The fabrication of short period multilayers for X-ray mirror applications

R.E.Somekh

Dept of Materials Science and Metallurgy, University of Cambridge, Pembroke
Street, Cambridge. CB2 3QZ, UK

The fabrication of short period multilayers for X-ray mirrors with periods
in the region of 1 to 2nm still represents a considerable technical
challenge.  It is known theoretically that few elemental combinations can
be envisioned which will give high reflectivities at these wavelengths,
particularly at normal incidence.  Starting with this poor prospect, the
problems of interface roughness and interface mixing both conspire to make
the fabrication of ideal multilayers difficult.  In this paper the issues
associated with these apparently insurmountable problems are addressed and
an attempt is made to define routes to further improve the performance of
existing multilayers.

As the period of a multilayer is reduced from tens of nanometers to these
extra short periods the interface sharpness increasingly becomes important.
A compromise must be reached as far as the mutual miscibility of the
layers used.  One expects the sharpest composition gradients for immiscible
layers, however at the extreme such layers are likely to wet one another so
poorly that small islands tend to form as one layer is grown on another and
this leads to an inherent roughness.  If, on the other hand, the elements
of the layers are miscible which results to good wetting of one layer on
the next, there will also be a tendency for the interface mixing.   A
second similar compromise is usually made in terms of the energetic atom or
ion bombardment which promotes surface mobility and smoothness.  High
bombardment (or ion polishing) promotes surface smoothness and adatom
mobility, however such bombardment is also likely to produce subplantation
effects and intermixing of the surface and sub-surface layers.
These issues are discussed in general terms with part reference to our own
work on magnetron sputtered multilayers with periods in the 1-3nm range
based on the model systems W/Ti, W/Si, W/C and the all amorphous system
a-TaIr/a-TiNi.  Detailed studies have been made of resputtering phenomena.
Speculations will be made for research routes to further improve the
reflectivity performance of these multilayers.