NL2011845C2 - A capacitive sensor for humidity and/or ammonia sensing. - Google Patents

Description

A capacitive sensor for humidity and/or ammonia sensing

The present invention relates to a capacitive sensor for humidity and/or ammonia sensing comprising: a substrate, a first- and a second- electrode, and a dielectric material permeable to water and ammonia, wherein the first- and second-electrodes and the substrate constitute a capacitance, and wherein the dielectric material is provided on the substrate and separates the first- and second- electrodes.

Such capacitive sensors have been described in the academic literature. See e.g. E.J. Connolly et al., Sensors and Actuators B, 2005, 109, 44.

The basic structure of a capacitive humidity and/or ammonia sensor of the prior art, such as of the above identified document comprises, as sequential layers and from bottom to top: a bottom electrode, a silicon substrate, a porous dielectric material and a top electrode. For the case of humidity sensing, variations in ambient humidity causes die condensation of water vapour in the porous dielectric material, changing its permittivity and therefore the capacitance. For the case of ammonia sensing, it is not yet clear exactly what the sensing mechanism is, but possibly, due to a small voltage applied during capacitance measurements, a thin depletion layer is formed on the surface of the dielectric material. Ammonia ' molecules passing over this depletion layer might be decomposed, and subsequently hydrogen atoms absorb onto this depletion layer, thus changirig the capacitance.

In applications such as pig-farming, automotive and process industry, there is a large market for ammonia and humidity sensors. These can be harsh environments for electronics and at present there are no sensors on the market that have the required long-term reliability.

It is an object of the present invention to provide a capacitive sensor for humidity and/or ammonia that is more reliable, compact and cheaper compared to sensors of the prior art.

In a first aspect, the present invention relates to a capacitive sensor for humidity and/or ammonia sensing comprising: a substrate, a first- and a second- electrode, and a die lectric material permeable to water and ammonia, wherein the first- and second- electrodes and the substrate constitute a capacitance, and wherein the dielectric material is provided on the substrate and separates the first- and second- electrodes, wherein the first and second- electrodes are buried within the dielectric material.

The term buried signifies that the electrodes are not at a surface of the sensor exposed to the direct environment of the sensor, but rather are within and are covered by the dielectric material.

By providing the first- and second- electrodes within the dielectric material, the electrodes are shielded from the environment in which the sensor is.being operated. As a result, the electrodes are less susceptible to corrosion and cheap electrode materials can be used without having consequence for the long-term reliability of the sensor.

Humidity and ammonia sensors of the prior art, such as the sensor of E.J. Connolly et al., Sensors and Actuators B, 2005, 109, 44 have an electrode at an environment-exposed surface thereof; in order to impart reasonable long-term reliability, chemically inert electrode materials are preferably used; these are typically very costly. Furthermore, even if the electrode is chemically inert, electrical connections between the electrode and a circuit of the sensor may be susceptible to corrosion, again resulting in poor long-term reliability of the sensor.

In a preferred embodiment, the dielectric material is porous silicon carbide.

Silicon carbide is well-known to be able to withstand harsh chemical conditions such as those found in the environments in which sensors for humidity and ammonia sensors are required. Furthermore, the inventors have found porous silicon carbide, when used as a dielectric in a capacitive sensing arrangement to be sensitive to the presence of both ammonia gas and humidity.

Silicon carbide can be made porous by e.g. electrochemical etching- of SiC in HF. Adjusting the processing allows the sensitivity to both ammonia and humidity to be tuned e.g. by controlling the pore-size. A preferred dielectric material is low temperature plasma enhanced chemical vapour deposition silicon carbide. Such is fully compatible with CMOS processing and is also low cost.

In a preferred embodiment, the substrate is impermeable to water and ammonia.

Such serves to further protect the first- and second-electrodes from corrosion.

In a preferred embodiment, the first and second electrodes are aluminium electrodes. Aluminium is a low-cost material that can be easily shaped into the correct electrode architecture .

In a preferred embodiment, the thickness of the dielectric material is in the range of 0.1 to 10 μιη and preferably in the range of 0.2 to 1 μιη. Layers of dielectric material having a thickness falling within these ranges have been shown to perform very well when incorporated into capacitive sensors and are compatible with sensor manufacturing processes.

