1 19 th ESCAMPIG, Granada, Spain 15-19 July 2008
Aim of this work • To study the energy distributions of ions reaching the cathode in hollow cathode dc discharges of different gas precursors at low pressures, containing Ar, H 2 , N 2 , O 2 and/or CH 4 . • To get information about ion production in the glow and collision processes in the 2 sheath from the shapes of the different
3 EXPERIMENTAL SET-UP Hollow cathode dc discharge Cathode dimensions ~ 10 cm × 34 cm Low plasma pressure ~ 0.5 - 50 Pa Electron gun to ignite the plasma = Diagnostic Techniques • • Double Langmuir Double Langmuir Probe Probe -HV Electron Gun • • Quadrupole Mass Spectrometry Quadrupole Mass Spectrometry of Neutrals and Ions of Neutrals and Ions + Ion Energy Distributions + Ion Energy Distributions • • Visible Emission Spectroscopy Visible Emission Spectroscopy 3
Ion Energy Distributions Hollow Cathode Ion Source Quadrupole Ion Energy Analizer SEM for neutrals Mass Analizer Negative Glow E ~ 0 V s PLASMA MONITOR Sheath: s ~ 1 cm , ∆ V ~ V Anode-Cath. ~ 300 V ⇒ • E = 0 ----------- in Negative Glow Ions diffuse without kinetic energy gain • ∆ V > 300 V --- in Cathode Sheath ⇒ Ions are acelerated towards the cathode and gain kinetic energy. Part of this energy can be lost in collisions with neutrals at high enough pressure (depending on each k(E i ) value) 4
Ion Energy Distributions Hollow Cathode Quadrupole Ion Energy Analizer SEM Ion Source Mass Analizer Negative Glow E ~ 0 V s PLASMA MONITOR Sheath: s ~ 1 cm , ∆ V ~ V Anode-Cath. ~ 300 V V ac Double Langmuir Probe -3 ) 3 • T e ~ 1 - 10 eV depending on Pressure 10 (cm 2 E = 0 ⇒ T e instead of E/N for Modelling N e /10 1 10 H 2 , 150 mA • T gas < 350 K ( i.e. ~ 0.025 eV ) T e Maxwell (eV) 8 discharge 6 Plasma far from Thermal Equilibrium ! 4 2 1 10 P (H 2 ) (Pa) 5
Ar plasmas 6 10 Low Pressure 0.7 Pa + 4 Ar Ion Intensity (cc/s) 5 10 Narrow peaks with E m ~ V AC Δ E ion = 1.2 eV Δ E FWHM / E m ~ 0.3% 4 10 2 ++ Ar + Ar 3 10 0 325 330 2 10 ++ Ar 100 200 300 (Ion Energy/q) (eV) Ion generation in the GLOW: Ar + e → Ar + + 2 e Ar + e → Ar ++ + 3 e Very low pressure ⇒ No collisions in the sheath between ions and neutrals 6
Ar plasmas 6 10 Low Pressure 0.7 Pa + 4 Ar 5 Ion Intensity (cc/s) 10 Narrow peaks with E m ~ V AC Δ E ion = 1.2 eV Δ E FWHM / E m ~ 0.3% 4 10 2 ++ Ar + Ar 3 10 0 325 330 2 10 ++ Ar 100 200 300 Ion Energy (eV) 3 10 4 Pa Higher Pressure Ion Intensity (cc/s) An entirely different distribution ! + Ar 2 10 ++ Ar 1 10 100 200 Ion Energy (eV) 7
