Soft X-Ray Photoemission Spectromicroscopy Project
at the Synchrotron Radiation Research Center in Taiwan
C.-H. Ko1, R. Klauser1, T. J. Chuang2, H.-H. Chan2, D.-H. Wie2
1
Synchrotron Radiation Research Center,
No. 1 R&D Road VI, Hsinchu Science-Based Industrial Park,
Hsinchu 30077, Taiwan
E-mail: chko@xsm1.srrc.gov.tw or ko@alpha2.srrc.gov.tw
2
Institute of Atomic and Molecular SciencesAcademia Sinica,
P.O. Box 23 - 166, Taipei 10764, Taiwan
Abstract. A scanning photoelectron spectromicroscopy end station is being
constructed at the Synchrotron Radiation Research Center (SRRC) in
Taiwan. High brightness soft x-rays will be provided by a U5 undulator
beamline. Zone plate based soft x-ray optics are used to focus the beam to
form the microprobe. A hemispherical sector analyzer with multi-channel
detection capability will collect the photoelectrons. Total of up to 32 images
can be acquired concurrently. The end station is also equipped with a sample
distribution system and a sample preparation/analysis chamber for in-situ
investigation of samples.
1 Introduction
A scanning photoelectron spectromicroscopy end station is being constructed at the
Synchrotron Radiation Research Center (SRRC) in Taiwan. High brightness soft xrays will be provided by a U5 undulator beamline.
Zone plate based soft x-ray optics are used to focus the beam to form the
microprobe. A hemispherical sector analyzer with multi-channel detection (MCD)
capability will collect the photoelectrons. Elemental and chemical mappings of material surfaces are formed by raster scanning the sample (with the focused beam fixed).
The system is also capable of simultaneously detecting total electron yield with a
channel electron multiplier, transmitted flux (using a silicon based photodiode) and
sample current. Total of up to 32 images can be acquired concurrently.
This project emphasizes on the in-situ investigation of samples, prepared and
analyzed in UHV. This requires flexibility in sample preparation and supplementary
analytical tools. A special transfer chamber will combine the spectromicroscopy
chamber with a fast entry air lock and an analysis/preparation chamber equipped
with LEED, XPS and scanning Auger electron microscope. The scientific programs
include chemical etching and deposition of semiconductors, interfacial interactions
on wide bandgap materials, metal clusters and thin films in the submicron spatial
regime.
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2 The U5 Beamline
The SRRC is a 1.3 GeV third generation synchrotron radiation facility with sixfold
symmetry triple-bend-achromatic lattice structure [1]. The spectromicroscopy end
station will be installed in a U5 beamline, which is currently under construction. The
U5 undulator with a total length of 4 m will provide high brightness radiation (>
10*17 ph/mm2/mrad2/0.1%BW/200 mA) in the energy range of 60 - 1500 eV. The
U5 beamline includes a vertical focusing mirror, moveable entrance and exit slits,
spherical grating monochromator with 4 different gratings and bendable vertical and
horizontal refocusing mirrors to steer the focal point. A maximum photon flux of
1014 ph/s/0.1%BW/200mA and a resolving power of 6000 to 15000 are predicted.
After this optics, the monochromatic beam with a size of 200 µm x 250 µm FWHM
at the focal point will go through a pinhole and beam position monitor chambers and
finally further focused by a zone plate in the spectromicroscopy end station.
3 The Spectromicroscopy End Station
In our design, the sample scanning scheme is completed by attaching a fine sample
scanning stage to a x-y-z sample manipulator used as a coarse scanning device. The
coarse scanning device is a x-y-z sample manipulator from VG (Vacuum Generators)
model Omniax. The VG Omniax manipulator will be motorized using a stepping
motor control with electrically adjustable damping and viscosity to achieve maximal
smoothing motion of the stepping motors. Resolution for the half-step movement is
0.5 µm. This manipulator provides ± 25 mm travel in the horizontal and 400 mm in
the vertical. The long vertical travel range enable us to translate the sample to the
lower level of the spectromicroscopy chamber for other analysis.
The fine sample scanning stage is based on PI's (Physik Instrumente) model P731.20 flexure stage modified for our UHV applications, which has a resolution of
1 nm (close loop with a capacitive sensor) and travel range of 80 µm x 80 µm.
Positioning device for the zone plate and order sorting aperture (OSA) is based
on the Inchworm-motor-driven UHV stages manufactured by Burleigh. The zone
plate positioning has 25 mm longitudinal (i.e., in the beam direction) travel range
and 10 mm x 10 mm transversal travel range. The positioning range for the OSA is
10 mm for x, y and z. Both the zone plate and OSA positioning stages will be
mounted on a linear translational stage (with 50 mm travel range) and a tilt/rotation
(small angle) stage for collinear adjustment. All of these components will be
integrated and mounted on a 10" flange as one unit (Fig. 1, 2).
