High-performance 2D electronic devices enabled by strong and tough two-dimensional polymer with ultra-low dielectric constant

As the feature size of microelectronic circuits is scaling down to nanometer order, the increasing interconnect crosstalk, resistance-capacitance (RC) delay and power consumption can limit the chip performance and reliability. To address these challenges, new low-k dielectric (k < 2) materials need to be developed to replace current silicon dioxide (k = 3.9) or SiCOH, etc. However, existing low-k dielectric materials, such as organosilicate glass or polymeric dielectrics, suffer from poor thermal and mechanical properties. Two-dimensional polymers (2DPs) are considered promising low-k dielectric materials because of their good thermal and mechanical properties, high porosity and designability. Here, we report a chemical-vapor-deposition (CVD) method for growing fluoride rich 2DP-F films on arbitrary substrates. We show that the grown 2DP-F thin films exhibit ultra-low dielectric constant (in plane k = 1.85 and out-of-plane k = 1.82) and remarkable mechanical properties (Young's modulus > 15 GPa). We also demonstrated the improved performance of monolayer MoS2 field-effect-transistors when utilizing 2DP-F thin films as dielectric substrates.

Engineering the Dielectric Constants of Polymers: From Molecular to Mesoscopic Scales

Polymers are essential components of modern-day materials and are widely used in various fields. The dielectric constant, a key physical parameter, plays a fundamental role in the light-, electricity-, and magnetism-related applications of polymers, such as dielectric and electrical insulation, battery and photovoltaic fabrication, sensing and electrical contact, and signal transmission and communication. Over the past few decades, numerous efforts have been devoted to engineering the intrinsic dielectric constant of polymers, particularly by tailoring the induced and orientational polarization modes and ferroelectric domain engineering. Investigations into these methods have guided the rational design and on-demand preparation of polymers with desired dielectric constants. This review article exhaustively summarizes the dielectric constant engineering of polymers from molecular to mesoscopic scales, with emphasis on application-driven design and on-demand polymer synthesis rooted in polymer chemistry principles. Additionally, it explores the key polymer applications that can benefit from dielectric constant regulation and outlines the future prospects of this field.

Achieving an Ultralow-κ yet Strong and Transparent Film by Antisolvent-Confined Nanowelding of Electrospun Polyimide Nanofibers

Developing ultralow-κ (dielectric constant) polyimides (PIs) that are mechanically robust while also being optically transparent is challenging. For the first time, we report a nanoporous PI film with an ultralow κ of 1.8 in combination with a tensile strength of up to 180 MPa, a Young's modulus of up to 6 GPa, and a transmittance of ∼88%. This is achieved by direct nanowelding of a porous electrospun PI nanofiber membrane using a simple mixture of ethanol-dominating DMAc. Benefiting from the effective evaporation of the antisolvent ethanol upon heating, the proposed nanowelding approach allows for the localized surface dissolution of the PI nanofibers, which enables the dissolved PI to "glue" the nanofibers and occupy vacant space in the membrane, resulting in the formation of a dense but nanoporous self-reinforced nanocomposite film. Our findings provide a renewed understanding of the potential of electrospun nanofibrous materials, and the underlying principle can hopefully be applied to other commodity polymers.

Synthesis of High-Performance Colorless Polyimides with Asymmetric Diamine: Application in Flexible Electronic Devices

