DEVITRIFYING GLASS FORMULATIONS FOR LOW EXPANSION PRINTED-CIRCUIT SUBSTRATES AND INKS

Abstract: Low-expansion devitrifying glass formulations useful in single layer or multilayer substrates for the fabrication of printed circuit boards that exhibit temperature coefficients of expansion that match that of large silicon or gallium arsenide chips. Such devitrifying glass formulations are also useful for overglaze, dielectric, copper via-fill and inner-layer thick film inks compatible with such substrates.

Description: DEVITRIFYING GLASS FORMULATIONS FOR LOW EXPANSION PRINTED-CIRCUIT SUBSTRATES AND INKS This invention relates to novel devitrifying glasses and to substrates and inks made therefrom. BACKGROUND OF THE INVENTION This is a continuation in part of copending application Serial No. 07/299,104 filed 1/19/1989. In the past, we successfully formulated novel devitrifying glasses which have temperature coefficients of expansion very closely matched to that of silicon and which are suitable for use as substrate material for direct mounting of very large silicon chips. The substrates are made by mixing the devitrifying glass with a ceramic filler. The mixture is then pressed and sintered into a fused body. The use of large silicon chips permits high density packaging of circuits, subassemblies and the like with the fewest number of processing steps. However, in operation, heat is generated in the chip, and thus large chips cannot be successfully mounted on a substrate unless the temperature coefficient of expansion of the chip material closely matches that of the substrate material. " Further, the devitrifying glasses which we formulated in the past in addition to having temperature coefficients of expansion essentially the same as that of silicon, also have a high flow temperature (i.e., from 800°-900°C) during initial firing and possess excellent reheat stability up to about 1000°C after devitrification. Specifically, the devitrifying glasses which were formulated in the past are comprised of zinc-barium-aluminum-silicate glasses that also contain minor amounts of phosphorous pentoxide, zirconium silicate and a member of the group of calcium oxide, magnesium oxide or mixtures thereof. Thus, the devitrifying glasses that we formulated in the past were made up of a mixture of seven or eight different materials. These different materials were mixed, on a weight basis, in the proper proportions. SUMMARY OF THE INVENTION The present invention is directed to new formulations for a devitrifying glass having a temperature coefficient of expansion which closely matches that of silicon, and which requires only five materials mixed in the proper proportions. Moreover the thermal coefficient of expansion of a substrate comprised of this new devitrifying glass may be adjusted by the addition of an appropriate dielectric filler material to match that of gallium arsenide as well as silicon. Further, this devitrifying glass is particularly suitable for use as a constituent of various inks including (1) an overglaze ink coating compatible with copper electrodes, (2) a hermetic dielectric ink compatible with copper electrodes, (3) a copper via-fill ink for copper-dielectric multilayers, and (4) an inner layer copper ink for copper-dielectric multilayers. The relatively high softening temperature of the subject glasses means their inks can be refired in making subsequent layers without undue adverse effects on previous layers. PREFERRED EMBODIMENTS OF THE PRESENT INVENTION The devitrifying glass formulations of the present invention comprise, on a weight basis: from about 26 to about 33 percent of zinc oxide (ZnO); from about 4 to about 8 percent of magnesium oxide (MgO); from about 2 to about 6 percent of barium oxide (BaO); or from about 2 to about 7 percent of strontium oxide (SrO); from about 12 to about 18 percent of aluminum oxide (A1 2 0 3 ); and from about 40 to about 50 percent of silicon oxide (Siθ2). In order to form the present devitrifying glasses, the above ingredients are mixed, dried and melted in an inert, e.g., platiunum, vessel at a temperature of about 1600°-1650°C . The melt is then quenched on counter rotating rollers to form vitreous glass flakes. These flakes are then made into a fine powder, having a particle size of about 1-5 micrometers, by a combination of dry and wet