Dielectrics, thick-film microstructure

IR spectra measurements as well as variation of the film thickness, shrinkage, and refractive index demonstrated substantial differences in the mechanisms of thermal decomposition of films prepared from the exclusively metal alkoxide precursor and from the metal alkoxides modified by 2-ethylhexanoic acid. These differences affect the evolution of film microstructure and thus determine the different dielectric properties of the obtained films. The dielectric permittivity of the films prepared from metal alkoxide solutions was relatively low (about 100) and showed weak dependence ofthe bias field. This fact may be explained by the early formation of metal-oxide network (mostly in the... 

Film Thickness. Film thickness is controlled by the viscosity of the precursor solution and speed of either spinning or withdrawal from the solution. In early work, films greater than 1 qm thick tended to peel off of the substrate (Ogawa, 1994). The films formed are smooth and optically transparent, so deposition of further layers is possible. Multiple layers 300-600 A thick were spin coated from an acidic CieTACl/TEOS solution to prepare films up to 3000 A thick for use as low dielectric constant films in microelectronic devices (Liu, 1996). Interference colors in the films as the number of layers increased indicated uniform thickness and microstructure. The mesostructure consisted of a randomly packed layer of micelles rf-spacing 54 A) near the substrate with ordered structures formed over the disordered layer rf-spacing 38 A). The structure was stable after calcination indicating formation of three dimensional rather than layered structures. 

Microstructural features of dielectric materials include the chemical composition and structure of the various phases as well as porosity and its distribution. These microstructural features, coupled with macroscopic defects such as pinholes, mesh marks, foreign matter, and interaction with conductor materials, determine dielectric properties. Very little information is published about composition of thick-film dielectrics, which are complex materials systems. The following section is a general discussion of dielectric microstructure development. 

Materials Analysis. The next level of complexity involves the measurement of dielectric properties for the determination of composition and microstructure as well as thicknesses. Thin films are typically microscopically inhomogeneous with substantial fractions of grain boundaries and voids, so their dielectric properties are rarely equal to those of the corresponding materials in bulk form. As an example, the pseudodielactric function <(> film deposited by low pressure... 

Tab.l Typical data for passive films taken from Ref. [1], density p, dielectric permittivity e, band gap energy g, flat band potential Ufb, equilibrium potential of oxide electrode Uqx (Reaction 2 [16]), donor concentration N, difference of electronegativity Ax, transference number of cations f+, formation factor dd/dU, and initial oxide thickness do. Because of the strong dependence of properties on the preparation technique, the microstructure and the sensitivity of thin films, the reliability of these data is less than for bulk, crystalline solids... 

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