Patent Publication Number: US-2017363556-A1

Title: Gas sensor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosures herein relate to a gas sensor. 
     2. Description of the Related Art 
     A gas sensor known in the art is capable of discriminating between gas components generated at the time of fire and carbon monoxide generated at the time of incomplete combustion or between a gas such as methane detected at the time of leakage of utility gas and miscellaneous gasses such as ethanol (see Patent Document 1, for example). 
     However, such a gas detector may fail to accurately detect a target gas in an atmosphere which includes both the target gas and other gases that are not a target for detection, as in the case of detecting acetone in an atmosphere which includes acetone and ethanol. 
     According to one aspect, there may be a need for a gas sensor that is capable of accurately detecting a target gas. 
     RELATED-ART DOCUMENTS 
     Patent Document 
     
         
         [Patent Document 1] Japanese Patent Application Publication No. 2001-175969 
       
    
     SUMMARY OF THE INVENTION 
     According to one embodiment, a gas sensor includes an adsorption layer configured to cause gas to adsorb thereon, the gas including a target gas and a non-target gas, and a sensor layer covered with the adsorption layer and having an electric characteristic thereof changing in response to density of the target gas passing through the adsorption layer, wherein the adsorption layer is made of a material whose primary component is a metal oxide having gold particles attached thereto. 
     According to at least one embodiment, a disclosed gas sensor is capable of accurately detecting a target gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a gas sensor according to an embodiment; 
         FIG. 2  is a drawing illustrating the sensitivity characteristics of a gas sensor of the first example; 
         FIG. 3  is a drawing illustrating the sensitivity characteristics of a gas sensor of the second example; 
         FIG. 4  is a drawing illustrating the sensitivity characteristics of a gas sensor of the third example; 
         FIG. 5  is a drawing illustrating the sensitivity characteristics of a gas sensor of the first comparative example; and 
         FIG. 6  is a drawing illustrating the sensitivity characteristics of a gas sensor of the second comparative example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the accompanying drawings. In the specification and drawings, elements having substantially the same functions or configurations are referred to by the same numerals, and a duplicate description thereof will be omitted. 
     A gas sensor according to an embodiment is capable of accurately detecting a gas that is a target for detection even when the atmosphere includes such a target gas and a gas that is not a target for detection. 
     In the following, an example will be described by referring to a semiconductor gas sensor capable of accurately detecting acetone in an atmosphere that includes acetone serving as an example of a target gas and ethanol serving as an example of a non-target gas. A gas sensor capable of accurately detecting acetone may preferably be used as a breath measuring device configured to detect acetone in the breath for use in health condition management. The target gas may be another gas that is not acetone, and the non-target gas may be another gas that is not ethanol. 
     &lt;Gas Sensor&gt; 
     A gas sensor of the embodiment will be described by referring to  FIG. 1 .  FIG. 1  is a schematic cross-sectional view of a gas sensor according to the embodiment. 
     The gas sensor illustrated in  FIG. 1 , which is a semiconductor gas sensor having a diaphragm structure, includes a substrate  10 , a thermally insulating support layer  20 , a heater layer  30 , an insulating layer  40 , and a gas detection layer  50 . The heater layer  30  is electrically coupled to a drive and process unit (not shown), which drives the heater layer  30  serving to operate as a heater. The gas detection layer  50  is electrically coupled to the drive and process unit (not shown), which reads the electric characteristics of a sensor layer  52  of the gas detection layer  50 . The sensor layer  52  will be described later. 
     The substrate  10  is a member made of semiconductor material such as silicon (Si), for example. The substrate  10  has a penetrating hole  11  penetrating the substrate  10  at the center thereof. 
     The thermally insulating support layer  20 , which is formed on the substrate  10 , has a diaphragm structure. The thermally insulating support layer  20  includes a thermally oxidized film  21 , a support film  22 , and a thermally insulating film  23 . 
