Fig. 1. The surface of a MEMS device is cleaned with bright, blue oxygen plasma in a plasma etcher to rid it of carbon contaminants. (100mTorr, 50W RF)

Plasma cleaning is the removal of impurities and contaminants from surfaces through the use of an energetic plasma or dielectric barrier discharge (DBD) plasma created from gaseous species. Gases such as argon and oxygen, 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.[1]

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, 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).[2] 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].

Fig. 2. Content of carbon over material depth z: before a sample treatment - diamond points and after the treatment during 1 s. - square points

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.[3]

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.[4] Results of the same applications to surfaces of glass samples coated with Cr and ITO layers are shown in Fig. 3.

Fig. 3. Contact Angle of Water Droplet of 5 μl on glass coated with different materials.

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.[5]

Fig. 4. Surface area of water droplet of 5 μl volume footprint on glass surface versus time t after its treatment. Droplet on untreated glass is shown in inset.

Applications

Fig. 5. Plasma beam cleaning a metal surface

Cleaning & Sterilization

Plasma cleaning removes organics contamination through chemical reaction or physical ablation of hydrocarbons on treated surfaces.[3] 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.[6] 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.[6]

Life Sciences

Cell viability, function, proliferation and differentiation are determined by adhesion to their microenvironment.[8] Plasma is often used as a chemical free means of adding biologically relevant functional groups (carbonyl, carboxyl, hydroxyl, amine, etc) to material surfaces.[9] 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,[10] tissue engineering,[11] implants and more.

  • Tissue Engineering Substrates[11]
  • Polyethyleneterephthalate (PET) cell adhesion[10]
  • Improved Biocompatibility of Implants: vascular grafts,[12] Stainless Steel Screws[13]
  • Long term cell confinement studies[14]
  • Plasma Lithography for Patterning Cell Culture Substrates[15]
  • Cell sorting by strength of adhesion[16]
  • Antibiotic removal by plasma activated steel shavings[17]
  • Single Cell Sequencing[18]

Materials Science

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[19]
  • Work function enhancement in polymer semiconductor heterostructures[20]
  • Improved adhesion of Ultra‐high modulus polyethylene (Spectra) fibers and aramid fibers[21]
  • Plasma Lithography for nanoscale surface structures and quantum dots[22]
  • Micropatterning of thin films[23]

Microfluidics

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.[24]

  • Blood plasma separation[25]
  • Single Cell RNA Sequencing[18]
  • Electroosmotic Flow Valves[26]
  • Wettability Patterning in Microfluidic Devices[27]
  • Long Term Retention of Microfluidic Device Hydrophilicity[28]
  • Improved adhesion to poly (propylene)[29]

Solar Cells & Photovoltaics

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[30]
  • Modify TiO2 Nanosheets to improve hydrogen generation[31]
  • Enhanced conductivity of PEDOT:PSS for better efficiency in ITO-free perovskite solar cells[32]

