WO2017108300A1 - Method for producing a flow sensor based on a thin film, and such a flow sensor - Google Patents

Abstract

The invention relates to a method for producing a flow sensor (1) based on a thin film, having a first heater and temperature measuring element (10) and at least one second heater and temperature measuring element (11). The heater and temperature measuring elements (10, 11) are spatially separated from each other, and a support structure (21) is provided which is designed to receive the heater and temperature measuring elements (10, 11). According to the invention, the production method has at least the following steps: providing a substrate (100); depositing a first photoresist (12) onto the substrate (100); depositing a second photoresist (13) onto the first photoresist (12); opening the first (12) and second photoresist (13) in order to produce a connection metallization; depositing a metal (14) in order to form the heater and temperature measuring elements (10, 11); structuring the deposited metal (14); and depositing a third photoresist (15) and removing the first photoresist (12) by means of at least one sacrificial layer etching process. The invention further relates to a flow sensor (1) produced using such a method.

Description

 Method of manufacturing a thin-film based flow sensor and such a flow sensor Description

The invention relates to a method for producing a thin-film based flow sensor, comprising a first heater and temperature measuring element and at least one second heater and temperature measuring element, wherein the heater and temperature measuring elements are spatially separated from each other, and wherein a support structure is provided, which for receiving the Heater and

Temperature measuring elements is formed. The invention is further directed to such a flow sensor formed on a thin film base. STATE OF THE ART

 Flow sensors or so-called mass flow sensors which operate on the principle of the hot wire anemometer are known. In this case, a hot wire is kept at a constant temperature according to a closed-loop method. The stronger the surrounding mass flow (e.g., airflow), the more power must be provided to keep the temperature of the hot wire constant. The latter is a measure of the air flow or the

incoming air mass per time. The disadvantage is the high required

Power consumption and the space-consuming and costly vectorial measurement. A use of mass flow sensors in small products is for many

in particular future applications indispensable, for example for mobile radiation detectors, air quality or pollen sensors or the like, in which an air volume in a chamber must be determined. In addition to the high power consumption commercially available

Mass flow sensors based on the principle of the hot-wire anemometer, above all, the high space requirement for integration into small devices hinder, such as smartphones. Sensors based on a MEMS (Micro Electro Mechanical System) require a separate heater placed between two thermocouples whose temperature is measured. A semiconductor sensor constructed flow sensor is known for example from DE 40 05 801 C2. The flow sensor has a first one

Resistance element and a second resistance element, which are connected in a Wheatstone bridge, wherein the resistance elements are spatially separated from each other and each a thin-film heating element is necessary to heat the resistance elements. The resistive elements as well as the thin-film heating elements are accommodated in a protective film. In order to provide an opening and thus to release the resistive elements, a semiconductor substrate has been opened by an etching process on which the resistive elements and the thin-film heating elements have been constructed by a thin-film technique. Disadvantageously, however, a larger etching volume is necessary, and furthermore, the separate structure of resistance elements and of the thin-film heating elements results in a complex construction. In this case, it is proposed in particular to indemnify a plurality of relatively existing bridge elements, whereby a considerable

Construction effort arises. Moreover, the openings created in the semiconductor substrate can be described as macroscopic, which may include depths of, for example, several micrometers, resulting in an additional considerable expenditure of time in the production.

OPENBARU NG D ER E RFIN DU NG

 The object of the invention is to provide an improved

Thin-film based flow sensor that is easy to manufacture and versatile. In particular, the flow sensor should

 Thin film base can be produced with few process steps, and should be used in standard installation situations application.

This object is achieved on the basis of a method for producing a thin film based flow sensor according to the preamble of claim 1 and starting from a thin film based flow sensor according to the preamble of claim 7 with the respective characterizing features. Advantageous developments of the invention are specified in the dependent claims. The method according to the invention has at least the following steps: providing a substrate; Depositing a first photoresist on the substrate; Depositing a second photoresist on the first photoresist; Opening the first and second photoresist to form a terminal metallization; Depositing a metal to form the heater and temperature sensing elements; Patterning the deposited metal; Depositing a third photoresist and removing the first photoresist by at least one sacrificial layer etch.

