Plasma Cleaning
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Plasma cleaning is the removal of impurities and contaminants from surfaces through the use of an energetic plasma or
dielectric barrier discharge Dielectric-barrier discharge (DBD) is the electrical discharge between two electrodes separated by an insulating dielectric barrier. Originally called silent (inaudible) discharge and also known as ozone production discharge or partial disc ...
(DBD) plasma created from gaseous species. Gases such as
argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
and
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
, as well as mixtures such as air and hydrogen/nitrogen are used. The plasma is created by using high frequency voltages (typically kHz to >MHz) to ionise the low pressure gas (typically around 1/1000 atmospheric pressure), although atmospheric pressure plasmas are now also common.


Methods

In plasma, gas atoms are excited to higher energy states and also ionized. As the atoms and molecules 'relax' to their normal, lower energy states they release a photon of light, this results in the characteristic “glow” or light associated with plasma. Different gases give different colors. For example, oxygen plasma emits a light blue color. A plasma’s activated species include
atoms Atoms are the basic particles of the chemical elements. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished from each other ...
,
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s,
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s,
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s,
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...
, metastables, and
photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that ...
in the short wave ultraviolet (vacuum UV, or VUV for short) range. This mixture then interacts with any surface placed in the plasma. 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
vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...
s and are evacuated from the chamber during processing. The resulting surface is ultra-clean. In Fig. 2, a relative content of carbon over material depth is shown before and after cleaning with excited oxygen /sup>. If the part consists of easily oxidized materials such as silver or copper, the treatment uses inert gases such as argon or helium instead. Plasma activated atoms and ions behave like a molecular sandblast and can break down organic contaminants. These contaminants vaporize during processing and are evacuated from the chamber. Most of these by-products are small quantities of gases, such as carbon dioxide and water vapor with trace amounts of carbon monoxide and other hydrocarbons. Whether or not organic removal is complete can be assessed with
contact angle The contact angle (symbol ) is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle between the surface tangent on the liquid–vapor interface and the tangent on the solid–liquid interfac ...
measurements. When an organic contaminant is present, the
contact angle The contact angle (symbol ) is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle between the surface tangent on the liquid–vapor interface and the tangent on the solid–liquid interfac ...
of water with the device is high. Contaminant removal reduces the
contact angle The contact angle (symbol ) is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle between the surface tangent on the liquid–vapor interface and the tangent on the solid–liquid interfac ...
to that characteristic of contact with the pure substrate. In addition, XPS and AFM are often used to validate surface cleaning and sterilization applications. If a surface to be treated is coated with a patterned conductive layer (metal,
ITO Ito, Itō or Itoh may refer to: Places * Ito Island, an island of Milne Bay Province, Papua New Guinea * Ito Airport, an airport in the Democratic Republic of the Congo * Ito District, Wakayama, a district located in Wakayama Prefecture, Japa ...
), treatment by direct contact with plasma (capable for contraction to microarcs) could be destructive. In this case, cleaning by neutral atoms excited in plasma to metastable state can be applied. Results of the same applications to surfaces of glass samples coated with Cr and
ITO Ito, Itō or Itoh may refer to: Places * Ito Island, an island of Milne Bay Province, Papua New Guinea * Ito Airport, an airport in the Democratic Republic of the Congo * Ito District, Wakayama, a district located in Wakayama Prefecture, Japa ...
layers are shown in Fig. 3. After treatment, the
contact angle The contact angle (symbol ) is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle between the surface tangent on the liquid–vapor interface and the tangent on the solid–liquid interfac ...
of a water droplet is decreased becoming less than its value on the untreated surface. In Fig. 4, the relaxation curve for droplet footprint is shown for glass sample. A photograph of the same droplet on the untreated surface is shown in Fig. 4 inset. Surface relaxation time corresponding to a data shown in Fig. 4 is about 4 hours. Plasma ashing is a process that uses plasma cleaning solely to remove carbon. Plasma ashing is always done with O2 gas.


