Tensile properties of aluminum/alumina multi-layered thin films

Thin, free standing aluminum and alumina films were produced by physical vapor deposition and tensile properties were measured. Young’s modulus of the aluminum was microstructure insensitive, but the plastic behavior was very structure sensitive. The natural surface oxide of the aluminum had no apparent affect on the measured value ofYoung’s modulus. The alumina films showed true brittle behavior, but Young’s modulus was lower than bulk. Impurities residing at the grain boundaries were observed in the aluminum films using transverse Auger electron spectroscopy (AES). The films were well characterized using AES, transmission electron microscopy, Rutherford backscattering spectroscopy, and secondary electron microscopy. Well characterized, thin three-layered aluminum/alumina compositionally modulated films were produced by alternate depositions and tensile properties were measured. Young’s modulus was found to be less than a weighted thickness average of Young’s modulus of the individual constituents. Otherwise, the mechanical measurements yielded typical bulk behavior.     

Morphological Control of Nanoporous Films by the Use of Functionalized Cyclodextrins as Porogens

Porous cyclic silsesquioxane (CSSQ) thin films containing nanopores (～ 2 nm) with low dielectric constant (k < 2.2), have been prepared by using various kinds of cyclodextrin (CD) derivatives as porogenic materials. The pore structure, including average pore size and interconnectivity, can be controlled by changing the functional groups of the cyclodextrin derivatives. The pore structure is found to be strongly related to the affinity of the functional groups between CD molecules. The electrical and mechanical properties of the porous thin films were monitored in order to determine the relationship between the pore structure and film properties. The mechanical properties of porous low‐k thin films (total porosity ～ 30 %) prepared with CD derivatives are found to be correlated with the pore interconnection length. The longer the deduced interconnection length in the thin film, the worse the mechanical properties (such as hardness and modulus) of the thin film, even though the porogen‐induced pore diameters are very small (～ 2 nm).     

Aqueous-Develop,Photosensitive Polynorbornene Dielectric: Optimization of Mechanical and Electrical Properties

The impact of thermal cure conditions on the mechanical and electrical properties of an epoxy cross-linked network incorporating a polynorbornene (PNB) dielectric polymer was studied. The cross-linking of the dielectric composition was achieved by an acid-catalyzed cationic cure reaction initiated by either thermal or photolytic activation of a photoacid generator. It is proposed that the observed mechanical and electrical properties of the fully cured polymer composition are the result of the development of a three-dimensional cross-linked network tying together the PNB polymer and multifunctional epoxy additives. The epoxy ring-opening reaction was measured using Fourier-transform infrared spectroscopy. The reduced modulus, internal film stress, dielectric constant, and swelling behavior of cross-linked films were studied as a function of curing temperature. Trends in the observed properties are explained by formation of a three-dimensional cross-linked network and degradation of the cross-links between the multifunctional epoxy additives at high temperature. It was also found that exposure of the film to aqueous base plays a role in the cure process and has a positive effect on the final properties. The optimum values of modulus, dielectric constant, residual stress, and moisture content were found for films cured at 160°C for 1 h. This relatively low cure temperature is potentially advantageous in device assembly and processing.     

Characterization study of time- and temperature-dependent mechanical behavior of polyimide materials in electronic packaging applications

The thermo-mechanical testing of the type HPP ST polyimide films with high performance, supplied by Dupont, was realized under different strain rates and temperature effects. Therefore, the rate-temperature-dependent stress-strain behavior of materials was investigated and the dependence of the Young’s modulus on temperature and strain rate was reported. In view of the uncertainty of the Young’s modulus determination, the specimens were tested with the unloading-reloading technique to verify the test results. The constant strain rate uniaxial tensile test and long-time creep test at various temperatures were performed to characterize the time-temperature-dependent mechanical property precisely. The cyclic loading test was also implemented on the specimen to investigate cyclic stress-strain behavior. In addition, the nanoindentation test was carried out at room temperature to validate the elastic modulus derived from the uniaxial tensile test. This research is expected to investigate the time-temperature-dependent mechanical behavior of the polyimide materials for different service regimes including tensile and cyclic mechanical loading under elevated temperature in a systematic manner.     

