CONTROLLED ATMOSPHERE FIRING PROCESS

Abstract: A process for firing oxidizable base metals in the presence of organic media in the presence of CO/CO₂ and water to avoid oxidation of the base metal.

Description: The invention is directed to a process for firing oxidizable base metals in a controlled atmosphere to avoid oxidation of the base metal. Multilayer circuits containing base metal conductors interleaved with insulating dielectric layers are made from base metal and ceramic powders, both mixed with organic binders and other organic constituents. When fired, the organic constituents must be removed while the metal must be retained in its unoxidized state. Multilayer base metal circuits are conventionally fired one layer at a time in a nitrogen atmosphere containing up to a few hundred ppm O₂ to remove the organics. The metal oxide and not the metal in its elemental form is usually thermodynamically stable under these conditions. However, the rate of oxidation of the metal is usually low, especially in the relatively reducing environment created by the organic decomposition products. After the metal powder has sintered oxidation requires diffusion of the oxygen through the metal oxide layer to the bulk metal. Most of the O₂ in the N₂ atmosphere is added at lower temperatures where the diffusion rate through the metal oxide layer into the metal is relatively low. Sintering, then, partially protects the metal from oxidation when the next layer is fired on top of it. However, a certain amount of oxidation is typical, especially when using organics that require high O₂ concentrations in the firing atmosphere for removal. Firing in an O₂/N₂ atmosphere often results in discoloration of the dielectric layer when over metal because of diffusion of metal ions and precipitation of metal atoms in the dielectric. The amount of metal oxidation and dielectric discoloration depend on the level of O₂ used which in turn depends on both the quantity and nature of the organics present. In general, low levels of O₂ give low metal oxidation but high dielectric discoloration, while high levels of O₂ give low dielectric discoloration but high oxidation. Optimizing the O₂ level under these circumstances is often difficult. Cofiring, or simultaneous firing, of dielectric and metal layers in an O₂/N₂ atmosphere is even more difficult than firing the layers one at a time because longer times are required to remove the organics and since the metal layers have not been previously sintered they have little protection against this oxidizing atmosphere. The extensive use and wide variety of controlled firing atmospheres is illustrated by the following prior art publications. U.S. 2,547,149, Beaudet The patent is directed to a method of firing porcelain in which the organic content of biscuit paste is burned off in a reducing atmosphere of CO/CO₂ and H₂O. U.S. 3,183,588, Mathieu Copper-alloy clad vessels are processed in a neutral or reducing atmosphere to avoid oxidation of the copper. U.S. 3,009,886, Wejnarth Silicon carbide-containing resistor compositions are sintered in a reactive hydrocarbonaceous gas at 2220-2450°C. U.S. 3,423,517, Arrhenius This patent is directed to monolithic ceramic electrical interconnecting structures which are fired in a reducing atmosphere such as argon containing 10% H₂. U.S. 3,615,980, Schuck et al. The patent is directed to the decal metallization of ceramic substrates in which the decal is fired in two steps. In the first, the decal is fired in an oxidizing atmosphere to burn off the decal binder and in the second the substrate is fired in a reducing atmosphere of H₂ and H₂O or H₂ and N₂ to effect bonding of the metal to the substrate. U.S. 3,770,529, Anderson The patent is directed to multilayer ceramic circuit structures made from green sheets in which the binder is burned off in either air or a reducing atmosphere. U.S. 3,852,877, Ahn et al. The patent is directed to making multilayer ceramic circuit structures from green sheets in which the binder is burned off in a reducing atmosphere of H₂ and H₂O. U.S. 4,089,813, Alexander The patent discloses firing BaTiO₃-based dielectrics in an atmosphere of N₂ and optionally either 2-10 mole % H₂O or H₂/CO₂. U.S. 4,097,911, Dorrian The patent is directed to a method of making monolithic ceramic capacitors in which the electrodes are fired at 2500°C in a low oxygen atmosphere comprising CO₂ and CO. U.S. 4,101,952, Burn The reference is directed to monolithic ceramic capacitors which are sintered at 950-1080°C in a CO₂/CO atmosphere to avoid oxidation of the electrode metals. U.S. 4,234,367, Herron et al. The patent discloses the firing of multilayer interconnected thick film circuits having copper conductors in an atmosphere of H₂/H₂O. U.S. 4,504,339, Kamehara et al. The patent discloses firing multilayered glass-ceramic structures having copper-based patterns in which firing is carried out in two