Electronegative Plasmas Basic Atomic Processes Basic Physics Aspects Eva Stoffels, Eindhoven...
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Electronegative Plasmas
Basic Atomic Processes
Basic Physics Aspects
Eva Stoffels, Eindhoven University of Technology
Negative ions: why bother?
• Most “interesting” chemical systems contain electronegative species
• Negative ions are “shy”, but… can influence the plasma
• Negative ions for energetic bundle preparation
• Negative ions are fun!
Where?
• atmosphere
• surface processing plasmas
• excimer lasers, halogene lamps…
O-, O2-, O3
-, CO2-, NOx
-, etc.
Etching (IC’s, cleaning):CF4, C2F6, C3F8, SF6,
O2…
Deposition (a-Si:H, diamond)CH4, SiH4, NH3
Excimer media: Ar, Kr, Xe + F2, Cl2
iodine lamp, XeCl lamp
Basic Atomic Processes
• Where do they come from?– Various kinds of electron attachment– Why doesn’t it work in the plasma?– Surface processes, energetic processes
• Where do they disappear?– Recombination (+/-)– Detachment– Transport
XY + e --> (XY-)* --> ???
• Non-dissociative attachment (XY- is stable)(XY-)* --> XY- + E
E = affinity(XY) + kinetic energy(e) - activation energy (XY-)*
Momentum conservation!
Stabilisation of the excited anion
• Autodetachment (XY-)* --> XY + e
• Radiative (XY-)* --> XY- + h(atomic species, interstellar space)
• Three-body (XY-)* + Z --> XY- + Z(Z carries out the energy, atm. pressure)
• Redistribution (XY-)* --> XY-()(polyatomic molecules, small excess energies)
Dissociative attachment (DA)
• (XY-)* --> X + Y- or X- + Y• process can be endo- or exothermic • released energyE = affinity(X or Y) + kinetic energy(e)
- activation energy (XY-)*- dissociation energy (XY)
carried out by product neutrals/anions, negative ions can be hot!!!
How does it work in practice?
(a) and (b) - activation energy needed
a) XY- unstable --> always DA (CF4)
b) XY- stable --> depends on electron energy and stabilisation (O2, H2)
nuclear separation
pote
ntia
l ene
rgy E2
E1
XYX+Y
X+Y
(a)
nuclear separation
pote
ntia
l ene
rgy
E1
E2
XY
X+Y
X+YE3
(b)
E
2
mainly endothermic
Typical cross-sections
• Resonant-like cross-sections• Threshold for electron energy
CF4: multiple
fragmentationpathways
possible 0
20
40
60
80
100
1 2 3 4 5 6 7 8 9 10 11
electron energy (eV)
coun
ts
F + CF3
F + CF2 + F
Strongly electronegative species
-6
-5
-4
-3
-2
-1
0
1
0 1 2 3 4 5
nuclear separation (A)
pote
ntia
l ene
rgy
(eV
)
Cl + Cl
Cl + Cl
Cl2
Cl2
o
(c)
-6
-5
-4
-3
-2
-1
0
1
0 1 2 3 4 5
nuclear separation (A)
pote
ntia
l ene
rgy
(eV
)
SF6 + F
SF5 + F
o
SF6
SF6
(d)
Cl2 - exothermic but small activation energy needed
SF6 - exothermic, no activation energy needed
Typical cross-sections
0.010.101.00
10.00100.00
1000.0010000.00
0 0.1 0.2 0.3 0.4
electron energy (eV)
coun
tsThe SF6 cross-section: no energy threshold
Langevin limit
• Theoretical maximum cross-section for electron capture
• based on electron-(induced) dipol interactions
E24exp1E2
a 20
max
- polarisability, E - electron energy
Typical electronegative gases
parentmolecule
Negative ions Te attachmentrate (m3/s)
reference
CF4 F, CF3
3 eV 410
18 Christophorou 1996C2F6 F
, CF3
3 eV 6.410
16 Christophorou 1998CHF3 F
300 K 510
20 Christophorou 1997O2 O
3 eV 3.510
17 Christophorou 1984H2 H
3 eV 10
20 Wadhera 1984
H2 (=2) H
3 eV 710 18 Wadhera 1984
CCl2F2 Cl
3 eV 8.510 16 Christophorou 1997a
CCl2F2 Cl
300 K 1.810 15 Christophorou 1997a
Cl2 Cl
300 K 2.010 15 Christophorou 1999
SF6 SF6 SF5
300 K 3.110
13 Smith 1984CCl4 Cl
300 K 3.910
13 Smith 1984limit 510
13
Why doesn’t it work in plasmas?
