Interlayer Dielectric Materials Research Articles

Effect of hydrogen on the integration process of photosensitive polyimide and its feasibility in advanced packaging

To reduce the resistive-capacitive (RC) delay, low-k materials are employed in advanced packaging systems that cover the redistribution layers (RDL). Owing to its remarkable properties, photosensitive polyimide (PSPI) is one of the most widely used interlayer dielectric (ILD) materials. Industries usually employ the forming gas annealing (FGA) (5 % H2 / 95 % N2 at 350 °C) in processes which help to reduce the dangling bonds at interfaces of Si/SiO2. Considering this, we have studied the effect of FGA onto photopatterned PSPI films and compared it with nitrogen gas annealing. The imidization in films at different ambientes is confirmed by Fourier Transform Infrared Spectroscopy (FTIR). X-ray photoelectron spectroscopy (XPS), and Elemental Dispersive Spectroscopy (EDS) implied to study the compositional change in PSPI after different ambient annealing. Our study reveals that FGA causes more imidization, but it forms impurities in PSPI leading to reduction in its mechanical strength and causes carbon from PSPI to diffuse inside copper interconnects. We hope the proposed research will help to understand the adverse effect of FGA annealing on the PSPI. Interestingly, N2 gas annealing is more feasible for PSPI than that of FGA 350 °C.

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.

Electrically tunable correlated domain wall network in twisted bilayer graphene

We investigate the domain wall network in twisted bilayer graphene (TBG) under the influence of interlayer bias and screening effect from the layered structure. Starting from the continuum model, we analyze the low-energy domain wall modes within the moiré bilayer structure and obtain an analytic form representing charge density distributions of the two-dimensional structure. By computing the screened electron–electron interaction strengths both within and between the domain walls, we develop a bosonized model that describes the correlated domain wall network. We demonstrate that these interaction strengths can be modified through an applied interlayer bias, screening length and dielectric materials, and show how the model can be employed to investigate various properties of the domain wall network and its stability. We compute correlation functions both without and with phonons. Including electron–phonon coupling in the network, we establish phase diagrams from these correlation functions. These diagrams illustrate electrical tunability of the network between various phases, such as density wave states and superconductivity. Our findings reveal the domain wall network as a promising platform for the experimental manipulation of electron–electron interactions in low dimensions and the study of strongly correlated matter. We point out that our investigation not only enhances the understanding of domain wall modes in TBG but also has broader implications for the development of moiré devices.

Micro-crosslinked polyimide nanocomposites with low dielectric constant and low dielectric loss for microwave antenna with molecular dynamics

Low dielectric polyimide (PI) is a crucial component of interlayer dielectric materials in modern electronic devices, which effectively mitigates signal delay and power loss issues arising from the high integration. Herein, low dielectric nanocomposite with micro-crosslinked structure is prepared through in-situ polymerization of fluorinated PI (f-PI) and polyhedral oligomeric silsesquioxane (POSS). Four f-PIs with different positions of trifluoromethyl and structure of backbone, and three functionalized POSS are studied by density functional theory (DFT) and classical molecular dynamics (MD) simulations to reveal the effects of the PI structure and the interfacial interaction of POSS with PI on dielelectric constant. Specifically, the dielectric constant and dielectric loss of 3 wt% 8NH2-POSS/PI1 at 1 kHz are reduced to 2.40 and 0.0017, respectively. The micro-crosslinked structure provides better thermal stability (513.3 °C), coefficient of thermal expansion (42.14 ppm °C−1), and acid resistance, meeting the requirements of integrated circuits (ICs) processing. In comparison to commercial epoxy boards (FR4), the 3 wt% 8NH2-POSS/PI exhibits lower S11 signal loss (−10.874 dB) and provides greater magnetic fields (2.00 G) near 3.0 GHz as the substrate of Nitrogen-Vacancy (NV) centers sensor. This well-performing low dielectric nanocomposite offers a promising medium for ICs.

