A Review Article on the Synthesis and Application of Low-Dielectric Polyimide was Published in Resources, Chemicals and Materials by the Team from the Key Laboratory of Rare Earth Chemistry and Applications of the School of Science, Liaoning Province

 With the advent of the 5G era, new technologies such as Fan-Shaped Wafer-Level Packaging have emerged to reduce signal loss and enhance signal transmission rates. Interlayer dielectric materials, as one of the crucial materials in telecommunications devices, directly affect signal transmission and device reliability. Polyimide (PI) has become an important interlayer dielectric material due to its excellent comprehensive properties (Figure 1). PI boasts outstanding electrical insulation properties (with a dielectric constant of approximately 3.0-4.0 and a dielectric loss of around 0.02), superior mechanical properties, and thermal resistance, making it widely used in flexible dielectric materials. To meet the demands of 5G networks for high-frequency and high-speed circuits, it is necessary to further reduce the dielectric constant and dielectric loss of PI.

 Figure 1 Synthesis Route and Structure of Polyimide (PI)

A comprehensive review of recent research on low-dielectric PI over the past five years has been conducted by the team led by Professor Li Wenze from the Key Laboratory of Rare Earth Chemistry and Applications of the School of Science, Liaoning Province, China.

1.Factors Influencing Low-Dielectric PI-Based Materials

Dielectric properties are crucial indicators for evaluating the electrical insulation performance of polymer materials. They describe the electrostatic energy storage and loss characteristics of dielectric materials under the influence of an electric field, encompassing two physical parameters: dielectric constant and dielectric loss. Typically, the dielectric constant of conventional PI films ranges from 3.1 to 3.6, which falls short of meeting the requirements for high-frequency and high-speed circuits (Figure 2). Consequently, research on low-dielectric PI materials has garnered increasing attention in recent years. One of the internal factors influencing the dielectric constant is the material's polarizability. According to polarization theory, polymer materials may exhibit four polarization modes. In the first part of the paper, the authors provide an overview of electronic polarization, atomic polarization, orientational polarization, and interfacial polarization.

Figure 2 Synthesis Route of Three-Layer Polyimide (PI) Films

2.Research Progress in Low-Dielectric PI

In recent years, low-dielectric constant PI materials have received increasing attention, and various research teams have employed different methods to prepare low-dielectric constant PI composites (Figure 3). In the second part of the paper, the authors provide an overview of the primary methods for reducing the dielectric constant of PI.

Figure 3 Process for Preparing Porous BN/PI Composite Films Based on HIPPE and Hot Pressing

3.Research Progress on fluorine-containing low dielectric PI

Introducing fluorine-containing structures in the main chain of PI molecules is one of the important methods to reduce dielectric constant PI. Under normal circumstances, the dielectric constant of fluorine-containing PI can be reduced to 1.8 to 2.9(figure 4). The possibility of this low-dielectric constant is attributed to two reasons: firstly, the fluorine possesses the highest electronegativity of all elements, so the polarization of the carbon-fluorine bond is very small; secondly, the volume of the fluorine-containing group is comparatively larger, and the introduction of fluorine atoms can increase the free volume of the polymer matrix. In the third part of this thesis, the author reviewed the research work on fluorine- containing low-dielectric PI.

Figure 4 preparation process of fluorine-containing polyimide (PI)

 In the end, the author pointed out that although PI-based materials have many ideal properties and are candidates for microelectronic devices. However, the following key challenges need to be addressed in order to enhance their applicability: (1) how to prepare the PI composites with low dielectric constant and low-dielectric loss. (2) How to fabricate non-porous PI composites with low-dielectric constant and low loss coefficient. (3) While low-dielectric PI displays strong application promise in many fields, research into these materials is still in its start-up phrase. There are still many hurdles to overcome when it comes to performance optimization and equipment stability. Significant progress has been made in the study of low dielectric PI, but there are also some technical problems. So far, the synthetic yield of low-dielectric PI-based materials has been very low. In order to achieve industrial-scale production, how to synthesize low-dielectric PI with higher efficiency and better stability remains a challenge.

This achievement was published in the English journal Resources Chemicals and Materials (RCM) sponsored by our university under the title of “Synthesis and Applications of Low Dielectric Polyimide”, and served as a back cover article for the current issue (Figure 5). Associate Professor Liu Yu of Shenyang University of Chemical Technology is the first author of this review paper, Professor Li Wenze and Dr. Luan Jian are the corresponding authors, and Professor Sun Yaguang, Associate Professor Zhang Xiaosa, and master’s degree student Zhao Xiaoyu participated in the writing.

Figure 5 Schematic diagram of the synthesis and application of low-dielectric polyimide (back cover image of the review article).

Y. Liu, X. Y. Zhao, Y. G. Sun, et al. Synthesis and applications of low dielectric polyimide [J]. Resources Chemicals and Materials, 2023, 2, 49–62.

DOI: 10.1016/j.recm.2022.08.001

https://doi.org/10.1016/j.recm.2022.08.001