PosterPDF Available

Abstract

In 2018, LNE’s metrological atomic force microscope (mAFM) performed its very first calibrations on standards developed at LNE (P600H60) in collaboration with C2N (Centre for Nanoscience and Nanotechnology). It provides a French traceability route to the SI meter for dimensional measurement at nanometer scale for calibration of standards commonly used in scanning probe and scanning electron microscopy. To measure in real time the position of the tip relative to the sample, the mAFM uses four double path differential interferometers whose He-Ne laser sources are frequency-calibrated. The measurement uncertainty of the mAFM was evaluated using a model of the metrology loop and a Monte Carlo approach and is estimated to +/- 2 nm and +/- 1 nm (k=2) for respectively P900H60 pitch and step height. The several studies conducted show that the main source of uncertainty for the positioning of the instrument comes from Abbe error. It represents 75% of the combined uncertainty and originates from residual misalignments of the interferometers (+/- 1 mm at k=2) combined with the parasitic rotations of the scanning stage (about 20 µrad in the worst case). In a perspective of continuous improvement of the measurement uncertainty, a more precise method to align the interferometer beams with the tip had to be found.
Minimization of Abbe error on the LNE’s metrological AFM by alignment of
interferometer laser beams using a CCD camera
S. Ducourtieux, D. Ahamed, A. Delvallée
LNE DMSI Nanometrology, 29 avenue Roger Hennequin, 78197 Trappes - France
Sources Evaluation
type Notation unity ui Rank
Re
-positioning of the alignment setup on the metrological AFM A u1 µm 101 2
Re
-positioning of the Zerodur prism on the metrological AFM B u2 µm 10
Re
-positioning of AFM tip on the tip holder B u3 µm 2
Positioning
of the tip holder on the metrological AFM head
Not evaluated
u4
Re
-positioning of the metrological AFM head A u5 µm 11
Screen
inclination with respect to Abbe plan
Not evaluated
u6
Screen shape
Not evaluated
u7
Determination of the measurement axis of
the interferometer B u8 µm 300 1
Interferometers
positioning with micrometer screw B u9 µm 5
Interferometers
misalignment during the clamping step A u10 µm 15
Tip/target alignment
B u11 µm 30 5
Mask/target
alignment B u12 µm 55.6 3
Camera pixel size calibration
B u13 µm 0,001
Fit of the beam profile for
position determination
Not evaluated
u14
Screen
thickness B u15 µm 50 4
Accuracy of the
spot position detection using the developed software B u16 µm 5.56
Reference prism
Measurement prism
Tip position
24.4 mm
7 mm
Motor
Stage
V
Screen
Camera
LNE’s metrological AFM:
In 2018, LNE’s metrological atomic force microscope (mAFM) performed its very first calibrations on standards developed at LNE (P600H60) in collaboration with C2N
(Centre for Nanoscience and Nanotechnology). It provides a French traceability route to the SI meter for dimensional measurement at nanometer scale for calibration of
standards commonly used in scanning probe and scanning electron microscopy. To measure in real time the position of the tip relative to the sample, the mAFM uses four
double path differential interferometers whose He-Ne laser sources are frequency-calibrated. The measurement uncertainty of the mAFM was evaluated using a model of the
metrology loop and a Monte Carlo approach and is estimated to +/- 2 nm and +/- 1 nm (k=2) for respectively P900H60 pitch and step height. The several studies conducted
show that the main source of uncertainty for the positioning of the instrument comes from Abbe error. It represents 75% of the combined uncertainty and originates from
residual misalignments of the interferometers (+/- 1 mm at k=2) combined with the parasitic rotations of the scanning stage (about 20 µrad in the worst case). In a
perspective of continuous improvement of the measurement uncertainty, a more precise method to align the interferometer beams with the tip had to be found.
Metrological AFM
P900H60
(2D grating standard)
Interferometer Detail of the metrology loop and laser beam alignment to meet Abbe principle:
6,75 mm
Measurement
beam
Reference prism (static)
Measurement prism
(mobile)
Sample holder with
an adjustable thickness
Sample
Reference
beam
Abbe plan
AFM tip
Setup used for laser beam alignment:
Z motor
CCD camera
Camera lens
Z translation
stage
White screen
Specific software developed under LabVIEW:
Continuous acquisition of images.
Pixel size calibration using the screen holder dimensions.
Possible adjustment of the camera exposure to improve spot detection.
Automatic display of crosshair to assist the beam alignment.
Pattern recognition to detect the spots and precisely determine their positions.
Gaussian profile fit on the detected pattern to increase the position determination
accuracy.
Results:
Uncertainty budget:
7 mm
14 mm
Theoretical alignment
Reference mirror
Mobile mirror
Alignment tool
used to translate
interferometers
Developed setup Setup mounted on the
instrument
Alignment achieved with an accuracy of 0,6 mm
Conclusion:
Mobile mirror
Distance between
spot: 7 mm
Possible angular
misalignment: 5°
Theoretical alignment Alignment uncertainty
The main source of uncertainty is due to the
impossibility to observe the second spot on the
measurement arm of the interferometer. Some
unfavourable assumptions are necessary to
estimate its position leading to the 300 µm
uncertainty for this components.
Before alignment After rough alignment After fine alignment
Target
Alignment of the
tip with the target
q
A specific protocol has been developed to reduce impact of Abbe error on the LNE’s mAFM by improving the accuracy of laser beam alignment. It uses
a specific setup and a CCD camera to visualize in the sample plane the tip position which is tracked by a target and the laser spots. A tool is then used
to translate and position the interferometer measurement axes with respect to the tip. With this protocol, the accuracy of laser beam alignment has
been improved from 4,5 mm to 0,6 mm and Abbe error reduced by a factor of 7 (the maximum parasitic rotations are still 20 µrad).
This alignment is limited by the lack of knowledge on the position of the secondary spots (dual path interferometer). Solutions are being studied to
overcome this limitation.
Abbe plan
Spot arrangement in
Abbe plan
... Most usually an additional interferometer axis (or axes for up to six degrees of freedom characterization) is used [17,18,[29][30][31][32] (which is costly). A different approach might be, for example, a CCD-assisted alignment [33] (which probably will not keep pace with the phase measurement) or in-situ estimation through physically "modulating " the pitch and yaw [34] (that require repeated measurement). ...
Article
Full-text available
Besides the environmental fluctuations, the typical sources of significant uncertainty in the laser interferometry systems are the geometrical errors. These are stemming, among others, from the misalignment of measurement axes, the thermo-mechanical influences of the system components and the mounting, guidance errors of the translation mechanism that carries the measurement mirror or vibrations. We report on a compact double-pass differential plane interferometer that features an original optical arrangement with four parallel and coplanar beams, where the beam pairs in the two arms are coaxial. The differential arrangement minimizes the dead path and shortens the metrological loop so that the sensitivity to thermal drifts and vibrations is reduced. The arm symmetry allows for the preservation of the Abbe principle, and the common path mitigates the influences of the environmental disturbances. The interferometer optics is designed as a self-contained single-piece assembly made using optical contacting from the low-expansion materials. The interferometer system integrates the homodyne receiver (even though the optical arrangement is well-suited for heterodyne detection too) and also a tilt-detection electronics that allows for detection of pitch and roll of the interferometer mirror so that the parasitic movement of the measuring mirror could be compensated for. The experimental characterization revealed a good optical performance of the interferometer with sub-nanometre cyclic error and the resolution of tilt detection in order of a few microradians.
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