DE102011081923A1 - Sensor chip, particularly thermal flow sensor chip for determining flow parameters, such as flow amount of gas, comprises sensor element for determining flow parameter, and condensate heater to heat substance by condensing temperature - Google Patents

Abstract

The sensor chip (5) comprises a sensor element (12) for determining the flow parameter, and a condensate heater (30), which is formed in the sensor chip to heat a substance contained in the gas by a condensing temperature. The condensation temperature corresponds to a dew point temperature of the water contained in the gas. The condensate heater has a heating element integrated in the sensor chip. The sensor element is a thermal membrane sensor element and the condensate heater is arranged symmetrically on opposite sides of the membrane sensor element. An independent claim is included for a method for determining a flow parameter of a gas.

Description

THE iNVENTION field

The present invention relates to a sensor chip for determining a flow parameter of a gas, for example a thermal flow sensor chip for determining a flow rate, a flow direction or a fluid state variable, as well as methods for determining a flow parameter.

Background of the invention

Flow or flow sensors for determining a flow parameter, such as a mass flow, a volumetric flow, a flow velocity of a fluid are known in various embodiments and with different measuring principles from the prior art. The fluid may be a gas or a liquid. The gas or the liquid may have several different components or substances. For example, air may be such a gas or fluid. For example, the air may be saturated with water vapor.

Flowmeters and the corresponding sensor chips are used in a variety of technical fields, such as aerospace, vehicle and motor control, heating and ventilation technology, medical technology and industrial process technology. If, for example, air is considered as a fluid, then in such a flow sensor it is often also referred to as an anemometer, a calorimeter, an air mass meter or else an air volumetric flow sensor or air mass flow sensor.

A flow sensor can be designed, for example, as an induction flow meter, ultrasonic flow meter, Coriolis mass flow meter, thermal flow sensor or as a vortex meter.

In the European patent application EP 1 512 948 A1 a gas flow sensor with flow diagnostics will be described. The gas is passed through a miniaturized thermal CMOS anemometer and a flow signal is measured. In this case, relatively high-frequency fluctuations in the flow signal of the CMOS anemometer are measured around an average flow signal, and pulsations in the gas flow are determined from the fluctuation signal and displayed.

The publication DE 10 324 292 A1 discloses a measuring element for a flow sensor, in particular an air mass sensor for internal combustion engines. It is proposed a measuring element for a flow sensor, so that the dimensions of the measuring element can be reduced.

From the WO 01/18500 A1 For example, a method and apparatus for precision mass flow measurement is known. It discloses a method and a sensor for improved mass flow measurement, wherein in known thermal mass flow sensors, a gas stream is heated with a heating element and from the temperature difference of two thermocouples, the mass flow is determined. In addition, at least one substance-specific parameter for characterizing the heat transfer behavior of the gas is measured and thus the mass flow quantity is corrected. For this purpose, a special sensor for measuring the substance-specific parameters is given, which is constructed similar to the mass flow sensor, but is exposed in a pitot tube to a flow-free portion of the medium.

Flowmeters with corresponding sensor chips are also used in medical technology, so for example in so-called spirometers a breathing gas measurement, i. H. determines the time-resolved measurement of the volume flow during inhalation and exhalation. Such a breathing gas measurement with temporal resolution of the volume flow during inhalation and exhalation can be carried out, for example, with thermal flow sensors, in particular with microtechnical membrane sensors in which the essential elements such as heaters and temperature sensors are located on a thin membrane. In general, thermal flow sensors have the advantage that they have a high sensitivity at low flow rate of a fluid. In the case of breathing gas measurement is a high sensitivity at low flow velocity in the important for artificial ventilation reversal point between expiration (exhalation) and inspiration (inhalation) important. At the same time, high measurement dynamics are desirable in order to detect or measure even high flow peaks.

Furthermore, so-called chamber head sensors are known, which would in principle also be suitable for flow measurement, for example in ventilation technology. In these sensors, the sensor chip is located in a small flow channel that is integrated at the tip of a probe. For measuring, the probe or its tip is placed in the gas flow, ie for example a pipeline, so that a defined part of the flow flows through the small flow channel and is measured there. In this arrangement, however, the sensor chip is not free-standing, which can lead to problems regarding a possible condensation of water on the probe, since the sensor chip integrated in the probe is. This arrangement may also be referred to as tube in tube bypass.

In the respiratory gas measurement or generally in a reliable measurement of a flow parameter of a fluid, such. As a volumetric flow measurement with a thermal sensor, there is a technical challenge is that an expiratory gas, for example, can be 100% saturated with water. As a result, under certain adverse operating conditions, condensation and increased aerosol formation may occur on the sensor as the temperature of the hoses and the meter may be below the dew point temperature of water. This can lead to a buildup of water in the form of drops on the flow sensor. In particular, in the case of a sensor membrane of the sensor, a non-compensatable influencing of the sensor signal and thus possibly incorrect measurements may occur. In extreme cases, it can lead to a complete signal collapse.

Since a working temperature of microtechnical silicon-based sensors, in contrast to thin-film sensors, can be below 100 ° C., water accumulations on the sensor generally remain stable and do not evaporate. The heating energy released by a heating element during sensor operation is insufficient to remove or heat away a drop of water within a reasonable period of time; H. to evaporate. Evaporation can only take place during inspiration due to the maximum humidity during expiration. Since a sensor failure during ventilation can be systemically critical, a faulty measurement due to condensate accumulation, such. B. water on the sensor chip only rarely and then only for a short time, from z. B. less than 5 s, 10 s or 15 s occur.

The object of the present invention is therefore to provide a sensor chip for determining a flow parameter of a gas, which prevents or eliminates condensation of a condensable substance contained in the gas and therefore enables reliable and accurate flow measurement over a wide measuring range. This object is made possible by the subject matter of patent claim 1 and by the methods according to claims 9, 12 and 14. Further advantageous embodiments are disclosed in the dependent claims 2 to 8, 10 to 11 and 13 to 16.

Embodiments of the present invention disclose u. a. a sensor chip for determining a flow parameter of a gas, wherein the sensor chip has a sensor element and a condensate heater which prevents or eliminates condensation on the sensor element, so that a reliable measurement of the flow parameter can take place.

According to further embodiments, the sensor chip may have a sensor element with a heater, wherein the condensate heater has a higher heat output than the heater for determining the flow parameter.

According to another aspect of the present invention, the flow parameter determination method of the present invention may include a step of heating the sensor chip or the sensor element above a condensing temperature of a condensable gas contained in the gas. As well as removing an already condensed condensate by means of the condensate heater. Furthermore, a step of detecting a moisture or condensation may be performed on the sensor chip.

In embodiments of the present invention, a flow sensor with sensor chip for determining a flow parameter of a fluid is further described with a sensor chip having a thermal membrane sensor element, and wherein the sensor chip is designed to freestanding flows around the fluid during operation of the flow measuring sensor as a "nude chip" become. The flow meter can be arranged free-standing in a main flow of the fluid.

According to a further aspect of the present invention, a sensor chip has two thermal membrane sensor elements which are designed to determine a flow parameter of the fluid, wherein the flow parameter of the fluid can be determined with the aid of the two different sensors in different measuring ranges and thus achieves a larger measuring range dynamics can be. In addition, in addition, a fluid temperature of the fluid flowing around the sensor chip or of the flow parameters can also be determined alternately.

According to further embodiments, the flow sensor may comprise a sensor chip with an integrated condensate heater, which is a condensation of a present in a gaseous fluid condensable material such. B. prevent water in air or can evaporate again when condensation has already occurred.

According to a further aspect of the present invention, in a method for determining a flow parameter of a fluid, a measuring head with a sensor chip attached on one side with at least one thermal membrane sensor element is free-standing, ie as a "nude chip" during the measurement of the flow parameter used in the flowing fluid.

Preferred embodiments of the present invention will be explained in more detail below with reference to the accompanying drawings. Show it:

1 the schematic plan view of a sensor chip according to an embodiment of the present invention;

2 the schematic plan view of a sensor chip according to another embodiment of the present invention;

3 a further schematic representation of a sensor chip according to an embodiment of the present invention.

4 the schematic representation of a flow sensor with sensor chip for determining a flow parameter of a fluid according to an embodiment of the present invention;

5a the schematic representation of a measuring head with flow sensor according to an embodiment of the present invention;

5b the schematic cross-sectional view of a flow sensor with sensor nude chip free-standing during operation in a fluid channel arranged according to embodiments of the present invention;

6 the schematic plan view of a measuring head with free-standing MEMS sensor in a flow channel, which is designed as Venturi nozzle with double-sided screens for Strömungslamamination;

7 a schematic detail view of the top view of a sensor head with sensor chip according to an embodiment;

8th a schematic representation of a sensor chip with an angular position to a fluid flow;

9 a schematic representation of the production of a sensor chip with a thermal membrane sensor element;

10 a schematic representation of a sensor chip for a flow sensor according to the invention and a thermal membrane sensor element and a thermopile thereon according to an embodiment of the present invention;

11 a diagram to illustrate the operation of a thermal flow sensor according to Sven Zinober ( Redundancy structures to increase the safety and reliability of microsystems, Dissertation, Albert-Ludwig-University Freiburg i. Breisgau, 09.07.2002 );

12a the schematic representation of a sensor chip with two thermal membrane sensor elements and a condensate heater according to embodiments of the present invention;

12b a schematic cross-sectional view of a sensor chip with encapsulated recess on the back of the thermal membrane sensor element;

13 a schematic representation of a flow measuring system with drive means and optional pressure measuring device according to an embodiment of the present invention.

