SENSOR AND METHOD OF PRODUCING SAME

Abstract: A sensor comprising a metal heat generating member formed with a heat-resistant insulating coating of Al₂O₃ or the like over its surface, and an atmosphere-­ sensitive layer of SnO₂ or the like supported on the coating. The insulating coating is formed, for example, by alumina sol coating and thermal decomposition, or plasma CVD of an aluminum compound. The sensitive layer is formed, for example, by the thermal decomposition of an organic compound of tin, vacuum evaporation or sputtering of tin, or printing of SnO₂ on the heat generating member when the member is in the form of a flat plate. At least one electrode is connected to the sensitive layer for deriving an output from the layer in response to a combustible gas, humidity or the like.

Description: FIELD OF THE INVENTION The present invention relates to a sensor for detecting components of atmospheres such as combustible gases, toxic gases, oxygen and water vapor, and a method of producing the same. More particularly, the invention relates to a sensor supported by a metal heat generating member serving as a heater and a method of producing the same. The invention sensor is used, for example, for the detection of oxygen, combustible gases, water vapor or the like utilizing variations in the resistance value of a metal oxide semiconductor, and for the detection of hydrogen, carbon monoxide, water vapor or the like using a proton conductor. PRIOR ART One of the basic problems in the field of gas sensors and humidity sensors is a reduction in power consumption. In this respect, U.S. Patent No. 4,343,768 proposes to subject a silicon substrate formed with an SiO₂ coating to undercut etching at its lower portion and to provide a bridge of SiO₂ coating at the etched portion. The bridge is thin and slender and is diminished in heat capacity and thermal conductivity. A metal oxide semiconductor, such as SnO₂, and a heater are provided on the bridge to afford a gas sensor. The documents US-A 4 007 435, FR-A 2 087 004 and GB-A-2 151 796 describe gas sensors comprising gas sensitive metal oxide semiconductor films supported on insulating ceramic substrates better illustrated in US-A-4 007 435. The power consumption of these sensors for heating the metal oxide films at their operating temperatures is not small. This is because ceramic substrates have large thermal capacity and large thermal conductivity, and because the insulating substrates may not be reduced in size below a certain lower limit. SUMMARY OF THE INVENTION An object of the present invention is to reduce the power consumption of sensors by a solution different from the above and to provide a method of producing such sensors. Another object of the invention is to produce such sensors with greater ease and to further reduce the power consumption of sensors. According to the invention, a metal heat generating member having a heat-resistant insulating coating on its surface serves as a support for a sensor. More specifically, a noble metal such as Pt or Pd-Ir alloy or a base metal such as Fe-Cr-Al alloy or Ni-Cr alloy is formed with a heat-resistant insulating coating over the surface to provide a metal heat generating member for use in this invention. The coating can be formed, for example, by plasma CVD, sputtering, ion plating, vacuum evaporation or CVD. When suitable coating conditions are selected, a compact coating can be obtained easily as firmly bonded to the metal. Subsequently, an atmosphere-sensitive layer is provided on the coating, whereby a sensor can be obtained. The atmosphere-sensitive layer is prepared from a substance whose resistance value or electromotive force varies on contact with the gas or water vapor to be detected. For example, the layer may be formed by a metal oxide semiconductor such as SnO₂, In₂O₃, TiO₂, LaCoO₃ or BaSnO₃ to detect a variation in its resistance value due to contact with a combustible gas, toxic gas, oxygen, water vapor or the like. Alternatively, a proton conductor may be used for the sensitive layer to detect a variation in its electromotive force due to hydrogen or carbon monoxide, or a variation in its resistance due to water vapor. At least one electrode is connected to the atmosphere-sensitive layer for detecting variations in resistance value or electromotive force. The atmosphere-sensitive substance can be separated from the metal heat generating member by the insulating coating. Many atmosphere-sensitive substances are metal oxides and are low in the strength of bond to metals. However, the insulating layer is usually formed of ceramics, almost completely eliminating the likelihood of the sensitive substance from dislodging or separating from the coating. Further if a heat generating member of base metal is brought into direct contact with the atmosphere-sensitive substance, there arises the problem of poisoning or deterioration of the sensitive substance by the base metal. Nevertheless, the heat-resistant insulating substance coating the surface of the base metal member obviates this problem. The configuration of the sensor is dependent on the metal heat generating member itself and can be made extremely