Dielectric ceramic body, method of producing the same and circuit board using the same

Abstract: A dielectric ceramic body prepared by firing a mixture of a cordierite glass powder and a TiO₂ powder, or a mixture of these powders and a RE₂Ti₂O₇ powder (RE representing a rare earth element), wherein particles of the TiO₂ powder or particles of the TiO₂ powder and the RE₂Ti₂O₇ powder are dispersed in a matrix phase of crystalline cordierite produced from the fired cordierite glass powder. In the first case, the cordierite glass powder and the TiO₂ powder have a proportion which satisfies 70 ≦ x < 100, 0 < y ≦ 30, and x + y = 100, where x and y represent contents in weight % of the cordierite glass powder and said TiO₂ powder, respectively. In the second case, the cordierite glass powder, TiO₂ powder and the RE₂Ti₂O₇ powder have a proportion which satisfies 50 ≦ x < 100, 0 < y ≦ 30, 0 < z ≦ 40, and x + y = 100, where z represents a content in weight % of the RE₂Ti₂O₇ powder.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates in general to a dielectric ceramic body, a method of producing the dielectric ceramic body, and a circuit board which uses the dielectric ceramic body. More particularly, the present invention is concerned with a dielectric ceramic body suitable for high-frequency circuit boards such as microwave integrated circuit (MIC) boards, a method of producing such a dielectric ceramic body, and such a microwave circuit board using a dielectric ceramic body. Discussion of the Prior Art Keeping pace with recent developments of various microwave devices such as pocket or portable telephones, there has been a growing demand for small-sized high-performance MIC boards used in such devices. In particular, a dielectric ceramic used to form a ceramic substrate for such MIC boards is required to have the following physical and strutural features: (1) The dielectric ceramic should have a sufficiently high specific dielectric constant (εr), to reduce the size of a resonance circuit, an inductance element and a capacitor of a MIC board, when the board is operated at a given frequency. (2) The dielectric ceramic should have a sufficiently high Q value, to increase the Q value of the resonance circuit and inductance element of the MIC board. (3) The temperature coefficient of the dielectric constant of the dielectric ceramic, and the temperature coefficient of the resonance frequency of the resonance circuit should be sufficiently low. (4) The dielectric ceramic should permit conductor patterns to be formed in a multi-layered or multi-level structure. (5) Further, an electrically conductive composition used to form the conductor patterns should have a sufficiently low resistivity. In this respect, it is noted that an increase in the resistivity of the conductor patterns reduces the Q value of the resonance circuit and inductance element of the MIC board, thereby resulting in an increase in transmission loss of the conductors. There have been known various dielectric ceramic compositions for the ceramic substrates for such MIC boards, such as: a dielectric ceramic composition as disclosed in laid-open Publication No. 58-20905 of examined Japanese Patent Application, whose major components consist of BaO and TiO 2 , a portion of which may be substituted by another element or other elements; a dielectric ceramic composition as disclosed in laid-open Publication No. 59-23048 of examined Japanese Patent Application, which has a composite perovskite structure such as Ba(Mg 1/3 Ta 2/3 )O 3 ; a dielectric ceramic composition as disclosed in laid-open Publication No. 54-35678 of examined Japanese Patent Application, whose major components consist of TiO 2 , ZrO 2 and SnO 2 , a portion of which may be substituted by another element or other elements; and a dielectric ceramic composition as disclosed in laid-open Publication No. 56-26321, whose major components consist of BaO, TiO 2 and RE 2 O 3 (RE: rare earth element). While the known dielectric ceramic compositions as described above have a relatively high dielectric constant, circuit boards using ceramic substrates formed of these ceramic compositions must use Pt, Pd or other electrically conductive materials which have a relatively high resistivity, where the internal conductor patterns are formed in a multi-level structure within the substrates. Namely, the firing or sintering temperatures of those dielectric ceramic compositions are so high that Ag, Cu, Au or other conductive materials having a sufficiently low resistivity cannot be used for the internal multi-level onductor patterns. For the ceramic substrates, there are also known inexpensive ceramic compositions such as alumina, steatite and