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)