steps. In the first step the binder is volatilized in an atmosphere of inert gas (N,Ar,He) and H₂O and the sintering is carried out in inert gas without water. U.S. 4,551,357, Takeuchi The reference is directed to a method of cofiring dielectric and base metal conductor pastes in which the organic medium of the conductor paste has a higher decomposition temperature than the binder in the dielectric paste. The cofiring is carried out in two steps: (1) an oxidizing step to decompose the binder without decomposing the organic medium and (2) a low oxygen atmosphere step to decompose the organic medium. An atmosphere of H₂ + N₂ + H₂O is disclosed as a suitable atmosphere for decomposing the organic medium. Hotchkiss et al., Protective Furnace Atmospheres, Metals Handbook, 1984 Edition This publication contains a general discussion of protective firing atmospheres and their function in annealing ferrous metals. Partly burned fuel gas is disclosed which presumably is saturated with water of combustion but also contains large amounts of N₂ (68.5 mole %). Other protective atmospheres discussed are hydrogen, dissociated ammonia, completely burned fuel gas and reacted fuel gas. In its primary aspect the invention is directed to a process for firing an oxidizable base metal in the presence of an organic medium to effect volatilization of the organic medium without substantial oxidation of the metal. The process comprises heating the dispersion in an atmosphere consisting essentially of CO, CO₂ and from 0.1 to 90 mole % H₂O basis CO₂, to a temperature above the thermal decomposition temperature of the organic medium and below the melting point of the base metal between the metal/metal oxide and carbon/carbon oxides phase transition lines of the resultant mixture of the atmosphere with gaseous products from decomposition of the organic medium for a time sufficient to burn out essentially all of the organic medium. In a second aspect, the invention is directed to the above-described process in which the base metal is comprised of an admixture of first and second base metals having lower and higher metal/metal oxide phase transition lines respectively and the firing temperature is between the metal/metal oxide transition lines of the base metals by which the first metal undergoes no substantial oxidation and the second metal undergoes essentially complete oxidation. The Drawing consists of two figures of which both Figure 1 and Figure 2 are graphical representations of the phase stability area of metallic copper in an atmosphere of CO/CO₂ as a function of firing temperature and the ratio of the partial pressures of CO to CO₂. In Figure 1, the effect of H₂O addition is shown and in Figure 2, lines of constant O₂ partial pressure are superimposed. Detailed Description of the Invention Firing in O₂/N₂ as described above is a kinetically-controlled process. That is, conditions are chosen that maximize the rate of carbon oxidation and minimize rates of metal oxidation, diffusion, and precipitation. Another approach is to use a thermodynamically-controlled process in which undesirable reaction rates are set to zero by choosing conditions under which the changes in the free energies of the undesirable reactions are positive. A CO/CO₂ atmosphere functions thermodynamically by controlling oxygen partial pressure. With the appropriate CO/CO₂ ratio, organics can be removed from the metal and dielectric layers with no metal oxidation or dielectric discoloration. There is no theoretical limit to the number of layers that can be cofired. In practice, however, relatively long times are required to remove the organics using this firing atmosphere. Adding water to the atmosphere substantially alters the rate of organic removal without significantly changing the thermodynamic conditions preventing metal oxidation or dielectric discoloration. In short, while CO/CO₂ by itself is sufficient for cofiring ceramic multilayers the addition of water makes the process a practical one. The amount of water to be added to the CO/CO₂ atmosphere is not itself narrowly critical. As little as 0.1 mole % water is beneficial and as much as 90 mole % H₂O does not appear to be detrimental. However, no further benefit of H₂O addition is apparent beyond about 30 mole % H₂O addition. Moreover, H₂O additions above about 5% yield only slight increases in firing rates. Thus, H₂O additions of 0.1-5 mole % are preferred and, at least for copper, about 3% appears to be optimum. The firing atmospheres for use in the invention are conveniently prepared by admixing CO and CO₂ gases in the desired proportions and then bubbling the admixture of gases through water in order to humidify the gases to the desired level. When using this method to add water to the CO/CO₂ mixture, the temperature of the water can be raised or lowered to increase or reduce the amount of water addition. When the water is added at room temperature (about 20°C), the addition of water to the CO/CO₂ admixture is approximately 3 mole %. It will be recognized that the process of the invention is applicable to a wide variety of oxidizable base metal so long as their metal/metal oxide phase transition lines in a CO/CO₂ atmosphere lie below the carbon/carbon oxides phase transition line. Thus suitable metals include Cu, Ni and Mo, the last only above about 700°C. Also suitable are base metals comprised of an admixture of first and second base metals having lower and higher metal/metal oxide phase transition bases respectively and the firing temperature is between the metal/metal oxide transition lines of the base metals by which the first metal undergoes essentially complete oxidation. By way of example, the oxide of the second metal may be an alloy of first and second base metals. The oxide of the second metal may be a glass former. However oxidizable base metals such as Ti and W are not suitable for use in the invention process because their metal/metal oxide phase transition lines lie above the carbon/carbon oxides phase transition line of CO/CO₂ atmospheres and thus the CO/CO₂ atmosphere itself will precipitate carbon under these conditions. The process of the invention can be used to fire liquid dispersions, thick film pastes or solid films such as green tapes or dry films. Such compositions may include, for example, particles of glass frit in admixture with base metal particles. The glass may be comprised substantially of oxides which are nonreducible at the firing conditions. Also when firing sequentially the process can be used to prevent the oxidation of already formed oxidizable metal layers adjacent to layers of other electrically functional material dispersions in organic medium. Examples of suitable organic media include a solid matrix of organic polymer and a liquid which can form a dispersion of paste consistency with base metal particles. Furthermore, it will be recognized that the process can be used in the manufacture of multilayer electronic elements such as multilayer interconnects, whether they are fired sequentially, semisequentially or cofired. The terms "electric functionality" and "electrically functional" refer to materials--either inorganic or organic--which when appropriately processed have conductive, resistive, or dielective functionality. The oxidizable metal may also be contained in a multi-layer composite element comprising a plurality of layers of oxidizable base metal dispersion alternating with layers of finely divided particles of materials having other electric functionality dispersed in organic medium and the layers are cofired. The layer of the element may be supported on an inorganic dielectric substrate. Preferably, the substrate is alumina. It is further preferred that at least one layer of oxidizable metal dispersion is adjacent to a layer of finely divided particles of dielectric materials dispersed in organic medium and the layers are cofired. It may also be possible that at least one layer of oxidizable metal dispersion is sandwiched between layers of finely divided particles of dielectric material dispersed in organic medium and the layers are cofired. Furthermore, the layer of finely divided dielectric material dispersed in organic medium may be sandwiched between layers of oxidizable metal dispersion. In each case it is necessary that the firing temperature be above the decomposition temperature of the organic medium in which the base metal is dispersed, but below the melting point of the base metal. The volatilization process, that is, the process of removing organic media during firing occurs in three steps: first, evaporation of lower molecular components; second, nonoxidative pyrolysis of heavier materials; and third, oxidation of the remaining materials. Thus, for most organic media the firing temperature for the process must be at least about 350 to 400°C and can, of course, be much higher to obtain more rapid and complete volatilization. As used herein the term "volatilization" means substantially complete removal of the organic medium which would normally take place as mentioned above by a combination of evaporation, oxidation and pyrolysis depending, inter alia, on the composition of the organic medium in which the base metal is dispersed. On the other hand, the process of the invention must be carried out below the melting point of the base metal. Preferably the process is carried out at a temperature at least 50°C below the melting point of the base metal being fired. For example, in firing copper which has a melting point of 1083°C the firing temperature will normally fall within the range of 900-1025°C. One of the advantages of the CO/CO₂ firing atmosphere is that it is an equilibrium-related or "buffered" system in which the O₂ concentration is fixed easily by adjusting the CO/CO₂ ratio. Thus the oxidation reduction potential of the