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.edumjk@iastate.edu http://uigelz.ece.iastate.eduhttp://uigelz.ece.iastate.edu April 2006 UCLA_0406_01

2. Iowa State University Optical and Discharge Physics 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. UCLA_0406_02

3. Iowa State University Optical and Discharge Physics AGENDA UCLA_0406_03  Introduction to Plasma Processing  Plasma surface functionalization  Description of the models  High Pressure:  Plasma dynamics in He/NH 3 /H 2 O 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.

4. Iowa State University Optical and Discharge Physics 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). UCLA_0406_04 http://www.plasmas.org/basics.htm  X-ray view of the sun, a plasma.

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

6. Iowa State University Optical and Discharge Physics TECHNOLOGICAL PLASMAS: PARTIALLY IONIZED GASES  Air plasma: N 2, O 2, N 2 +, O 2 +, O -, e where [e] << [M]. UCLA_0406_06  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.

7. Iowa State University Optical and Discharge Physics 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. UCLA_0406_07

8.  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 COLLISIONAL LOW TEMPERATURE PLASMAS UCLA_0406_08

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

10. Iowa State University Optical and Discharge Physics 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. UCLA_0406_10

11. Iowa State University Optical and Discharge Physics 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 Untreated PP Plasma Treated PP  M. Strobel, 3M  Polypropylene (PP) He/O 2 /N 2 Plasma  Massines J. Phys. D 31, 3411 (1998). UCLA_0406_11

12. Iowa State University Optical and Discharge Physics FUNCTIONALIZATION OF POLYMERS USING PLASMAS UCLA_0406_12  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.

13.  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/m 2. Iowa State University Optical and Discharge Physics SURFACE MODIFICATION OF POLYMERS UCLA_0406_13  Filamentary Plasma 10s – 200  m

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

15. Iowa State University Optical and Discharge Physics  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. UCLA_0406_17  E. Sachlos, European Cells and Materials v5, 29 (2003)  Tien-Min Gabriel Chu http://www.engr.iupui.edu/~tgchu PLASMAS FOR MODIFICATION OF BIOCOMPATIBLE SURFACES: TISSUE ENGINEERING

16.  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 LOW PRESSURE GLOW DISCHARGES UCLA_0406_16

17.  Low pressure  Low throughput  High precision  Grow expensive materials  High tech  High pressure  High throughput  Low precision  Modify cheap materials  Commodity Web Treatment of Films $0.05/m 2 $1000/cm 2 Microelectronics EXTREMES IN CONDITIONS, VALUES, APPLICATIONS Iowa State University Optical and Discharge Physics ISU_0105_11

18.  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/cm 2 or $1000/cm 2 ? Iowa State University Optical and Discharge Physics CREATING HIGH VALUE: COMMODITY PROCESSES $0.05/m 2 $1000/cm 2 ? ISU_0105_12

19. Iowa State University Optical and Discharge Physics FUNCTIONALIZING SMALL FEATURES UCLA_0406_18  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.  NH 3 plasmas for =NH x functionality for cell adhesion.  O 2 plasmas for =O functionality for improved wettability.

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

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

22.  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 HIGH PRESSURE: CHARGED PARTICLE TRANSPORT ELECTROSTATICS UCLA_0406_21

23. Iowa State University Optical and Discharge Physics  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. HIGH PRESSURE: NEUTRAL PARTICLE TRANSPORT UCLA_0406_22

24. Iowa State University Optical and Discharge Physics 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. UCLA_0406_23

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

26. Iowa State University Optical and Discharge Physics FUNCTIONALIZATION FOR BIOCOMPATIBILITY UCLA_0406_25 ( 1 K. Schroeder et al, Plasmas and Polymers, 7, 103, 2002)  Ammonia plasma treatment affixes amine (C-NH 2 ) groups on surfaces for applications such as cell adhesion, protein immobilization and tissue engineering. Micropatterned cell growth on NH 3 plasma treated PEEK 1

27. Iowa State University Optical and Discharge Physics GAS PHASE CHEMISTRY - He/NH 3 /H 2 O MIXTURES UCLA_0406_26  Electron impact reactions initiate dissociate NH 3 and H 2 O into radicals that functionalize surface.  H, NH 2, NH, O and OH are major radicals for surface reactions.

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

29. UCLA_0406_28 Iowa State University Optical and Discharge Physics SURFACE REACTION MECHANISM  Gas phase NH 2 and NH radicals react with surface alkyl sites creating amine (R-NH 2 ) groups and imine (R-NH  ) sites.

