This article has been supplied by PI (Physik Instrumente).

Tailor-made positioning solution for nanotomography: mechanical system approaches limit of technical feasibility

At the X-ray light source PETRA III at the DESY research center (German Electron Synchrotron) in Hamburg, Germany, the Helmholtz-Zentrum Geesthacht - Center for Materials and Coastal Research (HZG) operates the Imaging Beamline P05, which includes two experimental hutches, one for nanotomography and one for microtomography. In the nanoto-mography hutch, X-ray optics for three-dimensional micrographs with resolutions around 100 nm are used. The setup also includes microscopy optics for visible light, used for further magnification of the X-ray micrographs and their transfer to a camera.

With the aim to carry out as many different experiments as possible, the HZG provides two different X-ray optics configurations: An imaging setup, in which the sample is positioned in front of the objective optics , and a cone-beam setup, in which the sample is placed in the diverging beam behind the optics. In both cases, high mechanical stability and precision positioning are essential in order to obtain micrographs of high quality.

However, thanks to the close cooperation of the clients with the engineers and developers from PI (Physik Instrumente), this complex task could be solved in a practice-oriented manner.

A particular challenge was how to configure the control, which was based on an industrial controller. The challenge consisted in controlling almost 50 axes independently of one another while ensuring collision protection. The entire system was finally integrated into the TANGO interface customary for beamlines.

The Base:

Granite Platform Supported by Air Bearings

To minimize the effect of vibrations and securely fasten the individual components and stabilize them, relative to one another, a granite base 6.8 m in length forms the basis of the instrument. Another four moving granite platforms driven by linear motors are arranged on this base on air bearings. This makes it possible to position all components with high speed and precision: The sample stage, the X-ray optics, and the detector. The substructure itself, which weighs several tons, is also mounted on air bearings. This allows the entire assembly to be moved out of the X-ray beam with minimal effort when the second experimental station is to be used, while maintaining a stable position as soon as the air flow is switched off.

Complex Sequences during Sample Positioning

The basis of sample positioning is a horizontal positioning unit which moves the sample stage into the beam. It has a travel range of 20 mm, can be subjected to a load of 300 kg and works with a repeatability of 30 nm.

This displacement unit is equipped with three lifting elements which perform the height adjustment, tilt correction, and orthogonal alignment, relative to the beam. It is based on three identical, symmetrically arranged, and position-controlled stepper motors, combined with worm gears and spindle drives. Mounted on this Z stage is an air-bearing supported rotation stage. In developing this stage, the designers had to go push the limits of technical feasibility: What was required was a really “pure” rotary motion of the sample with minimal wobble, radial runout or eccentricity. Only in this case can sharp pictures over 360 degrees be made which all refer to the same volume element and can all be clearly assigned when reconstructing the picture. This is why the rotation stage, which rotates at a velocity of 36 °/s, works with flatness deviations of less than 100 nm at a resolution of 0.5 µrad. The air bearing does not produce any friction, which over time would lead to a deterioration of these values.

Parallel Kinematics for the Sample Holder and the Optics

The actual sample holder is located in the aperture of the rotation stage on the moving platform of a six-axis parallel kinematic system. The samples are positioned with six degrees of freedom. Essential features are the freely selectable pivot point of the parallel-kinematic system and its high stiffness. A six-axis parallel-kinematic system of this type is also used for the positioning of the optics. In nanotomography, which allows three-dimensional micrographs with resolutions below 100 nm, this machine is used to align compound refractive lenses (CRL) in the beam with high precision.

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