Fabrication of diffraction gratings for hard X-ray phase contrast imaging

Abstract

We have developed a method for X-ray phase contrast imaging, which is based on a grating interferometer. The technique is capable of recording the phase shift of hard X-rays travelling through a sample, which greatly enhances the contrast of low absorbing specimen compared to conventional amplitude contrast images. Unlike other existing X-ray phase contrast imaging methods, the grating interferometer also works with incoherent radiation from a standard X-ray tube. The key components are three gratings with silicon and gold structures, which have dimensions in the micrometer range and high aspect ratios. The fabrication processes, which involve photolithography, anisotropic wet etching, and electroplating, are described in this article for each of the three gratings. An example of an X-ray phase contrast image acquired with the grating interferometer is given.

Full text

Page 1

Fabrication of diffraction gratings for hard X-ray phase
contrast imaging
C. David*, J. Bruder, T. Rohbeck, C. Gru ¨nzweig, C. Kottler, A. Diaz, O. Bunk, F. Pfeiffer
Paul Scherrer Institut, CH 5232 Villigen-PSI, Switzerland
Available online 2 February 2007
Abstract
We have developed a method for X-ray phase contrast imaging, which is based on a grating interferometer. The technique is capable
of recording the phase shift of hard X-rays travelling through a sample, which greatly enhances the contrast of low absorbing specimen
compared to conventional amplitude contrast images. Unlike other existing X-ray phase contrast imaging methods, the grating interfer-
ometer also works with incoherent radiation from a standard X-ray tube. The key components are three gratings with silicon and gold
structures, which have dimensions in the micrometer range and high aspect ratios. The fabrication processes, which involve photolithog-
raphy, anisotropic wet etching, and electroplating, are described in this article for each of the three gratings. An example of an X-ray
phase contrast image acquired with the grating interferometer is given.
? 2007 Elsevier B.V. All rights reserved.
Keywords: Radiography; Medical imaging; Anisotropic silicon wet etching; Gold electroplating
1. Introduction
X-ray radiographic imaging is an invaluable tool to
investigate the inner structure of thick samples. The most
important applications are medical imaging and the inspec-
tion of products on production lines or luggage on airports.
The contrast mechanism presently used in such imaging
systems is based on the differences in absorption of the
samples constituents. However, for biological tissue sam-
ples, polymers or fibre composites, the use of conventional
X-ray radiography is limited due to their weak absorption.
By recording the phase shift of the X-rays passing through
the sample instead of the absorption, the contrast of radio-
graphs can be greatly enhanced. A number of techniques
have been developed in the past years to exploit X-ray
phase contrast [1,2], but as they all need a considerable
degree of coherence of the used radiation, none of them
can be used with the incoherent radiation of standard X-
ray tube sources. This is the reason why they have not
found any wide spread application in hospitals, factories,
or airports.
We have recently developed an interferometric X-ray
phase contrast imaging method based on diffraction grat-
ings [3,4]. The great advantage of the method is the fact
that it can be used with the polychromatic spectrum of
an incoherent X-ray tube [5]. The principle of the method
is based on detecting minute changes in the direction of
propagation, which are caused by refraction of the X-rays
passing through a phase shifting object. Equivalent to
refraction in the visible range, the change in direction is
proportional to the local gradient in phase shift, however,
it should be noted, that the refractive power of matter
for X-rays is many orders of magnitude weaker. The prin-
ciple of the experimental set-up is shown in Fig. 1. The
essential part of the interferometer consists of two gratings
placed between the object and the image detector, which
act as an array of collimating slits that have a transmission
depending strongly on the relative position of the two grat-
ings and the angle of incidence. Thus, any local phase gra-
dient in the object causes a local change in intensity
recorded on the detector. While the analyzer grating close
0167-9317/$ - see front matter ? 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.mee.2007.01.151
*Corresponding author. Tel.: +41 56 310 3753.
E-mail address: christian.david@psi.ch (C. David).
www.elsevier.com/locate/mee
Microelectronic Engineering 84 (2007) 1172–1177

