Micromechanical sensor for a water- or moisture-containing environment

Abstract

The invention relates to a micromechanical sensor (10) for detecting a measured quantity, comprising a substrate (14), a membrane (20) which is deflectable in a normal direction (16) depending on the measured quantity and which has at least one membrane material (18), which connects to the substrate (14) via a membrane edge (22) and which faces a moisture-containing environment (26) with a membrane surface (24), wherein at least a locally limited first region (32) of the membrane surface (24) has a hygromechanical material structure that differs from a second region (34) laterally offset thereto and / or a covering layer (64) with a layer thickness (68) of less than or equal to 1 µm is arranged on the membrane material (18) at least over the entire membrane surface (24).

Description

[0001] The invention relates to a micromechanical sensor according to the preamble of claim 1. State of the art

[0002] Micromechanical sensors comprise microelectromechanical structures that detect measured quantities through mechanical, electrical, and / or chemical interactions. However, in specific applications, micromechanical sensors can exhibit high cross-sensitivity to humidity or water, which can lead to unintended changes in the sensor signal.

[0003] Moisture or water affects micromechanical sensors through a variety of physical and / or chemical effects. For example, hydrophilic materials can have their physical properties altered by the adsorption or diffusion of water molecules. Surface adsorption, in turn, leads to changes in mechanical and electrical parameters. Additionally, water-bound layers can form on structures exposed to the environment, influencing their mechanical properties. Disclosure of the invention

[0004] According to the present invention, a micromechanical sensor with the features of claim 1 is proposed. This reduces the cross-sensitivity of the micromechanical sensor to humidity and water. The micromechanical sensor can measure the quantity more accurately and be made more moisture-resistant. The micromechanical sensor can be designed to be compact and cost-effective.

[0005] The membrane is mechanically and electrically connected to the substrate via its edge, either conductively or non-conductively. The membrane surface can be oriented perpendicular to the substrate.

[0006] The micromechanical sensor can be a microelectromechanical sensor. The micromechanical sensor can be a pressure sensor, a microphone, or an accelerometer. The pressure sensor can be an absolute pressure sensor or a differential pressure sensor. The pressure sensor can be a piezoresistive or capacitive sensor.

[0007] The hygromechanical material structure describes the entirety of the physical, chemical, and mechanical properties of a material that influence its mechanical behavior, particularly material stresses such as compressive and / or tensile stress, when exposed to moisture. The hygromechanical material structure thus indicates how the material behaves mechanically, for example, through material stresses or deformations, depending on the absorption, storage, or release of water or moisture.

[0008] In the preceding and following text, "lateral" is understood to mean a directional reference in a plane that has the normal direction as its normal.

[0009] If at least a locally limited first area of ​​the membrane surface has a hygromechanical material structure that differs in a targeted manner from a second area offset laterally, moisture-induced tensile and compressive stresses in the membrane can be balanced or avoided, thus reducing deformation of the membrane due to the action of moisture or water on the membrane.

[0010] The second area can be offset from the first area in at least one lateral direction or in two mutually perpendicular lateral directions. The second area can also be laterally offset from the first area by surrounding it.

[0011] When a covering layer with a thickness of less than or equal to 1 µm is placed on the membrane surface, which at least limits the absorption of water from the environment, it has been found that the moisture-related cross-sensitivity of the micromechanical sensor is significantly reduced.

[0012] Particularly in the case of silicon dioxide, the membrane material forming the membrane surface, an ice-like layer consisting of water molecules can form on the membrane surface through the adsorption of water molecules onto the silicon dioxide surface. This ice-like layer arises from the interaction between the water molecules and immobilized hydroxyl groups on the silicon dioxide surface, which forces the water molecules into an ordered arrangement. This ice-like layer can form several molecular layers, for example, up to 15.

[0013] The top layer can decouple the membrane surface from material stresses that would otherwise act on the top layer due to the ice-like layer. This reduces material stresses on the membrane surface caused by the ice-like layer.

