Synchrotron Laminograhy Technique to Examine Large Surface Objects

Computed tomography(CT) is a well-established technique for use with synchrotron radiation sources for obtaining bulk information from a number of sample types at micrometer resolution.

However, it is known that examining planar test objects or extended flat microsystems is mostly not satisfactory when the sample is significantly larger than the area of interest. The change in X-ray transmission during a scan mostly creates artefacts at the time of reconstruction.

In order to overcome this limitation, a new imaging technique was introduced that helps calculate 3D representations of widely extended, flat objects. In order to obtain meaningful data, it was required that the detector and sample be positioned with high stability and precision. This challenging task was solved by deploying a positioning system developed specifically for this application.

In a joint collaboration, the ANKA (Angströmquelle Karlsruhe) at the KIT (Karlsruhe Institute for Technology, Germany), the Fraunhofer IZFP (Institute for Non-Destructive Testing) Saarbrücken/Dresden, Germany, and the ESRF (European Synchrotron Radiation Facility), Grenoble, France as shown in Figure 1 developed synchrotron laminography, which helps examine large-surface objects. Since 2007, the instrument has been in operation, at the synchrotron radiation source ESRF at the imaging beamline ID19.

Figure 1. ESRF european synchrotron radiation facility

Maximum Demands on Positioning of Sample and Detector

In the case of conventional CT, reconstruction of volume information and enlarged asymmetrical bodies (such as plates, for example) is not possible as the various long radiation paths in the sample prevent consistent measurement of the projection data. The sample is scanned under rotation around an axis tilted relative to the beam direction. It is possible to reconstruct volume data from a different projection. As in Figure 2, the sample positioning is done between the detector and the X-ray source.

Figure 2. Principal arrangement of computer laminography at the Beamline ID 19 at the SRF.

In order to enable subsequent reconstruction of meaningful images, high stability and precision is required. For both sample and detector positional stability is required during imaging.

The detector weight is around 100kg, and is not at its center of gravity. It is a challenge to position the load with straight motion below 0.1µrad or 100nm resp. and 50nm resolution. Torque and leverage is eliminated at the same time. During sample positioning the angle at which the sample is exposed to the synchrotron X-ray beam should be adjustable.

It is important that sample positioning is adjustable individually, repeatably and securely. Further, the complete instrument must be easily maneuvered from the optical path while not being used or at the time of reference measurements.

Practical Solution for a Complex Task

A practice-oriented approach was adopted. Physik Instrumente has valuable expertise and several years of experience in beamline instrumentation. The research team, coordinated by PI miCos, develops application specific solutions which go beyond offering separate elements and include system integration as also the complete instrumentation. For computed laminography, this feature has again been demonstrated.

The detector and sample positioning include three co-operating systems as shown in Figure 3, a Z stage with granite base, a detector stage moveable in three directions, and sample positioning. The sample positioning system includes a six-axis positioning system and a rotation and tilting stage on which the actual sample carrier is held magnetically.

Figure 3. The detector and sample positioning consists of three complimentary systems: a Z stage with granite base, a detector stage moveable in three directions, and sample positioning. The latter consist of a six-axis positioning system and a rotation and tilting stage on which the actual sample carrier is positioned contact-free. (Image: PI, Fraunhofer EZRT, ESRF)

Details That Matter

The following issues were taken care of during the design of this positioning system:

• While designing the Z stage, the overall 2.5 ton weight was challenging as it had to be handles accurately and be lifted in parallel. A three-point bearing was used to achieve this. This enabled the unit to be shifted with minimal force and retain stability as soon as air supply is switched off.

• Granite base tilting can be readjusted. A controller manages the complete setup using joystick and a positioning display.

• The detector stage design is also a sophisticated one. The overhanging load of the 50kg heavy detectors must be moved over a range of 850x300x500mm. The absolute deviation must not be more than 100nm and tilting to +/- 30µrad is allowed.

• Other precisely matched components include the drive connected via centrally arranged ball screws, needle guidance and a very accurate optical linear encoder.

• A high transmission ratio in a zero-play drive enable self-locking of the vertical axis

Positioning Samples with Sub-Micrometer Accuracy

Now the samples must be accurately positioned. Here the six-axis positioning system plays its role (Image 4). This SpaceFAB is symmetrically designed, with three legs having a specific length are mounted on an XY stage in a ball joint.

The SpaceFAB platform is mounted to the legs via a cylindrical bearing in each case. The XY combination lower stages are combined into the granite base via guidings. Hence, the samples can be positioned with six degrees of freedom.

Important features are the freely selectable pivot point of the parallel-kinematic system and its high stiffness. The linear travel ranges are 150× 150×50mm, at 0.2µm position resolution, ±12.5° tilting is possible for the axial angle, and ±5° for the other directions.

Figure 4. Positions samples with micrometer precision: the six-axial parallel kinematic is symmetrically designed, where three "legs" of fixed length interact with the platform, which in turn are each supported by an XY stage combination (Image: PI)

On this parallel kinematic, a tilting stage and a combined rotation, is mounted. The rotating table enables 360° rotation at only 0.24µm absolute flatness deviation.

The repeatability of sample positioning with the SpaceFab has been specified and measured at below 0.5µm following reference measurement. Rotation eccentricity was below 0.5µm. This is essential so that the different projection angles have the same projected rotation center.

If the accuracy was not high enough, artefacts would occur during reconstruction. The compact construction height enables shallow tilting angles so that the synchrotron beam does not penetrate through the mechanical elements, hence making it impossible to do projection recording. An optical encoder ring assures high angular resolution.

Furthermore, the sample angle to the X-ray beam can be adjusted by up to 45° via the tilt stage at a resolution of 0.001°. The design includes a self-locking rack and pinion drive and is stable during examination.

The sample holder, which is a thin frame carrier, is also an excellent piece of technology. It is supported by Teflon cushions and is coupled magnetically. The sample holder is centered by two linear stages angled at 90° in relation to the rotating axis, which shift the sample holder over 150×150mm.

It is possible to switch the magnetic retention on and off, and a flexure joint and air cushion offer optimal parallelism.

Conclusion

The study results that can be achieved with this kind of a synchrotron laminography method can benefit a host of fields, from industry-oriented research to geology and life sciences. Figure 5 shows an example from microelectronics, where the quality of soldering points is crucial. A key contribution is provided by the bespoke positioning solution created by the specialists of the "Beamline Instrumentation", which can even align large samples and consequently rather high loads with micrometer precision.

Figure 5. 3D view of microstructure and voids inside soldering joints (Image: KIT)

This information has been sourced, reviewed and adapted from materials provided by PI (Physik Instrumente) LP, Piezo Nano Positioning.

For more information on this source, please visit PI (Physik Instrumente) LP, Piezo Nano Positioning.