US20030183000A1 - Gas flowmeter and manufacturing method thereof - Google Patents

Gas flowmeter and manufacturing method thereof Download PDF

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Publication number
US20030183000A1
US20030183000A1 US10/245,274 US24527402A US2003183000A1 US 20030183000 A1 US20030183000 A1 US 20030183000A1 US 24527402 A US24527402 A US 24527402A US 2003183000 A1 US2003183000 A1 US 2003183000A1
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United States
Prior art keywords
silicon
gas
thin film
heat element
hydrogen
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US10/245,274
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English (en)
Inventor
Masamichi Yamada
Junichi Horie
Izumi Watanabe
Keiichi Nakada
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HITACHI Ltd AND HITACHI CAR ENGINEERING Co Ltd
Hitachi Ltd
Hitachi Automotive Systems Engineering Co Ltd
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HITACHI Ltd AND HITACHI CAR ENGINEERING Co Ltd
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Assigned to HITACHI CAR ENGINEERING, CO., LTD., HITACHI, LTD. reassignment HITACHI CAR ENGINEERING, CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HORI, JUNICHI, NAKADA, KEIICHI, WATANABE, IZUMI, YAMADA, MASAMICHI
Application filed by HITACHI Ltd AND HITACHI CAR ENGINEERING Co Ltd filed Critical HITACHI Ltd AND HITACHI CAR ENGINEERING Co Ltd
Publication of US20030183000A1 publication Critical patent/US20030183000A1/en
Priority to US10/954,270 priority Critical patent/US6923053B2/en
Priority to US11/088,175 priority patent/US7228614B2/en
Abandoned legal-status Critical Current

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
    • G01F1/68—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using thermal effects
    • G01F1/684—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
    • G01F1/688—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow using a particular type of heating, cooling or sensing element
    • G01F1/69—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow using a particular type of heating, cooling or sensing element of resistive type
    • G01F1/692—Thin-film arrangements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
    • G01F1/68—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using thermal effects
    • G01F1/684—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
    • G01F1/6845—Micromachined devices
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T29/00—Metal working
    • Y10T29/49—Method of mechanical manufacture
    • Y10T29/49002—Electrical device making
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T29/00—Metal working
    • Y10T29/49—Method of mechanical manufacture
    • Y10T29/49002—Electrical device making
    • Y10T29/49007—Indicating transducer
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T29/00—Metal working
    • Y10T29/49—Method of mechanical manufacture
    • Y10T29/49002—Electrical device making
    • Y10T29/4902—Electromagnet, transformer or inductor

Definitions

  • the present invention relates to a gas flowmeter and a manufacturing method thereof and, more particularly, it relates to a thermal gas flowmeter in which a heat-generating resistor or heat element is made up of a silicon (Si) semiconductor thin film, as well as a manufacturing method thereof.
  • thermal gas flowmeters have been predominant as flowmeters for an amount of intake air provided in an electronic control fuel injection system of an internal combustion engine such as for automobiles, or a flowmeter for various kinds of gases used for the production of semiconductors and for hydrogen/oxygen for use in fuel cells since the thermal gas flowmeter can detect the amount of the mass gas directly.
  • a gas flowmeter manufactured by semiconductor micro-machining technique has been noted since it can reduce the cost and can be driven at a low power.
  • this invention provides a gas flowmeter comprising a semiconductor substrate formed with a cavity and at least a heat element formed by way of an insulating film above the cavity of the semiconductor substrate to measure the flow rate of a gas to be measured, in which the heat element is a silicon (Si) semiconductor thin film applied with impurity doping at high concentration, and the gas flow meter has barrier layers that disposed as an upper layer and a lower layer of the silicon (Si) semiconductor thin film and that are formed in a region to cover at least the cavity, the barrier layers less permeating and absorbing hydrogen in a heat generating temperature range of the heat element.
  • the heat element is a silicon (Si) semiconductor thin film applied with impurity doping at high concentration
  • the gas flow meter has barrier layers that disposed as an upper layer and a lower layer of the silicon (Si) semiconductor thin film and that are formed in a region to cover at least the cavity, the barrier layers less permeating and absorbing hydrogen in a heat generating temperature range of the heat element.
  • the barrier layer may comprise a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film.
  • the silicon (Si) semiconductor thin film is a polycrystalline silicon (Si) semiconductor thin film applied with impurity-doping, and the polycrystalline silicon (Si) semiconductor thin film is doped with phosphorus (P) or boron (B) as impurities at a high concentration.
  • the doping is applied at such high concentration that the resistivity of the silicon (Si) semiconductor thin film is 8 ⁇ 10 ⁇ 4 ⁇ cm or less.
  • this invention provides a gas flowmeter comprising a semiconductor substrate formed with a cavity and at least a heat element formed by way of an insulating film above the cavity of the semiconductor substrate to measure the flow rate of a gas to be measured in which the silicon (Si) semiconductor thin film is a polycrystalline applied with impurity doping, the polycrystalline silicon (Si) thin film is applied with doping of phosphorus (P) or boron (B) as impurities at high concentration, and the gas flowmeter has barrier layers that disposed as an upper layer and a lower layer of the silicon (Si) semiconductor thin film and that are formed in a region to cover at least the cavity, the barrier layers less permeating and absorbing hydrogen in a heat generating temperature range of the heat element.
