Presentation on theme: "Control of Ion Energy Distributions on Plasma Electrodes"— Presentation transcript:
1 Control of Ion Energy Distributions on Plasma Electrodes P. Diomede, D. J. Economou and V. M. DonnellyPlasma Processing Laboratory, University of HoustonDOE Plasma Science Center Teleseminar, February 8, 2013
2 Outline Introduction/Motivation 3 Methods to control IEDs on plasma electrodesPIC-MCC simulations and comparisons with experimentsModel for rapid calculation of IED and comparisons with experimentsConclusions
3 Introduction / Motivation Control of the energy of ions bombarding a substrate is important for both plasma etching and PECVD.The ion energy must be high enough to drive anisotropic etching, but not too high to induce substrate damage and/or loss of selectivity.As device dimensions keep shrinking, requirements on selectivity and substrate damage become ever more stringent.In addition, the ion bombardment energy is critical for controlling film microstructure and properties in PECVD.Such requirements impose strict limits not only on the mean ion energy but also on the ion energy distribution (IED).
4 IED on an electrode biased with sinusoidal RF voltage ti/trf <<1 wide (bimodal) IEDti/trf >>1 narrow (single peaked) IEDti = ion transit time through sheathtrf = period of the rf sheath E-field =2p/w
5 IED for Sinusoidal Sheath Voltage IEDs on the grounded electrode of a MHz Ar CCP at 75 mTorr. Peak separation is reduced for heavier ions. Single peak is centered at the DC sheath voltage. Impurity ions are used to avoid charge transfer collisions.1IEDs on the grounded electrodeof an Ar CCP at 50 mTorr. A singlepeak is obtained at high enoughfrequencies.2J. Coburn and E. Kay, J. Appl. Phys., 43, 4965 (1972).K. Kohler et al., J. Appl. Phys., 58, 3350 (1985).
6 Three principal ways to control IEDs 1. Plasma is generated by independent source power. Apply judicious bias voltage waveform on the substrate electrode immersed in the plasma.The substrate bias has minor effects on the plasma chemistry.The energy of ions is determined by the substrate voltage.The voltage appearing on the face of the substrate is at a constant negative value (Vfront), except for small excursions to positive values to neutralize the charge.M. A. Wank, R. A. C. M. M. van Swaaij, P. Kudlacek, M. C. M. van de Sanden, and M. Zeman, J. Appl. Phys. 108, (2010)
7 Effect of non-sinusoidal bias waveforms on IEDs Computational investigation by Agarwal and Kushner using an ICP reactor scale model.This voltage waveform on the substrate is positive during a=10% of the cycle.15 mTorr Ar/c-C4F8=75/25 gas mixture.500 MHz TVW frequencyA. Agarwal and M. J. Kushner, J. Vac. Sci. Technol. A 23, 1440 (2005)
8 Three principal ways to control IEDs 2. Produce the plasma with customized voltage waveforms.The Electrical Asymmetry EffectThe Electrical Asymmetry Effect (EAE) provides a new method to control the ion energy distribution (IED) on plasma electrodes. Importantly, the ion flux can also be controlled, independently of the ion energy.A voltage of the formis applied to an electrode of a capacitively-coupled plasma (CCP) reactor, with f2=2f1. The DC bias (thus the ion energy) can be varied simply by changing the phase q1.Adding higher harmonics enhances the EAE, but implementation becomes cumbersome.A DC bias can be imposed even on a geometrically symmetric system (equal electrode areas).8
9 Control of ion energy distributions using the electrical asymmetry effect Measured ion energy distribution functions in a geometrically and electrically asymmetric discharge at the powered (left) and grounded (right) electrode (Argon, 1 Pa, d = 4 cm, U1 = U2 = 100 V, f1 = MHz, f2 = MHz).U. Czarnetzki, J. Schulze, E. Schüngel and Z. Donkó, PSST, 20, (2011)
10 Independent Control of Ion Flux and Ion Energy Mean ion energy and ion flux as a function of q1 in an argon CCP discharge. 7.5 mTorr, U1=U2=100 V, f1=13.56, f2= 2f1.As the phase shift q1 is varied, ion energy varies but ion flux remains almost constant.U. Czarnetzki, J. Schulze, E. Schüngel and Z. Donkó, PSST, 20, (2011)
11 Gaussian Voltage Pulses Gaussian voltage pulses (repetition frequency of MHz) of the formV0 = voltage amplitudet0 = time of pulse max, Dt = FWHMIED depends on FWHM (Dt) of applied voltageT. Lafleur and J. P. Booth, J. Phys. D: Appl. Phys., 45, (2012)
12 Three principal ways to control IEDs 3. Apply synchronous bias on boundary electrode during afterglow of pulsed plasmaBoundary ElectrodeBoundary VoltagePlasmaPlasma PowerBias ONONOFFFor a grounded conductive substrate, the sheath voltage is equal to the plasma potential.Substrate
13 Te and Vp during a pulse Time-resolved Langmuir probe measurements Te is hardly affected by the application of the DC bias, while Vp is raised.The spread in the energy of ions entering the sheath scales with Te.11. K.-U. Riemann, Phys. Fluids (1981)13
14 IEDs in pulsed Ar ICP with synchronous DC bias voltage applied to the boundary electrode Separation of the peaks can be tuned by DC bias value and pressure.Narrow IED can be achieved in the afterglow.Full width at half maximum (FWHM) of the IED ranges from 1.7 to 2.4 eV.H. Shin, W. Zhu, L. Xu, D. J. Economou and V. M. Donnelly, PSST, (2011).
