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CONTROL OF ION ACTIVATION ENERGY TO SURFACES IN ATMO-

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1 CONTROL OF ION ACTIVATION ENERGY TO SURFACES IN ATMO-
SPHERIC PRESSURE PLASMAS USING POROUS DIELECTRICS FILMS Natalia Yu. Babaeva and Mark J. Kushner University of Michigan Department of Electrical Engineering and Computer Science Ann Arbor, MI USA 64th Gaseous Electronics Conference Salt Lake City, Utah November 2011 *Work supported by the Department of Energy Office of Fusion Energy Science GEC2011

2 University of Michigan Institute for Plasma Science & Engr.
AGENDA Atmospheric pressure DBDs for plasma medicine Description of the model Control of ion energies to surfaces Sub-surface methods On-surface methods Delivery of activation energy by ions and photons to rough surfaces and cracks. Concluding Remarks University of Michigan Institute for Plasma Science & Engr. GEC2011

3 ACTIVATION ENERGY IN PLASMA MEDICINE
Atmospheric pressure plasmas in contact with living tissue have therapeutic effects. Wound healing, melanoma treatment, sterilization Just as in microelectronics fabrication, the controlled delivery of activation energy (energetic ions, photons) will determine the “goodness” of the process. Even at atmospheric pressure, energetic particles can be delivered VUV photon fluxes can exceed ion fluxes. Electric fields of 100s kV/cm, ion mean free paths of 1 m  10s eV. Energies of 10s of eV exceed the sputtering threshold of lipid-layers (cell membranes). Can this be controlled? University of Michigan Institute for Plasma Science & Engr. GEC2011

4 University of Michigan Institute for Plasma Science & Engr.
MODEL: nonPDPSIM Plasma Hydrodynamics Poisson’s Equation Gas Phase Plasma Liquid Phase Plasma 2d-unstructured mesh with spatial dynamic range of 104. Fully implicit plasma transport. Time slicing algorithms between plasma and fluid timescales. Bulk Electron Energy Transport Kinetic “Beam” Electron Transport Neutral Transport Navier-Stokes Neutral and Plasma Chemistry Radiation Transport Surface Chemistry and Charging Ion Monte Carlo Simulation Circuit Model University of Michigan Institute for Plasma Science & Engr. GEC2011 4 4

5 MODELING PLATFORM: nonPDPSIM
Poisson’s equation: Transport of charged and neutral species: Charged Species:  = Sharffeter-Gummel Neutral Species:  = Diffusion Surface Charge: Electron Temperature (transport and rate coefficients from 2-term spherical harmonic expansion solution of Boltzmann’s Eq.): University of Michigan Institute for Plasma Science & Engr. AVS2011 5 5

6 MODELING PLATFORM: nonPDPSIM
Radiation transport and photoionization: Solid materials represented as lossy dielectrics with differentiated regions having specified permittivity  and conductivity . Poisson’s equation extended into materials. Solution: 1. Unstructured mesh discretized using finite volumes. 2. Fully implicit transport algorithms with time slicing between modules. University of Michigan Institute for Plasma Science & Engr. AVS2011 6 6

7 ION ENERGY-ANGULAR DISTRIBUTIONS TO SURFACE
IEADs to surfaces obtained with Plasma Chemistry Monte Carlo Module. Structured PCMC mesh super-imposed onto unstructured mesh – E-fields interpolated onto mesh. Pseudo-particles are launched from sites near surface determined by sources and fluxes into mesh. Monte Carlo techniques advance trajectories in time varying E-fields while accounting for collisions. The energy and angles of particles are recorded as they strike surfaces. University of Michigan Institute for Plasma Science & Engr. GEC2011

8 STREAMER DYNAMICS AND BOUNDARIES
A dielectric barrier discharge (DBD) interacts with its boundaries in a “circuit-like” manner. The applied voltage is divided between the Plasma column Avalanche front Sheath capacitance when striking surface Capacitance of surface layers as surface charges. Manipulating surface layer capacitance affects sheath voltage. University of Michigan Institute for Plasma Science & Engr. Animation Slide GEC2011

