Helium Ion Microscope
Scipioni, L., Stern, L. A., Notte, J., Sijbrandij, S., & Griffin, B. (2008). Helium Ion Microscope. Advanced
Materials & Processes, 166(6), 27-30.
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Download date: 15. Jun. 2017
HELIUM ION
MCROSCOP
Tiie physics of secondary electron
generation and ion backscattering
allows for the collection of image
information not possible with a
scanning electron microscope in
some applications.
First results are discussed here.
L Scipioni"", L A. Stern, j . Nolte,
S. Sijhrandij
H
tern such as that in Figure lc results, indicating the
position of the atoms and the crystal facets.
In the ALIS emitter, a tip reforming process causes
a small number of atoms to protrude more than the
others on the crystal axis (Fig. Ib). These become
tile only emitters; thus, they carry all of the ion current (Fig. Id). One emitter is selected to travel
through the ion optics, vielding a single atom emitter
with high brightness (B > 4x10" A - c m - srO- Tlie
electrostatic optics are theoretically capable of producing a focused probe with a diameter of 0.25 nm
(spot size down to 0.5 nm has been measured so
far.)
Carl Zeiss SMT Inc.
Peabody, Massachusetts
Material-sensitive imaging
In addition to imaging the topology of a surface or
particle, spatially resolved information is often
B. Griffin
needed for other properHes, such as chemical comUniirrsity of Western Australia
position, grain structure, density, conducti\'ity, and
ii, Atislnilia
the like. Acquiring such information in a charged
particle microscope requires that the beam interact
he Orion microscope is based on the first with the sample in specific ways.
Following are images that illustrate ways that the
commercially manufactured helium field
ion gun. Several attempts have been made helium ion microscope can provide high-sensitivity
to design a microscope based on the gas material information on surfaces.
Figiire 2a shows surface Information from gold
field ion source. Now, the Atomic Level Ion Source
(ALIS) technology provides a stable helium ion particles on carbon. The image shows a resolution
beam with extremely high brightness and thus a of about 2 nm. Notable is the amount of surface detail. This is due to the low energy of the secondary
sub-nanometer probe size on the sample.
The source is based on field ion emission from electrons (SE), mostly below 5 eV, so that the inforthe apex of a sharp needle held at a high positive mation is coming from just the top few nanometers.
voltage in the presence of a gas. The technology of Compare this to the SEM image in Figure 2b, where
the field ion microscope (FIM) is based on this phe- the surface character of the particles is completely
nomenon, and has itself been used in materials absent because of the higher energy SE's that
analysis. Figure 1 a shows an illustration of the typ- come from deeper below the surface, as well as tlie
ical hemispherical end-form of a needle of a single type II SE's. These serve to delocalize the probe
information.
crystal, and the multiplicity of ion beams emitted.
If these are collected on an imaging screen, a patThe helium ion microscope image also shows dis*Meinber of ASM International
tinct contrast levels corresponding to the grains in
T
Iff;
Fig. 1 — (a) Illustration (side viiiv) of the eiid-fonn of a typical field ion tip. (b) Illustration o/ the ALIS entitter. (c) Actual FIM image of the
geometry illustrated in la. (d) Actual FIM imageof ALIS emitter illustrated in Ib.
ADVANCED MATERIALS & PROCESSES/JUNE 2008
27
that involves modification of surface states.
An example of this is seen in Figure 3. This
sample (courtesy of A. Beyer, Bielefeld University,
Germany) is a chemically patterned, self-assembled
monolayer of nitrobiphenyl thiol (NBPT) formed
on a goldfilm.A contact eiectixm beam stencil mask
was placed over this layer. The mask had a 40-nm
frame within which were multiple electron transparent test windows in the form of circles and
ellipses.
Upon electron beam exposure, the terminal NO2
Fig. 2 — Gold particles U!U!\;CL! :ntli (a) a helium ion beam, (b) an electron beam. group at the NIBFT surface beneath these windows
Magnification is about 250.000X in each image. Vie arrows in the Orion image was reduced to NH2 — a change affecting just the
indicate afeiv ofthe many places ivhcre grain kmndaries are seen.
top atomic layer of the sample. Previously, this
change was orily detected in AFM in friction mixJe,
for there is neither topology nor detectable SEM contrast (Fig. 3b).
