An Improved Microprobe
Using Direct Undulator Radiation
M. R. Weiss1 , V. Wüstenhagen2 , C. Heske1 , R. Fink1 , E. Umbach1,2
1
Universität Würzburg, Experimentelle Physik 2,
Am Hubland, D-97074 Würzburg, Germany
2
Universität Stuttgart, 4. Physikalisches Institut,
Pfaffenwaldring 57, D-70550 Stuttgart, Germany
Abstract. The concept of a microspectrocope using direct undulator
radiation is described. This instrument utilizes the quasi monochromatic
beam from the first undulator at BESSY without monochromatization.
Thus X-ray induced Auger spectroscopy as well as photoemission using
the 2 eV wide first harmonic of the undulator are possible. Spatial resolution is achieved by an adjustable aperture in front of the sample yielding
a diffraction limited resolution of 3–4 µm. An improved concept using
an ellipsoidal mirror is described resulting in a spatial resolution in the
submicrometer range. As an example a microanalysis of Cu(InGa)Se2 , a
material used for thin film solar cells, is briefly discussed.
1
Introduction
Microscopes which provide spectroscopic information are presently under development because of their benefits in materials research. Especially photoelectron
spectroscopy has been implemented since one can analyze not only the composition but also the chemical state of a sample surface. The spatial resolution is
obtained either by imaging the emitted photoelectrons or by scanning focused
photons over the surface. Both methods, scanning or imaging, have specific advantages and disadvantages. The imaging method is very well suited for surveys
and spatially evolving patterns since it gives an image of the surface for a certain
electron energy in short time. Experimental setups based on this concept have
reached the highest spatial resolution today [1, 2, 3, 4]. This is mostly due to the
fact that in the past many efforts were concentrated on the imaging technique
in electron microscopy.
The scanning method, on the contrary, allows to direct all photons onto the
spot in question, thus providing faster spectroscopic analysis of this spot with
better signal-to-noise ratio. It is, in principle, as fast as imaging provided that
the time for the scanning motion is short. In the scanning mode one can also use
additional analysis techniques, for example mass spectroscopy of photo-desorbed
atoms [5].
X-Ray Microscopy and Spectromicroscopy
Eds.: J. Thieme, G. Schmahl, D. Rudolph, E. Umbach
c Springer-Verlag Berlin Heidelberg 1998
III - 16
M. R. Weiss et al.
The focusing of a UV- or X-ray spot can be achieved by Fresnel micro zoneplates [6], by normal incidence X-ray mirrors, e.g. in Schwarzschild geometry
with multilayer coatings [7, 8], or by grazing incidence, (often) non-spherical
X-ray mirrors [9, 10].
The photon sources for these microscopes are dipole magnets or insertion
devices at synchrotron radiation facilities. Present laboratory sources do not
provide the flux and brightness necessary for such low numerical aperture optics. For high spectral resolution X-ray monochromators are used which reduce
the flux considerably. To overcome the shortcomings of other concepts, namely
low flux, no adjustable photon energy and no availability of other methods of
analysis, we have developed a microspectroscope of the scanning type which is
described in the following.
For very high flux and hence short measurement times we use direct undulator light. This source is quasi-monochromatic by interference effects of its periodic magnet structure. Since the linewidth is rather broad (here: ∆E ≈ 2eV ),
this microspectroscope is best suited for X-ray induced Auger electron spectroscopy (XAES) for which monochromaticity is not required. Still, X-ray photoemission (XPS) analysis is possible, with reduced energy resolution. The specific advantages of this concept which we named Photon Induced Scanning Auger
Microscope (PISAM) are discussed in more detail in section 2.
For first tests, focusing was achieved by a simple aperture which demonstrated the possibilities of the concept and yielded first results. This apparatus
and one example of an application are also described in section 2. For improved
spatial resolution a focusing optics based on two grazing incidence mirrors was
designed (see Fig. 1).
Electron-analyzer
Storage Ring
eSample
Wiggler/Undulator
Condenser Mirror
Field Aperture
Focussing Mirror
Fig. 1. Scheme of the new PISAM II concept. The condenser mirror increases the
flux at the field aperture and the ellipsoidal focusing mirror provides the high spatial
resolution.
