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MAKING PLASMAS DO SMALL THINGS:

MAKING PLASMAS DO SMALL THINGS: FUNCTIONALIZING NOOKS-AND-CRANNIES IN POLYMERS AT LOW AND HIGH PRESSURE Mark J. Kushner Iowa State University Department of Electrical and Computer Engineering Department of Chemical and Biological Engineering 104 Marston Hall Ames, IA 50011

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MAKING PLASMAS DO SMALL THINGS:

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  1. MAKING PLASMAS DO SMALL THINGS: FUNCTIONALIZING NOOKS-AND-CRANNIES IN POLYMERS AT LOW AND HIGH PRESSURE Mark J. Kushner Iowa State University Department of Electrical and Computer Engineering Department of Chemical and Biological Engineering 104 Marston Hall Ames, IA 50011 mjk@iastate.eduhttp://uigelz.ece.iastate.edu April 2006 UCLA_0406_01

  2. ACKNOWLEDGEMENTS • Dr. Rajesh Dorai (now at Varian Semiconductor Equipment) • Dr. Natalie Babeva • Mr. Ananth Bhoj • Funding Agencies: • 3M Corporation • Semiconductor Research Corporation • National Science Foundation • SEMATECH • CFDRC Inc. Iowa State University Optical and Discharge Physics UCLA_0406_02

  3. AGENDA • Introduction to Plasma Processing • Plasma surface functionalization  Description of the models • High Pressure: • Plasma dynamics in He/NH3/H2O and humid air mixtures • Functionalization of rough and porous surfaces • Low Pressure: Ions and Shadowing • Concluding remarks • Work supported by National Science Foundation, 3M Inc and Semiconductor Research Corp. Iowa State University Optical and Discharge Physics UCLA_0406_03

  4. PLASMAS 101: INTRODUCTION • Plasmas (ionized gases) are often called the “fourth state of matter.” • Plasmas account for > 99.9% of the mass of the known universe (dark matter aside). • X-ray view of the sun, a plasma. http://www.plasmas.org/basics.htm Iowa State University Optical and Discharge Physics UCLA_0406_04

  5. TECHNOLOGICAL PLASMAS: PARTIALLY IONIZED GASES • A gas (collection of atoms or molecules) is neutral on a “local” and global basis. • An energetic free electron collides with an atom, creating a positive ion and another free electron. Iowa State University Optical and Discharge Physics UCLA_0406_05

  6. TECHNOLOGICAL PLASMAS: PARTIALLY IONIZED GASES • The resulting partially ionized gas (N+/N < 10-2-10-6) is not neutral on a microscopic scale, but is neutral on a global scale. • Partially ionized plasmas contain neutral atoms and molecules, electrons, positive ions and negative ions. • Air plasma: N2, O2, N2+, O2+, O-, e where [e] << [M]. Iowa State University Optical and Discharge Physics UCLA_0406_06

  7. TECHNOLOGICAL PLASMAS: REACTIVE SPECIES • Electron impact collisions on atoms and molecules produce reactive species. • These species emit photons, modify surfaces and create new materials. • These plasmas are called “collisional” because electrons impart energy to neutrals by physical impact. Iowa State University Optical and Discharge Physics UCLA_0406_07

  8. COLLISIONAL LOW TEMPERATURE PLASMAS • These systems are the plasmas of every day technology. • Electrons transfer power from the "wall plug" to internal modes of atoms / molecules to "make a product”, very much like combustion. • The electrons are “hot” (several eV or 10-30,000 K) while the gas and ions are cool, creating“non-equilibrium” plasmas. Iowa State University Optical and Discharge Physics UCLA_0406_08

  9. Thrusters • Lighting • Spray Coatings • Materials Processing • Displays COLLISIONAL LOW TEMPERATURE PLASMAS UCLA_0406_09

  10. MULTISCALE MODELING OF PLASMAS AND PLASMA-SURFACE INTERACTIONS • Our research group develops multi-scale, integrated reactor and feature scale modeling hierarchies to simulate plasma processing systems. • Fundamental plasma hydrodynamics transport • Plasma chemistry • Radiation transport • Plasma surface interactions • Materials modification and surface kinetics • We are very interested in the science of plasmas…but also interested in how plasmas can be used to optimally produce unique materials, properties and structures. Iowa State University Optical and Discharge Physics UCLA_0406_10

