**5.1. Linear structure**

Linear structured materials have been actively researched for various microelectronic applications. In the early stages of microelectronics development, IBM implemented a polyimide-based material in microchips based on its good thermal, mechanical, chemical, and electrical properties. However, as required properties have become stricter because of narrowing interconnect line distance, polyimide-based materials have been unable to satisfy device performance with the main reason due to its high water absorption. Despite its superior properties, it became apparent that a linear polymeric structure was unfeasible for application as more high performance devices were being demanded.

However, linear polymeric structures have given polymer scientists invaluable clues into the possible molecular content of low dielectric materials. According to the definition of a dielectric, the material density has a direct relationship with respect to its dielectric constant. Linear polymers occupy a free volume, derived from large steric hindrance compared to single small molecules. For this reason, linear structured materials such as organic polymers, polyethylene and polypropylene show quite low density (0.8~0.9), and thus low dielectric value (2.1~2.6). Unfortunately, these organic polymers suffer from critical disadvantages such as thermal instability such as low glass transition temperature and low degradation temperature.

Therefore, many scientists turned to polymeric materials having an aromatic moiety. This chemical structure showed enhanced thermal properties and was expected to have a low density due its rigid molecular structure. The high polarizability of these materials due to their relatively high dipole moment was expected to compensate for the inherently large free volume. Some of the various aromatic, linear polymers are outlined below.

### *5.1.1. Polyimides (PIs)*

Excellent thermomechanical properties can be obtained by incorporating a very stiff polymer. The classic example of a stiff polymer chain is aromatic polyimides, which have a rigid backbone due to the many aryl and imide rings along the chain. These structural characteristics give rise to excellent mechanical and thermal properties in the form high modulus (8–10 GPa) and high Tg (350 to 400<sup>o</sup> C) [10]. However, the rigid chain structure causes the PI chains to align preferentially parallel to the substrate, especially when deposited as thin films, which results in anisotropic properties [11-18]. For example, while the out-of-plane k value of BPDA-PDA is 3.1, the more important in-plane value is *>*3.5 [14].

The thermomechanical properties are likewise anisotropic. For instance, the CTE of thin films of rigid PIs is often <10 ppm/<sup>o</sup> C in the plane of the film, but can be more than ten times larger in the out-of-plane direction [14]. Another drawback to PIs is that they absorb water effectively owing to the carbonyl groups, which raises the dielectric constant further. The release of this water during processing can cause blistering of overlying layers [19].

Some of the drawbacks mentioned above can be ameliorated by tailoring the chemical structure of the PI. The k value and water adsorption can be lowered by incorporating fluorine into the material, while the anisotropy can be reduced by introducing single bonds between rings, making the chain less rigid. For example, PMDA-TFMOB-6FDA-PDA, which utilizes both of these design strategies, has an out-of-plane k=2.64 [20] and absorbs less moisture than unfluorinated PIs such as BPDA-PDA [10]. However, the in-plane k value is still *>*3.0, and the water uptake, although reduced, is significant enough to cause blistering in overlying layers during integration [19].
