**5.3. Network structure of low dielectric materials**

In defining the different types of network structure materials for low dielectric constant applications in microelectronics, two classifications may exist. One is organic networks based on elemental carbon including amorphous carbon (diamond-like-carbon(DLC)) [35,36] and interpenetrating polymer network (IPN). The other is inorganic networks based on silicon oxide bonds such as amorphous SiO2 and mesoporous crystalline silicon oxide.

Low Dielectric Materials for Microelectronics 71

aromatic starting compounds, which contain ortho-bisethinyl or -phenylethinyl groups [38]. The materials exhibit k values on the order of 2.6-2.7, with decomposition temperatures in excess of 500°C, no softening up to 490°C, good gap fill properties down to below 0.1 um, a maximum water uptake of 0.25%, and a coefficient of thermal expansion 4s

Benzocyclobutene (BCB) resins were developed by Dow in the 1980's [39], with a siliconcontaining derivative for microelectronics applications, and are commercially available under the name 'Cyclotene'. BCB resins with imide structures can be extremely tough, and the dielectric constant of cured films from this monomer is 2.6-2.7, with thermal stability up

Inorganic networks mainly consist of ceramics or amorphous silica. In various materials for microelectronics, the silicon oxides play a major role due to its low polarizability, superior thermal and mechanical properties. In addition, tunable microspores can be made to reduce the dielectric constant through control of microstructure under special

Microporous zeolite thin films were first investigated by Yan and co-workers. [43] These films offer good thermal stability (i.e. no pore collapse or unidirectional shrinkage) and inter-particle mechanical strength. They can be prepared via a simple spin-on method [43,44] or by in situ growth. [44,45] With simple spin-on methods a dispersion of small zeolite particles are prepared and cast onto a surface. The porosity originates from the interparticle porosity within the zeolite nanoparticles and the intraparticle porosity owing to the packing of the near-spherical nanoparticles in thin film

Some of the most promising materials for dielectric materials are poly(silsesquioxanes). Most common are polymethylsilsesquioxane (MSQ), e.g. Accuspin T-18 from Allied Signal [46], or poly(hydridosiLsesquioxane) (HSQ), e.g. FOx from Dow Corning [47]. Synthesis of these silsesquioxanes(MSQ and HSQ) have traditionally been through the sol-gel method, as its utility in being able to obtain highly cross-linked structures through acidic and or basic conditions has been well documented [48,49,50]. Dielectric constant values of around 2.6 can be achieved for HSQ and MSQ. But while MSQ exhibits this dielectric constant after curing at temperatures up to 450°C, HSQ must be cured at temperatures lower than 210°C [51]. Curing of HSQ at temperatures of 250°C or above results in dielectric constant around 3 or even higher 32°[51,52]. Gap fill and planarization properties are also acceptable and because of their chemical structure, which is closely related to SiO2, polymethylsilsesquioxanes are

of 66 ppm/K.

conditions.

format.

*5.3.2. Inorganic network* 

*5.3.2.1. Ordered Mesoporous Materials* 

*5.3.2.2. Network polysilsesquioxane* 

also compatible with existing lithographic procedures.

to 375oC and water adsorption of only 0.2% [40,42].

Intuitively, network structures have excellent thermal stabilities and mechanical properties, and chemical resistance, but have relatively high density which is a factor in elevated dielectric constants. Because of this, dielectric constant and mechanical properties should be carefully controlled and careful consideration of its crystalline structure, as crystalline structures tend to be easily polarized.
