Control of Plasma Uniformity using Phase and Waveform in a Multi-Frequency VHF Plasma Process Chamber

Summary form given only.VHF power is commonly used to generate plasmas for processing applications due to various benefits including low plasma potential and controllable dissociation. However, plasma uniformity can be difficult to control due to its strong dependence on frequency, load conditions (pressure, power, gas mixture electronegativity), reactor geometry and boundary conditions. This paper addresses the use of phase difference and voltage waveform in controlling plasma uniformity. VHF power of the same frequency is applied to 2 different electrodes, and voltage phase is controlled at the electrode. It is demonstrated that phase difference between electrodes (with respect to reference chamber ground) can be used to control plasma uniformity. A 180 degree out of phase condition maximizes the current between the electrodes, providing a relatively center high plasma density. On the other hand, a 0 degree in phase condition minimizes the current between electrodes, providing a center low plasma. In addition to phase, plasma uniformity can be controlled by changing the applied voltage waveform. A sinusoidal waveform at low frequencies predominantly generates edge high plasma. An asymmetric triangular waveform at low frequency generates higher harmonics (e.g., 2nd and 3rd harmonics) that lead to higher density and plasma tills in the chamber center. VHF frequency can be excited at a sub-harmonic of the desired frequency component. For example, if power at f MHz is desired for one of the VHF sources, it may be excited by driving an f/3 MHz generator. The f/3 MHz generator can then be used to excite the plasma with a higher frequency (in this case f MHz) to produce a center-high plasma uniformity profile. Alternatively, ratio of power of the two VHF sources can be selected to minimize the non-uniformity for a recipe or load condition. The two VHF generators can be coupled together to the same or different electrodes to control plasma uniformity.