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New polarization interferometry approaches for in-situ process control and product quality assessment

: Schreiber, J.

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South-African Insitute for Non-Destructive Testing -SAINT-:
18th World Conference on Non-Destructive Testing 2012. Proceedings. CD-ROM : WCNDT 2012, 16 - 20 April 2012, Durban, South Africa
Lynnwood Ridge: SAINT, 2012
ISBN: 978-0-620-52872-6
5 S.
World Conference on Non-Destructive Testing (WCNDT) <18, 2012, Durban>
Konferenzbeitrag, Elektronische Publikation
Fraunhofer IZFP, Institutsteil Dresden ( IKTS-MD) ()

Mass production in microelectronics and solar cell industry poses a problem of creating cheap and fast sensors being able to perform on-line quality testing of semiconductor wafers and bulk dielectric components in fab conditions. A challenging task is to determine areas of residual stress and associated high density of dislocation and microcracks in large format (up to 30 x 30 mm) but ultra-thin (less than 0.32 mm) polycrystalline Si wafers. In addition to microwave and ultrasound inspection methods, which only partially address the problem, the optical polarization heterodyne interferometry seems to be a promising solution. The method exploits the fact that the residual stress within silica wafer induces the anisotropy (birefringence) of the optical properties of the material due to the photo-elastic effect. The birefringence, in turn, can be evaluated by registering the phase difference between two orthogonally polarized waves passing through or reflecting off the wafer. Recently, the feasibility of polariscopy method was discussed for stress detection in Si, which exploits similar physical principles. These approaches determine depolarization analyzing the beam intensity for different polarization orientation. However, especially solar cell wafers have rough surfaces and hence a polarization dependent modification of the beam intensity will be observed. Hence the birefringence information will be masked by the surface effect. In this contribution we propose an alternative implementation of the polarization interferometry, where the birefringence phase shift of two beams with a polarization perpendicular to each other is measured directly independent on the intensity of both beams. The developed testing system provides a number of advantages over the polariscopy, in particular, the absence of mechanical movement of components, larger field of view, better accuracy for depolarizing non-polished wafers, and much higher operating rates. With the help of an industrial demonstrator it was able to analyse the mechanical stress state of a solar cell of 160x160 mm² in transmission as well as in reflection in less than 1 s with a resolution less than 80 µm.