Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
  • G01N27/307—Disposable laminated or multilayered electrodes

Definitions

• the present invention relates to electrochemical sensors intended to measure the concentration of a chemical substance in a liquid. Such devices find a particularly interesting, but not exclusive, application for the detection of chlorine levels in drinking water or swimming pool water.
• the invention relates, more particularly, to an electrode system for an electrochemical cell, as well as to its manufacturing method.
• Electrochemical sensors of the above type necessarily include a measurement electrode, a reference electrode and a counter electrode.
• Another type of such sensors is also known which further comprises an electrode, known as a generator, and its counter-electrode.
• the pH of the solution can be changed locally by applying a current to the generator electrode.
• a cathodic current will cause the production of OH- ions (the pH then becoming more basic) and, conversely, an anodic current will cause the production of H + ions (the pH then becoming more acidic).
• a counter electrode associated with the generator electrode, a counter electrode associated with the measurement (or working) electrode and a reference electrode are necessary for the production of a complete sensor.
• Document WO 02/095387 describes a structure, represented in FIG. 1, using an electrically conductive substrate 10, advantageously made of doped silicon and the lower face of which is covered with a metallization layer 11. Its upper face is covered with a passivation layer 12 formed of a stack of two sublayers of Si0 2 and Si 3 N, known to have excellent stability in an aqueous medium.
• the passivation layer 12 is pierced with a regular network of circular through openings receiving a conductive micro-disc 13 substantially thicker than the layer and slightly overflowing thereon in order to avoid any contact of the solution to be measured with the substrate.
• micro-discs are formed from the desired electrode material (s), for example, from a stack of layers of titanium, platinum and gold. These layers together constitute the measuring electrode of the system.
• Document WO 90/12314 proposes an arrangement of the same type, but in which the substrate is made of an inert material and the micro-electrodes are constructed on it by depositing successive layers. Contacts at these electrodes are made through openings made in the substrate.
• the object of the present invention is to provide an improved measurement electrode structure, not only from the point of view of its durability and efficiency, but also from that of its production cost. More specifically, the invention relates to an electrode system intended for an electrochemical cell, said cell being of the type which comprises a substrate and, linked to it and close to each other, on the one hand, a measuring electrode formed by a plurality of electrically conductive micro-discs connected to each other, and, on the other hand, a generator electrode formed by an electrically conductive plate pierced with circular openings of diameter greater than that of the micro-discs and arranged so that each opening is concentric with a micro-disk.
• This system is mainly characterized in that: - the substrate is made of an electrically conductive material and is pierced, on its upper face, with a regular network of cavities of substantially cylindrical shape, and - the micro-discs, forming the electrode are contained in these cavities.
• the electrode system according to the invention also has the following characteristics. - It comprises an electrically insulating layer deposited on the substrate and pierced with a plurality of circular openings centered on the cavities and having a diameter smaller than that of the cavities. -
• the micro-discs comprise a thin metallization, deposited at the bottom of each cavity and of diameter substantially equal to that of the openings of the insulating layer, and, optionally, a thick metallization filling, at least partially, the rest of the cavity.
• the thin metallization comprises a stack formed by an adhesion layer and a diffusion barrier layer, which can be respectively of titanium and platinum.
• the thick metallization is formed of the desired electrode material, which can be gold.
• the thick metallization can either be flush with the upper face of the substrate, or be covered with an active layer which is flush with the upper face of the substrate.
• the generating electrode can be either a thin layer of conductive diamond, or a thick conductive layer which forms, around the micro-electrodes, a rounded funnel-shaped wall defining a confinement volume protecting them from the hydrodynamic flow of the solution treat.
• the substrate is made of conductive silicon by doping.
• the present invention also relates to a method for producing the measurement electrode of an electrode system as defined above. It involves the succession of the following operations: - providing a conductive substrate; - deposit the insulating layer on its upper face; - Form on said insulating layer a mask with a network of circular openings whose arrangement and diameter correspond to the network of micro-discs to be produced; - etching the insulating layer through the mask so as to obtain said circular openings; - deeply etching the substrate through these openings so as to obtain said cavities; - depositing said thin metallizations at the bottom of each cavity; and - depositing said thick metallizations on the thin metallizations.
• the method according to the invention also has the following characteristics: - The etching of the insulating layer and the etching of the substrate are carried out by plasma or by wet chemical means. - The deposition of thin metallizations is carried out by vacuum evaporation. - The thick metallizations are deposited by galvanic growth or by catalytic precipitation.
• - Figure 2 is a plan view of the proposed system, without its generator electrode;
• - Figure 3 is a partial sectional view on a large scale along AA of this system;
• - Figure 4 illustrates the following operations for the realization of such a structure; and
• - Figures 5 and 6 illustrate two original ways of making the generator electrode.
• the structure shown in Figures 2 and 3 has an electrically conductive substrate 20 which is in the form of a square plate, typically 2 to 10 mm in side and 0.5 mm thick.
• This plate is advantageously made of silicon made conductive by doping according to techniques well known to those skilled in the art.
