Fine particle sensor with cascaded microscales

Abstract

A sensor for the continuous in-situ analysis of an aerosol flow for measuring the mass of the micron/submicron particles suspended in the air flow, including an aeraulic sorter allowing the particles to be sorted according to their size with an impactor body with a cascade of one or more stages; at least one MEMS microbalance per stage, with an oscillating silicon membrane located on the impaction zone of the particles; processor connected to each MEMS microbalance to determine the mass of all the particles on the impaction zone; a system for cleaning the MEMS microbalances allowing the evacuation of the particles from the MEMS microbalances; means for driving the aerosol flow.

Claims

1. A sensor for continuous in-situ analysis of an aerosol flow, allowing continuous measurement of a mass concentration, by size range of micron or submicron particles, suspended in the aerosol flow, comprising: an aeraulic sorter configured to carry out granulometry of the particles according to their size, by an impactor body with a cascade of at least two stages, with a flow pipe along a length of the impactor body, and at an inlet of the aeraulic sorter a particle sampling head allowing only PM10 particles to pass through, each stage consisting of at least: an axial nozzle through which the flow enters the stage, and lateral openings; a diameter and a number of lateral openings of the axial nozzle being chosen to allow control of an aerosol flow rate, and sorting of the particles according to their inertia, at each stage; at least one MEMS microbalance per stage, located at a distance from and facing the axial nozzle, with an active surface which is located in a particle impaction zone between lateral openings which allow the flow to pass through, to determine the mass less than or equal to one microgram of all the particles deposited on the particle impaction zone; each MEMS microbalance having at least one axial recess located under a membrane in order to suspend the membrane, the lateral openings being lateral to the axial recess and configured for the flow of the aerosol flow and located on either side of the axial recess, a processor connected to each MEMS microbalance to determine the mass of all the particles on the impaction zone; a system for cleaning each MEMS microbalance, at each stage, allowing evacuation of the particles from the MEMS microbalance, and means for flowing the aerosol flow, wherein the at least one MEMS microbalance has, as active surface, an oscillating silicon membrane with electrostatic actuation, said oscillating silicon membrane being: with a width comprised in a range between a hundred microns and ten millimeters, suspended from a plurality of silicon anchors to which the oscillating silicon membrane is fixed in tension, arranged between a plurality of electrodes suitable for electrostatic actuation, conformed to have a quality factor Q greater than 15000, and a mass m between 5 and 100 μg, conformed to have membrane oscillation frequencies between 1 and 20 MHz, and conformed to control adhesion and/or separation of particles on its surface by micro/nano structuring or physico-chemical functionalization.

2. The sensor according to claim 1, wherein the system for cleaning is realized by inlet channels and lateral outlet channels opening into the flow pipe, and means for circulating a cleaning fluid in these channels, the inlet lateral channels and outlet lateral channels fluidically communicating with the flow pipe and being arranged to circulate the cleaning fluid over the surface of the oscillating silicon membranes.

3. The sensor according to claim 1, wherein the oscillating silicon membrane is capable of deforming in its plane, in Lamé or extensional mode.

4. The sensor according to claim 1, wherein the means for flowing the aerosol flow are configured to allow a flow rate of one to ten L/min, and wherein the axial nozzles at each stage have an opening.

5. The sensor according to claim 1, wherein an alternating voltage combined with a direct voltage is applied to one or more actuating electrodes while the suspended oscillating silicon membrane is kept at zero electrostatic potential so as to deform the oscillating silicon membrane, the oscillating silicon membrane being electrostatically charged, a movement of which induces a charging movement and thus a current measured via contacts connected to the anchors of the oscillating silicon membrane or connected to an electrode not used for electrostatic actuation.

6. The sensor according to claim 1, wherein the oscillating silicon membrane has a surface area of one to 100 mm.sup.2.

7. The sensor according to claim 1, wherein a surface of the oscillating silicon membrane is black silicon.

8. The sensor according to claim 7, wherein the surface of the oscillating silicon membrane is covered with a layer of polymer.

