The Effect of Different Oxidizing Atmospheres on the Initial Kinetics of Copper Oxidation as Studied by In Situ UHV-TEM
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component of air. Moreover, steam or hydrogen within oxide scale is known to prevent loss of contact between the scale and the metal [8]. In situ TEM technique used here permits the visualization of structural changes during the oxidation process in real time and provides information on buried surfaces. There have been previous in situ studies by TEM, where the specimen was oxidized in another chamber and transferred in vacuum to the microscope[9-12]. However, the initial stages of Cu oxidation have not yet been studied by in situ TEM with ultra high vacuum conditions, which is essential for minimum contamination effects. EXPERIMENTAL Single crystal 99.999% pure 1000 A Cu films were grown on irradiated NaCl which was cleaved along the (001) plane in an UHV e-beam evaporator system, where the base pressure was 107° torr. 1000 A thickness was chosen so that the film was thin enough to be examined by TEM, but thick enough for the initial oxidation behavior to be similar to that of bulk metal. The Cu film was removed from the substrate and mounted on a specially prepared Si mount. The Si mount and the modified microscope specimen holder allow for resistive heating of the specimen up to 1000°C. The microscope used for this experiment is a modified JEOL 200CX with a spatial resolution of 2.5 A. An attached leak valve to the column of the microscope permits the introduction of gases directly into the microscope. In order to minimize the contamination, a UHV chamber was attached to the middle of the column, where the base pressure is less than 10V torr without the use of cryoshroud. The cryoshroud inside the microscope column can reduce the base pressure to approximately 10.9 torr when filled with liquid helium. A video camera mounted at the base of the column recorded real time pictures of the experiments. A 5tm objective aperture was used in order to enhance the contrast of the dark field images. For more details about the experimental apparatus, see McDonald et al [13]. The JEOL 200CX was operated at 100 keV. The Cu film forms a native oxide on the surface due to air exposure. To remove this oxide, the Cu film was annealed at 350 'C for 15 minutes [14]. To remove the copper oxide formed due to in situ oxidation, the specimen was annealed at 350'C and methanol gas was leaked into the TEM column at 5x10-5 torr column pressure. Methanol reacts with the oxygen atoms to form a methoxy species, which is bound to the surface by oxygen. At 350°C, the methoxy species decomposes to give gaseous formaldehyde and water vapor (eq. 1) [15]. CH 3 OH 9
+
[O].d
=
HCHO
g
+
H 20
(1)
To oxidize the copper film, scientific grade oxygen gas (99.999% purity) was leaked into the TEM chamber at 4x10- torr pressure. Later, the oxygen supply was discontinued and water vapor was leaked into the chamber with column pressure at 4 x 107' torr. The water vapor came
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from a glass tube, containing de-ionized water, attached to the side of the column. In another experiment, methanol cleaning was directly followed by water vapor exposure. Th
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