Elsevier

Microelectronic Engineering

Volume 84, Issues 5–8, May–August 2007, Pages 1172-1177
Microelectronic Engineering

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

https://doi.org/10.1016/j.mee.2007.01.151Get rights and content

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.

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 inspection 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 samples, 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 radiographs 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 gratings [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 principle 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 gratings and the angle of incidence. Thus, any local phase gradient in the object causes a local change in intensity recorded on the detector. While the analyzer grating close 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 sufficient 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 distances for reasons of required compactness and flux density. 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 p2 are the periods of the source grating and the analyzer grating, l is the source grating to beam-splitter grating distance and d is the beam-splitter grating to analyzer grating 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.

Section snippets

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 G0 is fairly easy to make, as its period p0 is typically on the order 15–150 μm, Moreover, the total area of G0 only needs to be large enough to cover the source size, i.e., a few square millimetres 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 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 interferometer reveals a great amount of additional information and detail.

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 fabricate suitable gratings using standard lithography techniques. 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 100 mm wafers. For typical applications in

Acknowledgement

This work was supported by the Swiss Commission for Technology and Innovation KTI/CTI under contact 7796.2 DCPP-NM.

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