Authors: Dr. Patrick McCarthy (GENCOA), Thomas Vartiainen (AVALUXE)
Ion sources are important components in physical vapor deposition (PVD) coating processes. They are being used for three main applications:
• Plasma treatment: To enhance surface reactivity
• Surface cleaning: Removal of organic contaminants
• Material etching (with metals)
These applications of ion sources in PVD coating help improve coating quality, adhesion, and control over film properties.
WORKING PRINCIPLE
An ion source works by ionizing neutral atoms or molecules to create positively charged ions.
Gas injection: A working gas (often an inert gas like argon) is introduced into the ion source chamber.
Electron generation & ionization: The generated electrons collide with the gas atoms, knocking off more electrons and creating positive ions.
Plasma formation: As more ions and electrons are created, a plasma forms within the source.
Ion extraction: An electric field is applied to extract the positive ions from the plasma. This is typically done using a set of electrically charged grids or electrodes.
Ion acceleration: The extracted ions are then accelerated by an electric field to the desired energy level.
Beam delivery: The ion beam is directed towards the target (for sputtering) or the substrate (for ion-assisted deposition).
All ion sources can be divided into two types, depending on their generation method:
Type 1 - Inverted magnetron (the main electrode where discharge is produced has a positive voltage). The term "inverted" refers to the reversal of the typical roles of inner and outer electrodes compared to conventional magnetron configurations
Type 2 - Magnetron-like-discharge (the main electrode where discharge is produced has a negative voltage).
Why carbon cathode and anode? Very low sputter yield hence long lifetime compared to metallic components. Carbon materials greatly enhance lifetime of anode and cathode – less expense on replacements
TYPE 1: INVERTED MAGNETRON-TYPE
Inverted magnetron: The main electrode - where discharge is produced - has a positive voltage. Graphics: GENCOA
This type of ion source is typically used on web & glass used to pre-treat before sputter coating. It is a powerful means to liberate moisture and burn-off hydrocarbons before the sputter coating of the flexible web.
Characteristics:
Low current (10-200 of mA) and high voltage (>1.000 V) operation
The linear ions sources work at sputtering pressures and with web speeds of <5m/min. For higher speed webs, magnetron-based plasma treaters are recommended.
A typical operation involves using DC power supply, with other power modes – such as pulsed DC – also suitable
TYPE 2 - MAGNETRON-LIKE-DISCHARGE
“Normal” magnetron: The main electrode - where discharge is produced - has a negative voltage.
A planar magnetron-based sputter device is configured to provide high power plasma activation of fast moving web materials in order to improve coating adhesion.
The target material here is stainless steel or titanium in order to reduce the sputtering rate.
Characteristics
Uses the same robust design as all planar magnetrons plus extra insulation for operation at high pressure
Average power density ~ 5-10W/cm2
Mid current (Amps) and mid voltage (>100V) operation
Many design and size options depending upon system / application
AC type dual cathode type plasma pre-treaters for plastic web pre-treatment
A more energetic type of plasma pre-treatment is a double electrode AC type of discharge. The Gencoa AC plasma treaters for example combine magnetic activation, high voltage AC type switching power for guiding electrons and plasma to the substrate surface. The dual electrode AC type plasma is more robust in dealing with heavy outgassing from the web material as they are stable in fully reactive modes.
They are typically being used for polymers and some cases in glass for adhesion improvements and surface activation.
Average power density ~ 5-10W/cm2
Mid current (Amps) and mid voltage (>100V) operation
Switching AC plasma with active anode enhancement produces a high intensity plasma for pre-treatment or O2 plasma production
more robust in ‘dirty’ environments
Highly scalable in terms of power - compared to microwave and radio frequency
plasma treaters/sources - can run at high substrate speeds and powers
The influence of power mode
DC power modes are less expensive than AC as a single cathode is used.
An AC power mode requires 2 electrodes and uses magnet enhancement for higher plasma density – AC better arc suppression than DC – AC better suited to environments with high ‘moisture’ content.
SUMMARY AND OUTLOOK
There are many choices for plasma pre-treatment devices depending upon the speed, type of substrates to be treated, and why the pre-treatment is required.
• DC based plasma sources are effective but AC plasma excitement offers many additional benefits.
• A new generation of etch, pre-treatment and oxidation devices based upon high current positive pulses are a reality.
• Positively pulsed HiPIMS type (hip3) plasma use ion irradiation to more rapidly pre-treat and etch materials compared to DC and AC methods.
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