FORMATION OF BRAGG DIFFRACTION IMAGES BY SIMPLIFIED SECTION TOPOGRAPHY

Field of the invention

This invention relates generally to the imaging of internal features in a single crystal, or in a multi crystal object with large crystal size, utilizing penetrating slit collimated x-ray radiation to form Bragg diffraction images of sections through the object. Although the technique of the invention is applicable to the formation of individual two-dimensional images, multiple such images derived from serial sections of diffraction through the object may be stacked by known techniques to derive three-dimensional reconstructions of internal features.

Background of the invention

Conventional Bragg diffraction section topography utilises a monochromatic x-ray source or takes specific characteristic lines from a polychromatic x-ray source; the angular position of the object and the relative position of the source and the detector may be selected to optimally receive Bragg diffractive radiation at the detector. Further, the incident x-ray radiation may be collimated using a very small aperture or very narrow slit to restrict the divergence angle of the incident beam which illuminates the object, and commonly a second set of slits are used to restrict the acceptance angle of the diffracted beam at the detector thus defining the Bragg diffraction within a very narrow range.

The present invention seeks to provide a useful alternative to, or at least a partial augmentation of, these conventional approaches.

Summary of the invention

The invention entails the concept of masking or occluding different Bragg trajectories for respective characteristic lines of incident radiation on an object by using a relatively wide slit with its opening extending in a direction normal to the diffraction vector to define the incident Bragg angle so as to allow only one characteristic line to illuminate the sample thereby avoiding overlapping multiple images from multiple characteristic

lines; and preferably to allow the crystal itself to select the Bragg diffraction from the single characteristic line according to the angular position of the object relative to the source and detector. Further, the invention entails the acquisition of multiple serial diffraction images by moving the object incrementally in small steps in a direction normal to the incident beam and in the plane of the diffraction vector.

The invention therefore provides a method of imaging internal features in a single crystal or multi crystal object, including:

directing toward the object penetrating x-ray radiation having a plurality of strong characteristic lines that exhibit different Bragg angles with respect to the crystal planes in the object;

collimating said radiation to spatially occlude Bragg trajectories from unwanted lines of said strong characteristic lines while passing Bragg trajectories for one said strong characteristic line;

recording an image of x-ray radiation diffracted by a section of the object, including at least one line of Bragg diffraction by the object; and

acquiring serial section diffraction images by relatively moving the object incrementally in steps in a direction normal to the diffracted beam and in the plane of the diffraction vector.

By "Bragg trajectory" is meant herein the wavefront trajectory that forms a Bragg angle with respect to a set of crystal planes in said object and for which the radiation will therefore be Bragg diffracted from that set of crystal planes.

Preferably, said penetrating x-ray radiation directed towards the object, hereinafter referred to as the incident x-ray radiation, is derived from an x-ray source sufficiently small to allow resolution of the internal features of interest in the object in the direction normal to the direction of the diffraction vector. Typically, for many applications, such a source will be of diameter in the range 10 to 50 microns. Preferably the source will be relatively distant from the object, typically for many applications in the range 100 to

300mm. Preferably the slit collimation will have a relatively wide separation, typically for many applications in the range 100 microns to 1 mm.

The invention further provides apparatus for imaging internal features in a single crystal or multi crystal object, including:

means to mount the object;

means to direct towards the object penetrating x-ray radiation having a plurality of strong characteristic lines that exhibit different Bragg angles with respect to the object;

means to collimate said radiation to spatially occlude Bragg trajectories from unwanted lines of said strong characteristic lines while passing Bragg trajectories for one said strong characteristic line;

a detector for recording an image of diffracted x-ray radiation, including at least one line of Bragg diffraction by the object; and

means to relatively move the object incrementally in steps in a direction normal to the diffracted beam and in the plane of the diffraction vector whereby to acquire serial section diffraction images.

There is preferably further included a source of said penetrating x-ray radiation directed towards the object, hereinafter the incident x-ray radiation, which is sufficiently small to allow resolution of the internal features of interest in the object. Such a source may be of a diameter in the range 10 to 50 microns.

Preferably, the source is sufficiently distant from the object to facilitate sufficient separation at the collimating means of the Bragg trajectories for said strong characteristic lines. Typically this separation would be in the range 100mm to 300mm.

The collimating means is advantageously a slit collimator, preferably adjustable to select different Bragg trajectories for occlusion or passage. Typically the separation of said slit would be in the range 100 microns to 1 mm.

