal, in one or more layers, but also resins or elastomers of different types. According to one feature, this bellows has a generally cylindrical shape, preferably of revolution but not necessarily, constituting a wall carrying peripheral corrugations providing a deformation capacity in an axial direction. Second embodiment According to a second embodiment of the invention, the sealed enclosure comprises a rigid housing sealingly closed by at least one deformable membrane, or several. That is to say, this or these membranes form a generally two-dimensional wall, which is deformable in a direction normal to the average surface of this wall. This is for example a flat surface of an elastically deformable material, or a flat surface in which are formed one or more corrugations, for example concentric, allowing such a deformation by bending. Different types of technologies can be used to produce this type of membrane: depending on the scale of dimensions concerned, this may be for example a metal sheet or a sheet of resin or elastomer; or a plastomer having a certain elasticity, for example by molding, machining and / or stamping; or a crystalline material such as silicon; or a low residual stress material such as silicon nitride or silicon oxide or a complex oxide of yttria-type zirconia or strontium titanate; or any known technique. It can be made of any material or combination of material that can be used in these types of processes. According to one particularity, of interest for many fields, in particular for miniaturized and / or integrated circuits, the membrane has a thickness less than one millimeter, preferably less than one hundred micrometers, or even less than ten micrometers. This type of membrane 35 may be made for example by various known technologies, such as those in the fields of integrated circuit electronics or electromechanical microsystems or "MEMS" (for MicroElectroMechanical Systems), or those used to realize Capacitive membrane sensors. This may be for example a combination of micro-deposition techniques, micromachining, and / or various forms of etching or etching or by plasma or radiation. Various embodiments of the invention are provided, integrating, according to all of their possible combinations, the various optional features set forth herein.

List of Figures Other features and advantages of the invention will become apparent from the detailed description of an embodiment which is in no way limitative, and the attached drawings in which: FIG. 1 is a diagram which illustrates according to the prior art a Pirani micro-gauge with open protection; FIGURE 2 is a side sectional diagram illustrating a mechanical modeling of a sensor according to the invention, according to a first embodiment with a housing made by a deformable bellows; FIGURE 3a and b are graphs of analytical curves from 20 modeling calculations which illustrate the behavior of the housing according to the external pressure Pext, for different encapsulation pressures Po and for a case constant Pg of 1170mbar: o in FIGURE 3a , for the internal pressure Pint, and o in FIGURE 3b, for the deformation in height bx; FIGURE 4a and b are graphs similar to FIGURES 3a and b, for a value different from the case constant Pg of 170mbar; FIGURE 5 is a flowchart illustrating the architecture of the operating models of the sensor according to the invention and the identified transfer functions involved; FIGS. 6a and b are graphs of analytical curves derived from modeling calculations which illustrate in output voltage and in sensitivity the response of the sensor according to the invention, compared to a Pirani gauge identical to that of the sensor but without a housing (c '). that is, non-packaged), which has been manufactured and measured at different case pressures and for a case constant of: o in FIGURE 6a: 170mbar, and o in FIGURE 6b: 1170mbar; FIGURE 7 is a graph illustrating the center dependence of the sensing pressure range of the sensor according to the invention as a function of the case constant, for different packaging pressure values; FIGS. 8a-b are graphs of analytical curves derived from modeling calculations which illustrate the evolution of the maximum sensitivity and the sensitive pressure range as a function of the case constant Pg and for different relative encapsulation pressures Po. to a Pirani gauge without case: o in FIGURE 8a, by the range of relative sensitivity, o in FIGURE 8b, by the value of the relative maximum sensitivity; illustrating an exemplary manufacture of a sensor according to the first embodiment: FIGURE 9 is a schematic diagram illustrating the manufacture of the bellows by electrolytic deposition of copper on an aluminum male mold, FIGURE 10 is a perspective view of the deformable bellows, after chemical dissolution of the male mold, FIGURE 11 is a photograph of the sensor once assembled and sealed, FIGURE 12a and FIGURE 12b are schematic side sectional views of the assembled and sealed sensor in the idle state and respectively compressed under the effect of an external pressure; FIGURE 13a and b are graphs which illustrate, in comparison with the same caseless Pirani gauge, for the sensor of FIGURE 12 provided with an interior volume reducer; o in FIGURE 13a, the unitary pressure responses of the Pirani gauge alone (light line) and the complete sensor (strong line) recorded experimentally, and o in FIGURE 13b, the sensitivity response respectively raised and calculated for these same sensors; FIGURE 14 is a side sectional diagram illustrating a sensor according to the invention, according to a second embodiment with a rigid housing 35 closed by a deformable membrane; FIG. 15 is a schematic perspective view illustrating a sensor according to the invention comprising a plurality of Pirani bridges encapsulated in a housing sealed by a micromembrane on silicon by a substrate transfer and soldering method.

EXAMPLES OF EMBODIMENTS OF THE INVENTION The inventors have carried out tests, by numerical modeling as well as by prototyping, which show that the solution of deformable waterproof capsule proposed by the invention does not necessarily bring the disadvantages and insufficiencies to which could be expected, while allowing 10 new and unexpected benefits a priori. It has been realized, presented here in a synthetic way, a modeling of the mechanical operation of the sealed package thus proposed, that is to say a package whose internal pressure is ensured and preserved by the tightness of the system.

