Plasma Diagnostics and Characterization Techniques

Plasma diagnostics encompass a suite of methods for measuring the fundamental parameters of ionised gases, including electron and ion densities, temperatures, energy distributions and chemical composition. Electrical probes, such as Langmuir probes, provide local measurements of electron temperature and density via current–voltage characteristics, while microwave and radio-frequency techniques exploit wave propagation and cutoff phenomena to determine global electron densities. Optical methods, notably optical emission spectroscopy and laser-induced fluorescence, yield information on excited species populations, electric fields and reaction kinetics without perturbing the discharge. Mass spectrometry and energy-resolved ion probes reveal detailed ion flux and energy distribution functions, essential for understanding surface interactions in materials processing. Advanced approaches such as Thomson scattering and cavity ring-down spectroscopy enable non-intrusive, high-resolution measurements of electron temperature and neutral species densities. Together, these methods underpin progress in fusion research, semiconductor fabrication, environmental control and space propulsion, offering real-time monitoring, process control and fundamental insight into plasma–surface interactions across diverse pressure, power and gas‐mixture regimes.

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Recent work has introduced a wafer-type ion energy monitoring sensor that integrates directly into semiconductor process chambers to map the spatial distribution of ion kinetic energies. By converting ion flux impacts into induced currents across patterned electrodes on a 150 mm wafer, this approach yields in situ ion energy distribution data without modifying existing wafer-handling systems, enabling real-time control of etching and deposition uniformity.

A novel flat microwave cutoff probe embedded in the substrate chuck or chamber wall has been demonstrated to measure electron density adjacent to processing wafers in real time and with minimal plasma perturbation. Electromagnetic simulations guided the optimal design of ring and bar-type probes, and experiments confirmed agreement with conventional probes under various gas chemistries and wafer materials, establishing a practical tool for continuous density monitoring during industrial etch, deposition and implantation processes.

Comprehensive experimental validation of one-dimensional particle-in-cell/Monte Carlo collision (PIC/MCC) simulations has underscored the importance of accurate input parameters—particularly gas temperature and surface emission coefficients—for predictive modelling of capacitively coupled radio-frequency plasmas. By systematically measuring gas temperature, electron density, spatio-temporal excitation rates and ion energy distribution functions in a reference reactor, researchers have calibrated simulation parameters to achieve quantitative agreement, paving the way for computationally assisted diagnostics and the determination of surface interaction coefficients in situ.

Plasma Diagnostics and Characterization Techniques publication trend

The graph below shows the total number of articles in plasma diagnostics and characterization techniques across all publications each year (not limited to Nature Index journals).

Technical terms

Ion energy distribution function (IEDF): Statistical distribution of ion kinetic energies impacting surfaces, critical for etch and deposition control.

Microwave cutoff probe (CP): Diagnostic that determines local electron density by detecting the cutoff frequency at which electromagnetic waves no longer propagate through the plasma.

Particle-in-cell/Monte Carlo collision (PIC/MCC): Numerical technique combining kinetic particle tracking with probabilistic collision processes to simulate plasma behaviour self-consistently.

Plasma sheath: Boundary layer adjacent to material surfaces where an electric field accelerates ions and repels electrons, shaping surface interactions.

Optical emission spectroscopy (OES): Non-intrusive technique analysing the wavelengths and intensities of light emitted by excited plasma species to infer species densities and excitation dynamics.

References

1. Wafer Type Ion Energy Monitoring Sensor for Plasma Diagnosis. Sensors (2023).
2. Flat cutoff probe for real-time electron density measurement in industrial plasma processing. Plasma Sources Science and Technology (2020).
3. Multi-diagnostic experimental validation of 1d3v PIC/MCC simulations of low pressure capacitive RF plasmas operated in argon. Plasma Sources Science and Technology (2021).