Ruhr University BochumInstitute for Electrical Engineeringand Plasma TechnologyProf. Dr.-Ing. Peter AwakowiczUniversitätsstraße 150 Tel: 0234/32-2306244801 Bochum Web: www.aept.rub.de

Optical Emission Spectroscopy (OES) - What You See Is What You Get

Marcel Fiebrandtdiagnostics@aept.ruhr-uni-bochum.de

1 Motivation

All plasmas emit radiation depending on their gas com-position and plasma parameters. The goal of opticalemission spectroscopy (OES) is measuring and analy-sing this radiation to get an insight into the plasma.

Fig. 1: Colors of plamas in a DICP due to different ga-ses (from left to right: H2; N2;O2)

The significant advantage of OES is its invasive mea-surement process. By just measuring the radiation fromthe plasma there is no need to bring a probe system incontact with the plasma (like Langmuir-Probe or Multi-pol Resonance Probe) which might influence the plas-ma itself. Furthermore, OES enables one to investigatevery small plasmas, often smaller as probes themsel-ves. Therefore, it is often the only usable measurementtechnique in addition to laser absorption spectroscopy.

Depending on the plasma, radiation from the vacuumUV (< 200 nm), UV (200 nm - 380 nm), visible (380 nm

- 780 nm) or infrared region (> 780 nm) can be detectedand analyzed.

Fig. 2: VUV and UV spectrum of an Ar/N2/O2 DICP-Discharge [1]

2 Measurement

Modern spectrometers with CCD-Chips allow fast mea-surements of spectra over a broad wavelength region.Combined with an optical fibre, the emitted plasma ra-diation is guided to the entrance slit of the spectrome-ter, making it very easy to measure spectra from diffe-rent positions and angles. Furthermore, if the spectro-meter is absolutely calibrated [2], it is possible to mea-sure the number of photons per wavelength, secondand volume emitted by the plasma. As the excitationof atoms and molecules in the plasma depends on theparameters describing a plasma, like electron tempera-

Optical Emission Spectroscopy (OES) - What You See Is What You Get

ture or electron density, one can determine these para-meters by investigating the radiation.

Additionally, it is possible to identify different speciesin the plasma by their specific radiation. By this, impu-rities or radicals in the plasma can be detected.

Fig. 3: Species produced in a pulsed corona plasma [3]

3 Spectrometers

Nowadays, it is possible to build sensitive, high-resolutionspectrometers in a compact design. Thus, spectrome-ters can be easily moved and aligned at different se-tups without much effort. A modern broadband echellespectrometer, for example, with an optical resolution of5 pm is as bis as a shoebox.

Fig. 4: Echelle Spectrometer ESA 4000 from LLA In-struments (Source: www.lla.de)

Spectrometers with a lower resolution and a broadwavelength region or with a high resolution in a smallwavelength region can be build much smaller and areconnected via a USB to a Laptop or PC. For example,the Ocean Optics QE Pro is just double the size of acomputer mouse.

Fig. 5: Compact Spectrometer QE Pro from Ocean Op-tics (Source: www.oceanoptics.com)

Literatur

[1] B. Denis, S. Steves, E. Semmler, N. Bibinov, W. No-vak, P. Awakowicz, "Plasma Sterilization of Pharma-ceutical Products: From Basics to Production", Plas-ma Processes and Polymers, 9, pp. 619-629, (2012)

[2] N. Bibinov, H. Halfmann, P. Awakowicz, K. Wie-semann, "Relative and absolute intensity calibrationof a modern broadband echelle spectrometer", Mea-surement Science and Technology, 18, pp. 1327-1337,(2007)

[3] R. Pothiraja, N. Bibinov, P. Awakowicz, "Pulsed co-rona plasma source characterization for film deposi-tion on the inner surface of tues", Journal of PhysicsD: Applied Physics, 43, (2010)

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