Striation quality

To prepare ultrathin polymer films on the surface of wafers, especially those of large diameter (6 or 8 inch), uniformity and defect density become important factors in determining the resist quality. The conventional spin coating method has been reported to introduce interference striations (11) and high defect densities (2.31 when used to prepare ultrathin polymer films. As an alternative approach, the LB technique has been proposed as being suited to the preparation of more uniform ultrathin polymer films (2). Using this technique monolayer polymer films can be transferred layer by layer to the surface of a solid substrate from the water surface. An important feature of the LB technique is that the accumulation of monolayer films allows the thickness of the built-up film to be controlled in a precise manner. Consequently, extremely uniform and ultrathin polymer films can be prepared. 

Figure E7.10b shows the SDF and compares it to that of circular tube flow of a Newtonian fluid. The SDF is broad with about 75% of the flow rate experiencing a strain below the mean strain. A better insight into the meaning of the SDF is obtained by following simultaneously the reduction of the striation thickness and the flow rates contributed by the various locations between the plates (Fig. E7.10c). The distance between the plates is divided into 10 layers. We assume for the schematic representation of the SDF that the strain is uniform within each layer. Let us consider in each alternate layer two cubical minor particles separated by a certain distance, such that the initial striation thickness is Tq. By following the deformation of the particles with time, we note that although the shear rate is uniform, since the residence time is different, the total strain experienced by the particle is minimal at the moving plate and increases as we approach the stationary plate. But the quality of the product of such a mixer will not be completely determined by the range of strains or striations across the flow field the flow rate of the various layers also plays a role, as Fig. E7.10c indicates. A sample collected at the exit will consist, for example, of 17% of a poorly mixed layer B and only 1%...

Figure 11.6 Striation thickrtess as a measure of mixture quality...

Here the total strain is calculated as the product of shear rate and residence time for the major component and /i, and 2 are the viscosities of the major and minor components respectively. It can be deduced from equation (11.10) that large striations (i.e. poor mixture quality) are favoured by a small volume fraction of the minor component and a high viscosity of the minor component. In other words, it is more difncult to mix by laminar shearing small volumes of a high viscosity minor component into a major component than it is to mix equal volumes of equal viscosity components. 

It is of vital importance that the distance between the lower edge of the feeder and the glass surface is set at a defined value. The striation and the bubble quality are decisively affected by the maintenance of a defined spacing. For this purpose, the casting mould is continuously lowered with a lifting platform. 

Testing for striation (stress-optical) if, due to the usage, optical quality in transmission is required, the shadow method is applied to detect striae. Detection of all surface damages. 

A key point to gather from the textured mixtures above is that the orientation of sampling can have a large impact on mixing measures as well as the sample size. Sampling perpendicular to a striated mixture must be undertaken with care to ensure that the sample size is smaller than the striation size to obtain accurate measures of mixture quality (or lack thereof). 

As mixing progresses, the interfacial area per unit volume increases and the striation thickness decreases. If there is a distribution of striation thickness then not only the mean but also the variance should be taken into consideration for the quality of mixing calculations. In Section 6.3.1 the interfacial area growth, or equivalently the striation thickness reduction, is calculated from geometrical arguments. 

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