Optical thin film and vacuum coating technology
Optical thin film and vacuum coating technology: Optical thin film vacuum coating technology generally adopts physical vapor deposition (PVD) technology. PVD includes thermal evaporation, sputtering, ion plating and other methods.
1. Thermal evaporation
Thermal Evaporation: When an evaporating material is heated in a vacuum chamber, its atoms or molecules escape from the surface.
(1) Saturated vapor pressure
At a certain temperature, the pressure exhibited by the vapor of the vaporized material in the vacuum chamber in the process of equilibrium with the solid or liquid is called the saturated vapor pressure Pv at this temperature.
lgPv=A-B/T The relationship between vapor pressure and temperature of evaporation material.
A=C/2.3 B=ΔH/2.3R A and B values can be determined experimentally.
ΔH=19.12B (Joule/Mole)
According to the saturated vapor pressure curves of various elements, it can be known that:
(1) The temperature required to achieve the normal film evaporation rate, that is, the temperature when the saturated vapor pressure is 1Pa;
(2) The sensitivity of evaporation rate to changes in temperature;
(3) Evaporation form, if the evaporation temperature is higher than the melting point, the evaporation state is molten, otherwise it is sublimated.
A homogeneous mixture of two or more substances, when evaporated, obeys the following laws:
(1) The law of partial pressure:
The total vapor pressure PT of the mixture is equal to the sum of the partial vapor pressures of each component, that is,
PT=P1+P2+…+Pi
(2) Ural's law:
When a certain component i exists alone, let its saturated vapor pressure at temperature be Pit, if the component occupies a molar fraction of Ni in the mixture, and the saturated vapor pressure of component i in the mixture state is Pi, then Pi=Ni× Pit
Velocity and energy of evaporating particles
Speed √ύ2=√(3kt/m)=√(3RT/M)
Energy Ě=3/2kT
When the evaporation temperature is in the range of 1000～2500℃, the average velocity of evaporating particles is about 105cm.s-1, and the corresponding average kinetic energy is about 0.1~0.2eV, that is, 1.6×10-20～3.2×10-20J
(2) Resistance heating evaporation
a. Selection of evaporation source material
(1) The melting point and vapor pressure of the evaporation source material;
A material with a high melting point is used as the heater, and at the same time, the amount of the evaporation source material entering the film as an impurity must be considered.
Evaporation source material Melting point (℃) Equilibrium temperature Vapor pressure (10-8) Torr10-5Torr10-2Torr (evaporation temperature)
Graphite C3700180021262680
Tungsten W3410211725673227
Tantalum Ta2996195724073057
Molybdenum Mo2617159219572527
Niobium Nb2468176221272657
Platinum Pt1772129216121907
(2) The reaction between the evaporation source material and the film material;
CeO2 reacts with Mo, Ta, W, and Pt is used as the evaporation boat;
Ge uses graphite crucible, or Ta boat lined with graphite paper;
W, Mo will also react with H2O or oxygen;
Some metals will form alloys with the evaporation source, and once alloyed, they will burn easily.
Such as: Ta and Au, Al and W, Ni and W, etc. form alloys at high temperatures
(3) Wetting characteristics of evaporation source material and thin film material.
Evaporation boat with tungsten wire generally needs to be a wet thin film material.
Here are some resistance evaporation boats:
A. Filamentous evaporation source
The wire diameter is generally 0.5 to 1.0 mm, with multiple strands (three strands).
Spiral wire evaporation source is often used to evaporate metals such as aluminum and nickel;
The Cone Blue Evaporation Source is used to evaporate bulk or filamentous sublimation materials and materials that are not easily wetted by the evaporation source.
B, foil evaporation source
The thickness of the evaporation source is usually 0.05-0.15mm, and the evaporation area is large. Pay attention to make good thermal contact between the coating material and the evaporation source, otherwise, the local heating will not only decompose the material, but also cause the film material to spray.
C, radiation evaporation source
The material is heated by the radiant heat of the tungsten wire, so that some materials with a low melting point are evaporated.
D. Evaporation source of chimney
Similar to radiation sources, evaporation is stable
E, flash evaporation
The alloy or compound is continuously sprinkled on the evaporation source, causing explosive rapid evaporation and preventing fractionation.
F. Graphite evaporation source
Used to evaporate germanium, silver and tantalum etc.
Spectrally pure, smooth inner surface,
Before use, it is treated with acid and alkali, and then impurities such as sulfur and phosphorus are removed in vacuum at about 2000 °C.
(3) E-beam heating evaporation
Principle: When the metal is in a high temperature state, a part of the electrons inside it escapes from the surface. After the electrons are accelerated by high pressure, they converge on the surface of the coating material, so that the kinetic energy becomes heat energy and the material evaporates. For details, see the introduction to the working principle of the electron gun ion source.
(4) Laser evaporation
A high-energy laser is used as the heat source to evaporate the thin film. The high-energy laser heats the evaporation material through the vacuum chamber window, and the laser beam power density can be increased to more than 106W/cm2 through the convergence.
Advantages: High melting point materials can be evaporated; non-contact heating is adopted, and the heat source is located outside the vacuum chamber, which reduces pollution. It is very suitable for the preparation of pure thin films in ultra-high vacuum, and obtains a high evaporation rate. It is suitable for the preparation of laser thin films.
Disadvantages: high cost; cannot show its superiority for some materials.
(5) Reaction evaporation
The metal or low-valent compound is evaporated in a certain reaction atmosphere, so that it undergoes a chemical reaction during the deposition process to generate the desired high-valent compound film.
Reactive evaporation is not only used for thermal evaporation with severe decomposition, but also for materials whose vapor pressure is too low to be evaporated with resistance heating.