gas admixture is easily established by adjusting the CO/CO₂. (As used herein the term "oxidation/reduction potential" means the tendency of the atmosphere under defining conditions either to oxidize or reduce other materials.) In most instances it will be preferred to utilize a controlled atmosphere of CO/CO₂ and H₂O alone so that the volume of gases which must be passed through the firing furnace can be minimized. Nevertheless it is quite feasible to use inert gas diluents in the CO/CO₂ and H₂O atmosphere such as N₂ and argon. Such inert gases in some instances be used to minimize undesirable reactions in the system after the organic medium is completely removed. It will, of course, be realized by those skilled in the art that the oxidation/reduction potential of the CO/CO₂ atmosphere might also be expressed in terms of O₂ content. However, CO/CO₂ ratio is a more convenient expression for the oxidation/reduction potential since the term incorporates both of the process variables which are used directly to adjust the oxidation/reduction potential. An advantage of the invention process is that it utilizes a high partial pressure of CO₂ (relative to O₂) which is the source of the oxidizing species of the CO/CO₂ system. The significance of this is that the diffusion transfer rate of CO₂ into the substrate is quite high and therefore volatilization is more complete in a given period of time. Concomitantly the CO/CO₂ system also has a high concentration of CO, the reducing species, which gives a high diffusion transfer rate of CO and thus maintains the base metal in the nonoxidized (bright) state. Furthermore, because the oxidative reaction of CO₂ is endothermic, the system is very unlikely to incur localized runaway reactions which can cause hot spots which in turn lead to unwanted entrapment of organic materials and other unwanted effects such as melting the base metal. Turning now to Figure 1 of the drawing, it can be seen that the addition of H₂O to the CO/CO₂ atmosphere at any given temperature results in a slight lowering of the ratio of the partial pressure of CO to CO₂. Furthermore, the lower the temperature the greater this lowering effect becomes. However, the amount of additional O₂ added to the firing atmosphere by this addition is not significant which can be seen in the following examples which show that a higher level of H₂O addition and the concomitantly higher oxidation potential do not thermodynamically contribute to better burnout characteristics. Test Procedures-Ink Adsorption The Ink Adsorption Test is a qualitative test for estimating the degree of dielectric porosity, and thus provides a rough measure of the completeness of organic removal. The test is carried out as follows. The test piece is held horizontally. A drop of nonsmear rubber-stamp ink (Ranger Products Company, Red Bank, NJ) is placed on the dielectric surface and allowed to stand 30 seconds. The test part is then held vertically in a stream of cold tap water for 30 seconds, followed by a stream of acetone for 15 seconds, in the attempt to wash off the ink. Areas of ink retention which are traceable to distinct defects, such as blisters or dirt, should be ignored. If there is no visible ink retention on the dielectric surface, the dielectric is deemed nonporous. If there is visible ink retention on the dielectric surface, the dielectric is deemed porous. EXAMPLES In the following examples, the solid film of dielectric material dispersed in photosensitive organic medium used is described in Example 14 of US-A-4,613,560 (U.S Patent Application S.N. 687,460 filed December 28, 1984). Examples 1-7 A series of seven tests was performed to observe the relative efficacy of several controlled firing atmospheres under prolonged firing times which approached equilibrium conversion levels. In each test, a solid film of copper dispersed in a photosensitive organic polymer medium and a solid film of dielectric material dispersed in a photosensitive organic polymer medium were laminated to 2.5 x 2.5 cm Al₂O₃ substrate squares and exposed to harden the laminated film layer. Several of these elements were then fired in each of the controlled atmospheres. The firing profile for this series of tests was two hours from room temperature to 650°C, 16 hours at 650°C and then slow cooldown by turning the furnace off. The results of these tests are given in Table 1 below: The results in Table 1 show that high-purity N₂ by itself has insufficient O₂ to volatilize the organic medium adequately. On the other hand, the addition of a small amount of H₂O as an O₂ source gave adequate volatilization of the organic medium, but substantially completely oxidized the copper. The same result was obtained when only 100 ppm O₂ was added to the N₂. Likewise, CO₂ by itself gave thorough burnout (volatilization) of the medium, but over-oxidized the copper. This same effect can still be seen when 0.01% volume CO