Fluid sensor for the detection of fluid media

Abstract

Fluid sensor (10) for detecting fluid media, in particular gases, comprising a sensor chip (12) with a chip surface (14) that can be exposed to the fluid medium, wherein the chip surface (14) has a measuring surface (16) designed as a membrane and a land surface (18), wherein conductor tracks (22, 24) of a sensor circuit (20) with at least one heating element (22) and at least one temperature sensor (24) are applied to the measuring surface (16), wherein a boundary structure (36) designed as a cap (38) is arranged on the sensor chip (12) that at least partially surrounds the measuring surface (16) on a top side (40), and wherein the sensor chip (12) has a supply channel (32) for the fluid medium to a bottom side (34) of the measuring surface (16) and the measuring surface (16) can be exposed to the fluid medium at least on the bottom side (34).wherein the limiting structure (36) is arranged on the upper side (40) of the measuring surface (16) opposite the feed channel (32), characterized in that the limiting structure (36) has at least one opening (44).

Description

State of the art The invention relates to a fluid sensor for detecting fluid media, in particular gases such as hydrogen. Such sensors are used, for example, in automotive engineering for the detection and / or concentration measurement of hydrogen in a hydrogen-air mixture. In many processes, for example in the fields of process engineering, chemistry, or mechanical engineering, gas concentrations must be reliably determined and / or a defined gas mass flow, particularly an air mass flow, must be supplied. This includes, in particular, combustion processes that take place under controlled conditions. An important example, to which the present invention is not limited, is the combustion of fuel in internal combustion engines of motor vehicles, especially with subsequent catalytic exhaust gas purification. The supply of gases of a precisely defined composition to fuel cells is another area of ​​application. Safety-related applications should also be mentioned. For example, a hydrogen sensor can be used in fuel cell vehicles to warn vehicle occupants of a slow hydrogen leak.Air becomes ignitable at a hydrogen content of approximately 4%, and even explosive at higher concentrations. Therefore, the hydrogen sensor can be coupled with a corresponding warning device or emergency control system. Other safety-related applications for such gas sensors are also conceivable. Various types of sensors are used to measure gas flow and / or gas concentration. One class of such sensors are sensors with a sensor chip. A sensor type of this class known from the prior art is the so-called hot-film air mass meter (HFM), which is described, for example, in one embodiment in DE 196 01 791 A1. In such hot-film air mass meters, a sensor chip is typically used, which has a thin sensor membrane, for example, a silicon sensor chip. At least one heating resistor is typically arranged on the sensor membrane, surrounded by two or more temperature measuring resistors (temperature sensors). In an airflow that passes over the membrane, the temperature distribution changes, which in turn can be detected by the temperature measuring resistors and evaluated by means of a control and evaluation circuit.For example, an air mass flow rate can be determined from the resistance difference of the temperature measuring resistors. Various other variants of this sensor type are known from the prior art. Besides detecting flow, the detection and measurement of the components that make up the respective gaseous fluid plays a crucial role. One sensor principle is based on the different heat capacities and / or thermal conductivities of the various fluid components and is described, for example, in M. Arndt: "Micromachined Thermal Conductivity Hydrogen Detector for Automotive Applications," Sensors, 2002. Proceedings of IEEE. For instance, the detection of hydrogen in an air-hydrogen mixture utilizes the fact that hydrogen has a higher thermal conductivity than air or its components. In a sensor setup similar to that of hot-film air mass meters (HFM), an air-hydrogen mixture diffuses, for example, through a thin membrane or a fine mesh into a measuring chamber of the sensor.The presence of hydrogen in the gaseous fluid changes the temperature of the heated measuring membrane or its heat output, which is released into the surrounding air. This, in turn, generates a measurement signal that reflects the hydrogen concentration. DE 10 2006 010 901 A1 describes a fluid sensor for detecting fluid media, comprising a sensor chip with a chip surface that can be exposed to the fluid medium. This chip surface includes a measurement surface and a base surface. Conductive traces of a sensor circuit with at least one heating element and at least one temperature sensor are applied to the measurement surface. Despite the numerous advantages of existing fluid sensors, there is still room for improvement. In the aforementioned state of the art, the measurement surface is exposed, allowing external flows or convection currents to reach the sensor chip. Such influences lead to measurement effects, known as cross-sensitivities, which are not related to the concentration change being measured. DE 691 17 694 T2 discloses a fluid sensor for the detection of fluid media with the features of the preamble of claim 1. Other fluid sensors are known from US 2006 / 0000272 A1, EP 1 816 444 A2 and US 4 389 876 A. Disclosure of the invention Therefore, a fluid sensor for the detection of fluid media is proposed which can at least largely avoid the disadvantages of known fluid sensors and in which, in particular, external flows or convections are kept away from the sensor chip by limiting the structures in order to avoid measurement effects that are not related to a concentration change to be measured. The fluid sensor according to the invention for detecting fluid media, in particular gases, comprises