  11. Untreated PP • Polypropylene (PP) Plasma Treated PP He/O2/N2 Plasma • M. Strobel, 3M PLASMA FUNCTIONALIZATION OF SURFACES • To modify wetting, adhesion and reactivity of surfaces, such as polymers, plasmas are used to generate gas-phase radicals to functionalize their surfaces. • Example: atm plasma treatment of PP • Massines J. Phys. D 31, 3411 (1998). Iowa State University Optical and Discharge Physics UCLA_0406_11

  12. FUNCTIONALIZATION OF POLYMERS USING PLASMAS • Functionalization of surfaces such as polymers occurs by their chemical interaction with plasma produced species - ions, radicals and photons. • Example: H abstraction in an oxygen containing plasma enables affixing O atoms as a peroxy site. • Functionalization usually only affects the surface layer. Iowa State University Optical and Discharge Physics UCLA_0406_12

  13. Filamentary Plasma 10s – 200 mm SURFACE MODIFICATION OF POLYMERS • Pulsed atmospheric filamentary discharges (coronas) are routinely used to web treat commodity polymers like poly-propylene (PP) and polyethylene (PE). • Due to the low value of these materials, the costs of the processes must by low, < $0.05/m2. Iowa State University Optical and Discharge Physics UCLA_0406_13

  14. COMMERCIAL CORONA PLASMA EQUIPMENT • Sherman Treaters • Tantec, Inc. Iowa State University Optical and Discharge Physics UCLA_0406_14

  15. PLASMAS FOR MODIFICATION OF BIOCOMPATIBLE SURFACES: TISSUE ENGINEERING • Tissue engineering requires “scaffolding”; substrates with nooks and crannies 10s -1000s m in which cells adhere and grow. • Scaffolding is chemically treated (functionalized) to enhance cell adhesion or prevent unwanted cells from adhering. • E. Sachlos, European Cells and Materials v5, 29 (2003) • Tien-Min Gabriel Chu http://www.engr.iupui.edu/~tgchu Iowa State University Optical and Discharge Physics UCLA_0406_17

  16. LOW PRESSURE GLOW DISCHARGES • Low pressure plasmas (< 1 Torr) are typically “glows” and not streamers. • Technology used to fabricate microelectronics devices to functionalize features to a few nm. • Diffusive transport and long mean-free-paths provide inherently better uniformity. • Energy of ions is typically larger (many eV) and controllable (100’s eV) • GEC Reference Cell, 100 mTorr Ar • Ref: G. Hebner Iowa State University Optical and Discharge Physics UCLA_0406_16

  17. $1000/cm2 $0.05/m2 EXTREMES IN CONDITIONS, VALUES, APPLICATIONS Web Treatment of Films Microelectronics • High pressure • High throughput • Low precision • Modify cheap materials • Commodity • Low pressure • Low throughput • High precision • Grow expensive materials • High tech Iowa State University Optical and Discharge Physics ISU_0105_11

  18. $0.05/m2 ? $1000/cm2 CREATING HIGH VALUE: COMMODITY PROCESSES • Can commodity processes be used to fabricate high value materials? • Where will, ultimately, biocompatible polymeric films fit on this scale? Artificial skin for $0.05/cm2 or $1000/cm2? Iowa State University Optical and Discharge Physics ISU_0105_12

  19. FUNCTIONALIZING SMALL FEATURES • Using atmospheric pressure plasmas (APPs) to functionalize small features is ideal due to their low cost. • Low pressures plasmas (LPPs), though more costly, likely provide higher uniformity. • Can APPs provide the needed uniformity and penetration into small features? • Are LPPs necessarily the plasma of choice for small feature functionalization. • In this talk, the functionalization of small features using APPs and LPPs will be discussed using results from computer simulations. • NH3 plasmas for =NHx functionality for cell adhesion. • O2 plasmas for =O functionality for improved wettability. Iowa State University Optical and Discharge Physics UCLA_0406_18

