Patent Description:
The disclosure especially refers to a multi-gas sensor.

The detection of environmental parameters, e.g. gas concentrations, in the ambient atmosphere is becoming increasingly important in the implementation of appropriate sensors within mobile devices, but also in the application in home automation, such as smart home and, for example, in the automotive sector. However, with the evermore extensive use of gas sensors, there is also a particular need to be able to monitor the quality of the ambient atmosphere, i.e. to determine the air quality using mobile devices. Such an air quality monitoring process should be inexpensively and cost-effectively implemented. To be more specific, for a best efficiency the entire gas sensor should act as an active gas sensing area. The more area the gas sensitive device provides, the higher is its sensitivity and therewith its efficiency. Usually, a graphene area is used to monitor the ambient air quality.

An ongoing trend in the evolution of mobile devices (smart phones, etc.) is the implementation of more and more additional features. One of the highly expected next steps will be the integration of gas sensors measuring air quality and/or detecting and warning of toxic air pollution. Therefor "simple" low cost sensor-devices with a small footprint and a very low power consumption are needed.

The deposition of an active sensor layer is a printing process of a solvent consisting of ink, graphene flakes and e.g. nanoparticles for functionalization. During drying, the fluid ink droplets form a so-called coffee-ring at the edge of the sensor area. The result is a thicker sensor layer at the sensor edge. During resistance measurement the current is located at the sensor edge and not homogeneously distributed over the whole sensor area. Thus, only the small current path of the "Coffee-Ring" is contributing to the sensitivity and not the whole sensor area.

Thus, two effects reduce the use of the whole gas sensor area. Firstly, locally clustered graphene flakes between electrodes lead to a reduction of the area usable for resistance measurements. Because of the clustered graphene flakes between electrodes printed on top of a substrate area a conductance only between a few electrodes usually result. Secondly, during the manufacturing process of producing the electrodes on the top surface of the substrate coffee stain arises. Coffee stain cause a surrounding shortage between the incoming metal lines, so that the conductance is only given at outer sensor edges. Both effects suppress the use of the whole sensing area and reduce the efficiency of the sensor.

An established concept is a printed or dispensed active sensor-layer consisting of graphene flakes and functional nanoparticles on top of an interdigital structure (as shown in <FIG>). The adsorption or desorption of gas molecules are reflected by the change of the electrical resistance of the device.

In addition to the mostly dominating coffee-ring-effect, the metal lines of the interdigital structure can cause accumulation and clumping of ink ingredients between two metal fingers. In this case, also only a small area is defining the sensitivity and not the whole sensor area.

Thus, a conventional method of gas measurement is measuring the resistance-change of a gas sensitive layer between the electrodes of an interdigital structure. This Method has two disadvantages:.

Both effects avoid the use of the whole sensor area because any grain or lump with a higher conductance between two metal fingers or higher deposition on a sensor edge will determine the resistance of the whole sensor area.

For example, <CIT> discloses a humidity-sensing device and a method of producing the humidity-sensing device. The humidity-sensing device is of capacitive type and includes a dielectric material with low permeability to moisture and an electrode with permeability to moisture greater than that of the dielectric material with which it is in contact. Further, <CIT> discloses an integrated circuit comprising a capacitive gas sensor on a semiconductor substrate using a pair of interdigitated capacitor electrodes and a gas sensitive material deposited on the electrodes.

<CIT> discloses a gas sensing method including: a step of supplying measured gas to a capacitor including a first electrode, a dielectric formed to be electrically connected to the first electrode, a graphene formed on the dielectric, and a second electrode formed to be electrically connected to the graphene; and a step of measuring a capacitance of the capacitor <NUM> after the measured gas is brought into contact with the graphene.

Generally, there is a need in the art for an approach to implement an improved gas sensitive device and an improved multi-gas sensor and a method of operating the gas sensitive device, that are independent from locally clustered graphene flakes and/or from coffee stain.

Such a need can be solved for the subject-matter of the independent claim. Further specific implementations of the present concept (method and apparatus) for providing an improved gas sensitive device for a multi-gas sensor in the dependent claims. A method of operating the gas sensitive device is also disclosed.

In an embodiment, a gas sensitive device comprises a substrate structure, and a gas sensitive capacitor. The gas sensitive capacitor comprises a first capacitor electrode in form of a gas-sensitive layer on a first main surface region of an insulation layer, and a second capacitor electrode in form of a buried conductive region below the insulation layer. The insulation layer is arranged between the first and second capacitor electrode. The gas sensitive capacitor comprises a first contact region for electrically contacting the first capacitor electrode, and a second contact region for electrically contacting the second capacitor electrode; wherein the gas-sensitive layer comprises a sheet impedance which changes in response to the adsorption or desorption of gas molecules. On top of the insulation layer a cover layer is disposed, so that the cover layer has at least a common plane with the first contact region and/ or with the first capacitor electrode so that the cover layer surrounds completely the first contact region or the first contract region and the first capacitor electrode in the at least one common plane; wherein, a thickness of the first capacitor electrode being the gas-sensitive layer is thinner than a thickness of the cover layer, both thicknesses extending along a z-direction perpendicular to the cover layer, wherein the gas-sensitive layer is formed by dropping an ink drop onto the insulation layer. The first capacitor electrode is preferably provided on top of the gas sensitive device. Therefore, the first capacitor electrode may be called top electrode. The second capacitor electrode is preferably buried and is provided in the gas sensitive device, in particular above a substrate structure carrying the gas sensitive device. Therefore, the second capacitor electrode may be called buried electrode.

