Resistance paste and resistor comprising the material

Abstract: An organic vehicle is added to and kneaded with a solid component comprising from 5 to 65 % by weight of a resistance material having a composition of Ca x Sr 1-x RuO 3 (x is from 0.25 to 0.75 moles) and from 35 to 95 % by weight of a non-reducible glass frit to obtain a resistance paste. A substrate is coated with the resistance paste and baked to produce a resistor. The resistance paste can be baked in a neutral or reducing atmosphere. The resistor has any desired resistance value within a broad range including even high resistance values of higher than 10 KΩ, and the reproducibility of the resistor with a desired resistance value is good.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to a resistance paste which can be baked in a neutral or reducing atmosphere, and a resistor to be formed by the use of the resistance paste. Description of the Related Art: In general, a ceramic substrate comprising alumina, zirconia or the like has circuit patterns for electrodes, resistors, etc., in order that various electronic parts can be mounted thereon. Electrodes (electrode patterns) are generally formed on the substrate by screen-printing a noble metal paste comprising silver, a silver-palladium alloy or the like followed by baking the thus-printed paste in air. However, since a noble metal paste such as that mentioned above is not only expensive but also problematic in its migration resistance, the tendency for such an expensive noble metal paste to be replaced by a base metal paste comprising, as the conductive component, copper, nickel, aluminum or the like has become accepted in this technical field. Such a base metal paste can be screen-printed on a substrate and then baked in a neutral or reducing atmosphere to give an inexpensive and good electrode pattern. In this case, it is desirable that the resistance paste which is to form resistors (resistor patterns) on the substrate, by which the plural base electrodes as formed by baking the printed base metal paste are connected with each other, can also be baked in a neutral or reducing atmosphere. Therefore, various resistance pastes that can be baked in a neutral or reducing atmosphere to form resistors (resistor patterns) have heretofore been proposed. Such resistance pastes includes, for example, resistance pastes comprising LaB 6 such as those described in Japanese Patent Publication Nos. 59-6481 and 58-21402, resistance pastes comprising NbB 2 such as those described in Japanese Patent Laid-Open No. 63-224301, resistance pastes comprising solid solutions of Nb x La 1-x B 6-4x such as those described in Japanese Patent Laid-Open No. 2-249203, etc. However, conventional resistance pastes such as those mentioned above are problematic in that, when they are desired to have a resistance value that is variable within a broad range by varying the mixing ratio of the resistance material consisting the paste to glass frit to be added thereto, even a slight variation in the amount of the glass frit in the paste often results in a rapid variation in the resistance value of the paste (that is, the resistance value of the paste greatly depends on the composition of the paste) and therefore the reproducibility of the desired resistance value of the paste is poor. Therefore, such conventional resistance pastes are still problematic in that they can be formed into practicable resistors having a resistance value only within a narrow range between 10 Ω/square and 10 KΩ/square. Apart from the above, other resistance pastes comprising a resistance material of RuO 2 , SrRuO 3 , CaRuO 3 or the like have also been proposed. However, resistance pastes comprising RuO 2 are problematic in that, when they are baked in a neutral or reducing atmosphere, RuO 2 is reduced to Ru metal with the result that they cannot be formed into resistors. Resistance pastes comprising SrRuO 3 or CaRuO 3 are also problematic in that, if the proportion of glass frit to be therein is increased more than a certain degree, the resistance value of the pastes suddenly becomes too great and therefore the reproducibility of the desired resistance value of the pastes is extremely poor. The publication: "MATERIALS RESEARCH SOCIETY SYMPOSIUM PROCEEDINGS; SAN FRANCISCO, CA, USA; vol. 341, 5-7 April 1994, pages 229-240, XP000608066; EOM C.B. et al: "Epitaxial thin films and heterostructures of various isotropic metallic oxides for device application" discloses a resistance material having a composition of the formula: Ca 0.5 Sr 0.5 RuO 3 . SUMMARY OF THE INVENTION The present invention seeks to solve the above-mentioned problems in the prior art and to provide a resistance paste which can be baked in a neutral or reducing atmosphere to surely give resistors having any desired resistance values within a broad range including values of even greater than 10 KΩ, a resistance material which constitutes the resistance paste, and a resistor which can be formed by the use of the resistance paste and which can realize resistance values within a broad range while the reproducibility of the realizable resistance values is good. The resistance material which the present invention uses so as to attain the above-mentioned object is characterized in that it has a composition of a general formula: Ca x Sr 1-x RuO 3 wherein x is from 0.25 to 0.75 moles. The resistance paste which the present invention provides so as to attain the above-mentioned object is characterized in that it comprises a solid component consisting of from 5 to 65 % by weight of the resistance material and from 35 to 95 % by weight of a non-reducible glass frit and an organic vehicle. The resistor which the present invention also provides so as to attain the above-mentioned object is characterized in that it is formed by coating the resistance paste on a substrate and then baking it thereon. The invention further provides a method defined by the features of claims 5 and 6, respectively. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a graph showing the relationship between the sheet resistance value of the resistors as produced in the examples and the comparative examples mentioned hereinunder and the amount of the glass frit added to the resistance pastes from which the resistors were produced. DETAILED DESCRIPTION OF THE INVENTION The resistance material used in the paste of the present invention has a composition corresponding to the general formula: Ca x Sr 1-x RuO 3 wherein x is from 0.25 to 0.75 moles. The resistance paste of the present invention comprises a solid component consisting of from about 5 to 65 % by weight of the resistance material and from about 35 to 95 % by weight of a non-reducible glass frit and an organic vehicle. One embodiment of the resistance paste is such that the non-reducible glass frit is a B 2 O 3 -SiO 2 -BaO-CaO-Nb 2 O 5 glass frit. The resistor of the present invention is formed by coating the resistance paste on a substrate and then baking it thereon. In the resistance material of the present invention, x falls between 0.25 moles and 0.75 moles. This is because if x is less than 0.25 moles, it is impossible to prevent the resistance material from having a too much increased resistance value when the glass frit content of the material is increased. On the other hand, if it is more than 0.75 moles, the increase in the resistance value of the material is also large. If so, therefore, the reproducibility of the desired resistance value of resistors comprising the resistance material is bad. In the resistance paste of the present invention, the content of the resistance material in the solid component falls between 5 % by weight and 65 % by weight and that of the non-reducible glass frit in the same falls between 35 % by weight and 95 % by weight. This is because if the content of the non-reducible glass frit in the solid component is less than 35 % by weight, the adhesiveness between a fired resistor and the substrate is lowered, but if it is more than 95 % by weight, the glass component flows out of the paste to worsen the weldability of the fired resistor to electrodes. To prepare the resistance paste of the present invention, an organic vehicle is added to and kneaded with a mixture (solid component) comprising the resistance material and the glass frit, so that the resulting resistance paste has the necessary printability. For this, employable are various organic vehicles which are generally used in ordinary resistance pastes for forming thick film resistors and which are prepared, for example, by dissolving an ethyl cellulose resin or acrylic resin in a terpene solvent such as α-terpineol or in a high-boiling point solvent such as kerosene, butyl carbitol, carbitol acetate or the like. If desired, additives may be added to the paste so as to make it thixotropic. Next, the present invention is explained in more detail with reference to the following examples, which, however, are not intended to restrict the scope of the present invention. Examples: First, a copper paste was screen-printed on an insulating substrate of alumina and baked in a nitrogen atmosphere to form electrodes thereon. Next, as raw material substances for resistance materials, powdery RuO 2 , CaCO 3 and SrCO 3 were weighed at predetermined proportions to have a composition of Ca x Sr 1-x RuO 3 (where x is a predetermined molar ratio) and wet-mixed in a pot mill. Then the resulting mixture was dried and subjected to milling of the resulting particles to have a predetermined mean particle size, and thereafter the particles were put into an alumina crucible and heated therein in a nitrogen atmosphere (reducing atmosphere) at a temperature falling between 900°C and 1300°C for 2 hours to produce the Ca x Sr 1-x RuO 3 composition. Next, this was ground in a solvent of acetone, using a shaking mill, into particles having a mean particle size of about 1 µm, and then dried. Thus, various resistance material samples were produced. Other resistance material samples were also produced in the same manner as above, except that the nitrogen atmosphere was replaced by an air atmosphere. Table 1 shows the various resistance material samples of Ca x Sr 1-x RuO 3 where x has a value indicated. In Table 1, the samples with asterisk (*) are comparative samples which are not within the scope of the present invention. **1 B 2 O 3 , SiO 2 , BaO, CaO and Nb 2 O 5 were mixed at a molar ratio of 36.05:31,67:18.02:9.26:5.00 and melted at a temperature falling between 1200°C and 1359°C to obain a fused glass of B 2 O 3 -SiO 2 -BaO-CaO-Nb 2 O 5 . This fused glass was rapidly cooled by putting it into pure water and then ground, using a shaking mill, into particles having a mean particle size of 5 µm. Thus was obtained a non-reducible glass frit sample. The resistance material sample prepared above and the non-reducible glass frit sample were mixed at various ratios shown in Table 1, and an organic vehicle as obtained by diluting an acrylic resin with α-terpionel was added to