JPS60166854A - Capacity type humidity sensor and manufacture thereof - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Application The present invention provides a moisture-sensitive insulating substrate and a moisture-sensitive insulating substrate disposed between two electrodes configured to form an intermediate space on the substrate. TECHNICAL FIELD The present invention relates to a capacitive humidity sensor having a thin layer from a composite and a method of manufacturing such a humidity sensor.

Prior Art This type of humidity sensor is known, for example, from West German Patent No. 23.
Specification No. 39545, Specification No. 2547389, West German Patent Publication No. 2920808 and No. 29
It is known from the description in No. 27634. In the case of all these known humidity sensors, the two configured electrodes are formed directly on the substrate as comb electrodes with interlocking teeth, and the moisture sensitive polymer is subsequently applied to both comb electrodes. or at least fill the intermediate space between the comb-shaped electrodes. This configuration does not allow a very fine configuration of the electrodes at all, and also in terms of the necessary insulation between the configured electrodes, the interelectrode spacing must be relatively large, which is achievable for a given surface expansion. The basic capacity of the humidity sensor that can be used is relatively small. Furthermore, the sensitivity and speed of response of the humidity sensor is limited due to the relatively large thickness of the polymer present between the electrodes. used (1 coalescence is mainly thermoplastic,
They are often polyamides and are therefore almost mechanically, chemically and thermally irresistible.

l'Handbook of Materials and Technology for Electronics
k of Materials and Processes
ses for Filectronics)” Editor C
har1. es A, Harper.

MCGraw-Hllll Book company
It is known that the electrical properties of polyimide, particularly the dielectric constant and dielectric loss factor, change depending on the water content, as described in 1970, New York, 1970, pages 1 to 66.

Accordingly, EP 0 010 771 A1 describes a capacitive humidity sensor, the moisture sensitive layer of which is a polyimide sheet, which is simultaneously bonded by a thin adhesive layer. One electrode is glued on a metal substrate, the other electrode is used as
A gold coating is applied to a polyimide sheet, and the gold coating is thin enough that it is water permeable. The polyimide layer can also be obtained by imidization from a polyimide precursor which is initially applied as a solution onto the metal substrate. Polyimide as a moisture-sensitive material has the advantage that it is chemically resistant, mechanically stable and heat-loadable. However, the sensitivity and response speed of this known humidity sensor are also limited. This is because the layer i1j of the polyimide layer on the negative side cannot be made arbitrarily thin in order to eliminate all DC conductivity between the electrodes, and the gold coating covering the polyimide layer on the other side is This is because water dispersion and at the same time the rapid equilibration of water vapor partial pressures are disturbed in the polyimide and in the atmosphere. Moreover, very thin gold layers are mechanically, metallurgically and electrically unstable, making it difficult to make connections with such thin gold layers.

Problem to be Solved by the Invention The object of the present invention is to provide a first method which has high sensitivity, high response speed, high long-term stability and low hysteresis with good mechanical stability, chemical resistance and thermal stability. The object of the present invention is to obtain a sensitive sensor of the type described in .

According to the invention, one structured electrode is formed on the substrate as a bottom electrode, the thin moisture-sensitive layer is made of polyimide, and the bottom electrode is formed of polyimide. The solution is that the second electrode is arranged as an upper electrode on the polyimide layer and is constructed complementary to the lower electrode in terms of structure.

The humidity sensor described according to the invention has the advantages of using polyimide for the moisture sensitive layer, namely the good mechanical, chemical, thermal and electrical properties of this material. The polyimide layer can be formed with only a few layers M. Since the upper electrode is disposed on the polyimide layer, the distance between the electrodes is determined by the layer thickness, and a small layer thickness results in a large initial capacitance. The arrangement of the polyimide layer between two complementary configured electrodes results in high sensitivity and high response speed. This is because the polyimide is in direct contact with the surrounding atmosphere in the intermediate space of the constructed upper electrode, and therefore the extremely thin polyimide layer can quickly become saturated with respect to the prevailing moisture. can. Therefore,
The constructed upper electrode need not be water permeable;
This upper electrode can therefore be formed with a thickness that provides good mechanical and electrical stability and that allows the connecting contacts to be attached without any problems, for example by soldering.

