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Development of an experimental device for laser beam alignment on the LNE's metrological AFM

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Abstract

The LNE has developed its own metrological atomic force microscope (mAFM) which is now the French reference instrument for the measurement of AFM and SEM standards. This instrument uses four double path differential interferometers to measure the XYZ position of the tip relative to the sample. The measurement uncertainties of the mAFM were evaluated and the uncertainty budget shows that the main source of uncertainty for the positioning of the instrument originates from Abbe error. This error results from two main components: the parasitic rotations generated by the stage during the sample scan and the misalignment of the laser beams with respect to the tip. The objective of the internship was to work on the reduction of the laser beam misalignment.
Master CDIM 2018-2019 Dourati AHAMED
Development of an experimental device for laser beam
alignment on the LNE’s metrological AFM
D. Ahamed, S. Ducourtieux, A. Delvallée
Introduction
The LNE has developed its own metrological atomic force
microscope (mAFM) which is now the French reference instrument
for the measurement of AFM and SEM standards. This instrument
uses four double path differential interferometers to measure the
XYZ position of the tip relative to the sample. The measurement
uncertainties of the mAFM were evaluated and the uncertainty
budget shows that the main source of uncertainty for the
positioning of the instrument originates from Abbe error. This error
results from two main components: the parasitic rotations
generated by the stage during the sample scan and the
misalignment of the laser beams with respect to the tip.
Tasks
The objective of the internship was to work on the reduction of the
laser beam misalignment. For this purpose, the needs for a better
alignment were identified. A device was designed in Solidworks to
address those needs, machined and then assembled. This device
(figure 2) is composed by a CCD camera that detects the laser beams
projected on a screen translated vertically thanks to a motorized
stage. A code has been developed under Labview to control the
device. Then an alignment protocol respecting as well as possible the
perfect alignment configuration represented on figure 1 has been
written. Finally, the performances achieved with the device (resulting
Abbe error before and after alignment) have been evaluated.
Results
The developed device showed on figure 2 uses a CCD camera with a
resolution of about 45 µm and a motorized thin screen that can be
translated in Z direction to detect the path and position of the
beams. When localized, the beams can be translated to their ideal
position by moving interferometers with a specific tool and with an
accuracy of about 50 µm.
Conclusion
In the end, the laser beam alignment have beam improved by a
factor of 10. The beams are now Abbe compliant at 0.3 mm and Abbe
error reduced from several tens of nanometer to only 6 nm (figure 3).
Journal of Internship - 2019
Keywords: metrological atomic force microscopy, Abbe error, uncertainty, beam alignment, interferometer
Figure 1: Perfect alignment configuration
Figure 2: Experimental device mounted on
mAFM.
Figure 3: beam alignment realized with device
on Abbe plan
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