9. The sensor according to claim 1, wherein the impactor body has a stack of wafers of silicon or other micro/nano-structured materials, supported and fixed to each other.

10. The sensor according to claim 9, wherein each stage has four wafers: a first wafer with an axial nozzle; a second wafer with an axial recess and lateral flow openings; and two wafers so-called “spacers” arranged between the first and second wafers.

11. The sensor according to claim 10, wherein the system for cleaning each MEMS microbalance comprises input channels and lateral output channels located between one of the so-called “spacers” and the first wafer, the first wafer and the one “so-called” spacer each comprising silicon.

12. The sensor according to claim 1, wherein the diameter and the number of the lateral openings and of each axial nozzle are chosen to allow the control of the aerosol flow rate and the sorting of the PM10 particles, with a diameter of 300 nm to 10 μm, according to their inertia at each stage in order to measure a thoracic fraction of particles.

13. The sensor according to claim 1, wherein the system for cleaning each MEMS microbalance comprises an element for heating, located under each oscillating silicon membrane and/or in the impactor body at each stage.

14. The sensor according to claim 1, wherein the system for cleaning each MEMS microbalance comprises the oscillating silicon membrane designed to oscillate at a given frequency, allowing the separation of particles from the oscillating silicon membrane during cleaning.

15. The sensor according to claim 1, wherein the impactor body comprises means for opening/closing: axial openings; and/or axial nozzles; for controlling the flow rate of the aerosol flow circulating into the impactor body.

16. The sensor according to claim 1, wherein the axial nozzles and axial recesses are of decreasing size along the axis of the aerosol flow, so as to impact particles of decreasing size at each stage.

17. The sensor according to claim 1, wherein the diameter and the number of lateral openings allowing flow are determined to impact: particles between 10 micrometers and 2.5 micrometers or more, at a first stage; and particles between 2.5 micrometers and one micrometer at a second stage; and particles between one micrometer and 0.3 micrometers at a third stage, depending on a direction of the aerosol flow.

18. The sensor according to claim 1, wherein the flow pipe comprises silicon and is coated with a conductive metal.

19. The sensor according to claim 1, wherein the width of the oscillating silicon membrane is between 500 microns and ten millimeters.

20. The sensor according to claim 1, wherein each MEMS microbalance comprises electrostatic actuators, the distance between the oscillating silicon membrane and the electrostatic actuators being less than or equal to 1 micrometer.

21. The sensor according to claim 1, wherein the oscillating silicon membrane is a rectangular or square membrane.

22. The sensor according to claim 1, wherein the MEMS microbalance is configured to allow the mass measurement of particles between the attogram and the nanogram.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other objectives, features and benefits will emerge from the detailed description that follows with reference to the drawings given for illustrative and non-limitative purposes, among which:

(2) FIG. 1 represents a vertical sectional view along axis 1 shown in FIG. 3, of the device according to the invention;

(3) FIG. 2 represents a vertical view along axis 2 perpendicular to axis 1, of the device according to the invention;

(4) FIG. 3 represents a top view of the device according to the invention, with axes 1 and 2;

(5) FIG. 4 represents a SEM image of a MEMS microbalance consisting of a silicon resonator with an electrostatically actuation and measuring principle with recovery of the output signal via a sectional anchor;

(6) FIG. 5 shows the resonance curve of a microbalance before (AV curve) with a mean frequency of 4100297.7 Hz, and after (AP curve) aerosol impaction with a mean frequency of 4100222.5 Hz;

(7) FIG. 6 shows a SEM image of a MEMS microbalance after aerosol deposition;

(8) FIG. 7 represents the main functions of the device according to the invention;

(9) FIG. 8 shows the manufacturing process of a membrane used in the device; device:

(10) FIG. 9 represents a result of mechanical simulation by finite elements with the COMSOL @ software, showing the maximum deformation of a square membrane and its 4 suspension anchors for an oscillation frequency corresponding to the Lamé resonance frequency. The elastic deformations shown in the figure vary from 0 (for the W mark) to a few nanometers (for the X mark); and

(11) FIGS. 10a and 10b represent scanning electron microscopy images of a square membrane with a black silicon area in the center.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(12) The present invention presents a sensor 1 for the continuous in-situ analysis of an aerosol flow, allowing the measurement of the mass of micron/submicron particles 2 suspended in the air flow and represented in FIGS. 1 to 3.

