GB2491413A

GB2491413A - Ionization gas sensor

Info

Publication number: GB2491413A
Application number: GB1109353.1A
Authority: GB
Inventors: Thomas Walewyns; Laurent Francis
Original assignee: Universite Catholique de Louvain UCL
Current assignee: Universite Catholique de Louvain UCL
Priority date: 2011-06-03
Filing date: 2011-06-03
Publication date: 2012-12-05
Also published as: GB201109353D0

Abstract

An ionization gas sensor for analyzing at least one gas comprises a first electrode 20, a second electrode 30, a first power source 50 electrically connected to said first and second electrodes and able to generate an electric field between them, a current measurement device for measuring a current flowing between said first and second electrodes through said at least one gas. The ionization gas sensor is characterized in that it further comprises means for moving said first electrode 20 with respect to said second electrode 30. This may be achieved with a third electrode 40 connected to another power supply V-bias providing a field to distort the first electrode 20 (dashed line) and change the spacing between electrodes 20 and 30. Nanowires may be used on the electrodes. A method of producing a sensor is also disclosed.

Description

IONIZATION GAS SENSOR

TECHNICAL FIELD

[0001] The present invention relates generally to the field of gas sensors. More precisely, according to a first aspect, the invention relates to an ionization gas sensor. According to a second aspect, the invention related to a method for measuring properties of at least one gas or gas mixture. According to a third aspect, the invention relates to a method for making an ionization gas sensor.

DESCRIPTION OF RELATED ART

[0002] Gas sensors are increasingly used in chemical, medical, and environmental industries, as well as in protecting systems against terrorism attacks. Current portable devices with catalyst-based electrochemical cells show limited sensitivity, low durability and selectivity issues. Furthermore, solid-state resistive materials, more sensitive, require elevated working temperatures.

[0003] Ionization of gases results in a unique fingerprint depending on their nature concentration, and pressure. Moreover, according to Paschen's law, the determination of different species of gases is possible by modifying the distance between electrodes when such species of gases are comprised between such electrodes. More precisely, for a specific gas or for a gas mixture trapped between two metal electrodes, the current flowing through the gas or gas mixture that is ionized as a function of the applied voltage results in a unique fingerprint at known humidity level. In order to enhance electric field and reduce a gaseous breakdown voltage (VbreakdQwfl), sharp nanotips, such as multi-walled carbon nanotubes (MWCNT5) (such as the ones described in the article by Y. Zhang et al. entitled "Novel Gas Ionization Sensors Using Carbon Nanotubes", Sensors Letters, Vol. 8, No. 2, 2010, pp.219-227), metallic (such as the ones described in the article by Fl. B. Sadeghian et al., entitled "A Novel Miniature Gas Ionization Sensor Based on Freestanding Gold Nanowires", IEEE Sensors Journal, Vol. 8, No. 2, 2008, pp.161 -1 69) or semiconductor (such as the ones described in the article by Fi. B. Sadeghian et al., entitled "Ultralow-voltage field-ionization discharge on whiskered silicon nanowires for gas-sensing applications", Nature Materials, Vol. 10, 2011, pp.135-140) nanowires, are incorporated on an electrode (cathode or anode, depending on the device configuration).

[0004] The gas sensor proposed in the article by Yong Zhang et al. entitled "Study of improving identification accuracy of carbon nanotube film cathode gas sensor", Sensors and actuators, A 125, 2005, pp.15-24 is particularly interesting. By using several distances d between electrodes, the authors have shown that is possible to measure different species of gases.

However, such a gas sensor has some drawbacks. The gas sensor described in this publication needs a network of couples of electrodes where each couple of electrodes has a specific distance d between them. As a consequence, the miniaturization of such a gas sensor is not easy when different species of gases are wanted to be analyzed. As the possible distances d between electrodes is determined by the geometry of the gas sensor, only a limited number of gases can be analyzed with such gas sensors.