[0014] The coating layer can prevent the membrane surface from absorbing water from the environment. The coating layer can be formed by atoms diffused into the membrane material, for example, from metals, metal oxides, and / or metal silicides. The coating layer can comprise a friction-reducing layer, a silicone gel, a hydrogel, aluminum oxide, a metal (gold, copper, platinum, chromium, nickel, or others), a metal alloy, oxides (silicon dioxide, aluminum oxide, zinc oxide, indium tin oxide, hafnium dioxide, titanium dioxide, niobium pentoxide, gallium oxide, zirconium dioxide, yttrium oxide, or others), metal silicides (titanium disilicide, tantalum disilicide, tungsten disilicide, cobalt disilicide, nickel disilicide, platinum disilicide, palladium disilicide, or others), and / or gold-silicon alloys. The coating layer can have a thickness of ≤ 100 nm, preferably ≤ 50 nm.

[0015] In a preferred embodiment of the invention, the top layer is provided to have a material structure that reduces compressive stress and / or tensile stress.

[0016] The membrane surface can have a surface roughness of less than or equal to 20 Ra.

[0017] The membrane can be rectangular, polygonal, round, oval, elliptical, or square with respect to a plane having the normal direction as its normal. The membrane can have a constant or variable thickness with respect to a plane having the normal direction as its normal in at least one lateral longitudinal direction and / or transverse direction.

[0018] The membrane material can be silicon, in particular monocrystalline, polycrystalline, and / or amorphous silicon. The membrane can be composed of at most one membrane material or of at least two different membrane materials. The membrane material forming the membrane can be or contain silicon oxide. One membrane material can be silicon, and the membrane material encompassing the membrane surface can be silicon oxide.

[0019] The first or second region can have a hygromechanical material structure corresponding to the main part of the membrane material. The first region can be locally limited in that it spans at most a portion of the membrane surface. The first region can also be offset from the second region in the normal direction. The second region can be located on a rear surface of the membrane opposite the main surface. The first region can have a hydrophobic or hydrophilic material property.

[0020] An elastic protective material, such as a protective gel, can be applied in the normal direction above the membrane or on an upper side of the membrane facing away from the substrate.

[0021] In a preferred embodiment of the invention, it is advantageous if the first region has a material structure that, compared to the second region, increases or decreases compressive stress due to hygromechanical factors. In the first case, this means that, assuming constant humidity, the first region develops a higher moisture-related compressive stress per unit volume or unit area than the second region, which develops no or a lower moisture-related compressive stress. In the second case, assuming constant humidity, this means that, compared to the second region, the first region develops a lower moisture-related compressive stress per unit volume or unit area than the second region, which develops a higher moisture-related compressive stress.

[0022] Alternatively or additionally, the first area can have a material structure that increases or decreases tensile stress hygromechanically compared to the second area. In the first case, this means that, assuming constant humidity, the first area develops a higher moisture-related tensile stress per unit volume or area than the second area, which develops no or lower moisture-related tensile stress. In the second case, assuming constant humidity, this means that the first area develops a lower moisture-related tensile stress per unit volume or area than the second area, which develops a higher moisture-related tensile stress.

[0023] In a preferred embodiment of the invention, it is advantageous if the second region has a material structure that increases compressive stress hygromechanically compared to the first region. Given constant humidity, this means that the second region, compared to the first region, develops a higher compressive stress per unit volume or unit area due to humidity than the first region, which develops no or a lower compressive stress due to humidity.

[0024] Alternatively or additionally, the second area can have a material structure that increases tensile stress hygromechanically compared to the first area. Given constant humidity, this means that the second area develops a higher moisture-related tensile stress per unit volume or unit area than the first area, which develops no or a lower moisture-related tensile stress.

[0025] In a preferred embodiment of the invention, it is advantageous if the first and / or second region has a material structure that, compared to the majority of the membrane material, increases or decreases compressive stress hygromechanically. Alternatively or additionally, the first and / or second region can have a material structure that, compared to the majority of the membrane material, increases or decreases tensile stress hygromechanically.