small. Consequently, the member can be such as is sufficient to support itself against gravity, making the sensor compacter and resulting in a reduced heat capacity. Generally, such sensors are fabricated by: 1) forming a heat-resistant insulating coating at least partially on a metal heat generating member, 2) providing an atmosphere-sensitive layer over the surface of the metal heat generating member so as to be supported at least partially on the coating, and 3) connecting at least one electrode to the atmosphere-sensitive layer. The electrode may be provided before or after the layer is formed. In the case where the heat generating member is made of base metal, it is desirable to separate the sensitive layer from the member by the insulating coating and to connect at least two electrodes to the sensitive layer. When the heat generating member is made of noble metal, the member may serve also as one of the two electrodes for the sensitive layer. In this case, the sensor can be such that a major portion of the sensitive layer is supported on the heat generating member, with only a minor portion of the layer supported on the insulating coating. Since it is difficult to attach a lead wire to the heat generating member which is small, the electrode is connected directly to the external electrode. The lead connecting work can then be dispensed with. When the heat generating member itself is made small-sized and lower in thermal conductivity, noticeably increased power consumption will not result. To assure diminished power consumption, it is desirable to make the metal heat generating member smaller. This nevertheless renders the member difficult to handle. It is therefore preferable to prepare a frame having a multiplicity of heat generating members attached thereto as arranged in series, form an insulating coating and atmosphere-sensitive layer on the frame, position the frame properly for external leads, and thereafter connect each member to the desired leads and separate the member from the frame. The generating member can then be positioned properly for the external leads and connected thereto with ease. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a fragmentary plan view of an embodiment; and Fig. 2 is an enlarged fragmentary view in section taken along the line I-I in Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS Construction of Sensor The heat generating member 14 is made of a noble metal material such as Pt, Ir, Pd 80-Ir 20 or Pt-ZGS (wherein ZrO₂ particles are separated out at Pt grain boundary), or a base metal material such as W, Fe-Cr-Al or Ni-Cr. Preferably, the member 14 is made of a material which is high in durability against atmospheres, heat resistance and electric resistance. Especially preferable is a high-resistance base metal heat generating material. In this embodiment, an Fe-Cr-Al alloy wire was used (Kanthal wire, product of Kanthal, Sweden; Kanthal is a trademark of the company). The member is in the form of a wire having such a diameter as to support the sensitive layer 20 thereon. More specifically, it is sufficient that the wire diameter be about 10 micrometers, but in view of the difficulty to be encountered in wire drawing, the diameter is preferably about 20 to about 80 micrometers. The heat-resistant insulating coating 18 is made of a material which is stable to the atmosphere to which the sensor is exposed and nonreactive with the member 14 or with the sensitive layer 20, and exhibits high strength to bond to the member 14 and affords high insulation resistance between the member 14 and the layer 20. Examples of useful materials for the coating are metal oxides such as alumina and silica, ceramics such as SiC, Si₃N₄ and BN, high-melting-point glasses such as borosilicate glass, etc. Of these, alumina, silica and like metal oxides are desirable. The term "insulating" as used herein means that the coating 18 has sufficiently great insulation resistance relative to the internal resistance of the sensitive layer 20. For example, when the sensitive layer is made of SnO₂ of low resistance, TiO₂ of high resistance may be used for the coating 18. The coating 18 further acts to shield the member 14 from the atmosphere for protection. The thickness of the coating 18, which is dependent on the bond strength and insulating properties, is preferably 50 angstroms to 10 micrometers, more preferably 100 angstroms to 5 micrometers. The atmosphere-sensitive layer 20 is made of a material which is determined according to the component of atmosphere to be detected. For example, metal oxide semiconductors such as SnO₂, In₂O₃ and Fe₂O₃ are used for detecting combustible gases and toxic gases. Metal oxide semiconductors such as BaSnO₃, LaNiO₃ and NiO are useful for detecting O₂. Ceramics such as MgCr₂O₄ and TiO₂ are useful for detecting the humidity of atmospheres. These materials physically adsorb water vapor and detect humidity from the electric conductivity of the adsorbed water. While the sensitive layer 20 is usable at room temperature when detecting humidities, the layer 20 is heat-cleaned using the member 14 to remove dust and oils from the layer 20. Examples of other materials