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Coating Densification for Stability

Multilayer thin film coatings selectively transmit and reflect different wavelengths based on the thickness and refractive index of the individual layers. The most widely utilized optical thin film coating technology is thermal evaporation, which uses either resistive heating or electron beams to vaporize the source material(s). Evaporation is popular because it is compatible with a wide range of source materials, can be used to produce thin films from the deep ultraviolet through the far infrared, and is highly cost effective.

Figure 3. Specialized tooling within the DSI MicroDyn® sputtering chamber causes continuous part rotation during coating, resulting in highly uniform deposition around the entire tube circumference.

However, evaporation is a relatively low energy process. As a result, the coating material atoms or molecules don't pack in tightly in the resultant thin film, making the layers somewhat porous. This means the coating layers can subsequently absorb moisture, which changes their effective refractive index, resulting in a shift in the coating's transmission or reflection properties. This problem is frequently of concern in military and aerospace applications, because the optic may be exposed to large swings in ambient temperature and humidity.

Sputtering is a coating process which increases the energy with which molecules impact the substrate surface. Consequently, the various forms of sputtering result in coatings that are substantially more densified than is possible using evaporation. This makes sputtered coatings essentially impervious to water absorption and its attendant performance shifts.

Deposition Sciences utilizes a proprietary variant of magnetron sputtering, called MicroDyn®, which is performed in a chamber under low vacuum conditions. Here, parts for coating are held on a rotating drum within the chamber and several electrically conductive (metal or semiconductor) coating targets are arranged around the circumference of the chamber. These targets are biased with a negative voltage and immersed in a magnetic field. Electrons leaving the target are contained in the vicinity due to this magnetic field. The electrons collide with the sputtering gas atoms and ionize them, and these ions are accelerated towards the target(s) because of their electrical potential. When these ions impact the target, they cause atoms or molecules to be ejected (sputtered) and deposited on to the optics. Because the sputtered target atoms are ejected with a large amount of energy, they pack densely into the thin film. Thus, this approach enables production of coatings that have both the spectral performance characteristics and the necessary environmental stability essential for countermeasures and other military applications.

Coating Highly Curved Surfaces

Another limitation of most thin film coating techniques, including both evaporation and sputtering, derives from the fact that they are performed under moderate to high vacuum conditions. This produces a relatively long mean free path for the coating material molecules, making deposition occur essentially in a line of sight to the material source. As a result, any surfaces that are “shadowed” by others from the material source, or which don't face perpendicular to the direction of material travel, don't experience deposition at the same rate (or any deposition at all) as surfaces which are perpendicular to the material travel. The resultant spatial variations in layer thickness across a part caused by this situation lead directly to unwanted variations in the wavelength characteristics of the coating.

Because of this, uniformly coating any kind of highly curved optic (domes, steep aspheres, tubes, spheres, etc.) using traditional deposition technology is challenging. Traditional coating of a shape like a cylindrical tube or full sphere also requires first coating one half of the part, and then turning the parts and performing a second coating run. This essentially doubles production costs and time.

The circular symmetry of the tubes that DSI coats for countermeasure applications provided an opportunity for the company to engineer customized tooling for the MicroDyn® sputtering system that would enable production of a uniform coating around the entire circumference of the cylinder in a single coating run. In this case, each individual tube is mounted on a rod within the coating chamber, and this tooling is configured so that the rods rotate throughout the coating process in a way that is synchronized with both chamber motion and the sputtering deposition process. The result of this motion is that every area on the outer surface of the tube is exposed to the coating material source for the same amount of time, producing uniform layer deposition around the entire tube circumference.

Conclusion

Coatings for military and aerospace applications often require performance over a very wide range of ambient conditions, necessitating fabrication methods that yield highly dense and stable thin films. Plus, the size and weight constraints of many of these applications also drive designers to use odd shaped components. Combining sputtering technology with innovative tooling concepts can meet the challenges of providing thin film coatings that meet these requirements.

This article was written by Ryan McDaniel, Material and Process Engineer Sr., Deposition Sciences, Inc., (Santa Rosa, CA). For more information, visit here .