Colorless polyimides (CPIs) are widely used as high-performance materials in flexible electronic devices. From a molecular design standpoint, the industry continues to encounter challenges in developing CPIs with desired attributes, including exceptional optical transparency, excellent thermal stability, and enhanced mechanical strength. This study presents and validates a method for controlling 2-substituents, with a specific emphasis on examining how these substituents affect the thermal, mechanical, optical, and dielectric characteristics of CPIs. The presence of two CF3 groups on the same side of the diamine structure ensured the transmittance of the film. The charge transfer effect and the molecular distance are dynamically regulated by changing the 2-substituent (-OCH3/-CH3/H/F). The polyimide exhibited a well-maintained equilibrium between transparency and thermal stability, with a T500nm value ranging from 86.2 to 89.6% in the visible region, and a glass transition temperature (Tg) ranging from 358.6 to 376.0 °C. Additionally, the 6FDA-2-MTFMB compound, when combined with methyl, excels as a protective layer and base material, exhibiting excellent performance in various aspects. It has been verified as an appropriate option for flexible photodetectors and wearable piezoresistive sensors. In summary, this systematic investigation will provide a comprehensive and demonstrative methodology for developing CPIs that are capable of adapting to flexible electronic devices.

Preparation and Properties of Fluorinated Poly(aryl ether)s with Ultralow Water Absorption and Dielectric Constant by Cross-Linked Network Strategy

Poly(aryl ether) materials are used in a wide range of applications in the communications and microelectronics fields for their outstanding mechanical and dielectric properties. In order to further improve the comprehensive performance, this work reports a series of cross-linkable poly(aryl ether)s (UCL-PAEn) containing trifluoroisopropyl and perfluorobiphenyl structures using 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 2,2'-diallyl bisphenol A, and perfluorobiphenyl as starting materials. Their chemical structures and the effect of changes in the allyl content on the properties are thoroughly investigated. Owing to the introduction of fluorine atoms and cross-linked networks, the cross-linked poly(aryl ether) films present low dielectric constants (Dk = 1.93-2.24 at 1 MHz), low water absorption (0.14% -0.25%), and hydrophobic film surfaces (94.3-99.4°). Additionally, because of the presence of cross-linked networks, the CL-PAEn films exhibit superior thermal stability, with the 5% weight loss temperatures all above 445 °C and the maximum thermal decomposition rate temperatures all above 550 °C. The cross-linked films also demonstrate excellent mechanical properties, with tensile strength in the range of 57.1 -146.7 MPa and tensile modulus in the range of 1.8 GPa-4.5 GPa.

Versatile Landscape of Low-k Polyimide: Theories, Synthesis, Synergistic Properties, and Industrial Integration

The development of microelectronics and large-scale intelligence nowadays promotes the integration, miniaturization, and multifunctionality of electronic and devices but also leads to the increment of signal transmission delays, crosstalk, and energy consumption. The exploitation of materials with low permittivity (low-k) is crucial for realizing innovations in microelectronics. However, due to the high permittivity of conventional interlayer dielectric material (k ∼ 4.0), it is difficult to meet the demands of current microelectronic technology development (k < 3.0). Organic dielectric materials have attracted much attention because of their relatively low permittivity owing to their low material density and low single bond polarization. Polyimide (PI) exhibits better application potential based on its well permittivity tunability (k = 1.1-3.2), high thermal stability (>500 °C), and mechanical property (modulus of elasticity up to 3.0-4.0 GPa). In this review, based on the synergistic relationship of dielectric parameters of materials, the development of nearly 20 years on low-k PI is thoroughly summarized. Moreover, process strategies for modifying low-k PI at the molecular level, multiphase recombination, and interface engineering are discussed exhaustively. The industrial application, technological challenges, and future development of low-k PI are also analyzed, which will provide meaningful guidance for the design and practical application of multifunctional low-k materials.

Facile strategy for intrinsic low-κ dielectric polymers: molecular design based on space charge conservation

