Plasma cleaning

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, molecules, ions, electrons, free radicals, metastables, and photons 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).{{cite book|title=Handbook of Adhesive Technology, Revised and Expanded|date=2003|publisher=CRC Press|isbn=978-0824709860|page=1036|url=https://books.google.com/books?id=XYbHqiapRjsC|author=A. Pizzi|edition=2, illustrated, revised|author2=K. L. Mittal}} These species react with organic contaminants to form H₂O, CO, CO₂, and lower molecular weight hydrocarbons. These compounds have relatively high vapor pressures 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 ^[1].

File:ShunkoDBDFig6.tif

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 measurements. When an organic contaminant is present, the contact angle of water with the device is high. Contaminant removal reduces the contact angle to that characteristic of contact with the pure substrate. In addition, XPS and AFM are often used to validate surface cleaning and sterilization applications.{{Cite journal|last1=Banerjee|first1=K. K.|last2=Kumar|first2=S.|last3=Bremmell|first3=K. E.|last4=Griesser|first4=H. J.|date=2010-11-01|title=Molecular-level removal of proteinaceous contamination from model surfaces and biomedical device materials by air plasma treatment|journal=Journal of Hospital Infection|volume=76|issue=3|pages=234–242|doi=10.1016/j.jhin.2010.07.001|pmid=20850199|issn=0195-6701}}

If a surface to be treated is coated with a patterned conductive layer (metal, ITO), 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.{{cite journal |author=Evgeny V. Shun’ko |author2=Veniamin V. Belkin |name-list-style=amp|title=Treatment Surfaces with Atomic Oxygen Excited in Dielectric Barrier Discharge Plasma of O₂ Admixed to N₂ |journal= AIP Advances|volume= 2 |issue= 2|pages= 022157–24|doi=10.1063/1.4732120|bibcode= 2012AIPA....2b2157S|year=2012 |doi-access=free}} Results of the same applications to surfaces of glass samples coated with Cr and ITO layers are shown in Fig. 3.

File:ContactAngleWaterWiki.tif

After treatment, the contact angle 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.Plasma Treatment Basics - http://www.plasmaetch.com/plasma-treatment-basics.php

File:ShunkoDBDFig7.tif

File:Plasma beam cleaning a metal surface.jpg

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.{{Cite journal|last1=Raiber|first1=Kevin|last2=Terfort|first2=Andreas|last3=Benndorf|first3=Carsten|last4=Krings|first4=Norman|last5=Strehblow|first5=Hans-Henning|date=2005-12-05|title=Removal of self-assembled monolayers of alkanethiolates on gold by plasma cleaning|journal=Surface Science|volume=595|issue=1|pages=56–63|doi=10.1016/j.susc.2005.07.038|bibcode=2005SurSc.595...56R|issn=0039-6028}} 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{{Cite journal|last1=Sun|first1=Tong|last2=Blanchard|first2=Pierre-Yves|last3=Mirkin|first3=Michael V.|date=2015-04-21|title=Cleaning Nanoelectrodes with Air Plasma|journal=Analytical Chemistry|volume=87|issue=8|pages=4092–4095|doi=10.1021/acs.analchem.5b00488|pmid=25839963|issn=0003-2700}}

Cell viability, function, proliferation and differentiation are determined by adhesion to their microenvironment.{{Cite journal|last1=Khalili|first1=Amelia Ahmad|last2=Ahmad|first2=Mohd Ridzuan|date=2015-08-05|title=A Review of Cell Adhesion Studies for Biomedical and Biological Applications|journal=International Journal of Molecular Sciences|volume=16|issue=8|pages=18149–18184|doi=10.3390/ijms160818149|issn=1422-0067|pmc=4581240|pmid=26251901|doi-access=free}} Plasma is often used as a chemical free means of adding biologically relevant functional groups (carbonyl, carboxyl, hydroxyl, amine, etc) to material surfaces.{{Cite journal|last1=Lerman|first1=Max J.|last2=Lembong|first2=Josephine|last3=Muramoto|first3=Shin|last4=Gillen|first4=Greg|last5=Fisher|first5=John P.|date=October 2018|title=The Evolution of Polystyrene as a Cell Culture Material|journal=Tissue Engineering. Part B, Reviews|volume=24|issue=5|pages=359–372|doi=10.1089/ten.TEB.2018.0056|issn=1937-3376|pmc=6199621|pmid=29631491}} As a result, plasma cleaning improves material biocompatibility or bioactivity and removes contaminating proteins and microbes. Plasma cleaners are a general tool in the life sciences, being used to activate surfaces for cell culture,{{Cite journal|last1=Pratt|first1=Kerri J.|last2=Williams|first2=Stuart K.|last3=Jarrell|first3=Bruce E.|date=1989|title=Enhanced adherence of human adult endothelial cells to plasma discharge modified polyethylene terephthalate|journal=Journal of Biomedical Materials Research|language=en|volume=23|issue=10|pages=1131–1147|doi=10.1002/jbm.820231004|pmid=2530233|issn=1097-4636}} tissue engineering,{{Cite journal|last1=Beardslee|first1=Luke A.|last2=Stolwijk|first2=Judith|last3=Khaladj|first3=Dimitrius A.|last4=Trebak|first4=Mohamed|last5=Halman|first5=Justin|last6=Torrejon|first6=Karen Y.|last7=Niamsiri|first7=Nuttawee|last8=Bergkvist|first8=Magnus|date=August 2016|title=A sacrificial process for fabrication of biodegradable polymer membranes with submicron thickness: A SACRIFICIAL PROCESS FOR FABRICATION OF BIODEGRADABLE POLYMER MEMBRANES|journal=Journal of Biomedical Materials Research Part B: Applied Biomaterials|language=en|volume=104|issue=6|pages=1192–1201|doi=10.1002/jbm.b.33464|pmid=26079689}} implants and more.

