The Use of Oxygen in SEM Plasma Cleaning Equipment - AZoM

In the past, optimum usage of SEM meant baking out one’s column and being cautious about wearing gloves when dealing with samples but as feature sizes decrease this may not be enough.

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Hydrocarbon buildup in SEM vacuum chambers can result in less than ideal resolution. The electron beam reacts with contamination and then deposits carbon on the sample which can result in charging and degrade the image contrast. Traditionally, oxygen plasma has been employed for cleaning surfaces in a vacuum. O2 is split into oxygen ions and radicals which effectively removes organic contaminants.

Low Energy Plasma Cleaners

Low energy plasma cleaners that pull oxygen ions and radicals out of “thin air” have been produced by manufacturers. Although this may seem like “snake oil”, it actually does work. Although the approach is very slow, it really works, has very little setup, and is easy to use. A “dirty” system will involve days or even weeks of the initial cleaning.

The Evactron® De-Contaminator was modified to use welding-grade oxygen. The investigation performed denotes that there are a day and night comparison for speed and effectiveness. When compared to using ambient air for weeks, one or two four-hour sessions with oxygen were shown to be more effective.

Figure 1. Oxygen tank attached to Evactron Decontaminator low energy plasma cleaner. Red tubing connects the bottle/regulator to the intake port of the Evactron.

Figure 2. Before (left) and after (right) 4 hours of chamber cleaning with oxygen plasma.

Working Principle

Plasma works by transforming gas atoms into radicals and ions. Gas atoms are oscillated into a frenzy by high-frequency magnetic fields, breaking bonds and emitting a luminescent glow. The radicals and ions then perform all the work. Oxygen ions and radicals are very effective as cleaning agents. During the cleaning process, the oxygen gas radicals and ions react with the hydrocarbons within the SEM. These reactions produce CO2, CO, and H2O that are then eliminated from the system by the vacuum pumps.

When air is used as the process gas, nitrogen ions are also produced in addition to oxygen ions and radicals. Nitrogen ions contribute to the cleaning process and also recombine with oxygen ions and radicals. However, the probability of long-lived oxygen reactive species is less, as air is 70% nitrogen. When their lifetime is reduced, their mean free path is also reduced, making cleaning farther away from the plasma less likely. When using oxygen as the process gas, there is no predatory nitrogen to get in the way. As oxygen ions and radicals have fewer restrictions, they can move farther and clean better.

Conclusion

It is interesting to know how a low-energy plasma system can be transformed to work with oxygen. A trip to a welding supply store produced a regulator and a bottle of O2. XEI Scientific was all set to start with a bit of tubing and some fittings. With the regulator in place, the bottle is opened and the regulator is hardly opened to generate a constant wisp of oxygen flow. Once the tubing is attached to the leak port of the plasma system, it is ready to go.

References and Further Reading

1. Neal Sullivan, et al., Microscopy and Microanalysis
2. T.C. Isabell AND P.E. Fischione, Plasma Science, .
3. http://en.wikipedia.org/wiki/Plasma_cleaning

This information has been sourced, reviewed and adapted from materials provided by XEI Scientific.

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/ atmospheric pressure), although atmospheric pressure plasmas are now also common.[1]

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.

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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].

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.

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 oxygen gas.[5]

Applications

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]

• Removal of Self Assembled Monolayers of alkanethiolates from gold surfaces[6]
• Residual proteins on biomedical devices[3]
• Nanoelectrode Cleaning[7]

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.

• Tissue Engineering Substrates[11]
• Polyethyleneterephthalate (PET) cell adhesion[10]
• Improved Biocompatibility of Implants: vascular grafts,[12] Stainless Steel Screws[13]
• Long term cell confinement studies[14]
• Plasma Lithography for Patterning Cell Culture Substrates[15]
• Cell sorting by strength of adhesion[16]
• Antibiotic removal by plasma activated steel shavings[17]
• Single Cell Sequencing[18]

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:

• Enhanced Thermopower of Graphene Films[19]
• Work function enhancement in polymer semiconductor heterostructures[20]
• Improved adhesion of Ultra‐high modulus polyethylene (Spectra) fibers and aramid fibers[21]
• Plasma Lithography for nanoscale surface structures and quantum dots[22]
• Micropatterning of thin films[23]

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]

• Blood plasma separation[25]
• Single Cell RNA Sequencing[18]
• Electroosmotic Flow Valves[26]
• Wettability Patterning in Microfluidic Devices[27]
• Long Term Retention of Microfluidic Device Hydrophilicity[28]
• Improved adhesion to poly (propylene)[29]

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

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• Reduction of Molybdenum Oxide (MoO3) enhances short circuit current density[30]
• Modify TiO2 Nanosheets to improve hydrogen generation[31]
• Enhanced conductivity of PEDOT:PSS for better efficiency in ITO-free perovskite solar cells[32]