Low-k 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.

A facile strategy for preparing low dielectric polybutadienes with ultra-low dielectric loss and enhanced thermal stability

With the rapid development of high speed and high frequency communication, organic low dielectric (low-k) materials with both ultra-low dielectric constant (Dk) and ultra-low dielectric loss (Df) are urgently required to ensure signal transmission speed and reliability. 1,2-polybutadiene (1,2-PB) is renowned for its excellent dielectric properties, making it used in high-frequency printed circuit boards (PCBs). However, its poor thermal stability and mechanical properties hinder its application which requires persistent stability. To address this issue, a series of thermal-crosslinkable benzocyclobutene (BCB)-modified PBs were synthesized through a hydrosilylation reaction. These cured PBs exhibited significantly enhanced thermal stability with the glass transition temperature (Tg) exceeding 400 °C. In particular, they displayed ultra-low Dk (2.32–2.41) and ultra-low Df (<0.001). It was found that the properties of cured PBs were related to BCB loadings. Among the cured polymers, PB-g-B30 demonstrated the optimal dielectric properties with a Dk of 2.34 and a Df of 3.6 × 10−4. Subsequently, a PB-g-B30/glass fiber laminate was fabricated, exhibiting superior dielectric and comprehensive properties when compared to commercial low-k laminates. This study provides a facile method to develop PBs with ultra-low Dk and Df and enhanced thermal stability. The outstanding comprehensive properties indicate their potential as low-k materials in high-frequency communication applications.

Heat transport in SiCOH thin films: an experimental and molecular dynamics study

Carbon-doped silicon dioxide (SiCOH) film is currently regarded as one of the most promising low-k materials in the integrated circuits (ICs) industry for advanced technology nodes. However, there have been limited studies on the thermal properties of SiCOH compared to its electrical and mechanical properties. In this study, we investigate the thermal conductivity of SiCOH thin films through molecular dynamics simulations (MD) and experimental characterizations. Our findings indicate that the size effect on thermal conductivity at 300 K is negligible when the thickness of SiCOH film is less than 20 nm. Additionally, we observe a contrasting temperature dependence law for the thermal conductivity of SiCOH thin films compared to crystal SiO2 thin films. Furthermore, we demonstrate a significant decrease in thermal conductivity with increasing porosity in SiCOH films; specifically, an increase in porosity from 5.35% to 42.77% results in a 60% reduction in thermal conductivity. Moreover, we validate our simulation results by characterizing the thermal conductivity of SiCOH using 3 ω method.

Comprehensive Review on the Impact of Chemical Composition, Plasma Treatment, and Vacuum Ultraviolet (VUV) Irradiation on the Electrical Properties of Organosilicate Films.

Organosilicate glass (OSG) films are a critical component in modern electronic devices, with their electrical properties playing a crucial role in device performance. This comprehensive review systematically examines the influence of chemical composition, vacuum ultraviolet (VUV) irradiation, and plasma treatment on the electrical properties of these films. Through an extensive survey of literature and experimental findings, we elucidate the intricate interplay between these factors and the resulting alterations in electrical conductivity, dielectric constant, and breakdown strength of OSG films. Key focus areas include the impact of diverse organic moieties incorporated into the silica matrix, the effects of VUV irradiation on film properties, and the modifications induced by various plasma treatment techniques. Furthermore, the underlying mechanisms governing these phenomena are discussed, shedding light on the complex molecular interactions and structural rearrangements occurring within OSG films under different environmental conditions. It is shown that phonon-assisted electron tunneling between adjacent neutral traps provides a more accurate description of charge transport in OSG low-k materials compared to the previously reported Fowler-Nordheim mechanism. Additionally, the quality of low-k materials significantly influences the behavior of leakage currents. Materials retaining residual porogens or adsorbed water on pore walls show electrical conductivity directly correlated with pore surface area and porosity. Conversely, porogen-free materials, developed by Urbanowicz, exhibit leakage currents that are independent of porosity. This underscores the critical importance of considering internal defects such as oxygen-deficient centers (ODC) or similar entities in understanding the electrical properties of these materials.

Reduced subthreshold swing in a vertical tunnel FET using a low-work-function live metal strip and a low-k material at the drain.

