# Patent

## Title
Thermistor made of metal nitride material, method for producing same, and film type thermistor sensor

## Abstract

Provided are a metal nitride material for a thermistor, which has a high reliability and a high heat resistance and can be directly deposited on a film or the like without firing, a method for producing the same, and a film type thermistor sensor. The metal nitride material for a thermistor consists of a metal nitride represented by the general formula: (Ti1-wCrw)xAlyNz (where 0.0<w<1.0, 0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, and x+y+z=1), wherein the crystal structure thereof is a hexagonal wurtzite-type single phase.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a U.S. National stage application of PCT International Application No. PCT/JP2013/082518 filed Nov. 27, 2013, which claims the benefit of Japanese Patent Application No. 2012-279148, filed Dec. 21, 2012, the entire contents of the aforementioned applications are hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a metal nitride material for a thermistor, which can be directly deposited on a film or the like without firing, a method for producing the same, and a film type thermistor sensor.
Description of the Related Art
There is a demand for a thermistor material used for a temperature sensor or the like having a high B constant in order to obtain a high precision and high sensitivity thermistor sensor. Conventionally, oxides of transition metals such as Mn, Co, Fe, and the like are typically used as such thermistor materials (see Patent Documents 1 and 2). These thermistor materials need firing at a temperature of 600° C. or higher in order to obtain a stable thermistor characteristic/property.
In addition to thermistor materials consisting of metal oxides as described above, Patent Document 3 discloses a thermistor material consisting of a nitride represented by the general formula: MxAyNz (where “M” represents at least one of Ta, Nb, Cr, Ti, and Zr, “A” represents at least one of Al, Si, and B, 0.1≦x≦0.8, 0<y≦0.6, 0.1≦z≦0.8, and x+y+z=1). In Patent Document 3, only a Ta—Al—N-based material consisting of a nitride represented by the general formula: MxAyNz (where 0.5≦x≦0.8, 0.1≦y≦0.5, 0.2≦z≦0.7, and x+y+z=1) is described in an Example. The Ta—Al—N-based material is produced by sputtering in a nitrogen gas-containing atmosphere using a material containing the element(s) listed above as a target. The resultant thin film is subject to a heat treatment at a temperature from 350 to 600° C. as required.
Other than thermistor materials, Patent document 4 discloses a resistance film material for a strain sensor, which consists of a nitride represented by the general formula: Cr100-x-yNxMy (where “M” is one or more elements selected from Ti, V, Nb, Ta, Ni, Zr, Hf, Si, Ge, C, O, P, Se, Te, Zn, Cu, Bi, Fe, Mo, W, As, Sn, Sb, Pb, B, Ga, In, Tl, Ru, Rh, Re, Os, Ir, Pt, Pd, Ag, Au, Co, Be, Mg, Ca, Sr, Ba, Mn, Al, and rare earth elements, the crystal structure thereof is composed of mainly a bcc structure or mainly a mixed structure of a bcc structure and A15 type structure, 0.0001≦x≦30, 0≦y≦30, and 0.0001≦x+y≦50). The resistance film material for a strain sensor is employed for measuring strain and stress from changes in the resistance of the sensor made of a Cr—N-based strain resistance film, where both of the amounts of nitrogen (x) and an accessory component element(s) M (y) are 30 at % or lower, as well as for performing various conversions. The Cr—N-M-based material is produced by reactive sputtering in a deposition atmosphere containing the accessory gaseous element(s) using a material containing the above-described element(s) or the like as a target. The resultant thin film is subject to a heat treatment at a temperature from 200 to 1000° C. as required.
PRIOR ART DOCUMENTS
Patent Documents


    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-226573
    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2006-324520
    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-319737
    [Patent Document 4] Japanese Unexamined Patent Application Publication No. H10-270201


SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
The following problems still remain in the conventional techniques described above.
In recent years, the development of a film type thermistor sensor made of a thermistor material formed on a resin film has been considered, and thus, it has been desired to develop a thermistor material that can be directly deposited on a film. Specifically, it is expected that a flexible thermistor sensor will be obtained by using a film. Furthermore, it is desired to develop a very thin thermistor sensor having a thickness of about 0.1 mm. However, a substrate material using a ceramic material such as alumina that has often been conventionally used has a problem that if the substrate material is thinned to a thickness of 0.1 mm for example, the substrate material is very fragile and breaks easily. Thus, it is expected that a very thin thermistor sensor will be obtained by using a film.
However, a film made of a resin material typically has a low heat resistance temperature of 150° C. or lower, and even polyimide, which is known as a material having a relatively high heat resistance temperature, only has a heat resistance temperature of about 200° C. Hence, when a heat treatment is performed in a process of forming a thermistor material, it has been conventionally difficult to use such a thermistor material. The above-described conventional oxide thermistor material needs to be fired at a temperature of 600° C. or higher in order to realize a desired thermistor characteristic, so that a film type thermistor sensor that is directly deposited on a film cannot be realized. Thus, it has been desired to develop a thermistor material that can be directly deposited on a film without firing. However, even the thermistor material disclosed in Patent Document 3 still needs a heat treatment on the resultant thin film at a temperature from 350 to 600° C. as required in order to obtain a desired thermistor characteristic. Regarding this thermistor material, a B constant of about 500 to 3000 K was obtained in an Example of the Ta—Al—N-based material, but the heat resistance of this material is not described and therefore, the thermal reliability of a nitride-based material is unknown.
In addition, the Cr—N-M-based material disclosed in Patent document 4 has a low B constant of 500 or lower and cannot ensure heat resistance to a temperature of 200° C. or lower unless a heat treatment in the range of 200° C. to 1000° C. is performed, and thus, a film type thermistor sensor that is directly deposited on a film cannot be realized. Therefore, it has been desired to develop a thermistor material that can be directly deposited on a film without firing.
The present invention has been made in view of the aforementioned circumstances, and an object of the present invention is to provide a metal nitride material for a thermistor, which has a high reliability and a high heat resistance and can be directly deposited on a film or the like without firing, a method for producing the same, and a film type thermistor sensor.
Means for Solving the Problems
The present inventors' serious endeavor carried out by focusing on an Al—N-based material among nitride materials found that the Al—N-based material having a good B constant and an excellent heat resistance may be obtained without firing by substituting the Al site with a specific metal element for improving electric conductivity and by forming it into a specific crystal structure even though Al—N is an insulator and difficult to provide with an optimum thermistor characteristic (B constant: about 1000 to 6000 K).
Therefore, the present invention has been made on the basis of the above finding, and adopts the following configuration in order to overcome the aforementioned problems.
Specifically, a metal nitride material for a thermistor according to a first aspect of the present invention is characterized by a metal nitride material used for a thermistor, which consists of a metal nitride represented by the general formula: (Ti1-wCrw)xAlyNz (where 0.0<w<1.0, 0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, and x+y+z=1), wherein the crystal structure thereof is a hexagonal wurtzite-type single phase.
Since this metal nitride material for a thermistor consists of a metal nitride represented by the general formula: (Ti1-wCrw)xAlyNz (where 0.0<w<1.0, 0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, and x+y+z=1), wherein the crystal structure thereof is a hexagonal wurtzite-type single phase, a good B constant and an high heat resistance can be obtained without firing.
Note that, when the value of “y/(x+y)” (i.e., Al/(Ti+Cr+Al)) is less than 0.70, a wurtzite-type single phase cannot be obtained, but two coexisting phases of a wurtzite-type phase and a NaCl-type phase or a single phase of only a NaCl-type phase may be obtained, so that a sufficiently high resistance and a high B constant cannot be obtained.
When the value of “y/(x+y)” (i.e., Al/(Ti+Cr+Al)) exceeds 0.95, the metal nitride material exhibits very high resistivity and extremely high electrical insulation, so that the metal nitride material is not applicable as a thermistor material.
When the value of “z” (i.e., N/(Ti+Cr+Al+N)) is less than 0.4, the nitridation amount of metals is too small to obtain a wurtzite-type single phase. Consequently, a sufficiently high resistance and a high B constant cannot be obtained.
In addition, when the value of “z” (i.e., N/(Ti+Cr+Al+N)) exceeds 0.5, a wurtzite-type single phase cannot be obtained. This is because the stoichiometric ratio of N/(Ti+Cr+Al+N) in a wurtzite-type single phase in the absence of defects at the nitrogen site is 0.5.
Regarding the value of “w” (i.e., Cr/(Ti+Cr)), if the condition of 0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, and x+y+z=1 is satisfied, a wurtzite-type single phase can be obtained in a wide composition range of 0.0<w<1.0, thus a sufficiently high resistance and a high B constant can be obtained.
A metal nitride material for a thermistor according to a second aspect of the present invention is characterized in that the metal nitride material for a thermistor according to the first aspect of the present invention is deposited as a film, and is a columnar crystal extending in a vertical direction with respect to the surface of the film.
Specifically, since this metal nitride material for a thermistor is a columnar crystal extending in a vertical direction with respect to the surface of the film, the crystallinity of the film is high, so that a high heat resistance can be obtained.
A metal nitride material for a thermistor according to a third aspect of the present invention is characterized in that the metal nitride material according to the first or second aspect of the present invention is deposited as a film and is more strongly oriented along a c-axis than an a-axis in a vertical direction with respect to the surface of the film.
Specifically, since this metal nitride material for a thermistor is more strongly oriented along the c-axis than the a-axis in a vertical direction with respect to the surface of the film, a high B constant as compared with the case of the strong a-axis orientation and further an excellent reliability in heat resistance can be obtained.
A film type thermistor sensor according to a fourth aspect of the present invention is characterized by including an insulating film; a thin film thermistor portion made of the metal nitride material for a thermistor according to any one of the first to third aspects of the present invention on the insulating film; and a pair of pattern electrodes formed at least on the top or the bottom of the thin film thermistor portion.
Specifically, since the thin film thermistor portion made of the metal nitride material for a thermistor according to any one of the first to third aspects of the present invention is formed on the insulating film in this film type thermistor sensor, an insulating film having a low heat resistance such as a resin film can be used because the thin film thermistor portion is formed without firing and has a high B constant and a high heat resistance, so that a thin and flexible thermistor sensor having an excellent thermistor characteristic can be obtained.
