Residual strain in cadmium telluride/gallium arsenide (001) heterostructures as a function of temperature

Optical studies of residual strain in cadmium telluride (CdTe) films grown using molecular beam epitaxy on gallium arsenide (GaAs) substrate have been performed using photoreflectance techniques. Measurements have been conducted to determine the fundamental transition energy, heavy-hole and light-hole transition energy critical-point parameters in a range of temperatures between 12 and 300 K. There are problems inherent in the fabrication of optoelectronic devices using high-quality CdTe films, due to strain effects resulting from both the lattice mismatch (CdTe: 14.6%) and the thermal expansion coefficient difference. The CdTe film exhibits compressive stress causing valence-band splitting for light and heavy holes. We have used different models to fit the obtained experimental data and, although the critical thickness for the CdTe has been surpassed, the strain due to the lattice mismatch is still significant. However, the strain due to the thermal expansion is dominant. We have found that the fundamental transition energy, E₀, is affected by the compressive strain and the characteristic values are smaller than those reported. In addition, the total strain is compressive for the full measured range, since the strain due to the lattice mismatch is one order of magnitude higher than that calculated from the thermal expansion.     

Analysis of the stress and interfacial reactions in Pt/Ti/SiO2/Si for use with ferroelectric thin films

The microstructure of the Pt/Ti/SiO₂/Si structure has been investigated by scanning and transmission electron microscopy. Pt films of 100 nm thickness deposited by sputtering or evaporation onto unheated substrates gave complete coverage of the underlying Ti layer and showed a granular and faceted structure with grains ∼20 nm in diameter. They did not exhibit hillocks or surface TiO_x formation. X-ray diffraction was used to examine the film stress through use of the sin²ψ method with bulk values for the elastic constants (v=0.39, E=162 GPa). The as-deposited sputtered film had a compressive stress of ∼540 MPa, while the evaporated films had tensile stresses of ∼630 MPa. The films then received a 400°C rapid thermal anneal (RTA) for 90 s and a subsequent RTA of 650°C for 30s. Further investigation of the film stresses and microstructure were made after each annealing step. After the low temperature anneal, the film stress for the sputtered film became tensile. Plan-view sections examined by transmission electron microscopy (TEM) showed that the as-deposited sputtered films were dense but became porous after annealing. Initially, the evaporated films had a less dense microstructure, but were more stable with annealing. Little change in the stress for the evaporated film was observed after this initial low temperature annealing step. Additional annealing of the evaporated and sputtered samples caused complete consumption of the Ti layer including some TiO_x formation from the underlying SiO₂ layer and marked interaction with the Pt; however, little change in the stress was found. The surface of the Pt film revealed larger grains, but otherwise remained unaffected. The underlying phase changes were minimized once the Ti layer had reacted with the Pt. Due to the ratio of the layers, Pt:Ti of 2:1, the surface of the Pt was unaffected.     

热处理对双离子束溅射SiO2薄膜力学及热力学特性的影响

光学薄膜的力学及热力学特性决定了光学系统性能的优劣。采用双离子束溅射的方法在硅110和肖特石英Q1基底上制备了SiO2薄膜,并对制备的膜层进行退火处理。系统研究了热处理前后SiO2薄膜的力学及热力学特性。研究结果表明,750℃退火条件下SiO2薄膜的弹性模量（Er）增加到72 GPa,膜层硬度增加到10 GPa。镀完后未经退火处理的SiO2薄膜表现为压应力,但是应力值在退火温度达到450℃以上时急剧降低,说明热处理有助于改善SiO2薄膜内应力。经退火处理的SiO2薄膜泊松比（vf）为0.18左右。退火前后SiO2薄膜的杨氏模量（Ef）都要比石英块体材料大,并且750℃退火膜层杨氏模量增加了50 GPa以上。550℃退火的SiO2薄膜热膨胀系数（f）从6.7810-7℃-1降到最小值5.2210-7℃-1。     

