WO2007082977A1 - Wire-free chemical sensor based on the connection of an inductor to an eis or emis capacitor, and applications thereof - Google Patents

Wire-free chemical sensor based on the connection of an inductor to an eis or emis capacitor, and applications thereof Download PDF

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Publication number
WO2007082977A1
WO2007082977A1 PCT/ES2007/070009 ES2007070009W WO2007082977A1 WO 2007082977 A1 WO2007082977 A1 WO 2007082977A1 ES 2007070009 W ES2007070009 W ES 2007070009W WO 2007082977 A1 WO2007082977 A1 WO 2007082977A1
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sensor device
eis
wireless sensor
capacitor
solution
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PCT/ES2007/070009
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Spanish (es)
French (fr)
Inventor
Antonio Baldi Coll
Jesús GARCÍA CANTÓN
Angel Merlos Domingo
Carlos DOMÍNGUEZ HORNA
Cecilia JIMÉNEZ JORQUERA
Lia Moreno Codinachs
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Consejo Superior De Investigaciones Científicas
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Publication of WO2007082977A1 publication Critical patent/WO2007082977A1/en

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    • 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
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance
    • G01N27/227—Sensors changing capacitance upon adsorption or absorption of fluid components, e.g. electrolyte-insulator-semiconductor sensors, MOS capacitors
    • 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
    • G01N27/403—Cells and electrode assemblies
    • G01N27/413—Concentration cells using liquid electrolytes measuring currents or voltages in voltaic cells

Definitions

  • the present invention is related to the field of chemical sensors, and more specifically, to wireless sensors for pH measurement and other concentrations of chemical species in solution.
  • Wireless sensors are very useful in applications where electrical connection to the measurement point is not possible or impractical.
  • wireless sensors are being used for monitoring in biomedical and clinical applications. Other fields such as food processing and distribution could also benefit if the manufacturing cost is sufficiently reduced.
  • Typical implementations of these sensors include a sensor device, amplification and transmission electronics, an antenna and a battery [Bravo TM Catheter-free pH Monitoring System, Medtronic Inc., CorTemp TM temperature pill, HQ Inc.].
  • the use of the battery can be avoided if inductive coupling power is used [NORIKA3, RF SYSTEM lab].
  • a simpler alternative, suitable for short ranges, is the wireless sensors based on passive resonators.
  • LC sensors which are based on the variation of inductance or capacitance of the components of an inductive-capacitive resonator (LC), and therefore of their resonance frequency, in response to changes in magnitude to measure.
  • Wireless sensor measurement is achieved by measuring the impedance of an external inductor magnetically coupled to the sensor. At the resonance frequency of the sensor the impedance of this inductor suffers a disturbance that can be detected. By scanning in frequency it can be observed, for example, that the phase of said impedance has a minimum that coincides with the resonant frequency.
  • Another form of detection consists in subtracting the impedance measured in the presence of the measurement sensor in the absence thereof and obtaining the maximum of the resulting module.
  • the only LC type chemical sensors that have been proposed previously are based on stimulus sensitive hydrogels ["Implantable medical sensor system", US Patent 6,201,980 Bl, 13-3-2001, M. Lei, A. Baldi, T. Pan, Y. Gu, RA Siegel, B. Ziaie, "A hydrogel-based wireless chemical sensor", IEEE MEMS 2004 Technical Digest, Maastricht 2004, pp. 391-394], and have very long response times.
  • the sensor proposed in the present invention is based on variable capacitors type EIS (Electrolyte-Insulator-Semiconductor), which are pH sensitive [MJ. Sch ⁇ ning, M. Thust, M. Müller-Veggian, P. Kordos, H.
  • One aspect of the invention relates to a wireless sensor device, to determine the pH of a solution, constituted, at least, by the following elements: i) An LC type resonator formed by an inductor and a capacitor with a type structure
  • EIS Electrode-Insulator-Silicon
  • the EIS capacitor has, in turn, a reference electrode that makes the electrical connection between the inductor and the solution whose pH is to be measured. In addition, it may have an additional low impedance electrode, which is decoupled in direct current from the reference electrode.
  • An external inductor magnetically coupled to the resonator inductor, but without the need for physical contact between the two, which is used to measure the resonance frequency.
  • This device can be modified, to measure the concentration of chemical species in solution, depositing, on the surface of the EIS capacitor insulator, an additional layer, said layer being constituted by a conductive material at whose interface with the solution a difference of Variable electrochemical potential with the chemical species to be determined.
  • Another aspect of the invention relates to the use of this device as a wireless pH sensor, or chemical species in solution, or as a capacitive type chemical sensor.
  • the present invention protects a device based on the use of capacitors type EIS (Electrolyte-Insulator-Semiconductor) - pH sensitive - and type EMIS (Electrolyte-Membrane-Insulator-Semiconductor) - sensitive to other chemical species in solution - in wireless sensors type LC, by connecting in parallel with an inductor.
  • EIS Electricallyte-Insulator-Semiconductor
  • EMIS Electrometic-Membrane-Insulator-Semiconductor
  • the advantages of the wireless chemical sensor proposed here compared to other wireless chemical sensors existing or proposed previously are: 1) simplicity of technology, 2) low cost, 3) speed of response, and 4) little or no material toxicity (no batteries needed).
  • An object of the invention consists of a wireless sensor device, hereinafter wireless sensor device of the invention, to determine the pH, or the concentration of ions in solution, comprising at least the following elements: i) A type resonator LC formed by an inductor and a capacitor with type structure
  • EIS Electrode-Insulator-Silicon
  • the EIS capacitor has, in turn, a reference electrode that makes the electrical connection between the inductor and the solution whose pH is to be measured, and ii) An external inductor magnetically coupled to the inductor of the resonator, but without the need for physical contact between the two, which is used to measure the resonance frequency.
  • Figure 1 shows the basic scheme of the sensor device.
  • the capacitor capacity of i) varies depending on the pH due to its structure type EIS (Electrolyte-Insulator-Silicon) where the electrolyte is the solution to be measured.
  • An EIS type structure responds to a potential change applied between the silicon and the solution according to a C-V curve (Capacity depending on Voltage) that has a steep slope area. Said zone of the C-V curve, called the emptying zone, coincides with the voltages for which the silicon closest to the insulator is empty of load carriers.
  • the emptying zone of the C-V curve of an EIS structure moves towards higher or lower voltage values when the pH increases or decreases.
  • the EIS structure When the EIS structure is polarized at a fixed voltage within the emptying zone, it behaves like a capacitor in which the capacity varies depending on the pH. When the EIS capacitor and the inductor are connected in parallel, the direct current (DC) voltage applied to the capacity automatically becomes OV. It is necessary, therefore, that the EIS structure has the emptying area centered on OV for a neutral pH, so that when the pH increases or decreases there is a change in the capacity of the structure as shown in Figure 2 EIS capacitors have previously been used as capacitive sensors, but never as part of a wireless sensor.
  • the present invention enables its use, for the first time, as wireless sensors, with the functional advantages that this entails, by connecting in parallel with an inductor.
  • the capacitor's series resistors and the resonator inductor described in i) must be sufficiently small for the resonator quality factor to be greater than 1.
  • a particular object of the invention is the wireless sensor device of the invention characterized in that the insulator of the EIS capacitor of i) is constituted by one or more layers of pH sensitive material, by way of illustration and without limiting the scope of the invention, belonging to the following group: silicon oxide, silicon nitride, tantalum oxide or aluminum oxide.
  • a particular embodiment of the invention is the wireless sensor device of the invention in which the reference electrode of the EIS capacitor is of the Ag / AgCl type, with internal 3M KCl solution.
  • the series resistance associated with the EIS capacity is mainly formed by the resistance of the liquid junction of the reference electrode and the resistance of the solution between this liquid junction and the capacity insulator surface.
  • the resistance of a liquid junction typically takes values around 5 k ⁇ and that of the solution depends on the geometry and relative arrangement of the reference electrode and the capacity, being able to acquire values of the order of kilo-ohms for little conductive solutions.
  • Another particular object of the invention is the wireless sensor device of the invention in which the EIS capacitor has, in addition to the reference electrode, a low impedance electrode, in order to obtain a lower series resistance and thus increase the power factor. resonator quality.
