Charging Effects in Soft X-Ray Microscopy
A. Ranck, K. Berens v. Rautenfeld, M. Fornefett, J. Friedrich, M. Pretorius,
M. Schroeder, V. Wedemeier, J. Voss
II. Institut für Experimentalphysik, Universität Hamburg, Germany
E-mail: ranck@x4u2.desy.de
Abstract. We show how charging effects can be used as a contrast mechanism
in X-ray microscopy. With the tunable discharging of the sample surface by
X-ray produced electrons charging can be limited. Utilizing the charging
effect in combination with different incident photon energies, dynamic effects
occur which can be used to measure the attenuation length of the sample
material within an absorption line. Techniques to image conducting structures
buried under an insulating protective layer are discussed as well as to image
lateral thickness homogeneities of insulating protective layers on top of a
conducting substrate. Attention has to be paid when taking excitation spectra
of insulators due to dynamic effects.
1 Charging Effects in Electron Microscopy and XPS
Charging effects do occur if materials with low electrical conductivity are investigated
with charged particles or particles leading to photoelectron emission. In electron
microscopy charging effects are complex phenomena since positively and negatively
charged zones can exist depending on the electron energy applied [1]. Non-conducting
samples prepared for electron microscopy usually have to be evaporated with a thin,
conducting coating and are investigated with low currents. These techniques lead to
images with very good resolution, but it is difficult to gain information about the inner
structure of a thick, insulating sample.
In X-ray Photoelectron Spectroscopy (XPS) non-conducting samples become
positively charged due to escaping photoelectrons. During illumination the charge
increases and further photoelelctrons are retracted by the sample. Since spectral
structures would shift during the measurement and destroy the spectral information,
also in XPS dynamic charging effects have to be avoided.
Different techniques are used in order to reduce an existing charge. A flood gun
can neutralize the surface but might not reach charges inside the sample produced by
X-rays because of the different attenuation lengths of photons and electrons. Thin
samples on a conducting substrate can be used in order to avoid charges in deeper
areas. But in spite of this they might have a high potential gradient ∂V/∂z normal to
the surface which can lead to photoelectron deflection or even ion migration [2].
2 Charging Effects in Scanning X-Ray Microscopy
2.1 Experimental Setup
The X-ray microscope at HASYLAB uses monochromatic radiation between 15 eV
and 1500 eV. Two mirror objectives can be interchanged, a Schwarzschild objective
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working below 30 eV and producing a focus of about 100 nm diameter and an ellipsoidal mirror providing a lateral resolution of 1 µm in the full energy range. The
sample is scanned inside a UHV chamber with a measured accuracy of 40 nm. The
microscope works in a microspectroscopic as well as in a spectromicroscopic mode.
This means, it is possible to gain spectroscopic information from single spots of focus
size as well as to image the sample using any contrast mechanism visible in these
spectra.
In order to investigate dynamic effects like charging it is necessary to have well
defined start and stop events of illumination. For this reason a computer controlled
beamchopper has been implemented just in front of the entrance pinhole of the
microscope as shown in Fig. 1. In single-switch mode it reaches up to 20 beam
switches per second, in resonance mode more than 200 Hz can be achieved. The
typical time constant for charging effects in our intensity range of about 108 to 109
photons per second turned out to be in the order of seconds. Thus the achieved time
between a fully closed beam cross-section and a fully opened cross-section of about
10 - 250 µs dependeing on the pinhole size is short enough.
Fig. 1. Setup of the X-ray microscope for detection of charging effects
The charging process is observed using the photoelectron signal of a time-of-flight
detector. By this way electrons of all energies are detected simultaneously and with a
high, constant efficiency. In most cases the charging process is observed by detecting
all electrons moving into the opening angle Ω of the detector (total electron yield
signal without light peak). By selecting a section of the time spectrum images can be
taken using only photoelectrons of certain energies. This can lead to strong contrast
variations in the images. Even the light peak produced by scattered X-rays can be used
as an imaging contrast showing topographic structures of the sample. An overview of
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the experimental setup is given in Fig. 1; further details are described in our
publication [3] of this volume.
Photons of the incident beam hit the entrance aperture of the X-ray objective and
produce a cloud of photoelectrons. These electrons are attracted by a charged sample,
discharging ist surface. Therefore the objective has been installed electrically
insulated and can be set to a potential VObj. In addition a metal shielding of the optics
is implemented. Most of these electrons can be retracted by applying a potential of
about 10 to 30 V without disturbing the measurement. Potentials below 10 V allow
some electrons to reach the sample surface. Thus the charging process of an insulator
can be limited and artifacts due to electrons being deflected by high potential
gradients can be avoided. Compared to a flood gun this method has the advantage that
charging and discharging processes are coupled, thus an overcompensation during a
break of illumination can be avoided. Still it must be kept in mind, that according to
[2] only surface charges can be neutralized whereas buried charges might remain,
leading to internal potential gradients.