SUMMARY OF THE INVENTION

[0005] According to a first aspect, it is an object of the present invention to provide a gas sensor that allows one to analyze different species of gases or mixtures of gases and that can be miniaturized. To this end, the invention proposes according to this first aspect a gas sensor comprising -afirst electrode -a second electrode -a first power source electrically connected to said first and second electrode and able to generate an electric field between said first and second electrode; -a current measurement device for measuring a current flowing between said first and second electrode through said at least one gas; and characterized in that said ionization gas sensor further comprises means for moving said first electrode with respect to said second electrode.

[0006] As the first electrode can move with respect to the second electrode, various separating distances d between these two electrodes can be imposed when using an ionization gas sensor according to the invention for analyzing a gas or a gas mixture. Only one couple of electrodes is needed for analyzing a gas or a gas mixture with various distances d contrary to the gas sensor proposed by Yong Zhang et al. where a network of couples of electrodes with various distances d is needed. As a consequence, the miniaturization of a gas sensor according to the invention is easier. As the first electrode can move with respect to the second electrode, the number of possible distances d between electrodes is not limited to the number of couples of electrodes with various distances d, contrary to the gas sensor proposed by Yong Zhang et al. [00071 The miniaturization of a gas sensor presents several advantages: lowered working voltages allowing compatibility with standard CMOS electronics, faster scanning rates and reduced gas sampling, reduced energy consumption are examples of such advantages.

[0008] Preferably, the means for moving said first electrode with respect to said second electrode comprises a third electrode and a second power source electrically connected to said first and third electrode and able to apply different voltages between said first and third electrode inducing the first electrode to move with respect to the second electrode because of capacitive actuation.

[0009] Preferably, the means for moving said first electrode with respect to said second electrode comprises a piezoelectric actuation device.

[0010] Preferably, the means for moving said first electrode with respect to said second comprises a thermal actuation device.

[0011] Preferably, nanowires stand on said second electrode. The integration of nanowires further reduces the power consumption and the dynamic of the sensor because of the dropping of the ionization voltage.

[0012] According to a second aspect, the invention relates to method for measuring properties of at least one gas comprising the steps of: i. providing a sample of said at least one gas between a first and a second (30) electrode of an ionization gas sensor; ii. positioning said first electrode at a given distance d from said second electrode; iii. applying a voltage between said first and second electrode; iv. measuring and recording a current value flowing between said first and second electrode; v. repeating steps iii and iv; vi. repeating steps ii to iv; vii. deducing for each distance d and from step ii to step iv measured current versus voltage curves; viii. providing a set of predetermined current versus voltage curves; ix. determining said properties of said at least one gas by applying a method of pattern recognition between the current versus voltage curves of step vii and step viii.

[00131 According to a third aspect, the invention relates to a method for producing an ionization gas sensor characterized in that it comprises the steps of: a) providing a substrate made of an insulating material; b) depositing a layer of conductive material on said substrate, for forming a pattern of second and third electrodes; c) depositing a first layer of polyimide and curing said first layer of polyimide; d) irradiating said first layer of polyimide for forming a set of tracks; e) etching said tracks for forming a corresponding set of nanopores; f) electrodepositing a metal or a semi-conductor in said nanopores for forming a corresponding set of nanowires; g) depositing one or more second layers of polyimide and curing said one or more second layers of polyimide; h) Depositing a layer of metal above said second layer or layers to form a top electrode; i) Etching said one or more second layers in an area between said top electrode and said pattern of electrodes;

BRIEF DESCRIPTION OF THE DRAWING

[0014] Fig.1 shows a typical current vs. voltage curve of a discharge in a gas.

[0015] Fig.2 is a schematic diagram of an ionization gas sensor according to the invention.

[0016] Fig.3 is a schematic diagram of a preferred embodiment of an ionization gas sensor according to the invention.

[0017] Eig.4 is another schematic diagram of another preferred embodiment of an ionization gas sensor according to the invention.

[0018] Fig.5 is a perspective view of a first design of said preferred embodiment.

[0019] Fig.6 is a perspective view of a second design of said preferred embodiment.

[0020] Fig.7 is a perspective view of a third design of said preferred embodiment.

[0021] Fig.8 is a block diagram showing the enhancement factor 13 as a function of geometrical parameters and distribution of nanowires.