In a preferred embodiment, the dielectric material has a first humidity sensing population of pores having diameters in the range of 1 nm to 100 nm and/or a second ammonia sensing population of pores having diameters in the range of 100 nm to 500 nm. It has been found that for humidity sensing, the reguirements are to have a pore size in the range of 1 nm to 100 nm. Wherein the dielectric material of the sensor has substantially only pores having diameters > 100 nm, an ammonia sensor results. Having first- and second- populations permits the relative sensitivities to ammonia and humidity to be tuned and opens up' the possibility of having a dual sensor. A dual sensor preferably has first- and second- populations of pores in a ratio of 1:1.

In a preferred embodiment, the thickness of the substrate is in the range of 0.1-10 μιη. The substrate is preferably thin in the region of the dielectric material to facilitate cleaning. As an example, heat can be applied via a heating element through the substrate. The heating element may also be used to control the temperature for constant capacitance measurements .

In a preferred embodiment, the first- and second-electrodes are on the substrate. Such facilitates manufacture of the device.

In a preferred embodiment, the first- and second-electrodes extend substantially over a cross-section of the dielectric material. By applying a short pulse of current to the electrodes, the electrodes can serve as a heating element as described above for cleaning the sensor and/or to control the temperature for constant capacitance measurements. Thus a separate heating element does not need to be provided and furthermore the heat will be more effectively transferred to the dielectric material since the electrodes are buried therein.

In a preferred embodiment, a surface of the first-and second- electrodes is provided with a shielding layer for protecting the first- and second- electrodes from corrosion.

In a preferred embodiment, the capacitive sensor further comprises a circuit for determining the capacitance.

In a second aspect, the present invention relates to a humidity and ammonia sensing device comprising at least a first- and a second- capacitive sensor according to the foregoing, wherein a dielectric material of the first capacitive sensor has pores having diameters in the range of 1 to 100 nm and wherein a dielectric material of the second capacitive sensor has pores having diameters in the range of 100 to 500 nm.

The present invention is hereafter further elucidated with reference to the Drawing. The Drawing relates to an exemplary embodiment of the sensor of the invention only and should not be considered to limit the scope of the claims.

Fig. 1 shows a basic structure of a capacitive sensor for humidity and/or ammonia according to the invention.

Fig. 2 shows a preferred electrode configuration.

Fig. 3 shows a preferred embodiment of a capacitive sensor for humidity and/or ammonia according to the invention. '

With reference to Fig. 1, a basic structure of a capacitive sensor for humidity and/or ammonia sensing 1 is shown comprising: a substrate 2, a first- and a second- electrode 3,4, and a dielectric material 5 permeable to water and ammonia, wherein the first- and second- electrodes 3,4 and the substrate 2 constitute a capacitance, and wherein the dielectric material 5 is provided on the substrate 2 and separates the first- and second- electrodes 3,4, wherein the first- and second- electrodes 3,4 are buried within the dielectric mate rial 5.

The electrodes 3,4 are preferably arranged as shown in Fig. 2. This configuration enables the device to be used as a capacitor and also a heating resistor. The two electrodes are in a horizontal plane of the device at an interface with the substrate 2. The first- and second- electrodes 3,4 extend substantially over a cross-section X of the dielectric material 5 as shown in Figures 1 and 3.

