X-Ray Diffraction on Surface Acoustic Waves
and Application to X-Ray Imaging
R. Tucoulou1, I. A. Schelokov1,2, D. V. Roshchupkin2, M. Brunel1
1
CNRS, Laboratoire de Cristallographie,
BP 166X, F-38042 Grenoble Cedex 09, France
2
Institute of Microelectronics Technology and Superpure Materials,
Russian Academy of Sciences, 142432 Chernogolovka,
Moscow District, Russian Federation
Abstract. New optics based on X-ray diffraction on surface acoustic waves
are being developed to perform X-ray space scannings with high resolution.
First results and optical properties of this system will be discussed here.
1 Introduction
X-ray lithography, X-ray local analysis of microstructures are strongly developing to
fullfill new requirements issued from many different fields such as medicine, biology,
microelectronics, etc.. Some great improvements have been done in X-ray optics,
particularly in the field of X-ray focussing with Bragg-Fresnel zone plates. But up to
now, it has to be noted that the easy monitoring of electron beams through
electromagnetic fields for example, has no equivalent for X-ray beam control. In any
case of X-ray local analysis, it is necessary to move the sample with motors instead of
deviating the X-ray beam direction.
It has been reported in previous articles [1, 2, 3, 4] that space-time modulation of
an X-ray beam can be achieved by diffraction on surface acoustic waves providing a
new tool for the design of X-ray beam monitoring systems.
We present here some general features and the first attempt to carry out a X-ray
imaging of a fixed sample.
2 Fundamentals of X-Ray Diffraction on Surface Acoustic Waves
The interaction of X-rays with surface acoustic waves (SAW) has already been studied
and presented in a number of articles [1, 2]. The periodic modulation of a piezoelectric crystal surface (LiNbO3) introduces some constructive interferences in the
reflected radiation which produce some satellites around the specular reflected beam,
whose angular directions are given by the grating equation:
cos α m = m
λ
+ cos α 0
Λ
(2.1)
with α0 the incident angle, αm the emergent angle of the mth order satellite, λ the X-ray
wavelength, Λ the acoustic wavelength (see Fig. 1 for the experimental set-up).
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Fig. 1. Experimental set-up
According to relation (2.1), the angular deviations of the satellites with respect to
the reflected beam are determined by the acoustic wavelength. Since the SAW is
generated by a transducer (IDT) fed by a RF signal whose frequency determines the
acoustic wavelength, it is easy to control electronically the positions of the satellites
by controlling the electric frequency.
Fig. 2. Detector scan around the reflected beam
X-ray wavelength: 1.54 Å, acoustic wavelength: 30 µm,
acoustic amplitude ≈ 13 Å, incident angle: 0.22o
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A detector scan showing the first order satellites around the reflected beam is presented in Fig. 2. The efficiency of the production of the first order satellites can reach
15 % of the intensity of the direct beam in the case of total external reflection.
3 Space Scans of Satellites
The standard acoustic emitter is called: interdigital transducer (IDT) and works
efficiently around a resonance frequency and has hence a frequency bandpass (in
terms of acoustic amplitude) inside which the production of satellites will be strong.
∆f
is estimated to be around 10 %. Varying the electric frequency fed to the IDT
f0
inside its bandpass involves a shift in the position of the satellite positions according
to the relation (2.1). At one meter from the piezoelectric crystal, a standard IDT with a
resonance frequency of 116 MHz will be able to bring about a space scanning of the
+1 order satellite of the order of 200 µm (∆α ~ 0.016°).
The resolution of this scan (the smallest step which can be performed) depends on
the precision of the RF generator. At one meter from the crystal, this resolution is
estimated to be very good, around 1 nm ! In fact, for hard X-rays, the size of the beam
will be the restricting factor since the smallest size achieved is about 1 µm2.
In order to increase the size of the scanned area, it is necessary to deposit on the
piezoelectric surface several standard IDTs whose bandpasses overlap.
4 X-Ray Imaging of a Fixed Sample
We report here the first attempt to characterize a fixed sample along one direction by
shifting step by step the +1 satellite. The sample to be tested is a regular metallic
grating made of cylindrical wires of diameter 50 µm separated by 50 µm each.
A scintillator totally opened is placed behind the grating and record the intensity of
the +1 satellite transmitted through the grating. It is expected to reproduce the shape
of the grating in the recorded intensity. Fig. 3 shows the principle of the experiment.
In order to preserve the detector from the 0 and -1 order radiation, it is necessary
to use a stop beam as it is shown in the Fig. 3.
9 IDTs have been deposited on the crystal surface in order to obtain a global
frequency bandpass running from 95 MHz to 170 MHz. At 30 cm from the crystal
(where the grating is placed), it should provide a scan of 280 µm for the +1 satellite.
The experimental mode consists in repeating twice a frequency scan over the
bandpass with a step of 1 MHz. During the first scan, the sample is removed in order
to record the apparatus function (since the +1 satellite intensity depends on the
frequency). Figure 4 presents the result of the two frequency scans.
It is clear that the resonance frequencies of the 9 IDTs should have been chosen
more closely one from another.
The reconstruction of the grating profile is obtained by doing the ratio between the
two recorded intensities (see Fig. 5).
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Fig. 3. Experimental set-up
Fig. 4. White circles: Scan without grating,
lack circles: Scan with grating
Three periods of the grating are clearly visible and correspond to the expected size
of the scanned zone. The obtained profile is not very sharp and the intensity does not
cancel when the +1 satellite runs into a wire. This is partly due to the fact that it was
not possible to precisely align vertically the grating and that the wires have not a good
definition.
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5 Conclusion
This experiment shows the feasibility of a beam monitoring system based on the
diffraction on surface acoustic waves. The main advantages of this system is the
opportunity to keep the sample fixed and the extremely high precision of the X-ray
scan.
On the other hand, it is difficult to design some large band systems diffracting
some constant intensity towards the satellites.
It has to be noted that 2D scans have already been achieved using two electroacoustic devices placed in a Kirkpatrick-Baez scheme.
Focusing optics are compatible with the insertion of an electro-acoustic device in a
optic chain [3] and can provide the possibility to carry out microanalysis of fixed
samples.
References
1
D.V. Roshchupkin, M. Brunel, F. de Bergevin, A.I. Erko, Nucl. Instr. and Meth.
B72 (1992) 471.
2
D.V. Roshchupkin, I.A. Schelokov, R. Tucoulou, M. Brunel, IEEE Trans. Sonics
and Ultrasonics 42 (1995), 127.
3
D.V. Roshchupkin, M. Brunel, Rev. Sci. Instrum. 64 (1993), 379.
4
R. Tucoulou, I. A. Schelokov, D.V. Roshchupkin, B. Brunel, L. Ortega,
P. Chavallier, Optics Comm. 118 (1995), 175.