Download Chapter 4 Submicron Soft X-ray Spectroscopy

Chapter 4
Submicron Soft X-ray
Spectroscopy
4.1
Introduction
The Max Planck Institute for Chemical Physics of Solids in Dresden, Germany (L. H. Tjeng, Zhiwei Hu) and the NSRRC proposed the construction of
a new and unique soft X-ray (400−1500 eV) undulator beamline at the TPS.
The main characteristics of this beamline are (1) a beam spot size smaller
than 1 µm × 1 µm, and (2) a photon energy resolution better than 20 meV
at 700 eV.
One of the missions of the Max Planck Institute in Dresden is to design new solid state materials with interesting and new physical properties.
Guided by various theoretical and chemical concepts, numerous new systems
are being explored with samples synthesized preferably in the form of welldefined single crystals. Most often these new single crystals are not larger
than 20−150 µm in size, a typical size which allows for sufficiently accurate
single crystal X-ray diffraction characterization. Special efforts are also being made to generate millimeter size crystals, but homogeneity is often a
potentially serious issue.
The small beam spot allows also of carrying out high calibre X-ray emission experiments at a side-branch beamline as proposed by the Tamkang
University (W.F. Pong).
4.2
Scientific Opportunities
The scientific motivation for the proposed beamline is to enable electron
spectroscopic measurements on small single crystals, and to obtain reliable
151
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CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
spectroscopic data from selected homogenous parts of larger crystals. A high
intensity beam with a beam spot size smaller than 1 µm × 1 µm will allow
angle-resolved or angle-integrated photoemission (PES) and polarization dependent X-ray absorption (XAS) experiments to investigate the electronic
structures of these new systems with unprecedented quality. The beamline
optical design will be such that a 15 meV photon resolution at 700 eV is
achievable, making possible highly detailed band mapping and Fermi edge
studies with an overall energy resolution of 20 meV. The use of 700 eV photons will provide reasonable bulk sensitivity as well as an optimal sensitivity
for the 3d, 4d and 5d spectral weights of the transition metal constituents,
important especially for the study of new strongly correlated transition metal
materials.
4.3
4.3.1
Photon Source
Source Parameters
For the soft X-ray range, we choose an undulator EPU46, named after its
period length, as the source for the first phase of a submicron-resolution soft
X-ray beamline at the TPS. The source parameters of the EPU46 are shown
in Table 4.1.
Table 4.1: EPU46 source parameters
E: 3 (GeV)
Photon energy (eV)
Stored electron current (A)
Magnet period length λ (mm)
Number of period, Nperiod
Peak field (T)
Deflection parameter Kmax
Total magnet length L (m)
Minimum magnet gap (mm)
EPU46
400 - 1,500
0.5
46
83
0.59
3.57
3.8
13
The length of this straight section is up to 7 m and the EPU46 operation
energy range is from 400 eV to 1500 eV. This energy range can cover the
oxygen K -edge (530 eV), 3d transition metal LII,III -edges and most of the
4.3. PHOTON SOURCE
153
rare earth M IV,V -edges (51500 eV). It will be an ideal source very well suited
for the study of the scientific issues mentioned above.
4.3.2
Source Brilliance and Flux
1.8
1.5
2
Brilliance ( 10 ph/s/mr /mm /0.1%bw )
Due to the low emittance of the TPS ring, at 500 mA storage ring current
and a minimum gap of 13 mm, the brilliance between 400 and 1500 eV calculated by SPECTRA is greater than 6 × 1019 photons·s−1 ·mr−2 ·mm−2 ·(0.1%bw)−1 ,
as shown in Figure 4.1. For an operating energy range of 400−1500 eV using the 1st harmonic of the undulator, the photon flux is above 1 × 1015
photons·s−1 ·(0.1%bw)−1 , as shown in Figure 4.2.
20
2
1.2
0.9
0.6
0.3
st
1 harm.
0.0
300
600
900
1200
1500
1800
Photon Energy ( eV )
Figure 4.1: Brilliance of the EPU46 as a function of photon energy.
4.3.3
Source Size and Divergence
Figures 4.3 and 4.4 show the source size (σ) and divergence (σ 0 ) of the soft
X-ray source. The horizontal source size varies insignificantly in the energy
range 400−1500 eV, with a RMS value of ca. 120 µm (H). The RMS value
of the vertical source size varies from 14 to 10 µm. The change of divergence
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
Flux ( photons/s/0.1%bw )
154
15
10
14
10
st
1 harm.
13
10
12
10
300
600
900
1200
1500
1800
Photon Energy ( eV )
Figure 4.2: Photon flux of the EPU46 as a function of energy.
over this energy range is 27 to 20 µrad in horizontal and 20 to 10 µrad in
vertical directions.
Photon Beam Size ( m )
4.3. PHOTON SOURCE
155
2.0x10
-4
1.8x10
-4
1.6x10
-4
x
1.4x10
-4
y
1.2x10
-4
1.0x10
-4
8.0x10
-5
6.0x10
-5
4.0x10
-5
2.0x10
-5
0.0
400
600
800
1000
1200
1400
1600
1800
Photon Energy ( eV )
Figure 4.3: Source sizes of the EPU46 in the horizontal (red) and vertical
(black) directions.
Photon beam divergences (rad.)
3.0x10
-5
x'
y'
2.0x10
-5
1.0x10
-5
400
600
800
1000
1200
1400
1600
1800
Photon Energy (eV)
Figure 4.4: Source divergences of the EPU46 in the horizontal (red) and
vertical (black) directions.