was added to the CO₂. This was not expected thermodynamically but is believed to have been caused by oxygen intrusion into the system. However, Examples 6 and 7 both show that when enough CO is added to the CO₂ the reduction potential of the gaseous mixture is sufficient completely to protect the copper from oxidation. In comparing Examples 6 and 7 there was no apparent difference in the effectiveness of CO/CO₂ and Co/CO₂/H₂O at equilibrium. This is of course because of the long firing time used in the three tests. However, as can be seen from Examples 8 and 9 below, the addition of water has a dramatic effect in speeding up the volatilization process without altering in any significant way the favorable thermodynamics of the underlying CO/CO₂ atmosphere. Examples 8 and 9 A further test was performed in the same manner as Examples 6 and 7 in which atmospheres of CO/CO₂ 1/99 and CO/CO₂/H₂O 1/96/3 (by volume) were used. The firing profile for these tests was one hour from room temperature to 650°C, 4 hours at 650°C and then slow cooldown by turning the furnace off. The copper surfaces in both tests were shiny. However, from the fact that the dielectric was dirty with the use of CO/CO₂ alone but clearly white with CO/CO₂/H₂O, it was apparent that complete burnout of organic medium was accomplished more rapidly by the addition of H₂O to the firing atmosphere. Examples 10-13 To observe the effect of various levels of H₂O addition on the volatilization of organic media, a series of tests was run in which H₂O addition was varied from 0 to 30 vol. %, with the same materials fired under otherwise identical conditions. In each test, a solid film of copper dispersed in a photosensitive organic medium and a solid film of dielectric material dispersed in a photosensitive organic polymer medium were laminated to 2.5 x 2.5 cm Al₂O₃ substrate squares and exposed to harden the laminated film layer. Several of these laminated elements were then fired in CO/CO₂ atmospheres containing various levels of H₂O using a firing profile of two hours from room temperature to 650°C and one hour dwell time at 650°C before turning off the furnace to effect a slow cooldown. the results of these tests are given in Table 2 below. These results show that at the short firing times used in this series of tests, the addition of 3% H₂O to the CO/CO₂ atmosphere gave substantially better burnout of organics from both the copper and the dielectric. The data also show that the addition of higher amounts of H₂O, while not deleterious, is nevertheless not required. Though the copper surfaces were not yet shiny at these conditions, it is evident that somewhat longer firing times would yield sufficiently better burnout to obtain a completely nonoxidized copper surface. Example 14 This example illustrates the use of the firing process of the invention in the manufacture of multilayer composite elements consisting of four copper layers alternating with three dielectric layers supported on an alumina substrate. The same conductor and dielectric materials were used as in the other Examples. A layer of copper film dispersion was laminated to the alumina substrate and a layer of the dielectric film was laminated to the copper film and the composite structure was fired in an atmosphere of CO/CO₂/H₂O 1/96/3. The firing profile was 5° per minute from room temperature to 650°C, about 16 hours at 650°C, 5° per minute up to 900°C and 10 minutes dwell time at 900°C before turning off the furnace to effect slow cooldown. This sequence of applying two functional layers and firing was repeated three times and then a final copper layer was added by the same method. Thus, the eight-layer (including the substrate) device was made in four firing steps. The finished device had no interlayer shorts or line breaks, had no detectable stain and was hermetic as measured by the Ink Adsorption Test described hereinabove. Example 15 This Example again illustrates the use of the firing process of the invention in the manufacture of multilayer composite elements as in Example 14. However, in this Example, the structure was made with only two firings. For the first firing, a composite supported on alumina was built up by lamination on the alumina substrate of two layers each of copper and dielectric (DE) films in the sequence Cu/DE/Cu/DE and firing the five layered laminate. For the second firing, three further layers were laminated atop the exposed dielectric layer of the first fired laminated structure in the sequence Cu/DE/Cu and the entire eight layer structure was cofired. Again, the finished device had no interlayer shorts, had no interlayer stain and was hermetic as measured by the Ink Adsorption Test. Examples 14 and 15 along with the other examples show clearly that the controlled atmospheric firing process of the invention can be used both for sequential and cofiring of multicomponent electrically functional structures.

