Coating Materials News Vol 9 Issue 2
Second Annual Reader Response Issue
CERAC has always encouraged feedback from readers on the topics that are covered each quarter in Coating Materials News (CMN) and we regularly receive suggestions for future editions. Often the recommended topics are too broad or have been covered in-depth in previous editions but those unique engaging questions, often voiced by a large segment of today's coating professionals, go into a special stack. We are making it an annual tradition to dust off that stack and do our best to provide comprehensive answers to as many questions as space will allow.
This year we're addressing the following issues:
Techniques for Coating Thickness Monitoring
We hope you find this to be an interesting and insightful issue of CMN and we look forward to your continued feedback.
Techniques for Coating Thickness Monitoring
The controversy between crystal and optical monitoring is similar to that of summer barbecue techniques, and each side's adherents often battle endless wars over the topic. Rather than trying to decide which is a "better" way to monitor and control layer thickness, the more sensible approach should be to decide which is needed. The general rule is that crystal monitoring is used for metals and for IR coatings at wavelengths longer than 1 µm where optical detectors become expensive. Optical monitoring is most useful between ~300 nm and 1 µm where many coatings are designed with layers that are a quarter wave (QW) or more optical thickness. Designs employing sub-QW layers present problems for optical monitoring and therefore crystal monitoring must be used.
Because optical monitoring senses the optical path of a layer, i. e., the product of index and physical thickness, it directly measures the optical effect of the layer. Crystal monitoring is based on the measurement of frequency change with deposited mass. Area density and acoustic mass of the thin-film form of materials are input parameters. The temperature of the oscillating quartz crystal must be maintained within a specific range to insure proper operation, and the total mass load must be within set limits to provide linear response. The frequency change is used to compute layer thickness under the assumptions that the condensed density is known (or at least repeatable after calibration). Deduction of the optical path in the layer is based on assuming knowledge and repeatability of the refractive index, which implicitly assumes compositional knowledge. Crystal monitoring is essential for deposition rate determination, especially for materials that require reaction to achieve desired stoichiometric composition such as oxides. A consistent rate is required to insure homogeneous composition and thereby constant index through the layer depth.
With careful calibration and consistency in parameter control it is possible to deposit complex multi-layers relying entirely on crystal monitoring. Current crystal systems contain sophisticated electronics that adjust for the influence of the growing mass on the oscillating crystal. Some provide multiple crystals for redundancy and large layer number.
Optical monitors have great difficulty determining deposition rate, so a companion crystal monitor is provided in most, if not all coating systems. Optical monitors require recognition of a slope change in the reflectance or transmittance curve that they are following. While zero slope is relatively easy to detect on peaks or valleys of QWs, the slope at any other point on the curve (as for non-QW layers) requires a complicated program, so many optical monitors need a human observer present to decide end points for layers. Noise is a problem in this decision process, another reason for the human operator.
We have presented some of the arguments for each type of monitoring, but our opinion is that an optical coating system should have both available for highest accuracy and reproducibility.
Coating Advances for Automobile Lighting
New headlights incorporating xenon-arc and metal-halide lamps provide greater luminance in a smaller volume. Lenses constructed of polymers and coated with reflectors have experienced shorter lifetimes because of problems with heating. Water degassing, reflective coating crazing, and loss of adhesion are problems that had to be solved. The historical use of lacquer to protect the aluminum reflector has been discontinued in the interest of cost savings, and new processes developed. Polymer surfaces must often be pretreated with a polymerizing plasma, generally containing oxygen, to establish adherent bonds with aluminum. The process often is an RF plasma exposure of a few seconds duration. A simultaneous benefit is surface cleaning of adsorbed water and other contaminants. Immediately following the surface conditioning, ~100 nm of aluminum is sputter deposited to form the reflective surface. The aluminum film can be deposited thermally, but as we have discussed in previous issues, sputtering produces higher packing densities and the kinetic energy discourages rough or columnar growth. Thus the film is smoother and less porous.
The aluminum film must be protected from the environment to preserve its reflectivity. Corrosion can be prevented with an over coat of vapor deposited silicon monoxide or a SiOx layer created by plasma polymerization of hexamethyldisiloxane (HMDSO). The latter deposits not as a continuous film, but as nano-particles with only moderate packing density. Headlights and fog lights often exhibit a condensed haze of water droplets. A hydrophilic coating must be developed that possesses the benefits of lacquer to prevent light scatter and initiation of corrosion from liquid water. Research is on-going in plasma polymerization deposition to perfect the process.
