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WO1993000581A1 - Detecteur de gaz - Google Patents

Detecteur de gaz Download PDF

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Publication number
WO1993000581A1
WO1993000581A1 PCT/GB1992/001148 GB9201148W WO9300581A1 WO 1993000581 A1 WO1993000581 A1 WO 1993000581A1 GB 9201148 W GB9201148 W GB 9201148W WO 9300581 A1 WO9300581 A1 WO 9300581A1
Authority
WO
WIPO (PCT)
Prior art keywords
gas sensor
semiconductor
semiconductor material
chemico
particles
Prior art date
Application number
PCT/GB1992/001148
Other languages
English (en)
Inventor
Carl John Sofield
Patrick Timothy Moseley
Original Assignee
United Kingdom Atomic Energy Authority
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by United Kingdom Atomic Energy Authority filed Critical United Kingdom Atomic Energy Authority
Publication of WO1993000581A1 publication Critical patent/WO1993000581A1/fr
Priority to GB9324390A priority Critical patent/GB2271643B/en

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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

Definitions

  • This invention relates to a gas sensor and more especially to a semiconductor gas sensor and its method of manufacture.
  • semiconductor gas sensor is meant a gas sensor which comprises a body of a semiconductor material typically a metal oxide, which is deposited on an electrically insulating substrate and is in contact with at least two electrodes.
  • the semiconductor material is such that its electrical resistance changes in the presence of gases to be detected. This change in resistance may be an increase or a decrease.
  • the change in resistance of the semiconductor material on exposure to a gas can be measured via the electrodes using a suitable amplifier and detector.
  • the electrodes are typically referred to as resistance electrodes.
  • Semiconductor gas sensors which operate in this manner may be referred to as resistance effect gas sensors.
  • Japanese laid-open patent application No. 61-104070 describes a method of vacuum depositing a thin film of material containing oxygen in which the material is irradiated with an electron beam during its deposition in the presence of a gaseous atmosphere containing oxygen as a constituent element.
  • the object of the invention is to introduce oxygen into the material being deposited without causing a deterioration in the crystallinity of the thin film. It is stated that thin films composed of substances containing oxygen or oxides may be used, amongst other things, in various gas sensors.
  • a semi-conductor gas sensor comprising, an insulating substrate, a body of semiconductor material formed on the insulating substrate chemico-radiolytically activated in situ, and at least two electrodes in electrical contact with the chemico-radiolytically activated semiconductor material.
  • a method of manufacturing a semiconductor gas sensor including the operations of contacting an active surface of a body of semiconductor material forming part of the semiconductor gas sensor with an interactive material capable of interacting with the semiconductor material under the influence of ionising radiation or high energy particles to alter the electrical properties of the semiconductor material, and exposing the surface of the body of semiconductor material to the ionising radiation or high energy particles thereby to activate the semiconductor material chemico-radiolytically and alter the electrical properties thereof.
  • the semiconducting material may be an oxide, for example ZnO, SnO 2 , ln 2 O 3 , MoO 3 , Ga 2 O 3 , Nb 2 O 5 , WO 3 , TiO 2 , Fe 2 O 3 , Ta 2 O 5 , bronzes such as Bi 6 Fe 4 Nb 6 O 30 , niobates such as K 2 Nb 6 O 16 , pyrochlores such as Bi 2 Sn 2 O 7 , tantalates such as KTaWO 6 , perovskite such as SrSnO 3 or tungsten oxides such as Na 0.1 Nb 0.1 W 0.9 O 3 .
  • the semiconducting material may be a non-oxide such as CdS.
  • the semiconductor may be an n type, p type or a mixture of semiconductor materials or it may be an organic semiconductor such as a semiconducting polymer.
  • the substrate may be of any material which is electrically insulating such as alumina.
  • chemico-radiolytically activated is meant a modification or change in the electrical properties which occurs at the active surface of a semiconductor material forming part of a semiconductor gas sensor as a consequence of that surface being treated with ionising radiation or high energy particles in the presence of an interactive material in its gaseous or liquid form.
  • ionising radiation radiation which induces ionisation on impinging upon a semiconductor material for example ⁇ radiation, ⁇ radiation or UV light.