     The thermally oxidized film  21  is formed on the substrate  10  and implemented as a thermally oxidized SiO 2  film, for example. The thermally oxidized film  21  serves to reduce thermal capacity such as to prevent the heat generated by the heater layer  30  from propagating to the substrate  10 . The thermally oxidized film  21  also has high etching electivity relative to the substrate  10 . 
     The support film  22  is formed on the thermally oxidized film  21  and implemented as a CVD-Si 3 N 4  film, for example. The support film  22  serves as a support film for the diaphragm structure. 
     The thermally insulating film  23  is formed on the support film  22  and implemented as a CVD-SiO 2  film, for example. The thermally insulating film  23  exhibits strong adhesion to the heater layer  30 , and provides electrical insulation between the substrate  10  and the heater layer  30 . 
     The heater layer  30  is formed on the thermally insulating support layer  20  at the center of the position of the penetrating hole  11  in a plan view. With this arrangement, the heater layer  30  is thermally isolated from the substrate  10 . The heater layer  30 , which is coupled to a power supply (not shown), generates heat upon receiving electric power from the power supply to heat the gas detection layer  50 . More specifically, the heater layer  30  heats an adsorption layer  53  to the temperature (e.g., 200 to 250 degrees Celsius) that oxidizes ethanol such as to reduce ethanol reaching the sensor layer  52 . The adsorption layer  53  will be described later. With the thermal isolation between the heater layer  30  and the substrate  10 , virtually no heat generated by the heater layer  30  dissipates into the surroundings. This arrangement effectively serves to increase the temperature of the gas detection layer  50 . The heater layer  30  is implemented as an alloy film of platinum (Pt) and tungsten (W), which will hereinafter be referred to as a “Pt—W film”. 
     The insulating layer  40  is formed on the thermally insulating support layer  20  to cover the heater layer  30  and implemented as a sputtered SiO 2  film, for example. The insulating layer  40  provides electrical insulation between the heater layer  30  and the gas detection layer  50 . The insulating layer exhibits strong adhesion to the gas detection layer  50 . 
     The gas detection layer  50  is formed on the insulating layer  40  at the center of the position of the penetrating hole  11  in a plan view. Namely, the gas detection layer  50  is formed on the insulating layer  40  at the same position as the position of the heater layer  30  in the plan view. The gas detection layer  50  includes an electrode layer  51 , the sensor layer  52 , and the adsorption layer  53 . In the case of adhesion being not sufficient between the insulating layer  40  and the electrode layer  51 , a bond layer made of a tantalum (Ta) film or a titanium (Ti) film, for example, may be disposed between the insulating layer  40  and the electrode layer  51 . 
     The electrode layer  51 , which is a pair of metal films formed on the insulating layer  40  at the opposite ends of the sensor layer  52  in a plan view, is implemented as a Pt film or a gold (Au) film, for example. 
     The sensor layer  52  is formed on the insulating layer  40  in contact with the pair of films of the electrode layer  51 . The sensor layer  52  is made of a metal oxide such as SnO 2 . The sensor layer  52  has the electric characteristics such as a resistance value thereof changing in response to the density of acetone passing through the adsorption layer  53 . The sensor layer  52  may be made of a material different from SnO 2 , which may be a material containing a metal-oxide semiconductor as a primary component such as In 2 O 3 , WO 3 , ZnO, TiO 2 , or the like. In the disclosures herein, the term “primary component” means that the proportion of the component in the material is 50% or more. 
     The adsorption layer  53  is formed on the electrode layer  51  and the sensor layer  52  such as to cover the surface of the sensor layer  52  for adsorption of gasses inclusive of acetone and ethanol. The adsorption layer  53  is made of Al 2 O 3  having gold (Au) particles attached thereto (Au—Al 2 O 3 ), for example. The Au particles of the adsorption layer  53  serve as a catalyst, which facilitates oxidation (i.e., combustion) of ethanol while bringing about virtually no oxidation of acetone at the temperature range of 200 to 250 degrees Celsius, for example. With this arrangement, the adsorption layer  53  enables the removal of ethanol through selective oxidation relative to acetone. In this manner, the adsorption layer  53  reduces ethanol reaching the sensor layer  52  while selectively allowing acetone to reach the sensor layer  52 . As a result, the sensor layer  52  can detect acetone separated from ethanol, thereby enabling an accurate detection of acetone. The adsorption layer  53  may be made of a material different from Au—Al 2 O 3 , which may be a material containing as a primary component a metal oxide insulating material such as ZrO 2 , CeO 2 , SiO 2 , Cr 2 O 3 , Fe 2 O 3 , Ni 2 O 3 , or the like having Au particles attached thereto (or contained therein, or mixed therewith). The average diameter of Au particles may preferably be 5 nm or smaller from the viewpoint of achieving high catalytic activation. 