156
4.4
4.4.1
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
Beamline Optical Design
General
The floor space of the new TPS ring has been mainly planned according
to the space requirements of new beamlines and end stations. The submicron
soft X-ray spectroscopy beamline will occupy port 45 of the TPS ring. One
3.8 m EPU46 undulator will be installed in the 7 m long straight section as
the photon source. The effective source size will increase as the source depth
is increased.
The total length of the submicron soft X-ray spectroscopy beamline is
around 52 m from the center of the undulator to the hallway. To simplify
maintenance, all the optical elements are designed to be outside the shielding wall. Distance from the source center to the first optical element is 24.5 m.
Based on the focusing condition and the photon flux, a KirkpatrickBaez (K-B) mirrors system is preferable for the submicron soft X-ray spectroscopy beamline. Cylindrical mirrors are chosen as they satisfy the required
beam size. This beamline is a dragon type beamline with an active grating
monochromator. The position of the exit slit is fixed by varied active grating, thus the spot size will remain submicron in the photon energy range
of 700−1000 eV. Details of the submicron soft X-ray spectroscopy beamline
overall concept and optical design will be discussed in the following sections.
4.4.2
10 m Active Grating Property
The grating of this monochromator is an active grating with a 3rd -order
polynomial profile. The shape of this grating is varied as the photon energy
is tuned. The center groove density (n0 ) is 1200 groves/mm, chosen to satisfy most users in delivering photons of 400−1500 eV with proper grating
efficiency and resolving power. The total arm of this monochromator is 10
m. The distance from the entrance slit to the grating is 4 m and that from
the grating to the exit slit is 6 m.
The grating holder is designed to be a bender of multiple adjustments to
fix the exit slit position and eliminate thermally induced local bump of the
grating surface. This will keep the beam spot-size almost constant at the
sample position.
4.4. BEAMLINE OPTICAL DESIGN
4.4.3
157
Prefocusing and Refocusing Mirrors System
The Au coating reflectance is shown in Figure 4.5 with an incident angle
varying from 1.5 ◦ to 3.0 ◦ . It shows that the reflectance is more than 60%
where the incident angle is less than 2.0 ◦ for the energy range 400−1500
eV. Mirror slope error apparently is one of the major factors degrading the
mirror function. Even though a mirror slope error of better than 0.5 µrad
(R.M.S) is commercially available, under the high heat load condition it is
difficult to maintain such a low slope error due to the thermally induced
local distortion. Thus, a cooling scheme is critical to reducing the thermal
distortion to achieve an ultra-high energy resolution.
1.0
o
o
Incident angle: 1.5 -3.0
Au
Reflectivity
0.8
s_polarization (1.5)
s_polarization (2.0)
s_polarization (2.5)
s_polarization (3.0)
p_polarization (1.5)
p_polarization (2.0)
p_polarization (2.5)
p_polarization (3.0)
0.6
0.4
0.2
0.0
200
400
600
800
1000
1200
1400
1600
1800
2000
Photon energy (eV)
Figure 4.5: Dependence of Au reflectivity on the incident photon angle.
The first mirror is the horizontal focusing mirror (HFM) and absorbs the
most heat from the source. Under such a high heat load condition, internal
water cooling is used for cooling of the HFM. Water side cooling scheme is
used for the vertical focusing mirror (VFM).
For submicron focusing, two stage focusing is necessary for the horizontal
direction. The demagnification ratio of the two horizontal mirrors for the
MPI (Max Planck Institute) branch is 0.192 × 0.023. The calculated spot
size is 1.2 µm× 0.4 µm (H × V) when the slits are set at 50 µm (horizontal
slit) × 1 µm (both entrance and exit slits). The demagnification ratio of the
two horizontal mirrors for the TKU (Tamkang University) branch is 0.192 ×
0.023. The calculated spot size is 2.2 µm × 1.2 µm (H × V) when the exit
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CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
slits are set at 50 µm × 10 µm with an entrance slit of 1 µm.
4.4.4
Optical Layout
The optical layout of the submicron soft X-ray spectroscopy beamline
is shown in Figure 4.6. The first optic is an HFM located at 24.5 m from
the EPU46 source center. The distance between the HFM and the VFM is
2.5 m. The demagnification of the VFM is 20. The focal point is at the
entrance slit of the monochromator. Parameters of the optical elements are
listed in Table 4.2. After the 10 m arm of the monochromator, a movable
deflection mirror is placed 1 m after the exit slit. After that, two branches are
selectable by a deflection mirror. Each branch is equipped with a set of K-B
mirrors to refocus the beam onto a sample. The calculated performances
of both branches are shown in Figure 4.7. The demand of users for the
MPI branch is a high energy resolution. As such it is designed to have an
energy resolution better than 35,000 with a photon flux greater than 1 ×
1011 photons/sec. The beam size at the sample is 2.8 µm × 1 µm (H × V).
The TKU branch is targeted for a high photon flux. Thus it is designed to
have a photon flux greater than 1 × 1012 photons/sec in the energy range of
600−1200 eV. The beam size at the sample is 3 µm × 3 µm (H × V). The
calculated performance satisfies the demands of both branches.
27.0
1.35
177
Cylindrical
200×30×30
Au
Si
24.5
4.7
177
Cylindrical
180×30×30
Au
GlidCop
Bendable
r1 (m)
r2 (m)
Deviation
angle (◦ )
Type
Size
(L×W×T)
mm3
Coating
Substrate
Surface
profile
Bendable
Vertical
pre-focusing
mirror
(VFM)
Horizontal
pre-focusing
mirror
(HFM)
Beamline branch
Plane
Si
Au
180×30×50
Plane
176
-
-
Deflection
mirror
(DM)
Elliptical
Si
Au
180×30×30
Cylindrical
177
1.2
6.4
Vertical
refocusing
mirror
(VRFMa)
Elliptical
Si
Au
180×30×30
Cylindrical
176
0.7
16.05
Horizontal
refocusing
mirror
(HRFMa)
MPI branch
Table 4.2: Optical parameters of the K-B mirrors.