The core of the invention is a method for producing a flow sensor, which method is based on the application of a number of photoresists on a substrate. The thin film base is formed by a plurality of photoresists, which are simple and flexible to produce and structure. The photoresists form a carrier layer for receiving the heater and

Temperature measuring elements, so that the etching of semiconductor substrates, for example consisting of silicon and silicon oxide, omitted. This inventive method results in a drastic reduction of

Production costs and it is a simple integration on the back-end CMOS-based possible because no high-temperature steps such as Epi-Si growth processes are necessary. This is not the particular

Characteristics of a CMOS wafer changed, since low process temperatures are needed for the production of the sensor. Moreover, the flow sensor produced according to the invention is also suitable for operation with current and voltage measurements based on standard components.

The structuring of the deposited metal can be effected by means of a mask or by means of etching, for example by means of the use of an etching stop layer which has been previously optically and / or thermally modified.

Advantageously, a CMOS ASIC wafer, a glass wafer or a polymer film is used as the substrate, so that for later use of the

Flow sensor no restrictions are the result.

With further advantage, a photoresist is deposited as a second photoresist which is chemically resistant to the sacrificial layer etching of the first photoresist. Furthermore, a photoresist is deposited as a third photoresist, the chemically also resistant to sacrificial layer etching of the first photoresist. The application of the individual photoresists is carried out with suitable

Application method, for example by spin coating or spraying, and structuring by means of photolithography. Advantageously, the support structure is constructed, for example, from SU-8, because this paint after the post-exposure beacon is insensitive to most etching methods and solvents and accordingly freestanding structures with the sacrificial layer etching of sacrificial layers, consisting of less

solvent-resistant photoresists, are possible. Furthermore, in SU-8 photoresists, structures with high edge steepness can be produced, making these coatings particularly suitable for the construction of suspension structures.

The invention is further directed to a flow sensor

A thin film base comprising a first heater and temperature sensing element and at least one second heater and temperature sensing element, and wherein the heater and temperature sensing elements are spatially separated, and wherein there is a support structure configured to receive the heater and temperature sensing elements. According to the invention, the

Carrier structure on at least one photoresist, and to form an exemption, the support structure has a free space to the substrate.

With further advantage, the support structure has a free space between the temperature measuring elements, which is produced by a sacrificial layer etching of the first photoresist. The free space extends in particular between the support structure and the surface of the substrate, so that the support structure receives the heater and temperature measuring elements freely suspended above the substrate.

With further advantage, the support structure has connecting arms for contacting the heater and temperature measuring elements. The connecting arms in this case comprise electrical connections for the heater and temperature measuring elements, which are enclosed up to the surface of the substrate with at least one photoresist. With even further advantage, the support structure has a structuring between the two heater and temperature measurement elements. The heater and

Temperature measuring elements can be enclosed in the photoresists and have a meandering structure, so that the largest possible

Convection surface of the heater and structural elements is created.

PREFERRED EMBODIMENT OF THE INVENTION

Further, measures improving the invention will be described in more detail below together with the description of a preferred embodiment of the invention with reference to FIGS. It shows:

FIG. 1 is a plan view of a thin film based flow sensor made by the method of the present invention;

Figure 2 is a cross-sectional view of the flow sensor along the

 Cutting line A-A ',

FIG. 3 shows a further plan view of the flow sensor,

Figure 4 is a cross-sectional view of the flow sensor along the

 Cutting line B-B ',

Figure 5 shows an arrangement of two flow sensors, with a

 Air flow have flowed from a horizontal flow direction and

Figure 6 shows an arrangement of two flow sensors, with a

 Air flow have flowed from a vertical flow direction.