  • high energy particles is meant ions, atoms, nuclei or molecules which have sufficient energy to induce ionisation on impacting upon a semiconductor material for example H + , Cl 6+ or ⁇ particles.
  • interactive material any material which in the presence of ionising radiation or high energy particles interacts physically or chemically with the active surface of a semiconductor material forming part of a semiconductor gas sensor during chemico-radiolytic activation.
  • Suitable interactive materials include for example N 2 , O 2 , Air, H 2 O, S and sulphur-containing materials, hydrocarbons and other organic materials, catalytic metals such as Pd and Pt in organometallic compounds and any material which can be used to produce a mixed metallic oxide.
  • These interactive materials may be used in their gaseous or liquid forms and may be chemisorbed or physisorbed onto the active surface of the semiconductor material before or during its chemico-radiolytic activation.
  • the ionising radiation or high energy particles may ionise the active surface of the semiconductor material or the interactive material or both and that the interactive material in gaseous or liquid form interacts physically or chemically with the active surface of the semiconductor material to produce a chemico-radiolytically activated surface.
  • the chemico-radiolytic activation of a semiconductor gas sensor may be carried out at ambient, sub-ambient or elevated temperatures , ambient or elevated pressures or in a partial vacuum.
  • the semiconductor gas sensor and the interactive material in gaseous or liquid form may be at the same or different temperatures during chemico- radiolytic activation.
  • high energy particles When high energy particles are used their energy may be between 1 keV up to 40 MeV, most preferably between 2 MeV and 30 MeV. When ionising radiation is used, its energy will depend on the nature of the semiconductor material, but should be sufficient to induce ionisation of the semiconductor material in the active surface region of the semiconductor material.
  • the fluence of high energy particles may be up to 1 ⁇ 10 20 particles cm -2 and is typically in the range 1 ⁇ 10 12 - 1 ⁇ 10 16 particles cm -2 .
  • the semiconductor gas sensor according to the present invention has superior properties compared to known semiconductor gas sensors.
  • the activated semiconductor gas sensor may have increased sensitivity, increased selectivity, and/or improved high temperature stability. Chemico-radiolytic activation may increase or decrease the background resistance in air of an activated gas sensor.
  • the semiconductor gas sensor according to the present invention may be used to detect any material which induces a change in the resistivity of the chemico-radiolytically activated semiconductor material.
  • Gaseous materials which may be detected include for example; O 2 , NH 3 , C 2 H 4 , CO, H 2 and CH 4 .
  • semiconductor gas sensors may need to be operated at elevated temperatures for example 400-600oC or greater.
  • the semiconductor gas sensor can be brought to these operating temperatures by a resistance heating element which is in contact with the electrically insulating substrate.
  • the chemico-radiolytically activated semiconductor material may have reduced resistivity compared to that before chemico-radiolytic activation. This may be as a consequence of radiation damage to the bulk semiconductor material and/or as a consequence of chemico-radiolytic activation. If reduced resistivity is observed it is preferred that the chemico-radiolytic activation is followed by a thermal annealing stage.
  • the thermal annealing stage requires the chemico-radiolytically activated gas sensor to be held at an elevated temperature e.g. 800°C until the resistivity of the sensor reaches at least 90% of the resistivity before chemico-radiolytic activation and more preferably 100%.
  • Figure 1 shows a schematic longitudinal sectional view of a semiconductor gas sensor suitable for chemico-radiolytic activation
  • Figure 2 shows graphically the variation of Log resistance ( ⁇ ) with elapsed time (minutes) for the exposure of the semiconductor gas sensor of Figure 1 , before ( - - - - ) , and after , chemico-radiolytic activation, to a variety of gases at an operational temperature of 450°C;
  • Figure 3 shows graphically the variation of the resistance in air minus the resistance in 1% CO divided by the resistance in air of the semiconductor gas sensor of Figure 1 ( ⁇ ) and the same semiconductor gas sensor after chemico-radiolytic activation ( ⁇ ) with temperature (°C) for the detection of 1% carbon monoxide in air;
