Gas Sensor with a Gas Permeable Region

Abstract

In an embodiment a method includes forming a dielectric membrane on a semiconductor substrate comprising a bulk-etched cavity portion, forming a heater within or over the dielectric membrane, forming a material for sensing a gas on a side of the dielectric membrane, forming a support structure near the material, wherein the support structure comprises an inorganic material and forming a gas permeable region coupled to the support structure in order to protect the material.

Claims

1. A method for manufacturing a gas sensing device, the method comprising: forming a dielectric membrane on a semiconductor substrate comprising a bulk-etched cavity portion; forming a heater within or over the dielectric membrane; forming a material for sensing a gas on a side of the dielectric membrane; forming a support structure near the material, wherein the support structure comprises an inorganic material; and forming a gas permeable region coupled to the support structure in order to protect the material.

2. The method according to claim 1, wherein the support structure is attached by wafer bonding.

3. The method according to claim 1, wherein a gas permeable layer is attached to the support structure, before the support structure is attached by wafer bonding.

4. The method according to claim 1, wherein forming the dielectric membrane comprises forming the dielectric membrane such that it is supported along its entire perimeter by the semiconductor substrate.

5. The method according to claim 1, wherein forming the dielectric membrane comprises using an etching technique to back-etch the semiconductor substrate to form the bulk-etched cavity portion.

6. The method according to claim 5, wherein the etching technique is selected from the group consisting of deep reactive ion etching (DRIE), anisotropic or crystallographic wet etching, potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH).

7. The device manufactured according to claim 1, wherein the support structure comprises a semiconductor material.

8. The method according to claim 1, wherein the support structure comprises a material comprising a glass or a ceramic.

9. The method according to claim 1, wherein the semiconductor substrate forms the support structure.

10. The method according to claim 1, wherein the dielectric membrane is only supported by one or more dielectric beams to connect the dielectric membrane to the semiconductor substrate.

11. The method according to claim 1, wherein the material is a gas sensing material.

12. The method according to claim 11, wherein the gas sensing material comprises a metal oxide material or a combination of metal oxides.

13. The method according to claim 1, wherein the material is deposited as a gate electrode, or is electrically connected to the gate electrode of a field effect transistor (FET).

14. The method according to claim 1, wherein the dielectric membrane is formed using an etching technique for back-etching the semiconductor substrate, the etching technique being selected from the group consisting of deep reactive ion etching (DRIE), anisotropic or crystallographic wet etching, potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH).

15. The method according to claim 1, wherein the dielectric membrane is formed by a front side etch of the semiconductor substrate.

16. The method according to claim 1, wherein the heater is a resistive heater comprising a CMOS material comprising aluminium, copper, titanium, molybdenum, polysilicon, single crystal silicon tungsten, or titanium nitride.

17. The method according to claim 1, wherein the semiconductor substrate is a bulk silicon substrate or an SOI substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

[0066] FIG. 1 shows a gas sensor with a gas permeable layer;

[0067] FIG. 2 shows an alternative gas sensor in which through silicon vias (TSVs) are used;

[0068] FIG. 3 shows an alternative gas sensor;

[0069] FIG. 4 shows an alternative gas sensor in which the gas permeable layer has holes;

[0070] FIG. 5 shows an alternative gas sensor in which the sensing material is below the membrane, and the gas permeable layer is on the back side, supported by the silicon substrate itself,

[0071] FIG. 6 shows an alternative gas sensor which is bonded by a flip chip; and

[0072] FIG. 7 shows an alternative gas sensor where the membrane is formed by a front side etch.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0073] FIG. 1 shows a gas sensor with a sensing material 6, a silicon substrate 1 with a gas permeable layer 8 attached on top. The heater 2 and heater tracks or metallization 3 are embedded within the dielectric membrane 4 supported on the substrate 1. Electrodes 5 on top of the membrane connect to a sensing material 6 which has been grown or deposited on the membrane. An additional patterned semiconductor 7 (or the support structure) is attached at the top by wafer bonding and the gas permeable layer 8 is on top of this support structure 7. The dielectric membrane 4 and the passivation can include one or more combinations of silicon dioxide and silicon nitride, or other dielectric layers. In one example, the gas permeable layer or region is a metal, dielectric and/or semiconductor layer with multiple holes. This can be formed for example, by depositing a dielectric or metal layer on a substrate. Then making holes within the metal or dielectric layer. And then back etching a selected part of the substrate and joining this structure to the gas sensing hotplate by wafer bonding.

