Development of High Temperature Polymers Possessing Low Dielectric Constant, Low Coefficient of Thermal Expansion, and High Transparency and Their Application to Electronic Devices.

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In microelectronic applications, polyimides (PIs) used as interlayer dielectrics are demanded to decrease both the dielectric constant (K) and the coefficient of thermal expansion (CTE). There are so far few practical low-K and low- CTE PIs. The present scientific research reports that a wholly cycloaliphatic (alicyclic) polyimide (PI) derived from cyclobutanetetracarboxylic dianhydride (CBDA) and trans-1,4-cyclohexanediamine (CHDA) accomplished a combined properties of a high Tg, a low CTE, and a low dielectric coefficient (K) simultaneously, although the PI showed no solubility in common organic solvents. It is believed that the low CTE generation mechanism is based on the in-plane chain orientation induced by thermal imidization. The solution-casting process (without imidization) of polyimide solutions does not usually cause pronounced in-plane orientation. In this work, we synthesized a novel cycloaliphatic tetracarboxylic dianhydride, 1,3-dimethyl-CBDA from photodimerization of citraconic anhydride. The results revealed that a cycloaliphatic copolyimide system derived from 1,3-dimethyl-CBDA with TFMB and cycloaliphatic diamines leads to a low CTE (24 ppm/K), a low K (2.67), high transparency (T% = 86% @400nm), and good film flexibility by the simple solution-casting of its PI varnish without imidization process. The low CTE corresponds to a high value of birefringence (An > 0.05) reflecting a high extent of in-plane orientation. The new materials developed in this work can be applied to interlayer dielectrics in semiconductor devices.