GAS SENSOR

Abstract

A gas sensor includes a support structure with a cavity, a sensing element sensitive to a gas and arranged in the cavity, and a filter spanning the cavity. The filter is a size selective filter.

Claims

1. A gas sensor, comprising a support structure comprising a cavity, a sensing element sensitive to a gas and arranged in the cavity, a filter, and a carrier, wherein the filter or the carrier and the filter in combination span the cavity wherein the filter is a size selective filter which is permeable for the gas to be detected by the sensing element and non-permeable for one or more other gases, wherein the size selective filter filters gas molecules subject to their size and wherein a size of a majority of pores in the carrier material exceeds a size of a majority of pores in the filter material.

2. The gas sensor of claim 1, wherein the size selective effect of the filter is determined by a size of pores in the filter material, in particular wherein the size of the pores in the filter material is dimensioned dependent on a size of a molecule of the gas to be detected, in particular wherein the size of a majority of the pores in the filter material is dimensioned to let molecules of the gas to be detected pass and to block molecules of one or more other gases, in particular wherein the size of a majority of the pores in the filter material is dimensioned to exceed the size of a molecule of the gas to be detected, and is dimensioned smaller than the size of a molecule of the one or more other gases to be blocked, in particular wherein the size of a majority of the pores in the filter material is 1 nm or less.

3. The gas sensor of claim 1, wherein the filter material comprises a fluoropolymer.

4. The gas sensor of claim 3, wherein the filter material comprises an amorphous fluoropolymer with a free fraction per volume of at least 19%.

5. The gas sensor of claim 2 wherein the filter material comprises a homopolymer.

6. The gas sensor of claim 3, wherein the filter material comprises 2,2,4-trifluoro-5-(trifluoromethoxy)-1,3-dioxole.

7. The gas sensor of claim 3, wherein the filter material comprises a copolymer, in particular wherein a second component of the filter material comprises tetraflouroethylene.

8. The gas sensor of claim 7, wherein the first component has a mole fraction between 20% and 99%, and wherein the second component has a mole fraction between 1% and 80%.

9. The gas sensor of claim 7, wherein the 2,2,4-trifluoro-5-(trifluoromethoxy)-1,3-dioxole has a mole fraction of 80%, and wherein the tetraflouroethylene has a mole fraction of 20%.

10. The gas sensor of claim 7, wherein the 2,2,4-trifluoro-5-(trifluoromethoxy)-1,3-dioxole has a mole fraction of 60%, and wherein the tetraflouroethylene has a mole fraction of 40%.

11. The gas sensor of claim 3, wherein the filter material comprises 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole.

12. The gas sensor of claim 3, wherein the filter material comprises a copolymer.

13. The gas sensor of claim 12, wherein the first component has a mole fraction between 20% and 99%, and wherein the second component has a mole fraction between 1% and 80%.

14. The gas sensor of claim 13, wherein the 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole has a mole fraction of 87%, and wherein the tetraflouroethylene has a mole fraction of 13%.

15. The gas sensor of claim 13, wherein the 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole has a mole fraction of 65%, and wherein the tetraflouroethylene has a mole fraction of 35%.

16. The gas sensor of claim 1, wherein an average thickness of the filter is less than 20 μm.

17. A gas sensor, comprising a support structure comprising a cavity, a sensing element sensitive to a gas and arranged in the cavity, a filter, and a carrier wherein the filter or the carrier and the filter in combination span the cavity; wherein an average thickness of the carrier is less than 1 mm and wherein the filter faces the cavity and the carrier faces an environment of the gas sensor.

18. The gas sensor of claim 17, wherein the carrier is one of: gas permeable; of size selective filtering property.

19. The gas sensor of claim 1, wherein the size of the majority of the pores in the carrier material exceeds the size of the majority of the pores in the filter material by a factor of at least 20.

20. The gas sensor of claim 17, wherein the size of the majority of the pores in the carrier material is 40 nm or more, and in particular is between 50 nm and 200 nm.

21. The gas sensor of claim 17, wherein the carrier material comprises a fluoropolymer.

22. The gas sensor of claim 21, wherein the carrier material comprises or consists of one of Polytetrafluoroethylene PTFE or Polyethylenetetrafluoroethylene ETFE.

23. The gas sensor of claim 17, wherein an average thickness of the carrier is between 1 μm and 500 μm.

24. The gas sensor of claim 17, wherein the combination of the filter and the carrier is attached to the support structure by means of an adhesive, in particular wherein the adhesive is one of: gas tight; at least of the same size selective filtering property as the filter.