  • Figure 4 shows graphically the variation of resistance (kohms) with elapsed time (minutes) for the exposure of tin oxide gas sensors, (a) irradiated with 4 MeV protons in an air atmosphere; and (b) irradiated with 4 MeV protons in a nitrogen atmosphere, to one pulse of 1% methane in air and one pulse of 1% carbon monoxide in air at an operational temperature of 400°C;
  • Figure 5 shows graphically the variation of resistance (kohms) with elapsed time (minutes) for the exposure of tin oxide gas sensors (a) comprising 3 coats of tin oxide from a sol and (b) 1 coat of tin oxide from a sol, to one pulse of 1% methane in air and one pulse of 1% carbon monoxide in air at an operational temperature of 450°C;
  • Figure 6 shows graphically the variation of resistance (kohms) with elapsed time (minutes) for the exposure of the tin oxide gas sensors (a) and (b) of Figure 5, after treatment with 4 MeV protons in an air atmosphere, to one pulse of 1% methane in air and one pulse of 1% carbon monoxide in air at an operational temperature of 450oC;
  • Figure 7 shows graphically the variation of resistance (kohms) with elapsed time (minutes) for the exposure of a tin oxide gas sensor comprising 3 coats of tin oxide from a sol (a) before; and (b) after treatment with 4 MeV protons in air, to one pulse of 1% methane in air and one pulse of 1% carbon monoxide in air at an operational temperature of 600°C;
  • Figure 8 shows graphically the variation of resistance (kohms) with elapsed time (minutes) for the exposure of a tin oxide gas sensor comprising 1 coat of tin oxide from a sol (a) before; and (b) after treatment with 4 MeV protons in air, to one pulse of 1% methane in air and one pulse of 1% carbon monoxide in air at an operational temperature of 600°C;
  • Figure 9 shows graphically the variation of Log resistance (ohms) with elapsed time (minutes) for the exposure of gas sensors 1, 2, 3 and 4 of Table 3 to a variety of gases at an operational temperature of 450°C;
  • Figure 10 shows graphically the variation of Log resistance (ohms) with elapsed time (minutes) for the exposure of gas sensors 3 and 4 of Table 3 (a) before; and (b) after ageing at 860°C and 780°C for 60 hours in air for sensor 3 and 4 respectively, to a variety of gases at an operational temperature of 600°C;
  • Figure 11 shows graphically the variation of Log resistance (ohms) with elapsed time (minutes) for the exposure of gas sensor 1 of Table 3 to a variety of gases at an operational temperature of 600°C;
  • Figure 12 shows graphically the variation of Log resistance (ohms) with elapsed time (minutes) for the exposure of gas sensor 2 of Table 3 to a variety of gases at an operational temperature of 600°C.
  • a semiconductor gas sensor 11, suitable for chemico-radiolytic activation includes an electrically insulating substrate 12 which acts as a support for a layer 13 of a semiconductor material, interdigitated electrodes 14 and a resistance heating element 15.
  • the semiconductor gas sensor of Figure 1 is irradiated or bombarded such that the ionising radiation or high energy particles impinge or impact upon an active surface 16 of the layer 13 of semiconductor material.
  • Alumina substrates in the form of thin wafers were coated on one side with a Pt based coating to form a resistance heater element.
  • the reverse side of each wafter was coated with a Pt based coating to form a series of interdigitated resistance electrodes.
  • SnO 2 semiconductor material was then sputter coated onto the substrates to a thickness of approximately 200nm in order to cover the interdigitated electrodes whilst maintaining the resistance heater element side of the substrate free from SnO 2 semiconductor material.
  • Semiconductor gas sensors as prepared above were chemico-radiolytically activated by treatment with either 4 MeV protons in air at a flux of up to 10 16 protons per cm 2 or 30 MeV Cl 6+ ions in a vacuum (10 -6 Torr) at a fluence of up to 5 x 10 14 ions per cm 2 .
  • the resistivity of the sensors in air was measured, if this was less than 90% of that recorded before treatment the sensors were annealed at 800°C until the resistivity was more than 90% of the resistivity of the untreated sensors.
  • the chemico- radiolytically activated gas sensors were used at 450°C to detect various gases by monitoring the change in resistivity of the modified sensors on exposure to the gas.
  • the responses for various gases are illustrated in Figure 2 and the effect of activation or sensitivity to a number of gases are listed in Table 1.
  • the results in relation to CO 2 are an indication of O 2 sensitivity as the sensors were exposed to nominally 100% CO 2 .
