new generation nuclear microprobe systems: a new look at old problems by david n. jamieson...
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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–
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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)
© David N. Jamieson 1999
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
© David N. Jamieson 1999
1 m
wal
l
Spatial Resolution Required:Applications published at the Last Conference 1998
“Pile up”
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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!!
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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!
© David N. Jamieson 1999
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
)
© David N. Jamieson 1999
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!
© David N. Jamieson 1999
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?
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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!
© David N. Jamieson 1999
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?
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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
© David N. Jamieson 1999
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
© David N. Jamieson 1999
© David N. Jamieson 1999
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