Plasma Treatment of Polymer Surfaces
Several types of polymer films and rigid substrates are used in food packaging, window laminates, data storage (CD), plastic optics, and related applications. Polymer substrates are prepared for metallization, laminating or printing. In the case of food packaging, the coating must retard the permeation of oxygen and water into the package. Aluminum layers are frequently used, but transparent coatings such as SiOx and Al2O3 are preferred to provide viewing the package contents. Aluminum layers ~200 nm thick provide adequate barriers. Oxide layers 1 - 6 µm thick are used to improve the abrasion resistance of polymer surfaces. Adhesion and equivalent barrier properties are more difficult to obtain with the oxide films. Polymer surfaces must be treated to insure adhesive bonding and to promote other good mechanical and optical properties, as in the example above. Surface modification to a depth of a few molecules is achieved via interaction with energetic ions, free radicals, electrons, and neutral particles. The surface modification can involve removal of adherent contaminates such as organic materials, generation of functional polar groups which promote covalent bonding, and densification through cross-linking. Corona discharge is often used in web coating, and is effective and economical for many polymers. Newer processes of surface treatment involve ionization in a RF field of various gas mixtures depending on the application and DC magnetron sputtering. Surface oxidation, nitration, amination or hydrolyzation can be produced to enhance surface energy to promote bonding. Fluorination, on the other hand, reduces surface energy giving a non-wettable, inert surface. .
Film Density and Refractive Index vs. Temperature
Fluoride compounds characteristically grow with packing density significantly lower than bulk (1.0). For example, MgF2 on substrates at 50° C has a packing density near 0.75. Temperature near 300° C is required for packing density to exceed 0.95. CaF2 at 50° C has only 0.6 packing density, and reaches 0.9 at ~300° C. Cerrous fluoride (CeF3), the main ingredient of IRX® starts out at 0.8 at 50° C, and reaches 0.95 at 300° C. IRX®, however, can be deposited below 150° C and provide good hardness and stable index. At higher temperatures or dryer pumping (cryo-pumping or dry nitrogen purging) the residual water absorption bands near 3 µm and 6 µm can be virtually eliminated. Some oxides, particularly ZrO2, require temperatures in excess of 250° C for >0.95 packing density. In that case, an increase in index is also achieved. Further data can be found in Ref. 2.
High temperature growth often is accompanied by greater scatter, and in those applications where scatter is important, compromises must be made with hardness. Coating of polymer substrates must be done at temperatures below 100° C. For these reasons, low temperature deposition techniques are more frequently employed. These include sputtering and ion assisted deposition (IAD) for oxide compounds. IAD can be used with fluoride compounds if the ion energy is kept below the value where the compound is dissociated and an absorbing species is formed. Only in the UV region does this become a significant concern.
What are Some Structural Differences Between E-beam and Sputter Deposited Films?
It is well known that observable structural differences exist between e-beam and sputter deposited layers. We briefly discuss these differences and their causes.
Electron-beam deposition produces essentially line-of-sight characteristics, and for this reason it is necessary to move the substrate to sample a variety of arrival angles. In spite of this randomization process, the e-beam films grow with a columnar structure. While substrate adhesion might be strong, the coating might exhibit poor coherence because inter-column void space can permit water and other gases to permeate, and the structure as a whole is subject to fracture because the bonding between columns is weak.
The columns can be the result of self shadowing in the line-of-sight growth, nucleation on substrate surface features such as polishing marks or other sources of surface roughness, and limited surface mobility. Structural growth models have illustrated how column size and density are affected by substrate temperature and adatom energy.
Sputtered coatings can also exhibit columnar grain structure, but there is a greater capacity to influence the size of the columns. The operative parameters that determine structure are: kinetic energies of the sputtered atoms (in excess of ~5 eV, compared to <1 ev="" for="" e-beam),="" energy="" transfer="" from="" energetic="" ions="" in="" the="" plasma,="" and="" scatter="" by="" the="" relatively="" high="" density="" of="" the="" gas="" atmosphere.="" these="" parameters="" discourage="" columnar="" growth="" by="" providing="" greater="" mobility="" at="" the="" substrate="" surface="" and="" scatter="" into="" many="" directions.="" energy="" and="" gas="" pressure="" must="" be="" balanced,="" however,="" because="" high="" background="" pressures="" can="" lead="" to="" porous="" coatings,="" such="" as="" with="" argon="" adsorption="" to="" grain="" surfaces.="" at="" very="" high="" bombarding="" energies,="" near="" 500="" ev,="" columnar="" structure="" can="" be="" suppressed,="" and="" fine="" grained="" layers="" result.="" these="" conditions="" can="" be="" achieved="" in="" "high"="" vacuum="" e-beam="" depositions="" using="" ion="" bombardment="" (iad)="" as="" well="" as="" in="" sputtering.="" for="" both="" techniques,="" sputtering="" and="" iad,="" there="" is="" a="" minimum="" ion="" atom="" ratio="" effective="" in="" discouraging="" columnar="" grain="">
|(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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