4.1 Optical Emission Spectroscopy

Optical emission spectroscopy is a powerful tool for analyzing plasmas. By detecting light emitted from excited particles, it reveals plasma composition and properties without disturbing the system. This non-invasive technique provides real-time insights into atomic and molecular species present.

However, OES has limitations. It only detects excited particles that emit light, missing ground-state species. Line-of-sight measurements can lack spatial resolution in non-uniform plasmas. Despite these constraints, OES remains a valuable method for plasma characterization in manufacturing processes.

Optical Emission Spectroscopy in Plasma Characterization

Principles of optical emission spectroscopy

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• Optical emission spectroscopy (OES) analyzes composition and properties of plasmas by detecting and analyzing light emitted by excited atoms, ions, and molecules
• Plasma species become excited through collisions with energetic electrons and emit characteristic wavelengths of light when returning to lower energy states
• Emitted light is collected, dispersed, and analyzed using a spectrometer to identify atomic and molecular species, determine plasma parameters (electron temperature and density), monitor plasma stability and uniformity, and detect impurities and contaminants

Interpretation of optical emission spectra

• Emission spectra contain discrete lines or bands corresponding to specific transitions between energy levels, with each atomic or molecular species having a unique set of emission lines or bands
• Atomic emission lines appear as narrow, well-defined peaks at specific wavelengths determined by the electronic structure of the atoms and can be identified by comparing observed lines with reference data
• Molecular emission bands appear as broad, often overlapping features arising from transitions between vibrational and rotational energy levels and can be identified by analyzing band structure and intensity distribution

Instrumentation for plasma diagnostics

• Main components of an OES system include:
  • Light collection optics (lenses or fiber optics) to collect light emitted by the plasma
  • Spectrometer to disperse the collected light into its constituent wavelengths (commonly Czerny-Turner and Echelle configurations)
  • Detector to convert the dispersed light into electrical signals for analysis (charge-coupled devices (CCDs) and photomultiplier tubes (PMTs))
• Spectral resolution determines the ability to resolve closely spaced emission lines and is influenced by factors such as slit width, grating dispersion, and detector pixel size
• Wavelength range depends on the spectrometer design and the choice of gratings or filters
• Calibration and data acquisition involve wavelength calibration, intensity calibration, and collection and processing of detector signals

Advantages vs limitations in characterization

• Advantages of OES:
  1. Non-invasive technique does not disturb the plasma during measurements
  2. Provides real-time, instantaneous information about plasma composition and properties
  3. Can probe different regions of the plasma using light collection optics for spatially resolved measurements
  4. Can identify a wide range of atomic and molecular species in the plasma
• Limitations of OES:
  1. Line-of-sight technique measures integrated emission along the observation path, limiting spatial resolution in non-uniform plasmas
  2. Requires the plasma to be optically thin for accurate quantitative analysis, as self-absorption and opacity effects can complicate interpretation of emission spectra
  3. Only detects excited species that emit light and cannot directly observe ground-state species and non-radiative processes
  4. Overlapping emission lines or bands from different species can cause spectral interferences, complicating identification and quantification of individual components