As proposed a gas sensitive layer on top of a flat surface forms the first capacity electrode, i.e. the top electrode, of a gas sensitive capacitor. The second capacity electrode of the capacitor is a buried electrode under a, preferably thin, insulation layer. The top electrode is contacted at the edge. Therefore, the inner part of the top electrode is contacted via the rather high ohmic gas sensitive layer. If gas molecules are adsorbed at the surface by the top electrode, the sheet resistance or sheet impedance will change. This can be measured with the methods described below.

According to an example, a multi-gas sensor comprises one or more gas sensitive devices according to any of the gas sensitive devices disclosed herein. With the disclosed multi-gas sensor it may be possible to distinguish two or more gases with a sensor response of one gas sensitive device as proposed herein.

According to an example, a method for operating the gas sensitive device according to any of the gas sensitive devices disclosed herein is proposed. However, such a method is an illustrative example only and is not covered by the appended claims. The method comprises applying an AC signal to the first capacitor electrode; reading out a signal between the first capacitor electrode and the second capacitor electrode, wherein the signal read out comprises an information on the sheet resistance or the sheet impedance of the gas-sensitive layer of the first capacitor electrode due to the adsorbed or desorbed gas molecules. The relation between the sheet resistance or the sheet impedance of the sensitive layer and the capacitive impedance causes a frequency dependency of capacity measurement. At low frequencies the capacitance of the whole area can be measured. At high frequencies only the capacitance of the sensor edge can be measured.

In contrast to the usual interdigital structure the proposed gas sensing capacitor is an averaging device. This means local non-homogeneities of the sensing layer are negligible. Also, the coffee ring effect has no negative effect to the gas detection.

The proposed multi-gas sensor and the disclosed method can be implemented with any of the proposed gas-sensitive device described within this application.

Embodiments of the present gas sensor device are described herein making reference to the appended drawings and figures.

Before discussing the present embodiments in further detail using the drawings, it is pointed out that in the figures and the specification identical elements and elements having the same functionality and/or the same technical or physical effect are usually provided with the same reference numbers or are identified with the same name, so that the description of these elements and of the functionality thereof as illustrated in the different embodiments are mutually exchangeable or may be applied to one another in the different embodiments.

In the following description, embodiments and examples are discussed in detail, however, it should be appreciated that the embodiments and examples provide many applicable concepts that can be embodied in a wide variety of semiconductor devices. The specific embodiments and examples discussed are merely illustrative of specific ways to make and use the present concept, and do not limit the scope of the embodiments. In the following description of embodiments and examples, the same or similar elements having the same function have associated therewith the same reference signs or the same name, and a description of the such elements will not be repeated for every embodiment. Moreover, features of the different embodiments described hereinafter may be combined with each other, unless specifically noted otherwise.

It is understood that when an element is referred to as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or intermediate elements may be present. Conversely, when an element is referred to as being "directly" connected to another element, "connected" or "coupled," there are no intermediate elements. Other terms used to describe the relationship between elements should be construed in a similar fashion (e.g., "between" versus "directly between", "adjacent" versus "directly adjacent", and "on" versus "directly on", etc.).

The embodiments shown in the Figs. are presented with a coordinate system, so that a thickness of the different layers extend along the z-direction, while the extension of the different layers extends parallel to a x-y plane.

<FIG> shows a schematic top view of a gas sensitive device <NUM> according to an embodiment; and <FIG> shows a schematic cross sectional view of the gas sensitive device <NUM> as shown in <FIG>. The gas sensitive device <NUM> comprises a substrate structure <NUM>, and a gas sensitive capacitor. The gas sensitive capacitor comprises a first capacitor electrode <NUM> in form of a gas-sensitive layer <NUM> on a first main surface region of an insulation layer <NUM>, and a second capacitor electrode <NUM> in form of a buried conductive region below the insulation layer <NUM>. The insulation layer <NUM> is arranged between the first and second capacitor electrode <NUM>, <NUM>. The gas-sensitive layer <NUM> comprises a sheet resistance or a sheet impedance which changes in response to the adsorption or desorption of gas molecules. The first capacitor electrode <NUM> may for example be called a top electrode, because it is disposed on top of the gas sensitive device <NUM>. The first capacitor electrode <NUM> is in contact with the ambient atmosphere allowing for adsorbing or desorbing gas molecules from the ambient atmosphere. The second capacitor electrode <NUM> is provided directly below the insulation layer <NUM> or directly below a stack of insulation layers <NUM> or between a stack of first insulation layers <NUM> and second insulation layers <NUM>. It is possible that the insulation layer <NUM> and/or insulation layer <NUM> comprises a stack of different insulation layers <NUM>, <NUM>. As for example shown in <FIG>, the second capacitor electrode <NUM> may be disposed above the substrate structure <NUM>, so that the second capacitor electrode <NUM> may be in contact on one side with the insulation layer <NUM> and on the opposite side may be in contact with the substrate structure <NUM>. The first capacitor electrode <NUM> may be in contact on one side with the insulation layer <NUM>, wherein the insulation layer <NUM> is a first insulation layer <NUM>. On the opposite side the first capacitor electrode <NUM> may have an external surface region being in contact with the ambient atmosphere (see <FIG>). The second capacitor electrode <NUM> may also be called the buried electrode. For example, the gas sensitive layer <NUM> may have a thickness along the z-direction depending of the material, the thickness of the gas sensitive layer <NUM> can vary over a wide range. A preferred thickness would be in the range of a real two dimensional layer (e.g. a graphene monolayer) up to several nanometers, for example, about <NUM>.

