EP1348121A1

EP1348121A1 - Method for producing thin film sensors, especially hot film anemometers and humidity sensors

Info

Publication number: EP1348121A1
Application number: EP01271537A
Authority: EP
Inventors: Wolfgang Timelthaler
Original assignee: E&E Elektronik GmbH
Current assignee: E&E Elektronik GmbH
Priority date: 2000-12-21
Filing date: 2001-12-05
Publication date: 2003-10-01
Also published as: DE10063794A1; WO2002050527A1; US20040113751A1

Abstract

The invention relates to a method for producing thin film sensors. The aim of the invention is to provide a method for the economical and effective large scale production of thin film sensors. To this end, sensor structures are applied (S10) to a glass substrate and the front of the composite from the glass substrate and the sensor structures is immovably linked (S20) with a support. Large areas of the substrate material are then substantially removed (S31, S32, S33) from the back down to a final thickness (d) of the glass substrate. The optional steps of removal (S32; S33) serve to reduce the roughness of the back. The link between the support and the composite from the glass substrate and the sensor structures is then dissolved (S50). The inventive method is characterized in that comparatively thick glass substrates are coated, thereby substantially reducing the reject rate caused by breakage or deformation of the glass substrates, even if the glass substrates of the finished thin film sensors are very thin. The inventive method also allows increasing the degree of automation in the production of thin film sensors.

Description

METHOD FOR PRODUCING THIN FILM SENSORS, IN PARTICULAR HOT FILM MOMETERS AND HUMIDITY SENSORS

The invention relates to a method for producing thin-film sensors which have a glass substrate. The invention also relates to hot-film anemometers and moisture sensors which have been produced by this method. 5

Thin-film sensors are used in large numbers, for example in the automotive industry to measure the intake air mass flow of internal combustion engines or as moisture sensors.

10 Due to the high requirements regarding chemical resistance and the high temperature loads, hot film anemometers for the determination of gas mass flows, for example, are often made with a substrate made of glass. Thin, approximately 1 μm thick layers, often made of molybdenum or platinum, are applied to this glass substrate by

15 Example vacuum evaporation or cathode sputtering (sputtering) applied. In many cases, this composite is then provided with a protective layer and a contact layer. The necessary sensor structures are then worked out of the applied layers, for example by a selective photo-etching process.

20

Capacitive moisture sensors are also manufactured as thin-film sensors. For this purpose, electrodes which intermesh with one another in comb-like fashion are applied to the glass substrate. A moisture-sensitive layer is placed on top, which is usually made of a polymer material.

25 material exists. The capacity of the electrode arrangement changes due to the water absorption of the moisture-sensitive layer, which is dependent on the relative air humidity. The relative humidity can then be determined by measuring the capacity. To further miniaturize and reduce the thermal response time of these sensors, one is possible

30 to aim for a small thickness of the substrate. With hot film anemometers in particular, it is important that a fast response speed is ensured, or that these sensors have a short thermal time constant. For these reasons, the hot-film anemometers are made with the thinnest possible substrate. There are usually two measuring resistors on this glass substrate, one of which is designed as a heating resistor. During operation at overtemperature, the temperature or the resistance of the downstream heating resistor is kept constant, which is achieved by tracking the sensor current. The sensor current also serves as a measurement for the flow rate of the intake air. Pulsations in the intake manifold of the engine and any flow reversal, in combination with averaging over the nonlinear characteristics of a hot film anemometer, lead to incorrect measurements that can no longer be accepted in modern engine management systems. The aim is therefore to implement a hot film anometer that has a lower thermal time constant. Substrates with a small thickness reduce the thermal capacity of the substrate, thereby minimizing the disturbing heat flow between the substrate and the sensor structures that falsifies the measurement result in thermally unsteady operation.

In order to achieve sufficiently low time constants, a maximum substrate thickness of 150 μm is necessary. Substrate thicknesses of 100 μm down to about 50 μm and below should preferably be achieved. The production of thin-film sensors in large numbers on such thin substrates, in particular glass substrates, is particularly difficult from a manufacturing point of view and is accompanied by high reject rates.

Normally, commercially available glasses with a thickness of 100 μm to 150 μm are used as substrates in the production of thin-film sensors. The sensor structures of several sensors are applied to these glass substrates. After that, protective and contact layers are usually applied. The coating creates considerable stresses in the thin glass substrate. In manufacturing, these tensions do not lead to the undivided glass substrates neglecting rejects due to breaks and cracks. However, the sawing of the assembled glass substrates to the final dimensions of the sensors is particularly critical in this context. In this step, additional tensions are inevitably introduced into the glass material, which then often leads to the glass breaking. In order to keep the reject rate within limits, glass substrates with a maximum size of 2 inches x 2 inches could previously only be used. In addition, the stresses described above, if they do not break, create warping of the glasses. These unsystematic deformations are the limiting factor for the automation efforts of thin-film sensor production with glass substrates.

