METHOD OF MAKING CERAMIC DIELECTRIC BODIES

Abstract: A method of making a ceramic dielectric body suitable for use in a laminar electrical component com­ prises (a) mixing a ceramic powder containing from 85 to 100 mole % of magnesium orthoborate (Mg₃B₂O₆) and a suitable liquid carrier to form a slurry, (b) forming a layer of the slurry from step (a) and drying the layer, (c) depositing a film of an electroding ink on at least a part of the surface of the dry layer from step (b), (d) firing the coated layer from step (c) at such a temperature that the ceramic material is sintered to give a dense ceramic layer, and (e) firing the coated layer from step (d) at such a temperature that the film of electroding ink is sintered to form a conductive film adhering to the ceramic layer.

Description: This invention is concerned with a method of making ceramic dielectric bodies which are suitable for use in laminar electrical components, such as wafer capacitors, multi-dielectric layer capacitors, and printed circuit assemblies. It is known to make dielectric ceramic bodies from mixtures of a major proportion of a high firing ceramic and a minor proportion of a borate flux, such materials being described, for example, in U.S. Patents 3,885,941, 4,027,209, 4,101,952 and 4,308,570. The borate fluxes used as sintering aids in the prior art (with the exception of U.S. Patent 4,533,974) are glasses rather than being crystalline and they have melting temperatures which are less than the characteristic minimum sintering tempera­ ture of the high firing ceramic with which they are mixed. The addition of such low melting glass fluxes reduces the temperature necessary for sintering by providing a vehicle for a liquid phase sintering mechanism. The only exception to this type of process known to us is the use of crystalline cadmium silicates as described in U.S. Patent 4,266,265. This flux is not, of course, a borate and the patent contains no disclosure or suggestion as to the use of crystalline borates as a sintering aid or flux. We have now found that magnesium orthoborate (Mg₃B₂O₆), which is one of several sintering aids described in U.S. Patent 4,533,974, provides exceptional dielectrical properties when employed as the predominant crystalline constituent of a ceramic dielectric body in a laminar electrical component such as a wafer capacitor, a mono­ lithic capacitor, or a printed wiring substrate. The ceramic material is very dense, has high Q and high frequency characteristics, and has a low dielectric constant. It is also capable of being "fluxed" with high melting materials such as barium oxide. Reaction of the boron in the magnesium orthoborate in a limited surface reaction with the "flux" produces eutectic which leads to liquid phase sintering at temperatures well below the melting point of either the "flux" or the magnesium ortho­ borate. For no other high firing dielectric than magnesium orthoborate is this known to be true. Furthermore, the compound of central interest here, magnesium orthoborate, is a very obscure material. Except for its having received academic attention as one of the three single-phase magnesium borates, it has apparently found no use. According to the present invention, therefore, there is provided a method of making a ceramic dielectric body suitable for use in a laminar electrical component, which comprises (a) mixing a ceramic powder containing from 85 to 100 mole % of magnesium orthoborate (Mg₃B₂O₆) and a suitable liquid carrier to form a slurry, (b) forming a layer of the slurry from step (a) and drying the layer, (c) depositing a film of an electroding ink on at least a part of the surface of the dry layer from step (b), (d) firing the coated layer from step (c) at such a temperature that the ceramic material is sintered to give a dense ceramic layer, and (e) firing the coated layer from step (d) at such a temperature that the film of electroding ink is sintered to form a conductive film adhering to the ceramic layer. The final assembly may form the nucleus for numerous laminar electronic components such as ceramic wafer capacitors, monolithic ceramic capacitors, and printed circuit boards. Magnesium orthoborate possesses a unique com­ bination of properties that can lead to high performance electrical components. Briefly, the dielectric constant (7.2) of magnesium orthoborate is quite low, making it possible to build small capacity capacitors to tight tolerances, and in printed circuit substrates reduces interwiring capacitances. This feature is combined with a high Q (low DF) at very high frequencies, a high density and a high bond strength with adjacent co-fired ceramics of a different composition which gives great versatility of design in multi-layer printed wiring substrates. It is also capable of being fired in air or co-fired with buried base metal electrodes, such as those of copper or nickel. In order that the invention may be more fully understood, the following Examples are given by way of illustration. Reference will be made in the Examples to the accompanying drawings, in which: Figure 1 is a side sectional view of a wafer capacitor having a dominantly magnesium orthoborate ceramic body; Figure 2 shows in side sectional view a mono­ lithic ceramic capacitor having a dominantly magnesium orthoborate ceramic body; Figure 3 shows in side sectional view a printed circuit substrate having a body that is primarily a magnesium orthoborate-containing ceramic, and Figure 4 shows a multilayer