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Types of Vacuum Deposition

To vaporize the evaporation material, most evaporation materials must be evaporated at a temperature of 1100~50℃ during vacuum evaporation. Therefore, various heating methods must be used to heat the evaporation material. The parts in the vacuum equipment that heat the evaporation material are usually called evaporation sources. There are many evaporation sources for evaporation coating, such as resistance heating sources, electron beam heating sources, high-frequency induction heating sources, etc. Resistance heating source is commonly used for evaporation coating of large-area glass substrates.

Resistor evaporation deposition method

An evaporation source of appropriate shape is made using high-melting-point metals such as tantalum, molybdenum, and tungsten. The material to be evaporated is placed on it, and airflow is passed through to heat and evaporate the evaporation material directly. Alternatively, the material to be evaporated is placed in a crucible, such as alumina or beryllium oxide, for indirect heating and evaporation. This is the resistance heating evaporation method.
(1) Resistance heating source
The resistance heating source is made of high-impedance material. When the current passes through the heating source, a large amount of heat energy is generated, which is used to heat the evaporation material. The heating source is a heating body that supports the evaporated material. The shapes include an ingle-strand spiral, a multi-strand spiral, a “concave” boat, a square boat, a conical single-strand wire, etc. They are direct heating evaporation sources. The indirect heating evaporation source consists of a crucible and a heating body. The powdered evaporation material is placed in the crucible, and the heating body is installed outside the crucible. The crucible is heated by the heating body to heat the powdered evaporation material indirectly. To accelerate the heating of the powdered evaporation material and improve the heating efficiency, the heating body is divided into two parts: one part is installed outside the crucible, and the other part is inserted into the powdered evaporation material to heat the powdered evaporation material together. This type is a comprehensive heating evaporation source. Schematic diagram of the shape of the resistance evaporation source
There are several requirements for the evaporation source material:
① To prevent the evaporation source material and the evaporation material (film material) from evaporating together, the material of the evaporation source heating body must have a sufficiently low vapour pressure at the evaporation vacuum and heating temperature of the evaporation material.
② The melting point of the evaporation source material must be higher than the melting point of the evaporation material to ensure that the evaporation source material can maintain a particular strength and not deform when the evaporation material evaporates rapidly.
③ The evaporation source material should have stable chemical properties and not react chemically with the evaporation material during the evaporation process to avoid affecting the quality of the film.
④ It must be able to load the evaporation material to be evaporated. For example, when a filamentary evaporation source is used, the evaporation material must be able to adhere to the heating body of the resistance heating source during the melting process.
Resistance heating evaporation is generally used for evaporation coating of materials with a relatively low melting point, especially for large-scale production with relatively low requirements for coating quality. To date, the resistance heating evaporation process is still widely used in producing aluminium-plated mirrors. Several shapes of commonly used evaporation sources and typical application examples are shown in Table 4-4.
Evaporation Sources Table

