Compositions: ceramic – Ceramic compositions – Refractory
Patent
1991-02-26
1994-02-01
Group, Karl
Compositions: ceramic
Ceramic compositions
Refractory
501 89, C04B 3558
Patent
active
052832147
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
As the electronics industry advances toward higher circuit densities, efficient thermal management will assume increasing importance. The removal of heat from critical circuit components through the circuit substrate is directly dependent on the thermal conductivity of the substrate. Beryllium oxide (BeO) has traditionally been the ceramic of choice for applications requiring electrically insulating materials having high thermal conductivity. Unfortunately, beryllium oxide is toxic to a small fraction of the general population, thus leading to a significant reluctance to use it.
Alumina (Al.sub.2 O.sub.3) is nontoxic and is easily fired to full density at 1500.degree.-1600.degree. C.; however, its thermal conductivity of between about 20 to about 30 W/m.degree.K is about one order of magnitude less than that of BeO (which has a thermal conductivity of about 260 W/m.degree.K). Additionally, the coefficients of thermal expansion (CTE) over the range of 25.degree.-400.degree. C. for alumina (6.7.times.10.sup.-6 /.degree.C) and beryllia (8.0.times.10.sup.-6 /.degree.C) are not well matched to those of semiconductors such as silicon (3.6.times.10.sup.-6 /.degree.C), and gallium arsenide (5.9.times.10.sup.-6 /.degree.C). Thus, alumina and beryllia provide less than ideal results when used in applications such as integrated circuit substrates through which heat transfer is to occur. In contrast, the CTE for aluminum nitride (AlN) is 4.4.times.10.sup.-6 /.degree.C, a value which is well matched to both of the previously described semiconductor materials.
In addition to having a CTE which makes it compatible with materials such as silicon and gallium arsenide, AlN can be sintered to provide shaped ceramic articles. Additionally, AlN articles are amenable to a variety of metallization processes. As such, AlN has repeatedly been suggested as a ceramic substrate for semiconductor applications. Although a variety of attempts to produce sintered AlN parts having high thermal conductivity are described in the literature, these generally have achieved limited success.
There is extensive literature on the sintering of AlN using a variety of sintering or densification aids. The bulk of the literature centers around the use of oxides of either rare earth elements (i.e., yttrium and lanthanide series elements), oxides of alkaline earth elements (i.e., the Group IIA elements), and mixtures thereof. These include compounds such as Y.sub.2 O.sub.3, La.sub.2 O.sub.3, CaO, BaO, and SrO. A system using Y.sub.2 O.sub.3 and carbon is described in a variety of patents, such as U.S. Pat. No. 4,578,232, U.S. Pat. No. 4,578,233, U.S. Pat. No. No. 4,578,234, U.S. Pat. No. 4,578,364 and U.S. Pat. No. 4,578,365, each of Huseby et al.; U.S. Pat. Nos. 3,930,875 and 4,097,293 of Komeya; and U.S. Pat. No. 4,618,592 of Kuramoto. Additionally, there is a wide variety of patents using Y.sub.2 O.sub.3 and YN including U.S. Pat. No. 4,547,471 of Huseby et al.
In the Huseby et al. patents which relate to the Y.sub.2 O.sub.3 and carbon system, described above, AlN samples which are doped with Y.sub.2 O.sub.3 and carbon are heated to 1500.degree.-1600.degree. C. for approximately one hour. The carbon serves to chemically reduce Al.sub.2 O.sub.3 phases contained in the AlN, thereby producing additional AlN and lowering the overall oxygen level in each part. The patents state that the Y.sub.2 O.sub.3 sintering aids are unaffected by this process. The parts are then sintered at about 1900.degree. C. Thermal conductivities as high as 180 W/m.degree.K have been reported for carbon treated samples produced by the methods described in these patents. Some evidence indicates, however, that these methods may introduce residual, free carbon within the sintered AlN piece, and this residual carbon can act to decrease the dielectric constant and loss throughout the piece. These effects may be undesirable in electronic applications, although acceptable in many other applications. Additionally, two other patents of Huseby et al. (U.S. Pat.
REFERENCES:
patent: 3113879 (1963-12-01), Foster et al.
patent: 4578364 (1986-03-01), Huseby et al.
patent: 4578365 (1986-03-01), Huseby et al.
patent: 4618592 (1986-10-01), Kuramoto et al.
patent: 4642298 (1987-02-01), Koramoto et al.
patent: 4666873 (1987-05-01), Morris, Jr. et al.
patent: 4778778 (1988-10-01), Mallia
patent: 4952535 (1990-08-01), Merkel
Chemical Abstract vol. 109, No. 14, Abstract No. 115034a (1988) pp. 296-247.
Gallo Chris
Group Karl
The Dow Chemical Company
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