溅射压力对制备a-GaAs1-xNx 薄膜光学常数的影响

采用反应磁控溅射法在室温条件下制备了a-GaAs_1-xN_x 薄膜。实验测定了薄膜厚度、氮含量、载流子浓度和光学透过率及并研究了其随溅射压的变化。系统研究了溅射压对所制备薄膜的光学带隙、折射率和色散参数的影响。所制备的薄膜为直接带隙材料,利用Cauchy和Wemple模型能够很好地拟合所制备薄膜的折射率色散曲线。     

Residual stress in silicon nitride films

The mechanical stress caused by Si₃N₄ films on (111) oriented Si wafers was studied as a function of the Si₃N₄ film thickness, deposition rate, deposition temperature and film composition. The Si₃N₄ films were prepared by the reaction of gaseous SiH₄ and NH₃ in the temperature range 700–1000°C. The curvature of the Si substrates caused by the Si₃N₄. films is related to the film stress; the substrate curvature was measured by an optical interference technique. The measured Si₃N₄. film stress was found to be highly tensile with a magnitude of about 10¹⁰ dynes/cm². For the thickness range of 2000–5000Å, there was no change in the measured stress. The total film stress was observed to decrease for decreasing deposition rate and increasing deposition temperature. A large change in film stress was observed for films containing excess Si; the stress decreased with increasing Si content. Based on published values for the thermal expansion coefficients for Si and Si₃N₄, a published value for Young’s Modulus for Si₃N₄, and the measured total stress values, a consistent argument is developed in which the total stress consists of a compressive component due to thermal expansion coefficient mismatch and a larger tensile intrinsic stress component. Both the thermal and intrinsic stress components vary with film deposition temperature in directions which decrease the total room temperature stress for higher deposition temperatures.     

Microstructure and stress in high-k Hf-Y-O thin films

We investigated the microstructure and the stress of high-k Hf-Y-O thin films deposited by atomic layer deposition (ALD). These hafnium oxide based films with a thickness of 5-60 nm stabilized in crystal structure with yttrium oxide by alternating the Hf- or Y-containing metal precursor during deposition. The microstructure was investigated by XRD and TEM in dependence of substrate and deposition temperature. The film stress was monitored during thermal cycles up to 500 °C using the substrate curvature method on (1 0 0)-Si wafer material with or without 10 nm TiN bottom electrode as well as on fused silica. It was observed that crystallinity and phases are depending on deposition temperature and film thickness. During thermal treatment the films crystallize depending on deposition temperature, yttrium content and substrate material at different temperatures. Crystallization of the films depends strongly on yttrium content. The highest reduction of 720 MPa was observed for films deposited with a Hf:Y cycle ratio of 10:1 where 6.2% of all metal atoms are replaced by yttrium. These Hf-Y-O films also show the highest k-value of 29 and have the smallest thermal expansion coefficient mismatch to TiN electrodes. Therefore we conclude that Hf-Y-O films are candidates for application in next generations of microelectronic MIM-capacitor devices or metal gate transistor technology.     

Driving force for the formation of Sn whiskers: compressive stress-pathways for its generation and remedies for its elimination and minimization

Compressive stress is widely accepted as the driving force for tin whisker formation. There are several pathways for compressive stress buildup in Sn coatings, which include the following: residual stress generated during plating; stress formation due to interfacial reactions between tin and copper substrate; mechanical stress; and thermal mechanical stress due to coefficient of thermal expansion (CTE) mismatch between the tin layer and substrate during thermal cycling. In order to prevent or reduce whisker growth in tin deposits, compressive stress has to be eliminated or minimized. This paper discusses the pathways for compressive stress formation and various remedies for its elimination and minimization. Particularly, a novel approach for dealing with thermal mechanical stress due to CTE mismatch is discussed.     