  • Example 2 An alumina substrate in the form of a thin wafer was coated on one side with a Pt based coating to form a resistance heater element. A gold based coating was deposited on the reverse side of the wafer to form an interdigitated electrode array. Tin oxide was then deposited by radio frequency (RF) sputtering to produce films of thickness of approximately 1 ⁇ m in order to cover the interdigitated electrodes whilst maintaining the resistance heater element side of the substrate free from SnO 2 semiconductor material.
  • RF radio frequency
  • the tin oxide gas sensors as prepared above were chemico-radiolytically activated by treatment with either 4 MeV protons in an air atmosphere at a dose of 3 ⁇ 10 16 particles cm -2 or 4 MeV protons in a nitrogen atmosphere at a dose of 3 ⁇ 10 16 particles cm -2 .
  • the gas response behaviour of the activated sensors was measured at 400°C, 450°C, 500°C, 550-C and 600°C for 1% methane in air and 1% CO in air.
  • the activation increased the background resistance (i.e. in air) of the sensors.
  • Activation with protons in a nitrogen atmosphere for one sample increased the background resistance by approximately 21% from 3.48 M ⁇ to 4.2 M ⁇ and for another sample with protons in air by approximately 37% from 2.70 M ⁇ to 3.70 M ⁇ .
  • the gas response behaviour as measured at 400°C is illustrated in Figure 4.
  • the normal response to methane (a resistance decrease) was not seen; instead the sensor irradiated in air (a) shows little net response, and that irradiated in nitrogen (b) shows a small resistance increase.
  • the sample irradiated in nitrogen shows a significantly greater response to carbon monoxide than the sample irradiated in air. This difference was maintained at operating temperatures up to 500°C.
  • Example 3 An alumina substrate was prepared as in Example 2 with the exception that the tin oxide was deposited from a tin oxide sol, of concentration 100 gl -1 . by dipping the substrate into the sol, drying, and firing in air at 600°C. Specimens were prepared with either 1 coat or 3 coats of tin oxide and each coat was estimated to be greater than l ⁇ m thick on the basis of a comparison with results of deposition onto a glass substrate.
  • Specimens were irradiated with a beam of 4 MeV protons to a dose of 3 ⁇ 10 16 particles cm -2 in an atmosphere of air.
  • the resistance of the 1 coat was higher than the 3 coat; this relationship was repeated at temperatures up to 600°C.
  • the activated 3 coat sensor exhibited higher resistance over the non-irradiated sensor at all temperatures up to 600°C. This was also true for the 1 coat sensor up to 500°C but at 600°C the difference was virtually lost.
  • Alumina substrates were preared as in Example 2 to produce a tin oxide gas sensor.
  • the SnO 2 films after deposition were black due to a small deficiency of oxygen which was readily restored by firing in air at up to 850°C for 20 minutes.
  • These sensors were then chemico-radiolytically activated with various particle beams, doses and exposure atmospheres using a 6 MV tandem accelerator. The resistance (ohms) in air was measured at 450°C for each sensor and the results are listed in Table 3.
  • the gas response of these sensors was determined as described in Example 2, for methane, carbon monoxide, hydrogen, ethene and ammonia. These gases were used as 1% mixtures in dried air with the exception of ethene which was used as a 0.7% mixture in dried air and hydrogen which was used as a 100 ppm or 1% mixture in dried air. Intermediate purges of dried air were also used. In addition nominally 100% CO 2 (which is equivalent to 10 -6 atmospheres of O 2 ), and nominally 100 ppm N 2 in Argon were also included to test for O 2 sensitivity. For each test the gas response cycle was repeated to ensure that no irreversible modification of the gas sensing surface had occurred during the first exposure. The gas response results are illustrated in Figure 9, 10, 11 and 12.
  • the gas response data also illustrates that chemicoradiolytic activation can improve the selectivity of gas sensors.
  • sensor 1 irradiation with 4 MeV protons in air increased its sensitivity to oxygen, carbon monoxide, hydrogen and ammonia when measured at 600°C but had little effect on the sensitivity to 1% CH 4 , 100% carbon dioxide and 0.7% C 2 H 4 (see Figures 11 and 12).
  • the activation has caused a decrease in resistance and for certain combinations of conditions an increase in gas response which may not apply to all gases equally and hence the activation improves selectivity.
  • chemico-radiolytic activation is capable of substantially modifying the gas sensing properties of semiconductor gas sensors such as those made from tin oxide.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
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  • Pathology (AREA)
  • Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)