Elliptical
Si
Au
360×30×50
Cylindrical
177
2.6
12.3
Vertical
refocusing
mirror
(VRFMb)
Elliptical
Si
Au
360×30×50
Cylindrical
176
1
23.05
Horizontal
refocusing
mirror
(HRFMb)
TKU branch
4.4. BEAMLINE OPTICAL DESIGN
159
160
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
Figure 4.6: The optical layout of the submicron soft X-ray beamline
4.4.5
Performance Requirements
This beamline is designed primarily for X-ray absorption and photoemission studies of 3d, 4d and 5d transition metals and Rare Earth related
materials that require an energy range of 400−1500 eV. In particular, the
performance is optimized at 700 eV. Because we plan to perform highly detailed band mapping and to resolve structures around the Fermi edge, the
photon resolving power needs to reach a value of 35,000 at 700 eV . The use
of 700 eV photons will provide a reasonable bulk sensitivity as well as an
optimal sensitivity for the 3d, 4d and 5d spectral weights of the transition
metal constituents, important especially for the study of new strongly correlated transition metal materials. A higher photon flux improves the signal
to noise ratio of a spectrum, while the photon resolving power is an inverse
function of the beamline flux. It is necessary to find an equilibrium point to
4.4. BEAMLINE OPTICAL DESIGN
8
60000
(a)
55000
6
5
50000
Resolving Power
4
Photon Flux
11
45000
3
40000
2
1
Resolving Power
Photon Flux ( 10 photon / sec )
7
161
35000
0
400
500
600
700
800
900
1000
1100
1200
1300
Photon Energy ( eV )
15000
12
(b)
3.0
12500
2.5
2.0
Resolving Power
10000
Photon Flux
1.5
1.0
7500
Resolving Power
Photon Flux ( 10 photon/sec )
3.5
0.5
0.0
400
500
600
700
800
900
1000
1100
1200
5000
1300
Photon Energy ( eV )
Figure 4.7: The calculated performance of the submicron soft X-ray spectroscopy beamline. (a) MPI branch: entrance slit / exit slit = 1 µm / 1 µm,
and (b) TKU branch: entrance slit / exit slit = 1 µm / 10 µm.
balance the resolving power and the flux.
162
4.5
4.5.1
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
Beamline Detailed Design
Beamline Overview
This beamline is of the dragon-type design, and offers 2-stage K-B mirror
focusing. Figure 4.8 shows all components of the beamline. All vacuum
chambers are UHV-compatible and all pumps are ion pumps.
Shown bellow are brief descriptions of the components:
1. The mask defines the acceptance angle.
2. The photon absorber (PAB) protects all components downstream from
the radiation damage.
3. The aperture controls the incoming power and acceptance of the synchrotron radiation.
4. The white beam screen monitors the position of the incoming photons.
5. The HFM support system allows stable and precise optical adjustments.
6. The VFM support system provides a vibration-free foundation and
allows high precision optical adjustments.
7. The entrance slit limits the source size of the monochromator.
8. The horizontal slit limits the source size of the HRFM.
9. The monochromator chamber provides a vibration-free housing for the
monochromator and allows ultra high precision adjustments of the grating.
10. The movable exit slit selects the monochromatic beam.
11. The deflection mirror switches the beam between the two branches.
12. Each of the MPI and the TKU branches has a refocusing system to
focus the monochromatic beam onto a sample.
4.5.2
Front end
The EPU46 is a high flux and high brilliance light source, which produces
a high power density and incurs a high heat load for the downstream beamline components. The first optical component needs a high capacity cooling
system to dissipate the high heat load. Moreover, the high power density
Figure 4.8: The side view of the submicron soft X-ray beamline.
4.5. BEAMLINE DETAILED DESIGN
163
164
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
will introduce a large temperature gradient on the optical element. It will
generate a thermal bump on the optical surface of a mirror, degrading its
focusing capability. We thus utilize an aperture to lessen the heat load.
The safety factor is set as 1.2 in the heat load calculation under an operation current of 500 mA in the TPS phase I plan. To estimate the effect
of the highest heat load, we have used the maximum k value (kmax = 3.57),
which corresponds to a 13 mm gap in our analysis.
An adjustable aperture, located at 18 m, is used to limit the source divergence (δ 0 ). When the angular acceptance is larger than 4 δ 0 , the flux
throughput tends to saturate but more heat is collected by the optic. With
this aperture opening, we will obtain a very high flux throughput with a
relatively low heat load. The effective source size for the central cone (> 4
δ 0 ) can be described by the following equation:
δx,y =
q
2 + (18σ 0 )2 .
σx,y
x,y
(4.1)
When the aperture is varied from 4 δ 0 to 6 δ 0 , photon flux and heat power
increase by 5.9% and 200%, respectively. From this consideration, we set the
aperture at 4 δ 0 .
With this aperture, the EPU46 heat power allowed through is 300 W.
The power density is 29 kW/mrad2 . The cooling scheme calculation for the
HFM is based on this condition.
4.5.3
Photon Absorber
The PAB is a protection device for vacuum components from synchrotron
radiation illumination. It’s used mainly to block the synchrotron radiation
and its heat load during maintenance and switching branches. The schematic
of the PAB is shown in Figure 4.9. The blocking part is made of OFHC copper
with water cooling and mounted onto a pneumatic linear motion stage. To
reduce maintenance frequency, a welded bellows with a service lifetime of one
million cycles is used.