According to one preferred embodiment of the invention, between the configured lower electrode and the moisture-sensitive polyimide layer there is provided a moisture-sensitive electrical layer, which is an oxide layer formed by surface oxidation of the configured lower electrode. An insulating layer is arranged. Furthermore, the polyimide layer can be formed with very little layer 1, which can be less than 1 μm, without there being a risk of direct current conductivity between the electrodes. The humidity sensor thus formed has particularly high sensitivity and high response speed.

One preferred method for manufacturing a humidity sensor according to the invention is to provide a thin layer of metal of the lower electrode on a moisture-sensitive insulating substrate, configured and configured to correspond to the desired color of the lower '1L pole. Applying a thin layer of polyimide precursor on the surface of the substrate, covered by a bottom electrode, structured corresponding to the desired shape of the moisture sensitive layer, and converting the structured layer of polyimide precursor to ttB wet polyimide. For conversion, a thin layer of the metal of the upper electrode is provided on the surface of the substrate, covered by an imidized lower electrode and a moisture-sensitive polyimide layer, to be configured corresponding to the desired shape of the upper electrode. be.

In the case of the method according to the invention, the layer consisting of the polyimide precursor is still soluble and can therefore be easily constructed, which is not possible as is if the final moisture-sensitive layer is an insoluble polyimide. The fact that this is the case is exploited. Constructing the moisture-sensitive layer has advantages over its use in conjunction with configured electrodes; in particular, increasing even further the response speed of the moisture-sensitive layer This is possible with appropriate configuration.

One method of forming polyimides from acid-modified polyamides used as polyimide precursors is described in the periodical,
“Jour Op Polymer Science”
nal of PolymerScience)”,
Part A, Volume 3, 1965, pp. 1673-139
Page 0, Sroog, En
dre'Y), Abramo, Berr, EdwardS
and the paper “Aro” by Olivia (01ivior)
matic Polypyromellitimide
s from Aromatic Polyamic A
c1ds”.

Preferred features and other features of the humidity sensor according to the invention and of the method for its production are set out in claims 2 to 9.
or any one of paragraphs 11 to 16 of the same.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Further characteristics and advantages of the invention are apparent from the following description of an embodiment with reference to the drawings.

Flowcharts of Figures 1a and 1b and 2a
The fabrication of a preferred embodiment of a humidity sensor is described with reference to the cross-sectional views in FIGS. 2-2h, which humidity sensor is represented in plan view in FIG.

In process step 1, a thin layer of tantalum is applied by cathodic sputtering onto a substrate 1 of moisture-sensitive, mechanically stable electrically insulating material (FIG. 2a). For the substrate 1, an alkali-free glass of particularly high purity and highest surface quality is used. The tantalum layer can be sputtered to a thickness of 0.15μ, for example.

Corresponding to conventional thin film technology, a number of humidity sensors are manufactured simultaneously on the substrate 1, which are then separated from each other after completion. To simplify the process, the manufacturing of each humidity sensor will be described below.

In case of process step 2, the tantalum layer 2 is configured to form the bottom electrode. This can be done photolithographically in a conventional manner. To do this, first apply an etching mask on the tantalum layer 2 to a thickness of approximately 1.5 mm over the entire surface.
The shape is developed by coating with a layer of .mu. photolacquer, which is then exposed through a stencil, developed and cured. Subsequently, Iθ11 of layer 2
The tantalum not covered by the etching mask is removed by a wet chemical etching process.

After etching and removal of the etching mask, the tantalum layer 2 has the desired shape of the lower electrode 10, which is shown in plan view in FIG. The lower electrode 10 is a comb-shaped electrode with a web 11 from which a number of narrow, longitudinally extending teeth 12 project, between which intermediate spaces 13 are formed.
exists. The web 11 transitions at one end into an enlarged connecting terminal 14 . As is obvious, the drawing of FIG. 4 is greatly simplified. In reality, the tooth part 1 of the comb-shaped electrode
The number of 2 is significantly greater: the width of both parts 12 and the intermediate space 13
The zero width is approximately 5 to 10 microns each.