14a shows a block diagram of the method for determining a flow parameter of a gas according to an embodiment of the present invention;

14b shows a further block diagram of a method for determining a flow parameter of a gas according to another embodiment of the present invention.

Detailed description of the figures

With respect to the following description of the embodiments of the present invention, it should be noted that the same reference numerals are used throughout the specification in the different figures for functionally identical or functionally equivalent equivalent elements or steps for simplicity.

1 shows the schematic representation of a sensor chip 5 for determining a flow parameter of a gas, wherein the sensor chip is a sensor element 12 for determining the flow parameter. In the sensor element 12 For example, it may be a thermal membrane sensor element configured to determine a flow parameter of the fluid in a measurement region or to determine a fluid temperature of the fluid. For example, the thermal membrane sensor element may include two spaced apart thermocouple junctions, or generally temperature sensors, and may include a heating element disposed between the thermocouple junctions. The sensor chip may be designed to operate during the operation of a flow sensor, for example on one side attached to a measuring head to be flowed around as free-standing sensor finger of the fluid. Furthermore, the sensor chip, as in 1 shown, a condensate heater 30 on, which is formed around the sensor chip 5 to heat over a condensation temperature of a condensable substance contained in the gas, so as to prevent condensation of the substance on the sensor chip. The condensable substance may be, for example, water or water vapor in the gas. The condensate heater 30 may be formed to the sensor chip 5 to heat above a dew point temperature of the water vapor and thus to prevent condensation or droplet formation of the water on the sensor chip. Thereby, a reliable, accurate and reproducible determination of the flow parameter of a gas can be ensured. In the case of moist gases whose flow parameters are to be determined, the condensation or dew of water or other condensables contained in the gas can result in a malfunction or a wrong determination of the flow parameter.

As in 2 is shown, the sensor chip 5 a sensor element 12 for determining the flow parameter of a fluid, e.g. B. thus have a gas, wherein the sensor element 12 a heater 3 which is designed to generate a temperature gradient for a determination of the flow parameter. The heater 3 So in contrast to the already mentioned above condensate heater 30 needed or used to allow determination of the flow parameter by the sensor chip. The condensate heater 30 In contrast, it can be used to heat the sensor chip to a temperature above a condensation temperature of a condensable substance contained in the gas or to a condensate located on the sensor chip, for. B. to remove a drop of water or an aerosol. With the help of the heater 3 Thus, the actual measuring task of the sensor chip can be fulfilled while using the condensate heater 30 Condensation of a condensable substance contained in the fluid is prevented or such a condensate can be removed by means of heating. The heating capacity of the condensate heater 30 may be greater than the heating power of the heater according to embodiments of the present invention 3 , which refers to a sensor element 12 of the sensor chip 5 located.

3 shows a further embodiment of a sensor chip 5 for determining a flow parameter of a gas. In this embodiment, the condensate heater 30 symmetrical about the sensor element 12 arranged. The condensate heater 30 Here, for example, is designed as a resistance heater where it can be integrated in the sensor chip. The sensor chip can be, for example, a silicon chip, wherein the condensate heater is integrated in the silicon chip as a resistance heater. In other embodiments, the condensate heater may be disposed on the sensor chip, for example, in the form of a poly-silicon resistor structure or a heater structure or a platinum resistor structure. The condensate heater can be controlled by means of a drive device (not shown in FIG 3 ) via electrical feeders 19 be heated so that condensation is prevented or a condensate can be evaporated quickly.

4 shows a flow sensor 10 for determining a flow parameter of a fluid 15 , The flow sensor has a sensor chip 5 on, with the sensor chip 5 at least one thermal membrane sensor element 12 formed to be a flow parameter of the fluid 15 in a measuring range or to determine a fluid temperature, for example a relative fluid temperature or an absolute fluid temperature. The thermal membrane sensor element 12 has two temperature sensors. These temperature sensors can be, for example, thermocouples, resistance strips, for example with platinum or nickel, or thermopiles with contact pairings. The thermal membrane sensor element has a membrane 9 , the two spaced thermocouple junctions, that is, in series with a thermal stack or "Thermopile" alternately interconnected contact pairings 8a . 8b (For example, transitions between aluminum and poly-silicon or between two metals), and arranged between the thermocouple junctions heating element 3 having. The sensor chip 5 So has a thermal membrane sensor element 12 with two spaced-apart temperature sensors, for example, two thermocouples, each of which thermocouple thermocouple junction is disposed on the membrane and the respective second thermocouple junctions of the two thermocouples are arranged on the sensor chip bulk or -Sensorchip frame. Between the thermocouple junctions of the two thermocouples is on the membrane 9 a heating element 3 arranged. The sensor chip is designed to be in a mode for measuring the flow sensor as a freestanding sensor finger of the fluid 15 to be flown around. During operation, the flow sensor may, for example, be in a pipe or in a fluid channel 17 be introduced, leaving a fluid 15 in the fluid channel 17 freestanding sensor chip 5 flows around.

According to one aspect of the present invention, therefore, the part of the sensor chip required for flow measurement is in direct contact with the medium or the fluid. In particular, is nearby the sensor membrane 9 or the thermal membrane sensor element 12 no housing part or any other device available that favors an accumulation of condensate on the sensor chip. The condensate can be, for example, water droplets dissolved in air, which can form particularly well, for example, due to capillary forces in the case of existing housing parts in the vicinity of the part of the sensor chip required for the flow measurement. That is, you can call this arrangement as a freestanding nude chip in the measuring or fluid channel.

The flow sensor according to the invention can be used, for example, in medical technology, air travel technology, industrial process technology, automotive engineering and heating and environmental technology. One application in medical technology is z. B. in the breathing gas measurement, ie for time resolved measurement of the volume flow during inhalation and exhalation. This can be done by means of thermal flow sensors, in particular by means of microtechnical membrane sensors as in the present embodiment, which can be very advantageous because the essential elements on a thin membrane 9 Thus, among other things, a high measuring sensitivity, a high dynamic range, a fast response time and a relatively low energy consumption can be realized during the measurement.

The flow sensor 10 Thus, it can advantageously have a high measuring sensitivity at a low flow velocity in the reversal point between expiration (exhalation) and inspiration (inhalation), which is important for artificial respiration. Furthermore, a high dynamic range can be achieved both for the flow measuring range and for the time-accurate resolution of the highly dynamic breathing curves. The lower the thermal mass of the membrane sensor element, the higher temporal resolution can be achieved. For example, in the case of a microtechnical sensor, a temporal resolution of about 1 ms can be achieved. Another advantage of the above-mentioned flow sensors with thermal membrane sensor element is the relatively low energy consumption, which is why the sensors are also well suited for battery operation in portable devices.

The flow sensor, as in 4 For example, compared with the above-mentioned chamber head sensor, it has the advantage that the flow sensor has a lower thermal mass, since only the sensor chip is in contact with the housing of the flow sensor on one side. In contrast, in the case of the abovementioned chamber head sensor, the entire sensor chip is located in a housing, so that a significantly higher energy expenditure is required when the sensor chip is heated to a temperature which is higher than a condensation temperature or a dew point temperature in the case of water.

Furthermore, the relatively small flow or bypass channel in the chamber head sensor favors the accumulation or condensation of water droplets in the region of the sensor chip by the capillary effect. In contrast, the flow sensor according to the invention is placed freely in the main flow of a fluid, so that no "small" flow channels or housing parts are present in the region of the flow sensor, the an accumulation or condensation or condensation with a fluid contained in a condensable material, such as For example, water droplets in air, be favored.

Furthermore, a possible adhesion of a condensate, such. B. of water droplets on such a freestanding nude chip, as in 4 is substantially less due to the lower surface area without undercuts, so that also condensate adhesion is counteracted. The flow sensor is thus constructed so that a temperature-dependent condensate separation is difficult.

In the case of the flow sensor 10 As already mentioned above, certain flow parameters may, for example, be a flow velocity, a mass flow, a volume flow, a state variable or another substance-related parameter of the fluid. The fluid may be a gas or a liquid. With the flow sensor, a flow velocity of the fluid can be determined with high temporal resolution. The flow meter may be a flow of a gas or a liquid, ie a fluid, for example through a pipe or a fluid channel, 17 determine or measure. A parameter can be determined with the aid of which a flow parameter in the above sense can then be calculated in an evaluation device. The evaluation device can be electrically coupled to the flow sensor. It may, for example, be part of a control or regulating device that controls the flow sensor. For example, when determining a volume flow rate, the flow rate of the medium can be measured and then the flow rate can be determined by the formula volumetric flow = pipe cross-sectional area x velocity. The result can be given in liters / minute, for example.