Figure 1 shows an example of a possible embodiment of a

Flow sensor 1, which is constructed on a substrate, not shown. The flow sensor 1 has a carrier structure 21, which is constructed from a plurality of layers of photoresists. In the support structure 21 are two spatially separated from each other arranged heater and Temperature measuring elements 10 and 11 were added, which are connected via electrical connections 19 to the substrate. The outgoing electrical connections 19 are received in connection arms 17, which are part of the support structure 21 and which surround the electrical connections 19. In the middle between the heater and temperature measuring elements 10, 11 is a structuring 18, which is shown in simplified form as a free space.

The heater and temperature measuring elements 10 and 11 are constructed by deposited metals 14 and have a meandering structure in the middle.

Figure 2 shows a cross-sectional view of the flow sensor 1 along the section line A-A ', as shown in Figure 1. The support structure 21 is composed of a second photoresist 13 and a third photoresist 15, and between the two photoresists 13 and 15 is shown in cross-section an applied metal 14, which forms the heater and temperature measuring elements 10, 11, as in the plan view in Figure 1 shown. Below the photoresists 13 and 15 is a free space 16, which was created by a first photoresist 12, wherein the first photoresist 12 is a sacrificial layer, which was etched away to create the free space 16. As a result, a floating arrangement of the support structure 21 arises, based on the remaining photoresists 13 and 15. In this case, the free space extends between the second photoresist 13 as the lower support side of the support structure 21 and the substrate 100, for example comprising a CMOS circuit.

FIG. 3 shows a further plan view of a flow sensor 1, in which a section line B-B 'is shown, and the associated sectional view is shown in FIG. 4.

FIG. 4 shows the sectional view of the flow sensor 1 along the section line BB 'according to FIG. 3. In addition, the substrate 100 is illustrated, on which the carrier structure 21 with the photoresists 13 and 15 is constructed. Between the two photoresists 13 and 15 is the deposited metal 14 to form the heater and temperature sensing elements 10, 11 as shown in Figure 3. Due to the course of the section line BB 'the deposited metal of the second heater and temperature measuring element 11 is shown lying in section, the interruptions due to the meander structure of the heater and temperature measuring element 11 arise. At the end sides, the deposited metal 14 has electrical terminals 19 to the substrate 100, and the electrical terminals 19 are enclosed in terminal arms 17 formed by the second photoresist 13 and the third photoresist 15. By a sacrificial layer etching of the first photoresist 12 has been removed, whereby the space 16 is provided below the support structure 21.

The deposited metal 14 forms the heater and temperature sensing elements 10, 11 and has a linear relationship between the resistance value and the temperature. With constant energization can so over the

measured voltage are at least indirectly converted into an operating temperature of the resistor. The cooling as a result of a flow around the

Metals 14 leads to cooling of the heater and temperature sensing elements 10 and 11, and thus the voltage changes as a result of the resistance change. Now both heater and temperature measuring elements 10, 11 of

Sensor element 1 spaced from each other and flowed with a flow which is applied laterally to the heater and temperature measuring elements 10, 11, by means of the measured voltage directly to the

Flow rate of the flow to be closed. The measuring principle is based on the fact that the lying in the flow direction heater and

Temperature sensing element is cooled more than the subsequent heater and temperature measuring element behind the first heater and

 Temperature measuring element located with the flow direction.

Figure 5 shows, for example, two flow sensors 1 with a 90 ° to each other rotated arrangement. The first flow sensor 1 has an orientation, so that the first heater and temperature measuring element 10 first from the

Flow 20 is flowing, and only then the second heater and temperature measuring element 11 is flown. This results in a temperature Tl of the first heater and temperature measuring element 10, which is smaller than the temperature T2 of the second heater and temperature measuring element 11. This results in a voltage difference, for example via a Wheatstone ash Bridge can be measured, and over on one

Flow rate of the flow 20 can be closed.