This modeling makes it possible to evaluate and quantify the transfer function provided by the sealed capsule that can be deformed under the effect of an external pressure. We will see that the dreaded drop in sensitivity when a Pirani gauge is encapsulated can theoretically be limited and the package can manipulate its performance with adequate control of the system dimensions. First Embodiment: Modeling and Example With reference to FIG. 2, more particularly for the first embodiment, we take as a model that of a deformable spring 25 having a biasing force F.zasnc tending to oppose the deformation induced by a pressure difference in the x direction. The internal volume of the spring being hermetic, two distinct pressures can be separated: the external pressure P.xt and the pressure of the sealed cavity. The spring has a transverse section S, a characteristic height to which is added a deformation eX which appears when PilYt. The spring being supposed perfect, the deformations in directions other than x are neglected. The first step is to determine the balance of forces by the fundamental principle of the static. The forces involved are the return force of the spring F = and the force exerted by the pressure of the atmospheres internal and external to the bellows. These forces are: - (Pi nt) xS (1) With: the stiffness constant of the spring representing the stiffness of the bellows (N.rn-1). At equilibrium it is considered that the gas contained in the capsule is a perfect gas, which is the case with most low-pressure gases. We will see that in our case this condition is all the more justified that the invention allows even justify often keep a low internal pressure, even for larger external pressures. When sealing the capsule, the internal pressure is exactly the packaging pressure Po and the following equality is satisfied: (3) With: -: the so-called packaging pressure (in Pa), 15 - f0: the volume of the undeformed capsule (m3) and I ° = sx ds - R: the universal constant of perfect gases (8,3144621 JK-1.mo1-1), - T: the packaging or operating temperature (K), - n: the number of moles contained in the encapsulated gas (moles). In the case where it is desired to operate this sensor in a temperature range wide or far from the sealing temperature, it is necessary to take into account the error induced by the increase in pressure due to the temperature variations of the gas. If necessary, this error is corrected by modifying the sealing temperature to approximate the average temperature of use: for example by controlling the ambient temperature upon sealing and / or by using a sealing method producing or permitting a specific temperature. Alternatively or in combination, it is intended to use a substrate heating system during measurement to artificially increase this average temperature of use. We can continue the analysis by determining the internal volume inside the capsule: tua [= ax x + We obtain then - according to the equality (3): P t And (by combining (1 ), (2) and (5) and then developing the calculation) a first version of the package transfer function is obtained: (6) With, in the form of a deformable bellows: p9 = 51. 0 (7) ) Characteristic pressure Pg. This quantity is a numerical constant characteristic of the transfer function, called the case constant. Pg is expressed in units of pressure (Pa) and is given here for a capsule of cylindrical form. We can rewrite the equation of the transfer function (6) in a polynomial form: + (8) 15 Whose all the solutions are: This expression makes it possible to establish the useful transfer function in pressure of the capsule, it is that is, the function expressing the internal pressure with respect to the external pressure. This expression being quadratic, it is necessary to select only the solutions presenting a physical reality, namely those giving and -7, positive only, and verifying the following packaging condition: ) = A set of analytical solutions can be obtained with the dimensions taken into account hereinafter in this analysis: with a bellows 25 having a diameter of 4 mm, a height of 7 mm and a stiffness constant k of 840 N / m. The constant Pa can be calculated from all these dimensions (and by the relation (7)), which gives us a value of 1170 mbar. Behavior of the Case FIG. 3a and b illustrate the behavior of such a case according to the external pressure Pext, for different initial pressures of encapsulation PO and for a case constant Pg of 1170 mbar: FIG. 3a, for the internal pressure Pie , and in FIGURE 3b, for deformation in height bx. Case constant Pg. 1170mbar With such a value of Pg, the transfer function is used, for example the relation (9) to plot the dependence of the internal pressure as a function of the external pressure, for different packaging pressures illustrated here. in FIGURE 3a. The 5x height deformation of the capsule is always extracted (from relation (5)) according to the analytical models, again as a function of the external pressure and for different encapsulation pressures o, as illustrated here in FIG. 3b. It may be noted in FIG. 3a that the transfer function is linear for all POs and for external pressure values Pext that are sufficiently high and exceed a certain pressure threshold.

By decreasing one of the constants Pg or Po, for example by reducing the initial height of the spring, it is thus possible to obtain such a linearity of the response in internal pressure Pie at lower external pressures Pext. Coefficient A, or Pressure Gain 25 Within this linear response range, for example for an external pressure Pext greater than 1400 mbar, the relation between - - and can be modeled as a linear equation: Pim (Pexi) = Ax Pexi + D (10) With A and D: two coefficients dependent on Pr t and Pg (without unit). It is further noted that low packaging pressures, here for 1 mbar and the neighboring values, result in better sensitivity to the external pressure, which is reflected here by a greater slope of the curve. Indeed, the coefficient A is 0.97 for Po = 1 mbar and is equal to 0.59 for PO = 1500 mbar, still in the linear regime. The pressure gain is therefore more advantageous for low packaging pressures. The operating limit of the system in linear mode at low external pressures is determined by Pg, the value around which a deflection of the curve is created. Depending on the values of Po, this deflection 5 will be more or less pronounced, even negligible at high values of encapsulation pressure. The offset D in pressure, which constantly changes the Pint response to the pressure Pext, is positive in the first case (at high Po), and negative in the second case where the packaging pressure Po is very low, in particular when it is much lower than Pg.

