Page 290 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology En012c-604 July 26, 2001 16:2

Polymers, Thermally Stable 799

CO CO CO CO
N N O N N
CO CO x CO CO
(XLVII) C C

y
(CF 3 )
CO C(CF 3 ) 2 CO CO CO
O O O O
CO CO CO CO
(CF 3 )
CF 3
H 2 N NH 2
e.g.,
C(CF 3 ) 2 H 2 N NH 2
CF 3

H 2 N NH 2
C(CF 3 ) 2
H 2 N C NH 2
O O
CF 3

Two aspects of polyimide development that have re- considered for use as very low dielectric constant insulat-
cently received strong research attention are, first, the use ing materials.
of polymer thin films as gas separation membranes (per-
meability and permselectivity) and, secondly, their use as
b. Addition polyimides. Addition polyimides are
low dielectric constant material for microelectronic de-
based on short pre-imidized units end-capped with reac-
vices. In both of these applications, the most recent ad-
tive (unsaturated) aliphatic or cycloaliphatic groups. Poly-
vances have featured the introduction of fluoro groups
merization occurs at relatively low temperature and pres-
as links between phenylene rings and/or as phenyl-ring
sure by chain extension/cross-linking processes. Good
substituents in the polmer chain. Examples of polyimide
flow/wetting characteristics of the low-molecular-weight
dianhydride and diamine precursors are shown below.
prepolymers have played a key role in the successful de-
In both applications, the high thermal stability and ex-
velopment of such systems as matrix resins in advanced
cellent mechanical properties observed in polyimides are carbon fiber reinforced composites.
important. In the former case, simultaneous improvements Currently, bismaleimides (XLVIII) are considered to
to permeabilities and permselectivities have been achieved be the most successful of the addition polyimides based
by the reducing packing and local motion of polymer on their performance-to-cost characteristics. Alone, they
chains in the fluoro-polyimides. are most effectively thermally polymerized, in the pres-
It has been suggested that the presence of, for exam- ence of free-radical catalysts, to produce the cured resin
ple, —C(CF 3 ) 2 groups in the polyimide chain lowers the directly. Alternatively, using nonstoichiometric mixtures
dielectric constant by decreasing interchain electronic in- of BMI and aromatic diamine, under carefully controlled
teractions. Certainly, the key to the successful production conditions, cured resins are produced via the initial for-
of microelectronic components is the lowest attainable di- mation of the polyaspartimide (XLIX).
electric constant insulating material. The fluorinated poly-
imides referred to above have emerged as a favored class CO CO
of such materials. N R N
CO CO
Applications that require this high level of performance (XLVIII)
include thin-film wiring in high-density electronic pack-
aging such as large-scale and very-large-scale (LSI and R = CH 2 CH 2 C(CH 3 ) 2 CH 2
VSLI) integrated circuits; fibers used in low dielectric con-
stant laminates for high-speed multilayer printed wiring CH 3
boards (PWBs); and foamed films (“nanofoams”), incor- or
porating nanometer-length scale voids, which have been R = ( Aryl-sulfone-ether ) n ; ( Aryl-carbonate-sulfone ) n