With the rapid development of microelectronic industry, electronic components tend to be miniaturized and chip interconnection density has also increased sharply [1–3]. Fan-out wafer-level packaging (FOWLP) has been one of the latest packaging forms with the ultrathin redistribution layer (RDL) and higher I/O density[4, 5].Thereinto, polyimide (PI) has become one of the most important dielectronic materials in RDL with superior thermal, mechanical property, low dielectronic constant and high chemical resistance and adhesion on various substrates [6–8]. However, new requirements have been appeared with the up-coming applications of 5G, AI, automate drive, internet of thing. One important challenge is about the data transmission loss in the high frequency communication which would result in high signal transmission loss, destroy signal integrity[9–11]. At the same time, the dielectric loss of the insulating dielectric materials will also increase in the high frequency[12–14]. Therefore, polymers with low dielectric constant and low dielectric loss have become an important research direction. In addition, the traditional precursor of poly (amic acid) (PAA) or poly (amic ester) (PAE) are typically imidized at a relatively high temperature of about 350 oC for the preparation of polyimide, which limits the application in semiconductor devices of FOWLP[15, 16]. In details, a high thermal curing process would cause a warpage of ultrathin silicon wafer because of the coefficient of thermal expansion (CTE) mismatch and even bringing about irreversible damage to chips. Therefore, low-dielectric (< 3.0) and low-temperature (< 250 oC) curable PI materials are highly required for advanced electronic package in FOWLP[17–19].
Firstly, there are several methods to realize the low temperature curing of polyimide. For example, the control of the monomer structures: this method is usually carried out by introducing flexible structures to destroy the conjugated structure of the polyimide molecular chain and increasing the movement of its molecular chain, which helps to realize the low temperature curing[20]. However, the introduction of flexible structures would influence the original mechanical and thermal properties of polyimide resulting in the high CTE and limited thermal stability[21]. Secondly, one-step preparation of PI in solution: with high boiling solvents (p-chlorophenol, benzoic acid, polyphosphoric acid), the polyimide molecular chains can realize fully imidization at a low temperature as well. Nevertheless, the synthesized polyimide must be soluble in the limited kinds of solvents, and its adhesion is also restricted as there are no more carboxyl groups in the following application process[22, 23].Thirdly, curing catalysts for low-temperature curing: the imidization of PI can also be effectively promoted by the introduction of curing catalysts[24].The catalyst, serving as a nucleophile, will attack the carbon atom on the carboxyl group in PAA and transfer the hydrogen atom to the carboxyl group, thus trapping hydrogen on the amide bond and promoting the cyclization and dehydration of the acid group to complete the imidization reaction. This method has been widely used in various low-temperature curing PI, which is of great practical importance to improve industrial production efficiency and reduce energy consumption[25]. For example, Chao Huang et al[26].adopted pyromellitic dianhydride (PMDA) and 4,4,-oxydiphenylamine (ODA) and 6 kinds of low-temperature curing accelerators were introduced to investigate their low-temperature imidization effect, including the quinoline (QL), isoquinoline (IQL), 2-methylquinoline (2-QL), 4-methylquinoline (4-QL), 8-methylquinoline (8-QL), and 5,6,7,8-tetrahydroquinoline (THQL). The results proved that high imidization rates of 98% could be achieved for PI-4-QL, PI-8-QL and PI-QL at 200°C. Although low-temperature curing was successfully achieved, the large amount addition of the low-temperature curing accelerators would result in a risk of volatilization in the imidization and the usage which should be completed avoided in the semiconductor industry. Then, Yuying Sui et al[27]. prepared the low temperature cured polyimide by introducing a 5-aminobenzimidazole through covalent bonding with PI chains. The results showed a complete imidization at 200 oC, within 2 h, the imidization index could be as high as 1.03. Furthermore, the as-prepared PI also exhibited superior thermal and mechanical properties, but the dielectric properties are not ideal. More recently, Changqing Li [28]et al. designed and synthesized 4,4,-(pyrazine-2,5-diyl)dianiline (PRZ) monomer to study properties of low temperature cured polyimide. This study revealed that PRZ monomer can effectively improve imidization degree of low temperature cured PI films of 10–13% under 200 oC, which was calculated by FTIR spectrum. In addition, the low temperature cured polyimide with improved imidization degree possess excellent mechanical, thermal and dielectric properties. However, even though the low temperature curing of PI could be achieved by the design of the polymer system or its structure, the relative high dielectric constant (> 3.35@5 G) is still limited which can’t meet the demand for the high frequency applications.
Up-to-now, there are still very limited works reported for the low temperature curing and low dielectric PIs. For example, Guoping Zhang’s group[29] introduced aminopropyl isobutyl polysilsesquioxane (POSS) with single vertex activity by in situ polymerization resulting in the PI-POSS nanocomposites. Low dielectric constant (k < 2.6) and low-temperature curing at only 200°C (99.4% imidization) with the blending of quinoline was also successfully achieved. However, the addition of POSS is as much as 10% and the blending curing catalysts would remain in the PI films after the imidization and resulted in the problem of gas release in their application. In addition, Tao Wang et al[30]. prepared aminoquinoline-functionalized graphene oxide (AQL-GO), which ingeniously achieved the low temperature imidization and low dielectric requirements of polyimide nanocomposites by nucleophilic attack of QL and chaotic packing of the polymer chain. The results show that the nanocomposite can be fully cured at 200 oC and exhibit a low dielectric constant (2.96@1 MHz) with 0.5 wt% addition of AQL-GO. However, the modification process of GO is complicated and micron-sized AQL-GO is prone to agglomeration, resulting in a limited reduction of dielectric properties.
In this work, low temperature curable and low dielectric PI nanocomposites are successfully prepared by covalent bonding of 6-aminoquinoline (AQL) and in-situ polymerization of fluorinated nanocarbon (FC). The as-prepared fluorinated nano carbon/polyimide (FCPI) composites realized fully imidization at 200 oC by very few introduction (2.76%) of AQL and low dielectric constant (2.75 @ 1MHz) with only 1 wt% addition of FC. Besides, FCPIs also prove excellent mechanical and thermal performance, showing a wide range of application prospects in advanced electronic package.