Intrinsic low‐dielectric‐constant polyimides based on a novel diamine having −CF3 groups and kinkable 1,4‐diisopropylbenzene units

AbstractPolyimides (PIs) with low dielectric constants and optical transparency are of great interest in 5G/6G communication applications and in the microelectronics industry, where they are used as interlayer dielectric materials, wearable display devices and so forth. In this study, we designed and synthesized a novel fluorinated diamine called 4,4′‐(1,4‐phenylenebis(propane‐2,2‐diyl))diphenoxybis(2‐(trifluoromethyl)‐4‐aminobenzene) (BPBTAB) monomer from bisphenol‐P having diisopropyl benzene units and fabricated four novel BPBTAB‐X PIs. These PI films possess excellent properties such as a low dielectric constant (1.13–1.84 at 1 MHz), high 5% loss temperature (395–491 °C), moderate glass transition temperature (223–254 °C), good tensile modulus (0.827–1.076 GPa), excellent optical properties (T450 up to 83.37%), low moisture absorption (&lt;0.63%) and good solubility in organic solvents. Hence, the as‐synthesized PI films exhibit excellent comprehensive properties for high‐frequency 5G/6G communication applications. © 2023 Society of Chemical Industry.

Fluorinated polyimide with triphenyl pyridine structure for 5G communications: Low dielectric, highly hydrophobic, and highly transparent

With the rapid development of 5G communications, the traditional interlayer dielectric material polyimide (PI) requires modification due to its intrinsically high dielectric constant (Dk), high dielectric loss (Df), and high moisture absorption, which make it difficult to meet the needs of the high-frequency communications industry. In this paper, a new strategy for the preparation of novel PI with intrinsically low Dk and Df is proposed from the perspective of molecular structure design, the modification effect of superimposing bulky triphenyl pyridine structures and strong electronegativity groups, combined with long-chain anhydride to reduce the density of the imide structure. Therefore, three bulk diamine monomers (BPAB, PBAB3F, PBAB6F) containing triphenyl pyridine structures were designed in this paper and polymerized with 4,4′-(4,4′-isopropyldiphenoxy) phthalic anhydride (BPADA) to obtain three new PIs (PI-H, PI-CF3, PI-2CF3) with different fluorine contents, focusing on the relationship between structure and properties, especially the effect of fluorine content on dielectric properties. The results of the study show that the dielectric and hygroscopic properties of PI improve with increasing fluorine content. Among them, PI-2CF3 has the best overall performance, showing low dielectric (Dk = 2.72@10 GHz & Df = 0.00233@10 GHz), high transparency (Cut-off wavelength (λ0) = 389 nm, Maximum transmittance (Tvia) = 89%) and low hygroscopicity (Water absorption (Ma) = 0.246%, Contact angle (Ca) = 111°).

Polyimides with low dielectric constants and dissipation factors at high frequency derived from novel aromatic diamines with bistrifluoromethyl pendant groups

Fan-out wafer-level packaging (FOWLP) urgently demands low dielectric constant and dissipation factor interlayer dielectric materials to mitigate high transmission losses at high frequencies.

Emerging ionic liquid-induced porous polyimide films toward ultralow permittivity, improved hydrophobic and electromagnetic wave absorption

There is an urgent need for interlayer dielectric materials with ultralow dielectric properties, high temperature resistance and hydrophobicity in the field of microelectronics due to the development of high-density integration and miniaturization of electronic components. Herein, porous polyimide (PI) films were successfully prepared by using ionic liquid (1-Butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4]) as porogen for the first time. Based on the inherent good compatibility between [BMIM][[BF4] and polyimide, this ionic liquid could be uniformly dispersed in the PI matrix, thus creating a favorable prerequisite for the formation of pores. It was demonstrated that the optimized rearrangement of PI molecular chains induced by etching process of [BMIM][BF4] led to the generation of pores and the reduction of chain spacing, which endow the porous films excellent dielectric, thermal and mechanical properties. The film with the porosity ratio of 85 vol% exhibited minimum permittivity of 1.5 at a wide range from 102 to 106 Hz, high temperature of 5% weight loss about 532 °C. The porous PI films not only provide the promising ultralow permittivity dielectric materials for the development of modern integrated circuits, but also present potential application in adsorption separation and special electromagnetic detection.

