Ultra Low-k Materials Research Articles

Enabling Durable Ultralow-k Capacitors with Enhanced Breakdown Strength in Density-Variant Nanolattices.

Ultralow-k materials used in high voltage devices require mechanical resilience and electrical and dielectric stability even when subjected to mechanical loads. Existing devices with organic polymers suffer from low thermal and mechanical stability while those with inorganic porous structures struggle with poor mechanical integrity. Recently, 3D hollow-beam nanolattices have emerged as promising candidates that satisfy these requirements. However, their properties are maintained for only five stress cycles at strains below 25%. Here,wedemonstrate that alumina nanolattices with different relative density distributions across their height elicit a deterministic mechanical response concomitant with a 1.5-3.3 times higher electrical breakdown strength than nanolattices with uniform density. These density-variant nanolattices exhibit an ultralow-k of ≈1.2, accompanied by complete electric and dielectric stability and mechanical recoverability over 100 cyclic compressions to 62.5% strain.Weexplain the enhanced insulation and long-term cyclical stability by the bi-phase deformation where the lower-density region protects the higher-density region as it is compressed before the higher-density region, allowing to simultaneously possess high strength and ductility like composites. This study highlights the superior electrical performance of the bi-phase nanolattice with a single interface in providing stable conduction and maximum breakdown strength.

Significantly Enhanced Porosity of Silicone Rubber Nanocomposite Foams via Cross-Linking Structure Regulation and Heterogeneous Nucleation by CNTs for Promising Ultralow-k Dielectrics

Ultralow-k materials with excellent thermal stability and good electrical insulation are urgently required taking into consideration the rapid development of 5G communication systems. Increasing the porosity of materials is the most effective way to reduce their dielectric constant. On this basis, with the help of cross-linking structure regulation and heterogeneous nucleation of CNTs, a polymethyl-vinyl siloxane (PMVS)/CNT foam with porosity up to 93.9% (density, 0.069 g/cm3) was fabricated via supercritical CO2 foaming. Therefore, promising dielectric properties were observed for the PMVS/CNT foam (0.5 wt % CNTs) with an ultralow dielectric constant (k = 1.52) and a low loss (tan δ = 0.0017) at 1 kHz. Additionally, the PMVS/CNT foams exhibited improved insulativity (RV > 1014 Ω·cm), prominent thermal stability (T5 > 444 °C), and good mechanical properties. These excellent PMVS/CNT foams afford a promising ultralow-k dielectric for modern integrated circuit development.

Thermal Conductivity of Two-Dimensional Benzobisoxazole-Linked Covalent Organic Frameworks with Nanopores: Implications for Thermal Management Applications

Thermal management is essential for maintaining the optimal performance of electronic devices. Although covalent–organic frameworks (COFs) have emerged as a platform for gas and energy storage applications, their thermal transport properties are greatly understudied. Herein, we report the thermal conductivities of three benzobisoxazole (BBO)-linked COFs with nanpores ranging from 1.3 to 2.5 nm over a wide temperature range (80–300 K) using the longitudinal, steady-state heat-flow method. In doing so, thermal conductivity values as high as 0.677 W m–1 K–1 at 300 K were obtained, and no relationship between the thermal conductivity and pore size was observed. These results were supported by density functional theory calculations. The thermal conductivities of the BBO-COFs doped with poly(3-hexylthiophene) were also investigated. The BBO-COFs could be useful as ultralow-k materials for thermal management applications.

On the relationship between SiF4 plasma species and sample properties in ultra low-k etching processes

The temporal behavior of the molecular etching product SiF4 in fluorocarbon-based plasmas used for the dry etching of ultra low-k (ULK) materials has been brought into connection with the polymer deposition on the surface during plasma treatment within the scope of this work. For this purpose, time-resolved measurements of the density of SiF4 have been performed by quantum cascade laser absorption spectroscopy. A quantification of the non-linear time dependence was achieved by its characterization via a time constant of the decreasing SiF4 density over the process time. The time constant predicts how fast the stationary SiF4 density is reached. The higher the time constant is, the thicker the polymer film on top of the treated ultra low-k surface. A correlation between the time constant and the ULK damage was also found. ULK damage and polymer deposition were proven by Variable Angle Spectroscopic Ellipsometry and X-ray Photoelectron Spectroscopy. In summary, the observed decay of the etching product concentration over process time is caused by the suppressed desorption of the SiF4 molecules due to a more dominant adsorption of polymers.