The flowmeter may additionally comprise integrated or external sensor subunits with which a temperature, a pressure, a substance-specific size or a moisture of the Fluids is determined or measured. The device 10 or the flow sensor may, for example, a measuring head 13 and an immersion probe, which is preferably immersed to a middle of the tube or fluid channel center and is made with its measuring surface, for example in an angular range of 5 ° to 75 ° to the fluid flow.

As in the 5a and 5b can be shown, a flow sensor 10 with a sensor chip on a measuring head 13 be attached, the corresponding electrical connections 11 wherein, during operation of the flow sensor, the sensor chip 5 with the thermal membrane sensor element 12 is flowed around as free-standing sensor finger of the fluid. The flow sensor can taper from the measuring head coming to the sensor chip. The flow sensor with the sensor chip can be in a fluid channel 17 , as in 5b is shown free-standing as a "nude chip" flowed around by the fluid so as to better a flow parameter of the fluid, so z. B. with a lower error rate, in a larger measuring range or with a higher dynamic range to be able to determine. The flow sensor may be a microelectromechanical semiconductor (MEMS) sensor. That is, the sensor chip may be, for example, a silicon semiconductor chip, which has been structured microsystemically. Also conceivable is the use of other semiconductor materials, such as silicon carbide, gallium arsenide, indium phosphide, etc., which are used in semiconductor technology and in microelectromechanics. Glass, ceramic or similar materials can also serve as a substrate for the flow sensor or sensor chip according to the invention.

The device 10 can according to some embodiments for tube cross-sections (see 5b ), which are larger than the tube cross-section covered by the sensor. For example, the tube cross-section or -Fluidkanalquerschnitt be at least three times, four times or ten times as large as the hidden by the sensor cross-section of the fluid channel 17 , In 5b is the schematic representation of a flow sensor with measuring head 13 in a flow channel 17 positioned is shown. In addition, at the measuring point, the determined sensor signal should still allow sufficient information about the flow profile of the total cross section of the fluid channel.

In some embodiments, the actual sensor element may be a microtechnical silicon chip which is mounted free-standing as a bare chip on one side connected to the measuring head or the immersion sensor and flows around the measuring medium. In this case, the sensitive area, so the sensor membrane, z. B. so the thermal membrane sensor element ideally be centrally located in the measuring tube to measure there, for example, the mass or volume flow. The sensor finger or the sensor chip of the flow sensor, which is free during operation, can be at least four times, five times or ten times the distance corresponding to a sensor chip width from a fluid channel wall of the fluid channel 17 , In which the freestanding sensor finger is flowed around by the fluid, have.

At the sensor chip 5 For example, it may be a rectangular sensor chip. Generally, a length L of the sensor chip may be at least three times, four times or five times as long as a width W of the sensor chip. That is, a longitudinal side of the sensor chip may be at least three times, four times or five times longer than a width of the sensor chip. The sensor chip can thus be formed elongated, wherein the longitudinal edge is three times or longer than the narrow side of the chip to the sensitive elements, ie z. B. the thermal membrane sensor element 12 with the heater 3 and the two temperature sensors in the flow of the fluid freestanding and outside of a flow disturbing transition to a mechanical anchoring with the measuring head 13 to be able to place. The electrical leads 19 (please refer 10 ) for the sensor chip can on a front side of the sensor chip 5 lie and the active sensory elements such. B. the thermal membrane sensor element 12 in a first third of the chip area at the opposite end of the anchorage to the measuring head 13 or be arranged at the opposite end of the chip. It can thereby be achieved that the flow measurement is influenced as little as possible by the connection to the fuselage of the measuring head or to the sensor finger housing.

The flow sensor may be, for example, a thermal flow sensor, a flow rate sensor or a flowmeter. The flow sensor 10 can for example consist of silicon. The sensitive element of the flow sensor is in some embodiments, a thin silicon nitride membrane, for example, a size of 100 to 500 microns × 400 to 1000 microns, ie z. B. 300 × 600 microns ² may have.

With the flow sensor 10 becomes the heat flow necessary in some embodiments for determining a flow parameter by a heater 3 provided, for example, as a line-shaped heater on the membrane. Correspondingly arranged thermocouples with the corresponding thermocouple junctions on both sides of the heating element can register the heat distribution across the membrane. This heat distribution changes as gases, liquids or generally a fluid flow across the membrane. Thus, both the direction and the flux density of the substance quantity can be concluded. For example, it may be a flow direction, a mass flow, a volume flow or a flow velocity of the membrane membrane, ie the thermal membrane sensor element 12 , flowing fluid can be determined.

The device according to the invention 10 can be used to measure dynamically changing and water-saturated air flows. For an accurate measurement of the flow velocity or the mass flow, the measuring location can be located in a pipe-bound measuring section, which contributes to a smoothing of the flow profile with its execution. This is schematically in the 6 shown. A holder with measuring head shown in plan view 13 and the freestanding MEMS sensor 10 in a flow channel 17 , which in this embodiment as Venturi nozzle with bilateral sieves 37 designed for flow laminarization. The MEMS sensor 10 is at an angle α to a main flow direction 15 (Arrows) to be flowed around optimally rectified by the fluid.

The flow sensor 10 can be placed in the pipe-bound measuring section such that it with its sensitive measuring surface 10a at an angle α to the main flow direction 15 the measuring fluid is employed. This ensures that the fluid flows directed in the thermal boundary layer region over the measuring diaphragm. The angular adjustment reduces Strömungsabschattungen or up to the membrane area propagating flow vortex, which can be generated by a leading edge. This enables the acquisition of reliable flow measurements in the membrane area of the MEMS sensor.

According to one embodiment, the flow sensor 10 a sensor chip 5 exhibit, on both a front 5a as well as on an opposite back 5b at least one thermal membrane sensor element 12 having. This can be done for example by structuring on both sides or z. Example, by a chip stack, made of two single-sided structured sensor chips, which are joined together at their respective backs. In order to be able to measure in a straight tube in both directions according to the calorimetric principle, a measuring diaphragm can therefore be located both on the front and on the back of the MEMS sensor. The then each of the flow direction averted sensor membrane can be used to additional measurement tasks, such. As the temperature, the humidity or the thermal determination of the volume fractions of a (known) binary gas mixture in the main stream.

With the device according to the invention 10 can be determined point by point, ie at a location of a pipe cross section or fluid channel cross section, a flow parameter, therefore, for a sufficiently accurate flow measurement or flow parameter determination a constant distribution of the flow velocity profile over the pipe cross section in the entire longitudinal region should be ensured. This can be achieved by suitable installation elements for Strömungslamaminarisierung before the measuring point, such. B. by the installation of sieves and / or by the advantageous tube design at the place of measurement. In this way, an even more accurate determination of a flow parameter of the fluid can be achieved by the sensor chip flowing around in the operation of the flow sensor as a freestanding sensor finger.

7 shows a schematic detail view of the top view of a flow sensor 10 with measuring head 13 and sensor chip 5 according to an embodiment of the present invention. The sensor chip 5 with his active front 5a on which the thermal membrane sensor element is located is over tapers 33 with the measuring head or the sensor housing 13 connected. The active front side 5a of the sensor chip may be inclined relative to the main flow of a fluid by an angle α, so that the fluid flows in the thermal boundary layer region directed over the measuring diaphragm.

8th shows the schematic enlarged flow surface of the sensor chip 5 an immersion sensor. By the angular adjustment by the angle α with respect to a flow direction 15 the fluid forms a rectified largely vortex-free fluid layer on the flown sensor side 5a off (solid arrows). If the flow (dashed arrows) in the straight measuring tube from the other side, ie the back 5b of the sensor chip, so may in embodiments on the back 5b of the MEMS chip 5 similar sensor structures as on the front 5a So, for example, both on the front 5a , as well as on the back 5b of the Senor chip 5 , in each case at least one thermal membrane element 12 be educated. This can, as already mentioned above, for. B. can be achieved by a chip stack or a back structuring. Due to the structuring on both sides with the thermal membrane sensor elements, a flow direction in both possible flow directions can then also be determined in each case. When using the flow sensor as a spirometer So z. B. inhalation and exhalation can be distinguished.

In 9 the manufacturing process and the structure of a flow sensor according to the invention is briefly explained. On a silicon wafer or silicon substrate 20 First, an adhesion layer of dry oxide and then a layer of silicon nitride (Si ₃ N ₄ ) 22 applied ( 9A ). The dry oxide layer may, for. B. have a thickness of 50 nm and the silicon nitride layer has a thickness of 100 nm. Next, as in the 9B is shown, the heater 3 and the temperature sensors 8th generated. At the temperature sensors 8th they may be, for example, thermocouples or thermopiles, each thermocouple 8th . 8th' each having two thermocouple junctions. The thermocouple 8th indicates the thermocouple junctions 8a and 8c on and the thermocouple 8th' the thermocouple junctions 8b and 8d , The heating element 3 is structured on the membrane in the form of two linear heating wires.