15 IEDs in pulsed Ar ICP with different synchronous DC bias voltages applied to the boundary electrode DC Bias applied continuously on the BE.Peak at high energy is shifted by the energy of the applied dc bias for positive biases.Low energy peak not detected by the measurements which discriminate low-energy ions having a broad angular distribution.M. D. Logue, H. Shin, W. Zhu, L. Xu, V. M. Donnelly, D. J. Economou and M. J. Kushner, PSST, (2012)
16 PIC-MCC SimulationsSimulation of pulsed plasma with synchronous boundary voltage.Comparison with experimental data.
17 Simulation of Pulsed CCP Reactor with DC Bias in Afterglow Pulsed plasma is sustained in capacitively coupled plasma (CCP) reactor.50 V DC bias is applied on the upper (boundary) electrode in the afterglow to modify the IED on the lower (substrate) electrode.
18 Application of DC Bias in the Afterglow of a Pulsed Plasma After plasma power turn off (afterglow), Te and Vp decay rapidly.Apply synchronously tailored positive bias voltage Vdc during specified time window in the afterglow.Bias raises plasma potential, modifying the IED on the wafer.
19 PIC simulation of Ar CCP: IED without Bias Ar plasma, VRF = 300 V, nRF = MHz, p = 10 mTorr, d = 6 cm10 kHz pulse frequency 50% duty ratioIED for continuous wave (cw) plasma w/o biasIED for pulsed plasma(10 kHz, 50% duty cycle) w/o biasBimodal distribution centered around 1/2 VRF.Tail to the left of the main peak due to ion-neutral collisions.Multiple secondary peaks given by ions created by CE collisions.Bimodal IED is retained, originating from the power ON fraction of the cycleNew peak appears at very low energies due to ions bombarding the substrate during the afterglow.
20 IEDs with Staircase DC Bias Applied in Afterglow Afterglow starts at time t = 50 ms.Additional peaks appear in the IED.Peak location can be controlled by the value of the applied bias voltage.
21 IEDs with Staircase DC Bias Applied in Afterglow (2) Peak strength can be controlled by the duration of the respective DC bias voltage.The relative strength of the other two peaks can be controlled by the duty ratio.P. Diomede, V. M. Donnelly and D. J. Economou, J. Appl. Phys., 109, (2011)
22 EEPF and Electron Density Evolution In the afterglow of pulsed CCP, apply 50 V DC during t = ms, followed by 300 V DC during t = ms.EEPF is temporarily heated when the bias is applied but the electron density evolution is hardly affected.
23 Comparison of PIC Simulation with Experimental Data IEDs predicted by the PIC simulation of the afterglow (right) compared to data# (left). The low energy peak of the data is due to the active glow (not simulated by PIC).Pulsed Ar plasma, 10 KHz modulation, 20% duty, 14 mTorr, 120 W average power, 24.4 V DC bias applied in afterglow during time windows shown above.Electron temperature in theafterglow predicted by PIC (line),compared to data# (points).# H. Shin, W. Zhu, L. Xu, D. J. Economou and V. M. Donnelly, PSST, (2011).
24 Simulation of Pulsed H2 CCP Reactor with DC Bias in Afterglow The Bari hybrid model for H2 CCPsNeutral species: fluid model for H atoms and H2(v = 0,…14).Plasma kinetics: PIC/MCC applied to electrons and four ionic species (H3+, H2+, H+, H-).
