Thin-film thermistor element and method of manufacturing it

Abstract

A thin-film thermistor element (1, 10) comprising: a base substance (2), a thermistor thin film (5, 5A, 5B) formed on the base substance (2); and at least one pair of electrodes (3, 4) formed on, under or within the thermistor thin film (5, 5A, 5B), wherein one pair of electrodes (3, 4) each comprises an electrode layer made of platinum or an alloy thereof, and the electrode layer is crystalline, wherein the electrode layer contains at least one of oxygen and nitrogen and the content of the at least one of oxygen and nitrogen in the electrode layer is inclusive of 0.01 wt% to inclusive of 4.9 wt%.

Description

Technical FieldThe present invention relates to a thin film thermistor element used as a sensor such as a thermo sensor or an infrared sensor, and a method of manufacturing the thin film thermistor element.Background ArtThin film thermistor elements have been used as thermo-sensors or infrared sensors for devices such as information devices, communication devices, medical devices, home appliance devices and automobile transmission devices. The thin film thermistor element is a sintered molded article of an oxide semiconductor having a large negative temperature coefficient. In general, in such a thin film thermistor element, electrodes are formed on a substrate, then a thermistor thin film is formed thereon, and the result is heat-treated at a temperature of 1400°C or below.Here, in the case of forming electrodes made of platinum (Pt), an alloy thereof, or the like directly on an underlayer provided in the substrate, film deposition is performed with the substrate heated to 100° C. or higher, and then the electrodes made of platinum (Pt), the alloy thereof, or the like are formed by patterning by vapor etching. In this case, a film deposition apparatus must have a mechanism for heating a substrate. In addition, since the gas phase etching does not use corrosive gas, a general gas phase etching apparatus performs pattern forming using a resist as a mask. In this process, there is a problem that the insulating underlayer, the thermistor thin film, and the metal such as platinum tend to easily separate from each other due to weak adhesion therebetween.Thus, in order to achieve a strong adhesive force between the underlayer and platinum or the like, an electrode is formed in a two-layer structure including an adhesive layer made of a metal, an alloy, or the like for achieving the adhesive force and a conductive layer made of platinum, an alloy thereof, or the like (Patent Literatures 1, 2, and 3).As conventional techniques of this kind, those described in the following literature are known (Patent Literature: 1. JP 2000-348 906 A, 2. JP H03-54841 B2, 3. JP H06-61012 A, 4. JP 4811316 B2, 5. JP 2008-294 288 A).Further conventional techniques for this are known from U.S. Pat. No. 2002-0 189 342 A1 and JP H07-201 521 A.SUMMARY OF THE INVENTIONHowever, as shown in FIGS. 3 and 4, in the conventional manufacturing method, electrodes 3, 4 each having an adhesive layer 3B, 4B and a conductive layer 3A, 4A and a thermistor thin film 5 are formed on a substrate 2 having an adhesive sublayer 2A, and then heat-treated. Since the conductive layer made of Pt, the alloy thereof, or the like is made of a noble metal, the conductive layer has a problem of light-off because the conductive layer has extremely weak adhesiveness with the underlayer and the thermistor thin film made of oxides.For this reason, the thermistor thin film 5 formed on the electrodes 3, 4 peels off, and the peeling of the electrodes causes an increase in resistance value. In the conventional method, an adhesive layer containing at least one of titanium and chromium is provided to improve adhesiveness. However, providing the adhesive layer containing at least one of titanium and chromium creates another problem that the properties deteriorate due to the reaction patterns with the thermistor thin film and the oxidation of titanium or chromium.The present invention has been made in view of the above circumstances, and has an object to provide a thin film thermistor element and a method of manufacturing the thin film thermistor element, which achieve sufficient adhesion force between a thermistor thin film and electrodes while maintaining the adhesion force between a substrate and the electrodes.To achieve the above object, the present invention provides a thin film thermistor element according to claim 1 and a method of manufacturing a thin film thermistor