FR2536423A1 - Process for the deposition of electrodes on a substrate made of organic material and devices obtained by this process. - Google Patents

Abstract

Principally a device allowing electrodes to be formed on the surface of a fluoro polymer 19, the electrode deposited in this way having three layers, a bonding layer 16, a conductive layer 17 and a protective layer 18. The most advantageous applications of the invention are electroacoustic transducers.

Description

METHOD OF DEPOSITING ELECTRODES ON MATERIAL SUPPORT
ORGANIC AND DEVICES OBTAINED BY THIS PROCESS
The present invention relates to devices using
work electrodes to induce an electric field within a
electrode support made of dielectric polymer material. Among these
devices, we find piezoelectric transducers whose
external electrodes must have good resistance to
external aggressive agents, be firmly anchored on their support
and offer low electrical resistance.
In particular, an electrode manufacturing process allows
both to submit an organic element, for example a polymer
fluorinated, to an electric field, as well as the three-layer electrodes
that we get by this process and electromechanical transducers
containing organic elements equipped with these electrodes.

Organic materials playing an increasingly important role in
electricity, we are brought to solve the problem of the deposit of electricity
trodes ensuring good electrical conduction, in a safe way, and do not
not degrading over time.

The deposition of electrodes on fluoropolymers presents
particular difficulties.

Fluoropolymers generally have a high sensi
bility in heat and especially when they constitute
materials made piezoelectric by mechanical treatment and / or
electric. Properties may be temporarily altered
or permanent by a rise in temperature. So it can be
produce in particular a fusion, a creep, a withdrawal, a depolari
electrical station, a phase change. Chemical degradation
electrodes are also a problem.

Attachment of metals is difficult on fluoropolymers
like polyvinylidene fluoride, hereinafter referred to as PVF2,
polyvinyl fluoride PVF, polyfluoroethylene propylene PFEP,
polytetrafluoroethylene PTFE, polypropylene PP or poly
vinyl chloride PVC. It is the same for their various copoly mothers and alloys.

 Various types of electrode deposition by vacuum evaporation are known. They all have drawbacks.

 Thus, gold alone does not adhere well to fluorinated polymers, for example PVF2.

 It is therefore necessary to provide a chromium sublayer, to deposit a layer of gold, but this latter deposit, given its duration, requires thermal screening of the source of metal to be evaporated to protect the substrate, gold being a source. very hot. This increases the cost of the operation, because a large part of the evaporated gold is deposited on the screen.

 If we consider the evaporation of nickel-chromium alloys, we are again in the presence of a very hot source. There is still a need for heat shielding. In addition, the layer, once deposited, has a poor electrical conductivity.

 Aluminum has average adhesion to fluorinated organic materials. In addition, this adhesion degrades quickly in the presence of moisture. The layer is completely removed on contact with a basic medium. By cons aluminum has a number of qualities. Its evaporation takes place from a cold source. Aluminum has a low resistivity.

This quality is essential for transmitter transducers, of the loudspeaker or ultrasound transmitter type, as well as for applications using high frequencies. Aluminum has a low density. This quality is particularly important for transducers whose electrodes must have a low inertia such as, for example, the membranes of microphones or loudspeakers.

 The subject of the invention is a method of depositing an electrode on a support made of fluorinated organic material making it possible to collect the electric charges induced on the surface of said support or to induce an electric field therein, characterized in that this process comprises at minus the following three steps: deposition by evaporation under vacuum of a metallic bonding layer, ensuring adhesion of the electrode to its support, deposition by evaporation under vacuum of a second metallic layer, constituting the element main conductor, and the deposition of a third layer ensuring the protection of the second layer. The invention also relates to an electrode making it possible to collect induced electrical charges on the surface of a support made of fluorinated organic material or there induce an electric field characterized by its three-layer structure> a first bonding layer ensuring the bonding of the electrode on said support, a second layer constituting the conductive element principal, and a third layer ensuring the protection of the second layer.

The invention will be better understood by means of the description below and the appended figures, given as nonlimiting examples, among which
- Figure 1 is a sectional view of a vacuum evaporation device for ia realization of electrodes according to the invention;
- Figure 2 is a top view of the device of Figure 1
- Figure 3 shows an electrode structure according to the invention;
- Figure 4 shows the photogravure aune electrode according to the invention.

In Figure 1, we can see a vacuum bell 15. An element of fluorinated organic material to be metallized 1 is attached to a plate 6 acting as a heat sink whose regular rotation is used to standardize the deposit. This rotation favors the evacuation of heat since the element to be metallized can cool when one moves away from the zone where it heats up. The plate 6 is supported by columns 12 which rest on a crown 21 rotated by pulleys 11 also playing the role of supports. In this example, sources 3 and 4 are hot sources intended respectively for depositing a thin bonding layer and a final protective layer. The sources 3 and 4 are associated with a heat shielding means 5. The evaporation device also contains a cold source 2 intended for depositing a layer
metallic interposed between the bonding layer and the protective layer. Source 2 is not provided with screening means. The movable cover 7 is used to hide each source during the preheating period. A means for controlling the thickness of the deposit is also provided, for example, a quartz resonator 8. Masks 9 on the elements to be metallized make it possible, if necessary, to delimit the contour of the electrodes. A effluvation rod 10 is connected to a high-voltage source 14. The sources of metals to be evaporated 2, 3, 4 comprise electrical heating means supplied by an electric generator 13.

