Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
  • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
  • G01N27/121—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid for determining moisture content, e.g. humidity, of the fluid
• • 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
  • G01N27/223—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance for determining moisture content, e.g. humidity
  • G01N27/225—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance for determining moisture content, e.g. humidity by using hygroscopic materials

Definitions

• the invention relates to a device for measuring the degree of moisture in gases, with a support body on which a base electrode, a counter electrode and a porous, non-conductive layer are arranged.
• the porous, non-conductive layer is either attached between the electrodes or the electrodes are arranged on the surface of the porous layer in such a way that current can only flow through this layer.
• the change in the capacitance or the electrical resistance due to the moisture penetrating into the porous layer can be used to infer the change in the degree of moisture of a gas.
• US Pat. No. 3,523,244 describes a moisture level meter in which an aluminum substrate is converted by an electrolytic etching process on the surface into a porous aluminum oxide layer, onto which a metal layer is applied as a counter electrode will wear.
• the published patent application DE 27 29 249 discloses a moisture level meter in which an area of a silicon semiconductor body is converted into a silicon dioxide layer of high porosity by an electrolytic etching process. A counter electrode is applied to an area of this layer in such a way that the ambient moisture can penetrate into the porous layer under the electrode.
• the known moisture level meters have a relatively long response time, i.e. H. a high inertia until the stable final value is reached and a not insignificant hysteresis behavior.
• the known devices require a longer recovery time in the case of condensation. Especially at low temperatures ( ⁇ 5 ° C), low air humidity ( ⁇ 10%) or reduced pressure, it is desirable to have measuring devices with higher accuracy.
• the invention is therefore based on the object of providing a hysteresis-free, condensation-insensitive moisture meter with a low response time.
• the device for measuring the moisture level of gases is characterized according to claim 1 in that the elements required for the measurement are attached to an area of the support body which is designed as a membrane.
• the low heat capacity thus achieved enables the porous layer to be heated and cooled very quickly.
• the membrane with the porous layer with the heating element provided and a suitable control circuit is heated above the ambient temperature until the measured capacitance or the measured resistance assumes a constant value which corresponds to the value for an unheated porous layer and almost moisture-free gas.
• the heating power consumed, or the current supplied to the heating resistor is then a measure of the degree of moisture in the gas examined.
• the support body and the membrane are made of a silicon single crystal according to claim 2 by means of lithography steps and backside etching. This makes use of a method that has been tried and tested in micromechanics for producing an ultra-thin membrane with a supporting edge.
• the ultra-thin membrane serves as a support element and at the same time as a base electrode.
• the con- This electrode is clocked over the supporting edge of the carrying body.
• the porous layer is applied to the surface of the membrane and the counter electrode above it, so that it forms the plates of a capacitor together with the membrane.
• the porous layer acts as a dielectric and changes the capacitance of the capacitor depending on the moisture absorbed.
• the porous layer is an area etched into the surface of the membrane, which extends into a predeterminable depth of the membrane and which is converted into a non-conductive layer by thermal oxidation. This means that a porous layer can be dispensed with. In this embodiment, the thermal coupling of the porous layer to the heated membrane is particularly good.
• the counter electrode consists of a moisture-permeable, electrically conductive film. This further development proves to be particularly advantageous when using a porous layer of silicon, since in such a layer
• the moisture-permeable film consists of a very thin gold layer, which is characterized by high corrosion resistance.
• the electrodes are each comb-shaped and interdigitated with their teeth applied to the surface of the porous layer.
• the electrodes are galvanically separated from each other in such a way that a current flow only through the porous layer can be made. This configuration can be used advantageously if contacting of the silicon semiconductor body is to be avoided.
• an insulation layer is applied to the surface of the support body, which has a recess in the area of the porous layer.
• This insulation layer protects the silicon surface when the recessed area is converted into a porous layer by etching and thermal oxidation. At the same time, it galvanically decouples the counter electrode from the support body and optionally serves as a base for the heating element.
• the heating element is designed as a conductor loop which surrounds the region of the porous layer.
• the heating resistor can be applied to the insulation layer in a particularly simple manner as an annular or rectangular metal film.
• the metal film is interrupted between the contact points of the power supply for the heater.
• the geometry of the heating resistor is chosen so that the porous layer can be heated as quickly and uniformly as possible.
• other types of heating elements for example a silicon heating resistor, can also be used to heat the porous layer.
• a conductor track is provided on the insulation layer in the immediate vicinity of the porous layer.
• the counterelectrode which is designed as a thin layer, covers both the porous layer and the conductor track and can therefore be easily contacted via the conductor track.