It can be noted on the bellows displacement curve, in FIGURE 3b, that low packaging pressures cause a large deformation at low pressures at low pressures Pext, ie in the pressure range for which the transfer function is no longer available. linear. It should be noted that this range of pressures below the Pg value is also exploitable, although the behavior is more difficult to define analytically because of the large nonlinear variations of λ. This example illustrates the need to find compromises in performance when designing the active package. In FIGURE 3b, it is furthermore noted that the values of the deformation of the spring with these dimensions remain less than 5% of the initial height, that is to say less than 0.35 mm in the present case. This value allows us to ensure that the capsule will remain in the limit of the elastic regime for a large number of materials. Case constant Pg. 170mbar (FIGURE 4) In the case of the same bellows whose height has been reduced to 1 mm instead of 7 mm, the constant g: g. is now worth 170 mbar. The external pressure dependence of this bellows is illustrated in FIGURE 4a and b. In this case, the system is linear over a larger external pressure range than in the previous case and the coefficient A is about 1 30 for most packaging pressures Po. It is nevertheless noted that the sensitivity is better for low encapsulation pressures Po, and we find a behavior for the shift in pressure D identical to that observed previously. The maximum deformations experienced by the bellows are much larger and reach 100% to 200% of the initial height. Nevertheless, the use under restricted conditions of pressures is achievable. For example, for a torque Po = 500 mbar and Pg = 170 mbar, it is quite possible to envisage operating the case in a range of external pressures ranging from 300 mbar to about 1000 mbar, while ensuring maximum deformation of the case of 0.5 mm, and while ensuring overall that the gain of the transfer function remains close to unity. This type of range is for example targeted in many embedded applications and / or outdoors, for example in aeronautics and / or meteorology. These are, for example, ranges of about 10 hundreds of mbar around or below the pressure at sea level, for example a range from 350 mbar to 1070 mbar. The set of values tested for the pair Po and Pg, illustrated through these two analytical examples, shows that it is possible to adapt such a case in different ways to provide new and varied performances, even under conditions where its architecture is subject to design constraints, for example by the maximum space that it must occupy in an embedded application, or for example by the type of material acceptable for its manufacture, since Pg depends on the rigidity coefficient of the system, and Therefore, by the systematic analysis of the transfer function with a fixed Pg, it is possible to play, for example, on the packaging pressure Po to achieve the targeted performances, or to play simultaneously on these two values to determine an optimized geometry in response to the need. Overall, it will be noted that the case allows a constant offset of the internal pressure value with respect to the external pressure and that the pressure gain is less than or equal to unity in the so-called linear regime, with limited deformation in the elastic regime; while it shows a gain greater than unity in the non-linear regime, with a behavior of the box at the limit of the operating conditions.

Composite Sensor As shown in FIGURE 2 and FIGURE 5, the sensor according to the invention comprises a sealed package or "package" which encloses a Pirani gauge. The pressure of the tested environment Pext acts on the package. This is deformed which causes its internal pressure Pint to vary according to the extent to which the inner pressure results from the external pressure through a transformation which constitutes the transfer function of the housing. This can be described as "active": unlike known partial packages that mitigate certain spurious effects or stabilize measurements within the same sensitivity range and for unchanged sensitivity, this active package can provide a complete transformation. information, for example with a full shift of the sensitivity range or even a local increase in sensitivity. In the case of the bellows as modeled here, this transfer function can be modeled (by the relation (9)): CP t) ± 1 (10 t) Inside the case, the Pirani gauge is influenced by the internal pressure: it thus receives a transformed version of the external pressure, int p (P -, ext) - The resistance of its sensitive element varies depending on the internal pressure, in a manner that depends on the characteristics of the gauge. This gauge returns an ohmic resistance information reflecting the internal pressure, in a way that can be evaluated for example according to one of the known mathematical models. By integrating, for example, the logarithmic model of a Pirani gauge, which is a generic and empirical analytical model for a logarithmic response gauge, the dependence Rp (Pext) of the resistance Rp as a function of the external pressure Pext can then be write: Rp (Pext) = B x TCR (T) xk, P0 With: - B: a calibration coefficient (without unit), 25 - TCR (T): the temperature coefficient of the material constituting the Pirani gauge, varying according to the temperature considered (in K-1). The sensor according to the invention thus forms a "composite" pressure sensor which contains two distinct but interconnected sensitive elements: A first sensitive element is formed by the active housing, which is modified by the variation of external pressure to be tested, which form thus the first measurand. A second or intermediate sensitive element is formed by the + 4P 2 Pint (Pext) 1 + 3037652 - 24 - gauge Pirani, which is modified by the variation of internal pressure, which thus forms an intermediate measurand. The Pirani gauge can thus be connected to a signal conditioner which produces and converts an electrical signal to measure the variation of the resistance of the sensing element, and thus provides a readable value representing the initial measurand Pext. These conditioners can for example be Wheatstone bridge type DC voltage as shown in FIG 5, or constant current, or heating or resistance or constant power, which allows for example to annihilate 10 possible drifts in temperatures generated by the term TCR ( T), or by other types of conditioners adapted to the type of signal that is desired. By combining the transfer function of the package with that of the pressure gauge, the response of the encapsulated Pirani gauge connected to a signal conditioner can be simulated. For example, as illustrated in FIG. 5, the signal conditioner comprises a constant voltage Wheatstone bridge, which outputs an electrical signal representing the dependence Vout (Pext) of an output voltage Vout as a function of the external pressure Pext, according to the relation: Von PÉxt) = True! Thus, it can be seen that the invention makes it possible to modify and adapt the response of the Pirani gauge by choosing the parameters of the casing to obtain a transfer function which allows the Pirani gauge to behave in a determined manner while using the composite sensor. in a different environment and potentially incompatible with the gauge and / or the behavior chosen for it. Combined behavior Numerical simulation was applied to the combined behavior of a complete composite sensor, comprising a real Pirani gauge and measured based on La0.80Ba0.20MnO3 (i.e., LBMO) a mixed valence manganite of the class of perovskite oxides, a gauge formed of 16 bridges in parallel of 10 μm long by 2 μm wide each, and having a sensing element-to-substrate distance of the order of 500 nm, manufactured according to standard methods laser ablation epitaxy, optical lithography, combined with direct ion beam physical etching and so-called release etching by a known chemical process such as reactive plasma etching or gaseous xenon difluoride. By way of non-exclusive example, a Pirani gauge of this type is proposed by the inventors in patent application FR 14,556,223, not yet published. The present invention is however applicable to all types of Pirani gauges, made in a known manner and from different materials, for example based on silicon only, or a metal such as platinum or tungsten, and for all the dimensions. This study has been applied to the case studied above, in its two bellows height versions, that is: - = 1170 mbar for a bellows with a height of 7 mm, and - Pg = 170 mbar for a bellows of a height of 1 mm.

