composition and thickness dependence of secondary electron yield for mcp detector materials slade j....
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Composition and thickness dependence of secondary electron yield for MCP detector materialsSlade J. Jokela, Igor V. Veryovkin, Alexander V. Zinovev, Jeffrey W. Elam, Anil U. Mane, and Qing Peng
Argonne National Laboratory
TIPP 2011 June 9-14 - Slade J. Jokela
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Motivation
Large Area Picosecond Photo-Detector Project (LAPPD)– http://psec.uchicago.edu
“Frugal” Micro-Channel Plate (MCP) detectors– Modern technologies allow for the construction of MCPs from more
cost-effective components and synthesis techniques• Porous, non-leaded glass (not effective as MCP on its own)• Atomic Layer Deposition (ALD) to functionalize the glass with secondary
emissive material (MgO and Al2O3)
– ALD allows for conformal coatings to be placed on any exposed surface (not a line of sight technique)
TIPP 2011 June 9-14 - Slade J. Jokela
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Purpose
Characterize ALD-synthesized MgO and Al2O3
– Determine how secondary electron emission (SEE) is influenced by:• Surface chemical composition
– Direct effects of elements/compounds on secondary emission– Effects that dopants/defects have on secondary emission
• Surface structure/morphology– Effects from electron or ion bombardment
TIPP 2011 June 9-14 - Slade J. Jokela
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Instrumentation
Repurposed surface analysis equipment– Low Energy Electron Diffraction module (LEED)
• Surface structure/morphology• Auger Electron Spectroscopy (AES)• Modified for Secondary Electron Emission (SEE) studies
– Pulsed electron source and secondary electron collection– X-ray Photoelectron Spectroscopy (XPS)
• Mg Kα– Ultraviolet Photoelectron Spectroscopy (UPS)
• He I & He II UV emission– Hemispherical Energy Analyzer
• Acquisition of XPS and UPS electron energy spectra– Ar+ source (up to 5 keV)
• Surface cleaning and depth profiling
TIPP 2011 June 9-14 - Slade J. Jokela
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Instrumentation
Ar+ Gun(obscurred)
X-ray Source(Mg Kα)
HemisphericalElectron Energy
Analyzer
HeatedSample Stage
He UV Source
Electron Gun(0 to 1 keV)
LEED Module(Repurposed
For SEY)
TIPP 2011 June 9-14 - Slade J. Jokela
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Study of Secondary Electron Yield vs.Material and Thickness
ALD-synthesized MgO vs. Al2O3
– Maximum gain and its corresponding energy generally increase with thickness
0 25 50 75 100 125 150 175 200 2252
3
4
5
6
7
Max. Gain vs. Thickness MgO Al
2O
3
Ele
ctro
n G
ain
(sec
onda
ries
/pri
mar
y)
Sample Thickness (Å)
0 200 400 600 8000
1
2
3
4
5
Ele
ctro
n G
ain
(sec
onda
ries
/pri
mar
y)
Primary Electron Energy (eV)
MgO 40Å 30Å 20Å
Al2O
3
40Å 30Å 20Å
TIPP 2011 June 9-14 - Slade J. Jokela
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Study of Secondary Electron Yield vs.Material and Thickness
Maximum in emission vs. thickness– Maximum escape length for a
secondary electron Simulation Effort
– Parameters from my measurements are used in MCP testing and simulation
• Matthew Wetstein (ANL)• Zeke Insepov (ANL)• Valentin Ivenov (FNAL) 0 25 50 75 100 125 150 175 200 225
2
3
4
5
6
7
Max. Gain vs. Thickness MgO Al
2O
3
Ele
ctro
n G
ain
(sec
onda
ries
/pri
mar
y)
Sample Thickness (Å)
TIPP 2011 June 9-14 - Slade J. Jokela
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Secondary Electron Yield vs. Surface Composition
5 keV Ar+ was used to sputter-clean the sample
0 200 400 600 800
0
1
2
3
4
5
6
MgO
Before Ar+ Sputter
After Ar+ SputterAl
2O
3
Before Ar+ Sputter
After Ar+ Sputter
E
lect
ron
Gai
n (s
econ
dari
es/p
rim
ary)
Primary Electron Energy (eV)
TIPP 2011 June 9-14 - Slade J. Jokela
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Secondary Electron Yield vs. Surface Composition
XPS spectra show a decrease in surface carbon– Double oxygen and carbon peaks in MgO imply a C-O-type compound
800 600 400 2000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Before Sputter After Sputter
Nor
mal
ized
Int
ensi
ty
Binding Energy (eV)
295 290 2850.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Carbon 1s
540 538 536 534 532 530 528 5260.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 Oxygen 1s
800 600 400 200
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Before Ar+ Sputter
After Ar+ Sputter
Nor
mal
ized
Int
ensi
ty
Binding Energy (eV)
295 290 285 2800.30
0.32
0.34
0.36
0.38
0.40
0.42
0.44Carbon 1s
540 538 536 534 532 530 528 526
0.5
1.0
1.5
Oxygen 1s
XPS Spectra of ALD-grown MgO XPS Spectra of ALD-grown Al2O3
O
O
CAl
O
O
C
Mg
TIPP 2011 June 9-14 - Slade J. Jokela
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Secondary Electron Yield vs. Surface Composition
Carbon contamination seems to affect SEY– Sources of carbon include atmospheric and remnants of ALD
precursors.– Emission increases when it is removed from MgO– Emission decreases when it is removed from Al2O3
– The difference can be explained by:• Carbon species is less emissive than MgO but more emissive than Al2O3
• The difference in bonding on MgO, as is evident in XPS spectra, results in an increase in work function
• Surface structure and composition is drastically altered by ion bombardment
– Preferential ion-sputtering– Ion Mixing– Recrystallization?