[0022] Fig.9 is block diagram representing a first embodiment of a method according to the invention for measuring at least one property of a gas.

[0023] Fig.10 is block diagram representing a second embodiment of a method according to the invention for measuring at least one property of a gas.

[0024] Fig.1 1 is a schematic diagram of a method for making an ionization gas sensor according to the invention.

Detailed description ot the invention

100251 The ionization gas sensor 10 of the invention allows analyzing at least one gas. Examples of gases are helium, argon, oxygen, air, nitrogen, carbon monoxide, carbon dioxide, nitric dioxide. As the gas sensor of the invention is able to analyze at least one gas, it also can analyze a mixture of different gases. The term analyzing' means that the ionization gas sensor 10 of the invention allows determining for instance the types of gas of a mixture comprising different gases, and a partial gas pressure (or equivalently, a specific gas concentration).

[0026] The ionization gas sensor 10 according to a first aspect of the invention and the method for determining properties of at least one gas according to a second aspect of the invention use two physical principles known by the one skilled in the art: Paschen's law and ionization phenomena.

[0027] Paschen's law describes a breakdown voltage of a gas species as a function of the product between a partial gas pressure and a distance between two electrodes, pd, given by equation (Eq. 1) -Bpd breakdown ( ( iV (Eq. 1).

1n(Apd)-1n 1n 1+-K 7 A and B are named Paschen coefficients, depending on the gas, and y is a second Townsend coefficient, see for instance "Electrical Breakdown of Gases" by J. M. Meek, J. D. Craggs, John Wiley & Sons, 1978.

[0028] Ionization phenomena: Each gas or gases mixture, depending on its concentration, shows a particular current vs. voltage curve (or I-V curve) when it is sandwiched between two electrodes, see for instance "Electrical Breakdown of Gases' by J. M. Meek, J. 0. Craggs, John Wiley & Sons, 1978.

An example of such a curve is shown in figure 1. The current vs. voltage curve of gas ionization can be separated into three distinct regions as shown on Figure 1: I. Firstly, in the quasi-ohmic area, a gas exhibits an ohmic behavior. The discharge current I depends on the velocity of carriers and is almost proportional to the electric field, which depends on the applied voltage V: J = CEgff = ("efle + nJi)eiz (Ea.2) where is a gas electrical conductivity, Eeff is an effective electric field, tie and n1 are electron and ion concentration, Ye and p are mobilities (constants

at small fields), and e is the electron charge.

II. Between regions I and II of figure 1, the effective carrier recombination increases and therefore reduces the total of charged particles. The rate of increase of current with voltage decreases until a saturation current, I is reached, given by: I3=Ade (Eq.3) where A is an electrode area, d is the inter-electrode distance, and dn/dt is the total rate of carrier production. The current density does not depend on Eeg nor on the mobilities, and is limited by the rate of carrier production.

Ill. If Ee11 is further increased, gas ionization takes place by electron impact on neutrals, raising the current at an increasing rate until breakdown happens at Vbreakdown. This current, called pre-breakdown Townsend discharge current, is expressed by: -I0e" (Eq.4) 1-e -i) where a and y are respectively Townsend's primary and generalized secondary ionization coefficients, representing a number of ionizing collisions per electron, and a number of electrons liberated per incident ion at the cathode.

[0029] Current vs. voltage curves such as the one shown in figure 1 with varying the distance d between two electrodes allows one to determine specific gas concentration of a gas mixture sandwiched between these electrodes.

[0030] Figure 2 shows a schematic diagram of an ionization gas sensor according to a first aspect of the invention. A first electrode 20 and a second electrode 30 are connected to a power source 50 that can apply a voltage between these two electrodes. Two examples of power sources 50 are: current source and voltage source. A resistor may be inserted in series with the power source 50 in order to limit the current and avoid any damage to the gas sensor 10. The at least one gas is sandwiched between the two electrodes and a current measurement device 60 can measure the current flowing between the two electrodes. The distance d between first 20 and second 30 electrodes can vary. In the example shown in figure 2, the first electrode 20 can move with respect to the second electrode 30 but it is possible to have a fixed first electrode 20 and a movable second electrode 30.

