FR2554237A1 - MOISTURE SENSOR AND METHOD FOR MANUFACTURING SAME - Google Patents

Abstract

THE CAPACITIVE HUMIDITY SENSOR IS SHAPED BY THE THIN LAYER TECHNIQUE ON AN INSULATING SUBSTRATE 1 INSENSITIVE TO HUMIDITY. A STRUCTURED ELECTRODE 12 IS FORMED ON THIS SUBSTRATE AS A BASIC ELECTRODE. THIS BASIC ELECTRODE 12 IS COVERED WITH A THIN LAYER 7, SENSITIVE TO HUMIDITY, OF POLYIMIDE. ON THIS POLYIMIDE LAYER 7 IS PLACED A STRUCTURED COVERING ELECTRODE 22 WHOSE STRUCTURE IS SENSITIVELY COMPLEMENTARY TO THAT OF THE BASE ELECTRODE. BETWEEN THE STRUCTURED BASE ELECTRODE 12 AND THE HUMIDITY SENSITIVE LAYER 7, AN ELECTRICAL INSULATING LAYER 3, INSENSITIVE TO HUMIDITY, PREFERREDLY FORMED BY SURFACE OXIDATION OF THE STRUCTURED BASE ELECTRODE, MAY BE PLACED. IN THE MANUFACTURING PROCESS, TO FORM THE MOISTURE SENSITIVE LAYER 7, WE START BY APPLYING AN INTERMEDIATE OF SYNTHESIS OF THE POLYIMIDE, THEN THE LAYER IS STRUCTURED ACCORDING TO THE DESIRED SHAPE OF THE MOISTURE SENSITIVE LAYER, AND THEN IT IS IMIDIZED TO TRANSFORM THE INTERMEDIATE INTO A MOISTURE SENSITIVE POLYIMIDE. USE TO MEASURE HUMIDITY LEVELS.

Description

The present invention relates to a capacitive moisture sensor comprising

  a moisture-insensitive insulating substrate and a thin layer of moisture-sensitive polymer placed on said substrate between two structured electrodes for gap formation, and a

  yielded manufacturing of this sensor.

  Humidity sensors of this type are known for example from German patents 23 39 545 and 23 47 389, and German patent applications 29 20 808 and 29 27 634. In all these known moisture sensors, two structured electrodes are formed directly on the substrate in the form of nested finger combs, and the moisture-sensitive polymer is then applied so that it covers the two electrodes in the form of

  comb or at least fill in the gaps between them.

  This arrangement does not allow a very fine structuring of the electrodes, and as for the necessary isolation between the stuctured electrodes, the intervals thereof must be relatively large, the basic capacity of the humidity sensor that can be obtained for

  a given surface area is relatively small.

  In addition, because of the relatively large thickness

  of the polymer between the electrodes, the sensi-

  The speed and response speed of the sensor are limited.

  The polymers used are most often thermo-

  plastics, usually polyamides, and consequently

  not very resistant mechanically, chemically and thermally. It is known from the "Handbook of Materials and Processes for Electronics", edited by Charles A. Harper, McGraw-Hill Book Company, New York 1970, pages 1-66,

  that the electrical properties of polyimides in parti-

  their dielectric constant and their factor of

losses, vary with humidity.

  In European Patent No. 0 010 771 there is therefore described a capacitive moisture sensor whose moisture-sensitive layer is a polyimide sheet which is adhered by a thin layer of adhesive to a metal substrate serving at the same time as an electrode. while the other electrode is a gold deposit made on the polyimide sheet and sufficiently thin to be permeable to water. The polyimide layer can also be obtained by imidization from a synthetic intermediate of the polyimide first applied in solution on

  the metal substrate. Polyimide, as a sen-

  moisture-resistant, has the advantages of being chemically resistant and mechanically stable and of being able to be thermally loaded. However, sensitivity and

  The response speed of this known sensor is also limited.

  because the thinness of the polyimide layer is limited so that any direct current conductivity is excluded between the electrodes, and on the other hand, the gold deposit covering the polyimide layer hinders the diffusion of the water and thus prevents a rapid equalization of the partial pressure of water vapor in the polyimide and in the atmosphere. The very thin gold layer is also mechanically, metallurgically and electrically unstable, and the application of the connection contact

  on this thin layer of gold creates difficulties.