Ultralow-k silicon containing fluorocarbon films prepared by plasma-enhanced chemical vapor deposition

Low dielectric constant materials as interlayer dielectrics (ILDs) offer a way to reduce the RC time delay in high-performance ultra-large-scale integration (ULSI) circuits. Fluorocarbon films containing silicon have been developed for interlayer applications below 50-nm linewidth technology. The preparation of the films was carried out by plasma-enhanced chemical vapor deposition (PECVD) using gas precursors of tetrafluorocarbon as the source of active species and disilane (5 vol.% in helium) as a reducing agent to control the ratio of F/C in the films. The basic properties of the low dielectric constant (low-k) interlayer dielectric films are studied as a function of the fabrication process parameters. The electrical, mechanical, chemical, and thermal properties were evaluated including dielectric constant, surface planarity, hardness, residual stress, chemical bond structure, and shrinkage upon heat treatments. The deposition process conditions were optimized for film thermal stability while maintaining a relative dielectric value as low as 2.0. The average breakdown field strength was 4.74 MV/cm. The optical energy gap was in the range 2.2–2.4 eV. The hardness and residual stress in the optimized processed SiCF films were, respectively, measured to be in the range 1.4–1.78 GPa and in the range 11.6–23.2 MPa of compressive stress.     

Measuring the through-plane elastic modulus of thin polymer films in situ

Due to the ongoing increase of the transistor density on a chip, industry has replaced the silicon oxide dielectric layers, traditionally used in the back-end interconnect stack, by low-K polymer films with a thickness down to several hundred nanometers. The use of these polymer dielectric films has introduced new failure modes. To have a better understanding of these failures, knowledge of the mechanical properties is necessary. Due to surface effects, the material properties of thin films may differ in the in-plane and trough-plane direction. Most techniques available for measuring these properties are only capable of obtaining the in-plane modulus. To have an in situ measurement of the through-plane modulus, a parallel plate capacitor (PPC) under hydrostatic pressure is used in combination with an interdigitated electrode (IDE) to capture the change in dielectric constant. Since it is believed to be mechanical isotropic, a benzocyclobutene (BCB) film is used to provide a reference measurement. The through-plane elastic modulus and change in permittivity for a 1 μm thick film sandwiched by two aluminum electrodes on a silicon wafer are reported. Two circular PPCs and four IDEs were tested at a pressure of 0, 5, 7.5 and 10 MPa. An initial relative dielectric constant of the film of 2.66 ± 0.05 was obtained. This yields a change in constant equal to 1.241 × 10⁻⁴ ± 2.1 × 10⁻⁵ per MPa pressure at room temperature. The through-plane modulus showed a linear elastic behavior equal to 4.73 ± 0.46, 4.11 ± 0.39 and 3.64 ± 0.31 GPa for 20°, 50° and 75 °C, respectively. The modulus at room temperature is in good agreement with the values found in literature.     

Photodefinable Epoxycyclohexyl Polyhedral Oligomeric Silsesquioxane

A photodefinable dielectric was developed using epoxycyclohexyl polyhedral oligomeric silsesquioxanes (POSS) and a photocatalyst. POSS is a hybrid organic/inorganic dielectric which has favorable mechanical and chemical stability for use as a permanent dielectric in microfabrication. Sharp, 10 μm wide features were formed from POSS using 365 nm radiation. The optical contrast was 1.51. POSS films were thermally stable to 350°C and demonstrated chemical stability in a variety of solvents and oxidants. The polymer film had an elastic modulus of 5.3 GPa and a hardness of 0.64 GPa. The POSS had high etch selectivity compared with organic films for pattern transfer.     