Different actuation means, such as mechanical means, piezoelectrical means, thermic means, can be used for modifying the distance d.

[0031J Figure 3 is a schematic diagram of a preferred embodiment of the ionization gas sensor 10 of the invention. In this embodiment, actuation is performed by electrical means: the ionization gas sensor 10 further comprises a third electrode 40 and a second power source 52. This second power source 52 is able to apply different voltages between the first 20 and third 40 electrode inducing an electrostatic force between said electodes and a deformation of at least one of these electrodes, whereby the distance d between the first 20 and second 30 electrode is varied. In this embodiment, the second 30 and third 40 electrodes are preferably golden (Au) electrodes.

Preferably, the first electrode 20 is an aluminium (Al) electrode. The thickness and material of the first electrode 20 is selected such that a deformation thereof is obtained by the electrical means. The value of the distance d can be controlled by varying the voltage applied by the second power source 52.

[0032] Figure 4 shows another possibility where the second electrode is the middle electrode.

[0033] Figure 5 shows a first design of said preferred embodiment where a silicon wafer 90 acts as a substrate. On this support, a golden second electrode 30 (ionization electrode) is deposited as a linear track having a width of typically 40 lAm. In fact, any type of conducting material can be used for the second electrode 30. On the sides of this ionization electrode, at a distance of typically 30 m, a third electrode 40 is deposited as two parallel tracks 40a and 40b, each having a width of typically 30 pm. The linear forms of the second and third electrodes allow an easier fabrication. These two tracks 40a and 40b are electrically connected together to the second power source 52. A layer of polyimide 100 (any spacer can be used in fact) supports the first electrode 20 at a distance d above the second 30 and third 40 electrodes.

Said distance d is typically 12 pm. The length of the first electrode 20 between the two polyimide supports 100 is typically 220 pm. The width of the second electrode 30 is typically 10 or 40 pm, and its thicknesses may be from 100 to 800 nm. Vertical displacements around 10 to 12 nm/V have been observed above lateral electrodes. In the design shown in figure 4, second 30 (and optional third 40) electrodes are preferably deposited directly on a silicon substrate. This allows an easier co-integration of the sensor interface electronics, and therefore an easier miniaturization when compared with other known gas sensors.

[0034] Figure 6 shows a second design. In this design, a single third electrode 40 is located between two parallel second electrodes 30a and 30b.

In this design, the electrostatic actuation is performed at the most efficient location, i.e. in the middle of the first electrode 20.

[0035] Figure 7 shows a third design. In this design, the first electrode 20 is wider e.g. has a width of 200 pm. A series of holes 22 are provided for allowing a better penetration of the gas or gas mixture to be measured at the sensitive region between the first 20 and second electrodes 30. This third design may built with the central second electrode 30 (as shown on figure 5) or with a central third electrode 40 with second electrodes 30a and 30b on the sides (as shown on figure 6 and 7). By using a wide first electrode 20, a larger current may be measured.

[0036] As represented on figures 5, 6 and 7, the second electrode 30 may support freestanding nanowires 80. In order to enhance electric field and reduce the gaseous breakdown voltage (Vbreakdown), sharp nanotips, such as multi-walled carbon nanotubes (MWCNT5), metallic or semiconductor nanowires, may be incorporated on the second electrode 30 (which may be a cathode or anode, depending on the device configuration). By using nanowires, the breakdown voltage Vbreakdown in air at atmospheric pressure (21 °C, 40% relative humidity) in a gap of 5.5 pm may be reduced at least from 320 V to 80 V. With such approach, sensors with high sensitivity, high selectivity, long durability and (ultra)-Iow-power operation (P < 1 0 pW with ionization currents «= 1 pA) are made possible.

L0037] By increasing locally the electric field, nanowires 80 have a high impact on the ionization system with a reduced operating voltage.