4.5.4
Aperture
Two rectangular apertures made of OFHC copper with cooling water
channels, shown in Figure 4.10, will take out the synchrotron radiation outside the central cone. The footprint of light allowed through is determined
by the relative positions of these two apertures. To allow such adjustment,
4.5. BEAMLINE DETAILED DESIGN
165
Figure 4.9: The schematic of the PAB. 1. Tungsten shielding plate, 2. Vacuum feedthroughs, 3. Pneumatic linear stages, 4. UHV chamber, 5. Stand.
a welded bellows is used in between these apertures. The movement resolutions for these apertures are 2 µm in the horizontal direction and 0.5 µm in
the vertical direction.
4.5.5
White Beam Screen
The white beam screen monitors the position of the synchrotron radiation.
The schematic of the white beam screen is shown in Figure 4.11. The screen
is made of Diamond/Yag and mounted on a linear vacuum feedthrough.
Position of fluorescence on the screen is observed with a micro-inspection lens
system with 0.35−4.3 X magnification. The spatial resolution of this system
is 2−9 µm and the field of view is 19.0 mm × 25.4 mm. This provides a good
guide for optics alignment.
igure 9: The schematic of the PAB.
1. Tungsten shielding plate, 2. Vacuum
eedthroughs, 3. Pneumatic linear stages, 4. UHV chamber, 5. Stand.
166
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
Aperture
Figure Figure
10: The
schematic
of theofaperture.
4.10:
The schematic
the aperture.
4.5.6 Cooling Scheme
5.5 White Beam Screen
The
chematic
The heat power on the HFM is still considerable (up to 300 W), even
though
apertures
limit the
to the
4 δ 0 .synchrotron
A high capacity
cooling The
white beamthe
screen
monitors
the acceptance
position of
radiation.
system is necessary to dissipate the heat on the HFM. However, the residual
heat load still induces a thermal bump on the mirror surface. The power disof the
white on
beam
screen
is shown
in Figure
11.origin
Theofscreen
is made of
tribution
the mirror
is shown
in Figure
4.12, with
the coordinates
located at the center of the HFM. The unit of the color bar is W/mm2 . To
simplify this model, the heat load is regarded as uniform over the footprint
and the power density is 1.08 W/mm2 .
We choose internal water cooling instead of liquid nitrogen (LN2 ) cooling
for the mirror because the latter induces unpredictable shrink of holders and
mirrors. A finite element analysis program (ANASYS) is utilized to simulate
our cooling scheme. We choose a bendable mirror made of Glidcop which
can withstand up to 350 W of heat. The calculation model of the HFM is
shown in Figure 4.13. The dimensions of the HFM are 450 mm × 40 mm ×
50 mm (L × W × T). With these conditions, the thermally induced slope
error is shown in Figure 4.14. This calculation shows the slope error on the
edge of the X-rays is large (up to ∼ 50 µrad). Such a large slope error can
be ruled out from the engaged area by opening up the aperture of the front
4.5. BEAMLINE DETAILED DESIGN
167
Figure 4.11: The schematic of the white beam screen, 1. Diamond/YAG
screen, 2. Cooling water feedthroughs, 3. Precision linear stages, 4. Compact
UHV chamber, 5. Micro-inspection zoom lens system.
end. Furthermore, we have used a ray tracing program (SHADOW) to simulate this scheme with the K value of the EPU46 chosen as 3.5. By changing
the curvature of the HFM, the slit can be fixed. The FWHM of the beam
size is slightly enlarged by the thermal bump and this causes a 12% beam
loss when the horizontal slit is set at 50 µm. Figure 4.15 displays the spot
diagram of light falling on the horizontal slit obtained from ray tracing. The
result shows that our cooling solution is working in this case.
The heat power over the VFM is up to 17 W. We will utilize a thermoelectric cooling module to take out the heat of the VFM. The X-ray footprint
is shown in Figure 4.16. The induced slope error with this cooling scheme
is shown in Figure 4.17. To reduce the effect of thermal distortion, a multicontact bender has been under testing as shown in Figure 4.18. This bender
is supposed to be able to mitigate the slope error due to the heat load below
168
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
Figure 4.12: The power distribution of X-rays on the HFM.
Figure 4.13: The cooling scheme of the HFM.
4.5. BEAMLINE DETAILED DESIGN
169
Figure 4.14: The thermally induced slope error of HFM.
0.3 µrad. Thus, we will be able to maintain an ultrahigh resolution without
wasting photons. The simulated spot diagram of light falling on the entrance
slit is shown in Figure 4.19, with the slope error of the VFM set at 0.1 µrad.
To achieve a resolving power of 35,000, the entrance slit of the monochromator will be set at 2 µm or below. The maximum heat load on the entrance
slit is 10 W and the beam size is 0.4 × 0.002 mm. We did our cooling scheme
simulation based on these conditions.
In this simulation, we used tungsten knife blades for the slit. The blades
were cooled by OFHC copper with cooling water channels. The design of the
entrance slit is shown in Figure 4.20. The thermal stress and deformation
of the blades were calculated using finite element analysis. The thermal
deformation of the knife blades is shown Figure 4.21.
The stress simulation result shows that the tensile strength is 2.6 GPa.
This value is smaller than the tungsten yield strength (7.5 GPa). Thus the
blade will not be damaged as the beam illuminates it. The blade deformation
is around 1.25 µm. To avoid collision of the blades, two blades are offset by
a few micrometers along the beam direction.