Thermomechanical property of diffusion barrier layer and its effect on the stress characteristics of copper submicron interconnect structures

Thermal stresses of thin titanium nitride (TiN) films on Si and Ge substrates have been measured by the bending beam method. The biaxial modulus and coefficient of thermal expansion of TiN thin film were deduced from the thermal stress data and found to be 355 GPa and 7.4 ppm/°C, respectively. Finite element analysis was used to study the effect of TiN diffusion barrier on the stress state of Cu lines in submicron damascene interconnects. The diffusion barrier was found to have a significant effect on the stress state of the Cu lines, especially for those embedded in interlevel dielectrics of low dielectric constant (k) materials. For the Cu/oxide interconnect structure, the metal lines with diffusion barrier were found to have a high near-hydrostatic triaxial stress state as expected. For the Cu/low k interconnect structure, a near-hydrostatic stress state was found to exist in the presence of the diffusion barrier; without the diffusion barrier, the stress state was not near-hydrostatic, instead it was dominated by a shear behavior. The implication of the diffusion barrier effect on the thermomechanical reliability of Cu interconnects is discussed.     

脉冲激光沉积法制备的PZT铁电薄膜的残余应力

根据压电本构方程和细观力学统计平均法，采用X射线衍射（XRD）测量Pb（Zr0.52Ti0.48）O5（PZT）铁电薄膜的残余应力。考虑激光沉积生长过程中，薄膜相变应力、热应力和本征应力对自由能的贡献，分析薄膜晶胞在晶体坐标系上的应力应变状态。由坐标转换将晶胞残余应力从晶体坐标系转换到样品坐标系得到任意取向晶粒的残余应力，通过取向平均得到薄膜样品坐标系上的残余应力。用脉冲激光沉积法（PLD）制备了不同厚度的PZT薄膜。利用X射线衍射分别采用细观力学统计平均法和传统sin^2φ法测量了PZT薄膜的残余应力。结果表明，两种结果在数值上是比较接近的（绝对差范围0．3～16．6MPa），残余压应力随着膜厚的增加从96MPa左右减少到45MPa左右。最后讨论了细观力学统计平均法的优缺点。     

Residual stress effect on electron mobility in ZMR-SOI

N-channel enhancement-mode MOSFETs have been fabricated in silicon-on-insulator (SOI) films prepared by both infra-red and laser zone-melting recrystallisation (ZMR). The SOI films are subjected to a lateral tensile stress due to the thermal expansion coefficient difference between silicon and silicon dioxide. The devices in the stressed films exhibit higher surface electron mobilities than those in bulk single crystal silicon. This phenomenon has been attributed to the influence of the stress through the change of the band structure as well as redistribution of carriers in k space.<>     

Effect of thermal annealing on physical properties of vacuum evaporated In2S3 buffer layer for eco-friendly photovoltaic applications

The present communication reports the effect of thermal annealing on the physical properties of In₂S₃ thin films for eco-friendly buffer layer photovoltaic applications. The thin films of thickness 150 nm were deposited on glass and indium tin oxide (ITO) coated glass substrates employing thermal vacuum evaporation technique followed by post-deposition thermal annealing in air atmosphere within a low temperature range 150–450 °C. These as-deposited and annealed films were subjected to the X-ray diffraction (XRD), UV–vis spectrophotometer, current–voltage tests and scanning electron microscopy (SEM) for structural, optical, electrical and surface morphological analysis respectively. The compositional analysis of as-deposited film is also carried out using energy dispersive spectroscopy (EDS). The XRD patterns reveal that the as-deposited and annealed films (≤300 °C) have amorphous nature while films annealed at 450 °C show tetragonal phase of β-In₂S₃ with preferred orientation (109) and polycrystalline in nature. The crystallographic parameters like lattice constant, inter-planner spacing, grain size, internal strain, dislocation density and number of crystallites per unit area are calculated for thermally annealed (450 °C) thin films. The optical band gap was found in the range 2.84–3.04 eV and observed to increase with annealing temperature. The current–voltage characteristics show that the as-deposited and annealed films exhibit linear ohmic behavior. The SEM studies show that the as-deposited and annealed films are uniform, homogeneous and free from crystal defects and voids. The grains in the thin films are similar in size and densely packed and observed to increase with thermal annealing. The experimental results reveal that the thermal annealing play significant role in the structural, optical, electrical and morphological properties of deposited In₂S₃ thin films and may be used as cadmium-free eco-friendly buffer layer for thin films solar cells applications.     

