In Situ Study of the Effects of Heavy-Ion Irradiation on Co-Evaporated Cosi 2 Films
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INSITU STUDY OF THE EFFECTS OF HEAVY-ION IRRADIATION ON CO-EVAPORATED CoSi2 FILMS CHARLES W. ALLEN* AND DAVID A. SMITH" * Argonne National Laboratory, Argonne, IL 60439 USA
**IBM Watson Research Center, Yorktown Heights, NY 10598 USA ABSTRACT The inasitu ion irradiation capability of Argonne's HVEM-Tandem User Facility has been employed to study effects of 1.5 MeV Kr+ irradiation and 300 kV electron irradiation on the crystallization of as-deposited and of partially crystallized 40 nm thick films of CoSi 2. Ion fluxes ranged from 8.5x10 14 to 6.8x10 15 m- 2 s- 1 for which beam heating effects may be neglected. The maximum electron flux at 300 kV was 0.8x10 23 m-2 s-1 . The maximum temperature at which crystalline CoSi 2 is amorphized by the ion irradiation of flux = 6.8x1015 m-2 s- 1 is between 250 and 280 K. At higher temperatures amorphous material crystallizes by growth of any preexisting crystals and by classical nucleation and growth, with radial growth rates which are proportional to ion flux. The average degree of transformation per ion is 4x10- 26 m3 per ion. Thermally induced crystallization of as-deposited films occurs above approximately 420 K. For ion doses at least as low as 3.4x1 016 m- 2 ion irradiation at 300 K promotes thermal crystallization at 450 K, by virtue of enhanced apparent nucleation and at large doses, by enhanced growth rate. INTRODUCTION A variety of experiments involving combinations of ion irradiation, electron irradiation and thermal annealing has been conducted. The results of the most important of these experiments have been presented in some detail elsewhere [1,2] and are summarized below. The specimens were coevaporated thin films of CoSi 2 -40 nm thick over a 5 nm thick film of amorphous silicon nitride, covering -2.3x10-8 M2 windows in thick Si substrates. The ion irradiation experiments were performed employing the iaasitu ion beam capability of Argonne's HVEM-Tandem Facility, whereby the response of an individual area of specimen is continuously observed during a given ion irradiation. In order to avoid Frenkel pair production due to electron irradiation during such observation, the HVEM was operated at 130-144 kV, well below threshold for electron-induced displacement of Si or Co. For ion irradiations 1.5 MeV Kr was employed at an rather small incident flux of (1.7 to 3.4)x10 15 M-2 s- 1 to ensure that beam heating was negligible. For electron irradiations 300 kV was employed to duplicate the irradiation experienced in a 300 kV TEM. The electron flux in this case was 0.8xl 023 m- 2 s- 1 at the center of electron probe. On the basis of TRIM 89 calculations [3] for the experimental conditions and an assumed displacement threshold of 18 eV which is a rather small value, an ion dose of 1018 M-2 corresponds to approximately 1 displacement per atom (dpa). These calculations also indicate that approximately one of every thousand Kr ions is implanted, which for the largest ion dose (3x1019 M-2 ) and the specimen thickness employed corresponds to about 0.5 atomic ppm. Both of these
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