References

  1. ↑ Evgeny V. Shun’ko & Veniamin V. Belkin (2007). "Cleaning Properties of atomic oxygen excited to metastable state 2s22p4(1S0)". J. Appl. Phys. 102 (8): 083304–1–14. Bibcode:2007JAP...102h3304S. doi:10.1063/1.2794857.
  2. ↑ A. Pizzi; K. L. Mittal (2003). Handbook of Adhesive Technology, Revised and Expanded (2, illustrated, revised ed.). CRC Press. p. 1036. ISBN 978-0824709860.
  3. 1 2 3 Banerjee, K. K.; Kumar, S.; Bremmell, K. E.; Griesser, H. J. (2010-11-01). "Molecular-level removal of proteinaceous contamination from model surfaces and biomedical device materials by air plasma treatment". Journal of Hospital Infection. 76 (3): 234–242. doi:10.1016/j.jhin.2010.07.001. ISSN 0195-6701. PMID 20850199.
  4. ↑ Evgeny V. Shun’ko & Veniamin V. Belkin (2012). "Treatment Surfaces with Atomic Oxygen Excited in Dielectric Barrier Discharge Plasma of O2 Admixed to N2". AIP Advances. 2 (2): 022157–24. Bibcode:2012AIPA....2b2157S. doi:10.1063/1.4732120.
  5. ↑ Plasma Treatment Basics - http://www.plasmaetch.com/plasma-treatment-basics.php
  6. 1 2 3 Raiber, Kevin; Terfort, Andreas; Benndorf, Carsten; Krings, Norman; Strehblow, Hans-Henning (2005-12-05). "Removal of self-assembled monolayers of alkanethiolates on gold by plasma cleaning". Surface Science. 595 (1): 56–63. Bibcode:2005SurSc.595...56R. doi:10.1016/j.susc.2005.07.038. ISSN 0039-6028.
  7. ↑ Sun, Tong; Blanchard, Pierre-Yves; Mirkin, Michael V. (2015-04-21). "Cleaning Nanoelectrodes with Air Plasma". Analytical Chemistry. 87 (8): 4092–4095. doi:10.1021/acs.analchem.5b00488. ISSN 0003-2700. PMID 25839963.
  8. ↑ Khalili, Amelia Ahmad; Ahmad, Mohd Ridzuan (2015-08-05). "A Review of Cell Adhesion Studies for Biomedical and Biological Applications". International Journal of Molecular Sciences. 16 (8): 18149–18184. doi:10.3390/ijms160818149. ISSN 1422-0067. PMC 4581240. PMID 26251901.
  9. ↑ Lerman, Max J.; Lembong, Josephine; Muramoto, Shin; Gillen, Greg; Fisher, John P. (October 2018). "The Evolution of Polystyrene as a Cell Culture Material". Tissue Engineering. Part B, Reviews. 24 (5): 359–372. doi:10.1089/ten.TEB.2018.0056. ISSN 1937-3376. PMC 6199621. PMID 29631491.
  10. 1 2 Pratt, Kerri J.; Williams, Stuart K.; Jarrell, Bruce E. (1989). "Enhanced adherence of human adult endothelial cells to plasma discharge modified polyethylene terephthalate". Journal of Biomedical Materials Research. 23 (10): 1131–1147. doi:10.1002/jbm.820231004. ISSN 1097-4636. PMID 2530233.
  11. 1 2 Beardslee, Luke A.; Stolwijk, Judith; Khaladj, Dimitrius A.; Trebak, Mohamed; Halman, Justin; Torrejon, Karen Y.; Niamsiri, Nuttawee; Bergkvist, Magnus (August 2016). "A sacrificial process for fabrication of biodegradable polymer membranes with submicron thickness: A SACRIFICIAL PROCESS FOR FABRICATION OF BIODEGRADABLE POLYMER MEMBRANES". Journal of Biomedical Materials Research Part B: Applied Biomaterials. 104 (6): 1192–1201. doi:10.1002/jbm.b.33464. PMID 26079689.
  12. ↑ Valence, Sarra de; Tille, Jean-Christophe; Chaabane, Chiraz; Gurny, Robert; Bochaton-Piallat, Marie-Luce; Walpoth, Beat H.; Möller, Michael (2013-09-01). "Plasma treatment for improving cell biocompatibility of a biodegradable polymer scaffold for vascular graft applications". European Journal of Pharmaceutics and Biopharmaceutics. 85 (1): 78–86. doi:10.1016/j.ejpb.2013.06.012. ISSN 0939-6411. PMID 23958319.