grinding in known manner. Compositions containing a devitrifying glass of the present invention together with suitable ceramic fillers known in the art • may be fabricated directly into a fused body for use as a substrate for semiconductor chip fabrication by pressing and sintering. The combination of devitrifying glass, suitably having a particle size of about 3-8 micrometers and ceramic filler, such as alumina, are blended with a suitable organic binder, solvent, surfactant and plasticizer, and tape cast and sintered to form a substrate. The ceramic filler aluminum oxide powder useful herein has a mean particle size of 0.62 micrometer. A commercially available useful product is AKP-20 from Sumitomo Chemical Company, Ltd. of Japan. By employing celesian, forsterite or alpha quartz as suitable fillers in the formulation of the aforesaid substrate, the thermal coefficient of expansion can be increased to match that of gallium arsenide. Similarly, the thermal coefficient of expansion can be decreased by adding willemite, cordierite or silicon dioxide as fillers in the fabrication of the substrate. The devitrifying glass formulations of the present invention, like other devitrifying glass formulations known in the art, are superior to co-fired substrates based on alumina ceramics for several reasons. First, alumina ceramics require very high processing temperatures (1400° - 1 550° C), thus making it necessary to utilize more refractory metallizations, such as tungsten and molybdenum in making electrical contacts. In contradistinction, substrates comprised of devitrifying glass based dielectrics can be processed at relatively low temperatures (900°C), permitting more conductive metals (copper, silver, etc.) to be used as conductors. Secondly, alumina ceramics have thermal expansion coefficients higher than that of silicon or gallium arsenide, and, hence, are not suitable for large area chip bonding. The devitrifying glass of the present invention is also suitable for use in preparing various types of inks employed in the fabrication of printed circuits by adding the glass particles to suitable metal powders and adding a suitable organic vehicle. Such printing inks can then be used for forming multilayer circuit boards on aluminum nitride or silicon carbide. Suitable metals for use as inks can be copper, for firing in nitrogen, or silver, silver-platinum alloy, gold or gold-platinum alloy, which can be fired in air. The organic vehicles are solutions of resin binders such as, for example, cellulose derivatives, particularly ethyl cellulose commercially available as ET-100 from Hercules, Inc.,. synthetic resins such as poly aery lates, polymethacrylates, polyesters, polyolefins and the like in a suitable solvent. A preferred binder is poly(isobutylmethacrylate). In general, conventional solvents utilized in inks of the type described herein may be used. Preferred commercially available solvents include, for example, pine oil, terpineol, butyl carbitol acetate, 2,2,4- trimethyl-2,3-ρentanediol monisobutyrate, available from Texas Eastman Company under the trademark Texanol and the like. The above resin binders may be utilized individually or in any combination of two or more. A suitable viscosity modifier can be added to the resin material if desired. Such, a modifier can be, for example, a castor oil derivative available from N. L. Industries under the trademark Thixatrol. The organic vehicles also suitably contain a surfactant such as olelyamine, available as Armeen O, or a high molecular weight N-alkyl-l,3-diaminopropane dioleate, available as Duomeen TDO, both from AKZO Chemie America. Regardless of the vehicle utilized, it is important that the homogeneity of the ink be maximized. Therefore, mixing is suitably carried out in a conventional apparatus which mixes in combination with subjecting the dispersion to high shearing action, such as a three roll mill. Various type of ink formulations may be made by varying the type and amount of the above ingredients, as is known. For example, overglaze inks comprise between 50-90% by weight of the devitrifying glass of the invention and about 10- 50% of an organic vehicle. Dielectric inks comprise between 50- 90% of a mixture of ceramic fillers and the devitrifying glass of the invention, preferably 50-85% of the mixture being the devitrifying