Low-k Materials Research Articles

Etch Characteristics of Low-K Materials Using CF3I/C4F8/Ar/O2 Inductively Coupled Plasmas

In this study, pulsed CF3I/C4F8/Ar/O2 inductively coupled plasmas have been studied for low-k etching, and the effects of CF3I addition to C4F8/Ar/O2 on the plasma characteristics and etch characteristics of low-k materials were investigated. The increased ratio of CF3I/(CF3I+C4F8) in the gas mixture increased CF3 radicals while decreasing CF2 radicals in the plasma, and which are related to the etching and polymerization, respectively. Therefore, the etch rates of SiCOH increased with increasing the CF3I ratio. However, the etch selectivity over an amorphous carbon layer and photoresist was the highest at the ratio of 0.5 because the CF2/F flux ratio from the plasma and the C/F ratio on the polymer layer were the highest at the CF3I ratio of 0.5. The SiCOH damage was decreased with increasing CF3I ratio and the SiCOH damage appeared to be very low, particularly when the CF3I ratio was ≥0.5 by showing low Si–CH3 bond loss, low F penetration, and a low surface roughness. Therefore, it is believed that, as opposed to the C4F8/Ar/O2 gas mixture only, mixing 50% CF3I into the C4F8/Ar/O2 gas mixtures resulted in not only a high etch selectivity over mask materials, but also a potentially reduced etch damage.

Facile Strategy for Preparing a Rosin-Based Low-k Material: Molecular Design of Free Volume.

Low-k dielectrics are urgently needed in modern integrated circuits. The introduction of free volume instead of porous structures has become a powerful strategy to reduce the k value. According to this strategy, the biomass resource rosin-containing hydrogenated phenanthrene ring was introduced into benzocyclobutene (BCB) resin to reduce the k value; then a rosin-based BCB monomer was successfully synthesized. Meanwhile, the BCB monomer without a rosin skeleton was prepared. After converting the monomers into thermo-crosslinked materials, notably that the rosin skeleton has a great influence on the free volume and k value of the material. The fractional free volume and k value of the former are 26% and 2.44, respectively, and those of the latter are 14% and 2.84, respectively. In addition, the distances between molecular chains and the density of the former are 0.60 nm and 1.06 g cm-3, respectively; those of the latter are 0.56 nm and 1.28 g cm-3, respectively. These data show that introducing hydrogenated phenanthrene rings occupies part of the space and hinders the packing of molecular chains, which increases the distance between molecular chains and reduces the density of the polymer, resulting in an increasing free volume and a reducing k value. Notably that introducing hydrogenated phenanthrene rings cannot affect other properties of the material. Therefore, this research indicates that introducing rosin skeletons can prepare high-performance materials, which provide some promising low-k materials for the development of electronics and microelectronics.

The Dielectric Behavior of Protected HKUST-1

We investigated the adsorption properties and the dielectric behavior of a very well-known metal-organic framework (MOF), namely Cu3(BTC)2 (known as HKUST-1; BTC = 1,3,5-benzenetricarboxylate), before and after protection with some amines. This treatment has the purpose of reducing the inherent hygroscopic nature of HKUST-1, which is a serious drawback in its application of as low-dielectric-constant (low-κ) material. Moreover, we investigated the structure of HKUST-1 under a strong electric field, confirming the robustness of the framework. Even under dielectric perturbation, the water molecules adsorbed by the MOF remained almost invisible to X-ray diffraction, apart from those directly bound to the metal ions. However, the replacement of H2O with a more visible guest molecule such as CH2Br2 made the cavity that traps the guest more visible. Finally, in this work we demonstrate that impedance spectroscopy is a valuable tool for identifying water sorption in porous materials, providing information that is complementary to that of adsorption isotherms.