Low-K Dielectric Compatible Wafer-Level Compliant Chip-to-Substrate Interconnects

Performance, power, size, and cost requirements in the microelectronics industry are pushing for smaller feature size, innovative on-chip dielectric materials, higher number of interconnects at a reduced pitch, etc., without compromising the microelectronics reliability. Compliant off-chip interconnects show great potential to address these needs. G-Helix is a lithography-based electroplated compliant interconnect that can be fabricated at the wafer level. G-Helix interconnects exhibit excellent compliance in all three orthogonal directions, and can accommodate the coefficient of thermal expansion (CTE) mismatch between the silicon die and the organic substrate without requiring an underfill material. These compliant interconnects are beneficial for integrated circuits (ICs) with low-K dielectric material. They are also potentially cost effective as they can be fabricated using conventional wafer fabrication infrastructure. In this paper we discuss the assembly and experimental reliability assessment, through thermal cycling, of G-Helix interconnects assembled on an organic substrate. Results from mechanical characterization experiments are also presented. It is shown that the proposed interconnects are not likely to delaminate or crack the low-K dielectric material. Also, a unique integrative approach is discussed, with interconnects having varying compliance for optimum electrical and mechanical performance.     

Mechanical processes in chemical-mechanical planarization: Plasticity effects in oxide thin films

The role of plastic behavior of normally brittle oxide films in controlling materials removal mechanisms in chemical-mechanical planarization (CMP) is discussed. Particular attention is given to how material removal mechanisms are sensitive to fundamental changes in surface materials properties. It is suggested that the synergism between chemical and mechanical effects in CMP can be framed in the context of an environmentally sensitive fracture process. The concept of “fracture” in the case of CMP, however it is argued, occurs at the nanometric or ultimately at the atomistic scale. This type of paradigm for chemical-mechanical planarization is developed through an analysis of the different types of materials behavior associated with surface changes where environment and mechanical effects are coupled.     

Study of nano-mechanical properties for thin porous films through instrumented indentation: SiO2 low dielectric constant films as an example

Much research has been focused on the mechanical properties of porous materials such as films of silica xerogels because of their potential for application to microelectronic interconnects. To accurately probe the film properties, one has to challenge with the porosity as well as the large differences between film and substrate properties. In this paper, a study is presented for the investigation of Young’s modulus and yield stress of these porous films by instrumented indentation under complete consideration of the substrate influence by using the approach of the ‘effectively shaped indenter concept’. This concept provides the basis of a more appropriate analysis for thin films in case of elastic-plastic contact situations as given for porous low-k films. It was found that the ratio of yield stress to Young’s modulus, which equals the yield strain of the stress-strain curve, is not constant and changes with porosity.     

Ultralow Dielectric Constant Tetravinyltetramethylcyclotetrasiloxane Films Deposited by Initiated Chemical Vapor Deposition (iCVD)

Simultaneous improvement of mechanical properties and lowering of the dielectric constant occur when films grown from the cyclic monomer tetravinyltetramethylcyclotetrasiloxane (V4D4) via initiated chemical vapor deposition (iCVD) are thermally cured in air. Clear signatures from silsesquioxane cage structures in the annealed films appear in the Fourier transform IR (1140 cm^?1) and Raman (1117 cm^?1) spectra. The iCVD method consumes an order of magnitude lower power density than the traditional plasma‐enhanced CVD, thus preserving the precursor's delicate ring structure and organic substituents in the as‐deposited films. The high degree of structural retention in the as‐deposited film allows for the beneficial formation of intrinsically porous silsesquioxane cages upon annealing in air. Complete oxidation of the silicon creates ‘Q’ groups, which impart greater hardness and modulus to the films by increasing the average connectivity number of the film matrix beyond the percolation of rigidity. The removal of labile hydrocarbon moieties allows for the oxidation of the as‐deposited film while simultaneously inducing porosity. This combination of events avoids the typical trade‐off between improved mechanical properties and higher dielectric constants. Films annealed at 410 °C have a dielectric constant of 2.15, and a hardness and modulus of 0.78 and 5.4 GPa, respectively. The solvent‐less and low‐energy nature of iCVD make it attractive from an environmental safety and health perspective.     