The detection of a membrane heating to determine a flow parameter of a fluid is z. B. on thin-film thermocouples at the edges, ie at the edges of the membrane 9 , The thermocouples 8th . 8th' exploit the Seebeck effect, whereupon an electrical voltage occurs as soon as two different interconnected metals or conductive materials experience a temperature difference. One contact point of each thermocouple 8th . 8th' namely the contact point or the thermocouple junction 8a for the thermocouple 8th and the contact point 8b for the thermocouple 8th' is located on the by the heater 3 heated sensor membrane 9 while the other contact point of each thermocouple 8th . 8th' namely the contact point 8c for the thermocouple 8th and the thermocouple junction 8d for the thermocouple 8th' are located on the silicon frame or on the Siliziumbulk. The silicon frame or the silicon bulb is, as in the 9D is shown schematically, not undercut.

The high thermal thermal conductivity of the silicon ensures a nearly constant reference temperature. In order to be able to detect smaller temperature differences, 5 thermocouples or contacts per side may be arranged on each side of the heater in some embodiments of the present invention and connected in series to a so-called thermopile. For example, a voltage of 180 μV / K can be tapped per contact. That is, according to some embodiments of the present invention, the thermal membrane sensor element 12 not only have two spaced-apart thermocouple junctions, but a plurality of such spaced-apart thermocouple junctions and arranged between this plurality of mutually spaced thermocouple junctions heating element 3 , A thermocouple junction may consist of two different metals or at least conductive or semiconducting materials. For example, the thermocouple may comprise at least one of the following metals: bismuth, constantan, nickel, platinum, aluminum, rhodium, copper, gold, silver, iron, titanium-tungsten or chromium. At the thermopiles 7 . 7 ' can each be a thermocouple junction on the sensor membrane and the respectively associated second thermocouple junction on the Sensorchipbulk, so z. B. the Siliciumbulk.

As a material for the heater 3 and a first thermoelectric partner of a thermocouple junction 8a . 8b respectively. 8c . 8d may be a doped and thus highly conductive polysilicon on the membrane 9 be deposited and structured. The thickness of the polysilicon may be, for example, about 100 nm. In a process step can therefore, as in the 9B is shown schematically, as a thermoelectric partner for the thermoelectric transition polysilicon 25 partly on the membrane 9 or the future membrane area 9 and partially deposited on the Siliziumbulk. In a further process step is then made of aluminum, for example 24 each associated with a thermocouple second thermocouple junction and the electrical traces and contact surfaces 29 (Pads) applied for later wire bonding of the chips. The aluminum can be applied in a thickness of, for example, about 300 nm.

Subsequently, the wafer, as in 9C is shown with a layer stack 27 of silicon oxide and nitride coated over the entire surface. The layer stack 27 can z. B. have a thickness of for example about 1 micron. Thus, a sufficient passivation of the functional elements of the sensor chip against aggressive media can be achieved. After applying the passivation 27 can from the back 5b forth, so from the side, the functional elements, such as the heater 3 and the thermocouples 8th . 8th' opposite, for example by wet-chemical anisotropic etching with potassium hydroxide (KOH) below the later membrane 9 a pit or recess 28 be structured.

As in 9D is shown, the silicon nitride serves 22 as an etch stop from the back, leaving the sensor membrane 9 stops after etching. The backside 9a the sensor membrane is therefore free. As in 9E is shown schematically, can thereafter by locally removing the passivation 27 and exposing the bond pads 29 a Bonding for the electrical connections of the thermocouples 8th . 8th' and the heater 3 be performed.

In 10 is the top view of a sensor chip 5 represented in a different level of detail of the individual elements. It shows the 10A the supervision of a microscope image of the flow sensor 5 and the 10B a schematic representation of the top view of a membrane 9 or the thermal membrane sensor element 12 with the heaters 3 and the two opposing thermoelectric junctions, here as serially connected thermopiles 7 . 7 ' are formed. Furthermore, in the 10C a detailed view of a thermopile 7 ' represented with the series-connected thermocouples.

At the sensor chip 5 It may be, as in the 10A is shown to act an elongated sensor chip in which, for example, the longitudinal side L is 6 mm and the width W 2 mm. In some embodiments, therefore, a longitudinal edge of the sensor chip 5 three times or more than the narrow side W of the chip to the sensitive elements 12 in the flow free-standing and outside of the fluidically disturbing transition to the mechanical anchoring of a measuring head to be able to place. The electrical leads 19 lie on one end face of the sensor and are there with the measuring head (not shown in 10A ) connected. The active sensory elements 12 can be in the first third of the chip area at the opposite end 5c be arranged of the sensor chip. As a result, a flow measurement is influenced as little as possible by the connection to the fuselage or measuring head of the sensor finger housing.

In the supervision of 10B is the thermal membrane sensor element 12 with the associated heater 3 shown. The thermocouples 8th respectively. 8th' are connected in series on their respective sides and in each case to a thermopile 7 . 7 ' summarized. The individual thermocouples 8th . 8th' or the thermopiles 7 . 7 ' each have thermocouple junctions 8a and 8b on the sensor membrane 9 are arranged and respectively associated second thermocouple junctions 8c and 8d , which are arranged on the chip frame or the Siliziumbulk. Both the heating elements 3 as well as the thermocouples 8th . 8th' have corresponding electrical leads 19 on. In 10C is a partial view of the Thermopile 7 ' with the thermocouple junctions 8b . 8d shown. The figure shows on the one hand the thermocouple transitions 8b , the aluminum 24 and polysilicon 25 and that on the membrane 9 are arranged, and on the other hand, the thermocouple junctions 8d which also polysilicon 25 and aluminum 24 have, and which are formed on the Siliziumbulk or sensor chip frame.

In operation, if so the heater 3 the surroundings are heated, the thermocouple junctions 8b of the thermopile 7 ' have a higher temperature T _warm than the associated thermocouple junctions 8d of the thermopile 7 ' which have a lower temperature T _cold . The membrane 9 has a low thermal mass, so that when heating the heating element, the membrane 9 relatively quickly assume the temperature generated by the heater, whereas the temperature on the silicon chip or the sensor chip frame will be lower.

In 11 is the functional principle of a thermal flow sensor 5 in a diagram from the dissertation by Sven Zinober, "Redundancy structures to increase the safety and reliability of microsystems", Dissertation, Albert-Ludwig University Freiburg im Breisgau, 2002 shown. The solid curve in 11 shows the schematic temperature profile along a fluid channel wall on a sensor surface for a resting medium or fluid and the dashed curve with an assumed flow of a fluid in the direction indicated by the arrow flow direction from left to right.

In a quiescent medium forms when heating the sensor membrane at the point X ₀ through the heater 3 a uniform decaying temperature distribution in the direction of the thermocouples 8th and 8th' out. That is, at a fixed flow, at an identical distance, -X ₁ and + X _{1 of} the thermocouples 8th and 8th' from the heater 3 measured an identical temperature. So rested the flow medium, the active sensor surface is heated by heat conduction in the membrane and in the medium. The corresponding temperature is therefore from the thermocouples 8th . 8th' detected. When an onset of flow in the flow direction indicated by the arrow then forms a temperature distribution as shown by the dashed curve. When flow is thus the heat distribution field in the direction of the flow thermocouple facing away 8th' postponed.

11 So shows the schematic temperature profile on the sensor plane with the two thermocouples 8th . 8th' and the heater in between 3 and the displacement of this course as a function of the flow velocity of the fluid flowing over it, wherein 10 the case shown is that of the heater 3 heated to a constant temperature (also referred to as constant temperature anemometry, CTA). According to the requirements or the flow parameter to be measured, the heater 3 also in different ways, ie with constant power, constant voltage or constant current (Constant current anemometry, CCA), or z. B. periodic or pulsed heated. A measuring head with a freestanding sensor finger can thus be arranged in a flowing fluid such that the flowing fluid firstly a first thermocouple 8th flows around, then the heater 3 and continue the second thermocouple 8th' flows around.

The temperature differences ΔT ₁ and ΔT ₂ give the difference between that to the heater 3 facing respective hot contacts 8a . 8b the thermocouples 8th . 8th' and the heater 3 opposite ends 8c . 8d the thermocouples 8th . 8th' , which are located on the silicon bulb and can therefore be described as a cold contact. ΔT ₁ is the temperature difference at the upstream (upstream) sensor 8th and ΔT ₂ at the downstream (downstream) sensor 8th' , If the flow medium is at rest, the active sensor surface is heated by heat conduction in the membrane and the medium. When the flow starts, here in the flow direction from left to right, this heat distribution field is shifted to the right. The thermocouples 8th . 8th' can be connected back to Thermopiles again 7 . 7 ' be summarized. Thus, the flow medium or fluid first reaches the upstream thermopile 7 , cool this, then it is at the stoker 3 heats and carries this heat. This will be the hot contacts 8b the outflow downstream thermopile 7 ' heated up more. The difference of the temperature differences at the thermopiles 7 . 7 ' , that is ΔT ₂ - ΔT ₁ is then the temperature-dependent temperature difference between the downstream thermopile 7 ' and the upstream thermopile 7 , It can therefore be measured, the temperature profile between the center of the membrane with the heater and the Siliziumbulk.

