**5.2. Branched structures**

66 Dielectric Material

of polar interactions.

polynorbornene [25,26]

temperature (<250<sup>o</sup>

*5.1.5. Polysilsesquioxane* 

polarized or have weak thermal stability.

*5.1.4. Polytetrafluoroethylene (PTFE)* 

other reliability problems in the interconnect structure.

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

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

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

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

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

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

C), and high thermal expansion coefficient (CTE) (*>*100 ppm/<sup>o</sup>

C).

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 application as low dielectric material.

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 offset one or more deficient property of the singular component.
