Integrated quantum efficiency, reflectance, topography and stress metrology for solar cell manufacturing

Abstract

We report application of phase shifting interferometric measurements to study of the spatially resolved quantum efficiency (QE) of the semiconductor solar-cells. In our method solar-cell is illuminated by two sets of mutually spatially orthogonal fringe patterns of known frequency, and varying phase (shifted phase). We report theoretical results obtained using simple analytical model describing properties of small spot size defects, and preliminary experimental results validating this method. The new method and new apparatus can be also used for studies of spectrally resolved QE. I TRODUCTIOFast characterization of solar cell photo-response attracted recently some attention [1]. In one of the approaches [1] the laser beams are moved on the wafer surfaces using a galvanometer xy scanner system and the beam size on the focus has a diameter of about 65 um. This approach allows to obtain quantitative information about the cell quality in time of 3 s using the photocurrent maps with a good correlation with the efficiency data.In our recent paper we reported development of structured light profilometer Zebra Optoprofiler [2] described in some detail below. Recently we developed structured light projection based profilometer for rapid characterization of silicon solar cell wafers described in our earlier paper [2], and subsequent advances in its calibration algorithms [3]. In our earlier paper we demonstrated that structured light projection metrology is capable in meeting throughput requirements for solar cell manufacturing [2]. In this paper we report further advancements in structured light projection metrology: the integration of spectrally, and spatially resolved quantum efficiency measurement. In our approach the quantum efficiency (QE) measurement is accomplished by using the same projector which is used for solar cell topography measurement as a source of illumination. It is possible to use projector to define a light spot on the surface of the sample and to use such spot for investigation of local characteristics of the measured wafer. We demonstrated that the very same hardware as used in profilometer [4], with new modified software application can be used to measure diffused spectrally resolved reflectance. Method described here applies structured light pattern of spectrally filtered beam on the surface of the wafer to measure photo-current. The dimensions of the "structured light pattern" are defined by the software and projector while the spectral characteristics are defined by projector and can be further defined by use of the appropriate narrow band pass filters. This approach does not only eliminate costly motion stages, but also increases speed of data acquisition. We show that using our system it is possible to vary lateral spot size of the impinging light beam in order to identify and study in detail solar cell anomalies.If the light excitation has known spectrum, and intensity then photo-current can be expressed in terms of quantum efficiency. Therefore we will focus in this paper on photo-current me...

Applications of projection fringes and structured light surface metrology for semiconductor packaging and their challenges

“…In addition, we investigated the possibility of combining FPT metrology with photo conductivity mapping metrology [2]. The results showed that the same illumination system used for the topography metrology can be used for probing of the photoelectric properties of the solar cell.…”