An Improved Microprobe Using Direct Undulator Radiation
III - 17
Although the highest possible spatial resolution has not yet been demonstrated for a grazing incidence ellipsoidal mirror it was chosen because it allows
to focus a broad range of photon energies. Thus the proper conditions can be
selected by the experimentalist, e.g. to gain higher count rates or additional information by resonant spectroscopy. The design considerations and first results
of the improved setup can be found in section 3.
2
2.1
The First Apparatus: PISAM-1
Description of the Setup and Its Performance
The first experiment using this concept was conducted with a set of apertures
which reduced the direct undulator beam down to a width of approximately
4 µm. The sample was mounted on a scanning stage motorized by steppermotors
(1.25 µm per step). The excited photoelectrons were detected by a VG CLAM2 spherical sector analyzer. A preparation chamber with standard equipment
(sputter gun, mass spectrometer, etc.) and a sample transfer system completed
the apparatus [11, 12, 13].
We performed several experiments to characterize the potential of this simple
setup [14, 15, 16]. Measurements of microstructured samples demonstrated a
photon energy-dependent spatial resolution of 15 µm at 45 eV and 3-4 µm at
280 eV. The accessible photon energies ranged from 40 eV to 550 eV covering
the absorption edges of sulfur, carbon, nitrogen, and oxygen.
The surprisingly good resolution regarding the simple setup and the diffraction limit is partly due to the proximity of the sample to the aperture (12
mm). The resulting diffraction is of the Fresnel type which leads to a smaller
spot than expected by Fraunhofer diffraction calculations [13, 14]. One might
object that the spatial resolution of conventional electron-induced Auger microscopes (SAM) is well beyond this resolution (typically 50-200 nm, even 5 nm
reported [17]), but the comparison of spatial resolutions leaves out other important aspects. SAM resolutions of this kind can only be reached by highly accelerated electrons (10 keV up to several 100 keV). Unfortunately, these electrons
contribute to a large background of inelastically scattered electrons which reduce the signal-to-background ratio (S/B) considerably. For good spectroscopic
analysis longer sampling times are hence required. Additionally, sensitive adsorbates and samples cannot withstand the required high fluxes and energies in
SAM instruments. Photons as excitation source have the advantage that much
less damaging secondary electrons are produced, and hence that less background
and a better S/B ratio result. This can even be improved by tuning the photon
energy to a resonance or threshold which also leads to additional spectroscopic
information [14].
Furthermore, we performed experiments to utilize the microanalytical potential of this setup (for an example see the next section). Additionally, we tested
microfabricational applications, for example writing of microwires either by depositing molybdenum from an organometallic precursor [15] or by photolytic
polymerization of condensed monothiophene[14, 16].
III - 18
2.2
M. R. Weiss et al.
Analysis of Cu(In,Ga)Se2 Thin Films for Solar Cells
Thin CuInGaSe2 (CIGS) films are very promising absorber materials for low
cost and high efficiency solar cells. A typical cell consists of a soda lime glass
substrate, a 1µm thick Mo back-electrode, a 1-2µm CIGS absorber, a CdS buffer
layer, and a ZnO top-electrode. The efficiency of such a solar cell is found to correlate with the XPS-derived sodium content segregating from the glass substrate
through the Mo back-electrode into the CIGS absorber film [18]. A laser-cutting
technique is applied on the Mo electrode for microstructuring and connecting
the single absorber cells in a module. Because this laser-cut drastically influences
crystal growth and segregation chemical microanalysis of the CIGS film on top
of this microstructure and its correlation with the electrical performance of a
solar cell in operation is of particular interest.
An microscopic image of the vicinity of a laser-cut in the Mo back-electrode
(width ≈30 µm) written prior to CIGS deposition and formation is shown in
Fig. 2 a). The film has a changed morphology near the cut indicated by differ-
a)
b)
Counts
190 m
In 4d
Na 2p
VB(Cu)
400 m
30
35
40
45
50
55
60
65
70
kin. energy [eV]
Fig. 2. (a) Optical microscopic image of the vicinity of a laser-cut. Brighter areas show
locations of different morphology. The analyzed position, line, and area are indicated.