9 University of Michigan Institute for Plasma Science & Engr.
STREAMER DYNAMICS: O2+ Two layers of polymer – top sheet and substrate. Electron and ion densities in the streamer channel approach 1015 cm-3. Upon intersection the dielectric, the streamer charges and spreads on the surface and a wave progates outward. Positive corona, N2/O2/H2O = 79.5/19.5/1.0, 1 atm, r1= r2= 2.2 University of Michigan Institute for Plasma Science & Engr. Animation Slide GEC2011

10 University of Michigan Institute for Plasma Science & Engr.
STREAMER DYNAMICS In the streamer head, E-fields are 200 kV/cm when far from surfaces. Upon intersection of the streamer with the surface, the electric fields in the resulting sheath can exceed kV/cm. 800 kV/cm x  (0.5 m) = 40 eV Spreading of streamer on surface produces a wave of sheath electric field away from point of intersection with E-fields of kV/cm. Fields are radially outward at first, then oriented towards surface. Positive corona, N2/O2/H2O = 79.5/19.5/1.0, 1 atm, r1= r2= 2.2. Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX GEC2011

11 ION ENERGY DISTRIBUTIONS vs r2
As r2 of sub-layer increases, E-field is expelled (larger capacitance takes longer to charge) - more potential left for sheath. Higher ion energies are incident on top layer. Although lasting only a few ns per pulse, above threshold ions induce reactions. Positive corona, N2/O2/H2O = 79.5/19.5/1.0, 1 atm. University of Michigan Institute for Plasma Science & Engr. MIN MAX GEC2011

12 CONTROL IEADS TO SURFACES WITH THIN FILMS
O2+ Density (humid air) Animation Slide In plasma medicine applications, it is difficult to modify materials in or under tissue – however one can modify environment on top of tissue. Porous thin film on top of tissue with selected , and through which plasma can penetrate provides means to manipulate the sheath. Control IEADs by the capacitance and thickness of film, and the size of pore. University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC2011

13 CONTROL OF ELECTRIC FIELDS TO SURFACES
ε/ε0 = 2 ε/ε0 = 32 E-field (flood) and potential (lines) A large permittivity film expels electric field, compressing more voltage in sheath penetrating into hole. If size of pores are smaller or commensurate with filament, sheath is molded over pore. Expelled potential is partly retained in sheath. Higher compression of potential lines, larger E-field in transient sheath leads to controllable IEAD. Substrate ε/ε0 = 32 University of Michigan Institute for Plasma Science & Engr. Animation Slide GEC2011

14 TIME EVOLUTION IEADs TO SURFACES: ε/ε0 = 2, 8, 16, 32, 80
1.9 ns 2.1 ns 2.5 ns 2.9 ns 3.3 ns TIME EVOLUTION IEADs TO SURFACES: ε/ε0 = 2, 8, 16, 32, 80 ε/ε0 = 2 As with controlling dielectric properties below the surface, IEADs (O2+) can be controlled with on-surface porous membranes. More energetic ions with narrow scattering angles to surface with increasing ε/ε0 as conformal sheath contains more of expelled potential. V-shape of IEADs due to curvature of filament front. Saturation of IEADS for ε/ε0 >30. ε/ε0 = 8 ε/ε0=16 ε/ε0=32 MIN MAX Log scale ε/ε0=80 University of Michigan Institute for Plasma Science & Engr.

15 IEADs vs SUBSTRATE HOLE: ε/ε0 = 2, 8, 16, 32, 80
50 μm 25 μm IEADs and activation energy through porous membrane can be controlled with ε/ε0 of membrane and diameter of pore. Larger ε/ε0  higher ion energy due to concentration of potential in sheath. Smaller hole  higher ion energy due to more sheath molding. MIN MAX Log scale University of Michigan Institute for Plasma Science & Engr. GEC2011