4-4 Evaporation Sources, Materials, and Applications

Evaporation Source Shape Material Typical Application
Single-Strand Spiral Tungsten Direct evaporation of refractory alloys; used for heat emission glass coating. Prone to erosion, shortening lifespan.
Multi-Strand Spiral Tungsten, Molybdenum Used for uniform aluminum evaporation in mirror coating; improves efficiency over single-strand designs.
Concave Boat Graphite Continuous evaporation of aluminum materials.
Square Boat Boron Nitride Continuous aluminum evaporation.
Conical Single-Strand Wire Tungsten, Molybdenum Used for evaporating granular metal materials.
Crucible with Spiral Heating Element Tungsten, Molybdenum Ideal for evaporating granular metals with stable heating.
(2) Evaporation materials
Vacuum evaporation coating is limited by vacuum evaporation conditions, and has the following requirements for evaporation materials:
a. When using a filamentary evaporation source, it must be wetted with the evaporation source material.
b. The evaporation temperature must be lower than the maximum temperature that the evaporation source can withstand, and the impurity content must be
c. It has good chemical stability and vacuum thermal stability.
d. It has a firm bond with the substrate.
e. It has little outgassing in a vacuum, and there are no volatile substances.
d. It has a firm bond with the substrate.
e. It has little outgassing in a vacuum, and there are no volatile substances.
f. It does not corrode the evaporation source and equipment.
g. It is easy to use and easy to obtain.
The evaporation materials for vacuum evaporation coating include metals, metal alloys, and compounds. When glass is used as a substrate for vacuum evaporation coating, the most commonly used evaporation material is metal, followed by alloys, and compounds are rarely used. Some commonly used metal evaporation materials are introduced below.
① Aluminum
Aluminum begins to evaporate rapidly above 1100℃, becoming a highly fluid liquid that wets and penetrates refractory surfaces. In a vacuum environment, its chemical activity intensifies, leading to reactions with crucible materials or causing the crucible itself to evaporate. To mitigate these issues, tungsten or tantalum wire resistance heating is favored over electron beam heating. Due to aluminum’s low surface tension at high temperatures, continuous aluminum wire feeding is often used to ensure uniform evaporation and prevent droplet aggregation.
② Chromium
Chromium, with a melting point of 1900℃, can evaporate at 1397℃ due to achieving a vapor pressure of 1 Pa. Its exceptional adhesion properties make it ideal for use as a bonding layer on glass and ceramic substrates. Chromium is typically evaporated using concave boats, square boats, or conical wires, with tungsten often used as the heating material to withstand high temperatures. Electroplating chromium onto tungsten wires improves thermal contact and evaporation efficiency, but thorough degassing of the wires is necessary to maintain process stability during heating.
Evaporation Materials and Sources

Evaporation Materials, Uses, and Sources

Evaporation Material Use Evaporation Source
Aluminum (wire or sheet) Silver-colored coating Multi-strand spiral
Gold and Copper Alloys (sheet or granule) Gold or copper-colored coating Multi-strand spiral or conical single-strand wire
Titanium Compounds (powder) Protective film Boat-shaped source
Oxides Protective film Boat-shaped source
Reflective Film Alloys (wire) Brown reflective film Multi-strand spiral
The disadvantage of the resistance heating method is that the maximum temperature that can be reached by heating is limited, and the heater’s life is short. In recent years, to improve the heater’s life, conductive ceramic materials synthesized by boron nitride have had a long life and have been used as heaters at home and abroad. It is a conductive ceramic material made of nitrides, borides, and other materials with excellent corrosion and heat resistance through hot pressing and coating. This evaporation source has stable performance and long service life. According to Japanese patent reports, a material composed of 20% to 30% boron nitride and refractory materials that can melt with it can be used to make a crucible. A layer of Zirconium 62% to 82% is coated on the surface, and the rest is zirconium-silicon alloy material.

Electron beam evaporation method

In vacuum evaporation coating, materials are placed in a water-cooled steel crucible and heated directly using an electron beam. The heat causes the materials to vaporize, and the vapor condenses on the substrate to form a thin film. This method overcomes many limitations of traditional resistance heating, making it especially suitable for producing high-melting-point materials and high-purity films. The electron beam provides precise control over evaporation, which is crucial for advanced coating technologies.
Electron beam vacuum deposition can be categorized based on the type of electron beam sources, such as ring guns, straight guns, E-type guns, and hollow cathode guns. The ring gun emits an electron beam from a circular cathode, which is then focused and directed onto the material inside the crucible, causing it to evaporate. Despite its simplicity, the ring gun has low power and efficiency, limiting its use to laboratory applications. As a result, more efficient electron beam sources are now preferred for industrial-scale production.
The straight electron gun is an axially symmetric linear acceleration gun. Electrons are emitted from a filament cathode, focused into a fine beam, and accelerated through an anode to heat and evaporate the coating material inside the crucible. Straight guns can operate at power levels ranging from several hundred watts to hundreds of kilowatts, making them suitable for vacuum evaporation and smelting. However, they have limitations, such as material contamination of the gun structure and sodium ions escaping from the filament, which can pollute the thin film. A recent improvement by a German company involves installing a deflection magnetic field at the gun’s exit, unseating an independent exhaust system near the filament to prevent contamination and extend the gun’s lifespan.
The E-type electron gun, which deflects the electron beam by 270°, overcomes many of the straight gun’s drawbacks and is now widely used. It generates high power density, enabling the melting of metals with high melting points and producing evaporated particles with substantial energy, which ensures strong adhesion between the film and the substrate. However, this gun requires high vacuum conditions and negative high voltage, necessitating pressure monitoring plates inside the chamber. These requirements increase the equipment’s complexity, cost, and maintenance difficulties while reducing safety.
The hollow cathode electron gun generates a plasma electron beam using low voltage and high current through a hollow tantalum cathode, with the crucible as the anode and an auxiliary anode positioned nearby. This configuration produces high-energy evaporated ions, achieving high ionization rates and excellent film quality. Compared to the E-type gun, it requires a lower vacuum, operates at low voltage, and has more straightforward, safer, and affordable equipment. Both E-type and hollow cathode electron guns have been successfully employed in China’s vapor deposition and ion plating equipment, producing high-quality films for mechanical and electronic industries. The electron beam technology offers significant advantages, including high beam density for rapid evaporation of materials above 3000°C, water-cooled crucibles to prevent contamination, and efficient heat transfer with minimal thermal losses.

