Plasma cleaning

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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) Oxygen Plasma Cleaning.JPG
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]

Contents

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 ShunkoDBDFig6.tif
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 ml on glass coated with different materials. ContactAngleWaterWiki.tif
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 ml volume footprint on glass surface versus time t after its treatment. Droplet on untreated glass is shown in inset. ShunkoDBDFig7.tif
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 Plasma beam cleaning a metal surface.jpg
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.

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:

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]

Solar Cells & Photovoltaics

Plasma has been used to enhance the performance of solar cells and energy conversion within photovoltaic devices:

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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.
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  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.
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  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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