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What is cathode magnetron sputtering？

Cathode Magnetron Sputtering is a vacuum-based physical vapor deposition (PVD) technique used to deposit thin films onto a substrate. This method involves ejecting material from a target (cathode) and depositing it onto a substrate using a combination of electric and magnetic fields to enhance the process. It builds on one of the earliest methods of vacuum film deposition, known as cathode sputtering, which dates back to discoveries in the mid-19th century.

In 1852, Grove in the UK and, later in 1858, Plücker in Germany independently observed cathode erosion during gas discharge experiments in glow discharge tubes. By 1877, the sputtering process was applied to mirror production, using rare gas ions to bombard metal surfaces. Over the next century, researchers refined the technique, culminating in the maturation of cathode sputtering technology by 1955.

Magnetron sputtering enhanced this traditional approach by incorporating magnetic fields to confine electrons near the target, increasing plasma density and ionization efficiency. This results in higher deposition rates and improved film quality.

What is the sputtering principle?

The principle of magnetron sputtering involves a process conducted in a vacuum chamber. The chamber is evacuated, and a negative voltage between -500 and -800 volts is applied to the cathode, while a positive voltage between 0 and 100 volts is applied to the anode. Argon gas is introduced into the chamber, and when the pressure reaches approximately 10^-5 Pa, a glow discharge occurs near the cathode. Argon atoms are ionized in the discharge, forming argon ions and free electrons that create a plasma region.

The electric field generated by the cathode voltage interacts with the magnetic field from permanent magnets, forming a perpendicular electromagnetic field. Under this influence, argon ions are accelerated toward the cathode, gaining significant energy in the potential drop zone near the cathode. These high-energy ions bombard the target material, transferring kinetic energy to neutral atoms or molecules on the target surface, causing them to eject. These ejected particles are then deposited onto the substrate, such as glass, forming a thin film layer.

Secondary electrons generated during the sputtering process reenter the plasma region, contributing to further ionization and maintaining the supply of argon ions. Once their energy is depleted, these electrons are attracted to the anode and expelled from the vacuum chamber. This process efficiently deposits high-energy particles onto the substrate while recycling electrons, forming the fundamental principle of magnetron sputtering.

Figure 4-14 Principle diagram of magnetron sputtering

Magnetron Sputtering Process

Magnetron sputtering is a process where ions formed in a plasma (or glow discharge, as shown in Figure 4-15) are accelerated toward a target to transfer momentum. Plasma is created by ionizing gases like argon, oxygen, nitrogen, or other reactive gases that are introduced into a vacuum system. The ions gain energy primarily from the negative DC voltage applied to the target surface. This energy is then transferred to the target atoms, allowing some atoms with sufficient energy to escape from the target surface.

The efficiency of energy transfer depends on the relative mass of the ions and the target atoms. The energy required for atoms to leave the target surface depends on the latent heat of evaporation of the target material. In reactive sputtering, where oxygen or nitrogen is used as the working gas, more energy is needed to break chemical bonds, resulting in lower efficiency. The sputtering yield, defined as the number of atoms ejected per ion, can vary for different materials and ion energies.

Sputtering is not an efficient process, as most of the energy is converted into heat and absorbed by the target. This necessitates water cooling to prevent the target material from bending or melting. Despite these challenges, sputtering remains an ideal method for depositing thin and uniform multilayers on large glass surfaces.

Figure 4-15 Schematic diagram of glow discharge

Magnetic Field, Plasma Density, and Efficiency in Magnetron Sputtering

The sputtering rate in magnetron sputtering depends heavily on the effective ion density within the plasma. To achieve a dense plasma, a perpendicular electric field (E) and magnetic field (B) configuration, known as an “EB field,” is employed. The magnetic field, generated by permanent magnets positioned behind the target, is aligned parallel to the target surface. This arrangement enhances ionization efficiency and plasma density.

Electrons in the plasma are accelerated away from the cathode under the influence of the electric field. As they pass through the magnetic field, they experience a perpendicular force, causing them to move in circular trajectories. With each circular motion, the electrons gain energy from the magnetic field. These high-energy electrons collide with gas atoms or sputtered atoms, ionizing them and generating additional ions. This cascade effect significantly increases plasma density, enhancing sputtering rates and improving deposition efficiency.

 

Optimization of the magnetic field configuration is crucial for maximizing ion generation and achieving higher plasma densities. A denser plasma enables faster sputtering rates and greater coating efficiency. This optimization has dramatically improved the productivity of magnetron sputtering, especially in the manufacturing of coated glass.