(13) This sensor 1 includes a device of aeraulic sorting allowing to perform the granulometry of the particles 2 according to their size while integrating the measurement systems.

(14) The device of aeraulic sorting according to the present invention:

(15) an impactor body 3 with cascade of one or more stages 4a, 4b, 4c, presenting at the entry a sampling head of the particles (the size, more particularly the aerodynamic diameter which corresponds to the average diameter, of these particles being advantageously lower than 10 micrometers, that is to say particles called in the state of the art PM10), followed by a flow pipe 6 formed for each stage and following the direction of the aerosol flow: at least one axial nozzle 10, followed by lateral openings 9 at the axial nozzle to allow the flow of the aerosol flow; the diameter of the axial nozzles 10, advantageously from one to several hundred micrometers, being chosen to allow the control of the flow rate of the aerosol flow and the sorting of the particles 2 according to their inertia; the duct 6 is also formed of walls 5 extending in the length of the impactor body 3 between the axial nozzle 10 and the lateral openings 9.

(16) Sensor 1 also has: at least one MEMS 7 microbalance per stage 4, with silicon oscillating membrane 7a with an actuator preferably electrostatically located in the impaction zone of the particles 2 facing an axial nozzle and between two or more lateral openings 9; this MEMS 7 microbalance is particularly designed to measure a shift, preferably of the order of a few tens of hertz, of the oscillation frequency in order to determine the mass less than or equal to one microgram of all the particles 2 deposited on the MEMS 7 microbalance; means for calculating connected to each MEMS 7 microbalance to determine the mass of all the particles 2 on the impaction zone, as a function of the offset between the oscillation frequency of the oscillating membrane 7a with particles 2 and the oscillation frequency of the membrane without particles 2; a system for cleaning MEMS microbalance allowing the evacuation of the particles 2 out of the MEMS microbalances; means for flowing aerosol flow.

(17) The system for cleaning allows the sensor to operate in continuous/real time, the MEMS 7 microbalances being cleaned before any new measurement of sensor 1.

(18) As an example only and in a non-limitative way, it can be chosen time steps of every hour, every quarter of an hour, every minute, every 10 seconds, to carry out the cleaning, thus allowing periodic measurements, real time or near real time measurements. This will typically depend on: the concentrations encountered; the use made of the system.

(19) If the regeneration of a sensor is an identified problem is known, the difficulty is to integrate and operate it in situ without “maintenance” intervention on the sensor and without damaging it. It is described in the state of the art the use of a supercritical fluid which is inapplicable without maintenance on the sensor and therefore for the present integrated system of the invention.

(20) The solution we propose is easy to implement and can be combined with a mechanical action (by vibrating the membrane vertically or horizontally at a frequency higher than the particle detachment frequency) or with a thermal action (by integrating resistive elements under the membrane).

(21) The schematic diagram of the system is shown in FIGS. 1, 2 and 3.

(22) The sampling head (filter with opening, vortex or other state of the art system) allows only PM10 particles to pass through, i.e. particles whose size, in particular the diameter, is less than 10 micrometers (PM10). A description and measurement of PM10 particles is in particular detailed in standard NF EN 12341.

(23) The diameter of the axial nozzles at each sampling stage can be of the order of a few hundred micrometers with flow rates from 1 to several L/min. Thus, this flow rate corresponds roughly to the human ventilatory flow rate, which is between 5 and 10 L/min, whereas flow rates<0.3 L/min such as those of the system described by Maldona-Garcia et al, 2017—IEEE SENSORS JOURNAL, Vol. 17, No. 6, Mar. 15, 2017 are not very representative. Moreover, this corresponds to cut-off diameters between 100 nm and a few μm (e.g. 10 μm) in order to measure particles with an average diameter smaller than 10 micrometers. For example to measure particles from a few 100 nm to 10 μm, preferably PM10 particles with a diameter of at least 100 nm, more preferably at least 200 nm.

