Deposition and in situ reflectance measurements of Y/Mo XUV multilayer mirrors

C.  Montcalm,*?  B.T.  Sullivan* and H.  Pepin?

* Institute for Microstructural Sciences, National Research Council of Canada
Montreal Road, Ottawa, Ontario, K1A OR6, Canada.

?  INRS-Energie et Materiaux, Universite du Quebec, 1650 Montee Ste-Julie,
Varennes, Quebec, J3X 1S2, Canada.

Preliminary studies of new material pairs for XUV multilayer mirrors have 
shown that Y/Mo multilayers can theoretically have high reflectances in the
8 to 15 nm wavelength region with reflectances between 25 and 35% being 
achieved around l=11.5 nm.[1,2]  These results were nearly a factor of two 
lower than expected even after taking into account the interface roughness 
estimated from the low-angle XRD studies.  Possible explanations for these 
apparently low XUV reflectances include contamination during deposition or 
oxidation of the surface layers prior to measurement.  Alternatively, the 
theoretical reflectance could have been overestimated if the optical 
constants derived from atomic scattering factors were not accurate because 
the assumed film density was incorrect.

To distinguish between the above different possible causes of the lower than 
expected XUV reflectances, Y/Mo multilayer mirrors were fabricated using a 
newly constructed UHV deposition/reflectometer system.  This system can be 
attached directly to a synchrotron beamline and multilayer samples can be 
characterized directly after deposition and before exposure to ambient air.  
With this system, we were able to confirm that both layer contamination 
during deposition and surface oxidation after deposition were responsible for 
the lower than expected reflectances.  The best Y/Mo multilayers were 
fabricated with base pressures below the 10-8 Torr range and had in situ near-
normal incidence reflectances of 46%, 35% and 21% at wavelengths of 11.4, 9.7 
and 7.9 nm, respectively.  After several days of exposure to ambient air 
these samples typically had a relative reflectance loss of ~10% as a result 
of the Mo surface layer oxidizing.[3]

In this presentation, the construction of the deposition/reflectometer 
system and the optimization of the deposition parameters of the Y/Mo 
multilayer mirrors will be described along with the in situ reflectance 
measurements.

1. C. Montcalm, P. Kearney, J.M. Slaughter, B.T. Sullivan, M. Chaker, H. 
Pepin, and C.M. Falco,  Survey of Ti-, B- and Y-based soft x-ray/extreme 
ultraviolet multilayer mirrors for the 2 to 12 nm wavelength region,  
submitted to Appl. Opt., November 1996.

2. C. Montcalm, B.T. Sullivan, M. Ranger, J.M. Slaughter, P.A. Kearney, C.M. 
Falco, and M. Chaker,  Mo/Y multilayer mirrors for the 8-12-nm wavelength 
region,  Opt. Lett. 19, 1173-1175 (1994).

3. C. Montcalm, B.T. Sullivan, S. Duguay, M. Ranger, W. Steffens, H. Pepin, 
and M. Chaker,  In-situ reflectances measurements of soft x-ray/extreme-
ultraviolet Mo/Y multilayer mirrors,  Opt. Let. 20, 1450-1452 (1995).