     As was described above, the gas sensor of the embodiment has an adsorption layer made of a material whose primary component is a metal oxide having Au particles attached thereto, thereby being capable of accurately detecting acetone. 
     &lt;Method of Manufacturing Gas Sensor&gt; 
     A description will be given of a method of making the gas sensor of the embodiment. 
     First, the thermally insulating support layer  20  is formed on the substrate  10 . Specifically, the substrate  10  is subjected to thermal oxidation, so that the thermally oxidized film  21  is formed in the surface of the substrate  10 . Next, silicon nitride (Si 3 N 4 ) is deposited on the thermally oxidized film  21  by a plasma CVD process, which forms the support film  22 . Then, silicon oxide (SiO 2 ) is deposited on the support film  22  by a plasma CVD process, which forms the thermally insulating film  23 . 
     The heater layer  30  is then formed on the thermally insulating support layer  20 . To be more specific, a Pt—W film is formed by sputtering on the thermally insulating film  23  at the center of the position of the penetrating hole  11  in a plan view, thereby forming the heater layer  30 . In so doing, a metal mask or the like may be used that has an opening at the position where the Pt—W film is to be formed. 
     Subsequently, the insulating layer  40  is then formed on the thermally insulating support layer  20  to cover the heater layer  30 . More specifically, SiO 2  is deposited by sputtering on the thermally insulating support layer  20  to cover the surface of the heater layer  30 , thereby forming the insulating layer  40 . 
     The gas detection layer  50  is then formed on the insulating layer  40 . Specifically, Pt is deposited by sputtering on the insulating layer  40  to form a pair of films as the electrode layer  51 . SnO 2  is then deposited by sputtering on the insulating layer  40  such as to be in contact with the pair of films of the electrode layer  51 , thereby forming the sensor layer  52 . A paste made by adding together, in equal weight proportion, organic solvent and γ-alumina with an Au-particle additive and by further adding silica-sol binder thereto is then pasted by screen printing on the electrode layer  51  and the sensor layer  52  in such a manner to cover the sensor layer  52 , followed by calcination to form the adsorption layer  53 . The adsorption layer  53  may be formed as a layer stacked on the surface of the sensor layer  52 , or may be formed to cover the electrode layer  51  and the surface of the sensor layer  52 . 
     The penetrating hole  11  is then formed through the substrate  10 . More specifically, the substrate  10  is etched from the back surface (from the surface on which the gas detection layer  50  is not formed) by plasma etching to remove Si in the area inclusive of the position of the gas detection layer  50  in a plan view, thereby forming the penetrating hole  11 . The high etching selectivity of the thermally oxidized film  21  relative to the substrate  10  allows only the substrate  10  to be etched while avoiding etching the thermally oxidized film  21 . Namely, the thermally oxidized film  21  serves as an etching stop film when etching the substrate  10 . 
     The above-described processes produce a gas sensor having the diaphragm structure as illustrated in  FIG. 1 . This is only an example of a method of making a gas sensor, and is not intended to be limiting. For example, the electrode layer  51  and the sensor layer  52  may be formed, followed by forming the penetrating hole  11  through the substrate  10 , and then forming the adsorption layer  53  to cover the sensor layer  52 . 
     EXAMPLE 
     In the following, a specific example of a gas sensor will be described. 
