Abstract:
The present invention provides a PTC thermistor element low in electric resistance at room temperature and suitable for monolithic incorporation with an integrated circuit. According to the present invention, the PTC thermistor film is subjected to rapid heating by heat irradiation in the annealing step. An n-type semiconductor is interposed between the electrodes and the PTC thermistor film, and a PTC thermistor film is also interposed between the n-type semiconductor and the electrode. Further, a plurality of such thermistor elements are parallel-connected to each other, and at least one of them is connected opposite to the other elements.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to PTC thermistor elements used in switching elements for example. and methods for producing the same. 
     Barium tltanate-type ceramics have been widely used as a material for ceramic filters and capacitors because of the high resistivity and dielectric constant. Also, barium titanate-type semiconductors, which contains a small amount of rare earth elements or the like as a dopant, have been used in temperature sensor elements and switching elements as a positive temperature coefficient (PTC) thermistor due to their PTC characteristics or the characteristic that the electric resistance thereof undergoes a sudden change around the Curie point where a phase transition occurs between the ferroelectric phase and the paraelectric phase. 
     In recent years, there has a proposal of an application of semiconductor film in the PTC thermistor element, in place of a bulk or sintered block of the semiconductor in a conventional PTC thermistor element. Such application of the thin PTC thermistor film enables the element a decrease in heat capacity thereof and Improvement in response characteristic. Further, such application of film could improve the quality and reliability of small-sized PTC elements since control in thickness of the film in producing process can be better exercised than in fine working of the condenser bulk. Another advantage is that the film is excellent in mechanical strength. 
     The film PTC thermistor is opening up various new possibilities including monolithic incorporation of thermistor elements on an Integrated circuit. 
     A typical PTC thermistor element using the film is shown in FIG.  10 . 
     As shown in FIG. 10, the film PTC thermistor element has an insulating substrate  53 , a PTC thermistor film  52  of barium titanate-type semiconductor formed thereon, and a pair of comb-shaped electrodes  51   a  and  51   b  made of aluminum films formed on the PTC thermistor film  52 . 
     The barium titanate-type semiconductor film is generally formed by the sputtering method. The sputtering technique can control a quantity of rare earth elements such as Sr, Mn and Y to be added to impart PTC characteristlcs to the film better than the vacuum deposition method or the chemical vapor deposition (CVD) method. 
     An addition of Sr, Sn, Zr or Pb changes the temperature at which the film sharply changes in resistance An addition of Y, Nb, Ta, Si, Sb, W. La, Ce, Pr, Nd, Sm, Gd or Ho makes the film semiconductive up to the inside of the crystal grain An addition of Mn, Fe, Cu or Cr Increases the change ratio in electric resistance with temperature rise. An addition of Si makes crystal grains in the film uniform in diameter 
     The PTC thermistor element is produced as the following process, for example. 
     First, the PTC thermistor film  52  is formed on the alumina substrate  53  by the sputtering technique, followed by heat treatment for three hours at 1.000° C. 
     Then, the comb-shaped electrodes  51   a  and  51   b  is formed on the surface of thus formed PTC thermistor film  52 . Here, single base metals such as Al, Ni, Cu, Au, In, Ag, Hg, Ga or Zn and their alloys, which can have an ohmic contact with the PTC thermistor film  52 , are employed as the materials of the comb-shaped electrodes  51   a  and  51   b . 
     Those electrodes  51   a  and  51   b  are formed by such techniques as vacuum deposition, printing, plating and sputtering. 
     If noble metals such Pt excellent in high-temperature durability are used in place of the aforesaid base metals, a shottky barrier which sends up the apparent electric resistance of the element will be inevitably formed and no ohmic contact will be produced between the PTC thermistor film and electrode. 
     There is a growing demand for film PTC thermistor elements. But such elements are too high in electric resistance to build in the integrated circuit. 
     The electric resistance of PTC thermistor element at room temperature is decreased when the thickness of the PTC thermistor film is increased. But, a PTC thermistor film as thick as tens of microns formed on a substrate would peel off from the substrate because of an internal stress thereof. That is, a firmly formed film would have a thickness of only several microns. 
