Coating Materials News Vol 9 Issue 4
Before You Coat – A Materials Perspective
There are many steps involved in the transition of a solid material into a thin film that exhibits desired optical, mechanical, and chemical properties. Each one of the dozens of compounds or elemental materials that is used to grow thin film layers requires individual preparation by CERAC and by the user. The user, our reader, is quite familiar with the process steps he or she must go through to produce the films desired, but is probably unaware of the initial preparation that CERAC does to insure high consistent quality of the starting material.
Evaporation grade materials are processed to produce the required purity for optical applications and the best chemical and physical forms suitable for deposition, whether by evaporation or by sputtering. The term “purity” has a meaning all its own when it comes to coating materials. Some impurities are detrimental to the performance of a deposited layer, while others are not. For example, the percentage of transition metals in a material used in the UV must be very low to avoid extrinsic absorption. However, the presence of a chemical neighbor metal such as scandium in hafnium oxide will not affect the final hafnium dioxide film in an appreciable way. Another example is the residual CaF2 in MgF2, typical for optical grade. Although in principal it seems best to buy the highest purity material available, one has to remember that this is changed significantly with the first evaporation of the material which has a tendency to pick up contamination from the chamber. Reducing the residual concentration of harmless materials of similar chemical properties is often very difficult and unnecessarily expensive, and usually without advantage. The exception is for similar materials that might have very different evaporation temperatures, leading to film growth inhomogeneity. In fact, this class of “impurity” can be beneficial and for some material compounds foreign materials are deliberately introduced. Newer evaporation materials such as CERAC's fluoride mixtures CIROM-IRX*, IRB™ , and oxide mixtures [2,3] are successful because the presence of the foreign component discourages any tendency to grow large crystallite (or columnar) microstructure. Such structures possess high void volume available for water absorption that creates variable stability by modifying the optical and mechanical properties of the layer. IRX® and IRB™ films are found to be amorphous and much denser than the pure materials they are derived from. For those critical applications where impurities are detrimental, the material might be further purified by recrystallization from the melt or solution. This is most often the approach followed in processing fluoride compounds where excess fluorine is admitted to prevent reduction.
The energetics and environment present in resistance-heated or electron-beam heated evaporation are different from those present in sputtering. This fact requires attention to the preconditioning of the particular material. Some materials are ready to use as received; others require some amount of pre-deposition processing by the user. We discuss here the differences specific to common thin film materials.
Compound materials differ greatly in their behaviors under high vacuum evaporation and sputter conditions. In the case of metal oxide compounds, most require reaction with added oxygen to achieve the desired final oxidized state. Exceptions include SiO2 and Al2O3, which do not experience decomposition during evaporation. Metal oxides of the desired stoichiometry can be produced by reacting sputtered metal ions with oxygen ions. In evaporation, the starting material is a metal oxide compound whose state of oxidation is controlled to cause complete oxidation of the film by reaction with supplied oxygen partial pressure. This reactive oxidation process takes place at a relatively low oxygen partial pressure, generally in the E-04 Torr to E-05 Torr range. In the sputtering environment, oxygen is mixed with the working gas argon at a pressure of 1-10 E-03 Torr or higher. Sputtering of pure metal targets is much more efficient than sputtering oxide compound targets, and achieving correct deposited oxide composition requires careful monitoring and control of the process.
Preparatory to being vaporized, the starting material must undergo specific processing to make it suitable for its intended application. The starting forms for oxide compounds are generally tablets or pellets (grains) of specific sizes. Depending on the properties of the compound, it might be hot pressed in an inert atmosphere such as argon or nitrogen or pressed into tablets and sintered. Control of the oxidation state, porosity, and crystalline state of the oxide compound is important for ease of evaporation and repeatability. Many oxide compounds are supplied in sub-stoichiometric form to reduce the tendency to spit and outgas when heated. Thus they are gray or black when received. Some oxides adsorb water on their surfaces and retain it as volatile water while others form hydrates that dissociate and release water during heating. So the density of the tablet or pellet must be made high enough to reduce its porosity to a minimum. In spite of the careful preparation of the starting material to minimize water emission, ever present sources of water that is available for incorporation in the film layer are the surfaces of the tooling and wall lining of the coating chamber. These sources of water are difficult to reduce to insignificant levels without extended preheating under vacuum conditions. They are often the unrealized guilty party rather than the starting material. Some materials, ITO and certain hygroscopic fluoride compounds, are naturally water attractants and are best stored in a heated dry atmosphere until used. Homogeneity of crystalline state in the starting material is important in preventing varying evaporation rates and inhomogeneous film properties with depth, such as refractive index and density. In the evaporation stage, consistent source temperature (elimination of hot spots) is also important to preserve crystal state.
