Thin Film Condensation on Nanostructured Surfaces

Water vapor condensation is a ubiquitous process in nature and industry. Over the past century, methods achieving dropwise condensation using a thin (<1 µm) hydrophobic “promoter” layer have been developed, which increases the condensation heat transfer by ten times compared to filmwise condensation. Unfortunately, implementations of dropwise condensation have been limited due to poor durability of the promoter coatings. Here, thin‐film condensation which utilizes a promoter layer not as a condensation surface, but rather to confine the condensate within a porous biphilic nanostructure, nickel inverse opals (NIO) with a thin (<20 nm) hydrophobic top layer of decomposed polyimide is developed. Filmwise condensation confined to thicknesses <10 µm is demonstrated. To test the stability of thin‐film condensation, condensation experiments are performed to show that at higher supersaturations droplets coalescing on top of the hydrophobic layer are absorbed into the superhydrophilic layer through coalescence‐induced transitions. Through detailed thermal‐hydrodynamic modeling, it is shown that thin‐film condensation has the potential to achieve heat transfer coefficients approaching ≈100 kW m^?2 while avoiding durability issues by significantly reducing nucleation on the hydrophobic surface. The work presented here develops an approach to potentially ensure durable and high‐performance condensation comparable to dropwise condensation.     

Fluctuation phenomena in nanoscale nickel films near the melting point

Voltage fluctuations in thin nanoscale nickel films which arise during current application through a film sample upon slow heating are studied. It is shown that positive voltage fluctuations (surges) arise due to an increase in the resistance of local regions of the film under study, caused by its local thinnings and discontinuities which result from the film melting onset. The temperature of the melting onset of nanoscale nickel films on oxidized silicon was experimentally determined as 740, 815, and 875 K for films 5, 20, and 40 nm thick, respectively.     

Stability of conductor metallizations in corrosive environments

Multi-layer thin-film conductor metallizations are subject to degradation by a variety of mechanisms. Besides chemical interdiffusion and electro-migration, atmospheric corrosion of thin films in service is a potential threat if components are utilized in uncontrolled environments without hermetic packaging. Galvanic corrosion is a particular possibility when gold conductor films overlie “active” metal adhesion layers. Loss of adhesion and conductivity degradation are produced by low temperature aging in an atmosphere containing water vapor and as little as 1 ppm chlorine. All “active” metals investigated, including Ti, Zr, Mo, Nb, Ta, W, Cr and NiCr, are subject in varying degrees to corrosive attack in which atmospheric species obtain access to the active metal via pinholes in the overlying gold, and at the edges of conductor patterns. In the chlorine-water vapor atmosphere the rate of attack is a maximum at 200–250°C and is lower at both higher and lower temperatures. Sus-ceptibility to attack of large continuous film areas is shown to be reduced greatly by: (1) the introduction of an intermediate layer of Pt, Pd, or Rh between the “active” metal and the gold; (2) use of very thick (hence impermeable) gold; or (3) pinhole occlusion treatments such as vibratory ball burnishing.     

Large‐Area,Periodic, Hexagonal Wrinkles on Nanocrystalline Graphitic Film

Sinusoidal wrinkles develop in compressively stressed film as a means to release stored elastic energy. Here, a simple way to fabricate large‐area, periodic, hexagonal wrinkled pattern on nanocrystalline graphitic films grown on c‐plane sapphire (<50 nm thick) by the spontaneous delamination–buckling of the as‐grown film during cooling is reported. According to the continuum mechanics calculation, strain‐relief pattern adopting the hexagonal wrinkled pattern has a lower elastic energy than that of the telephone cord wrinkle at thickness regime below 50 nm. A high‐fidelity transfer method is developed to transfer the hexagonal wrinkled films onto arbitrary substrates. Nanoindentation studies show that hexagonal wrinkle film engineered this way may act as shock absorber. The hexagonal wrinkled carbon film is able to selectively promote the differentiation of human mesenchymal stem cell toward the osteogenic lineage in the absence of osteogenic inducing medium.     

Interesting Evidence for Template‐Induced Ferroelectric Behavior in Ultra‐Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates

The first evidence for room‐temperature ferroelectric behavior in anatase‐phase titanium dioxide (a‐TiO₂) is reported. Behavior strongly indicative of ferroelectric characteristics is induced in ultra‐thin (20 nm to 80 nm) biaxially‐strained epitaxial films of a‐TiO₂ deposited by liquid injection chemical vapor deposition onto (110) neodymium gallium oxide (NGO) substrates. The films exhibit significant orthorhombic strain, as analyzed by X‐ray diffraction and high‐resolution transmission electron microscopy. The films on NGO show a switchable dielectric spontaneous polarization when probed by piezoresponse force microscopy (PFM), the ability to retain polarization information written into the film using the PFM tip for extended periods (several hours) and at elevated temperatures (up to 100 °C) without significant loss, and the disappearance of the polarization at a temperature between 180 and 200 °C, indicative of a Curie temperature within this range. This combination of effects constitutes strong experimental evidence for ferroelectric behavior, which has not hitherto been reported in a‐TiO₂ and opens up the possibility for a range of new applications. A model is presented for the effects of large in‐plane strains on the crystal structure of anatase which provides a possible explanation for the experimental observations.     

等离子增强化学气相沉积法低温生长SiO_x薄膜的针孔缺陷修复

针对等离子增强化学气相沉积(PECVD)方法低温(60℃)生长氧化硅(SiOx)薄膜中存在的针孔缺陷,在SiOx薄膜上采取原子层沉积(ALD)方法生长氧化铝(AlOy),利用ALD方法材料保形生长的特点,进行SiOx薄膜的针孔缺陷修复工艺技术研究。实验结果表明:在SiOx薄膜上利用ALD方法保形生长氧化铝,可以明显降低AlOy/SiOx复合薄膜的水汽渗透率,提高薄膜封装性能。通过实验数据分析认为:复合薄膜的水汽阻隔能力是由于ALD方法及PECVD方法两种薄膜生长方法的综合作用,这种综合作用很有可能来自PECVD方法薄膜中针孔缺陷的修复,而ALD方法正是完成修复过程的技术手段。另外,ALD方法的工艺参数与针孔缺陷的修复效果相关,ALD生长周期时间延长,有利于提高针孔缺陷的修复效果,从而降低了复合薄膜的水汽渗透率。     

Ultrasmooth Gold Films via Pulsed Laser Deposition

A simple, one step technique for depositing ultrasmooth gold films using pulsed laser deposition is demonstrated by optimizing process para­meters. The smoothest film having a root‐mean‐square roughness of 0.17 nm (including the substrate roughness of 0.11 nm) for a 35 nm thick film on a silicon substrate are obtained by introducing a nitrogen flow in the chamber during deposition. We postulate that the reduction in surface roughness caused by nitrogen gas pressure in the chamber is due to the force of the gas flow acting against the flow of the plasma plume containing Au atoms. The gas acts as a filter that reduces the kinetic energy of the gold adatoms. This is the best result reported so far for a single step deposition of gold. It is a step towards low‐loss planar gold films for surface plasmon applications.     

Roll‐to‐roll gravure printing of organic photovoltaic modules—insulation of processing defects by an interfacial layer

Gravure printing as direct patterning roll‐to‐roll (R2R) production technology can revolutionize the design of thin‐film organic photovoltaic (OPV) devices by allowing feasible manufacturing of arbitrary‐shaped modules. This makes a distinction to coating methods, such as slot die coating, in which the pattern is limited to continuous stripes. Here, we analyze the thin‐film formation and its influence on OPV module performance as the gravure printing of hole transport and photoactive layers are transferred from laboratory to R2R pilot production environment. Insertion of a 0.8‐nm layer of lithium fluoride (LiF) as an interfacial layer between the active layer and the electron contact provided insulation against the detrimental pinholes formed in the R2R printing process. Using this device configuration, we produced well‐performing R2R‐printed monolithic modules with a mean efficiency of 1.7%. In comparison, reference modules with an efficiency of 2.2% were fabricated using laboratory‐scale bench top sheet‐level process. Surface energy and tension measurements together with optical microscopy were used to analyze the printability of the materials. The pinhole insulation was investigated in detail by processing R2R‐printed OPV modules with different interfacial layer materials and performing electrical measurements under dark and AM1.5 illumination conditions. Furthermore, we analyzed the LiF distribution using X‐ray photoelectron spectroscopy. The insulating nature of the LiF layer to improve module performance was confirmed by manufacturing lithographically artificial pinholes in device structures. The results show the possibility to loosen the production environment constraints and the feasibility of fabricating well‐performing thin‐film devices by R2R gravure printing. Copyright © 2014 John Wiley & Sons, Ltd.     

Confocal Raman Spectroscopy of α‐Sexithiophene: From Bulk Crystals to Field‐Effect Transistors

We report confocal micro‐Raman spectra of the organic semiconductor α‐sexithiophene (T6) on bulk crystals and on thin films grown on technologically relevant substrates and devices. We show that the two polymorphs, which are clearly identified by their lattice phonon spectra, may coexist as physical impurities of one inside the other in the same crystallite. Spatial distribution of the two phases is monitored by Raman phonon mapping of crystals grown upon different conditions. Raman microscopy has then been extended to T6 thin films grown on silicon oxide wafers. We identify the crystal phase in thin films whose thickness is just 18 nm. The most intense total‐symmetric Raman vibration is still detectable for a two‐monolayer thick film. Comparative analysis between micro‐Raman and AFM of T6 thin films grown on field effect transistors shows that electrode‐channel steps favour the nucleation and growth of T6 molecules on the substrate, at least below 50 nm.     

Fabrication of Superstrong Ultrathin Free‐Standing Single‐Walled Carbon Nanotube Films via a Wet Process

A one‐pot and readily practical approach is described for the preparation of superstrong, ultrathin, free‐standing single‐walled carbon nanotube (SWNT) films. The SWNT films, with controlled thicknesses of tens to hundreds of nanometers, are prepared from commonly commercialized SWNTs via a wet process. The SWNTs could be easily transferred onto any substrates after self‐releasing from filter membranes without further treatment. The obtained films exhibit excellent performances with sheet resistance of 223 Ω sq^?1 and a transparency of 90% at 550 nm was obtained. Most important is that the as‐prepared free‐standing SWNT ultrathin films showed extremely high tensile strength up to 850 MPa for only about a 20‐nm thick film, which has great significance for practical applications, for example, as flexible electrode materials. The SWNT film is used to construct a capacitive touch‐screen prototype, which has a highly sensitive and quick signal touch response.     

Silicon and Nanoscale Metal Interface Characterization Using Stress-Engineered Superlayer Test Methods

Thin film layers are utilized in emerging microelectronics, optoelectronics, and microelectromechanical systems (MEMS) devices. Typically, these thin film layers are composed of different materials with dissimilar properties. A common mode of failure for thin films is delamination caused by external loading or intrinsic stress present in the materials. To characterize bonded thin film material systems, it is necessary to measure the interfacial fracture toughness. When material thicknesses approach micro- and nanoscales, interfacial fracture toughness measurement is a challenging task. Accordingly, innovative test techniques need to be developed to study interfacial fracture parameters. The ongoing research at Georgia Institute of Technology is developing fixtureless delamination test techniques that can be used to measure interfacial properties of micro- and nanoscale thin films. The single substrate decohesion test (SSuDT) and the single-strip decohesion Test (SSDT) are such fixtureless tests under development. In these tests, a thin film interface material of interest is deposited on a substrate. Then, delamination is driven by a superlayer material on top of the interface material. This superlayer material is sputter deposited and has high intrinsic stress. A deposited release layer material allows for the contact area between the interface material and the substrate to be controlled. These tests differ in geometry, but share the same generic methodology and can be used for a number of material systems over a wide range of mode mixities. This paper presents the methodology and implementation of the SSuDT and SSDT tests and compares results to better understand their scope. A case study of the interfacial fracture toughness as a function of mode mixity for titanium and silicon interface was performed.      