a sensor chip with a chip surface that can be exposed to the fluid medium, wherein the chip surface has a measuring surface designed as a membrane and a base surface, wherein conductor tracks of a sensor circuit with at least one heating element and at least one temperature sensor are applied to the measuring surface, wherein a limiting structure designed as a cap is arranged on the sensor chip, which at least partially surrounds the measuring surface on a top side, and wherein the sensor chip has a supply channel for the fluid medium to a bottom side of the measuring surface, and the measuring surface can be exposed to the fluid medium at least on the bottom side, wherein the limiting structure is arranged on the top side of the measuring surface opposite the supply channel.According to the invention, the boundary structure has at least one opening. The boundary structure is arranged such that it at least partially surrounds the measuring surface on one side. The sensor chip is designed such that the measuring surface can be exposed to the fluid medium on at least one side. The measuring surface is designed as a membrane. The boundary structure is designed as a cap. The boundary structure can be arranged on a side of the measuring surface facing away from the fluid medium. According to the invention, the boundary structure has at least one opening. The sensor chip has a feed for the fluid medium to one side of the measuring surface. The feed is designed as a feed channel, for example, in a base on which the sensor chip is arranged. The boundary structure is arranged on the side of the measuring surface opposite the feed. The fluid sensor proposed according to the invention comprises a sensor chip with a chip surface that can be exposed to the fluid medium. This chip surface includes a measurement surface and a base surface. For example, as described above, this can be a silicon chip. In the area of ​​the measurement surface, the sensor chip can, for example, be designed such that it has a transverse thermal conductivity that is at least one order of magnitude lower than in the area of ​​the base surface. This can be achieved, for example, as with the hot-film air mass meter chips described above, by using sensor chips with a thin sensor membrane that is only a few micrometers thick. This takes advantage of the low thermal conductivity in the air (or an alternative gas) surrounding the sensor membrane. Alternatively, porous areas can be created within the chip as the measuring area, with a measuring surface facing the fluid medium being measured. This can be achieved, for example, by porosifying a silicon chip. In this way, measuring areas can be produced which, due to the enclosed air cavities, exhibit a transverse conductivity of 0.1 W / mK to 2 W / mK, compared to a silicon substrate with a thermal conductivity of approximately 156 W / mK. Conductive traces of a sensor circuit with at least one heating element and at least one temperature sensor are applied to or near the measuring surface. A fundamental concept of the present invention is to keep external currents or convections away from the sensor chip by means of a boundary structure. The sensor chip according to the invention can, for example, be a silicon chip comprising a membrane, a heating element, and a temperature sensor for measuring heat flow through a gaseous medium. The boundary structure is arranged such that it at least partially surrounds or encloses the measuring surface. In the context of the present invention, a boundary structure is understood to be any structure suitable for keeping flows or convection away from the sensor chip structure. The boundary structure can, in particular, be formed in the form of protrusions on the sensor chip that slow down, deflect, and / or shield fluid flows from the measurement surface. Since the boundary structure will also absorb some of the heating power, it is proposed to position the boundary structure as rigidly as possible relative to the measuring membrane to reduce unwanted cross-sensitivities. This is achieved by attaching the boundary structure directly to the sensor chip, rather than solely or not exclusively via a housing component. To further reduce cross-sensitivities, it is also proposed to make the boundary structure thermally conductive and to connect it to the sensor chip via a thermally conductive connection. Metallic materials or materials with metallic coatings are particularly suitable for this purpose. If less conductive materials such as silicon, glass, or plastic are to be used, good thermal conductivity can still be achieved, at least via small geometries or short heat transfer paths from the sensor chip through the boundary structure. The gas supply on the actuation side can simultaneously serve as the required boundary structure on that side. This supply can be connected to the sensor chip, for example, by adhesive bonding or by a wafer bonding process. The cap structure is open. This design achieves the most consistent conditions and the greatest signal amplitude when the sensor chip is exposed to light from both sides. With only one side exposed, such a semi-open cap ensures at least geometric and thermal stability on the side facing away from the measuring medium. Potential changes in humidity on the side facing away from the measuring medium can be at least partially compensated for by using an additional humidity sensor. Brief description of the drawings Further optional details and features of the invention will become apparent from the following description of preferred embodiments, which are shown schematically in the figures. Figure 1 shows a cross-sectional view of a fluid sensor according to a first embodiment, Figure 2 shows a cross-sectional view of a fluid sensor according to a second embodiment, Figure 3 shows a perspective view of a fluid sensor according to a first modification, Figure 4 shows a perspective view of a fluid sensor according to a second modification, and Figure 