• Experiments: negative ion densities much too high (10 times than expected)
• Trends do not reproduce at all…
• What attaches in the plasma?
• Is DA everything, don’t we miss some other formation channel?
What attaches in the plasma?
• Plasma is a complex mixture
• Conversion of parent species into more active/electronegative ones
– electronically excited– vibrationally excited– other molecules/radicals
Excitation
• Electronic: lowered attachment threshold
e.g. O2(a)
(a1g 1 eV exc.
energy) 4 x higher cross-section
0
500
1000
1500
2000
2500
3000
0 2 4 6 8 10
electron energy (eV)
Vibrational excitation
• Lowered threshold, molecule larger
• in non-thermal plasmas Tvib >> Tgas
• extreme example: H2
Hydrogen negative ions
• Important: additional heating source for fusion plasmas
• hot molecular beams prepared by acceleration of H- and neutralisation
• good sources needed
• H2 itself hardly attaches electrons…
• but cross-section for =4 is104 x cross-section for =1!
H- production enhanced
• ??? Less hydrogen, more H- ???
• Argon dilution:more electronsmore H2()
0
1
2
3
4
5
0 20 40 60 80 100
% H2 in argon
dens
ity (
1015
m-3
)
Molecular conversion
• Typical examples: fluorocarbons, silane
• Polymerisation!
0
100
200
300
400
0,5 2,5 4,5 6,5 8,5 10,5electron energy (eV)
coun
ts
0 W10 W15 W30 WC2F6
C3F8
CF4
This is the effective DA cross-section in CF4, and CF4 plasma
CHF3 chemistry
• Important for high aspect ratio etching (contact holes) because of side-wall passivation
1 2 3 4 5 6 7 8 9 10
050
100150200250
CO
UN
TS
ELECTRON ENERGY (eV)
CF4
C2F6
(b)
CHF3 itself does not attach, its conversion products do!
Other complications?
• This was only gas phase, but is there more?
• YES! Surface production X + e(s) --> X-
• Surface converters for H- production– metal surfaces with very low work
function used– plasma lowers the necessary energy
(negative surface charging!)
Between plasma and surface
• Sheath – high E field– positive ions accelerated up to 1000 eV– what happens if they collide with neutrals
• Rich sheath chemistry:– formation of excited species
X+ + O2 --> X+ + O2* (+ O2 --> O2+ + O2
-)
– ion pair formationX+ + O2 O+ + O-
Consequences
• Low-pressure plasmas for surface processing - plenty of surface
• Negative ions formed mainly in the sheath
• In O2 : both O- and O2- formed
• surface/sheath production channel for molecular ions (direct attachment does not work)
Oxygen DC and RF glow discharges
0
50
100
150
200
250
300
350
400
-100,00 0,00 100,00 200,00 300,00 400,00 500,00
energy (eV)
coun
ts
V
-
acceleration
High-energy tail
cathode anode
0
200
400
600
800
0,00 5,00 10,00 15,00 20,00
energy (eV)
coun
ts
V(anode)
thermal ions (glow)
“cathode” ions
Negative ions in oxygen
2000
4000
6000
8000
10000
12000
0,000 0,100 0,200 0,300
pressure (mbar)
cou
nts
0
3000
6000
9000
12000
400 600 800 1000
dc voltage (V)co
unt
s
O2- O-
Especially at low pressures, high-energy negative ions present (higher pressures - thermalisation, chemical destruction)
Destruction processes
• Ion-ion neutralisationX + Y+ X + Y*.
