The surface topology considerably exceeds the depth of focus of a microscopic objective, irrespective of whether this is due to unevenness of injection-molded plastic microwell plates used in biotechnology or the various different heights of the components on the board, or tilting of the entire wafer during the manufacturing of electronics. It is only possible to focus sharply on the surface when the focus is adjusted in a range of approximately 300 µm. A real-time autofocus function is therefore necessary for three-dimensional image recording; the focus must be adjusted dynamically in the direction of the optical axis with high precision.
This task is done by a PIFOC® piezo-based objective scanner. With a travel range of up to 500 µm, it is well suited to autofocus applications and, when taking accuracy and particularly dynamics into account, is far superior to stepper motors. The PIFOC® Z drives from PI (Physik Instrumente) are designed to be very small and stiff.
The advantage: The response times are short and, thanks to the stable guiding, precision positioning is possible even over comparatively large travel ranges. The zero-play and high-precision flexure guide ensure high focus stability.
Piezo systems stand for accuracy, but in these applications, they demonstrate yet another strong point. The repeatability and settling time of less than 10 milliseconds are important. The piezo-based drive therefore prevents the object from going out of focus at high scanning speeds.
In conjunction with direct metrology, capacitive sensors and digital controllers, the PIFOC® achieves the highest linearity at a maximum deviation of 0.06 %. The capacitive sensors measure the mechanical part in motion directly and without physical contact. Neither friction nor hysteresis interferes with the measurement. The position of the objective can be matched accurately to the individual image.