Sensor for sensing surface acoustic waves, and method for producing such a sensor

Abstract

Disclosed is a surface acoustic wave sensor (1), called SAW sensor (1), comprising, from a lower face (2) to an upper surface (3) of said SAW sensor: a piezoelectric substrate (4), at least one interdigital transducer (IDT) (5), and a layer (6) of polytetrafluoroethylene (PTFE), an upper face of which entirely forms the upper surface of the SAW sensor, said PTFE layer having a thickness of between 50 nm and 4 μm.

Description

[0001] DESCRIPTION

[0002] TITLE: Surface Elastic Wave Sensor and Method of Manufacturing Such a Sensor

[0003] Technical field

[0004] The present invention relates to the field of Surface Acoustic Wave (SAW) sensors.

[0005] The invention relates, in particular, to the use of the SAW sensor for the detection of species under flow in a gaseous medium for applications in the field of analytical chemistry.

[0006] A preferred field of application according to the invention is that of the detection of particles in gaseous environments and particularly that of the detection of polluting particles present in the atmosphere.

[0007] The invention also relates to a device for measuring the concentration of particles contained in a gas. A preferred application is that of detecting the concentration of fine particles in the atmosphere.

[0008] State of the prior art

[0009] Among the particles suspended in the air, whether in solid or aerosol form, are fine particles or particulate matter, known by the acronym PM for "particulate matter." Generated mainly by human activity, fine particles constitute a major problem, particularly in cities, towns, and urban areas. A high level of fine particles in the air is a significant health risk factor leading to cardiovascular disease, impaired lung function, and a reduced life expectancy.

[0010] Monitoring ambient air quality is therefore a key contemporary issue. Organizations are responsible for monitoring air quality, and there are several methods for measuring the amount of fine particles in the air.

[0011] The technique of filtration gravimetry is known in the state, which consists of accumulating particles in a filter and then quantifying the particles by weighing the filter. This is a reference technique but has the main disadvantage of not allowing continuous monitoring of the quantity of particles.

[0012] Dynamic weighing methods such as the Tapered Element Oscillating Microbalance (TEOM) and the beta gauge are also known. These techniques both allow for continuous monitoring. However, they are both expensive; TEOM does not have good selectivity and the beta gauge requires a radioactive source.

[0013] Finally, optical fine particle detectors are known in the state of the art, for example by laser remote sensing or LIDAR, for the acronym "Light Detection And Ranging", which are based on the quantification of the light scattered by particles or aerosols.

[0014] An aim of the invention is, in addition, to propose a Surface Elastic Wave sensor for measuring the concentration of particles or molecules contained in a gas:

[0015] - enabling continuous measurement of the quantity of particles contained in a gas, and / or

[0016] - allowing the quantity of particles contained in a gas to be measured in real time, and / or

[0017] - allowing the measurement, under a gas flow, of the quantity of particles contained in the gas, and / or

[0018] - allowing the quantity of particles contained in the gas to be measured precisely, and / or

[0019] - allowing sensitivity to be maintained, in terms of detection, constant over time.

[0020] Another aim of the invention is, in addition, to propose a device for measuring the concentration of particles contained in a gas:

[0021] - enabling the quantity of particles of a given size contained in a gas to be measured selectively, and / or, and / or

[0022] - allowing the measurement, in real time, of the quantity of particles, of a given size, contained in a gas, and / or

[0023] - allowing the measurement, under a gas flow, of the quantity of particles, of a given size, contained in the gas, and / or

[0024] - allowing the quantity of particles of a given size contained in the gas to be measured precisely. Presentation of the invention

[0025] For this purpose, a Surface Acoustic Wave (SAW) Sensor is proposed comprising, from a lower face to an upper surface of said SAW sensor:

[0026] - a piezoelectric substrate,

[0027] - at least one interdigital transducer (IDT),

[0028] - a layer of polytetrafluoroethylene (PTFE) of which an upper face forms, at least in part, the upper surface of the SAW sensor; said PTFE layer has a thickness of between 50 nm and 4 pm.

[0029] Preferably, the PTFE layer has a thickness:

[0030] - greater than or equal to 50 nm, preferably 100, more preferably 150, advantageously 200, even more advantageously 250, particularly advantageously 300, preferably 350, more preferably 400, even more preferably 450 and most preferably greater than or equal to 500 nm, and / or

[0031] - less than or equal to 4 pm, preferably 3.75, more preferably 3.5, advantageously 3.25, even more advantageously 3, particularly advantageously 2.75, more preferably 2.5, even more preferably 2.25, even more preferably 2, particularly advantageously 1.75 and most preferably less than or equal to 1.5 pm.

[0032] Preferably, the SAW sensor further comprises an intermediate layer, on which rests, at least in part, the PTFE layer, covering, at least in part, the at least one IDT and covering, at least in part, the piezoelectric substrate; said intermediate layer has a thickness of between 10 and 300 nm.

[0033] Preferably, according to the present description, the terms "lower" and "upper" are defined relative to the lower face and the upper face of the SAW sensor. Preferably, the SAW sensor is suitable for, more preferably is intended to, measure the concentration of particles or molecules contained in a gas intended to be brought into contact with the SAW sensor.

[0034] Preferably, the upper face of the PTFE layer entirely forms the upper face of the SAW sensor.

[0035] Preferably, the upper face of the PTFE layer constitutes the detection surface of the SAW sensor. In other words, preferably, the particles or molecules or species whose concentration is determined and / or measured are intended to interact with the upper face of the PTFE layer.

[0036] Preferably, the intermediate layer covering the entirety of the at least one IDT and covering the entirety of the piezoelectric substrate.

[0037] Preferably, the PTFE layer rests solely and / or entirely on the intermediate layer.

[0038] Preferably, the intermediate layer has a thickness:

[0039] - greater than or equal to 10 nm, preferably 20, more preferably 30, more preferably 40, even more preferably 50, particularly advantageously 75 and most preferably greater than or equal to 100 nm, and / or less than or equal to 300 nm, preferably 250, more preferably 200, more preferably 175, even more preferably 150, particularly advantageously 125 and most preferably less than or equal to 100 nm.

