Stable and compact VCSEL based multiple-wavelength digital holography for optical metrology

In-line inspection enables the reduction of manufacturing time, manufacturing material and consequently manufacturing costs. However, it imposes strong requirements on the in-line inspection systems, which are fast and non-tactile data acquisition and processing, environmentally stable setup, ability to investigate different surface materials and composition of different surfaces such as rough scattering and specular scattering. Two of the most established optical metrology techniques are white light interference microscopy and confocal microscopy. However, the measurement process is very time-consuming. Structured illumination enables faster data acquisition but is restricted to rough scattering surfaces only. Multiple-wavelength digital holography (MWDH) offers the possibility to overcome the aforementioned shortcomings. It can be applied on rough scattering and specular reflective surfaces in a lensless lightweight manner. However, the stability and exact knowledge of the wavelengths employed is crucial for the successful application of MWDH. In this paper, we describe a method to ensure wavelength stability for MWDH.