Properties of Liquid-Phase Deposited Silica Films for Low-k Dielectric Applications

Both the electrical and mechanical properties of silica thin films deposited by liquid phase deposition (LPD) have been evaluated in this study. Silica thin films have been prepared on glass surface by immersing it in a supersaturated Hexafluorosilicic acid (H₂SiF₆)-based solution at a low temperature of 50°C. The as-deposited LPD silica films exhibit a low dielectric constant ( k ) that varies from 1.7 to 2.7 depending on the film morphology and fluorine content of the film. Young's modulus of these films was measured in the range of 18.9–24.5 GPa by a nanoindentation technique. The combination of extremely low k and fairly high modulus made this low-temperature-processed LPD silica films a very promising candidate for an interlayer dielectric film for the next-generation semiconductor devices.     

超低介电常数纳米多孔SiO2薄膜制备技术进展

纳米多孔SiO2薄膜具有超低的介电常数，作为绝缘介质在超大规模集成电路互连系统有着巨大的应用潜力。文中概述了纳米多孔SiO2薄膜的孔隙度与介电常数的关系，指出所有模型均位于串联和并联模型之间，介电常数均随孔隙度的增加而下降。说明了多孔SiO2薄膜按孔隙度不同主要分气凝胶和干凝胶两类。介绍了从溶液前驱物中合成的原理，绘出了制备纳米多孔SiO2薄膜的一般流程。详细总结了用气凝胶／干凝胶法和模板法制备纳米多孔SiO2薄膜的技术进展。探讨了旋转涂覆制备多孔SiO2薄膜的弱点及改进办法，指明了超低介电常数纳米多孔SiO2薄膜制备技术的发展方向。     

Elastic modulus prediction based on thermal conductivity for silica aerogels and fiber reinforced composites

The speed of sound is a critical parameter in the test of mechanical and thermal properties. In this work, we proposed a testing method to obtain the elastic modulus of silica aerogel from the sound speed formulas. The solid thermal conductivity of the silica aerogel is experimentally measured for predicting the sound speeds, and then the elastic modulus is calculated based on the elasticity sound speed model. The experimental data of the solid thermal conductivity of silica aerogels with different densities are employed and the obtained elastic modulus is fitted as a power-law exponential function of the density. Two existing sound speed models and three groups of available experimental data are also employed to validate the present fitting relation, and good agreement is obtained for the silica aerogel in the density range of 150–350 kg/m³. The fitting formula can also be extended to estimate the elastic modulus of the glass fiber-reinforced silica aerogel composite. The results show that the elastic modulus of the aerogel composite is sensitive to the glass fiber volume fraction, while the thermal conductivity is weakly dependent on the glass fiber volume fraction at room temperature in the studied range of fiber volume fraction.     

Polyethylene/silica nanorod composites with reduced dielectric constant and enhanced mechanical strength

Reinforcement of materials with low dielectric constant by nanofillers has attained more and more attention. Conventional 1D nanofillers, such as carbon nanotubes, significantly enhance mechanical properties of polymers; however, they do not reduce dielectric constant. In this study, silica nanorods (SNR) were used to prepare polyethylene composites by mechanical blending followed by hot pressing. It was found that the incorporation of SNRs enables enhancement of strength and elastic modulus. Dielectric constant did not increase as predicted when SNRs were incorporated into polyethylene (PE); on contrary, it considerably decreased from ~2.4 to around ~2.2 at SNR loading of 5 wt %. However, when glass fibers were used as fillers, we did not observe such decrease. SEM image and density measurement indicated that the composites were compact with very low porosity. We believe that the incorporation of SNRs alters crystalline and chain arrangement of PE, thus reduces the polarization and dielectric constant. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47143.     

Synthesis and Characterization of Low-Dielectric Silica Aerogel Films

Thin films of silica aerogel on a silicon wafer have been synthesized via supercritical drying of wet gel films that were obtained by spin coating the polymeric silica sol, followed by aging in an isopropanol (IPA) or tetraethoxysilane/isopropanol (TEOS/IPA) solution. The viscosity of the sol and the spin rate required to form uniform coating layers were optimized. The film thickness and microstructures could be controlled by adjusting the concentration of the sol, the spin rate, and the aging conditions. The porosities and the dielectric constants of the thin films were in the range of 76%–90% (densities of 0.59–0.22 g/cm³) and 2.0–1.5, respectively. The degree of planarization in the aerogel thin film was ∼100%, and the gap-filling capability on a 0.2 μm tungsten patterning wafer was excellent. In particular, aging the wet gel film in the TEOS/IPA solution was very effective in improving the properties of the aerogel films.     

