MEMS device

Abstract

The invention relates to a MEMS device (10) having a substrate (12), a diaphragm (14), a cavity (16), which is arranged between the substrate (12) and at least a portion of the diaphragm (14), and a heating element (18), which is located in the cavity (16), characterized in that the heating element (18) is arranged between the substrate (12) and at least a portion of the diaphragm (14). The heating element (18) is at least partially connected, in particular mechanically connected, to the membrane (14) or the heating element is mechanically laid flat by means of the membrane.

Description

Technical Field

[0001] The present invention relates to a MEMS device having a substrate, a diaphragm, a cavity, and a heating element, wherein the cavity is disposed between the substrate and at least a portion of the diaphragm, and the heating element is located in the cavity. Background Art

[0002] MEMS devices are microelectromechanical devices, such as those used in MEMS sensor devices. Devices of this type are known, for example, from WO 2020 / 061121 A1. The MEMS sensor device has a deformable diaphragm that covers a cavity. To implement the thermal effect principle, a MEMS heating element is provided outside the cavity. Additionally, a MEMS device is known from US2023 / 194478 A1, which has a diaphragm, a substrate, and a cavity located between the diaphragm and the substrate. A heating element spaced apart from the diaphragm is provided in the cavity. Additionally, a reflective layer spaced apart from the heating element may be provided. Summary of the Invention

[0003] A MEMS device is achieved by means of the present invention. Preferred expansions are listed below.

[0004] Advantages of the Invention

[0005] One aspect of the present invention is that the proposed MEMS device according to the present invention has a substrate, a diaphragm, and a cavity located between the substrate and at least a portion of the first diaphragm. A heating element is provided in the cavity, which is at least partially connected to the diaphragm, in particular mechanically connected or mechanically lying flat. Thus, it is achieved that the heat loss due to heat conduction and heat radiation to the air on both sides of the diaphragm is significantly reduced. The MEMS device is characterized by a lower power consumption. This lower power consumption is achieved by overall lower heat dissipation, especially to the lower half space of the cavity.

[0006] The advantageous expansions and improvements of the above MEMS device can be achieved by the measures listed below.

[0007] In one embodiment of the present invention, the reduction of heat conduction and heat radiation can be achieved by the following: providing a radiation reflector spaced apart from the heating element in the direction of the substrate. The radiation reflector is used to reflect the heat radiation starting from the heating element. The spacing between the radiation reflector and the heating element reduces the undesired fluid heat conduction; this is especially the case when there is a fluid negative pressure or vacuum, such as <10 mbar, in the cavity relative to the typical external ambient pressure of approximately 1 bar of the MEMS device.

[0008] In another embodiment, one or more support elements (Stützelemente) can be provided, which connect the substrate and the diaphragm either indirectly or directly. Thereby, the mechanical stability of the cavity with respect to the external pressure applied to the MEMS device can be significantly improved. The support elements can be implemented, for example, as support walls or support pillars, which ensure the spacing between the radiation reflector and the diaphragm and thus between the heating element.

[0009] In another embodiment, thermal insulation trenches can be provided, which are arranged such that the diaphragm is interrupted at least in sub-regions. In this way, a stronger thermal insulation of the cavity from its environment can be achieved, such that the lateral heat dissipation via the diaphragm can be reduced again.

[0010] In another embodiment, the diaphragm can be at least partially formed of a non-conductive material on its side facing away from the cavity. In this way, it is very easy to implement electrically insulated lines or electrodes on the diaphragm.

[0011] In another embodiment of the present invention, the cavity of the MEMS device can be implemented in a hermetically sealed manner and can have a heating element made of a refractory material that is more sensitive to the medium. These materials have high reliability in a protected atmosphere and at the same time allow particularly high operating temperatures.

[0012] In addition, a radiation reflector can be used in the MEMS device, which includes a metal layer. Here, it is advantageous that the metal layer acts particularly broadband and efficiently in terms of its reflection properties.