Ultralow dielectric cross-linked silica aerogel nanocomposite films for interconnect technology

An increase in resistance and capacitance (RC) delay has been a critical issue in reducing integrated circuit scales. The delay is successfully reduced using low dielectric constant (κ) interconnects and porous low-κ materials adopted in real devices. However, an increase in porosity could lower κ values with a dramatic decrease in elastic modulus, which would not sustain the stress during the packaging process. Highly porous cross-linked silica aerogel thin film with an ultralow-κ value of 1.7 applied to nanodevice interconnect technology has been suggested. Cross-linking using an epoxide ring-opening reaction produced a high elastic modulus (> 5.1 GPa), stable enough to sustain the chemical mechanical polishing (CMP) and the plasma dry etching process. The thickness of aerogel films is approximately 700 nm with a hydrophobic surface. Closed-surface pores created by selective solvent evaporation can prevent metal penetration. Our cross-linked aerogel films are the first among the low-dielectric aerogel films validated as interlayer low-dielectric materials and robust candidates for ultralow -κ dielectric materials.

Polymer Concentration and Liquid-Liquid Demixing Time Correlation with Porous Structure of Low Dielectric Polyimide in Diffusion-Driven Phase Separation.

Porous polyimide (PI) films are a promising low-k dielectric material for high-frequency data transmission with low signal attenuation. Pores are generated by non-solvent induced phase separation (NIPS) during phase inversion of polymer solution via non-solvent accumulation and solvent diffusion. In this study, aromatic PI was employed as a matrix for NIPS, and the influence of polymer concentration and liquid—liquid demixing time on the morphology of pores in the PI films was investigated. This ensured control over the porous structure of the PI film and provided desirable dielectric properties in a broad frequency range of 100 Hz–30 MHz (1.99 at 30 MHz) and thermal stability (Td5% > 576 °C, Tg > 391 °C). This study addresses the effect of polymer concentration and coagulation time on the morphology and physical properties of PI sponge films and provides guidance on the design and optimization of architectures for polymeric materials requiring pore modification.

Large Thermal Expansion LTCC System for Cofiring with Integrated Functional Ceramics Layers.

We have studied the sintering behavior of CT708 LTCC tapes with large CTE of 10.6 ppm/K. This low-k dielectric LTCC material is a quartz-based glass ceramic composite system with partial crystallization of celsian upon firing. The shrinkage, densification and dielectric properties were examined using different heating rates and a sintering temperature of 900 °C. The maximum shrinkage rate is at 836 °C (for a heating rate of 2 K/min) with a sintering density of 95% and a permittivity of ε’ = 5.9 and tan δ = 0.0004 (at 1 GHz). Due to their similar shrinkage and thermal expansion properties, CT708 tapes may be cofired with functional ceramic layers. As an example, we report on cofiring of a multilayer laminate of CT708 and a Sc-substituted hexagonal ferrite for applications as integrated microwave circulator components. This demonstrates the feasibility of cofiring of functional ceramic tapes and tailored LTCC tapes and documents the potential for the realization of complex LTCC multilayer architectures.

Reliability Characterization of a Low-k Dielectric Using Magnetoresistance as a Diagnostic Tool

Device reliability predictions of the low-k amorphous dielectric material SiCOH using its negative magnetoresistance (MR) is demonstrated herein. The magnitude of the MR is related to the density of defects at the interface and within the bulk of the dielectric material. Moreover, the mean-free path of carriers in the bulk dielectric material within the conduction band is increased in the presence of an applied external magnetic field due to the electron spin-polarization relaxation time modification cooperative effect resulting in suppression of the formation of singlet states and trap-mediated spin-singlet state pair hopping due to carrier spin constraints supporting triplet states with a concomitant rise in current in the conduction band. Interelectrode charge carrier conduction due to trap states within the bulk of the material leading to dielectric breakdown is a major detractor in insulators. A direct correlation is shown between the MR in the amorphous dielectric material SiCOH (a-SiCOH) and failure rates of the insulator in time-dependent dielectric breakdown (TDDB) material lifetimes.

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.

Plasma Polymerized SiCOH Films from Octamethylcyclotetrasiloxane by Dual Radio Frequency Inductively Coupled Plasma Chemical Vapor Deposition System.