As can be derived from <FIG>, the first capacitor electrode <NUM> and second capacitor electrode <NUM> may be provided with contact regions <NUM>, <NUM>. The gas sensitive device <NUM> comprises a first contact region <NUM> for electrically contacting the first capacitor electrode <NUM>, and a second contact region <NUM> for electrically contacting the second capacitor electrode <NUM>. For contacting the second capacitor electrode <NUM> the gas sensitive device <NUM> may be provided with a contact hole <NUM> extending through the gas sensitive device <NUM> from the second contact region <NUM> to the second capacitor electrode <NUM>. The insulation layer <NUM> between the first and second capacitor electrode <NUM>, <NUM> may be a dielectric material, for example SiN.

The core of the invention is an intelligent designed new capacitive gas sensitive device, with a new capacitive measurement method between a top and a buried electrode, wherein the method is not covered by the appended claims. The top electrode material is the gas sensitive layer <NUM> (e.g. graphene). The change of adsorbed gas molecules on the top electrode results in a change of the charge carrier density, which leads to a change of the AC impedance behavior.

The proposed gas sensitive device <NUM> has an improved product performance expected due to the fact, that the gas detection with the new design will be no longer determined by lumps between the metal finger or the small coffee ring surrounding the sensor field, but by the much more bigger sensor area at the top of the gas sensitive device <NUM>.

<FIG> shows a schematic view of an equivalent circuit of the gas sensitive device <NUM> according to an embodiment. Supposing an AC-coupling of the first capacitor electrode <NUM>, an equivalent circuit may be given by an RC-network. Here, the first contact region <NUM> is ohmically coupled to the first capacitor electrode <NUM>. Such a RC-network may have a frequency behavior depending in the adsorbed gas molecules. This means the sheet resistance or the sheet impedance of the top electrode or the first capacitor electrode <NUM> changes by the adsorption or desorption of gas molecules, for example of NO<NUM> (nitrogen dioxide), Os (ozone) or CO (carbon monoxide).

<FIG> shows a schematic view of a graphene material, i.e. the two dimensional structure of the graphene doped with nanomaterials. Graphene is arranged in a two-dimensional honeycomb lattice, which may be doped with nanoparticles <NUM> or doped with salts. Depending on the chosen nanoparticles <NUM> or the salts, the graphene may become sensitive for the adsorption of specific gas molecules from the ambient atmosphere. By doping the graphene with nanoparticle and/or with salts the graphene becomes functionalized. The herein disclosed sensor principle also works with a not graphene based gas sensitive layer <NUM>. Subject to the condition, that the used material is showing the described behavior. Examples of not graphene based layers <NUM> may be an amorphous Carbon, thin poly silicon, tantalnitride, titannitride, AIScN, or every material which forms a thin or two dimensional layer, which electrical conductivity can be influenced by the interaction with gas molecules.

<FIG> shows a comparison between the proposed gas sensitive device (shown in <FIG>) having an AC-coupled sensitive layer versus an established impedance measurement technique (shown in <FIG>). For performing the established AC-measurement technique an interdigital structure is necessary. Then a lateral AC measurement can be performed in order to determine the real and the imaginary parts of the impedance Z. The established impedance measurement technique needs the interdigital structure in order to be performable. The proposed gas sensitive device <NUM>, however, has a simple structure and allows also for measuring the impedance.

<FIG> shows a schematic view of the gas sensitive device <NUM> coupled to an AC-measurement device and coupled to a DC voltage source. A sensitivity of the proposed gas sensitive device <NUM> can be adjusted by applying and adjusting a DC bias voltage VDC-bias. For doing so, the gas sensitive device <NUM> is connected with a DC voltage source <NUM> and with a AC measurement device <NUM> vie a bias T-component <NUM>.

<FIG> shows a theoretic explanation of the graphene band-structure. <FIG> also shows schematically the effect of adjusting the DC bias voltage until the so called "Dirac point" is reached. Depending on the position of Fermi's energy EF the graphene provides electron conduction or provides hole conduction. It was found that at the Dirac point the gas sensitive layer has its highest sensitivity for adsorbing gas molecules from the ambient atmosphere. It should be noted that the "Dirac point" is influenced by the functionalization of the gas sensitive layer <NUM>, the (target) gas concentration and/or temperature. By using one or all of the parameters the regime at which the gas sensitive devices are sensitive or have a maximum sensitivity to ambient gases can be identified and, thus, can used for detecting gas molecules in the ambient atmosphere. The Dirac point of the gas sensitive device <NUM> can be determined from the local maximum of the DC-bias voltage VDC-Bias versus the resistance R.