170
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
2000
Rays
1500
FWHM=58 m
1000
500
0
2
1
The heat power over the VFM is up to 17 W. We will utilize thermoelectric
Z (mm)
module to take out the heat of the VFM. The dimensions of the VFM are 200 mm × 30
0 mm × 30 mm (L × W × T). The X-ray footprint is shown in Figure 16. We assume a
Gaussian distribution for the thermal load of 13 W within a 2.5 mm × 120 mm (L × W)
-1 footprint. The mirror is cooled by OFHC copper with water channel along the whole
side surface. The water flow rate is 2 L/min. The induced slope error with this cooling
-2
scheme is shown in Figure 17. We will also apply the bender mentioned above to
-0.4slope error to below
-0.21 μrad.To reduce0.0
0.2distortion, a
reduce the
the effect of thermal
X in(mm)
multiple bender has been under testing as shown
Figure 18. This bender is supposed
able to modify the slope error below 0.3 μrad. Thus, we can perform an ultra-high
Figure 4.15: The ray-tracing result at the horizontal slit.
resolution without wasting photons. The spot diagram on entrance slit is shown in
Figure 19 with the slope error of VFM is set as 0.1 μrad.
Thermal load
Cooling surface
Figure 4.16: The
model of and the X-ray footprint on the VFM.
Figure 16: The model of and X-ray footprint on the VFM.
0.4
Slope error(rad)
4.5. BEAMLINE DETAILED DESIGN
6.0x10
-6
4.0x10
-6
2.0x10
-6
171
0.0
-2.0x10
-6
-4.0x10
-6
-6.0x10
-6
-80
-60
-40
-20
0
20
40
60
80
X(mm)
Figure 4.17: The heat-load induced slope error of the mirror cooled by a
thermoelectric module.
Figure 4.18: The multiple bender under a long-trace profile testing.
172
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
0.8
0.6
0.4
Z (m)
0.2
0.0
-0.2
FWHM=1.2 m
-0.4stress simulation result shows that the tensile strength is 2.6 GPa. This value
The
-0.6 than the tungsten yield strength (7.5 GPa). Thus the blade will not be
is smaller
damaged-0.8as -100
the beam
illuminates it.0 The
blade
deformation
is around
1.25 m. To
500 1000 1500
-80 -60 -40 -20
20
40
60
80 100 0
Rays
X (m)
avoid collision of the blades, two blades are offset by a few micrometers along the
Figure 4.19: The ray-tracing result at the entrance slit.
beam direction.
SUS304
OFHC Cu
Knife blade
(Tungsten)
Figure 20: The design of the entrance slit.
Figure 4.20: The design of the entrance slit.
climax : X = -33.008, Y = 1.244
1.25
4.5.7
Support
and Motion Specifications
1.24
Deformation (m)
We will use a novel
motion system of 6 degrees of freedom to control the
1.23
HFM and the VFM. In this design, the mirror is directly mounted on the
1.22 will adjust the mirror chamber to align the mirror. The
mirror chamber. We
adjustment mechanism is shown in Figure 4.22. The mirror is mounted on
1.21
a base plate. There are 6 ball-joints to connect the base plate to 6 linear
sliders. The motions
of the 6 linear sliders can produce all the x, y, z, pitch,
1.20
1.19
-33.4
-33.2
-33.0
-32.8
-32.6
X (m)
Figure 21: The thermal deformation of the entrance slit knife blade.
4.5. BEAMLINE DETAILED DESIGN
173
climax : X = -33.008, Y = 1.244
1.25
Deformation (m)
1.24
1.23
1.22
1.21
1.20
will adjust the mirror chamber to align the mirror. The adjustment mechanism is shown
1.19
-33.4
-33.2
-33.0
-32.8
-32.6
in Figure 22. The mirror is mounted on a base plate.
are 6 ball-joints to connect
X (There
m)
the base plate to 6 linear sliders. The motions of the 6 linear siders can produce all the
Figure 4.21: The thermal deformation of the entrance slit knife blades.
x, y, z, pitch, roll and yaw motions of the base plate. The motion specification of the
base plate
is shown
Table 3. of the base plate. The motion specification of the base
roll and
yawinmotions
plate is shown in Table 4.3.
Base Plate for mounting
mirror chamber
Figure 22: The adjustment mechanisms and supports of the mirror system.
Figure 4.22: The adjustment mechanisms and supports of the mirror system.
monochromator
is equipped
one water-cooled
grating
initially,
TheThe
monochromator
is equipped
with onewith
water-cooled
grating initially,
with
with room for more in the future. In phase I operation of the beamline, one
room grating
for more for
in the
In range
phase Iof
operation
of the
one grating
for the drive
thefuture.
energy
400−1500
eVbeamline,
is enough.
The scanning
is mounted horizontally on a granite block. The grating rotates around two
concentric bearings and is driven by a scanning motor. Figure 4.23 shows the
energy range of 400-1500 eV is enough. The scanning drive is mounted horizontally on
a granite block. The grating rotates around two concentric bearings and is driven by a
scanning motor. Figure 23 shows the side view of the monochromator. For the 10 m
active grating monochromator, we will install an interferometer with a 0.008 μrad
resolution as an angle encoder and an element of the feedback loop. The motion
specifications of the monochromator are shown in Table 4.
174
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
Table 4.3: The motion specification of the HFM and VFM supports.
