1. FLUORINATION WITH REMOTE INDUCTIVELY COUPLED PLASMAS SUSTAINED IN Ar/F 2 AND Ar/NF 3 GAS MIXTURES* Sang-Heon Song a) and Mark J. Kushner b) a) Department of Nuclear Engineering and Radiological Sciences b) Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109, USA a) ssongs@umich.edu, b) mjkush@umich.edu http://uigelz.eecs.umich.edu Mark Strobel and Seth Kirk 3M Company, St. Paul, MN 55144 USA June 2010 * Work supported by 3M Company.

2. AGENDA SHS_MJK_ICOPS2010_02  Fluorination of polymers by plasma treatment  Description of the reaction mechanism and the model  Typical plasma properties  Fluorination properties  Long exposure time  Short exposure time  Concluding remarks

3. PLASMA SURFACE FLUORINATION OF POLYMERS  Plasma surface treatment can customize the surface energy and adhesion properties of polymers by modifying surface resident chemical groups.  Fluorination of polypropylene (PP) decreases the surface potential and increases the hydrophobicity.  In this talk, results from a computational investigation of remote, low pressure plasma fluorination will be discussed:  F/C ratio and degree of cross-linking  Limits of fluorination with long and short exposure times  Hydrophobicity http://www.nanotechweb.org SHS_MJK_ICOPS2010_03 University of Michigan Institute for Plasma Science & Engr.

4.  Sequential reactions:  Abstraction of H by F: – (CH 2 )(CH)(CH 3 ) – + F g  – (CH 2 )(CH)(CH 2) – + HF g  Followed by passivation – (CH 2 )(CH)(CH 2) – + F 2g  – (CH 2 )(CH)(CH 2 F) – + F g – (CH 2 )(CH)(CH 2) – + F g  – (CH 2 )(CH)(CH 2 F) –  Reactions occur repeatedly until all H is substituted by F... However.... BASIC FLUORINATION PROCESS  Structure of polypropylene (PP) polymer: University of Michigan Institute for Plasma Science & Engr. Yang et al, Plasma Proc. Polymers 7, 123 (2010). SHS_MJK_ICOPS2010_04 C H H C CH 3 H C HH H CC HH H C

5. CROSS-LINKING University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ICOPS2010_05 C H H C CH 3 H C H H CC HH H C C H H C H C HH CC HH H C  The free radical sites produced by abstraction of H atoms by F atom can be cross-linked.  Cross-linking consumes C-bonding that might otherwise be fluorinated and so potentially reduces the F/C ratio from its maximum value of 2. Yang et al, Plasma Proc. Polymers 7, 123 (2010).

6. University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ICOPS2010_06 REMOTE PLASMA FLUORINATION  To reduce the likelihood of energetic ion modification of the PP, a remote plasma system is used.  An upstream inductively coupled plasma provides an effluent of F atoms injected into the processing chamber.

7. HYBRID PLASMA EQUIPMENT MODEL (HPEM)  Remote Plasma Source Simulation: Inductively Coupled Plasma Fluid-Kinetics Poisson’s Eq. Surface Kinetics Module Sticking Coefficients Fluxes  Fluorination Reactor Simulation: Neutrals Only  Fluid Kinetics Module: continuity, momentum, energy  Surface Kinetics Module: With surface reaction mechanism University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ICOPS2010_07 Electron Energy Transport Module Frequency Domain Wave Eq. Neutral Fluxes Fluid-Kinetics

8. ICP AND FLUORINATION REACTOR GEOMETRIES  2D in cylindrically symmetric for ICP tube.  2D in Cartesian coordinates for reactor.  Ar/F 2 = 60/40, Ar/NF 3 = 60/40  600 mTorr, 720 sccm  10 MHz, varying coil power from 40 W to 180 W  Remote Plasma  Fluorination Reactor University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ICOPS2010_08

9. REMOTE PLASMA PROPERTIES: Ar/F 2 MIN MAX Log Scale  [e] Max = 1.7 x 10 12 cm -3  T electron = 0.2 – 5 eV  [F] Max = 4.8 x 10 15 cm -3  T gas-Max = 910 K  Plasma is concentrated on the wall due to skin depth and rapid attachment.  F atoms are generated by the dissociation of F 2 passes through the plasma zone.  Some component of thermal dissociation. University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ICOPS2010_09  Ar/F 2, 600 mTorr, 720 sccm  180 W, 10 MHz

10. REMOTE PLASMA PROPERTIES: Ar/NF 3  [e] Max = 7.7 x 10 11 cm -3  T electron = 0.2 – 7 eV  [F] Max = 3.5 x 10 15 cm -3  T gas-Max = 1764 K  Vibrational excitation and V-T relaxation in NF 3 increases gas temperature – more thermal dissociation.  Electron densities are lower as more power goes into vibrational excitation. MIN MAX Log Scale University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ICOPS2010_10  Ar/NF 3, 600 mTorr, 720 sccm  180 W, 10 MHz

11. FRACTIONAL DISSOCIATION University of Michigan Institute for Plasma Science & Engr.  Fractional dissociation based on fluxes emerging from plasma tube.  Dissociation scales sub-linearly with power.  Ar/NF 3 = 60/40  Ar/F 2 = 60/40 SHS_MJK_ICOPS2010_11  600 mTorr, 720 sccm, 10 MHz, 0.1 cm/s F/(F+2F 2 ) F/(3NF 3 +2NF 2 +NF+2F 2 +F)