Synthesis of hybrid free and nanoporous silica aerogel‐anchored polystyrene chains via in situ atom transfer radical polymerization

Polystyrene nanocomposite with mixed free and anchored chains was synthesized by atom transfer radical polymerization. Attachment of 3‐(trimethoxysilyl)propyl methacrylate with a double bond on the nanoporous silica aerogel surface results in a double bond grafted silica aerogel which could be incorporated into the polystyrene chains by a grafting‐through process. Conversion and molecular weight evaluation was carried out using gas chromatography and gel permeation chromatography, respectively. Double bond containing silica aerogel has an inconsiderable effect on conversion. There is no considerable discrepancy between the molecular weights of the free and anchored chains. Addition of silica aerogel with pendant CC bonds leads to increase of apparent rate constant of polymerization and also molecular weights. This is mainly because of initiator trapping in silica aerogel pores. Every percent of double bond containing silica aerogel with respect to styrene results in trapping of about 0.08 mol of ethyl alpha‐bromoisobutyrate among the silica pores. POLYM. COMPOS., 34:1648–1654, 2013. © 2013 Society of Plastics Engineers     

Synthesis of Low-Dielectric Silica Aerogel Films by Ambient Drying

A new ambient drying process that is simple, effective, and reproducible has been developed to synthesize low- k SiO₂ aerogel thin films for intermetal dielectric (IMD) materials. The SiO₂ aerogel films having a thickness of 9500 Å, a high porosity of 79.5%, and a low dielectric constant of 2.0 were obtained by a new ambient drying process using n -heptane as a drying solvent.     

纳米级增强体复合硅气凝胶的研究进展

二氧化硅气凝胶的脆性大、力学强度过低,严重限制了其应用领域。常规微米级或大于微米尺寸的增强体复合硅气凝胶可以提升硅气凝胶性能,但难以在纳米尺寸范围内对凝胶孔隙增强。目前,可以采用合成聚合物纳米纤维、纤维素纳米纤维、纳米管、无机纳米纤维、石墨烯及氧化石墨烯等增强硅气凝胶,纳米级增强材料能够均匀分散在硅气凝胶纳米孔隙中,干燥收缩小、孔比表面积大、可有效提升硅气凝胶抗压强度、改进韧性。     

Derivation of the Exponential Relation for the Effect of Ellipsoidal Porosity on Elastic Modulus

An analytical basis for the exponential relationship between elastic modulus and fraction porosity is derived. The exponential constant in the equation is shown to be a function of the pore shape and the orientation of the pores with respect to the stress axis and to be independent of pore size. A similar exponential relationship between the modulus and the number of cracks per unit volume is crack-size dependent.     

Effect of Porosity on Physical Properties of Sintered Alumina

Porous structures having a continuous solid phase with isolated pores were prepared by the addition of different amounts of crushed naphthalene to an alumina casting slip. Samples of from 5 to 50% porosity were fired together for comparable grain development, eliminating structural variables except porosity. Effects of porosity and temperature on strength, elastic modulus, modulus of rigidity, and coefficient of thermal expansion were investigated. Effects of porosity on thermal stress resistance and tor-sional creep properties were studied at constant temperature.     

有序介孔二氧化硅薄膜的制备及性能

以十六烷基三甲基溴化铵(CTAB)为模板剂,正硅酸乙酯为无机前驱体,在酸性条件下,采用溶胶-凝胶技术,用蒸发诱导自组装(EISA)工艺制备了二氧化硅透明介孔薄膜. 透射电子显微镜图显示热处理后的薄膜具有高度有序的六方相结构孔道;由椭偏仪测得热处理后薄膜的折射率低至1.18,厚度在180 nm左右;阻抗分析仪测得薄膜的介电常数为2.14. 薄膜经过六甲基二硅胺烷(HMDS)表面修饰后具有良好的疏水性能和热稳定性,作为低介电材料能更好满足工业需求.     