(24) The aim is to provide a device compatible with the WHO standards concerning the categories of particles (PM) to be measured and their threshold values. In particular, this device allows the measurement of the thoracic fraction of particles.

(25) Using the cascade impactor, particles 2 of different diameters are impacted at different stages 4a, 4b, 4c of the system and measured using MEMS 7 microbalance located at the impaction zones.

(26) Indeed, depending on the opening diameter of an impactor, the air flow loaded with particles will be more or less accelerated, the particles 2 of lower inertia will follow the air flow while those of higher inertia will be impacted on a zone located under an aerosol flow nozzle located opposite the MEMS 7 microbalance.

(27) Each stage 4a, 4b, 4c presents on the same plan: at least one axial recess 8 through or not through (for example a notch in the silicon wafer) located under each microbalance to suspend the membrane 7a, and lateral openings 9 for the aerosol flow located on both sides of the axial recess 8, and at each stage entrance an axial nozzle 10 located under the axial recess 8.

(28) Advantageously, the lateral openings 9 are arranged symmetrically to the axial recess 8 located centrally in the impactor body 3.

(29) Axial nozzles 10 and axial recesses 8 are of decreasing size along the axis of the aerosol flow so as to impact particles 2 of decreasing size at each stage.

(30) Here, it can be chosen for example a diameter and a number of lateral openings 9 located in the flow passage to allow impact: particles 2 between 10 micrometers and 2.5 micrometers at a first stage; and particles 2 between 2.5 micrometers and one micrometer, at a second stage; and particles 2 between 1 micrometer and 0.3 micrometer at a third stage, depending on the direction of the aerosol flow.

(31) The impactor body 3 can have a modular stack consisting of wafers 11 of silicon or other micro/nano-structured materials, for example by means of micro or nano manufacturing in clean rooms, supported and attached to each other.

(32) For example, other possible materials are: glass, aluminum.

(33) If flow pipe 6 is made of silicon or other micro/nano structured material, it can be covered with a conductive metal.

(34) Here in FIG. 2, each stage 4a, 4b, 4c shows four wafers 11:

(35) a first wafer 11a with an axial nozzle 10; a second wafer 11c with an axial recess 8 and lateral openings 9 for the flow of the aerosol flow along the impactor body, this flow being axial through the axial nozzle 10 and stretching radially to reach the lateral openings 9 after exiting the axial nozzle 10; two wafers 11b, 11 d so-called “spacers” arranged between the first wafer 11a and the second wafer 11c.

(36) Here on FIG. 1, the system for cleaning MEMS microbalance is realized by lateral inlet channels 12a and lateral outlet channels 12b in the interleaved silicon wafers and opening into flow pipe 6 by lateral air inlets 12′a and 12′b, and means for circulating a fluid such as air in these channels 12a and 12b.

(37) The inlet lateral channels 12a and outlet lateral channels 12b fluidically communicate with the flow pipe 6 to circulate a cleaning fluid, such as air or solvent or solvent+air, over the MEMS microbalances from the inlet lateral channels 12a to the outlet lateral channels 12b.

(38) They lead to the flat surface of stage 4a, 4b, 4c, which has the axial openings 9 and the MEMS microbalances 7.

(39) The input lateral channels 12a and output lateral channels 12b of each stage 4a, 4b, 4c are located between a silicon wafer called “spacer” 11b, 11d and the first silicon wafer 11a.

(40) As an alternative, not shown in FIG. 2, the system for cleaning MEMS microbalance includes an element for heating.

(41) In a second variant, the system for cleaning MEMS microbalance includes the oscillating membrane 7a.

(42) In a third variant, the system for cleaning has a combination of at least two of the following three devices: lateral input channels 12a and lateral output channels 12b; the element for heating located at the impaction zone the oscillating membrane 7a; in this case the membrane 7a is driven by means for controlling to vibrate at a certain frequency in order to evacuate the particles at a desired period.

