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 -, O 2 -, O 3 -, CO 2 -, NO x -, etc.

Etching (IC’s, cleaning): CF 4, C 2 F 6, C 3 F 8, SF 6, O 2 … Deposition (a-Si:H, diamond) CH 4, SiH 4, NH 3

Excimer media: Ar, Kr, Xe + F 2, Cl 2 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 energy  E = 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 (CF 4 ) b) XY - stable --> depends on electron energy and stabilisation (O 2, H 2 ) E2E2 mainly endothermic

Typical cross-sections Resonant-like cross-sections Threshold for electron energy CF 4 : multiple fragmentation pathways possible

Strongly electronegative species Cl 2 - exothermic but small activation energy needed SF 6 - exothermic, no activation energy needed

Typical cross-sections The SF 6 cross-section: no energy threshold

Langevin limit Theoretical maximum cross-section for electron capture based on electron-(induced) dipol interactions  - polarisability, E - electron energy

Typical electronegative gases

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. O 2 (a) (a 1  g 1 eV exc. energy) 4 x higher cross-section

Vibrational excitation Lowered threshold, molecule larger in non-thermal plasmas T vib >> T gas extreme example: H 2

Hydrogen negative ions Important: additional heating source for fusion plasmas hot molecular beams prepared by acceleration of H - and neutralisation good sources needed H 2 itself hardly attaches electrons… but cross-section for =4 is 10 4 x cross-section for =1!

H - production enhanced ??? Less hydrogen, more H - ??? Argon dilution: more electrons more H 2 ( )

Molecular conversion Typical examples: fluorocarbons, silane Polymerisation! This is the effective DA cross-section in CF 4, and CF 4 plasma

CHF 3 chemistry Important for high aspect ratio etching (contact holes) because of side-wall passivation CHF 3 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 + + O 2 --> X + + O 2 * (+ O 2 --> O O 2 - ) –ion pair formation X + + O 2  O + + O -

Consequences Low-pressure plasmas for surface processing - plenty of surface Negative ions formed mainly in the sheath In O 2 : both O - and O 2 - formed surface/sheath production channel for molecular ions (direct attachment does not work)

Oxygen DC and RF glow discharges V - acceleration High-energy tail cathode anode V(anode) thermal ions (glow) “cathode” ions

Negative ions in oxygen O2-O2- O-O- Especially at low pressures, high-energy negative ions present (higher pressures - thermalisation, chemical destruction)

Destruction processes Ion-ion neutralisation X  + Y +  X + Y*. Coulomb process: very high cross- section (>10  16 m 2 ) Rate depends on ion temperature (  - red. mass in amu, E a - affinity X in eV)

Destruction processes Direct neutral detachment X  + 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 - n e too low

“Chemical” destruction Associative detachment X - + Y  XY + e X - + YZ  XY + Z + e Rate constants  10  16 m 3 /s Important in surface processing plasmas “Killer” in H - sources H - + H  H 2 + e Leads to plasma polymerisation

Associative detachment In O 2, CF 4 : 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 pressures C n F k - + CF m --> C n+1 F k+m + e In CF 4 /C 2 F 6 chemistry up to C 10 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 RFgnd 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 - /n e = 10 in O in C 2 F 6 >1000 in Cl 2, SF 6,… The latter are plasmas without electrons Kind of “afterglow” plasmas?

EEDF, ionisation rate, etc. Electron attachment causes decrease in n e DA depletes the plasma of low-energy electrons --> changes in EEDF, n e T e

Transport properties Ambipolar diffusion  + =  e +  -  + = - D +  n + + n +  + E  e,- = - D e,-  n e,- - n e,-  e,- E or  +,-,e = - D a +,-,e  n +,-,e

Electropositive case in electropositive case  << 1: D a + = D a e = D + +  + /  e D e or Thus, D + < D a < D e  = T e /T i >> 1

Spatial distribution of ions In parallel plate configuration: ionisation = diffusion = k ion n 0 n +

Now with negative ions  =   /  e << 1 Ambipolar diffusion coefficients (gas- phase D)

Moderately electronegative When  –Current is carried by electrons (I e / I - = n e  e / n -  - > 1) –Negative ions are trapped (D a -  0) –Positive ions are mildly accelerated (D a +  2 D)

Extremely electronegative  No ambipolar diffusion, D a = D Seldom occurs in active plasmas Common in afterglows (after relaxation of n e, two-component plasma left)

Spatial ion profiles – electronegative case Electron density profile almost flat because  n  =   n e (Boltzmann relation) constant production rate – parabolic profiles = const

Experimental data Indeed… At low pressures, T e 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)