In this research paper, a vertical tunnel field-effect transistor (TFET) structure containing a live metal strip and a material with low dielectric constant is designed, and its performance metrics are analyzed in detail. Low-k SiO2 is incorporated in the channel-drain region. A live molybdenum metal strip with low work function is placed in a high-k HfO2 layer in the source-channel region. The device is examined by the parameters I off, subthreshold swing, threshold voltage, and I on/I off ratio. The introduction of a live metal strip in the dielectric layer closer to the source-channel interface results in a minimum subthreshold slope and a good I on/I off ratio. The low-k material at the drain reduces the gate-to-drain capacitance. Both the SiO2 layer and the live metal strip show excellent leakage current reduction to 1.4 × 10-17 A/μm. The design provides a subthreshold swing of 5 mV/decade, which is an excellent improvement in TFETs, an on-current of 1.00 × 10-5 A/μm, an I on/I off ratio of 7.14 × 1011, and a threshold voltage of 0.28 V.

Fractures of low-k materials in a RF package with integrated passive device based on TGV

The radio frequency (RF) chips and passive devices integrated on the through-glass-via (TGV) substrate meets the demands of miniaturization, high performance and low losses in the application. The RF chip and integrated passive devices (IPDs) are interconnected electrically by a redistribution layer (RDL) on the TGV substrate with the isolation low-k materials. The low-k materials, however, are susceptible to fracture during the thermal process in packaging due to their weak mechanical properties. In this study, the fractures of the low-k material were studied by experiments and the finite element analysis (FEA) for a RF package with integrated passive device based on TGV. The mechanical properties of the low-k material used in the FEA were tested by fabricating freestanding low-k films using microfabrication techniques. Then the fracture behaviors of the low-k material in the package and its impact factors under thermal loadings were examined. The impact factors, such as the initial defect location, direction and length, were investigated by evaluating the stress intensity factors (SIFs) at the defect tips. The results revealed that the most hazardous location in the low-k material is the region below the micro-joint of RF chip. The vertical defects along thickness in low-k film are more likely to propagate than horizontal ones. The SIF value increases linearly with the defect length both in heating and cooling conditions.

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.

Non-contact near-field cavity perturbation method for quantitative dielectric measurement and metallic defect inspection

Low dielectric constant (low-k) materials are essential in electronic industry aiming at new generation high-speed communications. Dielectric constant of these materials are measured by standard procedures and instruments with conventional contact configuration. Non-contact and in-situ techniques for low-k dielectric measurement are highly demanded in practice and worth of investigation. We hereby proposed a near-field microwave probe (NFMP) to accurately measure dielectric constant of low-k samples in a non-contact manner with a quantitative electromagnetic (EM) cavity perturbation theory. An NFMP set was designed, optimized by finite-element method (FEM) and fabricated with handy materials in the lab. Frequency response of the NFMP was tested experimentally and further used for subsequent frequency displacement analysis. Various low-k samples were tested by the non-contact NFMP set and dielectric constant were extracted based on the field perturbation theory. The relative error was obtained less than 0.5 % for low-k materials. The NFMP was then fixed on a micrometer stage and worked in a line-scan mode to inspect conductive defects in a low-k substrate. Both surface and subsurface defects were inspected by analyzing displaced central frequencies. The NFMP system is promising to facilitate quantitative and non-contact dielectric property measurement in electronics.

Hetero-Structure Junctionless MOSFET with High-k Corner Spacer for High-Speed and Energy-Efficient Applications

In this research work, Hetero-structure Junction-less MOSFET having a Silicon-Germanium source and high-k inner corner spacer is proposed and investigated. In this article, we have shown that the introduction of a high-k dielectric material in the inner corner spacer and a low-k dielectric material in the rest of the spacer in the optimally designed device leads to a substantial reduction in parasitic capacitances, resulting in higher operating speed. It was also shown that proper doping in the drain-source underlaps regime, can improve the short channel performance (SCP) of the device by increasing the effective gate length. The optimally designed proposed device produces on current (ION) ~0.33 mA and off current (IOFF) ~ 5.55 fA along with ION/IOFF=6.08x1010, Subthreshold slope (SS)=59.6 mV/decade and drain induced barrier lowering (DIBL)=82.2 mV/V. This paper also highlights the performance improvement of the proposed device in terms of both speed and energy consumption, as compared to that of Junctionless Double Gate MOSFET when implemented as logic gates.

