Ionic Liquid Ion Sources in the Processing of Materials and Other Applications

Ionic Liquid Ion Sources in the Processing of Materials and Other Applications Paulo Lozano Massachusetts Institute of Technology Materials and Processes Far From Equilibrium Workshop November 3, 2010

Ionic Liquid Ion Sources (ILIS) ILIS are electrospray sources that operate in the pure ionic regime ionic liquid emitter!v! " 50 dyn/cm extractor r! 10 µm d! 1 mm 1. Low energy spreads / deficits 2. Ability to produce + or - ion beams 3. Low temperature operation 4. Stable over low and medium current levels 5. Operation at relatively low voltage 6. Large variety of ionic species available zero-vapor pressure ionic liquids (Room-temperature molten salts)

Formula 1 Formula 2 Name EMI-Im / EMI-Tf2N EMI-Beti C5MI- (C2F5)3PF3 EMI-GaCl4 EMI-N(CN)2 EMI-C(CN)3 BMI-FeBr4 HMI-FeBr4 HMI-FeCl4 BMI-FeCl4 EMIF2.3HF BMI-I EMI-BF4 HMI-PF6 BMI-Im / BMI-Tf2N C8H11F6N3O4S2 C10H11F10N3O4S2 C15H17F18N2P C6H11N2GaCl4 C8H11N5 C10H11N5 C8H15N2FeBr4 C10H19N2FeBr4 C10H19N2FeCl4 C8H15N2FeCl4 C6H13N2F3/ C6H14N2F4 C8H15N2I C6H11N2BF4 C10H19N2PF6 C10H15F6N3O4S2 Large Variety of Ionic Liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide 1-ethyl-3-methylimidazolium bis(pentafluoroethylsulfonyl) imide 1-pentyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate 1-ethyl-3-methylimidazolium gallium tetrachloride 1-ethyl-3-methylimidazolium dicyanamide 1-ethyl-3-methylimidazolium tricyanomethanide 1-butyl-3-methylimidazolium iron tetrabromide 1-hexyl-3-methylimidazolium iron tetrabromide 1-hexyl-3-methylimidazolium iron tetrachloride 1-butyl-3-methylimidazolium iron tetrachloride 1-ethyl-3-methylimidazolium hydrofluorogenate 1-butyl-3-methylimidazolium iodide 1-ethyl-3-methylimidazolium tetrafluoroborate 1-hexyl-3-methylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide Conductivity (Si/m) Viscosity (cp) Surface Tension (dyn/cm) Density (g/cm 3 ) + ion mass - ion mass Echem window (V) Melting point (C) Decomp. temp (C) Hydro phobic 0.84 28 (25 C) 41.6 1.52 111.2 280.1 4.1-15 450 Yes 0.34 28.7 28.75 1.6 111.2 380.2 4.1-1 - - 0.229 30.3 30.33 1.59 153.2 445.0 - - - Yes 2.2 13 48.6 1.53 111.2 211.5-11 130-2.8 14.5 (25 C) 7.8 (50 C) 49.05 1.08 111.2 66.0 3.3-12 240 No 2.2 18 47.9 1.11 111.2 90.1 2.9-11 240 No 0.55 62 (25 C) 47.1 1.98 139.2 375.5 - -2 290-0.28 95 (25 C) 42.01 1.86 167.3 375.5 - -82 270-0.47 45 (25 C) 39.37 1.33 167.3 197.7 - -86 320-0.89 34 (25 C) 46.5 1.38 139.2 197.7 - - 280-10 4.9-1.13 111.2 59 79 3-65 297-0.069 (23 C) 0.25 (50 C) 1110 (25 C) 54.7 1.44 139.2 126.9 - -72 265 No 0.73 (75 C) 1.3 (25 C) 34.1 (25 C) 2.3 (50 C) 15.6 (50 C) 52 1.24 111.2 86.8 4.5 12 412 No 4.6 (100 C) 4.7 (100 C) 0.11 680 (20 C) 363 (30 C) 43.4 1.29 167.3 144.9 - -61 417 Yes 0.406 (25 C) 0.924 (50 C) 69 (25 C) 37 1.43 139.2 280.1 4.1-25 439 Yes

