Mems-based fluid sensor having vertically spaced thermal structures

Abstract

The invention relates to a thermal fluid sensor (1) comprising: a membrane (7) which has an aperture (19); a heating element (9); and a temperature sensor (11). The heating element (9) and the temperature sensor (11) are positioned such that at least one property of a fluid flowing through the aperture (19) can be measured on the basis of a modulation of a heat flow between the heating element (9) and the temperature sensor (11). The thermal fluid sensor (1) has a support structure (13) which extends at least partly over the aperture (19) at a vertical distance from the membrane (7), wherein the temperature sensor (11) or the heating element (9) is attached to the support structure (13). The invention also relates to a method for producing the thermal fluid sensor and to a use of the thermal fluid sensor for measuring one or more properties of the fluid.

Description

[0001] MEMS-BASED FLUID SENSOR WITH VERTICALLY SPACED THERMAL STRUCTURES

[0002] DESCRIPTION

[0003] The invention relates to a thermal fluid sensor comprising a membrane having an aperture, a heating element, and a temperature sensor. The heating element and the temperature sensor are positioned such that at least one property of a fluid flowing through the aperture can be measured based on a modulation of a heat flow between the heating element and the temperature sensor. The thermal fluid sensor has a support structure that extends at least partially across the aperture at a vertical distance from the membrane, with the temperature sensor or the heating element being mounted on the support structure.

[0004] Furthermore, the invention relates to a method for producing the thermal fluid sensor and to a use of the thermal fluid sensor for measuring one or more properties of the fluid.

[0005] Background and state of the art

[0006] Microsystems technology is now used in many applications to manufacture compact, mechanical-electronic devices. The resulting microsystems (microelectromechanical systems, or MEMS) are extremely compact (in the micrometer range) while simultaneously offering excellent functionality and ever-decreasing manufacturing costs.

[0007] MEMS technologies offer a variety of mechanisms for actuation and sensing. MEMS designs and components can also be used for temperature measurements. Temperature measurement can also be used to infer the properties of a flowing fluid. A MEMS-based component that can be used to infer fluid properties through temperature measurements will be referred to below as a thermal fluid sensor, thermal flow sensor, or thermal flow sensor.

[0008] Thermal fluid sensors can be divided into different categories based on their basic physical principle. Thermoresistive sensors use resistive elements for thermal detection. Thermoelectric sensors detect thermal changes using thermopiles. In contrast, diode and transistor elements are used in thermoelectric sensors. Changes in the resonant frequency in mechanical structures due to temperature-induced stresses are used for frequency-analog sensors (see Kuo, Yu & Meng (2012)).

[0009] Thermal fluid sensors can be used in a wide variety of applications, for example for the detection of gases or as a microphone.

[0010] WO 2021 / 038099 A1 discloses a photoacoustic gas sensor which, in addition to the components commonly used in the prior art for utilizing the photoacoustic effect for gas detection, also includes a sensor based on a thermal operating principle. The sensor can comprise a heating element, for example a heating resistor, and a thermopile and is preferably arranged on a separating membrane with at least one aperture located between a detection and reference chamber of the gas sensor. Due to a modulation of the heat transfer between the heating element and the temperature sensor by the generated photoacoustic pressure (depending on the gas concentration), a flow occurs across the perforations, which modulates the heat transfer between the heating element and the sensor. A photoacoustic signal can therefore be detected via the modulation of the heat transfer.

[0011] WO 2019 / 135004 A1 discloses a gas sensor comprising a membrane, a heating element, and at least two temperature sensors. The membrane extends along a frame forming a carrier material. Furthermore, the membrane has at least two interrupted regions. The heating element is arranged on the membrane between the at least two interrupted regions. The temperature sensors are located at least partially on the membrane, on opposite sides of the heating element. The gas to be analyzed flows through the interrupted regions, allowing gas-specific heat to flow from the heating element to the temperature sensors. Properties of the gas can be detected based on the transit times of heat transport through the gas between the heating element and a temperature sensor.

[0012] Xu et al. (2022) discloses a sensor in a monolithic structure that can detect the sound velocity of particles or flows based on the measured temperature difference between closely spaced heating wires. For this purpose, heating wires distributed in a plane are arranged with an acoustically transparent heat sink. The acoustically transparent heat sink can be in the form of a grid. Four sensor arrays comprising a heating element and two detectors are placed along a plane on a silicon nitride membrane.

[0013] From the publication JPH 08320245 A, a flow sensor is known which is capable of measuring the flow rate and flow velocity of a fluid in a pipeline. For this purpose, the flow sensor comprises a heater and two temperature sensors. While one of the two temperature sensors is exposed to the fluid flow in the pipeline, the other temperature sensor is to be protected from the fluid flow. For this purpose, the flow sensor comprises a first substrate and a second substrate, which are positioned one above the other such that the second substrate is above the first substrate. The heater is located on an intermediate layer arranged between the first substrate and the second substrate. The intermediate layer can be provided by an oxynitride film. The temperature sensors are each located on a further oxynitride film, which extends above the second substrate and below the first substrate.Only the upper temperature sensor is exposed to the fluid flow, while the lower temperature sensor is protected from the fluid flow in a cavity formed by a substrate. Holes are provided in the oxynitride film so that the fluid to be measured can pass through the holes to both temperature sensors and the heater. The flow sensor of JPH 08320245 is intended to enable the measurement of the flow rate and flow velocity of the fluid in a pipeline. A measurement of the properties of a fluid flowing through the flow sensor or an aperture of the flow sensor is not disclosed.

[0014] Known thermal fluid sensors are characterized by their simple processing, compact design, and functionality. However, there is also a need for improvement. In particular, known arrangements can result in inefficient heat conduction from the heating element to the temperature sensor, particularly due to the arrangement of the components on the membrane and / or the design of the membrane, and thus in a loss of measurement accuracy. Reducing or eliminating measurement noise and increasing sensitivity would also be desirable. Furthermore, given the versatile applicability of thermal fluid sensors, there is still a drive toward progressive sensor miniaturization.

[0015] Object of the invention

[0016] The object of the invention is to provide a thermal fluid sensor and a method for its production that eliminates the disadvantages of the prior art. In particular, the object of the invention is to provide a thermal fluid sensor that is characterized by reliable functionality and high measurement accuracy while maintaining a compact design.

[0017] Summary of the invention

[0018] The object of the invention is achieved by the independent claims. Preferred embodiments of the invention are described in the dependent claims.

[0019] In a first aspect, the invention relates to a thermal fluid sensor comprising a membrane with an aperture, a heating element and a temperature sensor, which are positioned such that at least one property of a fluid flowing through the aperture can be measured based on a modulation of a heat flow between the heating element and the temperature sensor, wherein the thermal fluid sensor has a support structure which extends at least partially over the aperture at a vertical distance from the membrane, wherein the temperature sensor or the heating element is mounted on the support structure.

[0020] Preferably, the heating element and the temperature sensor in the thermal fluid sensor are vertically spaced from one another, with one component preferably being mounted on the membrane and the other component being mounted on the support structure. The thermal fluid sensor is preferably characterized in that the temperature sensor is mounted on the support structure, while the heating element is mounted on the membrane, or that the temperature sensor is mounted on the membrane, while the heating element is mounted on the support structure.

[0021] The thermal fluid sensor according to the invention has proven advantageous in a number of aspects. In particular, the arrangement according to the invention can effectively reduce measurement noise. In known thermal fluid sensors of the prior art, the arrangement of the temperature sensor and heating element on one level can lead to a significant heat transfer through material-bound heat conduction. The sensitivity of a preferred thermal fluid sensor, on the other hand, is based on heat transfer by convection via the analyzing fluid, which flows between the temperature sensor and heating element. In other words, the modulation of the heat flow between the heating element and the temperature sensor occurs through the fluid (to be analyzed), so that properties of the fluid can be determined based on the measurable influence of the fluid on the heat flow.For example, heat transfer by convection depends advantageously on the properties of the fluid to be analyzed, so that conclusions can be drawn about, for example, flow parameters, pressure fluctuation(s), composition and / or concentration of the fluid based on a modulation of the heat transfer.

[0022] In-plane heat conduction, for example, across a membrane on which the temperature sensor and heating element are arranged, results in heat transfer that is independent of the properties of the fluid being analyzed. Changes in the thermal conductivity of the membrane or other materials or components of the sensor therefore lead to thermal modulation between the temperature sensor and heating element, which is independent of the properties of the fluid but is reflected in the measurement results as noise. In particular, heat conduction across the membrane results in a higher temperature at the temperature sensor, which increases the thermal noise in the output signal, as this scales proportionally to the square root of the temperature.

[0023] According to the invention, this disadvantageous effect of the prior art is eliminated by reducing heat conduction in the plane between the heating element and the temperature sensor by locating the heating element or the temperature sensor on a support structure, thereby providing a vertical displacement between the heating element and the temperature sensor.

[0024] This arrangement advantageously ensures that heat transfer from the heating element to the temperature sensor occurs primarily through convection via the fluid being analyzed. The heat modulation thus reflects properties of the fluid being analyzed (flow parameters, pressure fluctuations, heat conduction, etc.). Noise caused by material-bound heat conduction, which is independent of the fluid's properties, is advantageously reduced.

[0025] A further advantage of the thermal fluid sensor according to the invention is that it enables a particularly controlled modulation of the heat flow transmitted from the heating element to the temperature sensor. In particular, the support structure, which extends at least partially at a vertical distance across the aperture of the membrane, allows a particularly small cross-section in the form of a channel for a flow direction of the fluid through the thermal fluid sensor to be achieved. The modulation of the heat flow depends on the design of the channel. However, the support structure advantageously enables the adjustment of a fluidic resistance for the fluid flowing through the heat flow. Thus, an acoustic resistance for the sound pressure waves can be advantageously adjusted, particularly in the case of an oscillating fluid flow.Depending on the application, the desired measurement sensitivity or acoustic properties of the fluid sensor can be easily adjusted. Advantageously, signal amplification can be easily achieved by simultaneously attaching multiple temperature sensors. For this purpose, a channel can be provided, along the sides of which temperature sensors and / or heating elements can be arranged, for example, in series. The support structure advantageously allows for a small cross-section of the channel even for such a geometry by adjusting the vertical distance of the support structure above the membrane accordingly.

[0026] The channel preferably refers to a spatial section that determines the flow behavior of the fluid between a section of the support structure and the membrane or a heating element or temperature sensor installed thereon. The channel is formed in particular by a vertical extension of the support structure above the membrane (or a temperature sensor or heating element installed thereon). The design of the channel is therefore determined in particular by the dimensions of the support structure in relation to the underlying membrane (or a temperature sensor or heating element installed thereon). The heat flow from the heating element to the temperature sensor leads through the channel, so that the modulation of the heat flow of the fluid depends on the flow behavior in the formed channel.

[0027] Advantageously, the design of the support structure allows for a virtually arbitrarily low channel height, for example, less than 10 pm, 5 pm, 1 pm, 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, or 50 nm. The channel height preferably refers to the lowest height or lowest vertical extension of a flow section within the thermal fluid sensor, which preferably exists between a temperature sensor or heating element installed on the membrane and a support structure extending above it.