  • This second low impedance electrode must be decoupled in direct current from the reference electrode to prevent it from interfering with the direct current potential set by the latter in the solution.
  • Another particular embodiment of the invention is the wireless sensor device of the invention with additional low impedance electrode, wherein said electrode is on the surface of the capacitor insulator and has a configuration consisting of a layer of conductive material, with such geometry. that minimizes the average distance between electrode surface points and capacitor insulator surface points.
  • FIG. 3 An example of such geometry, which is shown in Figures 3 and 4, consists of a set of long tracks or fingers arranged in parallel and with separation equal to that of its width, such that the areas of the capacitor insulator exposed to the The solution has dimensions equal to those of the electrode fingers, that is, the electrode and capacitor areas are interdigitated.
  • Another particular embodiment of the invention is the wireless sensor device of the invention with an additional low impedance electrode, wherein said electrode is covered with a thin layer of insulator to be decoupled in direct current. with respect to the reference electrode. This layer functions as a capacitor between the electrode and the solution.
  • the low impedance electrode is isolated from the capacitor's silicon substrate by another insulating layer, which will also have an associated capacity.
  • the total capacity of the EIS structure with a low impedance electrode, measured between said electrode and the capacitor's silicon substrate, is formed by several capacities in series and in parallel. Specifically, by the capacities of the insulating layer between electrode and the solution, the capacity of the solution itself, the capacity of the insulator between the solution and the substrate and the capacity of the silicon in the empty area of carriers, all of them in series, and the capacity associated with the insulating layer between the electrode and the substrate in parallel to the others. Of all of them, only the capacity of the empty area of silicon carriers depends on the potential applied to the structure. In order for the structure to have sufficient sensitivity and range of variation, the geometry of the different layers that compose it must be properly designed, so that the variations in the capacity of the silicon translate into measurable variations in the total capacity of the structure.
  • the capacity of the solution is in parallel with its resistance.
  • the resistance of the solution depends on its conductivity.
  • the sensor's resonance frequency must be chosen so that, for the range of conductivities in which you want to work, the impedance of the solution has a fully capacitive or fully resistive behavior.
  • the capacitor structure of i) described above is sensitive to the pH of the solution.
  • depositing on the surface of the insulator membranes sensitive to other chemical species, the capacity, as well as the wireless sensor that we build with it, can be sensitive to these other chemical species in solution.
  • the variations of the electrochemical potential established between the membrane and the solution are transmitted to the surface of the insulator, producing variations in the structure's capacity in the same way as with the membraneless sensors.
  • Most membranes developed so far have low dielectric constant and high resistivity. This could make the resistance
  • the capacitive structure series is excessively high and the necessary quality factor cannot be obtained.
  • alternating currents also pass through the capacity associated with the insulating layer between the electrode and the silicon substrate.
  • the total capacity that goes directly from the low impedance electrode to the capacitor's silicon substrate is also dependent on the electrochemical potential. That is, if the membrane is sufficiently resistive, it is the variations of this capacity that contribute mostly to the variation of the total capacity of the structure. In this way, ohmic losses by current passage through said membrane are avoided and a high quality factor of the resonator is achieved.
  • Another particular object of the invention is the wireless sensor device of the invention in which an additional layer is deposited on the surface of the EIS capacitor insulation, said layer being constituted by a conductive material at the interface of which the solution is It establishes a variable electrochemical potential difference with the chemical species to be determined.
  • the inductor and the reference electrode necessary to form the sensor device can be manufactured independently or can be integrated, by means of microfabrication techniques, into the same die or silicon chip with which the EIS capacity is formed.
  • a particular object of the invention is the wireless sensor device of the invention in which the reference electrode and the inductor of the LC type resonator are integrated in the same die or silicon chip with which the EIS capacity is formed, so that The inductor is formed by a spiral-shaped conductive track, the inner end of which is connected to the low impedance electrode and the outer end of which is connected to the silicon substrate, and the reference electrode is formed by a layer of Ag / AgCl in contact with the rear face of the silicon die, on which the internal KCl solution reservoir and the liquid bond towards the external solution are formed.
  • Another object of the invention is the use of the wireless sensor device of the invention as a chemical sensor.
  • Another particular object of the invention is the use of the wireless sensor device of the invention as a wireless pH sensor.
  • Another particular object of the invention is the use of the sensor device of the invention, in which an additional layer, as a wireless sensor of chemical species in solution, has been deposited on the surface of the EIS capacitor.
  • another particular object of the invention is the use of the sensor device of the invention, with double electrode and additional layer on the insulator surface of the EIS capacitor, as a capacitive type chemical sensor and, therefore, not wireless.
  • the capacity is measured with wires directly connected to it, so a resonator that can be measured remotely is not required.
  • FIG. 1 is a simplified diagram of the sensor where you can see the inductor (1), a section of the EIS capacity (2) formed by a silicon die covered on its front face by an insulator (3) below from which an empty region of load carriers (4) is formed and covered on its underside by an aluminum layer (5).
  • the inductor has one of its terminals (6) connected to the rear face of the silicon die, and the other terminal (7) connected to an Ag / AgCl reference electrode (8) formed by a chlorinated silver wire (9) in a cavity (10) filled with 3M KCl that communicates with the outside through a liquid junction of porous material (11).
  • FIG. 1 is a graph showing the characteristic characteristic curves of Capacity - Voltage for a capacitive structure of type EIS at different pH of the solution.
  • Figure 3 is a three-dimensional representation of the EIS capability with low impedance electrode (12) isolated from the solution.
  • the structure of said electrode is formed by tracks or fingers of conductive material (15) interspersed with areas of EIS capacity of the same dimensions (16). Additionally, the electrode has an area (18) without an insulating layer for interconnection.
  • Figure 4 is a representation of the section of the structure of Figure 3 in which the fingers of conductive material of the low impedance electrode (12), the insulating layer between it and the silicon substrate (13) can be seen, the insulating layer between the substrate and the solution and between the electrode and the solution (14), and the empty region of carriers that is formed under the EIS capacity (17) and also penetrates below the electrode fingers.
  • a wireless sensor device was constructed in the following way: By means of microfabrication techniques typical of microelectronic technologies, EIS-type capabilities with low impedance electrode such as those in Figures 3 and 4 were manufactured, where the materials and dimensions were as follows: Silicon die is 3mm x 3mm x 0.5mm p-type silicon with boron impurity concentration of 5 x 10 14 .
  • the electrode fingers (15) and the EIS capacity (16) are both 3 ⁇ m wide and 1.5mm long, with the electrode 218 fingers and the EIS capacity 217 fingers.
  • the low impedance electrode is made of highly doped polysilicon (until degeneration) and is 450nm thick.
  • the insulating layer between the electrode and the substrate is silicon oxide of thermal type and has a thickness of 800nm.
  • the insulator between the electrode and the solution is formed by a double layer of silicon nitride type LPCVD (Low Pressure Chemial Vapor Deposition) of lOOnm thick on silicon oxide of thermal type of approximately 120nm thick.
  • the insulator between the silicon substrate and the solution is formed by a double layer of silicon nitride of LPCVD type lOOnm thick over silicon oxide of thermal type 78nm thick.
  • the interconnection area of the low impedance electrode is lOO ⁇ m x lOO ⁇ m and is covered by a 0.5 ⁇ m layer of aluminum.
  • the face of the silicon die opposite the electrode is also covered by a 0.5 ⁇ m layer of aluminum.
  • the interconnection of the EIS capacity, that is, of the low impedance electrode and the silicon substrate, as well as the protection of said interconnection of the solution is Performed using the following encapsulation technique:
  • the silicon die was glued on its metallic face with aluminum to a PCB strip (printed circuit) on a copper track.
  • the bonding was done with a conductive epoxy to ensure the electrical connection between the substrate and the copper track of the PCB.
  • the interconnection area of the low impedance electrode was connected to another PCB track using the wire bonding technique.
  • an encapsulating polymer was applied on the edges of the silicon die and on the metal areas of the wire bonding and the PCB tracks, leaving only a small area of the tracks unencapsulated for the inductor weld.