25. The gas sensor of claim 17, wherein the filter is attached to a top surface of the support structure by means of an adhesive .

26. The gas sensor of claim 17, wherein a distance between the sensing element and the combination of the filter and the carrier is at least 100 μm.

27. The gas sensor of claim 1, comprising electrodes in electrical communication with the sensing element, a heater in thermal communication with the sensing element, and wherein the sensing element comprises metal oxide material, wherein the sensing element is configured to detect one of CO, Ethanol, H.sub.2, H.sub.2S.

28. The gas sensor of claim 1, wherein the support structure comprises a circuit board, a semiconductor chip arranged on the circuit board and supporting the sensing element, a cap for housing the semiconductor chip, wherein the cap contributes to forming the cavity and comprises an opening wherein the opening is spanned by the size selective filter.

29. An electronic device, which comprises one of a home automation device, a consumer electronics device, a mobile phone, a tablet computer—and a watch, comprising the gas sensor according to claims 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the invention will be better understood when consideration is given to the following detailed description thereof. This description makes reference to the annexed drawings, wherein:

[0045] FIGS. 1 to 9 each illustrate a schematic sectional view of a gas sensor in accordance with embodiments of the present invention;

[0046] FIG. 10 illustrates a schematic sectional view of a gas sensor structure contributing to a gas sensor according to an embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION

[0047] Prior to illustrating any embodiments of the gas sensor, attention is drawn to the materials used in or as filter and the carrier if any.

[0048] Preferably, the filter is made from a fluoropolymer, and preferably consists of a fluoropolymer. A fluoropolymer is a fluorocarbon-based polymer that exhibits multiple carbon-fluorine bonds. It usually has a high resistance to solvents, acids, and bases, so that it can advantageously be used for the present purpose.

[0049] For the filter, preferably an amorphous Teflon AF alike material is used according to row no. 4 of the following Table I showing preferred compositions for the filter material in each row. Row no. 4 thereby denotes an umbrella term, the other individual material compositions according to preferred embodiments in row no. 1 to row no. 3 can be subsumed under. Hence, the filter fluoropolymer may in one embodiment be a homopolymer, see row no. 3, while in the other embodiments, it is a copolymer, see row no. 1 and row no. 2.

TABLE-US-00001 TABLE I No Name Component 1 Component 2 1 Teflon AF 2400 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole tetrafluoroethylene 2 Teflon AF 1600 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole tetrafluoroethylene 3 FDD homopolymer 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole n/a 4 Teflon AF alike 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole tetrafluoroethylene

[0050] In the following Table II properties for the individual material compositions of above row no. 1 to 4 are illustrated, wherein [0051] Comp 1, mol % denotes the mole fraction of the Component 1; [0052] Comp 2, mol % denotes the mole fraction of the Component 2; [0053] FFV denotes the free fraction per volume; [0054] Tg °C. denotes the glass transition temperature of the resulting material, in °Celsius; and [0055] Tmax denotes a maximal temperature at which polymers do not show notable degradation (i.e. thermally stable), in °Celsius.

TABLE-US-00002 TABLE II No Name Comp 1, mol % Comp 2, mol % FFV, % Tg, ° C. Tmax, ° C. 1 Teflon AF 2400 87 13 33 240 360 2 Teflon AF 1600 65 35 30 160 360 3 PDD homopolymer 100 0 >33 335 360 4 Teflon AF alike 100-20 0-80 80-250 360

[0056] For the filter, alternatively, preferably a material according to one of the rows no. 5 to 9 of the following Table III is used. Row no. 8 thereby denotes an umbrella term, the other individual material compositions according to preferred embodiments in row no. 5 to row no. 7 can be subsumed under. Hence, the filter fluoropolymer may in one embodiment be a homopolymer, see row no. 7, while in the other embodiments, it is a copolymer, see row no. 5 and row no. 6. Row no. 9 indicates a further homopolymer that preferably can be used as filter material.

TABLE-US-00003 TABLE III No Name Component 1 Component 2 5 Hyflon AD 80 2,2,4-trifluoro-5-(trifluoromethoxy)-1,3-dioxole tetrafluoroethylene 6 Hyflon AD 60 2,2,4-trifluoro-5-(trifluoromethoxy)-1,3-dioxole tetrafluoroethylene 7 TTD homopolymer 2,2,4-trifluoro-5-(trifluoromethoxy)-1,3-dioxole n/a 8 Hyflon AD alike 2,2,4-trifluoro-5-(trifluoromethoxy)-1,3-dioxole tetrafluoroethylene 9 Cytop (homopolymer) perfluoro-butenylvinylether n/a

[0057] In the following Table IV properties for the individual material compositions of above row no. 5 to 9 are illustrated, with the legend according to Table II.

