*5.3.2.2. Network polysilsesquioxane*

Some of the most promising materials for dielectric materials are poly(silsesquioxanes). Most common are polymethylsilsesquioxane (MSQ), e.g. Accuspin T-18 from Allied Signal [46], or poly(hydridosiLsesquioxane) (HSQ), e.g. FOx from Dow Corning [47]. Synthesis of these silsesquioxanes(MSQ and HSQ) have traditionally been through the sol-gel method, as its utility in being able to obtain highly cross-linked structures through acidic and or basic conditions has been well documented [48,49,50]. Dielectric constant values of around 2.6 can be achieved for HSQ and MSQ. But while MSQ exhibits this dielectric constant after curing at temperatures up to 450°C, HSQ must be cured at temperatures lower than 210°C [51]. Curing of HSQ at temperatures of 250°C or above results in dielectric constant around 3 or even higher 32°[51,52]. Gap fill and planarization properties are also acceptable and because of their chemical structure, which is closely related to SiO2, polymethylsilsesquioxanes are also compatible with existing lithographic procedures.

#### 72 Dielectric Material

Efforts to further decrease the dielectric constant without decreasing mechanical strength, POSS skeletons have been introduced in MSQ. To suppress the phase separation, incompletely condensed methyl functionalized POSS precursors have been used to form chemical bonds with oligomeric sol precursors. These incompletely condensed POSS moieties functioned as coupling agents while expanding the free volume of the final sol after curing which was accomplished to 4 GPa of modulus and 2.3 of dielectric constant. [53](figure 7).

Low Dielectric Materials for Microelectronics 73

addition of various porogens such as the block copolymers, poly(styrerene-block-acrylic acid) [57], macromolecules of cyclodextrin [58], poly(caprolactone [59], and calix[4]arene

Many of these studies with porogens have reported materials that have excellent mechanical and electrical properties, but lack in other practical aspects for application in microelectronics. When porogens are introduced into a matrix, critical problems may occur, such as thermal degradation products acting as a poison or contaminant within the matrix or interfacial adhesion problems. Therefore, use of porogens has yet to remain a difficult

The search for materials with low dielectric constant in the microelectronics industry has and will continue feverishly into the future as the demand of faster processing speeds increases. Reduction of the dielectric constant of a material can be accomplished by selecting chemical bonds with low polarizability and introducing porosity. Integration of such materials into microelectronic circuits, however, poses a number of challenges, as the materials must meet strict requirements in terms of properties and reliability. The introduction of low-k materials in microelectronics research and development is a good example of how industrial needs drive new fundamental and applied research topics in science. Examples include pore structure characterization, deposition of thin films on porous substrates, mechanical properties of porous films, and conduction mechanisms in these materials. The substantial efforts made by materials and IC researchers to integrate the lowk films and continue historical device performance improvements have contributed to, and

process for practical applications in microelectronics.

are still leading to, innovative fundamental and applied science.

[1] Ray, GW. 1998. Mater. Res. Soc. Symp. Proc.511:199

[2] Fox, R, Pellerin JR. 1997. Res. Rep. Austin TX: SEMATECH

He Seung Lee, Albert. S. Lee, Kyung-Youl Baek and Seung Sang Hwang

*Center for Materials Architecturing, Korea Institute of Science Technology, Seoul, Korea* 

This work was financially supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea and Partially by a grant from Center for materials architecturing of Korea Institute of

[60].

**6. Conclusions** 

**Author details** 

**Acknowledgement** 

**7. References** 

Science and Technology (KIST)

**Figure 7.** Introduction of POSS moiety by sol-gel method
