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 PengArgonne 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 Al2O3

Elec

tron

Gai

n (s

econ

darie

s/pr

imar

y)

Sample Thickness (Å)

0 200 400 600 8000

1

2

3

4

5

Elec

tron

Gai

n (s

econ

darie

s/pr

imar

y)

Primary Electron Energy (eV)

MgO 40Å 30Å 20Å

Al2O3

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 Al2O3

Elec

tron

Gai

n (s

econ

darie

s/pr

imar

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+ Sputter

Al2O3

Before Ar+ Sputter After Ar+ Sputter

El

ectro

n G

ain

(sec

onda

ries/

prim

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

Inte

nsity

Binding Energy (eV)

295 290 2850.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14Carbon 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

Inte

nsity

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

1.5Oxygen 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

El

ectro

n G

ain

(sec

onda

ries/

prim

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

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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

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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

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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.