Detailed Introduction to Vacuum Evaporation Plating

Vacuum evaporation coating is achieved through a vacuum pump. This method involves vacuuming the vacuum chamber containing the substrate through a vacuum pump, heating the plating material with a gas pressure below 10-2Pa, causing its atoms or molecules to vaporize and escape from the surface to form a vapor stream, which is incident on the surface of the substrate and condenses to form a solid thin film.

1. Principle of Vacuum Evaporation Plating

(1) The evaporation characteristics of membrane materials under vacuum conditions. The mass of material evaporated per unit area per unit time is called the evaporation rate. The ideal maximum rate Gm (in kg/(m · s)): Gm=4.38 × 10-3Ps (Ar/T) 1/2, where T is the thermodynamic temperature of the evaporating surface, in K, Ps is the saturated evaporation pressure of the material at temperature T, in Pa, and Ar is the relative atomic or molecular mass of the membrane material. During vapor deposition, the vapor pressure of the film material is generally required to be between 10-2-10-Pa. The Cm of the material is usually between 10-4 and 10-10 Pa, so it is possible to estimate the required heating temperature for known evaporative materials.

(2) The spatial distribution of vapor particles. The spatial distribution of vapor particles significantly affects the deposition rate of evaporated particles on the substrate and the film thickness distribution on the substrate. This is related to the shape and size of the evaporation source. The simplest ideal evaporation sources come in two types: point and small flat.

2. Vacuum evaporation plating method

(1) Resistance heating evaporation. It is an evaporation source made of filamentous or sheet-like high melting point metals in an appropriate shape, where the film material is placed, connected to a power source, and heated by a resistor to evaporate. The basic requirements for evaporation source materials are high melting point, low vapor pressure, no chemical reaction or mutual dissolution with the membrane material at evaporation temperature, and certain mechanical strength. In addition, the resistance heating method also requires that the evaporation source material and membrane material are easily wetted to ensure stable evaporation state. Common evaporation source materials include tungsten, molybdenum, tantalum, graphite, boron nitride, etc.

(2) Electron beam evaporation. The film material in the resistance heating method is in direct contact with the evaporation source material, and the two are easily mixed, which needs to be avoided for coating of semi conductive substrate components. The electron beam heating method can solve this problem. Its evaporation source is an e-shaped electron gun. The film material is placed in a water-cooled copper crucible, and the electron beam is emitted from the source. A magnetic field coil is used to focus and deflect the electron beam, and the electron trajectory is magnetically deflected by 270 ° to bombard and heat the film material.

(3) High frequency heating. It involves placing alumina or graphite crucibles in a high-frequency induction coil to perform high-frequency induction heating on the membrane material. Induction coils are usually made of water-cooled copper tubes. This method is mainly used for large-scale evaporation of aluminum.

(4) Laser heating. It uses laser irradiation on the surface of the film material to heat and evaporate it. Due to the different wavelength ranges of laser absorption by different materials, it is necessary to choose the corresponding laser. After focusing, the power density of this method can reach 106W/cm, and it can evaporate any high melting point material that can absorb laser energy. The evaporation rate is extremely high, and the composition of the film produced is almost the same as that of the material.

3. The process of vacuum evaporation plating

The vacuum evaporation coating process is determined based on product requirements. The general process for non continuous coating is as follows: preparation before plating - vacuum pumping - ion bombardment - baking - preheating - evaporation - sample extraction - post plating treatment - testing - finished product.

Preparation before plating includes workpiece cleaning, production and cleaning of evaporation sources, cleaning of vacuum chambers and workpiece racks, installation of evaporation sources, cleaning and placement of film materials, and loading of workpieces. These tasks are important as they directly affect the quality of the coating. There are different cleaning methods for different substrates or components. For example, after removing surface dirt and oil stains, glass is rinsed or brushed with water, then rinsed with pure water, and finally dried or wiped dry with anhydrous alcohol; The metal is washed with water and then washed with acid or alkali. It is then washed and dried with water; For rough surfaces and substrates with holes. It is advisable to perform ultrasonic cleaning while cleaning with water, alcohol, etc. Plastic and other workpieces are prone to static electricity during molding. If not eliminated, it can cause pinholes in the film and reduce its adhesion. Therefore, it is often necessary to remove static electricity first.

After the workpiece is placed in the vacuum chamber, it is first vacuumed to 1-0.1 Pa using a vacuum pump for ion bombardment. That is, a certain amount of high-voltage electricity is applied to the aluminum rod in the vacuum chamber to generate a glow discharge, causing electrons to obtain a high speed. The surface of the workpiece quickly carries negative charges, which attract positive ions to bombard the surface of the workpiece. A chemical reaction occurs between the workpiece adsorption layer and the active gas, resulting in further cleaning of the workpiece surface. After a certain period of ion bombardment, turn off the high-voltage electricity, increase the vacuum degree, and heat and bake at a certain temperature to quickly release the gas adsorbed on the workpiece and workpiece frame. After reaching a certain vacuum degree, first apply a lower power current to the evaporation source to preheat or pre melt the film material, and then apply a specified power current to rapidly evaporate the film material. After evaporation, stop pumping and inflate again, open the vacuum chamber to remove the workpiece. Some film layers, such as aluminum plating, are soft and prone to oxidation and discoloration, and require topcoat to protect them.

