Roughness vs. thickness in Ru/C multilayers - a parametric study

C. C. Walton, H. Takenaka*, T. Kawamura* and J. B. Kortright

Ernest Orlando Lawrence Berkeley National Laboratory
1 Cyclotron Road 
Berkeley, California 94720  USA

*NTT Interdisciplinary Research Laboratories
9-22 Midori-cho 3-chome
Musashino-shi Tokyo 180  Japan

Review of measurements of interface roughness in multilayer mirrors has shown
a trend of increasing roughness with increasing thickness in various systems.
In order to study this dependence more thoroughly in a single system, a set of
multilayers was fabricated with a range of d-spacings from 15E to 150E by rf-
sputtering in argon.  The nearly-immiscible Ru/C system was chosen to
eliminate as much as possible the effects of interdiffusion.  To address the
increase of roughness with number of layer pairs, two sets of samples were
made, one with constant number of layer pairs and one with constant total
thickness.  Measurements of surface roughness by AFM, and of average roughness
by modeling of hard x-ray reflectivity, and their dependence on thickness will
be reported.  Growth of roughness with number of layers and implications about
the fundamental causes of roughness and contributions from the substrate will
be discussed.