  The aim of the invention is to propose a moisture sensor of the type mentioned above which, with good mechanical, chemical and thermal resistance, has a high sensitivity, a high speed of response,

  high long-term stability and low hysteresis.

  The object of the invention is achieved by virtue of the fact that a structured electrode is formed on the substrate as a base electrode, that the moisture-sensitive thin layer is made of polyimide and placed on the structured base electrode, and that the second electrode is placed on the polyimide layer as the cover electrode and is structured substantially

  complementary to that of the base electrode.

  The moisture sensor produced according to the invention has the advantages of using polyimide for the moisture-sensitive layer, that is to say the good mechanical, chemical, thermal and electrical properties of this material. The polyimide layer may be thin. As the cover electrode is applied to the polyimide layer, the distance between the electrodes is determined by the thickness of this layer, so that with this small thickness, a large initial capacitance is obtained. The disposition

  of the polyimide layer between two structural electrodes

  Complementary results give a high sensitivity and a high speed of response, because the polyimide, in the intervals of the structured cover electrode, is in direct contact with the ambient atmosphere, so that the very thin layer of polyimide can quickly

  saturate with ambient humidity. It is therefore not necessary

  that the saturated cover electrode is permea-

  ble with water, so that it can be of a thickness that gives good mechanical and electrical stability and allows trouble-free application of contacts

  connection, for example by welding.

  According to a preferred embodiment of the invention, between the structured base electrode and the moisture-sensitive polyimide layer is placed a moisture insensitive electrical insulating layer which is preferably an oxide layer formed by surface oxidation. of the structured base electrode. The polyimide layer

  can then be extremely thin,

  less than 1 um, without any danger of conductivity

  DC current between the electrodes. The humidity sensor thus arranged has a sensitivity and a speed

  particularly large response.

  A preferred method of manufacturing the moisture sensor

  of the present invention is characterized in that a thin layer of the metal of the base electrode is applied.

  on an insulating substrate insensitive to moisture and struc-

  according to the desired shape of the base electrode, that a thin layer of a synthesis intermediate of the

  polyimide is applied to the surface of the

  green by the structured base electrode and is structured

  according to the desired shape of the moisture-sensitive layer

  that the polyimide synthesis intermediate of the structured layer is imidized to be converted to moisture sensitive polyimide, and that a thin layer of the metal of the cover electrode is applied to the surface of the substrate covered by the base electrode and the moisture-sensitive polyimide layer and is structured according to the desired shape of the cover electrode. In the process of the invention is used the fact that the layer consisting of the synthetic intermediate of the polyimide is still soluble and can therefore easily be structured, while this would hardly be possible in the case of the insoluble polyimide of the final layer sensitive to moisture. The structuring of the moisture-sensitive layer is favorable for its use with structured electrodes; in particular, a

  appropriate structuring of the moisture-sensitive layer

  This allows you to further increase its response speed.

  A process for forming polyimide from an acid-modified polyamide which serves as a synthetic intermediate for polyimide is described in the article "Aromatic Polypyromellitimides from Aromatic Polyamic Acids" by Sroog, Endrey, Abramo, Berr, Edwards and Olivier appeared in the journal "Journal of Polymer Science", part A, Vol.

3, 1965, pages 1373 to 1390.

  Other features and advantages of the invention

  will emerge from the following description of examples

  embodiment with the aid of the appended drawings, in which: FIGS. 1a and 1b are a flowchart of the phases

  of a preferred method of manufacturing a humidity sensor

ity,

  Figures 2a to 2h are sectional views of various

  the stages of manufacture of the moisture sensor shown in Figure 5 by the method of Figures 1a and 1b; Fig. 3 is a sectional view corresponding to Fig. 2h of a modified embodiment of the humidity sensor; FIG. 4 is a view from above of the base electrode obtained at a stage of manufacture, and

  FIG. 5 is a top view of the humidity sensor.

finished.

