Ion Energy Distributions in Pulsed Plasmas with Synchronous DC Bias: Effect of Noble Gas W. Zhu, H. Shin, V. M. Donnelly and D. J. Economou Plasma Processing Laboratory University of Houston Acknowledgements: DoE Plasma Science Center and NSF AVS,58th International Symposium, Nashville, TN, USA UNIVERSITY of HOUSTON Plasma Processing Lab 1
Outline Motivation Experimental Apparatus Results and Discussion Ion energy distribution (IED) control Effect of noble gas (Ar, Kr, Xe) Summary and Conclusions UNIVERSITY of HOUSTON Plasma Processing Lab 2
Motivation: control of IED IED Flux with broad ion energy distribution Energy Monoenergetic ion flux Wide range of ion energies results in surface roughness and damage E i <E chemical sputtering E i > E physical sputtering Nearly monoenergetic ion bombardment results in higher level of control Low surface damage Accurate thickness control High selectivity Control of IED: Accurate control of ion energy peak position Narrow width of ion energy distribution UNIVERSITY of HOUSTON Plasma Processing Lab 3 3
Experimental apparatus Boundary electrode Ion energy analyzer at Z= 170mm Monochromator Differentially pumped ion energy analyzer (IEA) Movable Langmuir probe (LP) periscope Ion energy can be manipulated by applying DC bias to Boundary Electrode. UNIVERSITY of HOUSTON Plasma Processing Lab 4
Control of V p using DC boundary voltage Continuous DC bias on the boundary electrode in cw plasma V p (V) 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 IEA location +10V +5V 0V -5V 10V 160 180 200 220 240 260 280 axial location, z (mm) Ar The DC bias on the boundary electrode is superimposed to the plasma potential. With positive DC bias, V p is shifted by corresponding boundarybias bias voltage. Negative DC bias barely changed V p since ion flux to the -10V wall is limited. UNIVERSITY of HOUSTON Plasma Processing Lab 5
lized IED Norma IEDs in CW plasma with continuous DC 1.4 1.2 1.0 0.8 06 0.6 0.4-8V -4V Ground +4V +8V +12V boundary voltage V p measured by Langmuir probe Ar Ion energy peaks indicate plasma potential because the IEA entrance is ground (V p =V sh ). p V p is in excellent agreement with Langmuir probe measurement at the same location. Positive DC bias on boundary electrode shifts 0.2 plasma potential 0.0 0 2 4 6 8 10 12141618 202224 26283032 Energy (ev) IEDs from continuous wave plasma are broad UNIVERSITY of HOUSTON Plasma Processing Lab 6
Timing scheme for pulsed plasma with synchronousdc boundaryvoltage in afterglow rf power Positive DC bias (+24V) 10kHz Modulation 20µs 80µs ON OFF UNIVERSITY of HOUSTON Plasma Processing Lab 7
V p (V) Control of V p using DC boundary voltage Time resolved LP measurements in pulsed plasma 24 +10V continuous bias on BE 22 no bias on BE 20 18 16 14 12 10 8 6 4 2 0 0 10 20 30 40 50 60 70 80 90 100 time ( s) Continuous bias V p (V V) 24 +23V synchronous BE bias 22 during 70-97 s 20 18 16 14 Bias window 12 10 8 6 4 2 0 0 10 20 30 40 50 60 70 80 90 100 time ( s) Synchronous bias Synchronous DC bias on the boundary electrode can shift plasma potential only during biasing window UNIVERSITY of HOUSTON Plasma Processing Lab 8