• Tissue Engineering Substrates
• Polyethyleneterephthalate (PET) cell adhesion
• Improved Biocompatibility of Implants: vascular grafts,{{Cite journal|last1=Valence|first1=Sarra de|last2=Tille|first2=Jean-Christophe|last3=Chaabane|first3=Chiraz|last4=Gurny|first4=Robert|last5=Bochaton-Piallat|first5=Marie-Luce|last6=Walpoth|first6=Beat H.|last7=Möller|first7=Michael|date=2013-09-01|title=Plasma treatment for improving cell biocompatibility of a biodegradable polymer scaffold for vascular graft applications|journal=European Journal of Pharmaceutics and Biopharmaceutics|volume=85|issue=1|pages=78–86|doi=10.1016/j.ejpb.2013.06.012|pmid=23958319|issn=0939-6411}} Stainless Steel Screws{{Cite journal|last1=Kumar|first1=Sunil|last2=Simpson|first2=Darren|last3=Smart|first3=Roger St. C.|date=2007-12-15|title=Plasma processing for inducing bioactivity in stainless steel orthopaedic screws|journal=Surface and Coatings Technology|series=ICMCTF 2007|volume=202|issue=4|pages=1242–1246|doi=10.1016/j.surfcoat.2007.07.075|issn=0257-8972}}
• Long term cell confinement studies{{Cite journal|last1=Junkin|first1=Michael|last2=Wong|first2=Pak Kin|date=2011-03-01|title=Probing cell migration in confined environments by plasma lithography|journal=Biomaterials|volume=32|issue=7|pages=1848–1855|doi=10.1016/j.biomaterials.2010.11.009|pmid=21134692|pmc=3023939|issn=0142-9612}}
• Plasma Lithography for Patterning Cell Culture Substrates{{Cite journal|last1=Nam|first1=Ki-Hwan|last2=Jamilpour|first2=Nima|last3=Mfoumou|first3=Etienne|last4=Wang|first4=Fei-Yue|last5=Zhang|first5=Donna D.|last6=Wong|first6=Pak Kin|date=2014-11-07|title=Probing Mechanoregulation of Neuronal Differentiation by Plasma Lithography Patterned Elastomeric Substrates|journal=Scientific Reports|language=en|volume=4|issue=1|pages=6965|doi=10.1038/srep06965|pmid=25376886|pmc=4223667|bibcode=2014NatSR...4E6965N|issn=2045-2322}}
• Cell sorting by strength of adhesion{{Cite journal|last1=Blackstone|first1=B. N.|last2=Willard|first2=J. J.|last3=Lee|first3=C. H.|last4=Nelson|first4=M. T.|last5=Hart|first5=R. T.|last6=Lannutti|first6=J. J.|last7=Powell|first7=H. M.|date=2012-08-21|title=Plasma surface modification of electrospun fibers for adhesion-based cancer cell sorting|url=https://academic.oup.com/ib/article/4/9/1112/5204468|journal=Integrative Biology|language=en|volume=4|issue=9|pages=1112–1121|doi=10.1039/c2ib20025b|pmid=22832548|url-access=subscription}}
• Antibiotic removal by plasma activated steel shavings{{Cite journal|last1=Tran|first1=Van Son|last2=Ngo|first2=Huu Hao|last3=Guo|first3=Wenshan|last4=Ton-That|first4=Cuong|last5=Li|first5=Jianxin|last6=Li|first6=Jixiang|last7=Liu|first7=Yi|date=2017-12-01|title=Removal of antibiotics (sulfamethazine, tetracycline and chloramphenicol) from aqueous solution by raw and nitrogen plasma modified steel shavings|journal=Science of the Total Environment|volume=601-602|pages=845–856|doi=10.1016/j.scitotenv.2017.05.164|pmid=28578242|bibcode=2017ScTEn.601..845T|hdl=10453/114587|issn=0048-9697|hdl-access=free}}
• Single Cell Sequencing{{Cite journal|last1=Gierahn|first1=Todd M.|last2=Wadsworth|first2=Marc H.|last3=Hughes|first3=Travis K.|last4=Bryson|first4=Bryan D.|last5=Butler|first5=Andrew|last6=Satija|first6=Rahul|last7=Fortune|first7=Sarah|last8=Love|first8=J. Christopher|last9=Shalek|first9=Alex K.|date=April 2017|title=Seq-Well: portable, low-cost RNA sequencing of single cells at high throughput|journal=Nature Methods|language=en|volume=14|issue=4|pages=395–398|doi=10.1038/nmeth.4179|pmid=28192419|pmc=5376227|issn=1548-7105|hdl=1721.1/113430}}