Ionic Liquid Ion Sources (ILIS) in Materials Sciences 1. Focused ion beam (FIB) technology 2. Reactive ion etching 4. Implementation using porous metals

Focused Ion Beam (FIB) Applications ION SOURCE T~900 ºC LMIS Ion Implantation Lithography Scanning Ion Microscopy Localized engraving TEM sample preparation Mask repair T~20 C ILIS nanofabrication nano-imaging nanopores

Probe Size in FIB Probe size d defined as beam cross sectional dimension at target plane: C d 2 = I 3 s di + I ΔW 1/ 2 3 W πm 2 dω 2 C c 2 + M 2 D 2 di πm 2 dω I - current Ω - solid angle W - energy D - source size M - Magnification C s - spherical aberration coefficient C c - chromatic aberration coefficient In FIB, we want the smallest possible beam probe

ILIS Properties Adequate for FIB High brightness, contained beams Characterize influence of emitterextractor geometry and extraction polarity on ILIS beam profiles Beam stable with change in polarity ILIS in translational stage Visualization System Parabolic profile Single beam is required for FIB

ILIS Properties Adequate for FIB pure ionic regime narrow energy distributions (EMI-BF4) (BMI-I)I- I - ILIS operate at lower currents than LMIS EMI-BF 4 Ga (T=900 ºC)

Preliminary Work on ILIS FIB Ion optics column to scan beam over target Beam probe ~ 30 microns Potential for much smaller probes Non-optimized column (high aberration coefficients) Non-filtered ion beam Light ions for imaging, heavy ions for etching Negative ions to avoid target charging

Preliminary Work on ILIS FIB Retarding potential analysis (RPA) shows the signature of fragmentation (in this case, copious for the ionic liquid BMI-I): RPA data would look like the following if: No fragmentation at all No fragmentation during acceleration Because of polydispersity and heavy ion fragmentation, filtering would also be required for ILIS FIB: fragments

Reactive Ion Etching using ILIS LMIS (for FIB) or plasma (DRIE for semiconductor processing) require chemical assistance for enhanced etching LMIS beam plasma ions Volatile compounds Reactive gases Mask Sample Use ionic liquids with reactive ions (EMI-BF 4, BMI-I, EMI-GaCl 4 ) instead of reactive gas injection 15 kv Ion Gun

Reactive Ion Etching using ILIS Engraving on silicon through TEM mask using EMI-BF4 counts microns Native RMS: 0.32 nm Etched AFM on ILIS etched silicon RMS: 0.33 nm Sharp edge from crude, non-collimated ion beams No alteration of the as-grown silicon surface Uniform etching over beam area High etching rates (40 silicon atoms per incident ion)

Reactive Ion Etching using ILIS X-ray Photoelectron Spectroscopy (XPS) shows new chemical environment on the surface Evidence of reactive ion etching (explains enhanced rates) Potential for large throughput RIE applications using arrays of emitters Interesting experiments for the future (iodine-based IL on GaAs substrate)

Need to Understand ILIS and Interactions Based on field assisted ion evaporation: liquid vacuum j = σ kt h exp 1 kt ΔG e 3 E 4πε 0 E 1 10 9 V/m MD simulations to understand emission / fragmentation processes Interaction of ions and clusters with materials

Increase Throughput for Etching / Propulsion Each emitter produces very little current/thrust: I e 1 µa F e 0.1 µn An option is to manufacture densely packed arrays of single emitters on porous metals. High pure ionic current per emitter Fully passive flow with no valves/tubes/pressure Redundant high resistance liquid flow passages Large liquid/metal contact area for increased capacitance Expected performance as a thruster Electrospray (d = 300 μm) Electrospray (d = 100 μm) Ion Engine (NSTAR-class) Thrust density (N/m 2 ) Mass (kg/n) Volume (cm 3 /mn) 0.9 10 3.3 8.3 1 0.4 1.3 313 w/feed system 109 thruster only

Electrochemical Microfabrication on Porous Metals 1. Porous nickel substrate HCl moving cathode V 2. Deposit photo-definable resist UV light 5. Electrochemical etching 6. Remove mask 3. Define pattern with UV light 4. Develop resist and obtain mask