Motion
Parameter
Drive
Pitch
Range
Resolution
Repeatability
Drive
Roll
Range
Resolution
Repeatability
Drive
Yaw
Range
Resolution
Repeatability
Drive
Vertical
Range
Resolution
Repeatability
Drive
Lateral
Range
Resolution
Repeatability
Drive
Longitudinal
Range
Resolution
Repeatability
Specification
Linear slider, fitted with limit switches
and an encoder
Normal position ±8 mrad
<1 µrad for HFM
<0.2 µrad for VFM
<5 µrad
Linear slider, fitted with limit switches
and an encoder
Normal position ±8 mrad
<5 µrad
<10 µrad
Linear slider, fitted with limit switches
and an encoder
Normal position ±8 mrad
<5 µrad
<10 µrad
Linear slider, fitted with limit switches
and an encoder
Normal position ±5 mm
<1 µm
<5 µm
Linear slider, fitted with limit switches
and an encoder
Normal position ±5 mm
<1 µm
<5 µm
Linear slider, fitted with limit switches
and an encoder
Normal position ±5 mm
<2 µm
<10 µm
4.5. BEAMLINE DETAILED DESIGN
175
side view of the monochromator. For the 10 m active grating monochromator,
we will install an interferometer with a 0.008 µrad resolution as an angle
encoder and an element of the feedback loop. The motion specifications of
the monochromator are shown in Table 4.4.
Figure 4.23: The schematic of the grating chamber.
Table 4.4: The motion specifications of the monochromator
Motion
Parameter
Scanning drive
Scanning
Range
Motor step
Encoding
Specification
Linear slide, fitted with limit switches
and an encoder
±5 degrees from the horizontal direction
< 0.02 arc sec
< 0.04 arc sec with a laser interferometer
176
4.6
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
End Stations
Two end stations are devised for this beamline. One is dedicated to
soft X-ray angle-resolved/angle-integrated photoemission (PES) experiments
and the other optimized for polarization dependent X-ray absorption (XAS),
X-ray excited optical luminance (XEOL) and X-ray emission spectroscopy
(XES).
4.6.1
MPI End Station
The end station of the MPI branch mainly consists of a SPECS PHOIBOS 225 hemispherical analyzer with a combined delayline and four-channel
micro-Mott spin detector (DLD-Mott). The end station is capable of performing photoemission spectroscopy (PES), angle-resolved photoemission spectroscopy (ARPES), spin-resolved PES, and X-ray absorption spectroscopy
(XAS) experiments. Figure 4.24 shows the drawing of the end station. The
PHOIBOS 225 has a mean radius of 225 mm with 8 entrances and 3 exit
slits. The energy resolution of the analyzer is better than 7 meV. The angular resolution is smaller than 0.1 ◦ . The combined DLD-Mott detector is
situated along the dispersive direction of the analyzer as shown in Figure
4.25(a) [J. Electron Spectrosc. Relat. Phenom. 185, 47 (2012)]. Emitted
electrons from samples are crossing the exit plane of the analyzer at angles
close to 90 ◦ in this area. For spin-resolved measurements the electrons are
further accelerated to 20−25 keV and hit a gold target, as shown in Figure
4.25(b). The scattered electrons are recorded in four spin channels by microchannel plate detectors. Figure 4.25(c) shows the configuration of these
four spin channels; channels 1 and 2 are positioned perpendicular to the
dispersive direction of the analyzer, and channels 3 and 4 are along the dispersive direction. Each pair (1 and 2, or 3 and 4) allows determination in one
spin direction. The analysis chamber is equipped with a custom-made, highprecision, fully motorized OMICRON 5-axis (x, y, z, azimuth, and polar)
manipulator with a LHe or LN2 cooling cryostat. The high precision 5-axis
manipulator lends itself to the ARPES experiments. Alternatively, one can
also replace the DLD-Mott detector by a SPECS 2D-CCD detector with a
wide-angle span of ±30 ◦ , which covers the first Brillouin Zone of most of the
interesting oxides.
The end station belongs to the group of Prof. Claudia Felser in the
MPI CPfS institute in Dresden. The end station is currently located in the
beamline P09 at PETRA III in Hamburg, as shown in Figure 4.26. Dr.
Andrei Gloskovskii, Dr. Gerhard H. Fecher, and coworkers have constructed
and commisioned the end-station in 2011 and 2012. The whole instrument
4.6. END STATIONS
177
Figure 4.24: Drawing of the MPI end station.
Figure 4.25: (a) Schematic of an analyzer with a combined delayline and
four-channel micro-Mott spin detector. (b) Side view of the micro-Mott spin
detector. (c) Configuration of the four spin channels. [J. Electron Spectrosc.
Relat. Phenom. 185, 47 (2012)]
178
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
is mounted on a motorized high precision XYZ-platform made of monolithic
granite. The reproducibility of movements can be better than 2 micrometers.
The user team plans to move the whole system to the TPS in the middle of
2014.
The user team will mount a more versatile and user-friendly in-vacuum
transferring and sample preparation system when the end station is installed
at the MPI branch beamline at the TPS. Figure 4.27 shows a sketch of such
a system attached to the main analysis chamber. The main piece is a PREVAC transferring system. The same system has been used in the wide-angle
ARPES end station at the Dragon beamline of the NSRRC. Satellite chambers will be attached to it and be equipped with a LEED, an ion sputtering
gun, and e-beam heaters for surface science characterization and preparation.
A molecular-beam-epitaxy (MBE) film growth facility will also be docked
onto the PREVAC transferring system.
Figure 4.26: Photo of the end station located at beamline P09 at PETRA
III in Hamburg.
Figure 26: Photo of the end station located at beamline P09 at PETRA III in Hamburg.