The capacitive sensor for humidity and/or ammonia according to the invention 1 is fully compatible with CMOS device structure and technology, and can be incorporated onto standard silicon wafers (the substrate 2) as shown in Fig. 3. The substrate (2) is preferably etched back at the location of the dielectric material 5 to form a membrane having a thickness in the range of 0.1 to 10 pm (i.e. the substrate 2 has a thickness in the range of 0.1 to 10 pm over an interface with the dielectric material). The reason for using a membrane is so that a short current pulse through the electrodes 3,4 would heat the device 1 resulting in a reset to zero. The heater will also be used to control the temperature for constant capacitance measurements. Accurate temperature control can be achieved with the proposed structure since the temperature measurement can be incorporated into the membrane. Ambient temperature will be measured with a second temperature sensor in die bulk silicon. The membrane structure will lead to several advantages over die bulk devices in terms of lower power consumption and functionality. ' In the device shown in Fig. 3, the dielectric materi al 5 is silicon carbide (having a thickness in the range of 0.1 to 10 pm and preferably in the range of 0.2 to 1 pm), the electrodes 3,4 are aluminium electrodes and the substrate 2 is a standard p-type silicon wafer. The basic process is therefore that first the electronics 3,4 are processed on a standard wafer. The metal for the electrodes 3,4 can either be the metal used in the standard IC or an additional metal layer. Silicon carbide is then deposited over the whole wafer. This will form the dielectric material 5 permeable to humidity and ammonia for the device 1 and also a robust coating layer for the electronics. The wafer is then etched back in the areas of the sensor 1 stopping on the SiC and metal. An additional met- al layer is deposited, which will form the counter electrode for porous formation. After porous formation, this layer is removed and a final coating is placed on the back side of the membrane. Optionally, a SiN shielding layer (not shown) is provided on a top-surface only of the electrodes 3,4 as further protection against corrosion.

In the preferred embodiment of the invention shown in Figure 3, the dielectric material 5 has a first humidity sensing population of pores having diameters in the range of 1 nm to 100 nm and a second ammonia sensing population of pores having diameters in the range of 100 nm to 500 nm (in a ratio of 1:1) . This allows a single sensor 1 to be used for detecting both ammonia and humidity. As an alternative, two sensors 1 according to the invention can be incorporated into a humidity and ammonia sensing device wherein a dielectric material 5 of the first capacitive sensor has pores having diameters in the range of 1 to 100 nm and wherein a dielectric material 5 of the second capacitive sensor has pores having diameters in the range of 100 to 500 nm.

Although the invention has been discussed in the foregoing with reference to an exemplary embodiment of the sensor of the invention, the invention is not restricted to this particular embodiment which can be varied in many ways without departing from the gist of the invention. The discussed exemplary embodiment shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary the embodiment is merely intended to explain the wording of the appended claims without intent to limit the claims to this exemplary embodiment. The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible ambiguity in the wording of the claims shall be resolved using this exemplary embodiment.

Claims (13)