  The manufacture of a preferred embodiment of the humidity sensor, shown from above in FIG. 5, will be described using the flowchart of FIGS.

  Figures la and lb and sections of Figures 2a to 2h.

  In phase I of the process, a thin layer of tantalum 2 is applied by cathodic sputtering onto a substrate 1 of electrically insensitive and mechanically insensitive material insensitive to moisture (FIG. 2a). For this substrate 1 is preferably used a glass of high purity and excellent surface state containing no alkali metals. The layer of tantalum

  may have for example a thickness of 0.15 pm.

  According to the usual thin film technology, on the substrate 1 are simultaneously produced a large number of humidity sensors which are separated from each other.

  each other at the end of the manufacturing process. For simplicity

  proud, we describe below the manufacture of only one of

these sensors.

  In phase II, the tantalum layer 2 is structurally

  for the formation of the base electrode. it

  can be done in the usual way by photolithography.

  For this, we first form on the tantalum layer 2 an attack mask covering the entire surface of a layer of photoresist about 1.5 microns thick, then exposes the varnish to light at

  through a pattern, it is developed and hardened.

  Then, the layer 2 tantalum not covered by the mask is removed by wet etching. After etching and removing the mask, the tantalum layer 2 has the desired shape of the base electrode 10 shown in FIG.

  base 10 is a comb-shaped electrode comprising

  as a border 11 of o leave a large number of elongated narrow fingers 12 between which there are

  13. At one end, border 11 is

  forms an enlarged connection range. 14. The representative

  Figure 4 is very simplified for clarity. In reality, the number of fingers 12 of the comb-shaped electrode is much larger; the width

  fingers 12 and those of the intervals 13 are approximately

5 to 10 μm.

  Figure 2b shows a section of three fingers

12 of the base electrode 10.

  In phase III is made, after covering the connection pad 14 of an oxidation protection mask, a surface oxidation of the tantalum of the base electrode. This surface oxidation is done by anodizing with a voltage of about 100 V to a depth of about 0.05 μm, so that under the oxide layer a metal tantalum thickness of about 0.1 μm remains. The tantalum oxide Ta205 formed by the anodization occupies a larger volume than the metal, so that the tantalum oxide layer

  3 formed (Figure 2c) has a thickness of about 0.15 μm.

  The oxide layer 3 constitutes a passivating coating which is absolutely impervious and highly insulating. This provides the advantage that the base electrode 10 is practically

  chemically unassailable and therefore extremely stable.

  From an electrical point of view, in the finished moisture sensor, any direct current conductivity of the capacitor structure is avoided. From a physical point of view, this means that the finished moisture sensor has excellent linearity and stability,

  especially at high temperatures and at a humid

high relative density.

  In phase IV, the entire surface of the substrate 1 and the base electrode 10 formed thereon is covered with a layer 4 (FIG. 2d) of a synthetic intermediate of the polyimide from which will be formed later by imidization the moisture-sensitive polyimide. This polyimide synthesis intermediate is a high purity acid modified polyamide which is applied by spraying in a layer of about 1.0 to 2.0 μm thick. The layer 4 is then, in phase V, dried at 90 to 120 C. The small thickness of the layer is possible in particular thanks to the good

  surface condition of the glass substrate 1.

  Phases VI to VIII serve to structure the subsequent moisture-sensitive layer. This structuring is done again by the usual photolithographic process. For this, in phase VI, a layer 5 (FIG. 2e) of positive photoresist

  about 1.5 μm in thickness is applied by projection

  This layer of photoresist is, in phase VII, dried for about 30 minutes at 90 ° C. In phase VIII, the layer of photoresist is exposed to light through a pattern 6 in places where it must not be exposed. later exist a moisture-sensitive layer. It is in particular the connection pads 14 of all the base electrodes formed on the glass substrate which must be disengaged for the application of the electrical contacts, and also, in the example described here, the intervals 13 between the fingers 12. of the base electrode 10. After the exposure, the photosensitive varnish, in phase IX, is developed, that is to say that the exposed parts of the

  photoresist 5 are removed by dissolution.

