Properties Of Anisotropic Films Research Articles

3ω techniques for measurement of volumetric heat capacity and anisotropic thermal conductivity of a solution processable, hybrid organic/inorganic film, Te-PEDOT:PSS

Measuring the thermal properties of anisotropic films of hybrid materials poses a challenge to existing metrology techniques. We have developed a new approach for measuring the volumetric heat capacity and anisotropic thermal conductivity of these systems using the 3ω method. While there exist many avenues for measuring the thermal properties of thin films, most carry with them difficult requirements such as smooth surfaces or advanced lithography. Here, we present measurements of a film's in-plane and cross-plane conductance and its volumetric heat capacity using relatively simple sample configurations, each requiring a single heater. For the measurement of volumetric heat capacity, we present a new model fitting method, relying on a standard film-on-substrate configuration. For the measurement of in-plane thermal conductance by 3ω, we have developed the use of an embedded micro-wire heater in suspended drop cast films, allowing for a 12 μm wide heater without the need for advanced lithography. We also expose the surprisingly significant effect of thermal radiation in the suspended film measurement and its associated error. Our measurements reveal a large anisotropy in the thermal conductivity of our test material, Te-PEDOT:PSS, of kin-plane/kcross-plane = 19, consistent with the nanoscale morphology of the material.

In- and through-plane conductivity of 8YSZ films produced at room temperature by aerosol deposition

Aerosol deposition is a unique spray coating process to deposit dense ceramic films directly at room temperature just by spraying a dry powder, without any binder or additional sintering step needed. Therefore, it is a very promising coating method for functional materials that may be used to produce a variety of electrochemical devices, solid oxide fuel cells or gas sensors. However, functional properties like the electrical conductivity significantly depend on the microstructure of the produced film. Consequently, anisotropic film properties may occur even for isotropic functional materials. In this work, films of oxide ion-conducting yttria-stabilized zirconia (YSZ) were produced by aerosol deposition. The orientation-dependent electrical conductivity was measured using two different electrode configurations: one for the in-plane conductivity and another one for through-plane measurements. A slight anisotropy was observed, attributed to the present film morphology. In particular, the grain boundaries influenced the total conductivity. Furthermore, a thermal post-deposition treatment significantly affected the total conductivity of aerosol-deposited YSZ films.

Humidity resistant MoS2 coatings deposited by unbalanced magnetron sputtering

MoS2 is a suitable solid lubricant for environments free of oxygen or water vapor (i.e. vacuum). Humid air degrades film properties due to oxidation accompanied by high wear and increasing coefficients of friction. The present study aims at the further development of sputtered pure MoS2 coatings, extending their applicability to varying environmental conditions by increasing the resistance against humidity. The systematic coating development process is supported by using an experimental Box–Behnken design with variations of the deposition parameters cathode voltage, target/substrate distance, temperature and argon gas pressure. In contrast to common one-factor-at-a-time (OFAT) studies, this approach enables a determination of interactions between deposition process parameters and tribological–mechanical MoS2 film properties. The tribological improvement focuses on a maximization of wear resistance in air and vacuum measured in ball-on-disk experiments. The evaluated mechanical properties are hardness, elastic modulus and residual stresses. These stresses were determined by the substrate curvature method. The study reveals that the residual stress state in the films and the hardness-to-modulus ratio are crucial for their tribological performance in humid air and vacuum environments. After a detailed determination of the relationships between deposition conditions and film properties, some selected microstructural analyses are presented which show that a substantially basal orientation of the lattice has positive effects on wear but also causes anisotropic film properties which result in fissile fracture of the coating if strong shock or point loads occur.

Development of m-plane GaN anisotropic film properties during MOVPE growth on LiAlO 2 substrates

The anisotropic film properties of m-plane GaN deposited by metal organic vapour phase epitaxy (MOVPE) on LiAlO 2 substrates are investigated. To study the development of layer properties during epitaxy, the total film thickness is varied between 0.2 and 1.7 μm. A surface roughening is observed caused by the increased size of hillock-like features. Additionally, small steps which are perfectly aligned in (1 1 −2 0) planes appear for samples with a thickness of ∼0.5 μm and above. Simultaneously, the X-ray rocking curve (XRC) full width at half maximum (FWHM) values become strongly dependent on incident X-ray beam direction beyond this critical thickness. Anisotropic in-plane compressive strain is initially present and gradually relaxes mainly in the [1 1 −2 0] direction when growing thicker films. Low-temperature photoluminescence (PL) spectra are dominated by the GaN near-band-edge peak and show only weak signal related to basal plane stacking faults (BSF). The measured background electron concentration is reduced from ∼10 20 to ∼10 19 cm −3 for film thicknesses of 0.2 μm and ∼1 μm while the electron mobilities rise from ∼20 to ∼130 cm 2/V s. The mobilities are significantly higher in [0 0 0 1] direction which we explain by the presence of extended planar defects in the prismatic plane. Such defects are assumed to be also the cause for the observed surface steps and anisotropic XRC broadening.

Diffraction of picosecond bulk longitudinal and shear waves in micron thick films

Investigation of thin metallic film properties by means of picosecond ultrasonics [C. Thomsen et al., Phys. Rev. Lett. 53, 989 (1984)] has been under the scope of several studies. Generation of longitudinal and shear waves [T. Pezeril et al., Phys. Rev. B 73, 132301 (2006); O. Matsuda et al., Phys. Rev. Lett. 93, 095501 (2004)] with a wave vector normal to the film free surface has been demonstrated. Such measurements cannot provide complete information about properties of anisotropic films. Extreme focusing of a laser pump beam (≈0.5 μm) on the sample surface has recently allowed us to provide evidence of picosecond acoustic diffraction in thin metallic films (≈1 μm) [C. Rossignol et al., Phys. Rev. Lett. 94, 166106 (2005)]. The resulting longitudinal and shear wavefronts propagate at group velocity through the bulk of the film. To interpret the received signals, source directivity diagrams are calculated taking into account material anisotropy, optical penetration, and laser beam width on the sample surface. It is shown that acoustic diffraction increases with optical penetration, so competing with the increasing of directivity caused by beam width. Reflection with mode conversion at the film-substrate interface is discussed.

Picosecond acoustic diffraction in anisotropic thin film (µm); application to the measurement of stiffness coefficients

Investigation of thin metallic film properties by means of picosecond ultrasonics has been under the scope of several studies. Generation of longitudinal and shear waves with a wave vector normal to the film free surface has been demonstrated. Such measurements can not provide complete information about properties of anisotropic films. Acute focusing of the laser pump beam (≈ 0.5 µm) on the sample surface has recently allowed us to provide evidence of picosecond acoustic diffraction in thin metallic films (≈1 µm) such as aluminum, gold, copper. Waveforms have been experimentally recorded in a gold layer (2 µm thick) for several distances between pump and probe on the sample surface. Due to acoustic diffraction, the acoustic wavefronts propagate at a group velocity which differs from phase velocity in the anisotropic film. However, a specified signal processing allows us analyzing the space repartition of the acoustic wave-vectors for both longitudinal and shear waves. Four stiffness coefficients of the anisotropic gold layer could thus be recovered accurately, demonstrating the feasibility of the measurement.

Novel technique for measuring through-plane modulus in thin polymer films

Polymer thin films are widely used as coatings and interlevel dielectrics in microelectronic applications. In multilayer structures, stresses generated in the films due to interaction with adjacent layers and solvent evaporation induced shrinkage, causing the polymer chains to orient in the plane of the film resulting in anisotropic film properties. Characterization of properties in all directions is essential for accurate electrical and mechanical design and modeling. A new technique has been developed to measure, in-situ, the through-plane (z) stress-strain behavior of thin polymer films. A parallel plate capacitor device and an interdigitated electrode structure were used as sensors to detect changes in dielectric constant and thickness of thin polymer films under compression, Results are reported for 8-14 micrometer thick, Dow Chemical Cyclotene 3022 benzocyclobutene (BCB) films, The dielectric constant was found to change linearly with stress. Using this result, the through-plane stress-strain curve was obtained.

Novel Technique for Measuring Through-Plane Thermo-Mechanical Properties of Thin Polymer Films

ABSTRACTPolymer thin films (less than 20 ltm) are widely used as coatings and inter-level dielectrics in microelectronic applications. In multi-layer structures, stresses generated in the films due to interaction with adjacent layers and solvent evaporation induced shrinkage, cause the polymer chains to orient in the plane of the film resulting in anisotropic film properties. Characterization of properties in all directions is essential for accurate electrical and mechanical design and modeling.A new technique has been developed to measure, in-situ, the through-plane (z) properties of thin polymer films. A parallel plate capacitor device and an interdigitated electrode structure were used as sensors to detect changes in dielectric constant and thickness of thin polymer films under thermal or mechanical loading. The through-plane compression modulus and coefficient of thermal expansion were calculated and results are reported for benzocyclobutene (BCB) films.

Photo-Generation of Linearly Polymerized Liquid Crystal Aligning Layers Comprising Novel, Integrated Optically Patterned Retarders and Color Filters

For the first time photo cross-linking of linearly polymerizable polymers (LPPs) is shown to induce uniaxial planar alignment in adjacent liquid crystal polymer (LCP)-layers on single substrates. Ways and novel materials allowing integration of LPP-aligning layers with optical retarders in patterned, hybrid LPP-LCP-configurations with freely adjustable optical axes are presented. The novel multifunctional, anisotropic photopolymer configurations are shown to render in-situ optical retarders and polarization interference filters for black-white and color liquid crystal displays (LCDs) feasible. The molecular mechanisms inducing the anisotropic film properties and their thermal and optical stabilities are outlined. The photo-patternable, high resolution hybrid configurations are shown to exhibit excellent thermal and light stability.