Page 2

to the detector consists of an array of highly absorbing
gold lines, the beam splitter grating just downstream of
the object is made of phase shifting lines, which reduces
the losses of the set-up. Note that the described set-up does
not require monochromatic radiation. More details on the
optical considerations and the data acquisition procedure
can be found elsewhere [4].
It is evident, that such a set-up with only two gratings
(as shown in Fig. 1a) requires a source that provides suffi-
cient spatial coherence, i.e., which is small enough and far
away enough to provide a sufficiently narrow angular
uncertainty of the incoming rays. Whereas this is not a
problem at a synchrotron, where the source is usually less
than a millimeter in size and situated tens of meters away
from the experiment, this poses a severe restriction for
the use in laboratory equipment based on X-ray tubes,
which usually cannot be placed at sufficiently large dis-
tances for reasons of required compactness and flux den-
sity. This problem can be solved by introducing a third
grating just downstream of the source (see also Fig. 1b),
that essentially creates an array of spatially coherent line
sources. If the condition p0= p2· l/d is fulfilled, where p0
and p2are the periods of the source grating and the ana-
lyzer grating, l is the source grating to beam-splitter grating
distance and d is the beam-splitter grating to analyzer grat-
ing distance, then the images created by each line source
are superimposed in the image plane. The implementation
of such a source grating therefore makes it possible to use
incoherent radiation sources, resulting in an efficient use of
the available flux.
2. Grating fabrication
For each of the three gratings, a different fabrication
process was chosen, as the dimensions and X-ray optical
requirements are quite different. The source grating G0is
fairly easy to make, as its period p0is typically on the order
15–150 lm, Moreover, the total area of G0only needs to be
large enough to cover the source size, i.e., a few square mil-
limetres are sufficient. The main requirement of the grating
structures is a sufficient height of the gold absorber. In our
present setup, we use X-ray photon energies up to 30 keV.
It can be calculated from the X-ray optical constants of
gold [6], that 40 lm thickness corresponds to a transmis-
sion of only a few percent. We use a standard photo lithog-
raphy process for the pattern definition (see Fig. 2, left).
The samples are coated with a thin plating base of 15 nm
of titanium and 50 nm of silver. A 50 lm layer of SU8
resist (XP KMPR 1050 by Kayaku MicroChem Ltd.) is
spin-coated on to the plating base. After exposure and
development, the gold structures with heights of 40–
45 lm are plated onto the sample from a cyanide based
plating bath (Autronex CC-AF-B by Enthone Omi) using
a current density of 3.5 mA/cm2. After the deposition,
the SU8 resist can be stripped in N-methyl-2-pyrrolidone
(NMP) even though this is not required with respect to
the optical performance, as the resist is transparent to X-
rays. A scanning electron micrograph of the resulting struc-
tures is shown in the right part of Fig. 2.
The requirements for the beam splitter grating G1are
more challenging. It should consist of low absorbing,
Fig. 1. Schematic view of the grating-based X-ray interferometer for phase contrast imaging. The two-grating set-up (a) is used with spatially coherent
radiation such as synchrotron radiation, whereas the three-grating set-up (b) can be used with incoherent radiation from a standard X-ray tube.
C. David et al. / Microelectronic Engineering 84 (2007) 1172–1177
1173

Page 3

phase shifting structures with a period p1? 4lm. The best
performance is achieved when the structures have a duty
cycle of 0.5, and the height is chosen such that they intro-
duce a phase shift of p. Furthermore, the grating area lim-
its the field of view of the imaging device, so it should be
many centimetres in size, depending on the application.
Wechose 100 mm<110>
280 lm thick, both sides polished, as substrates. The wafer
size limits the area of our gratings to 64 mm · 64 mm. The
grating fabrication process is depicted in the left part of
Fig. 3. The grating structures are etched into the silicon
substrates. First, the photo resist pattern is transferred
oriented siliconwafers,
into a thin oxide layer which then serves as a mask for
the anisotropic wet etching process in 20% aqueous
KOH solution. At 76 ?C, we obtained an etch rate of
1.68 lm/min in the <110> direction. The etch rate along
the <111> directions is about 80· slower, resulting in
nearly perpendicular side walls of the structures. This is
confirmed by an inspection of the structures. The right
part of Fig. 3 shows a cross section of a beam splitter grat-
ing with a structure height of 22 lm, which corresponds to
a p-phase shift for 17.5 keV photon energy. The oxide
masking layer has been removed in buffered oxide etch
(BOE) before inspection.
Fig. 3. Left: fabrication process for the beam splitter grating G1. The <110> silicon substrate is anisotropically etched in a KOH solution resulting in high
aspect ratio structures with nearly vertical side walls. Right: cross section of a 4 lm period grating with a structure height optimized for 17.5 keV photon
energy.
Fig. 2. Left: fabrication process used to make the source grating G0. Right: cross section of a source grating with 127 lm period and a gold absorber
thickness of 42 lm. The SU8 resist has been removed in NMP solution. The silicon wafer thickness is 250 lm.
1174
C. David et al. / Microelectronic Engineering 84 (2007) 1172–1177