The degree of reactivity of reactive evaporation depends on the chemical properties of the reactive materials, the stability of the reactive gas, the free energy of forming compounds, and the decomposition temperature and substrate temperature of the compounds.
2. Sputtering
(1) Basic principles
The ion impact on the target transfers some of its momentum to the target atoms. If the kinetic energy gained by the atom is greater than the heat of sublimation, it can be ejected out of the lattice.
(2) Sputtering threshold and sputtering rate
The sputtering threshold is the minimum energy required by the incident ions to sputter the cathode target. Depending on the target material, it decreases as the atomic number increases.
The sputtering rate represents the average number of atoms that each positive ion can eject from the cathode when the positive ions hit the cathode. It is related to the type, energy, angle of the incident particles, the type of target, lattice structure, surface state, sublimation heat and other factors, and the single crystal material is also related to the surface orientation.
(3) Speed and energy of sputtered particles
When bombarded with He+, the velocity of most sputtered atoms is 4×105cm/s, and the average kinetic energy is 4.5eV;
When bombarded with Ar+, the average velocity of most metal atoms is 3-6×105cm/s, and the energy of the particles increases linearly with the increase of the mass of the target material factor.
(4) High frequency sputtering RF
The high frequency alternating current causes the target to be bombarded alternately with ions and electrons.
It can be used to sputter insulating dielectric materials, a capacitor can be connected in series, and metals can be sputtered.
(5) Magnetron sputtering
The use of the orthogonal electromagnetic field makes the electrons move from a straight line to a cycloid motion in the orthogonal electromagnetic field, which greatly increases the probability of collision with gas molecules and causes a significant change in the ionization rate.
Advantage:
(1) High sputtering rate can be obtained;
(2) When sputtering metal, secondary electron bombardment can be avoided and the substrate can be kept close to a cold state, which is of great significance to single crystal and plastic substrates;
(3) It can work with dc and rf discharge, and can prepare dielectric and metal films.
Shortcoming:
(1) Low temperature and high speed sputtering of ferromagnetic materials cannot be achieved;
(2) The insulating target will increase the temperature of the substrate;
(3) The utilization rate of the target is low (30%).
(6) Reactive sputtering
A necessary condition for a reaction between reactants is that the reactant molecules must have sufficient energy to overcome the intermolecular barrier.
The reaction process basically occurs on the substrate surface and the target surface during sputtering.
At this time, the target surface is undergoing two processes of sputtering and reaction to generate compounds at the same time. If the sputtering speed is greater than the compound production speed, the target is in the state of metal sputtering;
As the reactive gas pressure increases and the metal sputtering rate decreases, the target may stop sputtering as the rate of compound formation exceeds the rate of sputter removal.
Reactive sputtering technology can easily prepare metal oxide films such as Ti, Ta, Zn and Sn.
3. Ion plating
A new process developed by combining two technologies of vacuum thermal evaporation and sputtering.
DC ion plating: The thin film material is evaporated by resistance heating, and a DC electric field is applied between the evaporation source and the substrate, and the substrate is at a negative potential (1-5kV). When the vacuum chamber is pumped to 10-3 ~ 10-4Pa, Ar and other inert gases are filled to 1Pa (reactive gas is charged at the same time for reactive ion plating). Then a glow discharge is established between the substrate and the evaporation source to ionize the inert gas, and the positive ions generated by the ionization are accelerated to the substrate under the action of the electric field. The molecules or atoms of the evaporated material are also ionized as they pass through the plasma, gaining acceleration energy in the electric field. Most of the ions will become neutral particles due to collision, but they have very high energy, generally 1-100 eV depending on the applied voltage. The high-energy particles are incident on the surface of the substrate, on the one hand, the substrate is heated. If the voltage is 4KV and the current density is 0.5mA/cm2, the substrate temperature can reach about 300 ℃ after 15 minutes; on the other hand, the deposited film layer is sputtered . In order to ensure a certain deposition rate, the energy and evaporation rate of incident particles must be controlled so that the deposition rate is greater than the sputtering rate.
High-frequency ion plating: A high-frequency coil is installed between the substrate of the DC method and the evaporation source. Since the high-frequency electric field increases the electron movement path and the ionization rate, the discharge can be maintained and the ionization rate can be increased at a higher vacuum degree (10-1 ~ 10-2Pa) and a lower discharge voltage.
Agglomerated ion beam method: The crucible with small holes heats the evaporation material. Due to the high internal pressure of the crucible, the vapor gathers into clusters and sprays out of the small holes, ionizes in another ionization chamber, and accelerates to the substrate.
Advantages of ion plating:
(1) The film layer has strong adhesion;
The three functions of high-energy particle bombardment: clean the substrate and generate high temperature; make the molecules or atoms with poor adhesion re-sputter and leave the substrate; promote the surface diffusion and chemical reaction of the film material, and even produce the injection effect, thus attaching The focus is greatly enhanced.
(2) High film density
The high-energy particles not only have high surface mobility, but also re-sputtering overcomes the shadowing effect during deposition, so the density of the film is close to that of the bulk material.
(3) The uniformity of the film layer is good
Thin films can be deposited both in front of and behind the substrate. The charged ions move in the direction of the electric force line, and the film can be deposited wherever the electric force line touches; the higher working pressure causes the vaporized particles to produce gas-phase scattering, and the film thickness percentage on the rear/front surface increases with the increase of the discharge pressure and the evaporation rate decreases. . Parts of complex shapes can be plated.
(4) The film deposition rate is fast
The main uses of ion plating at present:
Manufacture of high-hardness machine tools and wear-resistant solid lubricating films, and durable decorative films on metal and plastic products.
It is also used to prepare high-strength optical films.
Low-pressure reactive ion plating has been able to deposit low-loss optical films.