IEDs of pulsed plasma with synchronous DC boundary voltage IED 0.05 7 mtorr 14 mtorr 28 mtorr 0.04 50 mtorr 0.03 0.02 Plasma potential peak Ar Due to DC bias Low energy broad peaks are due to active glow; High energy sharp peaks are due to DC bias in the afterglow. Separation of the peaks can be tuned by DC bias value and pressure Narrow IED can be achieved in the afterglow. 0.01 0.00 0 2 4 6 8 101214161820222426283032 Energy (ev) Full width at half maximum (FWHM) of the IED ranges from 1.7 to 2.4 ev and scales with T e. UNIVERSITY of HOUSTON Plasma Processing Lab 9
Broadening of IED Qualitative IEDs resulting from synchronous DC bias during part of afterglow. IED V p_f + V dc V p V p_i + V dc kt e_f kt e_i Energy (ev) The IED shifts to lower energies and becomes sharper with time. The measured IED in the afterglow is a time averaged distribution. Width of IED correlates with T e and variation of V p during bias window. UNIVERSITY of HOUSTON Plasma Processing Lab 10
T e and V p change during bias Time resolved Langmuir probe measurements T e (ev) 4.0 ON OFF 3.5 3.0 25 2.5 2.0 1.5 V p (V) 16 14 12 10 8 6 10 1.0 4 0.5 2 0.0 0 10 20 30 40 50 60 70 80 90 100 0 0 10 20 30 40 50 60 70 80 90 100 time ( s) time ( s) The smaller the electron temperature and the variation of plasma potential during the biasing window, the sharper the ion energy distribution. UNIVERSITY of HOUSTON Plasma Processing Lab 11
Early vs. late afterglow bias 0.04 0.03 22-60 s 32-60 s 42-60 s t = 38 s t = 28 s 0.03 0.02 00 50-98 s 60-98 s 70-98 s 80-98 s t = 48 s t = 38 s t = 28 s IED 0.02 0.01 t = 18 s IED 0.01 t = 18 s 0.00 0 2 4 6 8 101214161820222426283032 Energy (ev) 0.00 0 2 4 6 8 101214161820222426283032 Energy (ev) Early afterglow biasing (V p changes considerably and T e is high) vs. late afterglow biasing (V p and T e have decayed to low values). IED width is smaller with late afterglow biasing. UNIVERSITY of HOUSTON Plasma Processing Lab 12
Outline Motivation Experimental Apparatus Results and Discussion Ion energy distribution (IED) control Effect of noble gases (Ar, Kr, Xe) Summary UNIVERSITY of HOUSTON Plasma Processing Lab 13
n i (cm - 3 ) 1x10 12 9x10 11 8x10 11 7x10 11 6x10 11 5x10 11 4x10 11 3x10 11 2x10 11 1x1010 11 ON Temporal evolution of plasma density for different gases OFF Ar Kr Xe 0 0 10 20 30 40 50 60 70 80 90 100 time ( s) Plasma dens sities(cm - 3 ) 1.4x10 11 1.2x10 11 1.0x10 11 8.0x10 10 6.0x10 10 4.0x10 10 2.0x10 10 0.0 ni ne Fluid simulation 30 40 50 60 70 80 90 100 time ( s) At the edge of the plasma, the plasma density is nearly constant, even over a long afterglow duration of 80 µs. Transport of electrons from the higher density central region of the plasma to the edge region, is balancingthelossof loss of plasmadue to diffusion. Maintaining a nearly constant ion (and electron) density during the afterglow may be useful in processes employing pulsed plasmas. UNIVERSITY of HOUSTON Plasma Processing Lab 14
T e (ev V) 5.0 4.5 4.0 3.5 3.0 25 2.5 2.0 1.5 10 1.0 0.5 ON Temporal evolution of T e and V p for different noblegases OFF Experiment Model Ar Ar Kr Kr Xe Xe V p (V) 16 14 12 10 0.0 0 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 time ( s) time ( s) T e and V p are in the order of Ar < Kr < Xe T e and V p decay slower in Xe plasma (slower diffusion cooling). p Experimental data of T e are consistent with predictions from global model. 8 6 4 2 Ar Kr Xe UNIVERSITY of HOUSTON Plasma Processing Lab 15
