Low-temperature sinterable ceramic composition, and monolithic ceramic substrate using the same

Abstract: Disclosed are a low-temperature sinterable ceramic composition comprising 100 parts by weight of an essential component composed of from 60 to 85 % by weight of cordierite and from 15 to 40 % by weight of glass, and not larger than 25 parts by weight of a side component of TiO 2 , and a monolithic ceramic substrate comprising the composition. The composition can be sintered at relatively low temperatures, and the sintered substrate has a small thermal expansion coefficient and a small dielectric constant. The temperature-dependent variation in the electrostatic capacity of the substrate is small. Preferably, the glass component in the composition is composed of 20 to 60 % by weight of SiO 2 , 30 to 50 % by weight of B 2 O 3 , 5 to 30 % by weight of MgO, 0 to 15 % by weight of Al 2 O 3 , and 1 to 5 % by weight of R 2 O in which R is an alkali metal.

Description: The present invention relates to a ceramic composition, particularly to a low-temperature sinterable ceramic composition, and a monolithic ceramic substrate using the ceramic composition. With the popularization of small-sized electronic appliances, ceramic substrates have been being much used for mounting thereon various electronic parts to construct such electronic appliances. Recently, monolithic ceramic substrates have been developed in order to further increase the packaging density of electronic parts to be mounted thereon. Monolithic ceramic substrates are generally prepared by laminating a plurality of ceramic green sheets each having on its surface a circuit pattern made from an electroconductive paste, followed by baking them. Alumina has heretofore been used as the ceramic material for those monolithic ceramic substrates. However, monolithic ceramic substrates of alumina are problematic in the following points. 1 ○ Since alumina has a high sintering temperature falling between 1500 and 1600°C, laminated alumina green sheets require much energy for baking them, resulting in increase in the production costs of monolithic ceramic substrates of alumina. 2 ○ The electroconductive material for inner circuits to be formed inside the monolithic ceramic substrates of alumina is limited to a high-melting-point metal that is proof against high-temperature baking, such as W or Mo. However, since the metal has a high specific resistivity, the circuit pattern made of the metal shall have a high resistance value. 3 ○ Since the thermal expansion coefficient of alumina is larger than that of silicon chips constituting semiconductor parts to be mounted on the alumina substrates, thermal stress is imparted to the silicon chips on the substrates to crack them. 4 ○ Since alumina has a high dielectric constant of about 10, the time delay for the signal transmission through the circuits made inside the monolithic alumina substrates is large. Recently, there is increasing a great demand for monolithic substrates for LCR-integrated LSI capable of being driven at a high frequency over 10 MHz. Those monolithic substrates require not only the dissolution of the problems noted above but also small temperature-dependent variation in the dielectric capacity of the substrate material to be, for example, not higher than +/- 30 ppm/°C. From EP 0 480 468 a low-temperature sinterable ceramic composition is known comprising 1-25 weight percent of cryslalline ceramic material being selected from a group including cordierite and TiO 2 , 25-50 vol.