It is often necessary to conduct surface cleaning in order to remove organic contaminants and to optimize surfaces for ensuing processing. This article outlines the advantages of plasma cleaning, its applications, and some processing recommendations when making use of plasma instruments.
Advantages of Plasma Cleaning
Plasma cleaning eliminates biological contaminants through chemical reaction (O2 or air plasma) or physical ablation (Ar plasma). In addition to this, plasma treatment introduces chemical functional groups (carbonyl, carboxyl, hydroxyl) on the surface, making the majority of surfaces hydrophilic.
This can be seen in a decrease in water contact angle and increased wettability [Figure 1]. A surface that is both clean and hydrophilic is often vital to encourage adhesion and to improve bonding to another surface. Additionally, plasma can be applied to sterilize and eliminate microbial contaminants on the surface, which is of a great benefit to 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 AL, Menke EJ, Dubowski Y, Newberg JT, Penner RM, Hemminger JC, Wingen LM, Brauers T and Finlayson-Pitts BJ. "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 any impact on the main properties of the material. Consequently, plasma treatment can be carried out on a broad range of materials, as well as complex surface geometries. The below list contains applications and samples that have been treated with 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 generally used for plasma cleaning. An air or O2 plasma eliminates biological contaminants through chemical reaction with highly reactive oxygen radicals and ablation by energetic oxygen ions. The plasma also encourages hydroxylation (OH groups) on the surface, making the surface more hydrophilic and improving surface wettability.
As an alternative, an argon plasma may be favored for cleaning in order minimize additional oxidation of surfaces (e.g. metals). Argon plasma does not react with the surface directly, but instead, cleans through ion bombardment and physical ablation of contaminants off the surface. In some instances, a mixture of Ar and O2 may be applied to clean surfaces using a combined process of physical ablation with argon and chemical reaction with oxygen.
For applications that are susceptible to possible contamination from trace impurities in borosilicate glass, a quartz chamber is advised ahead of the standard Pyrex chamber.
Below are recommended process conditions for plasma cleaning in a Harrick Plasma cleaner. Please note that it may be necessary to experiment to determine optimal process conditions.
- Pressure: 500 mTorr to 1 Torr
- RF power: MEDIUM or HIGH
- Process time: One to three minutes
- Low RF power may be used to minimize surface roughening; the process time may need to be adjusted to offset the lower power.
This information has been sourced, reviewed and adapted from materials provided by Harrick Plasma.
For more information on this source, please visit Harrick Plasma.