Cleaning optical elements improves performance. Cleaning methods and equipment directly impact the surface quality of optical components.
What is surface quality?
Surface quality describes the level of imperfections that can be visually noted on the surface of an optical component. These imperfections consist of scratches, dig (little pits), bubbles, edge chips, and coating blemishes. Among them, the scratch and dig, clearly indicated in the standards of MIL-PRF-13830B, are widely used in the optical industry. a surface quality of 60-40, for instance, permits a scratch of 0. 06mm (60 microns, or 0.0024”) and a dig diameter of 0. 40mm (400 micron, or 0.0158”).
Why is surface quality important for optics manufacturing?
In modern optical technology, optical parts tend to become functional devices after coating process. They are usually required to have an excellent surface quality before and after coating, such as high-power laser thin film devices and high reflectors used on laser gyroscopes. The roughness of the substrate should be less than 1.5nm or even reach at 0.8nm.
Relevant data show that when the laser is injected into the scratch on the surface of K9 glass, the electric field intensity at the scratch increases by 2.3 times, which affects the performance of laser resistance. The high reflectivity of laser gyro mirror is required to be ≥ 99.94%. Any defects on the substrate and film surface will affect the reflectivity.
Optical coating performance determines the function of this device. Devices with high requirements in particularly are not allowed to have decreased performance due to surface defects. Therefore, paying attention to the surface quality should be maintained throughout the whole process of coating process.
How can manufacturers have and maintain high surface quality?
Cleaning is a key part of maintaining excellent surface quality for optic parts.
The surface quality of thin-film devices is determined by the combination of
1 Substrate Scratch&Dig, which was produced by the optical polishing.
2 Film quality, which would be affected by the following things,
- Pollutants left during substrate cleaning before coating;
- During coating, the vacuum chamber pollutes the substrate and film surface;
- Improper operation and other problems in the whole coating process.
After the finish of substrate polishing and before sending them to optical coating, therefore, cleaning plays a vital role in determining the result of coating.
Optics cleaning methods
Photonchina provides proper materials, techniques, and handling procedures to minimize the risk of damage. There are 5 methods below to have an efficient and reliable cleaning procedure in our production.
1) Blowing method
Avoid touching the surface of the optic with unprotected fingers. Always wear powder-free gloves or finger cots. The first step in cleaning any kind of optic is to remove dust or loosely held particles by blowing them off the surface using a dust-free blower or a camel hair brush. This reduces the chance of scratching the optic during the actual cleaning.
2) Wipe method
- Use the blowing method first.
- Place a drop of methanol or reagent-grade acetone on the side of a lens tissue paper. With the other hand, hold the optic by its edges.
- Gently place the wet area of the tissue onto the optic.
- Slowly drag the lens tissue across the surface applying equal pressure until the lens and tissue are dry.
If stronger cleaning is required, then the next step will be to wipe using lens tissue paper along with methanol and acetone.
- Fold a piece of lens tissue paper to create a folded edge that is a little longer than the size of the optic.
- Wet the folded edge of the tissue with acetone.
- Wipe the optic with the lens tissue paper with one continuous motion. Apply some pressure on the wipe to remove stubborn stains.
Repeat this process, always with new lens tissue paper to eliminate depositing any contamination on the optic.
A final wipe with methanol is recommended since methanol does not leave streaks on the surface like acetone. Isopropyl alcohol is also effective, but it too can leave streaks like acetone. It is usually better for cleaning laser optics exposed to particles (in an uncontrolled environment or outside of a vacuum chamber).
If the size of the optic is very small, cotton tips can be used instead of lens tissue paper with the same procedure.
3) Bath method
Small optics or optics that require milder cleaning than the wipe method can be cleaned by the immersion method.
- In this method a Petri dish is lined with lens tissue paper and filled with methanol.
- Once in the Petri dish, clean the optic gently with a soaked cotton ball.
- Once wiped, agitate the Petri dish for a few minutes.
- Repeat steps 1 through 3 with acetone instead of methanol.
- Next, remove the optic from the Petri dish and blow-dry.
4) Soap solution method
If the optic contains contamination in the form of grease (fingerprints, etc.), it will need to be cleaned with a soap solution. The soap solution should be a nonabrasive, mild solution without any additives.
- Use the blowing method first.
- Use a soap solution and clean with either the wipe method.
- Rinse thoroughly with deionized water in a Petri dish.
- Cleaning with methanol and acetone should be next.
- After rinsing, blow-dry with a dust-free blower to remove all water.
5) Ultrasonic cleaning method
Ultrasonic cleaning is a very efficient form of cleaning where dirt particles are completely and rapidly removed from the surface of optics. It has the following benefits,
- Batch parts can be cleaned at one time
- No damage to parts due to gentle cleaning
- High level of cleanliness
Photonchina offers full cleanroom environment ultrasonic cleaning, inspection, and vacuum packaging to ensure the highest level of cleanliness and protection for our customers.
The process of cleaning in Photonchina
Cleaning of optical components usually takes place in two stages: in-process cleaning and cleaning before coating.
In-process cleaning mainly refers to the time when optical parts have finished grinding and polishing. Grinding and polish are important process to determine the surface quality of optics and manufacturing efficiency. So, the cleaning for first time generally takes place after grinding and polishing.
The main pollutants in the grinding and polishing process are polish powder, asphalt, and paint residue, in some cases. The cleaning solution after grinding can be roughly divided into two types: organic solvent cleaning agent and semi water based cleaning agent.
Cleaning before coating. The main pollutants of cleaning before optical coating are oil, fingerprint, dust, etc. As the coating process has a stringent demand on the cleanliness of optical components, the choice of cleaning method and cleaning agent, if applicable, is critical.
In consideration of a certain cleaning agent, it is necessary to consider its corrosiveness and other characteristics that will impact on optics.