Symmetric Charge Transfer Effect of collisions in the sheath Ar + + Ar → Ar + Ar + 6 10 6 6 10 10 0.7 Pa + 0.7 Pa 4 Ar 0.7 Pa + + Ion Intensity (cc/s) 5 4 4 Ar 10 Ar 5 Ion Intensity (cc/s) Ion Intensity (cc/s) 5 10 10 Δ E ion = 1.2 eV Δ E ion = 1.2 eV Δ E ion = 1.2 eV 4 10 4 4 10 2 10 2 2 ++ Ar ++ ++ Ar Ar Pullins & Dressler, + Ar 3 + + Ar 10 Ar 3 3 10 10 Z. Phys. Chemie (2000) 0 0 0 325 330 325 325 330 330 2 2 2 10 10 10 ++ Ar ++ ++ Ar Ar Model of Davis and Vanderslice 100 200 300 100 100 200 200 300 300 Ion Energy (eV) Ion Energy (eV) Ion Energy (eV) Phys. Rev. 131, 219 (1963) 3 3 3 10 10 10 ⎛ ⎞ ⎡ ⎤ − 1 1 ⎜ ⎟ ⎛ − ⎞ ⎛ − ⎞ ⎢ ⎥ E 2 2 dN E s E 4 Pa 4 Pa 4 Pa ⎜ ⎟ ⎜ ⎟ 0 = − ⎜ ⎟ s/ λ =1 1 exp 1 1 Ion Intensity (cc/s) Ion Intensity (cc/s) Ion Intensity (cc/s) ⎜ ⎟ ⎜ ⎟ ⎢ ⎥ λ ⎝ ⎠ ⎜ ⎝ ⎠ ⎟ N dE V V ⎢ ⎥ Model 0 0 0 ⎣ ⎦ ⎝ ⎠ + + Ar + Ar Ar 2 s/ λ =4 Relevant parameter: s/ λ (sheath collisions) 2 2 10 10 10 s/ λ =9 S = sheath width s/ λ =9 ++ ++ Ar Ar ++ Ar λ = ion mean free path ~ 1.4 mm s/ λ =1 S = 1.3 cm (at 4 Pa, with σ (Ar + ) ~ 4 · 10 -15 cm 2 ) 1 1 10 10 1 10 100 200 100 200 σ (Ar ++ ) ~ 7 · 10 -16 cm 2 ⇒ s/ λ (Ar ++ ) ~ 1.5 100 200 Ion Energy (eV) Ion Energy (eV) Ion Energy (eV) 8
H 2 plasmas at low pressure Narrow peaks Ion Intensity (cc/s) H 2 , 2 Pa ( Δ E FWHM / E m < 1%) + H 3 4 10 H 2 + e → H 2+ + 2e - 3 + 10 H in the H + e → H + + 2e - + H 2 GLOW 2 10 + → H + + H 2 H + H 2 150 200 250 300 Ion Energy (eV) H generated by e - impact dissociation of H 2 Previous experiments based on optical emission spectroscopy ⇒ [ H ] / [ H 2 ] ~ 10% in these H 2 discharges. ( I Méndez , FJ Gordillo , VJ Herrero, I Tanarro & 2006, J. Phys. Chem. A ) 9
H 2 plasmas Collision Cross Sections of H 2 + + H 2 A V Phelps (1990) Ion Intensity (cc/s) H 2 , 2 Pa + H 3 J. Phys. Chem. Ref. Data 4 10 + H 3 3 + 10 H + H 2 2 10 150 200 250 300 Ion Energy (eV) + generated H 3 generated efficiently efficiently by by H + 3 + + H 2 → H 3 + + H H 2 only at low E i ⇒ in in the the GLOW GLOW 10
H 2 plasmas + + H 2 Collision Cross Sections of H 2 A V Phelps (1990) Ion Intensity (cc/s) H 2 , 2 Pa J. Phys. Chem. Ref. Data + H 3 4 10 + H 3 fast H 2 3 + 10 H + H 2 2 10 150 200 250 300 Ion Intensity (cc/s) + H 3 H 2 , 20 Pa 5 10 + H ⇐ Higher ⇐ Higher Pressure Pressure + H 2 x10 4 10 + lost H 2 lost of of energy energy by by symmetric symmetric H 2+ Model:s/ λ =13 charge transfer charge transfer: : 3 10 + + H 2 → H 2 (fast) + H 2 H 2 + 50 100 150 200 250 300 Ion Energy (eV) only at high E i ⇒ in the SHEATH. Important for modelling. σ ~ 9·10 -20 m 2 ⇒ s ~ 2 cm at 4 Pa 11
High Energy Region H 2 plasmas 4 4 E m Ion Intensity (cc/s) x 10 + H 3 H 2 , 2 Pa 3 Ion Intensity (cc/s) H 2 , 2 Pa + H 3 4 10 Δ E ion = 2 eV 2 + (x20) H 3 + 10 H 1 + H 2 8 eV 2 0 10 280 290 150 200 250 300 5 Ion Intensity (cc/s) x 10 E m 3 Ion Intensity (cc/s) + H 3 H 2 , 20 Pa 5 H 2 , 20 Pa 10 + + H 3 H 2 + H 2 x10 4 10 + (x3) H 1 3 10 Model:s/ λ =13 0 50 100 150 200 250 300 Ion Energy (eV) 240 280 320 Ion Energy (eV) 12