(43) The means for controlling and flowing the aerosol flow are, for example, one or more pumps.

(44) In a variant not shown in the diagrams, the impactor body 3 of the sensor 1 has means for opening/closing: lateral openings 9 and possibly axial recesses 8 if they are continuous; and/or axial nozzles 10.

(45) The silicon membranes 7a, with an electrostatically actuation (FIG. 4) of the developed microbalances, are vibrated at their resonance according to a deformation mode with a high quality factor.

(46) The actuation of membrane can also be: piezoelectric or magnetic.

(47) Each membrane 7a is suspended from several anchors 13, preferably made of silicon, to which the membrane is fixed in tension, the membrane 7a being arranged between several electrodes 14 suitable for electrostatic actuation or measurement. Advantageously, the membrane is surrounded on its circumference by the electrodes.

(48) Here in FIG. 4, four electrodes 14 have been made, which can be made of doped silicon, supplied with an input signal and receiving an output signal.

(49) In the case where the actuation is electrostatic, an alternating voltage combined with a direct voltage is applied to one or more actuation electrodes 14 while the suspended membrane 7a is maintained at a zero electrostatic potential. An electrostatic force oscillating at the same frequency as the actuation voltage is thus induced on the membrane 7a.

(50) The membrane 7a is then deformed in a planar oscillating movement. As membrane 7a is electrostatically charged, the movement of the membrane 7a induces a charging movement and thus a current proportional to the speed of movement of membrane 7a.

(51) This current can be measured via contacts connected to the anchors 13 of membrane 7a or to an electrode not used for actuation. There are particular actuation frequencies for which the deformation amplitude of membrane 7a is maximum.

(52) For a resonance frequency, the current measured at this frequency is maximum. With an addition of mass, this mechanical resonance frequency will vary.

(53) The measurement of the resonance frequency therefore provides information about the deposited mass.

(54) Advantageously, the surface of the MEMS 7a membranes of the MEMS 7 microbalances can be used to control the adhesion and/or separation (alternatively wetting/un-wetting of the cleaning fluid) of particles 2: micro/nano structured and/or functionalized
promoting the adhesion of particles 2 selectively according to their surface properties.

(55) Eventually it can have electrostatic/mechanical or thermal properties to control the adhesion and/or separation of particles 2, when charged. For example a possibility if some aerosols are negatively charged and others rather positively charged, using electrostatic properties can also improve the deposition efficiency.

(56) One or more layers of material can be placed on the surface of the 7a membranes of MEMS microbalances to allow preferential adhesion of target 2 particles.

(57) Depending on the application, the surface of the 7a membranes is configured differently.

(58) For example, the surface of the membranes 7a can be optimized to preferentially detect the mass of biological aerosols, pollens, fungi, bacteria thanks to their specific surface properties.

(59) The surface of the membranes 7a of MEMS microbalances can be micro or nanostructured silicon, such as black silicon (shown in FIGS. 10a and 10b).

(60) Advantageously, the surface of the membranes can be coated with a polymer. In one particular realization mode, the surface of the membrane 7a is black silicon coated with a thin layer of polymer to, for example, influence the adhesion of target particles 2 to the membrane 7a or the behavior of target particles 2 with respect to the 7a membrane.

(61) The oscillating membrane 7a was realized as shown in FIG. 4.

(62) The width of the oscillating membrane 7a range from several hundred microns to several millimeters. That means, at least 100 μm, or even 200 μm, 300 μm, 400 μm, preferably at least 500 μm. In addition, the oscillating membrane 7a can have a width of up to 10 millimeters, preferably up to 5 millimeters. Thickness dimensions can be between 30 μm and 50 μm or between 35 μm and 45 μm, for example.

(63) For each MEMS 7 microbalance, the distance between the oscillating membrane 7a and the electrostatic actuators which are the electrodes is less than or equal to 1 micrometer.

(64) The oscillating membrane 7a is preferably a rectangular or square membrane.