forsterite. However, these ceramic compositions have excessively high temperature coefficients, and undesirably limit the choice of the conductive material for the internal multi-level conductor patterns, to Pt, Pd, W, Mo or other conductors having high resistivity values. SUMMARY OF THE INVENTION The present invention was developed in the light of the prior art situations described above. It is accordingly a first object of the invention to provide a dielectric ceramic body which has a sufficiently high dielectric constant and a sufficiently low resistivity, and which permits the use of an electrically conductive material having a sufficiently low resistivity, for internal conductor patterns formed therein in a multi-level or multi-layered structure. A second object of the invention is to provide a method of producing such a dielectric ceramic body. A third object of the invention is to provide a circuit board which uses such a dielectric ceramic body. The present invention was completed based on findings obtained as a result of an extensive study on a variety of dielectric ceramic compositions. According to a first aspect of the present invention there is provided a dielectric ceramic body as set out in claim 1. According to a second aspect of the present invention, there is provided a method of producing a dielectric ceramic body, as set out in claim 3. The dieletric ceramic body according to the present invention may further comprise up to 30 parts by weight in total of at least one filler powder having a dielectric constant not higher than 30, per 100 parts by weight of the mixture of the cordierite glass powder, the TiO 2 powder and the RE 2 Ti 2 O 7 powder. BRIEF DESCRIPTION OF THE DRAWINGS The above object, features and advantages of the present invention will be better understood by reading the following detailed description of the invention, when considered in connection with the accompanying drawings, in which the single figure is an elevational view in cross section of one form of a circuit board embodying the present invention. DETAILED DESCRIPTION OF THE INVENTION The major components of the dielectric ceramic powder mixture include a cordierite glass powder, a TiO 2 powder and a RE 2 Ti 2 O 7 powder, and the contents x, y and z of these three components should be determined so as to satisfy the formulas indicated. If the contents x, y and z do not fall within the respective specified ranges, the object of the invention cannot be fully achieved, either. For example, if the content x of the cordierite glass powder is smaller than 50% by weight, it becomes difficult to sinter the dielectric ceramic powder mixture at a temperature not higher 1100°C, and accordingly difficult to sufficiently densify the powder mixture. Further, the sufficient densification of the powder mixture and the crystallization of the cordierite glass powder are difficult, if the content y of the TiO 2 powder exceeds 30% by weight, or if the content z of the RE 2 Ti 2 O 7 powder exceeds 40% by weight. The cordierite glass powder is used to give the produced dielectric ceramic body excellent microwave dielectric properties, and to permit the sintering of the dielectric ceramic powder mixture at a sufficiently low temperature. For this cordierite glass powder, any known cordierite glass composition which gives a primary crystal in the form of cordierite can be used as a rule. A preferred example of such cordierite glass composition consists of 25-65% by weight of SiO 2 , 7-25% by weight of MgO, 10-45% by weight of Al 2 O 3 , and 0-20% by weight of B 2 O 3 . With the contents of SiO 2 , MgO and Al 2 O 3 held within the respective ranges specified above, the powder mixture can be effectively sintered and densified at 1100°C or lower. If the content of B 2 O 3 exceeds 20% by weight, the water-proofing property of the sintered dielectric ceramic body will be deteriorated. Optionally, the cordierite glass composition for the cordierite powder may further contain other components such as oxides of alkali metals, oxides of alkali earth metals, and metal oxides such as ZrO 2 , ZnO and PbO, as needed to improve the sinterability of the dielectric ceramic powder mixture. Further, the cordierite glass composition may optionally contain Mn, Cr, Fe, Co,Ni, Cu, or oxides of rare earth elements, to suitably color the dielectric ceramic body produced., and also contain TiO 2 as a nucleus formation agent for promoting crystallization of the cordierite glass. This component TiO 2 of the cordierite glass powder should be distinguished from the TiO 2 powder, which cooperates with the cordierite glass powder to provide the dielectric ceramic powder mixture that gives the relevant