Profilometry – A Guide to Discovering Micron-Sized Objects

In numerous industrial and research applications, contactless and ultra-precise surface analyzes are of considerable interest to assure the quality of the material under scrutiny and to identify even the tiniest contour deviations.

One of the major benefits of attocube’s Industrial Displacement Sensor (IDS) is the large acceptance angle. Attocube’s IDS is an optical fiber-based, three-channel Fabry-Pérot interferometer. Thanks to its proprietary patented techniques, the system enables measurements on surfaces with more than 10° inclination with regards to the measurement direction. This article demonstrates such techniques, showing nanometerprecise 3D profilometry data of micron-sized metal cylinders.

Profilometry

Figure 1. Metal cylinders were mounted on a 3D stack consisting of positioners and precision scanners. A cylinder’s surface was measured from above (z-axis), all while its position (x- and y-axes) was simultaneously tracked.

Experimental Setup

The measured object itself was mounted on an attocube 3D positioner set (ANPxyz101) with a 3D scanner (ANSxyz100) on top. This enabled easy, fast positioning within a 5 x 5 x 5 mm3 travel range and fine, precise positioning within 50 x 50 x 24 µm3.

The experimental setup using three sensor heads connected to attocube’s IDS is shown in Figure 1. A 12 mm diameter focusing sensor head with a 2.8 mm working distance (D12/F2.8) and a spot size of better than 2 µm measured the surface of the cylinders. Perpendicular to this, two focusing D4/F8 sensor heads were used to measure the relative displacements along (x-axis) and perpendicular (y-axis) to the axial orientation of the cylinder. Therefore, all three coordinates of each measured surface point were recorded, enabling a full 3D reconstruction.

Once the setup was prepared, it took less than a minute to align the cylinder and bring it into the focus point of the D12/F2.8 sensor head. As soon as the cylinder was close to the focus point, the alignment signal promptly showed high contrast. The maximum allowable z-displacement was found to be 120 µm in total for the D12/F2.8 sensor head.

Profilometry

Figure 2. Profilometry measurements on two metal cylinders with diameters of 400 µm and 315 µm. The measured profiles (solid red and blue lines) are shown over a range of ± 230 µm (a), and over ± 50 µm (b). The nominal diameters of the cylinders are depicted with dashed lines. Red dotted lines (b) represent a diameter tolerance of ± 300 nm for cylinder 1.

Results

The measured z-displacements (solid red and blue lines) and the specified z-positions (dashed lines) along the y-position of two metal cylinders with diameters of 400 µm and 315 µm are depicted in Figure 2(a). Figure 2(b) shows the corresponding zoom-in plot. From this graph, the angular alignment tolerance of the D12/F2.8 sensor head is directly deduced to be greater than ±10°. Such wide angular tolerance explains why setting up the measurement could be done so quickly.

(a) 3D color plot showing the surface morphology of a 200 µm diameter cylinder. Each color represents a 100 nm height step in z-direction.

the surface morphology of a 200 µm diameter cylinder

Figure 3. (a) 3D color plot showing the surface morphology of a 200 µm diameter cylinder. Each color represents a 100 nm height step in z-direction. (b) The surface of the metal cylinder also shown as a 2D figure.

It is imperative to note that this alignment tolerance will be much larger when measuring on materials with lower reflectivity, such as sapphire or glass. Furthermore, curved objects with larger dimensions (for instance spherical touch probes of coordinate measuring machines or rotating shafts) can also be easily studied in detail.

The measured surface contours of two metal cylinders represented in Figure 2(b) (red and blue solid lines) can now be compared with their respective ideal circular forms (red and blue dashed lines): A deviation of approximately 300 nm (red dotted lines) can be seen at the top of cylinder 1 (red line), while the maximum deviation is less than 200 nm for cylinder 2 (blue line). Except these maximums, the measured surface profiles fit well to the specified diameters of 400 and 315 µm.

A 3D surface of a separately measured 200 µm metal cylinder was constructed using all interferometric axes simultaneously. The measured profile covering a 40 x 28 µm2 area is shown in Figure 3(a). In Figure 3(b), the corresponding two dimensional top view is plotted. Numerous deformations can be seen: The object profile clearly shows a dent on its surface with a depth of around 400 nm in the center position. Additionally, the diameter contour in the front part near x = 0 has a plateau over a length of around 10 µm.

Conclusion

To summarize, this article presents the IDS’ capability to accurately measure surface profiles of micron-sized objects. This technique is not limited to static analysis. Measurements of moving or vibrating microscale-objects (even under the influence of external force) have already been well executed.

Attocube Systems AG.

This information has been sourced, reviewed and adapted from materials provided by Attocube Systems AG.

For more information on this source, please visit Attocube Systems AG.

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