E-Beam Evaporation vs Magnetron Sputtering for Optical Coatings

Optical coating, as a core technology for enhancing the performance of optical components, is widely used in laser equipment, imaging systems, photovoltaic devices and other fields. Its quality directly determines key indicators of optical systems such as transmittance, reflectance and environmental stability. Electron Beam Evaporation (E-Beam Evaporation) and Magnetron Sputtering (magnetron Sputtering) are two mainstream physical vapor deposition (PVD) technologies at present, and there are significant differences between them in coating principles, performance and application scenarios. This article will start from the essence of technology, systematically compare the core advantages and limitations of the two technologies, and provide a scientific basis for the process selection of optical coating.

Both techniques achieve the migration and deposition of target material atoms/molecules in a vacuum environment, but the differences in energy excitation and transfer mechanisms lay the foundation for their subsequent performance differences.

Electron beam evaporation technology uses high-energy electron beams as energy carriers. Electrons generated by an electron gun are accelerated by high voltage and then precisely bombshell the surface of the target material placed in a water-cooled crucible under the focusing effect of a magnetic field. The kinetic energy of electrons is converted into thermal energy, causing the target material to form a high-temperature molten or evaporated state locally. After the gaseous target material atoms/molecules are detached from the surface of the target material, they move randomly in the vacuum chamber and eventually deposit on the surface of the pre-treated optical substrate, forming a uniform film. Throughout the entire process, water-cooled crucibles can effectively prevent chemical reactions between the target material and the crucible, reducing impurity contamination. This feature gives them an advantage in the preparation of high-purity films.

Magnetron sputtering is based on the principles of gas discharge and ion bombardment. Inert gas (usually argon) is introduced into a vacuum chamber and excited by a radio frequency or direct current electric field to form plasma. Under the action of an electric field, argon ions in the plasma accelerate and bombshell the surface of the target material, enabling the atoms of the target material to obtain sufficient energy to break free from the lattice constraints (i.e., the "sputtering" process). To enhance the sputtering efficiency, the device sets up a magnetic field behind the target material. Through the confinement effect of the magnetic field on electrons, the movement path of electrons in the plasma is prolonged, increasing the collision probability with argon molecules, thereby improving the plasma density and sputtering rate. According to the different types of target materials, it can be divided into DC magnetron sputtering (suitable for conductor targets) and RF magnetron sputtering (suitable for insulating targets).

Optical coating has strict requirements for the purity, uniformity, density and stress state of the film. The performance differences of the two technologies in these core indicators directly determine their application scope.

The purity of the film is a key factor affecting optical performance. Excessive impurity content will lead to increased light absorption and decreased transmittance. Electron beam evaporation ensures high purity through three points: First, the energy of the electron beam is concentrated on the surface of the target material, and the crucible only receives a small amount of heat through radiation, avoiding the melting and adhesion of the target material to the crucible. Second, it has a higher vacuum degree (usually reaching the level of 10^-6 Pa), reducing the contamination of evaporated particles by gas molecules. Thirdly, it can achieve precise evaporation of a single target material, avoiding cross-contamination of multiple target materials. Experimental data show that the impurity content of the SiO₂ anti-reflection film prepared by electron beam evaporation is less than 50ppm, while the impurity content of the magnetron sputtering process is usually 100-200 PPM due to the residual gas ions in the plasma.

The purity deficiency of magnetron sputtering mainly stems from the plasma environment. Argon ions may embed into the film lattice, and the oxide layer on the target surface will be mixed into the film during the sputtering process. Although it can be improved by increasing the vacuum degree and using pre-sputtering of target materials, for optical films with ultra-high purity requirements (such as laser resonant cavity lens coating), it is still difficult to match electron beam evaporation.

The uniformity of the film directly affects the surface shape accuracy of optical components, especially when coating large-sized substrates, it is even more crucial. Electron beam evaporation can achieve a film thickness deviation of less than ±1% on a 300mm diameter substrate by rotating the substrate stage and optimizing the electron beam scanning path. However, due to the "point source" characteristics of the evaporation source, thickness attenuation is prone to occur at the edge of the substrate. Magnetron sputtering performs better on large-sized substrates (such as 600mm*600mm photovoltaic glass) due to the "surface source" sputtering characteristics of the target material. The thickness uniformity can be controlled within ±2%, and the thickness distribution of the film layer is closer to a rectangle, with a weaker edge effect.

In terms of density, magnetron sputtering has an advantage. Sputtered particles have higher kinetic energy (typically 10 to 100 times that of electron beam evaporated particles), and when deposited on the substrate surface, they can produce stronger adsorption and diffusion effects, forming a more closely arranged film lattice with a density of over 98%. This compactness enhances the film's wear resistance and moisture and heat resistance. For instance, the TiO₂ high-reflection film prepared by magnetron sputtering has a reflectance attenuation of less than 0.5% after being placed at 85 °C /85%RH for 1000 hours. The density of the film evaporated by electron beam is usually between 90% and 95%, and subsequent annealing treatment is needed to improve its performance, but this may lead to changes in the stress of the film.