[0040] Preferably, the intermediate layer is a silica layer.

[0041] Preferably, the at least one IDT consists of a Titanium / Platinum bilayer.

[0042] At least one IDT can be made of aluminum.

[0043] Preferably, the SAW sensor comprises at least two IDTs (5), one of the IDTs, called the transmitter IDT, is arranged to generate an elastic wave intended to propagate on the upper surface of the piezoelectric substrate (4) and at least one of the other IDTs, called the receiver IDT, is arranged to detect the elastic wave propagating on the surface of the piezoelectric substrate. The SAW sensor may comprise at least one, preferably a single, IDT and one or more reflectors; the IDT is arranged to generate an elastic wave intended to propagate on the upper surface of the piezoelectric substrate and to detect the elastic wave reflected by the reflector(s).

[0044] Preferably, the upper surface of the piezoelectric substrate comprises an area, called a sensitive area, around which the at least two IDTs are arranged.

[0045] Preferably, the upper surface of the piezoelectric substrate comprises an area, called a sensitive area, around which are arranged the at least one, preferably the single, IDT and the reflector(s).

[0046] According to the invention, a device is also proposed for measuring the concentration of particles contained in a gas, said measuring device comprises:

[0047] - a gas inlet intended to receive a flow of gas containing particles,

[0048] - at least two stages connected in series or in cascade, one of the stages communicates with the gas inlet and two consecutive stages communicate with each other; at least two of the stages are arranged to retain particles whose size is greater than a given value, the size of the particles retained by the at least two stages decreasing according to the direction of the gas flow,

[0049] - at least two of the stages each comprise a SAW sensor according to the invention.

[0050] Preferably, each of the stages is arranged to retain particles whose size is greater than a given value.

[0051] The at least two stages arranged to retain particles and the at least two stages each comprising a SAW sensor may be identical or different stages.

[0052] Preferably, the size of a particle is understood to mean the diameter, the average diameter or the aerodynamic diameter of a particle. The aerodynamic diameter may be understood to mean the diameter of a spherical particle having the same density as water and having the same terminal fall velocity as the particle in question. Preferably, each of the stages comprises a SAW sensor according to the invention.

[0053] Preferably, the measuring device comprises means for controlling the flow of gas through the stages.

[0054] Preferably, the measuring device comprises three stages.

[0055] Preferably, the measuring device comprises a first stage communicating with the inlet and arranged to retain particles whose size is greater than 10 pm.

[0056] Preferably, the measuring device comprises a second stage, consecutive to the first along the direction of the gas flow, arranged to retain particles whose size is greater than 2.5 pm.

[0057] Preferably, each of the three stages includes a filter plate.

[0058] Preferably, each filter plate comprises at least one orifice.

[0059] Preferably, a diameter of the at least one orifice decreases from one stage to another according to the direction of the gas flow.

[0060] Preferably, the means for controlling the gas flow through the stages are arranged so that a gas flow rate is between 0.45 and 5.5 l / min.

[0061] Preferably, the at least one orifice of the first stage filter plate has a diameter of between 3 mm and 8 mm.

[0062] Preferably, the at least one orifice of the second stage filter plate has a diameter of between 1 mm and 3 mm.

[0063] Preferably, the at least one orifice of the filter plate of the third stage, consecutive to the second stage in the direction of the gas flow, has a diameter of between 0.7 and 0.9 mm.

[0064] Preferably, the SAW sensor according to the invention is suitable, more preferably is particularly suitable, more preferably is designed and particularly advantageously is specially designed, to cooperate with and / or to be mounted in and / or to form part of the device for measuring the concentration of particles contained in a gas according to the invention. According to the invention, a method for manufacturing a Surface Elastic Wave sensor according to the invention is also provided. The manufacturing method comprises the steps of:

[0065] - depositing at least one metal layer on a piezoelectric substrate, carrying out a succession of microfabrication steps to form at least one interdigital transducer (IDT) on the piezoelectric substrate,

[0066] - depositing a layer of polytetrafluoroethylene (PTFE) forming, at least in part, the upper surface of the SAW sensor; said PTFE layer has a thickness of between 50 and 4 micrometers.

[0067] Preferably, the manufacturing method further comprises, prior to the step of depositing the PTFE layer, the step of depositing an intermediate layer having a thickness of between 10 and 200 nm; said intermediate layer covers, at least in part, the at least one IDT and covers, at least in part, the piezoelectric substrate.

[0068] Preferably, the PTFE layer is plasma deposited in the presence of a fluorocarbon gaseous reagent.

[0069] Preferably, the manufacturing method according to the invention is suitable, more preferably is particularly suitable, more preferably is designed and particularly advantageously is specially designed, for implementing the SAW sensor according to the invention. Also, any characteristic of the SAW sensor according to the invention is directly transposable to the manufacturing method and vice versa.

[0070] According to the invention, a method for cleaning a Surface Acoustic Wave (SAW) Sensor is also provided. Preferably, the cleaning method is implemented by a SAW sensor comprising a piezoelectric substrate capable of generating and / or capable of propagating Rayleigh waves on its surface. Preferably, the cleaning method is implemented by a SAW sensor according to the invention. More preferably, the cleaning method is implemented by a SAW sensor according to the invention comprising a piezoelectric substrate capable of generating and / or capable of propagating Rayleigh waves on its surface. Particularly advantageously, the cleaning method is implemented by a SAW sensor comprising a piezoelectric substrate made of Lithium Niobate according to the invention.

[0071] Preferably, the method of cleaning the SAW sensor comprises the steps of:

[0072] - deposit one or more droplets of liquid on the surface of the SAW sensor,

[0073] - moving the liquid droplet(s) on the surface of the SAW sensor by means of elastic waves propagating on the upper surface of the piezoelectric substrate; preferably, the elastic waves propagating on the upper surface of the piezoelectric substrate are generated by a high-power Radio Frequency source.