We have fabricated porous plasma polymerized SiCOH (ppSiCOH) films with low-dielectric constants (low-k, less than 2.9), by applying dual radio frequency plasma in inductively coupled plasma chemical vapor deposition (ICP-CVD) system. We varied the power of the low radio frequency (LF) of 370 kHz from 0 to 65 W, while fixing the power of the radio frequency (RF) of 13.56 MHz. Although the ppSiCOH thin film without LF had the lowest k value, its mechanical strength is not high to stand the subsequent semiconductor processing. As the power of the LF was increased, the densities of ppSiCOH films became high, accordingly high in the hardness and elastic modulus, with quite satisfactory low-k value of 2.87. Especially, the ppSiCOH film, deposited at 35 W, exhibited the highest mechanical strength (hardness: 1.7 GPa, and elastic modulus: 9.7 GPa), which was explained by Fourier transform infrared spectroscopy. Since the low-k material is widely used as an inter-layer dielectric insulator, good mechanical properties are required to withstand chemical mechanical polishing damage. Therefore, we suggest that plasma polymerized process based on the dual frequency can be a good candidate for the deposition of low-k ppSiCOH films with enhanced mechanical strength.

Magnetoresistance Recovery in the Amorphous Dielectric Material SiCOH

The use of a magnetoresistance in the characterization of transport properties in the amorphous low-k dielectric material SiCOH is demonstrated. The double occupancy of charge carriers in trap states within the dielectric material can only exist in spin singlet formation due to Pauli Exclusion. The trap-assisted negative magnetoresistance (MR) in amorphous SiCOH, driven by an applied electric field that results in an observed increase in magnitude of the current in the conduction band is due to singly occupied trap spin-mixing suppression of carriers with the application of an external magnetic field. The material MR decays with time under electrical bias and temperature stress as traps are filled by charge carriers and from space charge accumulation. The MR can be reinstated by the ionization of these traps via the conduction mechanisms of nonthermally activated tunneling and thermal ionization with the assistance of an applied coulombic potential barrier lowering electric field. In this work a direct correlation is shown between a material MR and the trapping, de-trapping, and trap avoidance of singly occupied traps in the transport of charge carriers in the amorphous low-k dielectric material SiCOH (a-SiCOH).

Inkjet Printing of a Benzocyclobutene-Based Polymer as a Low-k Material for Electronic Applications.

Polymeric materials with a low dielectric constant are widely used in the electronic industry due to their properties. In particular, polymer adhesives can be used in many applications such as wafer bonding and three-dimensional integration. Benzocyclobutene (BCB) is a very interesting material thanks to its excellent bonding behavior and dielectric properties. Usually, BCB is applied by spin-coating, although this technology does not allow the fabrication of complex patterns. To obtain complex patterns, it is necessary to use a printing technology, such as inkjet printing. However, inkjet printing of BCB-based inks has not yet been investigated. Here, we show the feasibility of printing complex patterns with a BCB-based ink, reaching a resolution of 130 μm. We demonstrate that with a proper dilution, BCB-based inks enter the printability window and drop ejection is achieved without the formation of satellite drops. In addition, we present the conditions in which there is an appearance of the coffee ring effect. Inks that feature a too high interaction with the substrate are more likely to show the coffee ring effect, deteriorating the printing quality. We also observe that it is possible to achieve a better film uniformity by increasing the number of printed layers, due to redissolution of the BCB-based polymer that helps to level possible inhomogeneities. Our work represents the starting point for an in-depth study of BCB-based polymer fabrication using jet printing technologies, as a comparison of the bonding quality obtained with different materials and different technologies could give more information and broaden the perspective regarding this field.