% borosilicate glass. The object of the present invention is to provide a low-temperature sinterable ceramic composition, which can be sintered at relatively low temperatures, which has a small thermal expansion coefficient and a small dielectric constant, and of which the temperature-dependent variation in the electrostatic capacity is small, and to provide a monolithic ceramic substrate comprising the composition. To attain the object, the invention provides a low-temperature sinterable ceramic composition comprising 100 parts by weight of an essential component composed of from 60 to 85 % by weight of cordierite and from 15 to 40 % by weight of glass, and TiO 2 up to 25 parts by weight based on the essential component as a side component. Preferably, the glass component in the composition is composed of from 20 to 60 % by weight of SiO 2 , from 30 to 50 % by weight of B 2 O 3 , from 5 to 30 % by weight of MgO, from 0 to 15 % by weight of Al 2 O 3 , and from 1 to 5 % by weight of R 2 O in which R is an alkali metal. In the composition, the side component TiO 2 has a negative temperature coefficient of electrostatic capacity. Having been added to the essential component composed of cordierite and glass and baked, this TiO 2 acts to lower the absolute value of the temperature coefficient of the resulting sintered composition. The invention also provides a monolithic ceramic substrate comprising the low-temperature sinterable ceramic composition as a dielectric or insulating material. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a cross-sectional view illustrating one embodiment of the monolithic ceramic substrate of the invention. PREFERRED MODES OF CARRYING OUT THE INVENTION Now, preferred modes of carrying out the invention relating to the low-temperature sinterable ceramic composition and the monolithic ceramic substrate are mentioned below with reference to the following Examples. Example 1: Raw materials of cordierite, SiO 2 , MgO and Al 2 O 3 were weighed and mixed to prepare a mixture. This mixture was calcined at 1350 to 1400°C, and ground to obtain powder of cordierite having a composition of Mg 2 Al 4 Si 5 O 18 and having a mean grain size of from 1 to 3 µm. Next prepared were powdery raw materials of glass, SiO 2 , B 2 O 3 , MgCO 3 , Al 2 O 3 and Li 2 CO 3 . These were weighed and mixed to give glass of SiO 2 :B 2 O 3 :MgO:Al 2 O 3 :Li 2 O = 27:40:22:8:3 (by weight), and melted under heat at 1450 to 1550°C for 1 to 4 hours, and thereafter rapidly cooled and vitrified. The resulting glass pieces were milled in a zirconia ball mill to obtain glass powder having a mean grain size of from 0.5 to 3 µm. Next, the glass powder and the cordierite powder produced in the above were mixed with powder of TiO 2 that had been prepared previously, in varying ratios shown in Table 1 below, and then wet-milled in a zirconia ball mill or the like for 3 to 4 hours to obtain homogeneous powdery mixtures comprising glass powder and ceramic powder. In Table 1, the amount of TiO 2 added is in terms of parts by weight relative to 100 parts by weight of the total amount of cordierite and glass. Next, an organic binder and a solvent of toluene were added to and mixed with each powdery mixture, and then fully kneaded in a ball mill to obtain homogeneous dispersions. These were thereafter defoamed under reduced pressure to give slurries. Organic vehicles such as binder, solvent and plasticizer used herein are ordinary ones and their components are not specifically defined. Sample No. Ratio of Cordierite to Glass (wt.%) TiO 2 (wt.pts.) Cordierite Glass * 1 90 10 0 * 2 90 10 15 * 3 90 10 25 * 4 90 10 33 * 5 85 15 0 6 85 15 15 7 85 15 25 * 8 85 15 33 * 9 80 20 0 10 80 20 15 11 80 20 25 * 12 80 20 33 * 13 60 40 0 14 60 40 15 15 60 40 25 * 16 60 40 33 17 55 45 0 * 18 55 45 15 * 19 55 45 25 * 20 55 45 33 Each slurry was cast onto a film support, using a doctor blade, to form strips of ceramic green sheets having a thickness of 0.2 mm. After having been dried, these strips were peeled off from the film support and blanked out to give ceramic green sheets having a predetermined size. Next, a plurality of the thus-prepared ceramic green sheets were laminated and pressed to form a monolithic ceramic body, which was then heated at a heating rate of 200°C/hr, and baked at 980°C for 2 hours to obtain a sintered ceramic body. The thermal expansion coefficient, the dielectric constant and the temperature coefficient of dielectric constant of each of those sintered ceramic bodies produced