A enhancement factor p can be introduced for accounting for the increase of electric field because of a sharp geometry of nanowires 80. The enhancement factor p is mainly function of the nanowires aspect ratio v= L/r (where L and r are respectively the nanowires length and radius), but also of the gap between the nanowires apex and the counter electrode, d, and the average horizontal distance between nanowires, s (corresponding to their surface density), known as screening effect, see for instance the article by R. G. Forbes et al. and entitled "Some comments on models for field enhancement", Ultramicroscopy, Vol. 95, 2003, pp.57-65. Taking into account these influences, the following analytical expressions can be defined: fi =1.2(v+2.15)° .[1_exP[_2.3172JJ (Eq. 5), fl=fl[1-dLLJ (Eq. 6).

Figure 8 shows a 3D surface corresponding to equation (Eq. 6) for a 1 lAm long and 20 nm in diameter nanowires array. Nanowires or nanotubes densities reported so far are around 1 9 cm2 due to the bottom-up process used to obtain them. The corresponding value of the field ionization enhancement is low (about 20). Larger values (about 70) can be reached by a 2 pm mean separation, corresponding to a density of 2.5 1 7 cm2, as computed analytically. The increase of the effective electric field at the vicinity of tips of nanowires can be evaluated by the following equation: p = £fLL. (Eq. 7), app where Leff is the effective electric field and EaPP5 the apparent macroscopic field, corresponding to V/d for parallel-plate capacitor geometries (with V, the applied voltage between first and second electrode, and d, the distance between these two electrodes).

[0038] When nanowires 80 stand on the second electrode 30, this second electrode 30 preferably carries a positive voltage with respect to the first electrode 20 but the second electrode 30 can also be negative with respect to the same first electrode 20. Nanowires 80 are preferably deposited on the second electrode 30 by electrodeposition, a technique known by the one skilled in the art.

[0039] Contrary to the gas sensor described in the article by Y. Zhang et al. entitled Novel Gas Ionization Sensors Using Carbon Nanotubes", Sensors Letters, Vol. 8, No. 2, 2010, pp.219-227, or in the article "Miniaturized gas ionization sensors using carbon nanotubes" by Ashish Modi et al. in letters to Nature, 24, p171 (2003), a measurement with the ionization gas sensor of the invention does not need to apply an electric field that induces a breakdown of the at least one type of gas. With the ionization gas sensor 10 of the invention, the entire current vs. voltage curve is used for analyzing the at least one type of gas. As a consequence, nanowires 80 or nanotubes deposited on the second electrode 30 in a preferred embodiment are not destructed when using a gas sensor according to the invention.

[0040] In another preferred embodiment, the distance d between first and second 30 electrode varies because of thermal actuation means. As an example, a deflection of the first electrode 20 can be induced because of a rise of its temperature when a sufficiently high electrical current flows through it. Typically, the first electrode 20 is in aluminum. Thermal actuation means typically comprises a current source for providing a sufficiently high current to the first electrode 20 inducing its heating.

[0041] According to a second aspect, the invention relates to a method for analyzing at least one gas. Fig.9 is bock diagram representing a first embodiment of a method according to the invention for measuring at least one property of a gas. In a first step 200, a value for the bias voltage provided by second power source 52 is selected for obtaining an initial value of the distance d. In step 210, a value of the ionization voltage provided by first power source 50 is selected for producing a given voltage V on the curve of Fig. 1. The voltage may be selected in any or all of the ranges I, Il or Ill of the diagram of figure 1. The current is then measured in step 220 and stored, together with the corresponding value of the bias voltage. Step 210 and 220 are then repeated for j values of the ionization voltage. Steps 200, 210, 220 are then repeated i times for different values of the bias voltage. The set of measurements is then processed in step 230 for obtaining a set of curves similar to the diagram of figure 2. This processing step comprises the conversion of the ionization voltage provided by first power source 50 to the equivalent voltage V of parallel plates of Fig. 1. This set is then compared in step 240 with a database 250 of similar curves obtained in a calibration step 260. The properties of the measured gas or gas mixture are then obtained by a method of pattern recognition. The database 250 may be obtained by performing the above sequence of measurements for known gases or gas mixtures or by performing a theoretical model computation. Pattern recognition can use for instance a method such as the one described by Z Zikai et al. in IEEE (2010), pp 1-4, 4th International Conference on Bioinformatics and Biomedical Engineering (iCBBE). When experimental calibration is used for building the database 250, one will preferably use a similar gas sensor 10 as the one used for carrying the experiments.