  • the secular change of the resistance value of the heat element can be reduced by using the barrier layers and the secular change of the gas flowmeter can be reduced.
  • the doping treatment is applied at high concentration such that the resistivity of the silicon (Si) semiconductor thin film is 8 ⁇ 10 ⁇ 4 ⁇ cm or less.
  • a gas flowmeter comprising a semiconductor substrate formed with a cavity and at least a heat element formed by way of an insulating film above the cavity of the semiconductor substrate to measure the flow rate of a gas to be measured, in which the semiconductor substrate is a silicon semiconductor substrate, and the method includes the steps of: forming a first insulating film containing a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film formed on the surface of the silicon semiconductor substrate, as at least a first barrier layer which less permeates and absorbs hydrogen; forming a silicon semiconductor thin film on the first insulating film; applying impurity doping by thermal diffusion to the silicon semiconductor thin film thereby doping impurities at high concentration; patterning the silicon semiconductor thin film and, forming at least a pattern of the heat element; stacking a second insulating film containing a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film formed on the surface of the silicon semiconductor substrate, as at least a
  • the secular change of the resistance value of the heat element can be reduced by using the barrier layer and the secular change of the gas flowmeter can be reduced.
  • the heat treatment annealing is a heat treatment at 550° C. or higher and 900° C. or lower.
  • the heat treatment annealing is applied in an atmosphere at least containing a hydrogen gas in a case where hydrogen is contained in the gas to be measured.
  • the heat treatment annealing is applied in an atmosphere containing an inert gas in a case where hydrogen is not contained in the gas to be measured.
  • a gas flowmeter which comprises a semiconductor substrate formed with a cavity and at least a heat element formed by way of an insulating film above the cavity of the semiconductor substrate to measuring the flow rate of a gas to be measured.
  • the semiconductor substrate is a silicon semiconductor substrate.
  • the method includes the steps of: stacking a first silicon dioxide film, a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film as a first barrier layer which less permeates and absorbs hydrogen, and a second silicon dioxide film on the surface of the silicon semiconductor substrate; forming a silicon semiconductor thin film on the second silicon dioxide film; applying an impurity doping by thermal diffusion to the silicon semiconductor thin film to apply doping of phosphorus (P) at high concentration such that the resistivity is 8 ⁇ 10 ⁇ 4 ⁇ cm or less; patterning the silicon semiconductor thin film to form at least a pattern for the heat element; stacking a third silicon dioxide film, a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film as a second barrier layer which less permeates and absorbs hydrogen and a fourth silicon dioxide film further on the heat element, and surrounding the heat element with the first and the second barrier layer which less permeate hydrogen; applying a heat treatment annealing to the heat
  • the secular change of the resistance value of the heat element can be reduced by using the barrier layer and the secular change of the gas flowmeter can be reduced.
  • FIG. 1 is a plan view of a sensor device of a gas flowmeter according to a first embodiment of the invention
  • FIG. 2 is a cross sectional view taken along line A-A′ in FIG. 1;
  • FIG. 3 is a cross sectional view showing the state of mounting an air flowmeter for use in an internal combustion engine using the gas flowmeter according to the first embodiment of this invention
  • FIG. 4 is a circuit diagram of the gas flowmeter according to the first embodiment of this invention.
  • FIG. 5 is a step diagram showing manufacturing steps of a sensor device of the gas flowmeter according to the first embodiment of the invention
  • FIG. 6 is an explanatory view diagram showing a relation between resistivity ( ⁇ ) and impurity concentration of a silicon (Si) semiconductor thin film used in the gas flowmeter according to the first embodiment of this invention
  • FIG. 7 is an explanatory diagram showing a relation between temperature coefficient of resistance ( ⁇ ) and resistivity ( ⁇ ) of a silicon (Si) semiconductor thin film used in the gas flowmeter according to the first embodiment of this invention
  • FIG. 8 is an explanatory view of a barrier layer used in the sensor device of the gas flowmeter according to the first embodiment according of this invention.
  • FIG. 9 is an explanatory diagram of a barrier layer used in the sensor device of the gas flowmeter according to the first embodiment of this invention.
  • FIG. 10 is an explanatory view of a barrier layer used in the sensor device of the gas flowmeter according to the first embodiment of this invention.
  • FIG. 11 is an explanatory diagram of a barrier layer used in the sensor device of the gas flowmeter according to the first embodiment of this invention.
  • FIG. 12 is an explanatory diagram showing further method of preventing secular change in the sensor device of the gas flowmeter according to the first embodiment of this invention.
  • FIG. 13 is an explanatory diagram showing further method of preventing secular change in the sensor device of the gas flowmeter according to the first embodiment of this invention.
  • FIG. 14 is a cross sectional view showing the constitution of a gas flowmeter according to a second embodiment of this invention.