The further flow sensor 1 is arranged rotated by 90 °, so that both heater and temperature measuring elements 10, 11 are equally flowed by the flow 20. As a result, the temperature T3 of the first heater and temperature measuring element 10 is equal to the temperature T4 of the second heater and temperature measuring element 11. From the two voltages, the vector components of the flow can be determined and the amount calculated by vector addition.

Figure 6 shows the arrangement of the flow sensors 1 with a

Flow direction 20 from a vertical, the 90 ° turned to

Flow direction of the flow 20 according to Figure 5. As a result, the temperatures Tl and T2 of the heater and temperature sensing elements 10, 11 of the first flow sensor 1 are the same, and the temperature T4 of the first heater and temperature sensing element 10 is less than the temperature T3 of the second heater and temperature sensing element 11 of the second flow sensor 1.

A comparison of the temperature differences of the heater and

 Temperature measuring elements 10 and 11 of the flow sensors 1 in Figures 5 and 6 illustrates that can already be closed by the respective voltages resulting from the temperatures on the flow direction of the flow 20.

The invention is not limited in its execution to the above-mentioned preferred embodiment. Rather, a number of variants is conceivable, which makes use of the illustrated solution even with fundamentally different types of use. All from the

Claims, the description or the drawings

Features and / or advantages, including constructive details, spatial arrangements and method steps, can be essential to the invention, both individually and in the most diverse combinations. Above all, it is also possible to keep the voltage constant and to measure the current.

Claims

claims A method of fabricating a thin film based flow sensor (1) comprising a first heater and temperature sensing element (10) and at least one second heater and temperature sensing element (11), said heater and temperature sensing elements (10, 11) being spatially separated and wherein a support structure (21) is provided, which is designed to receive the heater and temperature measuring elements (10, 11), the method for the production comprising at least the following steps:  Providing a substrate (100),  Depositing a first photoresist (12) on the substrate (100), depositing a second photoresist (13) on the first photoresist (12), opening the first (12) and second photoresist (13) to form a contact metallization,  Depositing a metal (14) to form the heater and  Temperature measuring elements (10, 11),  Patterning the deposited metal (14),  Depositing a third photoresist (15) and  Remove the first photoresist (12) by at least one  Sacrificial. 2. The method according to claim 1, characterized in that a CMOS ASIC wafer, a glass wafer or a polymer film is provided as the substrate (100). 3. The method according to claim 1 or 2, characterized in that a  Photoresist is deposited as a second photoresist (13) which is chemically resistant to the sacrificial layer etching of the first photoresist (12). 4. The method according to any one of claims 1 to 3, characterized in that a photoresist is deposited as a third photoresist (15) which is chemically resistant to the sacrificial layer etching of the first photoresist (12). 5. The method according to any one of the preceding claims, characterized  in that at least one of the photoresists (12, 13, 15) is spin-coated or sprayed on. 6. The method according to any one of the preceding claims, characterized  in that at least one of the photoresists (12, 13, 15) is structured by means of a photolithographic process. 7. A thin film based flow sensor (1) comprising:  a first heater and temperature measuring element (10) and at least one second heater and temperature measuring element (11), and wherein the heater and temperature measuring elements (10, 11) are spatially separated, and wherein a support structure (21) is present, the Receiving the Heizerund temperature measuring elements (10, 11) is formed, and wherein the support structure (21) with the heater and temperature measuring elements (10, 11) on a substrate (100) is constructed,  characterized in that the carrier structure (21) has at least one photoresist (12) and has a free space (16) for the substrate (100) to form an exemption. 8. flow sensor (1) according to claim 7, characterized in that the support structure (21) has a free space (16) between the  Temperature measuring elements (10, 11), which by a  Opferschichtätzung the first photoresist (12) is generated. 9. flow sensor (1) according to claim 7 or 8, characterized in that the support structure (21) has connecting arms (17) for contacting the heater and temperature measuring elements (10, 11). 10. flow sensor (1) according to one of claims 7 to 9, characterized  characterized in that the support structure (21) has a structuring (18) between the two heater and temperature measuring elements (10, 11).