Ultra-low dielectric constant

Both xerogels and aerogels are characteristically high surface area materials (surface areas normally exceed 500 m2/g). Unlike wet gels, many uses exist for dried gels due to their high surface areas and small pore sizes (typically, < 20 nm diameters). Examples include catalyst supports (12.). ultrafiltration media (18), antireflective coatings (19-20), and ultra-low dielectric constant films. (Lenahan, P. M. and Brinker, C. J., unpublished results.)... 

Typical nanopore materials are porous oxides or polymers that have an ultra-low dielectric constant (k < 2). Ultra-low k is significant in producing highspeed electronic devices such as the interconnect structure shown in Figure 13.8. Si02, which has a dielectric constant of about 4, is currently used as a dielectric material between interconnects in most microelectronic devices. When the packing density between multilevel interconnects increases, a low... 

Solution and Solid-State NMR Characterization of Ethylene Oxide-Propylene Oxide Composites for Ultra-Low Dielectric Constant Applications... 

High-resoIution proton and silicon NMR has been used to study structure formation in solution mixtures of ethylene oxide/propylene oxide triblock copolymers and methyl silsesquioxane. These mixtures are precursors to ultra low dielectric constant filim used in the fabrication of integrated circuits. The solution NMR results show that micelle formation is suppressed during solvent casting and curing of the films, and that miscibility is enhanced by the interactions of both the ethylene oxide and propylene oxide blocks of the triblock copolymer with the methyl silsesquioxane matrix. 

Figure 1. A schematic diagram showing casting and curing of the ultra low dielectric constant films. The fdms for devices are cast on silicon wafers.

Liu J., Min Y., Chen J., Zhou H., and Wang C. Preparation of the ultra-low dielectric constant polyimide fiber membranes enabled by electrospinning. Macromol. Rapid Commun. 28 no. 2 (2007) 215-219. 

Liu Y-L, Fangchiang M-H (2009) Polyhedral oligomeric silsesquioxane nanocomposites exhibiting ultra-low dielectric constants through POSS orientation into lamellar structines. J Mater Chem 19(22) 3643-3647. doi 10.1039/b900141g... 

The chemical component of CMP slurry creates porous unstable oxides or soluble surface complexes. The slurries are designed to have additives that initiate the above reactions. The mechanical component of the process removes the above-formed films by abrasion. In most planarization systems the mechanical component is the rate-limiting step. As soon as the formed porous film is removed, a new one is formed and planarization proceeds. Therefore, the removal rate is directly proportional to the applied pressure. To achieve practical copper removal rates, pressures greater than 3 psi are often required. These pressures should not create delamination, material deformation, or cracking on dense or relatively dense dielectrics used in silicon microfabrication on conventional dielectrics. However, the introduction of porous ultra-low-fc (low dielectric constant) materials will require a low downpressure (< 1 psi) polishing to maintain the structural integrity of the device [7-9]. It is expected that dielectrics with k value less than 2.4 will require a planarization process of 1 psi downpressure or less when they are introduced to production. It is expected that this process requirement will become even more important for the 45-nm technology node [10]. 

Thin film super-low dielectric constant silica aerogel is investigated for application in ultra-large-scale integrated circuits. It is believed that aerogels could more than double the computer speed [ 11J. Such thin films can be made by using TMOS based solution mixed with dimethyl sulfoxide, and dried with supercritical carbon dioxide after coating. A relative dielectric constant as low as 1.1 can be obtained. 

Application of high external pressures influences the transition temperature to the antiferroelectric phase (Yasuda et al., 1979 Samara and Semmingsen, 1979). The Tq becomes lower as the applied pressure increases. Under an ultra-high pressure of about 3 GPa, the antiferroelectric transition itself disappears and the high dielectric constant of ca. 200 is maintained even at cryogenic temperatures (Moritomo et al., 1991). Since Raman diffraction measurements under 3-4.5 GPa revealed that squaric acid exists still as an alternating bond form, the tautomerization coupled with intermolecular proton transfer occurs even at low temperatures (Moritomo et al., 1990). 

Ethylene oxide/propylene oxide triblock copolymers have been successfully used to template pore formation in ultra low-k films, and dielectric constants as low as 1.5 have been observed with polymer loading levels of 50 wt% (5). These films have good mechanical properties, a high breakdown voltage and a low moisture uptake. We have characterized the films with high-resolution solid-state proton NMR and found that the triblock copolymers form nm-sized core-shell structures with the propylene oxide block at the interface between the ethylene oxide block and the methyl silsesquioxane matrix (14). 

Mechanical and thermal characterization of ultra low k dielecteics is very similar to the characterization of dense low k dielectrics however the introduction of porosity requires the development of new characterization techniques in order to understand the pore structure. The dielectric constant, dielectric breakdown and coefficient of diermal expansion can be measured using the same techniques used for dense low k electrics. Modulus and hardness can also be measured by die same techniques, however, if using nanoindentation, measurements firom porous dielectrics may have larger substrate contributions at equivalent film thicknesses. Therefore, the modulus values... 

Kofod G, Risse S, Stoyanov H et al (2011) Broad-spectrum enhancement of polymer composite dielectric constant at ultra-low doping caused by sihca-supported copper nanoparticles. Composites 5 1623-1629... 

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