Figure 4-16 Magnetic field and electron motion trajectory on the surface of magnetron sputtering target

Magnetron sputtering is particularly effective for depositing uniform thin films over large areas, making it a preferred method for applications such as architectural glass coatings, optical films, and functional coatings. Its ability to precisely control sputtering parameters ensures consistent performance and high-quality results.

What are the materials used for magnetron sputtering?

Magnetron sputtering involves several types of materials for effective thin-film deposition. These materials can be categorized based on their role in the process:

1.Sputtering Gases

Sputtering gases are divided into two main types: working gases and reactive gases. 

(1) Working Gases

Working gases are ionized under a strong electric field in the vacuum chamber at pressures between 0.1 and 1 Pa. The positive ions, attracted by the high negative voltage on the cathode, gain significant energy and bombard the target material, causing sputtering.

When selecting a working gas, key considerations include:

  • High sputtering yield.
  • Chemical inertness to the target material.
  • Cost-effectiveness and availability.
  • High purity.

Inert gases such as argon, helium, and neon are commonly used. While helium and neon achieve higher sputtering rates for the same ion energy, argon is preferred in industrial applications due to its availability, lower cost, and satisfactory performance. Argon eliminates the need for complex and expensive material vaporization processes, making it the standard working gas in industrial sputtering.

(3) Gas Pressure

Gas pressure significantly affects sputtering dynamics:

  • Higher pressure increases discharge current and reverse scattering, causing energetic particles to decelerate due to collisions. This reduces the energy of sputtered particles, which convert to heat upon reaching the substrate.
  • Lower pressure has the opposite effect, reducing collision rates but potentially impacting plasma density.

To maintain uniform coatings, the vacuum system must have a high pumping speed to remove residual gases and contaminants rapidly. Pressure gradients near the discharge area should be avoided to prevent non-uniform discharge and uneven film deposition.

(2) Reactive Gases

Reactive gases are introduced to produce compound films, such as oxides, nitrides, sulfides, and carbides. During sputtering, these gases (e.g., oxygen, nitrogen, hydrogen sulfide, methane) chemically react with atoms ejected from the pure metal target. This reaction forms compounds like oxides (e.g., TiO₂), nitrides (e.g., TiN), sulfides, or carbides, which deposit onto the substrate surface, creating functional coatings.

Oxide and nitride films are most commonly produced, with oxygen and nitrogen as the primary reactive gases in industrial applications.

(4) Gas Contamination

Sputtered films typically exhibit high purity, as the process avoids contamination from evaporation sources (e.g., crucible materials in vacuum evaporation). However, gas contamination in the sputtering chamber can introduce impurities into the film. Common contamination sources include:

  • Residual water vapor in the vacuum chamber.
  • Desorbed gases from chamber walls due to ion bombardment.
  • Adsorbed gases on the target material.
  • Leaks in the sputtering chamber.
  • Impurities in the working gas itself.

Most contamination can be minimized through:

  • High-quality vacuum technology.
  • Adequate pre-sputtering to remove surface contaminants.
  • Using high-purity gases to reduce external doping.

By carefully managing the gas selection, pressure, and contamination, magnetron sputtering can produce high-purity, uniform films suitable for advanced applications in optical coatings, electronics, and protective layers.

Table 4-6 Sputtering yields of various elements obtained using inert gas ions (500 eV)

Element He Ne Ar Kr Xe
Be 0.24 0.42 0.51 0.48 0.35
C 0.07 0.12 0.13 0.17
Al 0.16 0.73 1.05 0.96 0.82
Si 0.13 0.48 0.5 0.5 0.42
Ti 0.07 0.43 0.51 0.48 0.48
V 0.06 0.48 0.65 0.62 0.63
Cr 0.17 0.99 1.18 1.39 1.55
Mn 1.39 1.43
Mn 1.9
Bi 6.64
Fe 0.15 0.88 1.1 1.07 1
Fe 0.63 0.84 0.77 0.88
Co 0.13 0.9 1.22 1.08 1.08
Ni 0.16 1.1 1.45 1.3 1.22
Ni 0.99 1.33 1.06 1.22
Cu 0.24 1.8 2.35 2.35 2.05
Cu 1.35 2 1.91 1.91
Cu(III) 2.1 2.05 3.9
Cu 1.2
Ce 0.08 0.68 1.1 1.12 1.04
Y 0.05 0.46 0.68 0.66 0.48
Zr 0.02 0.38 0.65 0.51 0.58
Nb 0.03 0.33 0.6 0.55 0.53
Mo 0.03 0.48 0.8 0.87 0.87
Mo 0.24 0.64 0.59 0.72
Ru 0.57 1.15 1.27 1.2
Rh 0.06 0.7 1.3 1.43 1.38
Pd 0.13 1.15 2.08 2.22 2.23
Ag 0.2 1.77 3.12 3.27 3.32
Ag 1 1.7 2.4 3.1
Ag 3.06
Sm 0.05 0.69 0.8 1.09 1.28
Gd 0.03 0.48 0.83 1.12 1.2
Dy 0.03 0.55 0.88 1.15 1.29
Er 0.03 0.52 0.77 1.07 1.07
Hf 0.01 0.28 0.7 0.8
Ta 0.01 0.28 0.57 0.87 0.88
W 0.01 0.28 0.57 0.91 1.01
Re 0.01 0.37 0.87 1.25
Os 0.01 0.37 0.87 1.27 1.33
Ir 0.01 0.43 1.01 1.35 1.56
Pt 0.03 0.63 1.4 1.82 1.98
Au 0.07 1.08 2.4 3.06 7.01
Au 1.1 1.3 2.5 7.7
Pb 1.1 2.7
Th 0 0.28 0.62 0.98 1.05
U 0.45 0.85 1.3 0.81
Sb 2.83
Sb（固体） 1.2
Sb（液体） 1.4

