SLIDE 1
1
2
- To study the energy distributions of ions
reaching the cathode in hollow cathode dc discharges of different gas precursors at low pressures, containing Ar, H2, N2, O2 and/or CH4.
- To get information about ion production in
19 th ESCAMPIG, Granada, Spain 15-19 July 2008 Aim of this work To - - PowerPoint PPT Presentation
19 th ESCAMPIG, Granada, Spain 15-19 July 2008 Aim of this work To - - PowerPoint PPT Presentation
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
SLIDE 2
SLIDE 3
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 3
- HV Electron Gun
=
Diagnostic Techniques
- Double
Double Langmuir Langmuir Probe Probe
- Quadrupole Mass Spectrometry
Quadrupole Mass Spectrometry
- f Neutrals and Ions
- f Neutrals and Ions
+ Ion Energy Distributions + Ion Energy Distributions
- Visible Emission Spectroscopy
Visible Emission Spectroscopy
SLIDE 4
4
Ion Energy Distributions
Ion Energy Analizer
Quadrupole Mass Analizer
SEM Ion Source for neutrals
PLASMA MONITOR
Hollow Cathode
Negative Glow E ~ 0
s Sheath: s ~ 1 cm , ∆V ~ VAnode-Cath. ~ 300 V 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(Ei) value)
SLIDE 5
5
Ion Energy Distributions
Ion Energy Analizer
Quadrupole Mass Analizer
SEM Ion Source
PLASMA MONITOR
Hollow Cathode
Negative Glow E ~ 0
s Sheath: s ~ 1 cm , ∆V ~ VAnode-Cath. ~ 300 V V
1 10
2 4 6 8 10
1 2 3 Te Maxwell (eV)
P (H2) (Pa)
H2 , 150 mA discharge
Ne/10
10 (cm
- 3)
Vac
Double Langmuir Probe
- Te ~ 1 - 10 eV depending on Pressure
E = 0 ⇒ Te instead of E/N for Modelling
- Tgas < 350 K ( i.e. ~ 0.025 eV )
Plasma far from Thermal Equilibrium !
SLIDE 6
6
Ar plasmas
100 200 300
10
2
10
3
10
4
10
5
10
6
325 330 2 4
(Ion Energy/q) (eV)
Ar
++
Ion Intensity (cc/s)
Ar
+
0.7 Pa
Ar
++
Ar
+
Δ Eion= 1.2 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 Low Pressure Narrow peaks with Em ~ VAC ΔEFWHM / Em ~ 0.3%
SLIDE 7
7
100 200 300
10
2
10
3
10
4
10
5
10
6
100 200
10
1
10
2
10
3
325 330 2 4
Ion Energy (eV)
Ar
++
Ion Intensity (cc/s)
Ar
+
0.7 Pa 4 Pa
Ar
++
Ion Intensity (cc/s) Ion Energy (eV)
Ar
+
Ar
++
Ar
+
Δ Eion= 1.2 eV
Ar plasmas
Higher Pressure
An entirely different distribution !
Low Pressure Narrow peaks with Em ~ VAC ΔEFWHM / Em ~ 0.3%
SLIDE 8
8
100 200 300
10
2
10
3
10
4
10
5
10
6
100 200
10
1
10
2
10
3
325 330 2 4
Ion Energy (eV)
Ar
++
Ion Intensity (cc/s)
Ar
+
0.7 Pa 4 Pa
Ar
++
Ion Intensity (cc/s) Ion Energy (eV)
Ar
+
Ar
++
Ar
+
Δ Eion= 1.2 eV
Effect of collisions in the sheath
⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎣ ⎡ − ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − =
−
1 V E 1 s exp V E 1 dE dN N E
2 1 2 1
λ Relevant parameter: s/λ (sheath collisions) S = sheath width
λ = ion mean free path ~ 1.4 mm
(at 4 Pa, with σ(Ar+) ~ 4 · 10-15 cm2)
Model of Davis and Vanderslice
- Phys. Rev. 131, 219 (1963)
Symmetric Charge Transfer Ar+ + Ar → Ar + Ar+
Pullins & Dressler,
- Z. Phys. Chemie (2000)
100 200 300
10
2
10
3
10
4
10
5
10
6
100 200
10
1
10
2
10
3
325 330 2 4
Ion Energy (eV)
Ar
++
Ion Intensity (cc/s)
Ar
+
0.7 Pa 4 Pa
Ar
++
Ion Intensity (cc/s) Ion Energy (eV)
Ar
+
s/λ=9
Model
S = 1.3 cm
Ar
++
Ar
+
Δ Eion= 1.2 eV
100 200 300
10
2
10
3
10
4
10
5
10
6
100 200
10
1
10
2
10
3
325 330 2 4
Ion Energy (eV)
Ar
++
Ion Intensity (cc/s)
Ar
+
0.7 Pa s/λ=1 s/λ=1 s/λ=4 4 Pa
Ar
++
Ion Intensity (cc/s) Ion Energy (eV)
Ar
+
s/λ=9
Ar
++
Ar
+
Δ Eion= 1.2 eV
σ(Ar++) ~ 7 · 10-16 cm2⇒ s/ λ (Ar++) ~ 1.5
SLIDE 9
9
H2 plasmas at low pressure
150 200 250 300
10
2
10
3
10
4
H
+
H3
+
H2
+
Ion Intensity (cc/s)
H2, 2 Pa Ion Energy (eV)
H2 + e → H2+ + 2e- H + e → H+ + 2e- H + H2
+ → H++ H2
in the GLOW H generated by e- impact dissociation of H2
Previous experiments based on optical emission spectroscopy ⇒ [ H ] / [ H2 ] ~ 10% in these H2 discharges.
