Bulletin of the American Physical Society
64th Annual Gaseous Electronics Conference
Volume 56, Number 15
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session CT2: Sheaths and Plasma Boundaries |
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Chair: Ralf Peter Brinkmann, Ruhr University Room: 255E |
Tuesday, November 15, 2011 8:00AM - 8:30AM |
CT2.00001: LIF measurements of the ion distribution function in the sheath and pre-sheath of a biased capacitively coupled plasma reactor Invited Speaker: The time-dependent argon ion energy distribution function (IEDF) above and within the plasma sheath of a radio frequency (rf) biased substrate has been measured using laser induced fluorescence (LIF) in a commercial plasma processing tool. The measurements were acquired at eight different phases of the 2.2 MHz rf waveform and show the ion dynamics to vary dramatically throughout a cycle. Discharge parameters were such that the rf bias period was on the order of the ion transit time through the sheath (t$_{ion}$/t$_{rf}$ = 0.435). The first experiments measured the IEDF along a line parallel to the normal of the wafer with spatial resolution (dy=88 um). These measurements have been extended to an x-y plane (y is the height above the wafer) which includes the wafer edge so that f(r,v,t) can be measured within a sheet of light. These measurements include the option for multiple bias voltages (2.2, 60 MHz). A patented technology, which enables rapidly switching either bias or ICP source without a match circuit allows for time sequencing in any combination with (t$_{pulse}>$50 usec). The heat flux and plasma flow is derived from the ion distribution function. This work embodies the first time resolved measurement of ion velocity distribution functions (IVDFs) within an rf biased sheath over a large area (30 cm diameter) substrate. Additional probe measurements of the plasma parameters above the wafer will be presented as well. Comparisons will be made to ion energy and velocity distributions obtained from computer modeling. [Preview Abstract] |
Tuesday, November 15, 2011 8:30AM - 9:00AM |
CT2.00002: Control of the Ion Energy Distribution on Plasma Electrodes Invited Speaker: The energy of ions bombarding the substrate is of critical importance in plasma etching and deposition. For example, precise etching with high selectivity and no substrate damage requires a nearly-monoenergetic ion energy distribution (IED) with tight energy spread. The IED may be controlled by applying ``tailored'' bias voltages on the substrate, or on a nearby electrode (boundary electrode) immersed in the plasma. A PIC-MCC simulation was conducted of the application of DC waveforms on a boundary electrode, during the afterglow of a pulsed discharge. Staircase voltage waveforms with selected amplitudes and durations resulted in IEDs with distinct narrow peaks, having controlled peak energies and fraction of ions under each peak. A model was also employed to achieve ``designer'' IEDs, i.e., distributions with a desired (pre-selected) shape and energy spread. This was accomplished by solving the ``inverse'' problem, i.e., that of finding the voltage waveform that must be applied on the electrode to yield a desired IED. IEDs were measured in a Faraday-shielded inductively coupled plasma. Narrow distributions with well-controlled ion energy were obtained by pulsing the plasma and applying a synchronous DC bias on a boundary electrode during the afterglow. The peak ion energy was controlled by the DC bias, as the plasma potential and T$_{e}$ decayed drastically in the afterglow. IED measurements were performed in Ar, Kr and Xe plasmas. The full width at half maximum (FWHM) of the IEDs followed the order Xe$>$Kr$>$Ar. Higher electron temperature in the afterglow correlated with larger FWHM. The width of the IED could also be controlled by varying the pulsed plasma frequency and duty cycle, or the time window of the application of the DC bias during the afterglow. Model predictions in terms of IEDs or voltages to produce given IEDs were in agreement with experimental data. [Preview Abstract] |
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