Sub-micron/nanometer position measurement.

Vision system are also used for accurate position measurement of products or features. Especially in the micro assembly equipment this type of measurement is indispensable. It's getting difficult when the size of the product is getting smaller. Optics and illumination is getting more and more important and sub pixel measurement accuracy is needed.
J.A.I.M.S realized a measurement accuracy of 5 nanometer (3 sigma) in a field of view of 240 x 180 micrometer. The product size (or feature) is 20 x 3 micrometer. This accuracy is realized in one direction, the accuracy in the other direction was in this situation not important and is not measured

Measurement setup

The product is clamped on a 3D piezo stage (Physik Instrument) so the measurement can be performed in a 3D measurement area of almost the same size of the field of view.
In this situation a long working distance objective (Mitutoyo WD 20mm) is used so more working space in front of the optic is available (e.g. product grippers).
A blue led including diffusers, diafragm and optics are used for illumination the product.
In this setup a 5Mp camera is used and the measurement software is created using LabVIEW and special by J.A.I.M.S. developed software.
The measurement software contains 4 algorithms so the effect of the algorithm on the visible part of the product could be investigated. Measurements are performed by moving the product in the required measurement direction and in the focus direction.
Also stability measurements are performed without moving the product by the piezo actuator.
Also the difference between the piezo position (measured by the piezo system itself) and the (by vision) measured position is compared.

Measured Results

Of course a lot of data is created during the test and it is not possible to show all these results.
The Numerical aperture of the objective is decreased to create a larger focus depth.
SubM-Meas-stationary SubM-Meas- piezo vs vision Meas results in focus direction The first plot shows the result without piezo movement, so the product is in a stable position. Shown are the results of two algorithms including the linear fit of the position. The time span is about 1 minute and of course the whole mechanical measurement system does have a temperature drift.
The product position scale is in microns.

The second plot shows (blue) the position of the piezo measured by the piezo system itself and (red) the product position measured by vision system. The product position scale is in nanometers.
The third plot shows the position (in microns) with respect to the focus position. Also the linear fit is given. And of course the product is not accurately aligned with the optical axes. Good mechanical design and calibration solve this problem.
Remark: Not every product can be measured so accurately, the product has to match very restricted optical characteristics.