Reversible Electrochemically Triggered Delamination Blistering of Hydrogel Films on Micropatterned Electrodes

Stimuli responsive elastic instabilities provide opportunities for controlling the structures and properties of polymer surfaces, offering a range of potential applications. Here, a surface actuator based on a temperature and electrically responsive poly(N‐isopropyl acrylamide‐co‐sodium acrylate) hydrogel that undergoes a two‐step delamination and buckling instability triggered using micropatterned electrodes is described. The electrically actuated structures entail large out‐of‐plane displacements that take place on time‐scales of less than 1 s, in response to modest triggering voltages (?3–6 V). Alongside these experimental observations, finite element simulations are conducted to better understand the two‐step nature of the instability. In the first step, hydrogel films undergo delamination and formation of blisters, facilitated by electrochemical reduction of the thiol groups anchoring the film to the electrodes. Subsequently, at larger reducing potentials, the electrolytic current is sufficient to nucleate a gas bubble between the electrode and the gel, causing the delaminated region to adopt a straight‐sided blister shape. Finally, thermally induced deswelling of the gel allows the film to be returned to its flat state and readhered to the electrode, thereby allowing for repeated actuation.     

磁场退火对FePt/Ag薄膜磁性能的影响

采用磁控溅射法室温沉积获得FePt/Ag薄膜,然后在500℃下,于真空磁退火炉中对薄膜进行退火处理。利用XRD和振动样品磁强计(VSM),研究了磁场退火对薄膜结构和磁性能的影响。结果表明,500℃零磁场退火获得了矫顽力为0.763 4 MA.m–1、平均晶粒尺寸21 nm的L10-FePt薄膜。磁场提供了FePt成核生长的驱动力,0.8 MA.m–1磁场退火后FePt的平均晶粒尺寸为26 nm,矫顽力增大至0.804 3 MA.m–1。非磁性Ag的掺杂可有效抑制磁性FePt晶粒的团聚生长。     

Study of metal adhesion on porous low-k dielectric using telephone cord buckling

The interfacial adhesion energy between metal and porous low-k dielectrics is an important parameter for the reliability study of back-end of line integration. In this work, we have observed the spontaneous film delamination with telephone cord morphology after 130 nm thick Ta was sputtered onto methyl silsesquioxane (MSQ) low-k dielectric. The highly compressive stress inside the Ta film is the driving force for the spontaneous buckling. The adhesion failure was identified to be at Ta/MSQ interface by using focused ion beam and scanning electron microscopy. Pinned circular blister model was applied to fit the buckling morphology. The interfacial adhesion energy was extracted to be 7.90 J/m² at 87° phase angle. The Cu/MSQ interface was evaluated in a similar fashion by using a stressed overlayer Ta/Cu. The fracture energy was calculated to be 3.34 J/m² with the similar phase angle. The results suggest that an adhesion promoter between Cu and low-k dielectrics is essential for a mechanically stable structure.     

Low Temperature Stabilization of Nanoscale Epitaxial Spinel Ferrite Thin Films by Atomic Layer Deposition

In this work heteroepitaxial stabilization with nanoscale control of the magnetic Co₂FeO₄ phase at 250 °C is reported. Ultrasmooth and pure Co₂FeO₄ thin films (5–25 nm) with no phase segregation are obtained on perovskite SrTiO₃ single crystal (100) and (110) oriented substrates by atomic layer deposition (ALD). High resolution structural and chemical analyses confirm the formation of the Co‐rich spinel metastable phase. The magneto‐crystalline anisotropy of the Co₂FeO₄ phase is not modified by stress anisotropy because the films are fully relaxed. Additionally, high coervice fields, 15 kOe, and high saturation of magnetization, 3.3 μ_B per formula unit (at 10 K), are preserved down to 10 nm. Therefore, the properties of the ALD‐Co₂FeO₄ films offer many possibilities for future applications in sensors, actuators, microelectronics, and spintronics. In addition, these results are promising for the use of ALD compared to the existing thin‐film deposition techniques to stabilize epitaxial multicomponent materials with nanoscale control on a wide variety of substrates for which the processing temperature is a major drawback.     

