Characterization of Secondary Emission Materials for Micro-Channel Plates S. Jokela, I. Veryovkin, A. Zinovev
Secondary Electron Yield Testing Technique We have incorporated XPS, UPS, Ar-ion sputtering, and SEY measurements into one high-vacuum system Tested Materials Al 2 and MgO for emissive materials Au for calibration of our system Electron-Dose Effect Emission changes as a function of electron fluence Exploring different techniques to examine what s changing Chemistry and composition Morphology Discussion and Summary
SEY Testing Setup Low energy electron diffraction (LEED) setup Electrons are emitted at constant energy (95 ev) Sample is biased using a computer-controlled Keithley Sourcemeter Bias is adjusted to allow for primary electron energy ranges between and 95eV Beam current (I Beam ) is determined at beginning of scan and set as a constant E electron = 95eV I Beam Yield Yield I V I I Collector Collector Beam I Sample Beam V I Sample 1 Sample I I Beam I Collector -95 to V bias
Gold Standard Ar + -ion sputtering affects both surface composition and morphology C and O, as well as unobserved surface features, may be responsible for the difference in secondary electron yield SEY of Gold XPS of Gold 2. 1.8 4 3 Ar + -cleaned Au Secondary Electron Yield (per primary) 1.6 1.4 1.2 1..8.6.4.2 Gold Foil Uncleaned Ar-ion sputter cleaned Count Rate (s -1 ) 2 1 15 1 5 O Uncleaned C. 2 4 6 8 5 4 3 2 1 Binding Energy
Gold Standard Results are comparable to literature and calculations 2. 1.8 Secondary Electron Yield (per primary) 1.6 1.4 1.2 1..8.6.4.2. Gold Foil Uncleaned Ar-ion sputter cleaned 2 4 6 8
Gold Standard UPS spectrum using 21.22 ev helium emission and a -5V sample bias 5d 5/2 located at ~4.3eV binding energy (with respect to E f ) 5d 3/2 located at ~6.1eV binding energy (with respect to E f ) Work function = 4.42eV (does not account for detector work function ) Previous tests have shown analyzer resolution of about 1eV 6 Inelastic Peak 5-5V Bias UPS of Gold 14 Count Rate (s -1 ) 4 3 2 12 1 8 6 4 2 1 8 6 4 2 E f =4.42 ev 1 5d 3/2 5d 5/2 25 2 15 1 5 Binding Energy (ev)
MCP Secondary Electron Emission Materials Films are deposited using Atomic Layer Deposition (ALD). Deposited on conductive Si substrates. Various thicknesses have been and will continue to be studied. So far Al 2 and MgO have been tested.
Al 2 Emission vs. Thickness and Electron Fluence Al 2 was provided by Jeff Elam s group (Qing, Anil) 4. 4. Secondary Electron Yield (per primary) 3.5 3. 2.5 2. 1.5 1..5. Sample S1 Al 2 min 4 min 9 min 14 min 18 min 2 4 6 8 Secondary Electron Yield (per primary) 3.5 3. 2.5 2. 1.5 1..5. Sample S3 Al 2 min 4 min 9 min 13 min 19 min 24 min 28 min 33 min 37 min 43 min 2 4 6 8 Al 2 Film thickness S1 5.5nm, S3 11.3nm
Al 2 Emission vs. Thickness 3.5 Selected Data Averaged Si substrate may affect SEY, especially for films less than 1nm Long-term monitoring or high-fluence electron exposure will determine the final values of these curves. Secondary Electron Yield (per primary) 3. 2.5 2. 1.5 1..5 S1 S3 S1 Std. Dev. S3 Std. Dev.. 2 4 6 8 Al 2 Film Thickness S1 5.5nm, S3 11.3nm
XPS of Al 2 3 25 O 3 2 1 2 544 542 54 538 536 534 532 53 Count Rate (s -1 ) 15 1 F 6 55 5 45 4 35 Al in Al 2 3 3 295 29 285 28 275 5 Al 2 S3 Carbonaceous Species 7 6 5 4 3 2 1 Binding Energy (ev)
MgO 7 6 Secondary Electron Yield (per primary) 5 4 3 2 1 min 4 min 9 min 13 min 18 min Secondary Electron Yield (per primary) 6 5 4 3 2 1 min 4 min 13 min 18 min 23 min 28 min 32 min 41 min 2 4 6 8 2 4 6 8 MgO Film Thickness S5 (left) 7.2nm, S9 (right) 14.4nm
MgO Not nearly as large of a difference between samples as was seen in the Al 2 samples. This experiment should be pursued further to determine if the similarity in emission is real. Examining the electron-dose effect may help us here Secondary Electron Yield (per primary) 6 5 4 3 2 1 S5 S9 S5 Std. Dev. S9 Std. Dev. 2 4 6 8 MgO Film Thickness S5 7.2nm, S9 14.4nm
XPS of MgO Presence of multiple carbon compounds are evident One is most likely a carboxyl, based on double oxygen peak near 531 ev 16 14 12 O 15 1 5 Count Rate (s -1 ) 1 8 6 F 545 54 535 53 Carbonaceous Species Mg 4 2 MgO S9 7 6 5 4 3 2 1 Binding Energy (ev)
Overall Comparison MgO is clearly a better emitter, especially for higher primary electron energies. With the amount of variation seen in prior samples, MgO is comparable to Al 2 for lower primary electron energies. Sample charging appears to occur for MgO and Al 2 Increasing temperature will increase sample Secondary Electron Yield (per Primary) 5. 4.5 4. 3.5 3. 2.5 2. 1.5 1..5 7nm MgO 6nm Al 2 Gold foil >1 m thick. conductivity 2 4 6 8
Electron-Dose Effect Al 2 Emission decreases with increased fluence MgO Emission increases with increased fluence We will explore why this is the case initially using XPS and SEM Focused electron beam from LEED system does not cover a large enough area for our XPS system to detect any chemical or compositional changes. Defocused electron beam from separate gun has been used. However, an unexpected increase in fluorine is observed in XPS spectra for electron exposure. Mass spectrometry should be used to detect material liberated from the sample
Ultraviolet Photoelectron Spectroscopy of Al 2 and MgO Valence band edge MgO 6.97eV Al 2 8.4eV 4 35 3 MgO Al 2 25 Count Rate 2 15 1 5 2 15 1 5 Binding Energy (ev)
Future Work and Additional Techniques/Equipment Preparing for large-area, electron bombardment Monitor and study does effect Will monitor SEY as a function of sample temperature Writing control software (LabView) to integrate all systems into one control system Ideally, we would like to have complete control over the lens system for the hemispherical analyzer Optimize energy resolution of XPS and UPS Designing/preparing new sample holder that is compatible with transmission photocathodes, sample heating, and can hold at least one sample for long term storage (faraday cup) Exploring options for Secondary Ion Mass Spectrometer (SIMS) Examining of doping profiles in photocathodes Programmable thermal desorption Electron stimulated desorption