[0013] In another embodiment, it can be provided that the substrate has a substrate notch. The substrate notch is provided on the side facing away from the cavity. In this way, it is possible to cover the substrate notch with a layer serving as a radiation reflector. Thereby, the number of materials that can be used as radiation reflectors can be significantly broadened. In addition, in this way, the radiation reflector can also be introduced independently of the previous layers, cavities or process steps for producing the MEMS device or for introducing the heating element.

[0014] In another embodiment, it can be provided that the cavity is vertically delimited by two curved beams and laterally by delimiting walls above the substrate notch. Here, the curved beams have only one or two anchors on the substrate, which, similar to the thermal insulation trenches, limit the heat loss caused by heat conduction to the connected one or more ends and improve the efficiency of the heating element.

[0015] In another embodiment, a transducer structure can be provided on the diaphragm. In particular, a structure suitable for use as a sensor is suitable for this purpose. Here, it is advantageous that almost no heat is discharged into the substrate, and as much heat as possible is discharged in the direction of the transducer structure. Here, a metal oxide gas sensor with interdigital electrodes is mentioned by way of example. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the following, other features and advantages of the present invention are explained with reference to the drawings according to embodiments.

[0017] The drawings show:

[0018] Figure 1 a, 1b show a schematic top view and a cross-sectional view of a MEMS device according to a first embodiment of the present invention;

[0019] Figure 2 a, 2b show a schematic top view and a cross-sectional view of a MEMS device according to a second embodiment of the present invention;

[0020] Figure 3 a, 3b show a schematic top view and a cross-sectional view of a MEMS device according to another embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0021] In the drawings, the same reference numerals denote identical or functionally identical elements.

[0022] Figure 1 a schematically shows a top view of the MEMS device, Figure 1 b schematically shows a cross-sectional view of the MEMS device. The MEMS device 10 has a diaphragm 14 and a substrate 12 spaced apart from the diaphragm. Here, the diaphragm 14 is spaced apart from the substrate 12 such that a cavity 16 is formed in the sub-region therebetween. The diaphragm 14 spans the cavity 16 at the boundary with respect to the environment 30 of the MEMS device 10 and is fastened to the anchor 19. On the side of the diaphragm 14 facing the cavity 16, a heating element 18 is arranged such that the heating element is mechanically in contact with the diaphragm 14 in a manner facilitated by the external ambient pressure. Alternatively, the heating element 18 can be configured to be directly connected to the diaphragm 14. The heating element 18 can be implemented as a spiral infrared radiator.

[0023] Preferably, the MEMS device 10 is configured such that the internal pressure in the cavity 16 is significantly lower than the external pressure in the environment 30 of the MEMS device 10. For example, an ambient pressure of 1 bar may exist in the environment 30, while the pressure in the cavity 16 is significantly lower, for example, in the range of less than 700 mbar, preferably less than 10 mbar. With this arrangement and the reduced pressure in the cavity 16, accidental heat conduction and associated heat loss to regions outside the MEMS device 10 are reduced.

[0024] In one embodiment of the invention, a radiation reflector 20 may be provided. The radiation reflector 20 is arranged spaced apart from the heating element 18 in the direction of the substrate 12. For example, the radiation reflector 20 may be applied directly as a layer on the substrate 12. The following materials may be used for the radiation reflector 30: materials that reflect the radiation starting from the heating element 18. Here, it may be sufficient if the radiation is reflected at least in a sub-region of the layer provided with the radiation reflector 20. In particular, a metal layer may be used for the radiation reflector 20. With a metal mirror layer, incident heat radiation can be reflected particularly broadband and efficiently.