2.Sputtering Materials (Targets)

Sputtering materials, or targets, are typically manufactured in various shapes such as rectangular plates, circular plates, and cylindrical rolls. Smaller sputtering devices may use disk- or rod-shaped targets. Materials suitable for targets include metals, semiconductors, alloys, halides (commonly fluorides), oxides, nitrides, selenides, tellurides, and compounds of elements from Groups III-V and II-V in the periodic table. Frequently used materials include metals, alloys, semiconductors, and oxides, with occasional use of nitrides, silicides, carbides, and borides.

（1）Types of Sputtering Materials

Sputtering targets are essential for producing films with specific properties, and their selection should align with the desired performance, particularly in glass coatings. Efficiency depends on the sputtering yield, which varies by material and ion energy, with high-yield targets preferred for better efficiency. Common targets include chromium, titanium, tin, zinc, copper, aluminum, bismuth, stainless steel, and indium-tin-oxide (ITO) for sun-control coatings, mirrors, and conductive films, while gold, silver, copper, and lead are widely used for low-emissivity coatings. Due to the high-power currents used in sputtering, effective water cooling is necessary to prevent target overheating, diffusion, decomposition, melting, or evaporation.

（2）Installation Methods for Sputtering Targets

Sputtering targets can be installed using either planar mounting or rotational mounting.

Planar Mounting

Planar mounting, commonly known as the planar target configuration, is widely used in sputtering systems worldwide. In this setup, the target is installed at the lower edge of the sputtering source and supported by a metal plate equipped with permanent magnets arranged in specific patterns (Figure 4-18). Cooling water circulates through channels within the magnet assembly to maintain target temperature. The vacuum chamber is connected to a vacuum system, working gas (e.g., argon), and reactive gas supply. Substrates are positioned at a set distance from the target, either stationary or transported on rollers for continuous sputtering. However, planar targets exhibit circular wear patterns, with the most erosion occurring along the target’s centerline, resulting in a material utilization rate of approximately 40%.
Figure 4-18 Schematic diagram of magnetic pole plane layout

Figure 4-19 Schematic diagram of rotating target structure

Rotational Mounting

Rotational mounting, introduced in the 1990s by Boc Coating Technology, uses cylindrical (rotating) targets to enhance efficiency (Figure 4-19). In this configuration, cylindrical targets (or target tubes) surround a fixed assembly of magnets, cathodes, and mounting tubes. Deionized water circulates within sealed and insulated cooling jackets in the target assembly to prevent overheating. This design offers significant advantages, including higher material utilization, as the rotating target transforms erosion grooves into layered wear patterns, increasing utilization to over 70%. Additionally, the reduced frequency of target replacements minimizes downtime and improves operational efficiency.