The spectromicroscopy chamber is also equipped with an ion gun for simple Ar
sputter-cleaning, a flood gun for sample neutralization and an electron gun for
alignment. More dedicated sample preparation and analysis can be done by
transferring the sample to the specific chambers attached to the sample distribution
system. We will use a hemispherical sector analyzer (HAS) from PHI (Physical Electronics) using Omni V lens with 16-channel MCD for collecting and analyzing the
photoelectrons. The MCD can be used for applying the technique of "parallel imaging
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Fig. 1. Major components inside the spectromicroscopy chamber
on chemical shifts" [2]. Detection of sample current, total electron yield and
transmitted flux will also be implemented. Total electron yield will be detected by a
channel electron multiplier, while transmitted flux will be detected by a channel
electron multiplier or a silicon photodiode from IRD (International Radiation
Detection).
The Omni V lens of the PHI's HSA has a quadrupole deflection system in the
input lens. This deflection system is used to deflect the electrons from the imaged
area on the sample into the analyzer. By rastering the imaged area, an image can be
taken. This feature will greatly reduce the time in aligning the focused x-rays to the
analysis area of the analyzer. For the purpose of alignment, we will also setup a
photon beam position monitor based on AXUV-PSI1 silicon photodiode from IRD
(with a 0.3 mm diameter central hole) in-between the pinhole (the pinhole defines
the source for the zone plate focusing) and the zone plate. Also inside the
spectromicroscopy chamber and downstream of the zone plate focusing optics, a
Chevron detector (micro-channel plates) with phosphor screen will be used to inspect
the defocused radiation pattern of the zone plate/OSA optics for aligning the zone
plate and OSA. This defocused radiation pattern shown on the phosphor screen will
be examined by a long-distance microscope through a view port.
Attached to the spectromicroscopy chamber is a sample distribution chamber of
30" diameter manufactured by VG (Fig. 2). A transfer arm mounted at the center of
this "carousel" chamber can transport the sample carrier to any attached chamber. At
the present stage, we will connect a fast-entry lock, an analysis chamber for LEED,
XPS, scanning Auger spectroscopy (SAM) and thermal desorption (TDS)
experiments, a STM/AFM chamber and a small preparation chamber to the system.
The sample is in horizontal face-up position mounted on a modified sample plate
from Omicron. The sample plate sits inside two grooves held by two pins in the
sample carrier. The hook on one side of the sample plate can be easily picked up by
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Fig. 2. Scanning photoemission Spectromicroscopy end station at SRRC U5 Beamline
a wobble stick to be transformed into the manipulator of the attached chamber.
The design of this transfer and distribution system allows us an optimum in flexibility for future expansion. All connected chambers are separated by gate valves from
the carousel chamber and can operate as independent units. Outside users can even
bring their own preparation chamber for their specific in-situ experiments. This is
particular important for the study of material growth processes, where the growth
quality is very sensitive to the preparation parameters.
4 Image and Data Acquisition System
Our image and data acquisition system is designed to be able to collect total of up to
32 images concurrently. A 32 channel multi-channel scaler (MCS) is used for this
purpose. Among the 32 MCS channels, 16 MCS channels will be used for the 16
MCD channels, one for the sample current detection, one for the total electron yield
detection, one for the sum of all 16 MCD channels, one for the storage ring current
reading, one for the transmitted flux detection and one for the clock frequency input
(for image normalization).
There are two major merits in our electronic design, namely, the modularization
of the motion control unit and the hand-shaking scheme. We have configured our
motion control in the following way: the scanning motion is triggered-start and at
the end of the motion a end-of-motion pulse will be issued from the motion control
unit. All of the motion controls for the image scanning (for example, the
piezoelectric movement of the fine scanning, the stepping motor movement and the
Inchworm motor motions, etc.) will be configured in this way. Based on this
modularization scheme, our image acquisition electronic system is able to operate
with the hand-shaking scheme regardless of variations on the motion mechanisms
(for example, piezoelectric vs. Stepping motor). After setting the scan parameters, all
that is needed for a computer to do is to issue a trigger signal to start a 2-D (or 3-D)
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scan, then, the computer waits for the data acquisition to be finished. This scheme
greatly reduces the load on the software programming. The programs become very
easy to develop and maintain. The modularization also enable us to treat different
motion control mechanisms (for example, the piezoelectric motion vs. the stepping
motor motion) as if they are identical when seen by our electronic system.
5 Current Status
The U5 undulator will be shipped to SRRC around January, 1997 and start installation. Currently, the U5 beamline and components of the spectromicroscopy end
station are under construction. The spectromicroscopy system is expected to be in
operation before July, 1997.
Acknowledgments
We are very gratefully to those who are involved in the U5 undulator project and the
U5 beamline designing and construction projects at SRRC. The support we received
from SRRC are enormous and encouraging.
Work is supported in part by the SRRC, in part by the Institute of Molecular and
Atomic Sciences of Academia Sinica and in part by the National Science Council of
R.O.C. under Grant Nos. NSC 85-2613-M-001-001 and NSC 85-2613-M-001-004.
References
1 Y.-C. Liu, Rev. Sci. Instrum. 66, 2011 (1995).
2 C.-H. Ko, J. Kirz, H. Ade, S. Hulbert, E. Johnson, E. Anderson, K. Maier,
B. Winn, in Proc. SPIE, Vol. 2516 (1995).