Chromatic focus variation with projected pattern illumination for wide range surface measurements

Modern manufacturing technologies can generate complex surfaces with a wide texture range, from rough to smooth, which presents a challenge for surface metrology instruments. Optical surface metrology instruments are typically limited to measuring a particular texture scale; therefore, a hierarchy of instrumentation is needed. For example, a focus variation (FV) instrument proved its capability to measure rough surfaces, including steep and high-slope surfaces. Still, it fails to measure smooth surfaces (i.e., those with insufficient surface texture and a lack of contrast), such as mirrors and silicon wafers, which are usually measured with interferometer systems. With the increasing demand for in-situ measurements, there is a need for a technology that can measure a wide range of surface textures within a single instrument. This paper proposes an illumination pattern projection via a polarised pixelated mask placed through the light source to measure optically smooth surfaces with a novel, to our knowledge, chromatic focus variation (CFV) system. The CFV system employs a dispersive objective lens and wavelength scanning mechanism to replace mechanical motion stages that contribute to bulky equipment and usually require regular maintenance and calibration to ensure accuracy over time. The pattern projection generates local contrast between neighbouring pixels, enabling smooth and rough surface measurements. Combining the CFV system with the polarised mask pattern projection requires no energy/power at the scanning head, which simplifies metrology integration for on-machine measurement. The experimental results show that the proposed mask projection can successfully measure various smooth surfaces with features up to 180 nm step height.