• Coulomb process: very high cross-section (>1016 m2)
• Rate depends on ion temperature
0.4-a
1/2-2/1
13rec E
T
300 1034.5k
( - red. mass in amu, Ea - affinity X in eV)
Destruction processes
• Direct neutral detachmentX + Y X + Y + e
– Y must have energy X affinity (not likely in cold plasmas)
• Electron-induced detachment X + e X + e + e
– important in high-density sources (ICP, ECR, microwave)
– in DC/RF glows - ne too low
“Chemical” destruction
• Associative detachmentX- + Y XY + eX- + YZ XY + Z + e
• Rate constants 1016 m3/s• Important in surface processing
plasmas
• “Killer” in H- sources H- + H H2 + e
• Leads to plasma polymerisation
Associative detachment
• In O2, CF4: higher pressures, less negative ions
Modelling:
production against detachment
--> decrease
Associative detachment
Extra detachment by oxygen atoms
Plasma polymerisation
• Ion-induced: faster than neutral
• Works at low pressuresCnFk
- + CFm --> Cn+1Fk+m + e
• In CF4/C2F6 chemistry up to C10 detected
• In silane: dust formation channel!
Transport & surface losses
• In active plasmas: sheath keeps them away
• in DC: losses to the anode
• in afterglow: free diffusion
RF gnd
X
V
Summary I
• Negative ions are produced by DA, but…• Not to the parent molecules• Gas conversion, excitation extremely
important• Surface production!• Destruction processes – more or less as
expected.• Polymerisation via negative ions efficient
Basic Physics Aspects
• What if there are too many negative ions = n-/ne = 10 in O2
50-100 in C2F6
>1000 in Cl2, SF6,…
• The latter are plasmas without electrons• Kind of “afterglow” plasmas?
EEDF, ionisation rate, etc.
• Electron attachment causes decrease in ne
• DA depletes the plasma of low-energy electrons --> changes in EEDF, ne Te
Transport properties
• Ambipolar diffusion + = e + -
+ = - D+ n+ + n++E
e,- = - De,- ne,- - ne,-e,-E
or +,-,e = - Da+,-,e n+,-,e
Electropositive case
• in electropositive case << 1:Da
+ = Dae = D+ + +/e De or
Thus, D+ < Da < De
ee
a D1
1D)1(D
= Te/Ti >> 1
Spatial distribution of ions
• In parallel plate configuration:ionisation = diffusion
02
2
nnkdx
ndD eion
a = kion n0 n+
Now with negative ions
)1())(1(
)21(1
)2(11 DD
1
1
)21(
2(11DD
a
a
aae
a DDD
= /e << 1
Ambipolar diffusion coefficients (gas-phase D)
Moderately electronegative
• When
– Current is carried by electrons (Ie / I- = ne e / n- - > 1)
– Negative ions are trapped(Da
- 0)
– Positive ions are mildly accelerated(Da
+ 2 D)
Extremely electronegative
• No ambipolar diffusion,
Da = D• Seldom occurs in active plasmas• Common in afterglows(after relaxation of ne, two-
component plasma left)
Spatial ion profiles – electronegative case
• Electron density profile almost flatbecause n = ne (Boltzmann relation)
constant production rate – parabolic profiles
02
2
nnkdx
ndD eion
a = const
Experimental data
• Indeed…
• At low pressures, Te is also homogeneous
Higher pressures
• Source function (ionisation rate) not homogeneous, profiles distorted
Summary II
• Negative ions just exist in the plasma• Typically – trapped and not very active,
but…• When too many:
– Depletion of (low-energy) electrons– Different transport properties– Determine spatial charge distribution– Chemical reactions (polymerisation, dust
formation)