In Situ Hvem Study of Ion Irradiation-Induced Grain Growth in Au Thin Films
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IN SITU HVEM STUDY OF ION IRRADIATION-INDUCED GRAIN GROWTH IN Au THIN FILMS JOYCE C. LIU, JIAN LI AND J. W. MAYER Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853; CHARLES W. ALLEN AND LYNN E. REHN Materials Science Division, Argonne National Laboratory, Argonne, IL
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ABSTRACT In situ observations of 1.5 MeV Xe+ ion irradiated Au films at room temperature and at 150 0C reveal the evolution of grain growth: the average grain size increases by the mechanisms of grain boundary migration and grain coalescence. INTRODUCTION Ion irradiation induced grain growth has been observed in several materials, including Ni, Au, Si and Ge [1,2,3,4]. Similar to investigations of thermally induced grain growth, the objective is to establish a relationship between grain size and irradiation time or between grain size and ion dose when the dose rate is constant. Results from Ag implanted Ni thin films [1,2] show that the average grain diameter is proportional to the ion dose. For Au, Si, and Ge, it is found, however, that the increase of grain size with the dose exhibits a power law dependence [3]. Since previous studies of ion irradiation induced grain growth are all based on observation following irradiations [1,2,3, 4], a direct observation of the ion irradiation induced grain growth is useful for understanding the mechanism, kinetics and driving force of this process. The evolution of grain growth in ion irradiated Au thin films is observed in an electron microscope, and some of the results are discussed in the present paper. EXPERIMENTAL PROCEDURE Au films with thickness -55 nm were prepared by sputter deposition on NaCl substrates. The grain size distribution is uniform and the average size is about 25 nm. Moreover, grains in the as-deposited films were preferentially oriented in . Self-supporting films were then 2 irradiated with 1.5 MeV Xe+ ions at a dose rate of 1.7 x lOll/cm .sec to minimize Xe implantation in the films and the effect of beam heating. The experiments were carried out at the HVEM-Tandem User Facility at Argonne National Laboratory [5], where a 2 pMeV Tandem accelerator is interfaced to an AEI EM-7 1.2 MV high voltage transmission electron microscope (HVEM). The ion beam is introduced into the HVEM via a 330 ion-beam access tube; i.e., the ion beam is 330 from the electron beam in the microscope. During irradiations at room temperature and 150 0 C, microstructural changes of the films are examined in situ with the HVEM operated at 500 kV. The evolution of grain growth is followed by recording the images continuously on video tape and sequentially on photographic film. OBSERVATIONS AND DISCUSSION Direct observations Direct observation of the Au films during ion irradiation reveal a dynamic picture of the induced grain growth; i.e., a gradual increase of average grain size is associated with dynamic changes in lattice defect structures. Although individual defects were not resolved due to their high density, the change of defect structure is suggested by the continuous M
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