Abstract

Des détecteurs de gaz à semi-conducteurs d'une sensibilité ou d'une sélectivité ou d'une stabilité thermique modifiées sont préparés par activation chimio-radiolytique du détecteur; on effectue ceci en exposant le détecteur à un rayonnement ou à un bombardement, notamment à un rayonnement ionisant ou à un bombardement par des particules d'énergie élevée, en présence d'un matériau d'interaction sous sa forme liquide ou gazeuse.
PCT/GB1992/001148 1991-06-26 1992-06-24 Detecteur de gaz WO1993000581A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB9324390A GB2271643B (en) 1991-06-26 1993-11-24 Gas sensor

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB919114038A GB9114038D0 (en) 1991-06-26 1991-06-26 Gas sensor
GB9114038.4 1991-06-26

Publications (1)

Publication Number Publication Date
WO1993000581A1 true WO1993000581A1 (fr) 1993-01-07

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PCT/GB1992/001148 WO1993000581A1 (fr) 1991-06-26 1992-06-24 Detecteur de gaz

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AU (1) AU2153392A (fr)
GB (2) GB9114038D0 (fr)
WO (1) WO1993000581A1 (fr)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2754901A1 (fr) * 1996-10-21 1998-04-24 Sagem Capteur chimique de gaz et procede de fabrication d'un tel capteur
US5823044A (en) * 1997-04-30 1998-10-20 Ford Global Technologies, Inc. Method for selective gas sensors based on nonlinear gas reactions
US5831145A (en) * 1997-04-30 1998-11-03 Ford Global Technologies, Inc. Method of introducing selectivity to nonselective gas sensors
US5863803A (en) * 1996-12-20 1999-01-26 Ford Global Technologies, Inc. Method and apparatus for modulating a gas sample for a calorimetric gas sensor
US6131438A (en) * 1996-12-20 2000-10-17 Ford Global Technologies, Inc. Method of operating a calorimetric gas sensor
US6298291B1 (en) 1999-12-22 2001-10-02 Visteon Global Technologies, Inc. Method of establishing baseline filter for air quality