     As a solution to that problem, a PTC thermistor element as shown in FIG. 1 has been proposed. This thermistor element has an electrode  2  formed on the surface of an insulating substrate  1 , and a PTC thermistor film  3  and the other electrode  4  formed thereon. It is hoped that adopting such a lamination structure In which a PTC thermistor film is sandwiched between a pair of electrodes could lower the electric resistance of the element. 
     However, the PTC thermistor element of that lamination structure falls to exhibit fully desired characteristics on the following ground. 
     That is, to form a PTC thermistor film by the sputtering technique, it is necessary to raise the substrate temperature in forming a film and the subsequent heat treatment temperature up to 800° C. or higher as described in “Electronic Ceramics” (Sep. 1987, p. 28 to 33) and Japanese Laid-Open Patent Publication No. Hei 2-77102 . The electrode  2  to be formed before the PTC thermistor film  3  is exposed to that high temperature for one hour or more. Base metals used as the electrode material are oxidized at that high temperature, losing the dielectric constant or diffusing into the PTC thermistor film and deteriorating the PTC characteristics of the formed film. 
     An object of the present invention is to solve the above-mentioned problems thereby to provide a PTC thermistor element with excellent PTC thermistor characteristics. 
     BRIEF SUMMARY OF THE INVENTION 
     The method for making a PTC thermistor element of the present invention comprises the steps of: forming a first electrode on an insulating substrate; forming a PTC thermistor film containing barium titanate as a main component on the first electrode; heat-treating the substrate; and forming a second electrode on the film, wherein the heat treatment of the substrate is rapid heating using a heat irradiation. 
     In making a PTC thermistor element, a first electrode, PTC thermistor film and second electrode are formed on the substrate in this order. Therefore, it is observed that the material contained in the first electrode diffuses into the PTC thermistor film at forming the PTC thermistor film on the first electrode, or at annealing the formed PTC thermistor film. As a result, the PTC thermistor film fails to develop a perovskite crystal structure and to exhibit the desired characteristics. 
     The diffusion of the electrode material depends on the temperature and time of the heat treatment of the PTC thermistor film. Therefore, the diffusion into the PTC thermistor of the electrode material can be kept down when the PTC thermistor is heated rapidly and then cooled in a short time thereby to shorten the time in which the electrode is exposed to a high temperature. 
     Preferably, the maximum temperature of the substrate in the heat treatment is set in a range of 900 to 1,500° C., and the time of treatment including the heating and cooling is set at 0.5 to 5 minutes. The preferable treatment time depends on the diffusion rate of the electrode material at the maximum temperature. If the heating In one session of the above treatment is not enough for developing the desired PTC thermistor characteristics, the heating under the same conditions may be repeated a number of times For example, in a case of Ni as the electrode material, one heat treatment for 300 seconds is enough at 900° C., but a repetition of five times of the heat treatment for 30 seconds is desired at 1,500° C. With Al, it is desirable to conduct the heat treatment for 30 seconds at 900° C. for 20 times. 
     It is desirable to use a lamp heater for the rapid heating of the substrate. 
     A PTC thermistor element of the present invention includes: a barium titanate-containing PTC thermistor film; an n-type semiconductor connected to the PTC thermistor film; a first electrode connected to the PTC thermistor film; and a second electrode connected to the n-type semiconductor. 
     In an example of making the PTC thermistor element, the PTC thermistor film is formed on a surface of a substrate made of n-type semiconductor, and a pair of the electrodes are then formed on each side of the substrate so as to sandwich the substrate and the PTC thermistor film formed thereon therebetween. The pair of electrodes may also be formed on the same side. In this case, an insulating substrate provided with an n-type semiconductor film on the surface may be used in place of the substrate of n-type semiconductor. 