  13. ↑ Kumar, Sunil; Simpson, Darren; Smart, Roger St. C. (2007-12-15). "Plasma processing for inducing bioactivity in stainless steel orthopaedic screws". Surface and Coatings Technology. ICMCTF 2007. 202 (4): 1242–1246. doi:10.1016/j.surfcoat.2007.07.075. ISSN 0257-8972.
  14. ↑ Junkin, Michael; Wong, Pak Kin (2011-03-01). "Probing cell migration in confined environments by plasma lithography". Biomaterials. 32 (7): 1848–1855. doi:10.1016/j.biomaterials.2010.11.009. ISSN 0142-9612. PMC 3023939. PMID 21134692.
  15. ↑ Nam, Ki-Hwan; Jamilpour, Nima; Mfoumou, Etienne; Wang, Fei-Yue; Zhang, Donna D.; Wong, Pak Kin (2014-11-07). "Probing Mechanoregulation of Neuronal Differentiation by Plasma Lithography Patterned Elastomeric Substrates". Scientific Reports. 4 (1): 6965. Bibcode:2014NatSR...4E6965N. doi:10.1038/srep06965. ISSN 2045-2322. PMC 4223667. PMID 25376886.
  16. ↑ Blackstone, B. N.; Willard, J. J.; Lee, C. H.; Nelson, M. T.; Hart, R. T.; Lannutti, J. J.; Powell, H. M. (2012-08-21). "Plasma surface modification of electrospun fibers for adhesion-based cancer cell sorting". Integrative Biology. 4 (9): 1112–1121. doi:10.1039/c2ib20025b. PMID 22832548.
  17. ↑ Tran, Van Son; Ngo, Huu Hao; Guo, Wenshan; Ton-That, Cuong; Li, Jianxin; Li, Jixiang; Liu, Yi (2017-12-01). "Removal of antibiotics (sulfamethazine, tetracycline and chloramphenicol) from aqueous solution by raw and nitrogen plasma modified steel shavings". Science of the Total Environment. 601–602: 845–856. Bibcode:2017ScTEn.601..845T. doi:10.1016/j.scitotenv.2017.05.164. hdl:10453/114587. ISSN 0048-9697. PMID 28578242.
  18. 1 2 Gierahn, Todd M.; Wadsworth, Marc H.; Hughes, Travis K.; Bryson, Bryan D.; Butler, Andrew; Satija, Rahul; Fortune, Sarah; Love, J. Christopher; Shalek, Alex K. (April 2017). "Seq-Well: portable, low-cost RNA sequencing of single cells at high throughput". Nature Methods. 14 (4): 395–398. doi:10.1038/nmeth.4179. hdl:1721.1/113430. ISSN 1548-7105. PMC 5376227. PMID 28192419.
  19. ↑ Xiao, Ni; Dong, Xiaochen; Song, Li; Liu, Dayong; Tay, YeeYan; Wu, Shixin; Li, Lain-Jong; Zhao, Yang; Yu, Ting; Zhang, Hua; Huang, Wei (2011-04-26). "Enhanced Thermopower of Graphene Films with Oxygen Plasma Treatment". ACS Nano. 5 (4): 2749–2755. doi:10.1021/nn2001849. hdl:10220/7452. ISSN 1936-0851. PMID 21417404.
  20. ↑ Brown, Thomas M.; Lazzerini, G. Mattia; Parrott, Lisa J.; Bodrozic, V.; Bürgi, Lukas; Cacialli, Franco (2011-04-01). "Time dependence and freezing-in of the electrode oxygen plasma-induced work function enhancement in polymer semiconductor heterostructures". Organic Electronics. 12 (4): 623–633. doi:10.1016/j.orgel.2011.01.015. ISSN 1566-1199.
  21. ↑ Biro, David A.; Pleizier, Gerald; Deslandes, Yves (1993). "Application of the microbond technique. IV. Improved fiber–matrix adhesion by RF plasma treatment of organic fibers". Journal of Applied Polymer Science. 47 (5): 883–894. doi:10.1002/app.1993.070470516. ISSN 1097-4628.
