| AVS 55th International Symposium & Exhibition | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS2-WeA |
| Session: | Plasma Diagnostics, Sensors, and Control I |
| Presenter: | V.L. Brouk, Advanced Energy Industries, Inc. |
| Authors: | V.L. Brouk, Advanced Energy Industries, Inc. D.C. Carter, Advanced Energy Industries, Inc. R.L. Heckman, Advanced Energy Industries, Inc. |
| Correspondent: | Click to Email |
The presence of instabilities in low pressure (5 to 100 mT) electronegative plasmas is well documented. In inductively driven plasmas instabilities often exist between the stable low density, capacitive mode and the stable high density, inductive regions causing oscillations in particle density, optical emission and coil voltage. Instabilities also occur in capacitively coupled plasmas driven similarly by electron attachment to electronegative species. Oscillation frequency can range from less than a 100 Hz to greater than 10 kHz and can depend on multiple process parameters including pressure, gas flow, gas mixture and power level.1,2 Especially for high efficiency, switch mode power amplifiers, interaction between the power-dependent plasma impedance and the load-dependent amplifier response can promote or aggravate unstable behavior. This interaction involves complex impedance trajectories having both magnitude and angle components when displayed in the impedance plane. The common practice of adjusting transmission cable length shifts the phase angle of the amplifier portion of the interaction and thus minimizes the combined feedback to achieve a stable operating state. For a singular set of conditions, the point of transition from one mode to the next can be well defined and effectively addressed in this manner. But complete mapping of such behaviors across a broad process space is not practical due to the enormous number of variables present in modern plasma processes. Further, adjustments of physical cable length can be problematic in many applications. For these reasons a convenient means for predicting the onset of plasma instabilities and ideally a method for avoiding an unwanted transition to unstable operation is desirable. In this study we demonstrate a quantitative diagnostic for assessment of plasma stability providing a measure of margin from an unstable threshold. When used with a properly equipped RF amplifier, the technique provides context necessary to avoid the onset of instability in these plasmas. Using a fixed transmission cable and minor adjustments in RF frequency to adjust the electrical wavelength, we show how this method can be used to actively stabilize both low and high frequency plasma oscillations.
1 A. M. Marakhtanov, et. al., J. Vac. Sci. Technol. A 21 (6), Nov/Dec 2003, 1849-1864.
2 A. Descoeudres, et. al., Plasma Sources Sci. Technol. 12 (2003), 152–157.