With the above measuring principle can flow sensors with high dynamics, z. From 1:10 to 1: 10,000, e.g. So build from 1: 1000. When used for liquid measurements, the flow sensor should have a dense passivation of the thermopile structures and their supply lines so as to avoid an electrical short circuit of the active sensor elements. The flow sensor 5 is for gases and liquids or generally fluids z. B. can be used both in an anemometer and in a calorimeter operation.

In 12a is the schematic representation of a sensor chip 5 for a flow sensor 10 for determining a flow parameter of a fluid according to further embodiments of the present invention. The sensor chip 5 Therefore, not only a sensor element, for. B. a thermal membrane sensor element 12 have, but also two or more sensor elements, ie z. B. also a second thermal membrane sensor element 12 ' , The thermal membrane sensor element 12 may be configured to determine the flow parameter of the fluid in a particular or first measuring range or to determine a fluid temperature of the fluid. The fluid temperature may be, for example, an absolute or a relative fluid temperature. For example, the relative fluid temperature may be a temperature difference between the membrane 9 , which can quickly assume the fluid temperature due to their low thermal heat capacity and the temperature of the sensor chip frame or the silicon bulb act. The thermal membrane sensor element 12 has two mutually spaced thermocouple junctions 8a . 8b and a heating element disposed between the thermocouple junctions 3 on and where the sensor chip 5 is designed to be flowed around during operation of the flow sensor as a freestanding sensor finger from the fluid.

As in 12a 1, the sensor chip may be another thermal membrane sensor element 12 ' which is configured to determine a flow parameter of the fluid in a further or second measuring range or also to determine a fluid temperature. Also the further thermal membrane sensor element 12 ' can be two spaced thermocouple junctions 8a ' . 8b ' and a heating element disposed between the thermocouple junctions 3 ' exhibit. Opposite the first thermal membrane sensor element 12 can the second thermal membrane sensor element 12 ' to distinguish that a distance D1 (see 13 ) of the two thermocouple junctions 8a . 8b of the first membrane sensor element 12 to the heating element arranged therebetween 3 is different to a distance D2 of the two thermocouple junctions 8a ' . 8b ' the further membrane sensor element 12 ' to the heating element arranged therebetween 3 ' , That is, the thermal membrane sensor elements 12 and 12 ' differ in that the distance of the thermocouple junctions 8a and 8b respectively. 8a ' and 8b ' to the respective heaters 3 . 3 ' have a different distance D1 and D2. Due to the different distances D1 and D2 of the thermocouple junctions to the respective heaters, a flow parameter in different measuring ranges can be determined by the thermal membrane sensor elements. These different measuring ranges can be z. B. also overlap. With a smaller distance z. B. higher volume flows are measured, as with a greater distance.

According to some embodiments of the present invention, the sensor chip 5 So two complete trained thermal membrane sensor elements 12 . 12 ' exhibit. These membrane sensor elements 12 and 12 ' can For example, each a silicon nitride / oxide thin film membrane 9 . 9 ' have and a line-shaped heating resistor 3 . 3 ' of polysilicon, which is arranged in each case in the middle of the membrane transversely to a flow direction and can be controlled separately for each membrane sensor element. For this purpose, thermopile structures can again be symmetrical on both sides 7 . 7 ' from the corresponding thermocouples 8th . 8th' be arranged with contact pairs, for example, polysilicon / aluminum. The "warm contacts" 8a . 8b . 8a ' . 8b ' lie on the sensor membrane 9 . 9 ' and the cold contacts 8c . 8d . 8c ' . 8d ' as described above, for example on the silicon frame. This applies if the heater is in operation and the respective membrane 9 . 9 ' warmer than the silicon frame. In normal sensor operation this is also the case. If the membrane is colder than the frame, the cold contacts of the thermopiles are located 7 . 7 ' on the membrane and the warm on the silicon frame. Both sensor elements 12 . 12 ' Thus, for example, the two thermal membrane sensor elements can be used for flow measurement, but can be designed for different measuring ranges. Thus, by a corresponding control device or control device by switching between the two membranes in operation a total of a larger measuring range for a to be examined or determined flow parameters are covered. In embodiments in which the sensor chip 5 on the back side 5b is structured and also there each have two sensor elements or a chip stack of two single-sided structured sensor chips which are connected to their backs, so the flow sensor z. B. also have four sensor elements or thermal membrane sensor elements. For example, when using the flow sensor or sensor chip in medical technology for measuring respiratory gas, a simple pre-selection for the ventilation of adults or children can be made by a corresponding measuring range selection of the thermal membrane sensor element. Operated is usually only a membrane as a flow sensor for determining a flow parameter, while the second membrane for measuring the fluid temperature, for. B. the relative fluid temperature or the absolute fluid temperature is used. For measuring the fluid temperature needs in embodiments of the present invention, the heating element 3 ' the second membrane 12 ' not be operated. Due to the extremely low thermal mass of the unheated membrane, this very quickly assumes the temperature of the ambient medium, ie of the fluid, and is therefore ideal for the dynamic temperature measurement of the fluid. Due to the low thermal mass of the membrane, this is "thermally insulated" from the Sensorchipbulk, so z. B. against the Siliziumbulk. With the help of thermopiles 7 . 7 ' or temperature sensors 8th . 8th' Then, the differential temperature between the membrane - ie the fluid temperature - and the temperature of the chip frame measured. The temperature difference-proportional voltage of the thermopiles then expresses the relative temperature difference between the chip frame or silicon frame and the fluid temperature. Just as well, the absolute temperature measurement can be done with a sensor resistance element on the membrane, which is made for example of poly-silicon. In a flow measuring system, the control of the corresponding heaters and the thermocouples can be carried out so that in a first state, as already mentioned above, the first thermal membrane sensor element determines a flow parameter in a first measurement range, while the further thermal membrane sensor element or the second thermal membrane sensor element dynamic temperature measurement of the fluid temperature makes.

In a second state, the control device or the control device may be the first thermal membrane sensor element 12 and the second thermal membrane sensor element 12 ' so control that now the first thermal 12 Membrane sensor element performs a dynamic temperature measurement of the fluid while z. B. the heater 3 is not in operation and the second thermal membrane sensor element 12 ' in a second measuring range, which differs from the measuring range of the first thermal membrane sensor element, determines a flow parameter of the fluid.

In some embodiments of the present invention, a temperature sensor may be included in the chip frame or in the silicon frame or bulk region 35 integrated or arranged, which is designed to determine the absolute temperature of the sensor chip or the silicon frame. The temperature measurement can therefore be a static temperature measurement. The temperature sensor may, for example, be a diode integrated in the silicon frame 35 be.

The flow sensor 10 with the sensor chip 5 can be designed so that the sensor chip is flowed around during operation of the flow sensor as a free-standing sensor fingers of the fluid. For this purpose, the surface of the sensor chip, as related to the 9C is described to be formed by an appropriate passivation so that the fluid, such as a liquid, can cause no short circuits or other damage to the sensor chip.

Like in the 12a is further shown, the sensor chip 5 in further embodiments, in addition, a so-called condensate heater 30 exhibit. In the embodiment in 12a the condensate heater consists of two series-connected in the silicon frame of the sensor chip doped resistors. These can be symmetrical to a flow direction of the fluid before and after the two thermal membrane sensor elements 12 . 12 ' be arranged. The condensate heater 30 So it can be symmetrical in front of and behind the sensor membrane 9 be arranged to avoid strong thermal gradient on the sensor chip. With the condensate heater, the sensor chip 5 are heated to a temperature above a condensation temperature of a condensable gas contained in a gaseous fluid. For example, with the help of the condensate heater 30 the sensor chip to a temperature above a dew point of a moist, ie water vapor-containing gas, heated and maintained at this temperature. As a result, condensation of the substance which can be condensed in the fluid on the sensor chip can be avoided and thus a malfunction of the sensor chip due to condensation formation can be prevented.

The condensation temperature is the temperature at which a substance condenses at a given pressure, i. H. from the gaseous to the liquid state of aggregation. The condensation temperature is substance-specific and also dependent on the pressure, where pressure and temperature conditions together are referred to as the condensation point.

According to another approach, the sensor chip in a previously completed droplet coverage of the measuring membrane, for example by aerosol formation, inter alia, for a short time with high energy through the condensate heater 30 be heated to re-evaporate the aerosol. In the case of a gas having water vapor, therefore, in the case of a droplet or aerosol formation on the sensor chip, the water can be used with the aid of the condensate heater 30 be evaporated again very quickly, so that again correct flow parameters can be determined by the sensor chip. The dew point temperature is the temperature at which an equilibrium state of condensing and evaporating water is established on an object when moisture is present, in other words condensate formation is beginning. The heater 3 on the thermal membrane sensor element, in contrast to the condensate heater 30 Have a heating capacity that is insufficient to evaporate a condensate on the. For example, the membrane heater 3 have a heating capacity of 8 mW, while the condensate heater has a heating capacity of more than 100 mW.