     An n-type semiconductor placed between the PTC thermistor film and the electrodes would work as a part of the electrode. Even if, therefore, valiances are observed in workmanship and electric characteristics, the PTC thermistor characteristics of the element are hardly influenced as a whole. Further, no shottky barrier will be formed between the PTC thermistor film and the n-type semiconductor. 
     The n-type semiconductor may be a semiconductor formed with the barium titanate of the perovskite-type crystal structure as a main component. Alternatives may be semiconductors with the main component being Si, zinc oxide, titanium oxide, iron oxide, tin oxide, etc. 
     Under the above-mentioned arrangement, electrodes can be formed after a PTC thermistor film being formed. Therefore, the heat treatment of the PTC thermistor film can be performed at a high temperature and a film of a higher quality can be obtained. Besides, it is possible to prevent the electrode material from diffusing into the PTC thermistor film. Thus, the present invention can provide a PTC thermistor element which is low in electric resistance at room temperature. 
     The suitable electrode materials include aluminum, nickel, zinc, copper, silver, In—Ga and In—Hg. Another PTC thermistor element of the present invention has a plurality of PTC thermistor units, each provided with a PTC thermistor film containing barium titanate as well as a pair of electrodes, all the PTC thermistor units parallel-connected to each other, with at least one of them connected In reverse manner. 
     If a noble metal is used for the electrode, a shottky barrier will be formed between the PTC thermistor film and the electrode, exhibiting diode-like rectification characteristics. Therefore, when a plurality of PTC thermistor units are parallel-connected to each other and at least one of them is connected to the others in reverse manner, the rectification characteristics can be offset, and thus a PTC thermistor element with a low electric resistance at room temperature can be obtained. Therefore, it becomes possible to use noble metals as electrode materials—such noble metals as Pt, Ru, Rh, Pd, W, Re, Os, Ir and Au. 
     The above-mentioned PTC thermistor unit is of such a lamination structure that the PTC thermistor film is sandwiched between a pair of electrodes. It may also be so arranged that one of the electrodes and the PTC thermistor film are placed on the p-type semiconductor layer formed on the same side of the substrate while the other electrode is put on the PTC thermistor film. Another arrangement is that one electrode is provided on a side of a substrate made of a p-type semiconductor and a PTC thermistor film with another electrode provided thereon is provided on an opposite side of the substrate. That is, the both electrodes are arranged so as to face each other via the substrate. 
     Of the electrodes, the one that  1 s formed after the formation of the PTC thermistor film may be made of aluminum, nickel, In—Hg alloys and In—Ga alloys. 
     The aforesaid p-type semiconductor may be semiconductors formed with the main component being silicon, nickel oxide, cobalt oxide, iron oxide, manganese oxide, bismuth oxide or chromium oxide. 
     While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a perspective view of a PTC thermistor element of the lamination structure. 
     FIG  2  is a characteristic diagram showing PTC characteristics of a PTC thermistor element in an embodiment of the present invention. 
     FIG. 3 is a perspective view of the main part of a PTC thermistor element in another embodiment. 
     FIG. 4 is a perspective view of the main part of a PTC thermistor element in still another embodiment. 
     FIG. 5 is a perspective view of the main part of a PTC thermistor element in a further embodiment. 
     FIG. 6 is a characteristic diagram showing PTC characteristics of the PTC thermistor elements in the embodiments of the present invention. 
     FIG. 7A, FIG.  7 B and FIG. 7C are explanatory diagrams at different stages in production of a PTC thermistor element in a still further embodiment. FIG  7 A shows a state when one electrode is formed on a substrate. FIG. 7B shows a state when a PTC thermistor film being formed on the electrode FIG. 7C shows a state when the other electrode being formed thereon. 
     FIG. 8 is a block diagram showing an equivalent circuit of the same PTC thermistor element. 
     FIG. 9 is a characteristic diagram showing PTC characteristics of the same PTC thermistor element. 
     FIG. 10 is a perspective view of the main part of a prior art PTC thermistor element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following paragraphs, preferred examples of the present invention will be described with reference to the attached drawings. 
     EXAMPLE 1 
     In the present embodiment, an explanation will be made on an improved method for making the PTC thermistor of the lamination structure shown in FIG.  1 . 