[0028] This advantageously provides a particularly compact fluid sensor which is also characterized by excellent measuring properties.

[0029] The possibility of sensitive measurements and higher resolution results, among other things, from higher fluid flow velocities in the heat path between the temperature sensor and the heating element, which are made possible by the design of the support structure or the setting of a low-height channel. This allows for particularly high-precision determination of fluid properties, such as its flow velocity. Advantageously, this method also allows for the detection of particularly small pressure differences, making the sensor ideal for acoustic measurements as a sound detector, for example, in a photoacoustic gas sensor.

[0030] In particular, for dynamic measurements of pressure fluctuations, a low channel height also allows for a high acoustic resistance. This allows the measurement range to be extended with respect to low cutoff frequencies or a smaller back volume to be provided for desired measurement frequencies.

[0031] For the acoustic resistance, the relationship

[0032] (1 ) Rs ~ h / (lw 3 )

[0033] (see Lauwers, Gliere & Basrour (2020)), where Rs is the acoustic resistance, h is the length, I is the width and w is the height, where w is the channel height between the temperature sensor or heating element and the support structure (see Fig. 1 ).

[0034] The design of the support structure is advantageous due to a particularly low height and thus due to the antiproportional relationship between Rs and w 3 a particularly high acoustic resistance can be achieved.

[0035] In view of the achievable high acoustic resistance, a lower back volume can be advantageously provided for dynamic measurements of pressure fluctuations at relatively lower frequencies, as will be evident from the following explanations.

[0036] For the back volume V the following relationship applies:

[0037] (2) C bv = V / (y Po), where Cbv is the acoustic compliance or capacity, y is the adiabatic coefficient of the fluid and Po is the static pressure of the fluid, which is usually 1 atm (physical unit atmosphere).

[0038] For a lower acoustic cutoff frequency tu the following applies:

[0039] (3) w = 1 / (R s C bv ).

[0040] The capacity C bvcorresponds to a back volume, which is present in a spatial section between the carrier and a cover. The acoustic resistance Rs can advantageously be set particularly high by a small distance w (see equation (1)). Thus, a particularly low lower cutoff frequency can be advantageously ensured even with a relatively small back volume by selecting a sufficiently high acoustic resistance.

[0041] A lower limit frequency preferably refers to a frequency below which, in the case of dynamic pressure fluctuations, the thermal fluid sensor can measure properties of the fluid with reduced sensitivity (S).

[0042] As explained above, the lower cutoff frequency is determined primarily by the acoustic or fluidic resistance, the capacitance, and the back volume. Providing a support structure to form a channel with any desired height advantageously allows for a low lower cutoff frequency while maintaining a compact design.

[0043] An upper frequency limit preferably indicates a frequency above which, in the case of dynamic pressure fluctuations, the thermal fluid sensor measures fluid properties with reduced sensitivity. The upper frequency limit is influenced, for example, by the heat capacity of the temperature sensor or the thermal resistance of the membrane. Thus, at high frequencies of pressure fluctuations and a correspondingly rapidly fluctuating heat modulation, the temperature sensor may reach a measurement limit at which it can no longer resolve the modulation. In this regard, it should be noted that the modulation of the heat flow at a higher frequency preferably occurs twice as high as the frequency of an acoustic signal.This is particularly due to the fact that the temperature sensor detects a lower temperature in both a positive pressure cycle (when the pressure increases) and a low pressure cycle (when the pressure decreases), due to a corresponding modulation of the heat flow.

[0044] By providing a miniaturized temperature sensor and a thermally insulating support structure, high upper limit frequencies can also be advantageously achieved.

[0045] As a result, the described fluid sensor can preferably achieve both a particularly low lower limit frequency and a high upper limit frequency, so that a broad frequency band is available in which the thermal fluid sensor can make reliable measurements of pressure fluctuations and thus also of sound waves (particularly with regard to audio applications).

[0046] Furthermore, the support structure allows for a particularly close arrangement between the temperature sensor and the heating element. This close placement advantageously enables energy-efficient operation of the thermal fluid sensor, as even operating the heating element at lower power / temperature delivers good measurement results due to its proximity to the temperature sensor.

[0047] Preferably, the thermal fluid sensor is a thermal MEMS fluid sensor, which is based on MEMS technology and whose sensor structures are at least partially dimensioned in the micrometer range (1 pm to 1000 pm). For example, the heating element, the temperature sensor, the support structure, the membrane, or the formed channel can preferably have a height, width, and / or length in the range of less than 1000 pm. It may also be preferred that, for example, only the height of the channel is less than 1000 pm, while, for example, the width of the channel has a larger dimension.

[0048] Furthermore, the thermal fluid sensor according to the invention can preferably be manufactured with particularly efficient process efficiency. The thermal fluid sensor according to the invention can preferably be manufactured using means and process steps of the semiconductor and / or microsystem that have proven themselves in the prior art, ensure fast and simple processing, and are suitable for mass production. For example, to manufacture the thermal fluid sensor, it may be preferable to provide the membrane with the aperture and support structure in a (semiconductor) process. This further simplifies and reduces the cost of manufacturing, so that a compact and robust thermal fluid sensor can be provided cost-effectively.

[0049] For the purposes of the invention, a thermal fluid sensor refers to a sensor capable of measuring parameters of a fluid flowing between the heating element and the temperature sensor based on a modulation of heat or a heat flow. In other words, a thermal fluid sensor utilizes the fact that modulation of the heat flow between the heating element and the temperature sensor allows conclusions to be drawn about the properties of the fluid flowing between these components.

[0050] The heat flow refers to the heat that emanates from the heating element and is transferred to the temperature sensor. It is known to those skilled in the art that heat in itself is a form of energy. The fluid that flows between the temperature sensor and the heating element, and thus through the heat flow, influences the heat flow. This influence and the associated change in the heat flow through the fluid can be detected by the temperature sensor, whereby the change in the heat flow is referred to as modulation of the heat flow in the context of the invention. From a structural point of view, the thermal fluid sensor comprises the components essential for functionality, including a membrane with an aperture, a heating element, a temperature sensor, and a support structure. Other components that the thermal fluid sensor has can also be preferred.

[0051] The membrane has an aperture through which the fluid flows to the heat flow, with the heat flow being established between the heating element and the temperature sensor. Thus, the aperture of the membrane preferably enables a transition of a fluid from a cavity in the carrier into the channel in the direction of the heat flow. The cavity of the carrier refers to a recess in the carrier. The cavity of the carrier is located in a front volume. Thus, the membrane separates the front volume from a rear volume. The rear volume preferably refers to a spatial section that the fluid reaches after it enters the channel from the opening of the membrane and then passes through the heat flow. A rear volume in the sense of a MEMS microphone can be enabled by a rear cover that is attached to the carrier above the membrane. The rear volume can also have no boundary.

[0052] The aperture preferably refers to an opening provided in the membrane. The preferred support comprises, for example, two or more side regions between which the membrane is arranged. The membrane itself can preferably be applied to a sacrificial layer coated on the support. The design of the channel provided by the support structure can be regulated by the thickness of the sacrificial layer on which the membrane is coated. Furthermore, the temperature sensor or the heating element is preferably located on the membrane, whereas the respective other component is located on the support structure. If the heating element is mounted on the support structure, the temperature sensor is located on the membrane. If the temperature sensor is on the support structure, the heating element is located on the membrane.

[0053] The support structure preferably refers to a component of the thermal fluid sensor according to the invention to which the temperature sensor or the heating element is attached. Structurally, the support structure is designed such that it extends at least partially over the aperture at a vertical distance from the membrane. For example, the support structure can have a base and a longitudinal section for this purpose, wherein the longitudinal section preferably runs parallel to the membrane and is arranged at a vertical distance above the latter. In the context of the invention, the support structure can predetermine a vertical distance between the temperature sensor and the heating element, wherein the vertical distance is adjustable, for example, via the height of the base. In addition to the vertical distance or the vertical displacement between the heating element and the temperature sensor, an additional lateral distance may be preferred.As explained in more detail below, a lateral distance can be used, for example, to adjust a sensitivity by adjusting a heat path for the heat flow.

[0054] By arranging the temperature sensor or heating element on the support structure and membrane with a vertical spacing, heat conduction between the two components via the membrane can be advantageously reduced. This allows heat transfer to occur primarily by convection, while simultaneously avoiding or reducing heat losses due to heat conduction (through the membrane). The support structure can therefore also be viewed as a spacer or insulation element, which, on the one hand, reduces heat conduction and, on the other hand, enables an advantageous design of a channel with almost any desired low height (and thus high acoustic resistance) when the fluid flows through the heat flow.

[0055] In the context of the invention, the fluid preferably refers to a substance that can be continuously deformed by the action of shear forces. The fluid is thus, in particular, a gas or a liquid that can flow through the thermal fluid sensor and thus be measured. Since the influence of the fluid on the heat flow can be used to draw conclusions about the fluid properties, the thermal fluid sensor according to the invention can also be considered a liquid sensor or a gas sensor, depending on the fluid's phase.

[0056] In a further preferred embodiment, the thermal fluid sensor is characterized in that the at least one property of the fluid flowing through the aperture, which is measurable by means of a modulation of a heat flow between the heating element and the temperature sensor, is a flow parameter, a pressure fluctuation, a composition and / or concentration of the fluid.

[0057] Therefore, a multitude of properties of a fluid can advantageously be measured by means of the preferred thermal fluid sensor, which opens up a wide range of applications.

[0058] In a preferred embodiment, a flow parameter can be measured based on a modulation of the heat flow between the heating element and the temperature sensor. Preferably, a flow parameter is selected from a group comprising a flow velocity, a pressure, a volume flow, and / or a flow frequency (in the case of an oscillating flow) of the fluid.

[0059] The heating element can preferably be configured so that, at a constant heating temperature, it emits constant heat and thus a fixed heat flow, so that the temperature sensor detects a measuring temperature. The effect of the fluid modulates the heat flow, resulting in a change in the measuring temperature measured at the temperature sensor. The change in the measuring temperature is detected by the temperature sensor. In other words, the modulation of the heat flow is detected by a change in the measuring temperature at the temperature sensor. For example, the higher the flow velocity of the fluid, the lower the measurable temperature. In other words, at a high flow velocity, the difference to the base temperature is greater than at a lower flow velocity.The difference between the base temperature and the measured temperature can be used to determine the flow velocity.

[0060] It may also be preferred, for example, for the fluid sensor to be regulated to a constant measuring temperature at the temperature sensor. In this case, the heating element is preferably configured so that it emits heat, which ensures a constant measuring temperature at the temperature sensor. The effect of the fluid results in a modulation of the heat flow. However, a control signal for the heating element can be used to adjust the heat emission or heating temperature in such a way that the heat modulation is balanced out and a constant heat transfer to the temperature sensor occurs, so that the measuring temperature at the temperature sensor is kept constant. The modulation of the heat is therefore captured by the necessary control of the heating element. If the flow velocity is high, the control signal for providing the heat flow from the heating element must be intensified in order to maintain a constant heat transfer.

[0061] In both of these examples, the pressure and thus pressure fluctuations of the fluid flowing through the heat flow can also be measured. The higher the pressure or pressure fluctuations, the greater the modulation of the heat flow.