  • FIG. 15 is a cross sectional view showing the constitution of a gas flowmeter according to a third embodiment of this invention.
  • FIG. 1 is a plan view of a sensor device of the gas flowmeter according to the first embodiment of this invention.
  • FIG. 2 is a cross sectional view taken along line A-A′ in FIG. 1.
  • a sensor device 1 is formed entirely on a semiconductor substrate 2 as a base.
  • the semiconductor substrate 2 is a single crystal silicon (Si) plate.
  • a cavity 9 is formed on the side of the lower surface of the semiconductor substrate 2 , and a diaphragm 3 is formed on the cavity 9 .
  • the cavity 9 is formed as a hole having a substantially rectangular planer shape.
  • three-layered films comprising an insulating film 7 a , a barrier layer 8 a and an insulating film 7 b are stacked successively on one surface of the semiconductor substrate 2 .
  • the three-layered films have a structure of covering the entire surface of the semiconductor substrate 2 including the cavity 9 .
  • the insulating films 7 a and 7 b comprise silicon dioxide (SiO 2 ).
  • the barrier layer 8 a is formed of a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film that less permeates and absorbs hydrogen within a heat generation temperature range of a heat-generating resistor or heat element 4 .
  • the heat element 4 , and upstream-temperature measuring resistors 5 a and 5 b and downstream-temperature measuring resistors 5 c and 5 d are formed on the surface of the insulating film 7 b constituting the diaphragm 3 . Further, a gas temperature measuring resistor 6 is formed on the surface of the insulating film 7 b in a region outside the diaphragm 3 .
  • the heat element 4 , the gas temperature measuring resistor 6 , the temperature measuring resistors 5 a to 5 d , the gas temperature measuring resistor 6 and wiring connection portions 11 ( 11 a , 11 b , 11 c , 11 d , 11 e , 11 f , 11 g , 11 h , 11 j , 11 i , 11 k , and 11 l ) for each of the resistors are prepared each as fine stripe having predetermined conductivity (resistance value) from a polycrystalline or single crystalline silicon semiconductor thin film subjected to a phosphorus (P) or boron doping treatment at high concentration.
  • P phosphorus
  • Three-layered films of an insulating film 7 c , a barrier layer 8 b and insulating film 7 d are successively stacked on the upper layer of the insulating film 7 b and each of the resistors.
  • the three-layered films have a structure of covering the entire surface of the semiconductor substrate 2 including the cavity 9 .
  • the insulating film 7 a and 7 b comprise silicon dioxide (SiO 2 ).
  • the barrier layer 8 a is formed of a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film which less permeates and absorbs hydrogen within a heat generation temperature range of the heat element 4 .
  • the three-layered films of the insulating film 7 c , the barrier 8 b and the insulating film 7 d are provided to protect each of the resistors.
  • the upper layer and the lower layer for each of the resistors are constituted, respectively, so as to be surrounded by the barrier layers 8 a and 8 b which less permeates and absorbs hydrogen in the heat generation temperature region of the heat element 4 .
  • the thin silicon semiconductor film as each of the resistors is subjected to a heat treatment annealing at 550° C. or higher and 900° C. or lower in order to reduce the secular change of the resistance upon current supply at high temperature.
  • terminal electrodes 12 ( 12 a , 12 b , 12 c , 12 d , 12 e , 12 f , 12 g and 12 h ), through holes (not shown) are formed in the three-layered films of the insulating film 7 c , the barrier layer 8 b and the insulating film 7 d and then thin film pads (electrode films) such as of aluminum (Al) or gold (Au) are formed to establish electric connection with each of the resistors.
  • thin film pads electrode films
  • the heat element 4 is disposed substantially linearly along a direction perpendicular to the flowing direction of a gas to be measured depicted by an arrow 10 , the temperature measuring resistors 5 a , 5 b , 5 c and 5 d are formed so as to be paired with each other while being spaced apart in the directions of the upstream and the downstream, and the direction perpendicular to the flowing direction of the gas.
  • the targeted gas to be measured includes air, hydrogen, nitrogen, oxygen, gaseous carbon dioxide, city gas, methane, propane, butane and steams.
  • the diaphragm 3 comprising the insulating films 7 a , 7 b , 7 c and 7 d and the barrier layers 8 a , 8 b has a structure covering the entire surface of the cavity 9 , the mechanical strength of the diaphragm 3 can be kept, dusts, oil and water do not intrude into the cavity in the inside and measurement at high reliability is possible for a long period of time.
  • the gas flow rate and the gas flowing direction are measured by comparing the temperatures (resistance values) of upstream-temperature measuring resistors 5 a and 5 b disposed upstream the heat element 4 with the temperatures of the downstream-temperature measuring resistors 5 c and 5 d disposed downstream the heat element 4 .
  • the temperature of the downstream-temperature measuring resistors 5 c and 5 d is lower than the temperature of the upstream-temperature measuring resistors 5 a and 5 b , in which the sign representing the temperature difference between the upstream-temperature measuring resistors 5 a and 5 b and the downstream-temperature measuring resistors 5 c and 5 d is inverted.