element according to claim 5. Advantageous further developments are described in the dependent claims.In these inventions, the electrode layer is formed containing oxygen and nitrogen, and is then crystallized by heat treatment. Thus, the concentration of oxygen and nitrogen in the film of the conductive layer made of platinum (Pt), an alloy thereof, or the like can be prevented from varying even in the heat treatment after the film deposition for the one pair of electrodes and the thermistor thin film. Consequently, the surface state of the electrode layer before and after the heat treatment can be maintained advantageously. In contrast, in the case of an electrode layer which does not contain oxygen and nitrogen as in a conventional one, a rapid progress of oxidizing and nitriding the electrode layer in the heat treatment results in a phenomenon of peeling of the electrodes. In addition, an adhesive layer containing at least one of titanium and chromium, if provided, deteriorates the properties by reacting with the thermistor thin film.In the case of the electrode layer of the present invention formed in the method of crystallizing a film by heat treatment after the film is deposited containing oxygen and nitrogen, the content of oxygen and nitrogen is prevented from varying, so that the peeling of the electrodes and the property deterioration can be prevented.Further, the thin film thermistor element according to the present invention is characterized in another exemplary embodiment in that the electrode layer is deposited containing at least one of oxygen and nitrogen.Further, the method of manufacturing a thin film thermistor element according to the present invention is characterized in another exemplary embodiment in that the first step includes depositing the electrode layer with at least one of oxygen and nitrogen added. Then, after depositing the electrode layer, the one pair of electrodes is formed by patterning in the second step of patterning in a process such as etching.These inventions enable the electrode layer to contain at least one of oxygen and nitrogen during film deposition, and are capable of crystallizing the electrode layer advantageously into crystalline state grains having <111> orientation in the third step using the method of crystallizing by heat treatment.Further, the method of manufacturing a thin film thermistor element according to the present invention is characterized in another exemplary embodiment in that the first step includes depositing the electrode layer with at least one of oxygen and nitrogen added.In these inventions, the content of the at least one of oxygen and nitrogen is set to 0.01 weight percent to 4.9 weight percent, inclusive, which makes it possible to crystallize the electrode layer into crystalline state grains having <111> orientation and also to prevent a large increase in resistance value due to peeling of the electrode layer.Brief Description of the DrawingsFIG. 1 is a cross-sectional view and a plan view illustrating a thin film thermistor element according to an embodiment of the present invention. FIG. 2 is a flowchart illustrating a method of manufacturing a thin film thermistor element according to the embodiment of the present invention. FIG. 3 is a cross-sectional view and a plan view illustrating a thin film thermistor element according to a conventional thin film thermistor element. FIG. 4 is a flow chart illustrating a method of manufacturing a thin film thermistor element according to an embodiment of the conventional thin film thermistor element. FIG. 5 is a cross-sectional view and a plan view corresponding to FIG. 1 and illustrating another example which is a modified example of a thin film thermistor element according to the embodiment of the present invention. FIG. 6 is a flowchart corresponding to FIG. 2 and illustrating a method of manufacturing a thin film thermistor element according to the other example, which is the modified example of the embodiment of the present invention. FIG. 7 is a graph illustrating a resistance value change in a heat resistance test at 250° C., which represents an effect of the present invention. FIG. 8 is a graph illustrating a B value change in the heat resistance test at 250° C., which represents an effect of the present invention. FIG. 9 is a graph illustrating a resistance value change in a temperature change test of 40° C. to 250° C., which represents an effect of the present invention. Fig. 10 is an electron microscope photograph of a thin film thermistor element after