Plasma Induced Damage Reduction of Ultra Low-k Dielectric by Using Source Pulsed Plasma Etching for Next BEOL Interconnect Manufacturing

In order to reduce the interconnect resistance and capacitance (RC) time delay of a semiconductor integrated circuit, a more porous dielectric material is used in recent interconnection for lower dielectric constant. However, it is difficult to use highly porous low-k dielectric materials at the narrow pitch because it is easily damaged during the plasma etching processes. In this study, as one of the plasma induced damage reduction methods in the etching of porous low-k dielectric, RF pulsed plasma methods have been investigated by using a dual frequency capacitively coupled plasma etching system. RF pulsed plasmas generated more polymerizing species and less UV compared to continuous wave plasmas and showed reduced damaged layer compared to the conventional continuous wave plasma etching. Porous SiCOH dielectric patterned with a TiN hard mask was etched using the RF pulsed plasmas and the results showed more anisotropic etching profiles with less sidewall damages, which was estimated by the thickness loss of sidewall low-k material after dipping into a diluted HF solution. Therefore, it is believed that the RF pulsed plasma etching process of ultra low-k dielectric materials can improve the RC time delay related to plasma damage for the next interconnect manufacturing technology.

Etch Damage Reduction of Ultra Low-k Dielectric by Using Pulsed Plasmas

To reduce interconnect resistance and capacitance (RC) time delay of semiconductor integrated circuit, the dielectric material with more porosity is used in recent interconnection for lower dielectric constant. However, it is difficult to use highly porous low-k dielectric materials at the narrow pitch because it is easily damaged during the plasma etching processes. In this study, as one of the plasma induced damage reduction methods in the etching of porous low-k dielectric using C4F8-based gases, RF pulsed plasma methods have been investigated by using a dual frequency capacitively coupled plasma etch system. RF pulsed plasmas generated higher carbon-rich radicals in the plasma and less charging effect on the dielectric material surface compared to continuous wave plasmas and, therefore, showed a reduced damaged layer compared to the conventional continuous wave plasma etching. Porous SiCOH dielectric with a patterned TiN hard mask was etched using the RF pulsed plasmas and the results showed more anisotropic etching profiles with less sidewall damages. Therefore, it is believed that the RF pulsed plasma etching process of ultra low-k dielectric materials can improve the RC delay related with plasma damage for the next interconnect technology.

Low-k dielectric etch challenges at the 7 nm logic node and beyond: Continuous-wave versus quasiatomic layer plasma etching performance review

For logic nodes of 7 nm and beyond, back-end-of-line (BEOL) trench patterns have a critical pitch of less than 40 nm, directly affecting the plasma etch process window of the dual damascene etch process. Feature size dependent etch depth (reactive ion etch, RIE lag), hard mask selectivity, and ultra-low-k (ULK) damage have become significant challenges that must be overcome in order to meet target device performance. Recently, atomic layer etching has been used to widen the plasma etch process window in terms of selectivity and process control [S. Sherpa, P. L. F. Ventzek, and A. Ranjan, J. Vac. Sci. Technol. A 35, 05C310 (2017); T. Tsutsumi, H. Kondo, M. Hori, M. Zaitsu, A. Kobayashi, T. Nozawa, and N. Kobayashi, J. Vac. Sci. Technol. A 35, 01A103 (2017)]. In this work, the impact of a quasiatomic layer etch (QALE) process, a conventional continuous wave plasma, and a pulsed plasma process on ULK materials were investigated to determine the benefits of an ALE process approach for BEOL etching. Both blanket ULK film and patterned ULK samples were used for this study. The ULK etch damage from each process was characterized using Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy on three different ULK films. From patterned samples, it was determined that QALE could be used to successfully suppress RIE lag in low-k materials at advanced pitches, while keeping low-k damage to a minimum. In addition, the QALE technique showed improved hard mask selectivity and resulted in lower line edge pattern roughness. Based on this study, the authors concluded that QALE is a powerful plasma etch method to overcome BEOL etch challenges at advanced pitches.

