• PDF / 461,584 Bytes
• 6 Pages / 414.72 x 648 pts Page_size
• 100 Downloads / 302 Views

DOWNLOAD

REPORT

---

Mat. Res. Soc. Symp. Proc. Vol. 356 0 1995 Materials Research Society

EXPERIMENTAL PROCEDURE Sample preparation The samples were fabricated at Stanford University by sputter deposition in a ultra-high vacuum chamber on (001) MgO single crystal substrates. An epitaxial seed layer of Cr, 20 nm thick, was deposited at 3000 C. The multilayer structure was formed by alternatively depositing layers of Ag and Cr at room temperature. Pure (001) Ag and (001) Cr films were fabricated in a similar manner. Three sample sets were fabricated. For the first sample set, the bilayer period, was changed while the volume fraction of Cr was held at 50%. In the second set, the thickness of the Cr layer varied while the thickness of Ag remained 5 nm. For the third sample set, the volume fraction of Cr was changed while the bilayer spacing was held at 10 nm. Large stresses exist in these films, and for bilayer spacings around 3nm, some films debonded from the substrates. The bilayer spacing was measured from the low angle superlattice lines (LASLL) obtained from symmetric x-ray diffraction data. An example of the LASLL appears in Figure 1(a). Many higher order LASLL were observed for most samples, indicating that interfaces were well defined. High angle superlattice lines, indicating that the interfaces are crystalline, were also observed. Data from phi x-ray scans indicate that the samples are epitaxial single crystal films exhibiting the expected (001) rotated crystal structure. A typical phi x-ray scan, which was used to determine epitaxy and relationship between crystals, appears in Figure 1(b).

106 . . .. . . . . . . . . l.I'.. .. . (a)

10 7 i. l... . ..

(b)o

Z

,105

CA10

-A = 14.46 rnm,tcr /A =0.7

''f

0

104

S103 =

10

I

-

-

S_ S102

"•-4Ur-

.

. ... . 1 1 . 1 1..

I.. I 1 11..

20Cr(I 0) 4.392' 20A

(2)6446

o -

104

I

-Ar r•••

•

/

00

11

r_ A

103 1 0 1 r . , I. ..

1

2

. . .

3

.. . . .

4

5

, ,

6

7

. .

0

8

60

90

120 150 180

Phi (degrees)

Two theta (degrees) Figure 1.

30

Data from x-ray scans: (a)Symmetric 20 scan showing low angle super-lattice lines from the multilayer structure. (b)Phi scan showing the rotated 450 relationship between the single crystal layers of the multilayer structure.

Nanoindentation Testing The mechanical properties of the samples were measured by depth-sensing indentation testing with the Nanoindenter using a Berkovich tip. The typical loading sequence appears in Figure 2(a). The initial loading was at a constant displacement rate of 5 nm/sec. The rate for the final unloading segment was adjusted so that it was completed in 20 seconds. Indentations were conducted in a 6x6 array with the indentation depths ranging from 150 to 25 nm. Hardness measurements are reported for the 150 nm indentations. The bilayer spacing can have a dramatic effect on the strength of these multilayer films. The load verses displacement curves from representative experiments are shown in Figure 2(b) and clearly illustrate that the multilayer films are stronger than the single component film