AVS 51st International Symposium
    Plasma Science and Technology Wednesday Sessions
       Session PS2-WeM

Paper PS2-WeM10
Extraction of a Directional Ion Beam with Controlled Energy Using a Pulsed Plasma

Wednesday, November 17, 2004, 11:20 am, Room 213B

Session: Plasma Sources
Presenter: L. Xu, University of Houston
Authors: L. Xu, University of Houston
V.M. Donnelly, University of Houston
D.J. Economou, University of Houston
Correspondent: Click to Email
A 13.56MHz pulsed (typically 50µs ON/50µs OFF), capacitively coupled plasma reactor was developed to generate a nearly monoenergetic, directional ion beam. The DC bias on the target ranged from -150 to -270 volts depending on the input power and pressure. A Langmuir probe was employed to characterize the pulsed Ar plasma and a 3-grid ion energy analyzer was used to measure the ion energy distribution (IED). The EEDF during the power-ON fraction of the cycle was non-Maxwellian with a high energy tail, most likely due to secondary electrons emitted from the target. Beyond 4 µs into the power-OFF period, a Maxwellian EEDF was observed and the electron temperature (T@sub e@) decayed rapidly. The evolution of the EEDF during the power-OFF period was also used to verify the collapse of T@sub e@. After a specified delay in the power-OFF period, a positive voltage pulse was applied to a DC ring electrode surrounding the plasma to raise the plasma potential (V@sub p@) and "push" positive ions through a grounded grid out of the plasma. With the DC ring electrode voltage pulse on, a high-voltage shift of a Langmuir probe IV curve signified an increase of V@sub p@. With 50 V applied to the DC ring, the energy of the extracted ion beam peaked at 49.4 V, while the FWHM of the IED was 4.3 V, limited by the rise time of the homemade voltage pulse circuit. The ion beam is also expected to be very directional since the ion temperature depends on T@sub e@, which was very low during ion extraction. Finally, the sheath thickness during ion extraction was much larger than the holes of the grid, resulting in a vertical sheath electric field and minimal divergence of the ions as they traversed the grid holes. This was verified by a self-consistent PIC simulation. Work supported by NSF.