  • the terminals of a 130 ⁇ H inductor were then soldered to the two PCB tracks and the connecting wire of a miniaturized reference electrode was welded to the PCB track that connected to the low impedance electrode.
  • the EIS capacity set, inductor and reference electrode was re-encapsulated with polymeric material (19) to protect the electrical connections of the solution, leaving only the liquid junction of the reference electrode (10) and the PCB part exposed to the environment which contains the EIS capability.
  • the sensor was introduced consecutively in 4 plastic containers with buffer solutions of pH equal to 2.0, 4.0, 8.7 and 11.5.
  • buffer solutions of pH equal to 2.0, 4.0, 8.7 and 11.5.
  • an inductor magnetically coupled to the sensor inductor was used.
  • Said external inductor was connected to an impedance measurement circuit with which the frequency at which the phase of its complex impedance had a minimum was identified, and which coincides with the resonance frequency of the sensor.

Abstract

The present patent describes a wire-free chemical sensor based on LC resonators formed by inductors connected to EIS (Electrolyte-Insulator-Semiconductor) capacitors - which are pH-sensitive - and EMIS (Electrolyte-Membrane-Insulator-Semiconductor) capacitors - which are sensitive to other chemical species in solution. The electrical connection to the solution (and to the membrane) is achieved by means of two electrodes, one being a reference electrode for obtaining a stable DC potential and the other being of low impedance for obtaining a high resonator quality factor. An external inductor coupled magnetically to the resonator’s inductor is used to measure the resonant frequency remotely. The advantages offered by this wire-free chemical sensor are its simplicity, low cost, response speed and the non-toxic nature of the materials (it does not use batteries).

Description

TÍTULOTITLE
SENSOR QUÍMICO INALÁMBRICO BASADO EN LA CONEXIÓN DE UN INDUCTOR CON UN CAPACITOR TIPO EIS Ó EMIS Y SUS APLICACIONESWIRELESS CHEMICAL SENSOR BASED ON THE CONNECTION OF AN INDUCTOR WITH AN EIS OR EMIS TYPE CAPACITOR AND ITS APPLICATIONS
SECTOR DE LA TÉCNICASECTOR OF THE TECHNIQUE
La presente invención está relacionada con el campo de los sensores químicos, y más concretamente, con sensores inalámbricos para medida de pH y otras concentraciones de especies químicas en disolución.The present invention is related to the field of chemical sensors, and more specifically, to wireless sensors for pH measurement and other concentrations of chemical species in solution.
ESTADO DE LA TÉCNICASTATE OF THE TECHNIQUE
Los sensores inalámbricos son de gran utilidad en aplicaciones donde la conexión eléctrica al punto de medida no es posible o es poco práctica. Actualmente, los sensores inalámbricos están siendo utilizados para monitorización en aplicaciones biomédicas y clínicas. Otros campos como el de procesamiento y distribución de alimentos se podrían también beneficiar si el coste de fabricación se reduce suficientemente. Implementaciones típicas de estos sensores incluyen un dispositivo sensor, electrónica de amplificación y transmisión, una antena y una pila [Bravo™ Catheter-free pH Monitoring System, Medtronic Inc., CorTemp™ temperature pill, HQ Inc.]. El uso de la pila se puede evitar si se utiliza la alimentación por acoplamiento inductivo [NORIKA3, RF SYSTEM lab]. Una alternativa mas simple, adecuada para alcances cortos, son lo sensores inalámbricos basados en resonadores pasivos. Un ejemplo de ellos son los sensores LC, los cuales se basan en la variación de inductancia o capacitancia de los componentes de un resonador inductivo-capacitivo (LC), y por lo tanto de su frecuencia de resonancia, en respuesta a cambios en la magnitud a medir. La medida inalámbrica del sensor se consigue midiendo la impedancia de un inductor externo acoplado magnéticamente al sensor. A la frecuencia de resonancia del sensor la impedancia de este inductor sufre una perturbación que puede ser detectada. Haciendo un barrido en frecuencia se puede observar, por ejemplo, que la fase de dicha impedancia tiene un mínimo que coincide con la frecuencia de resonancia. Otra forma de detección consiste en restar la impedancia medida en presencia del sensor de la medida en ausencia del mismo y obtener el máximo del modulo de la resultante. Diversos sensores LC han sido propuestos a lo largo de las últimas décadas debido a la facilidad de realizar sensores capacitivos sensibles a diferentes parámetros ambientales. En la literatura se puede encontrar referencia a sensores LC sensibles a presión, temperatura, humedad y otras magnitudes físicas ["System and methodfor the wireless sensing of physical properties", patente US6111520, 29-08-2000, C. C. Collins, "Miniature passive pressure transensor for implanting in the eye, " IEEE Trans. Biomed. Eng., vol. 14, pp. 74-83, 1967, Timothy J. Harpster, Brian Stark and Khalil Najafi, "A passive wireless integrated humidity sensor", Sensors and Actuators A: Physical, vol. 95 (2002) pp.100- 107]. Los únicos sensores químicos tipo LC que se han propuesto con anterioridad se fundamentan en hidrogeles sensibles a estímulos ["Implantable medical sensor system ", patente US 6,201,980 Bl, 13-3-2001, M. Lei, A. Baldi, T. Pan, Y. Gu, R.A. Siegel, B. Ziaie, "A hydrogel-based wireless chemical sensor", IEEE MEMS 2004 Technical Digest, Maastricht 2004, pp. 391-394], y tienen tiempos de respuesta muy largos. El sensor que se propone en la presente invención se basa en capacitores variables tipo EIS (Electrolito- Aislante -Semiconductor), las cuales son sensibles al pH [MJ. Schδning, M. Thust, M.Müller-Veggian, P. Kordos, H. Lüth "A novel silicon-based sensor array with capacitive EIS structures", Sensors and Actuators B, vol. 47 (1998), pp. 225-230] y capacitores tipo EMIS (Electrolito- Membrana-Aislante-Semiconductor) que son sensibles a otras especies químicas [A. Poghossian, D.-T. Maia, Yu. Mourzina, MJ. Schδning, "Impedance effect of an ion- sensitive membrane: characterisation of an EMIS sensor by impedance spectroscopy, capacitance—voltage and constant—capacitance method", Sensors and Actuators B, vol. 103 (2004), pp. 423-428]. La estructura de los capacitores EIS y EMIS ha sido utilizada como sensores químicos de tipo capacitivo, pero nunca en sensores inalámbricos de tipo LC.Wireless sensors are very useful in applications where electrical connection to the measurement point is not possible or impractical. Currently, wireless sensors are being used for monitoring in biomedical and clinical applications. Other fields such as food processing and distribution could also benefit if the manufacturing cost is sufficiently reduced. Typical implementations of these sensors include a sensor device, amplification and transmission electronics, an antenna and a battery [Bravo ™ Catheter-free pH Monitoring System, Medtronic Inc., CorTemp ™ temperature pill, HQ Inc.]. The use of the battery can be avoided if inductive coupling power is used [NORIKA3, RF SYSTEM lab]. A simpler alternative, suitable for short ranges, is the wireless sensors based on passive resonators. An example of these are the LC sensors, which are based on the variation of inductance or capacitance of the components of an inductive-capacitive resonator (LC), and therefore of their resonance frequency, in response to changes in magnitude to measure. Wireless sensor measurement is achieved by measuring the impedance of an external inductor magnetically coupled to the sensor. At the resonance frequency of the sensor the impedance of this inductor suffers a disturbance that can be detected. By scanning in frequency it can be observed, for example, that the phase of said impedance has a minimum that coincides with the resonant frequency. Another form of detection consists in subtracting the impedance measured in the presence of the measurement sensor in the absence thereof and obtaining the maximum of the resulting module. Various LC sensors have been proposed over the last decades due to the ease of making capacitive sensors sensitive to different environmental parameters. In the literature, reference can be found to LC sensors sensitive to pressure, temperature, humidity and other physical quantities ["System and method for the wireless sensing of physical properties", US6111520, 08-29-2000, CC Collins, "Miniature passive pressure transensor for implanting in the eye, "IEEE Trans. Biomed Eng., Vol. 14, pp. 74-83, 1967, Timothy J. Harpster, Brian Stark and Khalil Najafi, "A passive wireless integrated humidity sensor", Sensors and Actuators A: Physical, vol. 95 (2002) pp. 1500 - 107]. The only LC type chemical sensors that have been proposed previously are based on stimulus sensitive hydrogels ["Implantable medical sensor system", US Patent 6,201,980 Bl, 13-3-2001, M. Lei, A. Baldi, T. Pan, Y. Gu, RA Siegel, B. Ziaie, "A hydrogel-based wireless chemical sensor", IEEE MEMS 2004 Technical Digest, Maastricht 2004, pp. 391-394], and have very long response times. The sensor proposed in the present invention is based on variable capacitors type EIS (Electrolyte-Insulator-Semiconductor), which are pH sensitive [MJ. Schδning, M. Thust, M. Müller-Veggian, P. Kordos, H. Lüth "A novel silicon-based sensor array with capacitive EIS structures", Sensors and Actuators B, vol. 47 (1998), pp. 225-230] and EMIS type capacitors (Electrolyte-Membrane-Insulator-Semiconductor) that are sensitive to other chemical species [A. Poghossian, D.-T. Maia, Yu. Mourzina, MJ. Schδning, "Impedance effect of an ion-sensitive membrane: characterization of an EMIS sensor by impedance spectroscopy, capacitance — voltage and constant — capacitance method", Sensors and Actuators B, vol. 103 (2004), pp. 423-428]. The structure of the EIS and EMIS capacitors has been used as capacitive type chemical sensors, but never in wireless LC type sensors.