dielectric ceramic body. The cordierite glass composition (cordierite glass powder) inevitably includes some amounts of impurities, during preparation of the cordierite glass. The cordierite glass powder may be prepared from the cordierite glass composition as indicated above, which is produced in a suitable known process for producing glasses. Usually, the individual components of the glass are mixed in a suitable proportion to provide the desired cordierite glass composition, and the mixture is melted. The molten mixture is rapidly cooled into a glass, which is then crushed or ground into the intended cordierite glass powder. For improving the sinterability of this cordierite glass powder, the particle size of the cordierite glass powder is preferably 20µm or smaller, more preferably about 1-10µm. According to the present invention, the TiO 2 powder is used to control or adjust the dielectric constant of the dielectric ceramic body produced, and the temperature coefficient of the resonance frequency of a resonance circuit formed by a conductor pattern carried by the dielectric ceramic body. The temperature coefficient of the resonance frequency increases as the content x of the TiO 2 power increases. If a dielectric ceramic body consists solely of a cordierite glass, the temperature coefficient of thr resonance frequency falls within a range between -80ppm/°C and -40ppm°C. However, the addition of the TiO 2 powder will increase the temperature coefficient, even to a positive value. Since TiO 2 per se is a ceramic having a dielectric constant as high as about 100, the use of the TiO 2 powder with the cordierite glass powder makes it possible to increase the dielectric constant of the dielectric ceramic body produced. Generally, a TiO 2 powder consists of extremely finely divided particles, and therefore tends to prevent the sinetering of the cordierite glass powder. If the content y of the TiO 2 powder exceeds 15% by weight, it is desirable to calcine this powder in order to enlarge the particle size to a suitable value. For example, the TiO 2 powder is calcined so that the particles of the calcined power have a BET specific surface area of not larger than about 2m 2 /g. The RE 2 Ti 2 O 7 powder serves to control the dielectric constant and the temperature coefficient of the resonance frequency of the dielectric ceramic body produced, like the TiO 2 powder. However, the RE 2 Ti 2 O 7 powder reduces the temperature coefficient of the resonance frequency, contrary to the TiO 2 powder which increases the temperature coefficient. Since the RE 2 Ti 2 O 7 powder has a dielectric constant as high as 40, the addition of this powder results in increasing the dielectric constant of the dielectric ceramic body produced. For the rare earth element RE of the RE 2 Ti 2 O 7 powder, rare earth elements such as Sm, Nd, La, Pr and Y may be used. More specifically, the RE 2 Ti 2 O 7 powder is prepared by mixing TiO 2 with an oxide or oxides of at least one of these rare earth elements or a compound that gives such oxide(s), so as to establish a suitable mole ratio, then calcining the mixture and finally crushing or grinding the calcined mixture. The RE 2 Ti 2 O 7 powder may contain two or more rare earth elements. The dielectric ceramic body is prepared by using the dielectric ceramic powder mixture which consists of the cordierite glass powder, the TiO 2 powder and the RE 2 Ti 2 O 7 powder. The contents x, y and z of the three major components are suitably determined so as to determine the formulas indicated. However, the dielectric ceramic powder may further include at least one filler powder which is selected for example from the group consisting of: alumina; quartz; mullite; forsterite; and quartz glass. Crystalline fillers such as alumina, quartz, mullite and forsterite have a dielectric constant not higher than 30, and will not considerably increase the dielectric constant of the dielectric ceramic body produced. However, alumina is effective to increase the strength of the dielectric ceramic body, while quartz, mullite and forsterite are effective to control the thermal expansion coefficient of the dielectric ceramic body. For this reason, these fillers are added as needed in suitable amounts. Further, a highly refractory glass such as quartz glass may be used like the above-indicated fillers above whose dielectric constant is not higher than 30. The total amount of the at least one filler powder is up to 30 parts by weight per 100 parts by weight of the mixture of the cordierite glass powder, TiO 2 powder and RE 2 Ti 2 O 7 powder. If the total content of the filler powder or powders exceeds 30 parts by weight, the dielectric ceramic composition cannot be