| A series of hyperbranched polycarbosilanes (PCSs) were prepared as precursors for hybrid organic/inorganic organosilicon network materials having low dielectric constants. These materials are of potential interest as interlayer dielectrics in semiconductor devices. The hyperbranched polycarbosilanes ([CH2Si(OR)2]n, [CH2SiMe(OMe)] n and [CH2C6H4Si(OMe)2] n) were synthesized by Mg-induced coupling of ClCH2SiCl(OR) 2, ClCH2SiCl(Me)(OMe) or p-ClCH2C6H 4SiCl(OMe)2. Hydrochloric acid was used to catalyze the sol-gel processing of these hyperbranched polycarbosilanes in water/alcohol mixtures. A coating-baking-curing process for the preparation of low-k films was developed that yields uniform, adherent films on Si wafers with controllable thickness. The resultant films were characterized by TGA, FT-IR, ellipsometry and AFM. The films showed low dielectric constants, low refractive index, good thermal stability and good mechanical and electrical properties. The triblock copolymer, poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-b-PPO- b-PEO), was used as the porogen to prepare porous films with lower dielectric constants.; Ways to control the structures and properties of these hyperbranched polycarbosilanes were also investigated. A series of chloromethylchloroalkoxysilanes (ClCH2SiCl3-x(OR)x, R = Me, Et; x = 1--3) were synthesized by the alkoxylation of ClCH2SiCl 3 and then used to prepare hyperbranched hydridopolycarbosilanes (HPCSs), which have the average composition of "[SiH2CH2]" but with different degrees of branching, through a Mg-induced Grignard coupling reaction followed by reduction with lithium aluminum hydride. Methyl hydridopolycarbosilanes (MHPCS) derived from chloromethylchloromethylalkoxysilanes (ClCH2SiClMe(OR), R = Me, Et) and a novel hyperbranched polymer with the average formula, "[CH 2-C6H4-SiH2]n", were also prepared. These polymers were characterized by IR, 1H, 13C and 29Si NMR spectroscopy, GPC and TGA. Both the relative amount and the nature of the alkoxy substituents on each Si atom in the starting material for the Grignard reaction were found to strongly influence the composition and structure of the resultant polymer, due to the lower reactivity of the Si-OR bond towards the Grignard reagents compared with that of the Si-Cl bond. This study provided insights into ways to control the structures and the molecular weights of the hyperbranched polycarbosilanes so as to improve the properties of these polymers as potential precursors to low-k materials and silicon carbide ceramics. |