the heat treatment, which represents an effect of the present invention. FIG. 11 is a graph of profiles obtained from the conductive layer of the thin film thermistor element by thin film X-ray diffraction (thin film XRD: X-ray Diffraction with Patting Incidence), which represents an effect of the present invention.DESCRIPTION OF EMBODIMENTSReferring to FIGS. 1 and 2, a description will be provided for an embodiment of a thin film thermistor element and a method of manufacturing a thin film thermistor element according to the present invention. It should be noted that the drawings used in the following description are provided with the components appropriately changed in scale to make the components have recognizable dimensions.A thin film thermistor element 1 according to the present embodiment is, for example, a sensor for temperature detection, and includes an Si substrate (base substance) 2 having a surface on which an SiO2layer 2A is formed as an underlayer; a pair of electrodes 3 and 4 formed by patterning on the SiO2layer 2A; a thermistor thin film 5 formed on the SiO2layer 2A, the electrode 3, and the electrode 4; and a passivation film 6 covering the thermistor thin film 5, as illustrated in FIGS. 1 and 2.The above thermistor thin film is formed on the pair of the electrode 3 and the electrode 4.The electrode 3 and the electrode 4 are formed on the SiO2layer 2A. The pair of the electrode 3 and the electrode 4 are disposed opposite to each other with a certain distance therebetween. The pair of the electrode 3 and the electrode 4 has an electrode terminal portion 7A and an electrode terminal portion 7B, respectively, which extend to the outside of the thermistor thin film layer 5.The pair of the electrode 3 and the electrode 4 is formed to contain at least one of oxygen and nitrogen in the film deposition in the method described below. In this case, the content of at least one of oxygen and nitrogen is set to 0.01 percent by weight to 4.9 percent by weight, inclusive, by heat treatment. Here, the content of at least one of oxygen and nitrogen means a total content of both oxygen and nitrogen, if both are contained.The thermistor thin film 5 is a composite metal oxide film made of a Mn-Co-based composite metal oxide (e.g., a Mn3O4-Co3O4-based composite metal oxide) or a composite metal oxide (e.g., a Mn3O4-Co3O4-Fe2O3-based composite metal oxide) which is at least one of Mn-Co-based composite metal oxide containing Ni, Fe and Cu, and has a spinel crystal structure.The passivation film 6 is made of an SiO2film. In place of the SiO2film, another insulating film such as a silicon nitride film (Si3N4), a silicon monoxide film (SiO), a glass film, a ceramic film, or a heat-resistant resin film may be used as long as the film has insulating properties and is capable of blocking the external atmosphere.Next, the method of manufacturing the thin film thermistor element 1 according to the present embodiment will be described.As illustrated in FIG. 2, the method of manufacturing the thin film thermistor element according to the present embodiment includes a step (S 01) of depositing a thin film made of platinum (Pt), an alloy thereof, or the like on the SiO2layer 2A in the Si substrate 2, a step (S 02) of forming the pair of the electrode 3 and the electrode 4 by patterning after the film deposition, a step (S 03) of heat-treating the electrode 3 and the electrode 4, a step (S 04) of depositing the thermistor thin film 5 on the electrode 3 and the electrode 4, a step (S 05) of patterning the thermistor thin film 5, a step (S06) of heat-treating the thermistor thin film 5, a step (S07) of depositing the passivation film 6, and a step of patterning the passivation film 6.First, an SiO2 / Si substrate 2 is prepared, and an upper surface of the Si substrate 2 is thermally oxidized to form the SiO2layer 2A, for example, with a layer thickness of 0.5 μm.Then, the first step (S 01) of depositing an electrode layer made of platinum (Pt), an alloy thereof, or the like is provided.The first step (S 01) uses a high-frequency sputtering apparatus, a DC sputtering apparatus, or the like to deposit the electrode layer using an atmospheric gas to which at least one of an oxygen gas and a nitrogen gas is added, applying a sputtering power of 100 W to 2000 W at an atmospheric pressure of 100 mPa to 1330 mPa and at an argon gas flow rate of 10 scm 3 to 50 scm 3. In this step, the gas concentration is adjusted so that at least one of oxygen and nitrogen may be contained after film deposition.In the second