The condensate heater 30 Thus, for example, as an integrated condensate heater in the sensor chip, so z. B. the silicon frame, be arranged or be formed as a separate heating element on the sensor chip. If the sensor chip, as in the 12a is illustrated as doped resistor is integrated in the silicon, these doped resistor structures can advantageously in the same process step as the temperature diode 35 be generated.

In some embodiments, the heating resistor has an electrical resistance of less than 500 ohms, 100 ohms or 50 ohms. If, for example, the heating resistor is below 100 ohms, sufficient energy can be made available even at low operating voltages of, for example, 3.3 V to 6 V for quickly heating up the sensor chip and thus for evaporating a condensate on the sensor chip. This makes battery powered applications possible.

The condensate heater 30 may be formed to the sensor chip 5 or the one or more thermal membrane sensor elements 12 . 12 ' to heat above a temperature which is in a metrologically safe range above the dew point of the fluid to be examined, for example 5 K, 10 K or 50 K above the dew point temperature.

The condensate heater 30 may, according to some embodiments, have a heating power greater than 50 mW, 100 mW or 500 mW. For example, the condensate heater may have a heating power between 50 mW and 500 mW, or between 80 mW and 200 mW.

The condensate heater 30 can be configured to heat the sensor chip to a temperature greater than 50 ° C, 130 ° C, 150 ° C, 180 ° C or 180 ° C. The condensate heater 30 , which is designed to prevent condensation of a condensable contained in the fluid and / or to remove or evaporate such a condensate from the sensor chip, has a greater or higher heating power than the heater 3 that on the one or more thermal membrane sensor elements 12 . 12 ' which is used to determine a flow parameter of the fluid.

The task of the condensate heater, the one compared to the heaters 3 on the thermal sensor element has greater heating power, it is firstly to prevent condensation and / or in the case of condensation forming the condensate by evaporation, for example, within a period of 5 s or 10 s to remove. According to some embodiments, the heater 3 a sensor element, for example the thermal membrane sensor element 12 be designed to apply a heating power less than 50 mW, 10 mW or 1 mW, and the condensate heater 30 can be designed to provide a heating capacity greater than 50 mW, 100 mW or 500 mW.

The condensate heater 30 may in some embodiments have a fivefold, tenfold or fifty times higher maximum heating power than a heater 3 . 3 ' a mounted on the sensor chip sensor element 12 . 12 ' This heater may be configured to determine or facilitate a flow parameter of the fluid. The condensate heater can be dimensioned with respect to its heating power so that it can heat the sensor chip above a condensation temperature of the substance contained in the fluid and / or on the other hand to allow rapid evaporation of a located on the sensor chip or the sensor element of the sensor chip condensate.

Heating a sensor chip 5 by means of a condensate heater arranged on the sensor chip 30 via a condensation temperature of a condensable substance contained in the gas to prevent condensation of the substance on the sensor chip, can be carried out so that the sensor chip to a temperature between 1 K and 50 K or between 5 K and 30 K above the condensation temperature of is heated in the condensable substance contained gas.

In embodiments of the present invention, therefore, a method for determining a flow parameter of a gas can be carried out so that before / during or after the step of determining the flow parameter of the gas by means of a sensor element arranged on the sensor chip, for. B. the thermal membrane sensor element, a heating of the sensor chip by means of a arranged on the sensor chip condensate heater 30 is performed via a condensation temperature of a condensable substance contained in the gas, so as to prevent condensation of the substance on the sensor chip.

According to further embodiments of the present invention, the method for determining a flow parameter of a fluid, e.g. As a gas, are performed so that a flow parameter of the gas by means of a thermal sensor element 12 which is a heater 3 , is determined, and before, during this determination or thereafter, a heating of the sensor chip by means of a arranged on the sensor chip condensate heater 30 via a condensation temperature of a condensable substance contained in the gas, so as to prevent condensation of the substance on the sensor chip. According to further embodiments of the method according to the invention, a condensation of the condensate from the sensor chip by heating the sensor chip via a condensation temperature of the substance by means of a condensate heater disposed on the sensor chip can take place in an already completed condensation of the condensable substance contained in the fluid.

The flow measuring system or the flow sensor can also be a drive / control device 40 ( 13 ), in which regardless of a control of a sensor element for determining a flow parameter of the fluid, the condensate heater 30 can be independently controlled or heated.

If, for example, a sensor chip is designed to be elongate and the sensor element is arranged on the sensor chip such that there is as great a distance as possible from mechanical anchoring to a measuring head of the flow sensor, the flow measurement of the fluid is influenced as little as possible by the connection to the fuselage or measuring head of the sensor finger housing and at the same time it can be achieved that when the condensate heating is switched on 30 the active sensory area can be effectively heated.

The condensate heater 30 is particularly advantageous if, as in some embodiments, it is integrated into a sensor chip of silicon, since silicon has a high thermal conductivity and thus a very homogeneous temperature distribution can be achieved. A version as doped resistors in silicon supports this effect particularly well. However, it is also conceivable a condensate heater 30 used as a thin film resistor of, for example, poly-silicon or platinum, such as on glass or ceramic substrates in conventional hot-film anemometers. Due to the low thermal mass of the freestanding sensor chip, a necessary excess temperature above a condensation temperature of a condensable substance contained in the fluid can be achieved with relatively little energy expenditure, or with less energy than in conventional Heißfilmanemometern, so that this measuring device is also well suited for battery operation. For example, a 1 μl water droplet with a heat output of 350 mW can be evaporated in 10 seconds.

In 12b is a schematic cross-sectional view of a sensor chip 5 as he related to the 9C described with a closure 35 the back etching pit 28 below the sensor membrane 9 shown. On a back 5b of the sensor chip, the one on the front 5a opposed thermal membrane sensor elements, for example, a glass plate 35 be arranged, which is the above-mentioned back etching pit, chamber or recess 28 , closes. That is, the back space under the membrane 9 is sealed from the back of the wafer. The closure 35 the backside etch pit below the sensor membrane may be protected by, for example, a glass wafer bonded to wafer level to protect the measurement membrane 9 before backside moisture buildup and to avoid vortex formation in the LEE area of the sensor. Furthermore, as a side effect, the mechanical stability of the etching with the pits 28 perforated sensor chips 5 through the glass lid 35 elevated.

In other embodiments, the glass wafer 35 through other materials that are used for sealing 35 the recess 28 suitable to be used. A backside 9a the arranged on the sensor chip thermal membrane sensor element 12 , which is a front of the sensor chip, which is the heating element 3 opposite, may be sealed so that an attachment of a condensate on the direct back 9a the thermal membrane sensor element 12 is prevented.

In further embodiments, the membrane cavern or the chamber 28 which a back 9a the membrane 9 , the seal 35 and the side walls 20a . 20b the etching pit is formed has a slight negative pressure. The inclusion of slight negative pressure in the membrane cavern 28 can be used to keep a membrane bulge in the working area of the sensor under ambient pressure always arched towards the pit. The negative pressure in the membrane cavern or the sealed etching pit 28 For example, it may be less than or equal to 800 mbar, 500 mbar or 300 mbar. That is, in the chamber 28 There may be a negative pressure of less than 800 mbar, 500 mbar or 300 mbar. By including the negative pressure, the sensor sensitivity can also be increased since the thermal mass of the gas volume on the back of the diaphragm is lower and thus also the parasitic signal which does not contribute to the measuring effect is reduced.

In 13 is the schematic representation of a flow measuring system 99 for determining a flow parameter of a fluid according to some embodiments of the present invention. The flow measuring system 99 has a flow sensor 10 with a sensor chip 5 on, with the sensor chip 5 a first thermal membrane sensor element 12 which is configured to determine the flow parameter of the fluid in a first measuring range or to determine a fluid temperature. The first thermal membrane sensor element 12 also has two spaced-apart thermocouple junctions 8a . 8b on and on between the thermocouple junctions 8a . 8b arranged heating element 3 , The sensor chip 5 is designed to operate during flow measurement 99 with the flowmeter 10 to be flowed around as a freestanding sensor fingers of a fluid to be examined. The sensor chip 5 further comprises a second thermal membrane sensor element 12 ' configured to determine the flow parameter of the fluid in a second measurement range or to determine a fluid temperature of the fluid. The second thermal membrane sensor element 12 ' again has two spaced thermocouple junctions 8a ' and 8b ' and a heating element disposed between the thermocouple junctions 3 ' , A distance D1 of the two thermocouple junctions 8a . 8b the first thermal membrane sensor element 12 to the heating element arranged therebetween 3a is different from a distance D2 of the two thermocouple junctions 8a ' and 8b ' the second thermal membrane sensor element 12 ' from the heating element interposed therebetween 3 ' ,

The flow measuring system 99 can also have a drive device 40 for the sensor chip 5 have, wherein the drive means 40 the thermal membrane sensor elements 12 . 12 ' so that, in a first state, the first thermal membrane sensor element 12 determines the flow parameter in the first measuring range and the second thermal membrane sensor element 12 ' the fluid temperature is determined. In a second state, the first thermal membrane sensor element determines 12 the fluid temperature and the second thermal membrane sensor element 12 ' the flow parameter in a second measuring range.