[0074] Description of figures

[0075] Other advantages and particularities of the invention will appear on reading the detailed description of implementations and embodiments which are in no way limiting, and the following appended drawings:

[0076] - FIGURE 1 is a schematic representation from a side view of an embodiment of the SAW sensor according to the invention,

[0077] - FIGURE 2 is a schematic representation of a side view section of the SAW sensor shown in FIGURE 1,

[0078] - FIGURE 3 is a schematic representation of a side view section of an embodiment of the device for measuring the concentration of particles contained in a gas comprising the SAW sensor according to the invention,

[0079] - FIGURE 4 is a schematic representation of the operating principle of an impaction device for the size sorting of particles contained in a gas flow,

[0080] - FIGURE 5 is a graph representing the evolution, for several thicknesses of the PTFE layer, of the gravimetric sensitivity of the SAW sensor as a function of the thickness of a gold layer deposited on the detection surface of the sensor,

[0081] - FIGURE 6 is a schematic representation of the SAW sensor cleaning process.

[0082] Description of the embodiments

[0083] The embodiments described below being in no way limiting, it will be possible in particular to consider variants of the invention comprising only a selection of the described characteristics, isolated from the other described characteristics (even if this selection is isolated within a sentence comprising these other characteristics), if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection comprises at least one characteristic, preferably functional without structural details, or with only a part of the structural details if this part only is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

[0084] With reference to FIGURES 1 and 2, an embodiment of the Surface Elastic Wave sensor 1 or SAW sensor 1 according to the invention is presented. The SAW sensor 1 comprises a lower face 2 and an upper surface 3. The SAW sensor comprises a piezoelectric substrate 4 whose lower face or surface 2 constitutes the lower face or surface 2 of the SAW sensor 1. According to the non-limiting embodiment, the piezoelectric substrate 4 is made of Lithium Niobate Y128 and has a size of 1 cm x 1 cm x 500 μm. The person skilled in the art knows the piezoelectric materials that can be used as piezoelectric substrate 4. By way of non-limiting example, the piezoelectric substrate is made of quartz, Lithium Tantalate (LiTaCh), Lanthanum Gallium Silicate, Aluminum Gallium Nitride (Al-GaN), Gallium Arsenide (GaAs), Aluminum Nitride (AIN), Aluminum Scandium Nitride (AIScN) or Lead Titano-Zarconate (PZT).

[0085] The SAW sensor 1 comprises at least one interdigital transducer (5) or IDT 5. The IDT 5 are known to those skilled in the art; they are two interlaced metal combs. According to the non-limiting embodiment, the SAW sensor 1 comprises two IDTs 5. One of the IDTs 5 is called the emitter and is arranged to generate, under the effect of an alternating electrical signal, periodic elastic waves propagating on the upper surface of the piezoelectric substrate 4. The other of the IDTs 5 is called the receiver and is arranged to detect the elastic wave, emitted by the emitter IDT 5, having propagated on the surface of the piezoelectric substrate 4. According to the non-limiting embodiment, the IDTs 5 consist of a Titanium / Platinum bilayer but can also be made of aluminum. The SAW sensor 1 further comprises a layer 6 of PTFE, Teflon™ according to the non-limiting embodiment presented, an upper face of which forms the upper surface of the SAW sensor 1.The PTFE layer 6 has a thickness typically between 50 and 4 μm. The thickness is approximately 600 nm depending on the embodiment. The upper surface 3 of the PTFE layer 6 therefore constitutes the upper surface 3 of the SAW sensor 1. The upper surface 3 of the SAW sensor 1 is the detection surface 3 on which the particles, the concentration of which in a gas is to be determined, will be deposited.

[0086] The SAW sensors of the prior art do not include a PTFE layer 6 covering the surface of the piezoelectric substrate and the IDTs. The inventors observed that the SAW sensors of the prior art, particularly when the species to be detected are in a gas stream brought into contact with the detection surface of the sensor, bounced off the upper surface of the SAW sensor. Thus, the particle concentration measured by the SAW sensor was distorted.

[0087] The inventors found that, surprisingly, the PTFE layer 6 according to the invention made it possible to attenuate, or even eliminate, this rebound effect of the particles on the surface of the SAW sensor 1. The results illustrating this effect are presented below, in particular in Table 1. They also noted that the presence of the PTFE layer 6 on the sensitive zone 8 of the upper surface of the piezoelectric substrate 4 does not cause a significant loss of the intrinsic sensitivity of the SAW sensor 1 but, on the contrary, provides a gain in sensitivity due to the “anti-rebound” effect provided by the PTFE layer 6.

[0088] The sensitivity, in terms of detection, of the SAW sensor 1 varies depending on the thickness of the PTFE layer 6. The sensitivity of the SAW sensor 1 varies depending on the thickness of the PTFE layer 6 according to a parabola-type function. The optimum sensitivity of the SAW sensor 1 depends on several parameters including, in particular, the type of material constituting the piezoelectric substrate 4. Typically, the optimum sensitivity of the SAW sensor 1 will be between 400 nm and 4 pm. This optimum is between 600 and 2 pm depending on the embodiment. The lower limit of 50 nm constitutes a technical limit in terms of deposition control and reproducibility. The desired effects, namely obtaining the anti-bounce effect and / or obtaining a sensitivity gain, are obtained at this lower limit. This anti-bounce effect increases with the thickness of the PTFE layer 6.Indeed, it has been observed that this anti-rebound effect is better for a PTFE layer 6 thickness of 10 μm than for a PTFE layer 6 thickness of 4 μm. Also, the optimum sensitivity of the SAW 1 sensor, as a function of the thickness of the PTFE layer 6, is a compromise between the reduction in the sensitivity of the SAW 1 sensor due to the attenuation / damping of the amplitude of the elastic waves in the PTFE layer 6 when the thickness of the latter is too large, typically beyond 4 μm, and the increase in the anti-rebound effect with the increase in the thickness of the PTFE layer 6.