High-frequency induction evaporation source deposition method

The high-frequency induction evaporation source is a graphite or ceramic crucible containing evaporation materials in the center of a water-cooled high-frequency spiral coil so that the evaporation materials will produce strong eddy current losses and hysteresis losses (for ferromagnetic materials) under the induction of the magnetic field in the high-frequency band, causing the evaporation materials to heat up until they are vaporized and evaporated. The smaller the volume of the film material, the higher the induction frequency. In large-scale equipment for continuous vacuum aluminum plating on steel strips, high-frequency induction heating evaporation technology has achieved satisfactory results.
Advantages of high-frequency induction evaporation source: ① The evaporation rate is high, about 10 times higher than that of resistance evaporation source; ② The temperature of the evaporation source is uniform and stable, and it is not easy to produce splashing; ③ When the evaporation material is metal, the evaporation material can generate heat; ④ The evaporation source is loaded once, and no feeding mechanism is required, so the temperature control is relatively easy and the operation is relatively simple.
Its disadvantages are: ① Boron nitride crucibles with good thermal shock resistance and stable high-temperature chemical properties must be used; ② The evaporation device must be shielded and requires a more complex and expensive high-frequency generator; ③ The pressure near the coil has a fixed value. If it exceeds this fixed value, the high-frequency field will ionize the residual gas and increase the power consumption.

Laser beam evaporation source evaporation method

Evaporation technology using a laser beam evaporation source is an ideal method for preparing thin films. This is because the laser can be installed outside the vacuum chamber, which not only simplifies the spatial layout inside the vacuum chamber and reduces the outgassing of the heating source but also wholly avoids the contamination of the plated material by the evaporator, achieving the purpose of the pure film layer. In addition, laser heating can reach incredibly high temperatures, and laser beam heating can “flash evaporate” certain alloys or compounds. This is also extremely useful for ensuring the film’s composition and preventing the film’s fractionation or decomposition. However, due to the high cost of making high-power continuous lasers, their application range is limited to a certain extent, and they cannot be widely used in the industry.

Vacuum evaporation process

The process flow of vacuum evaporation is as follows:
Glass Substrate→Manual Inspection→Cutting→Washing Drying→Mounting (Source + Film Material)→Coating(Vacuum Evaporation)→Unloading→Inspection→Packaging Storage
The production of coated glass by vacuum evaporation is currently intermittent.
After the glass substrate is transported to the workshop, it is visually inspected. If the appearance quality meets the requirements, it will be placed on the washing and drying machine; if the glass specifications do not meet the specifications required by the vacuum glass coating machine, it must be cut according to the specifications needed for the coating machine before being placed on the washing and drying machine. In the washing and drying machine, the glass is rinsed with washing water with detergent, brushed with a nylon brush, and then rinsed with tap and deionized water. The water on the glass surface is scraped off with a soft scraper, and then the water on the glass surface is blown dry with filtered dry air using a wind knife. The glass piece is removed and placed on the transfer rack for use.

Film formation process

In a vacuum environment, the evaporation material moves linearly in any direction in the state of atoms or molecules under the action of the evaporation source. When it encounters a surface with a lower temperature, it is adsorbed. As the particles continue to increase, a film layer is gradually formed. From the beginning of evaporation to the film formation on the substrate surface, it can be divided into four stages. The schematic diagram of the formation process is shown in Figure 4-12.

