Confocal microscopy is an optical imaging technique that increases resolution and contrast by using a spatial pinhole to block out-of-focus light. This allows precise imaging of surfaces with depth selectivity. In a typical confocal setup, the system scans the sample point by point and captures light only from the focal plane, which enables the reconstruction of high-resolution 3D surface profiles. The Zeta-20 from KLA uses a proprietary implementation of confocal microscopy called Z-dot technology. Unlike interferometric methods, Z-dot does not rely on interference fringes but instead captures focus information pixel by pixel as the objective lens moves along the Z-axis.

At each pixel location, the system identifies the point of maximum contrast or sharpness (i.e., best focus), which corresponds to the height of the surface at that point. This data is then used to build a 3D topography map of the sample. Because it analyzes focus locally, Z-dot is highly effective on complex surfaces, including rough or sloped regions, transparent or semi-transparent materials and samples with mixed optical properties.