(b) Valence spectrum using the PISAM microprobe set at the indicated small area on
the sample. The photon energy of the first harmonic was 70 eV.
ent grey scales. Measurements of the optical beam induced current (OBIC) [19]
yielded areas of increased efficiency at the position of the brighter strips. The microspectroscopical analysis of a spot (Fig. 2 b) displays the occurrence of sodium,
indium, and copper in the surface region indicated by the photoemission structures from Na 2p, In 4d and valence band, the latter being predominantly due
to copper, for hν=70 eV.
With the electron analyzer set at the corresponding energies we scanned
across the laser-cut as indicated by the white box in the left frame of Fig. 2.
An Improved Microprobe Using Direct Undulator Radiation
III - 19
intensity
Na 2p
In 4d
height [ m]
VB(CU)
2
1
profile
0
-1
0
100
200
300
400
500
position [ m]
Fig. 3. Line scans across the laser-cut with the analyzer set at the Na 2p, In 4d, and
valence (i.e. Cu VB) energies indicating the relative concentration of the elements. The
narrow rectangular box in the middle (position: 200-230 µm) indicates the actual size
of the cut, the grey filled areas show regions of high OBIC signal.
The resulting line scans are displayed in Fig 3. A close examination of these
and similar data (not shown) leads to the following conclusion. At the lasercut all intensities appear to drop strongly. This is probably an artefact of the
measurement which is due to due to charging effects since the absorber material
has no back-electrode there. An area of ≈100 µm width left and right from the
cut shows reduced indium and enhanced copper concentration indicating a new
Cu-rich phase. In the areas of increased OBIC signal (grey boxes in Fig. 3),
the sodium concentration shows a strong variation. This variation cannot be
explained by topographical effects alone, such as height variation of the film
(as indicated by the height profile in Fig. 3, bottom). We rather believe that
sodium influences the size of the crystallites [18], which are larger in this area
than elsewhere. Sodium is also expected to segregate predominantly to the grain
boundaries [18]. The distribution of elements displayed in Fig. 3 can be found all
along the cut as seen in area scans. Fig. 4 shows 2D grey-scale intensity images
for sodium and indium displaying the 2D-distribution of these elements along
the laser-cut.
Obviously, the patterning step of the Mo-back contact leads to compositional
variations on a significantly larger length scale than that of the actual laser-cut.
III - 20
M. R. Weiss et al.
a)
[ m]
150
100
50
0
0
100
200
300
400
300
400
[ m]
b)
[ m]
150
100
50
0
0
100
200
[ m]
Fig. 4. 2D-intensity maps for sodium (a) and indium (b) in the vicinity of a laser-cut
in the Mo back-electrode. The scanned area is about 400 × 190µm2 .
An improvement of the complete solar cell therefore must be combined with improved patterning methods. Moreover, new structuring techniques or substrate
modifications might be necessary in order to optimize the spatial distribution of
the sodium content in the absorber material.
3
Improved Design: PISAM-2
After successful testing of the performance of PISAM-1 we thought of ways
to improve the spatial resolution. An obvious solution is to image the aperture demagnified onto the sample. Several alternatives are available to focus
photons in the XUV region from 10 eV to 600 eV, as mentioned above. Microzoneplates have reached spatial resolutions down to 100 nm [6]. Unfortunately,
their efficiency is very low, and they focus only for a fixed preselected photon
energy. Bragg-Fresnel lenses provide higher efficiency but are still in the exploration phase and hence still limited in their spatial resolution. Normal incidence
mirrors are coated with a multilayer to improve their reflectivity at a certain
photon energy, e.g. 100 eV. These yielded the highest spatial resolution achieved
at present with mirrors: 100 nm [7]. This is mostly due to the fact that normal
incidence mirrors are rotational symmetric and spherical (or only little aspheric),
and hence can be manufactured with very high precision.
