WO2009094986A1

WO2009094986A1 - Hot film sensor

Info

Publication number: WO2009094986A1
Application number: PCT/DE2009/000067
Authority: WO
Inventors: Karin Bauer; Winfried Kupke; Xaver Riedl; Roland Wagner
Original assignee: Eads Deutschland Gmbh
Priority date: 2008-01-30
Filing date: 2009-01-21
Publication date: 2009-08-06
Also published as: DE102008006831A1

Abstract

The invention relates to a hot film sensor comprising a sensor element (2) that is formed by an electrically conductive structure, and a substrate (1) on which the sensor element (2) is arranged. A cavity (3) that reduces the heat transfer from the sensor (3) to the substrate (1) and that extends below the sensor element (2) is embodied in the substrate (1). According to the invention, the substrate (1) is formed by a flexible film that is made of an electrically insulating material, in which the cavity (3) extending below the sensor element (2) is embodied on the rear side (1b) facing away from the sensor element (2). A reducing thickness of the substrate (1) remains on the front side (1a) facing the sensor element (2).

Description

Hot-film sensor

The invention relates to a hot film sensor having a sensor element formed by a conductor structure according to the preamble of claim 1.

Hot film sensors and arrays of hot film sensors are used to measure fluid flow, for example, to measure airflow on aircraft components such as wings, tail units, or the like in the wind tunnel or test flight. In this case, a sensor element forming a thin conductor structure of suitable geometry is produced on a thin, electrically and thermally insulating substrate and connected to an electronic heating and evaluation circuit. For measurement purposes, the sensor is brought to a temperature which is higher than the fluid, the heating electronics being designed to keep the voltage (CV operation), the current (CC operation) or the temperature (CT operation) constant. The heat dissipated by the flow of the surrounding fluid serves to characterize the flow, in particular its velocity. The measuring principle as such has long been used, for example, in the field of meteorological hot wire anemometers or air mass sensors of internal combustion engines. The hot film sensors used to measure flows work the more effectively and accurately the more heat is transferred into the flowing fluid and the less is released by heat conduction through the substrate into the (test) carrier to be measured. A substrate with low heat conduction can be further improved by introducing into the substrate rear side a cavity (recess, cavity or recess) which further locally reduces the heat conduction.

From EP 0 958 487 B1, DE 199 61 129 A1, US Pat. No. 6,698,283 B2, US Pat. No. 5,237,867 A or JP 2003 021547 A, hot-film sensors are known in which sensor elements formed by an electrical conductor structure are arranged on a Substrate are arranged, wherein in the substrate an extended under the sensor element cavity for reducing the heat transfer from the heating element to the substrate and thus to the underlying (trial) carrier is formed. In EP 0 958 478 B1, the substrate is formed by a relatively solid support made of glass, for example with a thickness of 0.5 mm, in which a cavity forming the window, through the entire thickness of the support continuous window is recessed. A coating, which ultimately serves as a membrane carrying the actual sensor element, is formed by a plurality of partial layers, wherein a silicon nitride layer is embedded between two silicon carbide layers. The hot-film sensor described in DE 199 61 129 A1 is constructed in a similar manner, wherein a base film carrying the actual sensor element is arranged on a plate-like substrate in which a cavity forming the cavity is also provided as a window passing through the entire thickness of the plate-like substrate is. The hot-film sensors known from US Pat. No. 6,698,283 B1 and US Pat. No. 5,237,867 also use a plate-like substrate which consists of semiconductor silicon and has a window which also extends through its entire thickness, above which the actual sensor element forms on a membrane insulating film is arranged. The hot film sensor known from JP 2003 021547 A is constructed in a similar manner. From US 5484 517 a hot film sensor is known in which the sensor element is applied to a substrate formed by a polyimide film, which has a constant thickness. Finally, from DE 44 17 679 C1, a hot film sensor is known, in which with the actual sensor element electrically connected sensor leads in recesses of a carrier film are sunk so that they do not represent elevations in the film surface.

The object of the invention is to provide an improved hot-film sensor which allows the most accurate and unadulterated measurement of fluid flows. The object is achieved by a hot film sensor having the features of claim 1.

Advantageous embodiments and further developments of the hot-film sensor according to the invention are specified in the subclaims.