A substrate material employing a ceramic such as alumina that has often been conventionally used has a problem that if the substrate material is thinned to a thickness of 0.1 mm for example, the substrate material is very fragile and breaks easily. On the other hand, since a film can be used in the present invention, a very thin film type thermistor sensor having a thickness of 0.1 mm, for example, can be obtained.
A film type thermistor sensor according to a fifth aspect of the present invention is characterized in that at least a portion of a pair of the pattern electrodes that is bonded to the thin film thermistor portion is made of Cr in the film type thermistor sensor according to a fourth aspect of the present invention.
Specifically, since at least a portion of a pair of the pattern electrodes that is bonded to the thin film thermistor portion is made of Cr in this film type thermistor sensor, the bondability between the thin film thermistor portion made of Ti—Cr—Al—N and a portion made of Cr of a pair of the pattern electrodes becomes high. In other words, since Cr that is one of the elements constituting the thin film thermistor portion is used as the material for the bonding portion of a pair of the pattern electrodes, the bondability between both the portions becomes high, and thus a high reliability can be obtained.
A method for producing a metal nitride material for a thermistor according to a sixth aspect of the present invention is characterized in that the method for producing the metal nitride material for a thermistor according to any one of the first to third aspects of the present invention includes a deposition step of performing film deposition by reactive sputtering in a nitrogen-containing atmosphere using a Ti—Cr—Al composite sputtering target.
Specifically, since film deposition is performed by reactive sputtering in a nitrogen-containing atmosphere using a Ti—Cr—Al composite sputtering target in this method for producing a metal nitride material for a thermistor, the metal nitride material for a thermistor of the present invention, which consists of the (Ti,Cr)AlN described above, can be deposited on a film without firing.
A method for producing a metal nitride material for a thermistor according to a seventh aspect of the present invention is characterized by the method according to a sixth aspect of the present invention, wherein the sputtering gas pressure during the reactive sputtering is set to less than 0.67 Pa.
Specifically, since the sputtering gas pressure during the reactive sputtering is set to less than 0.67 Pa in this method for producing a metal nitride material for a thermistor, a film made of the metal nitride material for a thermistor according to a third aspect of the present invention can be formed which is more strongly oriented along the c-axis than the a-axis in a vertical direction with respect to the surface of the film.
A method for producing a metal nitride material for a thermistor according to an eighth aspect of the present invention is characterized in that the method according to a sixth or seventh aspect of the present invention includes a step of irradiating the formed film with nitrogen plasma after the deposition step.
Specifically, since the formed film is irradiated with nitrogen plasma after the deposition step in this method for producing a metal nitride material for a thermistor, nitrogen defects in the film are reduced, resulting in a further improvement in heat resistance.
Effects of the Invention
According to the present invention, the following effects may be provided.
Specifically, since the metal nitride material for a thermistor according to the present invention consists of a metal nitride represented by the general formula: (Ti1-wCrw)xAlyNz (where 0.0<w<1.0, 0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, and x+y+z=1), wherein the crystal structure thereof is a hexagonal wurtzite-type single phase, the metal nitride material having a good B constant and an high heat resistance can be obtained without firing. Also, since film deposition is performed by reactive sputtering in a nitrogen-containing atmosphere using a Ti—Cr—Al composite sputtering target in the method for producing the metal nitride material for a thermistor according to the present invention, the metal nitride material for a thermistor of the present invention, which consists of the (Ti,Cr)AlN described above, can be deposited on a film without firing. Further, since a thin film thermistor portion made of the metal nitride material for a thermistor according to the present invention is formed on an insulating film in the film type thermistor sensor according to the present invention, a thin and flexible thermistor sensor having an excellent thermistor characteristic can be obtained by using an insulating film such as a resin film having a low heat resistance. Furthermore, since a substrate material is a resin film rather than a ceramic material that becomes very fragile and breaks easily when being thinned, a very thin film type thermistor sensor having a thickness of 0.1 mm can be obtained.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a (Ti+Cr)—Al—N-based ternary phase diagram illustrating the composition range of a metal nitride material for a thermistor according to one embodiment of a metal nitride material for a thermistor, a method for producing the same, and a film type thermistor sensor of the present invention.
FIG. 2 is a perspective view illustrating a film type thermistor sensor according to the present embodiment.
FIG. 3 is a perspective view illustrating a method for producing a film type thermistor sensor in the order of the steps according to the present embodiment.
FIG. 4 is a front view and a plan view illustrating a film evaluation element for a metal nitride material for a thermistor according to an Example of a metal nitride material for a thermistor, a method for producing the same, and a film type thermistor sensor of the present invention.
FIG. 5 is a graph illustrating the relationship between a resistivity at 25° C. and a B constant according to Examples and a Comparative Example of the present invention.
FIG. 6 is a graph illustrating the relationship between a Al/(Ti+Cr+Al) ratio and a B constant according to Examples and a Comparative Example of the present invention.
FIG. 7 is a graph illustrating the relationship between a Cr/(Ti+Cr) ratio and a B constant according to Examples and a Comparative Example of the present invention.
FIG. 8 is a graph illustrating the result of X-ray diffraction (XRD) in the case of a strong c-axis orientation, where Al/(Ti+Cr+Al)=0.76 according to an Example of the present invention.
FIG. 9 is a graph illustrating the result of X-ray diffraction (XRD) in the case of a strong a-axis orientation, where Al/(Ti+Cr+Al)=0.76 according to an Example of the present invention.
FIG. 10 is a graph illustrating the result of X-ray diffraction (XRD) in the case where Al/(Ti+Cr+Al)=0.61 according to a Comparative Example of the present invention.
FIG. 11 is a graph illustrating the relationship between a Al/(Ti+Cr+Al) ratio and a B constant for the comparison of the material exhibiting a strong a-axis orientation and the material exhibiting a strong c-axis orientation according to Examples of the present invention.
FIG. 12 is a graph illustrating the relationship between a Cr/(Ti+Cr) ratio and a B constant for the comparison of the material exhibiting a strong a-axis orientation and the material exhibiting a strong c-axis orientation according to Examples of the present invention.
FIG. 13 is a cross-sectional SEM photograph illustrating the material exhibiting a strong c-axis orientation according to an Example of the present invention.
FIG. 14 is a cross-sectional SEM photograph illustrating the material exhibiting a strong a-axis orientation according to an Example of the present invention.