30. Iowa State University Optical and Discharge Physics TREATMENT OF POROUS POLYMER BEADS UCLA_0406_29  Functionalized Porous Bead for Protein Binding sites (www.ciphergen.com)  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.

31. Iowa State University Optical and Discharge Physics DBD TREATMENT OF POROUS POLYMER BEAD UCLA_0406_30  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/NH 3 /H 2 O=90/10/0.1  PRF – 10 kHz

32. Iowa State University Optical and Discharge Physics ELECTRON TEMPERATURE, SOURCE UCLA_0406_31  - 5 kV, 1 atm, He/NH 3 /H 2 O=90/10/0.1, 0-3.5 ns  Electron Temperature  Electron Source Animation Slide-GIF MIN MAX

33. Iowa State University Optical and Discharge Physics ELECTRON DENSITY UCLA_0406_32  Electron density of 10 13 - 10 14 cm -3 is produced.  Electron impact dissociation generates radicals that functionalize surfaces.  - 5 kV, 1 atm, He/NH 3 /H 2 O=90/10/0.1, 0-3.5 ns Animation Slide-GIF MIN MAX

34. Iowa State University Optical and Discharge Physics POST-PULSE RADICAL DENSITIES UCLA_0406_33  - 5 kV, 1 atm, He/NH 3 /H 2 O=90/10/0.1  NH  NH 2  OH MIN MAX

35. Iowa State University Optical and Discharge Physics ELECTRON DENSITY IN AND AROUND BEAD UCLA_0406_34  Corona treating a porous polymer bead placed on the lower dielectric.  - 5 kV, 1 atm, He/NH 3 /H 2 O=90/10/0.1, 0-3 ns.  In negative corona discharge, electrons lead the avalanche front and initially penetrate into pores. Charging of surfaces limit further electron penetration.  Electrons (3.7 x 10 13 cm -3 ) 50  m Animation Slide-GIF MIN MAX

36. Iowa State University Optical and Discharge Physics UCLA_0406_35  - 5 kV, 1 atm, He/NH 3 /H 2 O=90/10/0.1, 0-3 ns TOTAL POSITIVE ION DENSITY IN AND AROUND BEAD  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.  Ions (3.7 x 10 13 cm -3 ) 50  m Animation Slide-GIF MIN MAX

37. Iowa State University Optical and Discharge Physics [NH 2 ] INSIDE PORES UCLA_0406_36 (log scale)  - 5 kV, He/NH 3 /H 2 O=90/10/0.1, pore dia=4.5  m, 1 atm 90  m Bead 2x10 10 - 2x10 13 MINMAX 3 ns 80  s 9.1x10 12 - 9.3x10 12 7.5x10 12 - 8.5x10 12  Since electrons poorly penetrate into most pores, little NH 2 is initially produced inside bead.  NH 2 later diffuses into pores from outside. 30  m Bead 80  s 2x10 10 - 2x10 13

38.  [NH 2 ] within pores increases with pore diameter during the pulse and in the interpulse period. Iowa State University Optical and Discharge Physics UCLA_0406_37 [NH 2 ] INSIDE PORES : PORE DIAMETER 8.5  m4.5  m3  m (log scale) MINMAX [NH 2 ] cm - 3  - 5 kV, He/NH 3 /H 2 O=90/10/0.1, bead dia=90  m, 1 atm  t = 3 ns 2x10 10 - 2x10 13  t = 80  s 7.5x10 12 -8.7x10 12

39. Iowa State University Optical and Discharge Physics FUNCTIONALIZATION OF POROUS BEAD SURFACES UCLA_0406_38 [ALKYL] =C  MIN MAX 1.25x10 10 – 1.25x10 11 10 12 – 10 13 log scale, cm - 2 [AMINE] =C-NH 2  - 5 kV, 1 atm, 10 kHz, He/NH 3 /H 2 O=90/10/0.1, Bead size=90  m, Pore dia= 4.5  m, t=0.1 s A B C D E F G H I J K A B C D E F G H I J K Letters indicate position along the surface.

