Moshe Sarfaty

Langmuir probe in low temperature, magnetized plasmas: Theory and experimental verification

Langmuir probe theory, adapted to magnetized, partially ionized, low temperature processing plasmas with radial diffusion dominated by electron-neutral collisions, was verified in electron cyclotron resonance (ECR) plasmas. Plasma parameters such as plasma potential, electron temperature, plasma density, and the ratio of electron saturation current to ion saturation current (Ie*/Ii*) were measured by single-sided planar probe in various ECR plasmas (H2, He, N2, O2, Ar, and CF4). The neutral pressure was varied between 0.5 and 8.5 mTorr and the microwave power between 170 and 1250 W with good matching conditions; the reflected power was kept at less than 3% of the input power. The measured ratios of Ie*/Ii*, and other plasma parameters were consistent with the probe theory for pressures greater than 2.0 mTorr for various plasmas of Ar, He, H2, and N2. These results indicate that the electron-neutral collisional probe theory works well for magnetized ECR plasmas (magnetic flux densities of 0.8–1.0 kG).

Temporal fluctuations: A fingerprint of surface chemical reactions

A real-time fast Fourier transform of temporal fluctuations of optical emission and rf power signals captures surface chemical reactions. The observed fluctuations, previously considered as noise and traditionally time averaged, depend on process chemistry and surface materials. The coherent optical emission fluctuations are specific only to the etch by-products. Plausible explanations, such as plasma density or electron temperature modulations, plasma instabilities, rf coupling modulations, or external hardware effects are dismissed by the data. It has been confirmed that the observed frequency patterns are directly related to surface chemical reactions of the film exposed to the plasma. This finding can be used to diagnose, monitor, and control chemical surface reactions in real time.