| AVS 55th International Symposium & Exhibition | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS2-WeA |
| Session: | Plasma Diagnostics, Sensors, and Control I |
| Presenter: | M.A. Sobolewski, National Institute of Standards and Technology |
| Authors: | M.A. Sobolewski, National Institute of Standards and Technology D.L. Lahr, National Institute of Standards and Technology |
| Correspondent: | Click to Email |
When a plasma etch consumes one layer and exposes an underlying layer, changes are detected in measured electrical parameters, such as the dc self-bias voltage and the voltage, current, impedance, and phase at the fundamental and harmonic frequencies. Consequently, these electrical signals are widely used for endpoint detection, i.e., for determining when to terminate an etch. However, the mechanisms responsible for the observed electrical changes are not well understood. The electrical changes may indicate a change in plasma electron density and ion flux caused by changes in the gas-phase densities of etch products and reactants that occur as an etch proceeds to completion. Alternatively, changes in substrate electrical properties or surface properties such as work function and the yield of secondary and photoemitted electrons may be involved. To investigate these mechanisms, experiments were performed in an inductively coupled plasma reactor equipped with rf bias and a wave cutoff probe. The cutoff probe1,2 allowed small changes in the plasma electron density to be measured with good accuracy and resolution, on a time scale of a few seconds, regardless of the presence or absence of insulating layers on probe surfaces. Simultaneous measurements of electrical signals and cutoff probe data were made during CF4/Ar plasma etches of thermal silicon dioxide films on silicon substrates. Changes observed in the components of voltage, current, impedance and phase at the rf bias frequency were related to, and fully explained by, changes in plasma electron density measured by the cutoff probe. The dc self-bias voltage and harmonic signals showed more complicated behavior that cannot be explained solely by changes in plasma electron density. The results allow several general conclusions to be drawn about the relative reliability and usefulness of endpoints obtained from each of the different electrical signals.
1 M. A. Sobolewski and J.-H. Kim, J. Appl. Phys. 102, 113302 (2007).
2J.-H. Kim, S.-C. Choi, Y.-H. Shin and K.-H. Chung, Rev. Sci. Instrum. 75, 2706 (2004).