POLYMERISATION PROCESSES IN LOW-PRESSURE FLUOROCARBON PLASMAS

1
POLYMERISATION PROCESSESIN LOW-PRESSUREFLUOROCAR
BON PLASMAS

• Winfred Stoffels, Eva Stoffels
• PO Box 513, 5600 MB Eindhoven.
• stoffels_at_discharge.phys.tue.nl
• http//discharge.phys.tue.nl/stoffels/

2
FLUOROCARBON PLASMAS

• Reactive Ion Etching of semiconductors/photoresist
  
• Various chemistries CF4, C2F6, CHF3, C4F8, etc.
• Chemical effects essential for etching
  performance
• Important problems
• production/destruction of CFx radicals
• surface passivation - deposition of polymer film
• dust particle formation

3
WHY STUDY POLYMERISATION?

• Problem deposition mechanism of polymeric
  fluorocarbon film on the surface
• Deposition efficiency
• CF4 ltlt C2F6 lt C4F8
• Consequences
• RIE lag - surface passivation
• sputtering and flaking of the film
• dust particle formation
• new surface reactions production of small
  radicals (CF, CF2, CF3)

4
GAS PHASE POLYMERISATION

• Film deposition on the surface is not due to CFx
  radical sticking
• Is surface polymerisation related to gas phase
  polymerisation?
• A solution to deposition problem
• gas phase polymerisation
• formation of active unsaturated polymers
• polymer sticking to the surface
• film growth

5
OVERVIEW OF RIE CHEMISTRY

• Plasma
• CF4 e ? CFx (4-x)F e
• ?CFx (4-x)F 2e CnFk ?CF3-
  F, F- CF3
• Surface

gas phase polymerisation
ion neutralisation, etching SiFx
? polymer film formation
recombination sputtering
radical formation
6
CFx RADICAL DENSITIES
Spatial distribution of CF, CF2 and CF3 radical
densities in an rf discharge, measured by TDL
infrared absorption. Densities at the surface are
higher than in the plasma glow. Surface
production mechanism?
7
POLYMERISATION AT LOW PRESSURES

• Problems
• low densities - low reaction rates
• limited residence time
• only two-body reactions
• Possible mechanisms
• unsaturated species/radical polymerisation
• ion-assisted polymerisation
• Negative ions as polymer precursors

8
POLYMER DETECTION BY EAMS

• Large fluorocarbons are electronegative high
  electron attachment cross sections
• Mass spectrometry
• Ionisation Mass Spectrometry (classical)
• CnFk e (50 eV) ? (CnFk) ? smaller ions
• ? destruction of CnFk ? detection does not work
• Electron Attachment Mass Spectrometry (EAMS)
• CnFk e (0-5 eV) ? CnFk-1- F
• ? CnFk-1- detected ? good selectivity
  sensitivity

9
POLYMERS IN C2F6 PLASMA

• EAMS method CnF2n-k- ions detected, CnF2n-k1
  monitored.
• Species with n up to 10 detected (QMS mass
  limit!).
• Polymerisation is efficient at high plasma
  powers.

Low plasma power level High power level
10
POLYMER COUNT RATES IN CF4, C2F6 AND C4F8

• CF4 - fluorine rich, little polymerisation
• C2F6 - smaller FC ratio, larger parent molecule,
  more polymerisation
• C4F8 - large, unsaturated molecule, abundant
  polymerisation

? - neutral species ? - positive ions
11
FC RATIO OF POLYMERS

• CF4 plasma contains mainly saturated polymers
  CnF2n2. They are stable, not active and do not
  stick to the surface.
• In C2F6 and C4F8 plasmas more unsaturated species
  are formed. Unsaturated polymers are reactive.
  They stick to the surface and contribute to the
  polymer film growth.

12
CONCLUSIONS

• New mass spectrometry (EAMS) allows to detect
  large fluorocarbons in low-pressure plasmas.
• Polymerisation is enhanced by ion-assisted
  reactions.
• Polymerisation efficiency increases with
  increasing size of the parent gas and decreasing
  FC ratio
• CF4 ltlt C2F6 lt C4F8
• Gas phase polymerisation correlates with film
  growth on the surface
• CF4 ltlt C2F6 lt C4F8