TIPP 2011 June 9-14 - Slade J. Jokela
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Study of SEY for New Materials
Increased emission could be beneficial for MCPs– Other materials and structures should be considered
0 200 400 600 8000
1
2
3
4
5
6
7
E
lect
ron
Gai
n (s
econ
dari
es/p
rim
ary)
Primary Electron Energy (eV)
20nm MgO As Received Ar-ion Sputtered
20nm MgO + 1 Cycle TiO2
As Received Ar-ion Sputtered
20nm MgO+10% TiO2 Mixture
As Received Ar-ion Sputtered
TIPP 2011 June 9-14 - Slade J. Jokela
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Study of SEY for New Materials
TiO2 is reported to have an emission yield of about 1.2
Ar+ sputtering will not just remove material, but mix it– This Ti could behave as a dopant after mixing, decreasing work
function
800 600 400 200 00.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Rel
ativ
e In
tens
ity
Binding Energy (eV)
Before Ar+ Sputter
After Ar+ Sputter
545 540 535 530 525
0.0
0.5
1.0
1.5 Oxygen 1s
294 292 290 288 286 284 282 280
0.00
0.05
0.10
0.15
Carbon 1s
470 468 466 464 462 460 458 456 454 452 450
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
Rel
ativ
e In
tens
ity
Binding Energy (eV)
Before Ar+ Sputter
After Ar+ Sputter
XPS Spectra of ALD-SynthesizedMgO + 1 Cycle TiO2 Ti 2p XPS Peak
O
O
CTiMg
TIPP 2011 June 9-14 - Slade J. Jokela
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Summary
MgO clearly has higher emission than Al2O3
Ar+ sputtering produces different results in the two materials– A difference in carbon species on the surface may be responsible– Ion bombardment certainly changes surface morphology
An initial study on MgO and TiO2 layered materials proved interesting– As-synthesized samples showed lower emission than MgO alone– After Ar+ sputtering, all samples showed an increased emission
• MgO with 1 ALD-cycle of TiO2 showed the highest emission of all (post-ion-sputter)
• Ar+ sputtering induces mixing in the surface components– Structural changes or doping could result
TIPP 2011 June 9-14 - Slade J. Jokela
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Future Work
Inspection of surface composition and structure vs. electron exposure– Ageing in MCPs
Ion-induced surface reconstruction– Ion mixing or surface recrystaliztion (MgO + TiO2 study)
Instrumentation for this study (all-in-one)– STM– SEM– Scanning AES (Auger electron spectroscopy)– Heated stage for thermal desorption studies– Gas exposure to determine effects of atmospheric gasses on “cleaned”
surfaces
TIPP 2011 June 9-14 - Slade J. Jokela
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Thank you
ALD-Synthesis performed by:
Jeff Elam, Anil Mane, Qing Peng (Argonne National Laboratory)
Instrumentation and scientific expertise/support:
Igor Veryovkin and Alexander Zinovev (Argonne National Laboratory)
LAPPD Collaboration
Argonne National Laboratory's work was supported by the U. S.Department of Energy, Office of Science, Office of Basic EnergySciences and Office of High Energy Physics under contractDE-AC02-06CH11357.