step (S 02), the electrode layer is formed by patterning by general photolithography or etching after depositing the electrode layer, and thereby the pair of the electrode 3 and the electrode 4 is obtained.In the third step (S 03), the pair of the electrode 3 and the electrode 4 may be crystallized into grains having a crystalline structure in <111> orientation with containing the oxygen and the nitrogen by using a method of crystallizing the pair of the electrode 3 and the electrode 4 by being maintained in the atmosphere at a heat treatment temperature of 400° C. to 1000° C. for 1 to 10 hours.In the third step (S 03), the pair of the electrode 3 and the electrode 4 may instead be crystallized in columns having a <111> orientation crystal structure containing the oxygen and the nitrogen using the method of crystallizing the pair of the electrode 3 and the electrode 4 by being maintained in the atmosphere at a heat treatment temperature of 400° C. to 1000° C. for 1 to 10 hours.On the other hand, in the third step (S 03), the pair of the electrode 3 and the electrode 4 can be crystallized into grains and columns having a crystalline structure in <111> orientation with containing the oxygen and the nitrogen using the method of crystallizing the pair of the electrode 3 and the electrode 4 by being maintained in the atmosphere at a heat treatment temperature of 400° C. to 1000° C. for 1 to 10 hours.Next, the step (S 04) is performed in which the thermistor thin film 5 is deposited on the pair of the electrode 3 and the electrode 4.First, a composite metal oxide film to be the thermistor thin film 5 is deposited with a film thickness of 0.5 μm, for example, by sputtering. Here, it is preferable to set the composite metal oxide film to have a film thickness of 0.3 μm or greater at which the film thickness dependence on the volume resistivity becomes low.In this step, the sputter deposition conditions are set to, for example, an atmospheric pressure of 100 mPa to 1330 mPa, an argon gas flow rate of 10 scm 3 to 50 scm 3 and the use of a sputtering power of 100 W to 2000 W. Here, a sputtering method in which sputtering is performed while heating the SiO2 / Si substrate 2 on which the thermistor thin film 5 is to be formed may be applicable. In this case, the temperature of the substrate is preferably set in a range of 200 to 800°C.After the sputtering, the step (S 05) of performing pattern formation by etching is performed. Then, the step (S 06) of heat-treating the thermistor thin film 5 by a predetermined heat treatment is performed. This heat treatment is carried out for 1 to 24 hours in the atmosphere at a temperature of 400° C. to 1000° C.Instead, the above heat treatment may be performed in an atmosphere of an inert gas such as an argon gas or a nitrogen gas, or any of these gases to which O2is added at 0.1 volume percent to 25 volume percent.Finally, the processing proceeds to the step (S07) of depositing the passivation film 6. The SiO2 passivation film 6 serving as a protective film, infrared light absorbing film or the like is laminated on a first thermistor thin film 5A and a second thermistor thin film 5B. After the film deposition, the passivation film 6 is patterned (S08).In this way, the thin film thermistor element is manufactured as a temperature detection sensor.According to the method of manufacturing the thin film thermistor element, the pair of electrode 3 and electrode 4 are heat-treated after the film is deposited containing oxygen and nitrogen. In the case of the electrodes formed by the method in which the film containing oxygen and nitrogen is deposited and thereafter crystallized by heat treatment, in the heat treatment after the film deposition of the pair of the electrode 3 and the electrode 4 and the thermistor thin film 5, the content of oxygen and nitrogen is prevented from varying due to the heat.Consequently, since the content of oxygen and nitrogen in the pair of the electrode 3 and the electrode 4 after the heat treatment is prevented from varying, it is possible to maintain favorable conditions by preventing the occurrence of peeling, so that the adhesive force between the Si substrate 2 and the pair of the electrode 3 and the electrode 4 can be maintained even after the heat treatment. Since an adhesive layer containing at least one of titanium and chromium is not provided, the oxidized and nitrided states are stabilized, contributing to stabilization of thermistor characteristics.In addition, in the film deposition, the pair of the electrode 3 and