As one of the precision manufacturing fields, the precision of optical thin film preparation often reaches the nanometer or even micro-nano level. Therefore, the complexity of its preparation is self-evident. Sometimes a small mistake will lead to a complete loss, bringing great difficulties to the preparation of the thin film.  

Had the greatest influence on many process factors, and easy to observe the often is the film thickness uniformity, it can severely affect the physical properties of thin film, according to the experience of preparation, film thickness uneven leads to film spectral performance have great migration, for example, in the preparation of the coating process will lead to a low transmittance of thin-film,  Secondly, it will also affect the laser damage threshold of the film, the film stress and so on.  

The preparation of optical films is a complex process, so the film thickness monitoring system with high accuracy is often equipped in the film preparation equipment.  

1. The preparation of optical films

The preparation of optical thin film is a complex process, it is by putting a chunk of solid materials evaporation and sputtering, through gas-phase transmission, finally in the substrate condensation, in the process of its preparation, including vacuum conditions (vacuum), evaporation rate, multifaceted process factors such as substrate temperature, the influence of microstructure and chemical composition of the membrane and ideal condition appear large deviation,  Finally, the mechanical properties, laser damage resistance threshold and optical properties of the film are changed.  Several common optical film preparation methods are described below.  

(1) Physical vapor deposition  

Physical vapor deposition method simply said, under vacuum conditions, the use of physical methods, the material source – solid or liquid surface gasification into gaseous atoms, molecules or partial ionization into ions, and through low-pressure gas (or plasma) process, deposition in the substrate surface has a special function of the film technology.  Up to now, physical vapor deposition technology can deposit not only metal film, an alloy film, but also compound, ceramic, semiconductor, polymer film, and so on.  

The high vacuum environment was chosen because the film material would not react with active gases in the air during the deposition process, and the vapor molecules would not collide with gas molecules in the vacuum environment, but directly reach the substrate.  In the actual process of thin film deposition, there are many process parameters that need to be controlled, usually involving vacuum technology, material science, precision machinery manufacturing, photoelectric technology, computer technology, automatic control technology, and other fields.  

(2) Ion beam assisted deposition  

Ion-beam assisted deposition (ION-beam assisted deposition) is a process in which high-energy particles are used to bombard the surface of the deposited film while the film is deposited by the gas phase, so as to change the composition and structure of the deposited film.  This coating technology, which combines ion-assisted and reactive evaporation methods, can realize low-temperature film formation, improve the microstructure and mechanical properties of the film and improve the adhesion between the film and the matrix, so as to improve the comprehensive properties of the film.  

However, the application of ion beam-assisted evaporation technology is limited due to the uneven energy beam density of ion beam bombarding substrate and the reverse sputtering caused by high energy ions.  Generally, the fastness of soft films such as zinc sulfide and magnesium fluoride is improved obviously by ion-assisted technology, but the effect of laser damage resistance is not obvious either for soft films or oxide hard films vaporized by the electron beam.  

(3) Reactive ion plating method  

Using hot cathode arc source induced membrane material in coating indoor ion discharge plasma formed, evaporation coating materials by ionization ion part, in the floating potential of a workpiece formed under the action of the electric field in the substrate, with a range of kinetic energy of ion state of membrane material particle deposition after combining with the reaction gas into a membrane, the membrane layer and glass substrate adhesion strong,  The hardness and friction resistance of thin films have been improved significantly, so scientists in the field of optical thin films have paid more attention to it.  However, the equipment cost of this technology is high and its potential for improving laser damage resistance needs further study.  

(4) Gas-phase mixed evaporation method  

The gas-phase mixed evaporation method is a process in which two electric guns are used to evaporate two different materials at the same time, and two quartz probes are used to monitor the deposition rate of each gun respectively.  This kind of optical film can be used as a single-layer antireflective film for some substrate materials, instead of dozens of multilayers originally coated on the substrate, so as to improve the microstructure of the film, increase the strength of the film, and make it possible to prepare refractive index gradient change.  

In this technique, the abrupt interface between the film and air (or matrix) obtained by the conventional method is eliminated, and the abrupt interface is replaced by the gradual interface, which has stronger adhesion and less interface absorption. In addition, the thermal conductivity of the gradual interface is higher than that of the ordinary film interface.  This kind of nonuniform film has become an important branch of thin-film optics.

It breaks the traditional design method of thin-film system, and thus obtains excellent spectral properties which cannot be prepared by the traditional film system. Moreover, it is expected to improve the damage resistance of thin-film elements greatly (about 20%), so it arouses great interest.  

(5) Sol-gel method  

The sol-gel method is a metal alkoxide, or other metal inorganic salt solution as the precursor solution, hydrolysis, and polymerization of solution at low temperature by the chemical reaction, such as the first generation sol, and then generate a certain space structure of the gel, and then after heat treatment or decompression drying, at lower temperatures were all kinds of methods of inorganic material film or composite film.  

This technique can be used to prepare various optical films such as high reflection film, antireflection film, optical fiber, refractive index gradient material, organic dye-doped nonlinear optical material, waveguide grating, and rare earth luminescent material, etc.  With the further development of the research, it is expected that new nanomaterials which are difficult to be prepared by conventional methods can be prepared by the combination of the self-propagating method and self-propagating method.  

2. Conclusion  

The above are five methods of making optical films.  In the course of the development of optical thin film, various advanced film preparation technology has been applied to the technology of optical thin film preparation.  These technologies not only greatly broaden the range of materials that can be used for optical thin films, but also greatly improve the performance and function of optical thin films, thus providing a broader and far-reaching development space for optical thin films.