• the conductor track is as annular or rectangular metal film formed which surrounds the area of the porous layer.
• a particularly advantageous development of the invention is characterized in claim 11.
• the electronic circuit provided for evaluating the measurement signals is integrated on the same support body. This describes a miniaturized moisture level meter which can be monolithically integrated into a semiconductor chip using known methods from microstructure technology and microelectronics.
• the advantages achieved by the invention are, in particular, that the amount of moisture adsorbed in the heated porous layer is very small and constant during the measurement, so that no hysteresis behavior can occur if the degree of moisture changes. Since the heat capacity of the membrane is very low, the temperature is controlled with a short time constant and the response time of the moisture level meter is short. This also applies to low humidity, low temperature and low pressure. The temperature-dependent diffusion processes, which lead to a high response time in known moisture meters, are largely eliminated.
• the invention is described below using an embodiment for example with reference to the drawing, described in more detail.
• the drawing shows the three-dimensional representation of a cut moisture level meter.
• the moisture level meter in FIG. 1 consists of a support body 10, which is manufactured, for example, from a p-type silicon single crystal in a ⁇ 100> orientation.
• the surface of the support body 10 is covered with an insulation layer 11 made of silicon nitride.
• a region in the surface layer of the support body 10 is converted into a non-conductive porous layer 12 by an electrochemical etching process with subsequent thermal oxidation.
• the insulation layer 11 has a cutout 15.
• An inner and an outer conductor track 13 and 14 run around the recess 15.
• the outer conductor track 14 is opened at a point which is not visible in the figure and serves as a heating resistor.
• the inner conductor track 13 serves for contacting the counter electrode, which is not shown here for better clarity.
• This counter electrode consists of a thin gold layer, which extends over the conductor track 13 and the porous layer 12.
• the support body In the area 16, which serves as the support element of the porous layer 12 and the conductor tracks 13 and 14, the support body is thinned to an ultra-thin membrane.
• the membrane serves as the base electrode, which together with the counter electrode forms a capacitor.
• the porous layer between the electrodes has a different dielectric constant depending on the adsorbed moisture.
• the capacitance or resistance between the silicon membrane and the counter electrode measured with dry gas.
• the heating resistor 14 is heated by a suitable control circuit, which can also be integrated on the silicon semiconductor body, by ohmic heat until the value determined in the dry gas is reached again.
• This state is continuously maintained by the control even when the moisture level changes.
• the degree of moisture in the surrounding gas can be determined from the heating power consumed.
• Several moisture level meters can be produced simultaneously on one wafer.
• the surface of the wafer 10 is protected from the subsequent electrochemical etching process, for example, by epitaxially depositing a silicon nitride layer 11 that is a few micrometers thick. Small areas 15 with an area of a few 100 square micrometers are removed from the silicon nitride layer and the underlying silicon layer is converted into a highly porous silicon layer 12 by an electrochemical etching process to a depth of 0.1 to a few micrometers.
• the etching solution preferably consists of 10 to 33 percent by weight HF acid with 50 percent by weight ethanol.
• the current density is between 0.1 and 200 mA / cm, the specific resistance of the wafer used is between 0.001 and 1 ohm / cm.
• the entire structure with the porous areas is subjected to thermal oxidation in order to oxidize the porous layers 12.
• the oxidation preferably takes place 300 ° C. for a period of 2 hours in a pure oxygen atmosphere in order to produce a monomolecular silicon dioxide layer on the surface of the porous layers 12.
• the rectangularly arranged conductor tracks 13 and 14 are deposited on the silicon nitride layer serving as an insulation layer.
• the moisture-permeable counterelectrodes are then applied via a shadow mask which covers the wafer surface except for the areas of the conductor tracks 13 and the porous layers 12. This process is preferably carried out by sputtering or vapor deposition of an approximately 10 nm thin gold layer in a high vacuum.
• the back of the wafer is thinly etched in the areas 16 using the methods known in microstructure technology until a membrane thickness of approximately 0.5 micrometers to approximately 20 micrometers is reached.
• a potassium hydroxide solution for example, is used as the etching solution for the anisotropically acting etching process.
• the remaining silicon layer in ring-shaped areas can be completely removed.
• a metal layer is applied to the back of the wafer after the natural oxide has been removed in a dry chemical etching process.
• the contact can also be made from the top of the wafer. For this purpose, openings are etched into the insulation layer 11 and the exposed areas are metallized.
• the silicon wafer is suitably in divided the individual areas of the moisture level meter.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
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• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)

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Citations (3)

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• 1989
  • 1989-07-17 DE DE3923595A patent/DE3923595C1/de not_active Expired - Lifetime
• 1990
  • 1990-05-11 EP EP90906880A patent/EP0483155A1/de not_active Withdrawn
  • 1990-05-11 WO PCT/DE1990/000337 patent/WO1991001494A1/de not_active Application Discontinuation
  • 1990-05-11 FI FI920182A patent/FI920182A7/fi not_active Application Discontinuation

Patent Citations (3)

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