White light scanning interferometry permits three-dimensional profile mapping of rough surfaces without the 2p-ambiguity unlike conventional laser phase measuring interferometry. The achievable measurement resolution is in the nanometric range or even less, but practical measurement accuracy is far beyond the resolution being deteriorated by the scanning precision of micro-actuators in use. PZT ceramics are popularly employed for the scanning, of which non-linearity errors often reach up to 10 % of the overall operating range due to inherent hysteresis. Elaborate capacitive and LVDT displacement sensors may be incorporated to compensate for PZT scanning errors, but it is still not easy to obtain nanometric accuracy especially when a large measuring range over more than 100 um is required.
In this work, a self-compensation method of PZT scanning errors for effective implementation of white light interferometry is proposed. Without additional displacement sensors, the method directly determines scanning intervals from obtained interferograms. And So, actual scanning intervals are identified with nanometric accuracy not being affected by PZT scanning errors.
Also, a new white light interferometric algorithm is proposed. The proposed algorithm is capable of making the most of the phase-shifting technique well established in laser interferometry. The porposed alogorithm utilize above PZT self-calibration results and permits arbitrary scanning step of PZT actuator which is not adequate for conventional white light interferometric algorithms. Finally the proposed phase-shifting algorithm is tested to demonstrate that a measuring repeatability of less than 7 nanometers are practically achieved over 100 um measuring range.
