Preparation and Properties of Polysilsesquioxanes - Polysilsesquioxanes as a Candidate to a Low Dielectrics for Electron
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ABSTRACT Polysilsesquioxanes PSSQ as a candidate of coatings for interlayer low dielectric films were synthesized by acid and base catalyzed hydrolytic polycondensation of RSi(OMe)3 (R=methyl, vinyl, 3-methacryloxypropyl). Dip and spin coating of PSSQ on organic and inorganic substrates followed by curing at 100 'C, 400 'C, and 450 'C provided transparent and tough coating films of thickness 2000-9000 A. Polysilsesquioxanes adhered strongly to the substrates to form coating films of which the adhesion strength and hardness were 10 and 9H based on JIS K5400 comparable to ISO standard, as heating time and molecular weight increase. On curing at the elevated temperatures, they provided the films with a very smooth surface and the dielectric constants of 3.2-3.6 (R=3-methacryloxypropyl), 3.8-4.0 (R=vinyl), and 2.5-2.7 (R=methyl) depending on film thickness and molecular weight of PSSQ. The films from PSSQ (R=methyl) were found to be porous compared with those from PSSQ (R=vinyl). INTRODUCTION An increasing attention has been focused on interlayer low dielectric materials for semiconductor electronic devices with large scale integrated circuits in the coming new century. Interlayer films prepared by organic compounds have low dielectric constants, but they are not subjected to the device preparation process at high temperatures. Polysiloxanes can be a candidate because of the excellent chemical, physical, and electrical properties. However, the polysiloxanes, generally known as silicone, have a least intermolecular interaction, which implies that they show no spinnability and no film formation. To make it possible, at present, the main chain structure of siloxane linkage is modified to provide polysiloxane copolymers consisted of the unit structures T2, T3 , and Q 4 and the pendants such as silanol and alkoxy groups. Such polysiloxanes are conveniently prepared by sol-gel method [1-4] (cohydrolysis-condensation process) of one to three silane compounds such as dimethyldimethoxysilane (DMS), methyl- or vinyltrimethoxysilane (MTS, VTS), and tetraethoxysilane to provide coating solutions from which interlayer films with an appreciable low dielectric constant 2.7-3.5 [5] have been reported to be prepared. Polysilsesquioxanes PSSQ generally represented as (RSiO 3 ,2)n (R=methyl: MTS, vinyl: VTS, 3-methacryloxypropyl: MAS) are also an another potential candidate because they are the polymers with well-defined structure and an appreciable stability to self247 Mat. Res. Soc. Symp. Proc. Vol. 565 ©1999 Materials Research Society
condensation and provide tough free-standing films, which has been realized by our works. [6] In this work, therefore, we wish to report the coating properties of PSSQ, especially as an interlayer dielectric, and also the relation between the structure and dielectric constant of PSSQ. EXPERIMENTAL Into a 200 ml four necked flask equipped with a stirrer and the nitrogen inlet and outlet tubes, MTS, VTS, or MAS (0.1 mol, each) and methanol (14 ml) were placed and then cooled to 0 'C. Water and 6 N hydroc
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