A method is known from WO98 / 34084 in which an additional membrane layer is applied to a glass support. The glass carrier is then removed on a relatively small area from the back to the membrane by a selective etching process. The membrane, which can consist of several layers, then serves as a substrate for the sensor structures of the hot-film anemometer. In this way, thin-film sensors with a small substrate thickness can be produced. These extremely low substrate thicknesses have the advantage that the response time of the hot film anemometer is minimized, but they are extremely sensitive to mechanical loads. This design has proven to be insufficiently robust, especially for use in motor vehicles.

EP 0043001 B1 describes a method for producing moisture sensors using thin-film technology on a glass substrate. The initial thickness of the glass substrate is not further reduced after the sensor structures have been applied. Even the application of an etching protection layer actively prevents the removal of glass material from the substrate in the etching process. The production of thin-film sensors with thin glass substrates is therefore extremely delicate and uneconomical with this method. The invention is therefore based on the object of providing a method which enables economical production of thin-film sensors on thin glass substrates in large numbers.

This object is achieved by a method according to claim 1.

Advantageous embodiments of the method according to the invention result from the measures in the claims dependent on claim 1.

On the basis of the measures according to the invention, commercially available glass plates with an initial thickness D of 0.3 mm to 0.9 mm can be used as the starting material for the glass substrate. In contrast to the previously possible starting glass substrate formats of 2 inches x 2 inches, the new method means that corresponding plates with a square shape with a side length of 4 inches or round plates with a diameter of 6 inches can be used. The area that can be used when applying these plate sizes for the application of the sensor structures, which have an edge length of only a few millimeters, is accordingly increased by a factor of 4 to 7. Accordingly, 4 to 7 times as many sensor structures can be applied and structured out in one work step using the new method with a reasonable reject rate. At the same time, this procedure results in a significantly reduced risk of breakage.

Further advantages and details of the method according to the invention result from the following description of a possible exemplary embodiment with reference to the attached figures.

It shows

Figure 1 shows schematically the respective process steps for the production of thin-film sensors; Figure 2a shows a cross section through a composite

Glass substrate and sensor structures after the trenches have been scratched;

Figure 2b shows a cross section through a composite

Glass substrate and sensor structures in connection with the carrier;

Figure 2c shows a cross section through a composite of glass substrate and sensor structures in connection with the carrier after grinding;

Figure 2d shows a cross section through the reclamped finished thin-film sensors.

Figure 1 is essentially intended to explain the process flow. The production process is shown in FIGS. 2a to 2d using cross sections. Identical parts are identified in FIGS. 2a to 2d with identical reference symbols.

In the example shown, sensor structures 2 are first applied to a square glass substrate 1 with an initial thickness D of 0.5 mm and an edge length of 4 inches in step S10. The sensor structures 2 are in this case for a hot film anemometer and therefore consist of a measuring resistor and a heating resistor and the associated protective and contact layers.

As an alternative to this, the sensor structures 2 can also be comb-shaped electrodes with a corresponding moisture-sensitive layer and, if appropriate, additional protective layers for moisture sensors.

According to the definition, that side of the glass substrate 1 on which the sensor structures 2 are applied is referred to as the front side 1.1. The opposite side of the glass substrate 1 is consequently the back 1.2 named. Due to the comparatively large initial thickness D and the associated high mechanical stability of the glass substrate 1, its handling is unproblematic. Even after the sensor structures 2 have been applied, the comparatively thick glass substrate 1 practically never shows cracks or warps. The sensor structures 2 for a hot film anemometer consist, for example, of molybdenum conductor tracks, which are applied to the glass substrate 1 by sputtering. These tracks are then covered with a protective layer, which in turn is covered with a gold contact layer. The corresponding structures are then worked out using a selective photo-etching process.

As soon as the manufacture of the sensor structures 2, consisting of the above-mentioned layers, has been completed, the glass substrate 1 in the example shown is incised in two directions perpendicular to one another between the sensor structures 2 (FIG. 2a). The depth t of the trenches 1.3 thus introduced is selected so that after the grinding process of the rear side 1.2 of the glass substrate 1 described below, only the rectangular thin-film sensors remain in the glass substrate 1 without connecting bridges. In other words, the depth t of the trenches 1.3 is greater than the final thickness d of the glass substrate 1.