Preferably, in both the method and the apparatus, the collimator is in a direction normal to the diffraction vector. By this is meant that the preferred slit extends longitudinally in a direction normal to the diffraction direction (and the diffraction vector), and parallel to the diffracting plane.

Brief description of the drawing

The invention will now be further described by way of example only, with reference to the accompanying drawing which is an optical diagram depicting the functioning of the apparatus and method of the invention, and showing the Bragg trajectories for three typical characteristic lines of the radiation emitted by the x-ray source.

Embodiments of the invention

In the illustrated configuration, an x-ray source 12 with associated optics generates an incident beam of x-ray radiation 14 directed towards a single crystal or multi crystal object 20. Object 20 is on a mount 22 to permit its rotational and translational adjustment as further discussed below. Components of incident radiation 14 are Bragg diffracted in the general direction of a detector 30 that detects and records a two- dimensional image 32 of the Bragg diffracted x-rays 16 from the section of the object illuminated by the X-rays. This detector 30 may typically be a charge-coupled diode (CCD) array. A representative diffracting plane is indicated by line 24.

The incident beam 14 is collimated by an adjustable slit collimator 40 as further discussed below, and the undiffracted component intersects a beam stop 42 after emerging from the object.

X-ray source 12 is chosen to be sufficiently small to allow resolution of the internal features of interest in the object perpendicular to the direction of the diffraction vector, and for this purpose may typically be of a diameter in the range 10 to 50 microns.

Mount 22 is not illustrated in detail but is typically a ball and socket mount capable of being tilted in any direction without limitation of movement (that is, 360 degree continuous), which in turn is mounted in a rotation stage with 360 degree rotation. This complete assembly 22 is mounted on a linear translation stage 26, which is orientated at right angles to the incident beam and has a driver to move it incrementally in selectable steps in the plane of the diffraction vector, in the direction normal to incident beam 14, indicated by arrow-headed dotted line 27.

In general, source 12 includes a target that generates a number of strong characteristic lines. For example, for a molybdenum target, such lines include those designated Ka1 , Ka2, Kb1 and Kb2. Because the Bragg angle is dependent upon the wavelength and the crystal plane spacing to define the condition for constructive interference, the Bragg trajectories for the strong characteristic lines - that is the trajectories that will define a Bragg angle with respect to a set of planes in the crystal or a crystal of the object - will have a small angular separation to the source for a given set of crystal planes and for a given source to object distance. In conventional x-ray diffraction imaging, this collimator separation is typically less than 20 microns. In the illustrated configuration, however, if the distance of the source 12 from the object 20 is sufficient (eg 100 to 300mm, typically around 200mm) to separate the Bragg trajectories by a practical finite amount at collimator 40, the adjustable slit 41 of collimator 40 can have more than 100 microns separation to spatially occlude Bragg trajectories other than that of interest.

This is depicted in the diagram for three Bragg trajectories 14a, 14b, 14c for respective strong characteristic lines of the incident radiation 14. Detector 30 is optimised in its position to record a two-dimensional image 32 of the Bragg diffracted x-rays 16b generated by the trajectory 14b passed by collimator 40, having a Bragg angle θ _B with respect to a diffracting crystal plane set represented by line 24. This image may well include a representation 21a of an internal feature 21 in the object. Bragg trajectories 14a, 14c are occluded by collimator 40.

In a practical system, it will of course be understood that the collimation and recordal steps are repeated for a sequence of displacements of the object by mount 22, incrementally in steps, in direction 27 normal to the incident beam and in the plane of

diffraction to obtain a serial set of two-dimensional images 32 for successive parallel sections of the object. These images can then be stacked together by known reconstruction techniques to produce a three-dimensional representation of at least a segment of the object including the internal feature 21.

During the afore-described steps, the relative orientation of the object 20 to the incident x-ray beam 14b and the direction of detection may be set to optimise the characterisation of internal features by any one or more of the following known actions:

• selection of the 2-θ _B angle to illuminate the preferred crystal plane to give general maximum brightness of the diffracted image with suitable image size;

• rotation of the object about the axis normal to the 2-θ _B plane;

• tilting of the object to bring reflections into the 2-θ _B plane.

• imaging of the object using wide open slit collimation to allow likely reflections to be located in the CCD detector 30;

• tilting of the object to bring the diffraction images normal to the rotation axis;

• adjustment of the slit 40 to further more narrowly define the incident radiation reaching the object;

• fine-tuning of the image to optimise detection according to the requirements of internal feature imaging.

It will be further appreciated that the inventive method may be applied to successive orthogonal crystal directions as required to image the strain field around a feature of interest.