The above is a general process for vacuum deposition, which includes alloy deposition, compound deposition, and high melting point compound deposition based on the specific composition of the film layer; According to the process, there are also ion beam assisted deposition, laser beam assisted deposition, single crystal deposition, and amorphous deposition methods.

4. The scope of vacuum evaporation coating

Vacuum evaporation coating is widely used for coating various metal, alloy, and compound thin films, and is applied in industrial fields such as optics, electronics, light industry, and decoration.

① Vacuum evaporation aluminum film mirror making: Vacuum evaporation aluminum film mirror making has many advantages over traditional chemical plating silver mirrors. It can save a large amount of precious metal silver, and can save about 7g of silver per square meter of lens. The reflectivity of aluminum mirrors is similar to that of silver mirrors, but it has better corrosion resistance, clearer images, durability, and good product quality. Vacuum evaporation aluminum film process is simple, quality is easy to control, no environmental pollution, and low cost.

The vacuum aluminum plating mirror making process usually uses tungsten wire evaporation source. The tungsten wire grows and becomes brittle when heated for a long time, and is easily damaged due to aluminum erosion. Therefore, the amount of aluminum material used in the coating process and the temperature (heating current) of the evaporation source heating need to be strictly controlled to extend the life of the tungsten wire, which is a problem that should be noted in this process.

② Metallization of plastic products: The surface of plastic products such as knobs, handles, frames, cosmetic containers, lighting fixtures, handicrafts, etc. is coated with a layer of metal film using vacuum evaporation technology, which has been practical and widely used.

The most commonly used coating materials for plastic metallization are aluminum, nickel, chromium, etc. Among them, aluminum is inexpensive, the metal has good reflectivity, and can be colored into multiple colors, making it the most commonly used.

The plastic metalization process usually requires a primer to be applied before coating, and a topcoat to protect the coating after coating.

③ Metallization treatment of plastic film: steaming aluminum film on polyester or polypropylene plastic film, used for decorative film, pressure film, capacitor film, packaging film, etc., has already been widely used,

The metalization treatment of plastic films usually uses semi continuous winding coating machines. Currently, foreign coating machines can achieve a winding speed of 200-400m/min, a width of 1-2m, and a length of 6000-30000m.

The substrate materials for plastic films include polyester, polyvinyl chloride, nylon 6, polycarbonate, ethylene, polypropylene, polytetrafluoroethylene, fiberglass, styrene, etc.

The typical application of metalization treatment of plastic film for decorative film is to make gold and silver wire. The most commonly used substrate is polyester film, and the coating material is mostly aluminum. In advanced decorative applications, gold and silver are also used. Aluminum is plated on the polyester substrate and a transparent protective film is coated on the surface, and then cut into a single layer of silver wire structure. If transparent oil soluble dye is dyed on the film layer or film, it can be made into gold or other colored silk. The aluminum film layers of two aluminum plated polyester films are bonded together to form a double-layer structure of gold and silver silk.

Gold and silver wire is used to manufacture decorative materials such as fabrics, tablecloths, handicrafts, and curtain fabrics.

The metalization treatment of plastic film is also used for calendering film, which is used for surface hot pressing of plastic products, paper, artificial leather, etc., to obtain patterns with metallic luster. The coating material is mainly aluminum, and nickel or chromium is used when high corrosion resistance is required. The bottom coating of calendering film requires the use of peeled paraffin materials, followed by a transparent resin layer. The surface coating of the coating should use a widely applicable heat sensitive adhesive, so that it can be used for hot pressing on various material surfaces. Hot embossing has a wide range of applications in books; Cigarette boxes, labels, and decorative patterns of products can all be used.

After metalization treatment, plastic film can also be used for packaging film, which has moisture-proof, anti oxidation and deterioration, shading, fragrance preservation, and decorative effects.

Dielectric thin film materials coated with metal film can be used to manufacture capacitors. The manufacturing process of metal film capacitors is simple, and they have self recovery function after local breakdown, which has been widely used. The commonly used coating materials are zinc and aluminum, and the adhesion performance between zinc and substrate materials is poor. Generally, adding a small amount of silver, tin, copper, etc. can improve the adhesion performance.

The application of vacuum pumps for vacuum deposition, in addition to the above-mentioned aspects, is also widely used in many technological and industrial fields such as optics, electronics, semiconductors, automation, and solar energy utilization.