Page 4

The fabrication of the analyzer grating G2is the most
challenging part, as the grating period is only p2= 2 lm,
and large areas have to be patterned with good uniformity.
The required thickness of the gold absorber structures
should be sufficient to provide a high contrast. A transmis-
sion of the structures of less than 25% is acceptable. It can
be calculated [6], that 10 lm thickness is sufficient for pho-
ton energies below 20 keV. For photon energies of 30 keV,
about 25 lm of gold is required. We pursue two
approaches for the fabrication of high aspect ratio gratings
on 100 mm wafers with areas of up to 64 mm · 64 mm. The
first method is shown in the upper part of Fig. 4. A silicon
grating with a period of 2 lm and a duty cycle of 0.5 is used
as substrate, and the grating grooves are filled with gold by
electroplating. It is essential that the filling starts only at
the bottom of the grooves to ensure a complete, uniform
filling. For this purpose, a 200 nm thick sacrificial layer
of aluminium is evaporated under a sloped angle with
respect to the grating structures. Then the plating base con-
sisting of a 15 nm thick adhesive layer of chromium and a
Fig. 4. Top: fabrication steps to make the analyzer grating G2. A silicon grating is metallised with an Al sacrificial layer and a gold plating base, so that the
grooves of the grating can be selectively filled with gold by electroplating. Bottom: cross section of an analyzer grating with 2 lm period and 12 lm high
absorber structures.
C. David et al. / Microelectronic Engineering 84 (2007) 1172–1177
1175

Page 5

50 nm thick layer of gold is evaporated onto the sample
under normal incidence. By etching away the aluminium
in phosphoric acid, the gold is removed from the ridges
of the grating lines, whereas the plating base remains on
the bottom of the grooves. The same plating bath was used
as for the fabrication of the source grating G0, the current
density was again set to 3.5 mA/cm2. This procedure was
successful for the fabrication of structures with aspect
ratios up to 12, i.e., up to a structure height of up to
12 lm. The lower part of Fig. 4 shows the cross section
of an analyzer grating. For higher aspect ratios, the depo-
sition of the plating base in the grooves of the silicon grat-
ing becomes more and more difficult, as some of the metal
is deposited on the side walls. This results in an inhomoge-
neous, incomplete filling of the structures.
To obtain structures with even higher aspect ratios, the
method depicted in the left part of Fig. 5 was developed.
The basis of the process is a silicon grating, again produced
by anisotropic wet etching, which has a period of 4 lm and
a duty cycle of 0.25. A 15 nm thick adhesive layer of chro-
mium and a 50 nm thick layer of gold were evaporated
onto the grating. During the evaporation, the grating is
tilted by several degrees to assure that some of the metal
is also covering the side walls of the silicon structures.
The grating structures were then covered with a 1 lm thick
layer of gold by electroplating. Due to the high aspect ratio
of the silicon structures, most of the gold is deposited onto
the side walls resulting in absorber structures with an effec-
tive period of 2 lm. The high aspect ratio of the structures
inhibits a fast exchange of electrolyte solution, which leads
to a depletion of the electrolyte solution for high current
densities. In order to obtain a homogenous coverage of
the structures, it turned out to be necessary to reduce the
current density to only 0.2 mA/cm2. The right part of
Fig. 5 shows a grating with 24 lm high gold structures,
which provide sufficient contrast even for 30 keV photon
energy.
The main advantages of this method compared to the
one described in Fig. 4 are the bigger structure periods that
need to be made by photolithography, and the higher
aspect ratios that can be achieved. However, the control
of the duty cycle and the plated layer thickness is more crit-
ical in the latter process, as both have a direct effect on the
position of the gold structures.
3. X-ray imaging results
A number of samples have been imaged using the three-
grating set-up described in Fig. 1b [7]. As an example for
the phase contrast X-ray imaging, two images of a fish
are show in Fig. 6. Both the absorption contrast image
and the phase image were acquired with the same X-ray
dose. The phase image provided by the grating interferom-
eter reveals a great amount of additional information and
detail.
4. Conclusion and outlook
X-ray grating interferometry is a technique that can be
applied to obtain images of the local phase gradients of
large samples (several centimeters). It is possible to fabri-
cate suitable gratings using standard lithography tech-
niques. Future developments will focus on increasing the
field of view and the photon energy. From the point of view
of grating manufacturing it is feasible to produce gratings
on larger silicon substrates than the currently used
Fig. 5. Left: fabrication steps used to produce analyzer gratings with very high aspect ratio absorber structures. A silicon grating with a duty cycle of 0.25
is coated with a homogeneous gold layer. Right: cross section of an analyzer grating with an effective period of 2 lm and an aspect ratio of 24.
1176
C. David et al. / Microelectronic Engineering 84 (2007) 1172–1177