However, the helium ion microscope makes the
patterned areas highly visible, as demonstrated in
Figure 3a, Tbis capability opens tlie door to many
more opportunities to investigate such chemically
modified surfaces, Note also that it is possible with
both helium ion microscopy and SEM to image
through the organic monolayer to the gold film
below. Only the ion beam technology allows for deFig. 3 — NBPT monolayer overagoldfrlm. patterned with an electron beam mask,tection of both.
(a) Orion image, (b) SEM image.
Rutherford images
A unique feature of the Orion microscope is the
ability to acquire Rutherford backscattered ion images (RBI). This mode highlights materials in the
sample based upon atomic number "Z" contrast,
and provides complement<iry information to tlie SE
image. The helium ion microscope is capable of capturing both images simultaneously through two
parallel detection channels, so that identical conditions are assured for both.
For example, this capability may be utilized in a
materials failure analysis. Tlie images in Fig. 4 are
Fig. 4 — Metal whisker formed on a thi-coated copper electrical connector, (n) from a sample of a tin wliisker that has grown out of
Orion S£ image, shozrhig cimnges in contrast lei>els along the whisker, (b) Orion
an ordinary tin-coated copper electrical connector.
RBI image, showing little to no material contrast. Cren: arrows point at identical
These
whiskers grow over time and can eventtially
sites in each image where a transition is seen in the whisker.
extend from one electrical connector to another,
the material. Although grain boundaries are visible causing failures in electronics. Several space
in the SEM image, the resolution is compromised, satellites have even suffered partial or total loss
Acknowledgenients
and the contrast between the various grains is min- of functionality due to this phenomenon. Although
The authors thank
imal. Thus, an entirely new view of tliis material be- common and well-studied, the fundamentals
Korsten Rottond
behind the problem are still not completely
comes
possible with the helium ion microscope.
Andre Beyer of the
University of
Bielefeld, Germany,
who kindly
prepared and
provided the
patterned NBPT
monolayer somple
discussed herein,
and Gary Stupian of
The Aerospace
Corporation for the
tin whisker sample.
28
Another important aspect of imaging is chemical
or material contrast. How well can materials be differentiated in an image based on the SF signal? It
turns out that the SF yield is a more sensitive fujiction of material for an incident ion beam than for an
incident electron beam. Thus, in helium ion microscopy, the images of a given sample tend to display a greater range of gray levels than a corresponding SFM image. This may be attributable to the
fact tliat the energy loss for an ion beam at this energy is dominated by nuclear-nuclear scattering
rather than electron scattering.
The greatly increased surface sensitivity also is
beneficial for the investigation of surface chemical
states. Tliese factois could benefit the analysis of cataiysts, which depend on surface properties for their
behavior and also for any type of nano-patteming
undei-sttxjd.
The images in Fig. 4 a, b show complementary information from this whisker. The SE image from tlie
helium ion microscope shows the toptilogy of the object. The fact tliat tlie different sections ak>ng its lengtli
are at different contrast levels indicates that stime discrete changes take place as the whiskers grow. Tliese
changes were seen in several places, particularly
where the whiskers have a kink or bend.
The arrows ui the figuivs point to such areas. The
RBI image (also produced on the helium ion microscope), on the other hand, shows that the material
is uniform all along the length. Tliis hints that the
physical or electronic structure of tlie tin is changing
in some way, rather tlian a material variation. However, the RBI image does reveal that a different element (with a higher backscatter yield) is located di-
ADVANCED MATERIALS & PROCESSES/JUNE 2008
rectly at the transition points on the whisker. These
are seen at the two places indicated by arrows,
where the contrast is brighter in a small band. This
information could guide analysis of the growth dynamics, Note also that the many particles seen on
the whisker in the SE image are invisible in the RBI
image, indicating that they are most likely also tin,
the same as the whisker.
Penod Au
6
*
Elemental analysis
0.05
Tlie collection of backscattered helium ions also
10
20 30
40
50
60
70
80
90 too
Atomic number
provides the potential for quantitative analyses of
materials. Tlie intensity, energy spectrum, and an- Fig. 5 — This graph shows the helium backscatter rate for elemental targets.
gular distribution of the backscattered ions all cany Experimental (fwhits) and simulated (line) backscatter yields are shown for 25 keV
information about the sample under the primary helium ions in conducting elemental targets.
