Stresses in Mo/Si and W/Si multilayer films

David L. Windt

Bell Laboratories
Room 1D-456, 600 Mountain Ave, Murray Hill, NJ 07974
windt@physics.att.com


The stresses in sputtered Mo/Si and W/Si multilayer (ML) films have been
measured using the wafer curvature technique. In one experiment(1), a strong
dependance of  stress on background pressure was observed in Mo/Si ML films,
i.e., the ML stress becomes increasingly tensile with increasing background
pressure.  For example, in the case of ML films containing 40 periods of 4.3
nm Si layers and 2.6 nm Mo layers, the stress increases from approximately
-450 MPa (compressive) to -280 MPa as the background pressure in the
deposition chamber (i.e., measured just prior to deposition) increases from
6.0*10-8 torr to 1.0*10-5 torr.  For MLs of the same period but with thicker
Mo layers, the dependence on the background pressure is even stronger. These
observations indicate that while the Mo layers become more tensile with
increasing background pressure, the Si layers become more compressive.  We
also find that the variation in ML stress correlates with the concentration of
incorporated hydrogen in these films, i.e., increasing hydrogen concentrations
correspond to more tensile films, and the concentration of hydrogen increases
linearly with background pressure.  The most likely explanations for the
observed variation in ML stress with background pressure are (a) that the
stress is due to incorporation of hydrogen atoms, and (b) that the surface
mobility of adatoms is decreased with increasing background pressure due to
the affect of adsorbed residual gas atoms.

In another set of experiments, the stresses in Mo/Si and W/Si MLs were
measured as a function of the individual metal and Si layer thicknesses; the
stresses were measured for both as-deposited and 300 C-annealed films.  The
microstructure of the polycrystalline metal layers (i.e., grain size, lattice
parameter, and out-of-plane strain) was investigated using X-ray diffraction.
In the case of Mo/Si multilayers, the film stresses can be understood in terms
of the measured strains in the Mo layers, assuming a simple dependence of the
stresses in the amorphous Si layers as a function of thickness.  In contrast,
in the case of W/Si multilayers, two W phases are present, depending on the W
layer thickness, making the strain analysis significantly more complicated. 
In both the Mo/Si and W/Si systems large irreversible stress changes occur on
thermal cycling.   These stress changes can be either positive or negative,
depending on the specific metal/Si layer thicknesses involved, and are likely
the result of diffusion, i.e., resulting from a net volume transport from one
type of layer to the other, leaving one layer more compressive, and the other
more tensile. 


1. D. L. Windt, W. L. Brown, C. A. Volkert, and W. K. Waskiewicz, J. Appl.
Phys., 78, 2423-2430 (1995)