Thin films temperature effects

VDP of polyimides is usually performed in vacuum (pressures <10 Pa). Just as in any CVD process, the deposition parameters greatly influence the properties of the polyimide thin films. The effect of a few notable ones viz., substrate temperature and the relative fluxes of the precursor vapors is the focus of the next section. 

Figure 15. Time resolved infrared absorption spectra of Vs(N02) in 940 nm thick PVN films during shock loading, in reflectance units. Calculated plots include only thin film interference effects, excluding pressure and temperature shifts and chemical reaction, making the differences between the experimental and calculated spectra the primary subjects of interest. Shock pressures were determined by interferometry. Arrows show the time at which the shock has fully traversed the film and rarefaction begins.

Thin film field-effect transistors have been prepared of poly(3-alkyl-thiophenes) by using spin-coating techniques. The devices are used in the determination of the charge carrier mobility p, dc conductivity a, and the carrier concentration p. Poly(3-hexylthiophene) is characterized in a wide temperature range T = 130 - 430 and possible transport mechanisms are discussed. ... 

Electrical Properties. Generally, deposited thin films have an electrical resistivity that is higher than that of the bulk material. This is often the result of the lower density and high surface-to-volume ratio in the film. In semiconductor films, the electron mobiHty and lifetime can be affected by the point defect concentration, which also affects electromigration. These effects are eliminated by depositing the film at low rates, high temperatures, and under very controUed conditions, such as are found in molecular beam epitaxy and vapor-phase epitaxy. 

Economic and process considerations usually dictate that agitated thin-film evaporators be operated in single-effect mode. Veiy high temperature differences can then be used many are heated with Dowtherm or other high-temperature media. This permits achieving reasonable capacities in spite of the relatively low heat-transfer coefficients and the small surface that can be provided in a single tube [to about 20 m" (200 ft")]. The structural need for wall thicknesses of 6 to 13 mm (V4 to V2. in) is a major reason for the relatively low heat-transfer coefficients when evaporating water-like materials. 

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

Temperature dependent Hall effect measurements have also been carried out in the temperature range 30 to 260 K on a K3C60 thin film [116]. For three... 

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