( I Méndez , FJ Gordillo, VJ Herrero, I Tanarro & 2006, J. Phys. Chem. A )
Narrow peaks
(ΔEFWHM / Em < 1%)
SLIDE 10
10
A V Phelps (1990)
- J. Phys. Chem. Ref. Data
Collision Cross Sections of H2
+ + H2
H H3
3 + + generated
generated efficiently efficiently by by H 2
+ + H 2 → H 3 + + H
- nly at low Ei ⇒ in
in the the GLOW GLOW H 3
+
150 200 250 300
10
2
10
3
10
4
H
+
H3
+
H2
+
Ion Intensity (cc/s)
H2, 2 Pa Ion Energy (eV)
H2 plasmas
SLIDE 11
11
A V Phelps (1990)
- J. Phys. Chem. Ref. Data
H2 plasmas
Collision Cross Sections of H2
+ + H2
H H2
2+ + lost
lost of
- f energy
energy by by symmetric symmetric charge charge transfer transfer: :
σ ~ 9·10-20 m2 ⇒ s ~ 2 cm at 4 Pa H2
+ + H2→H2 (fast) + H2 +
- nly at high Ei ⇒ in the SHEATH.
Important for modelling.
fast H 2
150 200 250 300
10
2
10
3
10
4
50 100 150 200 250 300 10
3
10
4
10
5
H
+
H3
+
H2
+
Ion Intensity (cc/s)
H2, 2 Pa
H2, 20 Pa
H
+ 2x10
H3
+
H
+
Ion Intensity (cc/s)
Ion Energy (eV)
Model:s/λ=13
⇐ ⇐ Higher Higher Pressure Pressure
H 3
+
SLIDE 12
12
280 290 1 2 3 4 240 280 320 1 2 3 8 eV Δ Eion= 2 eV
H2, 2 Pa
Ion Intensity (cc/s) x 10
4
Ion Intensity (cc/s) x 10
5
H3
+
H
+(x20)
Ion Energy (eV) Em
H2, 20 Pa
H3
+
H
+(x3)
Em
150 200 250 300
10
2
10
3
10
4
50 100 150 200 250 300 10
3
10
4
10
5
H
+
H3
+
H2
+
Ion Intensity (cc/s)
H2, 2 Pa
H2, 20 Pa
H
+ 2x10
H3
+
H
+
Ion Intensity (cc/s)
Ion Energy (eV)
Model:s/λ=13
High Energy Region
H2 plasmas
SLIDE 13
13
Observations on H+ Energies
Narrow peak at Em + Secondary peak at: E ~ Em + 8 eV The secondary peak dissapears and a broad shoulder ( E < Em ) appears. 280 290 1 2 3 4 240 280 320 1 2 3 8 eV Δ Eion= 2 eV
H2, 2 Pa
Ion Intensity (cc/s) x 10
4
Ion Intensity (cc/s) x 10
5
H3
+
H
+(x20)
Ion Energy (eV) Em
H2, 20 Pa
H3
+
H
+(x3)
Em
WHY?
SLIDE 14
14
Experiments with electron beams, consistent with the Frack-Condon rule
G H Dunn & L J Kieffer, Phys. Rev. (1963)
280 290 1 2 3 4 8 eV Δ Eion= 2 eV
H2, 2 Pa
Ion Intensity (cc/s) x 10
4
H3
+
H
+(x20)
Ion Energy (eV)
1st: Low H2 Pressure
Potential-Energy Diagram for Ground H2 and Predissociative States
- f H2
+ and H2 ++
H2 + e- → H+
(fast)+ H (fast)+ 2e-
Dissociative H2 ionization
~8 eV ~16 eV
In the dc Discharge, fast H+ are generated in the GLOW by e- impact and acelerated towards the Cathode with their EXCESS OF ENERGY
SLIDE 15
15
Dissapearance of the secondary H+ peak at E>Em
80 90 240 280 320 1 2 3
Ion Intensity (cc/s) x 10
5
Ion Energy (eV)
H2, 20 Pa
H3
+
H
+x 3
2nd: Higher H2 Pressure
?