The drawing in FIG. 2b shows that the lower electrode 100 has six teeth 12vC.
This corresponds to a cross-sectional view.

In the case of treatment process B, the surface oxidation of tantalum in the lower electrode is
This is done after the connection terminals 14 have been covered with an oxidation protection mask. The surface oxidation is about I over a depth of about 0.05μ
This is done by anodizing at a voltage of D0V, so that under the oxide layer there remains a metallic tantalum approximately 1 μm thick. The tantalum oxide Ta2o5 formed by anodic oxidation occupies a larger volume than in the case of full bending, so the tantalum oxide layer 3 produced (FIG. 2C) has a thickness of about 0.15μ. Oxide layer 3
is a completely tight, highly insulating passive coating. This has the advantage that the lower electrode 10 is virtually unattackable chemically and at the same time is extremely stable. In terms of electrical aspects, it is achieved that in the case of the finished humidity sensor all direct current conductivity of the capacitor structure is suppressed. In terms of physical aspects, this means that the finished humidity sensor has outstanding linearity and stability, especially at high temperatures and high relative humidity.

For process step 4, the entire surface of the substrate 1 and the lower S electrode 10 formed thereon are coated with a layer 4 of polyimide precursor (
(FIG. 2d), from which a moisture-sensitive polyimide is subsequently formed by imidization. The polyimide precursor is an acid-modified, highest purity polyamide, which can be obtained by centrifugation from approximately 1.0 to 2.
A layer thickness of 0μ is provided. Subsequently, layer 4 is subjected to treatment step 5.
and dried at 90'C to 120°C.

A low layer thickness enables a particularly good surface quality of the glass substrate 1.

Process steps 6 to 8 are used for the subsequent construction of the moisture-sensitive layer. This construction is again carried out by conventional 7 otolithographic techniques. For this purpose, in process step 6 layer 5 (FIG. 2e) is centrifuged from the tenon photolacquer to a thickness of approximately 1.5 μm, and this photolacquer layer is removed in process step 7 at approximately 60°C at 90°C. Allow to dry for minutes. In process step 8, the single layer of photolacquer is exposed via the stencil 6 in locations where no photosensitive layer is subsequently present. These are, in particular, the connection terminals 14 of all lower electrodes, which are formed on the glass substrate and which must be exposed in order to attach the electrical connection contacts and which are then described. In this embodiment, the intermediate spaces 13 between the teeth 12 of the lower electrode 10 must also be exposed. After exposure, the photolacquer layer is developed in a process step 9, ie the exposed parts of the photolacquer layer 5 are removed. The developer used for this purpose also dissolves the underlying parts of the layer 4, and the parts of the layer 4 which lie below the unexposed positions of the photolacquer layer 5 remain present.

The necessary etching of the construction of the photosensitive layer is therefore carried out in step i with the development of the photolacquer layer.

After the remaining, unexposed parts of the photolacquer layer have been removed in process step 10, the still present acid-modified polyamide of layer 4 is imidized in process step 11 at 300%
This is carried out by heat treatment at ~400 DEG C. for about 1 hour, whereby a moisture-sensitive polyimide layer is obtained. The structure thus obtained corresponds to the cross-sectional view in FIG. 2f.

Teeth 12 of the comb-shaped lower electrode 10 made of tantalum in layer 2
The web 11 (as well as the invisible web 11 in the case of FIG. 2f) is completely surrounded by a layer 3 of tantalum oxide on the surface which is not in contact with the glass substrate 1, the tantalum oxide layer being It is surrounded on the sides by a layer 7 of moisture-sensitive polyimide. On the other hand, the tooth portion 12
A glass substrate is exposed in the intermediate space 13 between them. In the case of imidization, the layer thickness is reduced, so that from the previously described polyamide layer with a thickness of about 1-2 μm, a polyimide layer with a layer thickness of about 0.5-1.0 μm results.