K1-5型外壳共晶贴片热应力研究北大核心

针对采用10号钢为基材的K1-5型外壳的芯片裂纹问题，对其共晶应力进行了仿真，并尝试对工艺过程进行仿真优化。结果表明，无论采用何种缓慢或快速的散热方式，都不能从根本上改变10号钢与Si芯片因热膨胀系数的巨大差异而导致的热应力。通过比较三种不同的管壳材料可知，以可伐材料为基体的K1-5管壳的共晶热应力最低，为316 MPa,而以10号钢为基体的热应力最高，为19 800 MPa,远远超出了硅芯片的极限断裂强度544 MPa。根据应力的基本理论，可伐与Si芯片的热膨胀系数的差异最小，无氧铜次之，而10号钢为最大，这也是以10号钢为基体的K1-5管壳在共晶时芯片开裂的根本原因。将管壳基材更换为可伐材料，仿真分析和实际试验结果均证明该管壳能够有效解决芯片开裂的问题。     

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.     

Electrical and Structural Real-Time Changes in Thin Thermoelectric (Bi<Subscript>0.15</Subscript>Sb<Subscript>0.85</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films by Dynamic Thermal Treatment

A recent trend in thermoelectrics is miniaturization of generators or Peltier coolers using the broad spectrum of thin-film and nanotechnologies. Power supplies for energy self-sufficient micro and sensor systems are a wide application field for such generators. It is well known that thermal treatment of as-deposited p-type (Bi_0.15Sb_0.85)₂Te₃ films leads to enhancement of their power factors. Whereas up to now only the start (as-deposited) and the end (after annealing) film stages were investigated, herein for the first time, the dynamical changes of sputter-deposited film properties have been observed by real-time measurements. The electrical conductivity shows a distinct, irreversible increase during a thermal cycle of heating to about 320°C followed by cooling to room temperature. The interpretation of the Seebeck and Hall coefficients points to an enhancement in Hall mobility after annealing. In situ x-ray diffractometry shows the generation of an additional Te phase depending on temperature. This is also confirmed by energy-dispersive x-ray microanalysis and the corresponding mapping by scanning electron microscopy. It is presumed that the Te enrichment in a separate, locally well-defined phase is the reason for the improvement in the integral film transport properties.     

Phase transformation and paired-plate precipitate formation in Pb0.91La0.09Zr0.65Ti0.35O3 films grown on sapphire substrates

We report on the phase transformation behavior of Pb_0.91La_0.09Zr_0.65Ti_0.35O₃ (9/65/35) PLZT films grown on r-sapphire substrates via rf-magnetron sputtering. A complex microstructure results in these films depending on deposition and annealing conditions. A random equiaxed polycrystalline grain morphology was observed after rapid thermal annealing or furnace annealing when the as-deposited films were predominantly pyrochlore. Precipitate formation (100–150 nm) was observed in PLZT films that were deposited at temperatures in excess of 490°C with a perovskite structure, after furnace annealing at 700°C. We believe that this is related to internal stresses in the films due to both the lattice mismatch and the thermal expansion mismatch between the PLZT film and the sapphire substrate.     

Structural and interfacial properties of high-k HfOxNy gate dielectric films

The thermal stability and interfacial characteristics for hafnium oxynitride (HfO_xN_y) gate dielectrics formed on Si (1 0 0) by plasma oxidation of sputtered HfN films have been investigated. X-ray diffraction results show that the crystallization temperature of nitrogen-incorporated HfO₂ films increases compared to HfO₂ films. Analyses by X-ray photoelectron spectroscopy confirm the nitrogen incorporation in the as-deposited sample and nitrogen substitution by oxygen in the annealed species. Results of FTIR characterization indicate that the growth of the interfacial SiO₂ layer is suppressed in HfO_xN_y films compared to HfO₂ films annealed in N₂ ambient. The growth mechanism of the interfacial layer is discussed in detail.     