glass, and 10-15% of an organic vehicle. Copper via-fill inks can comprise at least about 50% by weight of copper, at least 25% by weight of the devitrifying glass of the invention and an organic vehicle. Copper inner layer inks can comprise copper, a small amount of the devitrifying glass of the invention, a small amount of metal oxides and an .organic vehicle. The following examples set forth illustrative examples of the preparation of devitrifying glasses of the invention, and various inks made with the subject devitrifying glasses. In the examples, percent is by weight and temperature is given in degrees Celsius. EXAMPLE 1. Preparation of a Barium Oxide Containing Glass. The following oxides were mixed and melted at 1600°- 1650°C in a platinum crucible and quenched in known manner. Constituents % TnO 29.07 MgO 6.00 BaO 4.90 Al 2 θ3 15.59 Siθ2 44.44 After devitrification for two hours at 900° C, the glass of the aforesaid example had a thermal coefficient of expansion of 43.9 x 10-7/oc at 895°C. This is quite close to the thermal coefficient of expansion of silicon. EXAMPLE 2. Preparation of a Strontium Oxide - Containing Glass. Constituents 2 O 29.27 MgO 6.04 SrO 4.29 AI2O3 15 = 68 Siθ2 44.72 The above oxides were mixed, melted at 1 00°-1650°C in a platinum crucible and quenched. Five thick film ink formulations were prepared, screen printed and fired in nitrogen at 900° C as detailed below EXAMPLE 3. Preparation of an Overglaze Ink. Constituents % Glass powder of Example 1 74.7 6% ET-100 in Texanol 14.9 10% Thixatrol in Texanol 0.9 Texanol 9.3 Duomeen TDO 0.2 The glass powder and the organic vehicle were mixed on a three roll mill. The ink was screen printed and fired in nitrogen at 900° C , and tested for hermeticity and resistance to mild acids and mild alkali. The results were satisfactory. Leakage current was less than 10 ~ 6 A/cm^, which was also satisfactory. EXAMPLE 4. Preparation of an Hermetic Dielectric Ink. Constituents % Glass powder of Example 1 67.2 Aluminum Oxide powder 7.5 6% ET-100 in Texanol 14.9 10% Thixatrol in Texanol 0.9 Texanol 9.3 Duomeen TDO 0.2 The ink was screen printed and fired in nitrogen at 900° C . The ink was inert to mild acid and mild alkali and had a satisfactory leakage current of less than 10" 6 A/cm^. EET EXAMPLE 5. Preparation of a Copper Via-Fill Ink. Constituents % Copper powder 50.2 Glass powder of Example 1 32.2 6% ET-100 in Texanol 1 1.3 10% Thixatrol in Texanol 2.5 Texanol 3.6 Duomeen TDO 0.2 This ink was formulated as in Example 3 and used in combination with the dielectric ink of Example 4 to make a copper - dielectric multilayer printed circuit. There was no cracking or blistering in the vias. EXAMPLE 6. Preparation of Copper Via-Fill Ink Constituents Copper powder 54.5 Glass powder of Example 1 27.8 6% ET-100 in Texanol 10.9 10% Thixatrol in Texanol 2.9 Texanol 3.6 Duomeen TDO 0.3 This ink was formulated as in Example 3 and used in combination with the dielectric ink of Example 4 to make a copper - dielectric multilayer printed circuit. There was no via cracking or blistering in multilayer structures. EXAMPLE 7. Preparation of an Inner Layer Copper Ink. Constituents % Copper powder 75.0 Glass powder of Example 1 3.8 Cadmium Oxide 0.6 Cuprous Oxide 0.6 Bismuth Oxide 3.8 6% ET-100 in Texanol 8.6 10% Thixatrol in Texanol 2.5 Texanol 4.8 Duomeen TDO 0.3 This ink was formulated as in Example 3, and used in combination with the dielectric ink of Example 4. The fired overlying dielectric coating showed excellent hermeticity. The above ink was also used for buried copper conductors in the copper/dielectric structure as above. No blistering was noted after firing, and the adhesion of a copper layer formed from the above ink was good on both the substrate and the dielectric. From the foregoing discussion, it is apparent that the devitirfying glass formulations of the present invention are useful both in making a substrate and in making multilayer copper-dielectric structures supported by a substrate of printed circuits on which silicon or gallium arsenide chips may be mounted, and then applying a hermetic-sealing overglaze to such fabricated printed circuits. Alternatively, such printed circuits may be comprised of co-fired buried copper layers within the substrate that make use of the devitrifying glass formulations of the present invention.