  • the gas flow rate can be measured based on the temperature difference, and the direction of the gas flow can be judged by the sign of the temperature difference.
  • FIG. 3 is a cross sectional view showing the state of mounting the air flowmeter for use in the internal combustion engine using the gas flowmeter according to the first embodiment of the invention.
  • the sensor device 1 shown in FIG. 1 is mounted, for example, to an air intake passage 13 of an automobile internal combustion engine.
  • the sensor device 1 including a support 15 and an external circuit 16 therefor, is disposed 16 in a sub-passage 14 provided inside the air intake passage 13 .
  • the external circuit 16 is electrically connected by way of the support 15 to the terminal electrodes (terminal electrodes 12 shown in FIG. 1) of the sensor-measuring device 1 .
  • the intake air of the internal combustion engine usually flows in the direction depicted by an arrow 10 (forward flow). Further, depending on the operation conditions of the internal combustion engine, it may sometimes flow in the direction opposite to the arrow 10 (backward flow).
  • forward flow the air flow flowing the air intake passage of the internal combustion engine includes the forward flow and the backward flow, and the air flow rate can be measured exactly either in the case of the forward flow and the backward flow and a distinguishment between them can be made by the use of the gas flowmeter in this embodiment.
  • the gas flow rate is measured by disposing the sensor device 1 in the gas passage as with the constitution shown in FIG. 3.
  • a circuit portion such as the external circuit 16 is constituted in such a way as to sufficiently keep air tightness so as to inhibit leakage of the combustible gas thereto in order that safety is protected.
  • FIG. 4 is a circuit diagram of the gas flowmeter.
  • identical reference numerals with those in FIG. 1 represent identical portions.
  • the heat element 4 and the gas temperature-measuring resistor 6 constitute a bridge circuit together with resistors 20 a and 20 b .
  • Power is supplied to the bridge circuit from a power source 18 by way of a transistor 19 .
  • the voltage at the terminals 12 a and 12 c of the bridge circuit are inputted to a control circuit 17 .
  • the control circuit 17 comprises an input circuit including an A/D converter or the like, an output circuit including a D/A converter or the like and CPU for conducting operational processing.
  • the control circuit 17 controls the transistor 19 such that the temperature (Th) of the heat element 4 is higher by a predetermined value ( ⁇ Th) than the temperature (Ta) of the gas temperature-measuring resistor 6 corresponding to the gas temperature. That is, when the temperature of the heat element 4 is lower than the set value, the transistor 19 is turned on by the output from the control circuit 17 to allow a heating current to flow the heat element 4 . On the contrary, when the temperature is higher than the set temperature, the transistor 19 is turned off to decrease the heating current for the heat element 4 to zero, whereby the temperature is controlled so as to keep the set temperature value.
  • the upstream-temperature measuring resistors 5 a and 5 b and the downstream-temperature measuring resistors 5 c and 5 c constitute a bridge circuit.
  • the potential difference at the terminals 12 g and 12 e of the bridge circuit is inputted to the control circuit 17 .
  • the control circuit 17 detects the temperature difference between the upstream-temperature measuring resistors 5 a and 5 b and the downstream-temperature measuring resistors 5 c and 5 d based on the potential difference between the terminals 12 g and 12 e of the bridge circuit.
  • a resistance value of an adjusting resistor (not shown) is previously adjusted such that potentials between the terminals 12 g and 12 e of the bridge circuit agree with each other when the gas flow rate is zero.
  • a potential difference between the terminals 12 g and 12 e when the gas flow rate is zero is previously stored in a memory 21 .
  • the relation between the gas flow rate (Q) and the potential difference between the terminals 12 g and 12 e of the bridge circuit are previously stored as a map in the memory 21 and, the measured value of the gas flow rate (Q) and the flowing direction of the gas flow are judged and outputted based on the potential difference and the magnitude relation between the terminals 12 g and 12 e , respectively.
  • the bridge circuit for detecting the temperature difference is constituted by using the four temperature measuring resistors 5 a , 5 b , 5 c and 5 d in a manner where the upstream-temperature measuring resistors 5 a and 5 b and the downstream-temperature measuring resistors 5 c and 5 d intersect each other and, accordingly, the potential difference appearing between the terminals 12 g and 12 e increase about twice a bridge circuit comprising a pair of temperature measuring resistors, so that the sensitivity is increased and the accuracy is improved.
  • the method of detecting the gas flow rate (Q) it is also possible to adopt a method of using a heating current (corresponding to the potential at the terminal 12 c in FIG. 4) flowing to the heating element 4 as the flow rate detection signal in addition to the method of detecting the temperature difference between the upstream-temperature measuring resistors 5 a and 5 b and the downstream-temperature measuring resistors 5 c and 5 d . Further, it is also possible to adopt a method of multiplying the signal for the temperature difference between the upstream-temperature measuring resistors 5 a and 5 b and the downstream-temperature measuring resistors 5 c and 5 d with a heating current (corresponding to the potential at the terminal 12 c in FIG. 4) flowing to the heat element 4 , thereby obtaining a flow rate detection signal.
