Silicon nitride sinter having high thermal conductivity and proc

Details Silicon nitride sinter having high thermal conductivity and proc Silicon nitride sinter having high thermal conductivity and proc

1999-03-22

2000-11-07

Jenkins, Daniel J.

Compositions: ceramic

Ceramic compositions

Refractory

264647, 264659, C04B 35583, C04B 3565

Patent

active

061436771

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to an Si.sub.3 N.sub.4 base sintered body which is useful not only as various parts for use in semiconductor devices, including insulating substrates and various radiating plates, but also as various structural parts for motor vehicles, machines, OA apparatuses, etc. and is excellent in productivity and especially in mechanical strength and radiating properties. This invention also relates to a process for producing the sintered body.


BACKGROUND ART

Ceramics comprising silicon nitride as the main component are superior in heat resistance, mechanical strength, and toughness to other ceramic materials, and are materials suitable for various structural parts such as automotive parts and OA apparatus parts. Attempts are being made to use them as insulating radiating substrates for semiconductor devices, etc. so as to take advantage of their high insulating properties.
Alumina and the like have conventionally been used extensively as ceramic substrates for semiconductors. However, with the trend toward higher speeds, higher degrees of integration, and higher outputs in semiconductor devices, materials having higher thermal conductivity and excellent radiating properties have come to be desired and the application of AlN and SiC has progressed. However, no high thermal conductive substrate has been obtained so far which is made of AlN or the like and is sufficient in strength and toughness, and the current substrates have drawbacks in product handling and shape because of breakage caused by external force, etc. There is hence a desire for the development of a ceramic material combining high-strength properties which enable the material to withstand external force with excellent radiating properties.
Silicon nitride (Si.sub.3 N.sub.4), which intrinsically has high strength, is expected to be used as insulating radiating substrates if its thermal conductivity can be improved. However, since the conventionally known silicon nitride sintered bodies have lower thermal conductivities than AlN and SiC, they have not been put to practical use as an insulating radiating substrate.
The thermal conductivity of insulating ceramics such as silicon nitride is mainly attributable to the transmission of phonons. Since phonons are scattered by phases having different impedances, such as lattice defects and impurities, present in the sintered body, the thermal conductivity .kappa. is defined by the following numerical formula 1: formula 1) l is the mean free path of phonons).
The specific heat capacity c and the group velocity V in numerical formula 1 each is a number which varies from material to material and can be regarded as almost the same in the same material. Consequently, the thermal conductivity of silicon nitride crystal grains is governed substantially by the mean free path of phonons. For example, when AlN or Al.sub.2 O.sub.3, which have conventionally been used generally, is added as a sintering aid, then aluminum ions or oxygen ions form a solid solution in Si.sub.3 N.sub.4 crystal grains and thus scatter phonons, resulting in a reduced thermal conductivity. Because of this, general silicon nitride base sintered bodies to which Al.sub.2 O.sub.3, AlN, Y.sub.2 O.sub.3, or the like has been added have a thermal conductivity as low as about 15 W/m.multidot.k.
Various investigations have hence been made in order to obtain a silicon nitride base sintered body having a high thermal conductivity. For example, the thermal conductivity of a silicon nitride base sintered body obtained through an HIP treatment after the addition of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 in combination as a sintering aid is discussed in "Paper Journal of Ceramics Society of Japan)," Vol.97 (1989), No.1, pp.56-62. The result given therein is that the thermal conductivity of the sintered body becomes higher as the proportion of .beta.-form crystal grains increases or as the proportions of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 in the sintering aid increases and decreases, respectively. There is a description

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"Thermal Properties of HIP Sintered Silicon Nitride", K. Watari, Journal of Ceramics Society of Japan, vol. 97(1989), No. 1, pp. 56-62.

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