DESCRIPTION OF THE EMBODIMENTS
Hereinafter, a description will be given of a metal nitride material for a thermistor, a method for producing the same, and a film type thermistor sensor according to one embodiment of the present invention with reference to FIGS. 1 to 3. In the drawings used in the following description, the scale of each component is changed as appropriate so that each component is recognizable or is readily recognized.
The metal nitride material for a thermistor of the present embodiment is a metal nitride material used for a thermistor, which consists of a metal nitride represented by the general formula: (Ti1-wCrw)xAlyNz (where 0.0<w<1.0, 0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, and x+y+z=1), wherein the crystal structure thereof is a hexagonal wurtzite-type (space group: P63mc (No. 186)) single phase. Specifically, this metal nitride material for a thermistor consists of a metal nitride having a composition within the region enclosed by the points A, B, C, and D in the (Ti+Cr)—Al—N-based ternary phase diagram as shown in FIG. 1, wherein the crystal phase thereof is wurtzite-type.
Note that the composition ratios of (x, y, z) at the points A, B, C, and D are A (15, 35, 50), B (2.5, 47.5, 50), C (3, 57, 40), and D (18, 42, 40), respectively.
Also, this metal nitride material for a thermistor is deposited as a film, and is a columnar crystal extending in a vertical direction with respect to the surface of the film. Furthermore, it is preferable that the metal nitride material for a thermistor is more strongly oriented along the c-axis than the a-axis in a vertical direction with respect to the surface of the film.
Note that the decision about whether a metal nitride material for a thermistor has a strong a-axis orientation (100) or a strong c-axis orientation (002) in a vertical direction (film thickness direction) with respect to the surface of the film is made by examining the orientation of the crystal axis using X-ray diffraction (XRD). When the peak intensity ratio of “the peak intensity of (100)”/“the peak intensity of (002)”, where (100) is the hkl index indicating a-axis orientation and (002) is the hkl index indicating c-axis orientation, is less than 1, the metal nitride material for a thermistor is determined to have a strong c-axis orientation.
Next, a description will be given of a film type thermistor sensor using the metal nitride material for a thermistor of the present embodiment. As shown in FIG. 2, a film type thermistor sensor 1 includes an insulating film 2, a thin film thermistor portion 3 made of the metal nitride material for a thermistor described above on the insulating film 2, and a pair of pattern electrodes 4 formed at least on the thin film thermistor portion 3.
The insulating film 2 is, for example, a polyimide resin sheet formed in a band shape. The insulating film 2 may be made of another material such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or the like.
A pair of the pattern electrodes 4 has a pair of comb shaped electrode portions 4a that is patterned so as to have a comb shaped pattern by using stacked metal films of, for example, a Cr film and an Au film, and is arranged opposite to each other, and a pair of linear extending portions 4b extending with the tip ends thereof being connected to these comb shaped electrode portions 4a and the base ends thereof being arranged at the end of the insulating film 2.
A plating portion 4c such as Au plating is formed as a lead wire drawing portion on the base end of each of a pair of the linear extending portions 4b. One end of the lead wire is joined with the plating portion 4c via a solder material or the like. Furthermore, except for the end of the insulating film 2 including the plating portions 4c, a polyimide coverlay film 5 is pressure bonded onto the insulating film 2. Instead of the polyimide coverlay film 5, a polyimide or epoxy-based resin material may be formed onto the insulating film 2 by printing.
A description will be given below of a method for producing the metal nitride material for a thermistor and a method for producing the film type thermistor sensor 1 using the metal nitride material for a thermistor with reference to FIG. 3.
Firstly, the method for producing the metal nitride material for a thermistor according to the present embodiment includes a deposition step of performing film deposition by reactive sputtering in a nitrogen-containing atmosphere using a Ti—Cr—Al composite sputtering target. The Ti—Cr—Al composite sputtering target is a Ti—Cr—Al sintered body that is prepared by mixing a Ti—Al alloy powder and a Cr—Al alloy powder together and then compacting them by hot pressing.
It is preferable that the sputtering gas pressure during the reactive sputtering is set to less than 0.67 Pa.
Furthermore, it is preferable that the formed film is irradiated with nitrogen plasma after the deposition step.
More specifically, the thin film thermistor portion 3 having a thickness of 200 nm, which is made of the metal nitride material for a thermistor of the present embodiment, is deposited on the insulating film 2 which is, for example, a polyimide film having a thickness of 50 μm shown in FIG. 3(a) by the reactive sputtering method, as shown in FIG. 3(b). The exemplary sputtering conditions at this time are as follows: an ultimate degree of vacuum: 5×10−6 Pa, a sputtering gas pressure: 0.4 Pa, a target input power (output): 200 W, and a nitrogen gas partial pressure under a mixed gas (Ar gas+nitrogen gas) atmosphere: 60%. In addition, the metal nitride material for a thermistor having a desired size is deposited on the insulating film 2 using a metal mask so as to form the thin film thermistor portion 3. It is preferable that the formed thin film thermistor portion 3 is irradiated with nitrogen plasma. For example, the thin film thermistor portion 3 is irradiated with nitrogen plasma under the degree of vacuum of 6.7 Pa, the output of 200 W, and the N2 gas atmosphere.
Next, a Cr film having a thickness of 20 nm is formed and an Au film having a thickness of 200 nm is further formed thereon by the sputtering method, for example. Furthermore, a resist solution is coated on the stacked metal films using a barcoater, and then pre-baking is performed for 1.5 minutes at a temperature of 110° C. After the exposure by an exposure device, any unnecessary portion is removed by a developing solution, and then patterning is performed by post-baking for 5 minutes at a temperature of 150° C. Then, any unnecessary electrode portion is subject to wet etching using commercially available Au etchant and Cr etchant, and then the resist is stripped so as to form a pair of the pattern electrodes 4 each having a desired comb shaped electrode portion 4a as shown in FIG. 3(c). Note that a pair of the pattern electrodes 4 may be formed in advance on the insulating film 2, and then the thin film thermistor portion 3 may be deposited on the comb shaped electrode portions 4a. In this case, the comb shaped electrode portions 4a of a pair of the pattern electrodes 4 are formed below the thin film thermistor portion 3.
