*5.1.3. Polynorbornene*

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

#### 66 Dielectric Material

despite its rigid backbond due to the randomly coiled nature of the polymer chains and lack of polar interactions.

Low Dielectric Materials for Microelectronics 67

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

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

In polymer chemistry, polymer branching induces a lower profile of material density without significant changes of in chemical properties. Because of this, many researchers have investigated polymer geometries such as graft and hyper branched structure for

Branched polymers are advantageous in that through chemical modification of the side or end groups can give unique functionality that could not be realized by composites of two or three different materials. Also, the chemical bonding between the two components may

Graft copolymers are a special type of branched copolymer in which the side chains are structurally distinct from the main chain. The Figure (3) depicts a special case where the

for polymer and ceramic scientists in the microelectronics industry.

offset one or more deficient property of the singular component.

main chain and side chains are composed of distinct homopolymers.

constant without curing process. [31]

application as low dielectric material.

**Figure 3.** Special cases of grafted polymer

**5.2. Branched structures** 

*5.2.1. Graft Polymers* 

This combination of properties makes polynorbornene an interesting candidate for ILD/IMD use. However, polynorbornenes exhibit insufficient adhesion to substrates with polar surfaces such as Si, oxides or metals and its rigid backbone results in a rather brittle material. To overcome these shortcomings, a copolymer with alkoxysilyl and aikyi side groups at the norbornane rings in the backbone of polynorbornene was developed by BFGoodrich (Avatre[). [23,24] The alkoxysilyl groups enhance adhesion to surfaces with hydroxyl groups and increase the relative mobility of the polymer chains, and hence the elongation at break of thin films [23]. However, the dielectric constant increases from 2.2 of the unsubstitituted polymer to 2.67 with an aikoxysilyl content of 20% [23]. Copolymers from alkoxysilyl norbornene and alkylnorbornene derivatives show improved elongationat-break and reduced dielectric constant [23] of E = 2.56. The glass transition temperatures and thermal stabilities of these materials are reduced compared to non-functionalized polynorbornene [25,26]