  20. ELECTROMAGNETICS AND ELECTRON KINETICS • The wave equation is solved using tensor conductivities: • Electron energy transport: Continuum and Kinetics • where S(Te) = Power deposition from electric fields L(Te) = Electron power loss due to collisions  = Electron flux • (Te) = Electron thermal conductivity tensor • SEB = Power source source from beam electrons • Kinetic: MCS is used to derive including e-e collisions using electromagnetic and electrostatic fields . Iowa State University Optical and Discharge Physics UCLA_0406_19

  21. LOW PRESSURE:PLASMA CHEMISTRY, TRANSPORT ELECTROSTATICS • Continuity, momentum and energy equations are solved for each species. • Semi-implicit solution of Poisson’s equation: Iowa State University Optical and Discharge Physics UCLA_0406_20

  22. HIGH PRESSURE:CHARGED PARTICLE TRANSPORT ELECTROSTATICS • Continuity: electron collisions, volume and surface chemistry, photo-ionization, secondary emission, Sharfetter-Gummel fluxes. • Optically thick photoionization sources (important for streamers) • Fully implicit solution of Poisson’s equation. • Unstructured mesh. Iowa State University Optical and Discharge Physics UCLA_0406_21

  23. HIGH PRESSURE: NEUTRAL PARTICLE TRANSPORT • Fluid averaged values of mass density, mass momentum and thermal energy density obtained in using unsteady algorithms. • Individual fluid species diffuse in the bulk fluid. Iowa State University Optical and Discharge Physics UCLA_0406_22

  24. NANOSCALE EVOLUTION OF SURFACE PROPERTIES • The Monte Carlo Feature Profile Model (MCFPM) predicts evolution of features using energy and angularly fluxes obtained from equipment scale models. • Arbitrary reaction mechanisms may be implemented (thermal and ion assisted, sputtering, deposition and surface diffusion). • Mesh centered identify of materials allows “burial”, overlayers and transmission of energy through materials. Iowa State University Optical and Discharge Physics UCLA_0406_23

  25. CELL MICROPATTERNING: MODIFICATION OF POLYMERS • PEO - polyethyleneoxide • pdAA – plasma deposited acrylic acid • Modification of polymer surfaces for specified functionality can be used to create cell adhering or cell repulsing regions. 1Andreas Ohl, Summer School, Germany (2004). Iowa State University Optical and Discharge Physics UCLA_0406_24

  26. FUNCTIONALIZATION FOR BIOCOMPATIBILITY Micropatterned cell growth on NH3 plasma treated PEEK1  Ammonia plasma treatment affixes amine (C-NH2) groups on surfaces for applications such as cell adhesion, protein immobilization and tissue engineering. (1K. Schroeder et al, Plasmas and Polymers, 7, 103, 2002) Iowa State University Optical and Discharge Physics UCLA_0406_25

  27. GAS PHASE CHEMISTRY - He/NH3/H2O MIXTURES • Electron impact reactions initiate dissociate NH3 and H2O into radicals that functionalize surface. • H, NH2, NH, O and OH are major radicals for surface reactions. Iowa State University Optical and Discharge Physics UCLA_0406_26

  28. SURFACE REACTION MECHANISM  Gas phase H, O and OH abstract H atoms from the PP surface producing reactive surface alkyl (R-) radical sites. Iowa State University Optical and Discharge Physics UCLA_0406_27

  29. SURFACE REACTION MECHANISM  Gas phase NH2 and NH radicals react with surface alkyl sites creating amine (R-NH2) groups and imine (R-NH) sites. Iowa State University Optical and Discharge Physics UCLA_0406_28

  30. TREATMENT OF POROUS POLYMER BEADS • Biodegradable porous beads are used for drug delivery and gene therapy. • Macroporous beads are 10s µm in diameter with pore sizes < 10 µm. • External and internal surfaces are functionalized for polymer supported catalysts and protein immobilization. • Penetration of reactive species into pores is critical to functionalization. • Functionalized Porous Bead for Protein Binding sites (www.ciphergen.com) Iowa State University Optical and Discharge Physics UCLA_0406_29

