Mo/Si multilayers produced using ion etching with energies
between 300 and 2000eV at various angles of incidence

H.-J. Voorma, E. Louis, N.B. Koster, D.J. Paff and F. Bijkerk

FOM-Institute for plasma physics 'Rijnhuizen', Edisonbaan 14, P.O.Box 1207,
3430 BE Nieuwegein, The Netherlands, Tel +31-(0)30-6096999, Fax:
+31-(0)30-6031204

For the production of Mo/Si multilayers produced with e-beam deposition,
the additional usage of ion polishing has been proved to be very beneficial
for the multilayer quality [1, 2, 3, 4, 5]. However the dependence of the
angle of incidence and the energy of the ions has hardly been explored.
Therefore, the influence of these parameters on the interface roughness of
the Si layer and consequently the quality of the multilayers is
systematically investigated. In previous studies it has been found that ion
etching of the Si layer is more effective compared to etching of the Mo
layer, therefore this study focuses on etching of the Si layer.
We produced multilayers using ion etching with Kr-ions, at an angle of
incidence of 20, 30, 40 and 50 degree with respect to the surface. For all
these angles we used ion energies between 300eV and 2000eV. The upper limit
of the energy is set by the power supply of the ion gun. The ion current
during etching for all these experiments is between 17-19 mA. With these
settings we are able to control the thickness of the Si layer during
etching using an in-situ reflection system.
All samples are analysed with small-angle reflectivity measurements to
determine the interface roughness. From these measurements it is found that
ion etching with 2000eV at 50 degrees angle of incidence is preferred,
because this results in a minimum interface roughness. Although there is a
strong angular dependence at low ion energies, this dependence is less
clear at higher ion energies. These findings are in contrast to earlier
reports in which it has been stated that ion etching of the Si layer with
Kr ions was optimal at 300eV and 45 degrees [3, 4, 5]. Our method of
optimizing the ion-erosion parameters is more comprehensive and covers a
larger parameter space.
Diffuse x-ray scattering measurements are used to model the ion erosion,
taking into account the penetration depth of the ions and the rotation of
the substrate during the production. Furthermore we will show data on the
etch yield as a function of the ion energy and angle of incidence.

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