|
In-line inspection during the manufacturing process to evaluate topography related parameters has increasingly become important. However, it imposes strong requirements and the in-line inspection systems. Two of the most established optical metrology techniques that can cope well with different surface materials are white light interference microscopy and confocal microscopy. However, the measurement process is very time-consuming. Another technique, which offers single-shot capability and hence fast data acquisition is structured illumination, but restricted to the inspection of 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. Two VCSELs are mounted on a common heat sink with a distance of a few mm only in order to eliminate the influence of wavelength drift between the two VCSELs caused by different temperature. Moreover, the temperature is stabilized to 20C via a Peltier Element. In addition, the light-emitting surface of the two VCSEL is directly bonded to two corresponding single-mode fibers. Further fiber couplers and a fiber combiner are applied to result in a lightweight, highly robust and flexible setup. Information multiplexing via different angles of the two wavelength corresponding reference waves is introduced to enable single-shot data acquisition. In that manner, disturbances caused by changing environmental conditions as encountered in sequential acquisition are eliminated. The advantages of the VCSEL based MWDH system are demonstrated in comparison to structured illumination on non-cooperative materials (scattering and/or specular samples, samples with different colour properties).
|