the electrode 4 are made to contain at least one of oxygen and nitrogen, the conductive layer 3B can be crystallized into granular crystals (or columnar crystals or granular and columnar crystals) having the <111> orientation advantageously containing oxygen and nitrogen. In particular, since the content of at least one of oxygen and nitrogen in the pair of the electrode 3 and the electrode 4 is set to 0.1 weight percent to 4.9 weight percent, inclusive, the pair of the electrode 3 and the electrode 4 can be crystallized into granular crystals (or columnar crystals or granular and columnar crystals) having the <111> orientation sufficiently containing oxygen and nitrogen, and a large increase in resistance value due to detachment of the pair of the electrode 3 and the electrode 4 can be prevented.A description is provided hereinafter for a reason why the content of at least one of oxygen and nitrogen in the pair of the electrode 3 and the electrode 4 is set to 0.1 weight percent to 4.9 weight percent, inclusive.Particularly in certain examples shown in FIG. 11, the oxygen content of a crystallized electrode is 1.3%, whereas the oxygen content of a non-crystallized electrode is 8.3%. The upper limit of 4.9 weight percent is approximately an average value in these data, and the lower limit is found to be 0.01 weight percent because the layer inevitably absorbs oxygen even if the argon of the sputtering gas is not made to contain oxygen.Note that in the case where the pair of the electrode 3 and the electrode 4 crystallized into grains (or column crystals or grains and columns) having the <111> orientation contains an oxygen or nitrogen element at 5 percent by weight or more, the pair of the electrode 3 and the electrode 4 made of Pt, an alloy thereof, or the like contains such an excessive amount of oxygen and nitrogen that the content tends to vary easily, and therefore it is difficult to achieve a sufficient effect of improving the adhesion force. In addition, when the oxygen or nitrogen element is contained in excess of 5 weight percent, the resistance value as the electrode material greatly increases. Accordingly, if the content is set within the above setting range, it is possible to obtain sufficient adhesion force between the thermistor thin film 5A and the electrode 3 to prevent peeling and thereby also maintain favorable electrical characteristics even after a heat resistance test at 250° C. and a temperature change test at, for example, 100,000 cycles have been performed.The above-mentioned Patent Literature 4, 5 proposes that the electrode layer made of platinum (Pt), an alloy thereof, or the like is made amorphous. However, the heat resistance is 150° C. or less. The present invention provides an effect of improving the heat resistance.Although the thermistor thin film 5A is deposited on the electrode 3 in the foregoing embodiment, for example, a thin film thermistor element 10 may be formed in another example of the foregoing embodiment in which a pair of an electrode 3 and an electrode 4 is formed within a thermistor thin film 5A as shown in Fig. 5.As illustrated in FIG. 6, manufacturing the above thin film thermistor element 10 includes: a step (S 101) of depositing the thermistor thin film 5A on an SiO2layer 2A in an Si substrate 2; a step (S 102) of depositing a thin film made of platinum (Pt), an alloy thereof, or the like; a step (S 103) of forming the pair of the electrode 3 and the electrode 4 by patterning after the film deposition; a step (S 104) of heat-treating the electrode 3 and the electrode 4 for crystallization; a step (S 105) of depositing a thermistor thin film 5B on the pair of the electrode 3 and the electrode 4; a step (S 106) of performing pattern formation of the thermistor thin film 5B; a step (S 107) of heat-treating the thermistor thin film 5A and the thermistor thin film 5B; a step (S 108) of depositing a passivation film 6 on these films; and a step (S 109) of patterning the passivation film 6.In addition, instead of the Si substrate 2 made of monocrystalline silicon, which is a typical semiconductor, a semiconductor substrate made of germanium (Ge), gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), gallium nitride (GaN), silicon carbide (SiC), gallium phosphide (GaP), or the like may be used as another semiconductor material.As a typical insulating substrate, an alumina (Al2O3) substrate or an insulating ceramic substrate made of silicon nitride (Si3N4), quartz (SiO2), aluminum nitride (AlN) or the like can be used.Instead of the