FIGS. 6a and b illustrate the calculated behavior of the system at the output voltage (top) and at the sensitivity (bottom), compared to a case-less and measured (thick curve) Pirani gauge, with different analytical result curves (FIG. fine curves) according to different packaging Po pressures, at fixed temperature. The case constant is 170mbar in FIGURE 6a, and 1170mbar in FIGURE 6b. It can be seen from these curves that, when the gauge is packaged, an offset of the pressure sensor response of an approximate value of Pg + Po occurs, especially for the low pressures Po. Thus the pressure of the maximum sensitivity is offset by nearly 170 mbar in FIGURE 6a and about 1070 mbar in FIGURE 6b, still at very low values of Po. This indicates that the packaging makes it possible to raise the sensitivity range of the Pirani gauge to higher values. even much higher than the sensitivity range of the same non-packaged Pirani gauge. The value of this offset is of the order of the value of the constant Pg and can be increased by the increase of the packaging pressure Po. Gold, the case constant Pg is determined solely by geometric factors, which can be chosen and controlled independently of the pressure values of the tested medium, which confirms the advantages of this composite sensor in terms of freedom of adaptation.

On the sensitivity curves, the signal gain at the level of the pressure at the center of the maximum sensitivity range P 2 is found, with a signal gain of 900% in FIGURE 6a and more than one decade in FIGURE 6b, for POE = 1 mbar. On the other hand, the sensitivity range width (obtained by calculating the half-height width of the sensor sensitivity peak) with the lowest Po pressures appears here reduced compared to those obtained at higher POE pressures, for which this range of sensitivity range is however higher compared to that of the gage Pirani without case. FIGURE 7 is a graph which illustrates the evolution of the external pressure of the maximum sensitivity of the sensor according to the invention as a function of the case constant / 1 and the initial internal pressure Po. FIG. 7 shows the evolution of the pressure ID, of maximum sensitivity as a function of the case constant Pg, for the same packaged gauge with different packaging pressures Po, that is to say of the internal pressure during encapsulation. It can be seen that Pgr and δ always play a preponderant role in the shift of the pressure of the maximum sensitivity P. This set of curves also shows that we can only increase the value of Ps since the two parameters will always be positive.