 In Figure 2, the screens 5 are partially shown for clarity of the figure. The references used correspond to those used in FIG. 1 to designate the same elements.

 The sectional view of FIG. 3 shows the element to be metallized 19. The bonding layer 16 deposited on the surface of the polymer element 19 comes from a high temperature source. This layer is limited in thickness, because the deposition must be stopped early enough to avoid excessive heating of the polymer element 19. The bonding layer can, for example, be produced by depositing chromium of Snm. The protective layer 18, if it is metallic, can be evaporated immediately after the deposition of the intermediate layer 17.

 The layer 17 ensuring the conduction which is located between the bonding layer and the protective layer is produced from a cooler source, for example an aluminum source, so that the deposition time can be extended without deterioration of the element to be metallized.

 Figure 4 illustrates the photoengraving technique. The photogravure resin 20 is chosen so that it resists the etching solution which must attack the three layers 16, 17, 18 of the electrode. Photoengraving is adopted for applications requiring fine patterns, for example, transducers having electrodes arranged in a network.

 With a view to depositing a bonding layer of chromium or nickel alloy on supports requiring not to prolong their exposure to heat, we opt for a thin deposit, the requirement of satisfactory electrical conductivity being met by virtue of the presence of the main conductive element in a thick layer.

 The conduction layer can advantageously be produced in the form of a thick layer with a metal of low electrical resistivity which can be evaporated at low temperature, for example aluminum or silver.

 The protective layer is not necessarily metallic, it can be. for example a varnish. When it is metallic, it can be made up of a very thin layer of gold, chromium or an alloy of nickel and chromium.

 The following description of the process is given only by way of example of a particularly advantageous embodiment of the invention. It is a three-layer process comprising the deposition of a chromium bonding layer, an electrically conductive layer of aluminum and a protective layer of chromium.

After cleaning, the parts to be metallized are fixed to the underside of the turntable 6, a good thermal conductor. The enclosure is then evacuated at a residual pressure of the order of 4
Pa. Under this pressure and possibly in the presence of neutral gas, the elements to be metallized undergo an electrical effluvation treatment ensuring a decontamination action on the surfaces to which is added, in the case of polymers, a surface degradation action favorable to good hanging of the metallic deposit. This preliminary treatment is carried out by creating a plasma in the enclosure. The enclosure being metallic and connected to earth, use is made of the insulated stripping rod 10 and connected to a high-voltage source 14. The stripping takes place under the following conditions: - pressure = 4 Pa; - electric discharge intensity = 30 mA; - duration of treatment = 3 m, - rotation of the stage: I revolution per second. A much higher current or duration of effluvation causes excessive degradation of the polymer, resulting in its yellowing. After effluvée, the enclosure is evacuated in secondary vacuum until the evaporation pressure of the order of 10 4 Pa. The plate still rotates at the angular speed of the order of one revolution per second. In this way, the heat-sensitive elements are exposed intermittently to the heat radiation of the sources, arranged near the edge of the plate of the enclosure. The rotation also allows uniform deposits to be obtained. These three layers can be deposited successively during a single pumping cycle of the vacuum enclosure.

This structure is particularly well suited to fluoropolymers. The bonding layer in chromium has a very small thickness, typically 5 x 10 ~ 9m. This allows it to be deposited in a short time, and therefore not to overheat the element to be metallized, despite the high temperature of the chromium source (1500 to 1600 K). The electrical conduction layer in this example consists of a deposit of aluminum which is thicker than the deposit of the bonding layer, typically 5 × 10 -8 m. The lower temperature of the source allows such a thickness. Thus a PVF2 element which cannot withstand temperatures above 80 "C without damage lends itself to the evaporation of a conduction layer without thermal shielding of the source.

 The protective layer may have a thickness comparable to that of the bonding layer. It consists of a layer of chrome typically 5 x 10 9m thick. The use of the same metal for the bonding layer and the protective layer allows the same source to be used for the framing layers of the conduction layer.

 Electrodes produced according to the invention, by depositing a layer of chromium, a layer of aluminum and a layer of chromium were compared to the monolayer electrodes of aluminum and of an alloy comprising 50% of nickel and 50 % chromium.

All the electrodes were deposited in a similar manner. In all cases, the total thickness of the layer, as measured by the quartz thickness meter, is between 50 and 60 nm.

 The conduction resistance of the different layers has been measured, using a 4-conductor method, so as to eliminate the influence of the contact resistances.

The following results were obtained square resistance electrodes R Cr / Al / Cr 1.1 Al 1.4
NiCr 41
It is therefore confirmed that the Cr / AL / Cr electrode is a good conductor and that the chromium layers do not disturb the resistance imposed by the central aluminum layer. The difference between the values of R for the Cr / AI / Cr and Al electrodes is probably linked on the one hand to the measurement error, on the other hand to a difference in thickness of the electrode on PVF2.