[0062] In Balakrishnan et al. (2017) the connection

[0063] (4) P / AT = A + B Vü, where A and B are material-dependent constants, P is the fluid pressure, AT is the temperature difference due to the fluid action, and u is the flow velocity. This shows that a higher pressure results in a greater difference in heat flow.

[0064] The pressure or the fluid can also propagate through the thermal fluid sensor through the heat flow due to pressure fluctuations. Since pressure fluctuations of an elastic fluid represent sound waves, it is also possible to detect sound pressure waves using the preferred thermal fluid sensor. As disclosed in more detail elsewhere herein, the preferred thermal fluid sensor can therefore also advantageously be used as a microphone. The composition and / or concentration of the fluid flowing through the heat flow can also be measured using the preferred thermal fluid sensor. This can preferably be done in the context of photoacoustic spectroscopy. For example, pressure fluctuations resulting from a photoacoustic effect due to the detection of a specific fluid and / or its detected concentration can be detected by the modulation of the heat flow.In a further preferred embodiment, the thermal fluid sensor is characterized in that a channel is formed between the heating element and the temperature sensor by a portion of the support structure and the membrane, wherein a flow path for the fluid can be provided through the channel. In other words, a channel is preferably formed between the heating element and the temperature sensor by a portion of the support structure and the membrane, wherein the flow path for the fluid is defined by the channel.

[0065] The channel preferably refers to a space between the temperature sensor or heating element on the membrane and the longitudinal section of the support structure, through which the fluid flows after it has penetrated through the aperture. The channel is defined in particular by the support structure, which extends vertically across the membrane. The channel is also determined by the design of the temperature sensor or heating element mounted on the membrane. Furthermore, the thickness of sacrificial layers, for example, applied between the carrier and the membrane, can influence the design of the channel, as these can reduce the distance between the membrane and the support structure.

[0066] The attachment of the support structure makes it possible to provide a flow section for the fluid through the thermal fluid sensor, wherein, in particular, a channel is formed in the flow section between a region of the support structure (e.g., a longitudinal section) and the membrane or components installed thereon. The channel is located, in particular, in the region of the fluid flow section through which the heat flow is transferred from the heating element to the temperature sensor. Therefore, in the context of the invention, the channel can also be referred to as a modulation channel.

[0067] The channel can be considered an exit region if the fluid flows from a front-side volume through the cavity of the carrier via the aperture of the membrane through the channel. Conversely, the channel can also be considered an entry region if the fluid flows from a rear-side volume through the channel to the aperture in the direction of the cavity to a front-side volume.

[0068] The channel preferably has dimensions in the three spatial dimensions of length, height, and width. The length of the channel refers to the spatial dimension that corresponds to the flow direction of the fluid within the thermal fluid sensor. If the flow direction is specified by a vector, the length of the channel is preferably parallel to this vector to indicate the flow direction. The height of the channel is a component orthogonal to the length and extends vertically orthogonal to the plane of the membrane. The height of the channel is determined in particular by the smallest vertical component of the flow section within the thermal fluid sensor. The smallest vertical component of the flow section, or the height of the channel, is the distance between the temperature sensor or heating element positioned on the membrane and the longitudinal section of the support structure located above it.

[0069] The width of the channel refers to a component that is perpendicular to the length and height of the channel. Width and length lie in the plane of the membrane, with the length corresponding to the direction of fluid flow. In Fig. 1, the length, width, and height are labeled for clarity. The acoustic resistance RS from equation (1) corresponds to the dimensions of the channel in terms of length h, width l, and height w. The cross-section of the channel through which fluid flows is calculated as the product of width and height (as a reference section for the flow direction of the fluid or from the fluid's perspective).

[0070] The length of the channel is preferably between the boundary of the membrane and the aperture and the boundary of the longitudinal section above the membrane. Thus, the length of the channel can also be considered a partial length of the support structure (preferably the longitudinal section) and the membrane. The length of the channel is preferably in a range between 1 pm and 200 pm, preferably 5 pm - 100 pm, or even 10 pm - 50 pm. The length of the channel is adjustable, in particular, by the length of a longitudinal section of the support structure above the membrane.

[0071] The channel height is preferably in a range between 50 nm and 5 pm, particularly preferably between 100 nm and 5 pm, 200 nm and 2 pm, or even 300 nm and 1 pm. Such low channel heights can preferably be formed by conformal sacrificial layer deposition.

[0072] The width of the channel is preferably between 1 pm and 50 mm, preferably between 5 pm and 10 pm, or even 10 pm to 1000 pm. In the cross-sectional views of the figures below, the length and height are shown schematically (not necessarily to scale), while the width dimension corresponds to a (non-visible) depth of field of the figures. In the case of an elongated slit of an aperture, the channel will preferably also have a greater width in the dimension of the depth of field.

[0073] In a further preferred embodiment, the thermal fluid sensor is characterized in that a flow direction of the fluid is changed as it flows through the thermal fluid sensor, at least in one region, wherein a change in the flow direction preferably has an angle of substantially 90°. The region preferably represents a flow section for the fluid within the thermal fluid sensor.

[0074] Terms such as substantially, approximately, about, approx., etc., preferably describe a tolerance range of less than ± 40%, preferably less than ± 20%, particularly preferably less than ± 10%, even more preferably less than ± 5%, and especially less than ± 1%, and always include the exact value. Partially, preferably, describes at least 5%, particularly preferably at least 10%, and especially at least 20%, in some cases at least 40%.

[0075] Thus, the change in flow direction can preferably also be 130°, 125°, 120°, 115°, 110°, 105°, 100°, 95°, 90°, 85°, 80°, 85°, 80°, 75°, 70°, 60°, 55°, or 50°. The angle for the change in flow direction is preferably predetermined by the support structure, preferably by an angle that exists between a base and a longitudinal section, which preferably determine the shape and / or configuration of the support structure. In particular, it is advantageously possible to direct the fluid particularly precisely in the direction of the heat flow, which is made possible by the channel. The fluid preferably flows through the aperture of the membrane and is deflected by the support structure so that the fluid flows through the channel within the flow section, wherein the heat flow between the temperature element and the heating element is preferably substantially orthogonal to the flow path of the fluid.

[0076] In a further preferred embodiment, the thermal fluid sensor is characterized in that the thermal fluid sensor is configured to measure a continuous or oscillating fluid flow, preferably sound pressure waves.

[0077] A continuous fluid flow preferably refers to a fluid flow that has instantaneous values ​​that do not change over time. An instantaneous value refers to a value of a time-dependent quantity at a given point in time. Thus, in analogy to electrical engineering, a continuous fluid flow is comparable to a direct current, which is why the English term "DC flow" can also be used to describe the continuous fluid flow. The preferred thermal fluid sensor is advantageously capable of regulating the flow velocity at which the continuous fluid flow propagates by changing the size of the aperture incorporated in the membrane. Thus, the flow velocity is higher when the aperture is designed to be smaller. Conversely, the flow velocity can be reduced when the aperture is larger.The aperture can therefore be used to adjust the flow velocity of the fluid into the channel of the thermal fluid sensor. The design of the channel itself can also adjust the flow velocity within the channel. In particular, the cross-section of the channel, i.e. the product of height and length (see Fig. 1, w * I, where I denotes the dimension in the image plane) determines the flow velocity of the fluid at which it can pass through the heat flow. The smaller the cross-section, the higher the flow velocity of the fluid within the channel. With a larger cross-section of the channel, the flow velocity can be reduced. The dimensions of the channel can in turn be determined by the support structure.Consequently, the flow velocity relevant for the modulation, which exists at the point where the flow crosses the heat flow, can be adjusted by the dimensions of the support structure.

[0078] An oscillating fluid flow preferably refers to a fluid flow that propagates in the form of a wave, resulting in a change in at least one location- and time-dependent variable of the fluid. Accordingly, an oscillating fluid flow is comparable to an alternating current in electrical engineering, which is why the English term AC flow can also be used to describe the oscillating fluid flow. In other words, an oscillating fluid flow propagates via pressure waves. Consequently, the preferred thermal fluid sensor can also be used as a microphone, since sound also propagates in the form of pressure waves or sound pressure waves. With an oscillating fluid flow, an acoustic resistance and a back volume are particularly relevant with regard to a lower cutoff frequency, as explained above.Preferably, the back volume refers to a volume formed on the back side between the membrane and a cover on the carrier. Due to the possibility of setting a high acoustic resistance, the thermal fluid sensor can advantageously be designed such that reliable performance is still achieved over a wide frequency bandwidth even with a low back volume. In this respect, a high degree of miniaturization can be achieved, particularly for acoustic applications.

[0079] Overall, the preferred thermal fluid sensor advantageously allows both continuous and oscillating fluid flows to be measured by modulating the heat flow. This makes the preferred thermal fluid sensor suitable for a wide variety of applications. The detection of a property of the fluid that modulates the heat flow can be achieved, in particular, by an electronic circuit or a processing unit.

[0080] In a further preferred embodiment, the thermal fluid sensor is characterized in that the thermal fluid sensor has an electronic circuit, wherein the electronic circuit is configured to control the heating element, read out a measurement signal of the temperature sensor and / or determine at least one property of the fluid based on the modulation of the heat flow between the temperature sensor and the heating element.

[0081] For the purposes of the invention, an electronic circuit preferably refers to a combination of individual electrical or electromechanical elements in a functional arrangement. The electronic circuit preferably allows data or electrical signals to be sent, received, and / or processed.

[0082] Preferred electronic circuits include, without limitation, an integrated circuit (IC), application-specific integrated circuits (ASIC), a programmable logic device (PLD), a field-programmable gate array (FPGA), a microprocessor, a microcomputer, a programmable logic controller, and / or other electronic, preferably programmable, circuit.

[0083] Preferably, the electronic circuit is configured to receive measurement data from the temperature sensor and to send control signals or commands to the heating element to generate the heat flow.

[0084] Preferably, the electronic circuit can be configured to regulate the heating element depending on the measurement data from the temperature sensor. The electronic circuit is preferably configured to issue a command to generate the heat flow to the heating element after a measurement from the temperature sensor, for example, to increase, maintain, or decrease the heat output.

[0085] In preferred embodiments, the electronic circuit can also be configured to process the measurement data recorded by the temperature sensor with regard to the modulation of the heat flow. Processing data preferably means that the electronic circuit is configured to perform computing operations and / or computational steps, for example, with recorded measurement data from the temperature sensor. In particular, the electronic circuit or an external data processing unit connected thereto can be used to determine the difference between a base value of the heat flow and the measurement data measured at the temperature sensor. The electronic circuit can thus be present as evaluation, readout, and / or control electronics, which enables control of the heating element, readout of the temperature sensor, and / or evaluation of the measurement data.If an evaluation is carried out, the electronic circuit can also be referred to as a computing unit.

[0086] The term "computing unit" preferably refers to any device or unit that can be configured to perform computing operations. The computing unit is preferably, for example, a processor, a processor chip, a microprocessor, and / or a microcontroller, which is preferably configured to evaluate the modulation of the heat flow. The computing unit can also preferably be a programmable circuit board. The computing unit can also preferably comprise a computer-usable or computer-readable medium, such as a hard disk, random access memory (RAM), read-only memory (ROM), flash memory, etc.