<FIG> and <FIG> show possible design variants for the gas sensitive area of the gas-sensitive device <NUM>. For example, a projection of the first capacitor electrode <NUM> vertically with respect to the first main surface region at least partially or completely overlaps with the second capacitor electrode <NUM>. The <FIG> and the <FIG> show for example that different kinds of circle or parts of circles are possible to choose for the first and/or second capacitor electrodes <NUM>, <NUM>. Furthermore the first and/or second capacitor electrodes <NUM>, <NUM> may be formed, for example, as ellipses or parts of ellipses, polygons or part of polygons, meander or parts of meander, combs or parts of a comb. It is also possible to choose a combination of different geometries or a combination of different shapes. Also a free hand designed shape may be chosen (see <FIG>). Also a different overlap sensing area to the underlying substrate structure <NUM> may be chosen (see <FIG>).

<FIG> also shows that a surrounding shape of the first electrode <NUM> may be different from a surrounding shape of the insulator layer <NUM> or wherein a surrounding shape of the first electrode <NUM> may be equal to a surrounding of the insulator layer <NUM>. If different surrounding shapes of the insulator layer <NUM> and the first electrode <NUM> a partial overlap may result (<FIG>). Also a complete overlap may result according to the example shown in <FIG>, when different surrounding shapes of the insulator layer <NUM> and the first electrode <NUM> are used. Different configuration in which surrounding shape of the first electrode <NUM> may be equal to a surrounding of the insulator layer <NUM> are shown in <FIG>. As shown in <FIG>, the first electrode <NUM> completely or only partially overlaps the surface of the insulator layer <NUM>. The insulator layer can be made of SiN and can have a thickness along the z-direction of about <NUM> to <NUM>. The insulator layer <NUM> can also be made of other dielectrics or can be made of a stack of different dielectrics, for example SiN/SiO/SiN or the like. The person skilled in the art is conscious about the correct molecule formula of SiOx and SiyNx, which are here abbreviated with SiO and SiN, respectively.

According to an embodiment, the first contact region <NUM> at least partially or completely surrounds and electrically contacts the first capacitor electrode <NUM>. Thereby, the first contact region <NUM> forms an area contact region or a point contact region with the first capacitor electrode <NUM>. The same may apply to the second contact region <NUM>, i.e. the second contact region <NUM> forms an area contact region or a point contact region with the second capacitor electrode <NUM> or with the second and a third capacitor electrode <NUM>, <NUM>. As shown in <FIG>, the first contact region <NUM> may be a full contact, surrounding fully or completely the gas sensitive layer <NUM>. Also, the first contact region <NUM> may be a partial contact, only partially surrounding the gas sensitive layer <NUM> (<FIG>). Furthermore, the first contact region <NUM> may an area contact, having an area that is in contact with or next to the gas sensitive layer <NUM> (<FIG>). Furthermore, the first contact region <NUM> may be generated by a point contact, wherein only a point is in contact with the gas sensitive layer <NUM> (<FIG>). Furthermore, the first contact region <NUM> may be generated by any combination of the disclosed different kinds of contact (<FIG>). The disclosed example of the different contact regions, as shown on <FIG> may be used for the first capacitor electrode <NUM> and/or for the second capacitor electrode <NUM>. The contact region <NUM> may be made of a metal or any other electrically conducting material.

According to an embodiment, on top of the insulation layer <NUM> a cover layer <NUM> is disposed. The cover layer <NUM> has at least a common plane with the first contact region <NUM> and/ or with the first capacitor electrode <NUM>, so that the cover layer <NUM> surrounds the first contact region <NUM> or the first contract region <NUM> and the first capacitor electrode <NUM> in the at least one common plane. Such embodiments are, for example shown in <FIG>, <FIG>, <FIG>, <FIG> and <FIG>. Stated differently, along the x-direction the cover layer <NUM> and at least the first contract region <NUM> and the first capacitor electrode <NUM> share a common plane, i.e. the cover layer <NUM> and at least the first contract region <NUM> and the first capacitor electrode <NUM> lie in the same common plane(s).

According to an embodiment, the first contact region <NUM> extends from the first capacitor electrode <NUM> in the at least one common plane to a position above the cover layer <NUM> being in at least a plane parallel to the at least one common plane. As for example shown in the <FIG>, <FIG>, <FIG>, <FIG> and <FIG>, starting from the first capacitor electrode <NUM> the first contact region <NUM> contacts a first border region of the first capacitor electrode and extends along the -x direction (here the contact region extends on top of the insulation layer <NUM> in -x direction). Then first contact region <NUM> extends along the z direction and then along again the -x direction, i.e. the first contact region <NUM> extends on top or above the cover layer <NUM>.

The extension along the z direction corresponds to a thickness of the cover layer <NUM> along the z-direction. <FIG>, <FIG>, <FIG> and <FIG> for example show that the first border region of the first capacitor electrode <NUM> has a sloped curvature (i.e. is ramped) along x-z-direction.