  • a heating current corresponding to the potential at the terminal 12 c in FIG. 4
  • FIG. 5 is step diagrams showing manufacturing steps of the sensor device for the gas flowmeter according to the first embodiment of this invention. Identical reference numerals with those in FIG. 1 show identical portions.
  • silicon dioxide (SiO 2 ) layers 7 a and 22 are formed on the upper and lower surfaces of the silicon semiconductor substrate 2 , respectively, by a thermal oxidation treatment with each having a thickness of about 0.2 ⁇ m. Then, a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film which less permeates and absorbs hydrogen in a heat generating temperature range of the heat element 4 ( ⁇ 550° C.) is formed to about 0.15 ⁇ m in thickness at a temperature of 770 to 800° C. by a vacuum CVD method (Chemical Vapor Deposition: hereinafter referred to as LPCVD method).
  • LPCVD method Physical Vapor Deposition
  • the stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film has a dense film quality, which is a film of high mechanical strength having residual tensile stresses and also has a characteristic of less permeating and absorbing hydrogen within the heat generating temperature range ( ⁇ 550° C.) of the heat element 4 .
  • a silicon dioxide (SiO 2 ) layer 7 b is formed to about 0.5 ⁇ m in thickness by an LPCVD method on the barrier layer 8 a .
  • the silicon dioxide (SiO 2 ) film formed by the LPCVD method has opposite compressive stresses in the film and heat conductivity smaller by one digit as compared with the silicon nitride (Si 3 N 4 ) thin film described above.
  • this provides a multi-layered structure of the silicon dioxide (SiO 2 ) layers 7 a and 7 b and the thin silicon nitride (Si 3 N 4 ) film 8 a , and the silicon nitride (Si 3 N 4 ) thin film has tensile stresses in the film while the silicon dioxide (SiO 2 ) film has compressive stresses in the film, the residual stresses act in the direction offset with each other, so that distortion caused by the residual stresses in the diaphragm 3 can be reduced to improve the strength.
  • a polycrystalline silicon (Si) semiconductor thin films 23 for forming the heat element 4 and the temperature measuring resistors 5 a to 5 d and 6 are formed on the insulating film 7 b with each having a thickness of about 1 ⁇ m by the method such as the LPCVD.
  • the polycrystalline silicon (Si) semiconductor thin film may also be formed by the LPCVD using plasmas, ECR-PCVD using electron cyclotron resonance or CVD using microwaves as other plasma using manufacturing methods. While the polycrystalline silicon (Si) semiconductor thin film is used for the silicon (Si) semiconductor thin film 23 in this embodiment, a silicon (Si) semiconductor thin film of an epitaxially grown single crystal structure may also be formed.
  • an impurity doping treatment is applied to the thus formed silicon (Si) semiconductor thin film 23 by a thermal diffusion treatment.
  • Phosphorus glass (POCl 3 ) is formed on the surface of silicon (Si) semiconductor thin film 23 and a heat treatment at 1000° C. for 30 min or more is applied to form a silicon (Si) semiconductor thin film 23 doped with phosphorus (P) at high concentration to provide a resistivity ( ⁇ ) of 8 ⁇ 10 ⁇ 4 ⁇ cm or less.
  • phosphorus (P) is used as the impurity in this step
  • high concentration doping can also be conducted by using boron (B) as the impurity.
  • B boron
  • the use of phosphorus (P) as the impurity can provide more effect.
  • the silicon (Si) semiconductor thin film 23 is patterned by a method, for example, of reactive dry etching to form a heat element 4 , temperature measuring resistors 5 a to 5 d , a gas measuring temperature resistor 6 (not shown) and wiring connection portions 11 ( 11 a to 11 l ) (not shown) for connecting each of the resistors and terminal electrodes 12 .
  • a silicon dioxide (SiO 2 ) layer is formed to about 0.5 ⁇ m in thickness as an insulating film 7 c
  • a stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film is formed at a temperature of 770 to 800° C. by the LPCVD method to about 0.15 ⁇ m in thickness as the barrier layer 8 b
  • a silicon dioxide (SiO 2 ) layer is stacked to about 0.2 ⁇ m in thickness as an insulating film 7 d.
  • a heat treatment is applied so as to enhance the stability of the resistance value (secular change) of the polycrystalline silicon (Si) semiconductor thin film.
  • the heat treatment annealing is applied in an atmosphere at least containing a hydrogen gas and in a case where hydrogen is not contained in the gas to be measured, a heat treatment annealing is applied in an inert gas atmosphere, at 550° C. or higher or 900° C. or lower.
  • the three-layered structure comprising the insulating films 7 a and 7 b and the barrier layer 8 a are formed as the lower layer of the heat element 4
  • the three-layered structure comprising the insulating films 7 c and 7 d and the barrier layer 8 b are formed as the upper layer of the heat element 4 . Since they provide a film structures symmetrical with respect to the upper and lower directions of the silicon (Si) semiconductor thin film 23 , distortion of the diaphragm 3 by thermal stresses can be reduced to improve the strength.
