High Strength Low Dielectric Constant Aromatic Thermosets
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High Strength Low Dielectric Constant Aromatic Thermosets
Yongqing Huang and James Economy Materials Science and Engineering Dept., Univ. of Illinois at Urbana-Champaign, 1304 West Green St., Urbana, IL 61801, U.S.A.
ABSTRACT Continuing miniaturization of microelectronic devices requires development of low dielectric constant materials to lower the RC delay, power dissipation and crosstalk noise. Although spin-on polymer dielectrics usually have better potential for extendibility to lower dielectric constant (k) values compared to chemical-vapor-deposited dielectrics, their low mechanical properties prevent them from being successfully integrated with copper metal lines. Recent evaluation of a new thermosetting oligomer shows high thermal stability, low moisture pick-up and low dielectric constant. Techniques to optimize the solubility and spin coating characteristics of the oligomer have been developed. The thermally cured polymer displayed a thermal stability up to 480°C in nitrogen and 400°C in air. The cured polymer displayed a dielectric constant of 2.7 at 1 MHz and a breakdown strength larger than 230 V/µm. Nanoindentation testing showed that it had an extraordinarily high Young's modulus of 16.8 GPa and a hardness of 3.5 GPa. By use of porogens, a dielectric constant as low as 1.85 was obtained while still maintaining an acceptable high Young's modulus of 7.7 GPa and hardness of 2.0 GPa. Nanoscratch testing indicated that this material had good adhesion to the Si substrate, and Ta which is a diffusion barrier for copper. These results appear unique compared to all commercially available low-k candidates.
INTRODUCTION Minimization in integrated circuit (IC) dimension allows faster device speed, higher device packing density, and the integration of more functions on a single chip. However, the propagation delays increase with increasing numbers of interconnects, and limit the overall performance of the device. From a materials point of view, in order to lower the propagation delays, higher conductivity metal and lower dielectric constant materials are required to replace the Al and SiO2 interconnect structure [1]. Although Cu has successfully replaced Al and become the current metallization material, designing low-k dielectrics remains one of the main challenges today. As early as 1999, the Internal Technology Roadmap for Semiconductors (ITRS) called for low-k materials of 2.2, a goal that was modified several times and ultimately pushed out to 2007 [2-3]. The reason for this lag is that low-k materials need to meet stringent material property requirements for successful integration into the structures. These include sufficient mechanical properties to survive chemical mechanical polishing (CMP) processing and to withstand the stress during packaging. Moreover, some properties, such as Young’s modulus, hardness and thermal conductivity, decrease with increasing porosity, which of course is
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essential to achieve lower k value [4]. This is especially critical for low mechanical strength spin
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