Ion Assisted Deposition Optical Coating

Optical Coatings are thin films adhered to the interfaces of optical substrates, the functions of optical coatings are diverse, such as optimizing the optical transmission, eliminating ghost images, maximizing reflectivities, and manipulating the polarization characteristics of light, common types of optical coatings include anti-reflection coatings (AR coatings), high reflective coatings (HR coatings), optical filter coatings, etc. 

There are multiple technologies and processes utilized to manufacture optical coatings; different processes are suitable for different types of optical coating preparation and have unique technical advantages and limitations. Among these optical coating technologies, the Ion Assisted  Deposition Electron Beam Evaporation (IAD E-beam) process, which Hangzhou Shalom EO adopts as a coating technique at our workshop, and which exhibits its unique advantage of a balance between cost, throughput, and higher film densities of the optical coatings will be discussed in as the main topic of this technical article. In this tutorial about Ion Assisted Deposition Electron Beam Evaporation (IAD E-beam) optical coating technology, we will offer a thorough introduction to what is ion-assisted deposition, the advantages, and disadvantages of IAD optical coatings, the actual process and facilities used for IAD coating, and the suitable application contexts for Ion Assisted E-beam optical coatings.

Overview- Different Optical Coating Technologies

Optical coating technologies, in general, can be divided into physical vapor deposition (PVD), and chemical vapor deposition (CVD). 

1. Physical Vapor Deposition (PVD)

Physical Vapor Deposition (PVD)  Coating is a thin-film deposition process that occurs in the vacuum chamber. The PVD coating process deposits material as individual atoms or molecules onto a substrate; therefore, the PVD process produces extremely thin and uniform coatings, often in the nanometer to micrometer range. Physical vapor deposition is characterized by a process in which the material goes from a condensed phase to a vapor phase and then back to a thin film condensed phase. Physical vapor deposition (PVD) coating is the most common optical coating method, including evaporation coating and sputtering coating.

a) Evaporation coating

Evaporation coating vaporizes the target material by heating it and then deposits the gas molecules on the surface of the substrate in a vacuum environment to form a thin film.  According to different heating methods, evaporation coating is divided into thermal evaporation and electron beam evaporation.

Thermal evaporation: Thermal evaporation coating uses resistance to heat the target material to cause it to sublimate or melt into vapor. Commonly used thermal evaporation materials include aluminum, silver, magnesium fluoride, etc. This method is simple and easy, but it has high requirements for temperature control and is prone to introducing impurities due to overheating, affecting the purity of the film.

Electron beam evaporation: Electron beam evaporation uses an electron gun to heat the target material and bombards the surface of the target material with a high-energy electron beam to sublimate it locally. Electron beam evaporation has high heating efficiencies and is suitable for coating high melting point materials (such as metal oxides and fluorides). It is widely used in the preparation of high-quality optical films (such as anti-reflection coatings).

Ion-Assisted Deposition Electron Beam Evaporation: The Ion-Assisted Deposition Electron Beam Evaporation (IAD E-Beam) optical coating technique combines ion-assisted process with the electron beam evaporation process, the ion assisted deposition e-beam technique can be regarded as a modified version of the average electron evaporation technique, in the coating procedure, besides the usage of electron beam, an separate ion source bombards ions at the growing film, leading to denser, more durable, and higher-quality coatings.

Advanced Plasma sputtering (APS): Advanced Plasma Sputtering (APS) is an enhanced thin-film deposition technique that combines elements of Ion-Assisted Deposition (IAD) and e-beam evaporation but replaces the ion beam with a hot cathode DC glow discharge plasma, this ensures uniform distribution of energies, leading to more consistent film properties across large areas. 

b) Sputtering coating

Sputtering coating is an optical coating process involving bombarding the target material with high-energy particles to make the material atoms detach from the target surface and deposit on the substrate. The main sputtering technologies include Ion Beam sputtering, Advanced Plasma Sputtering, and plasma-assisted reactive Magnetron Sputtering.

Ion Beam Sputtering (IBS): Ion Beam Sputtering is a high-precision PVD technique that uses a focused ion beam to bombard a target material, ejecting atoms that then deposit onto a substrate to form a thin film. This method provides exceptional film density, precise control over film thickness, excellent smoothness, and repeatable results, making it ideal for advanced optical coatings.

Plasma-Assisted Reactive Magnetron Sputtering (PARMS): Plasma-Assisted Reactive Magnetron Sputtering (PARMS) is an advanced thin-film deposition technique that combines reactive sputtering with plasma assistance to enhance film quality and deposition control. Just like APS (Advanced Plasma Sputtering), PARMS generates a glow discharge plasma to ionize gas atoms, creating a high-energy environment for sputtering. However, magnets behind the target create a magnetic field, which traps electrons near the target surface, which increases ionization efficiencies.

2. Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition (CVD) is a thin film deposition technique based on chemical reactions, which converts gaseous materials into solid films through chemical reactions. Common CVD technologies include low-pressure CVD (LPCVD) and plasma-enhanced CVD (PECVD).

a) Low-pressure CVD (LPCVD): Chemical vapor deposition performed at low pressure is suitable for the preparation of uniform and dense films. LPCVD is often used to deposit silicon oxide and silicon nitride films and is widely used in optical filters and waveplate films.

b) Plasma-enhanced CVD (PECVD): The reactive gas is excited by plasma and deposited on the substrate at a lower temperature. The PECVD process can achieve the preparation of high-quality film layers at lower temperatures and is suitable for preparing thin films on heat-sensitive substrates, such as optical film layers in display screens.

CVD technology is mainly used to prepare dielectric films in optical coatings, especially high-quality anti-reflection and wave plate films.

Section1. What is Ion Assisted Deposition Electron Beam Evaporation Optical Coating？

The term Ion Assisted Deposition Electron Beam Evaporation describes the combination of two optical coating technologies: electron beam (often abbreviated as e-beam) evaporation and ion-assisted deposition (IAD, often equally called ion beam assisted deposition/IBAD, Ion Assisted Coating/IAC, Ion Beam Enhanced Deposition/IBED).

In the IAD e-beam process, electron-beam evaporation is the core mechanism for depositing material onto a substrate. As explained above in the section, this is what happens during the electron beam evaporation process: A electron gun is used to generate a focused electron beam, which is directed towards the target material placed in a vacuum chamber. The electron beam strikes the target material (such as metals, oxides, or other evaporative substances), causing it to heat up and evaporate. The evaporated material forms a vapor cloud that condenses on the substrate, creating a thin film coating. The vacuum chamber ensures that the evaporated material can travel without interference from air molecules, maintaining the purity of the deposited film.

The ion-assisted deposition optical coating technique refers to the process of adding separate sources of ion beams to enhance the densification and adhesion of films. Ion beams can improve the microstructure of the film, making it denser and improving its mechanical properties.

In contrast with the traditional electron-beam evaporation process, the ion-assisted deposition electron beam evaporation enhances the basic electron-beam evaporation with the introduction of ion bombardment during the deposition process, where an individual ion source is used to generate ions in the vacuum chamber. The generated ions accelerate and collide with the growing optical thin film, providing extra energy to the film atoms. The energies change the microstructure of the thin films and help the film become denser and harder, promote better adhesion to the substrate, and control film stress. Ion beams can also react with vapor-deposited atoms or background reactive gases to form new compounds.

Section2. The Setup for IAD E-Beam Evaporation Optical Coating Process

The basic setup for ion beam-assisted deposition is a wide-beam ion source, which is responsible for generating ions in the existing vacuum coating equipment. The ion optics then focus and accelerate the ion beam with a high voltage or magnetic field into the target material or the evaporation stream deposited on the substrate.

The Ion Assisted Deposition IAD coating process is realized on the basis of the coating setup of the Electron Beam Evaporation, but the Ion-Assisted Deposition has an additional source of plasma, which is guided to the cap-like substrate and bombards the growing film, resulting in a denser microstructure, thus eliminating thermal drift, which is one of the advantages of IAD.

The plasma in the IAD is generated by a DC discharge with an RF-heated cathode. At an intense discharge of about 40A, molecules in the vapor are activated and ionized, and in addition, reactive argon and oxygen ions de-impact the substrate at energies of up to 150 eV, resulting in a relatively high deposition rate.

Section3. What are the Advantages and Disadvantages of Ion-Assisted Deposition Electron Beam Evaporation (IAD E-Beam) Compared to Other Methods?

· Advantages of IAD E-beam Optical Coatings:

Ion-Assisted Beam Deposition (IBAD) eliminates columnar structures and increases the aggregation density, thereby improving the film quality and enhancing the performance, lifespan, and reliability of the optics. 

Ion-assisted deposition facilitates high-quality PVD optical film formation process. The cleaning effect of the ion beam removes physically adsorbed impurities on the substrate and erases contamination caused by trace oil backflow from the diffusion pump system, enhancing the adhesion of the film. Simultaneously, during the deposition process, the momentum transfer from energetic ions to the film particles increases their energy and mobility, disrupting the columnar structure and filling voids, thereby improving the aggregation density of the thin film. This not only improves of the optical coating’s spectral consistencies but also enhances the stabilities of the coating process.

