WHAT IS PLASMA?

Plasma, the fourth state of matter, is a distinct processing medium for surface treatment and surface modification. This note discusses the nature of plasma and how plasma is formed, its unique advantages, and the types of surface interactions that are possible during plasma treatment.

Nature of Plasma

Plasma is a partially ionized gas consisting of electrons, ions and neutral atoms or molecules. Although the plasma electrons are at a much higher temperature (around 104 K ) than the neutral gas species, the plasma as a whole is at near-ambient temperature. The plasma electron density is typically around 1010 cm-3.

Plasma Formation

Plasma is generated when a radio frequency (RF) oscillating electric field is generated in the gas, either through the use of capacitive plates or through magnetic induction. At sufficiently low pressures, the combined effect of the electric field acceleration of electrons and elastic scattering of the electrons with neutral atoms or field lines leads to heating of the electrons. When electrons gain kinetic energy in excess of the first ionization threshold in the neutral gas species, electron-neutral collisions lead to further ionization, yielding additional free electrons that are heated in turn.

Plasma Advantages

Plasma treatment affects the surface of a material without altering the bulk material properties. In addition, the plasma forms at near-ambient temperature, minimizing the risk of damage to heat-sensitive materials. Depending on process gases and conditions, plasma can clean, activate, or chemically modify surfaces. As such, plasma treatment can be applied to many different materials as well as complex surface geometries, including glass coverslips and slides, semiconductor wafers, polymer fibers and fibrous scaffolds, oxide and metal nanoparticles, and porous membranes.

BENEFITS OF SURFACE CHEMISTRY MODIFICATION

With plasma treatment, surfaces can be modified by attachment or adsorption of functional groups to alter surface properties for specific applications. The functional groups introduced can be tailored depending on the process gas used and, in turn, the surface wettability may be altered to be hydrophilic [Figure 1] or hydrophobic [Figure 2] with the appropriate gas. Increased wettability prepares the surface for subsequent processing (e.g. film deposition or adsorption of molecules) by improving surface coverage and spreading of coatings and enhancing adhesion between two surfaces, while creating a more hydrophobic surface may be critical for self-cleaning or where water penetration is undesirable.

Figure 1. Water droplet contact angle measurements on 3 different borosilicate glass surfaces: (a) halocarbon wax-coated (92°), (b) untreated (32°), and (c) argon plasma-cleaned using a Harrick Plasma cleaner (<10°).
Source: Sumner, A. L., E. J. Menke, Y. Dubowski, J. T. Newberg, R. M. Penner, J. C. Hemminger, L. M. Wingen, T. Brauers, B. J. Finlayson-Pitts. “The Nature of Water on Surfaces of Laboratory Systems and Implications for Heterogeneous Chemistry in the Troposphere.” Phys. Chem. Chem. Phys. (2004) 6: 604-613 – Reproduced by permission of The Royal Society of Chemistry (http://www.rsc.org/pccp).
Figure 2. Water droplet contact angle as a function of O2 plasma treatment time, using a Harrick Plasma cleaner, on poly(tetrafluoroethylene) (PTFE), indicating increased hydrophobicity. Plasma treatment produces nanoscale roughness that increases hydrophobicity.
Data from Lee, S.-J., B.-G. Paik, G.-B. Kim, Y.-G. Jang. “Self-Cleaning Features of Plasma-Treated Surfaces with Self-Assembled Monolayer Coating.” Jpn. J. Appl. Phys. (2006) 45: 912-918.
Figure 3. Water droplet contact angle as a function of O2 plasma treatment time, using a Harrick Plasma cleaner, on poly(tetrafluoroethylene) (PTFE), indicating increased hydrophobicity. Plasma treatment produces nanoscale roughness that increases hydrophobicity.
Data from Lee, S.-J., B.-G. Paik, G.-B. Kim, Y.-G. Jang. “Self-Cleaning Features of Plasma-Treated Surfaces with Self-Assembled Monolayer Coating.” Jpn. J. Appl. Phys. (2006) 45: 912-918.

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