[0026] A preferred embodiment of the invention is advantageous in which the first region has a crystal structure and / or crystal orientation that differs from the second region and / or the predominant material structure of the membrane material. The first region can have a crystalline or at least partially crystalline material structure, and the second region can have an amorphous material structure, or vice versa. Alternatively or additionally, the second region can have a crystal structure and / or crystal orientation that differs from the predominant material structure of the membrane material.

[0027] The first area can have a (100) crystal orientation and the second area can have a (110) crystal orientation or vice versa.

[0028] A preferred embodiment of the invention is advantageous in which the membrane material comprises or is polycrystalline silicon and the first and / or second region has an amorphous material structure. The amorphous material structure can be formed by depositing silicon at a lower temperature than the deposition temperature to form polycrystalline silicon. The amorphous material structure can be formed by laser treatment. Preferably, only one of the two regions of the first and second region can have an amorphous material structure.

[0029] The membrane material can alternatively or additionally contain or be monocrystalline silicon.

[0030] A preferred embodiment of the invention is advantageous in which the first region and / or the second region is arranged in the membrane material or as an additional layer on the surface of the membrane, in particular on the surface of the membrane material. The additional layer can be applied by physical or chemical vapor deposition, in particular plasma-enhanced chemical vapor deposition, electrochemical or galvanic deposition, electron beam evaporation, molecular beam epitaxy, atomic layer deposition, injection processes and / or sputtering.

[0031] A preferred embodiment of the invention is advantageous in which the second region at least partially encloses the membrane edge and / or the first region encompasses the membrane center of the membrane surface. Alternatively, the second region may also enclose the membrane center and / or the first region at least partially enclose the membrane edge.

[0032] In a specific embodiment of the invention, it is advantageous if the cover layer is made of a layer material with greater elasticity than the membrane material. The cover layer can have a hydrophobic material property.

[0033] In a particular embodiment of the invention, it is advantageous if a water diffusion barrier layer is arranged in the normal direction above the cover layer, in particular on a top surface of the cover layer, especially on a side of the cover layer facing away from the substrate. This reduces or prevents the diffusion of water from the environment to the membrane material.

[0034] Further advantages and advantageous embodiments of the invention will become apparent from the description of the figures and the illustrations. Character description

[0035] The invention is described in detail below with reference to the illustrations. These show, in detail: Fig. 1: A cross-section of a micromechanical sensor in a special embodiment of the invention. Fig. 2: A cross-section of a micromechanical sensor in a special embodiment of the invention. Fig. 3, Fig. 4 to Fig. 5: A top view of a membrane of a micromechanical sensor in each further specific embodiment of the invention. Fig. 6: A top view of a micromechanical sensor in a special embodiment of the invention. Fig. 7, Fig. 8 to Fig. 9: A cross-section of a membrane of a micromechanical sensor in each further specific embodiment of the invention.

[0036] Fig. Figure 1 shows a cross-section of a micromechanical sensor in a specific embodiment of the invention. The micromechanical sensor 10 is designed as a pressure sensor 12 for detecting a measured quantity, for example, ambient pressure, and comprises a substrate 14, preferably made of silicon, and a membrane 20, which is deflectable in a normal direction 16 into a free space 17 (cavity) depending on the measured quantity. The membrane 20 is made of at least one membrane material 18, preferably polycrystalline silicon, and is mechanically connected to the substrate 14 via a circumferential membrane edge 22. A membrane surface 24, facing away from the substrate 14, is oriented towards a humid environment 26. The membrane 20 is cantilevered to the substrate 14 in the normal direction above the free space and can contain a gas or gas mixture and have an internal pressure lower than the ambient pressure of the environment 26.

[0037] Moisture can cause compressive stresses 28 in the membrane material 18, which in the region of the membrane edge 22 cause a downward deflection of the membrane 20 in the normal direction 16, here towards the substrate 14, and in the membrane center 30 cause an upward deflection of the membrane 20 in the normal direction 16, here away from the substrate 14. However, if a compressive stress 28 is applied over the entire membrane surface 24, the membrane 20 is deflected downwards, since the component of the compressive stress in the region of the membrane edge 22 predominates over the component of the compressive stress in the region of the membrane center 30.

