Self-calibrating lateral scanning white-light interferometer

Abstract

The concept of lateral scanning white-light interferometer (LSWLI) has been introduced nearly a decade ago [1] as an alternative to the conventional white-light (WL) interferometers [2-14], capable of improved speed and image stitching. The general principle of this type of measurement is shown in Figure 1. A conventional white light interferometer is equipped with an XYZ stage which can perform an accurate lateral (XY) translation. The interferometer objective is tilted with respect to this stage such that the zero optical path difference (OPD) makes an angle α with respect to the direction of the translation. By convention, the tilt angle will be measured from the direction of the translation. For the case when this angle is different than zero, an object placed on the stage will present a specific fringe pattern whose density is dictated by the magnitude of the angle. In Figure 2, a linear fringe pattern obtained from a flat surface is shown. As the profiled object is translated at a constant speed, the CCD will record interference frames at a constant rate. Figure 3 shows how different pixels of the object (marked by up or down pointing arrows) will be recorded in consecutive frames during the object translation. In the case when the CCD frame rate and the stage speed are properly correlated, a given point of the object will be translated by exactly one pixel from one CCD frame to the other. The correlogram of each object point can thus be recovered by taking a "diagonal section" through the stack of recorded frames (Figure 4). Because during the scan the optical path difference of each point of the sample changes continuously, the LSWLI correlogram looks similar with its counterpart obtained by using WL interferometers. As mentioned before, the LSWLI measurements allow for a continuous data acquisition process, eliminating thus the need for a cumbersome stitching procedure that must be done for large samples when measured by using a standard WL interferometer. It also allows for a faster data acquisition and, in principle, it is possible for very large samples to be measured during a single pass.