Next, as shown in FIG. 3(d), the polyimide coverlay film 5 with an adhesive having a thickness of 50 μm, for example, is placed on the insulating film 2, and then they are bonded to each other under pressurization of 2 MPa at a temperature of 150° C. for 10 minutes using a press machine. Furthermore, as shown in FIG. 3(e), an Au thin film having a thickness of 2 μm is formed at the base ends of the linear extending portions 4b using, for example, an Au plating solution so as to form the plating portions 4c. 
When a plurality of film type thermistor sensors 1 is simultaneously produced, a plurality of thin film thermistor portions 3 and a plurality of pattern electrodes 4 are formed on a large-format sheet of the insulating film 2 as described above, and then, the resulting large-format sheet is cut into a plurality of segments so as to obtain a plurality of film type thermistor sensors 1.
In this manner, a thin film type thermistor sensor 1 having a size of 25×3.6 mm and a thickness of 0.1 mm, for example, is obtained.
As described above, since the metal nitride material for a thermistor of the present embodiment consists of a metal nitride represented by the general formula: (Ti1-wCrw)xAlyNz (where 0.0<w<1.0, 0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, and x+y+z=1), wherein the crystal structure thereof is a hexagonal wurtzite-type single phase, a good B constant and a high heat resistance can be obtained without firing.
Since the metal nitride material for a thermistor is a columnar crystal extending in a vertical direction with respect to the surface of the film, the crystallinity of the film is high, so that a high heat resistance can be obtained.
Furthermore, since the metal nitride material for a thermistor is more strongly oriented along the c-axis than the a-axis in a vertical direction with respect to the surface of the film, a high B constant as compared with the case of a strong a-axis orientation can be obtained.
Since film deposition is performed by reactive sputtering in a nitrogen-containing atmosphere using a Ti—Cr—Al composite sputtering target in the method for producing the metal nitride material for a thermistor of the present embodiment, the metal nitride material for a thermistor, which consists of the (Ti,Cr)AlN described above, can be deposited on a film without firing.
In addition, when the sputtering gas pressure during the reactive sputtering is set to less than 0.67 Pa, a film of the metal nitride material for a thermistor, which is more strongly oriented along the c-axis than the a-axis in a vertical direction to the surface of the film, can be formed.
Furthermore, since the formed film is irradiated with nitrogen plasma after the deposition step, nitrogen defects in the film are reduced, resulting in a further improvement in heat resistance.
Thus, since the thin film thermistor portion 3 made of the metal nitride material for a thermistor described above is formed on the insulating film 2 in the film type thermistor sensor 1 using the metal nitride material for a thermistor of the present embodiment, the insulating film 2 having a low heat resistance, such as a resin film, can be used because the thin film thermistor portion 3 is formed without firing and has a high B constant and a high heat resistance, so that a thin and flexible thermistor sensor having an excellent thermistor characteristic can be obtained.
A substrate material using a ceramic such as alumina that has often been conventionally used has a problem that if the substrate material is thinned to a thickness of 0.1 mm, for example, the substrate material is very fragile and breaks easily. On the other hand, since a film can be used in the present invention, a very thin film type thermistor sensor having a thickness of 0.1 mm, for example, can be provided.
In addition, since at least a portion of a pair of the pattern electrodes 4 that is bonded to the thin film thermistor portion 3 is made of a Cr film, the bondability between the thin film thermistor portion 3 made of Ti—Cr—Al—N and the Cr film of a pair of the pattern electrodes 4 becomes high. Specifically, since Cr that is one of the elements constituting the thin film thermistor portion 3 is used as the material for the bonding portion of a pair of the pattern electrodes 4, the bondability between both the portions becomes high, and thus a high reliability can be obtained.
EXAMPLES
Next, the evaluation results of the materials according to Examples produced based on the above embodiment regarding the metal nitride material for a thermistor, the method for producing the same, and the film type thermistor sensor according to the present invention will be specifically described with reference to FIGS. 4 to 14.
<Production of Film Evaluation Element>
The film evaluation elements 121 shown in FIG. 4 were produced according to Examples and Comparative Examples of the present invention as follows.
Firstly, each of the thin film thermistor portions 3 having a thickness of 500 nm which were made of the metal nitride materials for a thermistor with the various composition ratios shown in Table 1 was formed on an Si wafer with a thermal oxidation film as an Si substrate (S) by using a Ti—Cr—Al composite target with various composition ratios by the reactive sputtering method. The thin film thermistor portions 3 were formed under the sputtering conditions of an ultimate degree of vacuum of 5×10−6 Pa, a sputtering gas pressure of from 0.1 to 1 Pa, a target input power (output) of from 100 to 500 W, and a nitrogen gas partial pressure under a mixed gas (Ar gas+nitrogen gas) atmosphere of from 10 to 100%.
Next, a Cr film having a thickness of 20 nm was formed and an Au film having a thickness of 200 nm was further formed on each of the thin film thermistor portions 3 by the sputtering method. Furthermore, a resist solution was coated on the stacked metal films using a spin coater, and then pre-baking was performed for 1.5 minutes at a temperature of 110° C. After the exposure by an exposure device, any unnecessary portion was removed by a developing solution, and then patterning was performed by post-baking for 5 minutes at a temperature of 150° C. Then, any unnecessary electrode portion was subject to wet etching using commercially available Au etchant and Cr etchant, and then the resist was stripped so as to form a pair of pattern electrodes 124, each having a desired comb shaped electrode portion 124a. Then, the resultant elements were diced into chip elements so as to obtain the film evaluation elements 121 used for evaluating a B constant and for testing heat resistance.
Note that the film evaluation elements 121 according to Comparative Examples, each having the composition ratio of (Ti,Cr)xAlyNz outside the range of the present invention and a different crystal system, were similarly produced for comparative evaluation.