[0042] Fig.1 0 is bock diagram representing a second embodiment of the method, similar to the method of the first embodiment, but where the iteration loops are performed in a different order. Preferably, a part of the current to voltage curve that corresponds to a pre-breakdown Townsend discharge current is used. The determination of a gas or of gas properties is more effective when considering this part as it stronger changes when the type of gas or the gas properties change. In such an embodiment of the method of the invention, the entire current to voltage curve does not need to be recorded.

[0043] Because of motion of first electrode 20 with respect to second electrode 30, capacitance between these two electrodes changes. This capacitance variation could be used as a first gas discrimination before applying the step of pattern recognition 240.

L0044] In a preferred embodiment, a varying periodic voltage is applied between the first 20 and second electrode 30. In a still preferred embodiment and when a third electrode 40 is used for modifying the distance d between the first 20 and the second 30 electrode, a varying periodic voltage is applied between the first 20 and third electrode 40. In such a case, the first electrode typically vibrates above the second electrode 30. The use of AC excitation signals rather than DC signals allows an increase of sensibility of the method of the invention. For example, by driving the moving electrode at mechanical resonance.

[0045] According to a third aspect, the invention relates to a fabrication process for producing an ionization gas sensor 10. This fabrication process is based on the microstructuration of ion-track etched polyimide used to synthesize nanowires and anchor the structures. Figure 11 summarizes the process. A silicon wafer 90 may comprise an oxidized silicon (Si02) top layer 310. A patterned gold (Au) layer 320 is evaporated on said substrate to form the bottom electrodes 30 and 40 (Figure 11.1). The wafer periphery (not shown) is fully covered by the gold layer to ensure homogeneous electric contact with the anode of an electroplating controller, whereas the pattern inside the periphery defines area where the nanowires growth will take place.

A layer of Titanium (Ti) may be deposited on top of the gold layer. Platinum (Pt) may be used instead of gold. Redox potential of the electrode for electrodeposition has to be higher than the one of the electrodeposited metal.

Following, a 2.5 im thick layer 330 of a non-photosensitive P12545 polyimide from HD MicrosystemsTM is spin-coated and cured by steps until 350°C (Figure 11.2). Au/Ti and Si02 thicknesses are respectively 100/5 nm and 300 nm. The first layer 330 of polyimide is irradiated from the top by a beam of heavy ions produced by a cyclotron in order to define tracks 340, which are then etched in order to create nanopores with fixed diameter (Figure 11.3).

The nanopores are then filled by electrodeposition of metal or semi-conductor to obtain nanowires 80 (Figure 11.4). A preferred metal for producing the nanowires is nickel (Ni). Nickel is an example of material that can be electrodeposited. Other examples are Cu, Ag, Au. The wafer 90 is placed in a homemade three-electrode electrodeposition cell with one side in the electrolyte covered by a spiral platinum counter-electrode (anode). The wafer periphery is connected to the working electrode (cathode). To ensure that the nanowires 80 do not collapse, the aspect ratio (length/radius) has to be limited to a maximum of 200. Several layer 350, 350' of polyimide are then spin-coated and cured to reach the desired thickness (Figure 11.5). The top electrode 20 is obtained by lift-off patterning (a method of microfabrication known by the one skilled in the art) of an aluminum layer with thicknesses from 100 to 800 nm (Figure 11.6). The polyimide spacer is then patterned in two steps through the top aluminum layer by reactive ion etching in oxygen plasma: an anisotropic etching followed by an isotropic etching to release the structures. Such techniques are known by the one skilled in the art. At the end, the top aluminum electrode forms a suspended microbridge 20 or membrane 20, from 100 to 800 nm thick, above freestanding nanowires 80, as shown on Figure 11.7.

[0046] The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention as defined in the following claims, and their equivalents, in which all terms are to be understood in their broadest possible sense unless otherwise indicated. As a consequence, all modifications and alterations will occur to others upon reading and understanding the previous description of the invention. In particular, dimensions, materials, and other parameters, given in the above description may vary depending on the needs of the application.