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5621320A (en) * 1979-07-28 1981-02-27 Fujitsu Ltd Manufacture of semiconductor device
DE3342230A1 (de) * 1983-02-21 1984-08-23 Veb Kombinat Robotron, Ddr 8012 Dresden Detektor zum nachweis von gasen
JPS61104070A (ja) * 1984-10-25 1986-05-22 Toshiba Corp 薄膜形成法
JPS62119193A (ja) * 1985-11-15 1987-05-30 Matsushita Electric Ind Co Ltd 半導体の製造方法
US4670291A (en) * 1984-10-05 1987-06-02 Osaka University Method of controlling supersaturated injection and concentration of exotic atoms into deep portions of a solid with a high energy electron beam
EP0230104A2 (fr) * 1985-12-13 1987-07-29 General Motors Corporation Semi-conducteur détecteur de gaz comportant un site thermiquement isolé
GB2194857A (en) * 1986-07-28 1988-03-16 Mitsubishi Electric Corp Apparatus utilizing charged particles
JPS63303899A (ja) * 1987-05-30 1988-12-12 Matsushita Electric Ind Co Ltd 半導体の製造方法

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5621320A (en) * 1979-07-28 1981-02-27 Fujitsu Ltd Manufacture of semiconductor device
DE3342230A1 (de) * 1983-02-21 1984-08-23 Veb Kombinat Robotron, Ddr 8012 Dresden Detektor zum nachweis von gasen
US4670291A (en) * 1984-10-05 1987-06-02 Osaka University Method of controlling supersaturated injection and concentration of exotic atoms into deep portions of a solid with a high energy electron beam
JPS61104070A (ja) * 1984-10-25 1986-05-22 Toshiba Corp 薄膜形成法
JPS62119193A (ja) * 1985-11-15 1987-05-30 Matsushita Electric Ind Co Ltd 半導体の製造方法
EP0230104A2 (fr) * 1985-12-13 1987-07-29 General Motors Corporation Semi-conducteur détecteur de gaz comportant un site thermiquement isolé
GB2194857A (en) * 1986-07-28 1988-03-16 Mitsubishi Electric Corp Apparatus utilizing charged particles
JPS63303899A (ja) * 1987-05-30 1988-12-12 Matsushita Electric Ind Co Ltd 半導体の製造方法

Non-Patent Citations (4)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 005, no. 070 (E-056)12 May 1981 & JP,A,56 021 320 ( FUJITSU LTD ) 27 February 1981 *
PATENT ABSTRACTS OF JAPAN vol. 010, no. 287 (C-375)30 September 1986 & JP,A,61 104 070 ( TOSHIBA CORP ) 22 May 1986 cited in the application *
PATENT ABSTRACTS OF JAPAN vol. 011, no. 349 (C-456)14 November 1987 & JP,A,62 119 193 ( MATSUSHITA ELECTRIC IND CO LTD ) 30 May 1987 *
PATENT ABSTRACTS OF JAPAN vol. 013, no. 137 (C-582)5 April 1989 & JP,A,63 303 899 ( MATSUSHITA ELECTRIC IND CO LTD ) 12 December 1988 *

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2754901A1 (fr) * 1996-10-21 1998-04-24 Sagem Capteur chimique de gaz et procede de fabrication d'un tel capteur
US5863803A (en) * 1996-12-20 1999-01-26 Ford Global Technologies, Inc. Method and apparatus for modulating a gas sample for a calorimetric gas sensor
US6131438A (en) * 1996-12-20 2000-10-17 Ford Global Technologies, Inc. Method of operating a calorimetric gas sensor
US5823044A (en) * 1997-04-30 1998-10-20 Ford Global Technologies, Inc. Method for selective gas sensors based on nonlinear gas reactions
US5831145A (en) * 1997-04-30 1998-11-03 Ford Global Technologies, Inc. Method of introducing selectivity to nonselective gas sensors
US6298291B1 (en) 1999-12-22 2001-10-02 Visteon Global Technologies, Inc. Method of establishing baseline filter for air quality

Also Published As

Publication number Publication date
GB2271643A (en) 1994-04-20
GB2271643B (en) 1994-08-24
GB9324390D0 (en) 1994-02-09
AU2153392A (en) 1993-01-25
GB9114038D0 (en) 1991-08-14

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