Micromechanical analysis of copper trace in printed circuit boards

Printed circuit boards (PCB) are designed and manufactured with a variety of polyamide materials such as solder mask, metallic material such as copper trace, composite materials such as prepreg and core material. Polyamide materials such as solder mask and composite materials such as prepreg play important factor on the total deformation of laminate package due to the large coefficient of thermal expansion (CTE). On the other hand, the patterning of the copper layers also exerts important influence to the thermal mechanical behavior of the substrate due to the consistent large Young’s modulus of copper at both room temperature and reflow temperature compared with the small Young’s modulus of polyamide materials. Some approximate methods based on rule of mixtures have been used for estimating material properties in layers of copper mixed with interlayer dielectric material, but few techniques include the effect of copper trace pattern. The detailed comparison of different approximate methods has been done in this paper and a modified homogenization method has been proposed to include the effect of copper trace pattern. A series of three point bending test are performed with the comparison of numerical prediction using the proposed homogenization method and the detailed copper trace pattern respectively. Finally, a micromechanical analysis is done for the copper trace crack problem in package-on-package (PoP).     

Aqueous-Develop,Photosensitive Polynorbornene Dielectric: Properties and Characterization

The properties of a new aqueous-base-develop, negative-tone photosensitive polynorbornene have been characterized. High-aspect-ratio features of 7:1 (height:width) were produced in 70-μm-thick films in a single coat with straight side-wall profiles and high fidelity. The polymer films studied had contrast of 12.2 and low absorption coefficient. To evaluate the polymer’s suitability to microelectronics applications, epoxy crosslinking reactions were studied as a function of processing condition through Fourier-transform infrared spectroscopy, nanoindentation, and dielectric measurements. The fully crosslinked films had an elastic modulus of 2.9 GPa and hardness of 0.18 GPa.     

Tera Tool [terahertz time-domain spectroscopy]

The terahertz differential time-domain spectroscopic method is applied to characterize the dielectric and optical properties of a variety of thin films at terahertz frequency. The results of several samples including silicon dioxide, parylene-n polymer film, tantalum oxide film, and protein thin layer samples were presented. The dielectric property of silicon dioxide thin film is well fitted to that of a bulk. The dielectric properties of parylene-n thin films show good agreement with the result measured by the goniometric terahertz time-domain spectroscopy. The dielectric and optical properties of the tantalum oxide show reasonable data with previously available data. Some properties in thin films are slightly different from the bulk materials. The origin of this discrepancy is considered due to fine grain formation, mechanical stresses, formation of interfacial layers, or rough interfaces during thin-film deposition process. The terahertz differential time-domain spectroscopy may be applied to the measurement of the dielectric and optical properties of thin films (nanometer to micrometer) of several materials, which cannot be done by any other method.     

Variable Frequency Microwave Curing of Amide-Epoxy Based Polymers

Irradiation of a dielectric material with microwave irradiation results in energy transfer due to rotational-vibrational transitions within the molecule. The energy transfer results in a local rise in temperature within the material. Microwave-induced reactions can occur at a lower average temperature than convective heating resulting in faster thermal curing of polymer dielectrics because the energy absorption is localized at particular sites. In this study, variable frequency microwave (VFM) curing of epoxy-based dielectric films was investigated. The microwave energy was swept through a range of frequencies to dissipate standing waves so that metallic conductors could be present with the dielectric films. The rate of reaction and film properties of polyamideimide (PAI) and bisphenol A epoxy resin (BPAEp) were studied. Benzanilide and BPAEp were used as model compounds for the reaction between the amide and epoxy. Compared to convective heating, the microwave reaction rates were higher at each isothermal cure temperature. The resulting mechanical properties of the films cured by microwave heated were superior to thermally cured materials. The elastic modulus of VFM cured PAI/BPAEp films was less than that of thermally cured films and the elongation to break was twice as high. Further, the adhesion to copper was improved with microwave processing. The chemical structure of the VFM cured polymer was different from the thermally cured polymer, based on Fourier transform infrared analysis, and is likely the origin of the improved properties.     

Effect of Microstructural Development on Mechanical and Electrical Properties of Inkjet-Printed Ag Films

The microstructural characterization of inkjet-printed Ag films sintered at various conditions was carried out to analyze the effect of microstructure on mechanical and electrical properties. As expected, the films became denser with grain growth with increasing sintering time and temperature, which resulted in improvement in mechanical properties. However, the resistivity of the films reached a minimum value of 3.0 μΩ cm before full densification. In order to improve the mechanical properties, pressure-assisted sintering was introduced. As a result, inkjet-printed Ag films sintered at 250°C under 5 MPa showed a tensile strength of 550 MPa, elongation of 2.4%, Young’s modulus of 55 GPa, and resistivity of ~3.0 μΩ cm.     

