Electronegative Plasmas

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- + DE DE = affinity(XY) + kinetic energy(e) - activation energy (XY-)* Momentum conservation!

Stabilisation of the excited anion • Autodetachment (XY-)* --> XY + e • Radiative (XY-)* --> XY- + hn (atomic species, interstellar space) • Three-body (XY-)* + Z --> XY- + Z (Z carries out the energy, atm. pressure) • Redistribution (XY-)* --> XY-(n) (polyatomic molecules, small excess energies)

Dissociative attachment (DA) • (XY-)* --> X + Y- or X- + Y • process can be endo- or exothermic • released energy DE = affinity(X or Y) + kinetic energy(e) - activation energy (XY-)* - dissociation energy (XY) carried out by product neutrals/anions, negative ions can be hot!!!

E2 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) mainly endothermic

Typical cross-sections • Resonant-like cross-sections • Threshold for electron energy CF4: multiple fragmentation pathways possible

Strongly electronegative species Cl2 - exothermic but small activation energy needed SF6 - exothermic, no activation energy needed

Typical cross-sections The SF6 cross-section: no energy threshold

Langevin limit • Theoretical maximum cross-section for electron capture • based on electron-(induced) dipol interactions a - 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. O2(a) (a1Dg 1 eV exc. energy) 4 x higher cross-section

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 n=4 is 104 x cross-section for n=1!

H- production enhanced • ??? Less hydrogen, more H- ??? • Argon dilution: more electrons more H2(n)

Molecular conversion • Typical examples: fluorocarbons, silane • Polymerisation! 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 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 formation X+ + 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)

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

Negative ions in oxygen O2- 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 m2) • Rate depends on ion temperature (m - red. mass in amu, Ea - 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 - ne too low

“Chemical” destruction • Associative detachment X- + Y  XY + e X- + YZ  XY + Z + e • Rate constants  10-16 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 pressures CnFk- + CFm --> Cn+1Fk+m + e • In CF4/C2F6 chemistry up to C10 detected • In silane: dust formation channel!

RF gnd V X Transport & surface losses • In active plasmas: sheath keeps them away • in DC: losses to the anode • in afterglow: free diffusion

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 a = 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 • G+ = Ge + G- • G+ = - D+n+ + n+m+E • Ge,- = - De,-ne,-- ne,-me,-E or G+,-,e = - Da+,-,en+,-,e

Electropositive case • in electropositive case a << 1: Da+ = Dae = D+ + m+/me De or Thus, D+ < Da < De g = Te/Ti >> 1

Spatial distribution of ions • In parallel plate configuration: ionisation = diffusion = kion n0 n+

Now with negative ions Ambipolar diffusion coefficients (gas-phase D) m = m+/me << 1

Moderately electronegative • When am < 1 • Current is carried by electrons (Ie / I- = neme / n-m- > 1) • Negative ions are trapped (Da- 0) • Positive ions are mildly accelerated (Da+ 2 D)

Extremely electronegative • am > 1, a > 1000 • 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 flat because n- = agne (Boltzmann relation) constant production rate – parabolic profiles = 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)

Electronegative Plasmas