There are currently many methods for monitoring thin films in vacuum coating machines, mainly including: visual monitoring, fixed (extreme) value monitoring, crystal oscillation monitoring, and time monitoring. This article will primarily introduce three methods: visual monitoring, fixed (extreme) value monitoring, and crystal oscillation monitoring. Visual monitoring, also known as direct monitoring, uses the eyes to monitor the film. During the film growth process, interference phenomena cause color changes, and we control the film thickness based on these color changes. This method has a certain degree of error and is not very accurate, requiring experience.
Fixed-value (extreme-value) monitoring: This mainly employs reflective (transmissive) optical monitoring. Extreme-value monitoring method: As the film thickness increases, its reflectivity and transmittance change accordingly. When the reflectivity or transmittance reaches an extreme point, the optical thickness ND of the coating can be determined to be an integer multiple of one-quarter of the monitoring wavelength (λ). However, the extreme-value method has a relatively large error because the reflectivity or transmittance changes very slowly near the extreme value; that is, the R/T only changes after a significant increase in the film thickness ND. The most sensitive position is at one-eighth of the wavelength. Fixed-value monitoring method: This method utilizes the fact that the stopping point of coating is not at one-quarter of the monitoring wavelength. Then, the computer calculates the reflectivity (or transmittance) of the total film thickness at wavelength 1, which is the stopping point of coating. Crystal oscillation monitoring: The working principle of crystal oscillation is based on the principle that the vibration frequency of a quartz crystal is inversely proportional to its mass. However, one drawback of quartz monitoring is that when the film thickness increases to a certain level, the vibration frequency is not entirely linearly related to the thickness due to the properties of quartz itself. In this case, a new quartz oscillator must be used.
Several monitoring methods each have their advantages and disadvantages, but for multi-layer coatings, optical monitoring is usually the primary method, supplemented by quartz crystal oscillation. In addition, for some processes requiring gas injection during coating, flow meters or pressure gauges are needed, which require precise valves and photoelectric sensing systems for control. Vacuum coating machines also require a rotation control system, where the umbrella's main shaft is placed inside a bearing, and a motor drives the bearing to rotate the umbrella. The rotation speed is then controlled by a PLC. The crucible rotation is driven by a motor and uses photoelectric induction counting, while the shielding plate rotates using a pneumatic switch. To accelerate the pumping rate and achieve a certain vacuum level, the vacuum chamber needs to be cooled, freezing the air inside to -130 degrees Celsius and freezing and removing any water vapor. The electrical control section mainly adopts PLC automatic control. The pre-designed program is first input into the PLC, and the main circuit of the processor is connected to each no-load system on the operation panel. When the switch on the operation panel is pressed, the information is transmitted to the central processor, and then the central control system analyzes and issues instructions to the branch circuits to execute and complete the action.
Vacuum coating machines are devices integrating multiple disciplines. They incorporate the most advanced electromechanical technologies, control technologies, electrical automation, IT technologies, refrigeration technologies, microcircuit integrated systems, high-voltage control systems, mechanical technologies, processing technologies, optoelectronic technologies, optical technologies, pneumatic control technologies, optoelectronic sensing technologies, communication technologies, vacuum technologies, thin-film optics and coating technologies, and more. Vacuum coating machines can be considered a representative of emerging industries. Today, vacuum coating machines are widely used, especially in the production of thin films. The various thin films produced are used in various optoelectronic systems and optical instruments, such as digital cameras, digital camcorders, telescopes, projectors, energy control, optical communication, display technology, interferometers, artificial satellites and missiles, semiconductor lasers, microelectromechanical systems, the information industry, laser manufacturing, various filters, the lighting industry, sensors, architectural glass, the automotive industry, decorative items, coins, eyeglass lenses, and more. Coating machines have become closely integrated into human life.
