Effect of surface structures upon ultrathin film interference fringes
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Tomoya Ogawa Department of Physics, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo, 171 Japan
Koichi Toyoda Laser Science Research Group, Institute of Physical and Chemical Research (RIKEN), Wako, Saitama, 351-01 Japan
Zhenguo Wang Institute of Semiconductors, Academia Sinica, Box 912, Beijing, 100083 China (Received 22 February 1993; accepted 26 March 1993)
Effect of surface structures upon ultrathin film interference fringes generated from extremely thin films or epitaxial layers grown on semiconductor wafers has been studied. Since dark regions of fringes correspond to the places where the thin films are destroyed or absent, the fringes are investigated to detect uneven surfaces with undesired structures. Therefore, surface microstructures can be detected and characterized effectively by the modification of the fringes.
I. INTRODUCTION With the development of epitaxially grown layers and/or thin films, nondestructive detection and characterization of the layers, surface microstructures, and interfaces between substrate and the layer and/or film become more and more important and necessary for the improvement of the planner devices. Recently, we developed successfully a new method called ultrathin film interference fringes, which is based on interference between light scattered by a rough interface and reflected light from a flat surface. The fringes are observed by extremely thin films which are a nondestructive detection to characterize the films and the interface properties. The formation principle of the interference and some qualitative observations have been reported previously by the present authors.1'2 The new interference fringes are remarkably different from other fringes such as Hidinger fringes observed by equal inclination, since the fringe interval between neighbors increases along the fringes' radius from the center. The fringes are located on the rough surface because the interference fringes are explained by interference between light reflected by a flat surface and light scattered from a rough interface between thin film and substrate. The visibility of the fringes is dependent upon light intensity balance between the reflected and scattered light. The reflected light is determined by the refractive index of the epitaxial layer/film, and light scattered from the interface is determined mainly by microroughness and refractive index difference between the films and the substrates. The quality of interfaces of a wafer with a smooth surface can be characterized judging from visibility and fine structures of the fringes.1'2 J. Mater. Res., Vol. 8, No. 9, Sep 1993
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In this paper, we mainly report and discuss some results on the effect of surface structures upon the interference fringes. II. EXPERIMENTAL A laser beam was focused onto a spot about 0.5-1.0 mm in diameter by a complex lens. The head of a laser diode was mounted on a rotation stage for adjustment of illumination angle. In order to make fringes into nearly concentric rings,
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