The invention provides a hot-film sensor having a sensor element formed by an electrical conductor structure and a substrate on which the sensor element is arranged, wherein a cavity formed underneath the sensor element is formed in the substrate for reducing the heat transfer from the sensor element to the substrate. According to the invention, the substrate is formed by a flexible film made of an electrically insulating material, in which the cavity underneath the sensor element is formed on the rear side facing away from the sensor element, with a reduced thickness of the substrate being left on the front side facing the sensor element.

According to advantageous embodiments of the invention, the substrate forming flexible film has a thickness of 10 to 1000 .mu.m, in particular a thickness of 25 to 250 microns.

The cavity formed on the rear side of the substrate facing away from the sensor element preferably has a depth of 20% to 80% of the thickness of the film forming the substrate.

According to an advantageous embodiment of the hot-film sensor according to the invention, a passivation layer which forms a smooth surface is provided on the conductor structure forming the sensor element and the substrate. According to an advantageous development of the invention, it is provided that the passivation layer has an area formed above the sensor element with a higher heat transfer and an area formed above the remaining substrate with lower heat transfer.

According to one embodiment of this, it can be provided that the passivation layer in the area formed above the sensor element consists of a material having a higher thermal conductivity and in the region formed above the remaining substrate a material having a lower thermal conductivity.

In this case, the material of higher thermal conductivity, which is provided above the sensor element, have a higher rigidity than the material of lower thermal conductivity provided over the remaining substrate.

The conductor structure forming the sensor element can be formed as a meander, spiral or as a double spiral.

According to a development of the hot-film sensor according to the invention, it is provided that the conductor structure is connected to electrical contacts which are made from the front to the back of the substrate.

According to advantageous embodiments of the hot-film sensor according to the invention, a number of sensor elements in a 1-dimensional or in a 2-dimensional arrangement may be arranged together on the flexible film forming the substrate, wherein in each case underneath the sensor elements extended cavities on the rear side facing away from the sensor element, the substrate forming flexible film are formed. The flexible film forming the substrate may advantageously be made of polyimide or of polyetheretherketone.

According to a development of the hot-film sensor according to the invention, the rear side of the substrate is electrically insulated by an additional insulation in the form of a further film or coating or by an adhesive layer.

In the following, embodiments of the invention will be explained with reference to the drawing.

Show it:

1 is a perspective greatly enlarged view of a hot film sensor according to an embodiment of the invention.

FIG. 2 is a perspective cross-sectional view of the hot film sensor according to the embodiment of FIG. 1; FIG.

FIG. 3 is a plan view of the hot film sensor according to the embodiment of FIG. 1; FIG.

FIG. 4 is a rear view of the hot film sensor according to the embodiment of FIG. 1; FIG.

5 shows a plan view of a further embodiment of the hot-film sensor according to the invention; and

Fig. 6 is a top view of a hot-film array as a still further embodiment of the invention. FIGS. 1 to 4 show a hot-film sensor according to an exemplary embodiment of the invention, in which a sensor element 2 formed by an electrical conductor structure is arranged on a substrate 1. The substrate 1 is formed by a flexible film of an electrically insulating material, for example by a polyimide film or by a film of thermoplastic polyetheretherketone (PEEK). In the substrate 1 formed by the flexible film, a cavity 3 which extends underneath the sensor element 2 is formed, which serves to reduce the heat transfer from the sensor element 2 to the substrate 1 and thus to the (test) carrier located underneath. This cavity 3 in the form of a cavity, recess or depression is formed on the rear side 1 b of the substrate facing away from the sensor element 2, so that a reduced thickness of the substrate 1 remains on the front side 1a facing the sensor element 2.

The flexible film forming the substrate 1 may for example have a thickness of 10 to 1000 μm, in particular a thickness of 25 to 250 μm, and the cavity 3 formed on the rear side 1b of the substrate 1 facing away from the sensor element 2 may have a depth of 20%, for example. to 80% of the thickness of the substrate 1 forming film.

On the sensor element 2 forming the conductor structure and the substrate 1, a passivation layer 4 is provided, which forms a smooth surface. In the exemplary embodiment illustrated in FIGS. 1 to 4, the passivation layer 4 has a region 4 a with higher heat transfer formed above the sensor element 2 and a region 4 b with a lower heat transfer formed over the remaining substrate 1. For this purpose, the passivation layer 4 in the region 4a formed above the sensor element 2 consists of a material having a higher thermal conductivity and in the region 4b formed over the remaining substrate 1 made of a material having a lower thermal conductivity. The higher heat transfer in the region 4a above the Sensor element 2 may additionally or alternatively also be effected by a smaller thickness of the passivation layer 4 in this region 4 a.