[0087] Method steps for determining the properties of the fluid, as described herein, are preferably carried out by the electronic circuit or computing unit. The phrase "configured to" preferably means that computer code and / or software is installed on the electronic circuit or computing unit to carry out the method step, for example, to check the extent to which a modulation of the heat flow was caused and which fluid parameters were decisive for this. The computer code and / or software for evaluating the modulation of the heat flow can be written in any programming language or a model-based development environment, e.g., in C / C++, C#, Objective-C, Java, Basic / VisualBasic, MATLAB, Simulink, StateFlow, Lab View, or Assembler.

[0088] The computer code and / or software, which is preferably installed on the electronic circuit or processing unit, in particular to evaluate the modulation of the heat flow, can be considered a technical feature, since a direct physical effect of the thermal fluid sensor, for example, the supply of a control signal for a heating element or the detection of a temperature change, is utilized. Functional descriptions of the computer code and / or software can therefore be considered preferred and defining embodiments of the invention. The computer code and / or software used in each case is available to the person skilled in the art and can be installed accordingly using standard knowledge.In a further preferred embodiment, the thermal fluid sensor is characterized in that the support structure comprises a base and a longitudinal section, wherein the base is connected to the membrane and the longitudinal section extends above the aperture. Preferably, the temperature sensor is mounted on or below the longitudinal section.

[0089] The base and the longitudinal section preferably designate sections of the support structure which determine its design. The base designates the section of the support structure which extends substantially vertically. In particular, the base preferably predetermines the vertical displacement across the aperture of the membrane. The longitudinal section is preferably arranged substantially perpendicularly on the base and extends at least across the aperture and preferably beyond the membrane or a temperature sensor or heating element mounted thereon. The temperature sensor or heating element is preferably arranged on or below the longitudinal section. If the heating element is located on the membrane, the temperature sensor is preferably arranged on or below the longitudinal section. If the temperature sensor is preferably mounted on the membrane, the heating element is located on or below the longitudinal section.

[0090] The design of the support structure comprising a base and a longitudinal section advantageously provides flexibility for the arrangement of the temperature sensor or the heating element on the support structure. The base can be used to specify the vertical positioning. In other words, the base can in particular determine the vertical displacement between the temperature sensor and the heating element. The longitudinal section makes it possible to specify a lateral positioning of the temperature sensor or the heating element. For example, it may be preferred that the temperature sensor and the heating element are located directly above one another vertically without any lateral displacement. It may also be preferred that, in addition to the vertical displacement between the temperature sensor and the heating element, there is also a lateral displacement.Furthermore, the width of the channel can be determined by the longitudinal section and thus also a flow section for the fluid, which is essentially transverse to the heat flow of the heating element.

[0091] In a further preferred embodiment, the thermal fluid sensor is characterized in that the base has a height between approximately 0.5 pm - 20 pm, preferably between approximately 1 pm - 10 pm, and / or the longitudinal section has a length between approximately 10 - 1,000 pm, preferably between approximately 50 - 800 pm, particularly preferably between approximately 50 - 500 pm, most particularly preferably between approximately 50 - 200 pm.

[0092] The preferred dimensions for the base and / or longitudinal section have proven advantageous in that they can provide a flow path which leads to particularly good measurement results.

[0093] In particular, the base of the support structure allows the vertical dimensions of the flow path to be determined. Consequently, the base can be used to adjust the smallest vertical distance between the temperature sensor or the heating element, depending on which component is attached to the membrane, and the longitudinal section of the support structure. The height of the channel is therefore less than the height of the base. The height of the base essentially corresponds to the sum of the height of the channel and a vertical component of the flow section between the channel and the aperture on the membrane. The height of the channel is preferably also influenced by the thickness or height of the longitudinal section, which preferably has a thickness between 10 nm - 20 pm, more preferably between 100 nm - 10 pm or even 1 - 5 pm. In particular, the height of the channel can also be adjusted by the layer thickness of the deposition of a sacrificial layer which is deposited on the membrane (oron the temperature sensor and heating element), and onto which the support structure is applied. After etching the sacrificial layer, the height of the channel corresponds to the original layer thickness of the sacrificial layer. The smallest vertical distance between the temperature sensor or heating element and the support structure (or the height of the channel) is crucial for the acoustic resistance (see equation (1)). By appropriately setting a low height for the channel, a particularly high acoustic resistance can advantageously be achieved.

[0094] The term acoustic resistance refers specifically to the characteristic sound impedance or acoustic field impedance. Acoustic resistance primarily describes the resistance to the propagation of sound and thus to pressure fluctuations. Acoustic resistance is determined by the quotient of sound pressure and sound velocity, or as the product of density and sound speed.

[0095] Furthermore, the aforementioned preferred dimensions for the height and / or length of the support structure have proven advantageous in that cross-sections can be adjusted to optimize the modulation itself, particularly by adjusting the flow velocity at the intersection point with the heat flow. Thus, the sensitivity of the thermal fluid sensor can be optimized for a wide variety of applications.

[0096] Furthermore, the dimensions of the base can be used to regulate the height between the temperature sensor or heating element on the membrane and the support structure. In combination with the dimensions of the base, a sacrificial layer applied to the carrier can also influence the height between the temperature sensor or heating element on the membrane and the support structure. The lowest height, or the lowest vertical component of the channel, is preferably located between the temperature sensor or heating element and the support structure. The lowest height is preferably between approximately 50 nm and 5 pm, more preferably between approximately 300 nm and 1 pm.

[0097] The support structure thus makes it possible to determine the dimensions of the channel in vertical (height) and lateral (length and width) terms. The width of the channel is preferably orthogonal to the height and orthogonal to the direction of fluid propagation within the channel in the direction of the heat flow (see Fig. 1, corresponding to component z in the image plane). It is preferred that dimensions between 1 pm - 50,000 pm, preferably 5 pm - 10,000 pm, or even 10 pm - 1,000 pm be used.

[0098] The length of the longitudinal section is essentially the sum of the length of the membrane aperture and the length of the channel. Therefore, the preferred length of the longitudinal section is suitable for the placement of multiple temperature sensors and / or heating elements to enable more intensive heat transfer and / or highly accurate measurement, depending on the application.

[0099] The preferred dimensions with regard to the longitudinal section and / or base have also proven advantageous in that they ensure sufficient stability of the support structure. The support structure is not impaired by the fluid flowing through the channel, thus ensuring long-term functionality of the measurement. In particular, vibrations are advantageously avoided, thus ensuring high reliability for measuring the properties of the fluid. In a further preferred embodiment, the thermal fluid sensor is characterized in that below the temperature sensor or the heating element on the support structure, preferably on a longitudinal section, there is at least one opening, preferably a plurality of openings, wherein the at least one opening is preferably circular, elliptical, triangular, square, polygonal or slit-shaped.

[0100] The opening advantageously enables better transfer of the heat flow from the heating element to the temperature sensor. Without the opening, the material of the support structure, which forms the longitudinal section, is exposed. The transfer of the heat flow may therefore be less effective, depending on the thermal conductivity of the material of the support structure. An opening below the temperature sensor or heating element can further improve direct heat transfer between the two components, enabling extremely precise temperature measurements with minimal energy consumption.

[0101] In a further preferred embodiment of the thermal fluid sensor, it may be preferable to provide a region with a reduced layer thickness and / or a material with increased thermal conductivity below the temperature sensor or the heating element on the support structure, preferably in a longitudinal section. These embodiments can also improve the transfer of the heat flow from the heating element to the temperature sensor.

[0102] For example, the layer thickness of the longitudinal section in an area below the temperature sensor or heating element can be reduced by 20%, 30%, 40%, 50% or more in order to ensure more efficient heat transfer. It may also be preferable to introduce a material with increased thermal conductivity in an area to simplify heat transfer. A material with increased thermal conductivity preferably means a material that has a higher thermal conductivity than the base material of the support structure. The thermally conductive material can preferably have, for example, a thermal conductivity of more than 200 W / (m K), more than 300 W / (m K) or more than 400 W / (m K), where W / (m K) corresponds to the unit watt per meter and Kelvin. Examples of thermally conductive materials include silver, copper, gold and / or aluminum.

[0103] In this respect, in the aforementioned embodiments, it is preferable to provide a region beneath the heating element or temperature sensor which is characterized by a material composition and / or thickness that differs from the rest of the support structure, thereby supporting the transfer of the heat flow. With regard to the shape of the region, the above statements apply to various conceivable shapes of the openings. The dimension is preferably adapted to the size of the heating element or temperature sensor, with a maximum extension being less than 50 pm, preferably less than 40 pm, 30 pm, 20 pm, 10 pm, or 5 pm in preferred embodiments.

[0104] In a further preferred embodiment, the thermal fluid sensor is characterized in that the support structure comprises a material selected from a group comprising polysilicon, silicon nitride and / or silicon dioxide. The preferred materials have proven particularly advantageous for possible losses that could occur due to the effect of heat conduction. In particular, the preferred materials for the support structure are characterized on the one hand by a sufficiently low thermal conductivity in order to support the function of the support structure of reducing material-bound heat conduction from the heating element to the temperature sensor. Furthermore, possible measurement noise that could occur due to heat transfer via the membrane and support structure is reduced. Instead, the heat is advantageously transferred essentially by convection through the fluid to be analyzed.On the other hand, the materials are not so insulating that heat transfer would be prevented by the layer thickness, for example, of a longitudinal section of the support structure on which a heating element is located (see Fig. 3). As explained above, in preferred embodiments, for example, regions of an opening or reduced layer thickness can be provided to assist heat transfer. With the aforementioned materials, in particular with a thickness of the longitudinal section of less than 100 pm, preferably less than 50 pm, more preferably less than 10 pm, most preferably less than 5 pm, good transfer can be ensured even without such regions.

[0105] Furthermore, the preferred materials can be processed using state-of-the-art semiconductor and microsystem technology processes to provide the support structure.

[0106] In a further preferred embodiment, the thermal fluid sensor is characterized in that the support structure comprises a material having a thermal conductivity between 0.1 - 40 W / (m*K), preferably between 1 - 30 W / (m*K).

[0107] The thermal conductivities mentioned are sufficiently low to keep losses due to thermal conduction of the material of the support structure low or minimize them, so that an optimal measurement of the heat flow through the temperature sensor can be enabled.

[0108] In a further preferred embodiment, the thermal fluid sensor is characterized in that the vertical distance between the heating element and the temperature sensor is in a range of approximately 100 nm - 10 pm, preferably approximately 100 nm - 1 pm.

[0109] The vertical distance between the temperature sensor and the heating element is preferably the sum of the height of the channel and the thickness of the longitudinal section on which the temperature sensor or the heating element is located.

