Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
  • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
  • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
  • G01D5/14—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
  • G01D5/24—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance
  • G01D5/2403—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance by moving plates, not forming part of the capacitor itself, e.g. shields
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
  • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
  • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
  • G01D5/14—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
  • G01D5/24—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance
• • 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
  • G01N27/02—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
  • G01N27/228—Circuits therefor

Definitions

• the present invention relates to a circuit for measuring a variable influencing the capacitance-voltage characteristic of a capacitive element according to the preamble of patent claim 1 and a method for measuring a variable influencing the capacitance-voltage characteristic of a capacitive element according to the preamble of patent claim 9.
• Capacitive MOS structures have a voltage-dependent capacitance value.
• the course of the capacitance-voltage curve is influenced, for example in the case of capacitive MOS structures, by further variables if the metallic layer consists of catalytic metals or at least partially comprises catalytic metals. This influence of certain sizes on the voltage-dependent capacitance curve of such capacitive structures is used to form sensors with which the relevant variable can be detected which influences the voltage-dependent capacitance curve.
• A are, for example, gas sensors that respond to certain gases, such as hydrogen sensors.
• gases such as hydrogen sensors.
• a constant bias voltage is applied to the capacitive MOS element, to which a high-frequency voltage signal of a predetermined amplitude is superimposed.
• the capacitance changes in the operating point determined by the preload. This change in capacitance can be recorded in a measuring bridge.
• the capacitive MOS element is acted upon with a bias voltage in a feedback control circuit such that it has a constant capacitance value.
• the readjusted pretension or the shift in the voltage operating point can be used to determine the variable influencing the capacitance-voltage characteristic.
• both of the techniques described a comparatively complex circuit is required to detect the variable influencing the capacitance-voltage characteristic.
• Both techniques is the detection of a comparatively small signal change in the operating point, therefore both techniques lack an adequate signal / noise ratio for the ascertained variable, that is to say the moisture or the gas concentration.
• the object of the present invention is to develop a circuit or a method for measuring a variable of the type mentioned at the outset which influences the capacitance-voltage characteristic of a capacitive element such that the accuracy in the determination the influencing size is further improved.
• Area under the curve of the voltage-capacitance characteristic is used in a predetermined range.
• measurement can be simply implemented by applying a periodic voltage signal of a predetermined amplitude to the capacitive element, an integration circuit integrating the output current of the capacitive element over a certain period of time.
• the resulting integration value has a predeterminable dependency on the influencing variable, so that it is possible, for example, to use the integer read out a table whose output value is the influencing variable.
• the resulting measuring circuit according to the invention for carrying out this measuring method essentially consists of a voltage source and an integration circuit with a capacitive element in the feedback branch that can be discharged via an electronic switch, so that the circuit has a very simple structure.
• the dynamic measurement is carried out with an AC voltage component which moves around an operating point.
• the influencing variable is determined on the basis of the integral over the shifted capacitance-voltage curve and not only on the basis of the detection of the change in the curve at a single voltage or capacitance point. This results in a signal / interference voltage ratio, which is considerably improved compared to the prior art, of the determined variable causing the shift in the capacitance voltage characteristic.
• FIG. 2 shows a circuit diagram of the circuit according to the invention for measuring the capacitance-voltage cha- size influencing characteristic
• Fig. 3 waveforms of voltage signals, as they occur in the circuit of FIG. 2;
• the voltage-dependent capacitance profile of a capacitive MOS structure, the metal layer of which contains a catalytic metal shifts depending on an influencing variable, for example the H 2 concentration or the concentration of a be ⁇ can be certain gas in the ambient atmosphere.
• an influencing variable for example the H 2 concentration or the concentration of a be ⁇ can be certain gas in the ambient atmosphere.
• This shift in the capacitance-voltage characteristic was, as explained at the outset, recorded in the prior art with constant bias by measuring the change in capacitance Ac or when the capacitance value was kept constant by measuring the change in bias ⁇ v .
• the invention provides, as can be seen from the detailed explanations below.
• the capacitive element C s in the preferred embodiment is a capacitive MOS element whose metal layer consists of palladium, which is also referred to as a MOS palladium gate sensor.
• the capacitive element C s is supplied by a first voltage source VS with a square wave voltage, the amplitude of which corresponds to +/- 1.5 V in a preferred embodiment with a duty cycle of 50%.
• the capacitive element C s is connected to the inverting input of an operational amplifier OPV.
• a second voltage source VR generates a second square-wave signal which corresponds to the first square-wave signal but is inverted with respect to it.
