Thermal  stability  of W_1-xSi_x/Si multilayers  for  X-ray
optics under rapid thermal annealing

E. Majkova, S. Luby, M. Jergel, R. Senderak

Institute   of  Physics,  Slovak  Acad.  Sci.,   842   28
Bratislava, Slovak Republic,

G. Haindl, F. Hamelmann, U. Kleineberg, U. Heinzmann

Universitat   Bielefeld,   Fakultat   fur   Physik,   4800
Bielefeld, Germany

     The thermal stability of multilayers (MLs) for X-ray
mirrors  can be increased by using the pair of  materials
in  thermodynamic  equilibrium. In  this  work  this  was
achieved  by  doping  of W layer by Si  to  decrease  the
driving  force  for interdiffusion. The W0.66Si0.33  /Si,
W0.5Si0.5  /Si,  W0.33Si0.66 /Si and for comparison  W/Si
MLs   with   ten  bilayers  were  fabricated  by   e-beam
deposition  in  UHV  onto  oxidized  Si  substrates.  The
nominal thickness was 5.5 nm for Si and 2.5 nm for  W  or
W1-xSix  layers. The samples were heat treated by halogen
lamp rapid thermal annealing and by standard annealing in
vacuum  up  to  1000 C for 30 s and 25 min, respectively.
Samples   were   analyzed  by  soft  X-ray   reflectivity
measurements, small and large angle X-ray diffraction and
by  the Rutherford backscattering spectrometry. From  the
results  it follows the increased stability of W1-xSix/Si
MLs  in comparison with W/Si ones. The temperature  which
the   sample  can  withstand  without  a  serious  damage
increases  from 500 to 850C with x increasing from  0  to
0.66 (this composition corresponds to the stable tungsten
disilicide).   As  deposited  MLs  were  amorphous.   The
crystalline  bcc W or WSi2 phases appeared  at  500C  for
x0.5.
     The increased thermal stability of W1-xSix/Si MLs is
attributed:
(a)  to  the  structural stability of amorphous  mixtures
against crystallization. For x=0.66, well developed  WSi2
phase  appeared only at 1000C/30 s annealing.  Hence,  Si
helps to keep W in the amorphous state;
(b)  to  the suppressed interdiffusion at the interfaces.
For  x=0.66 the stoichiometry of the structure  does  not
change due to the crystallization of WSi2 (like for lower
values of x) and there is no need for the diffusion of Si
for the further growth of the tungsten silicide.
     On the other hand the reflectivity of W1-xSix/Si MLs
is lower in comparison with the W/Si one because of lower
optical contrast.