1 10
2 4 6 8 10
20 Pa
Te Maxwell (eV)
P (H2) (Pa)
I Méndez, VJ Herrero, I Tanarro & FJ Gordillo
- J. Phys. Chem. (2006)
2 Pa 280 290 1 2 3 4 8 eV Δ Eion= 2 eV
H2, 2 Pa
Ion Intensity (cc/s) x 10
4
H3
+
H
+(x20)
Ion Energy (eV)
Te decreases with increasing pressure
I Méndez, VJ Herrero,I Tanarro & FJ Gordillo
- J. Phys. Chem. (2006)
1 10 10
- 13
10
- 12
10
- 11
10
- 10
10
- 9
10
- 8
H
+ 2+H H ++H2
H2+e H
++H+2e
k (cm3 s-1) Te (eV) Rate constants, for H
+ formation
H+e H
++2e
Rate constant for fast H+ generation decreases with Te much more than for the other H+ formation proceses.
SLIDE 16
16
80 90 240 280 320 1 2 3
Ion Intensity (cc/s) x 10
5
Ion Energy (eV)
H2, 20 Pa
H3
+
H
+x 3
Higher H2 Pressure
Collision Cross Sections for H3
+ + H2 reactions
(very small in general⇒ H3
+ very “stable” in H2 media)
A V Phelps (1990)
- J. Phys. Chem. Ref. Data
H 3
+ + H 2 → H + + 2 H 2
H+ “may be” generated in the SHEATH and accelerated towards the Cathode (small extrapolated cross sections)
Apearance of the broad shoulder at E < Em Important for modelling
SLIDE 17
17
10
4
10
5
10
6
10
7
300 320 340 10
3
10
4
10
5
10
6
Ar
++
H
+
3
ArH
+
Ion Intensity (cc/s) H2+Ar (7%), 2 Pa
Ar
+
H
+ 2
H
+
Ion Intensity (cc/s) (Ion Energy/q) (eV)
H2+Ar plasmas
ArH+ Formation in the GLOW High rate coefficients at low impact energies Ar + H2
+ → ArH+ + H
Ar+ + H2 → ArH+ + H Ar + H3
+ → ArH+ + H2
H3
+, ArH+ and Ar++:
Narrow peaks with no wings ⇒ No collisions in the Sheath H2
+, H+ and Ar+:
Broadening at low energies ⇒ Charge transfer and inelastic collisions H+ with energy excess: H2
+ dissociative ionization
See more in Poster 3-77
SLIDE 18
18
10
4
10
5
10
6
10
4
10
5
10
6
310 320 330 340 350 360 10
3
10
4
10
5
350
HO2
+
H3O
+
Ion Intensity (cc/s)
H3
+
OH
+
O
+
H
+
O2
+
H2O
+
H2
+
Ion Intensity (cc/s)
O
++
Ion Intensity (cc/s)
(Ion Energy/q) (eV)
OH
+x3
O
+x3
H
+
H2+O2 (20%) plasmas
Protonated ions: Narrow peaks with no wings ⇒ No collisions in the sheath Molecular ions: Broadening at low energies ⇒ Symmetric charge transfer in the sheath H+, O+, OH+ and O++: Broadening at low energies ⇒ Charge transfer and inelastic collisions Energy excess at high energy ⇒ Dissociative ionization or stripping reactions:
SLIDE 19
19
10
4
10
5
10
6
10
4
10
5
10
6
310 320 330 340 350 360 10
3
10
4
10
5
350
HO2
+
H3O
+
Ion Intensity (cc/s)
H3
+
OH
+
O
+
H
+
O2
+
H2O
+
H2
+
Ion Intensity (cc/s)
O
++
Ion Intensity (cc/s)
(Ion Energy/q) (eV)
OH
+x3
O
+x3
H
+
H2+O2 (20%) plasmas
Dissociative ionization, O+ & O++ : O2 + e → O+ (fast) + O (fast) + 2e O2 + e → O++ (fast) + O (fast) + 3e
- Norm. Ion Intensity
Ion Energy (eV)
SLIDE 20
20
H2+O2 (20%) plasmas
Stripping reaction, OH+: O+ (fast) + H2 → OH+ (fast) + H
300 320 340 10
4
10
5
10
6
10
7
Ar
++
H
+
3
ArH
+
Ion Intensity (cc/s) H2+Ar (7%), 2 Pa
(Ion Energy /q) (eV)
Compare with ArH+ Formation No fast ions are involved! Sharp fall at high energy Ar + H2
+ → ArH+ + H
Ar+ + H2 → ArH+ + H Ar + H3
+ → ArH+ + H2
10
4
10
5
10
6
10
4
10
5
10
6
310 320 330 340 350 360 10
3
10
4
10
5
350
HO2
+
H3O
+
Ion Intensity (cc/s)
H3
+
OH
+
O
+
H
+
O2
+
H2O
+
H2
+
Ion Intensity (cc/s)
O
++
Ion Intensity (cc/s)