Research progress in thermal expansion behavior of polyimide films

<p indent=0mm>As one of the most important flexible substrates and interlayer dielectric materials, the thermal expansion performance of polyimide films has always attracted great interest. Especially with the development of advanced electronic and microelectronic devices, the linear and volumetric thermal expansion of polyimide films are facing more and more stringent requirements. In order to explore new generation polyimide films with low thermal expansion, it is necessary to clarify the thermal expansion behaviors and influence mechanism. For polymer films, the thermal expansion actually includes in-plane and out-of-plane linear as well as volumetric thermal expansion. Most researches on thermal expansion of polyimide films are focused on the in-plane thermal expansion, but there are few studies on the out-of-plane and volume thermal expansion of films. There is no systematic understanding of the mechanism of thermal expansion behavior. The research on linear and volume thermal expansion behavior of polyimide films were firstly summarized in this article. The testing methods were introduced in detail, including thermomechanical analysis (TMA), variable temperature wide-angle X-ray diffraction (VT-WAXD), capacitance technique, optical interferometry, <italic>etc</italic>. The influencing factors of thermal expansion behavior of polyimide films were discussed, such as molecular backbone structures, local molecular motions, chemical crosslinking, aggregation structures and film preparation process. The relationship between structures and processes on in-plane, out-of-plane linear and volumetric thermal expansion behavior of polyimide films was clarified. The problems and challenges in the future research on thermal expansion were prospected. This review can provide a necessary guidance for the development of high-performance polyimide films with low thermal expansion in the wide temperature range.

Progress on Polymer Composites With Low Dielectric Constant and Low Dielectric Loss for High-Frequency Signal Transmission

The development of information transmission technology towards high-frequency microwaves and highly integrated and multi-functional electronic devices has been the mainstream direction of the current communication technology. During signal transmission, resistance-capacitance time delay, crosstalk, energy consumption increase and impedance mismatch restrict the high density and miniaturization of Printed circuit board (PCB). In order to achieve high fidelity and low delay characteristics of high-frequency signal transmission, the development of interlayer dielectric materials with low dielectric constant (Dk) and low dielectric loss factor (Df) has become the focus of researchers. This review introduces the dielectric loss mechanism of polymer composites and the resin matrix commonly used in several high-frequency copper-clad laminates, and mainly describes how to reduce the dielectric constant and dielectric loss of materials from the level of molecular structure design, as well as the effect of fillers on the dielectric properties of polymer substrates. As a kind of potential functional fillers for dielectric polymeric composites, the carbon nanofillers are used to tailor the dielectric properties of their composites via different dimensions and loadings, as well as their proper preparation methods. This review finally summarizes the interface bonding failure mechanism and a feasible idea to optimize the dielectric properties of polymer matrix composites is also proposed.

The Evolution of Organosilicon Precursors for Low-k Interlayer Dielectric Fabrication Driven by Integration Challenges.

Since the application of silicon materials in electronic devices in the 1950s, microprocessors are continuously getting smaller, faster, smarter, and larger in data storage capacity. One important factor that makes progress possible is decreasing the dielectric constant of the insulating layer within the integrated circuit (IC). Nevertheless, the evolution of interlayer dielectrics (ILDs) is not driven by a single factor. At first, the objective was to reduce the dielectric constant (k). Reduction of the dielectric constant of a material can be accomplished by selecting chemical bonds with low polarizability and introducing porosity. Moving from silicon dioxide, silsesquioxane-based materials, and silica-based materials to porous silica materials, the industry has been able to reduce the ILDs’ dielectric constant from 4.5 to as low as 1.5. However, porous ILDs are mechanically weak, thermally unstable, and poorly compatible with other materials, which gives them the tendency to absorb chemicals, moisture, etc. All these features create many challenges for the integration of IC during the dual-damascene process, with plasma-induced damage (PID) being the most devastating one. Since the discovery of porous materials, the industry has shifted its focus from decreasing ILDs’ dielectric constant to overcoming these integration challenges. More supplementary precursors (such as Si–C–Si structured compounds), deposition processes (such as NH3 plasma treatment), and post porosity plasma protection treatment (P4) were invented to solve integration-related challenges. Herein, we present the evolution of interlayer dielectric materials driven by the following three aspects, classification of dielectric materials, deposition methods, and key issues encountered and solved during the integration phase. We aim to provide a brief overview of the development of low-k dielectric materials over the past few decades.