  31. DBD TREATMENT OF POROUS POLYMER BEAD • Corona treatment of porous polymer beads for drug delivery. • How well are the internal surfaces of pores accessible to the plasma? • What is the extent of functionalization on internal surfaces? • Bead size ~ 10s m • Pore diameter ~ 2-10 m - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1 Iowa State University Optical and Discharge Physics  PRF – 10 kHz UCLA_0406_30

  32. MINMAX ELECTRON TEMPERATURE, SOURCE  Electron Temperature  Electron Source Animation Slide-GIF - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3.5 ns Iowa State University Optical and Discharge Physics UCLA_0406_31

  33. MINMAX ELECTRON DENSITY • Electron density of 1013-1014 cm-3 is produced. • Electron impact dissociation generates radicals that functionalize surfaces. Animation Slide-GIF - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3.5 ns Iowa State University Optical and Discharge Physics UCLA_0406_32

  34. MINMAX POST-PULSE RADICAL DENSITIES  NH  NH2  OH - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1 Iowa State University Optical and Discharge Physics UCLA_0406_33

  35. 50 m MINMAX ELECTRON DENSITY IN AND AROUND BEAD • Corona treating a porous polymer bead placed on the lower dielectric.  Electrons (3.7 x 1013 cm-3) • In negative corona discharge, electrons lead the avalanche front and initially penetrate into pores. Charging of surfaces limit further electron penetration. Animation Slide-GIF - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3 ns. Iowa State University Optical and Discharge Physics UCLA_0406_34

  36. 50 m MINMAX TOTAL POSITIVE ION DENSITY IN AND AROUND BEAD  Ions (3.7 x 1013 cm-3) • Ions lag electrons arriving at bead but persist at surfaces due to negative charging that makes the surfaces cathode like. • Lower surface (anode) is ion repelling. Animation Slide-GIF - 5 kV, 1 atm, He/NH3/H2O=90/10/0.1, 0-3 ns Iowa State University Optical and Discharge Physics UCLA_0406_35

  37. 90 mm Bead 3 ns 80 ms [NH2] INSIDE PORES • Since electrons poorly penetrate into most pores, little NH2 is initially produced inside bead. • NH2 later diffuses into pores from outside. 2x1010- 2x1013 7.5x1012- 8.5x1012 30 mm Bead 80 ms 3 ns  - 5 kV, He/NH3/H2O=90/10/0.1, pore dia=4.5 mm, 1 atm 9.1x1012- 9.3x1012 2x1010- 2x1013 Iowa State University Optical and Discharge Physics MIN MAX (log scale) UCLA_0406_36

  38. [NH2] INSIDE PORES : PORE DIAMETER 8.5 mm 4.5 mm 3 mm • t = 3 ns • 2x1010- 2x1013 • t = 80 s • 7.5x1012-8.7x1012  [NH2] within pores increases with pore diameter during the pulse and in the interpulse period. [NH2] cm- 3 MIN MAX (log scale)  - 5 kV, He/NH3/H2O=90/10/0.1, bead dia=90 mm, 1 atm Iowa State University Optical and Discharge Physics UCLA_0406_37

  39. FUNCTIONALIZATION OF POROUS BEAD SURFACES [ALKYL] =C 1.25x1010– 1.25x1011 C I K E A G F B D E G H J [AMINE] =C-NH2 F C I D H - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1, Bead size=90 mm, Pore dia= 4.5 mm, t=0.1 s J 1012– 1013 B A K Letters indicate position along the surface. log scale, cm- 2 Iowa State University Optical and Discharge Physics MINMAX UCLA_0406_38

  40. AMINE SURFACE COVERAGE: SIZE OF BEAD • Outer surfaces have significantly higher amine coverage than interior pores. • Smaller beads pores have more uniform coverage due to shorter diffusion length into pores. • Beads sitting on electrode shadow portions of surface. Pore dia = 4.5 mm - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1, t=1 s Iowa State University Optical and Discharge Physics UCLA_0406_39

  41. MINMAX BEADS IN DISCHARGE: ELECTRON DENSITY • Uniformity may be improved by dropping beads through discharge instead of placing on a surface. • He/O2/H2O = 89/10/1, 1 atm • Electrons produce a wake beyond the particle. Animation Slide-GIF • Electron Density (1.6 x 1014 cm-3), 0-2.5 ns Iowa State University Optical and Discharge Physics UCLA_0406_40