In addition, the sensor chip 5 as described above, a condensate heater 30 and a temperature sensor 35 , z. As a diode, have. These can also be from the control device or control device 40 be controlled so that an absolute fluid temperature can be determined and / or that the sensor chip, as described above, holds about a condensation temperature of a condensable substance contained in the fluid or if such condensation has already occurred, evaporates the corresponding condensate by heating the sensor chip or . away.

In the drive / control device 40 or as a separate device may further be formed an evaluation, based on the of the sensor chip and, for example, an external pressure measuring sensor 45 determined pressure value using the real gas equation can determine a mass flow or volume flow.

In embodiments of the present invention, the fluid may be a water vapor-bearing gas, and the condensation temperature may accordingly correspond to the dew point temperature of water. In some embodiments of the present invention, the sensor chip may include a sensor element, e.g. B. comprise a thermal membrane sensor element and also at least one further sensor element or membrane sensor element which is designed to determine only a fluid temperature. This further sensor element, for example, can not determine a flow parameter in a second measuring range, but instead have a temperature sensor or a thermocouple junction for determining a fluid temperature. That is, in some embodiments, the second sensor element on the sensor chip only for determining a fluid temperature, for. B. a relative fluid temperature or even an absolute fluid temperature can be used.

The drive device 40 For example, it may be configured to drive the sensor chip of a flow sensor in such a way that a thermal membrane sensor element 12 in a first state the flow parameter in the measuring range of the thermal membrane sensor element 12 determined and that in a second state, the thermal membrane sensor element 12 determines the fluid temperature of the fluid flowing around the sensor. In this embodiment, therefore, the measurement of fluid temperature and determination of the flow parameter can take place chronologically in succession from a single thermal membrane sensor element. As already described above, the sensor chip flows around the fluid to be examined during operation as a freestanding nude chip.

The sensor chip 5 The flow sensor can furthermore be operated in such a way that condensate formation of a material which can be condensed from the fluid is detected on the sensor chip. This may, for example, also during the step of determining a flow parameter by means of the thermal membrane sensor element by applying a time-variable electrical signal to the heater 3 and then comparing that by the temperature sensors 8th . 8th' measured response signal with an expected measured response signal without condensation on the thermal membrane sensor element 12 respectively.

That is, the flow sensor 10 with the sensor chip 5 may also be used to detect moisture on the sensor chip according to embodiments of the present invention. This can be achieved, for example, by applying a time-variable electrical signal to the heater 3 through the drive device 40 respectively. The time-variable electrical signals can be, for example, rectangular or sinewave signals. For the detection of critical humidity, the heater can also be used during the flow measurement 3 every now and then with a time-varying signal, such. B. a rectangular or sinusoidal signal to be modulated to the deformation of a response signal to the thermopiles 7 . 7 ' the presence of a capacitor such as. B. to detect water or impurities.

A control of the condensate heater 30 through the drive device 40 can for example be carried out so that a water droplet located on the measuring membrane can break the thermopile signal in an inadmissible manner and therefore the condensate heater 30 is operated at full power to quickly remove the water droplet or condensate and the thermopiles 7 . 7 ' or over the thermo-elements 8th . 8th' the two membranes to detect whether the droplet has evaporated. The apparatus may detect condensate formation during the process of determining a flow parameter by applying a time-varying electrical signal to the heater and comparing the response signal to an expected response signal.

14a shows a schematic block diagram of the method for determining a flow parameter of a gas according to an embodiment of the present invention. Thus, the method may include a step of determining 100 a flow parameter of a gas by means of a sensor chip arranged on the sensor element and further, before, during or after the determination 100 a flow parameter of a gas by means of a sensor element, a step of heating 110 of the sensor chip by means of a condensate heater 30 via a condensation temperature of a condensable substance contained in the gas. As a result, condensation of the substance on the sensor chip can be prevented or suppressed.

In 14b the schematic block diagram of the method for determining a flow parameter of a gas according to another embodiment of the present invention is shown. The method has a step of determining 120 a flow parameter of a gas by means of a thermal sensor element with its own heater 3 on. Furthermore, the method before, during or after the step of determining 120 a step of heating 130 of the sensor chip by means of a condensate heater via a condensation temperature of a substance contained in the gas to a condensation of the Substance on the sensor chip to prevent, or alternatively, if already a condensate or an aerosol is detected on the sensor chip, a step of removing 140 the condensate of a condensable substance contained in the gas by heating the sensor chip on the condensation temperature of the substance. If the condensable substance contained in the gas z. As to water vapor or water, so there is a warming over the corresponding dew point of water instead.

According to further embodiments of the present invention, the method may further comprise a step of detecting a humidity. For this purpose, for example, during the flow measurement or during the determination of the flow parameter of the gas, the heater 3 the sensor element now and then with a time-varying electrical signal, such. B. a square wave or sinusoidal signal to be modulated so as to the deformation of the response signal to a thermocouple or temperature sensor, the presence of a capacitor, for. B. to detect water or impurities. So lets one on the sensor element 12 landed water droplet break the response signal of a thermocouple, which is arranged on the sensor element, a heating power of the condensate heater is greatly increased and detected via the output signal of a thermocouple, which may be located on the sensor element, whether the condensate or the droplet is evaporated , During this time, the device can output to the outside the status message about incorrectly determined flow parameter values to a user.

In embodiments of the present invention, a flow sensor may include a sensor chip 5 have, wherein the required flow measurement part of the sensor chip 5 is in direct contact with the medium or fluid. In particular, in some embodiments of the present invention in the vicinity of the sensor membrane, no housing part or other device is present, the attachment of a capacitor such. B. of water droplets, z. B. due to capillary forces favored. That is, one can call this arrangement a freestanding nude chip in a fluid measurement channel. In some embodiments of the present invention, in addition to the thermal sensor membrane for flow measurement, the sensor chip may have one or more other membranes used for dynamic temperature measurement of, for example, a breathing gas in medical technology and for detection of moisture.

According to some embodiments of the present invention, the sensor chip can also have a condensate heater or it can be integrated in the sensor chip. With the help of the condensate heater, the necessary amount of heat can be entered to a present on the sensor chip condensate such. As water, for a short time to evaporate or the entire sensor chip to a temperature above a condensation temperature, for. As for water above a dew point temperature to heat.

In some embodiments of the present invention may also be used for temperature measurement, silicon-structured diode which, together with a dynamic temperature measurement, which is carried out by means of another thermal membrane sensor element, the determination of a fluid temperature, ie, for. B. allows a gas temperature.

From the mass flow, the gas temperature and the gas pressure, which for example via an external pressure sensor 45 can be determined, a volume flow can be calculated due to the timely accurate measurement on the real gas equation.

According to some embodiments, the method according to the invention with respect to the flow measurement on the known calorimetric measurement principle, with which a mass flow can be determined, orientate. In the simplest case, the heating structure can be used 3 on the sensor membrane 9 be operated with constant voltage or with constant power. The temperature distribution over the membrane, which is slightly shifted by the flow, is evaluated with the thermopiles 7 . 7 ' before and after the heater 3 is measured. The thermopiles 7 . 7 ' provide a differential voltage that is proportional to the temperature difference between the sensor membrane and silicon frame.

The resulting characteristic, ie the difference between the two thermopile voltage as a function of the mass flow is non-linear and has the highest sensitivity at low flow. The assignment of a mass flow value to a specific thermopile voltage is usually done by calibration. The advantage of the difference measurement (difference between thermopile signal before and after the heating structure) is now similar to a bridge circuit in that the measured variable freed from offset and at correct zero balance by sign change and a flow direction of the fluid can be displayed. Differential circuits are robust to interference signals, since parasitic effects usually affect both thermopiles equally (common-mode rejection).

In embodiments of the present invention, the determination of the absolute gas temperature or fluid temperature by adding the with the temperature sensor, such. B. the temperature diode 35 measured absolute temperature of the sensor chip 5 or silicon frame and the dynamic temperature difference between the second unheated thermal membrane sensor element and the silicon frame.

The temperature-voltage characteristics of the diode and thermopiles should be known. From the mass flow determined by the membrane sensor or the thermal membrane sensor element, the correct gas volume or the volume flow is determined using the real gas equation. In addition to the gas temperature, the gas pressure, which consists of an additional barometric and possibly a differential pressure sensor, also flows into this calculation 45 is won. This can be integrated on the sensor chip or it can be an external pressure gauge.

According to further embodiments of the present invention, the sensor chip 5 the condensate heater 30 on, with which the sensor chip 5 is kept in the simplest case at a constant above a condensation temperature of a condensable substance contained in the fluid, which is safe in all operating traps on the condensation point or in the case of water vapor contained in the gas above the dew point. At this excess temperature, the mass flow signal is calibrated. For a portable application in which it depends on low energy consumption, the condensate heater can be heated to the respective temperature just above a condensation temperature depending on the conditions of use, ie z. B. in a temperature range of 1 K to 20 K above the condensation point or dew point.

For this purpose, it is necessary to know the influence of the chip temperature on the characteristic field and to deposit this as a further parameter in the control electronics or the evaluation and control device.