4.6. END STATIONS
179
Figure 4.27: Sketch of a planned in-vacuum transferring and preparation
Figure
27: Sketch
of a to
planned
in-vacuum
system
attached
the main
analysistransferring
chamber. and preparation system attached to
the main analysis chamber.
Publications of results using the above end station in Hamburg in 2011
and 2012:
1. J. Nayak et al., Phys. Rev. Lett. 109, 216403 (2012)
2. T. Bertaud et al., Appl. Phys. Lett. 101, 143501 (2012)
3. A. M. Kaiser et al., Appl. Phys. Lett. 100, 262603 (2012)
4. A. X. Gray et al., Phys. Rev. Lett. 108, 257208 (2012)
5. K. Medjanik et al., J. Electron Spectrosc. Relat. Phenom. 185, 77
(2012)
6. A. Gloskovskii et al., J. Electron Spectrosc. Relat. Phenom. 185, 45
(2012)
7. C. Caspers et al., Phys. Status Solidi RRL 5, 441 (2011)
8. C. Caspers et al., Phys. Rev. B 84, 205217 (2011)
4.6.2
TKU End Station
The Tamkang University Team (TUT) proposes to construct a photonin/photon-out soft X-ray spectroscopy end station at the Taiwan Photon
Source (TPS) in collaboration with scientists and engineers at the NSRRC.
180
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
The proposed end station will carry out experiments on soft X-ray absorption
and emission, resonant inelastic X-ray scattering spectroscopy, and X-ray
excited optical luminescence (XAS, XES, RIXS and XEOL). It will also have
capabilities of a medium-high magnetic field (0−4 T) and a low-temperature
(∼ 5 K) for the sample environment. This proposal aims to utilize photonin/photon-out soft X-ray spectroscopy to study the electronic properties of
fundamental condensed matters, magnetic materials, low-dimensional/nanoscale materials, materials in the field of renewable energy, bio-materials, and
complex hydride systems. This facility will be unique in the coming years and
will provide an in-depth understanding of fundamental and novel materials,
as well as their potential applications.
Schematic for the SXF end station is shown in Figure 4.28. Variedline spacing spectrometer (VLS), XEOL, 7-element Ge detector, and liquid
cell are employed in this end station. The conceptual design of the VLS
spectrometer has been done. The optical layout is shown in Figure 4.29.
Figure 4.28: Schematic of the SXF end station.
Based on this design, a list of specifications is as follows:
Source-to-mirror distance: 1250 mm
Mirror-to-grating distance: 100.306 mm
4.6. END STATIONS
181
Grating-to-detector distance: from 1199 mm to 1275 mm
Mirror included angle (2θ): 174 ◦
Grating included angle (α + β): 174 ◦
Mirror size: 90 mm(L) by 90 mm(W) by 20 mm(T), identical to qRIXS
Grating size: 90 mm(L) by 100 mm(W) by 20 mm(T), identical to qRIXS
Vertical acceptance: ±2 mrad
Horizontal acceptance: ±5 mrad
To change the photon energy, rather than moving the detector, the grating
will be rotated instead. The energy range can be tuned by simply varying the
central line density while keeping the ratios between different VLS parameters
unchanged. To target at 640 eV, a central line density of 1350 lines/mm will
work, and the horizon energy now becomes ∼ 300 eV. If one increases the
line density to 1500 lines/mm, then the energy range becomes 375−1250 eV.
Figure 29: Optical layout of the X-ray emission spectrograph.
Figure 4.29: Optical layout of the X-ray emission spectrograph.
The To
TKU
team
has developed
a new
liquidtheand
gas will
cellsbe
change
the photon
energy, rather
thangeneration
moving the of
detector,
grating
and collaborated with Dr. J.-H. Guo (Advanced Light Source, Lawrence
rotated National
instead. The
energy range
can Figure
be tuned4.30
by simply
the concepts
central line
Berkeley
Laboratory,
USA).
shows varying
the design
of the liquid and gas cells.
density while keeping the ratios between different VLS parameters unchanged. To
This flow
liquid
cell may
even
widerlines/mm
application
of the
extarget
at 640eV,
a central
line offer
density
of 1350
will work,
andin-situ
the horizon
periment. For instance, the study of drugs or DNA-based solutions can be
energy
now
becomes
~ 300eV.
oneflow
increases
thecell
line due
density
1500 lines/mm,
then
carried
out
more
precisely
by Ifthe
liquid
to to
elimination
of the
sample damage induced by the X-rays.
the energy range becomes 375 – 1250 eV.
The components of the flow liquid cell are as follows:
1. 1 basic EC cell on a 2.75 inch CF flange, with a fast valve and an
electrical
The TKUfeedthrough.
team has developed a new generation of liquid and gas cells and
2.
8 valve rods
collaborated
with w/balls.
Dr. J.-H. Guo (Advanced Light Source, Lawrence Berkeley National
3.
3 valve body
of shows
the old
Laboratory,
USA).inserts
Figure 30
thedesign.
design concepts of the liquid and gas cells.
182
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
(a)
(b)
Figure 4.30: Sketch of the liquid and gas cells.
4. 3 valve body inserts of a new design.
5. 12 reaction cell cap of the old design.
6. 12 reaction cell cap of a new design.
7. 18 window caps, 6 each made of stainless steel, copper and PEEK.
8. 3 liquid flow adapter blocks.