<Film Evaluation>
(1) Composition Analysis
Elemental analysis was performed on the thin film thermistor portions 3 obtained by the reactive sputtering method by X-ray photoelectron spectroscopy (XPS). In the XPS, a quantitative analysis was performed on a sputtering surface at a depth of 20 nm from the outermost surface by Ar sputtering. The results are shown in Table 1. In the following tables, the composition ratios are expressed by “at %”. Some of the samples were also subject to a quantitative analysis for a sputtering surface at a depth of 100 nm from the outermost surface to confirm that it had the same composition within the quantitative accuracy as one in the sputtering surface at a depth of 20 nm.
In the X-ray photoelectron spectroscopy (XPS), a quantitative analysis was performed under the conditions of an X-ray source of MgKα (350 W), a path energy of 58.5 eV, a measurement interval of 0.125 eV, a photo-electron take-off angle with respect to a sample surface of 45 deg, and an analysis area of about 800 μmφ. Note that the quantitative accuracy of N/(Ti+Cr+Al+N) and Al/(Ti+Cr+Al) was ±2% and ±1%, respectively.
(2) Specific Resistance Measurement
The specific resistance of each of the thin film thermistor portions 3 obtained by the reactive sputtering method was measured by the four-probe method at a temperature of 25° C. The results are shown in Table 1.
(3) Measurement of B Constant
The resistance values for each of the film evaluation elements 121 at temperatures of 25° C. and 50° C. were measured in a constant temperature bath, and a B constant was calculated based on the resistance values at temperatures of 25° C. and 50° C. The results are shown in Table 1.
In the B constant calculating method of the present invention, a B constant is calculated by the following formula using the resistance values at temperatures of 25° C. and 50° C.
B constant (K)=ln(R25/R50)/(1/T25-1/T50)
R25 (Ω): resistance value at 25° C.
R50 (Ω): resistance value at 50° C.
T25 (K): 298.15 K, which is an absolute temperature of 25° C. expressed in Kelvin
T50 (K): 323.15 K, which is an absolute temperature of 50° C. expressed in Kelvin
As can be seen from these results, a thermistor characteristic having a resistivity of 20 Ωcm or higher and a B constant of 1500 K or higher is achieved in all of the Examples in which the composition ratios of (Ti,Cr)xAlyNz fall within the region enclosed by the points A, B, C, and D in the ternary phase diagram shown in FIG. 1, i.e., the region where “0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, x+y+z=1”.
A Graph illustrating the relationship between a resistivity at 25° C. and a B constant from the above results is shown in FIG. 5. A graph illustrating the relationship between a Al/(Ti+Cr+Al) ratio and a B constant is also shown in FIG. 6. From these graphs, the film evaluation elements 121, the composition ratios of which fall within the region where Al/(Ti+Cr+Al) is from 0.7 to 0.95 and N/(Ti+Cr+Al+N) is from 0.4 to 0.5 and each crystal system of which is a hexagonal wurtzite-type single phase, have a specific resistance value at a temperature of 25° C. of 100 Ωcm or higher and a B constant of 1500 K or higher, which is the region realizing a high resistance and a high B constant. In data shown in FIG. 6, the reason why the B constant varies with respect to the same Al/(Ti+Cr+Al) ratio is because the film evaluation elements 121 have different amounts of nitrogen in their crystals or different amounts of lattice defects such as nitrogen defects.
Further, a graph illustrating the relationship between a Cr/(Ti+Cr) ratio and a B constant is shown in FIG. 7.
As can be seen from the above measurement results, if the condition that Al/(Ti+Cr+Al) is from 0.7 to 0.95 and N/(Ti+Cr+Al+N) is from 0.4 to 0.5 is satisfied, a specific resistance value at a temperature of 25° C. of 100 Ωcm or higher and a B constant of 1500 K or higher, which is the region of a high resistance and a high B constant, can be realized in a wide composition range of 0.0<Cr/(Ti+Cr)<1.0.
In the materials according to Comparative Examples 2 and 3 as shown in Table 1, the composition ratios fall within the region where Al/(Ti+Cr+Al)<0.7, and the crystal systems are cubic NaCl-type. Thus, a material with the composition ratio that falls within the region where Al/(Ti+Cr+Al)<0.7 has a specific resistance value at a temperature of 25° C. of less than 100 Ωcm and a B constant of less than 1500 K, which is the region of low resistance and low B constant.
In the material according to Comparative Example 1 shown in Table 1, the composition ratio falls within the region where N/(Ti+Cr+Al+N) is less than 40%, that is, the material is in a crystal state where nitridation of metals contained therein is insufficient. The material according to Comparative Example 1 was neither NaCl-type nor wurtzite-type and had very poor crystallinity. In addition, it was found that the material according to this Comparative Example exhibited near-metallic behavior because both the B constant and the resistance value were very small.
(4) Thin Film X-Ray Diffraction (Identification of Crystal Phase)
The crystal phases of the thin film thermistor portions 3 obtained by the reactive sputtering method were identified by Grazing Incidence X-ray Diffraction. The thin film X-ray diffraction is a small angle X-ray diffraction experiment. The measurement was performed under the conditions of a Cu X-ray tube, the angle of incidence of 1 degree, and 2θ of from 20 to 130 degrees. Some of the samples were measured under the condition of the angle of incidence of 0 degree and 2θ of from 20 to 100 degrees.
As a result of the measurement, a wurtzite-type phase (hexagonal crystal, the same phase as that of AlN) was obtained in the region where Al/(Ti+Cr+Al)≧0.7, whereas a NaCl-type phase (cubic crystal, the same phase as those of CrN and TiN) was obtained in the region where Al/(Ti+Cr+Al)<0.65. Note that it is contemplated that a crystal phase in which a wurtzite-type phase and a NaCl-type phase coexist will be obtained in the region where 0.65<Al/(Ti+Cr+Al)<0.7.
Thus, in the (Ti,Cr)AlN-based material, the region of high resistance and high B constant can be realized by the wurtzite-type phase where Al/(Ti+Cr+Al)≧0.7. In the materials according to Examples of the present invention, no impurity phase was confirmed and the crystal structure thereof was a wurtzite-type single phase.
In the material according to Comparative Example 1 shown in Table 1, the crystal phase thereof was neither wurtzite-type nor NaCl-type as described above, and thus, could not be identified in the testing. In this Comparative Example, the peak width of XRD was very large, showing that the material had very poor crystallinity. It is contemplated that the crystal phase thereof was metal phase with insufficient nitridation because it exhibited near-metallic behavior from the viewpoint of electric characteristics.



















