1. POLYMERISATION PROCESSES IN LOW-PRESSURE FLUOROCARBON PLASMAS
Winfred Stoffels, Eva Stoffels
PO Box 513, 5600 MB Eindhoven.

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 << C2F6 < 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
recombination sputtering
 polymer film formation
radical formation

6. Surface production mechanism?
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-k+1 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 F:C ratio, larger parent molecule, more polymerisation
C4F8 - large, unsaturated molecule, abundant polymerisation
 - neutral species
 - positive ions

11. F:C RATIO OF POLYMERS
CF4 plasma contains mainly saturated polymers CnF2n+2. 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 F:C ratio:
CF4 << C2F6 < C4F8
Gas phase polymerisation correlates with film growth on the surface: