Thin film texture

Coercivity of Thin-Film Media. The coercivity ia a magnetic material is an important parameter for appHcations but it is difficult to understand its physical background. It can be varied from nearly zero to more than 2000 kA/m ia a variety of materials. For thin-film recording media, values of more than 250 kA / m have been reported. First of all the coercivity is an extrinsic parameter and is strongly iafluenced by the microstmctural properties of the layer such as crystal size and shape, composition, and texture. These properties are directly related to the preparation conditions. Material choice and chemical inborn ogeneties are responsible for the Af of a material and this is also an influencing parameter of the final In crystalline material, the crystalline anisotropy field plays an important role. It is difficult to discriminate between all these parameters and to understand the coercivity origin ia the different thin-film materials ia detail. 

The determination of particle size and stmctural iaformation for fibers and polymers, and the study of stress, texture, and thin films are appHcations that are growing ia importance and can be examined with x-ray iastmments. 

X-ray diffraction consists of the measurement of the coherent scattering of x-rays (phenomenon 4 above). X-ray diffraction is used to determine the identity of crystalline phases in a multiphase powder sample and the atomic and molecular stmctures of single crystals. It can also be used to determine stmctural details of polymers, fibers, thin films, and amorphous soflds and to study stress, texture, and particle size. 

As we have seen, the orientation of crystallites in a thin film can vary from epitaxial (or single crystalline), to complete fiber texture, to preferred orientation (incomplete fiber texture), to randomly distributed (or powder). The degree of orientation not only influences the thin-film properties but also has important consequences on the method of measurement and on the difficulty of identifying the phases present in films having multiple phases. 

Other excellent methods of phase identification include TEM and electron diffraction. These may be more useful for low-Z materials, ultrathin films, and for characterizing small areas, including individual grains. For multiphase films with incomplete texture, these methods and XRD are complementary, since in commonly used geometries, they probe atomic planes perpendicular and parallel to the thin film surface, respectively. 

The film thickness of epitaxial and highly textured thin films can be measured with XRD. Close to the usual or primary difftaction peaks there are secondary or subsidiary maxima in the difftacted intensity (see Figure 6), which are due to the finite film thickness. The film thickness is inversely proportional to the spacing between these maxima and is easily calculated. X-ray reflectivity is another accurate method for measuring a film s thickness. 

Strained set of lattice parameters and calculating the stress from the peak shifts, taking into account the angle of the detected sets of planes relative to the surface (see discussion above). If the assumed unstrained lattice parameters are incorrect not all peaks will give the same values. It should be borne in mind that, because of stoichiometry or impurity effects, modified surface films often have unstrained lattice parameters that are different from the same materials in the bulk form. In addition, thin film mechanical properties (Young s modulus and Poisson ratio) can differ from those of bulk materials. Where pronounced texture and stress are present simultaneously analysis can be particularly difficult. 

R. W. Smith. A kinetic Monte Carlo simulation of fiber texture formation during thin-film deposition. J Appl Physics 57 1196, 1997. 

Textured Tin Oxide Films Produced by Atmospheric Pressure Chemical Vapor Deposition from Tetramethyltin and Their Usefulness in Producing Light Trapping in Thin-Film Amorphous Silicon Solar Energy Mater., 18 263-281 (1989)... 

A thin film of tin oxide with a rough texture, produced by MOCVD from tetramethyl tin, (CH3)4Sn, deposited on an amorphous silicon cell provides a light-trapping surface, which enhances the efficiency of the device. 

I Schnitzer, E Yablonovitch, C Caneau, TJ Gmitter, and A Scherer, 30% External quantum efficiency from surface textured, thin-film light-emitting diodes, Appl. Phys. Lett., 63 2174—2176, 1993. 

Reflected light Microscopy can be used for examining the texture of solid opaque polymers. Materials which can be prepared as thin films are generally examined by transmitted light. Two common techniques used are (i) polarised-light Microscopy, and (ii) phase contrast Microscopy. 

Piezoelectric materials have a variety of commercial applications, and a large number of such materials are known. To examine such materials in thin-film form, highly textured crystalline thin films are likely to be needed, so microstructure control is therefore an issue. Screening approaches should be straightforward. 

In practice, the application of x-ray measurement techniques to thin films involves some special problems. Typical films are much thinner than the penetration depth of commonly used x-rays, so the diffracted intensity is much lower than that from bulk materials. Thin films are often strongly textured this, on the other hand, results in improved intensity for suitable experimental conditions but complicates the measurement problem. Measurements at other than ambient temperature, not usually attempted with bulk materials, constitutes additional complexity. Since typical strains are on the order of 1 X 10 , measurements of interplanar spacing with a precision of the order of 1 X 10 are needed for reasonably accurate results hence, potential sources of error must be kept to a low level. In particular, the sample displacement error can be a major source of difficulty with a heated sample. The sample surface must remain accurately on the axis of the instrument during heating. 

The required degree of understanding of the physical properties of metal thin films used for interconnects on chips is illustrated by the following example. It was found that the performance of conductors on chips, A1 or Cu, depends on the structure of the conductor metal. For example, Vaidya and Sinha (10) reported that the measured median time to failure (MTF) of Al-0.5% Cu thin films is a function of three microstructural variables (attributes) median grain size, statistical variance (cr ) of the grain size distribution, and degree of [111] fiber texture in the film. 

It is seen from the discussion above that Cu is electrodeposited in vias and trenches on a bilayer a barrier metaVCu seed layer. When the barrier layer is composed of two layers (e.g., TiN/Ti), Cu is electrodeposited as a trilayer a barrier bilayer/Cu seed layer. This type of underlayer for electrodeposition of Cu raises a series of interesting theoretical and practical questions of considerable significance regarding the reliability of interconnects on chips. In Section 19.1 we have noted that interconnect reliability depends on the microstructural attributes of electrodeposited Cu (for Cu-based interconnects). These microstractural attributes, such as grain size, grain size distribution, and texture, determine the mechanical and physical properties of the thin films. Thus, one basic question in the foregoing series of questions is the problem of the influence of the underlayer barrier metal on the microstructure of the Cu seed layer. The second question is the influence of the microstructure of the Cu seed layer on the structure... 

Astonishingly, the same ARUPS spectra have been observed for ex situ grown TTF-TCNQ thin films (Rojas et al, 2001). The hlms were obtained by thermal sublimation in HV ( 10 mbar) on cleaved KCl(lOO) substrates and consisted of highly oriented and strongly textured rectangular-shaped microcrystals as shown in the TMAFM image of Fig. 6.9. The molecular afc-planes are parallel to the substrate surface and the microcrystals are oriented with their a- and fc-axis parallel to the [110] and [110] substrate directions, respectively, due to the cubic symmetry of the substrates. ARUPS spectra taken on the as-received hlms, measured along the substrate equivalent [100] direchons at T 100 K, are shown in Fig. 6.10. 

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