Thus, a gauge already performing at high pressures can see its range of sensitivity shifted to even higher pressures. This represents an important advantage, since it is more complicated to design a high pressure gauge with a single Pirani gauge (not packaged), because of the technological requirement that imposes a smaller gap to measure a higher pressure. However pressure values close to atmospheric pressure often require a nanoscale gap, which represents a limiting technological difficulty. On the contrary, by carefully designing the package and adapted to the needs of the sensitivity pressure, it is possible to produce a high-performance gauge at high pressures by virtue of the range offset that it provides while using gauges with a coarse gap and / or or a lower precision, so easier to develop and manufacture. It thus becomes possible to circumvent the need to have a nanometric gap for high pressure Pirani gauges, for example sensitive to atmospheric pressure. For a given gauge, the parameters of the package are, for example, determined directly according to the value of the pressure Pext to be measured, typically by choosing a maximum sensitivity pressure ID, equal to or close to this pressure to be measured. This choice is made for example by inversion of the mathematical models, or by selection in a range of casings already studied and / or tested and whose effects are known. In FIGURE 8 are presented analytical curves of the dependence of the maximum sensitivity and the range of sensible pressure as a function of Pg and Po. The sets of curves of FIGURE 8a and FIGURE 8b generally evolve in opposite directions - a gain in maximum sensitivity is produced by decreasing Po and / or Pg, a gain in sensitivity range width is obtained by increasing Po and / or Pg - illustrating the need to find a compromise between the two sensitivity parameters. All values are given as the percentage relative to the sensitivity of the original gauge, i.e., not packaged. In FIGURE 8a, the relative sensitivity range is the variation of the half-height width of the sensitivity peak indicated on the curves in the lower part of FIGURE 6. The reference is the unpackaged gauge: if one goes up to more than 100% increase the width of the range and conversely less than 100%, it decreases the same range compared to the unpackaged gauge. In FIGURE 8b, the maximum sensitivity, i.e., the amplitude of the peak sensitivity of the curves in the lower portion of FIGURE 6 is illustrated, again relative to the non-packaged gauge for reference. It can be seen that the behavior with respect to Po and Pg is opposite to that of the curves of FIG. 8a, which makes it necessary to choose a compromise between these two performances: sensitivity and range of sensitivity. From FIGURE 8 and FIGURE 6, we can also see that the capsule will modulate both the voltage amplitude of the response and the width of the sensitive pressure range, but in the opposite direction and while changing the pressure offset. Thus, since the lower internal pressure curves have regions of steeply steeper slope, for example, 1 mbar and 10 mbar, it is seen that the sensitivity is increased in these regions, relative to the values. of Pie higher but also compared to the gauge 3037652 - 28 - not packaged. Thus, it is understood that the package can also be chosen and adapted to improve the sensitivity, in specific or even narrow regions, to values of the packaging pressure Po extremely low, for which the coefficient A pressure gain (in the relationship (10)) will be greater than unity. For higher packaging pressures, the packaging can degrade the sensitivity and this coefficient A becomes less than 1, but the range of pressures covered by the sensor is then much greater. Thus, by changing the packaging pressure, one can choose which quality, between the voltage sensitivity and the pressure sensitivity, will be increased to the detriment of the other, and the final response of the sensor can be completely modulated if necessary. the user, or even catch the irregularities of the design of the capsule defining Pg by modifying Po during assembly or adjust the final response according to the final need on the ground. For example, a low packaging pressure Po makes it possible to increase both the range of the sensible pressures and the maximum voltage sensitivity when the value of Pg is sufficiently high. For example, one can be very sensitive on 100 mbar only around a pressure of 1000 mbar, or in a wider way using a gauge which already has a very wide range of sensitive pressures when it is not packaged. On the contrary, by increasing the packaging pressure Po, the range of sensible pressures is greatly increased, even if the total signal variations at the sensor output as well as the value of the maximum sensitivity can be degraded. Thus, it will be understood that the combination of the deformable waterproof casing with the Pirani gauge, if it may have disadvantages on certain points, makes it possible (by the different geometrical and material characteristics) to choose and regulate a balance between the different objectives, including manufacturing costs, the range of sensitivity covered and sensitivity sought. In addition, the total signal variation will also be impacted and will decrease, i.e. the voltage change for 1, especially at low packaging pressures. This element can also be an advantage because a lower total variation can facilitate downstream acquisition by reducing the risk of voltage saturation of the amplification stage. Manufacturing Example 5 FIGURE 9, FIGURE 10 and FIGURE 12 illustrate an example of manufacture of a sensor according to the first embodiment, provided with a housing having the shape of a bellows of micrometric and millimetric dimensions, more particularly according to FIG. the dimensions of the previously studied example in its version with a height of 7 mm.

The bellows of this experimental sensor was made with the shape and dimensions of a bellows designed as a main element forming a mechanical energy recuperator, as described in the thesis "Toward an energy harvester for leadless pacemakers", M. Deterre, 2013, University Paris-Sud 11. This bellows has an active part, that is to say deformable, consisting of a thin layer of nickel protected on both sides by two micrometric layers of copper. This structure is interesting for the realization of the sensor according to the invention, in particular, because it has sufficient elasticity, a very good gas tightness and is compatible with the TO-8 type 20 supports often used for pressure sensors commercial. Its nickel structure also provides good protection against electromagnetic radiation, which is an additional advantage. In this example, the manufacture is carried out by successive deposition of material on the outer surface of a male mold, which is then destroyed by etching. Such a method is described, for example, in the M.Deterre thesis cited above. Depending on the choice of materials, other methods may be used for other materials, for example a coating of a male mold by dipping or spraying in the case of an elastomer or a plastomer or a resin any other, or by other known methods. The manufacture of the bellows illustrated in FIGURE 2 and FIGURE 10 comprises the following three steps: the design and production of the male mold by the formation of an aluminum cylinder and the etching of corrugations or corrugations, several substeps of metallic deposits which will constitute the final bellows, and the complete etching of the underlying aluminum mold. The male mold is produced in the form of a cylindrical hollow aluminum piece on the turn of which corrugations are cut, for example by turning. These corrugations will allow the deformation of the bellows and give it a significant elasticity, according to its height and 5 only in this direction. Different numbers and dimensions of corrugations are usable according to the mechanical characteristics concerned. This example is based on the preferred dimensions described in Martin Deterre's thesis, which are reported in the following table: Parameter Value Corrugation type square Number of corrugations 7 Outer radius 4 mm Inner radius 3.5 mm Total bellows height 7 mm 0.5 mm pitch (pitch) As illustrated in FIG. 9 for the copper layer, all of these layers are deposited by electrolysis. In this example, the electrolysis device comprises two electrodes, the anode (counter-electrode) and the cathode (working electrode), immersed in the same electrolyte solution and connected to a current source. More elaborate mounts include a third electrode (reference electrode) to measure the potential of the working electrode. In the case of the electrolytic deposition of metals, the cathode is the seat of the reduction of the metallic species in solution: + 'e - In the present example, the deposits are carried out in galvanostatic mode, that is to say a density current is imposed on the working electrode, the potential being free to evolve. The mold has a gripping base that protrudes from the region to form the bellows. This base is non-conductive, for example covered with a layer of resin, here of the type AZ5214 with annealing at 110 ° C for 3 minutes. The insulating resin thus limits the metal growth to areas in conductive contact with the electrolyte. The mold is kept in rotation to homogenize the deposit obtained and to standardize its thickness. The electrical contact on the mold is taken at the periphery of the mold where a conductive zone is systematically released outside the electrolyte. The experimental device used comprises three electrodes connected to a potentiostat capable of delivering up to 2 A of current, controlled by a computer. The conductive mold is the cathode and a large ultra-pure plate (10x15 cm2) of copper (or nickel depending on the step) forms the anode. The distance between the anode and the cathode is 16 cm. A third electrode, placed near the cathode, is a saturated calomel reference electrode (SCE), contacted with the electrolytic bath by means of a salt bridge of potassium chloride (saturated KCl). The solution is kept at room temperature (20 ° C +/- 1 ° C). Stirring is provided by means of a magnet bar (5 cm) and a magnetic stirrer whose rotation is maintained at 100 rpm. The various layers of copper, then of nickel and then of copper are deposited with the following thicknesses: Coated layer Thickness Copper (inner layer) 1 μm Nickel 10 μm Copper (outer layer) 1 μm The two layers of brass covering the nickel layer both sides protect the nickel layer during the etching of the aluminum mold. This etching is sufficiently inert with respect to the copper so that a thickness of 1 μm of copper is sufficient to protect the central layer of nickel in the time taken by the etching of the mold. The aluminum forming the bellows mold is dissolved by chemical etching, here in a solution of HNO3 (1% to 5% for the etching of aluminum), H3PO4 (65 to 75% for the etching of the oxide native present on the surface) and CH3COOH (5% to 10% for wetting and better homogeneity of etching) diluted in deionized water. The etching is carried out by simple immersion in the solution for a period of 30 minutes to 1 hour at room temperature. The photos in FIGURE 10 illustrate the bellows once released from its male mold. The mechanical behavior of the bellows is measured using a mechanical characterization bench including a piezoelectric force measuring module and a distance laser measurement which indicates its stiffness constant k and provides linear measurements, which indicates that the bellows has an elastic deformation over the entire range of deformation surveyed 3037652 - 32 -. The mechanical behavior of the bellows thus obtained was characterized experimentally to determine its stiffness constant k, three bellows of different thicknesses of nickel, 12 pm for B1, 8.5 pm for B2, and 9 pm for B3. This measurement, carried out using a mechanical characterization bench including a piezoelectric force measuring module and a distance laser measurement, indicates that the structures have a linear deformation; This indicates that the bellows has elastic deformation throughout the entire deformed strain range. The sensor Pirani gauge is made by a sensitive element 31, formed of a layer of 60 nm perovskite oxide La0.80Ba0.20MnO3, previously deposited on a layer of about 20 nm of another oxide, SrTiO3 itself epitaxially grown on a monocrystalline silicon substrate. To form the gauge, 16 parallel bridges of 10 μm long by 2 μm wide each, and having a sensitive element distance to substrate controlled to remain in a range of 500 nm to 1 μm, are manufactured according to standard methods, optical lithography, combined with direct ion beam physical etching and so-called release etching by a known chemical process such as reactive plasma etching or gaseous xenon difluoride. Three other gauges, identical and interconnected with the previous one, but not released from the substrate that carries them and therefore insensitive to gas pressure by Pirani effect, form a complete Wheatstone bridge assembly for a finer measurement at constant voltage.

