A clean surface with increased surface wettability is oftentimes beneficial to promote adhesion and enhance bonding to other surfaces. This article discusses the benefits of plasma treatment in altering surface wettability characteristics for adhesion and other applications, plasma processing guidelines, and examples of contact angle measurements on plasma-treated materials.
Plasma cleaning removes organic contaminants by chemical reaction (air or O2 plasma) or physical ablation (Ar plasma). Plasma treatment also introduces chemical functional groups (carbonyl, carboxyl, hydroxyl) on the surface, rendering most surfaces hydrophilic. This is typically observed as a decrease in water contact angle and increased wettability [Figure 1 and Figure 2]. 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.
Surfaces can be plasma treated to modify surface chemistry without affecting the bulk properties of the material. As such, plasma treatment can be applied to a wide variety of materials as well as complex surface geometries. Below are examples applications and samples that have been treated in our plasma instruments:
Air or oxygen (O2) gas is typically used for plasma cleaning and surface activation. An air or O2 plasma removes organic contaminants by chemical reaction with highly reactive oxygen radicals and ablation by energetic oxygen ions. The plasma also promotes hydroxylation (OH groups) on the surface, rendering the surface more hydrophilic and increasing surface wettability.
Alternatively, argon plasma may be preferred for surface activation to minimize further oxidation of surfaces (e.g. metals). Argon plasma cleans by ion bombardment and physical ablation of contaminants off the surface and can also increase surface hydrophilicity by reaction of the plasma-activated surface upon exposure to ambient air.
For applications that are sensitive to potential contamination from trace impurities in borosilicate glass, a quartz chamber is recommended over the standard Pyrex chamber.
Below are suggested process conditions for plasma cleaning in a Harrick Plasma cleaner (some experimentation may be required to determine optimal process conditions):
Figure 1. Water drop contact angle measurement on 316L stainless steel (a) as received, (b) after chemical clean (ultrasonication in 70% ethanol, acetone, and 40% nitric acid), and (c) after chemical clean and O 2 plasma treatment using a Harrick Plasma cleaner. Data from Mahapatro, A., D. M. Johnson, D. N. Patel, M. D. Feldman, A. A. Ayon, C. M. Agrawal. "Surface Modification of Functional Self-Assembled Monolayers on 316L Stainless Steel Via Lipase Catalysis." Langmuir (2006) 22: 901-905.
Figure 2. Water droplet contact angle measurement on ultrahigh molecular weight polyethylene (UHMWPE) as a function of O 2 plasma treatment time using a Harrick Plasma cleaner. Data from Widmer, M. R., M. Heuberger, J. Vörös, N. D. Spencer. "Influence of Polymer Surface Chemistry on Frictional Properties under Protein-Lubrication Conditions: Implications for Hip-Implant Design." Tribol. Lett. (2001) 10: 111-116.