(65) The oscillating membrane 7a is advantageously a rectangular membrane.

(66) It can also have a circular shape or any other shape.

(67) The oscillating membranes 7a of the MEMS 7 microbalances are conformed to present: a quality factor Q higher than 15000; this quality factor being obtained by using silicon, a well-chosen deformation mode of the microbalance, and a shape of the anchors 13; with m.sub.min=m/Q where m.sub.min is the minimum detectable mass of the solid particles to be measured by the microbalance, and m is the mass of the membrane or active surface. Masses m between 5 and 100 μg.

(68) The frequencies of the membranes 7a of the MEMS microbalances are advantageously between 1 and 20 MHz.

(69) Advantageously, the MEMS 7 microbalance is configured to allow the mass measurement of particles 2 between the attogram and the nanogram.

(70) The resolution or accuracy of the MEMS 7 microbalance in mass measurement is the nanogram, due to the choices made to take silicon, with a specific deformation mode of the membrane 7a, and a particular shape of the 13 anchors, which allow to maximize the quality factor of the resonator while keeping a large active surface.

(71) By the geometry of the microbalance (in particular square), its type of actuation (electrostatic) and its deformation mode (according to a volume mode in the plane) in Lamé or extensional mode, which makes it possible to guarantee high frequencies of use (of a few MHz) while maintaining a large active surface (of the order of mm.sup.2). A large active surface can for example correspond to an active surface from 100 μm.sup.2 to 10 mm.sup.2, for example between 200 μm.sup.2 and 9 mm.sup.2, between 200 μm.sup.2 and 6 mm.sup.2, between 200 μm.sup.2 and 4 mm.sup.2, between 500 μm.sup.2 and 2 mm.sup.2).

(72) The microbalance thus obtained shows a high-quality factor and thus a good sensitivity (about 50 Hz/ng) while being able to measure a large number of particles with a diameter ranging from 0.1 to 10 μm without saturation (corresponding to mass concentrations ranging from a few ng/m.sup.3 to a few 10 μg/m.sup.3).

(73) Thus, the microbalance is particularly designed to measure an offset of only a few tens of Hertz around the mechanical oscillation frequency of one to several MHz of the membrane. More particularly, the mechanical oscillation frequency of the membrane is between 1 MHz and 20 MHz, even more particularly the mechanical oscillation frequency of the membrane is between 1 MHz and 20 MHz and the measurement of the offset is a few tens of Hertz around the mechanical oscillation frequency of the membrane 7a, for example about 50 Hz.

(74) The deformation mode according to an in-plane volume mode means that the elastic deformations do not occur locally on the surface but are simultaneous throughout the entire thickness of the membrane 7a. Here the deformations are in-plane, i.e. the deformation is not vertical to the active surface but horizontal. Moreover, the electrostatic actuation allows the electrodes to be positioned not on the active surface of the MEMS membrane 7a, but outside it and act on the thickness of the active surface to deform it.

(75) Thus, these electrodes on the outside alternately cause the membrane 7a to expand and contract in the horizontal directions of the active surface, the membrane with its active surface and thickness moving only in the horizontal directions of the active surface.

(76) Thus, the MEMS membrane 7a is not actuated out of plane (in the direction of the thickness of the active surface).

(77) The sensor 1 may have means for controlling and communicating (wired or radio), electronic or software, allowing the user to acquire, visualize, and exploit data, integrated or remote.

(78) These means allow in particular the integration of the sensor 1 or of a system with a set of sensors 1 in a building air quality control system.

(79) FIG. 5 shows the frequency shift of the oscillating membrane 7a following the deposition of 2 micron/submicron particles suspended in the airflow.

(80) FIG. 6 shows the impact of the particles 2 on the membrane 7a.

(81) FIG. 7 represents the main functions of the device according to the invention. Thus according to a method of realization the device includes a particle separation module FP1, a particle deposition module FP2, an analysis module FP3.

(82) Advantageously, the FP1 separation module includes a selection head, for example a PM10 head. The particle separation module FP1 allows to sort the particles preferably according to their inertia, for example according to their size, and this on several impaction stages.