Furthermore, the material of higher thermal conductivity, which is provided in the area 4a above the sensor element 2, have a higher rigidity than the material of lower thermal conductivity provided over the remaining substrate 1.

The conductor structure forming the sensor element 2 is connected to electrical contacts 5 a, 5 b, which in the embodiment shown in FIGS. 1 to 4 are performed from the front side 1 a to the back side 1 b of the substrate 1.

The cavity 3 can be produced by various methods, for example:

1. A masking layer is applied to the back side 1b of the substrate 1 and patterned photolithographically. In an oxygen plasma, which may also be mixed with some fluorine, the cavity 3 is etched into the substrate 1. Thereafter, the mask layer is removed.

2. A process is carried out as in 1., but the etching is carried out in a suitable manner as wet etching.

3. For thick substrates 1 and large structures, the cavity 3 can also be produced by a fine milling cutter.

4. The cavity 3 is generated by laser ablation.

5. The cavity can be produced by laminating at least two films, wherein in one of these films, the geometry of the cavity was cut, while the other serves as a closed cover sheet. The cropping This film can be made by a Schneidpiotter, laser cutting, water jet cutting or plasma etching.

The hot film sensor with the finished substrate 1 is attached to the sensor element 2 upwards and the cavity 3 down on the (trial) carrier to be measured.

The production of the substrate 1 by a flexible film, such as a film of polyimide, allows attachment to curved or rounded surfaces, such as on components of wings, tail units or other bodies flowed around, in particular of aircraft.

The hot-film sensor described is characterized by a low height, so that flow properties can be determined on or in the immediate vicinity of the carrier surface to be examined, without the carrier would have to be changed, for example by attaching incisions, without affecting the flow to be examined significantly becomes.

The sensor has the described passivation layer 4 for protection against environmental influences and for electrical insulation above the sensor element 2. By structuring the passivation layer in the manner described above, a passivation layer 4 a with higher thermal heat transmission is provided over the sensor element 2 and a passivation at the other regions 4b is provided with a lower thermal heat transfer, an increase of the heat given off locally at the location of the sensor element 2 to the flow is achieved.

So that the flow to be measured is not influenced by the geometry of the sensor, the sensor has a small thickness and a smooth surface. In particular, the electrical contacts 5a, 5b are through the substrate 1 on the Back to leads or terminals 7a, 7b carried out. If the (test) carrier to be measured consists of a conductive material, for example aluminum, the back side 1b of the substrate 1, on which the leads or terminals 7a, 7b then run, is replaced by an additional insulation in the form of a further film or coating Adhesive layer electrically insulated from the substrate 1, which is indicated in Fig. 1 by the partially shown, dashed coating with the reference numeral 6.

The conductor structure forming the sensor element 2 may be formed as a meander, spiral or as a double spiral.

Fig. 5 shows in plan view another embodiment of the hot film sensor according to the invention, in which said conductor structure is formed as a double spiral. As a result, line crossings in the supply line are avoided and the result is a largely direction-independent sensitivity of the sensor element 2.

A number of sensor elements 2 in a 1-dimensional or in a 2-dimensional arrangement can be provided on a common substrate 1 formed by a flexible film, wherein cavities 3 of the type described on the rear side facing away from the sensor element 2 are located below the respective sensor elements 2 1b of the substrate 1 forming the flexible film are provided. Such a design allows high resolution flow measurement with small "pixels" and high sensor resistance. This makes it possible, with the same sensitivity of the sensor elements 2 to use lower currents for measurement than with conventional sensors.

The cavity 3 under the sensor element 2 may have an area of the order of 0.1 to 10 mm ² and a depth of 10 to 200 μm. Fig. 6 shows, as a further embodiment of the invention, a hot-film array with three rows of 16 sensors each. The individual sensors are arranged as a two-dimensional array. It is also possible for a plurality of arrays to be distributed on more than one film substrate

The above-described structured passivation with a passivation layer 4 a of higher thermal conductivity and a passivation layer 4 b of lower thermal conductivity is particularly advantageous in that also thermally conductive materials with higher rigidity can be used. These then only need to form individual islands in a matrix of a softer and / or stress-free passivation layer 4b with lower thermal conductivity. The coated sensor film remains relatively flexible and stress-free and can thus be handled better than if a tighter and / or more rigid thin film is applied to the entire surface. Otherwise, in the case of a high voltage thin film, the sensor film would throw or roll so much that it could no longer be mounted on a curved surface or handled further without the coating 4 locally peeling or cracking.