High-Tg porous polyimide films with low dielectric constant derived from spiro-(adamantane-2,9′(2′,7′-diamino)-fluorene)

Porous polyimide (PI) films with low dielectric constants and excellent thermal properties have been a pressing demand for the next generation of high-performance, miniature, and ultrathin microelectronic devices. A series of novel porous PI films containing fluorenyl-adamantane groups were prepared successfully via thermolysis of poly(ethylene glycol) (PEG) added in the PI matrix. The cross-sectional morphologies of porous PI films showed closed pores with diameters ranging from 135 to 158 nm, which were uniform and regular in shape without interconnectivity. These porous PI films exhibited excellent thermal properties with a glass-transition temperature at 376 °C whereas the 5% weight loss temperature in air excess of 405 °C due to enhanced rigidity afforded by fluorenyl-adamantane groups. Accompanied by thermolysis content of PEG increasing from 0 to 20 wt %, the density of porous PI films decreased, and the corresponding porosity grew significantly from 0 to 11.48%. Depending on porosity, the dielectric constant and dielectric loss of porous PI films significantly declined from 2.89 to 2.37 and from 0.050 to 0.021, respectively. These excellent properties benefit the as-prepared porous PI films for application as interlayer dielectrics, integrated circuit chips, or multichip modules in microelectronic fields. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47313.     

二氧化硅气凝胶复合隔热材料研究进展

二氧化硅气凝胶具有高孔隙率、低热导率等特点,使其成为新型超级隔热材料。然而,二氧化硅气凝胶的柔韧性、整体性差,并且常温干燥制备的气凝胶在高温时热导率迅速上升,这些都大大限制了二氧化硅气凝胶的应用。近些年,通过原位溶胶-凝胶法和模压成型法制备得到的二氧化硅气凝胶复合隔热材料,在一定程度上提高了其韧性、整体性和高温隔热性能,使得二氧化硅气凝胶作为单独块体隔热材料成为可能。本文阐述了二氧化硅气凝胶隔热材料的隔热机理,综述了近年来抗辐射型、纤维增强型和聚合物增强型二氧化硅气凝胶复合隔热材料的研究现状,最后讨论了该领域今后研究趋势。     

微组装基板材料的孔隙率与其抗折强度关系的研究

在玻璃陶瓷基板材料中加入PS有机球能有效地降低基板的介电常数,但同时影响基板的抗折强度。通过建立空心球力学模型,得到基板材料的有效体积模量和有效杨氏模量的表达式,从而推导出表达闭合孔孔隙率与抗折强度之间关系的公式。该公式与实验结果符合较好。     

Effects of silica aerogel particle sizes on the thermal–mechanical properties of silica aerogel – unsaturated polyester composites

Silica aerogels with a surface area as high as 773?m²?g^?1 and a density of 0.077?g?cm^?3 were produced from rice husk via sol–gel process and ambient pressure drying. A particulate composite material was prepared by adding silica aerogel particles of three different particle sizes (powder, granules and bead) to unsaturated polyester resin with a fixed volume fraction of 30%. Thermogravimetric and thermal conductivity studies revealed that silica aerogel composites were having higher thermal stability and thermal insulation than the neat resin. It was suggested that the preservation of aerogel pores from resin intrusion is important for better thermal properties. Larger silica aerogel particles have more porous area (unwetted region) which results in a lower degradation rate and lower thermal conductivity of the base polymer. However, the addition of silica aerogel into resin has reduced the tensile modulus of the polymer matrix where smaller particle size displayed higher toughness than those with bigger particle size.     

Synthesis of silica aerogel thin sheets and evaluation of its thermal,electrical, and mechanical properties

Silica aerogels are excellent thermal and acoustic insulators because of interconnected open nanopores with more than 90% porosity and higher surface area. Silica aerogel is derived by sol-gel process and dried under super-critical, sub-critical or ambient pressure conditions. Thin silica aerogel sheets could be effective thermal insulators but high fragility hinders the wider applications. We have successfully developed a synthesis method for thin, flexible, and non-fragile aerogel sheets with excellent hydrophobicity, lower thermal conductivity, and non-combustible properties via ambient drying method. The silica aerogel sheets prepared compose of silica aerogel powder, fiber glass chopped strands, and solvent-based binder. Aerogel thin insulation sheets of thickness 164 μm were prepared by pressing through rollers using aerogel paste composed of aerogel powder, fiber glass strands, and binders. The thermal conductivity values obtained were between 0.02~0.63 W/mK at temperature 25~400°C, contact angle θ = 121‘_, weight loss 3.91% when heated up to 800°C in air, dielectric voltage breakdown 3.67 kV, dielectric strength 6.37 kV/mm and tensile strength of 2.65 N/mm². The overall thermal, electrical, and mechanical evaluation of aerogel thin insulation sheet showed they have higher potential to replace existing thick and bulky aerogel composites as thermal and electrical insulators in aviation, automobiles, electronics, and high power batteries.     

