Hillock Formation in Tensile Loaded Films
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73 Mat. Res. Soc. Symp. Proc. Vol. 391 ©1995 Materials Research Society
Table 1: Specimen Annealing Histories Before/
Specimen
Annealing History
After Hillocks
1.
Before
96 hrs., 150'C; 192 hrs., 150°C
After
96 hrs., 150'C; 192 hrs., 150'C; 10 min., 4001C
2.
Before
24 hrs., 150'C; 48 hrs., 150'C; 96 hrs., 150'C
After
24 hrs., 150'C; 48 hrs., 150'C; 96 hrs., 150'C; 10 min., 400'C
3.
Before
24 hrs., 150'C; 96 hrs., 150'C
After
24 hrs., 150*C; 96 hrs., 150'C; 10 min., 400°C; 15 min. 400'C
Before
48 hrs., 150'C; 96 hrs., 150TC
After
48 hrs., 150'C; 96 hrs., 150TC
4.
MICROSTRUCTURAL ANALYSIS Grain size data were obtained by measuring individual grain areas using NIH Image, an image analysis program for Macintosh computers [2]. At least 700 grains were measured for each specimen. The mean grain sizes given are the mean equivalent cylindrical diameters obtained from the measured areas. Hillock sizes and densities were similarly obtained. Orientations of individual grains were obtained by convergent beam diffraction and conical dark field imaging was used to identify differently texture regions. EXPERIMENTAL RESULTS Hillock Formation The first instance of hillock formation occured during the TEM observation of a film annealed for 288 hours at 150'C and 10 minutes at 400'C (Specimen 1 listed in Table 1). TEM observation, two days after the 400'C anneal, revealed that the film, which had been continuous before the 400'C anneal, was torn in places, indicating a state of tensile stress. The hillocks formed during exposure of a plate, so the hillock formation was not witnessed. The total time for hillock formation is generously estimated to be less than 5 minutes. The film tore further during this hillock
formation. Figure 1 shows a sequence of TEM images of the film. The hillocks appear black because their increased thickness attenuates the electron beam. The hillocks formed during exposure of the image in Figure la. Small hillocks in Figure la can be identified and matched with the larger asperities in Figure lb, indicating their growth over a short period of time. After this initial observation, Specimens 2 and 3 were annealed to reproduce the conditions leading to hillock formation in Specimen 1. Hillocks were present in these films after annealing at 400'C. Hillocks were observed in Specimen 4 after nine months of storage at room temperature. In an attempt to witness the hillock formation, Specimen 3 was annealed for 30 minutes at 80'C using a TEM specimen holder with a heating element. Hillocks formed during this in situ experiment, but never in the region of the specimen being observed. This was true of hillock formation in all specimens. It was difficult to match hillock images with the same regions before hillocking because hillocks did not form in regions which had been extensively studied. 74
a 6 pLm b Figure 1. Hillock formation during TEM observation. The hillocks formed as the image shown in (a) was being exposed. (b) The same region after hillock formation.
Film Microstructure Grain s
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