Substrate anisotropic

Mechanism (8-94) dominates when the TEOS-O2 mixture is strongly diluted. While being adsorbed on the surface, the precursors are further oxidized with releasing CO2 and H2O into the gas phase and formation of the Si02 film. Ion bombardment of the substrate anisotropically stimulates the vertical deposition rate of Si02, providing directionality to the PECVD process. Details regarding the surface kinetics of the PECVD process can be found in Stout and Kushner (1993) and Calc et al. (1992). 

Although dating as far back as the 1960s, anisotropic adhesives have only recently been rediscovered and advanced to a state where they are being used in place of solder to interconnect fine-pitch, high I/O-count flip-chip devices, TAB devices, and high-density interconnect substrates. Anisotropic adhesives are also used to interconnect edge connectors and flex cable to PWBs and flat-panel displays (FPDs). ACAs are commercially available in either paste or film form and as either thermoplastic or thermoset types. 

The chapter begins with an overview of elastic anisotropy in crystalline materials. Anisotropy of elastic properties in materials with cubic symmetry, as well as other classes of material symmetry, are described first. Also included here are tabulated values of typical elastic properties for a variety of useful crystals. Examples of stress measurements in anisotropic thin films of different crystallographic orientation and texture by recourse to x-ray diffraction measurements are then considered. Next, the evolution of internal stress as a consequence of epitaxial mismatch in thin films and periodic multilayers is discussed. Attention is then directed to deformation of anisotropic film-substrate systems where connections among film stress, mismatch strain and substrate curvature are presented. A Stoney-type formula is derived for an anisotropic thin film on an isotropic substrate. Anisotropic curvature due to mismatch strain induced by a piezoelectric film on a substrate is also analyzed. 

Islands occur particularly with adsorbates that aggregate into two-dimensional assemblies on a substrate, leaving bare substrate patches exposed between these islands. Diffraction spots, especially fractional-order spots if the adsorbate fonns a superlattice within these islands, acquire a width that depends inversely on tire average island diameter. If the islands are systematically anisotropic in size, with a long dimension primarily in one surface direction, the diffraction spots are also anisotropic, with a small width in that direction. Knowing the island size and shape gives valuable infonnation regarding the mechanisms of phase transitions, which in turn pemiit one to leam about the adsorbate-adsorbate interactions. 

As the laminate industry grew, this anisotropic behavior was accepted and fabrication techniques adapted to it. For example, expansion and contraction space was left between wall panels, very strong adhesives were developed for bonding the product to substrates, special substrates were qualified, and where it was necessary to cut holes into the laminates the corners were radiused to prevent cracking from stress concentration. 

Conventionally RAIRS has been used for both qualitative and quantitative characterization of adsorbed molecules or films on mirror-like (metallic) substrates [4.265]. In the last decade the applicability of RAIRS to the quantitative analysis of adsorbates on non-metallic surfaces (e.g. semiconductors, glasses [4.267], and water [4.273]) has also been proven. The classical three-phase model for a thin isotropic adsorbate layer on a metallic surface was developed by Greenler [4.265, 4.272]. Calculations for the model have been extended to include description of anisotropic layers on dielectric substrates [4.274-4.276]. 

Isotropic carbon is obtained by the pyrolysis of a hydrocarbon, usually methane, at high temperature (1200-1500°C) in a fluidized bed on a graphite substrate.Under these conditions, a turbostratic structure is obtained which is characterized by very little ordering and an essentially random orientation of small crystallites. In contrast to graphite which is highly anisotropic, such a structure has isotropic properties (see Ch. 7). Isotropic carbon is completely inert biologically. Its properties are compared to alumina, another common implant material, in Table 17.8. Notable is its high strain to failure. 

Nucleation The formation of sohd crystalhne nuclei on a foreign substrate is basically subject to the same laws as the formation of noncrystalhne nuclei. Specific features found in the case of crystalline nuclei are (1) considerably higher ESE values (2) a faceted rather than spherical shape and (3) anisotropic properties (i.e., different ESE values o, for different crystal faces /) ... 

Bmne H, Roder H, Bromann K, Kem K, Jacobsen J, Stoltze P, Jacobsen K, Nprskov JK. 1996. Anisotropic comer diffusion as origin for dendritic growth on hexagonal substrates. Surf Sci 349 LI 15. 

Lithography With the STM Nonelectrochemical Methods. The prospect of atomic density information storage has spurred applications of the STM as a surface modification tool. In this application, the anisotropic current density distribution generated by an STM tip is exploited to "write" on a substrate surface. Features with critical dimensions < 5 nm have been written in UHV, in air, and under liquids. 

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