25 Application of DC Bias in the Afterglow of a Pulsed Plasma After plasma power turn off (afterglow), Te and Vp decay rapidly.Apply synchronously tailored positive bias voltage Vdc during specified time window in the afterglow.Bias raises plasma potential, modifying the IED on the wafer.
26 Number densities and electron KE Evolution mid-plane of the discharge (x = 3 cm)Plasma ON (t=0 ms): increase approaching quasi-steady-state values ~ 20 ms.Plasma OFF (t=50 ms): decay throughout the afterglow .H2+ ions disappear after 5 ms into the afterglow.Plasma ON (t=0 ms): rapidly rise to a peak early, quasi-steady-state ~ 10 ms.Plasma OFF (t=50 ms): plummet during the first few ms and decay at a much slower rate later in the afterglow.DC bias ON (t=70 ms): temporary heating.Hydrogen plasmaVRF = 300 V, nRF = MHz, p = 50 mTorrd = 6 cm, 10 kHz pulse, 50% duty ratioRF ONRF OFFDC ON
27 IEDs for continuous wave (cw) plasma w/o bias (1) simulationH+simulationBimodal structure and tail towards lower energies due to ion-neutral collisions.The H+ IED bimodal structure has a wider energy spread, due to the lower mass of H+.Predicted bimodal distribution, with a more intense peak at lower energy, is close to the experimental IED for H3+.H3+experimentH2, VRF = 300 V, nRF = MHz, 50 mTorr, 6 cmExperiments: D. O’Connell et al. Phys. Plasmas 14, (2007)
28 IEDs for continuous wave (cw) plasma w/o bias (2) simulationH2+H2+ IED exhibits multiple peaks, explained by symmetric charge exchange collisions during the sheath collapse.Ions thus created experience only a fraction of amplitude of the oscillating sheath voltage.Computed H2+ IED is in good agreement with experimental results.The energy dependence of the acceptance angle of the ion optics contributes to an artificial distortion of the IED in the low energy region.experimentH2+H2, VRF = 300 V, nRF = MHz, 50 mTorr, 6 cmExperiments: D. O’Connell et al. Phys. Plasmas 14, (2007)
29 Computed IEDs for pulsed plasma with 50 V DC bias in the afterglow Additional peaks appear in the H3+ and H+ IEDs.Peak location can be controlled by the value of the applied bias voltage.H2+ disappear in the afterglow due to the rapid decay of Te.H2, VRF = 300 V, nRF = MHz, 50 mTorr, 6 cm10 kHz pulse, 50% duty ratio
30 Model for Rapid Calculation of IED on Electrode in Contact with Plasma Bulk Plasma (n0, Te)SheathBlocking capacitor, CbElectrode (Target)Applied rf, VrfAssumptions: Bulk n0 and Te are not influenced by rf bias.Sheath is collisionless.Ion flux at sheath edge is time-independent.
31 Semi-analytic Model Schematic of the sheath region Electrode immersed in semi-infinite plasma of given electron (ion) density and electron temperature.Electron, ion and displacement currents flow through the sheath.Non-linear sheath capacitance Cs is calculated from the electric field at the electrode, E.A. Metze, D. W. Ernie, H. J. Oskam, J. Appl. Phys., 60, 3081 (1986).P. Miller and M. Riley, J. Appl. Phys., 82, 3689 (1997).T. Panagopoulos and D. Economou, J. Appl. Phys., 85, 3435 (1999).
32 Equivalent Circuit Model Subscripts T and G refer to “target” and“ground” electrodes, respectively.Ions respond toa “damped” potential VdVoltage Vrf is applied through blocking capacitor, Cb.Given n0, Te, Vrf and Cb, calculate VT , Vp, and Vd.A. Metze et al., J. Appl. Phys., 60, 3081 (1986).
33 Ion Energy Distribution y = ion energySample damped sheath potential waveformP. Diomede, M. Nikolaou, D. J. Economou, Plasma Sources Sci. Technol., 20, (2011).E. Kawamura, V. Vahedi, M.A. Lieberman, C.K. Birdsall, Plasma Sources Sci. Technol., 8, R45 (1999).