Enabling Simultaneous Extreme Ultra Low-k in Stiff, Resilient, and Thermally Stable Nano-Architected Materials

Low dielectric constant (low-k) materials have gained increasing popularity because of their critical role in developing faster, smaller, and higher performance devices. Their practical use has been limited by the strong coupling among mechanical, thermal, and electrical properties of materials and their dielectric constant; a low-k is usually attained by materials that are very porous, which results in high compliance, that is, silica aerogels; high dielectric loss, that is, porous polycrystalline alumina; and poor thermal stability, that is, Sr-based metal-organic frameworks. We report the fabrication of 3D nanoarchitected hollow-beam alumina dielectrics which k is 1.06-1.10 at 1 MHz that is stable over the voltage range of -20 to 20 V and a frequency range of 100 kHz to 10 MHz. This dielectric material can be used in capacitors and is mechanically resilient, with a Young's modulus of 30 MPa, a yield strength of 1.07 MPa, a nearly full shape recoverability to its original size after >50% compressions, and outstanding thermal stability with a thermal coefficient of dielectric constant (TCK) of 2.43 × 10-5 K-1 up to 800 °C. These results suggest that nanoarchitected materials may serve as viable candidates for ultra low-k materials that are simultaneously mechanically resilient and thermally and electrically stable for microelectronics and devices.

Damage and etching of ultra low-k materials in fluorocarbon plasma at lowered temperatures

SiOCH ULK films with k-value from 2.5 to 2.1 and porosity from 24 to 40% were etched in CHF3, CHF3 + Ar, CF4 and CF4 + Ar plasmas at +15…−120 °C with and without bias being applied. It was shown that the presence of Ar in gas mixture can significantly increase the damage of unetched ultra low-k (ULK) material (at sidewalls) due to the removal of −CH3 groups from the film by VUV photons. It was also shown that etching and damage of the sidewalls by F atoms can be partially prevented by lowering the temperature of the sample.

Leakage current induced by surfactant residues in self-assembly based ultralow-k dielectric materials

In this work, we studied low-field leakage currents in the self-assembly based spin-on low-k dielectrics (k = 2.2) as it may be affected by the degree of the organic template decomposition. The distinct role of the template residues could be examined due to the remarkably different rate of organosilica matrix cross-linking and template decomposition during the hard-bake process. We found that the incomplete decomposition of the sacrificial organic phase is responsible for increased low-field leakage current through the dielectric. The analysis of photocurrent spectra and the results of electron resonance spectroscopy suggest that the degradation of electrical performance can be attributed to the presence of defect states ∼5 eV below the insulator conduction band induced by the residues in the form of oxidized sp3-carbon chains. The lowest leakage current is attained in the template-free material obtained by introduction of additional broadband UV-assisted curing (λ > 200 nm).

A novel tri-layer nanoindentation method to measure the mechanical properties of a porous brittle ultra-low-k dielectric thin film

The mechanical properties of two porous ultra-low-k (ULK) dielectric thin films (porous carbon-doped silicon dioxide (pSiCOH); and octamethylcyclotetrasiloxane (OMCTS)) were measured by nanoindentation. Since a direct contact of a nanoindentation tip with a thin and brittle ULK film will likely induce damage to the film and thus alters its mechanical response, the mechanical properties measured from direct nanoindentation on an ULK film do not necessarily represent the behavior of an ULK thin film in service condition. To circumvent this difficulty, a novel tri-layer configuration was used for sample preparation. In this method, a thin layer of tetraethyl orthosilicate (TEOS) silica was deposited on an ULK thin film to protect it from direct contact with a nanoindenter tip. A nanoindenter tip was in contact with TEOS coating on the layered film in nanoindentation, and the collected data was subsequently analyzed by finite element analysis. The Ogden hyperfoam model was used as the constitutive law for the ULK film in simulations, and found to be appropriate. Results indicate that the linear elastic model is only applicable for very small strains. At large strains the ULK films show nonlinear behavior due to densification of the porous ULK material under compaction. The moduli for the pSiCOH and OMCTS dielectric films were determined as 2.6 GPa and 5.2 GPa, respectively.

Enhancement of seeding for electroless Cu plating of metallic barrier layers by using alkyl self-assembled monolayers

Tethering a self-assembled monolayer (SAM) on ultralow-k (porous) dielectric materials as a seed-trapping layer for electroless Cu plating has been extensively studied. By contrast, literature on direct electroless Cu plating of metallic barrier layers assisted by SAMs is scarce. Therefore, Ta, a crucial component of barrier materials for Cu interconnect metallization, was investigated as a model substrate for a new seeding (Ni catalyst formation) process of electroless Cu plating. Transmission and scanning electron microscopies indicated that catalytic particles formed on Ta films through TaOH groups tend to become aggregates with an average size of 14nm and density of 2×1015m−2. By contrast, Ta films with a plasma-functionalized SAM tightly bound catalytic particles without agglomeration, thus yielding a markedly smaller size (3nm) and higher density (3×1016m−2; one order greater than those formed by other novel methods). X-ray photoelectron spectroscopy clearly identified the types of material species and functional groups induced at each step of the seeding process. Moreover, the phase of the catalytic particles, either nickel alkoxide, Ni(OH)2, or metallic Ni, along with the seed-bonding mechanism, was also unambiguously distinguished. The enhancement of film-formation quality of Cu by the new seeding process was thus demonstrated.

