Ion energy distribution function measurements by laser-induced fluorescence in a dual radio frequency sheath

Abstract

Ion dynamics are investigated in a dual frequency radio frequency sheath as a function of radius above a 30 cm diameter biased silicon wafer in an industrial inductively coupled (440 kHz, 500 W) plasma etch tool. Ion velocity distribution (IVD) function measurements in the argon plasma are taken using laser induced fluorescence. Planar sheets of laser light enter the chamber both parallel and perpendicular to the surface of the wafer in order to measure both parallel and perpendicular IVDs at thousands of spatial positions. A fast (30 ns exposure) charge coupled device camera measures the resulting fluorescence with a spatial resolution of 0.4 mm. The dual-frequency bias on the wafer is comprised of a 2 MHz low frequency (LF) bias and a 19 MHz high frequency bias. The laser is phase locked to the LF bias and IVD measurements are taken at several different LF phases. Ion energy distribution (IED) function measurements and calculated moments are compared for several cases. IEDs were measured at two disparate phases of the phase-locked LF bias. IEDs were found to be multipeaked and were well-approximated by a sum of Maxwellian distributions. The calculated fluxes in the dual frequency case were found to be substantially more radially uniform than the single frequency bias case. For industrial applications, this radially uniform ion flux is evidently a trade off with the undesirable multipeaked structure in the IEDs.

Time-resolved laser-induced fluorescence spectroscopy using CW diode laser for diagnostics of argon-ion velocity distribution near AC-biased electrode

“…[46][47][48][49][50][51][52] To understand the dynamic behaviors of ions in plasmas, time-resolved LIF systems have been developed and applied to diagnose spatiotemporal fluctuations in plasma thrusters, [53][54][55][56][57] the temporal evolution of discharge and afterglow phases in pulsed plasmas, 58,59 and ion sheaths formed on AC-biased electrodes. [60][61][62][63][64][65] Time-resolved LIF measurements of ion sheaths were achieved by using pulsed dye lasers as the excitation-laser source and controlling the laser-pulse timing synchronized with the AC bias voltage applied to the electrode. To accurately evaluate the effects of tailored-waveform AC voltages on the IVDF, the temporal change in the IVDFs must be measured with sufficient data-acquisition timing in one cycle.…”

Laser induced fluorescence diagnostic for velocity distribution functions: applications, physics, methods and developments

“…Durot et al also demonstrated a transfer function averaging technique via band-pass filtering, phase-sensitive detection and transfer function averaging [83]. A pulsed laser, generally with a low repetition rate, can perform time-resolved measurements by phrase-locking the laser pulse trigger with the temporal phenomena if such phenomenon is repeatable [30]. A similar procedure can be used with CW lasers via AOM modulation [40].…”

“…It can be used to monitor the 2D IVDF distribution via planar LIF with a sheet of laser light. LIF has been used to measure ion properties within DC and RF sheaths in ICP devices, as described by Moore et al for a dual frequency (2.2 and 19 MHz) RF sheath [30]. Sadeghi et al also reported direct measurement of the argon ions and neutral VDFs in an ICP [163,164].…”

“…With the establishment of LIF as the higher resolution diagnostics, it became increasingly popular as a tool to measure ion flow [7], ion temperature [8], magnetic field strength [9] and ion density [10]. They are increasingly employed in basic plasma physics experiments to study sheath/presheath formation and other physical processes [11][12][13][14][15], in Hall thrusters to monitor IVDFs [16][17][18][19][20][21], in helicon devices to study ion beam driving mechanism [22][23][24][25][26][27][28][29], and in plasma processing to monitor evolution of IVDFs along the presheath [30], among other usages. The LIF diagnostic itself has advanced significantly since its invention: new LIF schemes have been discovered to expand the range of ions and neutral to be measured [7,[31][32][33][34][35], accurate evaluation of hyperfine and isotope effects [36], Zeeman split and atomic reference spectra improves LIF accuracy [37,38], the adaption of diode lasers improves costeffectiveness and operational safety [7,35,37], time-resolved techniques are formulated to observe periodic phenomena, and the physical limitations of LIF are being actively explored [28,39,40].…”

Probabilistic effect of metastable excitation on lock-in amplified plasma laser-induced fluorescence diagnostics at modulation frequencies comparable to the fluorescence frequency

“…2005), plasma processing (Zimmerman, Mcwilliams & Edrich 2005; Jacobs et al . 2007; Moore, Gekelman & Pribyl 2016), and basic plasma physics (Lee et al . 2006; Lee, Hershkowitz & Severn 2007 a ; Lee, Oksuz & Hershkowitz 2007 b ; Yip, Hershkowitz & Severn 2010; Yip et al .…”