  42. MINMAX ELECTRON DENSITY AND SOURCE • Ionization occurs around particle during initial avalanche and restrike. • Sheath forms above particle, wake forms below particle. • He/O2/H2O = 89/10/1, 1 atm • 0-2.6 ns • Electron Source (1023 cm-3s-1) Animation Slide-GIF • Electron Density (6 x 1013 cm-3) Iowa State University Optical and Discharge Physics UCLA_0406_41

  43. MINMAX POST-PULSE O and OH DENSITIES • Directly after the pulse, radicals have a similar wake below the particles. • He/O2/H2O = 89/10/1, 1 atm • 0-2.6 ns  [O] (8 x 1014 cm-3)  [OH] (5 x 1013 cm-3) Iowa State University Optical and Discharge Physics UCLA_0406_42

  44. BEADS IN DISCHARGE: SURFACE COVERAGE • Uniformity of functionalization, locally poor, is improved around the particle. • He/O2/H2O = 89/10/1, • 1 atm • Alkoxy (=C-O) and Peroxy (=C-OO) Coverage Iowa State University Optical and Discharge Physics UCLA_0406_43

  45. DBD TREATMENT OF PP SURFACE WITH MICROSTRUCTURE  E. Sachlos, et al. • Corona functionalization of rough polymer resembling tissue scaffold. • 1 atm, He/NH3/H2O, 10 kHz • Polypropylene. Iowa State University Optical and Discharge Physics UCLA_0406_44

  46. PENETRATION INTO SURFACE FEATURES – [e], [IONS] t = 2.7 ns t = 4 ns [e] cm- 3 1010 – 1013 [Positive ions] cm- 3 1010 – 1013 [Surface (-ve) Charge] mC 10-1 – 103  - 5 kV, 1 atm, He/NH3/H2O=98.9/1.0/0.1 Iowa State University Optical and Discharge Physics MINMAX UCLA_0406_45 log scale

  47. NH2 DENSITY: EARLY AND LATE [NH3]=10% [NH3]=30% 3x1012 - 3x1014 t =3 ns 1.8x1012 –1.9x1012, t =90 ms 2.25x1012 –2.35x1012, t =90 ms  NH2 is initially not produced inside the roughness, but later diffuses into the interior. [NH2] cm- 3  - 5 kV, 1 atm Iowa State University Optical and Discharge Physics MINMAX UCLA_0406_46

  48. SURFACE COVERAGE OF ALKYL RADICALS (=C)  Alkyl sites are formed by the abstraction reactions OH + PP  PP + H2O H + PP  PP + H2  Large scale and small scale uniformity improves with treatment. - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1 Iowa State University Optical and Discharge Physics UCLA_0406_47

  49. SURFACE COVERAGE OF AMINE GROUPS [=C-NH2] • Amine groups are created by addition of NH2to alkyl sites. NH2 + PP PP-NH2  Points with large view angles are highly treated. • - 5 kV, 1 atm, 10 kHz, He/NH3/H2O=90/10/0.1, t = 0.1 s Iowa State University Optical and Discharge Physics UCLA_0406_48

  50. OPTIMIZE CHEMISTRY, UNIFORMITY WITH GAS MIXTURE • Balance of peroxy (PP-OO), alkoxy (PP-O) and alcohol (PP-OH) groups can be controlled by composition of fluxes. • Example: He/O2/H2O e + O2  O + O + e e + H2O  H + OH + e O + O2 + M  O3 + M • Large f(O2), small f(H2O): Small OH fluxes, large O3 fluxes Small f(O2), large f(H2O): Large OH fluxes, small O3 fluxes • Impact on polypropylene surface chemistry PP + O  PP + OH (slow rate) PP + OH  PP + H2O (fast rate) PP + O2 PP-OO (slow rate but a lot of O2) PP + O3 PP-O + O2 (fast rate) PP + OH  PP-OH (fast rate) Iowa State University Optical and Discharge Physics UCLA_0406_49

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