Process of producing aluminum nitride multiple-layer circuit board

Abstract: A process of producing a multiple-layer circuit board of aluminum nitride, comprising the steps of: preparing green sheets of aluminum nitride, forming on the green sheets conductor patterns of a conductor paste containing tungsten as a main conductor component, laminating the green sheets with the conductor patterns formed thereon to form a lamination, and firing the lamination in a container made of boron nitride and in a pressurized nitrogen gas atmosphere.

Description: The present invention relates to a process of producing a multiple-layer circuit board of aluminum nitride. Aluminum nitride, AlN, has a high thermal conductivity and a thermal expansion coefficient comparable with that of silicon used in semiconductor devices, and therefore, is an advantageous replacement for alumina (Al 2 O 2 ) as a useful material of packages and circuit boards, particularly a circuit board on which silicon semiconductor devices are mounted. The circuit is formed of tungsten (W), which can be suitably fired together with aluminum nitride. A multiple-layer aluminum nitride circuit board having a circuit pattern of tungsten is produced by firing, usually in the following known sequence. An aluminum nitride powder, a sintering additive, a solvent, and an organic binder are mixed by milling to form a slurry, which is then subjected to a doctor blade process to form a green sheet. A plurality of green sheets are prepared in the same manner. Via-contact holes are formed in several green sheets and filled with a conductor paste or a tungsten paste, and circuit patterns of a conductor paste (tungsten paste) are formed on the remaining green sheets by screen printing. It is possible for via-contact holes to be formed in one green sheet and filled with a conductor paste, and then for circuit patterns to be screen printed on the green sheet. A predetermined number of green sheets are then laminated and pressurized to form an integral laminate. This green sheet laminate is dried, degreased by heating (or the binder is removed), and fired to form a multiple-layer aluminum nitride circuit board. When firing is carried out, special attention must be paid to the material of the container coexisting in the firing atmosphere, particularly the container accommodating the green sheet laminate, because of the following points. Because aluminum nitride is fired at a high temperature of 1600°C or higher, as disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 60-239366, 2-212365 and 2-243570, for example, the firing container containing the green sheet laminate during firing and the load setter must be made of a high heat-resistant material. Practically available materials having such a high heat-resistance include boron nitride (BN) and graphite (C). Graphite is less expensive and is preferable from the viewpoint of production cost, but generates a carbonaceous atmosphere during firing which thereby decomposes aluminum nitride and causes sublimation thereof. To eliminate this problem, the inner surface of a graphite container can be coated with a boron nitride powder, and the green sheet laminate packed together with aluminum nitride powder surrounding the laminate in the graphite container, as proposed in Japanese Unexamined Patent Publication (Kokai) No. 2-34568. This method, however, has a disadvantage in that the placing of the green sheet laminate in a graphite container occasionally causes non-uniform firing of the product aluminum nitride board, the setting of the green sheet laminate for firing requires a long time, and the aluminum nitride powder used for packing incurs an additional cost. Boron nitride (BN) can be advantageously used to eliminate these disadvantages of graphite, but when used as a firing container, causes a problem in that BN diffuses into W of the circuit conductor and forms tungsten boride (W 2 B), which increases the resistivity of the circuit conductor (Proceeding of the Ceramic Society of Japan, 1989, Autumn, p.82). One object of the present invention is to eliminate the above-mentioned problem of the conventional methods and provide a process of producing a multiple-layer aluminum nitride circuit board by using a firing container of boron nitride without causing an increase in the resistivity of a tungsten conductor. Also in the prior art, JP-A-3065566, JP-A-61084036, US-A-4608354 and EP-A-0276788 disclose processes for producing substrates having wiring patterns thereon. According to the