sufficiently densified at a sintering temperature not higher than 1100°C. To produce a dielectric ceramic body according to the present invention, a dielectric ceramic powder mixture is prepared by uniformly mixing the cordierite glass powder, the TiO 2 power and the RE 2 Ti 2 O 7 powder, together with the selected filler powder or powders added as needed. The thus prepared powder mixture is then formed into an unfired body in a known manner, and the formed unfired body is fired and densified into the dielectric ceramic body, such that the cordierite glass composition is crystallized into a crystalline phase of cordierite. The thus produced dielectric ceramic body has the matrix phase in the form of the crystalline cordierite, in which the TiO 2 and RE 2 Ti 2 O 7 particles are dispersed or distributed. The firing to sinter the unfired body formed from the dielectric ceramic powder mixture is effected at 1100°C or lower, for obtaining a highly dense fired body (dielectric ceramic body) which generally has a porosity of about 6%. The lower limit of the firing temperature is about 800°C. The crystallization of the cordierite glass composition occurs at a temperature between about 850°C and about 1100°C, during the firing process. However, this crystallization may be effected in a subsequent process following the firing process. As a result of the crystallization of the cordierite glass composition, the produced dielectric ceramic body has the crystalline cordierite matrix phase in which the TiO 2 and RE 2 Ti 2 O 7 particles are dispersed. While the primary crystal of the dielectric ceramic body consists of the cordierate, some crystalline phases of mullite, forsterite and spinel may be separated, depending upon the cordierite glass composition. The produced dieletric ceramic body unavoidably contains some amorphous phases. When a circuit board using the above dielectric ceramic body as a ceramic substrate may be produced in a suitable known manner. Generally, ceramic green sheets are formed of the dielectric ceramic powder mixture whose major components include the cordierite glass powder, the TiO2 powder and the RE 2 Ti 2 O 7 powder. On these ceramic green sheets, conductor patterns are formed by printing or other technique, using pastes of suitable electrically conductive compositions. The thus prepared ceramic green sheets are laminated or superposed on each other, and compacted together into a stack. The stack of the green sheets with the conductor patterns are then co-fired so that an intended circuit board is obtained. Since the firing and crystallization of the ceramic green sheets are effected at a relatively low temperature not higher than 110°C, the conductor patterns may be formed of an electrically conductive composition whose major component or components is/are selected from the group consisting of: Ag; Ag-based alloys such as Ag-Pt, Ag-Pd and Ag-Pt-Pd; Cu; commercially available Cu-based alloys; Au; and Au-based alloys such as Au-Pt, Au-Pd and Au-Pt-Pd. These Ag, Cu, Au, and Ag-, Cu- and Au-based alloyws have relatively low resistivity values, and can be co-fired with the dielectric ceramic composition at 1100°C or lower, at which those electrically conductive materials are not melted. Accordingly, the circuit board using the dielectric ceramic body as the substrate may contain internal conductor patterns whose resistivity is sufficiently low. Referring to the cross sectional view of Fig. 1, there is shown one form of a circuit board which is produced as described above according to the present invention. In this figure, reference numeral 2 denotes a dielectric ceramic body formed by firing of a dielectric ceramic composition whose major components include a cordierite glass powder, a TiO 2 powder and a RE 2 Ti 2 O 7 powder. Within the dielectric ceramic body 2 and on the surfaces of the body 2, there are provided various conductor patterns, which include internal conductors 4 embedded or buried in the body 2, external conductors 6 formed on the surfaces of the body 2, and earth conductors 8 buried in the body 2. These conductors 4, 6, 8-are formed in a multi-level or multi-layered fashion and carried by the dielectric ceramic body 2. The present circuit board further includes suitable resistors 10 which are embedded in or formed on the body 2. Since the dielectric ceramic body 2 of the thus constructed circuit board uses the dielectric ceramic mixture powder which can be sintered and crystallized at a relatively low temperature of 1100°C or lowe, the conductor patterns (4, 6, 8) may be formed of electrically conductive materials such as Ag- or Cu-based alloys having a sufficiently low resistivity, which can be co-fired with the