40. Iowa State University Optical and Discharge Physics AMINE SURFACE COVERAGE: SIZE OF BEAD UCLA_0406_39  - 5 kV, 1 atm, 10 kHz, He/NH 3 /H 2 O=90/10/0.1, t=1 s  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  m

41. Iowa State University Optical and Discharge Physics BEADS IN DISCHARGE: ELECTRON DENSITY UCLA_0406_40  Uniformity may be improved by dropping beads through discharge instead of placing on a surface.  He/O 2 /H 2 O = 89/10/1, 1 atm  Electrons produce a wake beyond the particle.  Electron Density (1.6 x 10 14 cm -3 ), 0-2.5 ns Animation Slide-GIF MIN MAX

42. Iowa State University Optical and Discharge Physics ELECTRON DENSITY AND SOURCE UCLA_0406_41  Ionization occurs around particle during initial avalanche and restrike.  Sheath forms above particle, wake forms below particle.  He/O 2 /H 2 O = 89/10/1, 1 atm  0-2.6 ns  Electron Density (6 x 10 13 cm -3 )  Electron Source (10 23 cm -3 s -1 ) Animation Slide-GIF MIN MAX

43. Iowa State University Optical and Discharge Physics POST-PULSE O and OH DENSITIES UCLA_0406_42  Directly after the pulse, radicals have a similar wake below the particles.  He/O 2 /H 2 O = 89/10/1, 1 atm  0-2.6 ns  [O] (8 x 10 14 cm -3 )  [OH] (5 x 10 13 cm -3 ) MIN MAX

44. Iowa State University Optical and Discharge Physics BEADS IN DISCHARGE: SURFACE COVERAGE UCLA_0406_43  Uniformity of functionalization, locally poor, is improved around the particle.  He/O 2 /H 2 O = 89/10/1,  1 atm  Alkoxy (=C-O) and Peroxy (=C-OO) Coverage

45. Iowa State University Optical and Discharge Physics DBD TREATMENT OF PP SURFACE WITH MICROSTRUCTURE UCLA_0406_44  Corona functionalization of rough polymer resembling tissue scaffold.  1 atm, He/NH 3 /H 2 O, 10 kHz  Polypropylene.  E. Sachlos, et al.

46. MIN MAX Iowa State University Optical and Discharge Physics PENETRATION INTO SURFACE FEATURES – [e], [IONS] UCLA_0406_45 [e] cm - 3 t = 2.7 nst = 4 ns 10 10 – 10 13 [Positive ions] cm - 3 10 10 – 10 13 [Surface (-ve) Charge]  C 10 -1 – 10 3  - 5 kV, 1 atm, He/NH 3 /H 2 O=98.9/1.0/0.1 log scale

47. Iowa State University Optical and Discharge Physics UCLA_0406_46 NH 2 DENSITY: EARLY AND LATE [NH 3 ]=10%  - 5 kV, 1 atm 3x10 12 - 3x10 14 1.8x10 12 – 1.9x10 12, t =90  s2.25x10 12 – 2.35x10 12, t =90  s  NH 2 is initially not produced inside the roughness, but later diffuses into the interior. MIN MAX [NH 3 ]=30% t =3 ns [NH 2 ] cm - 3

48. Iowa State University Optical and Discharge Physics SURFACE COVERAGE OF ALKYL RADICALS (=C  ) UCLA_0406_47  - 5 kV, 1 atm, 10 kHz, He/NH 3 /H 2 O=90/10/0.1  Alkyl sites are formed by the abstraction reactions OH + PP  PP  + H 2 O H + PP  PP  + H 2  Large scale and small scale uniformity improves with treatment.

49. Iowa State University Optical and Discharge Physics SURFACE COVERAGE OF AMINE GROUPS [=C-NH 2 ] UCLA_0406_48  - 5 kV, 1 atm, 10 kHz, He/NH 3 /H 2 O=90/10/0.1, t = 0.1 s  Amine groups are created by addition of NH 2 to alkyl sites. NH 2 + PP   PP-NH 2  Points with large view angles are highly treated.

50. Iowa State University Optical and Discharge Physics OPTIMIZE CHEMISTRY, UNIFORMITY WITH GAS MIXTURE UCLA_0406_49  Balance of peroxy (PP-OO), alkoxy (PP-O) and alcohol (PP-OH) groups can be controlled by composition of fluxes.  Example: He/O 2 /H 2 O e + O 2  O + O + e e + H 2 O  H + OH + e O + O 2 + M  O 3 + M  Large f(O 2 ), small f(H 2 O): Small OH fluxes, large O 3 fluxes Small f(O 2 ), large f(H 2 O): Large OH fluxes, small O 3 fluxes  Impact on polypropylene surface chemistry PP + O  PP  + OH (slow rate) PP + OH  PP  + H 2 O (fast rate) PP  + O 2  PP-OO (slow rate but a lot of O 2 ) PP  + O 3  PP-O + O 2 (fast rate) PP  + OH  PP-OH (fast rate)

51. Iowa State University Optical and Discharge Physics CONTROLLING FLUX OF OZONE TO SURFACE UCLA_0406_50  Pulsed corona discharge, 10 kHz  He/O 2 /H 2 O = 99-X /X/1  After short discharge pulse, flux of O atoms is large.  At end of interpulse period, flux of O atoms is negligible as most O has been converted to O 3.  Flux of O 3 increases by nearly 100 with increasing f(O 2 ).  Non-uniform O 3 fluxes results from reaction limited transport into microstructure.