High-Performance low-k Poly(dicyclopentadiene)-POSS nanocomposites achieved by frontal polymerization

Low dielectric constant (low-k, k < 2.5) thermoset polymers play a crucial role in high-frequency communication technology and the microelectronic industry. However, existing low-k materials often suffer from time- and energy-consuming preparation procedures and have k values greater than 2.5. Herein, we present the frontal ring opening metathesis polymerization (FROMP) of dicyclopentadiene and phenyl-functionalized POSS bearing three and four norbornene groups (i.e., T7N3 and T8N4), enabling the rapid formation of high-performance low-k nanocomposites through a low energy consumption approach. The results reveal that T7N3 and T8N4 exhibit excellent compatibility with poly(dicyclopentadiene) (PDCPD) without adversely affecting the materials’ FROMP behaviors. Consequently, by increasing IC-POSS content, both the dielectric and comprehensive properties of PDCPD can be significantly enhanced while the influence of IC-POSS type remains limited. In particular, both PT7-40 (40 wt% of T7N3, k = 2.1) and PT8-40 (40 wt% of T8N4, k = 2.12) demonstrate the lowest k values; moreover, PT8-40 exhibits the highest dimensional stability (with a coefficient of thermal expansion of 79.3 ppm oC−1), glass transition temperature (188 °C), elastic modulus (2.32 GPa), yield strength (70.4 MPa), and hydrophobicity among all samples. By programming the front velocity, these materials can be processed into complex structures through a combination of FROMP and three-dimensional printing techniques. This work offers a facile and energy-efficient approach for preparing low-k materials with on-demand structures and properties.

Fabrication of the low-k films with tunable k value as spacers in advanced CMOS technology

In advanced CMOS technology, a suitable spacer scheme is crucial to alleviate the effects of increasing parasitic resistance and capacitance on device performance as the critical dimensions shrinking. Low dielectric constant (low-k) films, possessing a tunable k value ranging from 3.5 to 6.5, were fabricated using plasma-enhanced atomic layer deposition in a single chamber. The fabrication process involved the deposition of the SiN film via SiH2I2 with N2 plasma, as well as the deposition of the SiOX, SiOCN, and SiON films using diisopropylamino silane with O2, Ar/O2, and N2/O2 plasmas, respectively. The introduction of groups containing carbon (C) tended to loosen the film structure, due to its weak bond strength with Si, thus made distinctions in structural and electrical stability. We developed such a process which can adjust the C-group concentration and O, N content to tune the film k value. The SiOx, SiOCN, SiON, and SiN films had high breakdown strength of 9.04, 7.23, 9.41, and over 11 MV cm−1, and meanwhile low leakage current density of 2.42 × 10−9, 4.78 × 10−8, 1.29 × 10−9, and 9.26 × 10−10 A cm−2, respectively. The films exhibited remarkable thermal stability, enhanced breakdown strength, and suppressed leakage with annealing treatment, which could be attributed to the desorption of —CHX groups. Moreover, the low-k materials demonstrated excellent step coverage both in the inner-spacer cavity and on sidewalls, exploring the potential application as spacers in advanced CMOS structure.

Glass-like thermal conductivity and phonon transport mechanism in disordered crystals.

Solid materials with ultra-low thermal conductivity (κ) are of great interest in thermoelectrics for energy conversion or as thermal barrier coatings for thermal insulation. Many low-κ materials exhibit unique properties, such as weak or even insignificant dependence on temperature (T) for κ, i.e., an anomalous glass-like behavior. However, a comprehensive theoretical model elucidating the microscopic phonon mechanism responsible for the glass-like κ-T relationship is still absent. Herein, we take rare-earth tantalates (RE3TaO7) as examples to reexamine phonon thermal transport in defective crystals. By combining experimental studies and atomistic simulations up to 1800 K, we revealed that diffusion-like phonons related to inhomogeneous interatomic bonding contribute more than 70% to the total κ, overturning the conventional understanding that low-frequency phonons dominate heat transport. Furthermore, due to the bridging effects of interatomic bonding, the κ of high-entropy tantalates is not necessarily lower than that of medium-entropy materials, suggesting that attempts to reduce κ through high-entropy engineering are limited, at least in defective fluorite tantalates. The new physical mechanism of multimodal phonon thermal transport in defective structures demonstrated in this work provides a reference for the analysis of phonon transport and offers a new strategy to develop and design low-κ materials by regulating the inhomogeneity of interatomic bonding.