TABLE-US-00004 TABLE IV No Name Comp 1, mol % Comp 2, mol % FFV, % Tg, ° C. Tmax 5 Hyflon AD 80 80 20 23 135 ca. 400 C. 6 Hyflon AD 60 60 40 23 129 ca. 400 C. 7 TTD homopolymer 100 0 170 ca. 400 C. 8 Hyflon AD alike 100-20 0-80 170 ca. 400 C. 9 Cytop (homopolymer) 100 0 21 108 400

[0058] The carrier, if any, preferably is made from a fluoropolymer, and preferably consists of a fluoropolymer e.g. Polytetrafluoroethylene (PTFE) or Polyethylenetetrafluoroethylene (ETFE) produced in the form of bulk, woven or nonwoven materials with the pore sizes specified above.

[0059] The sensing element preferably is represented by a patch of sensing material. In embodiments, the present sensing element can be operated as a chemiresistor, wherein a metal oxide material, the sensing element is made of, changes its electrical resistance in response to gas molecules having passed the filter, leading to chemical interactions between the material of the sensing element and the gas/analytes. In other embodiments, one may rely on a calorimetric determination of the gas/analyte. In still other embodiments, the patch of sensing material can be used for two purposes, namely: (i) as a chemiresistor that changes its electrical conductivity in the presence of the analyte, and (ii) as a catalyst in a calorimetric determination of the analyte.

[0060] In one embodiment, the latter comprises a metal oxide material, or MOX. Beyond the examples cited above, preferred MOX materials comprise SnO.sub.2 and/or WO.sub.3, and preferably comprises dopants too, the latter comprising one or more of Pd, Pt, Rh, Ir, Re, V, Ni, Au, and Co. The patch may be arranged on the support structure. It may for instance extend on an exposed surface of the support structure, such as a semiconductor substrate, e.g., overlaid flat on an upper surface thereof, or extend on substructures thereof, such as electrodes. Electrodes may be arranged in the gas sensor, so as to be in electrical communication with the patch of sensing material. They may be formed out of a platinum or gold layer, which metals are well suited for forming stable electrodes. Electrodes may for instance be in an interdigitated configuration. Thus, the patch in one embodiment may have a shape (e.g., convex) that spans a region that covers or includes interdigitated fingers of the electrodes.

[0061] In one embodiment, a heater is in thermal communication with the patch, to operate the sensing material at a required temperature. The heater may be a resistive heating element. For example, one may use a heater of tungsten, i.e., a heater comprising at least 50%, in particular at least 90%, of tungsten, to best withstand high temperatures. Several heaters may be provided, to heat a plate (e.g., a membrane, or a bridge), on which the patch is arranged. In variants, the heater may be embodied as a hotplate, which is resistively heated, without additional resistive elements being needed. The heater can be used to heat the patch and, if necessary, to furthermore control the temperature of. The semiconductor chip of the gas sensor preferably includes circuitry, integrated therewith, to heat the heater and perform resistive measurements, i.e., to measure an electrical conductivity and/or resistivity of the patch.

[0062] In the Figures, like elements are referred to by the same reference signs.

[0063] FIG. 1 shows a schematic sectional view of a gas sensor in accordance with an embodiment of the present invention. The gas sensor includes a sensing element 1, which is arranged on or integrated in a support structure. In this embodiment, the support structure includes a semiconductor chip 2, e.g. a silicon substrate, and an adhesive 5 which in the present embodiment is of sufficient thickness in order to form a cavity 6 in combination with a filter 3 and a carrier 4 for the filter 4.

[0064] The gas to be sensed can enter the cavity 6 and thereby reach the sensing element 1 through the carrier 4 and the filter 3. The combination of the filter 3 and the carrier 4 is attached to the semiconductor substrate 2 by means of the adhesive 5 with the filter 3 facing the cavity 6, and the carrier 4 facing the environment of the gas sensor.

[0065] FIG. 2 shows another embodiment of a gas sensor in accordance with the present invention. In this embodiment, the semiconductor substrate 2, which also can be any other support, has a recess 21 which recess contributes to the cavity 6, and in which recess 21 the sensing element 1 is located. In this embodiment, it is not only the thickness of the adhesive 5 that provides for a sufficient distance d between the sensing element 1 and the filter 3 or carrier 4 respectively. The recessed portion of the semiconductor substrate 2 contributes to this distance d.

