The Four Factors Affecting How Your Plasma Looks

A basic visual investigation of plasma can offer a large amount of practical diagnostic data.

Understanding the visual features of plasma can also assist in identifying early challenges before they become a concern. This article will outline some of the key elements that impact the appearance of plasma: (i) power, (ii) pressure, (iii) process gas and (iv) pump down curves.

Power

The main influence of power on plasma’s appearance is an inherent one; a glow with a higher intensity is created by higher powers.

The Four Factors Affecting How Your Plasma Looks

Image Credit: Henniker Plasma

Figures 1-4 presented above each demonstrate a basic air plasma process utilizing the Henniker HPT-200 benchtop system. In each example, the same pressure was used to maintain the plasma and the power rose from 25% to 100% in increments of 25%.

Higher powers are generally chosen to encourage adhesion and enhance wettability (plasma surface activation) or for the removal of hydrocarbons (plasma cleaning).

This is not applicable to every case. As an example, superior results are found at lower powers when bonding PDMS to glass for the production of microfluidic devices.

A brief observation of the plasma’s brightness is commonly sufficient to understand the degree of power, which can then be compared to the set output power if required.

Pressure

An unpredicted change in pressure during processing can suggest a high gas load from the samples is present (called outgassing).

It can additionally suggest a potential contamination source, a faulty pressure gauge or a vacuum link. Any of these unpredictable events has the potential to decrease the effectiveness of the plasma treatment.

The Four Factors Affecting How Your Plasma Looks

Image Credit: Henniker Plasma

Figures 5-8 displayed above demonstrate the influence on the air plasma’s appearance where pressure is increasing.

The plasma is diffuse, filling the space of the chamber in Figure 5 at 0.2 mbar. In figure 8 it becomes more restricted near the surfaces and edges of the walls and electrode.

As an example, if a specific process has been performed, but the appearance has modified as described, which would indicate high pressure, then the following observations and final conclusions could be made:

The Four Factors Affecting How Your Plasma Looks

Image Credit: Henniker Plasma

Process Gas

Plasma is also known as a ‘glow discharge’. The glow is generated by energetic species inside the plasma that emit energy in the expression of photons.

The color of the plasma is determined by the particular wavelength of these photons. Various gas mixtures and gases create particular colors that are characteristic to the gas (for example, in red neon lights).

The Four Factors Affecting How Your Plasma Looks

Image Credit: Henniker Plasma

Figures 9-12 presented above show plasma that has been produced with various gases, including air, hydrogen, oxygen and argon (all at 100% power and 0.4 mbar).

Air generates an intense pink or purple color, hydrogen is pink or red, oxygen is a faint white or grey color, and argon creates a violet color.

The practical application of this can be illustrated in the example shown in figure 10, which features an oxygen process. As shown in figure 12, if the color of the plasma is tinged with purple or pink, then it is probable that some air also exists in the chamber.

This may be caused by sample outgassing, a chamber leak, or if the starting pump downtime was not long enough to eliminate a sufficient amount of the residual air before the chamber was filled with oxygen.

A color alteration in the plasma can also be caused by samples that have a significant amount of surface contamination.

As the contamination is eliminated from the surface into the gas phase, it can mix with other species of gas to create distinct molecules, each of which can generate a specific wavelength of light.

There are expensive and complicated pieces of analytical equipment to quantify and measure the specific components in the gas phase, but a brief observation can also offer a lot of information.

Pump Down Curves

A related diagnostic test, beneficial for interpreting and monitoring system performance, is a sequence of pump-down vs. pressure time observations that can be recorded.

A pump-down curve is generally utilized to record the time taken to evacuate the chamber to a low pressure. If this is carried out often, then it can be employed to indicate whether a system is functioning correctly or if there is a potential issue.

The ‘standard’ pump down curve for the 5-liter volume HPT-200 chamber is illustrated in the plot of Graph (i) below. The same graph depicts the pump down curves for examples where a small leak is present, and the samples are outgassing.

Pump down curve HPT-200.

Graph (i) – Pump down curve HPT-200. Image Credit: Henniker Plasma

In this example, the origin of the leak in the above data was intentionally created by putting a strand of human hair across the vacuum chamber’s door seal, something which could be simply fixed by wiping it with isopropyl alcohol and a lint-free cloth and using visual inspection.

While it is challenging to identify a performance issue using a pump-down curve alone, with experience and consistent monitoring, the user can interpret and identify many challenges that have simple fixes with ease.

Read the article ‘How to Choose the Right Vacuum Pump’ for more detailed discussion and information regarding pump down curves’.

Summary

To conclude, visually inspecting the plasma itself and gaining familiarity with the performance of the system can provide an abundance of diagnostic data. 

The Four Factors Affecting How Your Plasma Looks

Image Credit: Henniker Plasma

Visit Henniker’s online Knowledge Base for more information on plasma treatment technology or contact its insightful and helpful team.

Supplementary Information

Introduction to Advanced Plasma Diagnostics

Several pieces of advanced equipment can be utilized for comprehensive plasma investigation. For the interested reader, some of these are detailed below. 

Langmuir probe current-voltage characteristic.

Figure 1. Langmuir probe current-voltage characteristic. Image Credit: Henniker Plasma

Langmuir probes are small metallic wires that are pushed into the plasma. The probe, or wire, is fixed to a source of voltage and the current is quantified as the voltage sweeps from negative to positive values.

As shown in figure 1, the current-voltage curve produced can be evaluated to provide quantitative measurements of electron and ion number densities, electron energy distribution function and electron temperature.

Thomson scattering experiment measured spectrum

Figure 2. Thomson scattering experiment measured spectrum. Image Credit: Henniker Plasma

Thomson scattering is where light is scattered from charged particles in the plasma utilizing high power lasers. The scattered light’s wavelength depends on the particle’s velocity and, therefore, energy. 

Figure 2 presents an example of the quantified spectrum from a Thomson scattering investigation. The electron temperature is determined by the width of the spectrum, and the charged particle number density can be deciphered from its area.

Typical argon plasma wavelength scan from 300-900 nm

Figure 3. Typical argon plasma wavelength scan from 300-900 nm. Image Credit: Henniker Plasma

Optical emission spectroscopy is an analytical method where the wavelengths of light generated from the plasma is monitored to establish the component species.

Figure 3 presents a standard argon plasma wavelength scan from 300-900 nm, distinct atomic or molecular transitions inside the plasma can be identified by the spikes in wavelengths.

The above has provided a brief and practical overview of the analytical techniques employed to measure and diagnose plasmas, and there are a number of other available techniques.

Contact Henniker today to learn more about any of the topics outlined in this article.

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This information has been sourced, reviewed and adapted from materials provided by Henniker Plasma.

For more information on this source, please visit Henniker Plasma.

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