> We need to fill deep trenches (1*10um) etched in the silicon layer of
SOI
> wafer with a material that has a refraction index close to but
different
> from that of Si (3.5).


You might want to try polysilicon.  It has excellent step coverage
(LPCVD or RTCVD) and has an index close to but not equal to that of
crystalline silicon (see "Optical study of undoped, B or P-doped
polysilicon", Y. Lahla and E. Scheid, Thin Solid Films 306 (1997) pp.
67-73).  RTCVD can generally fill trenches more completely than LPCVD
since the dep rate is higher and the films are smoother.

Cheers,

Greg Mattiussi
Spectalis Corp.