Dielectrics, thick-film properties, measurement

Concerning the two-layer model, the thickness and properties of each layer depend on the nature of the electrolyte and the anodisation conditions. For the application, a permanent control of thickness and electrical properties is necessary. In the present chapter, electrochemical impedance spectroscopy (EIS) was used to study the film properties. The EIS measurements can provide accurate information on the dielectric properties and the thickness of the barrier layer [13-14]. The porous layer cannot be studied by impedance measurements because of the high conductivity of the electrolyte in the pores [15]. The total thickness of the aluminium oxide films was determined by scanning electron microscopy. The thickness of the single layers was then calculated. The information on the film properties was confirmed by electrical characterisation performed on metal/insulator/metal (MIM) structures. 

It is claimed to be able to measure thicknesses with an accuracy of about 0.3 A, approximately the thickness of an atomic layer. Because the method relies on polarized fight, ellipsometry is a nondestructive technique, which makes it suitable for in situ measurements in some cases. One disadvantage is that the substrate must be reflective, so gold or sUicon wafer is often used. However, a major disadvantage with the technique is interpretinging the data, which is not trivial models of the air—thin film-substrate must be used. The dielectric and optical properties of the thin film must be known accurately to calculate the thickness of the film, and even then the modelling usually assumes a homogeneous layer which may not always be justified. 

Test Pattern. The basic conductor properties can be measured using a single test pattern, as illustrated in Fig. 8.15. These include resistivity, print definition and film thickness, film density, solder leach resistance, wettability, adhesion, and wire bondability. Each property will be discussed individually with reference to Fig. 8.15. Many applications require functional use tests which usually require specific test patterns and even multilayer construction processes. Similarly, numerous applications require standard conductor tests on thick-fihn dielectrics instead of the bare substrate. 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

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... 

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... 

In the case of a metallic substrate, Cs and Ch are much higher than Cj meaning that Cl is close to that of the dielectric film only. In the case of a flat covering layer, its thickness is simply derived by the plane capacitor relation h = eeo/Ci where Ci is deduced from relation (34) and e is the dielectric constant of the deposited material. When the film does not completely cover the substrate, the measured capacitance depends on the covering properties of the film and is no longer related to the film thickness. The equivalent thickness hz, defined from the total measured capacitance ... 

The samples have been prepared by e-beam evaporation of a dielectric layer followed by thermal evaporation of the silver fraction, which builds the island film, while the sandwich is completed by a further dielectric film. In every sample, intentionally the same amount of silver (corresponding to an average thickness of 4 nm, as recorded by quartz monitoring) has been embedded in a 6 nm thick dielectric film, formed from either Mgp2, LaFs, Si02, or AI2O3. The optical transmittance T and reflectance R of all films have been measured by a Perkin Elmer Lambda 19 spectrophotometer. To correlate the optical properties with the sample morphology, transmission electron microscopy (TEM) has been applied. 

It is also very important to measure and control the rate of deposition since film structure and properties are affected by the rate to some extent. Ratemeters can usually be used for all film materials. This is a distinct advantage in multilayer deposition of metals and dielectrics. The simultaneous measurement of thickness and rate with one and the same measuring principle is certainly of advantage, as is having a thickness and rate monitoring system which can be actively integrated in an automatic process control. 

A technique for calculating these properties to an adequate approximation from dielectric data has been suggested [1]. Precise measurements of Ilm in thin films of nonpolar liquids are in good agreement with the theoretical predictions [1]. However, the latter theory does not apply to films so thin as to have dielectric properties that vary with thickness. 

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