     First Example 
     An Si substrate serving as the substrate was subjected to thermal oxidation, thereby forming a thermally oxidized SiO 2  film serving as the thermally oxidized film  21  on the Si substrate. A CVD-Si 3 N 4  film serving as the support film  22  and a CVD-SiO 2  film serving as the thermally insulating film  23  were formed in this order on the thermally oxidized SiO 2  film by a plasma CVD process. 
     Subsequently, a Pt—W film serving as the heater layer  30  was formed on the CVD-SiO 2  film by a sputtering process performed by an RF magnetron sputtering apparatus. An SiO 2  film serving as the insulating layer  40  was then formed on the Pt—W film by a sputtering process performed by the RF magnetron sputtering apparatus. 
     After this, a sputtering process was performed by the RF magnetron sputtering apparatus to form a Pt film having a film thickness of 200 nm serving as the electrode layer  51  on the SiO 2  film under the conditions of an Ar gas pressure of 1 Pa, a substrate temperature of 300 degrees Celsius, and an RF power of 2 W/cm 2 . 
     A reactive sputtering process was thereafter performed by the RF magnetron sputtering apparatus to form an SiO 2  film having a film thickness of 400 nm serving as the sensor layer  52  on the SiO 2  film under the conditions of an Ar+O 2  gas pressure of 2 Pa, a substrate temperature of 150 to 300 degrees Celsius, and an RF power of 2 W/cm 2 . In so doing, SnO 2  containing 0.1 wt % antimony (Sb) was used as a target material. 
     A paste was then produced by adding together, in equal weight, organic solvent and γ-Al 2 O 3  (with an average particle diameter of 1 to 2 micrometers) with an additive of 0.7 wt % Au particles having an average diameter of 3.5 nm and by further adding 5 wt % to 20 wt % of silica-sol binder. The paste was then applied by screen printing on the Pt film and the SnO 2  film to form a circular shape with a thickness of approximately 30 micrometers and a diameter of 250 micrometers, followed by calcination at 300 degrees Celsius for 12 hours to form an Au—Al 2 O 3  film serving as the adsorption layer  53 . 
     Plasma etching was then performed to etch the Si substrate from the back surface to remove Si in the area inclusive of the position where the Pt—W film, the Pt film, the SnO 2  film, and the Au—Al 2 O 3  film were formed, thereby forming the penetrating hole  11 . 
     The above-described processes produced a gas sensor having the diaphragm structure as illustrated in  FIG. 1 . 
     Second Example 
     In the second example, a gas sensor was produced in the same manner as in the first example, except that different method and materials than in the first example were used to form the adsorption layer. Specifically, a paste was then produced by adding together, in equal weight, organic solvent and CeO 2  (with an average particle diameter of 2 to micrometers) with an additive of 0.6 wt % Au particles having an average diameter of 3 nm and by further adding 5 wt % to 20 wt % of silica-sol binder. The paste was then applied by screen printing on the Pt film and the SnO 2  film to form a circular shape with a thickness of approximately 30 micrometers and a diameter of 250 micrometers, followed by calcination at 300 degrees Celsius for 12 hours. With these processes, a CeO 2  (Au—CeO 2 ) film having Au particles attached thereto was formed to serve as the adsorption layer. 
     Third Example 
     In the third example, a gas sensor was produced in the same manner as in the first example, except that different method and materials than in the first example were used to form the adsorption layer. Specifically, a paste was then produced by adding together, in equal weight, organic solvent and ZrO 2  (with an average particle diameter of 2 to 3 micrometers) with an additive of 0.6 wt % Au particles having an average diameter of 3 nm and by further adding 5 wt % to 20 wt % of silica-sol binder. The paste was then applied by screen printing on the Pt film and the SnO 2  film to form a circular shape with a thickness of approximately 30 micrometers and a diameter of 250 micrometers, followed by calcination at 300 degrees Celsius for 12 hours. With these processes, a ZrO 2  (Au—ZrO 2 ) film having Au particles attached thereto was formed to serve as the adsorption layer. 