Biomimetic Carbon Nanotube Films with Gradient Structure and Locally Tunable Mechanical Property

Naturally existing materials often acquire unique functions by adopting a gradient structure with gradual change in their microstructure and related properties. Imparting such an elegant structural control into synthetic materials has been a grand challenge in the field. Here, the concept of gradient structure into macroscopic carbon nanotube (CNT) films is employed and the CNT arrangement from well‐aligned array to completely random distribution, in a continuous and smooth way, is changed. Gradient films with tailored aligned‐to‐random transition rate or multilevel hierarchical structures with repeated transition have been fabricated. Local deformation and mechanical properties are directly related to the arrangement of CNTs and can be tailored by Herman's orientation factor; in particular, the elastic modulus and stiffness span over several orders of magnitude from aligned to random regions within a single monolithic film. Controlled synthesis of macroscopic CNT gradient structures with tunable mechanical properties opens a potential route toward manufacturing biomimetic functional materials with locally optimized design.     

Mechanical and tribological properties of interlayer films for the damascene-Cu chemical-mechanical planarization process

Chemical-mechanical planarization (CMP) has emerged as the most preferred method to achieve excellent global and local planarity in the damascene-Cu process. As the feature sizes shrink, understanding the fundamentals of CMP is critical for successful implementation of the CMP process in sub 0.35-μm technology. It is also important to understand the effects of mechanical and tribological properties of the interlayer films on the CMP process to conduct successful evaluation and implementation of these materials. In this paper, we present the mechanical and tribological properties of various interlayer films (SiO₂, SiC, low-k B, low-k C, Ta, and Cu) and discuss the CMP process of the films in an alumina-based Cu slurry. Mechanical properties were evaluated using a nanoindentation technique. A micro-CMP tester was used to study the fundamental aspects of the CMP process. The coefficient of friction (COF) was measured during the process and was found to decrease both with downward pressure and with platen rotation. An acoustic sensor, attached to the substrate carrier, was used to monitor the process, and the signal was recorded to examine the difference in polishing behavior of these films. The acoustic emission (AE) signal was found to increase with the increase in platen velocity and pressure. Effects of machine parameters on the polishing behavior of the interlayer films and the correlation of mechanical properties with tribological properties have been discussed.     

The synthesis and dielectric study of BaTiO3/polyimide nanocomposite films

Polymer-ceramic dielectric composites have been of great interest because they combine the processability of polymers with the desired dielectric properties of the ceramics. A novel, simple method to make thin, homogeneous composite films with varying amounts of ceramic filler content has been proposed. Nanocomposite thick films of barium titanate (BaTiO₃) in polyimide (PI) from 3,3′,4,4′-benzophenone tetra carboxylic dianhydride, 4,4′-oxy dianiline have been successfully prepared by in situ imidization using n-methyl pyrrolidinone as the solvent. The dielectric properties of the nanocomposite films were discussed for various filler volume contents. These nanocomposite films exhibited stable dielectric properties in the various frequency ranges. Films were characterized by fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermo gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The curing cycle was optimized using FTIR. XRD results confirm BaTiO₃ particles remain unchanged in the PI matrix and no undesired secondary phases are present in the films. Their glass transition behavior and thermal stability were investigated by DSC and TGA, and the films were stable to 300 °C. Scanning electron microscope images show that the BaTiO₃ phase is well dispersed in the polymer matrix.     

应用纳米压痕法测试类金刚石薄膜力学性能的研究

介绍一种被称为材料机械性质微探针的新型纳米压痕仪，藉助于加荷-卸荷过程中压痕对负荷和压入深度，即压头位移的敏感关系，测试材料的硬度及弹性模量等力学性质。纳米和纳牛顿量级的压头位移与作用力，使得测试始终在大多数材料，特别是薄膜材料的弹性限度内，克服了维氏法、努氏法等传统方法引起压痕边缘模糊或者碎裂的缺点，保证了测试的正确性、可靠性和重复性。应用纳米压痕法成功地对具有脆性的类金刚石薄膜的硬度及弹性模量进行了测试和分析。