Tunable dielectric performance of porphyrin based metal−organic frameworks with polar molecule confinement

With the rapid development of 5G communications, the designs of metal-organic frameworks (MOFs) with high porosity, tunable structural features as well as low dielectric constant have been gradually focused and developed with potential to be used as an interlayer dielectric (ILD) material in integrated circuits (IC). In this work, porphyrin based metal-organic frameworks (AlOH)2H2TCPP, denoted as Al-TCPP-MOFs, were synthesized by hydrothermal reaction. The structural, thermal and dielectric properties of Al-TCPP-MOFs, Al-TCPP-MOFs-H2O (polar water molecular confinement on Al-TCPP-MOFs), and Al-TCPP-MOFs-DMF (polar DMF molecular confinement on Al-TCPP-MOFs) were characterized systematically. The results showed that the Al-TCPP-MOFs material exhibited high porosity, as well as uniform lamellar structure. The role of polar molecular confinement effect of different coordination systems on the dielectric performances of MOFs were investigated, and the relative dielectric constant of Al-TCPP-MOFs was found to be as low as 2.3 at 103 Hz. Moreover, the Al-TCPP-MOFs exhibited excellent stability of dielectric properties under a wide temperature range and reliable thermal stability up to 420 °C. The preparation of low dielectric constant porphyrin-based metal–organic framework with a high thermal stability opens up a new direction for the application of MOFs in electronic devices of 5G era.

Interface design and dielectric response behavior of SiO2/PB composites with low dielectric constant and ultra-low dielectric loss

The resistance-capacitance time delay, crosstalk interference and poor dimensional stability would damage the signal intact transmission. However, it was still a tough challenge to fabricate an interlayer dielectric material with low dielectric constant and low dielectric loss as well as excellent dimensional stability at the same time. In this work, for the first time, we constructed novel silicon dioxide/polybutadiene (SiO2/PB) composites with different SiO2 morphologies and surface function groups, which showed extremely low dielectric constant and dielectric loss in both low and high frequency bands. The crosslinking reaction between surface surficial vinyl groups of the SiO2 and PB macromolecules tremendously suppressed the interfacial polarization of the SiO2/PB composites. Therefore, the SiO2/PB composites exhibited low dielectric constant of 2.66 and ultralow dielectric loss of 0.0022 at high frequency band (10 GHz), and the coefficient of thermal expansion decreased to 154 × 10−6 /°C, which was three orders of magnitude less than that of the pure PB. The interface polarization response behavior of SiO2/PB composites was also elucidated by double layer theory, and the "resonance" phenomenon was firstly proposed to enhance the interface polarization. This work could provide a robust and simple strategy for the design of composites with ultralow dielectric loss and excellent dimensional stability for high-speed communication in different frequency bands.

Optimization of zero-level interlayer dielectric materials for gate-all-around silicon nanowire channel fabrication in a replacement metal gate process

In this work, the influences of zero-level interlayer dielectric (ILD0) materials on the fabrication of silicon nanowire (SiNW) gate-all-around (GAA) transistors based on conventional mainstream FinFET replacement metal gate (RMG) technology were investigated. We find that the fins in the source/drain (SD) regions covered by conventional plasma-enhanced chemical vapor deposition (PECVD) of SiO2 are seriously eroded after NW release and surface processing. To achieve desirable device fabrication, new fabrication technologies with ILD0 materials formed by PECVD SiNx and low pressure chemical vapor deposition (LPCVD) SiNx are introduced to replace the conventional PECVD SiO2. The results demonstrate that the PECVD SiNx ILD0 device overcomes the eroding issue at SD in NW release process but causes spacers exfoliation. In contrast, the LPCVD SiNx ILD0 provides excellent preservation of the SD fins as well as the adjacent spacers. Fully isolated SiNW GAA transistors with the LPCVD SiNx ILD0 film are successfully fabricated with well electrical characteristics and higher yield. The proposed optimized approach provides a robust processing window for the fabrication of GAA NW transistors.