[0089] The inventors have noticed that, in the particular case of a piezoelectric substrate 4 made of Lithium Niobate, the PTFE layer 6 does not adhere, little or too little to the piezoelectric substrate 4. Furthermore, it has been noticed that, in the particular case of a piezoelectric substrate 4 made of Lithium Niobate, the PTFE layer 6 does not adhere, little or too little to the piezoelectric substrate 4, regardless of the method of deposition of the PTFE layer 6.

[0090] This weak adhesion or absence of adhesion is not observed on all piezoelectric substrates 4. For example, the PTFE layer 6 adheres directly to a quartz piezoelectric substrate 4.

[0091] Also, in the case of a piezoelectric substrate 4 made of PTFE, the SAW sensor 1 according to the invention advantageously comprises an intermediate layer 7 on which the PTFE layer 6 rests. The intermediate layer 7 covers the at least one IDT 5 and covers the piezoelectric substrate 4. The intermediate layer 7 has a thickness of between 10 and 200 nm. According to the non-limiting embodiment, the intermediate layer 7 is made of silica.

[0092] The inventors observed that the presence of the intermediate layer 7 significantly increased the adhesion of the PTFE layer 6 to the piezoelectric substrate 4 made of Lithium Niobate and therefore to the SAW sensor 1.

[0093] An embodiment of the method for manufacturing the SAW sensor 1 according to the invention is also described. The method for manufacturing the SAW sensor 1 comprises the step of depositing at least one metal layer on a piezoelectric substrate 4. The thickness of the deposited metal layer is between 30 and 300 nm. The thickness of the metal layer will depend on the type of metal used for manufacturing the IDTs 5. According to the non-limiting embodiment, the metal layer is a Titanium / Platinum bilayer consisting of a Titanium layer of a few nanometers and a Platinum layer of 130 nm. The method then comprises the step of carrying out a succession of microfabrication steps to form the at least one IDT 5 on the piezoelectric substrate 4. For the manufacture of the at least one IDT 5, a photolithography technique is used to define the geometry of the IDTs 5.This is a method of structuring by lift-off using a negative resin (AZ-nLof) with a thickness of 1.3 μm adapted to the thickness of the metal chosen for the IDTs 5. The metal constituting the IDTS 5 is then deposited on the surface of the piezoelectric substrate 4 by an electron gun-assisted vacuum evaporation technique. In the case of the present invention, this is a stack of two metal layers. The first metal layer is a titanium layer of the order of a few nanometers thick intended to promote the adhesion of the second platinum layer of 130 nm thick which constitutes ITDT 5 itself.

[0094] The at least one IDT 5 is therefore produced from the previously deposited metal layer. In other words, the metal layer is structured, by microfabrication, to form the at least IDT 5.

[0095] Finally, the method then comprises the step of depositing a layer 6 of PTFE forming the upper surface 3 of the SAW sensor 1. The PTFE layer has a thickness of between 50 nm and 4 μm. The PTFE layer has a thickness of approximately 600 nm according to the non-limiting embodiment.

[0096] According to the non-limiting embodiment, the PTFE layer 6 is deposited by plasma. The gaseous precursors used during the plasma deposition of the PTFE layer 6 are fluorocarbon species. The gaseous precursor used according to the embodiment is tetrafluoromethane C4F8.

[0097] According to an advantageous but non-limiting embodiment, the PTFE layer 6 is deposited by deep reactive ion etching (DRIE). The use of the DRIE deposition technique cleverly allows the step of depositing the PTFE layer 6 to be carried out in continuity with the usual micro-manufacturing steps of the SAW sensor 1.

[0098] According to the non-limiting embodiment presented, the step of depositing the PTFE layer 6 by DRIE is carried out under a pressure of 40 mTorrs, a power of 2700 Watts and a mass flow rate of 300 standard cubic centimeters per minute or Standard Cubic Centimeters per Minute (seem) of octafluorocyclobutane (C4F8). The IDTs 5 of the state of the art are made of Aluminum. However, the deposition of the PTFE layer 6 by plasma can, depending on the embodiment, cause etching of the previously microfabricated IDTs 5 when they are made of Aluminum. The thickness of the aluminum IDTs 5 can be between 100 and 400 nm. The use of Titanium / Platinum IDTs 5 makes it possible to avoid this phenomenon.

[0099] In addition, during the cleaning step, high Radio Frequency (RF) power is used to enable the movement of particles. This has the effect, under certain conditions, of causing the migration of aluminum atoms, which results in the degradation of IDTs 5.

[0100] The absence or weak adhesion of the PTFE layer 6 on the piezoelectric substrate 4 made of Lithium Niobate has been cleverly resolved by means of an intermediate layer 7 arranged between the piezoelectric substrate 4 and the anti-rebound PTFE layer 6.

[0101] Advantageously, in order to increase the adhesion of the PTFE layer 6 to the SAW sensor, the method comprises, prior to the step of depositing the PTFE layer 6, the step of depositing an intermediate layer 7 having a thickness of between 10 and 200 nm. Thus, the intermediate layer 7 covers the at least one IDT 5 and covers the piezoelectric substrate 4. According to the embodiment, the intermediate layer 7 has a thickness of 50 nm. According to the embodiment, the intermediate layer 7 is a silica layer 7. According to the embodiment, the silica layer 7 is deposited by evaporation.

[0102] An embodiment of the SAW sensor cleaning method is also described, comprising a piezoelectric substrate 4 capable of propagating Rayleigh waves on its surface. According to the non-limiting embodiment, the cleaning method is implemented on a Lithium Niobate SAW sensor 1 according to the invention.

