Ni/Si based multilayer systems for the water window

M. Cilia and J. Verhoeven

FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098SJ
Amsterdam, The Netherlands.

We developed a process to make a multilayer system, based on nickel
as an absorber and silicon as a spacer. The system is to be applied in
optics as well as spectroscopy in a wavelength region between 3.1 and 4.4
nm.

A disadvantage of the pure Ni / Si system is the high chemical reactivity,
resulting in alloy formation. Several investigations have shown that an
ultra thin mixed layer, with a composition close to Ni2Si, can be formed at
room temperature at the interface of both components1,2). We investigated
the formation of silicon nitride by implantation of nitrogen ions. During
ion implantation, a part of the silicon layer will be removed by
sputtering. As observed before, smoothing of the surface can be expected3).
By using silicon nitride as a spacer we expect to prevent silicide
formation at the interface.

By evaporation we deposited 4 nm silicon on top of a nickel layer
and subsequently removed the silicon layer by  sputter etching with 10 mA /
cm2 300 eV nitrogen ions. During deposition and sputtering we used Auger
Electron Spectroscopy to determine the surface composition. Interference of
reflected C Ka radiation was used to monitor the layer thickness. We
observed that during growth, an interlayer of approximately .5 nm silicide
was formed. Sputter etching showed an initial shift of the Si LVV Auger
peak, ascribed to the formation of a nitride. The Si LVV peak shifted back
to its original position during continuing sputtering. This indicates that
the initially formed nitride is ultimately removed and that silicide is not
converted into nitride under nitrogen ion bombardment. As expected, a
reduction of the surface roughness could be observed from a change of the
average intensity of reflected C Ka radiation. From our investigation of
the deposition of nickel on silicon nitride we learned that no observable
formation of a silicide interlayer occurs.

We applied implantation of silicon by nitrogen to make a multilayer
system with a 3.8 nm period. A cross section TEM picture revealed that
nitrogen bombardment had induced the formation of a homogeneously mixed
layer of 3 nm, acting as the absorber. From the high chemical reactivity of
nickel and silicon we expect this interlayer to be a silicide.
We bombarded a silicon-nickel layer pair with 300 eV nitrogen ions as well
as neon ions. We learned that neon bombardment induces faster intermixing.
Apparently the formation of nitride competes with the formation of
silicide.

Our following investigations have been focused on the formation of
a stoichiometric nickel silicide of a controlled thickness by intermixing,
using noble gas ion bombardment. The intention has been to combine a more
stable nickel silicide as an absorber with silicon nitride as a spacer.
In our first successful attempt, deposition of silicon on nickel is
followed by a complete removal of the silicon overlayer by neon ion
bombardment, resulting in an intermixed surface. A new overlayer of silicon
is deposited and subsequently implanted by nitrogen ions. Additionally both
treatments by energetic ions form smooth interfaces. The recorded
reflection of C Ka radiation, monitored during deposition, showed less
interface mixing than where only nitrogen was implanted into silicon. This
has to be confirmed by cross section TEM.

We expect a maximum thermal stability of the system for a
combination of stoichiometric silicide and nitride. That requires a better
understanding of the processes that take place during ion enhanced silicide
formation. Therefore we will investigate in more detail the effects of the
ion mass and the ion energy as well as the amount of nickel that is
available to form silicide.

1)      K. Holloway, R.Sinclair and M. Nathan, J. Vac. Sci. Technol. A7,
p.1479 (1989)
2)      W.J. Schaffer, R.W. Ben=E9 and R.M. Walser, J. Vac. Sci. Technol. 15,
p.1325 (1978).
3)      R. Schlatmann, C. Lu, J. Verhoeven E.J. Puik and M.J. van der Wiel,
Appl. Surface Sci., 78,     p.147 (1994)