Plasma Cleaning

Plasma cleaning removes organic contaminants by chemical reaction (O₂ or air 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 observed as a decrease in water contact angle and increased wettability [Figure 1]. A clean, as well as hydrophilic surface, is often critical to promote adhesion and enhance bonding to other surfaces. In addition, plasma can be used to sterilize and remove microbial contaminants on the surface, beneficial for biomedical and biomaterials research applications.

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).

Surfaces can be plasma cleaned 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:

• AFM cantilever tips for surface morphology and frictional force measurements
• Glass and semiconductor wafers prior to subsequent deposition
• Patterned Polydimethylsiloxane (PDMS) substrates for microfluidic device fabrication
• Electron microscopy (EM) grids
• Gold surfaces for self-assembly experiments
• Fibrous polymer scaffolds for cell culturing and tissue engineering
• Carbon nanotubes for use as electrodes
• Quartz crystals for quartz crystal microbalance (QCM) measurements
• Optics and crystals (quartz, Ge, ZnSe) for spectroscopic measurements (ATR-FTIR)
• Nanoparticles plasma-treated to tune particle size and alter surface chemistry

Air or oxygen (O₂) gas is typically used for plasma cleaning. 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, an argon plasma may be preferred for cleaning to minimize further oxidation of surfaces (e.g. metals). Argon plasma cleans by ion bombardment and physical ablation of contaminants off the surface, but does not react with the surface. In some cases, a mixture of Ar/O₂ may be used to clean surfaces with a combination of physical ablation with argon and chemical reaction with oxygen.

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).

• Pressure: 500 mTorr to 1 Torr
• RF power: MEDIUM or HIGH
• Process time: 1-3 minutes
• Low RF power may be used to minimize surface roughening; the process time may require adjustment to compensate for the lower power