12. FLUORINATION REACTOR PROPERTIES MIN MAX  Effluents from upstream ICP source are injected through nozzles in downstream reactor.  Ar/F 2 mixture has more F radicals for a given plasma power.  Factional dissociation in the injected flow University of Michigan Institute for Plasma Science & Engr.  600 mTorr, 720 sccm, 180 W, 10 MHz SHS_MJK_ICOPS2010_12  Ar/F 2, 0.4  Ar/NF 3, 0.3

13. FLUORINATION: LONG EXPOSURE TIME

14. SATURATION OF F/C RATIO Cross-linking = CL Hydrogen atoms = 6 Carbon atoms = 3 University of Michigan Institute for Plasma Science & Engr.  Relationship between F/C and cross-linking SHS_MJK_ICOPS2010_14  Full fluorination of PP C HH HCH 3 C n C FF FCF 3 C n

15. REMOTE PLASMA FLUORINATION: POWER University of Michigan Institute for Plasma Science & Engr.  Move web slowly for long exposure to F atom fluxes.  F/C ratio (2 is full fluorination) saturates at about 1.9 at earlier times at higher powers though not linearly with power.  Ar/NF 3 = 60/40  Ar/F 2 = 60/40 SHS_MJK_ICOPS2010_15  600 mTorr, 720 sccm, 10 MHz, 0.1 cm/s

16.  Ar/NF 3  Ar/F 2  Plasma power to flow rate ratio is constant (0.25 W/sccm).  F/C ratio saturates at earlier times at higher flow rate. REMOTE PLASMA FLUORINATION: FLOW RATE University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ICOPS2010_16  600 mTorr, 10 MHz, 0.1 cm/s

17. DIRECT PLASMA SOURCE PROPERTIES  [e] with a peak value of 2.6 x 10 10 cm -3 has a maximum near the edge of the powered electrode.  T e in the bulk plasma is 2.5 eV, peaking near powered electrode.  Fractional dissociation is about 0.2 based on reactor averaged F and F 2 density.  [e] TeTe FF F2F2 University of Michigan Institute for Plasma Science & Engr. Yang et al, Plasma Proc. Polymers 7, 123 (2010).  700 mTorr, 1500 W, 10 MHz  Ar/F2 = 60/40, 600 sccm

18. DIRECT PLASMA FLUORINATION  When plasma is in direct contact with polymer, ion bombardment sputters surface sites.  Cross-linked and fluorinated sites are sputtered, exposing fresh PP, thereby decreasing F/C. University of Michigan Institute for Plasma Science & Engr.  CCP, 700 mTorr, 600 sccm, 1500 W, 0.1 cm/s SHS_MJK_ICOPS2010_18

19. University of Michigan Institute for Plasma Science & Engr. REMOTE vs DIRECT FLUORINATION SHS_MJK_ICOPS2010_19  Cross-linking consumes carbon-bonding that might otherwise be fluorinated.  With remote plasma F/C is limited only by cross-linking.  In direct plasma treatment, fluorination is limited by ion sputtering and cross-linking.  For direct CCP  Web speed 0.1 cm/s

20. FLUORINATION: SHORT EXPOSURE TIME

21. FLUORINATION PROPERTIES: POWER  600 mTorr, 720 sccm, 10 MHz, 9 cm/s  Ar/NF 3  Ar/F 2 University of Michigan Institute for Plasma Science & Engr.  F/C ratio increases with power, but limited by availability of F atoms.  F/C ratio in Ar/NF 3 is lower than in Ar/F 2 due to lower F atom fluxes. SHS_MJK_ICOPS2010_21

22. FLUORINATION PROPERTIES: FLOW RATE  Fluorination rate increases with flow rate with constant power to flow rate ratio.  Maximum F/C: Ar/F 2 = 1.8, Ar/NF 3 = 1.74  Ar/NF 3  Ar/F 2 University of Michigan Institute for Plasma Science & Engr.  600 mTorr, 10 MHz, 9 cm/s SHS_MJK_ICOPS2010_22

23. CONCLUDING REMARKS  Fluorination properties of PP are similar for remote Ar/F 2 and Ar/NF 3 plasmas.  Fluorination properties with long exposure time  F/C ratio is limited only by cross-linking in remote plasma fluorination system.  In direct plasma fluorination system, F/C ratio is limited by the combination of ion sputtering and cross-linking.  Fluorination properties with short exposure time  F/C ratio is limited by reaction rate.  Fluorination rate is ultimately a function of plasma power and flow rate to increase fluence of F atoms. University of Michigan Institute for Plasma Science & Engr. SHS_MJK_ICOPS2010_23

24. BACKUP FOR PROBABILITY  Sequential reactions:  Abstraction of H by F: – (CH 2 )(CH)(CH 3 ) – + F g  – (CH 2 )(CH)(CH 2) – + HF g Probability = 5 x 10 -5  Followed by passivation – (CH 2 )(CH)(CH 2) – + F 2g  – (CH 2 )(CH)(CH 2 F) – + F g Probability = 0.2 x 10 -4 – (CH 2 )(CH)(CH 2) – + F g  – (CH 2 )(CH)(CH 2 F) – Probability = 1 x 10 -4