In this example, the method of manufacturing a Pirani gauge, is that proposed by the inventors in the patent application FR 14 55623 not yet published. The sensitive element, that is to say the Pirani gauge combined with a Wheatstone bridge, and its silicon substrate, is mounted on a support 41, preferably of a standard type, here of the TO-8 Sur type. its face which carries the Pirani gauge, this support 41 is then assembled with the housing forming, the sealing cap 21. This operation is carried out or finalized inside a frame where the pressure of an inert gas, here the nitrogen, can be maintained at a given value, which pressure forms or determines the parameter PO of the calculations presented above. As shown in FIGURE 12a, the contacts of the sensing element 31 are electrically connected with the pins 42 of the carrier, thereby enabling electrical measurement. The pins of the TO-8 support are isolated from the rest of the support by glass 43 providing excellent electrical insulation and sealing the assembly against gas leakage. FIGURE 11 is a photograph showing the sensor once assembled and sealed. The capsule is assembled on the support, for example by embedding their diameters. A seal 49 then made at the base of the bellows and under the support 41, remaining in a zone separated from the corrugations 22 or the deformable part, here with a gas-tight polymerizing glue, for example based on epoxy resin, or by any known method. The whole is placed under a vacuum and then heated at 80 ° C for 12 hours, here at a pressure of 300 mbar, for solidification of the sealing resin, and to maintain the internal pressure of the housing at the pressure Po target. As shown in FIGURE 12b, if the sealed sensor is disposed in a tested environment with a higher pressure than its encapsulation pressure, the housing compresses and decreases in height. Adjustment by volume reducer The gauge included in the composite sensor thus obtained, when used alone, gives a maximum sensitivity Pg of about 100 mbar, a maximum observable in FIG. 13b for the so-called "only" gauge, c 'that is to say, not packaged. As seen previously in the analytical definitions (for example with reference to relation (7)), with the dimensions of the capsule used here and for a stiffness k being equal to 840 Nm-1, it can be calculated that the value of the constant Pg is equal to 1170 mbar. For this composite sensor, the maximum sensitivity of the gauge alone is shifted by at least the constant value Pg, which gives a value of 1270 mbar.

In the case of aiming for a lower maximum sensitivity, for example around 700 mbar, ie at the center of the range from 350 mbar to 1070 mbar, it may be desirable to modify the characteristics of the packaging. In addition, such an offset of Pg = 1170 mbar produces a degradation of the maximum sensitivity, as shown by the lower curves of FIG. 6b. Rather than modifying the characteristics of the gauge and / or the case, and therefore to start a manufacture again, one possibility is to reduce the internal volume of the bellows by placing an element of which one chooses the 5 dimensions, possibly at the moment of the assembly, for example a solid element of small volume such as a piece of plastic or metal or other solid solid. This reducing element is here fixed only at the top of the housing and does not touch corrugations or the base of the housing, it does not change the mechanical response of the housing.