52. Iowa State University Optical and Discharge Physics CONTROLLING FLUX OF OZONE TO SURFACE UCLA_0406_51  O 2 fluxes at any finite mole fraction; peroxy PP-OO formation dominates.  Large O 2 produces large O 3 fluxes which favors alkoxy PP-O.  Small O 2 increases OH fluxes by H 2 O dissociation and so alcohol PP-OH fractions increase.  Small scale uniformity is dominated by reactivity of O 3 and in ability to penetrate deep into crevices.  Low O 3 but moderate OH optimizes uniformity.  He/O 2 /H 2 O = 99-X /X/1

53. Iowa State University Optical and Discharge Physics CAN FLOW BE USED TO YOUR ADVANTAGE? UCLA_0406_52  Forced flow through the gap will redistribute radicals across the polymer.  Can this redistribution be used to customize functionalization?  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1 Inter-electrode gap = 2 mm Reactor depth = 1 m

54. Iowa State University Optical and Discharge Physics RADICAL DENSITIES FOLLOWING A SINGLE PULSE – 10 slpm UCLA_0406_53  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 10 slpm, 0 - 0.01 s  OH O3O3 OO  Radicals are produced in 5-10 ns pulse.  Flow advects radicals downstream; and diffuse upstream.  Radicals undergo gas phase reactions whoe flowing. 10 16 cm -3 10 14 cm -3 Animation Slide-GIF

55. Iowa State University Optical and Discharge Physics RADICAL FLUXES AT POLYMER SURFACE UCLA_0406_54  -5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0.01 s 10 13 10 15 10 17 10 16 10 18 10 20  10 slpm Flux (cm -2 s -1)  OH O3O3 Position along Surface 10 13 10 15 10 17 10 16 10 18 10 20  1 slpm  OH O3O3 Position along Surface Flux (cm -2 s -1) Animation Slide-GIF

56. TIME EVOLUTION OF SURFACE GROUPS ON PP– 10 slpm UCLA_0406_55  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1,0- 0.09 s  10 slpm 10 8 10 10 12 Position along the surface Iowa State University Optical and Discharge Physics Surface Coverage (cm -2)  Alkoxy PP-O  Peroxy PP-OO  Alcohol PP-OH  Alkoxy coverage initially increases as they are formed from alkyl sites and then decreases as they are react to form alcohol groups.  Peroxy sites monotonically increase as terminal species. Animation Slide-GIF

57. Iowa State University Optical and Discharge Physics SURFACE COVERAGE OF FUNCTIONAL GROUPS UCLA_0406_56  - 5 kV, 1 atm, He/O 2 /H 2 O=89/10/1, 0.09 s  1 slpm  10 slpm Position along the surface 10 9 10 11 10 13 Surface Coverage (cm -2) Position along the surface  Ratio of functional groups can be controlled by transport. 10 9 10 11 10 13 PP-OO* PP-O* PP-OH PP-OO* PP-O* PP-OH

58. Iowa State University Optical and Discharge Physics “LAB ON A CHIP”  “Lab on a Chip” typically has microfluidic channels 10s -100s  m wide and reservoirs for testing or processing small amounts of fluid (e.g., blood)  Internal surfaces of channels and reservoirs must be treated (i.e., functionalized) to control wetting and reactions.  Desire for mass produced disposable units require cheap process.  Ref: Calipers Life Sciences, Inc. http://www.caliperls.com UCLA_0406_57

59. Iowa State University Optical and Discharge Physics PLASMA PENETRATION INTO DEEP 50  m SLOTS : ELECTRONS UCLA_0406_58  Slow penetration through dielectric results from surface charging.  Rapid “restrike” through conductive and precharged slot. MINMAX Animation Slide-GIF  100  m  500  m  1000  m  -15 kV, 1 atm, N 2 /O 2 /H 2 O=79.5/19.5/1 2 mm

60. Iowa State University Optical and Discharge Physics UCLA_0406_59  Electron impact ionization in deep slots is augmented by photoionization.  Fully charging top surface reduces electric penetration. MINMAX Animation Slide-GIF  100  m  500  m  1000  m PLASMA PENETRATION INTO DEEP 50  m SLOTS : IONIZATION  -15 kV, 760 Torr, N 2 /O 2 /H 2 O=79.5/19.5/1