Theoretical investigation of an in situ k-restore process for damaged ultra-low-k materials based on plasma enhanced fragmentation

The authors present theoretical investigations of a k-restore process for damaged porous ultra-low-k (ULK) materials. The process is based on plasma enhanced fragmented silylation precursors to replace k-value damaging, polar Si-OH and Si-H bonds by k-value lowering Si-CH3 bonds. The authors employ density functional theory to determine the favored fragments of silylation precursors and show the successful repair of damaged bonds on our model system. This model system consists of a small set of ULK-fragments, which represent various damaged states of ULK materials. Our approach provides a fast scanning method for a wide variety of possible repair reactions. Further, the authors show that oxygen containing fragments are required to repair Si-H bonds and fragments with dangling Si-bonds are most effective to repair polar Si-OH bonds.

Exploration of Chemical Bonding Transformation Mapping to Assist Low-k Dielectric Nanostructure Fabrication

In leading edge integrated circuit manufacturing, reduction of RC time delay by incorporation of porous ultra low-k interlayer dielectrics into Cu interconnect nanostructure continues to pose major integration challenges. The main challenge is that porous structure renders interlayer dielectrics mechanically weak, chemically unstable and more susceptible to the RIE plasma etching damages. Besides the challenge of handling weak porous ultra low-k materials, a lack of sensitive metrology to guide systematic development of plasma etching, restoration and cleaning processes is the major stumbling block. We explored Multiple Internal Reflection Infrared Spectroscopy and associated IR techniques as a sensitive (sub-5 nm) characterization tool to investigate chemical bonding modification across fluorocarbon etch residues and low-k dielectric interface after plasma etching, ashing, UV curing and post-etch cleaning. The new insights on chemical bonding transformation mapping can effectively guide the development of clean-friendly plasma etch for creating ultra low-k dielectric nanostructures with minimal dielectric damages.

Effect of Laser Annealing on the Structure of Amorphous Porous SiCOH Materials

The inherent trade-off of porosity and mechanical properties in ultralow dielectric constant SiCOH materials is a critical challenge in semiconductor processing. Numerous postdeposition processes have been studied to achieve simultaneous low-k materials with adequate mechanical rigidity, including thermal annealing. Typically, these thermal anneals are characterized by times on the order of seconds to minutes at relatively low temperatures (below 500 °C). In this work, we report on potential advantages of submillisecond time frame anneals at extreme temperatures up to 1200 °C. The effects of such processing on the structure of ultralow-k materials were studied using molecular dynamics computer simulations employing a force field that facilitates the determination of bond rearrangements, coupled with direct experimental measurements during CO2 laser-induced spike annealing. Results show structural evolution with increasing temperature leading to densification of the SiOx network, reduction in the concentration of suboxides, and loss of remnant organic moieties from original SiCOH structures. In molecular dynamics simulations, organic structural units decompose at simulated temperatures above 850 °C, followed by an increase in Si–O networking at 900 °C. These observations are consistent with complementary experiments. These results provide atomic-scale structure and chemical intuition to guide the development of future low-k materials.

Exploration of Chemical Bonding Transformation Mapping to Assist Low-k Dielectric Nanostructure Fabrication

In leading edge integrated circuit manufacture, reduction of RC time delay by incorporation of porous ultra-low k (ULK) interlayer dielectrics (ILD) into Cu interconnect nanostructure continues to pose major integration challenges. The main challenge is porosity induced to lower the dielectric constant of the ILD has the undesirable side effect of compromising its mechanical and chemical stability. As a result, these materials are more susceptible to the damages caused by RIE plasma etching processes. Besides the challenge of handling weak porous ULK materials, a lack of sensitive metrology to guide systematic development of plasma etching, restoration and cleaning processes is the major stumbling block. To assist in the overall integration effort, we have been actively exploring Multiple Internal Reflection Infrared Spectroscopy (MIR-IR) and associated IR spectroscopic techniques as a sensitive (sub-5nm) characterization tool to assist the fabrication process of ULK nanostructure with targeted critical dimension. In this paper, we will report new findings obtained on chemical bonding modification across fluorocarbon etch residues and low-k dielectric interface after manufacture relevant processing treatments including plasma etching, ashing, UV curing and post-etch cleaning. MIR-IR metrology showed useful spectroscopic details of chemical bonding transformations taking place during plasma etch and strip processing on both blanket and dielectric trench structures. The observed increase of Si-OH and C=O and the decrease of Si-CH3 IR absorption bands correlated well with increasing plasma induced damage, K increases verified by MIS structure, especially for more aggressive stripping process. With functional group-specific chemical derivatization, chemical bonding structure of 1-2 nm post-etch residue on porous low-k trench structure was established. UV-induced modification of fluorocarbon polymer residue to improve its subsequent wet cleaning removal was studied by FTIR, XPS and SEM. The overall goal is to apply new insights on chemical bonding transformation mapping to achieve cleans-friendly plasma etches on creating ULK dielectric nanostructures with minimal dielectric damages.