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{{Cite journal|last1=Xiao|first1=Ni|last2=Dong|first2=Xiaochen|last3=Song|first3=Li|last4=Liu|first4=Dayong|last5=Tay|first5=YeeYan|last6=Wu|first6=Shixin|last7=Li|first7=Lain-Jong|last8=Zhao|first8=Yang|last9=Yu|first9=Ting|last10=Zhang|first10=Hua|last11=Huang|first11=Wei|date=2011-04-26|title=Enhanced Thermopower of Graphene Films with Oxygen Plasma Treatment|journal=ACS Nano|volume=5|issue=4|pages=2749–2755|doi=10.1021/nn2001849|pmid=21417404|hdl=10220/7452 |issn=1936-0851|hdl-access=free}}
• Work function enhancement in polymer semiconductor heterostructures{{Cite journal|last1=Brown|first1=Thomas M.|last2=Lazzerini|first2=G. Mattia|last3=Parrott|first3=Lisa J.|last4=Bodrozic|first4=V.|last5=Bürgi|first5=Lukas|last6=Cacialli|first6=Franco|date=2011-04-01|title=Time dependence and freezing-in of the electrode oxygen plasma-induced work function enhancement in polymer semiconductor heterostructures|journal=Organic Electronics|volume=12|issue=4|pages=623–633|doi=10.1016/j.orgel.2011.01.015|issn=1566-1199}}
• Improved adhesion of Ultra‐high modulus polyethylene (Spectra) fibers and aramid fibers{{Cite journal|last1=Biro|first1=David A.|last2=Pleizier|first2=Gerald|last3=Deslandes|first3=Yves|date=1993|title=Application of the microbond technique. IV. Improved fiber–matrix adhesion by RF plasma treatment of organic fibers|journal=Journal of Applied Polymer Science|language=en|volume=47|issue=5|pages=883–894|doi=10.1002/app.1993.070470516|issn=1097-4628}}
• Plasma Lithography for nanoscale surface structures and quantum dots{{Cite journal|last1=Junkin|first1=Michael|last2=Watson|first2=Jennifer|last3=Geest|first3=Jonathan P. Vande|last4=Wong|first4=Pak Kin|date=2009|title=Template-Guided Self-Assembly of Colloidal Quantum Dots Using Plasma Lithography|journal=Advanced Materials|volume=21|issue=12|pages=1247–1251|doi=10.1002/adma.200802122|s2cid=19900235 |issn=1521-4095}}
• Micropatterning of thin films{{Cite journal|last1=Kim|first1=Hyejin|last2=Yoon|first2=Bokyung|last3=Sung|first3=Jinwoo|last4=Choi|first4=Dae-Geun|last5=Park|first5=Cheolmin|date=2008-07-15|title=Micropatterning of thin P3HT films via plasma enhanced polymer transfer printing|journal=Journal of Materials Chemistry|language=en|volume=18|issue=29|pages=3489–3495|doi=10.1039/B807285J|issn=1364-5501}}

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.{{Cite journal|last=Chen|first=Cheng-fu|date=2018-06-03|title=Characterization of fracture energy and toughness of air plasma PDMS–PDMS bonding by T-peel testing|journal=Journal of Adhesion Science and Technology|volume=32|issue=11|pages=1239–1252|doi=10.1080/01694243.2017.1406877|s2cid=139954334|issn=0169-4243}}

