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

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

Low Dielectric Materials for Microelectronics 65

C, respectively. Resistance to

C) [10]. However, the rigid chain structure

C in the plane of the film, but can be more than ten times

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

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

The thermomechanical properties are likewise anisotropic. For instance, the CTE of thin

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

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

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

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

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

Polynorbornene [22] is a pure hydrocarbon polymer without any polar or polarizable groups. Known for their high thermal stability among organic polymers (Tg ~365oC) and low dielectric constant [23] (~2.2), polynorbornenes are soluble in common organic solvents

C, which is lower than many of the thermal treatment

chains, which results in a more isotropic material than is obtained for PIs.

values for the modulus and CTE are 2.0 GPa and 50-60 ppm/<sup>o</sup>

release of this water during processing can cause blistering of overlying layers [19].

modulus (8–10 GPa) and high Tg (350 to 400<sup>o</sup>

films of rigid PIs is often <10 ppm/<sup>o</sup>

in overlying layers during integration [19].

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

relatively low Tg of <275<sup>o</sup>

*5.1.3. Polynorbornene* 

temperatures of microelectronic devices.

[14].

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