[0110] The preferred distance ranges regarding the vertical displacement have proven advantageous in that they allow for particularly reliable temperature detection by the temperature sensor using convection. In particular, they enable essentially targeted convection to the temperature sensor without the heat flow having to suffer losses for the temperature measurement. This promotes the accuracy and sensitivity of the heat flow measurement and a modulation of the heat flow. The preferred vertical distance ranges also have a positive effect on the operating efficiency of the preferred thermal fluid sensor, since the proximity of the heating element and the temperature sensor allows for a reduction in energy consumption.In a further preferred embodiment, the thermal fluid sensor is characterized in that there is no lateral displacement between the heating element and the temperature sensor, so that preferably the heating element and temperature sensor have a maximum lateral overlap or there is a lateral displacement, wherein preferably the heating element and temperature sensor have a partial overlap or wherein the heating element and temperature sensor do not have a partial overlap but are laterally spaced apart.

[0111] In other words, there may preferably be a maximum lateral overlap (no lateral displacement), a partial lateral overlap, or a lateral spacing between the heating element and the temperature sensor.

[0112] In a further embodiment, the thermal fluid sensor is characterized in that there is a lateral displacement between the heating element and the temperature sensor, wherein the lateral displacement is preferably in a range of 1 pm - 20 pm or there is no lateral displacement or a lateral displacement of at most 1 pm between the heating element and the temperature sensor.

[0113] The lateral displacement preferably characterizes a displacement orthogonal to the vertical displacement, preferably along a longitudinal section of the support structure or with respect to the fluid flow along the path of the fluid flowing in the channel. The lateral displacement preferably comprises a displacement along a length (but can also occur along a width). The vertical and lateral displacement between the components is preferably determined by the geometric center of the active regions, for example, the geometric center of a heating structure and / or a sensor. In the case of a thermopile, the active region preferably corresponds to the hot end.

[0114] The sensitivity of the thermal fluid sensor can be adjusted by a lateral displacement between the temperature sensor and the heating element in combination with a vertical displacement. In a preferred embodiment, the heating element and the temperature sensor are located vertically essentially directly above one another, without a lateral displacement. In this case, the path of the heat flow, i.e. the heat path, is reduced to a minimum, so that potential heat flow losses are reduced or avoided. The heat path can be lengthened by an additional lateral displacement (in addition to the vertical displacement). It is advantageous that the longitudinal section of the support structure and thus also the channel can be designed with reduced dimensions, but an extended heat path is still possible.Thus, by means of the preferred thermal fluid sensor, it is possible to adjust both the dimensioning of the channel for a flow section of the fluid, for example by adjusting a height or cross-section as explained above, and to adjust the heat path independently thereof.

[0115] The lateral displacement thus advantageously provides a further degree of freedom, which can be used to influence the modulation of the heat flow. Thus, depending on the application, the flow velocity of the fluid in the channel and the heat path between the temperature sensor and the heating element can be optimized independently of one another in order to measure the properties of the fluid. For this purpose, it may be preferable for there to be a vertical displacement and a lateral displacement, with there being no lateral intersection or overlap between the temperature sensor and the heating element. In this case, the temperature sensor and the heating element are therefore laterally spaced apart. Furthermore, it may be preferable for there to be a vertical displacement and a lateral displacement, with there being a partial lateral intersection or overlap between the temperature element and the heating element.In a lateral plane of the two components, there can preferably be a lateral displacement, in the sense of a lateral offset, while still maintaining a partial overlap between the temperature element and the heating element. It can also be preferred that there is only a vertical displacement, with a substantially complete lateral overlap between the temperature sensor and the heating element. In this case, there is preferably no lateral offset between the two components; instead, the overlap is maximum, preferably 100%.

[0116] For this purpose, it is preferred that the temperature sensor or the heating element is located on the support structure, preferably on or below the longitudinal section, and the corresponding other component is located on the membrane.

[0117] In a further preferred embodiment, the thermal fluid sensor is characterized in that the membrane is held by a carrier, wherein the carrier preferably has a cavity which is located below the aperture of the membrane.

[0118] The support is preferably a frame structure, which is essentially formed by a continuous outer border in the form of side walls of a free, flat area. The frame structure is preferably stable and rigid. In the case of an angular frame shape (triangular, square, hexagonal, or generally polygonal outline), the individual side areas, which preferably essentially form the frame structure, are particularly referred to as side walls.

[0119] The membrane is preferably held by at least two side walls of the carrier. Preferably, the carrier comprises four side walls on which the membrane is mounted. The membrane is preferably suspended flatly within the remaining free area. The flat extension of the membrane characterizes a lateral direction of the thermal fluid sensor. The membrane is preferably located above the cavity of the carrier.

[0120] The flat design of the membrane preferably means that dimensions such as a length and / or width are many times greater than a height or thickness orthogonal thereto. Thus, the length and / or width can be greater than the thickness by a factor of 1.5, 2.5, 10, 100, 1,000, 10,000, or more. The thickness of the membrane is preferably between 0.1 and 1,000 μm, preferably between 0.5 and 500 μm, particularly preferably between 1 and 300 μm, and most particularly preferably between 5 and 200 μm.

[0121] The membrane is preferably configured in such a way that any vibrations caused by the fluid flow do not occur. Accordingly, the membrane is sufficiently stable so that the measurement result cannot be distorted by the modulation of the heat flow. In addition to the aperture, the membrane can preferably have one or more structures arranged perpendicular to the heating element. For example, the membrane can have a grid structure comprising webs arranged successively in a row and offset from one another. This advantageously reduces or prevents parasitic heat conduction in the membrane.

[0122] In a further preferred embodiment, the thermal fluid sensor is characterized in that the membrane comprises a material selected from a group comprising silicon nitride, silicon dioxide, polyimide and / or porous silicon.

[0123] The preferred materials for the membrane have proven advantageous in that, in addition to their excellent design options, they also offer thermal insulation, thus reducing or eliminating heat conduction through the membrane. This further improves the accuracy of the measurement results. Silicon nitride, for example, can be used as a preferred material for the membrane, which has proven particularly advantageous in terms of processing, stability, and thermal properties.

[0124] In a further preferred embodiment, the thermal fluid sensor is characterized in that the aperture has a shape selected from a group comprising circular, elliptical, triangular, quadrangular, pentagonal, hexagonal, heptagonal, octagonal or polygonal, wherein the aperture preferably has a surface area between 1 pm 2 - 10 000 pm 2, preferably between 1 pm 2 - 10:00 pm 2 or 10 pm 2 - 100 pm 2 .

[0125] The preferred design of the aperture allows a reliable introduction of the fluid into the preferred channel of the flow sensor without having to accept flow losses.

[0126] Particularly preferably, the aperture attached to the membrane has a shape that corresponds to a cross-section of the channel, thus enabling an efficient transition of the fluid from the aperture into the flow section to the channel. The cross-section of the channel, which is determined by the length and width, directly adjoins the flow section. For a channel in the form of a slit, for example with a width (in the drawing depth in the figures below) of more than 100 pm or more than 1 mm, the aperture will preferably also have the shape of a slit. Likewise, it may also be preferable to incorporate a plurality of apertures into the membrane, which are arranged along the channel.

[0127] In a further preferred embodiment, the thermal fluid sensor is characterized in that the heating element is an electrical heating element, preferably a heating resistor, wherein the heating element preferably comprises a material selected from a group comprising silicon, polysilicon, aluminum, copper, gold, platinum, nickel, silver and / or tungsten, wherein the material preferably has a doping as semiconductor material.

[0128] The heating element preferably refers to the component of the thermal fluid sensor from which the heat flow emanates by releasing thermal energy. An electric heating element preferably refers to a heating element capable of converting electrical energy into thermal energy, which in the context of the invention is used as a heat flow and transferred to the temperature sensor. In an electric heating element, the thermal energy or heat flow is preferably generated by using an electrically conductive material through which an electric current flows and which heats up due to Joule heating.

[0129] The preferred materials for the heating element, especially for the electric heating element, advantageously allow for particularly high temperatures without causing melting and / or chemical reactions (such as oxidation) of the heating element and / or other components. Furthermore, the preferred materials exhibit a high resistivity, allowing for a particularly compact heating element design. Doping, in the case of a semiconductor, allows for better electrical conductivity and thus also improved heat generation and convection from the heating element.

[0130] In a further preferred embodiment, the thermal fluid sensor is characterized in that the temperature sensor is selected from a group comprising a thermocouple, a diode, a thermopile or a thermistor.

[0131] The temperature sensor is preferably the component of the thermal fluid sensor that is capable of measuring the heat flow, in particular the modulation of the heat flow through the fluid.

[0132] A thermistor preferably refers to a component comprising an electrical resistance whose value changes reproducibly with temperature. Thermistors are divided into thermistors and thermistors based on their temperature behavior. A thermistor has a negative temperature coefficient and can conduct electrical current better when hot than when cold. A thermistor has a positive temperature coefficient and can conduct electrical current better when cold than when hot.

[0133] In a further preferred embodiment, the thermal fluid sensor is characterized in that the temperature sensor is a thermistor, wherein the thermistor is located on a section of the support structure, preferably on a longitudinal section, or on the membrane.

[0134] Using a thermistor, a changing electrical resistance can be used to easily and precisely determine the modulation of heat flow and thus the properties of the flowing fluid. Furthermore, the provision of a thermistor can be particularly easily integrated into the fluid sensor's manufacturing process (see Figures 2 and 4).

[0135] A thermocouple preferably refers to an arrangement comprising a pair of electrically conductive materials that are connected to one another at one end and, due to the thermoelectric effect, allow a conclusion to be drawn about the measured temperature. Electrical energy is provided when there is a temperature difference along the arrangement. A thermopile refers to an element that has several thermocouples. The thermocouples of the thermopile are preferably connected electrically in series and thermally in parallel. The temperature sensor can preferably be operated using a Wheatstone bridge circuit (also Wheatstone bridge). A Wheatstone bridge is used to measure the temperature; the voltage can be measured and related to the resistance of a thermistor, for example.

[0136] The Wheatstone bridge can be configured in various modes, for example, in constant temperature mode (CT). The heating element is preferably controlled for constant heat dissipation to the temperature sensor. Furthermore, the Wheatstone bridge can also be configured, for example, to detect a constant temperature difference (CTD), for example, between two sections of the temperature sensor to detect the heat flow. The latter is particularly preferred when a thermocouple is used as the temperature sensor.

[0137] The measuring point of the thermocouple or thermopile is referred to as the hot end or hot junction and forms one end of a thermocouple. The other end, which is analogously referred to as the cold end or cold junction, represents a reference point for detecting the temperature difference. Preferably, an electronic circuit and / or a computing unit for processing the temperature measurement is connected to the cold junction. A Wheatstone bridge in CTD mode can measure the temperature between the hot end and the cold end, allowing parameters to be determined based on the effect of the fluid on the heat flow.

[0138] In a further preferred embodiment, the thermal fluid sensor is characterized in that the temperature sensor is a thermocouple or a thermopile comprising a hot junction and a cold junction, wherein the hot junction is preferably located on a section of the support structure, preferably on a longitudinal section.

[0139] The cold junction can preferably also be located on the support structure. However, the cold junction is preferably located outside the support structure, preferably on the membrane or the carrier. For example, if the cold junction is located on a carrier material with high thermal conductivity, the carrier material (e.g., silicon with a high thermal conductivity of 150 W / (m*K)) can serve as a heat sink, allowing a temperature difference to occur within the thermocouple or thermopile, which can be used to detect the heat transfer from the heating element to the temperature sensor with particular sensitivity.