Chip Packaging Interaction (CPI) with Cu Pillar Flip Chip for 20 nm Silicon Technology and Beyond

Chip packaging interaction (CPI) has drawn great attention to advanced silicon technology nodes due to the introduction of Low-K (LK) and Ultra Low-K (ULK) materials in back end of line (BEOL) and Cu pillar in chip package interconnects. This paper summarizes GLOBALFOUNDRIES's activities in CPI studies for the 20 nm technology node, which includes CPI structure design, BEOL process characterization and optimization, assembly process optimization and reliability tests. The key issues and challenges were identified, analyzed, and overcome through a Design of Experiment (DOE) study in BEOL Q-time and reflow parameters in assembly process. It has been found that 1) BEOL ULK layer Q-time before SiCN capping is critical for moisture diffusion into BEOL materials, which affects both electrical performance and reliability; 2) Reflow temperature plays key role to control the package warpage to avoid non-wet issue. Package level reliability evaluation was performed and the results were reported. Through this study, it has been demonstrated that GLOBALFOUNDRIES's 20 nm technology successfully passed all CPI reliability tests. CPI challenges for future technology nodes and emerging packaging integration solutions are also discussed in the paper.

Chemical Mechanical Planarization Processes in Cu Interconnects: Invention to Implementation

Chemical mechanical planarization (CMP) is one of the most critical semiconductor process technologies that enabled the fabrication of Cu interconnects and helped to achieve the performance goals of modern microprocessor and memory chips. Cu interconnects were introduced at the 130 nm technology node with oxide ( k ≈ 4) as the dielectric material. Since then, It has been the industry standard and has been extended to 22 nm technology node with low-k /ultra low-k ( k < 3) materials as dielectrics. The relentless drive to improve processor performance accelerated the introduction of new materials along with simultaneous reduction in minimum wiring dimensions and increase in the number of interconnects levels. The aggressive interconnect scaling coupled with the necessity to introduce new materials resulted in several unique challenges for the CMP processes. This paper presents examples of the innovative approaches that were used in solving some of these problems.

Dielectric Properties of Selected Metal–Organic Frameworks

The electronic structure of a class of [Zn4O(CO2)6] based metal–organic frameworks (MOFs) is theoretically analyzed by means of density functional perturbation theory. The calculated static dielectric constants vary in a range between 1.33 and 1.54, characterizing the structures as ultralow-k dielectric materials and confirming earlier estimates qualitatively. We also present the results of first-principle calculations of the real and imaginary parts of the dielectric function and give the frequency-dependent dielectric constant up to the near-ultraviolet, which is important for high frequency semiconductor and optical applications of MOFs. The dielectric and electronic properties are governed by the linker molecules, so that the band gap and the dielectric constant can be engineered.

Chip-Package Interaction in 3D stacked IC packages using Finite Element Modelling

Chip Package Interaction (CPI) gained a lot of importance in the last years. The reason is twofold. First, advanced node IC technologies requires dielectrics in the BEOL (back-end-of-line) with a decreasing value for the dielectric constant k. These so-called (ultra) low-k materials have a reduced stiffness and reduced adhesion strength to the barrier materials, making the BEOL much more vulnerable to externally applied mechanical stress due to packaging. Secondly, advanced packaging technologies such as 3D stacked IC’s use thinned dies (down to 25μm) which can cause much higher stresses at transistor level, resulting in electron mobility shifts of the transistors. Also the copper TSV (through-silicon-via) generates local stress which affects the device performance. This paper considers both the packaging impact on BEOL integrity and transistor mobility shifts for 3D stacked IC (integrated circuit) technologies.