  • the terminal electrodes 12 ( 12 a to 12 h ) are formed, for example, with aluminum or gold to establish electric connection between the terminal electrodes and the resistors.
  • a mask material 22 for etching is patterned in a predetermined shape to expose only the etched portion of the semiconductor substrate 2 .
  • silicon dioxide or silicon nitride of higher etching selectivity is used for the mask material 22 .
  • the cavity 9 is formed by applying anisotropic etching from the rear face of the silicon semiconductor substrate 2 by using silicon dioxide or silicon nitride as the mask material and using an etching solution, for example, of potassium hydroxide (KOH).
  • KOH potassium hydroxide
  • heat treatment annealing at a temperature of 400° C. or lower for 1 hour or more is applied in a hydrogen or an inert gas atmosphere to enhance the film quality of the electrode material such as aluminum or gold and to ensure electric connection.
  • a gas to be measured is air, nitrogen, oxygen, gaseous carbon dioxide or steams
  • the heat treatment annealing is applied in a gas atmosphere, for example, of a measured gas or a nitrogen, argon, helium as an inert gas or fluorine.
  • the heat treatment annealing is applied in the gas atmosphere of a measured gas or a hydrogen gas.
  • the stability of the resistance value (secular change) of the silicon (Si) semiconductor thin film can be improved further.
  • the heat element 4 and the temperature measuring resistors 5 a to 5 d , and 6 are doped at a high concentration of phosphorus (P) or boron (B) so that the resistivity ( ⁇ ) is 8 ⁇ 10 ⁇ 4 ⁇ cm or less, the temperature coefficient of resistance ( ⁇ ) of the heat element 4 and the temperature measuring resistors 5 a to 5 d , and 6 can be kept relatively high. Therefore, a gas flowmeter that has a reduced cost and improved temperature measuring sensitivity can be provided.
  • FIG. 6 is an explanatory diagram of a relation between resistivity ( ⁇ ) and impurity concentration of a silicon (Si) semiconductor thin film used in a gas flowmeter according to the first embodiment of this invention.
  • FIG. 7 is an explanatory view for a relation between temperature coefficient of resistance ( ⁇ ) and resistivity ( ⁇ ) of a silicon (Si) semiconductor thin film used in a gas flowmeter according to the first embodiment of this invention.
  • a silicon (Si) semiconductor film generally shows resistance-temperature characteristics like a thermistor. However, it has a relatively narrow temperature range and when applied with an impurity doping treatment at high concentration, it shows resistance-temperature characteristics like a metal for example as shown in the following equation (1).
  • R is a resistance value of a semiconductor film at a temperature (T)
  • R0 is a resistance value of a semiconductor film at a temperature (T0)
  • ⁇ is a temperature coefficient of resistance
  • the temperature measuring resistors 5 a to 5 d and 6 have large temperature coefficient of resistance ( ⁇ ) in order to improve the detection sensitivity.
  • the heat element 4 has a small resistivity ( ⁇ ) in view of reducing a voltage for driving the heat element 4 when it is intended to be heated to a desired temperature (for example, 200° C.).
  • the solid line 26 in FIG. 7 shows a relation between the temperature coefficient of resistance ( ⁇ ) and the resistivity ( ⁇ ).
  • the temperature coefficient of resistance ( ⁇ ) increases in a region 27 where the resistivity ( ⁇ ) is 8 ⁇ 10 ⁇ 4 ⁇ cm or less. It is preferred that the temperature measuring resistors 5 a to 5 d and 6 have a large temperature coefficient of resistance ( ⁇ ) in view of the improvement in the detection sensitivity.
  • the heat element 4 has a small resistivity ( ⁇ )
  • a large temperature coefficient of resistance ( ⁇ ) 1000( ⁇ 10 ⁇ 6 /° C.) or more) can be attained at a low resistivity ( ⁇ ) in a region 27 where the resistivity ( ⁇ ) is 8 ⁇ 10 ⁇ 4 ⁇ cm or less.
  • the thickness of the silicon (Si) semiconductor film is a possible measure to lower the resistance value.
  • the limit for the thickness of the polycrystalline silicon (Si) semiconductor film capable of obtaining etching at high accuracy is about 1 ⁇ m, and the resistance value of the heat element 4 that can be driven by a driving voltage of 10 V or lower at this thickness is 1 k ⁇ or less.
  • the resistance value of the heat element 4 is 80 ⁇ . Further, including as far as the resistance of the wiring connection portions 11 to be connected with the heat element 4 , the entire resistance value is about 2000 ⁇ , which is less than 1 k ⁇ .
  • the impurity concentration when the resistivity ( ⁇ ) is 8 ⁇ 10 ⁇ 4 ⁇ cm or less is in a region of 2 ⁇ 10 20 (cm ⁇ 3 ) or more shown by a region 25 in FIG. 6.