The main purpose of developing the flow liquid cell, in short, is to provide access to studies on electronic properties of the liquid, liquid-solid interfaces, and in-situ chemical reactions. Additionally, we can understand the
mechanism of the ferromagnetism in oxide nanoparticles induced by electron
vacancy. It is possible to simultaneously adjust the pH value to control the
electron vacancy and monitor the variation of the electronic structure by
spectroscopic measurements. Currently, the flow liquid cell is under fabrication. Based on the design, a list of specifications follows:
1. A liquid reservoir and waste container is outside the UHV chamber.
2. Interlock valves will secure a small volume of liquid inside the UHV
chamber in the event of a broken membrane window.
3. Experimental conditions will be changed outside the vacuum chamber,
such as pH, concentration, temperature, etc.
4.7. RADIATION SAFETY
183
Sample holders are attached to the tips of rotary drives, which provide
rotation of the sample and connection for sample services such as heating or
cooling. Sample holders are fully UHV compatible. The construction of each
sample holder varies to give the required axes of rotation. The materials
used are stainless steel, alumina, beryllium copper, and OFHC copper. The
holders will be equipped with liquid nitrogen/liquid helium cooling accessories. The very low swept volume of these sample holders allows samples
to be positioned very close to the analysis equipment. Most sample holders
have low magnetic properties for use in sensitive surface science applications.
The sample size is normally 5 mm × 5 mm with an option for 25 mm × 25
mm. Figure 4.31 shows the sketch of a sample holder of such a design.
Figure 4.31: Sketch of the sample holder.
4.7
Radiation Safety
Bremsstrahlung Shielding
We use two stoppers to shield the Bremsstrahlung. The thickness is 300
mm and the safety margin is 50 mm in both vertical and horizontal directions.
184
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
The design of the Bremsstrahlung shieldings is shown in Figure 4.32. The
first stopper is placed after the HFM chamber with dimensions of 180 mm
× 180 mm × 300 mm (L × W × T). The second stopper is placed after the
VFM chamber, with dimensions of 200 mm × 180 mm × 300 mm (L × W
× T).
Synchrotron Radiation shielding
An optical hutch is built to block the stray synchrotron radiation and
residual Bremsstrahlung, the layout of which is shown in Figure 4.33. The
thickness of Pb contained in the side walls is 3 mm and that of the end wall
is 5 mm. This optical hutch encloses the HFM, the VFM, the entrance slit,
and the horizontal slit. We are designing the details of this hutch, and will
finish the design in March, 2014. Construction of the optical hutch will be
finished in July, 2014.
Figure 4.32: The design of the Bremsstrahlung shieldings.
4.7. RADIATION SAFETY
185
Slits
VFM
HFM
top view of the optical hutch with the roof removed.
Figure 33:Figure
The4.33:
topThe
view
of optical hutch (The roof was hided).
in July 2014.
186
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
4.8. SCHEDULE
4.8
187
Schedule
Figure 4.34 shows the schedule of construction and commission of the
beamline and end stations. The beamline optical design was started in 2011
and completed in the summer of 2012. The hardware design, fabrication, and
testing were started in the summer of 2012 and are expected to be completed
at the end of 2013. The installation will start in the summer of 2013 and be
finished in the fall of 2014. Commissioning will then start in the fall of 2014.
Schedule
2011
2012
2013
2014
optical design
hardware design
fabrication and testing
installation
commission
Figure 4.34: Schedule of construction and commission of the beamline and
end stations
4.9
Commissioning Plan
Commission of the TKU end station will focus on high-resolution performance. The impressive progress of XAS/XES/RIXS in the last decade has
ushered in new applications of these techniques.
Below is a description of the commissioning plan:
Nanoscale Materials: Among semiconductors, ZnO has been recognized as one of the most favorable photocatalytic materials because of its
high photosensitivity, lack of toxicity, a large bandgap, and chemical stability. The photocatalytic activities of such semiconductors were found to be
enhanced when noble metals such as Au nanoparticles were deposited, because these metal nanoparticles could store electrons. Figure 4.35(a) displays
the O K α -emission RIXS spectra obtained at various excitation energies Ex ;
Figure 4.35(b) presents the XES and corresponding XANES spectra of O 2p
states of various concentrations of nano-crystalline (nc)-Au/ZnO-nanorods
188
CHAPTER 4. SUBMICRON SOFT X-RAY SPECTROSCOPY
and pure ZnO nanorods. The results reveal that the contact of nc-Au particles with ZnO nanorods promotes interfacial charge transfer, although the
Fermi level EF in nc-Au/ZnO-nanorods is the same as that of pure ZnO
nanorods under the photon resolving power of 7000 around 520 eV.
With a higher resolving power (= 10,000) from this submicron soft X-ray
spectroscopy beamline, we expect to resolve the Fermi level shifts for different
sample sizes to reveal the mechanism of interface charge transfer vs. the size
effect.
Figure 4.35: (a) Comparison of XES spectra of nc-Au/ZnO-nanorods with
those of pure ZnO nanorods at selected excitation energies Ex ; (b) XES and
corresponding XANES spectra of O 2p states of nc-Au/ZnO-nanorods and
pure ZnO nanorods [J. W. Chiou et al., Appl. Phys. Lett. 90, 192112.]
4.10. CONSTRUCTION TEAM
4.10
189
Construction Team
NSRRC (C. T. Chen, S. C. Chun, H. J. Lin, L. J. Huang, D. J. Wang, H. S.
Fung, J. M. Jung, C. Y. Liu, H. W. Fu, H. M. Tsai, C. S. Lee, C. Y. Hua,
C. H. Kuo, K. Y. Kao, Y. Y. Chin)
Max-Planck-Institute for Chemical Physics of Solids in Dresden-Germany
(L.H. Tjeng, C. F. Chang)
Tamkang University (W. F. Pong)
National Kaohsiung University (J. W. Chiou)