Gas sensor

Abstract

We disclose herein a sensing device comprising a substrate, a dielectric layer located on the substrate, a heater located within the dielectric layer; a material for sensing a gas. The material comprises an alumina (Al.sub.2O.sub.3) doped conductive metal oxide.

Claims

1. A sensing device comprising: a substrate; a dielectric layer located on the substrate; a heater located within the dielectric layer; and a material for sensing a gas, wherein the material comprises an alumina (Al.sub.2O.sub.3) doped conductive metal oxide, wherein the sensing device is an NO.sub.2 sensing device, wherein the conductive metal oxide is any one of the group consisting of tungsten oxide, zinc oxide, indium oxide, titanium oxide, chromium oxide, copper oxide and tin oxide, and wherein an alumina doping concentration is between 1% and 10% within the conductive metal oxide.

2. The sensing device according to claim 1, wherein the conductive metal oxide comprises a combination of the metal oxides.

3. The sensing device according to claim 1, wherein the alumina doping concentration is between 2.6% and 3% within the conductive metal oxide.

4. The sensing device according to claim 1, wherein a capacitance and/or a resistance of the material for sensing a gas is sensitive to a presence of a gas.

5. The sensing device according to claim 1, wherein the heater has an interdigitated configuration.

6. The sensing device according to claim 1, wherein the substrate comprises an etched portion and a substrate portion, wherein the dielectric layer comprises a dielectric membrane, wherein the dielectric membrane is adjacent to the etched portion of the substrate; and wherein the heater is located within the dielectric membrane.

7. The sensing device according to claim 6, wherein the material for sensing a gas is located in one side of the dielectric membrane.

8. A sensing array device comprising: a two dimensional array of a plurality of sensing devices according to claim 1.

9. The sensing array device according to claim 8, wherein the sensing array device comprises: at least one sensing device comprising a material for sensing a gas comprising an alumina doped metal oxide, wherein a metal oxide is a first metal oxide; and at least one sensing device comprising a material for sensing a gas comprising an alumina doped metal oxide, wherein a metal oxide is a second metal oxide, and wherein the first metal oxide and the second metal oxide are different metal oxides.

10. The sensing device according to claim 1, wherein the sensing device is configured to operate at a temperature of approximately 200 C.

11. The sensing device according to claim 1, further comprising an electrode underneath the material for sensing gas.

12. The sensing device according to claim 11, wherein the electrode is configured to measure a resistance and/or a capacitance of the material for sensing gas.

13. The sensing device according to claim 1, wherein the sensing device is a CMOS based micro-hotplate in which the heater comprises a CMOS interconnect metal and the dielectric layer comprises a CMOS dielectric layer.

14. A method of sensing a gas using the sensing device according to claim 1, the method comprising: measuring a value of a capacitance and/or a resistance of the material for sensing a gas, wherein the material comprises an alumina (Al.sub.2O.sub.3) doped conductive metal oxide.

15. A method of manufacturing a sensing device, the method comprising: forming a substrate; forming a dielectric layer disposed on the substrate; forming a heater within the dielectric layer; and forming a material for sensing a gas, wherein the material comprises an alumina (Al.sub.2O.sub.3) doped conductive metal oxide, wherein the sensing device is an NO.sub.2 sensing device, wherein the conductive metal oxide is any one of the group consisting of tungsten oxide, zinc oxide, indium oxide, titanium oxide, chromium oxide, copper oxide and tin oxide, and wherein an alumina doping concentration is between 1% and 10% within the conductive metal oxide.