[0140] A diode can also advantageously function as a temperature sensor. This can be exploited, for example, by the fact that a diode's forward voltage changes essentially linearly with temperature. The higher the temperature, the lower the forward voltage, and vice versa. Accordingly, the change in the forward voltage can be used to draw conclusions about a change in temperature.

[0141] In a further preferred embodiment, the temperature sensor, preferably the thermocouple or thermopile, is mounted on the membrane. Accordingly, the heating element is preferably located on the support structure, preferably on or below the longitudinal section of the support structure. It may be preferred for the thermocouple or thermopile to act as a temperature sensor on an intermediate layer mounted on the membrane (see Fig. 3). The intermediate layer may preferably comprise electrical lines that enable temperature measurement through the hot junction and cold junction. In particular, the intermediate layer functions as a thermal insulation layer, so that possible temperature losses due to heat conduction from the membrane are reduced or avoided.

[0142] The wording a heating element or a temperature sensor means in the sense of the invention in particular at least one heating element or at least one temperature sensor, wherein preferably two, three, four or more heating elements or two, three, four or more temperature sensors can also be provided.

[0143] In a further preferred embodiment, the thermal fluid sensor comprises at least two temperature sensors, wherein a combination signal of the at least two temperature sensors, preferably a difference signal, is formed for measuring an oscillating fluid flow, preferably caused by sound pressure waves.

[0144] The at least two temperature sensors can, for example, be present as two ends of a thermocouple. However, they can also be provided by at least two independent temperature sensors. In preferred embodiments, the at least two temperature sensors are located on the support structure, preferably a longitudinal section of the support structure, with the heating element mounted on the membrane (see Fig. 6A). A reverse arrangement is also conceivable, with the at least two temperature sensors preferably being mounted on the membrane and the (at least one) heating element being mounted on the support structure, preferably on a longitudinal section of the support structure.

[0145] By providing two temperature sensors for measuring a first temperature measurement value Ti and a second temperature measurement value T2, a combined signal can be generated. The combined signal can, for example, relate to a difference between the two measured values ​​T2 and T1. Advantageously, by generating the combined signal, preferably a difference signal, a measurement signal can be obtained that is essentially in phase with an oscillating fluid flow flowing through the thermal fluid sensor (see Fig. 6B). Particularly with regard to measuring sound pressure waves, generating a phase-accurate measurement signal proves advantageous in order to improve measurement quality.

[0146] In a further aspect, the invention relates to a method for producing a preferred thermal fluid sensor, comprising the following steps: a) providing a carrier substrate, b) applying a membrane starting from a front side to the carrier substrate, optionally with application of a first sacrificial layer, c) structuring the membrane to provide an aperture, d) attaching a heating element or temperature sensor to the membrane, e) attaching the support structure to the membrane, optionally with application of a second sacrificial layer, f) attaching a temperature sensor or heating element to the support structure, g) etching the carrier substrate starting from a back side to provide a cavity and to obtain a carrier, such that the aperture lies above the cavity and the membrane is held by the carrier, h) providing a channel, optionally etching sacrificial layers.

[0147] One of ordinary skill in the art will recognize that technical features, definitions, and advantages of preferred embodiments of the described thermal fluid sensor also apply to the preferred method of manufacturing the thermal fluid sensor, and vice versa.

[0148] Preferably, the described manufacturing method serves to provide a thermal fluid sensor which has a support structure so that there is at least one vertical displacement between the temperature sensor and the heating element.

[0149] For this purpose, it is preferable to first provide a carrier substrate. The carrier substrate refers to a substrate that will later serve as a support for the membrane. This preferably involves restructuring the carrier substrate into a support during the preferred process. The carrier substrate can, for example, comprise a material selected from a group comprising monosilicon, polysilicon, silicon dioxide, silicon carbide, silicon germanium, silicon nitride, nitride, germanium, carbon, gallium arsenide, gallium nitride, indium phosphide, and / or glass.

[0150] A membrane is then preferably applied to the carrier substrate, starting from a front side. It may be preferable to apply a sacrificial layer to the carrier substrate beforehand. A membrane is then coated onto the sacrificial layer. Silicon, silicon oxide, silicon nitride, and / or polyimide are preferably used as the material for the membrane. The membrane can preferably be applied in a pre-structured form or structured after coating, so that the subsequent aperture is present as an interruption within the membrane.

[0151] For coating components and / or materials of the preferred thermal fluid sensor, one or more coating methods can be used selected from a group comprising spray coating, mist coating, vapor coating, physical vapor deposition (PVD), chemical vapor deposition (CVD), low pressure CVD (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), epitaxial coating, sputtering and / or atomic layer deposition.

[0152] It is then preferable to apply a heating element (or a temperature sensor) to the membrane. When using a first sacrificial layer, it is preferable that the heating element or temperature sensor be applied to the first sacrificial layer.

[0153] In a preferred embodiment, the heating element is applied to the membrane. A support structure is preferably provided after the heating element (or a temperature sensor) has been applied to the membrane. For this purpose, it may be preferable to apply a second sacrificial layer that covers both the aperture of the membrane and the heating element. The layer thickness of the second sacrificial layer above the later aperture preferably defines the height of the base of the later-applied support structure. The layer thickness of the second sacrificial layer above the heating element (or the temperature sensor) preferably defines the height of the later-obtained channel.

[0154] The support structure is preferably coated on the second sacrificial layer, wherein the support structure preferably has a base and a longitudinal section that extends at least over the aperture of the membrane. The length of the longitudinal section preferably defines the length of a flow section in the flow direction that projects beyond the membrane and temperature sensor (or heating element).

[0155] After attaching the support structure, the temperature sensor (or heating element) is preferably mounted on the support structure, preferably on or below the longitudinal section. It goes without saying that a corresponding component from the temperature sensor and heating element pair is always mounted on the support structure and the membrane. Once the temperature sensor has been attached to the membrane, the heating element is mounted on the support structure, or vice versa.

[0156] If the temperature sensor is preferably mounted on the support structure, particularly in the case of a thermopile, further optional intermediate steps can preferably be performed. For example, a metal layer can be used to contact a contact pad. The metal layer can, for example, extend along the base and the temperature sensor. Alternatively, the metal layer can extend along the temperature sensor and be applied to the base and the longitudinal section.

[0157] The carrier substrate can then be etched starting from the back side. For this purpose, one or more etching processes known from the prior art can be used, selected from a group comprising wet-chemical etching processes and / or dry etching processes, preferably physical and / or chemical dry etching processes, particularly preferably reactive ion etching and / or reactive ion deep etching (Bosch process).

[0158] The etching creates the cavity and thus the support, so that the aperture of the membrane lies above the cavity and the membrane is held by the support. Furthermore, the first and / or second sacrificial layer is preferably etched, thus creating the channel of the thermal fluid sensor.

[0159] In a further preferred embodiment, a temperature sensor, in particular a thermocouple, is applied to the membrane. For this purpose, the hot junction and the cold junction are preferably located on an intermediate layer that acts as a thermal insulator layer.

[0160] This further reduces or eliminates heat conduction through the material, allowing the modulation of heat flow to be measured with increased precision.

[0161] It may then be preferable to apply a second sacrificial layer and structure it. The support structure is applied to the second sacrificial layer. The thickness of the second sacrificial layer can determine the height of the base of the support structure. The thickness of the second sacrificial layer above the heating element (or the temperature sensor) preferably defines the height of the channel subsequently obtained. This channel is then also preferably structured. By structuring the support structure, in particular, the dimensions of the longitudinal section of the support structure can be determined.

[0162] After the support structure has been structured, the heating element is preferably arranged on the support structure, in particular on the longitudinal section. This can then preferably be followed by a backside etching to create the cavity on the carrier substrate and thus also the support for the membrane. To create the channel, the first and / or second sacrificial layer can be etched thereon. The fluid can flow through the channel through the heat flow, so that, as described, conclusions about the properties of the fluid can be drawn based on the modulation of the heat flow.

[0163] Optionally, it may be preferable to attach a cover to the carrier. The cover provides a back volume so that the support structure, the heating element, and the temperature sensor are located within the back volume. Attaching the cover is particularly preferred in the context of use as a microphone.

[0164] In a further aspect, the invention relates to a use of a preferred thermal fluid sensor for measuring one or more properties of a fluid, preferably a flow parameter, pressure fluctuations, preferably sound pressure waves, a composition and / or concentration of a fluid.

[0165] Flow parameters can, for example, be selected from a group comprising flow velocity, pressure and / or temperature of the fluid.

[0166] Typically, the higher the flow parameter, the greater the heat flow modulation. Regarding temperature, the greater the temperature deviation between the fluid temperature and the heat flow temperature, the greater the heat flow modulation.

[0167] The pressure, and thus pressure fluctuations, of the fluid flowing through the heat flow can also be measured. The higher the pressure or pressure fluctuations, the greater the modulation of the heat flow. Since pressure fluctuations are characteristic of sound waves, it is also possible to detect sound pressure waves using the preferred thermal fluid sensor. This makes the preferred thermal fluid sensor suitable for use as a sound detector or microphone.

[0168] Therefore, in a further preferred embodiment, the invention also relates to a sound detector or microphone comprising a sound inlet opening and an electronic circuit, wherein vibrations of a fluid in the form of sound waves pass through the sound inlet opening and an electrical signal dependent on the sound waves can be generated, characterized in that the microphone has a temperature sensor, a heating element and a support structure, wherein the support structure extends at least partially over the aperture at a vertical distance from a membrane, wherein the temperature sensor or the heating element is mounted on the support structure and the electronic circuit is configured to carry out a measurement of the sound waves based on a modulation of a heat flow between the heating element and temperature sensor.

[0169] One of ordinary skill in the art will recognize that technical features, definitions, and advantages of preferred embodiments of the described thermal fluid sensor also apply to the preferred microphone or sound detector, and vice versa.

[0170] The microphone preferably has a cover. The cover is preferably attached to the carrier and encloses the parts of the diaphragm, the support structure, the heating element, and the temperature sensor within a rear volume. The rear volume thus preferably refers to a spatial section provided by the cover and, in particular, located between the diaphragm and the cover.

[0171] Advantageously, a particularly low back volume can be achieved, which is particularly useful for miniaturization. Furthermore, the low height of the channel at the intersection point with the heat flow allows for a low acoustic resistance.

[0172] In a preferred embodiment, the microphone is a bottom-port design. The sound inlet corresponds to the aperture on the membrane located on the carrier. The sound propagates through the cavity, through the aperture, flows through the channel, and modulates the heat flow between the heating element and the temperature sensor, propagating further toward the rear volume and / or the rear volume. However, the microphone can also be a top-bottom design.

[0173] Advantageously, the preferred thermal fluid sensor enables measurements with a high bandwidth. An upper cutoff frequency can be determined, in particular, by a heat capacity of the temperature sensor or a thermal resistance of the membrane. By providing a miniaturized fluid sensor or temperature sensor and a thermally insulating support structure, a high upper cutoff frequency can advantageously be ensured. A lower cutoff frequency, in particular, can be determined by the design of the channel. The provision of a support structure to form a channel with arbitrarily small height advantageously allows a low lower cutoff frequency while maintaining a compact design. Thus, the proposed design can advantageously provide a thermal MEMS microphone with high dynamics.