DESCRIPCIÓN BREVEBRIEF DESCRIPTION
Un aspecto de la invención se refiere a un dispositivo sensor inalámbrico, para determinar el pH de una disolución, constituido, al menos, por los siguientes elementos: i) Un resonador tipo LC formado por un inductor y un capacitor con estructura tipoOne aspect of the invention relates to a wireless sensor device, to determine the pH of a solution, constituted, at least, by the following elements: i) An LC type resonator formed by an inductor and a capacitor with a type structure
EIS (Electrolito-Aislante-Silicio), conectados en paralelo, que produce una señal en forma de frecuencia de resonancia. El capacitor EIS dispone, a su vez, de un electrodo de referencia que realiza la conexión eléctrica entre el inductor y la solución cuyo pH se pretende medir. Además, puede disponer de un electrodo adicional de baja impedancia, el cual esta desacoplado en corriente continua del electrodo de referencia. ii) Un inductor externo acoplado magnéticamente al inductor del resonador, pero sin necesidad de contacto físico entre ambos, que se utiliza para medir la frecuencia de resonancia.EIS (Electrolyte-Insulator-Silicon), connected in parallel, which produces a signal in the form of resonance frequency. The EIS capacitor has, in turn, a reference electrode that makes the electrical connection between the inductor and the solution whose pH is to be measured. In addition, it may have an additional low impedance electrode, which is decoupled in direct current from the reference electrode. ii) An external inductor magnetically coupled to the resonator inductor, but without the need for physical contact between the two, which is used to measure the resonance frequency.
Este dispositivo se puede modificar, para medir la concentración de especies químicas en disolución, depositando, sobre la superficie del aislante del capacitor EIS, una capa adicional, estando dicha capa constituida por un material conductor en cuya interfase con la solución se establece una diferencia de potencial electroquímico variable con la especie química a determinar.This device can be modified, to measure the concentration of chemical species in solution, depositing, on the surface of the EIS capacitor insulator, an additional layer, said layer being constituted by a conductive material at whose interface with the solution a difference of Variable electrochemical potential with the chemical species to be determined.
Otro aspecto de la invención se refiere al uso de este dispositivo como sensor inalámbrico de pH, o especies químicas en disolución, o como sensor químico de tipo capacitivo.Another aspect of the invention relates to the use of this device as a wireless pH sensor, or chemical species in solution, or as a capacitive type chemical sensor.
DESCRIPCIÓN DETALLADADETAILED DESCRIPTION
La presente invención protege un dispositivo basado en la utilización de capacitores tipo EIS (Electrolito-Aislante-Semiconductor) - sensibles al pH - y tipo EMIS (Electrolito- Membrana-Aislante-Semiconductor) - sensibles a otras especies químicas en disolución - en sensores inalámbricos tipo LC, mediante su conexión en paralelo con un inductor.The present invention protects a device based on the use of capacitors type EIS (Electrolyte-Insulator-Semiconductor) - pH sensitive - and type EMIS (Electrolyte-Membrane-Insulator-Semiconductor) - sensitive to other chemical species in solution - in wireless sensors type LC, by connecting in parallel with an inductor.
Las ventajas de el sensor químico inalámbrico aquí propuesto, respecto a otros sensores químicos inalámbricos existentes o propuestos con anterioridad son las siguientes: 1) simplicidad de la tecnología, 2) bajo coste, 3) rapidez de la respuesta, y 4) poca o nula toxicidad de los materiales (no necesita pilas).The advantages of the wireless chemical sensor proposed here, compared to other wireless chemical sensors existing or proposed previously are: 1) simplicity of technology, 2) low cost, 3) speed of response, and 4) little or no material toxicity (no batteries needed).
Un objeto de la invención consiste en un dispositivo sensor inalámbrico, en adelante dispositivo sensor inalámbrico de la invención, para determinar el pH, o la concentración de iones en disolución, que comprende, al menos, por los siguientes elementos: i) Un resonador tipo LC formado por un inductor y un capacitor con estructura tipoAn object of the invention consists of a wireless sensor device, hereinafter wireless sensor device of the invention, to determine the pH, or the concentration of ions in solution, comprising at least the following elements: i) A type resonator LC formed by an inductor and a capacitor with type structure
EIS (Electrolito-Aislante-Silicio), conectados en paralelo, que produce una señal en forma de frecuencia de resonancia, y donde el capacitor EIS dispone, a su vez, de un electrodo de referencia que realiza la conexión eléctrica entre el inductor y la solución cuyo pH se pretende medir, y ii) Un inductor externo acoplado magnéticamente al inductor del resonador, pero sin necesidad de contacto físico entre ambos, que se utiliza para medir la frecuencia de resonancia.EIS (Electrolyte-Insulator-Silicon), connected in parallel, which produces a signal in the form of resonance frequency, and where the EIS capacitor has, in turn, a reference electrode that makes the electrical connection between the inductor and the solution whose pH is to be measured, and ii) An external inductor magnetically coupled to the inductor of the resonator, but without the need for physical contact between the two, which is used to measure the resonance frequency.
La Figura 1 muestra el esquema básico del dispositivo sensor.Figure 1 shows the basic scheme of the sensor device.
La capacidad del capacitor de i) varía en función del pH debido a su estructura tipo EIS (Electrolito-Aislante-Silicio) donde el electrolito es la solución que se quiere medir. Una estructura tipo EIS responde a un cambio de potencial aplicado entre el silicio y la solución según una curva C-V (Capacidad en función del Voltaje) que presenta una zona de pendiente pronunciada. Dicha zona de la curva C-V, denominada zona de vaciamiento, coincide con los voltajes para los cuales el silicio más próximo al aislante queda vacío de portadores de carga. La zona de vaciamiento de la curva C-V de una estructura EIS se desplaza hacia valores mayores o menores de voltaje cuando el pH aumenta o disminuye. Cuando la estructura EIS es polarizada a un voltaje fijo dentro de la zona de vaciamiento, se comporta como un capacitor en el que la capacidad varía en función del pH. Cuando el capacitor EIS y el inductor se conectan en paralelo, el voltaje en corriente continua (DC) aplicado a la capacidad pasa a ser automáticamente OV. Es necesario, por lo tanto, que la estructura EIS tenga la zona de vaciamiento centrada en OV para un pH neutro, de manera que cuando el pH aumente o disminuya se produzca un cambio en la capacidad de la estructura como se muestra en la Figura 2. Los capacitores EIS han sido utilizados previamente como sensores capacitivos, pero nunca formando parte de un sensor inalámbrico. La presente invención posibilita su utilización, por primera vez, como sensores inalámbricos, con las ventajas funcionales que eso supone, mediante se conexión en paralelo con un inductor.The capacitor capacity of i) varies depending on the pH due to its structure type EIS (Electrolyte-Insulator-Silicon) where the electrolyte is the solution to be measured. An EIS type structure responds to a potential change applied between the silicon and the solution according to a C-V curve (Capacity depending on Voltage) that has a steep slope area. Said zone of the C-V curve, called the emptying zone, coincides with the voltages for which the silicon closest to the insulator is empty of load carriers. The emptying zone of the C-V curve of an EIS structure moves towards higher or lower voltage values when the pH increases or decreases. When the EIS structure is polarized at a fixed voltage within the emptying zone, it behaves like a capacitor in which the capacity varies depending on the pH. When the EIS capacitor and the inductor are connected in parallel, the direct current (DC) voltage applied to the capacity automatically becomes OV. It is necessary, therefore, that the EIS structure has the emptying area centered on OV for a neutral pH, so that when the pH increases or decreases there is a change in the capacity of the structure as shown in Figure 2 EIS capacitors have previously been used as capacitive sensors, but never as part of a wireless sensor. The present invention enables its use, for the first time, as wireless sensors, with the functional advantages that this entails, by connecting in parallel with an inductor.