1. Een capacitieve sensor (1) voor vocht en/of ammoniameting omvattende: een substraat (2), een eerste en een tweede elektrode (3, 4), en een diëlektrisch materiaal (5) doorlaatbaar voor water en ammonia, waarbij de eerste en tweede elektroden (3, 4) en het substraat (2) een condensator (C) vormen, en waarbij het diëlektrisch materiaal (5) voorzien is op het substraat (2) en de eerste en tweede elektroden (3, 4) van elkaar scheidt, met het kenmerk, dat de eerste en tweede elektroden (3, 4) begraven zijn binnen het diëlektrisch materiaal (5) .A capacitive sensor (1) for moisture and / or ammonia measurement comprising: a substrate (2), a first and a second electrode (3, 4), and a dielectric material (5) permeable to water and ammonia, the first and second electrodes (3, 4) and the substrate (2) form a capacitor (C), and wherein the dielectric material (5) is provided on the substrate (2) and the first and second electrodes (3, 4) apart separates, characterized in that the first and second electrodes (3, 4) are buried within the dielectric material (5). 2. Capacitieve sensor (1) volgens conclusie 1, met het kenmerk, dat het diëlektrisch materiaal (5) poreus silicium carbide is.Capacitive sensor (1) according to claim 1, characterized in that the dielectric material (5) is porous silicon carbide. 3. Capacitieve sensor (1) volgens één of meer van de voorgaande conclusies, met het kenmerk, dat het substraat (2) ondoorlaatbaar is voor water en ammonia.Capacitive sensor (1) according to one or more of the preceding claims, characterized in that the substrate (2) is impermeable to water and ammonia. 4. Capacitieve sensor (1) volgens één of meer van de voorgaande conclusies, met het kenmerk, dat de eerste en tweede elektroden (3, 4) aluminium elektroden zijn.Capacitive sensor (1) according to one or more of the preceding claims, characterized in that the first and second electrodes (3, 4) are aluminum electrodes. 5. Capacitieve sensor (1) volgens één of meer van de voorgaande conclusies, met het kenmerk, dat de dikte van het diëlektrisch materiaal (5) in het bereik 0.1 to 10 pm ligt en bij voorkeur in het bereik van 0.2 tot 1 pm.Capacitive sensor (1) according to one or more of the preceding claims, characterized in that the thickness of the dielectric material (5) is in the range 0.1 to 10 µm and preferably in the range of 0.2 to 1 µm. 6. Capacitieve sensor (1) volgens conclusie 5, met het kenmerk, dat het diëlektrisch materiaal (5) een eerste vochtgevoelige verzameling poriën bezit met diameters in het bereik van 1 nm tot 100 nm en/of een tweede ammonia gevoelige verzameling poriën met diameters in het bereik van 100 nm tot 500 nm.Capacitive sensor (1) according to claim 5, characterized in that the dielectric material (5) has a first moisture-sensitive collection of pores with diameters in the range of 1 nm to 100 nm and / or a second ammonia-sensitive collection of pores with diameters in the range of 100 nm to 500 nm. 7. Capacitieve sensor (1) volgens conclusie 6, met het kenmerk, dat de eerste en tweede verzamelingen van poriën in een ratio 1:1 aanwezig zijn.Capacitive sensor (1) according to claim 6, characterized in that the first and second sets of pores are present in a ratio of 1: 1. 8. Capacitieve sensor (1) volgens één of meer van de voorgaande conclusies, met het kenmerk, dat de dikte van het substraat in het bereik 0.1 tot 10 pm ligt.Capacitive sensor (1) according to one or more of the preceding claims, characterized in that the thickness of the substrate is in the range 0.1 to 10 µm. 9. Capacitieve sensor (1) volgens één of meer van de voorgaande conclusies, met het kenmerk, dat de eerste en tweede elektroden zich op het substraat (2) bevinden.Capacitive sensor (1) according to one or more of the preceding claims, characterized in that the first and second electrodes are on the substrate (2). 10. Capacitieve sensor (1) volgens één of meer van de voorgaande conclusies, met het kenmerk, dat de eerste en tweede elektroden (3, 4) in hoofdzaak zich uitstrekken over een dwarsdoorsnede (X) van het diëlektrisch materiaal (5).Capacitive sensor (1) according to one or more of the preceding claims, characterized in that the first and second electrodes (3, 4) extend substantially over a cross-section (X) of the dielectric material (5). 11. Capacitieve sensor (1) volgens één of meer van de voorgaande conclusies, met het kenmerk, dat een oppervlak van de eerste en tweede elektroden (3, 4) voorzien is van een af-schermlaag voor het beschermen van de eerste en tweede elektroden (3, 4) tegen corrosie.Capacitive sensor (1) according to one or more of the preceding claims, characterized in that a surface of the first and second electrodes (3, 4) is provided with a shielding layer for protecting the first and second electrodes (3, 4) against corrosion. 12. Capacitieve sensor (1) volgens één of meer van de voorgaande conclusies, met het kenmerk, dat de capacitieve sensor (1) verder een schakeling omvat voor het bepalen van de capaciteit.A capacitive sensor (1) according to one or more of the preceding claims, characterized in that the capacitive sensor (1) further comprises a circuit for determining the capacitance. 13. Een vocht- en ammoniagevoelige inrichting omvattende ten minste een eerste en tweede capacitieve sensor volgens één of meer van de voorgaande conclusies, waarbij een di-elektrisch materiaal van de eerste capacitieve sensor poriën bezit met diameters in het bereik van 1 tot 100 nm en waarbij een diëlektrisch materiaal van de tweede capacitieve sensor poriën bezit met diameters in het bereik van 100 tot 500 nm.A moisture and ammonia sensitive device comprising at least a first and second capacitive sensor according to one or more of the preceding claims, wherein a dielectric material of the first capacitive sensor has pores with diameters in the range of 1 to 100 nm and wherein a dielectric material of the second capacitive sensor has pores with diameters in the range of 100 to 500 nm.