SiO2layer 2A as the underlayer, a silicon nitride (Si3N4) film, a silicon monoxide (SiO) film, or the like may be used.Note that in the case of an insulating substrate, the SiO2layer 2A as the underlayer may or may not be deposited not on the entire surface but only on a part of the surface.ExamplesNext, with reference to Figs. 7 to 9, a description is provided for evaluation results of a thin film thermistor element according to the present invention actually manufactured in the method of the aforementioned embodiment.The thin film thermistor elements of a present example were prepared.These examples were subjected to a heat resistance test at 250° C. to measure an electric resistance value and a B value thereof. Further, the electric resistance value was measured and evaluated after 100,000 cycles of a temperature cycle of 40° C. to 250° C.As is apparent from the above evaluation results, the change rates of the electric resistance value and the B value of the thin film thermistor elements of the present examples are much lower than those of conventional elements even after continuous tests are performed.Here, FIGS. 7 to 9 present the evaluation results of the thin film thermistor element of the present examples.FIG. 10 presents an electron microscope observation of a platinum film after the heat treatment. In this photograph, the platinum is found to be crystallized in grains.As shown in Fig. 11, an acute precipitate indicative of a crystallized state is detected from the heat-treated electrode sheet, and therefore the electrode sheet is found crystallized.Industrial applicabilityAccording to the thin film thermistor element and the method of manufacturing the thin film thermistor element of the present invention, it is possible to achieve sufficient adhesion force between the thermistor thin film and the electrodes while maintaining adhesion force between the base substrate and the electrodes.

Claims

A thin film thermistor element (1, 10) comprising: a base substance (2), a thermistor thin film (5, 5A, 5B) formed on the base substance (2); and at least one pair of electrodes (3, 4) formed on, under, or within the thermistor thin film (5, 5A, 5B), the one pair of electrodes (3, 4) each having an electrode layer made of platinum or an alloy thereof, and the electrode layer being crystalline, wherein the electrode layer contains at least one of oxygen and nitrogen, and a content of the at least one of oxygen and nitrogen in the electrode layer is 0.01 weight percent to 4.9 weight percent, inclusive.The thin film thermistor element (1, 10) according to claim 1, wherein the electrode layer is in a crystal state of a granular crystal having <111> orientation.The thin film thermistor element (1, 10) according to claim 1, wherein the electrode layer is in a crystal state of a columnar crystal having <111> orientation.The thin film thermistor element (1, 10) according to claim 1, wherein the electrode layer is in a crystal state of a granular crystal and columnar crystal with <111> orientation.A method of manufacturing a thin film thermistor element (1, 10) according to any one of claims 1 to 4 for forming a pair of electrodes (3, 4) by patterning on, under or within a thermistor thin film (5, 5A, 5B) formed on a base substance (2), the method comprising: a first step (S01, S101) of depositing an electrode layer, at least one of oxygen and nitrogen being added, a content of the at least one of oxygen and nitrogen in the electrode layer being 0.01 weight percent to 4.9 weight percent inclusive, a second step (S02, S103) of forming at least one pair of electrodes (3, 4) by patterning, and a third step (S03, S 104) of heat treating the electrode layer to transition the electrode layer to a crystalline state.The method of manufacturing a thin film thermistor element (1, 10) according to claim 5, wherein the heat treatment process in the third step changes the electrode layer to the crystalline state of a granular crystal having <111> orientation.The method of manufacturing a thin film thermistor element (1, 10) according to claim 5, wherein the heat treatment process in the third step changes the electrode layer to the crystalline state of a columnar crystal having <111> orientation.The method of manufacturing a thin film thermistor element (1, 10) according to claim 5, wherein the heat treatment process in the third step changes the electrode layer to the crystalline state of a granular crystal and columnar crystal with <111> orientation.

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