The process efficiency is mainly reflected in the deposition rate and production capacity. The deposition rate of electron beam evaporation varies greatly with the type of target material. For metal targets (such as aluminum and silver), it can reach 50nm/s, while for oxide targets (such as SiO₂ and TiO₂), it is only 1-5nm/s. Moreover, the amount of target loaded at one time is limited, and frequent shutdowns for target changes are required. It is suitable for small-batch and high-precision production. The deposition rate of magnetron sputtering is more stable. The deposition rate of metal targets can reach 20nm/s, and that of oxide targets through reactive sputtering can reach 3-8nm/s. It also supports simultaneous sputtering of multiple targets, enabling continuous deposition of multi-layer films. The single-batch production capacity is 3-5 times that of electron beam evaporation.

In terms of cost, the initial investment in electron beam evaporation equipment is relatively high (about 1.5 to 2 times that of magnetron sputtering equipment of the same specification), and the maintenance cost of the electron gun is high, with the filament and cathode needing to be replaced every 1,000 hours. The structure of magnetron sputtering equipment is relatively simple, and the target material utilization rate can reach 70-80% (while that of electron beam evaporation is only 50-60%). The long-term operating cost is lower, making it more suitable for large-scale industrial production.

Based on the above performance differences, the two technologies have formed clear application divisions in the field of optical coating, respectively corresponding to different performance requirements and production scales.

In fields where extremely high purity and optical accuracy of thin films are required, electron beam evaporation is an irreplaceable choice. For instance, in high-power laser lenses used in laser nuclear fusion devices, it is necessary to prepare anti-reflection films with extremely low loss. The SiO₂/Ta₂O₅ multilayer films prepared by electron beam evaporation can have a light absorption coefficient lower than 10^-6, which is far superior to that of magnetron sputtering products. In the infrared detection systems of the aerospace field, the Ge-based anti-reflection films prepared by electron beam evaporation can effectively enhance the infrared transmittance and maintain stable performance at extreme temperatures (-60°C to 120°C).

In addition, electron beam evaporation has obvious advantages in the preparation of precious metal films. Au reflective films used in high-end optical instruments can achieve a mirror reflectivity of up to 99.5% by electron beam evaporation process, with good uniformity of the film layer and no pinhole defects. In contrast, Au films prepared by magnetron sputtering are prone to surface roughness due to residual argon ions.

In large-scale production fields such as photovoltaics, display panels, and automotive glass, magnetron sputtering dominates with its advantages in efficiency and cost. In the production of photovoltaic solar cells, the ITO transparent conductive film prepared by magnetron sputtering can control the block resistance within 10Ω/sq, with a transmittance of over 90%, and the daily production capacity of a single production line can reach 100,000 pieces. In the coating of automotive windshields, the heat insulation film prepared by magnetron sputtering can effectively block over 90% of infrared radiation, and the film layer has strong adhesion. It has not peeled off after 2,000 friction tests.

In the display field, magnetron sputtering is the core technology for electrode coating of OLED devices. The Ag alloy conductive film prepared by it not only ensures high conductivity but also has good flexibility, which can meet the bending requirements of foldable screens. In addition, the reactive sputtering technology of magnetron sputtering can directly prepare oxide films without subsequent oxidation treatment, simplifying the process flow and being suitable for the mass production of consumer electronic optical components such as mobile phone camera lenses.

With the development of optical technology, a single coating technology has become difficult to meet the complex performance requirements, and the integrated application of the two technologies has become a new trend. For instance, in the coating of high-end camera lenses, a composite process of "electron beam evaporation + magnetron sputtering" is adopted: electron beam evaporation is used to prepare the core high-purity optical film layer, and magnetron sputtering is used to prepare the surface wear-resistant protective layer. This not only ensures the optical performance but also enhances the mechanical strength of the film layer.

In addition, both technologies are constantly being upgraded. Electron beam evaporation achieves precise control of deposition rate by introducing a pulsed electron gun. Magnetron sputtering has developed high-power pulsed magnetron sputtering (HiPIMS) technology, which can significantly enhance the kinetic energy of sputtered particles and prepare films with purity and density close to those of electron beam evaporation. These technological innovations are narrowing the performance gap between the two processes and providing more options for optical coating.

Electron beam evaporation and magnetron sputtering are not in a competitive relationship but rather complementary technologies tailored to different optical coating requirements. Electron beam evaporation, with its advantages of high purity and high precision, is suitable for the small-batch production of high-end precision optical components and special functional films, and is especially irreplaceable in high-end fields such as lasers and aerospace. Magnetron sputtering, with its high production capacity and low cost, has become the preferred choice in large-scale industrial fields such as photovoltaic and consumer electronics.

In the actual process selection, three core factors - optical performance requirements, production scale, and cost budget - need to be comprehensively considered. For high-end applications that prioritize precision, electron beam evaporation should be given priority. For large-scale production where cost and efficiency are prioritized, magnetron sputtering is more suitable. For complex performance requirements, a combined process of the two technologies can be adopted. In the future, with the continuous innovation of coating technology, the integrated application of the two processes will further expand the performance boundaries of optical coating and provide stronger impetus for the development of the optical industry.