[0103] The principle of the cleaning method is illustrated in FIGURE 6. In part A) of FIGURE 6, the presence of particles 18 and / or gas molecules 18 absorbed on the upper surface 3 of the SAW sensor 1 (preferably on the upper surface 3 of the PTFE layer 6) is observed. In part B) of FIGURE 6, a droplet 19 is illustrated, deposited (before movement of the droplet 19) on the upper surface 3 of the SAW sensor 1 (preferably on the upper surface 3 of the PTFE layer 6). In part C) of FIGURE 6, the droplet 19 is observed, after movement of the droplet 19, under the effect of the Rayleigh waves propagating from one IDT 5 to the other IDT 5, in the direction denoted 21.It can be observed in part C) of FIGURE 6 that the particles 18 and / or gas molecules 18 located on the path of the droplet 19 have been desorbed from the surface 3 of the SAW sensor 1 and then transported, along the surface 3 of the SAW sensor 1, by the droplet 19. This is due to the fact that, in addition to causing the movement of the droplet 19, the Rayleigh waves also cause agitation of the droplet 19 and / or convection of the fluid forming the droplet 19; and therefore a desorption of the species adsorbed on the surface 3 of the SAW sensor 1 and / or a mixture of the species (in particular particles 18 and / or gas molecules 18) inside the droplet 19. The effect of the Rayleigh waves is also sufficient to evacuate the droplets 19 from the surface 3 of the SAW sensor 1, or at least to move the droplets 19 from the area of ​​the surface 3 located between the two IDTs 5.In the particular case where the action of the Rayleigh waves does not allow the droplets 19 to be evacuated from the surface 3 of the SAW sensor 1, it could be envisaged to use or put into operation the pump of the impaction device and to use the air flow generated by the pump to support, participate in or contribute to the evacuation of the droplets 19 from the surface 3 of the SAW sensor 1, or at least from the area of ​​the surface 3 located between the two IDTs 5.

[0104] Exposure to particles and gases of the SAW sensor 1, and in particular of its upper surface 3, causes a saturation, fouling or passivation effect of the upper surface 3 of the SAW sensor 1 by the particles and / or gases to which it is exposed. Over time, this effect causes a reduction in the sensitivity of the SAW sensor 1. It is therefore necessary to clean the upper surface 3 to remove the particles and / or gases that have deposited there. In the state of the art, “conventional” cleaning methods consisting of washing the upper surface 3 of the SAW sensor 1 with a liquid are used.

[0105] The method for cleaning the SAW sensor 1 according to the invention comprises a step consisting of depositing one or more droplets of liquid on the surface of the SAW sensor 1. A person skilled in the art will know how to choose the type of liquid to be used depending on the use of the SAW sensor 1 that is made of it. For example, an organic solvent can be used. Preferably, water is used.

[0106] For example, according to the non-limiting embodiment, a single droplet is deposited according to the non-limiting embodiment. The liquid droplets are deposited using capillaries or microchannels. Preferably, the liquid droplets may be deposited using a microfluidic device.

[0107] The droplet(s) advantageously have a deposited volume of less than 15 microliters. Preferably, the droplet(s) advantageously have an average volume of between 1 and 15 microliters.

[0108] The cleaning method also comprises a step of moving the liquid droplets on the surface of the SAW sensor 1 by means of elastic waves propagating on the upper surface of the piezoelectric substrate. In practice, elastic waves similar or identical to those generated by the IDT(s) 5 during operation, i.e. during detection, of the SAW sensor 1 are used.

[0109] The elastic waves propagating on the upper surface 3 of the SAW sensor 1 are coupled to the liquid present on the upper surface 3. This has the effect of moving the droplets on the upper surface 3. The combined effects of the elastic waves and the movement of the droplets will make it possible to effectively remove the particles and / or gas molecules adsorbed on the detection surface 3 of the SAW sensor 1. The particles and / or gas molecules are then trapped in the droplets.

[0110] The movement of the droplets on the upper surface 3 of the SAW sensor 1 is possible thanks to the hydrophobic character of the upper surface 3 conferred by the layer 6 of PTFE.

[0111] It has been observed that the cleaning process is particularly effective in the case of a piezoelectric substrate 4 made of Lithium Niobate. In this case, the movement of the droplets on the upper surface 3 of the SAW sensor 1 is caused by the Rayleigh waves generated on the upper surface 3 of the SAW sensor 1. In order to increase the amplitude of the Rayleigh waves, a high-power source is used. This can reach power values ​​greater than 35 dBm where dBm, for decibel-milliwatts, is the power value emitted by the source in logarithmic scale. In addition, it has also been observed that when the IDTs 5 are made of Aluminum, during the cleaning step, carried out at high RF power, has the effect, under certain conditions, of causing the migration of aluminum atoms out of the IDTS 5, which results in the degradation of the IDTs 5.

[0112] With reference to FIGURE 5, the evolution of the gravimetric efficiency of the SAW sensor 1 is illustrated as a function of the thickness of a layer of gold deposited, progressively, on the detection surface 3 of the SAW sensor 1. The gravimetric efficiency was evaluated for several thicknesses of the PTFE layer 6. In FIGURE

[0113] 5 shows four curves: curves 14, 15, 16 and 17 representing SAW sensors 1 having, respectively, a Teflon™ layer 6 of 100, 150, 300 and 400 nm. The thickness of the gold layer is used to simulate the mass of particles depositing on the sensor.

[0114] It is noted that the lower the thickness of the PTFE layer 6, the more constant the sensitivity is during the deposition of the particles. That is to say that the sensitivity of the SAW sensor 1 changes little as a function of the thickness of the gold layer (and therefore the thickness of the layer of particles) deposited on the detection surface 3 of the SAW sensor 1. A high thickness of the PTFE layer 6), that is to say a thickness less than or equal to 300 nm, preferably 200 nm, will have the effect of allowing high sensitivity to be obtained at the start of the measurement but this will decrease when the thickness of the deposited particles increases. On the contrary, a low thickness of the PTFE layer 6, i.e. a thickness greater than or equal to 400 nm, preferably 300 nm, will have the effect of giving the SAW sensor 1 a sensitivity which does not vary or varies little when the thickness of the deposited particles increases.

[0115] The effectiveness of the cleaning process does not depend on the thickness of the layer

[0116] 6 of PTFE. As a non-limiting example, the thickness of the PTFE layer 6 of the SAW sensor 1 arranged to implement the cleaning method is between 50 nm and 4 micrometers. The range of optimal values ​​of the thickness of the PTFE layer 6 depends on the piezoelectric substrate 4.