present invention, there is provided a process of producing a multiple-layer circuit board of aluminum nitride, comprising the steps of: preparing green sheets of aluminum nitride; forming on said green sheets conductor patterns of a conductor paste containing tungsten as a main conductor component; laminating said green-sheets with said conductor patterns formed thereon to form a laminate; and primarily firing said laminate, a container made of boron nitride at a temperature of at least 1600°C and in a nitrogen gas atmosphere, characterised in that said nitrogen gas atmosphere is a pressurised nitrogen gas atmosphere, and after said primarily firing said laminate is subjected to a secondary firing at a temperature lower than 1600°C and in a non-oxidising gas atmosphere not containing carbon, without using said container. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a graph showing the relationship between the firing temperature and the sintered density of a fired AlN circuit board; Fig. 2 is a graph showing the relationship between the firing temperature and the thermal conductivity of a fired AlN circuit board; Fig. 3 is a graph showing the sintered density and the thermal conductivity of a fired AlN circuit board as a function of the firing time at a firing temperature of 1800°C; Fig. 4 is a graph showing the relationship between the pressure of the pressurized nitrogen gas atmosphere used for firing and the volume resistivity of the product circuit conductor of W; Fig. 5 is a graph showing the relationship between the amount of the AlN powder added and the shrinkage factor of the W conductor; Fig. 6 is a graph showing the relationship between the amount of the AlN powder added and the volume resistivity of the W conductor; Fig. 7 is a graph showing X-ray diffraction patterns of the surface of the W conductors obtained by conventional firing carried out under the normal pressure of 1 atm and firing carried out under a raised pressure of 3 atm; Fig. 8 is a graph showing X-ray diffraction patterns of the surface of the W conductors formed by primary and secondary-firing steps, in which the secondary firing atmosphere does not contain carbon; and Fig. 9 is a graph showing X-ray diffraction patterns of the surface of the W conductors formed by primary and secondary-firing steps, in which the secondary firing atmosphere contains carbon. As mentioned above, an increase in the resistivity of a W conductor can be caused by BN being released from a BN container during firing and reacting with the conductor W to form W 2 B in the conductor. After a primary high temperature firing carried out at a temperature of 1600°C or higher in a BN container, a secondary low temperature firing is carried out without using a BN container at a temperature lower than 1600°C in a non-oxidizing gas atmosphere not containing carbon, to decompose any W 2 B formed during the primary high temperature firing to return to W, and thereby make the resistivity of the W conductor lower than that created by the primary high temperature firing. A firing atmosphere not containing carbon is used in order to prevent a reaction of the conductor W with atmospheric carbon and the resulting formation of W 2 C having a high resistivity, and to prevent oxidization of AlN and the resulting formation of Al 2 O 3 . Such non-oxidizing gas atmospheres not containing carbon include hydrogen, nitrogen, argon, and the like. The present invention uses a firing atmosphere of a pressurized nitrogen gas to suppress the release of BN from the BN container, and in turn the reaction between BN and W, and thereby prevents the increase in the resistivity of the W conductor. When firing is carried out in a pressurized nitrogen gas atmosphere of 3 atm (1 atm = 1.013 × 10 5 Pa) or more, the increase in the resistivity of the W conductor is substantially completely prevented. The upper limit of the pressure of the pressurized nitrogen gas atmosphere need not be specified, but a nitrogen gas pressure higher than 10 atm does not provide a further advancement in the prevention of the increase in the conductor resistivity and only raises the production cost. Accordingly, the nitrogen gas pressure is suitably limited to not higher than 10 atm, in practice. According to one embodiment of the present invention, care is taken to prevent further the distortion of the aluminum nitride circuit board and the exfoliation of the W conductor, which unavoidably occur in the conventional firing process. In a conventional process, a green sheet laminate