Selecting source materials for evaporative and sputter thin film deposition is a critical process engineering requirement. The purity of these materials is a consideration in determining if a run results in a high-quality, uniform film, and involves the specific application of the coating. Highest quality evaporation and sputter target impurities are often in the form of chemical element contamination and require expensive refinement procedures. Refinement requirements vary with material compound and its supplied form.
Coatings used in many commercial applications can use lower, less expensive purities, while UV, high-energy laser, and semiconductor applications require very high purities. Purity grades range from 99.9% (3N) to 99.9999% (6N). Even trace impurity levels above 0.01% can degrade film performance. The right grade depends on your application. Many precision optical coating applications utilize 4N (99.99%) purity or higher. Advanced applications in defense optics and photonics often require 5N (99.999%) or higher.
Through careful selection of material purity level, coating engineers ensure that essential physical and optical characteristics, including stoichiometry, refractive index, and absorption and density, are consistently optimum across the entire production cycle.
Particulate "Spitting" in Thin Film Deposition
Larger particles can be explosively ejected from materials during resistance-heated, e-beam, or sputter deposition. These particles land on the substrate surface, become embedded in the growing film, and create localized defects such as nodules and pinholes.
Impurity contamination, in the form of voids, partially oxidized surfaces, metallic and non-metallic inclusions, and trace impurities have different evaporation temperatures, leading to explosive ejection.
How Impurities and Contamination Affect Film Quality
Surface and volume contamination has quality impacts beyond mere aesthetic flaws; they significantly compromise coating integrity and manufacturing efficiency. The following quality issues can be attributed to contamination and particulate inclusions and render a coating run unacceptable:
- Structural voids, including nodules and pinholes from particulates.
- Loss of Adhesion. Contaminants present at the interface between the substrate and the film can lead to bonding failures.
- Elevated Optical Absorption. Contamination and impure materials can increase absorption
- Inconsistent Thickness. Localized splatter can cause optical scatter
- Waste and Scrapped Materials. This is especially costly when defects occur during the final stages of complex multi-layer depositions.
Pressure and rate of deposition may mitigate spitting in some cases but can affect productivity. A better alternative is to manage impurity levels directly at the source.
Additional source material properties to consider beyond purity include:
- Grain size. Coarser grains can contribute to inconsistent melting and sublimation behavior
- Surface condition. Rough or oxidized source surfaces introduce defect-generating nucleation points
- Density. Dense pellets and tablets produce a more uniformly consistent rate and reduce internal void volume
- Form factor. Pellets, tablets, and rods behave differently in crucible geometries
- Form factor. The specific physical format, whether rods, tablets, or pellets, influences behavior within various crucible geometries.
Supply Chain Sourcing and Qualification
Simply acquiring high-purity materials is insufficient; the supply chain must be actively qualified beyond merely accepting a vendor's certificate of analysis.
Due to the inherent risk of lot-to-lot variation in evaporation materials, procurement from a qualified supplier must offer full batch traceability, including purity analysis certifications, trace element concentration, and results from thin film deposition validation tests.
Performance verification and rigorous process qualification are especially critical for defense and aerospace applications.
Key qualification steps include:
- Evaluating analytical data for every incoming lot, such as chemical assay, glow discharge mass spectrometry, or ICP-MS.
- Utilizing witness samples during incoming inspection to verify deposition characteristics prior to production.
- Developing vendor traceability and qualification records that correlate material lot numbers with specific process performance data.
Build Your Process on a Reliable Foundation
If your coating process shows run-to-run variability, check your material supply chain consistency and quality control before you adjust your process parameters. Many "process problems" are material problems in disguise.
At Tecport Optics, our systems are engineered to work with the full range of high-purity evaporation materials used in precision optical coating. Our process engineers understand how material form, purity grade, and crucible design interact in real production environments.
Whether you are commissioning a new system or troubleshooting an existing process, we can help you build the foundation your coating process needs to perform consistently. Reach out to our team and let's talk through your specific application.