Porous reaction-sintered AlN tapes for high-performance microelectronics application
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W.J. Clegg University of Cambridge, Department of Materials Science & Metallurgy, New Museums Site, Pembroke Street, Cambridge CB2 3QZ, United Kingdom (Received 6 February 2001; accepted 8 November 2001)
A novel approach was undertaken in producing porous AlN microelectronics tapes with high thermal conductivity and low dielectric constant. This method involved polymer microspherical powders used as a sacrificial mold to introduce controlled porosity into the green tapes during pyrolysis. The Al2O3-rich porous green tapes were then reaction sintered at 1680 °C for 12 h to create porous AlN tapes. This work builds upon a previously developed novel reaction sintering process that densified and converted Al2O3-rich tapes (Al2O3–20 wt% AlN–5 wt% Y2O3) to AlN tapes at a relatively low sintering temperature of 1680 °C. The sintering behavior of the porous tapes was investigated, and the effects of the microsphere particle size and volume addition were studied. The microspheres successfully contributed to the significant reduction of tape density by porosity, and this contributed to lowering its dielectric constant. Dielectric constants of the AlN tapes were reduced to about 6.8 to 7.7 while thermal conductivity values were reasonable at about 46 to 60 W/mK. Coefficient of thermal expansion (CTE) values showed a linear trend according to phase composition, with the porous AlN tapes exhibiting CTE values of 4.4 × 10−6 to 4.8 × 10−6/°C, showing good CTE compatibility with silicon at 4.0 × 10−6/°C. The added porosity did not significantly affect the CTE values.
I. INTRODUCTION
As integrated microelectronics perform at increasingly faster speeds and higher temperatures, the choice of the dielectric material has become crucial.1,2 Key factors here include the need for a low dielectric loss and optimal range permittivity to minimize signal delay time, a matching coefficient of thermal expansion with silicon, and a high thermal conductivity. Conventional dielectric materials include Al2O3 (high purity and with magnesia as a second phase), glass–ceramics, and glasses,3 while AlN and BeO have been identified as high performance dielectric materials due to their high thermal conductivity. Existing substrates employ thin ceramic tapes cast using solid powders. With the current trend towards miniaturization and higher device counts in integrated circuit packages, a significant increase in the usage of both multilayer ceramic packages has been apparent. The demand for the development of interconnection and packaging techniques that accommodate faster system signal propagation, low dielectric loss, and medium-range permittivity is also increasing. Delay time () of an electrical 306
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J. Mater. Res., Vol. 17, No. 2, Feb 2002 Downloaded: 06 Jul 2014
signal can be reduced by decreasing the dielectric constant (⑀) of a substrate material by the following relationship,4 where c is the velocity of light: ⳱ ⑀0.5/c
.
(1)
It is noted here that by reducing the dielectric constant by half, the propagation s
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