The composite of glass substrate and sensor structures is then connected to a carrier 3 on its front side in step S20. This is done, for example, by means of a releasable clamping adhesive connection. As a releasable clamping adhesive connection, a hot melt adhesive in the form of a wax 4, which is applied to the front side 1.1 of the composite of sensor structures 2 and glass substrate 1 in liquid or viscous form, is suitable, for example in accordance with the example shown. The front side 1.1 is then brought into contact with the carrier 3, which preferably also consists of glass, according to FIG. 2b. The wax 4 is then allowed to cool, whereby it solidifies, thus forming an immovable connection between the support 3 and the composite of sensor structures 2 and glass substrate 1. In addition to waxes, other hotmelt adhesives, such as rosin, or synthetic material compounds, for example from the category of polymer compounds, can also be used. At a later point in time, the connection can be released again by heating to a temperature above the melting point of the adhesive.

Adhesive films coated with adhesive on both sides can be used as an alternative detachable mounting adhesive connection. The use of these adhesive films has the advantage that the sensor structures 2 can dig into these films under the pressure of the subsequent removal processes, so that local pressure peaks in the sensors to be produced can be avoided. Of course, the term adhesive films also means flat materials with the same effect, such as adhesive fabric tapes or adhesive foam films, etc. With these adhesive foils, foils with an adhesive coating can preferably be used, the adhesive force of which diminishes significantly or disappears under the action of UV light. In this way, the adhesive effect can be deactivated at the desired time by suitable radiation.

In a further embodiment of the invention, the connection between the composite of glass substrate 1 and sensor structures 2 with the carrier 3 can also be produced by negative pressure. In this case, air is sucked in, for example, through a perforated carrier 3. As soon as the glass substrate 1 with the sensor structures 2 is connected to the carrier 3, the pressure on the suction side of the vacuum source drops, so that a contact pressure arises as a result of the pressure difference between the environment and the contact surface. So that the sensor structures 2 are not damaged when they are clamped onto the carrier 3, it is expedient to provide a suitable intermediate layer made of soft material.

Then, in accordance with the exemplary embodiment shown, the substrate material is removed in three partial steps (S31, S32, S33) from the rear side 1.2 up to the resulting final thickness d of the glass substrate 1 (FIG. 1). The entire rear side 1.2 of the stretched glass substrate 1 is first processed in step S31 with a relatively coarse grinding tool. The aim of step S31 is that the vast majority of the substrate material to be removed is removed here, or that after S31 the initial thickness D is scarcely greater than the resulting final thickness d of the glass substrate 1 to be aimed for.

The invention is not restricted to processing the entire rear side 1.2 of the glass substrate 1, but rather in this connection means a largely large-area removal of the rear side 1.2 up to the resulting final thickness d of the glass substrate 1. This means that at least 75% of the rear side 1.2 of the starting glass substrate is exposed to the removal process. Not only grinding, but also, for example, polishing or etching methods can be used as the method for carrying out the removal of substrate material.

The first removal step is often used to remove the majority of the volume of the substrate material to be removed, approximately 60% to 75% or more. The removal process can already be completed after this step, provided that the final thickness d of the glass substrate 1 has been reached and the processed surface is of sufficient quality in terms of roughness.

However, after the first relatively rough removal step, the reverse side 1.2 is advantageously subjected to a further processing to reduce the roughness in a second step (S32) as part of the removal process. This is to reduce voltage peaks due to micro-notches on the surface. The glass substrates 1 treated in this way are then mechanically relatively insensitive despite their small thickness. In step S32 (FIG. 1), the previously roughly ground back 1.2 is therefore subjected to a fine grinding process in the present exemplary embodiment.

As an alternative to fine grinding, a polishing process could also be carried out, for example. To reduce the roughness of the back 1.2 other suitable surface treatment methods can also be used. These steps can be, for example, those mentioned above, which can be carried out individually, overlaid or in any combination.

A further increase in the mechanical load-bearing capacity of the thin sensors is possible by means of a further removal step, in the present example according to FIG. 1, an etching process S33 on the rear side 1.2. As a result of the etching in this area, the surface of the rear side 1.2 becomes extremely smooth, which largely eliminates the notch stress peaks. In this step, the back 1.2 of the glass substrate 1 is treated with hydrofluoric acid. As a result, micro elevations on the rear side 1.2 are removed until the final thickness d of the glass substrate 1 is reached (FIG. 2c). The glass substrates 1 treated in this way then have a very smooth back 1.2. Alternatively or in addition to the example shown, dry etching processes or polishing etching processes can also be used as etching processes.

