Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Liquid phase sintering
Patent
1994-11-07
1996-07-30
Jordan, Charles T.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Liquid phase sintering
419 5, 419 57, 428551, 501 96, C04B 3558
Patent
active
055408842
DESCRIPTION:
BRIEF SUMMARY
The present invention generally concerns preparing co-fired, multilayer substrates with little or no discernable warpage. The present invention more particularly concerns preparing such structures by liquid phase sintering. The present invention still more particularly concerns preparing co-fired, multilayer aluminum nitride (AlN) substrates wherein at least a portion of the layers have conductive, refractory metal paste patterns deposited thereon.
Yasuhiro Kurokawa et al., in "Characterization of the AlN--W Interface in a Cofired Multilayer AlN Substrate", Journal of the American Ceramic Society, 72[4] pages 612-16 (1989), characterize the structure and chemistry of the AlN--W interface. They laminate tape-cast AlN green sheets screen-printed with tungsten thick-film paste to prepare a multilayer structure. The AlN sheets contain two weight percent calcium carbide (C.sub.a C.sub.2) as a sintering aid. The tungsten paste contains 3 weight percent (wt. % AlN powder to make the fired shrinkage of the tungsten close to that of the AlN. They fire the laminate at 1900.degree. C. under 0.1 megapascal (MPa) nitrogen gas for 4 hours (hrs). The sintering conditions employed by Kurokawa et al. approach those used in solid phase sintering.
European Patent Application 329,973 describes a number of problems encountered in attempting to produce electronic substrates by pressureless sintering. One problem is uneven sintering due to inhomogeneities in green ceramic sheets or uneven heating thereof. Uneven sintering can, in turn, lead to warping in the fired product. A second problem is that AlN is difficult to sinter. This can lead to insufficiently high sintered densities. A third problem is an inability to control lateral shrinkage during sintering of both the metal and ceramic layers. This can result in substantial residual stresses between layers, warpage or delamination of the substrate. and failure to meet required dimensional tolerances.
European Patent Application 329,973 proposes to solve the aforementioned problems by hot pressing ceramic green sheets containing ceramic powder and organic binders that leave no undesirable residue upon pyrolysis in the absence of oxygen. Boron nitride (BN) sheets made of BN powder in a similar binder are placed on each side of the ceramic green sheet to form a composite. The BN sheets are subsequently removed. Multilayer ceramic electronic substrates with internal interconnections between metal circuits deposited on various green sheets by conventional methods, such as screen printing, may also be produced by hot pressing.
One aspect of the present invention is a method of preparing a cofired, multilayer aluminum nitride substrate that is substantially free of warpage or delamination, the method comprising:
a. preparing at least two ceramic green sheets from a composition containing aluminum nitride, a combination of at least one first liquid phase sintering aid and at least one second liquid phase sintering aid, and, optionally, aluminum oxide (Al.sub.2 O.sub.3), the first and second sintering aids being selected from subgroups 1) oxygen-containing alkaline earth metal compounds, 2) oxygen-containing derivatives of lanthanide series elements, 3) oxygen-containing derivatives of yttrium, 4) alkaline earth metal halides, 5) halogen-containing derivatives of lanthanide series elements and 6) yttrium halides, provided, however, that the first and second sintering aids are selected from different subgroups, the combination of sintering aids being present in an amount sufficient to initiate liquid phase sintering of aluminum nitride at a temperature within a range of from 1400.degree. C. to 1600.degree. C. and substantially complete said liquid phase sintering at a temperature within a range of from 1600.degree. C. to 1900.degree. C.;
b. depositing a desired pattern of conductive metal paste on at least one major planar surface of at least one ceramic green sheet, the metal paste containing a refractory metal and at least one sintering additive selected from the group consisting of ni
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Chem Abstracts, vol 115, No 20.
Jenkins Daniel J.
Jordan Charles T.
The Dow Chemical Company
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