that are usable are proton conductors such as antimonic acid (H₂Sb₂O₆) and antimony phosphate (HSbP₂O₈). These proton conductors produce an electromotive force due to a difference in the concentration of H₂ or carbon monoxide and are usable for detecting H₂ and carbon monoxide. In this case, it is desirable, for example, to shield one of the electrodes from the atmosphere in order to create a difference in the concentration of hydrogen or the like between the two electrodes and produce an electromotive force. Thus, materials are useful for the layer 20 provided that they undergo variations in electrical characteristics due to a gas component of the ambient atmosphere. The thickness of the sensitive layer 20 is such that the heat generating member 14 is capable of supporting the layer 20. For example when the layer 20 is formed by vacuum evaporation or sputtering, the thickness is preferably 100 angstroms to 5 micrometers. When the layer is formed by powder coating, dipping or printing, the thickness is preferably 1 micrometer to 50 micrometers. Any known material or method is usable as desired for forming the sensitive layer 20. The insulating coating 18 may be formed over the entire surface of the member 14. Most preferably, the coating 18 is formed by plasma CVD, sputtering or ion plating. In the case of plasma CVD, an organic compound such as an aluminum alkoxide is injected into a plasma of Ar or the like, and the mixture is applied to the member 14 to complete the coating 18. In the case of sputtering, the coating is formed, for example, by applying aluminum by reactive sputtering in a low oxygen pressure. Ion plating can be practiced similarly. The insulating coating 18 can also be formed, for example, by the oxidation of a vacuum-deposited film of aluminum, alumina sol coating followed by sintering, or thermal decomposition of an organic compound such as aluminum isopropoxide Al(O-CH-(CH₃)₂)₃. These methods, i.e. plasma CVD, coating with alumina sol and sintering, and thermal decompostion of Al isopropoxide were checked for the bond strength and insulation strength of the resulting coating 18. When plasma CVD is resorted to, an alumina coating, even if 50 angstroms in thickness, gave insulation resistance of at least 1 megaohms between the electrode 28 and the heat generating member 14. In the case of sol coating or thermal decomposition of the isopropoxide, the coating, when obtained in the form of a single coat, had many pinholes. The application and sintering of sol, when repeated three to five times, gave a coating free from pinholes and having insulation resistance of at least 1 megaohms. In the case of the thermal decomposition of Al isopropoxide, the member 14 was dipped in an isopropoxide solution and then subjected to thermal decomposition to obtain an alumina coating. In this case, about 7 to about 10 repeated dipping-decomposition cycles afforded a pinhole-free coating having insulation resistance of at least 1 megaohms. In the case where the sensor whose insulating coating 18 is made of a metal oxide such as alumina or silica, is to be used in a neutral non-oxidizing atmosphere, SiC, Si₃N₄, BN, TiC or the like is also usable as a material for the coating 18. Although the above examples are given with respect to alumina, coatings 18 of other corresponding materials can be obtained using silica sol in place of alumina sol or tetraethyl silicate in place of Al isopropoxide. When plasma CVD was resorted to, the resulting alumina coating, even if 50 angstroms in thickness, exhibited sufficient bond strength and insulation resistance. Accordingly, the thickness of the coating 18 should preferably be at least 50 angstroms, more preferably at least 100 angstroms. The upper limit of the thickness of the coating 18, although not particularly significant, is preferably up to 10 micrometers, more preferably up to 5 micrometers. The member 14 is masked, for example, by applying nylon or like resin to the portion thereof where the coating 18 need not be formed. For the same purpose, the coating formed is alternatively be etched away from the portion. For example when the alumina coating is to be etched away, a photoresist is applied to the coating, followed by exposure to light, removal of the resist and etching. Preferred etchants for alumina are HF (etching at room temperature) and warm phosphoric acid. It is also desirable to use HF at room temperature for silica. The atmosphere-sensitive layer 20 is formed by vacuum evaporation, sputtering, powder coating or printing, or by dipping the member 14 in a dispersion of particles. The layer 20 may alternatively be formed by the thermal decomposition of an organic compound such as tin octylate. The member 14 is coated with a heat-resistant insulating coating 18, on which an atmosphere-sensitive layer 20 is provided as by printing. The heat generating member 14 itself is small-sized, and the electrodes joined to the sensitive layer 20 can be connected directly to contacts without using lead wires. Figs. 1 and 2 show such an embodiment. External leads 202, 204, 206 are secured to an unillustrated housing. An alumina or like insulating substrate 208 is