The growing need for high-power and compact-size microelectronic integrated circuits (ICs) in modern microelectronic industries and 5G communication systems demands low dielectric constant (κ) polymer dielectrics with excellent temperature capability, mechanical property and processability. However, conventional molecular design strategies often face difficulties of a trade-off between optimizing the dielectric performance of polymers and maintaining the aforementioned properties. Herein, we present an innovative and facile strategy that utilizes the space charge distribution characteristics of the target co-monomer to solve this trade-off. Based on this design strategy, a novel polyaryl ether ketone (PAEK) with two different charge distribution units (BAF and SBI) was designed and synthesized. Both the experimental results and computational simulations confirm that these two components serve to weaken the polarization of molecular chains in the electric field, induce higher molecular chain packing density and fewer weaknesses, and synchronously regulate the κ, dielectric loss (tan δ), thermal and mechanical properties and processability by generating a strong inter-chain electrostatic interaction. The resultant copolymer, PAEK-4F6S, exhibits exceptional low κ and tan δ values of 1.98 and 0.0024 at 1 MHz, respectively, and these values remain stable over a broad frequency (1-106 Hz, 8.2-12.4 GHz) and temperature range (30-150 °C). Furthermore, the resultant copolymer demonstrates excellent thermal stability and mechanical properties, with a glass transition temperature (Tg) of 195 °C, 5 wt% decomposition temperature (Td5%) of 498 °C under N2, tensile strength of 63.5 MPa and tensile modulus of 1011.2 MPa, respectively. The synthesis procedure of these resultant copolymers is facile, and they are found to have favorable solution and melt processing properties, making them suitable for processing and scalable production. More importantly, this design strategy is beneficial for lowering the κ and tan δ values, and simultaneously enhancing the comprehensive performances of the objective polymers, which provides a completely novel and facile approach for the design and fabrication of high performance low-κ polymers suitable for the needs of microelectronics and communication fields.

Crown Ether Copolymerized Polyimide Film: Enhanced Mechanical, Thermal Properties and Low Dielectric Constant under High Frequency

Polymer materials with a low dielectric constant and low dielectric loss have the potential to be applied to high-frequency signal transmissions, such as mobile phone antennas and millimeter wave radars. Two types of diamines, 4,4'-diamino-p-tetraphenyl (DPT) and crown ether diamine (CED), were prepared for ternary copolymerization with BPDA in this study. Cross-links with molecular chains were formed, increasing molecular chain distance by utilizing rings of CED. The MPI films exhibit a good thermal performance with the increase in CED addition, with Tg > 380 °C and CTE from -4 × 10-6 K-1 to 5 × 10-6 K-1. The Young's modulus can reach 8.6 GPa, and the tensile strength is above 200 MPa when 5% and 7% CED are introduced. These MPI films exhibit good mechanical performances. The dielectric constant of PI-10% film can go as low as 3.17. Meanwhile, the relationship between dielectric properties and molecular structure has been demonstrated by Molecular Simulation (MS). PI molecules are separated by low dielectric groups, resulting in a decrease in the dielectric constant.

Progress of research on the bonding-strength improvement of two-layer adhesive-free flexible copper-clad laminates

The arrival of the 5G era has placed high demands on the electronic products. Developing thin, light, and portable electronic products capable of simultaneously improving the transmission rate and reducing the signal delay and transmission loss is necessary to meet such demands. The traditional three-layer, adhesive, flexible copper-clad laminate (3L-FCCL) cannot satisfy these demands because of its adhesive component. The large thickness and poor heat resistance disadvantages of 3L-FCCL can be avoided with a two-layer, adhesive-free, flexible copper-clad laminate (2L-FCCL). However, 2L-FCCL has low bonding strength. This work introduces the selection of conductor materials and insulating base films for flexible copper-clad laminates. Modification studies aimed at increasing the bonding performance of 2L-FCCL are summarized based on three aspects. These modification techniques include the surface treatment of copper foils, modification and surface treatment of polyimide films, and surface treatment of liquid-crystal polymers. Prospects are further provided.

Rising of Dynamic Polyimide Materials: A Versatile Dielectric for Electrical and Electronic Applications

Polyimides (PIs) are widely used in circuit components, electrical insulators, and power systems in modern electronic devices and large electrical appliances. Electrical/mechanical damage of materials are important factors that threaten reliability and service lifetime. Dynamic (self-healable, recyclable and degradable) PIs, a promising class of materials that successfully improve electrical/mechanical properties after damage, are anticipated to solve this issue. The viewpoints and perspectives on the status and future trends of dynamic PI based on a few existing documents are shared. The main damage forms of PI dielectric materials in the application process are first introduced, and initial strategies and schemes to solve these problems are proposed. Fundamentally, the bottleneck issues faced by the development of dynamic PIs are indicated, and the relationship between various damage forms and the universality of the method is evaluated. The potential mechanism of the dynamic PI to deal with electrical damage is highlighted and several feasible prospective schemes to address electrical damage are discussed. This study is concluded by presenting a short outlook and future improvements to systems, challenges, and solutions of dynamic PI in electrical insulation. The summary of theory and practice should encourage policy development favoring energy conservation and environmental protection and promoting sustainability.

Polyimide/crown ether composite film with low dielectric constant and low dielectric loss for high signal transmission

Dielectric properties of polyimide (PI) are constrained by its inherent molecular structure and inter-chain packing capacities. The compromised dielectric properties of PI, however, could be rescued by introducing trifluoromethyl and forming a host-guest inclusion complex with the introduction of crown ethers (CEs). Herein, we report PI/crown ether composite films as a communication substrate that could be applied under high frequency circumstances. In this work, three kinds of bisphenol A-containing diamine (2,2'-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(2-methyl-4-aminophenoxy)phenyl]propane, and 2,2-bis[4-(2-trifluoro methyl-4-aminophenoxy)phenyl]propane) are synthesized and polymerized with 4,4'-(hexafluoroisopropylidene)diphthalic anhydride to prepare low-dielectric PI films by means of thermal imidization. Crown ethers are introduced into the PI with different mass fractions to obtain three series of PI films. Following the combination of trifluoromethyl into the molecular chain of PI, high frequency dielectric loss of modified PI films can be effectively reduced. The properties of these materials (especially the dielectric properties) are thoroughly explored by crown ether addition. The results show that the crown ether addition process can offer crown ethers with increased free volume of PI matrix, thus allowing them to generate a special necklace-like supramolecular structure, which makes the crown ether disperse more uniformly in the PI matrix, resulting in improved dielectric properties. Importantly, the dielectric constant and dielectric loss of the composite films at high frequencies are remarkably reduced to 2.33 and 0.00337, respectively. Therefore, these composite films are expected to find extensive use as a 5G communication substrate at high frequencies in the future.

Design and synthesis of novel poly (aryl ether ketones) containing trifluoromethyl and trifluoromethoxy groups

The high-frequency and high-speed communication in the 5 G era puts forward requirements for the dielectric properties of polymers. Introducing fluorine into poly(ary ether ketone) can improve its dielectric properties. In this work, by introducing the fluorine group strategy, we successfully designed and synthesized three novel trifluoromethyl (-CF₃) or trifluoromethoxy (-OCF₃)-containing bisphenol monomers and their F-substitution PEK-based polymers (PEK-Ins). All these PEK-Ins exhibited good thermal, mechanical and dielectric properties. The T d5% of the three polymers is all higher than 520℃. The free volume fraction of novel polymers increased from 3.75% to 5.72%. Among the three polymers, exhibited the lowest dielectric constant of the films is 2.839, and the dielectric loss is 0.0048, ascribing to the increasing free volume. The Young's modulus of the polymer film is as high as 2.9 GPa and the tensile strength is as high as 84 MPa. PEK-Ins reduced the dielectric constant by introducing a low fluorine content. This study provides a new way to design PEK to synthesize low dielectric constant polymers.

Dual cross-linking strategy to prepare fluorine-containing poly(arylene ether nitrile) films with a low dielectric constant and ultra-low water uptake

Dual cross-linked networks based on fluorine-containing poly(arylene ether nitrile) were constructed by a simple thermal treatment for obtaining flexible low-dielectric materials with excellent comprehensive performance.