Dependence of the dielectric constant on the fluorine content and porosity of polyimides

The trend toward miniaturization in integrated circuit fabrication demands good interlayer dielectric materials. This need can be met by polyimide (PI), which has extreme thermal and chemical stability and, most importantly, a low dielectric constant. Four porous PIs with symmetrically substituted fluorine contents were synthesized. Different porosity levels were achieved with a sol–gel technique through the incorporation of 10 or 20% tetraethyl orthosilicate into the polymer matrix and then acid etching. Their dielectric constants were correlated with the fluorine contents and porosity levels. High porosity levels and higher fluorine contents induced substantial decreases in the PI dielectric constants (2.4–2.7). The resultant values were within the applicable range for dielectric materials in integrated circuits. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

影响二氧化硅气凝胶隔热涂料热导率的因素

采用溶胶凝胶法及雾化技术制备了二氧化硅气凝胶微球,同时制备了二氧化硅气凝胶隔热涂料。利用扫描电镜（SEM）对涂料的微结构进行观测,采用激光粒度检测仪对二氧化硅气凝胶微球的尺寸进行检测,采用Hot Disk热导率仪测量了二氧化硅气凝胶隔热涂料的热导率。结果显示：根据SEM 图像,气凝胶微球在涂料中形成明显团聚,且在气凝胶体积分数较高时,涂料中气孔增多。此外,小粒径气凝胶微球更容易形成团聚。由于气凝胶微球热阻极大,气凝胶隔热涂料的热导率随气凝胶微球含量的增加而下降。气凝胶微球的团聚相比均匀分散不利于热导率的降低,而孔隙的增多则有利于涂料热导率下降,因为空气的热阻高。小粒径微球的界面热阻比大粒径微球更大,导致小粒径微球制备的隔热涂料热导率低,混合粒径使气凝胶微球堆积密度增大,有利于降低涂料的热导率。     

二氧化硅气凝胶及其在保温隔热领域应用进展

二氧化硅气凝胶是目前已知最轻的固体材料，具有热导率低、孔隙率高和比表面积大等优点，被誉为新型超级保温隔热材料。然而，二氧化硅气凝胶自身存在力学性能差和制备成本高的问题，大大限制了其在保温隔热领域大规模推广应用。本文简述了二氧化硅气凝胶合成技术和力学性能增强方法，从制备过程控制、老化条件优化、热处理、纤维复合和高分子聚合物复合等方面分析了其对气凝胶性能和工艺的影响，重点介绍了近年来二氧化硅气凝胶保温隔热材料应用在航空航天、军工领域、工业管道、建筑保温以及新能源汽车等领域的研究进展，总结了其在各领域应用的技术挑战。指出未来需进一步拓展二氧化硅气凝胶的使用温区，利用共前体和化学交联等方法增强高温下的隔热性能，同时解决气凝胶纤维复材“掉粉”和微米级粉体分散不均匀等难题，尤其是新能源汽车等新兴应用领域发展迅猛，未来仍需针对新的应用需求对其合成技术进行设计和优化。     

Elastic Properties of Silica Xerogels

To calculate elastic constants, longitudinal and tranverse acoustic wave velocities were measured for silica xerogels as a function of relative humidity (rh). The silica xerogels studied are microporous with open porosity of 53 vol%. The longitudinal wave velocity exhibits a minimum at about 35% rh. The transverse wave velocity decreases to a constant value for 35% rh. Consequently, Young's modulus is a minimum at about 35% rh, whereas the shear modulus decreases to a constant value at 35% rh. The bulk modulus and Poisson's ratio exhibit minimum values at about 15% rh. Young's modulus decreases from 4.91 to 3.42 GPa at 35% rh and then increases to 3.60 GPa at 55% rh. Poisson's ratio decreases from 0.184 to 0.164 at 15% rh and then increases to 0.272 at 55% rh. Below 35% rh, silica xerogels adsorb a monolayer of hydroxyls, whereas above 35% rh silica xerogels show pore filling.