Chemistry Reference
In-Depth Information
1. INTRODUCTION
Low dielectric constant materials are widely used as interlayer dielectric films in
ultra-large-scale integrated (ULSI) circuit multilevel interconnections [1-2]. The
dielectric constant of the interlayer dielectric film controls signal propagation de-
lay time and cross-talk in multilevel interconnections, and the dielectric constant
of currently used silicon dioxide (SiO
2
) films is too high for the sub-quarter mi-
crometer devices and beyond. Therefore, considerable attention has been focused
on the replacement of SiO
2
with new materials having low dielectric constant [1-
2]. It is well known that the generation of pores in materials should substantially
reduce the dielectric constant because the dielectric constant of air is only unity.
However, one of the problems is that it is difficult to maintain the desired thermal
and mechanical properties required for the ULSI manufacturing environment.
Furthermore, the incorporated air voids must be much smaller than the dimension
of microelectronic devices. Recently, Hedrick and co-workers have found that the
reduction in the dielectric constant could be simply achieved by incorporation of
nano-size air voids into PIs or poly(phenylquinoxalines) matrix [3-6]. The ap-
proach they took involved the preparation of block copolymers capable of self-
assembly in which the continuous phase was thermally stable polymer and the
dispersed phase was thermally unstable. Upon high temperature treatment, the un-
stable component undergoes thermolysis, leaving behind pores with size and
shape dictated by its initial morphology. While this technique has been demon-
strated to produce nanofoams with low dielectric constant, the synthesis proce-
dures and processing are relatively complicated. Furthermore, thermal degrada-
tion of the labile component reduces the molecular weight as well as certain
critical mechanical properties of the resulting nanofoam films.
The use of organic templates to control the structure of inorganic solids has
been proven very successful for designing porous materials with pore sizes rang-
ing from angstroms to micrometers [7-9]. In the case of microporous silicates, re-
cent reports illustrate that techniques using latex spheres can be adopted to create
silica structures with pore sizes ranging from 5 nm to 1
m. An improved proce-
dure was used by Park and Xia [10] for the fabrication of three-dimensional mi-
croporous films of polyurethane with spherical pores whose dimensions could be
precisely controlled in the range from ~ 0.2 to ~ 3
µ
m. The primary aim of their
work was to prepare films with three-dimensionally interconnected networks of
pores in the bulk and completely exposed pores on both top and bottom surfaces
of the films.
On the other hand, membrane separation of gases has emerged as an important
unit operation technique offering specific advantages over more conventional
separation procedures (e.g. cryogenic distillation and adsorption) [11-12]. Al-
though polymeric nanocomposite systems containing inorganic nanoparticles are
well known as barrier systems [13], however, Kusakabe and co-workers [14] re-
ported that the permeability of CO
2
in a PI/SiO
2
hybrid nanocomposite membrane
was 10 times larger than in the corresponding PI membrane.
µ
Search WWH ::
Custom Search