Mesoporous silica MCM-41-reinforced cardanol-based benzoxazine nanocomposites for low-k applications

The two different benzoxazine (Bz) monomers were synthesized from two different bisphenol-AF (BF)- and bisphenol-S (BS)-based diamines with cardanol (CL) precursor, whose molecular structures were confirmed using spectroscopy tools. Further, 2.5 wt%, 5 wt% and 7.5 wt% of phenol-3-aminopropyltriethoxysilane benzoxazine-functionalized MCM-41 (APMS) were reinforced with cardanol-bisphenol AF core diamine-based benzoxazine (BFCL-Bz) and cardanol-bisphenol-S core diamine-based benzoxazine (BSCL-Bz) and cured, which leads to increased thermal stability. Successfully developed polybenzoxazines (PBz) nanocomposites dielectric properties were studied. It was observed that the incorporation of 7.5 wt% of APMS into BFCL-PBz polybenzoxazine results an ultra-low dielectric constant value of about 1.78 at 1 MHz, whereas the APMS/BSCL-PBz composites showed 2.16 for the same weight % concentration of APMS. This may be attributed to the presence of fluorine moieties in APMS/BFCL-PBz system. Data resulted from different studies, and it is concluded that the hybrid composites developed in the present work can conveniently be used in the form of adhesives, sealants and encapsulants for low k interlayer dielectric materials for high-performance microelectronics applications.

Synthesis and characterization of novel fluorinated naphthalene-based poly(arylene ether ketone)s with low dielectric constants

At present, in the integrated circuit technology, the requirements for polymeric interlayer dielectric materials are getting higher and higher, such as high thermostability, high modulus, low dielectric constant, excellent processability. In this paper, some soluble fluorinated naphthalene-based poly(arylene ether ketone)s were prepared from three types of novel fluorinated naphthalene-based bisphenol monomers. With the introduction of fluorine atoms and naphthalene rings, the obtained polymers exhibited more excellent thermal stability than traditional poly(ether ether ketone),which the glass transition temperatures (Tg) were above 167 °C, and decomposition temperatures at 5% (Td5) were above 528 °C under nitrogen. The tensile strengths of membranes were in the range of 62.86–104.13 MPa, tensile modulus in the range of 1.96–3.83 GPa. Moreover, the poly(arylene ether ketone)s with 1-(3,5-ditrifluoromethylbenzoyl) (NPEEK-3) exhibited the lowest dielectric constant at 1 MHz and water uptake which were 2.57 and 0.31%, respectively. The dielectric constant is much lower than commercialized poly(ether ether ketone) of 3.3.

Investigation on Electrical Properties of Hydrogen Doped Sputtered Boron Carbon Nitride Thin Films

Interlayer dielectric materials (ILD) play a crucial role of isolating electrically conductive and semiconducting features on the devices [1]. Due to constant scaling down of device dimensions, the interconnect delay (τ = R x C) can be reduced by decreasing resistance of conductor and capacitance of ILD. Reduced capacitance can be achieved by high quality insulating film with low dielectric constant. Boron carbon nitride (BCN) films have achieved attention due to their complicated crystalline structure and unique properties. Atomic bonding similarities amongst boron, carbon and nitrogen allow formation of compounds with wide compositional range. BCN compounds are expected to combine the excellent properties of diamond, B4C and BN with their properties adaptable depending on their composition and structure [2, 3]. In this study, electrical properties of hydrogen doped BCN thin films will be investigated. BCN films are sputtered by B4C and BN targets. Films are deposited at different temperatures in presence of hydrogen/argon to investigate the influence of temperature and hydrogen gas on the electrical properties of BCN thin films.