Subsequently, the upper electrode is formed, and this upper electrode is
The second electrode of the capacitor, the first electrode is the lower electrode 10, and the dielectric is formed by the moisture sensitive layer γ. To form the upper electrode, first the entire surface of the structure obtained so far is coated with a layer 8 of the material of the upper electrode (FIG. 2g). Layer 8 can be the only metal layer or it can consist of a number of successively applied metal layers. If the upper electrode is made of gold, in order to achieve good adhesion of the gold layer, a thin nickel-chromium layer (thickness 0.02 μm) is first provided, and then a gold layer (thickness 0.02 μm) is first applied on top of this. 2μ) is required. For simplicity, the various positions of layer 8 are not shown in the drawings.

In case of process step 15, metal layer 8 is configured to form a top electrode. This is carried out in the manner previously described, again by conventional photolithographic methods. After etching and removal of the etching mask, the metal layer 8 has the desired shape of the upper electrode 20, which shape is represented in plan view in FIG. The upper 'electrode 20 is likewise a comb-shaped electrode with a web 21 from which a number of narrow, longitudinally extending fingers 22 project, between which intermediate spaces 23 are present. A connecting terminal 24 is connected to the web 21 of the upper electrode 20 and is present alongside the connecting terminal 14 of the lower electrode 10 . A cross-sectional view is shown in Figure 2h.
The teeth 22 of the upper electrode are above the intermediate spaces 13 of the lower electrode, and the intermediate spaces 23 of the upper electrode are the teeth of the lower electrode, as shown in the multiple teeth 22 of the two VA saturations. Part 12
exists above. The teeth 22 of the upper electrode are located in the intermediate space 13 between the teeth 12 of the lower electrode on the glass substrate, and the southern covering II8 wet layer 1 of the lower electrode is exposed in the intermediate space 23 of the upper electrode. ing. The moisture-sensitive layer 1 is interposed between the mutually opposite edges of the two electrode teeth 12 and 22 in each case at the transition between the edges of this region. Electrode tooth part 1
The edges of 2 and 22 may substantially coincide or may overlap slightly.

In case of treatment step 14, the resulting structure is heat treated for 30
It is carried out for 2 hours at a temperature of 0°C.

The high imidization temperature between 600°C and 400°C is 600°C.
C. allows the use of high heat treatment temperatures, thereby achieving significant aging resistance of the finished humidity sensor.

In process step 15, disassembly of the individual humidity sensors is carried out by making a crack in the glass substrate and breaking it. Subsequently, in process step 16, electrical connection contacts 15 or 25 are soldered to the two contact terminals 14 and 24 of all humidity sensors. The humidity sensor is thus ready for use.

This formation of a humidity sensor results in significant advantages in terms of manufacturing as well as properties in use.

One essential feature of the production method is that, in order to form the moisture-sensitive polyimide layer, a polyamide layer is first provided, this polyamide layer is subsequently structured and only then converted into a moisture-sensitive polyimide. This method, which is known per se, of course makes possible a significantly simpler construction of the subsequent moisture-sensitive layer using the usual methods of thin-film technology. This is because the acid-modified polyamides used as polyimide precursors are soluble and
Construction of polyimide layers would not be possible in this way, since polyimide is extremely chemically, thermally and mechanically stable. This property of polyimide, which makes it amenable to easy processing with thin-film technology, again results in remarkable long-term stability of the finished humidity sensor.

Furthermore, acid-modified polyamides are obtained in the highest purity. This is unavoidably required in order to ensure that humidity sensors can be produced with reproducible properties independent of the amount of charge.

Furthermore, the polyimide can be applied with very low layer thicknesses, for example from 0.5 to 1.0 .mu., without a reduction in mechanical strength using the method described. The speed of response with which the humidity sensor can track changes in relative humidity is significantly increased by the small layer thickness.

The characteristic curve of the humidity sensor can be influenced within the desired range by the imidization parameters, in particular the imidization temperature and the imidization atmosphere used. The optimum electrical properties, quality and physical properties of the humidity sensor are between 500°C and 40°C.
It has been found that this can be obtained at an imidization humidity of 0'C.