We decided to use grazing incidence optics. Although still difficult to manufacture they are the only focusing elements with energy-independent focus-
An Improved Microprobe Using Direct Undulator Radiation
III - 21
ing. Therefore, one needs not to sacrifice the tunability of the photon energy
which can be an important experimental parameter in photoemission and allows
to perform X-ray absorption experiments. Thus, we implemented an ellipsoidal
mirror for demagnification and also added a condenser mirror between undulator and aperture to even increase the photon flux (see Fig. 1). To find optimized
parameters for the system we calculated the expected focus for each possible
configuration, characterized by demagnification factor and grazing angle of incidence and limited by the manufacturing errors: tangential error and microroughness [20, 21]. Based on this consideration an optimized mirror was selected with
a grazing angle of 6◦ and a demagnification factor of 20. Larger angles would
lead to lower transmission at high photon energies, smaller angles to increased
curvature and therefore to even more problems to obtain the required surface
quality. Higher demagnification also leads to unsurmountable problems in producing the mirror. The described evaluation process resulted in a design with
the following properties:
– focus width: 0.2 − 0.3 µm
– focus height: 0.9 − 1.1 µm
– photon flux: >2 · 1012 phot/s 1%BW in the smallest possible focus
(0.2 × 0.9 µm2 ).
From the ray tracings a focal intensity-distribution as shown in Fig. 5 was derived. Also, the sensitivity to maladjustments was determined by ray tracing. It
1
0.8
0.6
0.4
0.2
0
2
y [ m]
intensity, norm.
grey scale image
-2
-1
x[
1
0
-1
-2
0
m]
1
2
-2
-1
0
1
]
y[ m
2
-2
-1
0
1
2
x [ m]
Fig. 5. Calculated intensity distribution of the PISAM-2 in the image plane including
all broadening effects for a photon energy of 600 eV .
resulted in stringent requirements for the mirror mount and fine adjustment: the
mirror must be positioned with an accuracy of 1 µm and oriented within 1µrad
for optimum conditions.
III - 22
M. R. Weiss et al.
Manipulator
Electron Analyzer
Ellipsoidal Mirror
Piezo Stage
Sample Holder
Mirror Holder
Fig. 6. Cut open view of the central experimental PISAM-2 chamber showing details
of the sample stage and mirror holder.
The recently completed PISAM II apparatus (see Fig. 6) appears to meet
these requirements and is briefly described in the following. To reach the goal of
high accuracy and stability, a complete reconstruction of the original PISAM-1
apparatus was neccessary: The sample positioning was divided in coarse movements with stepper motors and fine movements by piezo-stacks in flexure hinge
frames (see Fig. 6), thus giving a full scan range of 25 mm in 50 nm steps. The
ellipsoidal mirror is held by a mirror mount with 5 degrees of freedom, each of
which acts directly on the mirror, thus increasing the stability. The whole setup
is mounted on a marble plate to reduce vibration.
With a prototype ellipsoidal mirror of reduced surface quality first tests of
the new apparatus were recently performed on an integrated circuit. We scanned
across wire-edges on a chip. High count rates indicate the conductor material as
opposed to the adjacent insulator (SiO2 ). From the steep edges of the linescan a
spatial resolution of better than 1µm in both directions can be derived (Fig. 7).
This result was unexpected because of the large tangential error of this prototype
mirror. It is probably due to the fact that only a very small part of the mirror
was used which has a much higher surface quality than the rest of the mirror.
4
Conclusion
With this project we have further developed the idea of spatially resolved spectroscopy. The result is an apparatus that has reached submicron resolution utilizing the direct undulator radiation. The microspectroscope can be operated in
An Improved Microprobe Using Direct Undulator Radiation
3000
2500
2500
2000
2000
intensity
intensity
III - 23
1500
1500
1000
1000
500
500
0
7550
7575
7600
7625
x-position [ m]
7650
0
7575
7600
7625
7650
7675
y-position [ m]
Fig. 7. Line-scans across a wire of an integrated circuit. The steep edges indicate a
resolution of better than 1µm in both directions.
the Auger or photoemission mode with variable photon energies and provides
the highest photon density ever achieved in the sampling spot of such an instrument. Thus PISAM-2 is an interesting microprobe that may be useful for
several applications one of which, a structured film of a Cu(In,Ga)Se2 solar cell,
has been sketched as example.
Acknowledgements
We like to thank Dr. W. Riedl, Siemens Corp., for the solarcell samples. We
also like to thank the BESSY-crew and especially Prof. W. Peatman for support
regarding all stages of development. Financial support by the BMBF (projects
05 644WWA and 05 5WWAXB) is gratefully acknowledged.
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