ABSTRACT
In the previous chapter we considered what is probably the most difficult aspect of thin-film coating and filter production, that of materials. As we saw, these are not always satisfactory, and there are still problems associated with their stability. Once the materials have been chosen, and their properties are known, the thin-film designer, using the methods discussed in chapters 3-7, can usually produce a design to meet a given specification. Given suitable materials and an acceptable design, however, there are still further difficulties to be overcome in the construction of a practical filter. The two most important remaining factors are, first, controlling the uniformity of layer thickness over the area of the substrate, and second, controlling the overall thickness of each layer. Lack of uniformity causes a shift of characteristic wavelength over the surface of the filter, without necessarily affecting the performance in other ways, while thickness errors usually cause a reduction in performance. The magnitude of the errors which can be tolerated will vary from one design to another and the estimation of this is dealt with briefly. The bulk of this chapter is concerned with the general problem of minimising these two sources of error. One other important topic is substrate preparation, and that is considered on pages 497-9.
