The concept of film uniformity:
Uniformity in thickness can also be understood as roughness. In terms of optical film scale (i.e. 1 / 10 wavelength as unit, about 100a), the uniformity of vacuum coating has been quite good, and the roughness can be easily controlled within 1 / 10 of visible light wavelength, that is to say, there is no obstacle to the optical property of the film.
However, if it refers to the uniformity of atomic layer scale, that is, to achieve 10A or even 1A surface leveling, it is the main technical content and technical bottleneck in vacuum coating. The specific control factors will be explained in detail according to different coatings.
Uniformity of chemical components
That is to say, in the thin film, the atomic composition of the compound will easily produce non-uniformity due to the small scale. If the coating process of the sitio3 film is not scientific, then the composition of the actual surface is not sitio3, but other proportions. The coating is not the chemical composition of the desired film, which is also the technical content of vacuum coating.
The uniformity of lattice order degree is as follows
This determines that the film is single crystal, polycrystalline and amorphous, which is a hot issue in vacuum coating technology.
For sputtered coatings:
It can be simply understood as bombarding the target with electrons or high-energy laser, sputtering the surface components in the form of atomic clusters or ions, and finally depositing on the surface of the substrate, experiencing the film-forming process, and finally forming the film.
There are many kinds of sputtered coatings. The difference between sputtered coatings and evaporation coatings is that sputtering rate will be the main parameter.
The composition uniformity of PLD is easy to maintain, but the thickness uniformity at atomic scale is relatively poor (because it is pulse sputtering), and the control of crystal growth (outer edge) is not so good. Taking PLD as an example, the main factors are: the lattice matching degree of target and substrate, coating atmosphere (low pressure gas atmosphere), substrate temperature, laser power, pulse frequency, sputtering time.
For different sputtering materials and substrates, the best parameters need to be determined by experiments, which are different. The quality of coating equipment mainly depends on whether the temperature can be accurately controlled, whether the vacuum degree can be guaranteed, and whether the vacuum cavity cleanliness can be guaranteed. MBE molecular beam outer edge coating technology has solved the above problems, but it is basically used for experimental research. Ion evaporation coating and magnetron sputtering coating are mainly used in industrial production.