TABLE 1










CRYSTAL AXIS














EXHIBITING






























STRONG DEGREE







RESULT OF




XRD PEAK
OF ORIENTATION 







ELECTRIC




INTENSITY
IN VERTICAL







PROPERTIES
















RATIO OF
DIRECTION WITH 

COMPOSITION RATIO

SPECIFIC





















(100)/(002)
RESPECT TO 

Ti/
Cr/
Al/
N/



RESIS-




WHEN
SUBSTRATE SUR- 

(Ti + 
(Ti + 
(Ti + 
(Ti + 
Al/


TANCE




CRYSTAL
FACE WHEN 
SPUTTER-
Cr +
Cr +
Cr +
Cr +
(Ti + 
Cr/

VALUE 




PHASE IS
CRYSTAL PHASE 
ING GAS
Al +
Al +
Al +
Al +
Cr +
Ti + 
B CON-
AT



CRYSTAL
WURTZITE
IS WURTZITE TYPE
PRESSURE
N)
N)
N)
N)
Al)
Cr)
STANT
25° C.



SYSTEM
TYPE
(a-AXIS OR c-AXIS)
(Pa)
(%)
(%)
(%)
(%)
(%)
(%)
(K)
(Ω cm)






















COMPAR-
INSUFFI-



4
14
50
32
74
78
7
9.E−02


ATIVE EX-
CIENT













AMPLE 1
NITRIDA-














TION













COMPAR-
NaCl TYPE



18
4
35
43
61
18
821
4.E+01


ATIVE EX-














AMPLE 2














COMPAR-
NaCl TYPE



5
18
34
43
60
79
1171
6.E+01


ATIVE EX-














AMPLE 3














EXAMPLE 1
WURTZITE 
0.43
c-AXIS
<0.67
3
0.3
48
49
94
10
4581
1.E+06



TYPE













EXAMPLE 2
WURTZITE 
0.79
c-AXIS
<0.67
0.3
3
48
49
94
91
4035
1.E+06



TYPE













EXAMPLE 3
WURTZITE 
0.05
c-AXIS
<0.67
4
3
44
49
86
49
3439
6.E+05



TYPE













EXAMPLE 4
WURTZITE 
0.10
c-AXIS
<0.67
4
4
46
46
85
50
3057
1.E+05



TYPE













EXAMPLE 5
WURTZITE 
0.19
c-AXIS
<0.67
10
2
39
49
76
17
2532
9.E+04



TYPE













EXAMPLE 6
WURTZITE 
0.31
c-AXIS
<0.67
11
2
38
49
75
17
2701
6.E+04



TYPE













EXAMPLE 7
WURTZITE 
0.58
c-AXIS
<0.67
12
2
37
49
73
15
2452
4.E+03



TYPE













EXAMPLE 8
WURTZITE 
4.04
a-AXIS
≧0.67
13
3
42
42
72
19
1773
7.E+02



TYPE













EXAMPLE 9
WURTZITE 
2.96
a-AXIS
≧0.67
10
2
39
49
76
36
2132
2.E+05



TYPE













EXAMPLE
WURTZITE 
1.75
a-AXIS
≧0.67
2
12
38
48
73
86
2377
9.E+04


10
TYPE













EXAMPLE 
WURTZITE 
2.04
a-AXIS
≧0.67
4
0.4
54
42
93
10
3312
6.E+05


11
TYPE









Next, since all the materials according to the Examples of the present invention were wurtzite-type phase films having strong orientation, whether the films have a strong a-axis orientation or c-axis orientation of the crystal axis in a vertical direction (film thickness direction) with respect to the Si substrate (S) was examined by XRD. At this time, in order to examine the orientation of the crystal axis, the peak intensity ratio of (100)/(002) was measured, where (100) is the hkl index indicating a-axis orientation and (002) is the hkl index indicating c-axis orientation.
As a result of the measurement, in the Examples in which film deposition was performed at a sputtering gas pressure of less than 0.67 Pa, the intensity of (002) was much stronger than that of (100), that is, the films exhibited stronger c-axis orientation than a-axis orientation. On the other hand, in the Examples in which film deposition was performed at a sputtering gas pressure of 0.67 Pa or higher, the intensity of (100) was much stronger than that of (002), that is, the films exhibited stronger a-axis orientation than c-axis orientation.
Note that it was confirmed that a wurtzite-type single phase was formed in the same manner even when the thin film thermistor portion 3 was deposited on a polyimide film under the same deposition condition. It was also confirmed that the crystal orientation did not change even when the thin film thermistor portion 3 was deposited on a polyimide film under the same deposition condition.
An Exemplary XRD profile of the material according to the Example exhibiting strong c-axis orientation is shown in FIG. 8. In this Example, Al/(Ti+Cr+Al) was equal to 0.76 (wurtzite-type, hexagonal crystal), and the measurement was performed at 1 degree angle of incidence. As can be seen from the result, the intensity of (002) was much stronger than that of (100) in this Example.
An Exemplary XRD profile of the material according to the Example exhibiting strong a-axis orientation is also shown in FIG. 9. In this Example, Al/(Ti+Cr+A) was equal to 0.76 (wurtzite-type, hexagonal crystal), and the measurement was performed at 1 degree angle of incidence. As can be seen from the result, the intensity of (100) was much stronger than that of (002) in this Example.
The asterisk (*) in the graphs shows the peak originating from the device or the Si substrate with a thermal oxidation film, and thus, it was confirmed that the peak with the asterisk (*) in the graphs was neither the peak originating from a sample itself nor the peak originating from an impurity phase. A symmetrical measurement was performed at a 0 degree angle of incidence, confirming that the peak indicated by (*) is lost in the symmetrical measurement, and thus, that it was the peak originating from the device or the Si substrate with a thermal oxidation film.
An exemplary XRD profile in the Comparative Example is shown in FIG. 10. In this Comparative Example, Al/(Ti+Cr+Al) was equal to 0.61 (NaCl type, cubic crystal), and the measurement was performed at 1 degree angle of incidence. No peak which could be indexed as a wurtzite-type (space group: P63mc (No. 186)) was detected, and thus, the film according to this Comparative Example was confirmed as a NaCl-type single phase.
Next, the correlations between a crystal structure and its electric properties were further compared with each other in detail regarding the Examples of the present invention in which the wurtzite-type materials were employed.
As shown in Table 2 and FIG. 11, the crystal axis of some materials is strongly oriented along a c-axis in a vertical direction with respect to the surface of the substrate and that of other materials is strongly oriented along an a-axis in a vertical direction with respect to the surface of the substrate among the materials having nearly the same Al/(Ti+Cr+Al) ratio.
When both groups were compared to each other, it was found that the materials having a strong c-axis orientation had a higher B constant by about 200 K than that of the materials having a strong a-axis orientation provided that they have the same Al/(Ti+Cr+Al) ratio. When focus was placed on the amount of N (i.e., N/(Ti+Cr+Al+N)), it was found that the materials having a strong c-axis orientation had a slightly larger amount of nitrogen than that of the materials having a strong a-axis orientation. Since the ideal stoichiometric ratio of N/(Ti+Cr+Al+N) is 0.5, it was found that the materials having a strong c-axis orientation were ideal materials due to a small amount of nitrogen defects.
In addition, since three kinds of elements, Ti, Cr, and Al, are added to the site of the metal element in a wurtzite-type material, these three kinds of metal elements, Ti, Cr, and Al, can eliminate the lattice distortion due to nitrogen defects, and thus a flexible nitride thermistor material having a higher reliability (heat resistance) can be provided.
A graph illustrating the relationship between a Cr/(Ti+Cr) ratio and a B constant for the comparison of the material exhibiting a strong a-axis orientation and the material exhibiting a strong c-axis orientation according to Examples of the present invention is shown in FIG. 12. In addition, the data on the materials having nearly the same Al/(Ti+Cr+Al) ratio as that of the materials shown in FIG. 11 are plotted in FIG. 12. It has been found that the materials exhibiting a strong c-axis orientation have a higher B constant by about 200 K than the materials exhibiting a strong a-axis orientation provided that they have the same Al/(Ti+Cr+Al) and Cr/(Ti+Cr) ratios.



















TABLE 2










CRYSTAL AXIS














EXHIBITING






























STRONG DEGREE







RESULT OF




XRD PEAK
OF ORIENTATION 







ELECTRIC




INTENSITY
IN VERTICAL







PROPERTIES
















RATIO OF
DIRECTION WITH 

COMPOSITION RATIO

SPECIFIC





















(100)/(002)
RESPECT TO 

Ti/
Cr/
Al/
N/



RESIS-




WHEN
SUBSTRATE SUR- 

(Ti + 
(Ti + 
(Ti + 
(Ti + 
Al/


TANCE




CRYSTAL
FACE WHEN 
SPUTTER-
Cr +
Cr +
Cr +
Cr +
(Ti + 
Cr/

VALUE 




PHASE IS
CRYSTAL PHASE 
ING GAS
Al +
Al +
Al +
Al +
Cr +
Ti + 
B CON-
AT



CRYSTAL
WURTZITE
IS WURTZITE TYPE
PRESSURE
N)
N)
N)
N)
Al)
Cr)
STANT
25° C.



SYSTEM
TYPE
(a-AXIS OR c-AXIS)
(Pa)
(%)
(%)
(%)
(%)
(%)
(%)
(K)
(Ω cm)






