• This second square-wave voltage signal is applied to a reference capacitor C r , the output of which is also connected to the inverting input of the operational amplifier OPV.
• the capacitance of the reference capacitor C r is selected such that an offset voltage appearing at the output of the operational amplifier is compensated for with the capacitance-voltage characteristic of the capacitive element C s unaffected by the variable to be measured.
• the operational amplifier OPV In the feedback branch of the operational amplifier OPV, ie between its output and its inverting input, there is an integration capacitor Cf, to which an electronic switch S is connected in parallel.
• the electronic switch S ⁇ is actuated in a specific, fixed phase dependence on the phase of the first square-wave signal by means of a reset signal (cf. FIG. 3), as a result of which the integration is ended ⁇ and a voltage value at the output ⁇ A is reset.
• the output value of the operational amplifier OPV ⁇ VA before the reset is taken over in a sample and hold circuit 1.
• the influencing variable which is the H 2 concentration in the example, can be derived from this initial value.
• the H 2 concentration is expediently assigned to the output voltage £ V via a table, which is determined by a D / A converter and a ROM 2 can be implemented in which the dependency of the quantities mentioned, which is shown in FIG. 4, is stored.
• the curve shown in FIG. 4 is the result of a measurement with a palladium sensor, the catalytic metal layer of which is a 21.1 ran thick palladium layer which is arranged on a 34 ran thick silicon dioxide layer.
• a capacitive MOS structure with a 104.5 n thick gold gate on a 34 nm thick silicon dioxide insulator layer was chosen as the reference capacitor.
• the capacitance of the integration capacitor C is 1 nF.
• the influencing variable is detected in the circuit according to the invention by integrating the capacitance-voltage curve of the capacitive element to determine the area under this curve, which can be expressed by the following equation:
• the current through the capacitive sensor element C s is given by:
• the capacitance value Cf must be chosen so that the operational amplifier OPV is not overdriven at the maximum measured variable.
• FIG. 5 shows the two voltage-dependent capacitance curves of the reference element C r and the sensor element C s .
• the non-optimal adaptation of the two curves leads to an offset voltage of approximately 2 V.
• this offset voltage was taken into account.
• the method according to the invention for measuring the variable influencing the capacitance-voltage characteristic enables compensation of disturbance variables, which affect both the actual sensor element or capacitive element C s and the reference capacitor C. influence r in the same way in their capacitance-voltage characteristics.
• the capacitive (sensor) element C s has both a dependence of the capacitance-voltage characteristic on a variable A and on a disturbance variable B
• this undesirable sensitivity to the disturbance variable B can be compensated for be that a capacitive MOS element is used as a reference capacitor, which is only sensitive to the disturbance variable B.
• a large number of reference elements can be used to compensate for a corresponding number of undesirable cross-sensitivities.
• the frequency of the square-wave signals is preferably in the kHz range.
• the period of the rectangular voltage signals is longer than the minority carrier reaction time, so that the minority carrier accumulation is in a state of equilibrium, while the period of the rectangular voltage signals is shorter than the minority carrier generation recombination time, so that the MOS structure does not get into the equilibrium inversion range. This leads to an amplification of the signal, since the voltage-dependent capacitance curve runs downward into the area of low charge carrier depletion.
• the behavior of the MOS capacitor in the region of low charge carrier depletion must be taken into account. After applying a jump voltage the MOS capacitor only remains in the region of deep charge carrier depletion for a certain time T and then changes into the HF inversion curve.
• N B the doping and nn ⁇ ddiiee iinnttrriinnssiisscchhee Kconcentration for the relevant MOS capacitor.
• the clock rate can now either be chosen so high that there is still no movement influencing the accuracy in the direction of the HF inversion curve, or it can be chosen so that the HF inversion curve is reliably reached.
• FIG. 6 in which
• a capacitive MOS structure with a palladium gate is used as the moisture sensor.
• the method according to the invention can also be used for gas sensors with a MOS structure for evaluating the gas concentration as well as for measuring other variables influencing the capacitance-voltage characteristic of a capacitive element.

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• Chemical & Material Sciences (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Analytical Chemistry (AREA)
• Electrochemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• Power Engineering (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
• Measurement Of Resistance Or Impedance (AREA)

Priority Applications (2)

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Publications (1)

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Family Applications (1)

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Citations (1)

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• 1989
  • 1989-05-12 DE DE3915563A patent/DE3915563C1/de not_active Expired - Lifetime
• 1990
  • 1990-04-17 EP EP90906167A patent/EP0471684B1/de not_active Expired - Lifetime
  • 1990-04-17 DE DE90906167T patent/DE59003905D1/de not_active Expired - Fee Related
  • 1990-04-17 KR KR1019900702097A patent/KR930011421B1/ko not_active Expired - Fee Related
  • 1990-04-17 US US07/768,440 patent/US5235267A/en not_active Expired - Fee Related
  • 1990-04-17 WO PCT/EP1990/000612 patent/WO1990013793A1/de not_active Ceased
  • 1990-04-17 JP JP2505963A patent/JPH0740057B2/ja not_active Expired - Lifetime

Patent Citations (1)

Non-Patent Citations (4)

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