Film Properties of Low-Density and Ultra-Low-Dielectric-Constant Material
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ABSTRACT We studied the effect of pore diameter on the film properties of low-density porous material with special interest in the pore diameter range below 5nm. A novel low-density material, Interpenetrated SOG (IPS), was designed to realize such porous character. It is composed of a pyrolic template, which is an organic olygomer, and framework, made from SOG polymer and silica sol, forming an interpenetrated structure. Thermal decomposition of the organic component successfully resulted in films with pore diameter range below 5nm. It is found that the dielectric constant and the pore diameter depended on the density and the film strength was affected by the average pore diameter. Practically, films with the average pore size below 5nm possessed dielectric constants as low as 2.2-2.0 and sufficient strength. INTRODUCTION Low-dielectric-constant materials for insulating layer are increasingly demanded to break through the problem of RC-delay in integrated circuit that is scaling down to sub-half-micron region [1]. As materials whose dielectric constants are 3.0 and below, HSQ (Hydrogensilsesquioxane) which is an inorganic spin-on glass (SOG) material [2][3] and organic materials [43[5] have been reported. And for dielectric constant below 2.5, Sol-Gel film [6][7][8], Nanoform film [9] and Sol film [10] that are porous materials have been studied. Porous materials are regarded as the most promising candidates for such materials because of their potentially very low dielectric constants. But they have some serious problems to be
solved such as mechanical weakness, weak adhesion, and moisture absorption on the pore surface. Mechanical strength is particularly important considering the recent process technology such as CMP and Cu damascene process. Of porous materials reported[8][9], any material which has controlled pores smaller than 5 nm has not been reported as yet. We were motivated to investigate the properties of such material. We propose to call such a material, which has smaller pores than 5nm in diameter, a "low-density material", in order to distinguish it from other porous materials. In this paper, we report a novel low-density material, Interpenetrated-SOG (IPS), and the effect of pore diameter on the film properties, especially, the mechanical strength in detail. EXPERIMENTAL Preparation of IPS IPS was prepared by the hydrolysis of SOG precursor and silica sol in the presence of an organic oligomer. Silica sol is obtained by hydrolysis of alkoxysilane, and is 20-25nm in diameter. Preparation of sample films A Si substrate was spin-coated with solution of IPS in methylisobutylketone at 3000 rpm 63 Mat. Res. Soc. Symp. Proc. Vol. 565 ©1999 Materials Research Society
for 10 seconds, baked at 350 degrees Celsius(C) on a hot plate in a nitrogen house followed by curing at 400 degrees C in nitrogen atmosphere.
Evaluation Pore diameters were measured by BJH method using an adsorption of nitrogen. The film density was measured by gravimetric analysis. The film thickness and refractive index were estimated by
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