[0117] However, as detailed above, the thickness of the PTFE layer 6 has effects on the sensitivity of the sensors when detecting particles. Also, to obtain a good compromise between detection sensitivity and cleaning efficiency, the thickness of the PTFE layer 6 is between 50 nm and 4 μm. Preferably, the thickness of the PTFE layer 6 is between 600 nm and 1.5 μm for the SAW sensor 1 comprising a piezoelectric substrate 4 made of Lithium Niobate according to the invention.

[0118] With reference to FIGURES 3 and 4, a device 100 for measuring the concentration of particles contained in a gas is also described. The detection and measurement of the particle concentration is carried out by the SAW sensor 1 according to the invention. The SAW sensor 1 according to the invention is part of the measuring device 100 according to the invention. The measuring device 100 comprises a gas inlet 9 intended to receive a flow of gas containing particles whose concentration is to be measured. The measuring device 100 comprises at least two stages 10, 20, 30 connected in series. One of the stages, stage 10, communicates with the gas inlet 9. Two consecutive stages 10, 20, 30 communicate with each other. At least two of the stages 10, 20, 30, each of the stages 10, 20, 30 according to the embodiment, are arranged to retain particles whose size is greater than a given value.The size of the particles retained by the stages 10, 20, 30 decreases along the direction 11 of the gas flow. At least two of the stages 10, 20, 30, each of the stages 10, 20, 30 according to the embodiment, comprise a SAW sensor 1 according to the invention.

[0119] The measuring device 100 according to the invention makes it possible to retain at each stage particles having a size greater than a given value. This type of device can be described as an inertial weighing device or impaction device. The operating mode of these devices, which is illustrated in FIGURE 4, is known to those skilled in the art.

[0120] The method of "sorting" particles in this type of device is based on the inertial properties of aerosols. Some particles with an inertia greater than a certain limit value do not follow the streamlines of the gas flow 13 in the measuring device.

[0121] Thus, particles having a size greater than the cut-off diameter will collide with the SAW sensor 1 while others, of smaller diameter and higher speed, will continue their trajectory to the following (lower) stages. Also, the non-limiting embodiment presented relates to the detection of fine particles contained in air. More precisely, the objective of this embodiment is to determine the concentration of particles having a diameter greater than 10 pm, the concentration of particles having a diameter between 10 and 2.5 pm and the concentration of particles having a diameter less than 2.5 pm.

[0122] Also, stages 10 and 20 have the function of retaining particles having a diameter greater than or equal to a given value; according to the non-limiting embodiment: stage 10 retains particles having a diameter greater than or equal to 10 pm and the second stage 20 retains particles having a diameter greater than 2.5 pm, on the third stage 30 it is the concentration of all the particles having a diameter less than 2.5 pm which is measured by the SAW sensor 1.

[0123] The air inlet 9 brings the air whose particle concentration is to be measured into the measuring device 100. According to the embodiment, the measuring device 100 therefore comprises three stages 10, 20, 30 in series.

[0124] Stage 10, connected to the air inlet 9, includes the air flow acceleration plate 102. This plate 102 makes it possible to stop particles with a diameter greater than 10 pm.

[0125] Stage 10 also includes the impaction plate 101, on which is mounted a SAW sensor 1, intended to accommodate particles with a diameter greater than 10 pm.

[0126] An advantage of this embodiment is that it allows real-time measurement of the concentration of particles having a diameter greater than 10 pm.

[0127] Stage 20 comprises plate 203 for accelerating the air flow and stopping particles with a diameter between 10 pm and 2.5 pm (PM10). These particles are measured at stage 20 by the SAW sensor 1. Stage 20 also comprises element 202 connecting element 203 and plate 201 supporting the SAW sensor 1.

[0128] An advantage of this embodiment is that it allows real-time measurement of the concentration of particles having a diameter between 10 pm and 2.5 pm. These PM10 particles are particularly important for checking air quality. Stage 30 comprises a plate 303 for accelerating the air flow and stopping particles with a diameter of less than 2.5 pm. Stage 30 also comprises a connecting element 302 between element 303 and the plate 301 supporting the SAW sensor 1.

[0129] An advantage of this embodiment is that it allows real-time measurement of the concentration of particles with a diameter of less than 2.5 pm (PM2.5).

[0130] PM2.5 particles are particularly important for air quality monitoring.

[0131] Each stage includes connectors (not shown) allowing the SAW 1 sensors to be connected to a processing and control unit (not shown) for the SAW 1 sensors.

[0132] Element 12 is the outlet for the air whose particle concentration has been measured. This element makes it possible to connect the measuring device 100 to the pump which ensures the circulation of the air flow inside the measuring device 100.

[0133] According to a particular embodiment presented, the air flow inside the stages in series is adjusted to have a flow rate of between 0.45 and 5.5 l / min. The air flow range is not limiting and is to be adapted according to the dimensions of the measuring device 100 or the concentration of particles to be measured contained in the gas.

[0134] According to the particular embodiment presented, the air flow is adjusted to have a flow rate of 3 l / min.

[0135] An advantage of this embodiment is that it has low energy consumption, the flow rate being relatively low.

[0136] For particle size selection to be truly effective, the openings 105, 205, 305 in the accelerator plates 102, 203, and 303 must be correctly sized for the chosen air flow rate.

[0137] According to one embodiment of the invention, the first acceleration plate 102 comprises at least one opening 105 with a diameter between 3 mm and 8 mm, the second acceleration plate 203 at least one opening 205 with a diameter between 1 mm and 3 mm and the third acceleration plate 303 at least one opening 305 with a diameter between 0.7 and 0.9 mm. An advantage of this embodiment is to dimension the number and size of the openings 105, 205, 305 at each stage so that the filtration and size selection of the particles is effective for a given air flow rate.