By this means and in our case, leaving a free height internally of 1 mm instead of 7 mm of the case alone, the value of Pg is thus reduced to bring it down to 170 mbar and fall back onto the curves of FIG. 6a. . This modification is thus obtained without modifying the value of k and the other dimensions of the system, or the structure of the housing or its materials. The complete sensor will have a better sensitivity than the Pirani gauge at high pressures, thanks to the slight shift of Ps of nearly 200 mbar to high pressures, while ensuring that the range of 300 mbar-1070 mbar will be fully covered by the enlargement of the range of 20 sensitive pressures. A chosen packaging pressure is then used so as not to excessively reduce the sensitivity, such as 100 mbar, for example from data such as those of FIG. 6a or FIGURE 6b, and / or depending on the manufacturing conditions.

The composite sensor thus obtained thus comprises an enclosure having a constant α = 170 mbar at an internal pressure of PO = 100 mbar. FIGS. 13a and b then illustrate the pressure response and the sensitivity, respectively, from measurements made with this composite sensor called "packaged gauge", in comparison with the same box-less Pirani gauge called "gauge alone". The ordinate AV is the total variation obtained in voltage with respect to a reference, formed here by the voltage recorded at 1000 mbar. This makes it possible to appreciate the relative voltage variations of the system which are directly correlated with the Pirani response. In FIGURE 13b, the sinusoidal curve represents the calculated sensitivity (curve) and raised (points) for the gauge alone. The lower bell-shaped curve represents the calculated sensitivity (curve) and raised (points) for the sensor made above, from the same gauge.

As can be seen, the gauge alone thus has a sensitivity which is the lowest in the target range of 300-1070 mbar, and a much better sensitivity at low pressures around 100 mbar. On the contrary, the complete sensor has a sensitivity which is more stable over a very large range, and a maximum increased sensitivity of 130 mbar. In the overall range of the graph, between 100 and 1000 mbar, the maximum sensitivity of the composite sensor is much lower than that of the gauge alone. However, in a high range such as the range of 400 to 1070 mbar shown on the right of the graph, it can be seen that the maximum sensitivity of the composite sensor is stable, and much higher than that of the gauge alone on most of this high beach. In this range in particular, the sensitivity of the composite sensor compared to the gauge alone is even increased by nearly 100% to 1000 mbar, pressure at which the Pirani gauge alone is no longer sensitive to gas pressure. This results in both an enlargement of the sensitive pressure range and a sensible pressure maximum offset. These measurements on a prototype validate the established analytical models. It should be noted that this improvement is not obvious in the first instance, since the maximum sensitivity to the pressure ps decreases by almost 70% by the contribution of the active packaging, in agreement with the analytical models, and in agreement with the curves in lower parts of FIGURE 6a and FIGURE 6b. Thus, despite a decrease in its maximum sensitivity, it can be seen that the combination of the gauge with the deformable sealed enclosure has made it possible to move and adjust the maximum sensitivity obtained, while improving it in ranges hitherto difficult to achieve technologically, and while effectively protecting the sensitive element from all the aggressions of the environment tested. These attacks include the spurious ventilation effects, as seen in the curves of FIG. 13b, for which the measured data are much less dispersed with respect to the model in the case of the so-called packaged gauge than in the case of the gauge 3037652 - 36 - alone, despite the rapid pressure cycling shown by the arrows in FIGURE 13a. The invention makes it possible, for example, to obtain a sensor that is sensitive to the Earth's atmosphere without having to further reduce the gap of the Pirani gauge, with the advantages of the Pirani gauge and by eliminating many of its disadvantages, in particular its fragility. inherent to external parasitic phenomena. It is therefore an improvement in the manufacture of low cost pressure sensors based Pirani gauges robust and ultra-sensitive in the range of atmospheric pressure.

Embodiment 2 FIGURE 14 and FIGURE 15 illustrate a second exemplary embodiment of the invention, which will only be described in its differences. In this embodiment, the enclosure is formed by a cavity of fixed volume which surrounds the Pirani gauge, for example a cylinder of revolution, one end of which is closed by a deformable membrane. For very small deflections, the displacement w along the transverse section of the membrane, whatever the direction if it is circular, has been described in the publication "Factors affecting silicon membrane burst strength", Henning AK, et al. 2004. This displacement is expressed by the relation: (11) (, -4) With: the pressure difference exerted on the membrane (in Pa) Using the theories developed for the suspended membranes used in the technologies membrane pressure sensors, it is possible to extract the value of the rigidity coefficient D which is equal to: = fff t3 (12) 12 (1 -v12) 25 It is also known that the deflection of a circular membrane is expressed in all point by: 4 rr W (7 ") = Li-E) With: R: the radius of the circular membrane (m), defining the case, or capsule, as consisting of a membrane suspended above a cavity sealed as illustrated in FIG. 14, from which it can be deduced by integrating the new e value of Vint: (13) 3037652 - 37 - (14) With if the cavity is cylindrical with h the initial height of the fixed cavity. Considering that the relation (4) is still valid, we obtain: ## EQU1 ## Knowing that P falls back by developing on the expression (6) of the function of transfer: -I- Fg With this time a new expression of / 1.: 4ahD (16) The following calculations are identical to those presented above for the case of a bellows.