dielectric ceramic composition for the dielectric ceramic body 2, even where the conductor patterns (4, 8) are embedded or buried in the dielectric body 2. EXAMPLES To further clarify the concept of the present invention, some examples of the invention will be described. It is to be understood that the invention is not limited to the details of the illustrated examples, but may be embodied with various alterations, modifications and improvements, which may occur to those skilled in the art. Example 1 Measured amounts of raw materials such as MgO, Al 2 O 3 , SiO 2 and H 3 BO 3 which give each of cordierite glass compositions No. 1 through No. 18 as indicated in TABLE 1 were sufficiently mixed in an alumina pot, and the mixture was melted in a platinum crucible at 1600°C. The melt was introduced into water and was thereby rapidly cooled into a cordierite glass. Each of the thus obtained glasses was wet-ground into a powder form in an alumina pot using alumina balls, and the powder was dried. Thus, the cordierite glass powders having the respective compositions of TABLE 1 were prepared. In the meantime, various oxides of rare earth elements such as neodymium oxide were wet-mixed with titanium oxide (TiO 2 ), in proportions as indicated in TABLE 2. The mixtures were dried, and then calcined in an alumina crucible at 1300°C. The calcined mixtures were wet-crushed in an alumina pot using alumina balls, and the obtained powdered mixtures were dried. The prepared cordierite glass powders of TABLE 1 were wet-mixed with the powdered mixtures of TiO 2 and RE 2 Ti 2 O 7 such as Nd 2 Ti 2 O 7 , in various proportions indicated by the contents x, y, z (parts by weight) in TABLE 2, whereby dielectric ceramic compositions No. 1 through 34 were prepared. For the dielectric ceramic compositions Nos. 10-12 and 16-18, a filler powder as indicated in TABLE 2 was added. The mixing was conducted in a polyethylene pot using alumina balls. After each mixture was dried, polyvinyl alcohol (PVA) was added as a binder, which was uniformly mixed with the mixture. The thus obtained mixtures were passed through a 40-mesh shieve, whereby dielectric ceramic mixture powders Nos. 1-34 as indicated in TABLE 2 were prepared. These dielectric ceramic mixture powders were subjected to a press-forming operation under a surface pressure of 1 ton/cm 2 to prepare respective samples of circular discs, of diameter 20 mm and thickness 12 mm. The samples were air-dried and then fired for 30 minutes at 900-1000°C. The fired samples were ground to a find size of 16 mm diameter and 8 mm thickness. The thus prepared samples (Nos. 1-34) of dielectric ceramic body in the form of discs were tested to measure the dielectric constant and the non-load Q value, according to the Hakki & Coleman method. Also, the temperature coefficients (τf) of the resonance frequency over a range of -25 to 75°C were measured. The measured values are given in Table 2. The measurements were made over the frequency range 8-12 GHz. The Q values in the table are those at 10 GHz. It will be understood from TABLE 4 that the dielectric ceramic bodies according to the present invention, namely, samples Nos. 1-30, are sintered bodies which were sufficiently densified by firing at 900-1000°C and showed improvements in the dielectric constant, Q value and resonance frequency temperature coefficient (τf). On the other hand, the comparative samples Nos. 31-34 were not sufficiently densified at the firing temperature of 900-1000°C. Example 2 The dielectric ceramic mixture powder No. 5 of Table 2 in Example 1 was well mixed with acrylic organic binder, plasticizer, toluene and alcohol solvent, in an alumina pot using alumina balls, whereby a slurry was prepared. By using this slurry, ceramic green tapes having thickness of 0.1-1.0mm were formed by doctor-blading technique. In the meantime, a conductor paste to be used for printing conductive wiring patterns on the green tapes was prepared by well mixing an Ag-based powder, acrylic organic binder and terpineol organic solvent, by a kneader using three rolls. For forming resistor patterns, a commercially available resistor paste for thick-film circuits was prepared. By using these conductor and resistor pastes, suitable conductive wiring patterns, earth layers and resistor patterns were printed on the appropriate green tapes indicated above. Then, suitable conductor patterns which form a desired resonance circuit of stripline type or ring type and a desired inductance element were printed on the green tapes, according to the pattern of a capacitor and the principle of a distributed constant circuit. Thereafter, the green tapes on which these conductor and resistor patterns were printed were