     The PTC thermistor element shown in FIG. 1 was produced in the following way. 
     A thin nickel film of 0.2 μm thick in a specific configuration was formed on a substrate  1  of alumina of 5 mm width and 10 mm length by the sputtering technique, thus obtaining an electrode  2 . The temperature of the substrate  1  was set at 150° C. during the formation. Then a PTC thermistor film  3  in a specific configuration was formed on the electrode by the rf magnetron sputtering method using a ceramic target of Ba 0.77 Sr 0.23 Y 0.002 Mn 0.007 O 3 . 
     In this process, the temperature of the substrate  1  was set at 350° C., and a mixed gas of Ar and O 2  (Ar/O 2 )=9.5/0.5) was introduced into the system. The degree of vacuum was set at 1 Pa; the rf power at 150 W; the film forming time at one hour. A thickness of the PTC thermistor film  3  obtained was 2 μm. 
     The substrate  1  thus obtained was heated by using a lamp heater under an oxygen atmosphere. Six seconds after turning the lamp heater on, the substrate  1  was heated up to 1,200° C. The substrate  1  was held for 30 seconds at 1,200° C. and then the lamp heater turned off and the substrate was allowed to be cooled. Six seconds after turning the lamp heater off, the substrate  1  was cooled down to room temperature. This cycle of heat treatment was repeated 10 times. 
     Then. an electrode  4  of aluminum of 0.2 μm thick was formed in a specific configuration on the PTC thermistor film  3  to produce a PTC thermistor element. It is noted that a metal mask was used in forming each film. The area in which the two electrodes were faced with each other was set to 25 mm 2 . 
     As a comparative example, a PTC thermistor film formed in the same way was heated under an oxygen atmosphere In an electric furnace for 9 hours to produce a similar PTC thermistor element. In the electric furnace, the substrate was heated up to 800° C. at the rate of 200° C./hour and maintained at that temperature for one hour. Then, the substrate was allowed to cool down to room temperature in the electric furnace. This shall be called “Comparative Example 1A”. 
     Likewise, a PTC thermistor element as another comparative example was constructed using comb-shaped electrodes as shown in FIG. 10 in the following way. 
     A PTC thermistor film  3  was formed on an alumina substrate  1  of 5 mm width and of 10 mm length by the rf magnetron sputtering technique in the same way as in Example 1 to form a PTC thermistor film  3 . The PTC thermistor film thus formed was heated for 9 hours under an oxygen atmosphere in the electric furnace in the same way as in Comparative Example 1. Then an aluminum film of 0.2 μm thick was formed on the PTC thermistor film by the vacuum deposition method using a metal mask. The aluminum film thus formed was processed by photoLithography into comb-shaped electrodes  51   a  and  51   b  to obtain a PTC thermistor element. The comb-shaped electrodes  51   a  and  51   b  both had 10 teeth, each 5 min long and 0.35 mm wide, and the two combs separated by 8.0 μm from each other. This shall be called “Comparative Example 1B”. 
     The PTC thermistor thus obtained were put to tests to study their PTC characteristics. Here, it was impossible to measure the resistance value of Comparative Example 1A, because nickel in the electrode  2  diffused into the PTC thermistor film  3 . The measurements of the element of Example 1 and that of Comparative Example 1B are shown in FIG.  2 . 
     As shown in FIG. 2, the PTC thermistor element of the present embodiment exhibits good PTC characteristics and low electric resistance at room temperature as compared with the prior art PTC thermistor element (Comparative Example 1B) 
     EXAMPLE 2 
     In the present embodiment, an explanation will be made on an improvement in structure of a PTC thermistor element. 
     The outline of the structure of the PTC thermistor is shown in FIG.  3 . 
     In the present example, a 2.5 μm-thick PTC thermistor film  13  containing barium titanate as a main component is formed on one side of an n-type semiconductor substrate  14  also containing barium titanate as a main component (yttrium-doped barium titanate sintered block of 0.3 mm thick and 3 ohms cm in specific resistance). A 0.2 μm-thick electrode  11  made of an aluminum film is formed on the PTC thermistor film  13 . On the other side of the n-type semiconductor substrate  14 , also, there is formed an electrode  12  of a 0.2 μm-thick aluminum film. This PTC thermistor element was prepared in the following way. 
     First, the PTC thermistor  13  was formed on the n-type semiconductor substrate  14  by the rf magnetron sputtering method under the following conditions. 
     