Claims: We Claim: 1. A devitrifying glass comprising on a weight basis: (a) from about 26 to about 33 percent of zinc oxide (ZnO); (b) from about 4 to about 8 percent of magnesium oxide (MgO); (c) from about 2 to about 6 percent of barium oxide (BaO) or from about 2 to about 7 percent of strontium oxide (SrO); (d) from about 12 to about 18 percent of aluminum oxide (AI2O3); and (e) from about 40 to about 50 percent of silicon oxide (Siθ2). 2. The devitrifying glass of claim 1 comprising: (a) about 29 percent of zinc oxide; (b) about 6 percent of magnesium oxide; (c) about 4.9 percent of barium oxide; (d) about 15.6 percent of aluminum oxide; and (e) about 44.5 percent of silicon oxide. 3. The devitrifying glass of claim 1 comprising: (a) about 29.3 percent of zinc oxide; (b) about 6 percent of magnesium oxide; (c) about 4.3 percent of strontium oxide; (d) about 15.7 percent of aluminum oxide; and (e) about 44.7 percent of silicon oxide. 4. A composition comprising the devitrifying glass of claim 1 and an organic vehicle. 5. A composition of claim 4 suitable as an overglaze ink comprising from about 50-90% by weight of said glass and about 10-50% by weight of an organic vehicle. HEET 6. A composition of claim 4 suitable as a dielectric ink comprising from about 50-90% of said glass composition, from 5-10% by weight of a ceramic filler and an organic vehicle. 7. A composition of claim 4 suitable as a copper via fill ink comprising at least about 50% by weight of copper powder, at least 25% by weight of said glass composition and an organic vehicle. 8. A composition of claim 4 suitable as an inner layer copper ink comprising copper powder, from 2 to 15% by weight of said glass composition, from 0.5 to 8% by weight of metal oxides and an organic vehicle. 9. A fused body comprising the devitrifying glass of claim 1 and a ceramic filler. 10. An article comprising a body as in claim 9 having a patterned layer of a conductor thereon. 11. An article comprising a body as in claim 9 having a semiconductor chip mounted thereon. 12. An article comprising a body as in claim 10 having a semiconductor chip mounted thereon. 13. An article according to claim 10 having a patterned layer of copper thereon and an overglaze thereon, said overglaze comprising a fired ink composition comprising between about 50-90% by weight of a devitrifying glass comprising on a weight basis: (a) from about 26 to about 33 percent of zinc oxide (ZnO); (b) from about 4 to about 8 percent of magnesium oxide (MgO); (c) from about 2 to about 6 percent of barium oxide (BaO) or from about 2 to about 7 percent of strontium oxide(SrO); and (d) from about 12 to about 18 percent of aluminum oxide (AI2O3); and (e) from about 40 to about 50 percent of silicon oxide (Siθ2), and about 10-50% of an organic vehicle. 14. A multilayer article comprising a substrate having alternating dielectric and conductor layers wherein said dielectric layer is made from the ink composition of claim 6. 15. A multilayer article comprising a substrate having alternating dielectric and conductor layers wherein said conductor is copper and said layers are based on a glass composition of claim 1. TE SHEET

Inventors: CONLON EDWARD JAMES, HANG KENNETH WARREN, PRABHU ASHOK NARAYAN, CONLON, EDWARD, JAMES, HANG, KENNETH, WARREN, PRABHU, ASHOK, NARAYAN
Applicants: SARNOFF DAVID RES CENTER, DAVID SARNOFF RESEARCH CENTER, INC.
Application year: 1990
Main classification: H05K1
IPC codes: C03C3, C03C8, C03C10, H01B3, H01L21, H01L23, H05K1