The NiCr electrode has a much higher resistance. It should be noted that the ratio R (NiCr) / R (Al) is substantially equal to the ratio of the resistivities of these metals:

 The adhesion of the electrodes was tested by peeling off an adhesive tape ("scotch test"), previously applied using a roui on the surface of the electrode.

The results are reported in the table below using the following notation: ++ no alteration
+ some particles torn off punctually
- rags torn off, on the scale of a few mm2 - tearing at least 50% of the surface.

These comparative tests also relate to an electrode
Al / Cr, that is to say without an under layer of chromium.

Conditions under which
The sample was exposed before the Al AlCr Cr / AI / Cr NiCr test of the tape
Crude metallization + - ++ ++ 40 min stay in steam - - + ++ water stay
îOOhà-400C - ++ + stay of
100h at 30 C, 90% - - ++ + relative humidity stay from 100h at 70 "C, 60% - - + ++ relative humidity
This table first of all highlights the poor resistance to humidity of the aluminum electrode. It also appears that the Al / Cr electrode has very poor adhesion, even worse than that of the Al electrode. This is explained by the fact that the Al-PVF2 bond is degraded by the deposition of an upper layer of chromium, probably due to the slight shrinkage which the
PVF2 heated by the chromium source.

 This interpretation is validated by the results obtained on these two electrodes after exposure of the sample to -400 C, or an analogous effect of loss of adhesion occurs due this time to thermal contraction.

In order to evaluate the effectiveness of the upper chromium layer as a protective layer, samples provided with Al or Cr / Al / Cr electrodes were immersed in a potassium solution. The following results were obtained:
Time Concentration Electrodes
the KOH solution for immersion of the electrode
Al I g / l 2 min Disappearance
partial of
the electrode
Al 2 g / I 2 min Disappearance
total of
the electrode Cr / A1 / Cr 2 g / l 2 min Intact Cr / A1 / Cr - 4 g / l 2 min Intact Cr / A1 / Cr 8 g / l 2 min Aspect intact,
resistivity
increased Cr / Aì / Cr 16 g / l 2 min "Quilted" appearance
resistivity X 10 Cr / Aì / Cr 16 g / l 1 GH Only the
bottom layer
from Cr
These results show that the Cr / A1 / Cr electrode remains intact where the AI electrode is completely eliminated. Under the more severe conditions (concentration or immersion time), the upper chromium layer is not completely sufficient to protect aluminum from hydroxide formation. This reaction, however, tends to occur not at the surface, but through the edge where the aluminum is exposed, as shown by the formation of gas bubbles on the edges of the sample. - Note that this action cannot occur in applications of fluoropolymers where the edges of the sample are protected from the adjacent environment, for example by embedding.

 The transducers equipped with electrodes according to the invention have durable electrodes, having the desired geometry, having low electrical resistance and moderate cost. This last characteristic is very important for PVF2 transducers, the price of which is itself moderate.

Claims (10)

R - E V E N D I C A T I O N S  1. A method of depositing electrodes on a support made of fluorinated organic material (1, 19) making it possible to collect induced electric charges on the surface of said support or to induce an external electric field in a support made of fluorinated organic material (1, 19 ), characterized in that this process comprises at least the following steps: deposition by evaporation under vacuum of a bonding layer (16), ensuring the adhesion of the electrode to its support, deposition by evaporation under void of a second layer (17) constituting the main conducting element, the deposition of a third layer 18 ensuring the protection of the second layer.  2. deposition method according to claim 1, characterized in that the layer constituting the main conductive element (17) is made of aluminum.  3. deposition method according to claim 1, characterized in that the layer constituting the main conductive element (17) is made of silver.  4. deposition method according to any one of claims 1, 2 and 3, characterized by the fact that the bonding layer (16) is made of chromium or an alloy of nickel and chromium.  5. deposition method according to any one of claims 1, 2, 3 and 4, characterized in that the protective layer (18) is made of chromium or an alloy of nickel and chromium.  6. deposition method according to any of claims 1, 2, 3 and 4, characterized in that the protective layer (18) is made of gold.  7. deposition method according to any of claims 1, 2, 3 and 4, characterized in that the protective layer (18) is produced by depositing a varnish.  8. deposition method according to any one of the preceding claims, characterized in that the deposition of metals is preceded by effluvage of the surface intended to receive said deposit.  9. Electrode for collecting induced electric charges on the surface of a fluorinated organic material or inducing an external electric field in a fluorinated organic material, characterized by its structure in three layers - superimposed, a first bonding layer (16) ensuring the adhesion of the electrode to its support, - a second layer (17) constituting the main conducting element, a third layer (18) ensuring the protection of the second layer.  10. Electromechanical transducer equipped with at least one electrode according to claim 9, characterized in that it comprises an active element made of piezoelectric polymer serving as a support for said electrode.