16 ACTIVATION ENERGY INTO MICROSCOPIC OPENINGS
The delivery of activation energy onto surfaces with roughness and cracks is important for sterilization. Charging of surfaces, geometrical obstructions, and ability of plasma to “turn corners” are all important. Example: DBD discharge in humid air treating rough surface. First order issue: How deep can atmospheric pressure plasma penetrate into deep cracks? University of Michigan Institute for Plasma Science & Engr. Animation Slide MIN MAX Log scale GEC2011

17 POSITIVE STREAMER PLASMA INTO TRENCHES
[e] (cm-3) Plasma spreads over the dielectric simultaneously with penetration into trenches. Larger losses to side-walls, less photo-ionization ahead of streamer, larger capacitance slows streamer. 20kV, Humid Air AR = aspect ratio [depth of trench]/[width] AR=20 AR=50 AR=50 AR=20 AR = 20 AR = 50 0.3 to 1.4 ns Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX Log scale GEC2011

18 POSITIVE STREAMER PLASMA INTO TRENCHES
[e] (cm-3) [ρ] (cm-3) Te (eV) AR=20 50 100 200 20 20 50 100 200 1x1016 to 2x1017 1x1013 to 2x1015 5 to 22 eV Full penetration for AR<100, partial for AR=100, no penetration for AR>100. Penetration is partly determined by the Debye length (1-2 μm). For AR>100 sheath is comparable with trench width (10 m) . Positive streamer, 20kV ns University of Michigan Institute for Plasma Science & Engr. Animation Slide MIN MAX Log scale GEC2011

19 NEGATIVE STREAMER PLASMA INTO TRENCHES
Penetration into trench starts earlier than for positive streamer, which slows spreading of plasma over the dielectric. Penetration of plasma into smaller trench is slower due to charging of walls. Negative streamer, -20kV AR = aspect ratio [depth of trench]/[width] AR = 20 AR = 50 ns University of Michigan Institute for Plasma Science & Engr. Animation Slide MIN MAX Log scale GEC2011

20 NEGATIVE STREAMER PLASMA INTO TRENCHES
[e] (cm-3) [-ρ] (cm-3) (flood) Te (eV) AR=20 50 100 200 20 50 100 200 20 50 100 200 1x1015 to 2x1016 1x1013 to 1x1014 4 to 10 eV Due to earlier plasma penetration, potential is compressed to the bottom and rarefied near the top of the trench: top-bottom streamer propagation. With less reliance on photoionization, negative streamer penetrates into AR = 100 (10 m) trench. However, no penetration for AR = 200 (5 m). ns University of Michigan Institute for Plasma Science & Engr. Animation Slide MIN MAX Log scale GEC2011

21 PHOTON FLUXES ON INSIDE SURFACES
Positive Streamer Negative Streamer UV fluxes are integrated until the streamer reaches the bottom of the trench. Due to higher electron densities in narrower trenches, integrated photon fluence inversely proportional to the trench width. Fluence of photons is commensurate to (in some cases larger than) fluence on top surface. University of Michigan Institute for Plasma Science & Engr. GEC2011

22 IEAD TO BOTTOM OF 50 μm TRENCH (AR=20)
[ρ] (cm-3) E-field (kV/cm) E-field Vectors N 50 μm High electric fields due compression of potential near the walls and bottom of the trench. Electric field vectors have ±30 degrees with the normal, resulting in “double peak” IEADs. Ion energies exceed 150 eV. Side walls of the trench also receive highly energetic ion flux. ns O2+, positive streamer, ε/ε0 =2 University of Michigan Institute for Plasma Science & Engr. Animation Slide MIN MAX Log scale GEC2011

23 University of Michigan Institute for Plasma Science & Engr.
CONCLUDING REMARKS Even at atmospheric pressure, ions up to 10s eV can be delivered to surfaces by DBDs. IEADs are controllable by managing division of applied potential between capacitance of substrate and sheath. Porous membranes enables this control “from the top” for applications in plasma medicine. Plasmas can be delivered to high aspect ratio trenches (AR up to ) provided the trench width is higher than the plasma Debye length. With the same aspect ratio, negative streamers allow deeper penetration than positive. Ion energies to the bottom of the trench can exceed 150 eV. University of Michigan Institute for Plasma Science & Engr. GEC2011 23 23


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