Review—Beyond the Highs and Lows: A Perspective on the Future of Dielectrics Research for Nanoelectronic Devices

High-dielectric constant (high-k) gate oxides and low-dielectric constant (low-k) interlayer dielectrics (ILD) have dominated the nanoelectronic materials research scene over the past two decades, but they have recently reached a state of maturity and perhaps the limits of their scaling. Based on this, there is a need for a systematic review summarizing not only the historic research and achievements on high-k and low-k dielectrics, but also emerging device applications as well as an outlook of future challenges. We begin by first reviewing the factors that drove the emergence of low-k and high-k materials in nanoelectronics as ILD and gate dielectric materials, respectively, and the challenges and limits these materials ultimately approached in terms of permittivity scaling. We then illustrate that gate dielectric and ILD applications represent just a small fraction of the numerous dielectrics utilized in present day nanoelectronic products where permittivity scaling is now being increasingly demanded for materials such as dielectric spacers, trench isolation, and etch stopping layers. We conclude by examining the numerous new applications for dielectric materials that are emerging as the semiconductor industry transitions to novel patterning schemes, prepares for life post CMOS scaling, and explores ways to natively embed device functionality in the metal interconnect. For the former, we specifically examine the “colorful” requirements for the various enabling dielectric hardmask and spacer materials utilized in pitch division-multi-pattern processes and then discuss the role that selective area deposition of dielectrics and metals could play in reducing the complexity of such patterning processes. For the latter, we review the use of both high-k and low-k dielectrics in various metal-insulator-metal (MIM) structures as Fermi level de-pinning layers, tunnel diodes, and back-end-of-line (BEOL) compatible capacitive and resistive switching random access memory (ReRAM) elements. We further examine how dielectrics can hinder or aid new forms of computing such as quantum and neuromorphic in reaching their full potential. In conclusion, we find that while the field of dielectrics has a long history, it remains vibrant with numerous exciting new and old research vectors awaiting further exploration.

Dynamic Temperature Aware Scheduling for CPU-GPU 3D Multicore Processor with Regression Predictor

The 3D stacked integration of CPU, GPU and DRAM dies is a rising horizon in chip fabrication, where dies are vertically interconnected by TSVs (Through-Silicon Vias) to achieve high bandwidth, low latency and power consumption. However, thinned substrate, high power density and low thermal conductivity of inter-layer dielectric material cause thermal management a crucial problem. Moreover, the vertically stacked dies are susceptible to tight thermal correlations. High temperatures which tend to show higher spatial/temporal localities can make a negative impact on the IC’s reliability and lifetime. To mitigate such problems on CPU-GPU 3D heterogeneous processors, a novel dynamic temperature-aware task scheduling approach for compute workloads using OpenCL framework is proposed in this work. The proposed scheduler predicts the future temperature of each core from a regression model based on its current temperature, the neighbors’ temperatures and the execution profile of each workgroup. The scheduler then selects a core to assign workgroups from task queue based on their predicted temperature to keep the 3D chip below certain threshold temperature. Our experimental results demonstrate that the proposed scheduling technique is a viable solution to address the hotspots and heat dissipation issue of 3D stacked heterogeneous processors under reasonable performance tradeoffs.

Scrutiny of Leakage Currents with Insulating Materials for Transistor Applications

Continuous reducing the size of transistor technology has enabled extraordinary improvements in the switching speed, density, functionality and cost of microprocessors. Integrated Circuit industry is pursuing Moore’s curve down to deep-nanoscale dimensions. Advanced transistor technology now faces many challenges that together result in static power consumption due to leakage currents. In fact, leakage currents are responsible for more than 50% of the total power consumption in nanoscale designs. In deep-nanoscale arena, this percentage will increase further. However, diagnosing of the interface quality and interaction between insulators and semiconductors is significant to reduce the leakage current and achieve the high performance of switching devices in the nanoscale domain. Continuous scaling down has required drastic decreases of the SiO2 dielectric film thickness to achieve ever-higher capacitance densities. Fundamental limits of SiO2 as a dielectric material, imposed by electron tunneling, will be reached as this SiO2 film thickness approaches ~1nm. Therefore, alternate high-k interlayer dielectric material will be needed to replace SiO2 as a capacitor and gate dielectric material. Numerous alternate high-k materials are being actively investigated, ranging from Al2O3 (k ~ 9) to HfO2 (k ~ 25). High-k materials hold the promise of achieving very high capacitance densities with relatively thick films.