Knowledge Base

Everything you need to understand plasma technology all in one place.

Knowledge Articles

Let’s Talk About… getting to know your plasma

Plasma diagnostics

There’s no denying that plasmas look good, but did you know that a simple visual inspection of the plasma can provide a great deal of useful diagnostic information. Getting to know the plasma’s visual characteristics can also help to diagnose early problems before they become an issue. Let’s talk about a few of the main factors that affect how the plasma looks: (i) power, (ii) pressure, (iii) process gas and (iv) pump down curves.

Power

The primary effect of power upon the appearance of the plasma is a fairly intuitive one; higher powers produce more intense glows.

PowerFigure 1-4

 

Figures 1-4 [above] each show a simple air plasma process using the Henniker HPT-200 benchtop system. The plasma was maintained at the same pressure in each case and the power was increased from 25% to 100% in 25% increments.

In general, higher powers are preferred for hydrocarbon removal (plasma cleaning) or to increase wettability & promote adhesion (plasma surface activation). However, this is not true in all cases. For example, bonding PDMS to glass for the fabrication of microfluidic devices yields better results at lower powers.

A quick glance at the brightness of the plasma is often enough to get a feeling for the power level, which can then be checked against the set output power if necessary.


Pressure

An unexpected pressure variation during processing can indicate the presence of a high gas load from the samples (known as outgassing). It can also point to a possible vacuum leak, the presence of contamination, or a faulty pressure gauge. Any of these unexpected incidents has the potential to render the plasma treatment less effective.

Pressure Figure 5-8

Figures 5-8 [above] show the effect upon the appearance of an air plasma of increasing pressure. In figure 5, at 0.2mbar the plasma is diffuse, filling the volume of the chamber. In figure 8 it becomes more constricted around the edges/surfaces of the electrode and walls.

Let’s suppose we are running a specific process but the appearance has changed as above, indicating high pressure. We could make the following observations and eventual conclusions:-

 Henniker plasma diagnostics flow chart


Process gas

Plasma is also referred to as a ‘glow discharge’, the glow is the result of energetic species within the plasma releasing energy in the form of photons. These photons have a specific wavelength, which determines the colour of the plasma. Different gases and gas mixtures produce specific characteristic colours (think red neon lights for example).

Gas colour

Figure 9-12

In figures 9-12 above the plasma has been generated with a different gas; Argon, Oxygen, Hydrogen and Air (all at 0.4mbar and 100% power). Argon produces a violet colour, oxygen a faint white/grey colour, hydrogen is pink/reddish and air produces an intense more pink/purple colour.

So how is this useful? Well let’s assume we are working with an oxygen process, see figure 10. If the plasma colour is tinged with pink/purple, see figure 12, then it is likely that there is some air in the chamber too. This could be the result of a chamber leak, sample outgassing, or simply because the initial pump down time wasn’t long enough to remove enough of the residual air before oxygen was admitted into the chamber.

Samples that have a considerable amount of contamination on the surface may also produce a colour change in the plasma. As the contamination is removed from the surface into the gas phase, it can combine with other gaseous species to produce unique molecules, each of which can contribute a characteristic wavelength of light. There are complex and expensive pieces of analytical equipment to measure and quantify the individual components in the gas phase, but a quick glance can reveal a lot too!


Pump down curves

A related diagnostic test, useful for monitoring and interpreting system performance, is simply a series of pressure vs. pump-down time observations that can be recorded. In general, a pump-down curve is used to measure the time taken to evacuate the chamber to some low pressure, and if performed regularly it can be used as an indication that the system is behaving as normal, or if indeed there may be a problem.

The plot in Graph (i) below shows the “standard” pump down curve for the 5 litre volume HPT-200 chamber. Shown on the same graph are the pump down curves for the cases where samples are outgassing and there is a small leak present.

Graph (i)

Graph (i) – Pump down curve HPT-200

The source of the leak in the data above, in this case, was made intentionally just by placing a single human hair across the door seal of the vacuum chamber, something that could easily be rectified by visual inspection and wiping with a lint free cloth and isopropyl-alcohol.

Although it’s tricky to establish a performance problem from a pump-down curve alone, with regular observation and experience, the user is easily able to identify and interpret numerous cases that have very simple resolutions.

For further detailed information and discussion about  pump-down curves please see ‘Let’s talk about which vacuum pump should I choose?’.