2.2 The Charging Effect
A sample is placed in the focal plane
of the objective. At a time t0 the
beamchopper is opened and photoelectrons are detected without moving the sample. As can be seen in
Fig. 2 in case of a grounded, conducting sample the signal stays
constant, whereas in case of an
insulator the signal decays to zero
level if Vobj is set to an appropriate
potential. A bad conductor would
produce a signal in between the two
curves ending up in a constant level
above zero due to internal currents.
Thus the time dependency of the
signal depends on the conductivity
of the sample, the photon intensity
and energy. An image of a sample
consisting of these two compounds
would have a low contrast if taken
during the first illumination of the
sample and a much better contrast if
taken after an illumination of several
seconds (assuming equal electron
yields at t0). However, if conducting
parts of the sample are not connected with ground level they also
become charged. But in many cases
Fig. 2. Measured photoelectron yield
Fig. 3. Photoelectron image of aluminum
islands on a glass substrate. Some islands are
grounded, others have a week or even no
electrical contact to ground level. The dark
lines and the dark area in the upper middle
show the glass surface
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these conducting areas are much larger
than the focus size, so they still can be
discharged by electrons originating from
the X-ray objective as described above, a
flood gun or by leak currents through the
sample.
The image in Fig. 3 shows a part of an
old mirror (Al on a glass substrate) with a
corroded surface. The dark lines show
areas where the aluminium has vanished,
the bright fields show grounded aluminium
areas, whereas the darker fields show
aluminium islands not grounded or in bad
Fig. 4. Simplified illustration of measucontact with ground leven and thus getting
red data
charged. The same principal can be applied
e.g. for testing chip architectures which will be done in future. Instead of grounding a
conducting structure different potentials can be applied to control the contrast between
the areas to be observed.
2.3 Sample Discharging
An insulating sample is irradiated beginning at time t0 and irradiated until an
equilibrium state is reached. If discharging is allowed, the photoelectron signal
decreases to an equilibrium level above zero. This level characterizes the discharge
current. At t1 the beamchopper is closed and the monochromator driven from energy
E1 to E2. Both energies should be close enough to each other in order to gain similar
photon intensities. At t2 the beamchopper is opened again. The signals observed are
shown in Fig. 4:
a) If E1= E2 (break of illumination) the total electron yields at t1 and t2 are equal.
Since the sample is a non-conductor the charge does not change during a break of
illumination.
b) If the attenuation lengths d of E1 and E2 are similar also no effect is visible.
c) If d(E2) < d(E1) the signal again starts from a high level and decreases
approximately to the old equilibrium state. An additional charging process
occurred due to a higher absorption coefficient in the surface.
d) If d(E2) > d(E1) the signal starts from a lower level and increases approximately to
the old equilibrium state. This means, a discharging process occurred in the surface.
The described effects can be used e.g. to measure the attenuation length of the
sample material within an absorption line. The next step will be to measure samples
having a non-conducting layer on top of a conducting substrate. If the thickness s of
the top layer is chosen in a way that d(E1)<s<d(E2) or d(E1)>s>d(E2), it should be
possible to measure time dependencies due to an internal discharging process. If the
attenuation length is large enough to reach the conducting structure, a photo current
should occur. If the attenuation length is well known, the thickness of the nonconducting layer might be measurable. In this case the contrast mechanism can be
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used to image the lateral thickness homogeneity of an insulating layer. On the other
hand it should be possible to image a conducting structure buried under an insulating
(protective) layer, which can not be achieved by electron microscopy. Charging
effects also become important if excitation spectra are taken by varying the incident
photon energy. Due to the charging and discharging effects, spectra taken with
different times per bin (different dE/dt) do look different. Only spectra taken slow
enough to keep the charge equilibrium yield reproducible information.
3 Conclusions
Insulators are interesting objects to be investigated by a soft X-ray scanning
microscope. Utilizing the charging effect as a contrast mechanism it is possible to
image zones of different electrical potentials or conductivities which might be
interesting for testing chip architectures. With the tunable discharging of the sample
surface by X-ray produced electrons, the amount of charges can be limited and thus
artifacts avoided. We have shown how the discharging effect can be used to measure
attenuation lengths of a material utilizing different photon energies. By applying the
discharging effect as a contrast mechanism it seems to be possible to image the lateral
thickness homogeneity of an insulating layer on top of a conducting substrate. It also
seems to be possible to image conducting structures buried under a thin, nonconducting protective layer. If excitation spectra of insulators are taken, dynamic
effects have to be avoided by long enough bin times (small dE/dt) in order to gain
reproducible information.
Acknowledgments
This project is supported by the German Minister of Education and Research (BMBF)
under contract number 05644GUA 9.
References
1
L. Reimer: Scanning Electron Microscopy, Springer Series in Optical Sciences
Vol. 45, 119, Springer Verlag (1985).
2
J. Cazaux, P. Lehuede: J. of Electron Spectroscopy and Related Phenomena 59,
49–71 (1992).
3
J. Voss, K. Berens v. Rautenfeld, M. Fornefett, J. Friedrich, C. Kunz,
M. Pretorius, A. Ranck, M. Schroeder, V. Wedemeier: X-ray Microscopy V,
Proceedings of the Conference XRM 96 (this volume).