ceramic printed circuit assembly having a plurality of strata, some of which are formed primarily of magnesium orthoborate, and having a silicon integrated circuit chip connected thereto. Example 1 Magnesium orthoborate was prepared by milling boric acid and magnesium carbonate. The two compounds were milled together in acetone and then dried, calcined at 1200°C and comminuted to form a powder of Mg₃B₂O₆, as confirmed by X-ray diffraction analysis. The powder had an orthohombic crystal structure with an X-ray theoretical ultimate density of 3.10 g/cm³. A number of experimental wafer type capacitors were made, as shown in Figure 1. The wafer bodies were first formed by the conventional steps of wet milling a start powder for about two hours and drying. The start powder in this example consisted of the Mg₃B₂O₆ referred to above. This was crushed and jet pulverised to an average particle size of about 1 to 1.5 microns. The fine powder was combined in an organic vehicle and cast squares (about 1 cm²) were formed to about 0.6 mm thickness. Examples 2 and 3 One group of these cast squares (Example 2) was sintered in air at 1200°C, while another group (Example 3) was sintered in an environment having a partial oxygen pressure of 10⁻⁹ atmospheres of oxygen at 1200°C. A silver electroding paste was applied to the two major faces of each mature ceramic cast square, or wafer, and the wafers were fired at 800°C to form wafer capacitors each having bodies 10 and electrodes 11 and 12, as shown in Figure 1. The wafer capacitors from the groups of Examples 2 and 3 were essentially identical: namely, milky white in colour, exhibiting an unusually high density of 97% of that theoretically possible, a dielectric constant of 7.2, and at 1 MHz and 25°C the DF was less than 0.01%. Example 4 Another portion of the calcined Mg₃B₂O₆ powder obtained in Example 1 was combined with an organic vehicle to form a slurry that was repeatedly deposited in 5 mil (0.13 mm) thick layer, each layer being dried and having an electroding ink film of 10Ag/90Pd deposited thereon prior to deposition of the next slurry layer. The last three or topmost slurry layers and the first three slurry layers were not coated with the electroding ink. There were six electrode films with five active dielectric layers therebetween. This assembly was sintered at 1200°C for 2½ hours to drive off all the organic materials from both the ink and the slurry layers, and to alloy the metal to 10Ag/90Pd alloy film electrodes buried in a sintered dielectric block 20 of Mg₃B₂O₆. In this way, a monolithic ceramic capacitor was formed, as shown in Figure 2, in which left-end extending buried electrodes 21 and right-end extending buried electrodes 22 are contacted at the left and right ends respectively by silver terminations 25 and 26. The density of the ceramic was 97.2% of theoreti­ cal and the dielectric constant was 7.2. These monolithic capacitors had a higher capacitance, viz. 32 pF, than the wafer capacitors obtained in Examples 2 and 3 and conse­ quently it was possible to measure the DF at 10 KHz and at 100KHz. At 100 KHz the DF was 0.05% and at 1 MHz it remained at less than 0.01%. The temperature coefficient of capacitance was a positive 100 parts per million (P 100ppm/°C) at -55°C and was P 150ppm/°C at +115°C. Various other wafer capacitors were made by the conventional method steps described above, employing as the start powder the Mg₃B₂O₆ powder of Example 1 to which was added a small quantity of one of two other powder compounds. Examples 5, 7 and 9 Dielectric bodies were prepared containing various amounts of baria and manganese carbonate, and were sintered in a non-oxidising atmosphere at 1100°C to a very dense mature ceramic. The dielectric constant was raised about 10%. The addition in Example 5 of 3.7 mole % of baria without manganese carbonate gave about optimum results, i.e. the sintering temperature reduction amounted to about 50°C and the DF remained extremely low. Examples 6, 8 and 10 These Examples were the same as Examples 5, 7 and 9 respectively except for being sintered in air and the results were very similar. Example 11 The additive in this Example was zinc cadmium borate, 2ZnO.CdO.B₂O₃. (Note that this compound expression is in mole % of the constituent oxides.) This material has been used as a flux additive in other dielectric ceramic bodies for reducing sintering temperature. In this instance, however, it is clearly not as satisfactory as baria. Although the DF was quite low, the density of the body was comparatively low and the sintering temperature was reduced less for the same amounts of additive. Examples 12 to 17 Various (molar) amounts of lithium oxide were added to the magnesium borate. The lithium was actually added as lithium carbonate. By sintering a few wafers of each composition at temperatures in 50°C intervals, the minimum sintering temperatures of each composition were determined. The efficiency of the lithium additive to reduce the sintering temperature was greatest in Example 13 at the 3 mole % level. The DF remained essentially unaffec­ ted except in Example 14, where sintering was effected in a partial oxygen atmosphere and achieved nearly the same density by slightly sacrificing DF. Example 18 Using the same 3 mole % Li₂O composition as used in Examples 13 and 14, a monolithic ceramic capacitor was built having five buried electrodes, four "active" 6 mil (0.15 mm) thick dielectric layers, and having a total capacitance of 32 PF. The