herein were measured. The data obtained are shown in Table 2 below along with the sintered condition of each body. In Table 1 and Table 2, the samples marked with asterisk, * are outside the scope of the invention. Sample No. Thermal Expansion Coefficient (ppm/°C) Dielectric Constant (1 MHz) Temperature Coefficient (ppm/°C) Sintered Condition * 1 - - - Not sintered. * 2 - - - Not sintered. * 3 - - - Not sintered. * 4 - - - Not sintered. * 5 3.0 5.1 240 Good. 6 3.5 6.3 30 Good. 7 3.9 6.9 5 Good. * 8 4.4 7.4 -20 Good. * 9 3.1 5.1 220 Good. 10 3.6 6.2 20 Good. 11 4.0 7.0 0 Good. * 12 4.5 7.4 -30 Good. * 13 3.5 4.9 180 Good. 14 4.0 6.0 15 Good. 15 4.3 6.7 -5 Good. * 16 4.9 7.3 -40 Good. * 17 - - - Fused. * 18 - - - Fused. * 19 - - - Fused. * 20 - - - Fused. As is obvious from Table 2, the samples of ceramic composition of the invention comprising 100 parts by weight of the essential component composed of from 60 to 85 % by weight of cordierite and from 15 to 40 % by weight of glass and not larger than 25 parts by weight of the side component TiO 2 were well sintered when baked at the relatively low temperature of 980°C, and as compared with alumina, those samples have a small dielectric constant falling between 6.0 and 7.0, a small thermal expansion coefficient falling between 3.5 and 4.3 ppm/°C, and a small temperature coefficient of dielectric constant falling between -5 and + 30 ppm/°C. As opposed to these, the reference samples Nos. 1 to 4 having a glass content of smaller than 15 % by weight could not be sintered well at the temperature lower than 1000°C, while the reference samples Nos. 17 to 20 having a glass content of larger than 40 % by weight were fused while being baked. This is because the baking temperature 980°C was too high for the latter samples Nos. 17 to 20. In addition, as containing too much glass, those samples Nos. 17 to 20 have poor mechanical strength. For these reasons, the glass content of the composition of the invention preferably falls between 15 and 40 % by weight. On the other hand, the reference samples Nos. 8, 12 and 16, of which the TiO 2 content is larger than 25 parts by weight, are unfavorable for monolithic substrates, since their absolute value of the temperature coefficient of dielectric constant is large and since their thermal expansion coefficient and dielectric constant are large. Example 2: Cordierite powder was prepared in the same manner as in Example 1. Powdery glass compositions as in Table 3 below were prepared also in the same manner as in Example 1. Next, the glass powder and the cordierite powder prepared herein were mixed with powder of TiO 2 that had been prepared previously, in varying ratios shown in Table 3, and then wet-milled in a zirconia ball mill or the like for 3 to 4 hours to obtain homogeneous powdery mixtures comprising glass powder and ceramic powder. In this, the ratio of glass to cordierite was 20/80 by weight, and the amount of TiO 2 added was 15 parts by weight relative to 100 parts by weight of the total of glass and cordierite. Next, an organic binder and a solvent of toluene were added to and mixed with each powdery mixture, and then fully kneaded in a ball mill to obtain homogeneous dispersions. These were thereafter defoamed under reduced pressure to give slurries. Using these slurries, green sheets were formed, laminated, pressed, and baked at 980°C for 2 hours in the same manner as in Example 1 to produce sintered ceramic bodies. Sample No. Glass (wt.%) Glass Composition (wt.%) Cordierite (wt.