[0174] The composition and / or concentration of the fluid flowing through the heat stream can also be measured using the preferred thermal fluid sensor. This can be achieved, in particular, when the fluid sensor is used as a photoacoustic gas sensor, preferably when the thermal fluid sensor is used as a microphone.

[0175] In a photoacoustic gas sensor, intensity-modulated radiation with frequencies in the absorption spectrum of the molecule to be detected in the fluid is preferably used. If this molecule is present in the beam path, modulated absorption occurs, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. The heating and cooling processes lead to expansion and contraction of the fluid, causing sound waves with the modulation frequency. These can reach the preferred thermal fluid sensor, which can measure the sound waves by modulating the heat flow. Thus, in a further aspect, the invention relates to a photoacoustic sensor comprising a preferred thermal fluid sensor.

[0176] In a further preferred embodiment, the invention thus relates to a photoacoustic gas sensor comprising a modulatable emitter, a gas-fillable detection chamber, and a reference chamber. A thermal fluid sensor as described herein is fluidically disposed between the detection chamber and the reference chamber, and the detection chamber is preferably arranged in the beam path of the emitter, so that the emitter can excite gas in the detection chamber to generate sound pressure by means of modulatably emittable radiation, and a compensation of the sound pressure through the aperture in the membrane into the reference chamber can be detected by means of the preferred thermal fluid sensor. The membrane to which the aperture is attached preferably serves to provide a gas-tight separation between the detection chamber and the reference chamber.The aperture enables gas flow between the detection chamber and the reference chamber, with the support structure providing a channel as described, which enables particularly reliable measurements.

[0177] The radiation modulated with a modulation frequency preferably impacts the detection chamber. If the modulated irradiation occurs at a wavelength that corresponds to the absorption spectrum of a molecule of a gas component present in the gas mixture, a modulated absorption occurs, leading to heating and cooling processes whose timescales reflect the modulation frequency of the radiation. According to the photoacoustic effect, the heating and cooling processes lead to expansion and contraction of the gas component, which excites the gas component to form sound pressure waves with essentially the modulation frequency. The sound pressure waves are measurable as gas flow through the aperture using the thermal fluid sensor by modulating the heat flow between the heating element and the temperature sensor.This advantageously enables rapid and reliable detection of a PAS signal (photoacoustic spectroscopy signal). Modulating heat transport by the gas flow advantageously represents a particularly sensitive measure for determining PAS signals, which can be optimized, in particular, by optimizing the dimensions of the aperture, the channel, and / or by selecting the temperature sensor and / or the heating element.

[0178] FIGURES

[0179] In the following, the invention will be explained in more detail with reference to figures, without being limited to these.

[0180] Short description of the characters

[0181] Fig. 1 Schematic representation of a preferred embodiment of the thermal fluid sensor according to the invention Fig. 2 Representation of preferred steps of a method for producing a preferred thermal fluid sensor

[0182] Fig. 3 Schematic representation of another preferred embodiment of the thermal fluid sensor

[0183] Fig. 4 Representation of preferred steps for producing a method of a preferred thermal fluid sensor

[0184] Fig. 5 Schematic representation of a preferred embodiment of the thermal fluid sensor as a MEMS microphone

[0185] Fig. 6 Schematic representation of another preferred embodiment of the thermal fluid sensor

[0186] Detailed description of the characters

[0187] Fig. 1 serves to schematically illustrate a preferred embodiment of a thermal fluid sensor 1 according to the invention.

[0188] The thermal fluid sensor 1 comprises a membrane 7 with an aperture 19, as well as a heating element 9 and a temperature sensor 11. These are positioned such that at least one property of a fluid flowing through the aperture 19 can be measured based on a modulation of a heat flow between the heating element 9 and the temperature sensor 11. The thermal fluid sensor comprises a support structure 13, which extends at least partially over the aperture 19 at a vertical distance from the membrane 7.

[0189] In the illustration according to Fig. 1, the temperature sensor 11 is mounted on the support structure. Accordingly, the heating element 9 is mounted on the membrane 7. A reverse arrangement, with the heating element on the support structure and the temperature sensor on the membrane, is also possible.

[0190] Advantageously, the thermal fluid sensor 1 makes it possible to effectively reduce measurement noise. In prior art thermal fluid sensors, the arrangement of the temperature sensor and heating element on or along a plane can result in a significant heat transfer through material-bound heat conduction. However, the sensitivity of the thermal fluid sensor 1 is based on heat transfer by convection via the fluid being analyzed, which flows between the temperature sensor and heating element. Thus, heat transfer by convection advantageously depends on the properties of the fluid being analyzed, so that modulation of the heat transfer can be used to draw conclusions about, for example, flow parameters, pressure fluctuation(s), composition, and / or concentration of the fluid.

[0191] In-plane heat conduction, for example, across a membrane on which the temperature sensor and heating element are arranged, disadvantageously results in heat transfer that is independent of the properties of the fluid being analyzed. Changes in the thermal conductivity of the membrane or other materials or components of the sensor thus lead to heat modulation between the temperature sensor and heating element, which is independent of the properties of the fluid and is reflected in the measurement results as noise.

[0192] The adverse noise is eliminated by reducing heat conduction in the plane between heating element s and temperature sensor 11 by locating the heating element s or the temperature sensor 11 on a support structure 13, thereby providing a vertical displacement between the heating element s and the temperature sensor 11.

[0193] This ensures that heat transfer through a heat path 35 from the heating element s to the temperature sensor 11 occurs primarily by convection via the fluid to be analyzed. The heat modulation thus reflects properties of the fluid to be analyzed (flow parameters, pressure fluctuations, heat conduction, etc.). Noise caused by material-bound heat conduction, which is independent of the fluid's properties, is advantageously reduced.

[0194] A further advantage of the thermal fluid sensor 1 is that it enables a particularly controlled modulation of the heat flow transmitted from the heating element 9 to the temperature sensor 11. The support structure 13, which extends at least partially at a vertical distance above the aperture 19 of the membrane 7, allows a particularly small cross-section in the form of a channel 33 for a flow direction of the fluid through the thermal fluid sensor to be achieved. The modulation of the heat flow is independent of the design of the channel. However, the support structure 13 and in particular the channel 33 advantageously enable the adjustment of a fluidic resistance (or acoustic resistance) for the fluid flowing through the heat flow. Thus, a desired measurement sensitivity or acoustic properties of the thermal fluid sensor 1 can be advantageously adjusted in a simple manner.Advantageously, a signal can be amplified in a simple manner by attaching several temperature sensors at the same time.

[0195] The channel 33, which represents a spatial section within which the modulation of the heat flow through the fluid is carried out, lies within a flow section provided by the support structure 13.

[0196] Advantageously, the design of the support structure 13 allows for the channel 33 to be set to almost any desired height. The height of the channel 33 preferably refers to the lowest height or lowest vertical extension of the channel 33 within the thermal fluid sensor, which preferably exists between a heating element 9 installed on the membrane 7 and the support structure 13 extending thereabove.

[0197] The thermal fluid sensor 1 thus offers the possibility of recording particularly precise measurement results while maintaining a compact design. The higher accuracy and thus also the higher resolution result, among other things, from higher flow velocities of the fluid due to the heat flow, which can be made possible by designing the support structure 13 or setting a channel 33 with a low height. In particular, even particularly small pressure differences can be recorded in this way. Furthermore, for dynamic measurements of pressure fluctuations, a high acoustic resistance can be set by setting the low height of the channel 33, whereby a lower lower limit frequency can be ensured even with relatively small back volumes. The lower limit frequency is, as explained above, determined in particular by the acoustic or fluidic resistance, the capacitance and the back volume (see equation (3)).The provision of a support structure 13 for forming a channel 33 with any desired low height advantageously allows a low lower limit frequency while maintaining a compact structure.

[0198] Furthermore, a close arrangement between the temperature sensor 11 and the heating element 9 can be achieved, thereby enabling energy-saving operation of the thermal fluid sensor 1.

[0199] Fig. 2 shows a schematic representation of preferred steps for producing a preferred thermal fluid sensor 1, preferably the embodiment shown in Fig. 1.

[0200] Fig. 2A shows a carrier substrate 2 which is provided and on which a first sacrificial layer 21 is applied.

[0201] Fig. 2B shows the coating of the membrane 7, which is applied to the first sacrificial layer 21. Fig. 2B shows that this layer is structured, which serves to provide the aperture on the membrane 7. The structuring can determine the shape and / or dimensions of the aperture.

[0202] Fig. 2 C shows the attachment of a heating element 9 to the membrane 7. This is done by a coating process and an optional structuring of the heating element 9.

[0203] Fig. 2D shows the coating of a second sacrificial layer 25, which can also be patterned. The second sacrificial layer 25 can cover both the aperture 19 of the membrane 7 and the heating element 9. The layer thickness of the second sacrificial layer 25 above the patterning for the subsequent aperture defines the height of the base of the subsequently applied support structure. The layer thickness of the second sacrificial layer 25 above the heating element 9 defines the height of the channel subsequently created.

[0204] The support structure 13 is mounted on the second sacrificial layer 25, as also illustrated in Fig. 2 E. The base 15 is located on the membrane, while the longitudinal section extends along the second sacrificial layer 23. The length of the longitudinal section defines the length of a flow section in the flow direction, which projects beyond the membrane and temperature sensor (or heating element).

[0205] In Fig. 2 F it is shown that the temperature sensor 11 (or the heating element) is applied to the support structure 13, which can also be done by a coating process and optional structuring.

[0206] An optional intermediate step is shown in Fig. 2 F1 and 2 F2. This can be particularly useful when a thermopile is used as the temperature sensor 11 and coated onto the support structure 13. In Fig. 2 F1, it can be seen that a metal layer 31 is used, which serves to contact a contact pad 29. The metal layer 31 extends along the base 15 and the temperature sensor 11. As shown in Fig. 2 F2, the metal layer 31 can alternatively extend along the temperature sensor 11, which is applied to the base 15 and the longitudinal section 17.

[0207] Fig. 2G illustrates the etching of the carrier substrate 2 starting from a rear side to provide a cavity 5 and to obtain a carrier 3, such that the aperture 19 lies above the cavity 5 and the membrane 7 is held by the carrier 3. For this purpose, one or more etching processes known from the prior art can be used, selected from a group comprising wet-chemical etching processes and / or dry etching processes, physical and / or chemical dry etching processes, reactive ion etching and / or reactive ion deep etching (Bosch process). The etching provides the cavity 5 and thus the carrier 3, such that the aperture 19 of the membrane 7 lies above the cavity 5 and the membrane is held by the carrier.

[0208] Subsequently, as shown in Fig. 2 H, a channel is provided in which the first sacrificial layer 23 and the second sacrificial layer 25 are etched so that the fluid can flow through the channel to modulate the heat flow.