Etching Regimes Given that double layers change in real-time, the potential between electrodes also changes. It is necessary to measure the stability of the potential with respect to a calibrated probe. Potentiostatic scan (50 mv/s) on solid Nickel anode with 2N HCl Part A: Faradic etching Part B: Transport limited single reaction Part C: Secondary reactions begin to occur. probe

Etching Regimes Faradic Regime (A): Given Controlled that double by conductivity layers change Characteristic in real-time dimension the actual ~ Debye layer potential between electrodes, it is necessary to measure the stability of the potential with respect to a calibrated probe L f Metal Potentiostatic scan (50 mv/s) on solid Nickel anode with 2N HCl Part A: Faradic etching Part B: Transport limited single reaction Part C: Secondary reactions begin to occur. λ D L f >> λ D Etching occurs conformally to surface finish probe

Etching Regimes Transport Limited Regime (B): Potentiostatic scan (50 mv/s) on solid Given Controlled that double by transport layers of reaction products Nickel anode with 2N HCl change Characteristic in real-time dimension the actual ~ diffusion layer Part A: Faradic etching potential between electrodes, Part B: Transport limited single reaction it is necessary to measure the Part C: Secondary reactions begin to occur. λ stability of the potential with diff respect to a calibrated probe L f Metal L f << λ diff Smoothing by etching preferentially on protrusions probe

Etching Regimes Faradic Regime (A): Effect on solid metals Given that double layers change in real-time the actual potential between electrodes, it is necessary to measure the stability of the potential with respect to a calibrated probe Potentiostatic scan (50 mv/s) on solid Nickel anode with 2N HCl Part A: Faradic etching Part B: Transport limited single reaction Part C: Secondary reactions begin to occur. probe

Etching Regimes Transport Limited Regime (B): Effect on solid metals Given that double layers change in real-time the actual potential between electrodes, it is necessary to measure the stability of the potential with respect to a calibrated probe Potentiostatic scan (50 mv/s) on solid Nickel anode with 2N HCl Part A: Faradic etching Part B: Transport limited single reaction Part C: Secondary reactions begin to occur. probe

Etching Regimes Faradic Regime (A): Effect on POROUS metals Given that double layers change in real-time the actual potential between electrodes, it is necessary to measure the stability of the potential with respect to a calibrated probe Potentiostatic scan (50 mv/s) on solid Nickel anode with 2N HCl Part A: Faradic etching Part B: Transport limited single reaction Part C: Secondary reactions begin to occur. probe

Etching Regimes Transport Limited Regime (B): Effect on POROUS metals Given that double layers change in real-time the actual potential between electrodes, it is necessary to measure the stability of the potential with respect to a calibrated probe Potentiostatic scan (50 mv/s) on solid Nickel anode with 2N HCl Part A: Faradic etching Part B: Transport limited single reaction Part C: Secondary reactions begin to occur. probe

Etching in the Transport-limited Regime Critical to control: Voltage profile Paddle speed / agitation Solution concentration Geometry (electrode spacing) Etching times

Given that double layers change in real-time the actual Porous Nickel potential between electrodes, it is necessary to measure the stability of the potential with respect to a calibrated probe Etching Regimes Transport Limited Regime (B): Etching Emitters Potentiostatic scan (50 mv/s) on solid Nickel anode with 2N HCl Part A: Faradic etching Solid Nickel Part B: Transport limited single reaction Part C: Secondary reactions begin to occur. probe

Summary The properties of ionic liquid ion sources are suitable for applications in FIB, reactive ion etching and propulsion. Interesting for the large variety of ionic species available, including negative ions from a point source and intrinsic reactivity of ionic compounds. Throughput can be substantially increased with emitter arrays. Compact, densely packed arrays possible using porous metal emitters. Electrochemical microfabrication enables shaping of porous metal substrate to create such arrays.

Future Work Explore the experimental and theoretical limits of ILIS in FIB applications. Analyze individual components of ion beams, including fragmentation products through filtering. Continue to use MD tools to understand better the emission physics and interactions with materials. Explore the limits in clustering individual emitters in dense arrays. Extend the parameter space of electrochemical microfabrication for improved etching control. Develop models of flow transport and distribution in porous metal emitters. Further work still required on the electrochemical stability of ILIS.