Areal surface measurement using chromatic focus variation

Optical metrology is an essential measurement technology across various research and inspection domains. Focus variation instruments are widely used in the industry due to their ability to measure steep and high-slope surfaces. The focus variation method extracts an areal surface map by analysing neighbouring pixels' contrast to determine the image focus position in a stack of images. Traditional focus variation instruments mechanically scan the sample surface by attaching the objective lens to a piezoelectric actuator (PZT) for image stack capture. The slow mechanical movement of the PZT poses a limitation for fast inspection in modern advanced manufacturing applications. Additionally, the PZTs' bulky mass and size, and their nonlinear hysteresis motion are other issues associated with the mechanical scanning process. To address these limitations, we present a chromatic focus variation approach that replaces mechanical scanning with a wavelength scanning mechanism. The system employs a chromatic objective lens that determines the image focus by axially shifting the light source wavelength without any mechanical movement. This paper presents the measurement of a 30 μm step height sample using the proposed system. The results show that wavelength scanning is capable of surface measurements, but the current version of the system has a low vertical resolution, which should be enhanced in future versions.