It is thus noted that a membrane, thanks to its flexional stiffness coefficient D can be likened to a deformable constant spring k such as that used to model a bellows above, and its transfer function should be identical since based on the same relationships. The mathematical analysis of the system is also identical and the expected effects on the sensitivity of an encapsulated Pirani gauge similar to the case of the bellows. In particular, the rigid box structure with deformable membrane is particularly suitable for miniaturization, including micrometric or even smaller dimensions. It is thus possible to make such a composite sensor by encapsulating one or more micro or nanometric Pirani gauges, for example by a known method of substrate transfer. As illustrated for example in FIGURE 15, such a sensor comprises a plurality of multi-wire Pirani bridges 61 in La0.8013a0.20MnO3 (or LBMO) 25 deposited on SrTiO3 (or STO) and tracks 62 formed on a silicon substrate 60 or 902, for example by a known method of lithography. The sealed enclosure is here produced by an elastic micromembrane 69, 3037652 - 38 - formed separately on a silicon substrate, for example in a manner similar to the capacitive sensor membranes. The membrane is then assembled on the main substrate 60 by substrate transfer. On its periphery, this micromembrane 69 is then welded to the substrate 60 by a bead of gold / indium with a thickness of about 500 db which seals the volume surrounding the bridges 61. The substrate 60 here also carries two bridges. multi-son 68 of temperature measurement, each formed between two gold contact pads outside the sealed enclosure.

Other types of resilient sealing structures are provided, of different shapes, for which it is also possible to determine a case constant Pg. Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention.

Claims (15)

REVENDICATIONS1. Heat loss type sensor implementing the principle of the Pirani gauge for measuring the pressure and / or the temperature of a gas in a tested environment, characterized in that it comprises a gas-tight enclosure, or housing which is intended to be exposed to said tested environment, and which encloses a gaseous interior environment in which a resistive sensing element is disposed so as to be exposed to said gaseous interior environment; in that said sealed enclosure has a deformable structure under the effect of a pressure difference between said interior environment and said environment tested; thereby producing a resistive behavior of said resistive sensing element which synergistically depends on a transfer function of a combination of factors including the pressure prevailing within said tested environment, the stiffness of the deformable structure and the distance between the resistive element and the heat bath managing the heat losses, thereby providing a measure of said external pressure. 2. Sensor according to the preceding claim, characterized in that the sealed enclosure comprises a rigid housing sealingly closed by at least one deformable membrane. 3. Sensor according to the preceding claim, characterized in that the membrane has a thickness less than one millimeter, and preferably less than one hundred micrometers. 4. Sensor according to claim 1, characterized in that the sealed chamber comprises at least one deformable bellows. 5. Sensor according to the preceding claim, characterized in that the bellows has a generally cylindrical shape, of revolution or not, constituting a _wall carrying peripheral corrugations providing a deformation capacity in an axial direction. -40- 6. Sensor according to any one of the preceding claims, characterized in that the sealed enclosure also contains at least one volume element called reducing element occupying part of the interior space of said sealed enclosure, thereby reducing the gaseous volaene occupied by the internal gaseous environment. Sensor according to any one of the preceding claims, characterized in that the sealed enclosure is determined to provide a maximum sensitivity to a pressure value of the tested environment which is shifted towards the high pressures with respect to the value. pressure that would provide maximum sensitivity if the same sensing element was exposed directly to said tested environment. 8. A method of designing or adjusting a sensor according to any one of the preceding claims, characterized in that it comprises the following steps: - choice of at least two pressure values forming a desired measuring range for the tested environment for which said sensor is intended; determination of a so-called virtual gap value adapted to carry out at least one measurement within this measurement range targeted by a Pirani gauge which would bathe directly in said tested environment; from said virtual gap value, determination or choice: on the one hand of a value of said real gap, and on the other hand of a transfer function representing a behavioral transformation from a gauge presenting said real gap to a gauge having said virtual gap, o allowing to use a Pirani gauge having said actual gap to achieve at least one measurement within said target range; Determining or selecting a sealed enclosure whose structure, or the internal pressure, or a pair of these two characteristics, provides said transfer function. 9. Method according to the preceding claim, characterized in that the sealed enclosure is determined to provide a maximum sensitivity to a pressure value of the environment tested which is shifted towards the high pressures relative to the a pressure value that would provide the maximum sensitivity if the same sensing element was exposed directly to said tested environment, an offset value controlled by a choice of enclosure stiffness and / or initial interior pressure. 10. A method according to any one of claims 8 to 9, characterized in that the sealed enclosure is determined to provide greater sensitivity than if the same sensing element were exposed directly to said tested environment, by choice of value of the stiffness of the enclosure and / or the initial internal pressure. 11. A method of manufacturing a sensor according to any one of claims 1 to 7, comprising a manufacture of all or part of the sealed chamber by a deposition process and / or etching resolution less than five micrometers or less than one micrometer. 12. A method for measuring a pressure and / or temperature in a gaseous environment tested, characterized in that it comprises a use of a sensor according to any one of claims 1 to 7 within said environment gaseous. 13. Method according to the preceding claim, characterized in that the sensor used comprises a deformable sealed enclosure which is adapted to transmit the external pressure to its internal environment in the same. 14. The method of claim 12, characterized in that the sensor used comprises a deformable sealed enclosure which is adapted to transmit the external pressure to its inner environment according to a specific transfer function. 15. Method according to any one of claims 12 to 14, characterized in that it comprises a change in the temperature of the interior environment of the sealed enclosure.