Computational Analysis of the Effects of Nanoscale Confinement on the Structure of Low-k Dielectric Hybrid Organosilicate Materials

Computational Analysis of the Effects of Nanoscale Confinement on the Structure of Low-k Dielectric Hybrid Organosilicate Materials

Significance of plasma-surface interactions in the etch behavior of low-k materials

Low-k materials are an integral component in the advancement of semiconductor device performance by reducing parasitic capacitance and enabling faster device switching for a given thickness compared to traditional dielectric materials such as SiO2. With the advances in logic scaling, low-k materials are increasingly more prominent in the structures of advanced devices. For example, low-k materials are essential as the spacer material to provide both etch selectivity between dielectric materials and electrical isolation in field effect transistors. Consequently, the integration of low-k materials requires that the etch behavior of these materials be well understood so that the device structures can be reliably and reproducibly fabricated. In this study, the authors used a high-density plasma reactor with benchmark CF4- and NF3-based process chemistries to etch low-k materials including SiCN, SiOCN, and SiBCN in addition to Si, SiO2, and SiN reference materials. Numerous characterization techniques were utilized to understand the relationships between the plasma conditions, the evolution of the surface chemistry of the materials, and the resulting etch behavior. These techniques consisted of optical emission spectroscopy, spectroscopic ellipsometry, x-ray photoelectron spectroscopy, and attenuated total reflection Fourier transform infrared spectroscopy. The etch behavior of low-k materials under a given etch process is vital for establishing the etch selectivities in multilayer structures that are required to yield complex device geometries. For example, a directly proportional correlation was observed between the etch rate and intrinsic nitrogen concentration of the low-k materials. Potential mechanisms for the observed etch behaviors were explored using modeling and found that the intrinsic nitrogen composition in the low-k materials can result in energetically favorable reactions that result in the weakening and volatilization of the Si–N bond. Identifying the underlying mechanisms for the etch behaviors of low-k materials will provide key guidance into the development of etch processes that integrate these materials in current and future device structures.

Fluoride- and Seed-Free Synthesis of Pure-Silica Zeolite Adsorbent and Matrix Using OSDA-Mismatch Approach.

The pure-silica zeolite plays a crucially important role in the gas separation of alkane/alkene, the low-k dielectric material, and the robust matrix for confining metal species during catalysis. However, the environmentally friendly synthesis of pure-silica zeolites is still challenging since (1) the toxic fluoride or dealuminum seeds are inevitably utilized through the hydrothermal synthesis and (2) it will also take a longer crystallization time. Herein, we present an efficient method called the OSDA-mismatch approach for the fluoride- and seed-free synthesis of pure-silica zeolites using Si-SOD (enriched 4-rings) as the sole silica source. This approach allows for the rapid and green synthesis of 15 pure-silica zeolites (CHA, *BEA, EUO, SFF, STF, -SVR, *-SVY, DOH, MTN, NON, *MRE, MEL, MFI, MTW, and *STO). Furthermore, distinct crystallization mechanisms of two significant pure-silica CHA- and *BEA-type zeolites (denoted as Si-CHA and Si-BEA) are investigated in detail by advanced characterization techniques such as FIB, 3D ED, 4D-STEM, HRTEM, Raman, and 29Si MAS NMR. More importantly, Si-CHA displays promising propane/propylene separation performance even better than the one synthesized in the presence of toxic HF. In addition, the incorporation of Zn species within Si-BEA fabricated by this approach also renders superior performance on propane dehydrogenation.

Investigating the degradation mechanisms of moisture on the reliability of integrated low-k stack

This study investigates the phenomenon of moisture diffusion within integrated circuits, focusing on low-k and SiCN dielectric materials. The research confirms the main pathway of moisture diffusion occurring through interfaces between materials. The various steps involved in the moisture diffusion mechanism are identified and linked to electrical characterizations, including variations in capacitance, modifications in leakage current behavior, and dielectric breakdown. A modification in the conduction mechanism is observed. The investigation also highlights different types of bonds formed between the dielectrics and moisture based on literature review and baking process. Saturated samples exhibit a partial reversibility after a 250 °C bake but do not fully recover. Nevertheless, reversibility is shown to be dependent on the moisture content reached prior to baking.