Applications


Cleaning & Sterilization

Plasma cleaning removes organics contamination through chemical reaction or physical ablation of hydrocarbons on treated surfaces. Chemically reactive process gases (air, oxygen) react with hydrocarbon monolayers to form gaseous products that are swept away by the continuous gas flow in the plasma cleaner chamber. Plasma cleaning can be used in place of wet chemical processes, such as piranha etching, which contain dangerous chemicals, increase danger of reagent contamination and risk etching treated surfaces. * Removal of Self Assembled Monolayers of alkanethiolates from gold surfaces * Residual proteins on biomedical devices * Nanoelectrode Cleaning


Life Sciences This list of life sciences comprises the branches of science that involve the scientific study of life – such as microorganisms, plants, and animals including human beings. This science is one of the two major branches of natural science, ...

Cell viability, function, proliferation and differentiation are determined by adhesion to their microenvironment. Plasma is often used as a chemical free means of adding biologically relevant functional groups (carbonyl, carboxyl, hydroxyl, amine, etc) to material surfaces. As a result, plasma cleaning improves material
biocompatibility Biocompatibility is related to the behavior of biomaterials in various contexts. The term refers to the ability of a material to perform with an appropriate host response in a specific situation. The ambiguity of the term reflects the ongoin ...
or
bioactivity In pharmacology, biological activity or pharmacological activity describes the beneficial or adverse effects of a drug on living matter. When a drug is a complex chemical mixture, this activity is exerted by the substance's active ingredient or ...
and removes contaminating proteins and microbes. Plasma cleaners are a general tool in the life sciences, being used to activate surfaces for
cell culture Cell culture or tissue culture is the process by which cell (biology), cells are grown under controlled conditions, generally outside of their natural environment. After cells of interest have been Cell isolation, isolated from living tissue, ...
,
tissue engineering Tissue engineering is a biomedical engineering discipline that uses a combination of cells, engineering, materials methods, and suitable biochemical and physicochemical factors to restore, maintain, improve, or replace different types of biolo ...
, implants and more. * Tissue Engineering Substrates * Polyethyleneterephthalate (PET) cell adhesion * Improved Biocompatibility of Implants: vascular grafts, Stainless Steel Screws * Long term cell confinement studies * Plasma Lithography for Patterning Cell Culture Substrates * Cell sorting by strength of adhesion * Antibiotic removal by plasma activated steel shavings * Single Cell Sequencing


Materials Science Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries. The intellectual origins of materials sci ...

Surface wetting and modification is a fundamental tool in materials science for enhancing material characteristics without affecting bulk properties. Plasma Cleaning is used to alter material surface chemistries through the introduction of polar functional groups. Increased surface hydrophilicity (wetting) following plasma treatment improves adhesion with aqueous coatings, adhesives, inks and epoxies: * Enhanced Thermopower of Graphene Films * Work function enhancement in polymer semiconductor heterostructures * Improved adhesion of Ultra‐high modulus polyethylene (Spectra) fibers and aramid fibers * Plasma Lithography for nanoscale surface structures and quantum dots * Micropatterning of thin films


Microfluidics Microfluidics refers to a system that manipulates a small amount of fluids (10−9 to 10−18 liters) using small channels with sizes of ten to hundreds of micrometres. It is a multidisciplinary field that involves molecular analysis, molecular bi ...

The unique characteristics of micro or nanoscale fluid flow are harnessed by microfluidic devices for a wide variety of research applications. The most widely used material for microfluidic device prototyping is polydimethylsiloxane (PDMS), for its rapid development and adjustable material properties. Plasma cleaning is used to permanently bond PDMS Microfluidic chips with glass slides or PDMS slabs to create water-tight microchannels. * Blood plasma separation * Single Cell RNA Sequencing * Electroosmotic Flow Valves * Wettability Patterning in Microfluidic Devices * Long Term Retention of Microfluidic Device Hydrophilicity * Improved adhesion to poly (propylene)


Solar Cells A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.
&
Photovoltaics Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commerciall ...

Plasma has been used to enhance the performance of solar cells and energy conversion within photovoltaic devices: * Reduction of Molybdenum Oxide (MoO3) enhances short circuit current density * Modify TiO2 Nanosheets to improve hydrogen generation * Enhanced conductivity of PEDOT:PSS for better efficiency in ITO-free perovskite solar cells


References

{{DEFAULTSORT:Plasma Cleaning Plasma processing Semiconductor device fabrication Cleaning methods