5 shows a perspective view of a fluid sensor according to a third modification. Embodiments of the invention Fig. 1 shows a cross-sectional view of a fluid sensor 10 according to a first embodiment of the present invention. The fluid sensor 10 is suitable for detecting fluid media, in particular gases such as hydrogen. The fluid sensor 10 comprises a sensor chip 12 with a chip surface 14 which can be exposed to the fluid medium. The chip surface 14 has a measurement surface 16 and a land surface 18. In the illustrated embodiment, the sensor chip 12 can, for example, be a silicon chip which has a membrane made of silicon oxide and / or silicon nitride in the area of ​​the measurement surface 16. A sensor circuit 20 with a heating element conductor 22 and a temperature sensor conductor 24 is applied to the measuring surface 16. In other words, the conductors 22 and 24 are configured as a heating element 22 and a temperature sensor 24. Connection pads 26 of the conductors 22 and 24 are connected to a control and / or evaluation circuit 28, which determines a hydrogen concentration according to the principle described above. Under constant ambient conditions regarding gas composition, pressure, and temperature, the parameters heating power, membrane temperature, and chip temperature are related to each other. A change in the ambient conditions, and in particular a change in concentration to be detected, leads to a change in this relationship, which in turn can be used to measure the concentration. In principle, it is irrelevant which of the three parameters—heating power, membrane temperature, or chip temperature—is specified and which is measured. For example, a constant heater voltage can be specified for the fluid sensor 10, and all three parameters will be measured. Another possibility would be to regulate to a defined temperature or temperature difference and measure the heating power required for this. The sensor chip 12 is arranged on a base 30. The base 30 is, for example, a glass base. The base 30 can be connected to the sensor chip 12, for example, by adhesive bonding or alternatively by a wafer bonding process. A feed channel 32 is formed in the base 30. The sensor chip 12 is arranged on the base 30 such that the fluid medium to be detected can reach a lower surface 34 of the measuring surface 16 via the feed channel 32. Consequently, the lower surface 34 of the measuring surface 16 can be supplied with the fluid medium by means of the feed channel 32. The feed channel 32 opens into a cavity 35 located below the lower surface 34 of the measuring surface 16. A boundary structure 36 is arranged on the sensor chip 12. The boundary structure 36 at least partially surrounds the measurement surface 16. For example, the boundary structure 36 is designed as a cap 38. The cap 38 at least partially surrounds the measurement surface 16 on a top surface 40 of the measurement surface 16 that faces away from the feed channel 32 and opposite the bottom surface 34. The base 30 also acts as a boundary structure 36 on the bottom surface 34 of the measurement surface 16. Fig. 2 shows a cross-sectional view of a fluid sensor 10 according to one embodiment of the present invention. Only the differences from the first embodiment are described below, and identical components are designated with the same reference numerals. In the fluid sensor 10 of the second embodiment, the boundary structure 36 has at least one opening 44. For example, the boundary structure 36 is designed as a semi-open cap 38, which has the opening 44. The opening 44 is designed such that the cap 38 has an L-shaped cross-section. In other words, the cap 38 is completely open at its front 46. This embodiment allows the sensor chip 12 to be supplied with the fluid medium from both sides. Thus, the fluid medium reaches the underside 34 of the measuring surface 16 through the supply channel 32 and the upper side 40 of the measuring surface 16 through the opening 44 in the cap 38. In this way, the most consistent conditions and, at the same time, the greatest signal amplitude are achieved.In the case of one-sided exposure at the semi-open cap 38 of the fluid sensor 10 according to the second embodiment, geometric and thermal constancy is ensured on the upper surface 40 of the measuring surface 16 facing away from the fluid medium. Potential changes in humidity on the upper surface 40 facing away from the fluid medium can be at least partially compensated for by using an additional humidity sensor. Fig. 3 shows a perspective view of a fluid sensor 10 according to the invention, based on a first modification of the embodiment. Only the differences from the second embodiment are described below, and identical components are designated with the same reference numerals. In the embodiment of the fluid sensor 10 shown in Fig. 3, the opening 44 is designed as a through-channel 48 that connects opposite side surfaces 50 of the cap 38. The opening 44 can thus be designed as a through-opening, so that it penetrates both side surfaces 50 of the cap 38. Fig. 4 shows a perspective view of a fluid sensor 10 according to a second modification of the embodiment. Only the differences from the second embodiment are described below, and identical components are designated with the same reference numerals. In the fluid sensor 10 shown in Fig. 4, the opening 44 is formed on the front 46 of the cap 38, but does not penetrate it completely, i.e. the opening 44 does not extend to a back 52 of the cap 38, but is formed similarly to a blind hole. Fig. 5 shows a perspective view of a fluid sensor 10 according to a third modification of the embodiment. Only the differences from the second embodiment are described below, and identical components are designated with the same reference numerals. In the fluid sensor 10 shown in Fig. 5, the opening 44 is formed in a top surface 54 of the cap 38. For example, the opening 44 is designed as a cylindrical channel.

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