Además, para que el dispositivo sensor de la invención funcione, las resistencias serie del capacitor y el inductor del resonador descrito en i) deben tomar valores suficientemente reducidos como para que el factor de calidad del resonador sea mayor que 1.In addition, for the sensor device of the invention to work, the capacitor's series resistors and the resonator inductor described in i) must be sufficiently small for the resonator quality factor to be greater than 1.
Un objeto particular de la invención es el dispositivo sensor inalámbrico de la invención caracterizado porque el aislante del capacitor EIS de i) esta constituido por una o varias capas de material sensible al pH, a título ilustrativo y sin que limite el alcance de la invención, pertenecientes al siguiente grupo: óxido de silicio, nitruro de silicio, óxido de tántalo o óxido de aluminio.A particular object of the invention is the wireless sensor device of the invention characterized in that the insulator of the EIS capacitor of i) is constituted by one or more layers of pH sensitive material, by way of illustration and without limiting the scope of the invention, belonging to the following group: silicon oxide, silicon nitride, tantalum oxide or aluminum oxide.
Una realización particular de la invención es el dispositivo sensor inalámbrico de la invención en el que el electrodo de referencia del capacitor EIS es de tipo Ag/AgCl, con solución interna de KCl 3M.A particular embodiment of the invention is the wireless sensor device of the invention in which the reference electrode of the EIS capacitor is of the Ag / AgCl type, with internal 3M KCl solution.
La resistencia serie asociada a la capacidad EIS esta formada principalmente por la resistencia de la unión líquida del electrodo de referencia y la resistencia de la solución entre esta unión liquida y la superficie del aislante de la capacidad. La resistencia de una unión líquida toma valores típicamente alrededor de 5 kΩ y la de la solución depende de la geometría y disposición relativa del electrodo de referencia y la capacidad, pudiendo adquirir valores del orden de los kilo-ohmios para soluciones poco conductoras.The series resistance associated with the EIS capacity is mainly formed by the resistance of the liquid junction of the reference electrode and the resistance of the solution between this liquid junction and the capacity insulator surface. The resistance of a liquid junction typically takes values around 5 kΩ and that of the solution depends on the geometry and relative arrangement of the reference electrode and the capacity, being able to acquire values of the order of kilo-ohms for little conductive solutions.
Otro objeto particular de la invención es el dispositivo sensor inalámbrico de la invención en el que el capacitor EIS dispone, además del electrodo de referencia, de un electrodo de baja impedancia, con el fin de obtener una resistencia serie menor y aumentar así el factor de calidad del resonador. Este segundo electrodo de baja impedancia debe estar desacoplado en corriente continua respecto al electrodo de referencia para evitar que interfiera con el potencial a corriente continua fijado por este último en la solución. Otra realización particular de la invención es el dispositivo sensor inalámbrico de la invención con electrodo adicional de baja impedancia, en el que dicho electrodo está en la superficie del aislante del capacitor y posee una configuración consistente en una capa de material conductor, con una geometría tal que minimice la distancia media entre puntos de la superficie del electrodo y puntos de la superficie del aislante del capacitor. Un ejemplo de dicha geometría, que se muestra en las figuras 3 y 4, consiste en un conjunto de pistas o dedos largos dispuestos en paralelo y con separación igual a la de su anchura, de manera que las áreas del aislante del capacitor expuestas a la solución tienen unas dimensiones iguales a las de los dedos del electrodo, es decir, que las áreas de electrodo y del capacitor quedan interdigitadas. Otra realización particular de la invención es el dispositivo sensor inalámbrico de la invención con electrodo adicional de baja impedancia, en el que dicho electrodo está cubierto con una capa fina de aislante para que esté desacoplado en corriente continua respecto al electrodo de referencia. Esta capa hace las funciones de un condensador entre el electrodo y la solución. Además, el electrodo de baja impedancia esta aislado del substrato de silicio del capacitor por otra capa aislante, la cual también tendrá una capacidad asociada. La capacidad total de la estructura EIS con electrodo de baja impedancia, medida entre dicho electrodo y el substrato de silicio del capacitor está formada por varias capacidades en serie y en paralelo. Concretamente, por las capacidades de la capa aislante entre electrodo y la solución, la capacidad de la solución misma, la capacidad del aislante entre la solución y el substrato y la capacidad del silicio en la zona vacía de portadores, todas ellas en serie, y la capacidad asociada a la capa aislante entre el electrodo y el substrato en paralelo a las demás. De todas ellas, solo la capacidad de la zona vacía de portadores del silicio depende del potencial aplicado a la estructura. Para que la estructura tenga suficiente sensibilidad y rango de variación, la geometría de las diferentes capas que la componen se ha de diseñar adecuadamente, de manera que las variaciones de la capacidad del silicio se traduzcan en variaciones medibles de la capacidad total de la estructura.Another particular object of the invention is the wireless sensor device of the invention in which the EIS capacitor has, in addition to the reference electrode, a low impedance electrode, in order to obtain a lower series resistance and thus increase the power factor. resonator quality. This second low impedance electrode must be decoupled in direct current from the reference electrode to prevent it from interfering with the direct current potential set by the latter in the solution. Another particular embodiment of the invention is the wireless sensor device of the invention with additional low impedance electrode, wherein said electrode is on the surface of the capacitor insulator and has a configuration consisting of a layer of conductive material, with such geometry. that minimizes the average distance between electrode surface points and capacitor insulator surface points. An example of such geometry, which is shown in Figures 3 and 4, consists of a set of long tracks or fingers arranged in parallel and with separation equal to that of its width, such that the areas of the capacitor insulator exposed to the The solution has dimensions equal to those of the electrode fingers, that is, the electrode and capacitor areas are interdigitated. Another particular embodiment of the invention is the wireless sensor device of the invention with an additional low impedance electrode, wherein said electrode is covered with a thin layer of insulator to be decoupled in direct current. with respect to the reference electrode. This layer functions as a capacitor between the electrode and the solution. In addition, the low impedance electrode is isolated from the capacitor's silicon substrate by another insulating layer, which will also have an associated capacity. The total capacity of the EIS structure with a low impedance electrode, measured between said electrode and the capacitor's silicon substrate, is formed by several capacities in series and in parallel. Specifically, by the capacities of the insulating layer between electrode and the solution, the capacity of the solution itself, the capacity of the insulator between the solution and the substrate and the capacity of the silicon in the empty area of carriers, all of them in series, and the capacity associated with the insulating layer between the electrode and the substrate in parallel to the others. Of all of them, only the capacity of the empty area of silicon carriers depends on the potential applied to the structure. In order for the structure to have sufficient sensitivity and range of variation, the geometry of the different layers that compose it must be properly designed, so that the variations in the capacity of the silicon translate into measurable variations in the total capacity of the structure.
