Recent Progress in Low-Temperature, Atmospheric Pressure Plasmas

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1 Recent Progress in Low-Temperature, Atmospheric Pressure Plasmas Michael Barankin and Robert Hicks Chemical Engineering and Materials Science University of California, Los Angeles, CA

2 Outline Radio frequency, low-temperature, atmospheric plasma sources Discharge physics & chemistry Plasma applications Conclusions

3 Atomflo Plasma Tool Handheld system developed by Surfx Technologies LLC. Generates intense beam of radicals at low temperature. Atomflo applicator Control Unit

4 High-Speed Linear Sources 5.0 cm Beam widths from 1.0 to 12 inches. Activates plastic at up to 1.0 m/s. Gas temp o C. Treats 3D objects. Atomflo -500R by Surfx

5 Versatile Chemistry Atomflo may be fed with up to 5.0 vol.% O 2, H 2, N 2, CF 4, N 2 O, NH 3, etc, in inert gas. Up to 20% of molecules are dissociated into atoms, O, H, N, F, etc. Chemicals may be injected downstream to deposit thin coatings.

6 Plasma Physics Need to determine the properties of the plasma: Breakdown voltage, V B Electron density, n e Electron temperature, T e Neutral gas temperature, T n

7 Current-Voltage Curves for Atomflo Source Voltage (V) Torr 500 Torr Breakdown 300 Torr 200 Torr 100 Torr Current (A) Helium breaks down at 170 V. Townsend region before breakdown. Abnormal glow discharge.

8 Current and Voltage Waveforms Voltage (V) Time (ns) RF at MHz Current (A) Smooth waveforms without spikes. Capacitive current precedes voltage in time. Phase angle: Argon ~ 80º. Helium ~ 83 o.

9 Electron Density J = en eμe E J = current density, A/cm 2 n e = plasma density, cm -3 μ e = electron mobility (α 1/P), cm 2 /V.s E = electric field (V/d), V/cm

10 Electron Temperature Power balance on free electrons: ε = n e P K T B g k 1 I 1 + n e n e σ ei υ e + P K T B g σ ea υ e 2m M e 3 2 K B ( T T ) e g Power loss due to ionization Total power input Loss from electron-ion collisions Loss from electron-atom collisions Electron temperature J. Jonkers et al., Plasma Sources Sci. Technol., 12, 30 (2003).

11 Physics of Atomflo Plasma Source Break down voltages: helium = 170 V; argon = 550 V. Electron density (n e ) = 1.0 x10 12 cm -3. Electron temperature = 1.2 ev. Neutral temperature = o C.

12 Plasma Chemistry Need to determine: Reaction mechanism. Concentrations of radicals in plasma and afterglow.

13 Experimental Apparatus Plexiglass Box MCT Detector Gas Inlet Quartz Tube Powered Electrode Grounded Electrode Titration with NO: NO + O + M NO 2 + M Titration Gas Inlet Ceramic Collar IR Beam Tube FTIR Source Sapphire Window Flow Cell Sapphire Window To Vacuum

14 Oxygen Atom Density Intensity (a.u) Titration Point [O] = 1.2± cm -3 Gas density = cm vol.% O atoms! NO Concentration (x10 17 cm -3 ) Conditions: 5.0 L/min Ar, 6.0 vol.% O 2, 150 W/cm 3, and 300±30 C.

15 Plasma Model Model inputs: Distance (cm) n e T e T g Feed gases Mechanism: 32 rxns plasma 11 rxns afterglow Concentration (cm -3 ) O 2 O 2 O O - O 2 O 3 O( 1 D) + O 2 O - O O 2 ( 1 2 ( 1 Δ g ) Δ g ) Time (ms) O 3 Plasma Afterglow

16 Comparison of Model and Experiment Oxygen atoms Exp.: 1.2± cm -3 Model: cm -3 Ozone Exp.*: 4.3± cm -3 Model: cm -3 *Determined experimentally by UV absorption.

17 Comparison of Atmospheric Plasmas Type of Discharge Torch Corona Dielectric barrier discharge RF capacitive discharge Plasma Density (cm -3 ) O Atom Density (cm -3 ) Atomflo 1000x more powerful than coronas!

18 Applications Activating polymers for bonding. Metal etching. Depositing thin films: SiO 2, Si 3 N 4, a-si:h, ZnO, DLC.

19 Surface Activation

20 Adhesion to Carbon-Fiber Composites Without plasma, adhesive failure. With Atomflo plasma, cohesive failure.

21 Treatment of 3-D Parts Robot-mounted plasma sources scan any 3-D object. any size of flat panel display.

22 Plasma Etching Results Rates exceed those obtained in lowpressure plasmas. Chemistry selection for disparate materials: Organics etched with O 2 plasma. Metals etched with CF 4 /O 2 plasma. Etching Rate (μm/min) Kapton SiO 2 Ta W

23 Tantalum Etching in CF 4 /O 2 Plasma Afterglow Etch Rate (μm/min) RF Power (W) Tantalum is a surrogate for plutonium. Etch rates up to 6.0 μm/min observed. Rate most sensitive to applied power.

