*5.1.4. Polytetrafluoroethylene (PTFE)*

Proposals to use fluorinated organic materials like PTFE are aimed toward minimizing the dielectric constant using the bonds of lowest polarizability. PTFE, which consists of singly bonded carbon chains saturated with fluorine atoms, has one of the lowest k values (<1.9) of any nonporous material, and is normally deposited by spin-on films [27]. One drawback of PTFE is that the flexible and uncrosslinked chain structure limits the thermomechanical stability of the material. For example, one PTFE material evaluated in our laboratory was found to have a low yield stress (12 MPa), low elastic modulus (0.5 GPa), low softening temperature (<250<sup>o</sup> C), and high thermal expansion coefficient (CTE) (*>*100 ppm/<sup>o</sup> C). Together these factors can cause buckling or wrinkling of the film during process integration. A second issue for PTFE, which is a concern for all fluorine-containing materials, is the potential release of fluorine atoms that can cause corrosion of metals or other reliability problems in the interconnect structure.

### *5.1.5. Polysilsesquioxane*

While research of linear, rigid, organic polymers have centered on lowering the density and enhancing thermal and mechanical properties [28,29], many of these materials tend to have elevated dielectric constants and decreased processability because the main frame of these polymers are composed of aromatics, double, and triple bonds, which can be readily polarized or have weak thermal stability.

Polysilsesquioxanes (RSiO3/2)n comprise a class of polymers that exhibit unique physical properties different from those of purely organic or inorganic compounds [30]. The various structures of polysilsesquioxanes, including ladder-like polysilsesquioxanes with double stranded backbones, cage-type polyhedral oligomeric silsesquioxanes (POSS), and sol-gel processed random branched structures have shown many inherent advantageous properties such as high thermal stability, low dielectric constant, good mechanical properties, and chemical resistance. Such properties have made polysilsesquioxanes a material of interest for polymer and ceramic scientists in the microelectronics industry.

In particular, polymethylsilsesquioxanes are of particular interest, as these materials exhibit low loss at high temperatures exceeding 500oC. However, to approach the favorable properties of polysilsesquioxanes in a reproducible manner, a regular structure with high molecular weight, such as ladder-structures, is to be favorable. This rigid ladder backbone with high molecular weight would support lower dielectric constants stemming from increase of inter-molecular space and high mechanical strength, as well as minimizing shrinkage during the ILD process. This material had 6.3 GPa of modulus and 2.7 of dielectric constant without curing process. [31]