Lx'am. We are just beginning
to delve into the details of
these processes to derive
chemical element information.
BRUKER
The most easily accessible
information is the total intensity of the backscatter signal
as a function of the Z of the
BrukerAXS Handheld
target. It Ls generally tRie tJiat
the prt)bability of a backscatter event increases with
Z. We carried out a set of experiments to evaluate this. A
sample was prepared that
consisted of a copper Lilock
with 44 holes into which were
pressed, with indium as a
cement, pure elemental material from every conducting
element from boron to bismuth.
Positive Material Identification (PMI)
Relative measurement of
tlie RBI yield could be taken
by comparing tlie image gray
level from each of the samples. After controlling for
beam current variations and
collector efficiency variation
across the stage (via collection
of a reference copper area in
each image), we determined
tlie RBI yield curve plotted in
Fig. 5. Although the yield
generally increases with Z,
pnmiinent peaky are obviovLs,
one each in periods 4,5, and
6 of the periodic table. Tliey
correspond to the samples of
copper, silver, and gold. This
phenomenon is not predicted
in TRIM simulations, which
cia* shown by tlie solid line in
die plot. This behavior is as of
yet unexplained. The maxima
preclude a unique identification of an element based on
signal intensity, but the images still provide stime information about the make-up of
the sample.
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ADVANCED MATERIALS & PROCESSES/JUNE 2008
J
Handheld
29
10.000
Oxygen
Silicon
Copper
Gold
Au/Cu
1000
10
10
12
Backscattered
helium energij
spectra from thin
gold films on
substrates of
copper, silicon,
and glass (SiOi).
Also marked is
the expected
kinematic edge
for each of the
elements found
in these sauiples.
Primanj beam
energi/ was
b
14
18
20
22
Heliumtonenergy
Quantitative information
For more quantitati\'e infonnation, we turn to the
measurement of low energy Rutherford backscattering spectra. Tlie ion has a probability of undergoing a large angle scattering event through which
it is ejected from the material. The maximum energy of a backscattered ion is found theoretically for
the case in which the particles collide at the first
monolayer. Then the recoil energy is a fraction of
the incoming energy, as determined by kinematic
calculations: The higher the target atom mass, the
larger the recoil energy.
Ions, which penetrate more deeply into the material before backscattering, lose energy at a rate determined by the stopping power. Thus, collecting a
sufficient number of these ions with an energysensitive detector will reveal a spectioim with an
edge at the kinematically defined energy, and a tail
extending theoretically down to zero energy. The
shape and position of such a cur\'e can be utilized
to identify materials and measure layer thickness.
An example of our initial explorations into this
teclinique is illustrated in Fig. 6. Here we kxiked at
thin films of gold on substrates of copper, silicon, and
glass (SiO2). Calculatijig from the primary beam energy of 30 keV and the mass ofthe various materials
being probed, one would ideally expect edges in the
spectra at the line positions indicated on the plot.
Although tlie instnimentation factors broadened
the spectral features in this experiment, it is obvious
that we can both detect the substrate and distinguish the materials one from another. See for example the evidence of the oxygen in the glass
sample, which is absent from the silicon sample.
The double humps in these peaks can also enable
gold layer thickness determination.
As the technology is utilized by more researchers
utilizing more of the analytical options on the ttiol,
our understanding of tlie potential of this technique
will grow even further.
ۥ
For more infonnation: L. Scipioni, Carl Zeiss SM T Inc.,
One Corporation Way, Peabody, MA 01960; I.scipioni®
smt.zeiss.com; www.smt.zeiss.com.
Bibliography
1. V. N. Tondare, 1. Vac. Sci. Technol. A, 73 (6), p.l498 {20)5).
2. J. Morgan et. a\.. Microscopy Today, 14 (4), p.24 (2(X16),
3. M. K. MiUer and C. D. W. SmitlV, "Atom Probe Microanalysis: Principles and Applications to Materials Problems", Materials Research StKiety, Pittsburgh, ]9H9.
4. J.H. Richardson, and B.R. Lasley, "Tin Whisker Initiated Vacuum Metal Arcing in Spacecraft Electronics,"
1992 Govemment Microcirctiit Applications Conference,
Vol. XVIII, pp. 119 -122, November 10 -12,1992.
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