Table 4-7 Sputtering yields of argon ions of different energies colliding with various materials

Target Voltage (V) 200.0 600.0 1000.0 2000.0 5000.0 10000.0
Ag (Atoms/Ion) 1.6 3.4 - - 8.8 -
Al (Atoms/Ion) 0.35 1.2 - - 2.0 -
Au (Atoms/Ion) 1.1 2.8 3.6 5.6 7.9 -
C (Atoms/Ion) 0.05 0.02 - - - -
Cr (Atoms/Ion) 0.7 1.3 - - - -
Cu (Atoms/Ion) 1.1 2.3 3.2 4.3 5.5 -
Fe (Atoms/Ion) 0.5 1.3 1.4 2.0 2.5 -
Ge (Atoms/Ion) 0.5 1.2 1.5 2.0 3.0 -
Mo (Atoms/Ion) 0.4 0.9 1.1 - 1.5 2.2
Nb (Atoms/Ion) 0.25 0.65 - - - -
Ni (Atoms/Ion) 0.7 1.5 2.1 - - -
Pb (Atoms/Ion) 1.0 2.4 - - - -
Pt (Atoms/Ion) 0.6 1.6 - - - -
Si (Atoms/Ion) 0.2 0.5 0.6 0.9 1.4 -
Ta (Atoms/Ion) 0.3 0.6 - 1.1 - 1.05
Ti (Atoms/Ion) 0.2 0.6 - 1.1 1.7 2.1
W (Atoms/Ion) 0.3 0.6 - - 1.1 -
Zr (Atoms/Ion) 0.3 0.75 - - - -
LiF (Molecules/Ion) - - - 1.3 1.8 2.2
CdS (Molecules/Ion) 0.5 1.2 - - - -
GaAs (Molecules/Ion) 0.4 0.9 - - - -
PbTe (Molecules/Ion) 0.6 1.4 - - - -
SiC (Molecules/Ion) - 0.45 - - - -
SiO₂ (Molecules/Ion) - - 0.13 0.4 - -
Al₂O₃ (Molecules/Ion) - - 0.04 0.11 - -

Magnetron Sputtering Production Methods and Processes

Magnetron sputtering offers two primary production methods: batch production and continuous production. Each method follows distinct processes to achieve high-quality coated glass products.

1.Batch Production Method

In batch production, the process flow is cyclic, as shown in Figure 4-20.

Glass substrates are inspected manually upon arrival at the workshop. Approved substrates are washed and dried using a cleaning machine, then mounted into loading frames, one sheet per frame. The frames are loaded onto a transport cart and pushed into the sputtering chamber for coating.
After the chamber is sealed, vacuum pumping, pre-sputtering, and sputtering processes commence. Pre-sputtering is performed with the cathodes directed toward a pre-sputtering plate to remove impurities. Once the specified current density and voltage are reached, the ion energy is sufficient for sputtering onto the substrate. The cathode assembly moves over the substrate to deposit a layer, then resets to the pre-sputtering plate for subsequent layers. Typically, three layers are applied: an oxide layer, a metallic layer, and another oxide layer.

After sputtering, the chamber is vented, and the frames are removed manually. Coated glass undergoes quality inspection, and protective anti-scratch layers are applied before packaging and storage.

Figure 4-20 Intermittent production process flow

2.Continuous Production Method

Continuous production methods include horizontal and vertical configurations, each suitable for specific industrial setups.

(a) Horizontal Continuous Production

This method involves horizontal transport of substrates through multiple sputtering chambers, as illustrated in Figure 4-21.

Glass substrates are loaded using cranes or vacuum suction systems, cleaned, dried, and inspected before entering the vacuum system.
The substrates pass through sequential vacuum chambers for sputtering:

  • Chamber I: Reactive sputtering with oxygen to deposit an oxide layer.
  • Chamber II: Sputtering with inert gases (e.g., nitrogen) for a metallic layer.
  • Chamber III: Reactive sputtering with oxygen for a final oxide layer.

Intermediary isolation zones with gas controls ensure no cross-contamination between chambers.
After coating, substrates pass through an exit system for cleaning, inspection, and packaging.

Figure 4-21 Horizontal continuous production process flow

(b) Vertical Continuous Production

Vertical production, as shown in Figure 4-22, involves substrates being transported vertically at an 80° inclination using vertical conveyors or mounting frames. The glass undergoes continuous processing as it moves vertically through various coating stations. This method is efficient for producing Low-E glass, offering space-saving advantages and consistent coating quality.

Figure 4-22 Vertical continuous production process of Low-E glass

Features of Magnetron Sputtering for Coated Glass Production

Magnetron sputtering is a precise and versatile technique for coated glass production, offering excellent film uniformity, strong adhesion, high purity, and efficiency for large-scale manufacturing.

Excellent Film Thickness Control and Repeatability

The ability to control anode and cathode currents independently ensures precise film thickness control and repeatability, even across large areas.

Strong Adhesion Between Film and Substrate

The energy of atoms deposited onto the substrate is 1–2 orders of magnitude higher than that of vacuum evaporation methods. Combined with plasma exposure that continuously cleans and activates the substrate, the film adheres strongly to the substrate.