Description

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) Some preferred embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a cross section of a gas sensor with an Al.sub.2O.sub.3 doped metal oxide gas sensing material;

(3) FIG. 2 shows a cross section of an alternative gas sensor with an Al.sub.2O.sub.3 doped metal oxide gas sensing material, in which the substrate does not have an etched portion;

(4) FIG. 3 illustrates an exemplary flow diagram outlining the manufacturing method of the gas sensor; and

(5) FIG. 4 shows results from a gas functionality test on sensors with Al.sub.2O.sub.3 doped SnO.sub.2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) Generally speaking, the disclosure relates to gas sensing devices, including a gas sensing material comprising an alumina (Al.sub.2O.sub.3) doped metal oxide. This allows improved sensitivity to specific gases in the environment, e.g. NO.sub.2, at a reduced sensor operating temperature. It will be understood that the gas sensing material is a powder and a material vehicle mixed together. When it is exposed to a sufficiently high temperature the material formulation can transform into a porous sensing structure or a sensing layer, which is referred to as the gas sensing material. Generally speaking, the metal oxide material is formed from a formulation comprising metal oxide powder (particulate) and a vehicle solvent. When the metal oxide powder and vehicle solvent is mixed together it forms a paste/ink which is then deposited over a sensing electrode. After the deposition, at a high temperature (e.g. about 300 C.), the vehicle solvent is evaporated and/or decomposed from the ink/paste. This is then followed by a ramp to a higher temperature, for example about 600 C., which forms a stable, porous sensing material including metal oxide. In this disclosure, the sensing material is doped with alumina (Al.sub.2O.sub.3) for NO.sub.2 detection.

(7) FIG. 1 shows a cross section of a gas sensor with an Al.sub.2O.sub.3 doped metal oxide gas sensing material. The gas sensor comprises a dielectric membrane 8 supported by a semiconductor substrate 1 which has an etched portion and a substrate portion. Generally speaking, the dielectric membrane area 8 is immediately adjacent to the etched portion of the substrate 1. The dielectric membrane area 8 corresponds to the area above the cavity of the substrate 1. The heater 2 and heater tracks (or metallization) 3 are embedded within the dielectric layer 5, wherein the heater 2 is formed within the dielectric membrane area 8. Electrodes 7 are formed on top of the dielectric membrane 8. The electrodes 7 connect to a gas sensing material 17 which has been grown or deposited on the membrane 8. A passivation layer 6 is formed on top of the dielectric layer 5.

(8) The gas sensing material 17 is disposed on the electrode 7. The electrode 7 is configured to measure resistance and/or capacitance of the gas sensing material 17.

(9) The gas sensing material 17 can be alumina (Al.sub.2O.sub.3) doped tin oxide (SnO.sub.2). Alternatively the gas sensing material can be an Al.sub.2O.sub.3 doped metal oxide such as tungsten oxide (WO.sub.3), zinc oxide (ZnO), indium oxide (In.sub.2O.sub.3), titanium oxide (TiO), or copper oxide (CuO). The doping concentration is a low doping concentration using a solid state doping technique. The doping concentration is preferably between 2.6% to 3% Al.sub.2O.sub.3, however the doping concentration can be anywhere between 1% and 10% Al.sub.2O.sub.3 within the metal oxide material.

(10) The sensing material 17 allows the detection of nitrogen dioxide (NO.sub.2) at low concentrations. The gas sensor can detect NO.sub.2 at concentrations as low as, for example, 50 parts per billion (ppb), due to the low concentration alumina doping of the metal oxide gas sensing material. At a temperature of around 200 C. the gas sensor is highly selective to NO.sub.2. Around this temperature the gas sensor has a low sensitivity to other Volatile Organic Compounds (VoC) and reducing gases present. As the gas sensing material does not comprise noble metals, and the gas sensor has a low operating temperature, the gas sensor has a reduced probability of poisoning due to Siloxane.

(11) FIG. 2 shows a cross section of an alternative gas sensor with an Al.sub.2O.sub.3 doped metal oxide gas sensing material. Many of the features of FIG. 2 are similar to those of FIG. 1 and therefore carry the same reference numerals. In this embodiment, the substrate 1 does not have an etched portion. The dielectric layer 5 does not have a dielectric membrane. The heater 2 and heater tracks (or metallization) 3 are embedded within the dielectric layer 5. Electrodes 7 are formed on top of the dielectric layer 5. The electrodes 7 connect to a gas sensing material 17 which has been grown or deposited on the dielectric layer 5. A passivation layer 6 is formed on top of the dielectric layer 5.