%) TiO 2 (wt.pts.) SiO 2 B 2 O 3 MgO Al 2 O 3 Li 2 O * 21 20 15 48 26 8 3 80 15 22 20 27 40 22 8 3 80 15 * 23 20 65 16 8 8 3 80 15 * 24 20 18 55 16 8 3 80 15 * 25 20 39 50 0 8 3 80 15 * 26 20 22 32 35 8 3 80 15 27 20 41 35 13 8 3 80 15 * 28 20 24 34 19 20 3 80 15 * 29 20 28 41 23 8 0 80 15 * 30 20 26 39 21 8 6 80 15 31 20 20 49 20 8 3 80 15 32 20 60 31 6 1 2 80 15 33 20 55 30 6 6 3 80 15 34 20 24 50 15 8 3 80 15 35 20 38 46 5 8 3 80 15 36 20 26 33 30 8 3 80 15 37 20 33 40 24 0 3 80 15 38 20 28 34 20 15 3 80 15 39 20 28 40 23 8 1 80 15 40 20 27 39 21 8 5 80 15 The thermal expansion coefficient, the dielectric constant, the temperature coefficient of dielectric constant, and the dielectric loss of each of those sintered ceramic bodies produced herein were measured. The data obtained are shown in Table 4 below along with the sintered condition of each body. In Table 3 and Table 4, the samples marked with asterisk, * are outside the scope a prefered embodiment of the invention. Sample No. Thermal Expansion Coefficient (ppm/°C) Dielectric Constant (1 MHz) Temperature Coefficient (ppm/°C) Dielectric Loss (%) Sintered Condition * 21 - - - - Foamed. 22 3.6 6.2 +20 0.1 Good. * 23 - - - - Not sintered. * 24 - - - - Foamed. * 25 - - - - Not sintered. * 26 - - - - Foamed. 27 3.3 6.0 +15 0.1 Good. * 28 - - - - Not sintered. * 29 - - - - Not sintered. * 30 - - - - Foamed. 31 4.0 6.3 +25 0.1 Good. 32 3.4 5.9 +10 0.1 Good. 33 3.5 5.9 +10 0.1 Good. 34 4.0 6.2 +20 0.1 Good. 35 3.4 6.1 +20 0.1 Good. 36 3.5 6.2 +20 0.1 Good. 37 3.1 6.2 +20 0.1 Good. 38 4.2 6.3 +25 0.1 Good. 39 3.5 6.1 +20 0.1 Good. 40 3.7 6.3 +25 0.1 Good. As in Table 4, the glass component is preferably composed of SiO 2 , B 2 O 3 , MgO, Al 2 O 3 and Li 2 O, and the proportions of these constituents are preferably as follows: If the amount of SiO 2 is smaller than 20 % by weight as in the reference samples Nos. 21 and 24, the viscosity of the glass composition is so low that the composition is foamed between the ceramic grains existing therein. On the other hand, if the amount of SiO 2 is larger than 60 % by weight as in the reference sample No. 23, the viscosity of the glass composition is so high that the sinterability of the ceramic composition is poor. Therefore, the SiO 2 content is preferably from 20 to 60 % by weight. If the amount of B 2 O 3 is smaller than 30 % by weight as in the reference sample No. 23 , the viscosity of the glass composition is so high that the sinterability of the ceramic composition is poor. On the other hand, if the amount of B 2 O 3 is larger than 50 % by weight as in the reference sample No. 24, the chemical stability of the glass composition is so poor that the composition is foamed. Therefore, the B 2 O 3 content is preferably from 30 to 50 % by weight. If the amount of MgO is smaller than 5 % by weight as in the reference sample No. 25, the viscosity of the glass composition is so high that the ceramic composition is not sintered. On the other hand, if the amount of MgO is larger than 30 % by weight as in the reference sample No. 26, too much MgO unfavorably reacts with cordierite and the composition is foamed. Therefore, the MgO content is preferably from 5 to 30 % by weight. Al 2 O 3 is added to improve the chemical stability of the glass composition. If, however, its amount is larger than 15 % by weight as in the reference sample No. 28, the viscosity of the glass composition is so high that the ceramic composition is not sintered. Therefore, the Al 2 O 3 content is preferably not larger than 15 % by weight. If the amount of Li 2 O is smaller than 1 % by weight as in the reference sample No. 29, the composition is not sintered. On the other hand, if its amount is larger than 5 % by weight as in the reference sample No. 30, the composition is foamed. Therefore, the R 2 O content is preferably from 1 to 5 % by weight. Example 3: A slurry of the sample No. 10 in Example 1 was prepared. Using a doctor blade, this was cast to form a ceramic green sheet strip having a thickness of 50 µm. After having been dried, this strip was blanked out to give ceramic green sheets having a predetermined size. Next, Cu paste was screen-printed on these ceramic green sheets to form a surface circuit pattern, a capacitor pattern and a coil pattern thereon. Using a puncher, via holes were formed through the sheets, and filled with Cu paste. Next, a plurality of the thus-processed green sheets were laminated and pressed to give a laminate, which was then baked in a nitrogen atmosphere at 990°C to obtain a monolithic ceramic