蓝宝石图形衬底上生长GaN的微区拉曼光谱研究(英文)

采用微区拉曼光谱对生长在湿法腐蚀获得的无掩膜周期性图形蓝宝石衬底上GaN材料做了研究,结果显示,侧向外延生长区域具有较低的压应力。采用湿法腐蚀结合原子力显微镜对材料的位错进行了表征,侧向外延区域显示了低的位错密度,具有较高的晶体质量。另外通过对不同生长区域的拉曼纵光学声子与等离子体激元形成的耦合模高频支进行拟合,结果显示侧向外延区域具有较低的背底载流子浓度。研究认为,由于采用图形衬底,侧向外延区域悬空生长降低了位错密度,同时侧向外延区域不与蓝宝石接触,因此采用该方法生长的GaN材料具有较低的压应力和较低的背底载流子浓度。     

The influence of dopant species on thermal stability of NiSi film

We investigated the relationship between thermal stability of NiSi films and the implanted dopant species on Si substrates. The most stable NiSi layer appeared on Boron-implanted Si substrate, where the formation of pseudo-epitaxial transrotational structure was observed, just in case that the dose of boron is more than 5e15 atoms/cm². This unique crystallographic orientation of NiSi film on Boron-implanted substrate is a key role of thermal stability because thermal stress at grain boundary can be diminished by peculiar arrangement of transrotational domains, owing to the anisotropy in coefficient of thermal expansion (CTE) of NiSi.     

高膨胀系数玻璃-陶瓷复合材料的性能研究

选用具有良好介电性能和膨胀性能的硼硅酸盐(SiO2-BaO-B2O3-Al2O3)和石英,采用固相法合成了一系列具有高热膨胀系数的玻璃-陶瓷复合材料,并对这些复合材料进行了XRD、SEM分析,及其热、力、电性能的测试。结果表明:所制复合材料的热膨胀系数和弯曲强度随着石英含量的增加而增大,其相对介电常数则随之减小。石英质量分数为40%的复合材料在980℃烧结时,析出了大量的方石英相,复合材料的热膨胀系数增大。最终制备的复合材料具有高的热膨胀系数[(10.3～25.5)×10-6/℃]、较高的弯曲强度(146MPa)、较低的相对介电常数(5.6～6.4)及介电损耗(0.10%～0.30%)。     

Stress Controllability in Thermal and Electrical Conductivity of 3D Elastic Graphene‐Crosslinked Carbon Nanotube Sponge/Polyimide Nanocomposite

Stress controllability in thermal and electrical conductivity is important for flexible piezoresistive devices. Due to the strength‐elasticity trade‐off, comprehensive investigation of stress‐controllable conduction in elastic high‐modulus polymers is challenging. Here presented is a 3D elastic graphene‐crosslinked carbon nanotube sponge/polyimide (G_w‐CNT/PI) nanocomposite. Graphene welding at the junction enables both phonon and electron transfer as well as avoids interfacial slippage during cyclic compression. The uniform G_w‐CNT/PI comprising a high‐modulus PI deposited on a porous templated network combines stress‐controllable thermal/electrical conductivity and cyclic elastic deformation. The uniform composites show different variation trends controlled by the porosity due to different phonon and electron conduction mechanisms. A relatively high k (3.24 W m^?1 K^?1, 1620% higher than PI) and suitable compressibility (16.5% under 1 MPa compression) enables the application of the composite in flexible elastic thermal interface conductors, which is further analyzed by finite element simulations. The interconnected network favors a high stress‐sensitive electrical conductivity (sensitivity, 973% at 9.6% strain). Thus, the G_w‐CNT/PI composite can be an important candidate material for piezoresistive sensors upon porosity optimization based on stress‐controllable thermal or electrical conductivity. The results provide insights toward controlling the stress‐induced thermal/electrical conductivities of 3D interconnected templated composite networks for piezoresistive conductors or sensors.