All disclosed different kinds of first contact regions <NUM> have in common that the first contact region <NUM> ohmically contacts a first border region of the first capacitor electrode <NUM>, in particular the gas sensitive layer <NUM> of the top electrode. As shown in <FIG>, the first contact region <NUM> contacts the first border region of the first capacitor electrode <NUM> directly.

As shown in <FIG> the second buried capacitor electrode <NUM> may capacitively contact a center region of the first capacitor electrode <NUM>. Also, the buried third capacitor electrode <NUM> may capacitively contact a second border region of the first capacitor electrode <NUM>. The second and third capacitor electrodes <NUM>, <NUM> may be given by different kinds of partial contact, point contact or any combination of different contact form with the first capacitor electrode <NUM>, i.e. the gas sensitive layer <NUM>. For example, the first capacitor electrode <NUM> may be given as a rectangle or square, as an ellipse or circle or any other geometry.

According to a different implementation of the same concept as proposed herein <FIG> show a further embodiment. As shown in <FIG>, the capacitor electrode <NUM> is capacitively contacted by the second capacitor electrode <NUM> and the third capacitor electrode <NUM>, i.e. by the two buried electrodes <NUM>, <NUM>. The buried second capacitor electrode <NUM> and the buried third capacitor electrode <NUM> are arranged in the same plane of the gas sensitive device <NUM>. The second contact region <NUM> is coupled to the second capacitor electrode <NUM> or to the second and third capacitor electrodes <NUM>, <NUM>. The second contact region <NUM> and the second capacitor electrode <NUM> may be arranged in the same plane or in different planes of the gas sensitive device. Preferably, if the second contact region <NUM> and the second capacitor electrode <NUM> or the second and third capacitor electrodes <NUM>, <NUM> are arranged in different planes of the gas sensitive device <NUM>, these planes are parallel to each other. As shown in <FIG> the second and the third capacitor electrodes <NUM>, <NUM> are separated along the x-y-directions by a second insulation layer <NUM> extending between the third capacitor electrode <NUM> and the second capacitor electrode <NUM>.

<FIG> show variations of the implementation of the gas sensitive device <NUM> as shown in <FIG>. The variations as shown in <FIG> show two buried electrodes <NUM>, <NUM>, i.e. the second capacitor electrode <NUM> and the third capacitor electrode <NUM>, which may have the same or a different geometry to each other. For example, <FIG> show implementations in which the second capacitor electrode <NUM> has a different geometry or shape from the third capacitor electrode. For example, <FIG>, 15f and <FIG> show in which the second capacitor electrode <NUM> and the third capacitor electrode have the same geometry or shape, the shapes are only mirrored along an y-axis, said y-axis is located in the middle between the both electrodes <NUM>, <NUM>. In all implementations, the two buried electrodes <NUM>, <NUM> may have a line shape or a line shape combined with a point shape or any circle shape or any circle shape combined with a point shape, wherein the circle shape may comprise a full circle or any sub-circle shape.

<FIG> shows a schematic view of a gas sensitive device <NUM> and of an equivalent circuit of an AC-coupled multi gas sensor <NUM> comprising four gas sensitive devices <NUM> according to an embodiment. The configuration of the gas sensitive device <NUM> is already described with reference to <FIG>. As shown in <FIG>, an equivalent circuit of the AC-coupled multi-gas sensor <NUM> is given by a RC-network. The sheet resistance is depending on the gas sensitive layer <NUM>, in particular depending on the graphene layer. However the resulting adsorption of gas molecules depends on the presence of the according gas molecules and the level of doping with nanoparticles or salts. Different chip layouts may be used for high frequency measurements or for low frequency measurements. For high frequency measurements, which are also called high frequency NWA measurement, the amount a connections is higher than the connections used in the low frequency lock-in measurement technique. For example, for the low frequency lock-in measurement technique each first capacity electrode <NUM> of each gas sensitive device <NUM> has one connection to an outer port <NUM>, while each of the second capacity electrode <NUM> of the gas sensitive device <NUM> has a common single connection to an outer port. This is different to the high frequency NWA measurement technique, where each of the first capacity electrodes <NUM> of each gas sensitive device <NUM> and each of the second capacity electrodes <NUM> of each gas sensitive device <NUM> has two connections to an outer port <NUM>. This means each gas sensitive device <NUM> has four connections to outer ports, two gate-ports and two source ports. The four connections to outer ports enable a <NUM>-Port NWA-measurement in "T-configuration", which neglects the impedances of the connection lines. Preferably, the first capacity electrodes <NUM> are connected to the first port and the second capacity electrodes <NUM> are connected to the second port For such a configuration the following equations are valid: <MAT> <MAT> <MAT> <MAT>.

Here U is the voltage, I the current and Z the impedance. The indices give the current, voltage and impedance according to the elements of the circuit shown in <FIG>. In detail, Z<NUM> is the impedance of the gas sensitive capacitor without any parasitics. Z<NUM> and Z<NUM> are the (parasitic) impedances of the connecting lines. The switching frequency is determined by the sheet resistance of the gas sensitive layer <NUM> and therefore by the adsorbed gas molecules. At low frequencies the sheet resistance of the sensitive layer is directly reflected in the real part of impedance. The gas sensitive capacitor enables different AC measurement capabilities (e.g. Lock-in technic, PPL, S-Parameter, as shown in <FIG>). Compared to simple DC-resistance measurements, one of the advantages of AC-measurement methods is noise reduction. Furthermore, AC-measurement allows to distinguish between real and imagery impedance of the gas sensitive layer <NUM> and delivers therefore more information.