[0038] A locally confined first region 32 of the membrane surface 24 has a different hygromechanical material structure compared to a second region 34 that is laterally offset from it. The first region 32 comprises the center of the membrane 30 and is arranged laterally to the membrane edge 22. The second region 34 comprises, in particular, the area surrounding the first region 32, including the membrane edge 22. The first region 32 is composed of a material structure that increases hygromechanical compressive stress compared to the second region 34. This allows the moisture-induced compressive stress of the membrane edge 22 to be balanced by the moisture-induced compressive stress of the first region 32 due to the material structure in the center of the membrane 30, which increases hygromechanical compressive stress compared to the membrane edge 22.

[0039] Fig. Figure 2 shows a cross-section of a micromechanical sensor in a special embodiment of the invention. The micromechanical sensor 10 compensates for the Fig. 1 except for the following deviations. On the surface 38 of the membrane material 18 facing the environment 26, an additional layer 40 forming a hygromechanically compressive stress-increasing material structure is applied in the first region 32, which in this region forms the membrane surface 24 facing the environment 26. In the laterally adjacent second region 34, the surface 38 of the membrane material forms the membrane surface 24.

[0040] In Fig. Figure 3 shows a micromechanical sensor 10 with a membrane 20. The membrane 20, made of membrane material 18, is connected to the substrate via the surrounding membrane edge 22, with the membrane surface 24 facing the moisture-containing environment.

[0041] The first region 32, which is locally confined and encloses the membrane center 30, is at least partially surrounded by the circumferential membrane edge 22 and arranged laterally at a distance from the membrane edge 22. The second region 34, which at least partially surrounds the first region 32 laterally and encloses the membrane edge 22, has a hygromechanical material structure that differs from that of the first region 32. In order to compensate for the moisture-induced material stresses in the membrane 20, the first region 32 has a material structure that increases hygromechanical compressive stress compared to the second region 34.

[0042] The micromechanical sensor 10 in Fig. 4 balances that out Fig. 3 with the following differences. The first area 32 spans the membrane edge 22 and, compared to the second area 34 enclosing the membrane center 30, has a preferably hygromechanically compressive stress-reducing material structure.

[0043] In Fig. 5 is a micromechanical sensor 10 similar to the one in Fig. Figure 3 shows that several strip-shaped first regions 32 are arranged offset from one another in a lateral longitudinal direction 42 across the membrane surface 24. This allows the equalization of moisture-induced material stresses in the membrane 20 to preferably occur in a lateral longitudinal direction 42 and a transverse direction 44. The second region 34 is formed by the remaining area of ​​the membrane surface 24 surrounding the first regions 32.

[0044] The length 46 of the individual first regions 32 in the longitudinal direction 42 can be up to 20% of the membrane length 48 in the longitudinal direction 42. It is also conceivable to extend the first regions 32 in the transverse direction 44 in at least one direction or in both directions beyond the membrane edge 20. Additionally or alternatively, the first regions 32 can be subdivided along the transverse direction 44.

[0045] Fig. Figure 6 shows a top view of a micromechanical sensor in a particular embodiment of the invention. The micromechanical sensor 10 is, for example, a capacitive pressure sensor 12 with a first diaphragm 50 and a second diaphragm 52 offset from it, spaced apart and adjacent to it, each of which is associated with a measuring capacitance and a reference capacitance. The micromechanical sensor 10 comprises a substrate 14 and several bond pads 54 for the electrical connection of the measuring capacitances and the reference capacitances.

[0046] A first covering layer 56, preferably less than or equal to 1 µm thick, is arranged on the membrane material of the entire membrane surface 24 of the first membrane 50. This covering layer at least restricts the absorption of water by the membrane surface 24 from the environment. A second covering layer 58, also at most 1 µm thick, is arranged on the membrane material of the entire membrane surface 24 of the second membrane 52. This covering layer at least restricts the absorption of water by the membrane surface 24 from the environment. The first and second covering layers 56, 58 are laterally spaced apart and, in particular, span a respective membrane edge 22.