Capability to Produce Specialized Films

Magnetron sputtering allows the deposition of films from nearly all solid materials, including metals, alloys, semiconductors, and more. For example, high-melting-point metals can produce durable films, and reactive sputtering with gases like nitrogen and oxygen can create films with properties different from the target material, such as TiN (gold-like) films from titanium targets.

High Sputtering Rates

Magnetron sputtering uses a magnetic control system that recirculates electrons in the plasma zone, maintaining a high ionization rate and ensuring a consistent supply of ions. This enables high sputtering rates while keeping substrate temperatures low. Modern magnetron sputtering systems support large-scale continuous production, coating substrates as large as 3200 mm × 6000 mm in just 1–2 minutes, with daily outputs of 5000–8000 m².

High Film Purity

Unlike vacuum evaporation, sputtering avoids contamination from crucibles or other evaporation sources, resulting in high-purity films.

Key Considerations for Magnetron Sputtering of Coated Glass

Key considerations for magnetron sputtering of coated glass include maintaining substrate cleanliness, precise process control, and stable parameters to ensure high-quality, durable coatings.

Fresh Glass

Fresh glass has higher surface chemical activity, enhancing both chemical and physical adsorption. Glass should be coated within one month of production.

Thorough Pre-Coating Cleaning

Substrates must be scrubbed with detergent and rinsed with deionized water meeting quality standards, followed by proper drying.

Avoid Moisture During Transport and Storage

Exposure to water can cause stains or defects on the glass surface that are difficult to remove and become visible post-coating.

No Bare-Hand Contact After Cleaning

Touching cleaned glass with bare hands leaves sweat, oil, and dirt, which can weaken the bond between the film and glass, leading to visible defects or eventual delamination.

Glow Discharge Cleaning

Pre-coating glow discharge treatment must follow process specifications (voltage, current, and time) to effectively remove adsorbed moisture and gases, typically requiring at least 20 seconds.

Precise Control of Reactive Gases

In reactive sputtering, an insufficient amount of gas results in incomplete reactions, while excess gas can weaken the film’s adhesion. Accurate control is critical for optimal film quality.

Stable Process Parameters

Fluctuations in sputtering current or voltage can affect sputtering rates and the adsorption of neutral atoms or molecules on the substrate. Stable process parameters are essential for producing uniform and durable coatings.

Comparison Between Evaporation and Sputtering Methods

Evaporation and sputtering are fundamentally different techniques for glass coating. Table 4-8 outlines a detailed comparison of their characteristics.

Table 4-8: Comparison Between Evaporation and Sputtering Methods for Thin Film Preparation

Aspect Evaporation Sputtering
Vapor Generation Phase
  • Thermal process:
  • Atoms have low kinetic energy (~0.1 eV at 1500°C).
  • Directional distribution follows cosine law (point source or small surface element).
  • Few or no charged particles.
  • Alloys exhibit fractional evaporation; compounds may dissociate.
  • Ion bombardment with momentum transfer:
  • Atoms have higher kinetic energy (1–40 eV, with varied energy distribution).
  • Cosine law distribution occurs primarily at higher bombardment energies.
  • Ion population per incoming ion includes reflected neutral ions, positive ions, and negative ions (10⁻³ to 10⁻¹).
  • Alloys sputter with relatively uniform composition; compounds may dissociate.
Transport Phase
  • Evaporated particles move in high or ultra-high vacuum.
  • Minimal or no collisions occur, as the mean free path is much larger than the distance between the evaporator and the substrate.
  • Sputtered particles move in a working gas environment at relatively higher pressure (10⁻² to 10⁻⁴ mbar).
  • Collisions reduce energy, as the mean free path is shorter than the distance from cathode to substrate.
  • Charged particles experience isotropic directional changes and are prone to chemical reactions involving excitation, ionization, or dissociation.
Condensation Phase
  • Incoming atoms have negligible impact on the substrate.
  • Nucleation conditions remain stable.
  • Weak residual gas atom/molecule incidence (~10¹³ impacts/cm²·s).
  • Low gas incorporation results in pure films, with minimal chemical reactions and negligible substrate/membrane temperature changes.
  • Incoming ions and high-energy neutral particles strongly affect the substrate (causing roughening, penetration, defects, temporary surface charges, and chemical reactions with residual gas).
  • Nucleation conditions change significantly (simplified nucleation center formation).
  • Strong incidence of working/residual gas particles (~10¹⁷ impacts/cm²·s).
  • Higher gas/material incorporation may reduce film purity. Chemical reactions (activation, ionization) are common. Collisions and high-energy ions can increase substrate/membrane temperature.

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