• Blood plasma separation{{Cite journal|last1=Rafeie|first1=Mehdi|last2=Zhang|first2=Jun|last3=Asadnia|first3=Mohsen|last4=Li|first4=Weihua|last5=Warkiani|first5=Majid Ebrahimi|date=2016-07-19|title=Multiplexing slanted spiral microchannels for ultra-fast blood plasma separation|journal=Lab on a Chip|language=en|volume=16|issue=15|pages=2791–2802|doi=10.1039/C6LC00713A|pmid=27377196|issn=1473-0189}}
• Single Cell RNA Sequencing
• Electroosmotic Flow Valves{{Cite journal|last1=Martin|first1=Ina T.|last2=Dressen|first2=Brian|last3=Boggs|first3=Mark|last4=Liu|first4=Yan|last5=Henry|first5=Charles S.|last6=Fisher|first6=Ellen R.|date=2007|title=Plasma Modification of PDMS Microfluidic Devices for Control of Electroosmotic Flow|journal=Plasma Processes and Polymers|language=en|volume=4|issue=4|pages=414–424|doi=10.1002/ppap.200600197|issn=1612-8869}}
• Wettability Patterning in Microfluidic Devices{{Cite journal|last1=Kim|first1=Samuel C.|last2=Sukovich|first2=David J.|last3=Abate|first3=Adam R.|date=2015-07-14|title=Patterning microfluidic device wettability with spatially-controlled plasma oxidation|journal=Lab on a Chip|language=en|volume=15|issue=15|pages=3163–3169|doi=10.1039/C5LC00626K|pmid=26105774|pmc=5531047|issn=1473-0189}}
• Long Term Retention of Microfluidic Device Hydrophilicity{{Cite journal|last1=Zhao|first1=Li Hong|last2=Lee|first2=Jennifer|last3=Sen|first3=Pabitra N.|date=2012-07-01|title=Long-term retention of hydrophilic behavior of plasma treated polydimethylsiloxane (PDMS) surfaces stored under water and Luria-Bertani broth|journal=Sensors and Actuators A: Physical|volume=181|pages=33–42|doi=10.1016/j.sna.2012.04.038|issn=0924-4247}}
• Improved adhesion to poly (propylene){{Cite journal |last1=Bhat |first1=N. V. |last2=Upadhyay |first2=D. J. |date=2002-10-24 |title=Plasma-induced surface modification and adhesion enhancement of polypropylene surface |journal=Journal of Applied Polymer Science |language=en |volume=86 |issue=4 |pages=925–936 |doi=10.1002/app.11024 |issn=0021-8995|doi-access=free }}

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{{Cite journal|last1=Sun|first1=Jen-Yu|last2=Tseng|first2=Wei-Hsuan|last3=Lan|first3=Shiang|last4=Lin|first4=Shang-Hong|last5=Yang|first5=Po-Ching|last6=Wu|first6=Chih-I|last7=Lin|first7=Ching-Fuh|date=2013-02-01|title=Performance enhancement in inverted polymer photovoltaics with solution-processed MoOX and air-plasma treatment for anode modification|journal=Solar Energy Materials and Solar Cells|volume=109|pages=178–184|doi=10.1016/j.solmat.2012.10.026|issn=0927-0248}}
• Modify TiO₂ Nanosheets to improve hydrogen generation{{Cite journal|last1=Kong|first1=Xiangchen|last2=Xu|first2=Yiming|last3=Cui|first3=Zhenduo|last4=Li|first4=Zhaoyang|last5=Liang|first5=Yanqin|last6=Gao|first6=Zhonghui|last7=Zhu|first7=Shengli|last8=Yang|first8=Xianjin|date=2018-08-15|title=Defect enhances photocatalytic activity of ultrathin TiO₂ (B) nanosheets for hydrogen production by plasma engraving method|journal=Applied Catalysis B: Environmental|volume=230|pages=11–17|doi=10.1016/j.apcatb.2018.02.019|s2cid=103280998|issn=0926-3373}}
• Enhanced conductivity of PEDOT:PSS for better efficiency in ITO-free perovskite solar cells{{Cite journal|last1=Vaagensmith|first1=Bjorn|last2=Reza|first2=Khan Mamun|last3=Hasan|first3=MD Nazmul|last4=Elbohy|first4=Hytham|last5=Adhikari|first5=Nirmal|last6=Dubey|first6=Ashish|last7=Kantack|first7=Nick|last8=Gaml|first8=Eman|last9=Qiao|first9=Qiquan|date=2017-10-18|title=Environmentally Friendly Plasma-Treated PEDOT:PSS as Electrodes for ITO-Free Perovskite Solar Cells|journal=ACS Applied Materials & Interfaces|volume=9|issue=41|pages=35861–35870|doi=10.1021/acsami.7b10987|pmid=28901734|issn=1944-8244}}

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