POLYMERISATION PROCESSES IN LOW-PRESSURE FLUOROCARBON PLASMAS

Winfred Stoffels, Eva Stoffels PO Box 513, 5600 MB Eindhoven.

[email protected]

http://discharge.phys.tue.nl/stoffels/

FLUOROCARBON PLASMAS

• Reactive Ion Etching of semiconductors/photoresist • Various chemistries: CF 4 , C 2 F 6 , CHF 3 , C 4 F 8 , etc.

• Chemical effects essential for etching performance • Important problems: – production/destruction of CF x radicals – surface passivation - deposition of polymer film – dust particle formation

WHY STUDY POLYMERISATION?

• Problem: deposition mechanism of polymeric fluorocarbon film on the surface • Deposition efficiency: CF 4 << C 2 F 6 • Consequences < C – dust particle formation 4 F 8 – RIE lag - surface passivation – sputtering and flaking of the film – new surface reactions: production of small radicals (CF, CF 2 , CF 3 )

GAS PHASE POLYMERISATION

• Film deposition on the surface is

not

due to CF x 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

OVERVIEW OF RIE CHEMISTRY

Plasma

CF 4 + e  CF x + (4-x)F + e gas phase polymerisation  CF x + + (4-x)F + 2e  CF 3 + F, F + CF 3 C n F k ion neutralisation, etching SiF x

Surface

recombination sputtering radical formation  polymer film formation

CF

x

RADICAL DENSITIES

Spatial distribution of CF, CF 2 and CF 3 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?

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

POLYMER DETECTION BY EAMS

• Large fluorocarbons are electronegative; high electron attachment cross sections • Mass spectrometry: Ionisation Mass Spectrometry (classical): C n F k  + e (50 eV)  destruction of C n F (C k n F  k + )*  smaller ions detection does not work Electron Attachment Mass Spectrometry (EAMS): C n F k  C n + e (0-5 eV) F k-1  detected  C n F k-1 + F good selectivity & sensitivity

POLYMERS IN C

2

F

6

PLASMA

Low plasma power level High power level • EAMS method: C n F 2n-k ions detected, C n F 2n-k+1 monitored.

• Species with n up to 10 detected (QMS mass limit!).

• Polymerisation is efficient at high plasma powers.

POLYMER COUNT RATES IN CF

4

, C

2

F

6

AND C

4

F

8 • CF 4 - fluorine rich, little polymerisation 100 10 CF 4 • C 2 F 6 - smaller F:C ratio, larger parent 1 100 10 C 2 F 6 molecule, more 1 polymerisation • C 4 F 8 - large, unsaturated molecule, 100 10 abundant polymerisation C 4 F 8 1 1 2 3 4 5 6 7 8 9 10  number of C atoms (n)  - neutral species - positive ions

F:C RATIO OF POLYMERS

• CF 4 plasma contains mainly saturated polymers C n F 2n+2 . They are stable, not active and do not stick to the surface.

• In C 2 F 6 and C 4 F 8 plasmas more unsaturated species are formed. Unsaturated polymers are reactive. They stick to the surface and contribute to the polymer film growth.

3 0 2 1 0 2 1 1 3 0 3 2 1 2 3 4 5 6 7 8 9 10

number of C atoms (n)

CF C 2 F 6 C 4 4 F 8

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: CF 4 << C 2 F 6 < C 4 F 8 • Gas phase polymerisation correlates with film growth on the surface: CF 4 << C 2 F 6 < C 4 F 8