34 Comparison of Semi-analytic Model with Experimental Data (1): Pulsed Argon Plasma with DC bias in the afterglowexperimentmodelIEDs predicted by the semi-analytic model (right) compared to data# (left).Pulsed Ar plasma, 10 KHz modulation, 20% duty, 14 mTorr, 120 W average power, 24.4 V DC bias applied in afterglow during∆tb = µs.Plasma potential w/o DC bias predicted by the semi-analytic model (line), compared to data# (points).# H. Shin, W. Zhu, L. Xu, D. J. Economou and V. M. Donnelly, PSST, (2011).
35 Comparison of Semi-analytic Model with Experimental Data (2): Helicon plasma with bias voltage waveform on the substrateExperimental Target voltage, 500 kHz, ne = 2.6×1016 m-3, Te = 3eVX. V. Qin, Y.-H. Ting, and A. E. Wendt, PSST (2010).Target voltage in the semi-analytic modelThe simulated voltage waveforms are quite representative of the measured waveforms except for the “ringing”.
36 Comparison of Semi-analytic Model with Experimental Data (2): Helicon plasma with bias voltage waveform on the substrateExperimental Ar+ IEDsX. V. Qin, Y.-H. Ting, and A. E. Wendt, PSST (2010).IEDs from the semi-analytic modelPredicted peak locations and heights of the IED are in agreement with the measurements.The FWHM of the experimental peaks is larger, because of the ringing of the applied voltage waveforms.
37 Comparison of Semi-analytic Model with Experimental Data (3): ETP with bias voltage waveform on a dielectric substrateThe substrate bias has minor effects on the plasma chemistry.The energy of ions is determined by the substrate voltage.The voltage appearing on the face of the substrate is at a constant negative value (Vfront), except for small excursions to positive values to neutralize the charge.M. A. Wank, R. A. C. M. M. van Swaaij, P. Kudlacek, M. C. M. van de Sanden, and M. Zeman, J. Appl. Phys. 108, (2010)
38 Comparison of Semi-analytic Model with Experimental Data (3): ETP with bias voltage waveform on a dielectric substrateElectrode downstream of expanding thermal hydrogen plasma (H3+).Biased through blocking capacitor, Cb = 166 pF.Vp~0.2 V, Te = 0.15 eV, p = 18 Pa, ne = 2 x 1010 cm-3Top figs.: Kudlacek et al.#Bottom figs.: Semi-analytic model prediction.CB = 1.66 nF, AG/AT =25The energy peaks location and the voltage waveform on substrate electrode are predicted.Voltage appliedto blocking cap.Voltage of substrate electrode.#P. Kudlacek, R. F. Rumphorst and M.C.M. van de Sanden, J. Appl. Phys., 106, (2009).
39 Comparison of Semi-analytic Model with Experimental Data (4): Control of IEDs using the electrical asymmetry effectMeasured ion energy distribution functions in a geometrically and electrically asymmetric dual frequency discharge at the powered (left) and grounded (right) electrode (Argon, 1 Pa, d = 4 cm, U1 = U2 = 100 V, f1 = MHz, f2 = MHz).U. Czarnetzki, J. Schulze, E. Schüngel and Z. Donkó, PSST 20, (2011)
40 Electrical Asymmetry Effect: Semi-Analytic Model Prediction Cb = 0.7 pF, AG = 2 AT , Te = 3 eV, n0 = 2 x 1015 m-3, M = 40 amu (Ar+), f1 = MHz, f2 = MHz, U1 = U2 = 100 V, instrumental broadening 2 eV.
41 Concluding RemarksSeveral methodologies can be implemented to tailor the ion energy distribution on plasma electrodes.PIC-MCC and hybrid simulations of a pulsed plasma with synchronous DC bias applied in the afterglow, showed that it is possible to tailor IEDs to have distinct energy peaks with controlled energies and fraction of ions under each peak.Simulations were in good agreement with measurements.Although PIC simulation provides detailed information (e.g., IAD in addition to IED), fast execution of semi-analytic model is advantageous for initial screening of tailored voltage waveforms.Models/simulations in synergy with experiments are critical to understand and predict the behavior of plasmas and to unravel new strategies for tailoring IEDs.
42 Acknowledgements Prof. M. Nikolaou, University of Houston Dr. H. Shin, University of Houston, currently at Lam Research Corp.W. Zhu, University of HoustonProf. S. Longo, University of Bari and CNR/IMIP, ItalyProf. M. Capitelli, University of Bari and CNR/IMIP, ItalyFinancial Support:DoE Plasma Science CenterNSF