Effects of Different Discharge Modes on the Microstructure of Coatings

In the vacuum coating process, the microstructure of thin films plays a crucial role in determining their mechanical properties, optical performance, and corrosion resistance. The microstructure is primarily influenced by factors such as film density, grain size, stress state, and surface roughness. These parameters, in turn, are largely governed by the discharge mode used during deposition. The most commonly used discharge modes in thin film deposition are Direct Current (DC) discharge, Radio Frequency (RF) discharge, Medium Frequency (MF) discharge, and Pulsed DC discharge. Each of these discharge modes influences the plasma characteristics and energy distribution, which significantly impacts the microstructure of the deposited film. This article discusses how different discharge modes affect the grain morphology, film uniformity, stress state, and film density.

Direct Current (DC) Discharge and Its Effect on Film Microstructure

DC discharge is one of the most widely used sputtering techniques, particularly in the deposition of metallic films. DC discharge operates by creating an electric field between the target and substrate, causing electrons and ions to collide and deposit material onto the substrate.

Technical Features:

High sputtering rate: Suitable for rapid deposition of metallic films.

Low plasma density: Results in films with relatively large grain sizes and a rougher structure.

High residual stress: The internal stress in the film can be relatively high, which may affect adhesion and film durability.

Effects on Microstructure:

Grain size: DC discharge typically results in films with larger grain sizes.

Film density: The film is usually less dense, with potential porosity and voids.

Internal stress: The film often exhibits higher internal stress, which can lead to issues like delamination or warping in certain applications.

Radio Frequency (RF) Discharge and Its Effect on Film Microstructure

RF discharge uses high-frequency alternating electric fields to generate plasma, and is commonly employed for sputtering insulating materials such as oxides and nitrides. RF discharge is advantageous for non-conductive target sputtering because it avoids charge accumulation on the target, ensuring stable plasma generation.

Technical Features:

Higher plasma density: Leads to more uniform coatings.

Suitable for non-conductive targets: RF discharge is ideal for sputtering insulating materials such as oxides and nitrides.

Lower deposition rate: Due to lower sputtering power, RF discharge typically results in slower deposition rates.

Effects on Microstructure:

Grain size: RF discharge produces films with smaller grain sizes, which enhances film density and optical performance.

Stress: The film typically has lower internal stress, as the plasma uniformity reduces stress variation.

Surface quality: The film tends to have a smoother surface, making it ideal for optical coatings, dielectric films, and functional thin films.

Medium Frequency (MF) Discharge and Its Effect on Film Microstructure

MF discharge operates in the range of 10–200 kHz and is commonly used in metallic coatings and reactive sputtering processes. MF discharge generates stronger plasma under higher power conditions and is capable of delivering higher deposition rates.

Technical Features:

Higher power density: Allows for faster deposition rates and stronger sputtering effects.

Lower ionization losses: Compared to RF discharge, MF discharge results in fewer ionization losses, improving deposition efficiency.

High deposition rate: MF discharge is suitable for large-area coatings in industrial-scale production.

Effects on Microstructure:

Grain size: The film typically exhibits smaller grain sizes and better density.

Uniformity: Films deposited with MF discharge generally have a more uniform microstructure.

Stress: Due to the higher power density, MF discharge films exhibit lower internal stress, which contributes to better surface quality and high deposition efficiency.

Pulsed DC Discharge and Its Effect on Film Microstructure

Pulsed DC discharge is a technique that involves pulsed power supply control, often used in high-energy ion bombardment applications. This discharge mode is particularly useful for achieving higher ion density and more efficient sputtering effects, while also providing a higher deposition rate.

Technical Features:

Pulsed power: The high peak power during the pulses enables high deposition rates.

Improved arc suppression: Pulsed DC discharge helps to reduce arcing effects, which is particularly beneficial for high-power sputtering.

Sputtering efficiency: Pulsed DC discharge is more energy-efficient, offering high sputtering rates with relatively low power consumption.

Effects on Microstructure:

Grain size: The films produced by pulsed DC discharge generally have medium grain sizes, balancing film density and uniformity.

Film adhesion: The films typically exhibit strong adhesion to the substrate, thanks to high-energy ion bombardment.

Wear resistance: Pulsed DC films often show superior wear resistance due to the high ion bombardment during deposition.

Comparison of Discharge Modes on Film Microstructure