[0047] Summarized, the invention can be described as follows.

Ionization gas sensor 10 for analyzing at least one gas and comprising: a first electrode 20, a second electrode 30, a first power source 50 electrically connected to said first 20 and second 30 electrode and able to generate an electric field between said first 20 and second 30 electrode, a current measurement device 60 for measuring a current flowing between said first 20 and second 30 electrode through said at least one gas. The ionization gas sensor 10 is characterized in that said ionization gas sensor 10 further comprises means for moving said first electrode 20 with respect to said second electrode 30.

Claims

CLAIMS1. Ionization gas sensor (10) for analyzing at least one gas and comprising: -a first electrode (20); -a second electrode (30); -a first power source (50) electrically connected to said first (20) and second (30) electrode and able to generate an electric field between said first (20) and second (30) electrode; -a current measurement device (60) for measuring a current flowing between said first (20) and second (30) electrode through said atleastonegas; characterized in that said ionization gas sensor (10) further comprises means for moving said first electrode (20) with respect to said second electrode (30).
2. Ionization gas sensor (10) according to claim 1 characterized in that said means for moving said first electrode (20) with respect to said second electrode (30) comprises: -a third electrode (40); and -a second power source (50) electrically connected to said first (20) and third (40) electrode and able to apply different voltages between said first (20) and third (40) electrode inducing the first electrode (20) to move with respect to the second electrode (30) because of capacitive actuation.
3. Ionization gas sensor (10) according to claim 1 characterized in that said means for moving said first electrode (20) with respect to said second electrode (30) comprises a piezoelectric actuation device.
4. Ionization gas sensor (10) according to claim 1 characterized in that said means for moving said first electrode (20) with respect to said second electrode (30) comprises a thermal actuation device.
5. Ionization gas sensor (10) according to any of previous claims characterized in that nanowires (80) stand on said second electrode (30).
6. Ionization gas sensor (10) according to claim 5 characterized in that said nanowires (80) are metallic.
7. Ionization gas sensor (10) according to claim 5 characterized in that said nanowires (80) are made of a semi-conducting material.
8. Ionization gas sensor (10) according to claim 5 characterized in that said nanowires (80) are carbon nanotubes.
9. Method for measuring properties of at least one gas comprising the steps of: i. providing a sample of said at least one gas between a first (20) and a second (30) electrode of an ionization gas sensor (10) according to any of previous claims; ii. positioning said first electrode (20) at a given distance d from said second electrode (30); iii. applying a voltage between said first (20) and second (30) electrode; iv. measuring and recording a current value flowing between said first (20) and second (30) electrode; v. repeating steps iii and iv; vi. repeating steps ii to iv; vii. deducing for each distance d and from step ii to step iv measured current versus voltage curves; viii. providing a set of predetermined current versus voltage curves; ix. determining said properties of said at least one gas by applying a method of pattern recognition between the current versus voltage curves of step vii and step viii.
1O.Method according to claim 9 characterized in that step ii and iii are inverted.
11. Method according to claim 1 0 or 11 characterized in that step ii comprises an AC electrostatic actuation.
12. Method for producing an ionization gas sensor (10) characterized in that it comprises the steps of: a) providing a substrate (90) made of an insulating material; b) depositing a layer of conductive material on said substrate, for forming a pattern of second and third electrodes (30,40) c) depositing a first layer (330) of polyimide and curing said first layer (330) of polyimide; d) irradiating said first layer (330) of polyimide for forming a set of tracks (340); e) etching said tracks for forming a corresponding set of nanopores; f) electrodepositing a metal or a semi-conductor in said nanopores for forming a corresponding set of nanowires (80); g) depositing one or more second layers (350, 350') of polyimide and curing said one or more second layers (350, 350') of polyimide; h) Depositing a layer of metal (20) above said second layer or layers to form a top electrode (20); i) Etching said one or more second layers (350, 350') in an area between said top electrode (20) and said pattern of electrodes (30, 40).