Summary

In summary, a visual inspection of the plasma itself and becoming familiar with your system’s performance can reveal a wealth of diagnostic information - it is worth getting to know your plasma!  

henniker plasma cirrus atmospheric plasma citation 700x278

For more insights into plasma treatment technology visit our online Knowledge Base, or feel free to contact us, we are passionate about plasma® and always happy to help.


Supplementary information

Introduction to advanced plasma diagnostics.

There are a number of sophisticated tools that we use for detailed plasma analysis. A few of these are outlined below for the interested reader.  

Electrostatic Langmuir Probes

Figure (i)

Figure (i) – Langmuir probe current-voltage characteristic

Langmuir Probes are small metallic wires inserted into the plasma. The wire, or probe, is connected to a voltage source and the current is measured as the voltage is swept from negative to positive values. The resulting current-voltage curve [Fig (i)] is shown above can be interpreted to give quantitative measurements of ion and electron number densities, electron temperature and electron energy distribution function.

Thomson Scattering

Figure (ii)

Figure (ii) – Thomson scattering experiment measured spectrum

Thomson Scattering utilises high power lasers to scatter light from charged particles in the plasma. The wavelength of the scattered light is dependent on the velocity of the particles and therefore energy.  The above example Figure (ii) shows the measured spectrum from a Thomson scattering experiment. Here the width of the spectrum determines the electron temperature, and from its area the charged particle number density can be calculated.

Optical Emission Spectroscopy

Figure (iii)

Figure (iii).  Typical argon plasma wavelength scan from 300-900nm

Optical emission spectroscopy is an analytical technique that involves monitoring the wavelengths of light produced from the plasma in order to identify the constituent species. In a typical argon plasma wavelength scan from 300-900nm [Fig.iii], the spikes in wavelengths correspond to specific molecular / atomic transitions within the plasma.

There are many more analytical techniques used to diagnose and measure plasmas, we hope that the above provides a useful quick introduction.

Please contact us today if you would like to learn more about any of the topics discussed above.

REQUEST A QUOTATION OR FURTHER INFORMATION

See what our customers have to say

  • "The technical team at Henniker are very knowledgeable and supportive and always approachable. I have found it a pleasure to work with them."

    Simon Baxter - BAE Systems, MAI

    BAE
  • "Henniker guided us to choose the most suitable plasma unit for our application, ensured an accelerated delivery time & guided us through the very easy setup. We obtained quality results with their unit within minutes of setup & consistent results thereafter. The support they have provided has been rapid and helpful."

    Dr Ravi Desai - Making Lab, Francis Crick Institute

    Francis Crick Institute
  • "Henniker provided our team with excellent service during the course of our work together, the plasma cleaner arrived quickly and was installed with ease, giving us visible results from the outset and confirming that we made the right decision in choosing a local UK manufacturer."

    Dr Panagiotis Manesiotis BSc MRSC - Queen’s University Belfast

    Queens Uni Belfast
  • "Our customers and operations demand reliability at every level and were a key factor in our decision to choose a UK based manufacturer of plasma treatment equipment."

    Tom Doak - Trak Microwave

    Queens Uni Belfast
  • "Henniker’s after sales support is first class. They have always been extremely responsive if we have ever had need to call on them."

    Steve Rackham - Teledyne

    Teledyne
  • "Henniker’s plasma systems have delivered tangible benefits to us right from day one. The team there are very easy to work with."

    Ian Bruce - Coopervision

    Coopervision
  • "Henniker really stood out, both in their product range and technical knowledge. They are a great company to work with."

    Karthik Nair - University of Bradford

    University of Bradford
  • "We are very impressed with the ease of use and reliability of our plasma unit and were producing results within minutes of setting it up."

    Dr Neil Wilson - Warwick University

    Warwick University
  • "Our collaborative work with the team at Henniker was a very positive experience and one that we look forward to developing further."

    Ewen Kellar - TWI

    TWI
  • "Henniker provided a tailored product to match our exact requirements. They are a pleasure to work with."

    Dr Will Shu - Heriot Watt University

    Heriot Watt University

  • Huf
  • Kingspan
  • BOC
  • Maclaren
  • Cambridge Uni
  • Warwick Uni
  • Morgan Ceramics
  • QMUL
  • University Bradford
  • BAE
  • Imperial College London
  • Heriot Watt Uni
Henniker Plasma Logo

ISO logo

 

 

Connect with us

Henniker Plasma 3 Berkeley Court, Manor Park, Runcorn, WA7 1TQ, UK

  +44 (0)1925 830 771     +44 (0)1925 800 035     info@plasmatreatment.co.uk