buried electrodes were made of the industry-standard electroding alloy 70Ag/30Pd. The dielectric constant was 7.2, DF at 1 MHz and was less than 0.01%, the temperature coefficient of capacitance averaged 75 ppm/°C over the temperature range -55°C to +125°C, and insulation resistance at 200 volts and 150°C was more than 10⁶ megohms. Additional measurements of Q were made by a transmission-line method. At 200 MHz, the Q was 150; at 400 MHz, the Q was found to be about 100, which is equivalent to a DF of 0.01. Example 19 Another monolithic ceramic capacitor was made by exactly the same process as were the monolithic capacitors of Example 18, except that a 95Ag/5Pd (by weight) silver­ rich alloy was used instead of the 70Ag/30Pd alloy. For this capacitor, the Q at 200 MHz was 4000 and at 400 MHz was 350. This demonstrates that at high frequencies, the resistance of the buried electrodes, and not the dielectric, tend to become the dominant factor affecting the capacitor losses. Measurements have been made of some of the mate­ rial properties of magnesium orthoborate, with and without a flux (3% lithium oxide); the ceramics were sintered in air at 1200°C and 1050°C respectively. Some of these properties are similar to those of the dense alumina bodies that are used as printed circuit substrates and multilayer circuit boards. For example, the thermal coefficient of expansion is about the same, as is the temperature coefficient of capacitance. However, the magnesium borates of this invention have characteristics that offer significant advantages for these applications. The sintering temperatures of the magnesium ortho­ borate dielectrics is substantially lower than the minimum sintering temperature for alumina, namely about 1450°C. This advantage increases the number of materials which do not melt on co-firing with the ceramic, and a much greater variety of electroding the resistor body metals may be buried in a ceramic multilayer circuit board. Another method for making magnesium orthoborate, that utilizes a technique of charring a dried emulsion, produces powders that are very fine and tend to sinter at lower temperatures without flux, and are expected to make strong low-firing substrates. It has also been discovered that although the magnesium and boron oxide precursors are hygroscopic, magnesium orthoborate is not, a very important feature in a dielectric ceramic. The dielectric constant of alumina is 9, about 26% higher than that of the magnesium orthoborate ceramic. This leads to a commensurate reduction in the interwiring capacitances of equal dimensional printed circuit sub­ strates and multilayer circuit boards made with magnesium orthoborate ceramics. Alternatively, the latter may be reduced in size before the interwiring capacitances reach the values exhibited by conventional alumina counterparts. Referring to Figure 3, a printed circuit substrate 10 made of a magnesium orthoborate material has a plurality of film conductors 32, 33, 34, 35, 36 and 37 formed by screen printing thereon. These conductors may be deposited before or after the sintering of the ceramic substrate. A high resistivity film 39 may be one such as is described in U.S. Patent 3,989,874. A monolithic ceramic capacitor 40, similar to that of Figure 2, is shown solder-mounted to conductors 33 and 34. Figure 4 shows a multilayer ceramic printed circuit board made up of magnesium orthoborate material regions, e.g. 42, 43 and 44, that are shown as unhatched areas; metal conductors, e.g. 46, 47, 48, 49, 50, 51 and 52; a high resisitivity resistor film 55; and a high diele­ ctric constant ceramic material 59, e.g. a lead barium lanthanum zirconate titanate such as described in U.S. Patent 4,324,750. The material of Example 7 of the U.S. Patent was used, which has a dielectric constant of over 1800, to give a buried capacitor having a high capacitance value between conductors 46 and 51. The interwiring capacitance between other conductors, e.g. 51 and 49, had very low values. The conductive vias, e.g. 54 may be made by standard techniques of punching holes in the green ceramic layers and later filling them with metal. A silicon integrated circuit chip 60 is shown face-bonded directly to the conductors, e.g. 49, 50 and 52. The face bonding may be accomplished by conventional means such as heating to reflow solder-bond solder bumps to the substrate conductors or by ultrasonically energising the assembly. Example 20 Some of the magnesium orthoborate slurry used to make the monolithic ceramic capacitors of Example 18 was cast on a plate using the doctor blade technique, and dried. 70Ag/30Pd electroding ink was printed on a surface of the dried slurry layer. Subsequently, a 1 mil (0.02 mm) slurry layer of the lead barium lanthanum zirconate tita­ nate referred to above was formed by the same technique over the printed surface, and dried. Again, a printed electroding ink layer was formed and a top layer of magnesium borate slurry was deposited thereon to form a sandwich about the two titanate electrode layers. This assembly was sintered at 1050°C in air. The sintered sandwich was sectioned and observed under a microscope. There was complete bonding of the adjacent layers with no evidence of separation or delamination at any point. Example 20 demonstrates the ability of magnesium orthoborate to co-fire with and bond well to other ceramic materials. It is, therefore, a demonstration of the basic feasibility of multi-layer circuits like that shown in Figure 4.

