POLYMERISATION PROCESSES IN LOW-PRESSURE FLUOROCARBON PLASMAS

1. POLYMERISATION PROCESSESIN LOW-PRESSUREFLUOROCARBON PLASMAS Winfred Stoffels, Eva Stoffels PO Box 513, 5600 MB Eindhoven. stoffels@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 << 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 gas phase polymerisation Plasma CF4 + e  CFx + (4-x)F + e CFx+ + (4-x)F + 2e CnFkCF3- + F, F- + CF3 Surface ion neutralisation, etching SiFx recombination sputtering  polymer film formation 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-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: CF4 << C2F6 < C4F8