is loaded with a weight during the firing thereof to prevent distortion of the fired board that occurs upon the removal of a sintering additive and an impurity oxygen from a fired or sintered product, as proposed in Japanese Unexamined Patent Publication (Kokai) No. 2-129076. This method, however, causes a problem that the patterned conductor of a green sheet laminate is transferred to a load setter for loading the weight, and that the factor of shrinkage of the fired board undesirably varies between different positions of the same board. The conventional process also has a problem that the AlN substrate and the W circuit conductor formed thereon have different shrinkage factors during firing, and therefore, have a poor bonding therebetween. This frequently causes an exfoliation of the W conductor from the AlN substrate. According to this embodiment of the invention, a conductor paste having a conductor component of W and supplemented with an AlN powder in an amount of 10 wt% or less can be advantageously used to solve these problems. In particular, the distortion of AlN board and the exfoliation of W conductor due to the difference between the shrinkage factors of the AlN board and the W conductor can be prevented. When the distortion of an AlN board must be primarily prevented, the AlN powder is preferably present in the conductor paste in an amount of 3 wt% or more. To ensure a high thermal conductivity of the fired AlN board, the AlN green sheet having a conductor pattern of a conductor paste formed thereon should be fired at a high temperature of 1600°C or higher. Usually, this firing is sufficiently effective when carried out for about 2 hours. Reference will now be made to the following Examples in which Example 4 describes one embodiment of the present invention and Examples 1-3 are illustrative of preferred features of the present invention. Example 1 An AlN powder containing 1% impurity oxygen was blended with 5 wt% of a Y 2 O 3 powder as a sintering additive, a solvent (MEK), and an organic binder (PMMA) and was milled in a ball mill to form a slurry, which was then spread by a doctor blade process to form a 300 µm thick, 90 by 90 mm, square AlN green sheet. A W powder was blended with a solvent (MEK, terpineol) and an organic binder (PMMA) and milled in a ball mill to form a slurry, which was then agitated in an automated mortar to scatter the MEK in the air and roll-milled to form a W paste. The W paste was screen-printed on the AlN green sheet to form a conductor pattern of the W paste having a width of 200 µm, a length of 40 mm, and a height of 25 µm. Eight pieces of the green sheet were laminated at a temperature of 60°C and a pressure of 50 MPa to form an 8-layer laminate. After being dried, the laminates were degreased in a nitrogen gas flow at 600°C for 4 hours to remove the organic binder, placed in a BN container, and fired at a temperature of 1700°C in pressurized nitrogen gas atmospheres of 1 to 10 atm for 9 hours to form multiple-layer AlN circuit boards. The surface of the W conductor of the multiple-layer AlN circuit board was subjected to a measurement of the resistivity and an X-ray diffraction analysis, the results of which are summarized in Table 1. It is also possible for AlN green sheets to be drilled to form throughholes, a W paste filled in the throughholes to form via-contacts, a W paste screen-printed on the AlN green sheets to form conductor patterns, and the AlN green sheets having the thus-formed via-contacts and conductor patterns being laminated, degreased and fired to form a multiple-layer AlN circuit board. Firing pressure (atm) Sheet resistivity (mΩ/square) Surface product 1 25 (63.5) W2B 2 15 (38.1) W2B + W 3 10 (25.4) W + W2B 5 10 (25.4) W + W2B 7 9 (22.8) W + W2B 10 8 (20.3) W Note) W2B: substantially W 2 B only detected. W2B + W: W 2 B and small amount of W detected. W + W2B: W and small amount of W 2 B detected. W: substantially W only detected. The sheet resistivities are expressed in terms of the converted values for a sheet thickness of 1 mil (25.4 µm). The parenthesized values are the volume resistivities, in µΩ-cm, converted from the sheet resistivities. It can be seen from Table 1 that the pressurized nitrogen gas atmosphere, particularly when pressurized to 3 atm or more, effectively suppresses the formation of W 2 B in the W conductor, and therefore, suppresses the increase in the resistivity of the W conductor. Example 2 An A1N powder containing 1% impurity oxygen was blended with 5 wt% of a Y 2 O 3 powder as a sintering additive, a solvent (MEK), and an organic binder (PMMA) and was milled in a ball mill to form a slurry, which was then spread by a doctor blade process to form a 300 µm thick, 90 by 90 mm, square A1N green sheet. An A1N powder was added to a W powder at various amounts up to 10 wt%. The W powder was blended with a solvent (MEK, terpineol) and an organic binder (PMMA) and milled in a ball mill to form a slurry, which was then agitated in an automated mortar to scatter the MEK in the air and roll-milled to form a W paste. A W paste without an addition of A1N powder was prepared for comparison. The W paste was screen-printed on the A1N green sheet to form a conductor pattern of W having a width of 200 µm, a length of 40 mm, and a height of 25 µm, to form a "conductor pattern sheet". Another A1N green sheet was drilled to form throughholes having a diameter of 200 µm and the W paste was filled in the throughholes, to form a "via-contact sheet". The conductor pattern sheets and the via-contact sheets were alternately laminated to form a 6-layer laminate. After being dried, the laminates were degreased in a nitrogen gas flow at 600°C for 4 hours to remove the organic binder, placed in a BN container, and fired at various temperatures of from 1400 to 1900°C in pressurized nitrogen gas atmospheres of 1 to 9 atm to form multiple-layer A1N circuit boards. The surface of the W conductor of the multiple-layer A1N circuit board was subjected to a measurement of the resistivity and an X-ray diffraction analysis. A non-laminated or single A1N green sheets were fired under the same condition as the above and used for the measurement of the sintered density and the thermal conductivity of the fired product board. The results obtained from these measurements are described in detail below. Firing Temperature and Time Figure 1 shows the relationship between the firing temperature and the sintered density of the fired A1N board, and Fig. 2 shows the relationship between the firing temperature and the thermal conductivity of the fired A1N board. It can be seen from Fig. 1 that, to obtain a sintered density comparable with the theoretical density, the firing temperature must not be lower than 1600°C and that a firing temperature of 1650°C or higher provides a sintered density of about 100% of the theoretical density. It can be seen from Fig. 2 that a firing temperature of 1600°C or higher also provides a high thermal conductivity of 180 W/mk or higher. Figure 3 shows the sintered density and the thermal conductivity of a fired A1N board as a function of the firing time at a firing temperature of 1800°C. It can be seen from Fig. 3 that, to obtain a high density and thermal conductivity, the firing is carried out for preferably 2 hours or more, more preferably 3 hours or more. Firing Pressure and A1N Amount in W Paste Figure 4 shows the relationship between the pressure of the pressurized nitrogen gas atmosphere used in firing and the volume resistivity of the obtained W conductor. The resistivity exhibits a clear decrease even under a pressurization of 1 atm and a pressurization of 3 atm or more stably ensures a low resistivity. It also can be seen from Fig. 4 that the greater the amount of A1N powder added, the higher the resistivity, and therefore, the amount of A1N powder added should be a necessary minimum when as low as possible a resistivity is required. Figure 5 shows the relationship between the amount of A1N added and the shrinkage factor of the W conductor. The shrinkage factor of the W conductor rapidly approaches the shrinkage factor of the substrate A1N at a small amount of A1N added in the range of up to 3 wt%, the rate of approach slows when the amount of A1N added exceeds 3 wt%, and in the range of 10 wt% or more, the shrinkage factor of the W conductor becomes substantially equal to that of the A1N substrate. The shrinkage factor of the W conductor is desirably as close as possible to that of the substrate A1N, to prevent the thermal distortion of the A1N substrate and the exfoliation of the W conductor. An amount of A1N added of more than 10 wt% does not provide a further approach of the two shrinkage factors. The amount of AlN added should not be more than 10 wt% because the amount of A1N added is desirably as small as possible, to ensure as low as possible a resistivity of the W conductor. Figure 6 shows the relationship between the amount of A1N added and the resistivity of the W conductor. The resistivity of the W conductor is substantially unchanged and kept at