A high accuracy focus sensor is developed for a dedicated high speed microscope measurement system.
The required focusing accuracy is 1 micro-meter, and must be in-sensitive to diffraction from the substrate.
The optical path of the focus sensor goes through the microscope objective, such that the focusing and the inspection spot coincide on the substrate. The full optical path and tolerances are modeled in ZEMAX as part of the design.

 Further a high dynamical signal range of 1:10.000 is needed to maintain the accuracy on high reflective to dark substrates.
Therefore, a dedicated very low noise photodiode amplifier is designed with on-board ADC conversion and a ENDAT digital bus interface to the focusing actuator in order to avoid additional noise and electric-magnetical interference on the interface cables.
The focus sensor gives direct feed-back to the focus control loop.