(Ion Energy/q) (eV)
OH
+x3
O
+x3
H
+
SLIDE 21
21
Similar behaviours observed in:
- H2+N2 plasmas,
- N2+O2 plasmas …
Castillo M, Méndez I, Islyaikin A, Herrero V J and Tanarro I 2005 J. Phys. Chem.A, 109, 6255-63
SLIDE 22
22
H2+N2+CH4 plasmas
Tanarro I, Herrero V J, Tabarés F L et al. 2007 , J. Phys. Chem. A, 111, 9003-12
Complex identification of ions with the same mass/charge (m/q) ratio but different chemical compositions Inhibition of a-C:H film growth by N2
- f interest for tritium retention in
nuclear fusion reactors Used to synthesize crystalline β−C3N4
14 15 16 17 18
10
- 3
10
- 2
10
- 1
10
P = 2 Pa
CH
+ 3+ NH +
CH
+ 2+ N +
H2+N2(5%)+CH4(5%) H2+CH4(5%)
H2+N2(5%)
Relative Ions Concentrations CH
+ 4+ NH + 2
NH
+ 4
CH
+ 5+ NH + 3
Mass (a.m.u)
SLIDE 23
23
10
2
10
3
10
4
10
1
10
2
10
3
10
4
- 15
- 10
- 5
5 10
Ion Intensity (cc/s)
H2+ CH4 H2+ N2
m/q=15
Em
Ion Intensity (cc/s) H2+ N2+ CH4 (H2+ N2) + (H2+ CH4) Ion energy (eV) 10
2
10
3
10
4
- 20
- 10
10
10
2
10
3
10
4
Ion energy (eV)
H2+ CH4 H2+ N2
Em
Ion Intensity (cc/s)
m/q=14
H2+ CH4+ N2 (H2+ CH4) + (H2+ N2)
Ion Intensity (cc/s)
Herrero V J, Islyaikin A M and Tanarro I 2008 J. Mass Spectr. 43 [on line in advance of print DOI: 10.1002/jms.1388]
The ion energy distributions can help greatly to discriminate species with the same mass/charge (m/q) ratio CH2
+ and N+ ions (m/q = 14)
Comparable amounts (∼ 50%) CH3
+ and NH+ ions (m/q = 15)
NH+ ~ 5%, CH3
+ ~ 95%
SLIDE 24
24
Summary and Conclusions 1
- 1. -
In hollow cathode dc discharges, ions diffuse without kinetic energy gain within the negative GLOW, where E ~ 0.
- 2. - In collision free SHEATHS (S/λ<1), ions reaching the cathode have very narrow
energy distributions, peaking at Em ~ some 100 eV, corresponding to the anode-cathode fall potential. Under these circumstances, the ion composition measured reflects that of the negative GLOW.
- 3. - The energy excess of ions generated in the GLOW by:
- Molecular dissociative ionizations:
( X2 + e → X+ (fast) + X (fast) +2e )
- Stripping reactions :
( X+ (fast) + H2 → XH+ (fast) + H ) is maintained by the ions traveling through the SHEATH, and can be detected.
SLIDE 25
25
Summary and Conclusions 2
- 4. -
The energy gained in the electric field of the SHEATH can be lost as pressure increases, through inelastic and charge transfer collisions. When the sheath width, S, is larger than their mean free path, λ, this can lead to large distortions in the original energy distributions.
- 5. -
The relative concentrations of some ions generated in complex plasma mixtures with the same m/q ratio but different composition and formation mechanisms can be obtained from the ion energy distributions.
- 6. -
For modelling, different processes in the negative GLOW and in the SHEATH, should be taken into account , with rate coefficients that depend on ion energies.
SLIDE 26
26
Thank you very much for your attention
SLIDE 27
27
10
4
10
5
10
6
10
7
10
4
10
5
10
6
260 270 280 290 300 310 320
10
3
10
4
10
5
300 310
N2H
+
NH4
+
H3
+
Ion Intensity (cc/s)
H3 NH4 N2H
H2
+
N2
+
NH3
+
NH2
+
Ion Intensity (cc/s) N
++
NH
+
N
+
Ion Intensity (cc/s) Ion Energy (eV) H
+
H
+
NH
+ x 6
N
+x 6