[0138] The plate 102 has a central opening 105. The central opening 105 serves to accelerate the airflow as it passes through this element. Particles of greater inertia are deflected from the path of the airflow. More particularly, particles with a diameter greater than 10 μm are stopped at the impaction plate 101.

[0139] The SAW sensor 1 is fixed on the plate 101.

[0140] According to a preferred embodiment, for an air flow of (3 ± 0.3) l / min, the opening 105 on the plate 102 has a diameter of (7 ± 1) mm. For example, for flows of (0.50 ± 0.05) and (5.0 ± 0.5) l / min the opening 105 has a diameter of (3.0 ± 0.3) and (8 ± 1) mm respectively.

[0141] The second part of stage 10 is composed of an impaction plate 101. The plate 101 comprises orifices 108. The orifices 108 allow the air flow containing particles with a diameter of less than 10 pm to flow towards stage 20.

[0142] The air flow acceleration plate 203 at the stage 20 has a central opening 205. The central opening 205 makes it possible to accelerate the air flow when it passes through this element. Particles of greater inertia are deflected from the path of the air flow. More particularly, particles with a diameter greater than 2.5 pm are stopped at the plate 201 on which the SAW sensor 1 is mounted.

[0143] This element allows particles with a diameter between 10 pm and 2.5 pm to be stopped at stage 20.

[0144] The impaction plate 201 comprises orifices 208. The orifices 208 allow the air flow containing the particles with a diameter of less than 2.5 pm to flow towards the stage 30.

[0145] As for element 102, in this case also the size of the openings 205 in the plate 203 must be chosen according to the air flow rate so that the filtering of particles according to their size is effective. According to a preferred embodiment, for an air flow rate equal to (3 ± 0.3) l / min the opening 205 on the plate 203 has a diameter of (3.0 ± 0.5) mm. If a flow rate of the order of (0.50 ± 0.05) l / min is chosen the opening 205 must have a diameter of (1.0 ± 0.3) mm. On the other hand for a higher flow rate of the order of (5.0 ± 0.5) l / min the plate 203 must have two openings 205 each with a diameter equal to (2.0 ± 0.3) mm.

[0146] The airflow acceleration plate 303 has two central openings 305. The two central openings 305 make it possible to accelerate the airflow as it passes through this element. Particles of greater inertia are deflected from the path of the airflow. More particularly, particles with a diameter between 2.5 and 0.3 pm are stopped at the plate 301 on which the SAW sensor 1 is fixed.

[0147] The impaction plate 301 comprises orifices 308. The orifices 308 allow the air flow containing the particles with a diameter of less than 0.3 pm to flow towards the outlet 12.

[0148] If an air flow rate of (3.0 ± 0.3) l / min is chosen, according to a preferred embodiment, the plate 303 has two openings 305, each with a diameter equal to (0.8 ± 0.1) mm. For an air flow rate of (0.50 ± 0.05) l / min the plate 303 has a single opening 305 with a diameter equal to (0.7 ± 0.1) mm. For an air flow rate equal to (5.0 ± 0.5) l / min the plate 303 comprises two openings 305 each with a diameter equal to (0.9 ± 0.1) mm.

[0149] According to a non-limiting embodiment, the measuring device 100 comprises a microfluidic device. The microfluidic device is arranged to deposit the droplet(s) 19 on the upper surface 3 of the SAW sensor 1.

[0150] By way of non-limiting example, the microfluidic device may comprise, among other things, one or more pumps, for example peristaltic pumps (or micro-pumps), fluid conduits, for example capillaries, and / or sensors (for example flow, pressure and / or liquid flow sensors).

[0151] One or more of the stages 10, 20, 30, preferably each stage 10, 20, 30 of the measuring device 100, comprises one or more droplet depositing elements 19 arranged to deposit one or more droplets 19 on the upper surface 3 of the SAW sensor 1. Preferably, the microfluidic device comprises the depositing element(s). According to the embodiment, the depositing element(s) are outlet orifices (preferably arranged or located opposite the upper surface 3 of the SAW sensor 1) of the capillary(s).

[0152] By way of non-limiting example, the capillaries are made of silica (for example having an external diameter of the order of 320 to 440 μm) and are arranged to deposit the droplet 19, on the upper surface 3 of the SAW sensor 1, in a determined and / or adjustable location, on the SAW sensor 1.

[0153] Preferably, the depositing element(s) are arranged so as not to disturb the flow of air in the measuring device 100.

[0154] In order to illustrate the effect and contribution of using the SAW sensor 1 of the invention in the measuring device 100 rather than using the state-of-state SAW sensors, the results obtained by a measuring device 100 of the invention are presented. SiC particles having different diameters in air were injected into the measuring device 100. The particle concentrations measured with a measuring device 100 comprising SAW sensors 1 of the invention (which are denoted "SAW with Teflon™") and identical SAW sensors but not comprising a PTFE layer 6 (which are denoted "bare SAW") are presented in Table 1.

[0155] A 0.0125% SiC solution by mass percentage was prepared with particles having an aerodynamic diameter of 5 and 7 pm. Also, in Table 1 are presented the concentrations measured by the SAW sensor located at stage 20 and by the SAW sensor located at stage 30. Since the gas flow does not include particles with a diameter greater than 10 pm, nothing is measured on the SAW sensor located at stage 10.

[0156] In test number 1, the SAW sensors located at levels 20 and 30 do not include a 6-layer of PTFE. In test number 2, the SAW sensor 1 located at level 20 includes a 6-layer of PTFE and the SAW sensor located at level 30 does not include a 6-layer of PTFE. In test number 3, the SAW sensor 1 located at level 20 includes a 6-layer of PTFE and the SAW sensor 1 located at level 30 includes a 6-layer of PTFE. In the last column of Table 1, the percentage of PM10 (particles with a diameter less than 10 pm and greater than 2.5 pm) that is measured at level 30 is calculated, whereas in principle all PM10 should be retained at level 20.