(12) FIG. 3 illustrates a flow diagram outlining the manufacturing method of the gas sensor. The steps are as follows: 1. In S100, begin with a semiconductor substrate. The substrate comprises an etched portion and a substrate portion. In an alternative embodiment, the substrate is not etched. 2. In S105, form a dielectric layer on the substrate. The dielectric layer comprises a dielectric membrane adjacent to the etched portion of the substrate. A heater is formed within the dielectric membrane. Alternatively, the heater is formed in the dielectric layer on the substrate. 3. In S110, Al.sub.2O.sub.3 doped metal oxide powder is formed using solid state powder synthesis. The powder is calcined at 700 C. for 4 hours to obtain the controlled grain size suitable for the NO.sub.2 detection and stability. A paste is made from the powder, by for example 40% solid loading of the vehicle. 4. In S115, the paste is deposited at 100 C. onto the dielectric membrane using an ink jet printer with a spot size of, for example, 220 microns. 5. In S120, the film is annealed electrically at 700 C. for 2 hours to obtain the desired texture of the film. This is followed by a gas test at various temperatures to optimize the sensor drive mode.

(13) In S110 the Al.sub.2O.sub.3 doped metal oxide powder is formed using solid state powder synthesis. The steps for this are as follows: Precursor gel is precipitated from SnCl.sub.4 and ammonia solution. For this Tin tetrachloride (SnCl.sub.4, extra pure) is hydrolyzed with water, the precursor gel is obtained by mixing the hydrolyzate with ammonia solution (extra pure). The mixture is washed repeatedly with deionized water to remove unwanted chloride and ammonium ions. The SnO.sub.2 content of this gel is determined by thermal gravimetric analysis. With this result, the needed amount of alumina is calculated and the respective amount of aluminum nitrate (Al(NO.sub.3).sub.3, extra pure) is weighted out. The aluminum nitrate Al(NO.sub.3).sub.3 is dissolved in deionized water. The Al(NO.sub.3).sub.3 solution is mixed with the stoichiometric amount of ammonia solution to obtain a white precipitate This gel is washed with deionized water to remove unwanted nitrate and ammonium ions. Both precipitated gels are mixed for 30 min with a stirrer This is followed by drying at 150 C. overnight in a drying oven to obtain white crystals The white crystals are (wet) ground thoroughly in a mortar for 15 min. This powder is calcined at 800 C. for several hours in a tube furnace under ambient atmosphere, to result in a white powder.

(14) FIG. 4 shows results from a gas functionality test on sensors with Al.sub.2O.sub.3 doped SnO.sub.2. Four sensor parts were tested for sensitivity to 7 gases including methane, NO.sub.2, acetone, toluene, CO, ethanol, and H.sub.2. The tests were done with 15% background relative humidity, and 85% background relative humidity. The results show that at that around 200 C. heater temperature the sensors are highly selective to a very low concentration of NO.sub.2 present in the test chamber and are insensitive to the other six gases present. The tests were carried out in the DC mode to understand the sensor behaviour properly. The humidity had little or no effect on both the selectivity of the sensor and the sensitivity of the sensor.

REFERENCE NUMERALS

(15) 1. Semiconductor substrate 2. Embedded heater 3. Heater tracks or metallization 5. Dielectric layer 6. Gas permeable layer 7. Electrodes 8. Dielectric membrane area 17. Sensing material

(16) The skilled person will understand that in the preceding description and appended claims, positional terms such as above, overlap, under, lateral, etc. are made with reference to conceptual illustrations of an apparatus, such as those showing standard cross-sectional perspectives and those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to a device when in an orientation as shown in the accompanying drawings.

(17) Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.