       
         
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Target: sintered block of barium titanium 
               
               
                   
                 (Ba 0.77 Sr 0.23 Ti 1.02 Y 0.004 Mn 0.03 Si 0.007 O 3 ) 
               
               
                   
                 Substrate temperature: 300° C. 
               
               
                   
                 Gas fed: 
               
               
                   
                 argon (flow rate: 10 SCCM) and 
               
               
                   
                 Oxygen (flow rate: 1 SCCM) 
               
               
                   
                 Gas pressure in sputtering: 1 Pa 
               
               
                   
                 Rf power: 150 W 
               
               
                   
                 Rotation of substrate: 25 rpm 
               
               
                   
                 Sputtering duration: 120 minutes 
               
               
                   
                   
               
             
          
         
       
     
     The formed film was heat-treated under the atmosphere for three hours at 1,000° C. 
     Then, the electrodes  11  and  12  were each formed by the vacuum deposition method with the substrate temperature also raised to 150° C., to obtain a PTC thermistor element. An area of the PTC thermistor film  13 , which was sandwiched between the upper and lower electrodes  11  and  12 , was set at 10 mm 2 . 
     EXAMPLE 3 
     The PTC thermistor element of this example is shown in FIG  4 . 
     A PTC thermistor element of the present example has a 2.5 μm-thick n-type PTC thermistor film  23  containing barium titanate as a main component is formed on one side of a semiconductor substrate  24  also containing barium tltanate as a main component, as same as the PTC thermistor element of Example 2. On the PTC thermistor film  23  is formed an electrode  21  made of 0.2 μm-thick aluminum film On the same side of the n-type semiconductor substrate  24 , in addition, there is formed an electrode  22  made of a 0.2 μm-thick aluminum film. 
     This PTC thermistor element was prepared in the following manner. 
     The PTC thermistor film  23  was formed on a part (a rectangular area of 5 mm ×1 mm) of one side of the n-type semiconductor substrate  24  by the rf magnetron sputtering method. As in the element of Example 2, the target was a sintered block of the barium titanate with a composition of: Ba 0.77 Sr 0.23 Ti 1.02 Y 0.004 Mn 0.03 Si 0.007 O 3 . 
     The other sputtering conditions were also the same as those in Example 2. 
     Then, a 0.2 μm-thick aluminum film was formed on the PTC thermistor film  23  by the vacuum deposition method using a metal mask, to obtain the electrode  21  with a 5 mm 2  area (a rectangular area of 5 mm ×1 mm). In another area on the surface of the n-type semiconductor substrate  24 , a 0.2 μm-thick aluminum film was formed to produce the electrode  22  (a 5 mm 2  rectangular area of 5 mm ×1 mm). The substrate temperature in the film-forming step was all set at 150° C. 
     EXAMPLE 4 
     The PTC thermistor element of the present example is illustrated in FIG.  5 . 
     A 10 μm-thick n-type semiconductor film  34  containing barium titanate as a main component is formed on one side of an alumina substrate  35  as used in Example 1. On the n-type semiconductor film  34  is formed a 2.5 μm-thick PTC thermistor film  33  also containing barium titanate as a main component. On the other side of the PTC thermistor film  33  is formed an electrode  31  made of a 0.2 μm-thick aluminum film. In another area on the same side of the n-type semiconductor film  34 , there is formed an electrode  32  made of a 0.2 μm-thick aluminum film. 
     This PTC thermistor element was prepared in the following manner. 
     First, a 2.0 μm-thick n-type semiconductor film  34  was formed on the alumina substrate  35  by the rf magnetron sputtering method under the following conditions. 
     