24 Surface Morphology After Tantalum Etching x30000 Surface is heavily fluorinated: TaF 3 and TaF 4 by XPS SEI 5.00kV X μm

25 Plasma-Enhanced Chemical Vapor Deposition Plasma Gas Mixture RF Power Plasma Zone Metal Precursor Substrate Heater Ceramic Spacer Metal Precursor Sample Stage

26 Plasma-Enhanced Chemical Vapor Deposition RF Power Argon & O 2 PLASMA Plasma Physics Afterglow Chemistry Volatile chemical precursor Thin Film Materials

27 Organosilane Precursors TMCTS HMDSO HMDSN TEOS TMDSO

28 Low Porosity Glass Grown with HMDSN -35 nm 50 μm Three-dimensional image of 650-nm-thick film grown at 0.24 μm/min using HMDSN.

29 Dielectric Strength HMDSN Dielectric Strength HMDSN Breakdown Volts Glass film on 316 SS Thickness (μm)

30 Diamond-Like Carbon Process conditions: 0.1 L/min C 2 H 2,0.5 L/min H 2, 30 L/min He, 180 W & 155 o C. Dep. rate = 0.2 μm/min Confirm DLC film by C 13 NMR with magic angle spinning. sp 2 sp 3

31 Conclusions RF atmospheric plasmas are powerful tools for surface treatment. Ideal for automated processing of 3-D plastics. New processes are being developed to greatly enhance the functionality of materials.

32 Acknowledgements Surfx Technologies Peter Guschl, Steve Babayan, Joel Penelon, Quoc Truong, & Jason Orsborn U. Delaware, Center for Composite Materials UCLA Joe Dietzel & Jack Gillespie Angela Ladwig, Eleazar Gonzalez, Greg Nowling, Xiawan Yang, Maryam Moravej, Vincent Tu, Sarah Habib, Franky Chan, Melanie Yajima, Vu Le, Jarrad Ghirdalucci, Guowan Ding, Andreas Schutze

33 Numerical Model of the Plasma Chemistry One-dimensional plug-flow simulation. Mechanism includes 39 elementary steps among 19 species: neutrals, ions and metastables. Results compared to titration experiment.

34 Profiles of the Charged and Metastable Species Electronegative plasma: n F = 5 x n e CF 3 + Gas flow Density (cm -3 ) T e = 2.5 ev CF F _ 3 He* CF 2 + He 2 + He 2 * He Distance (mm) _

35 Profiles of the Neutral Species Density (cm -3 ) F CF 2 CF 3 C 2 F 4 Plasma C 2 F 5 CF 10 8 C 2 F Distance (mm) Afterglow C 2 F 6 F 2 Gas flow Fluorine atom density: 1.3x10 15 cm -3. Good agreement with H 2 titration.

36 H 2 Titration of F Atoms in CF 4 Plasma Afterglow HF infrared absorbance Titration point Hydrogen concentration (x10 15 cm -3 ) F atoms = 2x10 15 cm cm downstream of electrodes Torr CF 4, 2.3 Torr O 2, 745 Torr He, 73 W/cm 3 and 100 C.

37 Effect of Process Conditions on Tantalum Etch Rate Etch Rate (μm/min) O 2 Partial Pressure (Torr) Etch Rate ( μ m/min) CF 4 Partial Pressure (Torr)

38 Plasma Etching of UO 2 Process conditions: Total Flow: 42 L/min, He/O 2 /CF 4 O 2 : 6 Torr CF 4 : 15 Torr RF Power: 300 W Temperature: 200 o C Nozzle-to-sample distance: 3 mm

39 Uranium Oxide Surface Morphology Scanning electron micrographs of UO 2 film before and after etching with CF 4 /O 2 /He plasma: 0.5 μm 0.5 μm 0.5 μm Before After a 2-min etch After a 5-min etch

40 Surface Composition X-ray photoelectron spectra of UO 2 film U 4d3/2 U 4d5/2 U 4f5/2 U 4f7/2 U 5d5/2 Counts O Auger F Auger Fe 2p F 1s1/2 Ni Auger O 1s1/2 C 1s1/2 After a 2-min etch Before etching Binding Energy (ev)

41 Dependence of U 4f Peak Intensity on Time 1.6e+5 1.2e+5 Rate accelerates due to increased film porosity. Area (a. u) 8.0e+4 Re-deposition may result in tail at >3 min. 4.0e Etch time (min)