  • the resistance value of the heat element is selected from 50 to 900 ⁇ , while the resistance values of the temperature measuring resistors 5 a to 5 d and 6 are selected from 1 to 10 k ⁇ in view of power source voltage and the amount of heat generation in this embodiment.
  • FIGS. 8 to 10 are explanatory views of a barrier layer used in the sensor device of the gas flowmeter according to the first embodiment of this invention. Identical reference numerals with those in FIG. 1 show identical portions.
  • FIG. 8 shows is an enlarged view for a portion B in FIG. 2 which lacks the barrier layers 8 a and 8 b shown in FIG. 1.
  • the insulating films 7 a , 7 b , 7 c and 7 d comprising silicon dioxide (SiO 2 ) are formed above and below the heat element 4 comprising the polycrystalline silicon (Si) semiconductor film. Crystal grains 29 of silicon (Si), grain boundaries 28 , and boundary faces 30 with silicon dioxide (SiO 2 ) are present in the polycrystalline silicon (Si) semiconductor film 4 .
  • FIG. 9A in the crystal grains 29 of silicon (Si) shown in FIG. 8, silicon (Si) atoms are regularly arranged and bonded in a single crystalline state to form a chemically stable ideal structure.
  • FIG. 9B regular arrangement and bonding of the silicon (Si) atoms are intervened into an instable state in the grain boundaries 28 , at the boundary faces with silicon oxide (SiO 2 ) and in the defects (not illustrated) in the crystal grains 29 .
  • FIG. 10 shows a constitution of forming the barrier layers 8 a and 8 b in this embodiment. Since the barrier layers 8 a and 8 b stop diffusion and dissociation of hydrogen, the Si—H dangling bonds are stabilized and the aging change of the resistance is decreased also in a case of measuring the flow rate for a long period of time by heating the heat element.
  • FIG. 11 is an explanatory view of a barrier layer used in the sensor device of the gas flowmeter according to the first embodiment of this invention.
  • FIG. 11 shows the result of the experiment for the secular change of the resistance of the heat element 4 in a case where the barrier layers 8 a and 8 b are formed above and below the heat element 4 comprising the polycrystalline silicon (Si) semiconductor film (solid line 32 ) and in a case of the constitution with no barrier layers (solid line 31 ).
  • the abscissa denotes a current supply time while the ordinate shows the change of resistance after current supply.
  • the heat element 4 is heated to 250° C. under current supply.
  • the secular change of the resistance after 1000 hours of the current supply time for heating is about 1% in the constitution with no barrier layers.
  • the solid line 32 it can be seen that the secular change of the heat element 4 can be improved greatly by forming the barrier layers 8 a and 8 b above and below the heat element 4 comprising the polycrystalline silicon (Si) semiconductor film.
  • barrier layer 8 a when only one barrier layer 8 a is formed, the result was substantially equal with the result of lacking the barrier layer. It is effective in the suppression of the secular change of the heat element 4 to form the barrier layers 8 a and 8 b for both the layers above and below the heat element 4 comprising the polycrystalline silicon (Si) semiconductor film, thereby surrounding the heat element 4 with the barrier layers.
  • FIGS. 12 and 13 are explanatory views of further methods of preventing secular change of the sensor device of the gas flowmeter according to the first embodiment of this invention.
  • a heat treatment annealing is applied in an atmosphere at least containing a hydrogen gas in a case where hydrogen is contained in the gas to be measured and a heat treatment annealing is applied in an inert gas atmosphere in a case where hydrogen is not contained in the gas to be measured, within a temperature range from 500° C. to 900° C., respectively.
  • FIGS. 12 and 13 show the effects of the heat treatment annealing of the resistance and the secular change of the resistance of the heat element 4 , respectively, in the inert gas (nitrogen, argon, helium, fluorine, etc.) atmosphere, the heat element 4 comprising the polycrystalline silicon (Si) semiconductor film formed with the barrier layers 8 a and 8 b in a case where hydrogen is not contained gas to be measured.
  • the inert gas nitrogen, argon, helium, fluorine, etc.
  • the stoichiometrically stable silicon nitride (Si 3 N 4 ) thin film of the barrier layers 8 a and 8 b has a characteristic of less permeating and absorbing hydrogen at 550° C. or lower but easily permeating hydrogen at a temperature of 550° C. or higher.
  • the secular change of the resistor can be further decreased in a heat generating temperature range of the heat element 4 ( ⁇ 550° C.) in the subsequent stage.
  • phosphorus (P) impurity-doped at high concentration into the polycrystalline silicon (Si) semiconductor film is activated and phosphorus (P) starts to diffuse from the grain boundaries and boundaries where phosphorus is precipitated in a great amount into crystal grains and the decrease of phosphorus (P) at the grain boundaries and the boundaries results in the increase of Si—H dangling bonds, so that the aging change of resistance rather increases.
  • a temperature range from 550° C. to 900° C. is optimal for the heat treatment annealing.
  • the resistance value and the temperature coefficient of resistance optimal for each of the resistors can be obtained and, since phosphorus (P) precipitated in a great amount at the grain boundaries, defects and the boundary faces of the silicon (Si) semiconductor film inhibit the formation of Si—H dangling bonds, the secular change of the resistance for each of the resistors can be reduced.