[0074] FIG. 2 shows an alternative gas sensor in which through silicon vias (TSVs) 9 are used to connect electrically to the device. The TSVs are generally connected to metallization or pads (not shown). The remaining features of the gas sensor are the same as those described in respect of FIG. 1 above and thus carry the same reference numbers.

[0075] FIG. 3 shows an alternative gas sensor in which the support structure 7 (or the additional semiconductor substrate) is smaller, so that the bond pads 11 are exposed and can be electrically connected by wire bonding to either a package or a printed circuit board (not shown in the figure). Furthermore, this figure also shows a passivation layer 10—which may or may not be present on the device. The remaining features of the gas sensor are the same as those described in respect of FIGS. 1 and 2 above and thus carry the same reference numbers.

[0076] FIG. 4 shows an alternative gas sensor in which the gas permeable layer 8 has holes, or is gas permeable even in regions which connect to the semiconductor support structure 7. The gas permeable layer can be for example a film such as gore-tex. It can also be a film of metal, dielectric and/or semiconductor with holes. The remaining features of the gas sensor are the same as those described in respect of FIGS. 1 to 3 above and thus carry the same reference numbers.

[0077] FIG. 5 shows an alternative gas sensor in which the sensing material 6 is below the dielectric membrane 4, and the gas permeable layer 8 is on the back side, supported by the silicon back-etched substrate 1 itself. In this example, no additional support structure is needed as the back-etched substrate 1 acts as the support structure. The remaining features of the gas sensor are the same as those described in respect of FIGS. 1 to 4 above and thus carry the same reference numbers.

[0078] FIG. 6 shows an alternative gas sensor with the sensing material 6 below the dielectric membrane 4, and a gas permeable layer 5 on the backside, bonded by flip chip with the bonds 13 connected to a printed circuit board (PCB) 12. The bond 13 can be generally connected to metallization or pads (not shown). In this example the chip is bonded on the front or top side of the chip. The remaining features of the gas sensor are the same as those described in respect of FIGS. 1 to 5 above and thus carry the same reference numbers.

[0079] FIG. 7 shows an alternative gas sensor, where the membrane 14 is a suspended membrane, formed by a front side etching of the substrate, and is supported by one or more beams (not shown). The membrane 14 includes dielectric material, for example, silicon oxide. The substrate 1 includes a triangle region 10 which is generally empty and is formed due to the front-side etching of the substrate. The remaining features of FIG. 7 are the same as those described above and thus carry the same reference numbers.

[0080] In the above mentioned embodiments, a gas sensing material 6 is disposed on an electrode 5. The electrode 5 is configured to measure resistance and/or capacitance of the gas sensing material 6. In an alternative embodiment, a catalytic material can be used instead of the gas sensing material. When the catalytic material is used, no electrode underneath it is generally necessary, since the detection is done by measuring the change in temperature of the membrane rather than the resistance or capacitance of the material. Alternately, the gas sensing material could be deposited as part of a gate, or an extended gate of a gas sensing FET.

[0081] In summary, the present invention provides a micro-hotplate based gas sensor chip that attaches the gas permeable layer onto the chip itself. The prior art reports typically have the gas permeable layer on the package, whereas the present invention provides the gas permeable layer in the chip level. The prior art devices are not for a membrane based device, whereas the present invention uses membrane based devices. The prior art devices generally have relatively larger holes for the purpose of allowing air flow, but do not stop water or particles. The prior art devices generally have a single hole (for example EP1775259). Alternately, the method of the present invention can allow a smaller package (or even a chip level package), easier handling during assembly, and lower cost. Furthermore, in the prior art device, the gas permeable layer can be formed on the sensing material itself which can affect the properties of the sensing material. This problem does not exist in the present invention as there is a support structure provided to create a gap between the sensing material and the gas permeable layer. Further, in some prior art devices, a plastic moulded cap is provided on which it is difficult to have the gas permeable layer. This problem does not exist in the present invention.

[0082] Although the invention 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 invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.