EXAMPLE 5 
WURTZITE 
0.19
c-AXIS
<0.67
10
2
39
49
76
17
2532
9.E+04



TYPE













EXAMPLE 6 
WURTZITE 
0.31
c-AXIS
<0.67
11
2
38
49
75
17
2701
6.E+04



TYPE













EXAMPLE 7
WURTZITE 
0.58
c-AXIS
<0.67
12
2
37
49
73
15
2452
4.E+03



TYPE













EXAMPLE 8
WURTZITE 
4.04
a-AXIS
≧0.67
13
3
42
42
72
19
1773
7.E+02



TYPE













EXAMPLE 9
WURTZITE 
2.96
a-AXIS
≧0.67
10
2
39
49
76
16
2132
2.E+05



TYPE













EXAMPLE 
WURTZITE 
1.75
a-AXIS
≧0.67
2
12
38
48
73
86
2377
9.E+04


10
TYPE









<Crystalline Form Evaluation>

Next, as an exemplary crystal form in the cross-section of the thin film thermistor portion 3, a cross-sectional SEM photograph of the thin film thermistor portion 3 according to the Example (where Al/(Ti+Cr+Al)=0.76, wurtzite-type, hexagonal crystal, and strong c-axis orientation), in which the thin film thermistor portion 3 having a thickness of about 250 nm was deposited on the Si substrate (S) with a thermal oxidation film, is shown in FIG. 13. In addition, a cross-sectional SEM photograph of the thin film thermistor portion 3 according to another Example (where Al/(Ti+Cr+Al)=0.76, wurtzite-type, hexagonal crystal, and strong a-axis orientation) is shown in FIG. 14.
The samples in these Examples were obtained by breaking the Si substrates (S) by cleavage. The photographs were taken by tilt observation at an angle of 45 degrees.
As can be seen from these photographs, the samples were formed of a high-density columnar crystal in both Examples. Specifically, the growth of columnar crystals in a vertical direction with respect to the surface of the substrate was observed both in the Example revealing a strong c-axis orientation and in the Example revealing a strong a-axis orientation. Note that the break of the columnar crystal was generated upon breaking the Si substrate (S) by cleavage. It was also confirmed that when each film having a thickness of 200 nm, 500 nm, or 1000 nm was deposited on a Si substrate (S) with a thermal oxidation film, a high-density columnar crystal was similarly formed as described above.
<Heat Resistance Test Evaluation>
For the thin film thermistor portion 3 according to the Examples and the Comparative Example shown in Table 3, a resistance value and a B constant before and after the heat resistance test at a temperature of 125° C. for 1000 hours in air were evaluated. The results are shown in Table 3. The thin film thermistor portion 3 according to the Comparative Example made of a conventional Ta—Al—N-based material was also evaluated in the same manner for comparison.
As can be seen from these results, although the Al concentration and the nitrogen concentration vary, the heat resistance of the Ti—Cr—Al—N-based material based on the electric characteristic change before and after the heat resistance test is more excellent than that of the Ta—Al—N-based material according to the Comparative Example when the comparison is made by using the same B constant. Note that the materials according to Examples 4 and 5 have a strong c-axis orientation, and the materials according to Examples 9 and 10 have a strong a-axis orientation. When both groups are compared to each other, the heat resistance of the materials according to the Examples exhibiting a strong c-axis orientation is slightly improved as compared with that of the materials according to the Examples exhibiting a strong a-axis orientation.
Note that, in the Ta—Al—N-based material, the ionic radius of Ta is very large compared to that of Ti, Cr and Al, and thus, a wurtzite-type phase cannot be produced in the high-concentration Al region. It is contemplated that the Ti—Cr—Al—N-based material having a wurtzite-type phase has a better heat resistance than the Ta—Al—N-based material because the Ta—Al—N-based material is not a wurtzite-type phase.
















TABLE 3













RISING RATE 











OF SPECIFIC
RISING RATE 









SPECIFIC
RESISTANCE
OF B CONSTANT









RESIS-
AT 25° C. AFTER
AFTER HEAT







Al/

TANCE
HEAT RESISTANCE
RESISTANCE



M



(M +
B25-
VALUE AT
TEST AT 125° C.
TEST AT 125° C.



ELE-
M
Al
N
Al)
50
25° C.
FOR 1,000 HOURS
FOR 1,000 HOURS



MENT
(%)
(%)
(%)
(%)
(K)
(Ω cm)
(%)
(%)
























COMPAR-
Ta
60
1
39
2
2671
5.E+02
25
16


ATIVE EX-











AMPLE











EXAMPLE 4
(Ti, Cr)
8
46
46
85
3057
1.E+05
<4
<1


EXAMPLE 5
(Ti, Cr)
12
39
49
76
2532
9.E+04
<4
<1


EXAMPLE 9
(Ti, Cr)
12
9
49
76
2132
2.E+05
<5
<1


EXAMPLE 
(Ti, Cr)
14
38
48
73
2377
9.E+04
<5
<1


10









<Heat Resistance Evaluation by Irradiation of Nitrogen Plasma>

After the thin film thermistor portion 3 according to Example 5 shown in Table 1 was deposited on the insulating film 2, the resulting film was irradiated with nitrogen plasma under the degree of vacuum of 6.7 Pa, the output of 200 W, and the N2 gas atmosphere. The results obtained by the heat resistance tests of the film evaluation elements 121 with and without nitrogen plasma irradiation are shown in Table 4. As can be seen from the result, in the Example with nitrogen plasma irradiation, change in the specific resistance and the B constant is small, resulting in an improvement in heat resistance of the film. This is because the crystallinity is improved by reduction in nitrogen defects in the film by nitrogen plasma. It is more preferable that nitrogen plasma is radical nitrogen.









TABLE 4






RISING RATE OF




SPECIFIC




RESISTANCE AT




25° C. AFTER
RISING RATE OF



HEAT RESISTANCE
B CONSTANT AFTER


NITROGEN
TEST AT 125° C.
HEAT RESISTANCE


PLASMA
FOR 1,000 HOURS
TEST AT 125° C. FOR


IRRADIATION
(%)
1,000 HOURS (%)







YES
<2
<1


NO (EXAMPLE 5)
<4
<1









It can be seen from the above evaluation that a metal nitride material may exhibit an excellent thermistor characteristic when it is produced within a N/(Ti+Cr+Al+N) ratio of from 0.4 to 0.5. However, the stoichiometric ratio of N/(Ti+Cr+Al+N) is 0.5. Hence, it is found that nitrogen defects are present in the material because the amount of nitrogen is less than 0.5 in this test. Therefore, it is preferable to add a process for compensating the nitrogen defects, and one preferred example thereof is the nitrogen plasma irradiation described above.
The technical scope of the present invention is not limited to the aforementioned embodiments and Examples, but the present invention may be modified in various ways without departing from the scope or teaching of the present invention.
REFERENCE NUMERALS
1: film type thermistor sensor, 2: insulating film, 3: thin film thermistor portion, 4 and 124: pattern electrode

## Claims

1. A thermistor made of a metal nitride material, the metal nitride material consisting of a metal nitride represented by the general formula: (Ti1-wCrw)xAlyNz (where 0.0<w<1.0, 0.70≦y/(x+y)≦0.95, 0.4≦z≦0.5, and x+y+z=1), wherein the crystal structure thereof is a hexagonal wurtzite-type single phase.

2. The thermistor according to claim 1, wherein the metal nitride material is deposited as a film and is a columnar crystal extending in a vertical direction with respect to the surface of the film.

3. The thermistor according to claim 1, wherein the metal nitride material is deposited as a film and is more strongly oriented along a c-axis than an a-axis in a vertical direction with respect to the surface of the film.

4. A film type thermistor sensor comprising:
an insulating film;
a thin film thermistor portion made of the thermistor according to claim 1 on the insulating film; and
a pair of pattern electrodes formed at least on the top or the bottom of the thin film thermistor portion.

5. The film type thermistor sensor according to claim 4, wherein at least a portion of a pair of the pattern electrodes that is bonded to the thin film thermistor portion is made of Cr.

6. A method for producing the thermistor according to claim 1, the method comprising a deposition step of performing film deposition by reactive sputtering in a nitrogen-containing atmosphere using a Ti—Cr—Al composite sputtering target.

7. The method for producing the thermistor according to claim 6, wherein the sputtering gas pressure during the reactive sputtering is set to less than 0.67 Pa.

8. The method for producing the thermistor according to claim 6, the method comprising a step of irradiating the formed film with nitrogen plasma after the deposition step.