This favorable property, which is based on the moisture-sensitive material used and the type of its manufacture, is even further improved by the structure of the humidity sensor. In particular, as can be seen from the cross-sectional view in FIG. 2h, a large part of the surface of the moisture-sensitive layer 7 is directly exposed to the atmosphere, so that moisture can flow into the moisture-sensitive layer without any hindrance. can be invaded. Upper part of the moisture sensitive layer 'II
The area covered by the J and O poles is relatively small and can be quickly saturated with respect to prevailing moisture due to the particularly small layer thickness. A small amount of layer 1 again creates a large capacitance between the electrodes that can face each other. On the other hand, despite the small thickness of the moisture-sensitive layer, all risks of electrical direct current paths between the electrodes are prevented by the oxide layer 3 on the bottom surface.

This means that the upper electrode can be tightly formed into a water valve permeable so that the moisture directly reaches the moisture sensitive layer.
On the one hand, it increases the mechanical strength and electrical stability, and on the other hand, it makes it possible to directly solder the electrical connection contacts at the upper electrode.

By soldering the connection contacts, in particular the long-term instability of adhesive bonds, especially at high temperatures and/or high humidity, is eliminated.

FIG. 6 shows a cross-sectional view of a slightly modified embodiment of the humidity sensor, corresponding to FIG. 2b. This humidity sensor
In processing steps 8 and 9, the polyamider is distinguished from the degree sensor in the intermediate space 13 between the teeth 12 of the bottom electrode. That is,
The stencil 6 used for the construction has an intermediate space 13
It has no perforations at all, but is only used to expose the connection terminals 14. Furthermore, the manufacturing process proceeds without changing the previously described method, and the plan view of the finished humidity sensor corresponds to the drawing in FIG. However, as a result of the described alternative, as can be seen from FIG. 6, the teeth 22' of the top electrode are placed on the glass substrate 1 in the intermediate space 13 between the teeth 12 of the bottom electrode. It is not placed on the moisture-sensitive polyimide layer γ. Although the manufacture of the humidity sensor is simplified by eliminating the need for fine construction of the polyimide layer, the response speed of the humidity sensor of FIG. 6 is modest. That's because of water waves! This is because the +fIi path is long under the top electrode.

The humidity sensor of FIG. 3 also has a smaller base capacitance and a smaller capacitance variation range than that of the second hlffl humidity sensor.

The humidity sensor described above has a lower electrode constructed from tantalum, which is coated with an insulating tantalum oxide layer by surface oxidation, which has optimal mechanical, physical, chemical and electrical properties. properties and can be obtained with reasonably good reproducibility by methods of thin film technology. However, variations in the humidity sensor and its manufacturing method are also possible. The bottom electrode can therefore be obtained from a metal other than tantalum, which can be coated with a surface oxidation (and thus an insulating oxide layer). In all cases, the moisture-sensitive layer can be made extremely thin, independent of the insulation, and thus high sensitivity and large response speeds can be achieved. The advantage of the
For this purpose, the moisture-sensitive polyimide layer can also be made somewhat thicker, which polyimide layer thus guarantees electrical insulation between the electrodes. The manufacturing method is thereby somewhat simplified. Again, by arranging the moisture-sensitive layer between two configured electrodes, a significantly better sensitivity and response speed is still obtained than in conventional humidity sensors.

Instead of comb-shaped electrodes, other mutually complementary electrode shapes with intermediate spaces can be used, for example the grid web of the upper electrode is above the perforations of the lower electrode, and conversely the grid web of the lower electrode is below the perforations of the upper electrode. It is also possible to use grid-like electrodes arranged in the same way.