  The developer used for this also dissolves the underlying parts of layer 4, while the parts of layer 4 located under non-coated areas

  exposures of the photoresist layer 5

  tent. The attack required to structure the moisture-sensitive layer is therefore made in one

  operation with the development of the photoresist.

  After the removal in the X phase of the remaining unexposed part of the photoresist, is made in phase XI the imidization of the polyamide modified at

  the acid remaining from layer 4 by heat treatment

  than 300 to 400 C for about 1 hour, thanks to which

  the moisture-sensitive polyimide layer is obtained.

  The resulting structure corresponds to the section of Figure 2f. The fingers 12 of the comb-shaped base electrode 10 which remain from the tantalum of the layer 2 (as well as the edge 11 not visible in FIG. 2f) are, on their surface which is not in contact with the substrate 1, entirely surrounded by the layer 3 of tantalum oxide, and this layer of tantalum oxide is itself wrapped with a layer 7 of moisture-sensitive polyimide. In the intervals 13 between

  fingers 12, on the other hand, the glass substrate is exposed.

  During the imidization, the thickness of the layer decreases, so that the polyamide layer having the thickness of about 1 to 2 pm indicated above gives a layer

  of polyimide of about 0.5 to 1.0 μm thick.

  Then takes place the formation of the electrode of

  cover which constitutes the second electrode of the con-

  densifier, whose first electrode is the base electrode 10 and the dielectric is formed by the moisture-sensitive layer 7. To form the cover electrode, it begins by coating the entire surface of the structure obtained so far 'a layer 8 of the

  cover electrode material (Figure 2g).

  This layer 8 may be a single metal layer or may consist of several metal layers

  applied successively. When the cover electrode

  must be in gold, it is necessary to have a better adhesion of the gold layer, first apply a thin layer of nickel-chromium (thickness 0.02 pm) and only

  then the gold layer (thickness 0.2 μm). For simplicity

  proud, the different layers of layer 8 are not

shown in the drawings.

  In phase XIII, the metal layer 8 is

  structured for the formation of the cover electrode

  ture. This is done again by the photolitho-

  usual graph of the manner explained above. After

  attack and removal of the mask, the metallic layer

  that 8 has the desired shape of the cover electrode

  which is shown from above in FIG.

  The cover electrode 20 is also an electrode

  comb-shaped having a border 21 of o par-

  a large number of elongated narrow fingers 22 between which there are gaps 23. At the edge 21 of the cover electrode 20 attaches a connection pad 24 which is adjacent to the connection pad 14 of the electrode 10. As shown in Figure 2h, showing the section of several fingers

  of the cover electrode, the fingers 22 of the elec-

  Cover trode are above the intervals 13 of the base electrode, while gaps 23 of the cover electrode are above the fingers 12 of the base electrode. The fingers 22 of the cover electrode are, in the gaps 13 between the fingers 12 of the base electrode, on the glass substrate, and the moisture sensitive layer 7 which covers the fingers of the electrode of the base is exposed in the intervals 23 of the electrode-cover. In the transition zones between the edges of these zones, the moisture-sensitive layer 7 is enclosed between the border zones opposite two fingers 12 and 22. The edges of the fingers 12 and 22 can roughly overlap.

or ride slightly.

  In phase XIV, a two-hour anneal is carried out at 300 ° C. of the structure obtained. The high imidization temperature of 300 to 400 ° C. makes it possible to use the high annealing temperature of 300 ° C., so that the finished moisture sensor will have an excellent resistance to aging. In phase XV is made division into different moisture sensors by splitting and breaking of the glass substrate. Then, in phase XVI, at the two contact pads 14 and 24 of each sensor are soldered electrical contacts 15 and 25. The sensor

  moisture is then ready for use.

  The described arrangement of the moisture sensor offers significant advantages as far as

  manufacturing that features employment.

