Traditionally, optical coatings are produced by evaporation technology.The technical possibilities of evaporation were sufficient for decades to meet the requirements of optical films. However, the evaporation technique has proved to be problematic in the production of high-precision optical coatings. The energy of the vapor molecules is relatively low (<1 eV), which is why the deposited layers are generally porous and have a columnar microstructure. Such layers exhibit low stability against environmental influences and are therefore prone to spectral shifts.
The film quality and thus the stability against environmental influences can be improved by additional ion sources. Nevertheless, the intrinsic properties of vaporizing technology remain as a challenge, such as varying deposition rates, vapor plume and ion distribution, process and residual gas pressure, substrate temperature, …
Sputtering processes are stable and highly predictable. They show less process influences and therefore they are much better controllable and reproducible. In addition, a variety of sputtering methods offers advantages for special film properties. Sputtered particles show an energy of 20-30 eV which is much higher than evaporated molecules. Therefore, sputtered films show amorphous structure, high density, low defect and mechanical stability resulting in shift-free environmental resistance. Minimal optical losses are common to all sputtering processes.
Solayer offers a wide range of sputtering methods including RF magnetrons, unipolar pulsed DC magnetrons, dual magnetrons for MF AC and bipolar pulsed DC, HIPIMS magnetrons and advanced reactive assist sputtering.
PULSED DC SPUTTERING TECHNOLOGY
Unipolar pulsed DC sputtering processes provide excellent results when dielectric films are deposited with single magnetron reactive sputtering configurations. Conventional reactive DC sputtering process suffer from a re-deposition area on the target material – a so called ‘poisoned zone’. This poisoned zone consists of dielectric material and charges up during the process until it comes to an electrical breakdown in the form of an arc. Each arc interrupts the reactive process, interferes with the process control and leads to the generation of particles.In addition, the structure, the properties and the composition of the coating are adversely affected. The avoidance of arcs is therefore an essential necessity for the production of high-quality dielectric films.
In the case of unipolar pulsed DC sputtering, the formation of arcs is prevented by periodic reversal of the voltage. By reversal of voltage, the resulting charge up is discharged by showering the cathode with electrons. Using reverse voltage magnitude and pulse duty cycle for process tuning, arc formation will be suppressed and higher deposition rates are achieved.
HIPIMS is a special type of magnetron sputtering characterized by the use of high instantaneous power pulses.These pulses produce extreme power and current densities that ignite a high-density plasma in front of the cathode. As a result, significant ionization of the sputtered target material is achieved, which leads to self-sputtering processes and improved film properties.
Depending on the target material, these pulses can reach peak power densities of up to 1000 W/cm² at a pulse duration of up to several milliseconds. By use of pulse duration, pulse frequency, duty cycle and peak power density, the process can be tuned to achieve films with superior film characteristics. Also arcing can be suppressed with the right equipment setup and process parameters.
Optical films produced by HIPIMS show an improved adhesion, dense microstructure characterized by low surface roughness values and high refractive index. Also improved mechanical properties such as scratch resistance, low residual stress can be achieved.
DUAL ROTATABLE MAGNETRON TECHNOLOGY
MF dual-magnetron AC sputtering has been used since the 1990’s in the architectural glass industries. This process setup has several advantages compared to DC reactive sputtering that suffers from “disappearing anodes” and target poisoning as well as the resulting low rates.
A dual magnetron setup consists of two magnetrons that operated with a single MF power supply. With each half wave of the AC current, the two magnetrons change from cathode to anode and vice versa. This more or less eliminates the disadvantages of DC reactive sputtering. Due to the constant change, there is less target poisoning and the anode disappears not any longer. The process runs much more stable with higher rates and better layer properties.However, reactive MF dual magnetron sputtering still requests a fast and effective arc suppression of the power supply.
Today, the use of dual-rotary-magnetron configurations is more less the industry standard. The advantages of rotary magnetrons are outstanding what leads to a greatly reduced arc tendency. Moreover, rotary targets have more than twice as much material utilization and very much larger material reservoir. The better material utilization and longer service life lead to significantly reduced costs per piece.
Co-sputtering means simultaneous coating with two different materials. Due to the mixing ratio of the two materials, compounds can be produced in any composition. With regard to optical layers, this means that it is possible to produce any refractive index that is between the refractive indices of the two pure materials (high, low). This option offers a higher degree of freedom during the design of optical stacks, since it is simple to produce layer stacks with three and more refractive indices.
REACTIVE SPUTTERING TECHNOLOGY
Advanced reactive sputtering is a logical continuation of the above mentioned advantages of dual-rotary magnetron arrangements without the still remaining challenges of optical coatings. It is not that they can not be mastered, but the advanced reactive assist sputtering process brings the production to the next level of reliability and predictability. By separation of the material deposition from the oxidation of the film, all the challenges arising from reactive sputtering are eliminated. Due to the complete absence of reactive gas in the sputter compartment, the coating process behaves like a purely metallic process with deposition rates comparable to evaporation- even if sub-stoichiometric composite targets are used.The stoichiometric oxidation of the film takes place in a separate plasma compartment.
Two things are necessary to bring the advanced reactive assist sputtering process to perfection. A high gas separation factor to keep control of the reactive gas in the plasma oxidation compartment and very high rotation speed to allow very thin perfect oxidized subsequent films.
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