According to an embodiment, the first main surface region of the insulation layer <NUM> having the gas-sensitive layer <NUM> is topology-free. For example, the gas sensitive layer <NUM> is manufactured by dropping an ink drop onto the insulation layer <NUM>. Because the gas-sensitive layer <NUM> according to the present disclosure has no topology, the influence of locally clustered graphene flakes and/or from coffee stain does not affect the gas-sensitive layer <NUM>. The topology-freeness of the gas-sensitive layer <NUM> is shown throughout the Figures except Figs. in <NUM>, <NUM>, <NUM> and <NUM>.

According to an embodiment, the first capacitor electrode <NUM> and the second capacitor electrode <NUM> are arranged parallel to each other and vertical with respect to the first main surface region of the insulation layer <NUM>. With respect to the figure, where a gas sensitive device <NUM> is shown the first capacitor electrode <NUM> and the second capacitor electrode <NUM> are each arranged in parallel x-y planes, i.e. horizontal planes. Vertical means here along a z-direction. The insulation layer <NUM> between the first capacitor electrode <NUM> and the second capacitor electrode <NUM> is also called the first insulation layer <NUM>.

According to an embodiment, a third capacitor electrode <NUM> is disposed apart from the second capacitor electrode <NUM>, wherein the third capacitor electrode <NUM> and the second capacitor electrode <NUM> are positioned so that both extend along in at least one common plane. <FIG>, for example, shows a gas sensitive device <NUM> having the second and the third capacitor electrodes <NUM>, <NUM> lying on top of a second insulation layer <NUM>. On top of the cover layer <NUM> a contact region <NUM> for the second capacitor electrode <NUM> and a contact <NUM> for the third capacitor electrode <NUM> is disposed. For contacting the second and the third electrode <NUM>, <NUM> contact holes <NUM> are provided. The contact holes <NUM> are, for example, etched. The contact holes <NUM> extend along the z-direction through the cover layer <NUM> and through the insulation layer <NUM>.

According to an embodiment, the third capacitor electrode <NUM> and the second capacitor electrode <NUM> are spaced apart from each other in the least one common plane by the second insulation layer <NUM> extending between third capacitor electrode <NUM> and the second capacitor electrode <NUM>. The third capacitor electrode <NUM> and the second capacitor electrode <NUM> are spaced apart from each other by a part of the second insulation layer <NUM> extending between the third capacitor electrode <NUM> and the second capacitor electrode <NUM> along the z-direction. As shown in <FIG>, the second and the third capacitor electrodes <NUM>, <NUM> are sandwiched between the first insulation layer <NUM> and the second insulation layer <NUM>.

According to an embodiment, the first capacitor electrode <NUM> being the gas sensitive layer <NUM> is a thin and/or two-dimensional layer <NUM>, the electrical conductivity of which is influenceable by an interaction with gas molecules. For example, the two-dimensional layer <NUM> may be a conductive graphene based layer. Graphene has a two-dimensional structure as shown in <FIG>. The two-dimensional structure comprises benzene rings, which may be doped with salts and/or nanoparticles <NUM>. The first capacitor electrode <NUM> may also be a not graphene based gas sensitive layer <NUM>. Subject to the condition, that the used material is showing the described behavior. Examples of not graphene based layers <NUM> may be an amorphous Carbon, thin poly silicon, tantalnitride, titannitride, AIScN, or every material which forms a thin or two dimensional layer, which electrical conductivity can be influenced by the interaction with gas molecules.

According to an embodiment, the gas sensitive layer <NUM> or the thin and/or two-dimensional layer <NUM>, in particular a conductive graphene based layer, is doped with nanoparticles and/or doped with salts for functionalizing the first electrode <NUM>. A concentration of the doped nanoparticle or of the salts correlate to a concentration accuracy with which the adsorbed gas molecules can be measured. The nanoparticle and/or the salt used for doping determine which gas may be adsorbed. Stated differently, by changing the dopant another gas may be detectable.

According to an embodiment, the gas sensitive device <NUM> comprises a heater <NUM> disposed or positioned in the gas sensitive device <NUM>. The heater is used for bringing the gas-sensitive layer <NUM> to a desired temperature or temperature profile for sensing, or for resetting the gas-sensitive layer <NUM>. The heater <NUM> and the second capacitor electrode <NUM> are separated from each other by the second insulation layer <NUM>. The gas sensitive device <NUM> comprises a third contact region <NUM> for electrically contacting the heater <NUM>, as for example shown in <FIG>, <FIG> or <FIG>. The heater <NUM> can be used for resetting or cleaning the gas sensitive layer <NUM>. Cleaning or resetting means in this context to undo the adsorption of the gas molecules. The heater <NUM> comprises at least one side that at least partially forms with at least a side of the substrate layer <NUM> a cavity <NUM>, as for example shown in <FIG>. The <FIG> and <FIG> show further examples of the cavity <NUM>. In <FIG>, for example, the cavity <NUM> comprises at least one side of the substrate structure <NUM>, at least one side of the heater <NUM> and at least one side of the second insulation layer <NUM>. In <FIG>, for example, the cavity <NUM> comprises at least one side of the substrate structure <NUM>, at least one side of the heater <NUM>, at least one side of the second insulation layer <NUM> and at least one side of the second capacitor electrode <NUM>. The cavity <NUM> is provided for thermal decoupling. In order to keep the necessary heating power small, to reach a target temperature. In <FIG>, for example, the cavity <NUM> is shown.