[0025] To prevent possible corrosion, for example, by oxygen or moisture in the air, an additional dielectric layer made of, for example, silicon nitride or a layer made of polysilicon may be applied to the radiation reflector 20 for passivation. That is, the radiation reflector 20 is used to reflect heat radiation, while the spacing between the heating element 18 and the radiation reflector 20, combined with the negative pressure in the cavity 16, prohibits heat conduction via the residual gas. When the pressure in the cavity 16 is less than 50 mbar, especially less than 10 mbar, this heat conduction is particularly effective. Thus, the overpressure present in the environment 30 can also cause the diaphragm 14 to mechanically contact the heating element 18 better. Good contact ensures good thermal connection. Additionally, the heating element 18 can support the diaphragm at least within certain limits.

[0026] To ensure that the negative pressure present in the cavity 16 can be stably maintained, the cavity 16 needs to be constructed such that it is closed relative to the environment 30. This can be achieved, for example, by implementing the cavity in a hermetically sealed manner. Thus, heating wires sensitive to media, such as tungsten, can also be used in the heating element 18. In this way, high reliability can be combined with high operating temperatures.

[0027] For further stabilization, a support element 22 may additionally be provided, which directly connects the diaphragm 14 to the substrate 12 or - if a radiation reflector 20 is provided - indirectly via the radiation reflector 20. This type of support element 22 can also be provided multiple times and implemented as support walls or support columns.

[0028] Figure 2 a schematically shows a top view of the MEMS device 10 in another embodiment. Figure 2 b schematically shows a cross-sectional view of the MEMS device 10 in another embodiment. Here, in order to further stabilize the MEMS device 10, a support element 22 is provided, which indirectly connects the diaphragm 14 to the substrate 12 via an additional inner diaphragm 15. The substrate 12 has a substrate notch 26, which extends at least partially through the substrate in a manner adjacent to the additional inner diaphragm. Thus, it becomes possible to arrange the radiation reflector 20 in such a way that the edge of the substrate notch 26 is covered with a layer that reflects radiation. In this way, the process for introducing the radiation reflector 20 can be separated from the remaining processes for constructing the other layers and the cavity 16, that is, the processes can be made independent of each other. Therefore, the radiation reflector 20 can also be introduced at the end of the entire process for manufacturing the MEMS device 10. In particular, when the separation of the processes can be dispensed with, the radiation reflector 20 can also be arranged on the inner diaphragm 15 in the direction towards the cavity 16 or on the side of the inner diaphragm 15 facing away from the cavity 16.

[0029] By additionally providing the inner diaphragm 15, the construction of a closed cavity 16 can be achieved. Thus, the following advantage is obtained: The radiation reflector 20 does not necessarily have to contribute to sealingly closing the cavity 16 and thus a wider variety of materials can be used.

[0030] Figure 3 a schematically shows a top view of another embodiment of the MEMS device 10. Figure 3 b schematically shows a cross-sectional view of another embodiment of the MEMS device 10. The diaphragm 14 spans the cavity 24, where the diaphragm is fastened to the anchor 19. The diaphragm has an insulating groove 24, which is implemented as a suitably shaped notch in the diaphragm 14 such that the insulating groove 24 is coordinated with the set position of the cavity 16. Thermal insulation of the diaphragm 14 can be achieved by means of the insulating groove 24.

[0031] As Figure 3 shown in b, it results therefrom that the diaphragm 14 spans the cavity 16, however, extends transversely to the cavity 16 on the left and right sides of the insulating groove 24, which interrupts the diaphragm 14 here. This results in minimizing the heat dissipation transversely through the diaphragm 14.

[0032] In this embodiment, one or more support elements 22 can also be provided. In order to laterally delimit the cavity 16, support walls 23 are respectively provided. On the one hand, these support walls support the diaphragm 14 in the region of the cavity 16. On the other hand, these support walls ensure that the cavity 16 is also tightly sealed laterally. In order to seal the cavity in the direction of the substrate 12, an inner diaphragm 15 can be provided, by means of which the cavity is tightly sealed in the direction of the substrate 12. In addition, the radiation reflector 20 can be provided below the inner diaphragm 15 which has the advantages of radiation reflection described above, especially also on the side of the substrate facing away from the cavity. As in the previously described embodiments, the radiation reflector can also be applied above the inner diaphragm 15, i.e., on the inner diaphragm 15 in the direction of the cavity 16. It should be noted here that the tight seal of the cavity 16 is maintained.

