oes (optical emission spectroscopy)

Optical Emission Spectroscopy (OES) analyzes the light emitted by plasma during etching to monitor process chemistry and detect etch endpoints. Different elements and molecules emit characteristic wavelengths when excited in the plasma. As etching progresses through material layers, the emission spectrum changes—for example, CO emission increases when etching reaches carbon-containing layers, while silicon emission appears when etching silicon. OES systems use spectrometers to continuously monitor specific wavelengths or full spectra. Endpoint detection algorithms identify the characteristic emission changes that indicate layer breakthrough or etch completion. OES provides real-time, non-contact process monitoring without requiring test structures. Multi-wavelength monitoring improves reliability by tracking multiple species simultaneously. OES data can also detect process excursions, equipment drift, or chamber conditioning state. Advanced systems use machine learning to interpret complex spectral patterns and predict endpoint more accurately than simple threshold detection.