The invention provides a hot film sensor whose heat loss to the (trial) carrier is significantly reduced, resulting in improved sensor sensitivity and improved time response of the sensor. Due to the small thickness and the smooth surface of the sensor, the flow to be measured is little disturbed and there is the possibility of mounting on curved or curved surfaces. Because of a

Carrying out the electrical contacts 5a, 5b to the back, the sensor has a smooth surface, so that the size to be measured is not affected. LIST OF REFERENCE NUMBERS

1 substrate 1a front side

1b back side

2 sensor element

3 cavity

4 passivation layer a passivation layer of higher thermal conductivity b passivation layer of lower thermal conductivity

5a contact

5b contact

Additional insulation a Connection b Connection

Claims

claims

1. hot film sensor having a sensor element formed by an electrical conductor structure (2) and a substrate (1) on which the sensor element (2) is arranged, wherein in the substrate (1) under the sensor element (2) extended cavity (3) designed to reduce the heat transfer from the sensor element (3) to the substrate (1), characterized in that the substrate (1) is formed by a flexible film of an electrically insulating material, in which the under the sensor element (2) extended cavity (3) on the side facing away from the sensor element (2) back (1b) is formed, wherein a reduced thickness of the substrate (1) on the sensor element (2) facing the front (1a) is left.

2. hot film sensor according to claim 1, characterized in that the substrate (1) forming flexible film has a thickness of 10 to 1000 .mu.m, in particular a thickness of 25 to 250 microns.

3. Hot film sensor according to claim 1 or 2, characterized in that on the sensor element (2) facing away from the back (1b) of the substrate (1) formed cavity (3) has a depth of 20% to 80% of the thickness of the substrate ( 1) has forming film.

4. hot film sensor according to claim 1, 2 or 3, characterized in that on the sensor element (2) forming the conductor structure and the substrate (1) has a

Passivation layer (4) is provided, which forms a smooth surface.

5. hot film sensor according to claim 4, characterized in that the passivation layer (4) over the sensor element (2) formed region (4a) with a higher heat transfer and over the remaining substrate (1) formed region (4b) having a lower heat transfer.

6. hot film sensor according to claim 5, characterized in that the passivation layer (4) formed in the over the sensor element (2) region (4a) of a material with higher thermal conductivity and in the above the other substrate (1) formed region (4b ) consists of a material with lower thermal conductivity.

7. hot film sensor according to claim 6, characterized in that the material of higher thermal conductivity, which is provided over the sensor element (2), has a higher rigidity than the above the other substrate (1) provided material of lower thermal conductivity.

8. Hot film sensor according to one of claims 1 to 7, characterized in that the sensor element (2) forming the conductor structure is formed as a meander, spiral or as a double spiral.

9. hot film sensor according to one of claims 1 to 8, characterized in that the conductor structure with electrical contacts (5a, 5b) is connected, which are carried out from the front side (1a) to the back (1b) of the substrate (1).

10. hot film sensor according to one of claims 1 to 9, characterized in that a number of sensor elements (2) in a 1-dimensional or in a 2-dimensional arrangement on the substrate (1) forming the flexible film are arranged together, wherein in each case under the sensor elements (2) extended cavities (3) on the sensor element (2) facing away from the back of the substrate (1) forming the flexible film are formed.

11. hot film sensor according to one of claims 1 to 10, characterized in that the substrate (1) is formed by a flexible film of polyimide or polyetheretherketone.

5 12. hot film sensor according to one of claims 1 to 11, characterized in that the back (1b) of the substrate (1) is electrically isolated by an additional insulation (6) in the form of another film or coating or by an adhesive layer.

10 13. hot film sensor according to one of the preceding claims, characterized in that a plurality of sensor elements (2) are arranged as a two-dimensional array.

14. hot film sensor according to claim 13, characterized in that one or more arrays of sensor elements 15 are arranged distributed on a plurality of substrates designed as a film (1).