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Knowledge Base Fact Sheet

Explanation of Low Pressure Plasma Cleaning (IKB-014)

Explanation of the process and use of low pressure plasma cleaning.

Low Pressure Plasma Treatment uses controlled vacuum plasma for plasma cleaning and etching, in order to alter the surface of a material and to improve bonding, printing, painting, coating, or wet-ability. The process is performed in a plasma chamber under vacuum pressure. It is commonly used in the manufacturing of electronic devices. Almost any dry material can be treated in a plasma chamber.

Low Pressure Plasma Cleaning involves the removal of impurities and contaminants from surfaces through the use of energised plasma created from gaseous species. Gases such as Argon and Oxygen are used, as well as mixtures such as air and Hydrogen / Nitrogen. The plasma is created by using high frequency (RF) voltages (typically >10MHz) or Microwave frequency voltages to ionize the low-pressure gas (typically around 150mTorr-400mTorr), although atmospheric pressure plasmas are now also common.

In plasma, gas atoms are excited to higher energy states and also ionised. As the atoms and molecules ‘relax’ to their normal lower energy states, they release a photon of light which this results in the glow or light associated with plasma. Different mixtures of process gases give different colours. For example, oxygen plasma emits a light blue colour where as an oxygen / argon mixture is pink.

Plasma’s activated species include atoms, ions, electrons, free radicals, metastables and photons in the short-wave ultraviolet range (vacuum UV, or VUV for short). This ‘mixture’, which incidentally is around room temperature, then interacts with any surface placed in the plasma chamber.

Depending on the power of the RF energy supplied, a side effect of the Plasma process is that the part being treated can rise in temperature. Although temperature controlled chambers can be used to control and increase the cleaning/etch rate (60-90 degrees Celsius can increase the etch rate by up to four times), temperature sensitive components can be processed at >15 degrees Celsius.

If the gas used is oxygen, the plasma is an effective, economical, environmentally safe method for critical cleaning. The VUV energy is very effective in the breaking of most organic bonds (i.e., C–H, C–C, C=C, C–O, and C–N) of surface contaminants. This helps to break apart high molecular weight contaminants. A second cleaning action is carried out by the oxygen species created in the plasma (O2+, O2-, O3, O, O+, O-, ionised ozone, metastable excited oxygen, and free electrons). These species react with organic contaminants to form H2O, CO, CO2, and lower molecular weight hydrocarbons. These compounds have relatively high vapour pressures and are evacuated from the chamber during processing. The resulting surface is ultra-clean.

PE-100-Benchtop Plasma Cleaner
Example Low Pressure Plasma Cleaning / Etching Equipment

Low Pressure Plasma Chamber View

If the part to be treated consists of easily oxidised materials such as silver or copper, inert gases such as argon or helium are used instead. The plasma activated atoms and ions behave like a molecular sandblast and can break down organic contaminants. These contaminants are again vaporized and evacuated from the chamber during processing.

Low Pressure Plasma Treatment systems are able to remove 100% of these organic contaminants. This increases the bond strength of a solder or glue, increases or decreases wettability, and ensures any type of printing, painting, or coating remains on the object’s surface.

Typically, Low Pressure Plasma Cleaning process can last between 2-20 minutes, upon completion of the plasma cycle, the process chamber is evacuated under vacuum finally to remove any contamination from the Plasma process.

For more information on Plasma Etch, plasma cleaning and etching equipment, please click HERE.

Author

Date

Version

Author

Adam Marshall

Date

23 April 2020

Version

IKB014 Rev. 5

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