[Brief explanation of the drawing]

FIGS. 1a and 1b are flowcharts illustrating the process steps of a preferred method of manufacturing a volumetric humidity sensor;
Figures a to 2h are cross-sectional views showing the various stages of completion obtained in Figures 1a and 1b, respectively, in manufacturing the humidity sensor shown in Figure 5, and Figure 6 is Figure 2h. FIG. 4 is a plan view showing the lower electrode obtained in the completed stage, and FIG. 5 is a plan view showing the completed humidity sensor. 1... Substrate, 3... Electrical insulating layer, γ... Moisture sensitive layer,
10... Lower electrode, 13, 23... Intermediate space, 20
...Top electrode. Engraving of drawings (no change in content) Fig, la Fig, lb Fig, 2a Fig, 2d Procedural amendment (method) March 71, 1985 Commissioner of the Patent Office 1. Indication of the case Showa 1959 Patent Application No. 220649 2
, Title of the invention Capacitive humidity sensor and its manufacturing method 3, Relationship with the case of the person making the amendment Patent applicant Connoisseur 4, Agent 6, Subject of amendment (3) Letter of appointment 7, Contents of the M (1) (2) and (3) are both as attached.

Claims (1)

[Claims] 1. A capacitor having a moisture-sensitive insulating substrate and a thin layer of a moisture-sensitive polymer arranged between two electrodes configured to form an intermediate space on this substrate. In a static humidity sensor, one structured electrode is formed on the substrate as the bottom electrode, a thin moisture sensitive layer made of polyimide is placed on the structured bottom electrode, and a second electrode is formed as the top electrode. Capacitive humidity sensor, characterized in that it is arranged on a polyimide layer as an electrode and is configured essentially complementary to the bottom electrode. 2. The capacitive humidity sensor according to claim 1, wherein a moisture-sensitive electrical insulating layer is disposed between the configured lower electrode and the moisture-sensitive polyimide layer. 3. The capacitive humidity sensor according to claim 2, wherein the electrical insulating layer is an oxide layer formed by surface oxidizing the lower electrode. 4. Claim 6, wherein the lower electrode is made of tantalum and is coated with a tantalum oxide layer by surface oxidation.
Capacitive humidity sensor 05 according to any one of Claims 1 to 4, wherein both electrodes are comb-shaped. 6. The capacitive humidity sensor according to any one of claims 1 to 5, wherein both electrodes slightly overlap along the edge of the intermediate space. Z. Any one of claims 1 to 6, wherein the thin polyimide layer is configured to correspond to the shape of the lower electrode such that it does not cover the substrate in the intermediate space of the lower electrode. Capacitive humidity sensor described in . 8. The thickness of the polyimide layer is 0.5 to 1.0μ.
A capacitive humidity sensor according to any one of claims 1 to 7. 9. The capacitive humidity sensor according to any one of claims 1 to 8, wherein the substrate is a glass plate of high purity and high surface quality. 10. Method for manufacturing a capacitive humidity sensor having a moisture-sensitive insulating substrate and a thin layer of moisture-sensitive polymer disposed between two electrodes configured to form an intermediate space on the substrate , a thin layer of metallic material of the lower electrode is provided on the moisture-sensitive insulating substrate and configured to correspond to the desired shape of the lower electrode;
Provide a thin layer of polyimide precursor on the surface of the substrate, covered by a configured bottom electrode, configured corresponding to the desired shape of the moisture sensitive layer, and make the polyimide precursor of the configured layer moisture sensitive. A thin layer of the metal of the upper electrode is provided on the surface of the substrate, imidized for conversion to polyimide and covered by a lower electrode and a layer of moisture-sensitive polyimide, configured corresponding to the desired shape of the upper electrode. A method for manufacturing a capacitive humidity sensor, characterized by: 11. The method of claim 10, wherein the constructed bottom electrode is coated with a moisture-sensitive electrically insulating layer before applying the polyimide precursor layer. 12. The method as claimed in claim 11, wherein the lower electrode, which has a 12.M structure, is coated with an insulating oxide layer by surface oxidation. 1. The construction of the polyimide precursor layer is carried out by exposing and developing a layer of photolacquer using a photolithographic method, as claimed in claims 1o to 12.
The method described in any one of the preceding paragraphs. 14. The method according to any one of claims 1o to 16, wherein an acid-modified polyamide is used as the polyimide precursor. 15. Imidization of polyimide precursors at about 600°C ~
15. A method according to claim 14, carried out by heating to a temperature of 400<0>C. 16. After providing the upper electrode, the heat treatment is carried out at a temperature of about 300 oo.
The method described in any one of the preceding paragraphs.