Optical coating A class of optical medium materials consisting of the thin layered medium that transmits light beams through an interface. The application of optical thin films began in the 1930s. Nowadays, optical thin films have been widely used in the field of optics and optoelectronics to manufacture various optical instruments.

The main optical thin film devices include reflection film, anti-reflection film, polarization film, interference filter, spectrometer, and so on. They have been widely used in the national economy and national defense construction and have been paid more and more attention by scientific and technical workers. For example, the light flux loss of the complex optical lens can be reduced tenfold by using the antireflective film, the output power of the laser can be doubled by using the reflector with a high reflection ratio, and the efficiency and stability of the silicon photocell can be improved by using the optical film. In this paper, we mainly explain the optical coating technology.

Optical coating technology

There are several techniques that apply optical coatings, including evaporative deposition, plasma sputtering, ion beam sputtering, and atomic layer deposition (Table 1).

	Evaporative	Evaporative with IAD	Plasma Sputtering	IBS	ALD
Spectral Performance	Low	Medium	High	High-Very High	Very High
Coating Stress	Low	Medium	High	Very High	High
Repeatability	Medium	Medium	High	Very High	Very High
Process Time	Slow	Slow	Intermediate	Very Slow	Very Slow
Non-Flat Geometry Capabilities	Better	Better	Good	Bad	Best
Relative Price	$	$	$$	$$$	$$$

1. Deposition by evaporation

During evaporation deposition, the source material in the vacuum chamber is vaporized by heating or electron beam bombardment. During evaporation, the steam condenses onto the optical surface and precisely controls heating, vacuum pressure, substrate positioning, and rotation, allowing uniform optical coatings of a specific design thickness. Evaporative deposition can accommodate larger coater sizes and is generally more cost-effective than other techniques described in this section.

The relatively mild nature of vaporization results in loose or porous coatings. These loose coatings have water absorption problems, which can change the effective refractive index of the layer and cause performance degradation. Evaporation cannot be precisely controlled during evaporation deposition, so the layer thickness cannot be precisely controlled as with other techniques such as ion beam sputtering. However, the advantage of these loose coatings is that they are relatively stress-free. The evaporation coating can be enhanced using ion beam assisted deposition (IBAD or IAD), where the ion beam is directly applied to the substrate surface, increasing the adhesion energy of the source material to the surface and resulting in a denser, more robust coating.

2. Plasma sputtering

Plasma sputtering includes a range of known technologies, including advanced plasma sputtering and magnetron sputtering. The general concept comes from the generation of plasma. The ions in the plasma then accelerate into the source material, knocking out loose high-energy ions, which then sputter onto the target optical element. Although each plasma sputtering has its unique characteristics, advantages, and disadvantages, the techniques are grouped together because they share a common concept of operation. The differences in this group are much smaller than the other coating techniques discussed in this section. Plasma sputtering achieves a trade-off between price and performance between evaporation deposition and ion beam sputtering.

3. Ion beam sputtering (IBS)

During ion beam sputtering (IBS), a high-energy electric field is used to accelerate the ion beam (Figure 5). This acceleration gives the ion significant kinetic energy (~10-100 eV). When the source material is impacted, source material ions “sputter” from the target and form a dense film upon contact with the optical surface. 5 A major advantage of using IBS coatings instead of evaporative deposition is the ability to more precisely monitor and control the growth rate, energy input, and oxidation levels of individual coatings.

This level of control enables highly reproducible coating batches and minimal layer thickness error, ensuring that coating performance is consistent with designed spectral and phase parameters. 5 IBS coatings are much smoother than coatings using other coating techniques, making IBS the only coating technology that can produce a “super mirror” with a reflectance of more than 99.99%, and with a lower roughness than the original substrate. The high density of IBS coating makes it strong and durable, improves its chemical resistance, prolongs the service life of the coating, and makes it able to withstand harsher environments.

During the IBS process, the refractive index of each layer can also be varied, which further improves the level of process control. Known for its accuracy and repeatability, IBS is the preferred coating deposition technology for high-performance laser optical coatings. The disadvantage of IBS is that the cost is higher than other technologies because of the longer cycle time and stress created in the optical element, which can lead to deformation and optical distortion.

4. Atomic layer deposition (ALD)

Unlike evaporative deposition, the source material for atomic layer deposition (ALD) does not need to be evaporated from the solid but is supplied directly as a gas. Despite the use of gases, high temperatures are often used in vacuum chambers. In ALD, the precursors are delivered as non-overlapping pulses, each self-limiting. The chemistry of the process is designed so that only a single layer can attach to each pulse, and the geometry of the surface is not a limiting factor.

This allows for extraordinary control over layer thickness and design. This results in a slow deposition rate and a higher cost per coating. However, the chambers used for ALD are usually quite large and can cover many optical elements in a single run. ALD is also line-of-sight independent, which means it can be used to paint optical elements with unusual geometry that would be difficult to paint by other means.

Coating process and equipment

1. Cleaning equipment:

Ultrasonic cleaning machine: refers to the integration of cleaning and drying, can be directly mounted and coated. And the machine must be used in a clean space;

2. Ultrasonic cleaning technology of optical lens

In optical cold processing, lens cleaning mainly refers to the cleaning of residual polishing liquid, adhesive and protective material after polishing. Clean the edging oil and glass powder after lens edging; Cleaning of fingerprinting, saliva, and various attachments before lens coating.

The traditional cleaning method is to use the wiping material (gauze, dust-free paper) with chemical reagents (gasoline, ethanol, acetone, ether) to soak, wipe, and other means for manual cleaning.

This method is time-consuming and laborious, has poor cleanliness, obviously not suitable for the modern-scale optical cold processing industry. This forced people to find a mechanized cleaning method instead. So ultrasonic cleaning technology gradually into the optical cold processing industry and show its skills, further promoting the development of the optical cold processing industry.

The basic principle of ultrasonic cleaning technology can be roughly considered to be the use of the huge force generated by the ultrasonic field, in cooperation with the washing medium, to promote a series of physical and chemical changes in substances to achieve the purpose of cleaning.

When the high-frequency vibration higher than the acoustic wave (28 ~ 40khz) is transmitted to the cleaning medium, the liquid medium generates near-vacuum vacuums under the high-frequency vibration. In the process of the collision, merger, and extinction of the vacuums, the local liquid can instantly generate thousands of atmospheres of pressure. Such a large pressure causes a series of physical and chemical changes in the surrounding substances.