1. Denser Film Formation:

IAD E-beam optical coating process can dramatically improve manifold critical factors, including density, hardness and adhesion, while providing better control of surface texture and microstructure. Although the ion beam just penetrates the top surface atoms of the growing film, the ions released by the ion beam make the film denser due to the more compact arrangement of the microcrystals being formed.

This denser film formation gives IBAD better mechanical durabilities and environmental stabilities against moisture and weathering.

2. Better Control of Film Composition: 

The introduction of ions can help create more precise stoichiometric films, which is essential for compound materials such as oxides, nitrides, or carbides.

High Laser Damage Threshold in the NIR range:

E-beam coatings have the highest laser induced damage threshold (LIDT) in the near infrared (NIR) spectrum out of the listed coating technologies, as well as the widest range of useable optical coating materials, which allows for the highest flexibilities in the coating design space. 

3. Reduced Scattering 

The ion-assisted deposition optical coating process also reduces the scattering of the deposited material, which can be a significant expense for metal coatings such as silver, gold or platinum.

4. Greater Process Control

The angle at which the ion beam bombards the substrate can be used to influence the roughness and texture of the surface of the film for greater bond strengths either to the substrate or the coating.

5. Medium Cost and Manufacturing Cycle

Compared with the average electron beam evaporation process, the IAD e-beam is more expensive and more time-consuming but produces optical coatings with enhanced optical quality and mechanical ruggedness. 

Compared with other optical coating technologies such as ion beam sputtering (IBS) or APS, e-beam IAD excels with higher flexibilities, lower costs and shorter manufacturing cycles when volumes and price are prior concerns. The IAD e-beam coating technique can be viewed as a more average-purpose alternative providing similar (although inferior) optical properties to IBS coatings at cheaper prices.

· Disadvantages of IAD E-beam Optical Coatings

1. Longer Deposition Cycle Than Traditional Electron Evaporation Coating

IAD introduces additional processing steps, such as ion bombardment, which enhances film densities and adhesion. However, this also slows down the deposition rate compared to traditional electron beam evaporation. The added ion energies improve film properties but extend the overall coating cycle, making production less time-efficient.

2. Less Precise Control of Film Thickness Than Ion Beam Sputtering or Magnetron Sputtering

While IAD E-beam deposition improves film densities, its thickness control relies on quartz crystal monitoring or optical monitoring, which may not be as precise as in sputtering techniques. Ion beam sputtering (IBS) and magnetron sputtering offer better uniformity and finer thickness control at the atomic level due to their more controlled deposition mechanisms.

3. Less Repeatable Optical Properties Depending on the Coating Materials Used

The ion-assisted process can lead to variations in film stoichiometry and stress, especially with materials sensitive to ion energies. Unlike IBS, which provides highly consistent refractive indices and absorption properties, IAD coatings can exhibit slight variations in optical performance due to changes in deposition conditions and material interactions with the ion beam.

Section4. What are the suitable application contexts for ion-assisted deposition Electron Beam Evaporation (IAD E-Beam)?

The IAD e-beam optical coating method has its specialties and drawbacks, therefore, it is suitable for some particular optical coating applications while disadvantageous for some optical requirements. Below are examples where the ion-assisted deposition electron beam evaporation optical coating process exhibits benefits:

1. High Laser Induced Damage Threshold Optical Coating:

Laser-Induced Damage Threshold (LIDT) of the optical coating is a measure of the optical coating’s ability to withstand laser power or fluence without being damaged, it is an important parameter for optical coatings since optical coatings with insufficient laser induced damage threshold can lead to catastrophic failures. IAD e-beam deposition can produce optical coatings with consistently higher LIDT compared to other methods, this is due to several factors: first, lower intrinsic coating stress reduces the likelihood of microcracking and defect formation, second, the process maintains high coating purity, third, the fluence resistance of defects (i.e., how well the defects withstand laser energies) can be close to that of the intrinsic coating material.

2. Low-Cost Large-Scale Optical Coating:

E-beam evaporation is a cost-effective method for the large-scale production of optical coatings. The IAD enhancement improves film densities and adhesion without significantly increasing production costs. This makes it ideal for the mass production of optical components where cost efficiencies are critical.

3. Optical Coatings for Ultrafast Pulsed Lasers:

For ultrafast laser applications, coatings with minimal Group Delay Dispersion (GDD) are essential to preserve the pulse profile of femtosecond and picosecond laser pulses. IAD e-beam deposition is particularly effective for producing metal-dielectric coatings (e.g. ultrafast enhanced silver coatings) with low GDD, making it a preferred method for ultrafast optics.

Tags: Optical coating