substrate 1 as in Fig. 1. In Fig. 1, 2 is a dielectric or insulating ceramic layer, 3 is a capacitor device, and 4 is a coil device, and those devices 3 and 4 were formed inside the monolithic ceramic substrate 1. Next, Si bare chips 6 were mounted on the surface of the substrate 1 via bumps 5 therebetween. In the Si chips-mounted, monolithic ceramic substrate produced herein, the thermal expansion coefficient of the substrate was small and was near to that of the Si bear chips. Therefore, the presence of no substantial difference in the thermal expansion coefficient between the substrate and the chips prevented the substrate from being cracked. In addition, since the dielectric constant of the substrate was small, the signal transmission speed through the substrate was high and the crosstalk noise between the conductor patterns was reduced. Moreover, since the temperature coefficient of electrostatic capacity of the ceramic layers was small, the temperature-dependent stability of the capacitor devices in the substrate was high. In Examples mentioned hereinabove, used were oxides and carbonates as the powdery raw materials for glass. Apart from these, hydroxides and others are also employable. In those, Li was used as the alkali metal R. Apart from this, any other alkali metal, such as Na, K, Rb, Cs or Fr is also employable to produce the same results. As has been clarified hereinabove, low-temperature sinterable ceramic compositions for monolithic ceramic substrates are obtained according to the present invention, which can be sintered even at low temperatures not higher than 1000°C to give sound substrates having a small thermal expansion coefficient and a small dielectric constant. The temperature-dependent variation in the electrostatic capacity of the sintered substrates is small. Using the low-temperature sinterable ceramic compositions of the invention, obtained are monolithic ceramic substrates having better properties than alumina substrates. Specifically, since the thermal expansion coefficient of the substrates of the invention is small, the difference in the thermal expansion coefficient between the substrates and silicon chips to be mounted thereon is negligible and the substrates with the chips thereon are prevented from being cracked. Therefore, the substrates have high reliability without being deteriorated by moisture. In addition, since the substrates have a low dielectric constant, the signal transmission speed through them is high. Moreover, since the substrates can be sintered at low temperatures not higher than 1000°C, electroconductive materials having a low melting point and having a low resistance value, such as Cu and Ag/Pd can be used in them, and the resistance of the electroconductive circuits formed in them is lowered. Further, since the temperature-dependent variation in the electrostatic capacity of the substrates is small, the substrates are most favorable for LCR integration applicable to high frequency driving systems. 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 spirit and scope thereof.

Claims: A low-temperature sinterable ceramic composition comprising a) 100 parts by weight of an essential component composed of from 60 to 85 % by weight of cordierite and from 15 to 40 % by weight of glass, and b) TiO 2 up to 25 parts by weight based on the essential component as a side component. The low-temperature sinterable ceramic composition as claimed in claim 1, wherein said glass is composed of: from 20 to 60 % by weight of SiO 2 , from 30 to 50 % by weight of B 2 O 3 , from 5 to 30 % by weight of MgO, from 0 to 15 % by weight of Al 2 O 3 , and from 1 to 5 % by weight of R 2 O, in which R is an alkali metal. A monolithic ceramic substrate comprising the low-temperature sinterable ceramic composition of claim 1 or 2 as a dielectric or insulating material.

Inventors: SUGIMOTO YASUTAKA, SUGIMOTO, YASUTAKA
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1998
Main classification: C04B35
IPC codes: H05K1, H01B3, C04B35, H05K3, C03C14