*5.1.2. Poly (aryl ethers) (PAE)*

64 Dielectric Material

**5.1. Linear structure** 

*5.1.1. Polyimides (PIs)* 

**5. Utilization of low dielectric materials in microelectronics** 

(<1.0 g/cc) and lower individual bond polarizabilities.

A particularly difficult challenge for low dielectric materials development has been to obtain the combination of low dielectric constant and good thermal and mechanical stability. Generally, the types of chemical structures that imbue structural stability are those having strong individual bonds and a high density of such bonds. However, the strongest bonds often are the most polarizable, and increasing the bond density gives a similar increase in polarization. For example, the rigidity and thermal stability of SiO2 is in part due to the dense (2.2–2.4 g/cc) chemical network. Unfortunately, the high bond and material density in SiO2 lead to a large atomic polarizability, and therefore a high dielectric constant. Organic polymeric materials often have a lower dielectric constant due to the lower material density

In this part, the relationship between molecular structure and low dielectric properties is discussed with consideration of factors such chemical bond, density, and polarizability.

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

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

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

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

instability such as low glass transition temperature and low degradation temperature.

free volume. Some of the various aromatic, linear polymers are outlined below.

application as more high performance devices were being demanded.

The utilization of spin-on PAE materials results from attempts to balance the dielectric and thermomechanical properties. The aryl rings in these materials provide better thermomechanical properties than do PIs, but the flexible aryl linkages allow bending of the chains, which results in a more isotropic material than is obtained for PIs.

Additionally, the lack of polar groups, such as carbonyl, results in a lower k value and lower water uptake than the PIs. Fluorinated versions of PAEs had a k value of 2.4 [21]. However, because of concerns about fluorine corrosion, the fluorine was removed from later versions of the material. The nonfluorinated PAEs typically have a k of 2.8–2.9, whereas typical values for the modulus and CTE are 2.0 GPa and 50-60 ppm/<sup>o</sup> C, respectively. Resistance to thermal decomposition can be quite good for PAEs as weight losses of only <2% over 8 h at 425oC have been reported. One drawback of uncrosslinked PAEs is that they have a relatively low Tg of <275<sup>o</sup> C, which is lower than many of the thermal treatment temperatures of microelectronic devices.