Por otra parte, se ha de tener también en cuenta que la capacidad de la solución esta en paralelo con la resistencia de la misma. La resistencia de la solución depende de su conductividad. Para evitar que las variaciones de conductividad de la solución se traduzcan en variaciones de la capacidad total de la estructura se debe escoger la frecuencia de resonancia del sensor de manera que, para el rango de conductividades en el que se quiere trabajar, la impedancia de la solución tenga un comportamiento totalmente capacitivo o totalmente resistivo.On the other hand, it must also be taken into account that the capacity of the solution is in parallel with its resistance. The resistance of the solution depends on its conductivity. To avoid that the variations in conductivity of the solution translate into variations in the total capacity of the structure, the sensor's resonance frequency must be chosen so that, for the range of conductivities in which you want to work, the impedance of the solution has a fully capacitive or fully resistive behavior.
La estructura del capacitor de i) hasta ahora descrita es sensible al pH de la solución. Sin embargo, depositando sobre la superficie del aislante membranas sensibles a otras especies químicas se puede conseguir que la capacidad, así como el sensor inalámbrico que construyamos con ella, sea sensible a estas otras especies químicas en disolución. En este caso, las variaciones del potencial electroquímico que se establece entre la membrana y la solución se transmiten a la superficie del aislante, produciendo variaciones de la capacidad de la estructura de la misma manera que con los sensores sin membrana. La mayoría de membranas desarrolladas hasta el momento tienen constante dieléctrica baja y resistividad elevada. Esto podría hacer que la resistencia serie de la estructura capacitiva sea excesivamente alta y no se pudiera obtener el factor de calidad necesario. Sin embargo, hay que tener en cuenta que las corrientes alternas pasan también por la capacidad asociada a la capa aislante entre el electrodo y el substrato de silicio. Dado que parte de la zona de vaciamiento penetra lateralmente por debajo de los dedos del electrodo, como se ve en la Figura 4, la capacidad total que va directamente del electrodo de baja impedancia al substrato de silicio del capacitor es también dependiente del potencial electroquímico. Es decir, si la membrana es suficientemente resistiva, son las variaciones de esta capacidad las que contribuyan mayoritariamente a la variación de la capacidad total de la estructura. De esta manera se evitan las perdidas óhmicas por paso de corriente por dicha membrana y se consigue un factor de calidad del resonador elevado.The capacitor structure of i) described above is sensitive to the pH of the solution. However, depositing on the surface of the insulator membranes sensitive to other chemical species, the capacity, as well as the wireless sensor that we build with it, can be sensitive to these other chemical species in solution. In this case, the variations of the electrochemical potential established between the membrane and the solution are transmitted to the surface of the insulator, producing variations in the structure's capacity in the same way as with the membraneless sensors. Most membranes developed so far have low dielectric constant and high resistivity. This could make the resistance The capacitive structure series is excessively high and the necessary quality factor cannot be obtained. However, it should be borne in mind that alternating currents also pass through the capacity associated with the insulating layer between the electrode and the silicon substrate. Since part of the emptying zone penetrates laterally below the electrode fingers, as seen in Figure 4, the total capacity that goes directly from the low impedance electrode to the capacitor's silicon substrate is also dependent on the electrochemical potential. That is, if the membrane is sufficiently resistive, it is the variations of this capacity that contribute mostly to the variation of the total capacity of the structure. In this way, ohmic losses by current passage through said membrane are avoided and a high quality factor of the resonator is achieved.
Así, Otro objeto particular de la invención es el dispositivo sensor inalámbrico de la invención en el que se deposita, sobre la superficie del aislante del capacitor EIS, una capa adicional, estando dicha capa constituida por un material conductor en cuya interfase con la solución se establece una diferencia de potencial electroquímico variable con la especie química a determinar.Thus, Another particular object of the invention is the wireless sensor device of the invention in which an additional layer is deposited on the surface of the EIS capacitor insulation, said layer being constituted by a conductive material at the interface of which the solution is It establishes a variable electrochemical potential difference with the chemical species to be determined.
El inductor y el electrodo de referencia necesarios para formar el dispositivo sensor pueden ser fabricados independientemente o pueden ser integrados, mediante técnicas de microfabricación, en el mismo dado o chip de silicio con el que se forma la capacidad EIS.The inductor and the reference electrode necessary to form the sensor device can be manufactured independently or can be integrated, by means of microfabrication techniques, into the same die or silicon chip with which the EIS capacity is formed.
Un objeto particular de la invención es el dispositivo sensor inalámbrico de la invención en el que el electrodo de referencia y el inductor del resonador tipo LC están integrados en el mismo dado o chip de silicio con el que se forma la capacidad EIS, de forma que el inductor está formado por una pista conductora en forma de espiral, el extremo interior del cual se conecta al electrodo de baja impedancia y el extremo exterior del cual se conecta al substrato de silicio, y el electrodo de referencia está formado por una capa de Ag/AgCl en contacto con la cara posterior del dado de silicio, sobre la cual se forma el reservorio de solución interna de KCl y la unión líquida hacia la solución externa. Otro objeto de la invención es el uso del dispositivo sensor inalámbrico de la invención como sensor químico. Otro objeto particular de la invención es el uso del dispositivo sensor inalámbrico de la invención como sensor inalámbrico de pH.A particular object of the invention is the wireless sensor device of the invention in which the reference electrode and the inductor of the LC type resonator are integrated in the same die or silicon chip with which the EIS capacity is formed, so that The inductor is formed by a spiral-shaped conductive track, the inner end of which is connected to the low impedance electrode and the outer end of which is connected to the silicon substrate, and the reference electrode is formed by a layer of Ag / AgCl in contact with the rear face of the silicon die, on which the internal KCl solution reservoir and the liquid bond towards the external solution are formed. Another object of the invention is the use of the wireless sensor device of the invention as a chemical sensor. Another particular object of the invention is the use of the wireless sensor device of the invention as a wireless pH sensor.
Otro objeto particular de la invención es el uso del dispositivo sensor de la invención, en el que se ha depositando, sobre la superficie del aislante del capacitor EIS, una capa adicional, como sensor inalámbrico de especies químicas en disolución.Another particular object of the invention is the use of the sensor device of the invention, in which an additional layer, as a wireless sensor of chemical species in solution, has been deposited on the surface of the EIS capacitor.
Además de la conexión en paralelo del inductor y el capacitor con estructura tipo EIS o EIMS, que posibilita la utilización del dispositivo sensor de la invención como sensor inalámbrico, otro aspecto novedoso que presenta esta invención es el electrodo adicional de baja impedancia del capacitor de i) que se ha descrito en uno de los objetos particulares de la invención. La utilización del dipositivo sensor de la invención con dos electrodos, sin necesidad de que estén conectados al inductor, introduce un nuevo tipo de sensores no inalámbricos.In addition to the parallel connection of the inductor and the capacitor with an EIS or EIMS type structure, which makes it possible to use the sensor device of the invention as a wireless sensor, another novel aspect presented by this invention is the additional low impedance electrode of the capacitor of i ) which has been described in one of the particular objects of the invention. The use of the optional sensor of the invention with two electrodes, without the need for them to be connected to the inductor, introduces a new type of non-wireless sensors.
Por tanto, otro objeto particular de la invención es el uso del dispositivo sensor de la invención, con doble electrodo y capa adicional sobre la superficie del aislante del capacitor EIS, como sensor químico de tipo capacitivo y, por tanto, no inalámbrico. En este caso se mide la capacidad con hilos conectados directamente a la misma, por lo que no se necesita un resonador que se pueda medir a distancia.Therefore, another particular object of the invention is the use of the sensor device of the invention, with double electrode and additional layer on the insulator surface of the EIS capacitor, as a capacitive type chemical sensor and, therefore, not wireless. In this case, the capacity is measured with wires directly connected to it, so a resonator that can be measured remotely is not required.
