Short Period (< 3 nm) Multilayers of BN/Ni and Si/W For X-Ray Applications. 

Joseph Pedulla

National Institute of Standards and Technology, Physics Laboratory, Bldg. 221,
Room A141, Gaithersburg, MD 20899 USA, Tel:  301/975-3740 and
J. Pedulla & Associates, Silver Spring, MD 20904, Tel:  301/438-8598. 

Richard D. Deslattes

National Institute of Standards and Technology, Physics Laboratory, Bldg. 221,
Room A141, Gaithersburg, MD 20899 USA, Tel: 301/975-4841


Increased effort is currently directed toward producing smaller d-spacing
x-ray multilayer optics with high reflectivity.  Although inherent atomic
roughness and interface diffusion are intrinsic limitations, optics with
bilayer periods less than 3 nm with well defined structure and optical
properties have been reported [1].  Of several material combinations (BN, B4C,
C, Si, Cr, Fe, Co, Ni, Ag, and W) that we have attempted to use for optics
with these repeat distanc es, BN/Ni and Si/W have yielded the best results to
date and are the focus of this report.

In our work, dual ion beam assisted deposition (DIBAD) methods at low ambient
pressure (~ 10-4 Torr) were used to produce short-period multilayer optics
with good results.  The optical elements produced were 20 to 100 layer pair
structures on high quality float glass, silicon wafers, and superpolished
fused silica substrates.  In the DIBAD technique, material is sputtered from
source target to sample substrate by an argon beam with energies up to 1200 ev
and currents up to 100 mA.  Simultaneously, a second argon beam, directed at
the sample, smoothes the surface and densifies the layered materials.
Computer controlled deposition parameters provide excellent run-duration
stability and day-to-day reproducibility.  Resulting d-spacings throughout the
multilayer stacks are within 0.01 nm and day-to-day targeted d- spacings are
within a 0.1 nm.

High resolution x-ray reflect ometry at 8.05 keV (lambda 0.154 nm) was used to
measure the specular and diffuse reflectivity for these multilayers.
Structure, optical properties, interface roughness, and density are determined
from these data.  Data from a series of similar depositions gave d-spacing
deviations from projected values allowing measurement of the development of
interface layers.  Results from diffuse x-ray scattering measurements provided
estimates of interface roughness.  At-wavelength performance was evalu ated
using selected optics as monochromators in 1 keV (lambda ~ 1 nm) x-ray
applications.

[1] Fedorenko, A.  I., V.  V.  Kondratenko, Yu, P.  Pershin, O.  V.  Poltseva,
E.  N.  Zubarev, L.  L.  Balakireva, V.  V.  Didyk, V.  V.Burtsev, Structure
and Optical Properties of Multilayer X-ray Mirrors for Long Wave Part (3.1-4.4
nm) of Water Window, Cryst.  Res.  Technol.  29, 8, 1139-1147, 1994.