and kneaded with the resulting mixture to obtain a resistance paste sample. The proportion of the solid component (mixture comprising the resistance material sample and the non-reducible glass frit sample) to the organic vehicle was 60:40 by weight. Next, the thus-obtained resistance paste was screen-printed between the electrodes that had been formed on the alumina substrate by baking a copper paste thereon. The resistance pattern thus printed was such that it partly covered the both terminal electrodes and had a length of 1.5 mm and a width of 1.5 mm. Next, the alumina substrate having the resistance pattern printed thereon was dried at 120°C for 10 minutes and then baked in a tunnel furnace having a nitrogen atmosphere at a peak temperature of 900°C for 10 minutes, whereby a resistor was formed on the substrate. Thus, resistor samples were prepared. The sheet resistivity of each of the resistor samples prepared as above (sample Nos. 1 to 20) was measured. Table 1 above shows the data thus measured. The sheet resistivity was measured at 25°C, using a digital volt meter. Fig. 1 shows the relationship between the sheet resistance value of the resistor samples as produced herein and the amount of the glass frit added to the resistance pastes from which the resistor samples were produced, in which the value of the molar ratio x was employed as the variable parameter. In Fig. 1, only the resistance materials for d (sample Nos. 19 to 22 with x = 0.50) were produced in an air atmosphere, while the others were produced in a nitrogen atmosphere. In Fig. 1, the samples a, (sample Nos. 6 to 8 with x = 0.75), b (sample Nos. 9 to 11 with x = 0.50), c (sample Nos. 12 to 14 with x = 0.25) and d (sample Nos. 19 to 22 with x = 0.50) are within the scope of the present invention, while the samples e (sample Nos. 1 to 3 with x = 1.00) and f (sample Nos. 17 and 18 with x = 0.00) are comparative samples which are outside the scope of the present invention. In this, the samples g (sample Nos. 4 and 5 with x = 0.80) and h (sample Nos. 15 and 16 with x = 0.20) are also comparative samples which are outside the scope of the present invention, since the molar ratio x in these samples is not within the scope of the present invention. From Table 1 and Fig. 1, it is known that the variation in the sheet resistivity of the resistor samples comprising any of the resistance materials having the composition that falls within the scope of the present invention (resistance materials of sample Nos. 6 to 14 produced in a nitrogen atmosphere and resistance materials of sample Nos. 19 to 22 produced in an air atmosphere), which depends on the variation in the composition of the resistance material, is smaller than that of the resistor samples not falling within the scope of the present invention (sample Nos. 1 to 5, and sample Nos. 15 to 18) or, that is, the composition dependence in the sheet resistivity of the former is lower than that of the latter. Specifically, as is obvious from Table 1 and Fig. 1, the increase in the resistance value of the resistor samples with x = 1.00 (CaRuO 3 of sample Nos. 1, 2, 3) is steep when the glass frit content of the resistance material therein is more than 80 % by weight or, that is, the resistance value of these resistor samples greatly varies even if the mixing ratio of the resistance material to the glass frit therein is varied only slightly. Therefore, it is extremely difficult to make these resistor samples have a large resistance value of, for example, not lower than 5 KΩ and, in addition, the reproducibility of desired resistance values for these resistor samples is poor. As is also known from Table 1 and Fig. 1, the resistor samples with x = 0.00 (SrRuO 3 of sample Nos. 17 and 18) have a much increased resistance value of not lower than 1 GΩ, when the glass frit content of the resistance material therein is 80 % by weight. Thus, the composition dependence in the resistance value of these samples with x = 0.00 is extremely large and the resistance value of these samples greatly varies even when the mixing ratio of the resistance material to the glass frit therein is varied only slightly. Therefore, not only it is extremely difficult to make these samples have a desired resistance value but also it is almost impossible to expect high reproducibility of desired resistance values for these samples. In addition, it is known that the samples with x = 0.80 (sample Nos. 4 and 5) and the samples with x = 0.2 (sample Nos. 15 and 16) also have extremely large composition dependence in the resistance value. As opposed to these comparative samples, the increase in the resistance value of the resistor samples comprising a fired resistor having a composition that falls within the scope of the present invention (sample Nos. 6 to 14, and sample Nos. 19 to 22) is gentle. According to the present invention, therefore, it is easy to realize resistors having a desired resistance value especially within a range of high resistance values, and it is possible to improve the reproducibility of the resistors having a desired resistance value. Accordingly, by suitably determining the value of x within the range as defined according to the present invention or, that is, by suitably determining the ratio of Sr to Ca in the resistance material of the present invention that has a composition of a general formula Ca x Sr 1-x RuO 3 while by suitably determining the mixing ratio of the resistance material to the glass frit in the resistance paste of the present