DESCRIPCIÓN DE LAS FIGURAS La Figura 1 es un esquema simplificado del sensor donde se puede apreciar el inductor (1), una sección de la capacidad EIS (2) formada por un dado de silicio cubierto en su cara anterior por un aislante (3) debajo del cual se forma una región vacía de portadores de carga (4) y cubierto en su cara inferior por una capa de aluminio (5). El inductor tiene uno de sus terminales (6) conectado a la cara posterior del dado de silicio, y el otro terminal (7) conectado a un electrodo de referencia de Ag/AgCl (8) formado por un hilo de plata clorinizado (9) en una cavidad (10) llena de KCl 3M que se comunica con el exterior a través de una unión liquida de material poroso (11). El conjunto esta encapsulado con un material polimerico (19) que deja expuestos al entorno solo la unión líquida y el aislante de la capacidad EIS. La Figura 2 es una gráfica que muestra las curvas característica típicas de Capacidad - Voltaje para una estructura capacitiva de tipo EIS a diferentes pH de la solución. La Figura 3 es una representación tridimensional de la capacidad EIS con electrodo de baja impedancia (12) aislado de la solución. La estructura de dicho electrodo esta formada por pistas o dedos de material conductor (15) intercaladas con áreas de capacidad EIS de mismas dimensiones (16). Adicionalmente el electrodo dispone de una área (18) sin capa aislante para la interconexión del mismo.DESCRIPTION OF THE FIGURES Figure 1 is a simplified diagram of the sensor where you can see the inductor (1), a section of the EIS capacity (2) formed by a silicon die covered on its front face by an insulator (3) below from which an empty region of load carriers (4) is formed and covered on its underside by an aluminum layer (5). The inductor has one of its terminals (6) connected to the rear face of the silicon die, and the other terminal (7) connected to an Ag / AgCl reference electrode (8) formed by a chlorinated silver wire (9) in a cavity (10) filled with 3M KCl that communicates with the outside through a liquid junction of porous material (11). The assembly is encapsulated with a polymeric material (19) that exposes only the liquid junction and the EIS insulator to the environment. Figure 2 is a graph showing the characteristic characteristic curves of Capacity - Voltage for a capacitive structure of type EIS at different pH of the solution. Figure 3 is a three-dimensional representation of the EIS capability with low impedance electrode (12) isolated from the solution. The structure of said electrode is formed by tracks or fingers of conductive material (15) interspersed with areas of EIS capacity of the same dimensions (16). Additionally, the electrode has an area (18) without an insulating layer for interconnection.
La Figura 4 es una representación de la sección de la estructura de la Figura 3 en la que se pueden apreciar los dedos de material conductor del electrodo de baja impedancia (12), la capa aislante entre este y el substrato de silicio (13), la capa aislante entre el substrato y la solución y entre el electrodo y la solución (14), y la región vacía de portadores que se forma bajo la capacidad EIS (17) y que penetra también por debajo de los dedos del electrodo.Figure 4 is a representation of the section of the structure of Figure 3 in which the fingers of conductive material of the low impedance electrode (12), the insulating layer between it and the silicon substrate (13) can be seen, the insulating layer between the substrate and the solution and between the electrode and the solution (14), and the empty region of carriers that is formed under the EIS capacity (17) and also penetrates below the electrode fingers.
EJEMPLO DE REALIZACIÓN DE LA INVENCIÓNEXAMPLE OF EMBODIMENT OF THE INVENTION
Se construyó un dispositivo sensor inalámbrico de la siguiente forma: Mediante técnicas de microfabricación típicas de las tecnologías microelectrónicas se fabricaron capacidades tipo EIS con electrodo de baja impedancia como los de las Figuras 3 y 4, donde los materiales y las dimensiones eran los siguientes: El dado de silicio es de 3mm x 3mm x 0.5mm de silicio tipo p con concentración de impurezas de boro de 5 x 1014. Los dedos del electrodo (15) y de la capacidad EIS (16) tienen ambos anchura de 3μm y longitud de 1.5mm, teniendo el electrodo 218 dedos y la capacidad EIS 217 dedos. El electrodo de baja impedancia es de polisilicio altamente dopado (hasta la degeneración) y tiene un grosor de 450nm. La capa aislante entre el electrodo y el substrato es de óxido de silicio de tipo térmico y tiene un grosor de 800nm. El aislante entre el electrodo y la solución esta formado por una doble capa de nitruro de silicio tipo LPCVD (Low Pressure Chemial Vapour Deposition) de lOOnm de grosor sobre óxido de silicio de tipo térmico de aproximadamente 120nm de grosor. El aislante entre el substrato de silicio y la solución esta formado por una doble capa de nitruro de silicio de tipo LPCVD de lOOnm de grosor sobre óxido de silicio de tipo térmico de 78nm de grosor. El área de interconexión del electrodo de baja impedancia es de lOOμm x lOOμm y esta cubierto por una capa de 0.5μm de aluminio. La cara del dado de silicio opuesta a la del electrodo esta también cubierta por una capa de 0.5μm de aluminio. La interconexión de la capacidad EIS, es decir, del electrodo de baja impedancia y del substrato de silicio, así como la protección de dicha interconexión de la solución se realizó mediante la siguiente técnica de encapsulado: Se pegó el dado de silicio por su cara metalizada con aluminio a una tira de PCB (circuito impreso) sobre una pista de cobre. El pegado se realizó con un epoxy conductor para asegurar la conexión eléctrica entre el substrato y la pista de cobre del PCB. Posteriormente se conectó el área de interconexión del electrodo de baja impedancia a otra pista del PCB mediante la técnica del "wire bonding". Manualmente se aplicó un polimero encapsulante sobre los bordes del dado de silicio y sobre las zonas metálicas del "wire bonding" y las pistas del PCB, dejando solo una pequeña área de las pistas sin encapsular para el soldado del inductor. A continuación se soldaron los terminales de un inductor de 130μH a las dos pistas del PCB y se soldó el hilo conector de un electrodo de referencia miniaturizado a la pista del PCB que conectaba con el electrodo de baja impedancia. El conjunto de capacidad EIS, inductor y electrodo de referencia se volvió a encapsular con material polimérico (19) para proteger las conexiones eléctricas de la solución, dejando expuestos al entorno solamente la unión líquida del electrodo de referencia (10) y la parte del PCB que contiene la capacidad EIS.A wireless sensor device was constructed in the following way: By means of microfabrication techniques typical of microelectronic technologies, EIS-type capabilities with low impedance electrode such as those in Figures 3 and 4 were manufactured, where the materials and dimensions were as follows: Silicon die is 3mm x 3mm x 0.5mm p-type silicon with boron impurity concentration of 5 x 10 14 . The electrode fingers (15) and the EIS capacity (16) are both 3μm wide and 1.5mm long, with the electrode 218 fingers and the EIS capacity 217 fingers. The low impedance electrode is made of highly doped polysilicon (until degeneration) and is 450nm thick. The insulating layer between the electrode and the substrate is silicon oxide of thermal type and has a thickness of 800nm. The insulator between the electrode and the solution is formed by a double layer of silicon nitride type LPCVD (Low Pressure Chemial Vapor Deposition) of lOOnm thick on silicon oxide of thermal type of approximately 120nm thick. The insulator between the silicon substrate and the solution is formed by a double layer of silicon nitride of LPCVD type lOOnm thick over silicon oxide of thermal type 78nm thick. The interconnection area of the low impedance electrode is lOOμm x lOOμm and is covered by a 0.5μm layer of aluminum. The face of the silicon die opposite the electrode is also covered by a 0.5μm layer of aluminum. The interconnection of the EIS capacity, that is, of the low impedance electrode and the silicon substrate, as well as the protection of said interconnection of the solution is Performed using the following encapsulation technique: The silicon die was glued on its metallic face with aluminum to a PCB strip (printed circuit) on a copper track. The bonding was done with a conductive epoxy to ensure the electrical connection between the substrate and the copper track of the PCB. Subsequently, the interconnection area of the low impedance electrode was connected to another PCB track using the wire bonding technique. Manually, an encapsulating polymer was applied on the edges of the silicon die and on the metal areas of the wire bonding and the PCB tracks, leaving only a small area of the tracks unencapsulated for the inductor weld. The terminals of a 130μH inductor were then soldered to the two PCB tracks and the connecting wire of a miniaturized reference electrode was welded to the PCB track that connected to the low impedance electrode. The EIS capacity set, inductor and reference electrode was re-encapsulated with polymeric material (19) to protect the electrical connections of the solution, leaving only the liquid junction of the reference electrode (10) and the PCB part exposed to the environment which contains the EIS capability.
