The parameters of Table II assume e-beam evaporation without high-energy ion bombardment. The behavior and properties can be changed with IAD. For example with IAD, the substrate temperature can be lowered significantly while maintaining or improving on the density, hardness, refractive index, and often the adhesion properties of the layer. The higher temperature materials tend to develop multiple crystal states that transition form with temperature. These crystal forms possess different refractive indices, stress levels, and evaporation temperatures, complicating the process by introducing uncontrolled variable rates and complicating the results. Constraining to one crystal morphology is possible by melting at a controlled, consistent temperature. The latter is not possible in a small focused e-beam configuration where temperatures can differ by several hundred degrees between areas separated by a few mm, and where melting and recrystallizing zones are mixed.

When the starting material, generally a dielectric, is granular in form, heat transfer is inefficient and rapid heating of dust or points often causes explosive emanation of material. Particulate spatter ranging from occasional blasts to showers is often observed, and included particulates or the voids caused when they abrade away are unacceptable coating defects that reduce production yield. The presence of particulates in or on the coating results in failures that might cause optical scatter, might initiate mechanical stress fracturing, admit moisture, or might initiate laser damage. Explosive emanation of particulates or trapped volatiles often causes pressure and rate surges that can contribute to the inhomogeneity of the optical properties of the growing layer even disrupt the deposition process by upsetting the thickness monitoring system.

The problem is not restricted to broken pieces of the compound crystals, but also occurs with materials prepared by pressing into tablets, with or without a binding material. Evaporation technicians have developed techniques for reducing spatter and pressure/ rate effects that consists of melting the broken pieces or tablets to preparing a denser form of metal oxides starting materials for deposition. The process of forming a dense plug suitable for smooth evaporation consists of filling the crucible, melting down under vacuum, venting, recharging, and melting down again is a non-productive use of chamber time and resources.

Another problem reported with these dielectrics is the deflection or defocus of the electron beam due to charging of the material. This problem is eliminated when the material is slightly conducting, as partially reduced oxides are.

The solution to obtaining particulate-free E-beam consistent and economical depositions is to pre-melt the difficult materials without tying up any production resources. CERAC now has available pre-processed / pre-melted forms of the materials listed in the Tables above. Using a high power e-beam furnace, the bulk material in crystalline starting form is completely melted under controlled oxidation conditions. The resulting refractory oxide materials are prepared in the forms of pre-molded cones for standard crucible liners, rods, plates, or broken pieces ready for evaporation. The metal oxide compounds can be prepared with completely oxidized composition or partially reduced. The choice is dependent on the process and might consider the composition or deposition parameters of the adjacent layers of the stack. For example, alumina and silica (SiO₂) layers require little or no oxygen backfill pressure, while titania layers require a high O₂ partial pressure be present during layer deposition to eliminate absorption.

Another advantage of using pre-processed materials is that they provide consistent results run after run. Materials with multiple oxidation states, such as the titanium oxide series, can change composition after the first few evaporations from the crucible. Obviously this variability would make it impossible to achieve a repeatable production process. In critical applications, for example AR coating designs for eyeglasses that use thin layers of TiO₂, it is important that the index of the layers remain at their design-prescribed value. Small deviations cause the average R to exceed the tolerable value and render the coating lot unusable.

Complex, multi-layer optical coating designs may require dozens of layers, some relatively thin. The evaporation of conventional materials requires time and some degree of conditioning of each material before the shutter can be opened to deposit each material in the sequence of alternating material layers. Modern evaporation systems operating under automated control use materials that require a minimum ramp-up to achieve a rapid and consistent state of readiness as the recipe calls out switching materials in the programmed sequence of layers. With the exception of hafnia and zirconia the oxide compounds listed evaporate from a melted surface created by the e-beam. Evaporation proceeds from the heated hafnia and zirconia surfaces primarily by sublimation because the melted surface layer is very thin. Pre-melted materials offer a more constant composition and surface topography compared with the irregular surface of a pile of randomly sized pieces, therefore evaporation proceeds more smoothly, especially for the non-melting materials.