Substrate Preparation
Before applying a coating, substrates must be meticulously cleaned. The forces binding thin films to substrates are short-range interatomic and intermolecular forces. Even a monomolecular layer of contaminants on the substrate surface can significantly weaken adhesion, leading to poor performance and unreliable optical properties. Therefore, proper substrate preparation is critical for achieving quality coatings.
Importance of Cleaning
Contaminants such as grease, dust, or other residues can adversely affect the condensation process and the resulting film’s characteristics. Poor cleaning often results in coatings with a mottled or oily appearance, weak adhesion, and suboptimal optical performance. Additionally, contamination from the coating machine itself, such as oil backstreaming from pumps, may exacerbate these issues. When such symptoms arise, it is prudent to address both substrate cleaning and machine maintenance.
Cleaning Techniques
Various cleaning methods are available depending on the type of contamination. A comprehensive account of cleaning methods is provided by Holland and Mattox.
- Basic Cleaning Process
For relatively clean substrates, such as microscope slide glass, the following steps are usually sufficient:- Washing: Thoroughly wash substrates with laboratory-grade detergent and warm water. Avoid household detergents, which often contain additives that can leave smears.
- Rinsing: Rinse substrates thoroughly under running warm water.
- Drying: Dry immediately using a clean towel, soft paper tissue, or a jet of clean, dry nitrogen. Allowing substrates to air-dry can lead to stains that are difficult to remove.
- Wax and Grease Removal
Use isopropyl alcohol or similar solvents to remove wax or grease. A cotton swab soaked in alcohol can aid cleaning. Ensure the alcohol is clean by dispensing it from dedicated wash bottles to avoid contamination. - Ultrasonic Cleaning
Ultrasonic scrubbing in detergent or alcohol is effective but should not be prolonged, as it can damage substrate surfaces. Keep substrates wet throughout the cleaning process and use vapor cleaning or deionized water rinses for final drying. - Vapor Cleaning
Expose substrates to vaporized alcohol or degreasing agents. The vapor condenses and runs off, carrying away contaminants. Once the substrates reach the vapor temperature, condensation ceases, leaving the substrates perfectly dry.
Assessing Cleanliness
Contamination is often difficult to detect with the naked eye. Dust is visible under oblique illumination, but wax and grease are not. An old technique to assess cleanliness is the “breath figure” test:
- Breathe on a substrate to condense a thin moisture layer. Contaminants create a smeared pattern, while clean surfaces appear even.
This test should be used sparingly to avoid reintroducing contamination.
Final Cleaning in the Chamber
Once substrates are loaded into the chamber, a glow discharge cleaning process can remove residual contaminants:
- A high-voltage DC supply creates a plasma at pressures around 0.06 mbar (6 Pa). Positive ions bombard the substrate surface, effectively removing light contamination.
- This process usually lasts 5–10 minutes. However, prolonged exposure can increase dust accumulation, which may affect high-performance coatings such as laser mirrors.
Timing and Precautions
The first layer of evaporation should begin promptly after glow discharge cleaning. Cox and Hass observed that delays exceeding 3 minutes between cleaning and coating could lead to decreased film adhesion and quality. If a coating process is interrupted, a short glow discharge cycle should be performed before resuming.
Ion-Assisted Deposition
Energetic processes, such as ion-assisted deposition, provide effective substrate cleaning via ion beam bombardment. This method allows cleaning at deposition temperature and pressure, enabling immediate coating application without a pause.