a low value when an A1N powder is added in an amount up to 3 wt%, and is increased when the amount of A1N added is more than 3 wt%. Figure 7 shows X-ray diffraction patterns of the surface of the W conductors obtained by a conventional process in which firing was carried out in a nitrogen gas atmosphere under a normal pressure of 1 atm, and by a process in which firing was carried out in a nitrogen gas atmosphere under a pressurization of 3 atm, respectively. The W conductor obtained by the conventional normal pressure firing exhibits a large diffraction peak from W 2 B, whereas the W conductor obtained by the pressurized firing only exhibits a minute diffraction peak from W 2 B, i.e., the formation of W 2 B is effectively suppressed by the present inventive process. Example 3 A1N green sheets and a W paste were prepared in the same manner as in Example 2, except that the amount of A1N added in the W paste was varied in the range of from 0.5, 3, 10 to 20 wt% and a W paste without an addition of A1N was also prepared, for comparison. In screen-printing of the W paste on the A1N green sheets, a circuit pattern was printed on an A1N green sheet to provide the second layer of a green sheet lamination, and a blank pattern was printed on a green sheet to provide the fourth layer of the laminate The thus-prepared green sheets were laminated to form a 8-layer laminate. The laminate was fired in a pressurized nitrogen gas flow at a pressure of 3 atm and a temperature of 1700°C for 9 hours. The fired product of multiple-layer A1N circuit board was subjected to measurements of the board distortion, the adhesive strength of conductor, and the resistivity of the conductor of an inside layer. The results are summarized in Table 2. Amount of A1N added (wt%) Not added 0.5 3 10 20 Distortion of circuit board B B A A A Adhesive strength C B A AA AA Volume resistivity (µΩ-cm) 8.5 8.5 10 16 38 Note) Distortion (µm/100 mm) B: 5-10 A: 0-5 Adhesive strength (kg/cm 2 ) C: 0-1 B: 1-2 A: 2-2.5 AA: 2.5 or more It can be seen from Table 2 that the board distortion is obviously suppressed when A1N is added in an amount of 3 wt% or more, the adhesive strength of the W conductor is improved at a smaller amount of A1N added of 0.5 wt%, and the conductor resistivity of an inside layer is gradually increased when A1N is added in an amount up to 10 wt% and rapidly increased when the amount of A1N added exceeds 10 wt%. To consider all of these situations, a W paste preferably contains an A1N powder in an amount of up to 10 wt%. The preferred features described in Examples 2 and 3 not only prevent the increase in the resistivity of the W conductor due to the formation of W 2 B when a firing container of BN is used, but also avoid the transfer of the conductor pattern from an A1N sheet to a load setter and the fluctuation of the shrinkage factor of an A1N board depending on a loaded weight. Example 4 An A1N powder containing 1% impurity oxygen was blended with 5 wt% of a Y 2 O 3 powder as a sintering additive, a solvent (MEK), and an organic binder (PMMA) and was milled in a ball mill to form a slurry, which was then spread by a doctor blade process to form a 300 µm thick, 90 by 90 mm, square A1N green sheet. An A1N powder was added to a W powder in various amounts up to 10 wt%. The W powder was blended with a solvent (MEK, terpineol) and an organic binder (PMMA) and milled in a ball mill to form a slurry, which was then agitated in an automated mortar to scatter the MEK in the air and roll-milled to form a W paste. A W paste without an addition of A1N powder was prepared for comparison. The W paste was screen-printed on the A1N green sheet to form a conductor pattern of W having a width of 200 µm, a length of 40 mm, and a height of 25 µm, to form a "conductor pattern sheet". Another A1N green sheet was drilled to form throughholes having a diameter of 200 µm and the W paste was filled in the throughholes, to form a "via-contact sheet". The conductor pattern sheets and the via-contact sheets were alternately laminated to form an 8-layer laminate. After being dried, the laminates were degreased in a nitrogen gas flow at 600°C for 4 hours, to remove the organic binder, placed in a BN container, loaded by a load setter of BN, and fired at a temperature of 1800°C for 3 hours in a pressurized nitrogen gas atmosphere of 1 atm. This firing is referred to as a primary or high temperature firing. The laminates were further fired at 1400°C for 1 hour to form multiple-layer A1N circuit boards. This firing is referred to