     First Comparative Example 
     In the first comparative example, a gas sensor was produced in the same manner as in the first example, except that different method and materials than in the first example were used to form the adsorption layer. Specifically, a paste was then produced by adding together, in equal weight, diethylene glycol monoethyl ether and γ-Al 2 O 3  (with an average particle diameter of 2 to 3 micrometers) with an additive of 7.0 wt % palladium (Pd) and by further adding 5 wt % to 20 wt % of silica-sol binder. The paste was then applied by screen printing on the Pt film and the SnO 2  film to form a circular shape with a thickness of approximately 30 micrometers and a diameter of 250 micrometers, followed by calcination at 500 degrees Celsius for 12 hours. With these processes, an Al 2 O 3  (Pd—Al 2 O 3 ) film with Pd particles attached thereof was formed to serve as the adsorption layer. 
     Second Comparative Example 
     In the second comparative example, a gas sensor was produced in the same manner as in the first example, except that different method and materials than in the first example were used to form the adsorption layer. Specifically, a paste was then produced by adding together, in equal weight, diethylene glycol monoethyl ether and γ-Al 2 O 3  (with an average particle diameter of 2 to 3 micrometers) without any catalyst and by further adding 5 wt % to 20 wt % of silica-sol binder. The paste was then applied by screen printing on the Pt film and the SnO 2  film to have a thickness of approximately 30 micrometers and a diameter of 250 micrometers, followed by calcination at 500 degrees Celsius for hours. With these processes, an Al 2 O 3  film with no catalysts was formed to serve as the adsorption layer. 
     &lt;Evaluation&gt; 
     The gas sensors produced in the first through third examples and the first and second comparative examples were subjected to intermittent activation having a 30-second period with 1-second activation while the temperature of the gas sensor was set at 300 degrees Celsius, followed by changing the sensor temperature, and then measuring the resistance value of the gas sensor (i.e., the sensor layer  52 ) in the state of stable resistance. Gas sensitivity was derived from the measured resistance value. Gas sensitivity is calculated as Rair/Rgas where Rair represents the resistance value of the gas sensor in a clean air atmosphere that includes neither acetone nor ethanol, and Rgas represents the resistance value of the gas sensor in a gas atmosphere that includes a predetermined density of acetone or ethanol. Gas sensitivity being equal to 1 means that gas sensitivity is completely nonexistent. A difference between the sensitivity of acetone and the sensitivity of ethanol may be obtained by using the difference in sensitivity at a predetermined temperature (i.e. at a fixed temperature point), or may be obtained by using a difference between the averages of respective sensitivities obtained over a plurality of points in a predetermined temperature range. It suffices to use an evaluation method that brings about a great sensitivity difference. 
       FIG. 2  is a drawing illustrating the sensitivity characteristics of the gas sensor of the first example.  FIG. 2  illustrates the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of acetone and the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of ethanol. In  FIG. 2 , the horizontal axis represents the temperature of the gas sensor in Celsius, and the vertical axis represents the sensitivity of the gas sensor. Open circles represent the temperature dependency of sensitivity with respect to acetone, and solid circles represent the temperature dependency of sensitivity with respect to ethanol. 
     As illustrated in  FIG. 2 , sensitivity for acetone and sensitivity for ethanol exhibit different tendencies when the temperature of the gas sensor is in the range of 200 to 250 degrees Celsius, such that sensitivity for acetone is higher than sensitivity for ethanol. This appears to be because γ-Al 2 O 3  with an additive of Au particles having an average diameter of 5 nm or smaller serves to oxidize ethanol and to oxidize virtually no acetone in the range of 200 to 250 degrees Celsius. Accordingly, the gas sensor of the first example utilizes a difference between sensitivity for acetone and sensitivity for ethanol to discriminate between acetone and ethanol for successful detection. 
       FIG. 3  is a drawing illustrating the sensitivity characteristics of the gas sensor of the second example.  FIG. 3  illustrates the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of acetone and the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of ethanol. In  FIG. 3 , the horizontal axis represents the temperature of the gas sensor in Celsius, and the vertical axis represents the sensitivity of the gas sensor. Open circles represent the temperature dependency of sensitivity with respect to acetone, and solid circles represent the temperature dependency of sensitivity with respect to ethanol. 