adhered to the upper surface of the external lead 202 by die bonding. The alumina substrate 208 is provided with three contacts 210, 212, 214, to which a sensor main body 216 is bonded. The sensor main body 216 comprises a metal heat generating member 14 in the form of a wire and almost entirely covered with a heat-resistant insulating coating 18 of alumina or the like and with an atmosphere-sensitive layer 20 of SnO₂ or the like. A pair of film electrodes 30, 30 is joined to the layer 20. One of the electrodes 30 is connected to the contact 210, and the other electrode 30 to the contact 212 with gold paste portions 28, 28. With reference to Fig. 2, a process for fabricating this sensor will be described. An insulating coating 18 and an atmosphere-sensitive layer 20 of SnO₂ are formed almost over the entire surface of a metal heat generating member 14. Aside from this step, a die is prepared which has bonded thereto contacts 210, 212, 214. The member 14 is welded at its opposite ends to the die. The member 14 is weldable without removing the coating 18 and SnO₂ layer 20. The die has slits 220, 222 for separating the contacts 210, 212, 214 from one another. Film electrodes 30, 30 are thereafter provided on the layer 20. Using a gold paste 28, the electrodes 30, 30 are connected to the contacts 210, 212, respectively. Subsequently, the die is bonded to an alumina insulating substrate 208 with an adhesive 224. The die and the alumina insulating substrate 208 are then cut to obtain one sensor in the form of a chip. The alumina insulating substrate 208 is used to prevent the sensor main body 216 from breaking when the die is cut. The chip can be prepared similarly also when the contacts 210, 212, 214 are printed on the alumina insulating substrate 208 as film electrodes before the welding of the metal heat generating member 14. The separated chip is bonded to an external lead 202 with an adhesive 226, and the contacts 210, 212, 214 are connected to the external leads 202, 204, 206 by wire bonding, whereby the sensor of Fig. 1 can be obtained. According to the present embodiment, a sample was prepared in the following manner and checked for characteristics. A wire as a heat generating member 14 of Fe-Cr-Al alloy (Kanthal alloy) having a diameter of 20 micrometers was coated with alumina sol, which was then subjected to thermal decomposition at 800° C. This step was repeated 10 times to form an alumina insulating coating 18, about 1 micrometer in thickness, over the entire surface of the wire serving as a heat generating member 14. An isobutanol solution of an organic compound of Sn, i.e. Sn(OCH₃)₃·(O(CH₂)₃NH₂), was sprayed on the insulating coating 18, followed by thermal decomposition at 500° C to form SnO₂. Subsequently, the wire was cut to a length of 1 mm, and the cut wire was directly welded to contacts 210, 214. The wire was weldable without removing the insulating coating 18 and SnO₂ atmosphere-sensitive layer 20 since they were removed by the welding pressure and welding current. With the welded wire masked, gold film electrodes 30, 30 were formed on the atmosphere sensitive layer 20 by vacuum evaporation. The film electrodes 30, 30 were thereafter connected to the contacts 210, 212 with a gold paste 28. Table 4 shows the characteristics of the sensor thus obtained when the temperature of the SnO₂ layer 20 was 270°C.

Claims: A sensor comprising a metal heat generating member (14), insulated by a heat resistant insulating coating (18) at least partially over the surface of the metal heat generating member (14), and an atmosphere-sensitive layer (20) formed at least partially on the heat resistant insulating coating (18), characterized in that an insulating substrate (208) is provided with at least three contacts (210, 212, 214), the heat generating member (14) being provided on the substrate, the heat generating member (14) being connected to two of said at least three contacts (210, 214), the heat generating member (14) having wire shape, and at least one sensing film electrode (30) is joined to the sensitive layer (20) and connected directly to one of the other of said at least three contacts (212) by an electric-conductive paste (28) upon said contact. A method of producing a sensor comprising the steps of: forming a heat-resistant insulating coating (18) at least partially over the surface of a wire shape metal heat generating member (14); forming an atmosphere-sensitive layer (20) at least partially over the coating (18); providing at least one sensing film electrode (30) joined to the sensitive layer (20); forming an insulating substrate (208) provided with at least three contacts (210, 212, 214); providing the metal heat generating member (14) over the insulating substrate (208); connecting the heat generating member (14) to two of said at least three contacts (210, 214); and connecting the sensing film electrode (30) directly to one of the other of said at least three contacts (212) by an electric conductive paste (28) upon said contact.

Inventors: HIROSHI, KODA, KIYONORI, ONO, MUNEHARU, SHIMABUKURO
Applicants: FIGARO ENGINEERING INC.
Application year: 1987
Main classification: G01N27
IPC codes: G01N27