61. Iowa State University Optical and Discharge Physics PLASMA PENETRATION IN 10  m SLOT  High impedance of small slot slows penetration and limits “restrike” through slot. UCLA_0406_60 MINMAX Animation Slide-GIF S-ee e  -15 kV, 760 Torr, N 2 /O 2 /H 2 O=79.5/19.5/1

62. Iowa State University Optical and Discharge Physics PLASMA PENETRATION INTO DEEPER 10  m SLOT UCLA_0406_61  Removal of charge from streamer to charge walls weakens ionization front and stalls streamer.  Charging of top dielectric shields voltage from penetrating. MINMAX Animation Slide-GIF  500  m Thick  [e]  Ionization  -15 kV, 760 Torr, N 2 /O 2 /H 2 O=79.5/19.5/1

63. Iowa State University Optical and Discharge Physics SHAPES OF SLOTS MATTER: ELECTRONS  Charging of internal surfaces of slots produce opposing electric fields that limit penetration.  Restrike fills smaller slot with plasma. UCLA_0406_62 MINMAX Animation Slide-GIF  20 and 30  m slots  -15 kV, 1 atm, N 2 /O 2 /H 2 O=79.5/19.5/1

64. Iowa State University Optical and Discharge Physics SHAPES OF SLOTS MATTER: ELECTRONS  Charging of surfaces and topology of slot determine plasma penetration.  Here plasma is unable to penetrate through structure.  Direction of applied electric field and charge induced fields are in the opposite direction of required penetration. UCLA_0406_63 MINMAX Animation Slide-GIF  20 and 30  m slots  -15 kV, 1 atm, N 2 /O 2 /H 2 O=79.5/19.5/1

65. Iowa State University Optical and Discharge Physics SHOULDN’T LOW PRESSURE BE BETTER? UCLA_0406_64  Low pressure discharges with more uniform fluxes, longer mean free paths should be better for functionalization of small features.  Results from HPEM.  ICP without bias, He/O 2 =75/25, 15 mTorr 300 W

66. Iowa State University Optical and Discharge Physics ACTIVATION OF SURFACE SITES AND SPUTTERING UCLA_0406_65  Large fluxes of O atoms in low pressure systems increase likelihood of alkoxy formation (=C-O)  Low energy ion activation of surface sites increases rate of reaction direct peroxy (=C-OO formation)  High energy ions sputter the polymer.

67.  Strands flex with age. Bottom surfaces may eventually be exposed.  Top surfaces subject to low energy ion fluxes have activated sites and larger peroxy coverage.  Results from Monte Carlo Feature Profile Model (MCFPM). DIRECTIONALITY OF ION FLUXES IS A PROBLEM Iowa State University Optical and Discharge Physics UCLA_0406_66  Polypropylene M. Strobel, 3M  ICP without bias, He/O 2 =75/25, 15 mTorr 300 W

68. FUNCTIONALIZATION: TOP vs BOTTOM OF STRANDS Iowa State University Optical and Discharge Physics  Alkoxy =C-O  Peroxy =C-OO  Undersides of strands are mostly alkoxy.  Topsides, which receive low energy ion activation, are mostly peroxy.  ICP without bias  He/O 2 =75/25, 15 mTorr 300 W UCLA_0406_67

69.  Even with moderate 35V bias, sputter begins and activation is lessened. Surfaces are almost exclusively alkoxy (=C-O). Iowa State University Optical and Discharge Physics UCLA_0406_68  ICP, 35v rf bias, He/O 2 =75/25, 15 mTorr 300 W MODERATE BIAS: SPUTTERING, LOW ACTIVATION

70.  With 85V bias, sputtering is significant and redeposition of sputtered polymer reshapes surface. Functionalized surfaces are almost exclusively alkoxy (=C-O). Iowa State University Optical and Discharge Physics UCLA_0406_69  ICP, 85v rf bias, He/O 2 =75/25, 15 mTorr 300 W HIGH BIAS: SPUTTERING, REDEPOSITION

71. Iowa State University Optical and Discharge Physics CONCLUDING REMARKS UCLA_0406_70  Functionalization of complex surfaces will have challenges at both high and low pressure.  High Pressure:  Penetration of plasma into small spaces is problematic.  Must rely on slower diffusion of neutral radicals.  3-body reactions deplete radicals  Low Pressure:  Directionality of activation energy, an advantage in microelectronics processing, leads to uneven functionalization.  Difficult to treat soft materials.  Developing high pressure processes will result in much reduced cost.