Claims: A process for firing an oxidizable base metal in the presence of an organic medium to effect volatilization of the organic medium without substantial oxidization of the metal comprising heating the dispersion of the metal in the organic medium in an atmosphere consisting essentially of CO, CO₂ and from 0.1 to 90 mole % H₂O basis CO₂, to a temperature above the thermal decomposition temperature of the organic medium and below the melting point of the base metal between the metal/metal oxide and carbon/carbon oxides phase transition lines of the resultant mixture of the atmosphere with gaseous products from decomposition of the organic medium for a time sufficient to volatilize the organic medium therefrom. The process of claim 1 in which finely divided particles of the oxidizable base metal are dispersed in organic medium. The process of claim 1 in which the base metal is Cu. The process of claim 2 in which the organic medium is a solid matrix of organic polymer. The process of claim 2 in which the organic medium is a liquid and the dispersion is of paste consistency. The process of claim 2 in which the base metal is comprised of first and second base metals having lower and higher metal/metal oxide phase transition lines respectively and the firing temperature is between the metal/metal oxide transition lines of the base metals by which the first metal undergoes no substantial oxidation and the second metal undergoes essentially complete oxidation. The process of claim 6 in which the oxide of the second metal is a glass former. The process of claim 2 in which the particles of oxidizable base metal are in admixture with particles of glass frit. The process of claim 8 in which the glass is comprised substantially of oxides which are nonreducible at the firing conditions. The process of claim 6 in which the base metal is an admixture of particles of first and second base metals. The process of claim 6 in which the base metal is an alloy of first and second base metals. The process of claim 1 in which the oxidizable base metal is in the form of a previously fired layer adjacent to a layer of finely divided particle of electrically functional inorganic materials dispersed in organic medium. The process of claim 2 in which the oxidizable metal is contained in a multilayer composite element comprising a plurality of layers of oxidizable base metal dispersion alternating with layers of finely divided particles of materials having other electric functionality dispersed in organic medium and the layers are cofired. The process of claim 13 in which a layer of the element is supported on an inorganic dielectric substrate. The process of claim 14 in which the substrate is alumina. The process of claim 13 in which at least one layer of oxidizable metal dispersion is adjacent to a layer of finely divided particles of dielectric materials dispersed in organic medium and the layers are cofired. The process of claim 13 in which at least one layer of oxidizable metal dispersion is sandwiched between layers of finely divided particles of dielectric material dispersed in organic medium and the layers are cofired. The process of claim 13 in which a layer of finely divided dielectric material dispersed in organic medium is sandwiched between layers of oxidizable metal dispersion.

Inventors: MCEWEN, CRAIG STEVEN, SCRIVNER, NOEL CHARLES
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1987
Main classification: C04B41
IPC codes: C04B41, H05K1, C04B35, B22F3, H05K3, H01L21