(1) Nucleation-island stage
When the substrate adsorbs the incident evaporation material particles, many crystal nuclei appear, forming a film layer on the substrate. The diameter of the nucleus is about 2nm, and the distance between the nuclei is about 30nm. More and more atoms are adsorbed, forming islands of different shapes. The process of atoms combining instantly is as follows: two atoms that exist independently → more and more atoms are adsorbed → the contact area gradually becomes larger → the atoms are connected into islands.
(2) Agglomeration stage

Due to the continuous generation of vapor of evaporating material, the original small islands expand and connect to form large islands. The atoms adsorbed on the surface of the substrate have condensed into small islands, and the vapor atoms that arrive later continuously fill the gaps between the islands and aggregate into large islands.
(3) The edges of the large islands are irregularly connected to form a mesh film, channel, passage, and hole stage

The vapor of evaporating material gradually increases, forming a honeycomb structure between the large islands. The gaps begin to connect, but the connection is not dense enough.
(4) Film formation stage
Based on the channel stage, as the vapor atoms of evaporating material are continuously adsorbed, they fill around the channel and gradually form a film layer. The thickness of this film is not uniform at the microscopic level but is uneven.

The film formation process is shown in Figure 4-13.

Fihure 4-12

Principle of Vacuum Evaporation

Vacuum evaporation is an industrial coating method that utilizes molecular motion characteristics under vacuum conditions. At a certain temperature, some molecules of a substance transition from a condensed state (solid or liquid) to a gaseous state and leave the surface. However, under normal temperature and pressure, the evaporation rate of solids is minimal. When a solid material is placed in a vacuum and heated to its evaporation temperature, the heat of vaporization provides enough thermal energy for the molecules or atoms to overcome surface atomic attraction, causing them to escape at a specific speed and transform into gaseous molecules or atoms. These vapor molecules disperse rapidly, and when the vacuum degree is high, with the molecular free path (λ) significantly greater than the distance (d) from the evaporator to the substrate, the vapor molecules travel in a straight line without obstruction.

The principle of glass vacuum evaporation coating

During this process, vapor molecules reach the substrate and adhere to its surface through chemical adsorption (driven by chemical bond forces) and physical adsorption (caused by intermolecular van der Waals forces). If the substrate temperature is below a critical level, these vapor molecules condense on the surface, initiating the nucleation process. As more vapor molecules accumulate, crystal nuclei form, and their number increases. The newly deposited molecules and small nuclei migrate and merge, forming larger grains. The grains continue to grow and coalesce, gradually shaping a network-like film structure.
As deposition continues, the film’s average thickness increases, ensuring strong adhesion to the substrate and forming a continuous coating. The high density of incident vapor molecules promotes rapid nucleation, facilitating the uniform growth of the film. The resulting thin film maintains structural integrity and provides functional properties essential for various industrial applications, such as optics, electronics, and protective coatings.

Effect of coating conditions on film

The effect of coating conditions on thin films is significant, as factors like temperature, vacuum level, deposition rate, and material purity directly impact film quality and performance.

Effect of vacuum chamber pressure on film layer

In vacuum evaporation coating, achieving optimal film deposition requires the mean free path of vapor molecules to exceed the distance between the evaporation source and the substrate. This condition is only possible by creating a high vacuum in the chamber, which reduces the number of residual gas molecules and minimizes collisions between vapor and gas molecules, ensuring better deposition efficiency.

As vacuum levels increase, the probability of vapor molecules colliding with residual gas molecules decreases, enhancing film quality. For example, maintaining a vacuum pressure above 6 × 10-2 Pa during aluminum coating ensures a smooth and durable film layer. However, if vacuum levels drop, two issues may arise: (1) Residual gas molecules adsorbed on the substrate surface react with vapor molecules, forming compounds that degrade the film’s quality. (2) Collisions between vapor molecules and residual gases during deposition reduce the vapor’s kinetic energy, leading to poor adsorption on the substrate, resulting in weak or powdery films that can easily detach.

In practice, aluminum films produced below10-1 Pa often appear gray with reduced adhesion, and the film turns black at even lower vacuum levels and becomes fragile. Maintaining a higher vacuum prevents these issues, ensuring the film adheres properly to the substrate and meets quality standards.