       
         
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 Target: sintered block of barium titanium 
               
               
                   
                 (Ba 0.77 Sr 0.23 Ti 1.02 Y 0.004 Mn 0.03 Si 0.007 O 3 ) 
               
               
                   
                 Substrate temperature: 600° C. 
               
               
                   
                 Gas fed: 
               
               
                   
                 argon (flow rate: 10 SCCM) and 
               
               
                   
                 oxygen (flow rate: 0.55 SCCM) 
               
               
                   
                 Gas pressure in sputtering: 1 Pa 
               
               
                   
                 rf power: 150 W 
               
               
                   
                 Rotation of substrate: 25 rpm 
               
               
                   
                 Sputtering duration: 100 minutes 
               
               
                   
                   
               
             
          
         
       
     
     Then, the film thus formed was heat-treated for two hours at 1,300° C. 
     This sputtering and heat treatment were repeated further four times so as to make the thickness of the n-type semiconductor film  34  to 10 μm. The specific resistance of the n-type semiconductor film  34  thus obtained, measured by the four probe method, was 0.8 ohms-cm. 
     The 2.5 μm-thick PTC thermistor film  33  was formed on a part (on a rectangular area of 5 mm ×1 mm) of the surface of the n-type semiconductor film  34  by the rf magnetron sputtering method in the same way as in Example 2. 
     Then. a 02 μm-thick aluminum film as the electrode  31  was formed on the PTC thermistor film  33  by the vacuum deposition method using a metal mask. Also on the n-type semiconductor film  34 , a 0.2 μm-thick aluminum film having a rectangular area 5 mm ×1 mm was formed as the electrode  32  In the same way. The electrode  31  and electrode  32  were separated each other by 0.1 mm. The substrate temperature in the film-forming steps was all set at 150° C. 
     The PTC characteristics of the PTC thermistor elements obtained in the above Examples 2 to 4 are shown in FIG. 6 along with those of the element  1 B prepared as a comparative example in Example 1. 
     As illustrated in FIG. 6, the PTC thermistor elements of Examples 2 to 4 were all low in electric resistance at room temperature and the value is 10 −3  to 10 −2  times of the value of the element  1 B. Especially, the PTC thermistor elements of Examples 2 and 3 in which the PTC thermistor film Is sandwiched between a pair of electrodes are low in electric resistance. In other characteristics, the PTC thermistor elements of Examples 2 to 4 are about equal to the element  1 B in the rate of change in electric resistance before and after the Curie point. A probable explanation for the low electric resistance of the PTC thermistor elements of Examples 2 to 4 is that the electric current path runs in the direction of film thickness as against the direction parallel with the substrate in the prior art element. 
     EXAMPLE 5 
     In the present embodiment, there will be explained a PTC thermistor element in which heat-resistant noble metals can be used in the electrode. 
     As mentioned above, noble metals such as platinum are excellent in heat resistance, and electrodes of those materials cause no worry about such problems as oxidation and atom diffusion in the step of forming PTC thermistor film. But the trouble is that a shottky barrier will be formed between the electrode and the PTC thermistor film, and an ohmic connection is impossible. In a case, therefore, an electric current runs from the PTC thermistor film to the electrode through the shottky barrier, the electric resistance value will rise temporarily or when the electric current is low. That is, the same rectification characteristics as diode are exhibited due the formation of shottky barrier. 
     In the present embodiment, th(ere will be described a PTC thermistor element which exhibits excellent PTC characteristics even if noble metals are used as the electrode material. 
     First, electrodes  41   a  and  41   b  made of a noble metal are formed on an insulating substrate  40  as illustrated in FIG.  7 A. In the next step, PTC thermistor films  42   a  and  42   b  are formed on the noble metal electrodes  41   a  and  41   b . respectively, as shown in PIG.  7 E. On top of that, base metal electrodes  43   a  and  43   b  are so formed that the PTC thermistor films  42   a  and  42   b  are connected with the noble metal electrodes  41   b  and  41   a  which have been connected to the other PTC thermistor films, respectively, as shown in FIG.  7 C. 
     A 20 mm-square surface of an alumina substrate  40  was divided into  16  equal sections. On each section, a pair of 0.2 μm-thick platinum films were formed as electrodes  41   a  and  41   b  in the pattern shown in FIG. 7A by the rf magnetron sputtering method using a metal mask under the following condition. Here, the electrodes  41   a  and  41   b  were separated by 1 mm. 
     