IEDs for different carrier gases IED 0.5 Ar Kr 0.4 Xe 0.3 02 0.2 IED shows the same order of T e and V p during bias window : Ar < Kr < Xe FWHM of the narrow peak is 1.6, 2.4, and 3.0 ev for Ar, Kr and Xe 0.1 0.0 The area under the IED peak resulting from afterglow biasing is proportional to the ion flux 0 2 4 6 8 1012141618202224262830 Energy (ev) UNIVERSITY of HOUSTON Plasma Processing Lab 16
IEDs for different Cl 2 additions IED 05 0.5 0.4 0.3 0.2 40 sccm Ar (14mTorr) 0% Cl 2 1% Cl 2 Similar IEDs were found with 2% Cl 2 trace amount of Cl 2 addition 3% Cl 2 (<5%) 4% Cl 2 5% Cl 2 Peak ion energy was lower by a few ev, possibly due to lower V p and T e from Cl 2 plasma. 0.1 0.0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Energy (ev) Such control of IED can be applied to plasma etching with high precision. UNIVERSITY of HOUSTON Plasma Processing Lab 17
Summary and Conclusions 1. Nearly monoenergetic IEDs can be achieved using synchronous DC bias in the afterglow of a pulsed plasma. 2. The peak values and the FWHM of the IED can be controlled by varying DC bias and the time window it is applied, operating pressure, and carrier gas. 3. FWHM depends on the electron temperature and plasma potential variation during the time of biasing. 4. Maintaining a nearly constant ion (and electron) density during the afterglow may be useful lin processes employing pulsed plasmas. 5. By adding small amounts of chlorine to an argon plasma the IEDs are shifted to slightly smaller energies. UNIVERSITY of HOUSTON Plasma Processing Lab 18
Questions?
Backup slides
Time resolved T e in pulsed plasma T e (ev) 5.0 ON 7mTorr 45 4.5 4.0 OFF 3.5 OFF 30 3.0 2.5 2.0 0.5 14mTorr 28mTorr 50mTorr 50% duty cycle at 14mTorr At higher pressure, T e is lower during active glow, but higher during afterglow. T e decays to low values in 15 20 ms after plasma is turned off. 1.5 Duty cycle has no effect 1.0 on the T e decay. 0.0 0 10 20 30 40 50 60 70 80 90 100 time ( s) UNIVERSITY of HOUSTON Plasma Processing Lab 21
V p variation during bias window IED V pf p_f + V dc V pi p_i + V dc kt ef e_f kt ei e_i Energy (ev) UNIVERSITY of HOUSTON Plasma Processing Lab
Time scheme for different mod. frequency 10kHz 7.5kHz 5kHz UNIVERSITY of HOUSTON Plasma Processing Lab
IED with different mod. frequency 0.05 0.04 5kHz 7.5kHz 10kHz Ar 0.05 0.04 5kHz 75kH 7.5kHz 10kHz Kr 0.03 0.03 IED 0.02 IED 0.02 0.01 001 0.01 0.00 0 2 4 6 8 1012141618202224262830 Energy (ev) 0.00 0 2 4 6 8 1012141618202224262830 Energy (ev) Decreasing area of high energy peaks is due to lower T e and ion density Overall, pulsed Ar plasma shows best FWHM in the afterglow UNIVERSITY of HOUSTON Plasma Processing Lab
Etching process monitoring A) A emission intensity ( Si 2881 5.0x10-8 4.5x10-8 4.0x10-8 3.5x10-8 3.0x10-8 2.5x10-8 2.0x10-8 1.5x10-8 1.0x10-8 5.0x10-9 0.0 kinetic energy of ions accelerated in plasma sheath = boundary electrode bias voltage 20% pulsed 1% Cl 2, 5% TRG, 94% Ar plasma, 10kHz 100W, 70-97 s synchronous boundary electrode bias 14mTorr 28mTorr 50mTorr 60mTorr ON Δt bias t=0 t=20 µs t=100 µs 0 10 20 30 40 50 Boundary electrode bias voltage (V) Si removal by synchronous biasing in the afterglow. Si removal during activeglow. Afterglow etching threshold was found ~16V Activeglow g p type Si etching with sub threshold ion energy The sub threshold etch rate is significant, compared to ionassisted etching, for process such as atomic layer etching. UNIVERSITY of HOUSTON Plasma Processing Lab 25