Claims: 1. A method of making a ceramic dielectric body suitable for use in a laminar electrical component, which comprises (a) mixing a ceramic powder containing from 85 to 100 mole % of magensium orthoborate (Mg₃B₂O₆) and a suitable liquid carrier to form a slurry, (b) forming a layer of the slurry from step (a) and drying the layer, (c) depositing a film of an electroding ink on at least a part of the surface of the dry layer from step (b), (d) firing the coated layer from step (c) at such a temperature that the ceramic material is sintered to give a dense ceramic layer, and (e) firing the coated layer from step (d) at such a temperature that the film of electroding ink is sintered to form a conductive film adhering to the ceramic layer. 2. A method according to claim 1, in which the ceramic powder used in step (a) contains up to 15 mole % of a sintering aid. 3. A method according to claim 2, in which the sintering aid is BaO or Li₂O. 4. A method according to any of claims 1 to 3, in which the slurry formed in step (a) comprises an additional ceramic powder. 5. A method according to claim 3 or 4, in which the additional ceramic powder has a minimum sintering temperature of more than 1150°C. 6. A method according to any of claims 3 to 5, in which the ratio by weight of ceramic powder: additional ceramic powder in the slurry formed in step (a) is less than 1 : 10 and sintering is carried out at a temperature of up to 1150°C. 7. A method according to any of claims 1 to 6, in which the conductive film adhering to the ceramic layer is in the form of one or more patches which are individually secured by means of permanent conductive connections to conductive bumps extending from one face of an integrated circuit chip. 8. A method according to any of claims 1 to 7, which further comprises mixing a powdered dielectric material with a suitable liquid carrier to form a second slurry and applying a layer of said slurry to the surface of the coated layer from step (c), drying said second layer, and sintering the resulting composite structure in one step to form a dense ceramic body having a conductive film within. 9. A method according to claim 8, in which the second slurry has the same composition as the slurry used in step (b) so that the resulting ceramic body has the same ceramic composition throughout. 10. A method according to claim 8, in which the second ceramic layer of the resulting ceramic body has a higher dielectric constant than the first ceramic layer. 11. A method according to any of claims 8 to 10, which further comprises depositing a second film of an electroding ink on at least a part of the surface of the dry second layer, and sintering the resulting composite structure in one step to form a dense ceramic body having a conductive film within and a conductive film on the surface. 12. A method according to claim 11, in which the two conductive films in the resulting ceramic body are so arranged that each film extends to and is exposed at a face of the body, the faces at which the films are exposed being on opposite sides of the body. 13. A method according to any of claims 1 to 12, in which the elctroding ink comprises a metal in a carrier liquid and sintering of the deposited ink is carried out in a low partial oxygen atmosphere so that the metal is not oxidised. 14. A method according to claim 13, in which the metal is copper or nickel. 15. A laminar electrical component comprising a ceramic dielectric body made by the method claimed in any of the preceding claims.

Inventors: MAHER, GALEB HAMID
Applicants: SPRAGUE ELECTRIC COMPANY
Application year: 1986
Main classification: H01B3
IPC codes: H01L23, H01G4, H05K1, C04B35