Suppression of pinhole defects in fullerene molecular electron beam resists

Molecular resists, such as fullerenes, are of significant interest for next generation lithographies. They utilize small carbon rich molecules, giving the potential for higher resolution and etch durability, together with lower line width roughness than conventional polymeric resists. The main problem with such materials has historically been low sensitivity, but with the successful implementation of chemical amplification schemes for several of the molecular resist families this is becoming less of a concern. Aside from sensitivity the other main obstacle has been the difficulty of preparing good quality thin films of non-polymeric materials. Here we present a study of pinhole defect density in fullerene films as a function of substrate cleanliness, post-application bake, and incorporation of chemical amplification components. Ultrathin (sub 30 nm) films of the previously studied fullerene resist MF03-01, and the polymeric resist PMMA were prepared on hydrogen terminated silicon by spin coating and the density of pinhole defects in the films was studied using atomic force microscopy. It was seen that pinhole density was strongly affected by the quality of the substrates, with the lowest densities found on films spun on freshly cleaned substrates. Aging of the film subsequent to spin coating was seen to have less effect than similar aging of the substrate prior to spin coating. Additionally, the use of a post-application bake significantly degraded the quality of the films. The addition of an epoxy crosslinker for chemical amplification was found to reduce defect density to very low levels.     

Hexagonal Boron Nitride–Enhanced Optically Transparent Polymer Dielectric Inks for Printable Electronics

Solution‐processable thin‐film dielectrics represent an important material family for large‐area, fully‐printed electronics. Yet, in recent years, it has seen only limited development, and has mostly remained confined to pure polymers. Although it is possible to achieve excellent printability, these polymers have low (≈2–5) dielectric constants (ε_r). There have been recent attempts to use solution‐processed 2D hexagonal boron nitride (h‐BN) as an alternative. However, the deposited h‐BN flakes create porous thin‐films, compromising their mechanical integrity, substrate adhesion, and susceptibility to moisture. These challenges are addressed by developing a “one‐pot” formulation of polyurethane (PU)‐based inks with h‐BN nano‐fillers. The approach enables coating of pinhole‐free, flexible PU+h‐BN dielectric thin‐films. The h‐BN dispersion concentration is optimized with respect to exfoliation yield, optical transparency, and thin‐film uniformity. A maximum ε_r ≈ 7.57 is achieved, a two‐fold increase over pure PU, with only 0.7 vol% h‐BN in the dielectric thin‐film. A high optical transparency of ≈78.0% (≈0.65% variation) is measured across a 25 cm² area for a 10 μm thick dielectric. The dielectric property of the composite is also consistent, with a measured areal capacitance variation of <8% across 64 printed capacitors. The formulation represents an optically transparent, flexible thin‐film, with enhanced dielectric constant for printed electronics.     

Effect of firing atmosphere on air-fireable glass-free electrically conductive thick film

The firing atmosphere (air, oxygen, and argon) was found to affect the electrical and mechanical properties of an air-fireable electrically conductive glass-free silver-based thick film. For the optimum firing temperature of 930°C, air results in the lowest resistivity, but a minor amount of pinholes; oxygen results in the largest thickness, the smoothest surface, and no pinhole; and argon results in the highest resistivity, large pinholes, the smallest thickness, vanishing of macroscopic parts of the film, and the poorest scratch resistance. Argon gives higher resistivity than air or oxygen at essentially all firing temperatures.     

Cu/Ta/SiO2/Si薄膜在纳米压痕下的分层现象研究

采用磁控溅射技术在热氧化单晶硅衬底上先后淀积了厚度分别为50nm的Ta膜和400 nm的Cu膜.使用纳米压入仪在样品表面进行压入测试,在薄膜表面制造出残留压痕.使用扫描电镜(SEM)、聚焦离子束(FIB)、透射电镜(TEM)和X射线能谱仪(EDX)对残留压痕形貌、剖面上的分层现象进行观察,确定分层所在的位置.发现在69 mN的最大载荷作用后,在TA/SiO2界面处发生分层.分层的原因主要归结为在应力作用下,多层膜中各种材料的应变、弹性恢复能力不同.     

钯膜上CVD法制备碳纳米管薄膜的研究

采用化学气相沉积法,以乙炔为碳源,在各种钯膜上制备了碳纳米管薄膜。通过电子显微镜观察了碳管薄膜和钯膜的表面形貌。结果表明,在真空气氛下磁控溅射的钯膜上无法生长碳纳米管。对溅射的钯膜进行大气气氛下的退火处理,则可生长出稀疏的碳纳米管团聚颗粒。采用在氧气气氛下磁控溅射的钯膜作为催化剂,则可显著提高碳管的生长密度和纯度,从而获得致密均匀的碳纳米管薄膜。