Microstructure and Stress in Mo Films Sputter-Deposited on Glass Substrates
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119 Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
RESULTS AND DISCUSSION Mo films were first deposited on borosilicate glass substrates by DC magnetron sputtering. Film stresses varied from compression to tension with the Ar gas pressures while DC power was kept at 40 W, see Fig. Ia. There exists a transition pressure at about 4 mTorr for growing the film with compressive stress. Using the same growth conditions, we were not able to prepare a Mo film with compressive stress on a sodalime glass substrate, see Fig. lb. X-ray diffraction data, as shown in Fig. 2a and 2b, indicate a preferred orientation of (211) in the film grown on borosilicate glass substrate and a (110) preferred orientation in the film grown on sodalime glass substrate. (b)
(a)
90T2
6
DCV=300 V
I,
"compression tension
2
Tu=275 °C sub=
08
8 DCV=300 V
0
o
2
0
-1
-0.5
0 0.5 1 Stress (GPa)
1.5
2
o 0
0.2 0.4 0.6 0.8 1 Stress (GPa)
1.2 1.4
Fig. 1 Film stress versus gas pressure for the film (a) grown on borosilicate glass substrate and (b) grown on sodalime glass substrate. Both films are prepared by DC sputtering using a constant power at 40 W.
DC sputtering on borosilicate glass T.b 275C
o
DC sputtering on sodalime glass 0
D.
20
30
40
T.b-
50
60
70
80
90
30
35
40
45
50
55
60
275 C
65
2 0 (degree)
2 0 (degree)
Fig. 2 X-ray diffraction data of the films DC-sputtered at 40 W on (a) borosilicate glass substrate and (b) sodalime glass substrate. 120
70
From the above results, it is obvious that the substrate material plays an important role in determining the stresses and the textures in the films. Considering the differences in thermal expansion coefficients of the substrates and Mo films, there is a good match between Mo (5.55 x 10.6 /C) and the borosilicate glass substrate (5.0 x 10.6 /C) but a large discrepancy between Mo and the sodalime glass substrate (>10 x 10.6 PC). The larger thermal expansion coefficient of sodalime glass, compared to Mo, should result in a compressive stress in the Mo film, which contradicts our experiments. Ten atomic percent Na has been detected on the surface of CuInSe 2 film grown on a Mo-coated sodalime glass substrate as reported by Hedstrom et. al. [5]. Na atoms may diffuse and incorporate in the Mo film, causing the expansion of the crystal lattice of Mo and generating tensile stress. A tensile stress may create pores in the film, as observed in an optical microscope. It has been known that a higher ion energy may promote compressive stress in the film. An increase in DC voltage or a decrease in gas pressure may result in an increase in ion energy [6]. Figures 3a and 3b show the film stress versus Ar gas pressure when a DC voltage is kept at 400V and 350V, respectively. The transition pressures are 7.5 and 6.4 mTorr as determined from Fig. 3a and 3b, respectively. It is obvious that a higher DC voltage may increase the transition pressure. (a)
(b) 9 8
11
Tb= 2 75 0C
10
V s DCV=400
Tý,b=
2 75
°C
DCV=350 V
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00
5 6
7
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