Near infrared low coherence speckle interferometry (NIR-LCSI) as a tool for the investigation of silicon in solar cell production

A setup for an NIR-LCSI instrument is introduced. It is based on a Superluminescence diode (SLD) (wavelength = 1280nm, FWHM = 50nm) and an InGaAs camera with VGA resolution. The paper presents a work in progress. The aim of the research work is to measure the chipping process in a Si wafer saw with high spatial resolution. Hereby the sawing channel inside the silicon block is investigated. The chipping generates a change of the interface topography of the sawing channel. NIR-LCSI will be applied to measure this topography change and contributes thus to determine the size and volume of the silicon chips and thus the cutting rate of the sawing process. This paper presents a novel concept to increase the spatial resolution of the imaging system. With the use of a new type of "immersion" optics the numerical aperture can be significantly increased and a spatial resolution close to, or even below, the nominal illumination wavelength can be obtained. The speckle size and the resulting spatial and depth resolution of the LCSI measurements are investigated.