  An essential feature of the manufacturing process is that in order to form the moisture-sensitive polyimide layer, a polyamide layer is initially applied, then the polyamide layer is formed and only then is it converted into the moisture-sensitive polyamide. . This known method allows a very simple structuring of the subsequent layer sensitive to moisture according to the usual technology of

  the thin film technique because the modified polyamide

  the acid used as a synthetic intermediate for the polyimide is soluble, while a structuring of the

  polyimide layer would not be possible from this material.

  because the polyimide is very stable chemically, thermally and mechanically. These properties of polyimide, which oppose the easy implementation according to the technology of the thin layers, give in

  However, excellent long-term stability of the

moisture end.

  In addition, very high purity acid-modified polyamide is available, which is absolutely necessary to be able to manufacture a moisture sensor having independent reproducible characteristics.

of the charge.

  In addition, the polyimide, in the process described, can be applied in a very thin layer, for example

  0.5 to 1.0 μm thick, without the mechanical resistance

  nique suffers. The small thickness of the layer greatly increases the speed of response with which the humidity sensor can track variations

relative humidity.

  The parameters of the imidization process, in par-

  In particular, the temperature and the imidization atmosphere used make it possible to influence the characteristic curve of the humidity sensor in the desired direction. It appeared that an imidization temperature of 300 to 400 C

  gave the sensor electrical and physical properties

optimal issues.

  These advantageous properties based on the moisture sensitive material used and the method of manufacture thereof are further enhanced by the sensor structure. As shown in particular in the curve of FIG. 2h, a large part of the surface of the moisture-sensitive layer 7 is exposed directly to the atmosphere, so that moisture can freely penetrate this layer. The part of the moisture-sensitive layer covered by the

  coverage is relatively small and can, in

  bind because of the thin layer thickness, saturate quickly with ambient humidity. The small thickness of the layer gives a large capacity between the electrodes opposite. On the other hand, despite the small thickness of the moisture-sensitive layer, the oxide layer 3 of the base electrode avoids any danger of

  direct current flow between the electrodes.

  Since moisture has direct access to the moisture-sensitive layer, the cover electrode can be thick and impervious to water, which on the one hand

  increases the mechanical strength and electrical stability

  that and on the other hand allows the direct welding of the electrical contact to the cover electrode. In particular, contact welding suppresses the long-term instability of bonded junctions in particular

  at high temperatures and / or in high humidity.

  FIG. 3 is a section similar to FIG. 2h of a slightly modified embodiment of the sensor

  humidity. This differs from the sensor described above.

  Only in that in phases VIII and IX of the process, the polyamide in the gaps 13 between the fingers 12 of the base electrode is not removed. The pattern 6 used for structuring therefore has no days at the intervals 13, but serves only to expose the connection pads

  14. Moreover, the production takes place without

  in the manner previously described, and the top view of the finished moisture sensor corresponds to the

  Figure 5. As a result of the change in

  However, as shown in FIG. 3, the fingers 22 'of the cover electrode, in the gaps 13 between the fingers 12 of the base electrode, rest not on the glass substrate 1, but on the moisture-sensitive polyimide layer 7. The removal of the fine structuring of the polyimide layer simplifies the manufacture of the sensor; however, the response speed of the sensor of FIG. 3 is lower because the diffusion paths of the water under the base electrode are longer. In addition, the sensor of Figure 3 has a smaller base capacity and a smaller range of capacitance variation than that

of Figure 2h.

  The above-described moisture sensor having a tantalum-structured tantalum base electrode coated with a tantalum oxide insulating layer has optimum mechanical, physical, chemical and electrical properties and can be rationally manufactured with very good reproducibility by

  thin film techniques. Modifications

  However, sensor cations and manufacturing processes of this sensor are possible. Thus, the base electrode can be metal other than tantalum which can

  be covered with a layer of insulating oxide by oxidation

  superficial In addition, the insulating layer may be formed other than by surface oxidation, for example by coating an insulating material. In any case, the advantage is that the ultra-thin moisture-sensitive layer can be used without consideration of the electrical isolation and thus to obtain a high sensitivity and a high speed of response. If it is possible to accept some deterioration of these properties, it is also possible to remove the insulating layer and instead make the layer of moisture-sensitive polyimide a little thicker, in which case it will ensure

  electrical isolation between the electrodes. This simplifies

  a little bit of the manufacturing process. In this case, too, because of the arrangement of the moisture-sensitive layer between two structured electrodes, there is still a much better sensitivity and speed of response.

  than in the case of conventional moisture sensors.