<FIG> show different possible examples of where the heater <NUM> may be located in the gas sensitive device <NUM>. A layout of the heater <NUM> can deviate from the examples as shown in the Figures, in particular the heater <NUM> may have different geometries. For example, <FIG> and <FIG> show that the heater <NUM> is located below the second capacity electrode <NUM>. According to <FIG>, the heater <NUM> extends in a x-y plane parallel to the second capacity electrode <NUM>. The heater <NUM> is at least as long and as broad as the second capacity electrode <NUM>. Stated differently, a diameter of the heater <NUM> in the x-y plane is either as large as or even larger than a diameter of the second capacity electrode <NUM>. According to <FIG>, the heater <NUM> extends also in a x-y plane parallel to the second capacity electrode <NUM>. However, the heater <NUM> forms a ring around the buried electrode. An inner perimeter of the heater ring is preferably lager than an outer perimeter of the buried electrode <NUM>. Stated differently, the heater-ring <NUM> does not contact the buried electrode <NUM>. In the cross section as shown in <FIG> the heater-ring seems like an interrupted heater structure. In the cross sectional view of <FIG>, a length of the heater, in particular in the x-y plane, may be shorter than a length of the buried electrode. Furthermore, the heater <NUM> is not directly located beneath the second capacity electrode <NUM>. Instead, the heater <NUM> and the second capacity electrode <NUM> build a kind of a chessboard pattern with the substrate structure <NUM>. According to <FIG> the heater <NUM> is again formed as a ring and surrounds the second capacity electrode <NUM>, i.e. the heater <NUM> and the second capacity electrode <NUM> are disposed in the same plane. Other constructions or any combination of the shown examples are also possible. As shown in <FIG>, the heater <NUM> and the buried electrode <NUM> may be electrically separated by the insulator layer <NUM> or <NUM>, for example by SiO or SiN.

According to an embodiment, the gas sensitive device <NUM> may be provided with a plurality of first electrodes <NUM> on top of the insulator layer <NUM> and/or may be provided with a plurality of second electrodes <NUM> below the insulator layer <NUM>. Of course, the gas sensitive device <NUM> may also be provided with both, a plurality of first and second contact regions <NUM>, <NUM>. For the sake of simplicity, however, the Figures except Figs. in <NUM>, <NUM>, <NUM> and <NUM> of the present application only show a single first contact region <NUM> and a single second contact region <NUM>.

According to another aspect of the present disclosure a multi-gas sensor <NUM> is proposed. The multi-gas sensor comprises two or more gas sensitive devices <NUM> as disclosed herein. <FIG> shows an exemplary embodiment of a multi-gas sensor <NUM>. The multi-gas sensor <NUM> shown in <FIG> for example is provided with four gas sensitive devices <NUM>. Each of the gas sensitive devices <NUM> may be provided with a graphene layer or gas sensitive layer <NUM> that may be doped with different kinds of nanoparticles and/or salts. In this way, the multi-gas sensor <NUM> may become sensitive to the detection of different gas molecules and/or may become sensitive to different kinds of concentration levels of a particular gas molecule.

According to another aspect of the present disclosure, a method for operating the gas sensitive device <NUM> is disclosed. <FIG> shows a flow chart of the principles of method for operating the gas sensitive device <NUM>. The method <NUM> comprises applying an AC signal to the first capacitor electrode <NUM> according to a step <NUM> and reading out a signal between the first capacitor electrode <NUM> and the second capacitor electrode <NUM> according to step <NUM>. The signal read out comprises an information on the sheet resistance of the gas-sensitive layer <NUM> of the first capacitor electrode <NUM> due to the adsorbed or desorbed gas molecules.

<FIG> shows simulation results of different AC-measurement capabilities of the AC-coupled multi gas sensor when performing the proposed method <NUM>. If measurements at low frequencies f are preformed, a change of the impedance Z will be detected. <FIG> show results for measurements or simulations at low frequencies f. In <FIG> the impedance Z is plotted against the applied frequency f of the AC-signal. In <FIG> the measured or simulated impedance Z is plotted against its corresponding sheet resistance R. The sheet resistance R is the real part of the sheet impedance Z that comprises an imaginary part and a real part. At low frequencies, i.e. about f=<NUM><NUM> Hz (see also the ellipse <NUM>), the impedance change z and/or the resistance change R can be measured. The impedance change Z is correlated to the amount of adsorbed gas molecules. The impedance change Z may give a hint on the level of adsorbed gas molecules that are adsorbed by the gas sensitive layer <NUM>. <FIG> shows the deviation of the sheet resistance in % (in percentage) form the normal value of the sheet resistance. For example, the normal values of the sheet resistance of the gas sensitive layer <NUM> is given by the sheet resistance in atmosphere without pollution. Without pollution means without adsorbed molecules of the pollution, for example NOx, Os, CO<NUM>, CO or volantile organic components. The sheet resistance itself is always positive, however the deviation of the reference sheet resistance with the presence of adsorbed pollution molecules may be positive, zero or negative as can be seen in <FIG>. The sheet impedance at a low frequency f is correlated with the deviation of the sheet resistance R. The size of the value of the deviation of the sheet resistance may depend on the amount of adsorbed gas molecules.