  22. ↑ Junkin, Michael; Watson, Jennifer; Geest, Jonathan P. Vande; Wong, Pak Kin (2009). "Template-Guided Self-Assembly of Colloidal Quantum Dots Using Plasma Lithography". Advanced Materials. 21 (12): 1247–1251. doi:10.1002/adma.200802122. ISSN 1521-4095. S2CID 19900235.
  23. ↑ Kim, Hyejin; Yoon, Bokyung; Sung, Jinwoo; Choi, Dae-Geun; Park, Cheolmin (2008-07-15). "Micropatterning of thin P3HT films via plasma enhanced polymer transfer printing". Journal of Materials Chemistry. 18 (29): 3489–3495. doi:10.1039/B807285J. ISSN 1364-5501.
  24. ↑ Chen, Cheng-fu (2018-06-03). "Characterization of fracture energy and toughness of air plasma PDMS–PDMS bonding by T-peel testing". Journal of Adhesion Science and Technology. 32 (11): 1239–1252. doi:10.1080/01694243.2017.1406877. ISSN 0169-4243. S2CID 139954334.
  25. ↑ Rafeie, Mehdi; Zhang, Jun; Asadnia, Mohsen; Li, Weihua; Warkiani, Majid Ebrahimi (2016-07-19). "Multiplexing slanted spiral microchannels for ultra-fast blood plasma separation". Lab on a Chip. 16 (15): 2791–2802. doi:10.1039/C6LC00713A. ISSN 1473-0189. PMID 27377196.
  26. ↑ Martin, Ina T.; Dressen, Brian; Boggs, Mark; Liu, Yan; Henry, Charles S.; Fisher, Ellen R. (2007). "Plasma Modification of PDMS Microfluidic Devices for Control of Electroosmotic Flow". Plasma Processes and Polymers. 4 (4): 414–424. doi:10.1002/ppap.200600197. ISSN 1612-8869.
  27. ↑ Kim, Samuel C.; Sukovich, David J.; Abate, Adam R. (2015-07-14). "Patterning microfluidic device wettability with spatially-controlled plasma oxidation". Lab on a Chip. 15 (15): 3163–3169. doi:10.1039/C5LC00626K. ISSN 1473-0189. PMC 5531047. PMID 26105774.
  28. ↑ Zhao, Li Hong; Lee, Jennifer; Sen, Pabitra N. (2012-07-01). "Long-term retention of hydrophilic behavior of plasma treated polydimethylsiloxane (PDMS) surfaces stored under water and Luria-Bertani broth". Sensors and Actuators A: Physical. 181: 33–42. doi:10.1016/j.sna.2012.04.038. ISSN 0924-4247.
  29. ↑ Bhat, N. V.; Upadhyay, D. J. (2002-10-24). "Plasma-induced surface modification and adhesion enhancement of polypropylene surface". Journal of Applied Polymer Science. 86 (4): 925–936. doi:10.1002/app.11024. ISSN 0021-8995.
  30. ↑ Sun, Jen-Yu; Tseng, Wei-Hsuan; Lan, Shiang; Lin, Shang-Hong; Yang, Po-Ching; Wu, Chih-I; Lin, Ching-Fuh (2013-02-01). "Performance enhancement in inverted polymer photovoltaics with solution-processed MoOX and air-plasma treatment for anode modification". Solar Energy Materials and Solar Cells. 109: 178–184. doi:10.1016/j.solmat.2012.10.026. ISSN 0927-0248.
  31. ↑ Kong, Xiangchen; Xu, Yiming; Cui, Zhenduo; Li, Zhaoyang; Liang, Yanqin; Gao, Zhonghui; Zhu, Shengli; Yang, Xianjin (2018-08-15). "Defect enhances photocatalytic activity of ultrathin TiO2 (B) nanosheets for hydrogen production by plasma engraving method". Applied Catalysis B: Environmental. 230: 11–17. doi:10.1016/j.apcatb.2018.02.019. ISSN 0926-3373. S2CID 103280998.
  32. ↑ Vaagensmith, Bjorn; Reza, Khan Mamun; Hasan, MD Nazmul; Elbohy, Hytham; Adhikari, Nirmal; Dubey, Ashish; Kantack, Nick; Gaml, Eman; Qiao, Qiquan (2017-10-18). "Environmentally Friendly Plasma-Treated PEDOT:PSS as Electrodes for ITO-Free Perovskite Solar Cells". ACS Applied Materials & Interfaces. 9 (41): 35861–35870. doi:10.1021/acsami.7b10987. ISSN 1944-8244. PMID 28901734.
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