invention, it is possible to reliably produce resistors having any desired resistance value within a broad range, including even resistance values of higher than 10 KΩ (for example, from 1 KΩ to several MΩ). As is known from Table 1 and Fig. 1, the resistor samples (sample Nos. 19 to 22 with x = 0.50 for d) each comprising the resistance material as produced in an air atmosphere are apt to have a sheet resistance value on a higher level than the resistor samples (sample Nos. 9 to 11 with x = 0.50 for b) having the same composition but having been produced in a nitrogen atmosphere. Therefore, according to the present invention, it is possible to desirably control the level of the sheet resistance value of the resistors to be produced, by suitably varying the atmosphere for the production of the resistance pastes to be in the resistors. In the present invention, it is meaningful to previously mix CaRuO 3 and SrRuO 3 in such a way that x in the resulting mixture, Ca x Sr 1-x RuO 3 may fall between 0.25 moles and 0.75 moles and thereafter to heat the mixture in a nitrogen or air atmosphere to thereby produce a resistance material comprising a solid solution of Ca x Sr 1-x RuO 3 . The resistance material thus produced exhibits the particular effects of the present invention mentioned hereinabove. In the above-mentioned examples, the glass frit used comprised B 2 O 3 , SiO 2 , BaO, CaO and Nb 2 O 5 at a molar ratio of 36.05:31.67:18.02:9.26:5.00, as the non-reducible glass frit. However, the components constituting the non-reducible glass frit to be employed in the present invention and the composition thereof are not limited to the above-mentioned ones. Needless-to-say, it is possible in the present invention to employ other non-reducible glass frits comprising other components and having other compositions than the illustrated ones. The above-mentioned examples have demonstrated the formation of the resistors on the alumina substrate. However, the substrate on which the resistors of the present invention are formed is not limited to only such alumina substrate but the present invention is applicable to the formation of the resistors on other various substrates or bases made of other various materials. The present invention is not limited to only the above-mentioned examples with respect to the other various aspects. For example, the proportion of the organic vehicle to the solid component comprising a resistance material and a non-reducible glass frit in the resistance paste of the present invention and the temperature conditions and the atmosphere conditions for baking the resistance paste can be variously changed or modified within the scope of the present invention as claimed. As has been described in detail hereinabove, the resistance paste of the present invention is formed by adding an organic vehicle to a solid component comprising from about 5 to 65 % by weight of the resistance material of the present invention which has a composition of a general formula Ca x Sr 1-x RuO 3 (where x is from 0.25 moles to 0.75 moles) and from 35 to 95 % by weight of a non-reducible glass frit, followed by kneading. By coating a substrate with the resistance paste of the present invention and baking it in a neutral or reducing atmosphere, it is possible to reliably produce a resistor whose increase in the resistance value is gentler than that of conventional resistors. In addition, the reproducibility of the resistor of the present invention with such gentle increase in the resistance value is good. Specifically, by using the resistance paste of the present invention which comprises a resistance material of Ca x Sr 1-x RuO 3 (where x is from 0.25 moles to 0.75 moles) with varying the value x in the material and by suitably determining the mixing ratio of the resistance material to the glass frit in the paste, it is possible to produce a resistor having any desired resistance value within a broad range including even high resistance values of higher than 10 KΩ (for example, from 1 KΩ to several MΩ). While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof as defined by the claims.

Claims: A resistance paste comprising an organic vehicle and a solid component comprising from about 5 to 65 % by weight of a resistance material having a composition of the formula: Ca x Sr 1-x RuO 3 wherein x is from 0.25 to 0.75 moles, and from about 35 to 95 % by weight of a non-reducible glas frit. The resistance paste as claimed in claim 1, wherein the non-reducible glass frit is a B 2 O 3 -SiO 2 -BaO-CaO-Nb 2 O 5 glass frit. A resistor comprising a substrate with the resistance paste of claim 1 baked thereon. A resistor comprising a substrate with the resistance paste of claim 2 baked thereon. A method of producing a resistor by applying a resistance paste to a substrate and baking, utilizing the resistance paste of claim 1 as said paste. A method of producing a resistor by applying a resistance paste to a substrate and baking, utilizing the resistance paste of claim 2 as said paste. The method of claim 5 in which the baking is in an oxidizing atmosphere. The method of claim 5 in which the baking is in a reducing atmosphere. The method of claim 5 in which the baking is in a neutral atmosphere.

Inventors: NAGATA KEISUKE, TANI HIROJI, NAGATA, KEISUKE, TANI, HIROJI, Nagata, Keisuke, c/o Murata Manufacturing Co.,Ltd., Tani, Hiroji, c/o Murata Manufacturing Co.,Ltd.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1995
Main classification: H01C17
IPC codes: H01C7, H01C17