• the underside of the substrate 20 is covered with a conductive layer 21 made, for example, of titanium or aluminum or formed of a stack of three sublayers of titanium, platinum and gold.
• the thickness of this layer 21 is approximately 0.2 to 0.3 ⁇ m.
• the substrate 20 is pierced, on its upper face, with a regular network of substantially cylindrical cavities 22, of axis perpendicular to the plane of the substrate.
• these cavities have a diameter of 2 to 20 ⁇ m, a depth of 2 to 20 ⁇ m and are spaced from each other by approximately 40 to 400 ⁇ m.
• each cavity 22 is partially covered with a thin metallization 23 formed by an adhesion layer 23a and a conductive layer 23b resting on the adhesion layer.
• This stack for example, of titanium and platinum, has a thickness of about 0.2 to 0.3 ⁇ m.
• the layer 23b also serves as a diffusion barrier.
• the metallization 23 has a diameter less than 0.5 to 5 ⁇ m than that of the cavity.
• All of the metallizations 23 constitute the measuring electrode of the system.
• the rest of the cavity 22 is filled with a thick metallization 24 formed of the desired electrode material, advantageously gold or any other metal capable of being deposited by galvanic growth, such as platinum, copper, etc.
• the gold deposit 24 only partially fills the cavity 22, the upper part of which then receives an active layer, for example made of Nafion or an electropolymerized conductive polymer, such as Polypyrrole, constituting a selective, catalytic membrane. or protective.
• an active layer for example made of Nafion or an electropolymerized conductive polymer, such as Polypyrrole, constituting a selective, catalytic membrane. or protective.
• the upper face of the substrate 20 is covered with an insulating layer 25, called a passivation layer, which is formed, for example, of a stack of two sublayers of Si0 2 and Si 3 N 4 and has a thickness of approximately 0.1 to 0.3 ⁇ m.
• This layer is pierced with a regular network of circular through openings 26 centered on the cavities 22 and of the same diameter as the thin metallizations 23, therefore of diameter less than that of the cavities.
• FIG. 4 illustrates, by way of nonlimiting example, the main steps of the process for manufacturing the structure shown in FIGS. 2 and 3.
• FIG. 4a The conductive silicon substrate 20 is covered with the passivation layer 25 by a thermal ionization operation followed by chemical vapor deposition, known to those skilled in the art under the name LPCVD.
• FIG. 4b A "photoresist" mask 27 is formed on the passivation layer 25. It has a network of circular openings 28 whose arrangement and the diameter corresponds to the network of thin metallizations 23 to be produced.
• a microelectrode system is thus formed, forming a measurement electrode which, compared with the structure of document WO 02/095387, has the following main advantages: -
• the diameter of the electrodes can be greatly reduced, which decreases accordingly hydrodynamic dependence of the sensor response.
• the cavity provided in the substrate makes it possible to produce thick discs, without increasing their diameter, which greatly prolongs their service life.
• the cavity allows, optionally, to define and anchor an active layer on the micro-electrode.
• Only the upper face of the electrodes is exposed, which reduces their erosion.
• Simple operations allow the etching of the passivation layer to form the openings 26, the etching of the substrate to form the cavities 22 and the elimination of metallization 29 and of the "photoresist" 27. It will be noted, in particular, that a only one mask is necessary (the "photoresist” mask 27) to produce the cavities 22 and to accurately size the micro-discs 23.
• the system according to the invention has the advantage, since its substrate is conductive, of allowing the interconnection of the micro-electrodes in parallel by their rear face, which has the effect amplify the output signal.
• Another notable advantage is that it is much easier, and therefore less expensive, to produce the cavities intended to receive the microelectrodes directly in the substrate.
• the invention makes it possible, by a simple and therefore inexpensive method, to obtain high-performance, thick electrodes of very small diameter defined with great precision.
• the structure which has just been described can be completed by a metal generating electrode disposed around the measurement electrodes, according to the teaching of document WO 02/095387.
• the present invention therefore also aims to eliminate this contamination by replacing the metal generator electrode with a diamond generator electrode 30 deposited, as shown in FIG. 5, on the passivation layer 25.
• the electrode 30 is formed of a thin layer of conductive diamond, which is pierced with circular openings 31 of diameter greater than that of the microelectrodes 23 and arranged so that each opening 31 is concentric with a microelectrode.
• the electrode 30 has a thickness of 0.5 to 5 ⁇ m, while the circular openings 31 have a diameter of 5 to 50 ⁇ m.
• the tests carried out confirmed that the diamond has the largest window of potential in water and makes it possible to generate on it strongly oxidizing species, such as OH radicals, capable of efficiently burning organic matter.
• FIG. 6 showing a structure provided with a thick generator electrode 32 which forms, around the microelectrodes, a rounded funnel-shaped wall defining a confinement volume protecting them from the hydrodynamic flow of the solution to be treated.
• This electrode 32 is advantageously made of gold and deposited by galvanic growth. Typically, its thickness is 10 to 100 ⁇ m and the funnel that it forms has, at the base, a diameter of 10 to 50 ⁇ m.
• Such a structure greatly increases the efficiency of the system, especially when it has to process agitated or high-flow liquids, because the space created around the micro-electrodes allows, not only, a concentration of the generated species, but also, provides a larger generator electrode area.
• the galvanic deposition of the thick electrode 32 is a simple and inexpensive operation.

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• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

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• 2004
  • 2004-01-21 EP EP04405039A patent/EP1557665A1/de not_active Withdrawn
• 2005
  • 2005-01-17 WO PCT/CH2005/000019 patent/WO2005071394A1/fr not_active Ceased
  • 2005-01-17 EP EP05700314A patent/EP1706733A1/de not_active Withdrawn
  • 2005-01-17 US US10/586,668 patent/US7833395B2/en not_active Expired - Fee Related

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