new generation nuclear microprobe systems: a new look at old problems by david n. jamieson...

New Generation Nuclear Microprobe Systems:

A new look at old problemsBy

David N. Jamieson

Microanalytical Research Centre

School of Physics

University of Melbourne

Parkville, 3010

AUSTRALIA

7th International Conference on Nuclear Microprobe Technology and Applications, Cité Mondiale, Bordeaux, France, September 11 2000

© David N. Jamieson 1999

Electron Emission from Surfaces

CVD B-doped diamond films are electrically conductive

Diamond has a negative electron affinity Potential applications as a cold cathode electron

emitter Measure : number of electrons emitted from

surface per ion impact Measure =15 to 30 (metals: = 1.5)

H H H

electrons

Incident ion

+

–

I–

H H H

electrons

Incident ion

+

–

I+

1.

.

RI

RI50 m

Yield

max

minRBSI–


Filiform Corrosion in Aluminium

200 m

Yield

max

min

Anticorrosion layer removed

Filiform growth

C- RBS

Al - RBS HeCl - PIXE O - RBS

Al- RBS

Cl growth head

Filiforms grow under breaches in the anticorrosion coating on Al 3 MeV H PIXE data confirms role of Cl in catalysing growth of the filiform


Menke’s Syndrome revisited

Menke’s Syndrome is a Cu deficency genetic disorder.

The gene responsible for the disorder has now been mapped.

Pathways for Cu metabolism within cells can now be controlled and studied with unprecedented precsion.

But can the nuclear microprobe cope?

Need to resolve Cu distributions within single cells to a spatial resolution of sub-micron.

Images here are by indirect immunofluorescence from anti-body labelled Menkes protein.

Cells are less than 10 micons in width


Outline

The quest for superior spatial resolution in the Nuclear Microprobe: Why has the probe resolution stalled at 1 micron for 2 decades?

Some new insights provide possible pathways to future progress Introduction to elementary ion optics

– Chromatic aberration - not a problem?

– Spherical aberration - not too much of a problem?

– Stray magnetic fields - definitely a problem

– Demagnification - the way forward

– Ion source brightness - small advances to be welcomed A review of the next generation systems Conclusion (Topics not addressed:

– High efficiency detectors, fast DAQ’s to handle high intensity beams,

– specimen damage,channeling convergence angle)


1 m wall

Chip feature size and NMP resolutionS

ize

(mic

ron)

Year

Moore’s Law

<1 pA

808

6

8038

6

P5

P6

P7

>100 pA


1 m

wal

l

Spatial Resolution Required:Applications published at the Last Conference 1998

“Pile up”


ImagePlane

Introductory Ion Optics

xi = (x/x)xo + (x/)o + (x/ )oo + (x/)o3 + (x/ 2)o o

2

yi = (y/y)yo + (y/) o+ (y/ )oo + (y/)o3 + (y/ 2)o

2o 2

…plus higher order terms

Object Plane Aperture Plane

(xo , o , yo , o , o )

(xi , yi )

Lens System

Magnification

(x/x)xo

(y/y)yo

Focusing

(x/)o

(y/) o

Chromatic

(x/ )oo

(y/ )oo

Spherical

(x/)o3 + (x/ 2)o o

2

(y/)o3 + (y/ 2)o

2o 2


Steps to evaluate lens system design: 1. Calculate magnification and coefficients from ion optics computer codes 2. Measure:

– Beam Brightness– Chromatic momentum spread from the accelerator (use nuclear resonance)

3. Set object size so that demagnified image is equal to desired probe resolution 4. Set aperture size so that beam current is equal to desired beam current 5. Calculate aberration contribution from maximum divergence and energy spread

6. Add contributions to probe size in quadrature (or similar) 7. Spot size is now greater than desired spot size so go back to 3 and choose a smaller object

size Repeat 4-7 until done.

dm = 2(x/x)xo|max

dc = 2(x/ )o |max o |max

ds = 2|(x/)o3|max + |(x/ 2)o o

2|max

How to calculate probe resolution?

di2 = dm

2+dc2+ds

2

Wrong!!


System (x/)m/mrad%mom.error

(y/)m/mrad%mom.error

MelbourneRussian Quadruplet

130 180

SingaporeOxford triplet

-340 870

LeipzigSeparatedQuadruplet

-470 -1500

Chromatic Aberration, A closer look

Singapore system achieves sub-micron probes with 15o switcher magnet that has low energy dispersion

Yet chromatic aberrations of this system should be large Skilled tuning of system is part of the answer, but not all! Maximum dc depends on getting maximum and in the same beam particle

dc = 2(x/ )o |max o |max

High excitation systems

© David N. Jamieson 1999 Divergence, o

En

erg

y S

pre

ad,

o

+

high

low

Chromatic Aberration, A closer look

Are and correlated? Use MULE* to find out. Here is a slice of object plane phase space taken along and System was the HIAF accelerator in Sydney (From the work of Chris Ryan)

Not much beam in the danger zone Beam intensity is peaked in the paraxial zone

Ionsource

Acc

eler

ator

Magnet Ray used in maximum dc calculation

Danger zone

Conclusions: Not much beam at

edge of phase space

Chromatic aberration is not a severe problem*Thank you G.W. Grime


Spherical Aberration, A closer look

Traditionally, spherical aberration is computed from the rectangular model (RM)

Rectangular model:

B(z) = 0 z < 0

B(z) = B0 0 < z < L

B(z) = 0 z > L Results from this model agree with ray tracing

codes that use B(r0 , z) measured at r = r0

Detailed studies have been done by Glenn Moloney

– Measured field profiles B(r , z) at several r– Provides 3-D profile of True Fringe Field (TFF)

Numerical raytracing from measured B(r , z) reveals different spherical aberration coefficients!