[0157] [Table 1]

[0158]

[0159] Comparing tests 1 and 2 illustrates the problem of particle rebound on SAW sensors. It can be seen that PTFE layer 6 increases the number of particles measured at stage 20 and reduces the number of particles measured at stage 30. This perfectly illustrates the fact that some of the particles impacting SAW sensor 1 at a given stage bounce off the upper surface 3 and are therefore not measured.

[0160] In the absence of PTFE layer 6, the concentration measured at a given stage is therefore underestimated due to the rebound of certain particles on the SAW sensor 1.

[0161] In addition, this particle rebound phenomenon also, to a lesser extent, increased the quantity of particles measured at the lower stage because some of the particles that bounced were carried to the lower stage by the gas flow. The concentration measured at a lower stage is therefore overestimated in the absence of PTFE layer 6.

[0162] The SAW sensor 1 according to the invention therefore makes it possible to improve the accuracy of the measurement of the particle concentration in a gas, in particular when the measurement is carried out under gas flow. The SAW sensor 1 according to the invention therefore makes it possible to obtain more reliable and more precise measurements. 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.

[0163] Thus, in variants which can be combined with each other of the embodiments described above:

[0164] - the PTFE layer has a thickness greater than or equal to 50 nm, preferably 100, more preferably 150, advantageously 200, even more advantageously 250, particularly advantageously 300, preferably 350, more preferably 400, even more preferably 450 and most preferably greater than or equal to 500 nm, and / or

[0165] - the PTFE layer has a thickness less than or equal to 4 μm, preferably 3.75, more preferably 3.5, advantageously 3.25, even more advantageously 3, particularly advantageously 2.75, more preferably 2.5, even more preferably 2.25, even more preferably 2, particularly advantageously 1.75 and most preferably less than or equal to 1.5 μm.

[0166] In addition, the various features, forms, variations and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive.

Claims

CLAIMS 1. Surface Elastic Wave Sensor (1), called SAW sensor (1), comprising, from a lower face (2) to an upper surface (3) of said SAW sensor: - a piezoelectric substrate (4), - at least one interdigital transducer (5) or InterDigital Transducer (IDT), - a layer (6) of polytetrafluoroethylene (PTFE) of which an upper face forms, in its entirety, the upper surface of the SAW sensor; said PTFE layer has a thickness of between 50 nm and 4 pm.

2. SAW sensor (1) according to claim 1, comprising an intermediate layer (7), on which rests, at least in part, the layer (6) of PTFE, covering, at least in part, the at least one IDT and covering, at least in part, the piezoelectric substrate; said intermediate layer has a thickness of between 10 and 300 nm.

3. SAW sensor (1) according to the preceding claim, in which the intermediate layer (7) is a silica layer.

4. SAW sensor (1) according to any one of the preceding claims, wherein the at least one IDT (5) consists of a Titanium / Platinum bilayer.

5. SAW sensor (1) according to any one of the preceding claims, comprising at least two IDTs (5), one of the IDTs, called the transmitter IDT, is arranged to generate an elastic wave intended to propagate on the upper surface of the piezoelectric substrate (4) and at least one of the other IDTs, called the receiver IDT, is arranged to detect the elastic wave propagating on the surface of the piezoelectric substrate.

6. SAW sensor (1) according to the preceding claim, in which the upper surface of the piezoelectric substrate (4) comprises a zone (8), called sensitive zone, around which the at least two IDTs (5) are arranged.

7. Device (100) for measuring the concentration of particles contained in a gas, said measuring device comprises: - a gas inlet (9) intended to receive a flow of gas containing particles, - at least two stages (10, 20, 30) connected in series, one of the stages communicates with the gas inlet and two consecutive stages communicate with each other; at least two of the stages are arranged to retain particles whose size is greater than a given value, the size of the particles retained by the at least two stages decreasing according to the direction (11) of the gas flow, - at least two of the stages each comprise a SAW sensor (1) according to any one of claims 1 to 6.

8. Measuring device (100) according to the preceding claim, comprising means for controlling the flow of gas through the stages.

9. Measuring device (100) according to the preceding claim, comprising three stages (10, 20, 30), a first stage (10) communicating with the inlet is arranged to retain particles whose size is greater than 10 pm and a second stage (20), consecutive to the first along the direction (11) of the gas flow, is arranged to retain particles whose size is greater than 2.5 pm.

10. Measuring device (100) according to the preceding claim, wherein each of the three stages (10, 20, 30) comprises a filtering plate (102, 203, 303), each filtering plate comprises at least one orifice (105, 205, 305), a diameter of the at least one orifice decreasing from one stage to another according to the direction (11) of the gas flow.

11. Measuring device (100) according to the preceding claim, in which: - the means for controlling the gas flow through the stages are arranged so that a gas flow rate is between 0.45 and 5.5 l / min, - the at least one orifice (105) of the filter plate (102) of the first stage (10) has a diameter between 3 mm and 8 mm, the at least one orifice (205) of the filter plate (203) of the second stage (20) has a diameter between 1 mm and 3 mm and the at least one orifice (305) of the filter plate (303) of the third stage (30), consecutive to the second stage in the direction (11) of the gas flow, has a diameter between 0.7 and 0.9 mm.

12. Method for manufacturing a Surface Elastic Wave sensor (1), called SAW sensor, comprising the steps of: - depositing at least one metal layer on a piezoelectric substrate (4), carrying out a succession of microfabrication steps to form at least one interdigital transducer (5) or InterDigital Transducer (IDT) on the piezoelectric substrate, - depositing a layer (6) of polytetrafluoroethylene (PTFE) forming, in its entirety, the upper surface of the SAW sensor; said PTFE layer has a thickness of between 50 nm and 4 pm.

13. Method according to the preceding claim, comprising, prior to the step of depositing the PTFE layer (6), the step of depositing an intermediate layer (7) having a thickness of between 10 and 300 nm; said intermediate layer covers, at least in part, the at least one IDT and covers, at least in part, the piezoelectric substrate.

14. Method according to claim 12 or 13, in which the layer (6) of PTFE is deposited by plasma in the presence of a fluorocarbon gaseous reagent.

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