laminated on each other in a predetermined order, and then compacted at a temperature of 100°C, under 100kg/cm 2 . The thus formed multi-level conductor and resistor patterns separated by the green tapes were interconnected through a conductive paste which fills through-holes formed through the individual green tapes by punching, for example. Subsequently, the stack of green tapes was heat-treated at 500°C so as to remove the binder therefrom, and then fired at 900°C for 30 minutes., whereby the cordierite glass composition was crystallized. Thus, a circuit board constructed as shown in the drawing was obtained. On the surfaces of the thus prepared circuit board, there were formed external conductor and resistor patterns, by using a commercially available thick-film forming Ag/Pt- or Ag/Pd-based paste, and a commercially available Cu paste. These external patterns were formed after the firing of the ceramic substrate consisting of the green tapes, for improving the patterning accuracy, adhesion force and soldering resistance. Thus, there was produced a multi-level microwave integrated circuit (MIC) board containing a resonance circuit, a capacitor and other elements buried within the dielectric ceramic substrate. The multi-level MIC board exhibited excellent high-frequency or microwave characteristics. It will be apparent from the foregoing description that the dielectric ceramic body produced according to the principle of the present invention has a sufficiently high dielectric constant, a considerably reduced temperature coefficient of resonance frequency, and therefore permits the use of an electrically conductive material having a low resistivity, as internal conductor patterns formed therein. The dielectric ceramic composition which includes a cordierite glass as a major component exhibits excellent microwave dielectric properties, and is accordingly suitably used as a dielectric ceramic substrate for a microwave intergrated circuit board (MIC board).

Claims: A dielectric ceramic body obtained by firing a mixture of a cordierite glass powder, a TiO 2 powder and a RE 2 Ti 2 O 7 powder (where RE represents one or more rare earth elements), wherein in the fired body particles of TiO 2 and particles of RE 2 Ti 2 O 7 are dispersed in a matrix phase of crystalline cordierite produced from the fired cordierite glass powder, and wherein in said mixture said cordierite glass powder, said TiO 2 powder and said RE 2 Ti 2 O 7 powder are in amounts which satisfy the following: 50 ≤ x < 100 0 < y ≤ 30 0 < z ≤ 40 x + y + z = 100 where x, y and z represent the amounts in weight % of said cordierite glass powder, said TiO 2 powder and said RE 2 Ti 2 O 7 powder respectively. A dielectric ceramic body according to claim 1 wherein said mixture contains up to 30 parts by weight in total of one or more filler powders per 100 parts by weight of the total of said mixture of said cordierite glass powder, said TiO 2 powder and said RE 2 Ti 2 O 7 powder, the filler powder or each said filler powder having a dielectric constant not higher than 30. A method of producing a dielectric ceramic body, comprising the steps of: preparing a mixture of a cordierite glass powder, a TiO 2 powder and a RE 2 Ti 2 O 7 powder (where RE represents one or more rare earth elements) in amounts satisfying 50 ≤ x < 100 0 < y ≤ 30 0 < z ≤ 40 x + y + z = 100 where x, y and z represent amounts in weight % of said cordierite glass powder, said TiO 2 powder and RE 2 Ti 2 O 7 , respectively; forming an unfired body using said mixture; and firing said unfired body at a temperature not higher than 1100°C, to thereby densify said mixture and crystallize said cordierite glass powder to produce a crystallized cordierite phase containing particles of TiO 2 and particles of RE 2 Ti 2 O 7 dispersed therein. A method according to claim 3 wherein said mixture contains up to 30 parts by weight in total of one or more filler powders per 100 parts by weight of the total of said cordierite glass powder, said TiO 2 powder and said RE 2 Ti 2 O 7 powder, the filler powder or each said filler powder having a dielectric constant not higher than 30. A circuit board having a dielectric body and at least one conductor pattern formed within said dielectric body, said dielectric body being according to claim 1 or claim 2. A circuit board according to claim 5 wherein said at least one conductor pattern is formed of an electrically conductive composition whose major component is Ag, Cu or Au.

Inventors: HIRAI TAKAMI, YAMAGUCHI HIROFUMI, YANO SHINSUKE, HIRAI, TAKAMI, YAMAGUCHI, HIROFUMI, YANO, SHINSUKE
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1991
Main classification: H01B3
IPC codes: H05K1, C04B35, H01B3