El sensor se introdujo consecutivamente en 4 recipientes de plástico con soluciones tampón de pH igual a 2.0, 4.0, 8.7 y 11.5. Para la medida a distancia de dicho sensor se utilizó un inductor acoplado magnéticamente al inductor del sensor. Dicho inductor externo se conectó a un circuito de medida de impedancia con el que se identificó la frecuencia a la que la fase de su impedancia compleja presentaba un mínimo, y que coincide con la frecuencia de resonancia del sensor. Los valores de frecuencia de resonancia obtenidos para las cuatro soluciones tampón nos mostraron una alta linealidad, con coeficiente de regresión de R2= 0.9997 y pendiente igual a 12.2 KHz por unidad de pH. Por lo tanto se comprobó que el dispositivo sirve para medir, de forma precisa, el pH de una disolución. The sensor was introduced consecutively in 4 plastic containers with buffer solutions of pH equal to 2.0, 4.0, 8.7 and 11.5. For the remote measurement of said sensor, an inductor magnetically coupled to the sensor inductor was used. Said external inductor was connected to an impedance measurement circuit with which the frequency at which the phase of its complex impedance had a minimum was identified, and which coincides with the resonance frequency of the sensor. The resonance frequency values obtained for the four buffer solutions showed a high linearity, with regression coefficient of R 2 = 0.9997 and slope equal to 12.2 KHz per unit of pH. Therefore it was found that the device is used to accurately measure the pH of a solution.

Claims

REIVINDICACIONES
1. Dispositivo sensor inalámbrico para determinar el pH, o la concentración de iones en disolución, constituido, al menos, por los siguientes elementos: o Un resonador tipo LC formado por un inductor y un capacitor con estructura tipo EIS (Electrolito-Aislante-Silicio), conectados en paralelo, que produce una señal en forma de frecuencia de resonancia, y donde el capacitor EIS dispone, a su vez, de un electrodo de referencia que realiza la conexión eléctrica entre el inductor y la solución cuyo pH se pretende medir, y o Un inductor externo acoplado magnéticamente al inductor del resonador, pero sin necesidad de contacto físico entre ambos, que mide la frecuencia de resonancia1. Wireless sensor device to determine the pH, or the concentration of ions in solution, constituted, at least, by the following elements: o An LC type resonator formed by an inductor and a capacitor with an EIS type structure (Electrolyte-Insulating-Silicon ), connected in parallel, which produces a signal in the form of a resonance frequency, and where the EIS capacitor has, in turn, a reference electrode that makes the electrical connection between the inductor and the solution whose pH is to be measured, I An external inductor magnetically coupled to the resonator inductor, but without the need for physical contact between the two, which measures the resonance frequency
2. Dispositivo sensor inalámbrico, según reivindicación 1, caracterizado por que el aislante del capacitor EIS esta constituido por una o varias capas de material sensible al pH, como óxido de silicio, nitruro de silicio, óxido de tántalo o óxido de aluminio. 2. Wireless sensor device according to claim 1, characterized in that the insulation of the EIS capacitor is constituted by one or several layers of pH sensitive material, such as silicon oxide, silicon nitride, tantalum oxide or aluminum oxide.
3. Dispositivo sensor inalámbrico, según reivindicación 1, caracterizado por que el electrodo de referencia es de tipo Ag/AgCl, con solución interna de KCl 3M.3. Wireless sensor device according to claim 1, characterized in that the reference electrode is of the Ag / AgCl type, with an internal KCl 3M solution.
4. Dispositivo sensor inalámbrico, según reivindicación 1, caracterizado por que dispone de un electrodo adicional de baja impedancia, el cual esta desacoplado en corriente continua del electrodo de referencia. 4. Wireless sensor device according to claim 1, characterized in that it has an additional low impedance electrode, which is decoupled in direct current from the reference electrode.
5. Dispositivo sensor inalámbrico, según reivindicación 1, caracterizado por que el electrodo de baja impedancia está en la superficie del aislante del capacitor.5. Wireless sensor device according to claim 1, characterized in that the low impedance electrode is on the surface of the capacitor insulator.
6. Dispositivo sensor inalámbrico, según reivindicación 6, caracterizado por que el electrodo adicional de baja impedancia posee una configuración consistente en una capa de material conductor, con una geometría interdigitada respecto a la capacidad y está aislado del substrato de silicio del capacitor por otra capa aislante.6. Wireless sensor device according to claim 6, characterized in that the additional low impedance electrode has a configuration consisting of a layer of conductive material, with an interdigitated geometry with respect to the capacity and is isolated from the silicon substrate of the capacitor by another layer insulating.
7. Dispositivo sensor inalámbrico, según reivindicación 6, caracterizado por que el electrodo adicional de baja impedancia está cubierto con una capa fina de aislante para que esté desacoplado en corriente continua respecto al electrodo de referencia.7. Wireless sensor device according to claim 6, characterized in that the additional low impedance electrode is covered with a thin layer of insulator so that it is decoupled in direct current with respect to the reference electrode.
8. Dispositivo sensor inalámbrico, según reivindicaciones 1-8, caracterizado por que se ha depositando, sobre la superficie del aislante del capacitor EIS, una capa adicional, estando dicha capa constituida por un material conductor en cuya interfase con la solución se establece una diferencia de potencial electroquímico variable con la especie química a determinar.8. Wireless sensor device according to claims 1-8, characterized in that an additional layer has been deposited on the surface of the EIS capacitor insulation, said layer being constituted by a conductive material in whose interface with the solution a difference of electrochemical potential variable with the chemical species to be determined is established.
9. Dispositivo sensor inalámbrico, según reivindicaciones 1-9, caracterizado por que el electrodo de referencia y el inductor del resonador tipo LC están integrados en el mismo dado o chip de silicio con el que se forma la capacidad EIS, de forma que el inductor está formado por una pista conductora en forma de espiral, el extremo interior del cual se conecta al electrodo de baja impedancia y el extremo exterior del cual se conecta al substrato de silicio, y el electrodo de referencia está formado por una capa de Ag/AgCl en contacto con la cara posterior del dado de silicio, sobre la cual se forma el reservorio de solución interna de KCl y la unión líquida hacia la solución externa.9. Wireless sensor device according to claims 1-9, characterized in that the reference electrode and the inductor of the resonator type LC are integrated in the same die or silicon chip with which the EIS capacity is formed, so that the inductor It is formed by a spiral-shaped conductive track, the inner end of which is connected to the low impedance electrode and the outer end of which is connected to the silicon substrate, and the reference electrode is formed by an Ag / AgCl layer in contact with the rear face of the silicon die, on which the internal KCl solution reservoir and the liquid junction towards the external solution are formed.
10. Uso del dispositivo sensor inalámbrico según reivindicaciones 1-9, como sensor químico.10. Use of the wireless sensor device according to claims 1-9, as a chemical sensor.
11. Uso del dispositivo sensor inalámbrico según reivindicación 10 como sensor inalámbrico de pH.11. Use of the wireless sensor device according to claim 10 as a wireless pH sensor.
12. Uso del dispositivo sensor inalámbrico, según reivindicación 10 como sensor inalámbrico de especies químicas en disolución.12. Use of the wireless sensor device according to claim 10 as a wireless sensor of chemical species in solution.
13. Uso del dispositivo sensor inalámbrico con doble electrodo y capa adicional sobre la superficie del aislante del capacitor EIS, según reivindicación 10, como sensor químico de tipo capacitivo. 13. Use of the wireless sensor device with double electrode and additional layer on the insulator surface of the EIS capacitor, according to claim 10, as a chemical capacitive sensor.
PCT/ES2007/070009 2006-01-19 2007-01-17 Wire-free chemical sensor based on the connection of an inductor to an eis or emis capacitor, and applications thereof WO2007082977A1 (en)

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ES200600118A ES2304196B1 (en) 2006-01-19 2006-01-19 WIRELESS CHEMICAL SENSOR BASED ON THE CONNECTION OF AN INDUCTOR WITH AN EIS OR EMIS TYPE CAPACITOR AND ITS APPLICATIONS.

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