Toward miniaturizing microelectronics using covalent organic framework dielectric

Toward miniaturizing microelectronics using covalent organic framework dielectric

Effects of Cu Metal Barrier on Electrical Characteristics of Porous Carbon-Doped Oxide Film

Porous carbon-doped oxide (CDO) film is a promising low-dielectric-constant (low-k) material that is used as an insulator in the back-end-of-line interconnects of integrated circuits for 45 nm or below technological nodes. In Cu/porous low-k integrated interconnects, a barrier is required to prevent the migration of Cu into the low-k insulating dielectric, avoiding electrical instability and degradation of reliability. Cobalt (Co) and ruthenium (Ru), deposited by physical vapor deposition (PVD), were evaluated as Cu barriers in this study. Their effects on the electrical characteristics of the porous CDO film were compared. Experimental results revealed that Co is a better barrier against Cu diffusion, enhances adhesion, and generates electric stress-induced traps at a lower rate. However, damage to the underlying porous CDO film during Co or Ru PVD sputtering deposition was observed. More damage was detected when a Co barrier was used because vacuum-ultraviolet light irradiation was emitted by Ar plasma for a longer time. Therefore, Co can serve as an alternative material for Cu barrier layer. However, the method of deposition of the Co barrier must be carefully chosen to alleviate damage to the contacting low-k insulating dielectric.

Comparative Analysis of Chemical Mechanical Planarization Performance on Low-K Films

Mirroring the pursuit for ever-shrinking feature size in memory architectures, the need for materials capable of preventing crosstalk between increasingly closer conducting lines has led to the replacement of silicon dioxide with alternative low-k materials. Within this subset of materials exist films with notably low dielectric constants; however, these materials present unique challenges for chemical mechanical planarization (CMP) in comparison to more traditional dielectrics, the least of which include undesirable scratching and erosion (e.g. dishing) of low-lying areas. Fortunately, some degree of tuning of film properties (e.g. modulus and hardness) is possible through various deposition methods and post treatment conditions, among others. In this study, two slurries containing common Ceria abrasives were utilized to examine a selection of films of varying properties and preparations across several CMP process conditions. A structural test vehicle having a series of feature sizes designed to feed CMP modeling was used for analysis. Static etch rate and material removal rate were measured using optical metrology. Dishing was quantified via atomic force microscopy (AFM), and scratching performance was analyzed via optical methods. Using the collected data, a mechanism for removal was proposed for each slurry. Figure 1

Nanopore engineered tortuosity towards thermo-mechanically enhanced low-k polymer-mesoporous organosilica composite membranes

The continuous development of microelectronics requires mechanically enhanced insulating materials with a lower dielectric (low-k) constant. To accomplish this, in this paper, low-k polymer nanocomposites were reinforced with hydrophobic ethane-bridged mesoporous organosilica (EMO) particles as fillers. In this study, three types of EMOs such as pure EMO, EMO functionalized with trimethoxyphenylsilane (PTS) (EMO-PTS) group, and EMO functionalized with 1-[3-(trimethoxysilyl)propyl]urea (UTS) (EMO-UTS) group were prepared via sol-gel self-assembly method. Mesostructures and surface areas of synthesized EMO particles were successfully characterized by XRD, TEM, and BET, whereas 13C and 29Si CP/MAS NMR spectra were also evidenced the presence of organic functional groups in the final EMO particles. Then, these filler materials were employed along with PMDA-ODA PAA (polyamic acid) solution for making efficient polyimide (PI) polymer composites. The good dispersion of EMO/EMO-PTS/EMO-UTS fillers into polyimide (PI) matrix were investigated using cross-section of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom force microscopy (AFM). UV–visible spectroscopy has also shown optical transmittance more than 80% in the visible range. Dielectric constant (DC) was also found to be reduced from 3.42 (in PI) to 2.51 (in polymer composites) upon addition of 2–3 wt% of filler materials. The addition of these fillers into polymer PI has also significantly enhanced modulus, hardness and glass-transition temperature of the final composites. These results strongly suggest that EMO and functionalized EMO with either PTS or UTS could be used as promising “reinforcing mesoporous filler and low-k materials” with polyimide.

