The full text is divided into five parts.
1) Category of film thickness monitoring methods.
2) Single-wavelength optical film thickness control principle.
iii) The origin of the single-wavelength proportional method.
4) Example of photo-controlled thickness compensation.
v) Summary.
a).Classification of film thickness monitoring methods
As a key technology in thin film preparation engineering, film thickness monitoring technology has always been the top priority of engineers. The film thickness monitoring in the coating process requires a non-contact, non-destructive, real-time measurement of the thickness of the optical film, and a timely control signal when the required thickness is reached.
Film thickness is generally characterized in two ways, namely physical thickness and optical thickness. The corresponding common film thickness monitoring methods are quartz crystal oscillator method (crystal control) and optical film thickness monitoring method (light control).
The advantages of light control are:
Optical thickness can be measured directly;
It reflects additional information related to the refractive index and absorption of the film;
It is possible to automatically compensate for the film thickness error to a certain extent.
In contrast, Crystal Control can only measure the physical thickness of the film layer, and cannot directly reflect the optical thickness. Therefore, in the current high-precision optical coating applications, crystal control is commonly used to monitor the film formation rate, and light control is used to control the film thickness.
Light control technology, also known as photoelectric detection method, can be divided into extreme value method, differential method, according to the specific thickness judgment method