The comparatively small thin-film sensors are now independently on the support after the last removal step (FIG. 2c). In order to combine the thin-film sensors into suitable quantities for further handling, in the example shown these are then clamped onto a so-called end product carrier 5 (S40, FIG. 1). For this purpose, the thin-film sensors on the rear sides 1.2 are connected to the end product carrier 5 by a transformer adhesive 6. The front sides 1.1 of the thin-film sensors are still connected to the carrier 3 by the wax as a mounting adhesive 4 at this time. Similar to the clamping glue 4, the transformer glue 6 is a removable glue. In this example, however, the transformer adhesive 6 can be deactivated with UV light, in contrast to as the adhesive 4 - in the present example, the wax.

It is fundamentally expedient that different adhesive types are used for the clamping and transformer adhesive 4, 6. In particular, it is advantageous if clamping and clamping adhesive 4, 6 are replaced by Different measures or principles of action can be deactivated (UV light, heat). Clamping and reclamping adhesives 4, 6 can also be used, the effect of which wears off, for example, at different temperature levels. In this way, the desired adhesive connection can be released selectively.

Finally, the connection between the carrier 3 and the composite of glass substrate 1 and sensor structures 2 is released again in step S50. For this purpose, the arrangement is heated so that the wax 4 experiences a temperature which is above its melting point. The transformer adhesive 6 remains effective despite this heating. The finished thin-film sensors are then only in contact with the end product carrier 5. If necessary, solder bumps can also be placed for the connection technology of the thin-film sensors. The finished thin-film sensors are shipped together with the end product carrier 5.

The dashed arrows in FIG. 1 are intended to express that corresponding working steps can also be skipped in other configurations of the invention. The example given is not intended to limit the invention to this embodiment. For example, only one removal step can be carried out in the method according to the invention, which is either one of the processes S31, S32, or S33 or comprises another removal method.

Claims

claims

1. A method for producing thin-film sensors, wherein

Sensor structures (2) are applied to the front (1.1) of a glass substrate (1), - the composite of glass substrate (1) and sensor structures (2) is connected to a carrier (3) on its front (1.1), then the substrate material of the back (1.2) is largely removed over a large area up to a resulting final thickness (d) of the glass substrate (1), - finally the connection between the support (3) and the composite of glass substrate (1) and sensor structures (2) is released again.

2. The method of claim 1, wherein the removal of the substrate material comprises at least two removal steps, a first removal step of which is a grinding process, and at least a subsequent removal step comprises reducing the roughness of the back.

3. The method according to claim 2, wherein a removal step following the first removal step comprises a polishing process.

4. The method according to claim 2, wherein a removal step following the first removal step comprises a fine grinding process.

5. The method according to claim 2, wherein a removal step following the first removal step comprises an etching process.

6. The method according to claim 1, wherein before the connection of the carrier (3) with the composite of glass substrate (1) and sensor structures (2) in the Glass substrate (1) from the front (1.1) trenches (1.3) are introduced.

7. The method according to claim 6, wherein the depth (t) of the trenches (1.3) is greater than the final thickness (d) of the glass substrate (1).

8. The method according to claim 1, wherein the connection between the carrier (3) and the composite of glass substrate (1) and sensor structures (2) is produced via a releasable clamping adhesive connection (4).

9. The method according to claim 8, wherein the connection between the carrier (3) and the composite of glass substrate (1) and sensor structures (2) is produced with the aid of a hot melt adhesive (4).

10. The method according to claim 8, wherein the connection between the carrier (3) and the composite of glass substrate (1) and sensor structures (2) is produced with the aid of an adhesive film.

11. The method according to claim 1, wherein the connection between the carrier (3) and the composite of glass substrate (1) and sensor structures (2) is produced with the aid of negative pressure.

12. The method according to claim 1 or 6, wherein after the largely large-scale removal of the glass substrate (1), the composite of glass substrate (1) and sensor structures (2) on the back (1.2) of one

End product carrier (5) reclamped and connected to the same via a releasable transformer adhesive connection (6).

13. The method according to claim 1 or 6, wherein after the largely large-scale removal of the glass substrate (1) the composite of glass substrate (1) and sensor structures (2) on the back (1.2) on an end product carrier (5), and over a releasable transformer adhesive connection (6) is connected to the same.

14. Hot film anemometer, produced according to at least one of claims 1 to 11.

15. Moisture sensor manufactured according to at least one of claims 1 to 11.