Ion-Induced Growth of Whiskers on Sn Films
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ION-INDUCED GROWTH OF WHISKERS ON Sn FILMSt D.B. POKER*, J. SCHUBERT**, AND B. STRITZKER** * Oak Ridge National Laboratory, Oak Ridge, TN 37831 **Institut fuer Festkoerperforschung der Kernforschungsanlage Juelich, D-5170 Juelich, Federal Republic of Germany ABSTRACT Whiskers have been observed to form on thin films (1000-2000 A) of Sn following implantation of 20-keV H or He at temperatures below 150 K. The morphology of the whiskers following growth was examined using scanning electron microscopy (SEM) to illuminate the possible growth modes. In an effort to obtain support for either of several models of the growth process, the region near the base of the whiskers was examined for evidence of depletion or extrusion. No evidence of depletion was observed. The whiskers could be classified into two types: those that were supported on pedestals, a short transition region at the base of the whisker; and those without pedestals, which rose abruptly from the film with no transition. A novel structure has been observed on Sn films that were coated with 500 A of Au/Pd to enhance the SEM image. The structure consists of a thin, planar surface several microns in length, oriented perpendicular to the substrate. The structures did not appear immediately after coating, but were present after storage in air for one month. INTRODUCTION The growth of whiskers on thin films of Sn following low-temperature ion implantation has been previously characterized with respect to implant temperature, ion dose, film thickness, and ion, film, and substrate species [1,2]. Most notable of these results is that ion-induced whiskers (i.e., whiskers that are induced to grow by ion implantation, but which only appear after, not during the irradiation) are observed only on Sn films, but not Sn foils or other materials. It has been speculated that the whisker growth may be due to a recrystallization mechanism that exists only in Sn due to the existence of a second, low-density phase below room temperature [2]. The whiskers may grow by extruding material to relieve internal stresses within the film due to the recrystallization, similar to the process that produces squeeze whiskers [3]. Alternate processes that may lead to whisker growth involve diffusion of atoms into the whisker from the area surrounding the base of the whisker. These processes draw material onto the whisker by providing an attractive force, e.g., oxidation, and deplete the film surrounding the whisker. A test of these models would be to examine the area surrounding the base of the whiskers for evidence of extrusion or depletion. This paper describes the results of such a study. WHISKER GROWTH CONDITIONS AND MASS TRANSPORT Samples were prepared at two laboratories, Kernforschungsanlage (KFA) and Oak Ridge National Laboratory (ORNL). The majority of films were produced at KFA, by evaporation of Sn (5N purity) from a resistance heated W boat in a vacuum of 3 x 10-4 Pa. The evaporation rates were typically tResearch sponsored by the Division of Materials Sciences, U.S. Department of En
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