     As illustrated in  FIG. 3 , sensitivity for acetone and sensitivity for ethanol exhibit different tendencies when the temperature of the gas sensor is in the range of 100 to 200 degrees Celsius, such that sensitivity for acetone is higher than sensitivity for ethanol. This appears to be because CeO 2  with an additive of Au particles having an average diameter of 5 nm or smaller serves to oxidize ethanol and to oxidize virtually no acetone in the range of 100 to 200 degrees Celsius. Accordingly, the gas sensor of the second example utilizes a difference between sensitivity for acetone and sensitivity for ethanol to discriminate between acetone and ethanol for successful detection. 
       FIG. 4  is a drawing illustrating the sensitivity characteristics of the gas sensor of the third example.  FIG. 4  illustrates the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of acetone and the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of ethanol. In  FIG. 4 , the horizontal axis represents the temperature of the gas sensor in Celsius, and the vertical axis represents the sensitivity of the gas sensor. Open circles represent the temperature dependency of sensitivity with respect to acetone, and solid circles represent the temperature dependency of sensitivity with respect to ethanol. 
     As illustrated in  FIG. 4 , sensitivity for acetone and sensitivity for ethanol exhibit different tendencies when the temperature of the gas sensor is in the range of 200 to 250 degrees Celsius, such that sensitivity for acetone is higher than sensitivity for ethanol. This appears to be because ZrO 2  with an additive of Au particles having an average diameter of 5 nm or smaller serves to oxidize ethanol and to oxidize virtually no acetone in the range of 200 to 250 degrees Celsius. Accordingly, the gas sensor of the third example utilizes a difference between sensitivity for acetone and sensitivity for ethanol to discriminate between acetone and ethanol for successful detection. 
       FIG. 5  is a drawing illustrating the sensitivity characteristics of the gas sensor of the first comparative example.  FIG. 5  illustrates the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of acetone and the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of ethanol. In  FIG. 5 , the horizontal axis represents the temperature of the gas sensor in Celsius, and the vertical axis represents the sensitivity of the gas sensor. Open circles represent the temperature dependency of sensitivity with respect to acetone, and solid circles represent the temperature dependency of sensitivity with respect to ethanol. 
     As illustrated in  FIG. 5 , sensitivity for acetone and sensitivity for ethanol exhibit substantially the same tendencies when the temperature of the gas sensor falls within the range (i.e., 100 to 300 degrees Celsius) in which sensitivity for acetone is high. This appears to be because virtually no oxidation of acetone and virtually no oxidation of ethanol occur in the temperature range in which sensitivity for acetone is high, despite the fact that Pd is added in the adsorption layer as a catalyst. Because of this, the gas sensor of the first comparative example is incapable of discriminate between acetone and ethanol for detection. 
       FIG. 6  is a drawing illustrating the sensitivity characteristics of the gas sensor of the second comparative example.  FIG. 6  illustrates the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of acetone and the temperature dependency of sensitivity in a gas atmosphere that includes 1 ppm of ethanol. In  FIG. 6 , the horizontal axis represents the temperature of the gas sensor in Celsius, and the vertical axis represents the sensitivity of the gas sensor. Open circles represent the temperature dependency of sensitivity with respect to acetone, and solid circles represent the temperature dependency of sensitivity with respect to ethanol. 
     As illustrated in  FIG. 6 , sensitivity for acetone and sensitivity for ethanol exhibit substantially the same tendencies when the temperature of the gas sensor falls within the range (i.e., 100 to 300 degrees Celsius) in which sensitivity for acetone is high. This appears to be because the adsorption layer including no catalysts results in virtually no occurrence of either oxidation of acetone or oxidation of ethanol. Because of this, the gas sensor of the second comparative example is incapable of discriminate between acetone and ethanol for detection. 
     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 2016-119265 filed on Jun. 15, 2016, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.