When using vacuum evaporation without baking the chamber and substrate, a vacuum level between 10-2 and 10-5 Pacan still achieve satisfactory film quality. However, vacuum levels exceeding 10-6 Papre-baking is essential to remove gases thoroughly. Contaminants may be introduced if the substrate is not adequately protected during baking, increasing the risk of reduced film quality despite the high vacuum conditions.

Choice of layer deposition rate

When the vapor molecules of the evaporating material fly to the substrate surface, they collide with the residual gas molecules in the vacuum chamber. They may also collide with the gas molecules adsorbed on the substrate surface. The longer the film deposition time is, the more times the above collisions will occur; increasing the evaporation rate can reduce the probability of collisions so that more evaporating materials are adsorbed on the substrate per unit of time, the fewer impurities are formed, and the better the quality of the film. To obtain a high-purity movie, it can be obtained by increasing the film deposition rate. The specific measures are to appropriately increase the evaporation source’s temperature and the evaporation area. Taking the vacuum aluminum-plated mirror as an example, under the premise that the number of aluminum wires and the surface area of ​​the multi-strand spiral evaporation source have been determined, the following three points should be mastered in operation: appropriately increase the temperature of the evaporation source, melt all the aluminum wire materials, and evaporate the aluminum wire quickly and entirely without dripping after melting, to shorten the evaporation process as much as possible. Through these measures, high-quality aluminum-plated mirrors can be obtained.

Surface conditions of evaporation source and evaporation material

The surface of the evaporation source and evaporation material must be clean. The surface of the newly installed evaporation source and each added evaporation material must be cleaned to remove the oil and oxide scale. For workpieces with high coating quality, the workpiece should be blocked with a baffle when the evaporation material melts. Since the vapor molecules of the evaporation material have low energy and many impurities at this time, these vapor molecules cannot be deposited on the surface of the workpiece. After some, the baffle is removed to obtain a high-quality film layer.
If there is dirt in the evaporation source, a large number of impurity gas molecules will be generated when it is powered on and heated, affecting the quality of the film layer and reducing the deposition rate; if there is dirt in the evaporation material, a large number of impurity gas molecules will be generated when it is heated; if the dirt is an oxide layer, it will also affect evaporation because it is not easy to gasify. Both reasons will lead to a reduction in the deposition rate of the belly layer.

Leading equipment for vacuum evaporation

There are many types of vacuum evaporation equipment (coating machines), but the basic structure is the same. The correct figure is a schematic diagram of vacuum evaporation equipment. The system comprises a well-sealed coating chamber two and a vacuum equipment (vacuum unit) 6. A sealed pipeline connects the two. The vacuum equipment is used to evacuate the coating chamber to maintain a high vacuum degree. Glass generally requires a vacuum degree of 10.2Pa when coating and 10.4~10.6Pa when a high vacuum is needed. An evaporation source three is installed at the bottom of the coating chamber, and a glass substrate or glass product is installed at the top. After the film layer material evaporates, it is deposited on the glass substrate or glass product. 4, 5, and 7 are vacuum valves, 1 is an inner lining plate, and 8 is a diffusion pump.
The vacuum evaporation coating method can coat single-layer films on glass, such as aluminum, silver, copper, etc. Although aluminum is cheap, chemical reduction and thermal spraying methods cannot coat aluminum films. Vacuum evaporation and cathode sputtering methods can be used to coat aluminum films. The thickness of aluminum reflectors is 100-200nm in China. The copper film is brown and can replace brown glass aluminum film decorative mirrors. The vacuum evaporation method can also coat multi-layer films, such as the sunshade film of the Cr/Ni/Fe three-layer film system, which is gray and has a total light transmittance of about 50%. The characteristics of vacuum deposition coating are low productivity, high price, poor adhesion between the film and the glass, and poor film uniformity. If high uniformity of the film is required, the number of vaporization sources needs to be increased, and the vaporization sources must be replaced frequently during coating. The equipment investment is relatively high, and there are not as many types of film materials and product varieties as there are for vacuum sputtering. Therefore, the development speed of evaporation coating is not as fast as that of sputtering coating.

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