The use of plasma is an effective way to clean without using hazardous solvents. Plasma is an ionized gas capable of conducting electricity and absorbing energy from an electrical supply. Manmade plasma is generally created in a low-pressure environment. (Lightning and the Aurora Borealis are naturally occurring examples of plasma.) When a gas absorbs electrical energy, its temperature increases causing the ions to vibrate faster and “scrub” a surface.
In semiconductor processing, plasma cleaning is commonly used to prepare a wafer surface prior to wire bonding. Removing contamination (flux) strengthens the bond adhesion, which helps extend device reliability and longevity.
In biomedical applications, plasma cleaning is useful for achieving compatibility between synthetic biomaterials and natural tissues. Surface modification minimizes adverse reactions such as inflammation, infection, and thrombosis formation.
- Wire bond surface preparation
- Removing contaminants (flux) or sterilizing a surface
- Promoting adhesion between two surfaces
- Controlling surface tension to achieve either a hydrophobic or hydrophilic surface
- Increasing biocompatibility
- Improving polymer performance through cross-linking to decrease friction that wears out devices
How Plasma Cleaning Works – Ion Excitation
When a gas absorbs electrical energy, its temperature increases causing the ions to vibrate faster. In an inert gas, such as argon, the excited ions can bombard a surface and remove a small amount of material. In the case of an active gas, such as oxygen, ion bombardment as well as chemical reactions occur. As a result, organic compounds and residues volatilize and are removed.
Radio frequency (RF), microwaves, and alternating or direct current can energize gas plasma. Energetic species in gas plasma include ions, electrons, radicals, metastables, and photons in short-wave ultraviolet (UV) range. The energetic species bombard substrates resulting in an energy transfer from the plasma to the surface. Energy transfers are dissipated throughout the substrate through chemical and physical processes to attain a desirable surface modification – one that reacts with surface depths from several hundred angstroms to 10µm without changing the material’s bulk properties.