as a secondary or low temperature firing. The secondary firing was carried out at a pressure of 1 atm in an atmosphere of hydrogen, nitrogen or argon gases not containing carbon. A secondary firing was also carried out in a nitrogen gas atmosphere containing carbon, for comparison. An X-ray diffraction analysis was carried out for the surface of the W conductor formed on the surface of the product A1N boards. Figures 8 and 9 show the X-ray diffraction patterns of the W conductors in the primary- and secondary-fired states for atmospheres containing and not containing carbon. Table 3 summarizes the resistivities of the W conductor in the primary- and secondary-fired states and the percent reduction of the resistivity from the primary- to the secondary-fired states. Secondary firing atmosphere Sheet resistivity Carbon Atmosphere gas Primary-fired (µΩ/square) Secondary-fired (µΩ/square) Reduction (%) Not contained Hydrogen 35.3 (53) 8.0 (12) 77 Nitrogen 34.6 (52) 8.6 (13) 75 Argon 38.6 (58) 9.3 (14) 76 Contained Nitrogen 36.7 (55) 31.3 (47) 15 Note: The parenthesized values of the resistivity refer to the corresponding volume resistivity in mΩ-cm. It can be seen from Fig. 8 and Table 3 that, when the secondary firing is carried out in a firing atmosphere of a non-oxidizing gas (hydrogen, nitrogen, or argon) not containing carbon, the W 2 B formed during the primary firing is thermally decomposed during the secondary firing to form a conductor substantially of W only, providing a further decreased resistivity of the W conductor. On the other hand, Fig. 9 and Table 3 show that, when the secondary firing is carried out in a firing atmosphere of a non-oxidizing gas (nitrogen, in this case) containing carbon, W 2 C is formed on the surface of the W conductor, and thus the resistivity decrease due to the decomposition of W 2 B is not sufficiently obtained. According to the process described in Example 4, the board distortion and the conductor exfoliation is prevented by the AlN addition to the W conductor, and simultaneously, the W 2 B formed during the primary firing is thermally decomposed during the secondary firing to further reduce the resistivity of the W conductor. As described above, the present invention provides a process for producing a multiple-layer aluminum nitride circuit board by preventing the resistivity increase of the W conductor that would otherwise occur when a firing container of boron nitride is used. A secondary, low temperature firing whereby the W 2 B formed during a primary, high temperature firing is thermally decomposed to promotes the decrease of the conductor resistivity. A further advantage is additionally provided by the addition of aluminum nitride in a tungsten paste, whereby both the thermal distortion of the aluminum nitride board and the exfoliation of the tungsten conductor are effectively prevented without using the conventional loading of weight.

Claims: A process of producing a multiple-layer circuit board of aluminum nitride, comprising the steps of: preparing green sheets of aluminum nitride; forming on said green sheets conductor patterns of a conductor paste containing tungsten as a main conductor component; laminating said green sheets with said conductor patterns formed thereon to form a laminate; and primarily firing said laminate in a container made of boron nitride at a temperature of at least 1600°C and in a nitrogen gas atmosphere, characterised in that said nitrogen gas atmosphere is a pressurised nitrogen gas atmosphere, and after said primarily firing said laminate is subjected to a secondary firing at a temperature lower than 1600°C and in a non-oxidising gas atmosphere not containing carbon without using said container. A process as claimed in claim 1, characterised in that nitrogen gas atmosphere is pressurized to a pressure of 3.039 × 10 5 Pa (3 atm)or more. A process as claimed in claim 1 or claim 2, characterised in that said conductor paste contains a powder of aluminum nitride in an amount of 10 wt% or less. A process as claimed in claim 3, characterised in that said conductor paste contains a powder of aluminum nitride in an amount of 3 wt% or more.

Inventors: KAMEHARA NOBUO, MAKIHARA HIROSHI, OMOTE KOJI, TSUKADA MINEHARU, KAMEHARA, NOBUO, MAKIHARA, HIROSHI, OMOTE, KOJI, TSUKADA, MINEHARU, KAMEHARA, NOBUO, FUJITSU LIMITED, MAKIHARA, HIROSHI, FUJITSU LIMITED, OMOTE, KOJI, FUJITSU LIMITED, TSUKADA, MINEHARU, FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H01L21
IPC codes: H01L21, C04B35, H01L23, H05K1, C04B41