Ultrasmooth, conducting multilayer films for advanced applications 

D. G. Stearns, S. Baker and M. Wall 

Lawrence Livermore National Laboratory, P.O. Box 808, Livermore CA 

The incorporation of three-dimensional thin film structures in
advanced applications such as magnetic recording is presenting
ever-increasing demands on the control of the microstructure and
physical properties of the thin films.  As an example, in certain
electrically active devices it is necessary to have a contact layer on
the bottom of the device.  The contact layer must have low resistance
and be very smooth, since it serves as the underlayer to a thin film
stack.  Our previous experience with x-ray reflectance coatings has
lead us to investigate Mo/Si multilayer (ML) structures as a promising
candidate for such a contact layer.  In this paper we demonstrate that
Mo/Si ML films can be grown relatively thick (0.5 micron) with
virtually no accumulation of roughness and, with the proper choice of
structural parameters, can exhibit low resistivity. 

A series of Mo/Si ML structures was grown to study the dependence of
the film properties on the individual Mo and Si layer thicknesses.
Films having a total thickness of ~0.5 micron were grown by depositing
alternating Mo and Si layers onto high resistivity (> 1000 Ohm-cm)
superpolished Si wafers using dc magnetron sputter deposition.  The
individual layer thicknesses were varied from sample to sample, with
values of 2.5, 5.0 or 10.0 nm for Mo, and 1.0, 2.0 or 5.0 nm for Si. 
In addition, a single 0.5-um-thick Mo film was grown.  Deposition
conditions were chosen that have been shown to yield smooth
polycrystalline Mo and amorphous Si layers with thin (~1 nm),
asymmetric interlayers of mixed composition [1].  The microstructure
of the ML films was characterized using x-ray diffraction and
transmission electron microscopy (TEM).  The surface roughness was
characterized using atomic force microscopy and the resistivity was
measured using a four-point probe technique. 

The surface roughness (s^2) of the film, as defined as the integral of
the power spectral density (PSD) over the frequency range of 10^-3  -
10^-1 nm^-1 , is found to scale linearly with the Mo layer thickness
and is fairly independent of the Si layer thickness. Cross-sectional
TEM images show that the roughness originates from polycrystalline
texture of the Mo layers. The film resistivity increases with
decreasing Mo thickness in good agreement with Sondheimer-Fuchs theory
of electron scattering at the multilayer boundaries. Modeling of our
results yields a value of ~20 nm for the electron mean-free-path in
the Mo layers at room temperature. 

The opposing dependency of the resistivity and surface roughness of
the ML films on the Mo layer thickness allows us to identify optimum
structural parameters for achieving an ultrasmooth, conducting film.
The Si layers should be just thick enough to disrupt the Mo
crystallite growth; 1 nm is found to be sufficient. An Mo layer
thickness of 2.5 nm produces a surface roughness of  s < 0.2 nm with a
resistivity of ~ 60 micro-Ohm-cm.  The resistivity can be decreased at
the expense of greater surface roughness. 

The PSDs of the surface roughness of the films exhibit qualitatively
the behavior predicted by a simple, stochastic model of thin film
growth with relaxation via surface diffusion [2].  However,
quantitative modeling shows that the fundamental growth parameters
change with the Mo layer thickness.  In particular, the growth unit
volume Omega increases approximately linearly with the Mo layer
thickness.  This is likely due to the increasing size of the Mo
crystallites with layer thickness, consistent with a picture of
columnar, competitive growth cones [3].

[1]  D. G. Stearns, R. S. Rosen and S. P. Vernon, Appl. Opt.  32, 6952 (1993)
[2]  D. G. Stearns, Appl. Phys. Lett.  62, 1745 (1993)
[3]  R. Messier and J. E. Yehoda, J. Appl. Phys.  58, 3739 (1985)