[0066] FIG. 3 illustrates another embodiment of a gas sensor in accordance with the present invention. In this embodiment, the support structure of the gas sensor contains a semiconductor substrate 2 and a spacer 22 on top of a semiconductor substrate 2, for example. The spacer 22 can be of a different material than the semiconductor substrate 2.

[0067] FIG. 4 shows another embodiment of a gas sensor device in accordance with the present invention. Basically, this embodiment is similar to the embodiment of FIG. 1 except for top elements 9 added on to edges of the carrier 4.

[0068] FIG. 5 illustrates another embodiment of a gas sensor in accordance with the present invention. In this embodiment, the gas sensor again comprises a semiconductor chip 2 with the sensing element 1. The semiconductor chip 2 is partly covered by an encapsulation 22 in form of a mold and a lead frame 23 serves for outside contacting. The support structure for the sensing element 1 hence includes the semiconductor chip 2, the encapsulation 22, the adhesive 5 and the lead frame 23. A cavity 6 is formed by the support structure which cavity 6 is closed by a combination of a filter 3 and a carrier 4 for the filter 3. The combination of filter 3 and carrier 4 extends over the entire top surface of the support structure 2 to which the combination of filter 3 and carrier 4 is attached by means of the adhesive 5.

[0069] FIG. 6 illustrates another embodiment of a gas sensor in accordance with the present invention. In this embodiment, the gas sensor comprises a semiconductor chip 2 with the sensing element 1. The semiconductor chip 2 is partly covered by a silicon cap 24. A cavity 6 is formed by the semiconductor substrate 2, the silicon cap 24 and the adhesive 5, thus contributing to the support structure.

[0070] FIG. 7 illustrates another embodiment of the gas sensor in accordance with the present invention. Again, a sensing element 1 is arranged on/in a suspended membrane portion of a semiconductor substrate 2 which suspended membrane portion, for example, is prepared by etching substrate material from a backside of the semiconductor chip 2. This results in a cavity 6. For this reason, the combination of the filter 3 and carrier 4 is attached to a backside of the semiconductor chip 2, again by means of an adhesive 5.

[0071] FIG. 8 illustrates another embodiment of the gas sensor in accordance with the present invention. This embodiment resembles the embodiment of FIG. 5. In contrast to the embodiment of FIG. 5, the combination of the filter 3 and the carrier 4 is attached vice versa to the top surface of the support structure by means of the adhesive 5. Hence, the carrier 4 faces the cavity 6 while the filter 3 faces an environment of the gas sensor. Therefore, it is desired to seal a direct path from the outside of the gas sensor through the carrier 4 into the cavity 6. Such path is sealed by covering any surface of the carrier 4 that is neither covered by the filter 3 nor by the adhesive 5. For this purpose, the adhesive 5 also covers the front end of the carrier 4, and, in one embodiment is even lifted on top of the carrier 4, in order to at the same time act as top element for one of the purposes previously listed for top elements.

[0072] FIG. 9 illustrates another embodiment of the gas sensor in accordance with the present invention. In this embodiment, a semiconductor chip 2 including the sensing element 1 is arranged, and preferably electrically connected to a circuit board 7 such as a printed circuit board. In addition, one or more other chips such as an integrated circuit 71 may be arranged on the circuit board 7, too. A cap 8, e.g. made from metal, may, in combination with the circuit board 7 build a housing for the semiconductor chip 2, enclosing a cavity 6. The cap 8 has an opening 81, which is spanned by a combination of a filter 3 and a carrier 4. The combination is attached to the cap 8 by means of an adhesive 5, from the inside of the cap 8.

[0073] FIG. 10 illustrates a gas sensor structure contributing to a gas sensor of an embodiment of the present invention. The present structure is to be finalized by attaching a filter or a combination of a filter and a carrier to the present structure. The present gas sensor structure comprises a sensing element 1 arranged on a semiconductor chip 2 that is etched from its backside thereby defining a recess 23. The sensing element 2 covers electrodes 10 for supplying an electrical signal to an evaluation and control unit 11 to be evaluated there. The evaluation and control unit 11 preferably is integrated into the semiconductor chip 2. The evaluation and control unit 11 preferably also controls a heater 12 integrated in the semiconductor substrate 2, e.g. in the membrane remaining from the etching of the recess 23.

[0074] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.