Surface and volume production of negative ions in a low-pressure plasma

1. Surface and volume production of negative ions in a low-pressure plasma E. Stoffels, W.W. Stoffels, V.M. Kroutilina*, H.-E. Wagner* and J. Meichsner*, Department of Physics, Eindhoven University of Technology*Institute for Physics, University of Greifswald • Negative ions in low-pressure plasmas: • formation, confinement, extraction • Negative ion detection by mass spectrometry • Where (and how) are the ions produced? • Implications for plasma chemistry

2. Volume production of negative ions(example: low-pressure oxygen plasma) • For O- : dissociative attachment to (excited) O2: O2 + e  O + O- { O2(n), O2(a), O2(b) etc.} • For O2- : non-dissociative attachment: O2 + e + X  O2- + X charge transfer: O- +O2(a)  O + O2- dissociative attachment to O3: O3 + e  O2- + O All these processes are inefficient at low pressures!

3. Some factsin radio-frequency plasmas in oxygen • Electron/ion density measurements: Negative ion density 1016 m-3 Electron density 1015 m-3 Main ion O- Substantial O2- density: 10-20% • Modelling of ion density: calculated < measured (about factor 2) less molecular ions expected • This is also valid for other electronegative gases • Do we miss an important formation process?

4. RF gnd V X Extraction of negative ions in RF plasmas • Ions are heavy and immobile they “feel” average potential • Ions have little energy (room temperature) • Positive ions are accelerated by the sheath field • Negative ions are confined in the glow • No negative ion extraction during normal plasma operation!

5. Mass spectrometry of negative ions problem: extraction • In DC discharges - no problem (extraction to a mass spectrometer through an orifice in the anode) • In RF discharges - sheath field must be (locally) cancelled • Experimental tricks: • positively biased extraction orifice (*disturbs plasma) • pulsed plasma & detection in the afterglow (*no information about “active” discharge) However... • We detect negative ions in RF without these tricks! • What is the origin of these ions?

6. RF gnd glow HIDEN QMS Negative ions in O2 RF plasmaenergy spectra by QMS • Direct negative ion fluxes recorded by mass spectrometer • O- detected • energy spectra obtained • ions arrive at low energies (at most a few eV) • potential barrier between glow and grounded electrode = 40 V • O- from the glow does not have enough energy to pass it!

7. Negative ions in O2 RF plasmadependence on plasma parameters • Where are the ions created? • not in the plasma glow • surface? sheath? • Surface conversion (O+ O-) unlikely • no forward velocity to the surface • ions would not reach the QMS • Signals of O- increase with increasing power • high energy particles (positive ions, neutrals) important? • plasma chemistry? • Signals increase with pressure • background gas important? • sheath process?

8. anode fall QMS 0 -V QMS cathode fall • Most sheath-produced ions are directed to the glow. • We detect only a small fraction. • Is this an important ion formation channel? • What does it mean for plasma chemistry? Case study: • Take a DC plasma • At the anode: ions from the glow • At the cathode: sheath ions? -V 0 (At reference -V)

9. DC glow in oxygenextraction at the anode O- • Both O- and O2- are observed • Typical energy spectra: • O- energy  eV(anode fall) • ions accelerated in the column • not well thermalised • O2-energy  eV(anode fall) • charge transfer collisions: O2- (E) + O2 (th)  O2- (th) + O2 (E) • efficient thermalisation • energy loss in the anode fall region O2-

10. V acceleration - V(anode) thermal ions (glow) High-energy tail “cathode” ions DC glow at low pressure • Normally [O-] > [O2-] • At p < 0,05 mbar O2- is dominant • High-energy ions observed! • ions created in the cathode fall • low pressures - less collisions • these ions arrive at the anode!

11. DC glow in oxygencathode side O- signal at 0,2 mbar: • Ion production takes place close to the cathode • Can we extract negative ions at the cathode side? • QMS frame at a reference voltage = V(cathode) • Large signals observed! • Energy spectra of O- have a large high-energy tail • O- velocity towards the cathode? • O2- ions have energy  0

12. DC glow in oxygencathode side O- signal at 0,15 • Both O- and O2- signals increase with increasing pressure and DC voltage • Signals are larger than in the RF plasma • Cathode voltages higher than sheath voltages in RF • High energy particles essential in negative ion generation!

13. Possible mechanismssurface/RF sheath/DC cathode fall • surface conversion of O2+ (O+) into O2- (O-) : unlikely (at normal incidence of X+, X- is reflected) • formation of fast (excited) neutrals O2+ (high E) + O2 O2+ + O2 (high E) and dissociative attachment O2 (high E) + e  O- + O : unlikely (low electron density in the sheath) • ion pair formation: X+ (high E) + O2 O+ + O- (explains high energy of O-) • for O2- - charge exchange between excited and ground state molecules: O2* + O2 O2+ + O2- • Problem: lack of cross-section data on ion-induced processes

14. Conclusions • Negative ions can escape from low-pressure RF plasma (observed in O2 as well as CF4 plasmas) • Ions are generated in the sheath region • Sheath chemistry is very rich • High energy particles are involved in ion production • Most likely mechanism: ion pair formation upon high-energy positive ion impact • Rough estimations: if s > 10-22 m-2, O- production by ion pair formation in the sheath comparable with electron attachment in the glow! • Sheath process may be the major formation channel for some molecular ions (O2-) at low pressures (attachment does not work)