Introduction of Laser Pi-Grooving as Breakthrough Solution to Enhance die Strength of 40 nm ulow-k CMOS Silicon Technology during Wafer Saw Process

Nowadays, semiconductors and electronics are becoming part of our everyday activities. As the Integrated circuits become more useful to people, it also requires more function, which contain more complex and compact components. Aligned to this package requirement, the more challenging it become to package development as Silicon technology becomes more critical and complex from bare silicon to conventional MOS technology to Ultra Low-K, which requires a different strategy. The new process development in the Semiconductor industry is a necessity to cope up with these new technologies. Low-k devices always pose a big challenge in achieving good dicing quality. This is because of the weak mechanical properties of the low-k dielectric material used. Mechanical Sawing is the most popular cutting method for silicon, but with Ultra low-K technology, using mechanical sawing will lead to various sawing defects such as chippings and delamination [1,2]. These leads to the introduction of Laser Grooving to get rid of these dilemmas. Laser grooving uses heat to eradicate metals on this very thin metal wafer dicing saw streets in preparation for wafer saw process to prevent topside chippings and delamination/metal peel off [3]. These defects are not acceptable especially since the product application is a chip card. Since chip cards must be flexible and durable, they require higher die and package strength to serve its purpose. To achieve such package requirement, different method was evaluated such as standard mechanical dicing, standard Laser Grooving and the PI laser groove. 
 The paper will discuss how we were able to achieve the quality requirement for Ultra Low-K and at the same time eliminating top reject contributor during startup of this device.

Thermally conductive ultra-low-k dielectric layers based on two-dimensional covalent organic frameworks.

As the features of microprocessors are miniaturized, low-dielectric-constant (low-k) materials are necessary to limit electronic crosstalk, charge build-up, and signal propagation delay. However, all known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips. Two-dimensional (2D) covalent organic frameworks (COFs) combine immense permanent porosities, which lead to low dielectric permittivities, and periodic layered structures, which grant relatively high thermal conductivities. However, conventional synthetic routes produce 2D COFs that are unsuitable for the evaluation of these properties and integration into devices. Here, we report the fabrication of high-quality COF thin films, which enable thermoreflectance and impedance spectroscopy measurements. These measurements reveal that 2D COFs have high thermal conductivities (1 W m-1 K-1) with ultra-low dielectric permittivities (k = 1.6). These results show that oriented, layered 2D polymers are promising next-generation dielectric layers and that these molecularly precise materials offer tunable combinations of useful properties.

In-Situ Repair Plasma-Induced Damage and Cap Dielectric Barrier for Porous Low-Dielectric-Constant Materials by HMDS Plasma Treatment

Plasma damage and metal ion penetration are critical issues for porous low-dielectric-constant (low-k) materials used in the back-end-of-line interconnects. This study proposed a novel process with in-situ repairing plasma-induced damage and capping a barrier for porous low-k materials by Hexamethyldisilazane (HDMS) plasma treatment. For a plasma-damaged porous low-k material, its surface hydrophilic state was transformed to hydrophobic state by HDMS plasma treatment, revealing that damage was repaired. Simultaneously, a dielectric film was capped onto the porous low-k material, and displayed better barrier capability against Cu migration. Additionally, the breakdown reliability of the stacked dielectric was enhanced by the means of HDMS plasma treatment. The optimized HDMS plasma treatment time was found to be 10 s. Therefore, this proposed HDMS plasma treatment processing is a promising technique for highly applicable low-k material used for advanced technology nodes.

Area-Selective Molecular Layer Deposition of a Silicon Oxycarbide Low-k Dielectric

Area-selective deposition (ASD) of low-k materials is desired in back-end-of-line processes for fabricating nanopatterns such as fully self-aligned vias. However, the high temperature and/or aggres...

MOF and GNS work synergistically for low-k materials with higher thermal conductivity

In this work, the low dielectric constant composites with higher thermal conductivity are prepared, combining ZIF-8 insulation and graphite nanosheets (GNS) thermal conductivity via designing suitable configuration and synergistically reacting in epoxy resin matrix. It demonstrated that ZIF-8@GNS/EP (ZGNS/EP) composites have decreased by 26.4% in low dielectric constant and increased by 300.7% in thermal conductivity compared with neat EP. The dielectric loss of composites is always less than 0.033(102–107 Hz). Simultaneously, the tensile strength of the 0.5 wt% ZGNS/EP composite is increased by 71.1% than before. Overall, this work presents new ideal to prepare low dielectric constant materials, which also possess low loss and excellent thermal conductivity.