Polymeric electronic materials for microelectronics manufacturing: A review

This review paper is part of a series of papers dedicated to the centenary celebration of the American Chemical Society Division of Polymeric Materials: Science and Engineering which has been the scientific home of many polymer scientists and engineers in the semiconductor industry. It is a review of parts of the research and development of advanced polymeric electronic materials for the fabrication and manufacturing of modern integrated circuits at the International Business Machines Corporation. This paper covers three types of advanced polymeric electronic materials: (1) photoresist materials used to “print” integrated circuits on silicon wafers, (2) on-chip low dielectric constant (low-κ) materials to insulate electrically conducting copper wires and (3) photo-patternable on-chip low-κ materials which function both as a photoresist and as an on-chip low-κ material. The photoresist materials are thin film imaging material systems of bilayer photoresist materials for 248 nm lithography and trilayer photoresist materials for 193 nm lithography. The low-κ dielectric material part focuses on fundamental understanding of plasma-induced damage to nanoporous organosilicate low-κ dielectric materials and the design of damage-resistant low-κ dielectric materials. Finally, the photo-patternable low-κ dielectric materials part describes the design and the development of dual-functional photo-patternable low-κ dielectric materials for both 193 nm and 248 nm lithography patterning of semiconductor Back-End-Of-the-Line. Several of these materials have been adopted for high-volume manufacturing of semiconductor chips for some of the most popular electronic devices in the world today.

Polybenzoxazine/organosilicon composites with low dielectric constant and dielectric loss

The evolution of electronic communication technology raises higher requirements for low dielectric constant (low-k) materials. For this, a benzoxazine functional organosilicon (HP-aptes) with dense Si—O—Si crosslinking networks and large sterically hindered tert-butyl groups was prepared by the sol–gel method. Then, a series of polybenzoxazine composites (PPHP) were prepared from intrinsically low dielectric constant bis-functional benzoxazine monomer (P-aptmds) and HP-aptes. The double crosslinking networks of polybenzoxazine and organosilicon further increased the crosslinking density and decreased the dipole density of composites, which endowed the composites with enhanced low-k properties. When the content of HP-aptes is 30% (mass), the crosslinking density was 2.05 × 10−3 mol·cm−3, while that of PP-aptmds was 3.31 × 10−3 mol·cm−3. In addition, the dielectric constant and dielectric loss of PPHP composite at 1 MHz could reach 2.61 and 0.0056, respectively.

Bio-based Low-k Polymers at High Frequency Derived from Anethole: Synthesis and the Relationship between the Structures and the Properties.

Bio-based polymers have been widely investigated as sustainable low dielectric (low-k) materials in past decades. Nevertheless, a few of the polymers with excellent comprehensive properties have been achieved to satisfy the requirements of high-frequency communication application. In this paper, two fluorinated monomers (BCB-F and 2BCB-F) have been designed and successfully prepared from biomass anethole. The thermal-cross-linkable benzocyclobutene and polyfluorobenzene groups were introduced in order to obtain low-k polymers with good comprehensive properties. A control monomer C1 was prepared from the estragole, the isomer of anethole, to study intensively the effect of structures on properties. Among the thermally cured polymers, cured BCB-F with higher fluoride content shows a comparable dielectric constant (Dk) of 2.62 and lower dielectric loss (Df) of 1.31 × 10-3 at a frequency of 10 GHz, as well as better hydrophobic properties with a water uptake of 0.18%. Such good hydrophobic properties enable it to maintain the good dielectric properties even after being soaked in boiling water for 96 h. Cured 2BCB-F with bifunctional benzocyclobutene groups displays excellent heat resistance with a high glass transition temperature (Tg) of 408 °C and a low coefficient of thermal expansion (CTE) of 52 ppm/°C in the temperature range 30-300 °C. Cured 2BCB-F also shows good dielectric properties with a Dk of 2.61 and a Df of 2.60 × 10-3 at a frequency of 10 GHz. The good comprehensive properties reveal that the anethole-based polymers are suitable candidates as matrix or encapsulation resins for application in electronics and microelectric fields.

The characterization of low-k thin films and their fracture analysis in a WLCSP device

The IC industry implements low dielectric constant (low-k) materials in copper/low-k interconnects to reduce the RC delay and crosstalk noises. As low-k dielectrics are weaker than traditional SiO2, measuring their mechanical properties is important for numerical simulations to evaluate the structural integrity of a package with low-k materials. In this study, we fabricated freestanding samples of low-k thin film with/without pre-cracks on both sides using lithography and exfoliation for uniaxial tension and coefficient of thermal expansion tests. The tension tests revealed that the average fracture toughness and Young's modulus of the low-k film are 1.41 MPa·m1/2 and ∼2.5 GPa, respectively. Using the measured mechanical properties, we performed a fracture analysis by a 2D finite element model for the low-k material in a wafer-level chip-size package under thermal loading. The computational results showed that the critical microcrack in the low-k layer occurs near the solder balls, where the KI increases rapidly with crack length and exceeds KIC. The results are consistent with the experimental findings in literature. The present methods can help evaluate the integrity of low-k material in a package and improve reliability.