(83) The particle deposition module FP2 allows particle adhesion. In addition, the particle deposition module is configured to target an area of interest on the MEMS sensor. Thus, the FP2 particle deposition module has a targeted area of interest on the MEMS sensor. In addition, the particle deposition module is configured for particle adhesion by functionalization and/or micro/nano structuring of the MEMS surface. Advantageously, the particle deposition module is configured for the regeneration of the sensor 1 including self-cleaning e.g. by air flow, vibration, temperature.

(84) The FP3 analysis module can be configured to measure the resonance frequency of MEMS sensors before and after particle deposition.

(85) The FP3 analysis module can also be configured to determine the mass of the particles 2 deposited on the MEMS as a function of the frequency shift. Advantageously, this FP3 analysis module allows automated signal processing. This analysis module also allows continuous in-situ analysis.

(86) MEMS 7 microbalances are realized by microfabrication processes in clean room, whose steps are shown in FIG. 8.

(87) As shown in FIG. 8, they are made from SOI (Silicon On Insulator) wafers with a stack of polished monocrystalline silicon several tens of micrometers thick, called “Device” and then “D”, a layer of silicon oxide SiO2 called “Box”, later “B” or “B′”, a few micrometers thick, and a layer of polished monocrystalline silicon several hundred micrometers thick called “Handle”, later “H”, and an oxide backside, called F in FIG. 8, 2 micrometers thick.

(88) The D layer is made of very low electrically resistive doped Silicon in order to guarantee the electrostatic actuation of the 7a membranes. The H layer is made of highly resistive Silicon to avoid short circuits. The membrane 7a forming the MEMS 7 microbalance is structured in the thickness of the D layer.

(89) A silicon nitride Si.sub.3N.sub.4 layer of about one hundred nanometer thickness (named N) and then a polycrystalline silicon layer of micrometer thickness (named M) are deposited by LPCVD (Low Pressure Chemical Vapor Deposition) during steps 1 and 2.

(90) A photosensitive resin is deposited on the top surface of the SOI and photolithography is then performed to define the structure of the membrane 7a and electrodes 14 in the resin in steps 3 and 4. At this step the difference between the patterns of the actuating electrodes 14 and the membrane is of the order of two micrometers.

(91) The patterns are then transferred into the polysilicon layer by etching in an RIE (Reactive Ion etching) frame in step 5.

(92) The SOI wafer is then placed in a high-temperature furnace (1100° C.) for several hours in order to oxidize the polysilicon in step 6.

(93) As a result of this oxidation, a silicon oxide almost two microns thick was formed and the openings between the drive patterns and the membrane pattern were reduced to a few hundred nanometers.

(94) During steps 8 and 9, a photolithography, followed by a DRIE etching (“Deep Reactive Ion etching”) is performed to open the oxide layer where the electrical contacts will be deposited.

(95) In a step 10, these patterns are then transferred by RIE etching into the polycrystalline Silicon and then the Silicon Nitride.

(96) In addition, as schematized in step 7, the parameters of steps 1 to 6 as well as those of the D-Silicon DRIE etching have been previously optimized on test plates in order to obtain submicron gap openings.

(97) During steps 11 and 12, a photolithography followed by an Aluminum deposition named “T”, by sputtering and an acetone stripping allow to form the Aluminum contact pads for the electrical signal inputs and outputs.

(98) In step 13, a photolithography is performed on the back side of the SOI to define opening areas under the membrane.

(99) This pattern is transferred into the Silicon oxide of the back side by DRIE etching in a step 14.

(100) The membrane and electrode patterns are transferred into the D-layer by DRIE etching in a step 15.

(101) A final DRIE step on the back side transfers the opening area under the membrane into the H-layer.

(102) The exposed silicon oxide is then dissolved by a hydrofluoric acid (HF) vapor etch in step 16. The membrane is now suspended.

(103) The remaining polysilicon and nitride are finally cleaned by RIE etching in a step 17.