The starting form of a material can influence its final properties as a film layer as well as its ease of evaporation. Titanium dioxide is a case in point. This material is popular because it has the highest index of available materials that are transparent from about 400 nm to beyond 1000 nm wavelength and exhibit durable mechanical properties. The material is difficult to deposit as low optical absorption and smooth hard films because it tends to dissociate during evaporation and grow with an open columnar structure. Control of the important deposition parameters: oxygen partial pressure, substrate temperature, and rate are not sufficient to insure repeatable quality. The starting composition has been shown to be equally important. Many oxidation states exist for titanium oxide, some requiring very high evaporation temperatures, and some requiring high background oxygen pressures to fully oxidize, which in itself leads to film density problems. Users have gone to extreme measures to condition the material to reduce spitting and outgassing, often resorting to hours of e-beam melting and refilling. Many studies with different compositions have been made by researchers seeking the best starting material composition. It is generally agreed that Ti3O5 is the easiest and most successful composition to start with because it melts, whereas the other compositions either do not melt or dissociate to the degree that low absorption films cannot be grown. Some compounds sublime or form fused surfaces, both leading to spatter and excessive outgassing and high absorption. Since Ti3O5 melts, there is no possibility of trapping moisture that can produce outgassing and spitting. The reduction that normally occurs during heating is within the acceptable range that is easily restored with a moderate background partial pressure of oxygen. The films have low optical absorption, low stress, and low roughness and relatively high density. It has been found beneficial to admix other oxides with TiO2 to obtain improved properties; one such is praseodymium oxide, another is zirconium oxide. In sputtering, adding silicon or aluminum oxides can reduce surface roughness and increase density.
Zirconium oxide is a refractory compound that is transparent from the UV to the Mid-IR. It forms hard films of index near 2. ZrO2 is, however, difficult to evaporate and sputter from the pure composition with repeatable properties because two crystal states with different evaporation temperatures coexist. With this material also, improved film properties have been demonstrated when aluminum oxide and / or magnesium oxide are added to make a solid solution. CERAC offers a three-component material that has very high packing density and low stress when deposited as film layers .
Most fluoride compounds are refined, then melted, crushed and sized. Fluoride compounds that sublime rather than melt are vacuum hot pressed then broken to size. During evaporation decomposition does not occur, but if the compound attracts water, outgassing can occur. Magnesium fluoride is prepared by precipitation from solution. Since it can be melted, it can be recrystallized for higher purity, or doped with small percentages of other fluoride compounds to produce a product that grows with greater packing density (smaller or absent columnar microstructure). Pure MgF2 films, even when grown on substrates heated to >250° C exhibit water absorption and desorption and associated unstable refractive index and stress level. Their packing density can be as low as 70%. These problems are reduced in the doped films, as Pulker demonstrated many years ago .
Zinc sulfide, zinc selenide and silicon monoxide, among others, are created by chemical reaction and condensed from a vapor. The solid is then crushed and either pressed into desired shapes or sorted by size. Sulfides and selenides (and tellurides) decompose during evaporation heating but recombine with correct stoichiometry if the substrate temperature, surface condition, and background composition are favorable. Improper evaporation conditions can undo the careful starting material preparation. To prevent unrecoverable decomposition due to local hot spots, these three materials are best contained in a baffle box for evaporation where near isothermal conditions exist.
CERAC, as a chemical material manufacturer, has extensive experience in preparing materials with properties tailored for a given application. Our stock coating materials (evaporation and sputtering) incorporate that experience. (Visit our on-line catalog for a complete listing of stock materials) Additionally, we have provided special, custom compositions when a stock materials is not quite right. Please call our sales and technical staff to discuss your particular needs.
1. CMN V5, Issue 1, Jan.-Mar. 1994.
If you have a question or a topic you would like us to consider for a future issue of CMN, e-mail your requests to Nora Biersack at firstname.lastname@example.org or fax them to 414-289-9804. We also encourage contributions from other writers. Contact Nora via e-mail or by phone at 414-289-9800 x206 for more details on submitting an article.
|(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information)||
Dr. Mitchell C. Colton, Editor
Samuel Pellicori, Principal Contributor
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