Effects of ion beam irradiation on the crystallization of Copper films
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EXPERIMENT The experiments of copper deposition, ion irradiation, oxygen adsorption were carried out in an UHV chamber equipped with a 15 keV RHEED (VG Microtec, LEGI 10). XPS measurements with Mg Kcx excitation (VG Scientific, ESCALAB 2(X)-X) were conducted by transporting samples under UHV. The base pressure of the system was less than 5x 10-8 Pa. Argon ions ( 'Ar+) generated by an ion accelerator (HVEE, 2 MV system) were introduced into the UHV chamber with a working pressure less than lx10 7 Pa. AFM images were taken in air by using a Si tip of 150 nm head radius (DI, Nanoscope II). Nb-doped SrTiO 3 single crystals of (100) orientation were used as substrates. The atomic smoothness of the substrate surfaces was achieved by heating at 1473 K in vacuo (< 10-5 Pa) for 2 h and in 105 Pa 02 for 10 h. No surface segregation of Nb by these heat treatments was confirmed by RBS analysis. Prior to copper deposition the substrates were heated at 873 K for 30 min by focused infrared light (Thermo-Rikoh, IR-1000 GVH) under I x 10- 3 Pa 02 atmosphere in the chamber in order to remove the surface contaminants. Metal copper films of thickness between 1 to 5 nm were evaporated onto the substrates at 300 K with the deposition rate of about 0.01 nm/sec. The irradiation with Ar+ of 2 MeV energy to the2 samples was performed at room temperature with the fluence ranging from Ix 10" to 5x 10 ions/ m and the ion current density of 5 mA/m 2 . The sample temperature was monitored by a thermocouple attached to a side of sample. The heating effect by ion beam was negligible under the beam condition examined because the temperature increase detected by the thermocouple was 195 Mat. Res. Soc. Symp. Proc. Vol. 396 © 1996 Materials Research Society
(a)
(b)
Fig. 1. RHEED patterns of the initial SrTiO 3 (100). The azimuth of the incident electron beam was (a) [100] and(b) [110] directions of the substrate, respectively.
Fig. 2. RHEED pattern of as-deposited 1 nm copper film on SrTiO 3 (100).
Fig. 3. AFM image of as-deposited I nm copper film on SrTiO 3 (100). bar = 200 nm
less than 10 K. Oxygen adsorption was carried out by exposing the sample surface to 1000 L oxygen at room temperature (where I L =3 10-6 Torr.sec). Oxygen atmosphere was dynamically pumped to maintain the pressure at lxl0- Pa. RESULTS AND DISCUSSION Figures 1(a) and (b) show RHEED patterns of the cleaned SrTiO 3 (100) substrate surface with the electron beam incident along the [100] and [110] directions, respectively. These patterns consist of the intense fundamental spots and the weak 1/2-order ones on the Laue circles, indicating that the substrate surface is considerably flat and is reconstructed in the 2x2 superstructure. Figure 2 shows the RHEED pattern of the as-deposited copper film of 1 nm thickness on the SrTiO 3 substrate. The surface morphology of the film is shown in Fig. 3. Diffused Debye rings indicate that the deposited copper film consists of very fine crystals with no-epitaxial relation to the substrate.
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The exposure to oxygen or increase of film
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