  • impurities such as phosphorus (P) at high concentration such that the resistivity ( ⁇ ) of the silicon (Si) semiconductor thin film is 8 ⁇ 10 ⁇ 4 ⁇ cm or less
  • barrier film materials having a characteristic of less permeating and absorbing hydrogen in the heat generating temperature range of the heat element 4 , for example, gold (Au) or silver (Ag).
  • the use of a hydrogen absorptive and storing type metal such as a titanium (Ti, Ti alloy) system or nickel (Ni, Ni alloy) system is not appropriate as the material for the barrier layers 8 a and 8 b .
  • a hydrogen absorptive and storing type barrier layer hydrogen once stored in the barrier layer repeats precipitation and storage with lapse of time of heat generation of the heat element 4 . Accordingly, formation of the Si—H dangling bonds in the silicon (Si) semiconductor film fluctuates with time and the resistance value of the heat element 4 is unstable and is not maintained constant.
  • silicon dioxide (SiO 2 ) is not suitable as the material for the barrier layers 8 a and 8 b since this can easily permeate hydrogen in the heat generating temperature range of the heat element 4 , and the silicon (Si) semiconductor film is neither suitable to the barrier layer material since this is a hydrogen absorptive and storing type material.
  • the gas to be measured not containing hydrogen can include, for example, air, nitrogen, oxygen, gaseous carbon dioxide and steams, while the gas to be measured containing hydrogen includes hydrogen, city gas, methane, propane or butane as the target.
  • silicon (Si) semiconductor film has described as the polycrystalline.
  • regular arrangement/bonding of silicon (Si) atoms are intervened into an instable state, as shown in FIG. 9B, at the boundary faces 30 with the silicon dioxide (SiO 2 ) and in the defects in the crystal grains 29 (not shown) as shown in FIG. 8, and bonding of silicon (Si) and hydrogen (H) (dangling bonds) in which regular arrangement/bonding are intervened are formed.
  • this embodiment also has a similar effect on the single crystalline silicon (Si) semiconductor film.
  • barrier layers which less permeate and absorb hydrogen are formed above and below the heat element as the silicon (Si) semiconductor thin film impurity-doped at a high concentration respectively so as to surround the heat element, the secular change upon measurement of the gas flow rate can be decreased and the gas flow rate can be measured at high reliability.
  • the heat treatment annealing is applied at a temperature of 550° C. or higher and 900° C. or lower in an atmosphere containing at least a hydrogen gas after forming the upper barrier layer described above in a case where hydrogen is contained in the gas to be measured, and in an inert gas atmosphere after forming the barrier layer in a case where hydrogen is not contained in the gas to be measured, by which the secular change upon measurement of the gas flow rate can be decreased and, accordingly, the gas flow rate can be measured at high reliability.
  • FIG. 14 is a cross sectional view showing the constitution of the gas flowmeter according to the second embodiment of this invention. Identical reference numerals with those in FIG. 1 show identical portions.
  • barrier layers which less permeate and absorb hydrogen are formed above and below the heat element as the silicon (Si) semiconductor thin film impurity-doped at a high concentration respectively so as to surround the heat element, the secular change upon measurement of the gas flow rate can be decreased and the gas flow rate can be measured at high reliability.
  • FIG. 15 is a cross sectional view showing a constitution of a gas flowmeter according to the third embodiment of this invention. Identical reference numerals with those in FIG. 1 show identical portions.
  • the principal constitution of the gas flowmeter in this embodiment is identical with that shown in FIG. 14 and it further has a feature in saving the insulating film 7 c provided in the second embodiment shown in FIG. 14. It is also possible to save the insulating film 7 b.
  • barrier layers which less permeate and absorb hydrogen in the heat generating temperature range of the heat element are disposed respectively above and below the heat element of the silicon (Si) semiconductor thin film impurity doped at high concentration so as to surround the heat element, so that the secular change upon measurement of the gas flow rate can be reduced and, accordingly, the gas flow rate can be measured at high reliability is possible.
  • the secular change of the gas flowmeter can be reduced.

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US7158895B2 (en) * 2004-02-12 2007-01-02 Denso Corporation Device for detecting hydrogen concentration and method of detecting hydrogen concentration
US20050182574A1 (en) * 2004-02-12 2005-08-18 Yukihiro Sano Device for detecting hydrogen concentration and method of detecting hydrogen concentration
EP1882912A2 (en) * 2006-07-24 2008-01-30 Hitachi, Ltd. Thermal gas flow sensor
EP1882912A3 (en) * 2006-07-24 2008-12-31 Hitachi, Ltd. Thermal gas flow sensor
US20080188027A1 (en) * 2007-02-05 2008-08-07 Denso Corporation Semiconductor device having impurity-doped resistor element
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US20110127584A1 (en) * 2008-07-25 2011-06-02 Naoki Ushiyama Method for manufacturing infrared image sensor and infrared image sensor
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