*5.3.1.1. Diamond-like carbon [DLC]*

Amorphous diamond-like carbon [DLC], which can be prepared by chemical vapor deposition [CVD] method, [35,36] are metastable materials composed of sp2, sp3, and even sp1 hybridized carbon atoms with hydrogen concentrations, *CH*, ranging from 1% to 50%, with the composition being primarily determined by the nature of the precursor and the corresponding deposition conditions.

These sp3 bonds can occur not only with crystals - in other words, in solids with long-range order - but also in amorphous solids where the atoms are in a random arrangement. In this case, there will only be bonding between a few individual atoms and not in a long-range order extending over a large number of atoms. The bond types have a considerable influence on the material properties of amorphous carbon films. If the sp2 type is predominant the film will be softer, if the sp3 type is predominant the film will be harder.

Under the right conditions, it is possible to deposit DLC films with compressive stress, spanning values from 200-800 MPa, and dielectric constants approaching 2.7.[36, 37]

### *5.3.1.2. SiLK and BCB resins*

A very promising class of network polymers has been developed by Dow under the name 'Silk'. The formulations presumably consist of a mixture of monomeric and/or oligomeric 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 of 66 ppm/K.

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