       
         
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 Target: platinum (5 inches in diameter) 
               
               
                   
                 Substrate temperature: 200° C. 
               
               
                   
                 Gas fed: argon (flow rate: 10 SCCM) 
               
               
                   
                 Gas pressure in sputtering: 1 Pa 
               
               
                   
                 Rf power: 100 W 
               
               
                   
                 Rotation of substrate: 25 rpm 
               
               
                   
                 Sputtering duration: 10 minutes 
               
               
                   
                   
               
             
          
         
       
     
     Those platinum electrodes  41   a  and  41   b  were heat-treated in the atmosphere for three hours at 1.000° C. 
     On the upper side of the platinum electrodes  41   a  and  41   b  thus obtained, the 2.5 μm-thick PTC thermistor films  42   a  and  42   b  were formed by the rf magnetron sputtering method under the following conditions. 
     
       
         
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 Target: sintered block of barium titanium 
               
               
                   
                 (Ba 0.77 Sr 0.23 Ti 1.02 Y 0.004 Mn 0.03 Si 0.007 O 3 ) 
               
               
                   
                 Substrate temperature: 400° C. 
               
               
                   
                 Gas fed: 
               
               
                   
                 argon (flow rate: 10 SCCM) and 
               
               
                   
                 oxygen (flow rate: 1 SCCM) 
               
               
                   
                 Gas pressure in sputtering: 1 Pa 
               
               
                   
                 Rf power: 150 W 
               
               
                   
                 Rotation of substrate: 25 rpm 
               
               
                   
                 Sputtering duration: 120 minutes 
               
               
                   
                   
               
             
          
         
       
     
     The PTC thermistor films  42   a  lad  42   b  thus obtained were heat-treated in the atmosphere for two hours at 1,000° C. On top of that, a 0.2 μm-thick aluminum film as the electrode  43   a  was so formed that the PTC thermistor film  42   a  might be connected with the noble metal electrode  41   b  which had been connected with the PTC thermistor film  42   b . Likewise, a 0.2 μm-thick aluminum film as the electrode  43   b  was so formed that the PTC thermistor film  42   b  might be connected with the noble metal electrode  41   a  which had been connected with the PTC thermistor film  42   a . The parts of the PTC thermistor films  42   a  and  42   b  being sandwiched between the upper and lower electrodes were each 5 mm The substrate temperature was set at 150° C. at that time The PTC thermistor element thus obtained is provided with a number of PTC thermistor element units of the same construction, all parallel-connected to each other, with at least one of them placed in reverse manner. An equivalent circuit of that element unit is shown in FIG.  8 . 
     As a comparison. a PTC thermistor of the lamination structure shown in FIG. 1 was produced using platinum used as the material for the lower electrode  4 . The thickness of the PTC thermistor film  3  was set at 2.5 μm— the same as with the PTC thermistor element of Example 5—, and the PTC thermistor film  3  sandwiched between the electrodes  2  and  4  was set to 5 mm 2 . This shall be called element  5 A. 
     The PTC characteristics of those PTC thermistor elements are shown in FIG.  9 . For further comparison, the PTC characteristics of the element  1 B used in Example 1 are also shown in the figures. 
     As is evident from FIG. 9, the PTC thermistor element of the present embodiment is superior to the comparative elements  5 A and  1 B in PTC characteristics. 
     The comparative element  5 A exhibits rectification characteristics because of the formation of a shottky barrier, raising the electric resistance. The PTC thermistor element of the present embodiment in which at least one of a number of parallel-connected element units of the same construction is placed in reverse manner can keep down rectification characteristics. Needless to say, the PTC thermistor element of the present embodiment can lower the resistance far more than the element  18  provided with comb-shaped electrodes. 
     It is understood that various other modifications will be apparent to, and can be readily made by, those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.