[0209] Fig. 2 I shows a close-up view of the cross-section AA' from Fig. 2 H and illustrates that the narrowest point, or the smallest distance, is between the support structure 13 and the heating element 9. The narrowest point corresponds to the height of the channel from equation (1).

[0210] Fig. 3 shows schematically a further embodiment of the thermal fluid sensor 1 according to the invention.

[0211] The thermal fluid sensor 1 has the membrane 7 with the aperture 19. Here, too, the heating element 9 and the temperature sensor 11 are positioned such that at least one property of a fluid flowing through the aperture can be measured. The thermal fluid sensor 1 comprises a support structure 13 which extends at least partially over the aperture 19 at a vertical distance from the membrane 7. In the preferred embodiment according to the illustration in Fig. 3, the heating element is present on the support structure 13, in particular on the longitudinal section 17. Accordingly, the temperature sensor 11 is arranged on the membrane. The membrane 7 lies on the first sacrificial layer 21, which is located on the carrier 3.

[0212] The temperature sensor 11 is a thermocouple comprising a hot junction and a cold junction. The thermocouple 11 is located on an intermediate layer attached to the membrane 7. The intermediate layer acts as a thermal insulation layer, reducing or preventing possible temperature losses due to heat conduction from the membrane 7.

[0213] Fig. 4 schematically illustrates preferred steps for manufacturing a preferred thermal fluid sensor 1, preferably the embodiment according to Fig. 3.

[0214] Fig. 4 A shows the preparation of the carrier substrate 2, on which the first sacrificial layer 21 is applied. Fig. 4 B illustrates the application of the membrane 7 starting from the front side onto the carrier substrate 2, with the membrane 7 being applied to the first sacrificial layer 21. To provide the aperture 19 for the thermal fluid sensor, structuring is performed on the membrane 7.

[0215] Fig. 4C illustrates the application of temperature sensor 11 to membrane 7. In the embodiment shown in Fig. 4C, temperature sensor 11 is represented as a thermocouple 11. This involves a hot junction and a cold junction on an intermediate layer, which acts as a thermal insulator layer. This makes it possible to reduce material-dependent heat conduction, so that the modulation of the heat flow depends essentially on the fluid properties to be measured.

[0216] Fig. 4 D shows the second sacrificial layer 23 being applied and patterned. The patterning is performed in order to attach a portion of the support structure to the membrane 7 in the subsequent step.

[0217] Fig. 4 E shows the attachment of the support structure 13 to the membrane 7, wherein a partial section of the support structure 13 extends along the second sacrificial layer 23.

[0218] In Fig. 4 F it is shown that the heating element s is mounted on the support structure 13, wherein the support structure is structured, in particular for the design of the longitudinal section 17 of the support structure 13.

[0219] Subsequently, as shown in Fig. 4 G, the carrier substrate 2 is etched starting from the back side, creating a cavity 5 and the carrier 3. Thus, the aperture 19 is located above the cavity 5 on the membrane 7, which is held by the carrier 3.

[0220] Finally, the channel is created by etching the first sacrificial layer 21 and the second sacrificial layer 23. The fluid can flow through the channel due to the heat flow emanating from the heating element 9, causing a modulation so that a property of the fluid can be measured.

[0221] Fig. 5 illustrates a preferred embodiment of the thermal fluid sensor as a MEMS microphone. For this purpose, a cover 25 is attached to the carrier 3. The cover 25 provides a back volume 27. The support structure 13, the heating element 9 and the temperature sensor 11 are thus located within the back volume 27. The drawn lines illustrate sound waves which can modulate the heat flow. The attachment of the cover 27 is particularly useful in the context of use as a microphone in order to protect components from environmental influences and to form a back volume. By providing a high acoustic resistance, the back volume can advantageously be kept particularly small, so that a compact thermal fluid sensor or microphone with a relatively large frequency bandwidth can be produced.

[0222] Fig. 6A shows another embodiment of the thermal fluid sensor. The thermal fluid sensor comprises two temperature sensors 11. In the illustrated embodiment, the temperature sensors 11 are provided as two ends of a thermocouple. However, they can also be provided by two independent temperature sensors. The two temperature sensors 11 are located on the longitudinal section 17 of the support structure 13. Accordingly, the heating element 9 is mounted on the membrane 7. A reverse arrangement is also conceivable.

[0223] By providing two temperature sensors, a temperature measurement can advantageously be performed at two sections of the thermal fluid sensor. A first temperature measurement value Ti is recorded at a first temperature sensor 11, and a second temperature measurement value T2 is recorded at a second temperature sensor 11. By recording the first temperature measurement value Ti and the second temperature measurement value, a combined signal can be generated. The combined signal can be generated, for example, from the difference between the two measured values ​​T2-T1. By generating the combined signal (here as a difference signal), a measurement signal can advantageously be obtained that is essentially in phase with an oscillating fluid flow flowing through the thermal fluid sensor.A substantially in-phase signal has proven advantageous for a particularly precise determination of at least one fluid property, in particular with regard to the measurement of sound pressure waves.

[0224] In Fig. 6 B, the phase equality of the sound pressure (p_audio), which flows as an oscillating current through the thermal fluid sensor, is schematically illustrated with respect to the difference signal T2-T1.

[0225] LIST OF REFERENCE SYMBOLS

[0226] 1 Thermal fluid sensor

[0227] 2 Carrier substrate

[0228] 3 carriers

[0229] 5 Cavity

[0230] 7 Membran

[0231] 9 Heating element

[0232] 11 Temperature sensor

[0233] 13 Support structure

[0234] 15 bases

[0235] 17 Longitudinal section

[0236] 19 aperture

[0237] 21 First sacrificial layer

[0238] 23 Zweite Opferschicht

[0239] 25 Abdeckung

[0240] 27 Rückvolumen

[0241] 29 Kontaktpad

[0242] 31 Metallschicht

[0243] 33 Kanal

[0244] 35 Wärmepfad

[0245] LITERATURVERZEICHNIS

[0246] Kuo, Jonathan TW, Lawrence Yu, and Ellis Meng. "Micromachined thermal flow sensors - A review." Micromachines 3.3 (2012): 550-573.

[0247] Xu, Xiang-Yuan, et al. "A monolithic three-dimensional thermal convective acoustic vector sensor with acoustic-transparent heat sink." JASA Express Letters 2.4 (2022): 044001 .

[0248] Lauwers, Thomas, Alain Gliere, and Skandar Basrour. "An all-optical photoacoustic sensor for the detection of trace gas." Sensors 20.14 (2020): 3967.

[0249] Balakrishnan, Vivekananthan, et al. "Thermal flow sensors for harsh environments." Sensors 17.9 (2017): 2061.

Claims

PATENT CLAIMS 1. Thermal fluid sensor (1) comprising a membrane (7) with an aperture (19), a heating element (9) and a temperature sensor (11), which are positioned such that at least one property of a fluid flowing through the aperture (19) can be measured based on a modulation of a heat flow between the heating element (9) and the temperature sensor (11), characterized in that the thermal fluid sensor (1) has a support structure (13) which extends at least partially over the aperture (19) at a vertical distance from the membrane (7), wherein the temperature sensor (11) or the heating element (9) is mounted on the support structure (13).

2. Thermal fluid sensor (1) according to the preceding claim, characterized in that the at least one property of the fluid flowing through the aperture (19), which is measurable by means of a modulation of a heat flow between the heating element (9) and the temperature sensor (11), is a flow parameter, a pressure fluctuation, a composition and / or concentration of the fluid.

3. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that a channel between the heating element (9) and the temperature sensor (11) is formed by a section of the support structure (13) and the membrane (17), wherein a flow path for the fluid can be provided through the channel, wherein the channel preferably has a height of 50 nm - 5 pm, preferably of 100 nm - 2 pm, particularly preferably of 300 nm to 1 pm.

4. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that a flow direction of the fluid is changed at least in one area when flowing through the thermal fluid sensor (1), wherein a change in the flow direction preferably has an angle of substantially 90°.

5. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that the thermal fluid sensor (1) is designed to measure a continuous or oscillating fluid flow, preferably caused by sound pressure waves.

6. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that the thermal fluid sensor (1) has an electronic circuit, wherein the electronic circuit is configured to control the heating element (9), a To read the measurement signal of the temperature sensor (11) and / or to determine at least one property of the fluid based on the modulation of the heat flow between the temperature sensor (11) and the heating element (9).

7. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that the support structure (13) has a base (15) and a longitudinal section (17), wherein the base (15) is connected to the membrane (7), the longitudinal section (15) extends above the aperture (19) and the temperature sensor (11) is mounted on or below the longitudinal section (15).

8. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that below the temperature sensor (11) or heating element (9) on the support structure (13), preferably on a longitudinal section (15), there is an area which is characterized by a material composition and / or thickness which differs from the rest of the support structure in order to support a transfer of the heat flow.

9. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that the vertical distance between the heating element (9) and the temperature sensor (11) is in a range of 100 nm - 10 pm, preferably 100 nm - 1 pm.

10. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that there is no lateral displacement between the heating element (9) and the temperature sensor (11), so that preferably the heating element (9) and temperature sensor (11) have a maximum lateral overlap or there is a lateral displacement, wherein preferably the heating element (9) and temperature sensor (11) have a partial overlap or wherein the heating element (9) and temperature sensor (11) do not have a partial overlap but are laterally spaced apart.

11. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that the membrane (7) is held by a carrier (3), wherein the carrier (3) preferably has a cavity (5) which is located below the aperture (19) of the membrane (7).

12. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that the temperature sensor (11) is selected from a group comprising a thermocouple, a diode, a thermopile or a thermistor, wherein preferably the temperature sensor (11) is a thermistor, wherein the thermistor is located on a section of the support structure, preferably on a longitudinal section (17), or the temperature sensor (11) is a thermocouple or a thermopile comprising a hot junction and a cold junction, wherein the hot junction is preferably located on a section of the support structure (13), preferably on a longitudinal section (17).

13. Thermal fluid sensor (1) according to one or more of the preceding claims, characterized in that the thermal fluid sensor comprises at least two temperature sensors (11) and, for measuring an oscillating fluid flow, preferably caused by sound pressure waves, a combination signal of the at least two temperature sensors (11), preferably a difference signal, is formed.

14. A method for producing a thermal fluid sensor according to one or more of the preceding claims, comprising the following steps: a) providing a carrier substrate (2), b) applying a membrane (7) starting from a front side to the carrier substrate (2), optionally with the application of a first sacrificial layer (21), c) structuring the membrane (7) to provide an aperture (19), d) attaching a heating element (9) or temperature sensor (11) to the membrane (7), e) attaching the support structure (13) to the membrane (7), optionally with the application of a second sacrificial layer (23), f) attaching a temperature sensor (11) or heating element (9) to the support structure (13), g) etching the carrier substrate starting from a rear side to provide a cavity (5) and to obtain a carrier (3), such that the aperture (19) lies above the cavity (5) and the membrane (7) is held by the carrier (3), h) Providing a channel,optional etching of sacrificial layers (21, 23)., 15. Use of a thermal fluid sensor (1) according to one or more of the preceding claims 1-13 for measuring one or more properties of a fluid, preferably a flow parameter, a pressure fluctuation, preferably sound pressure waves, a composition and / or concentration of a fluid.

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