L z0

Coefficient RM TFFM

(x/ 2) -130 -130

(x/ 2) -390 +10

(y/ 3) -220 -190

(y/ 2) -390 +2


Spherical Aberration, A closer look

Coefficients calculated from the TFF model give aberration figures of different shapes compared to the rectangular model

The figure is more intense in the paraxial region - good!


Ion Source Brightness: Flux Peaking

Legge et al (1993) showed a 1 order of magnitude decrease in probe size required a 5 orders of magnitude increase in brightness for uniform model

True situation more complicated: 1 order of magnitude decrease in probe size requires 2 orders of magnitude increase in brightness

Uniform phase space

Set 5 nA

For 5 nA divergence is 2.5 times less than uniform model so spherical aberration is reduced by a factor of 16

100 m200 m

75 m

2 MeV He+

Cu

rre

nt (

pA

)


shadow

130mm 525mm

grid

Without magnet

With Magnet

Stray DC Magnetic Fields: Parasitic aberration

Non-uniform stray DC fields are a problem

Shadows of a line focus on a fine grid should be straight line

Small bar magnet has severe effect See large sextupole field

component aberrations Sources of stray DC fields in the

MARC laboratory:– Iron gantry and stairway over

the beam line– Steel equipment racks– Gas bottles– Stainless steel beam tube itself!


shadow

130mm 525mm

gridDeflect here

beam

beam

beam

beam

BEAM

PIPE

Stray DC Magnetic Fields: Aberrations of a beam pipe

Type 316 stainless steel beam pipe through quadrupole lenses

10 mm internal diameter Beam diameter 6 mm Grid shadow pattern reveals

aberrations See strong effect from different

deflections of the beam pipe! Effect here produced by a few cm

length What effect does 8 m have?


Stray AC Magnetic Fields: Beam spot jitter

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

xx

Bstray(t)object

virtualobject

Stray AC field causes a shift in the virtual object position The beam spot is scanned by the stray field in a complex

fashion

imageshift

hMh

http://www.meda.com/fm3page.htm

lens


Stray AC fields cause virtual movement of the object collimator

Used a 2-D scanwith y-coilsdisconnected

Gives position asa function of timein map of Cu x-rays

-2000-1000

01000

0 50 100

Time (s)

By (

nT

)

By

(nT

)

Stray AC Magnetic Fields: Beam spot jitter

3 m


Stray AC Magnetic Fields

Where: M = Magnification =

1/Demagnification q = beam particle charge L = Length of beam line E = beam energy m = beam particle mass

Em

LMqBx stray

i22

2

It is good to have: High demagnification systems Short systems

On the Melbourne system it is required that:

Bstray < 20 nT for xi < 0.1 m


Stray AC fields in MARC laboratory: Where from?

Field as a function of time tells the story Start: 6pm April 18 2000 Place: MP2 beam line, MARC laboratory

To MARC lab 50 m


Modify RF Ion Source

Beam from ion source emerges with low energy

Gas leakage from ion source canal fills low energy end of accelerator

Gas scattering degrades ion source brightness

Solution: Add recirculating turbopump

gas gas

T.p.

old new

From the work of Roland Szymanski


Modify Accelerator Column

Remove corona needles and replace with resistors

(Have now increased brightness by a factor of 10)

So need to design a system optimised for a flux peaked beam…

High demagnification!


Selected new quadrupole systems

1970 Russian quadrupletDx=Dy=30

1998 Leipzig separated quadruplet Dx=80 Dy=80

1998 CSIRO/MARC high excitation quintuplet Dx=67Dy=71

1980 Oxford high excitation triplet Dx=25 Dy=90

2000 Oxford separated triplet Dx=240Dy=50

2001 New systemDx=Dy=200?


125 4 3

Strong demagnification in a long system

CSIRO quintuplet system Leipzig two stage system

Strong demagnification in a short system, 80 mm WD

Very intense beam spot into 1 m


3 m at 20 nA

Resolution Versus Beam Current: CSIRO/MARC quintuplet system

1.3 m at 0.5 nA

Accelerator brightness =1.2 pA.m-2.mrad-2.MeV-1

12.7 m

1.2 m x 0.9 m at 0.1 nA

CSIRO-GEMOC Nuclear Microprobe

2.0 m at 10 nA

3100 pA/m2 !

1.8 m at 8 nA

12.7 m

From the work of Chris Ryan


Future Developments

Conclusion: To break through the 1 micron wall

Install heavier magnetic shielding! But be sure to clean off DC fields (10 nT).

Don’t worry about chromatic and spherical aberration, they are not a severe as first though because of flux peaking (<0.1 m)

Make brighter ion sources by small tweaks, even a factor of 10 is helpful (x1/3)

Install an optimised system for a strongly flux peaked accelerator, this will have a large demagnification (of necessity a high excitation system) (M-1 > 200)

Need more radical lens design to reduce working distance and increase fields (40 mm)

Apply the new system to some interesting problems! (< 0.1 m resolution)

MP2Bochum

Leipzig

Oxford tripletCSIRO 5

New Ox


Spherical Aberration: A closer look

The TFF model also revealed the need for careful attention to the field overlap between adjacent lenses

Must have a linear field gradient as a function of beam direction to minimise aberrations

Need to shape pole ends to achieve this

z

Pole tip

?

N

S N

S

Poletip

z

N

NS

S

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