[0033] In another configuration of the present invention, a transducer structure 29 is applied on the surface of the diaphragm 14. The transducer structure 29 is arranged on the diaphragm 14 in the direction of the environment 30 and is positioned above the cavity 16. With the MEMS device 10 configured in this way, almost no heat is discharged into the substrate 12. More precisely, the main part of the heat generated by the heating element 18 is discharged towards the transducer structure 29, and the radiation reflector 20 is again beneficial for this.

[0034] The transducer structure 29 can be, for example, a sensor, such as a gas sensor, a humidity sensor or an IR sensor. The sensor can have electrodes 27, especially interdigitated electrodes. It is advantageous here that the diaphragm 14 is at least partially non-conductively implemented on its side facing away from the cavity, so that the electrodes 27 are not short-circuited. If the diaphragm 14 is configured to be conductive, alternatively, a thin dielectric layer can also be provided between the diaphragm 14 and the electrodes 27. In addition, the transducer structure 29 can have a sensor layer 28, which is implemented, for example, as a layer sensitive to gas or humidity, such as a metal oxide layer, a polyimide layer, etc. With the transducer structure 29 configured in this way, for example, a change in resistance can be read out from the provided interdigitated structure. Alternatively, it is also possible that the transducer structure 29 includes a catalytic layer and / or a temperature sensor.

[0035] With the MEMS device 10, various types of sensing devices can be realized. Examples of the sensing devices are: gas sensors (such as MOx gas sensors), heat dissipation sensors, catalytic oxidation sensors (Pellistors) or mass flow sensors.

[0036] In addition, the MEMS device 10 can also be used as an (M)IR radiation source.

Claims

1. A MEMS device (10), comprising a substrate (12), a membrane (14), a cavity (16), and a heating element (18), wherein the cavity is arranged between the substrate (12) and at least a portion of the membrane (14), and the heating element is located in the cavity (16), characterized in that: The heating element (18) is at least partially connected to the membrane (14), in particular mechanically connected, or the heating element is mechanically laid flat by means of the membrane.

2. The MEMS device (10) according to claim 1, characterized in that A radiation reflector (20) is provided which is spaced apart from the heating element (18) in the direction of the substrate (12).

3. The MEMS device (10) according to claim 1 or 2, characterized in that: At least one supporting element (22) is provided, which connects the substrate (12) and the membrane (14) indirectly or directly.

4. The MEMS device (10) according to any one of claims 1 to 3, characterized in that The thermal insulation groove (24) is arranged in such a way that the membrane (14) is interrupted at least in a subregion.

5. The MEMS device (10) according to any one of claims 1 to 4, characterized in that The membrane (14) is at least partially electrically non-conductive, in particular on its side facing away from the cavity (16).

6. The MEMS device (10) according to any one of claims 1 to 5, characterized in that The internal pressure in the cavity (16) is lower than the external pressure of the environment (30) of the MEMS device (10), in particular less than 50 mbar in absolute value.

7. The MEMS device (10) according to any one of claims 1 to 6, characterized in that The cavity (16) is hermetically sealed.

8. The MEMS device (10) according to any one of claims 2 to 7, characterized in that The radiation reflector (20) comprises a metallic layer, in particular a metallic mirror layer.

9. The MEMS device (10) according to any one of claims 1 to 8, characterized in that The substrate (12) has a substrate recess (26) at least partially on its side facing the cavity (16).

10. The MEMS device (10) according to any one of claims 1 to 9, characterized in that A transducer structure (28), in particular a sensing structure, is arranged on the diaphragm (14).

Images