Falling below the dew point can be done with the help of square wave signals on the heater and the evaluation of the typical signal rise behavior of the thermopile voltage. This is for example in the patent DE 10 113 190 described, the disclosure of which is hereby incorporated into the description. For this purpose, the unheated temperature measuring membrane is used and applied at longer intervals with the necessary signal forms.

If a water droplet deposited on the measuring membrane causes the signal to break impermissibly, a full heating power of the condensate heater is set in accordance with some embodiments and detected via the thermopiles of the two thermal membrane sensor elements whether the droplet has evaporated. During this time, the device can output the status message about incorrect flow measurement values to the outside.

According to further embodiments of the present invention, the flow sensor can also be used for the detection of critical humidity, wherein during the flow measurement of the heater 3 now and then with a time-varying signal, z. B. a rectangular or sinusoidal signal is modulated to detect the presence of water or contaminants via the deformation of a response signal to the thermopile.

In some embodiments of the present invention, a measuring device in the form of immersion probe and the MEMS sensor chip described above, which is designed as a "nude chip" is shown. In embodiments of the present invention, a flow sensor or a sensor chip may be used to measure a volumetric flow of moist gases using at least one, two or more thermal membrane sensor elements, a thermometer, e.g. B. the diode 35 , a passive membrane that is not heated and by means of a driving maneuver of a condensate heater 30 to prevent condensation. Further, the flow sensor may be used to control droplet deposition on the sensor chip.

According to some embodiments of the present invention, a substantial reduction of the thermal sensor mass is advantageously achieved. Namely, the membrane thickness of a sensor element, for. B. a thermal membrane sensor element, for example, only 1 micron to 5 microns, ie z. B. 2 microns.

Thus, in some embodiments of the present invention, the resulting membrane may have a very low thermal mass. As a result, compared to conventional sensors enormously reduced or low heating power for measuring operation of the membrane sensor element is required. The heating power of a heater 3 for a measuring operation of the flow sensor, for example, between 1 mW and 50 mW are, so z. At 30 mW or at 5 mW. Furthermore, due to the low thermal mass of the membrane, a fast response time of the sensor of, for example, 0.1 ms to 5 ms, so z. B. of 1 ms can be achieved. Advantageously, a higher sensor sensitivity of, for example, 2 mV / mW to 30 mV / mW, so z. B. of 16 mV / mW can be achieved. Due to the above advantages can be a battery operation of the flow sensor, as well as use of the sensor for portable applications and ultimately an energy and weight reduction are enabled.

The sensor chip may be formed according to some embodiments as a micro-electro-mechanical (MEMS) chip with a correspondingly high degree of integration. The sensor chip with the corresponding heater can thus be produced by microsystem technology. With the sensor finger or sensor chip, in embodiments of the present invention, flow quantities, flow directions and a fluid or media temperature can be measured highly dynamically. The sensor chip can more than one membrane sensor element, ie z. B. have two thermal membrane sensor elements. Thus, by a corresponding control or by a corresponding maneuver, for example on the second membrane, one or more additional state or substance-specific variables of the medium or of the fluid can be derived. Thus, according to some embodiments, for example, a moisture or a composition of the fluid can be determined.

In addition, in some embodiments of the present invention, a targeted temperature control of the sensor or sensor chip by means of a arranged in or on the sensor chip condensate heater 30 possible. This advantageously makes it possible to carry out measurements or to determine a flow parameter of the fluid under condensing conditions by controlling the temperature of the sensor chip to temperatures above a condensation point of a condensable substance contained in the fluid, ie, water above the dew point. Furthermore, by means of the condensate heater and a condensate or droplets on the sensor chip can be eliminated by short-term heating.

It should also be pointed out that all the subject features of the present application can also be implemented as process steps in the corresponding inventive process.

In summary, it should be noted that some embodiments according to the invention provide an apparatus and method for measuring dynamically changing and water saturated air flows.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1512948 A1 [0005]
• DE 10324292 A1 [0006]
• WO 01/18500 A1 [0007]
• DE 10113190 [0125]

Cited non-patent literature

• Redundancy structures to increase the safety and reliability of microsystems, Dissertation, Albert-Ludwig-University Freiburg i. Breisgau, 09.07.2002 [0032]
• "Redundancy structures to increase the safety and reliability of microsystems", Dissertation, Albert-Ludwig-University Freiburg im Breisgau, 2002 [0073]

Claims (16)

Sensor chip ( 5 ) for determining a flow parameter of a gas, comprising: a sensor element ( 12 ) for determining the flow parameter, and a condensate heater ( 30 ), which is formed, the sensor chip ( 5 ) to heat above a condensation temperature of a substance contained in the gas, in order to prevent condensation of the substance on the sensor chip ( 5 ) to prevent. Sensor chip ( 5 ) according to claim 1, wherein the condensation temperature corresponds to a dew point temperature of water contained in the gas. Sensor chip ( 5 ) according to one of claims 1 to 2, wherein the condensate heater ( 30 ) at least one in the sensor chip ( 5 ) integrated heating resistor ( 30 ) having. Sensor chip ( 5 ) according to claim 3, wherein the resistance of the heating resistor is less than 500 ohms, 100 ohms or 50 ohms. Sensor chip ( 5 ) according to one of claims 1 to 4, wherein the condensate heater ( 30 ) is adapted to the sensor chip ( 5 ) to a temperature which is at least 5 K above a dew point temperature of the fluid. Sensor chip ( 5 ) according to one of claims 1 to 5, wherein the condensate heater ( 30 ) has a heating capacity greater than 50 mW, 100 mW or 500 mW. Sensor chip ( 5 ) according to one of claims 1 to 6, wherein the condensate heater ( 30 ) is formed, the sensor chip ( 5 ) to a temperature greater than 130 ° C, 150 ° C or 200 ° C to heat. Sensor chip ( 5 ) according to one of claims 1 to 7, wherein the sensor element ( 12 ) is a thermal membrane sensor element and the condensate heater ( 30 ) is arranged symmetrically on opposite sides of the membrane sensor element. Sensor chip ( 5 ) for determining a flow parameter of a gas, comprising: a sensor element ( 12 ) for determining the flow parameter, wherein the sensor element is a heater ( 3 ), which is designed to generate a temperature gradient for a determination of the flow parameter, a condensate heater ( 30 ), which is formed, the sensor chip ( 5 ) to heat above a condensation temperature of a substance contained in the gas in order to prevent condensation of the substance on the sensor chip, or to a on the sensor chip ( 5 ) remove condensate of the substance, wherein a heating capacity of the condensate heater ( 309 is greater than a heat output of the heater ( 3 ) of the sensor element ( 12 ). Sensor chip ( 5 ) according to claim 9, wherein the heater ( 3 ) of the sensor element ( 12 ) has a heating power that is less than 50 mW, 10 mW or 1 mW, and wherein the condensate heater ( 30 ) has a heating capacity greater than 50 mW, 100 mW or 500 mW. Sensor chip ( 5 ) according to one of claims 9 to 10, wherein the condensate heater ( 30 ) has at least a five times higher, ten times higher or fifty times higher maximum heating power than the heater ( 3 ) of the sensor element ( 12 ), so that a condensate located on the sensor element with the condensate heater ( 30 ) can be removed faster than with the heater ( 3 ) of the sensor element ( 12 ). Method for determining a flow parameter of a gas, comprising the following steps: determination ( 100 ) of the flow parameter of the gas by means of a sensor element arranged on the sensor chip, and heating ( 110 ) of the sensor chip by means of a arranged on the sensor chip condensate heater on a condensation temperature of a substance contained in the gas to prevent condensation of the substance on the sensor chip. Process according to claim 12, wherein the heating ( 110 ) is performed so that the sensor chip is heated to a temperature range which is between 1 K and 50 K or between 5 K and 30 K above the condensation temperature of the condensable substance contained in the gas. Method for determining a flow parameter of a gas, comprising the following steps: determining ( 120 ) of the flow parameter of the gas by means of a thermal sensor element ( 12 ) with a heater ( 3 ) and, heating ( 130 ) of the sensor chip by means of a arranged on the sensor chip condensate heater ( 30 ) over a condensation temperature of a substance contained in the gas to prevent condensation of the substance on the sensor chip, or removing ( 140 ) condensate of a substance contained in the gas from the sensor chip by heating the sensor chip via a condensation temperature of the substance by means of a arranged on the sensor chip condensate heater. The method of claim 14, further comprising a step of detecting a condensate formation of a condensable substance contained in the gas on the sensor element by comparing the measured flow parameter with one expected flow parameter, wherein based on the comparison in a condensate formation on the sensor element, a removal ( 140 ) of the condensate is carried out by heating by means of the condensate heater. The method of claim 15, further comprising a step of detecting condensate formation of the condensable material from the fluid on the sensor chip during the step of determining (FIG. 120 ) of the flow parameter in a measuring range, wherein the step of detecting an application of a time-variable electrical signal to the heater ( 3 ) and comparing the response signal at the thermocouple junctions with an expected response signal without condensation.

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