In the lower Figures of <FIG> (<FIG>) the capacity C is plotted against the applied frequency f of the AC-signal. <FIG> show results for measurements or simulations at high frequencies f. High frequencies f are about <NUM><NUM> Hz. At high or higher frequencies a detection of capacity changes will appear. For example, at frequencies between <NUM><NUM> Hz and <NUM><NUM> Hz, the capacity change C is correlated with the change of the sheet resistance R. At a sheet resistance change of ± <NUM>% at about <NUM><NUM> Hz the capacity change C is measured to be the highest. Where no sheet resistance change R occurs, i.e. at a sheet resistance change of <NUM>%, no capacity change occurs.

From the results shown in <FIG> it is derivable that a relation between the sheet resistance of the first electrode <NUM> and the capacitive impedance of the insulator layer <NUM> that is directly disposed between the first electrode <NUM> and the second electrode <NUM> causes a frequency dependency of the capacitance and the impedance measurements. The proposed method may further comprise applying an AC-signal to the gas sensitive capacitor; and measuring the capacitance / impedance of the whole area of the first electrode <NUM> at low frequencies, or measuring the capacitance at the edge of the first electrode <NUM> at high frequencies. By reading out the signal information about a concentration of adsorbed or desorbed molecules may be derived.

The present proposed gas sensitive device, which may be incorporated into a multi-gas sensor, provides an easy and cheap approach to determine the pollution of the ambient atmosphere with different kinds of gases, such a ozone Os, carbon monoxide CO, nitrogen dioxide NO<NUM> and many more different gases. The principle of the present disclosure is to provide the top electrode with a functionalized graphene layer, so that depending on the doped nanoparticles and/or doped salts different kinds of gas molecules can be detected. The top electrode thereby is provided by an ink drop with no specific topology. In this way, a gas sensitive device is provided that is independent from locally clustered graphene flakes and/or from coffee stain. Therewith, an approach to implement an improved gas sensitive device is disclosed.

Although some aspects have been described as features in the context of an apparatus it is clear that such a description may also be regarded as a description of corresponding features of a method. Although some aspects have been described as features in the context of a method, it is clear that such a description may also be regarded as a description of corresponding features concerning the functionality of an apparatus.

Additional embodiments and examples are described which may be used alone or in combination with the features and functionalities described herein.

Depending on certain implementation requirements, embodiments of the processing device can be implemented in hardware or in software or at least partially in hardware or at least partially in software. Some embodiments comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

Generally, embodiments of the processing device can be implemented as a computer program product with a program code, the program code being operative for performing one of the not claimed methods when the computer program product runs on a computer. Other examples, not falling within the scope of the claims, comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier. In other words, an example of the not claimed method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.

A further example, not falling within the scope of the appended claims, of the methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or non-transitory. A further example, not falling within the scope of the claims, comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. A further example, not falling within the scope of the claims, comprises a computer having installed thereon the computer program for performing one of the methods described herein.

In some examples, not falling within the scope of the claims, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some examples, not falling within the scope of the claims, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein.

Claim 1:
A gas sensitive device (<NUM>), comprising:
a substrate structure (<NUM>), and
a gas sensitive capacitor, the gas sensitive capacitor comprising
a first capacitor electrode (<NUM>) in form of a gas-sensitive layer (<NUM>) on a first main surface region of an insulation layer (<NUM>), and
a second capacitor electrode (<NUM>) in form of a buried conductive region below the insulation layer (<NUM>), so that
the insulation layer (<NUM>) is arranged between the first and second capacitor electrode (<NUM>, <NUM>);
a first contact region (<NUM>) for electrically contacting the first capacitor electrode (<NUM>), and
a second contact region (<NUM>) for electrically contacting the second capacitor electrode (<NUM>); wherein
the gas-sensitive layer (<NUM>) comprises a sheet impedance which changes in response to the adsorption or desorption of gas molecules;
wherein
on top of the insulation layer (<NUM>) a cover layer (<NUM>) is disposed, so that the cover layer (<NUM>) has at least a common plane with the first contact region (<NUM>) and/ or with the first capacitor electrode (<NUM>) so that the cover layer (<NUM>) surrounds completely the first contact region (<NUM>) or the first contact region (<NUM>) and the first capacitor electrode (<NUM>) in the at least one common plane; wherein a thickness of the first capacitor electrode (<NUM>) being the gas-sensitive layer (<NUM>) is thinner than a thickness of the cover layer (<NUM>), both thicknesses extending along a z-direction perpendicular to the cover layer (<NUM>), wherein the gas-sensitive layer (<NUM>) is formed by dropping an ink drop onto the insulation layer (<NUM>).