Principle of vacuum evaporation coating
1、Equipment and physical process of vacuum evaporation coating
The vacuum evaporation coating equipment is mainly composed of vacuum chamber and evacuation system. Inside the vacuum chamber, there are evaporation source (i.e. evaporation heater), substrate and substrate frame, substrate heater, exhaust system, etc.
The coating material is placed in the evaporation source of the vacuum chamber, and under high vacuum conditions, it is heated by the evaporation source to evaporate. When the average free range of the vapor molecules is larger than the linear size of the vacuum chamber,after the atoms and molecules of the film vapor escaped from the surface of the evaporation source, are rarely impeded by the collision of other molecules or atoms, and directly reach the surface of the substrate to be coated. Due to the low temperature of the substrate, the film vapor particles condense on it and form a film.
In order to improve the adhesion of evaporation molecules and substrate, the substrate can be activated by proper heating or ion cleaning. Vacuum evaporation coating goes through the following physical processes from material evaporation, transportation to deposition into a film.
(1)Using various ways to convert other forms of energy into thermal energy, the film material is heated to evaporate or sublimate into gaseous particles (atoms, molecules or atomic clusters) with a certain amount of energy (0.1 to 0.3 eV).
(2)Gaseous particles leave the surface of the film and are transported to the surface of the substrate at a certain speed of motion, essentially without collision, in a straight line.
(3)The gaseous particles reaching the surface of the substrate coalesce and nucleate, and then grow into a solid-phase film.
(4)Reorganization or chemical bonding of the atoms that make up the film.
2、Evaporation heating
(1) Resistance heating evaporation
Resistance heating evaporation is the simplest and most commonly used heating method, generally applicable to coating materials with melting point below 1500℃, high melting point metals in wire or sheet shape (W, Mo, Ti, Ta, boron nitride, etc.) are usually made into a suitable shape of evaporation source, loaded with evaporation materials, through the Joule heat of electric current to melt, evaporate or sublimate the plating material, the shape of evaporation source mainly includes multi-strand spiral, U-shaped, sine wave, thin plate, boat, cone basket, etc. At the same time, the method requires the evaporation source material to have high melting point, low saturation vapor pressure, stable chemical properties, not have chemical reaction with the coating material at high temperature, good heat resistance, small change in power density, etc. It adopts high current through the evaporation source to make it heat up and evaporate the film material by direct heating, or put the film material into the crucible made of graphite and certain high temperature resistant metal oxides (such as A202, B0) and other materials for indirect heating to evaporate.
Resistance heating evaporation coating has limitations: refractory metals have low vapor pressure, which is difficult to make thin film; some elements are easy to form an alloy with the heating wire; it is not easy to get a uniform composition of the alloy film. Because of the simple structure, low price and easy operation of resistance heating evaporation method, it is a very common application of evaporation method.
(2) Electron beam heating evaporation
Electron beam evaporation is a method of evaporating the coating material by bombarding it with a high-energy density electron beam by placing it in a water-cooled copper crucible. The evaporation source consists of an electron emission source, an electron acceleration power source, a crucible (usually a copper crucible) , a magnetic field coil, and a cooling water set, etc. In this device, the heated material is placed in a water-cooled crucible, and the electron beam bombards only a very small portion of the material, while most of the remaining material remains at a very low temperature under the cooling effect of the crucible, which can be regarded as the bombarded portion of the crucible. Thus, the method of electron beam heating for evaporation could avoid contamination between the coating material and the evaporation source material.
The structure of the electron beam evaporation source can be divided into three types: straight guns (Boules guns) , ring guns (electrically deflected) and e-guns (magnetically deflected) . One or more crucibles can be placed in an evaporation facility, which can evaporate and deposit many different substances simultaneously or separately.
Electron beam evaporation sources have the following advantages.
①The high beam density of the electron beam bombardment evaporation source can obtain a far greater energy density than the resistance heating source, which can evaporate high melting point materials, such as W, Mo, Al2O3, etc..
②The coating material is placed in a water-cooled copper crucible, which can avoid the evaporation of the evaporation source material, and the reaction between them.
③Heat can be added directly to the surface of the coating material, which makes the thermal efficiency high and the loss of heat conduction and heat radiation low.
The disadvantage of the electron beam heating evaporation method is that the primary electrons from the electron gun and the secondary electrons from the surface of the coating material will ionize the evaporating atoms and residual gas molecules, which will affects the quality of the film sometimes.
(3) High frequency induction heating evaporation
High-frequency induction heating evaporation is to place the crucible with coating material in the center of the high-frequency spiral coil, so that the coating material generates strong eddy current and hysteresis effect under the induction of high-frequency electromagnetic field, which causes the film layer to heat up until it vaporizes and evaporates. The evaporation source generally consists of a water-cooled high-frequency coil and a graphite or ceramic (magnesium oxide, aluminum oxide, boron oxide, etc.) crucible. The high frequency power supply uses a frequency of ten thousand to several hundred thousand Hz, the input power is several to several hundred kilowatts, the smaller the volume of the membrane material, the higher the induction frequency. Induction coil frequency is usually made of water-cooled copper tube.
The disadvantage of high-frequency induction heating evaporation method is that it is not easy to fine adjust the input power, it has the following advantages.
①High evaporation rate
②The temperature of the evaporation source is uniform and stable, so it is not easy to produce the phenomenon of coating droplets splash, and it can also avoid the phenomenon of pinholes on the deposited film.
③The evaporation source is loaded once, and the temperature is relatively easy and simple to control.
Post time: Oct-28-2022