4 4 E m Ion Intensity (cc/s) x 10 Observations on + H 2 , 2 Pa H 3 H + Energies 3 Δ E ion = 2 eV 2 + (x20) H Narrow peak at E m + 1 Secondary peak at: E ~ E m + 8 eV 8 eV 0 280 290 5 Ion Intensity (cc/s) x 10 E m 3 H 2 , 20 Pa The secondary peak dissapears + H 3 2 and a broad shoulder ( E < E m ) + (x3) H appears. 1 WHY? 0 240 280 320 Ion Energy (eV) 13
Dissociative H 2 ionization 1 st : Low H 2 Pressure H 2 + e - → H + (fast) + H (fast) + 2e - 4 4 Ion Intensity (cc/s) x 10 + H 3 H 2 , 2 Pa 3 Δ E ion = 2 eV 2 + (x20) H 1 8 eV ~16 eV ~8 eV 0 280 290 Ion Energy (eV) Potential-Energy Diagram for Ground H 2 and Predissociative States + and H 2 of H 2 ++ In the dc Discharge, fast H + are Experiments with electron beams, generated in the GLOW by e - impact consistent with the Frack-Condon rule and acelerated towards the Cathode 14 G H Dunn & L J Kieffer, Phys. Rev. (1963) with their EXCESS OF ENERGY
T e decreases with increasing pressure 4 4 I Méndez, VJ Herrero, 10 Ion Intensity (cc/s) x 10 I Tanarro & FJ Gordillo T e Maxwell (eV) + H 3 H 2 , 2 Pa J. Phys. Chem. (2006) 3 8 I Méndez, VJ Herrero,I Tanarro & FJ Gordillo 2 Pa Δ E ion = 2 eV 20 Pa 2 J. Phys. Chem. (2006) 6 + (x20) H Rate constant for fast H + generation 1 4 8 eV decreases with T e much more 0 2 280 290 than for the other H + formation proceses. 1 10 Ion Energy (eV) P (H 2 ) (Pa) 2 nd : Higher H 2 Pressure -8 10 80 90 3 5 + + +H 2 Ion Intensity (cc/s) x 10 H 2 +H H -9 10 + H 2 , 20 Pa H 3 + +2e H+e H k ( cm 3 s -1 ) 2 -10 10 + +H+2e H 2 +e H -11 10 1 + x 3 H Rate constants, -12 10 + formation for H 0 ? -13 10 240 280 320 1 10 T e (eV) Ion Energy (eV) Dissapearance of the secondary H + peak at E>E m 15
Collision Cross Sections for H 3 + + H 2 reactions + very “stable” in H 2 media) (very small in general ⇒ H 3 H + “may be” generated in the SHEATH and accelerated towards the Cathode A V Phelps (1990) J. Phys. Chem. Ref. Data (small extrapolated cross sections) + + H 2 → H + + 2 H 2 H 3 Higher H 2 Pressure 80 90 3 5 Ion Intensity (cc/s) x 10 + H 2 , 20 Pa H 3 2 1 + x 3 H 0 240 280 320 Important for modelling Ion Energy (eV) Apearance of the broad shoulder at E < E m 16
H 2 +Ar plasmas + , ArH + and Ar ++ : H 3 7 10 Narrow peaks with no wings ⇒ Ion Intensity (cc/s) H 2 +Ar (7%), 2 Pa + H No collisions in the Sheath 3 6 10 ArH + Formation in the GLOW + ArH 5 10 + → ArH + + H ++ Ar Ar + H 2 Ar + + H 2 → ArH + + H 4 10 6 10 Ion Intensity (cc/s) + → ArH + + H 2 Ar + H 3 + H 2 5 10 High rate coefficients at low impact energies + H 4 10 + , H + and Ar + : + H 2 Ar 3 Broadening at low energies ⇒ 10 300 320 340 Charge transfer and inelastic collisions (Ion Energy/q) (eV) H + with energy excess: + dissociative ionization H 2 See more in Poster 3-77 17
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