[0047] The membrane 20 in Fig. The assembly comprises, for example, a membrane material 18 consisting of polycrystalline silicon and a further membrane material 62 made of silicon oxide arranged in the normal direction 16 on the side of the membrane material 18 facing away from the substrate. A cover layer 64 made of a layer material 66 is additionally applied to the further membrane material 62. This layer material exhibits greater elasticity than the membrane material 18 and the further membrane material 62. The thickness 68 of the cover layer 64 is, in particular, less than or equal to 1 µm, preferably less than or equal to 100 nm.

[0048] The membrane 20 in Fig. 8 includes, in comparison to the one from Fig. 7 only the membrane material 18 consisting of polycrystalline silicon and the cover layer 64 on it.

[0049] The membrane 20 in Fig.9 comprises the covering layer 64 on the side of the membrane material 18 facing away from the substrate, which forms the membrane surface 24 and is created by diffusion of metals, metal oxides and / or metal silicides into the membrane material 18. A water diffusion barrier layer 70 can in turn be applied to this covering layer 64 to limit or prevent the diffusion of water molecules to the membrane surface 24.

Claims

[1] Micromechanical sensor (10) for detecting a measured quantity, comprising a substrate (14), a membrane (20) which can be deflected in a normal direction (16) depending on the measured quantity and which has at least one membrane material (18), which connects to the substrate (14) via a membrane edge (22) and which is oriented with a membrane surface (24) towards a moisture-containing environment (26), characterized by , that at least a locally limited first area (32) of the membrane surface (24) has a hygromechanical material structure that differs from a second area (34) laterally offset to it and / or a covering layer (64) with a layer thickness (68) of less than or equal to 1 µm is arranged on the membrane material (18) at least over the entire membrane surface (24) and restricts the water absorption of the membrane surface (24) from the environment (26). [2] Micromechanical sensor (10) according to claim 1, characterized by, that the first area (32) has a material structure that is hygromechanically increasing or decreasing compressive stress compared to the second area (34). [3] Micromechanical sensor (10) according to claim 1 or 2, characterized by , that the second area (34) has a material structure that increases compressive stress compared to the first area (32). [4] Micromechanical sensor (10) according to any one of the preceding claims, characterized by , that the first and / or second area (32, 34) has a material structure that hygromechanically increases or decreases pressure stress compared to the majority material structure of the membrane material (18). [5] Micromechanical sensor (10) according to any one of the preceding claims, characterized by, that the first area (32) has a crystal structure and / or crystal orientation that differs from the second area (34) and / or the majority material structure of the membrane material (18). [6] Micromechanical sensor (10) according to any one of the preceding claims, characterized by that the membrane material (18) has or is polycrystalline silicon and the first and / or second region (32, 34) has an amorphous material structure. [7] Micromechanical sensor (10) according to any one of the preceding claims, characterized by , that the first area (32) and / or the second area (34) is arranged in the membrane material (18) and / or as an additional layer (40) on the surface (38) of the membrane (20). [8] Micromechanical sensor (10) according to any one of the preceding claims, characterized by, that the second area (34) at least partially encloses the membrane edge (22) and / or the first area (32) completely encloses a membrane center (30) of the membrane surface (24). [9] Micromechanical sensor (10) according to any one of the preceding claims, characterized by , that the cover layer (64) is made of a layer material (66) with greater elasticity than the membrane material (18). [10] Micromechanical sensor (10) according to any one of the preceding claims, characterized by , that the top layer (64) has a hygromechanically compressive stress-reducing and / or tensile stress-reducing material structure. [11] Micromechanical sensor (10) according to any one of the preceding claims, characterized by , that a water diffusion barrier layer (70) is arranged in the normal direction (16) above the top layer (64).

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