  Instead of comb-shaped electrodes, it is possible to use other forms of complementary electrodes at intervals, for example grid-shaped electrodes placed in such a way that the bridges of the grid of the cover electrode are above days of the base electrode and, conversely the base electrode grid bridges are under the days of

the cover electrode.

Claims (16)

  A capacitive moisture sensor comprising a moisture-insensitive substrate (1) and a thin moisture-sensitive polymer layer (7) placed on the substrate between two electrodes (10, 20) structured for the formation of moisture. intervals, characterized by the fact   a structured electrode (10) is formed on the substrate   trat (1) as a base electrode, that the moisture-sensitive thin layer (7) is made of polyimide and placed on the structured base electrode (10), and that the second electrode (20) is placed on the layer of polyimide as the cover electrode and is structured in a   substantially complementary to that of electricity. basic trode.   2. Capacitive moisture sensor according to the   cation 1, characterized in that between the electrode   structured base (10) and the sensitized polyimide layer   moisture is placed an electrically insulating layer that (3) insensitive to moisture.   3. Capacitive moisture sensor according to the claim   2, characterized in that the insulating layer   (3) is an oxide layer formed by oxidation   surface of the structured base electrode   4. Capacitive moisture sensor according to the claim   3, characterized in that the base electrode (10) is tantalum and is covered with an oxide layer   tantalum (3) formed by surface oxidation.   5. Capacitive humidity sensor according to one of the   claims 1 to 4, characterized in that   two electrodes (10, 20) are comb-shaped.   6. Capacitive humidity sensor according to one of   claims 1 to 5, characterized in that the   two electrodes (10, 20) overlap slightly on   along the edges of the intervals (13, 23).   7. Capacitive humidity sensor according to one of the   preceding claims, characterized in that   the thin layer of polyimide (7) is structured according to the shape of the base electrode (10) in such a way that   it does not cover the substrate (1) in the intervals   the (13) of the base electrode. 8. Capacitive humidity sensor according to one of   preceding claims, characterized in that   the thickness of the polyimide layer (7) is included between 0.5 and 1.0 μm.   9. Capacitive humidity sensor according to one of   preceding claims, characterized in that the   substrate (1) is a high purity glass substrate and high quality surface.   10. Method of manufacturing a humidity sensor   according to one of claims 1 to 9, characterized by   the fact that a thin layer (2) of the metal of the base electrode (10) is applied to a moisture insensitive insulating substrate (1) and structured according to the desired shape of the base electrode, a thin layer (4) of a polyimide synthesis intermediate is applied to the surface of the substrate (1) covered by the structured base electrode and is structured according to the shape   of the moisture-sensitive layer, that the   synthesis of the polyimide of the   is imidized to be converted to moisture sensitive polyimide (7), and a thin layer (8) of the metal of the cover electrode (20) is applied to the surface of the substrate covered by the base and the layer (7) of moisture-sensitive polyimide and is structured according to the desired shape of the electrode cover.   11. Method according to claim 10, characterized in that prior to the application of polyimide synthesis intermediate layer (4), the structured base electrode (10) is covered with a layer (3)   insulating electrical insensitive to moisture.   12. The method of claim 11, characterized in that the structured base electrode (10) is covered with a layer (3) of insulating oxide by surface oxidation.   13. Method according to one of claims 10 to 12,   characterized in that the structuring of the layer (4) of synthetic intermediate of the polyimide is made photolithographically by exposure to the   light and development of a layer of photo-lacquer sensitive..   14. Method according to one of claims 10 to 13,   characterized by being used as an intermediate   polyimide synthesis a polyamide modified to acid.   15. Process according to claim 14, characterized in that the imidization of the synthetic intermediate of the polyimide is carried out by heating at a temperature from about 300 to 400 C.   16. Method according to one of claims 10 to 15,   characterized in that after application of the cover electrode (20) is annealed about 300 C.