Schwarzschild type X-ray microscope

Katsuhiko Murakami

Main Research Laboratory, Nikon Corporation
1-6-3 Nishi-ohi, Shinagawa-ku, Tokyo 140, Japan

       I will present the results of x-ray microscope imaging experiments
using a Schwarzschild objective as an application of x-ray multilayer
mirrors. X-ray microscopes using an x-ray optics, which consist of
multilayer mirrors, zone plates or grazing incidence mirrors, are being
developed by various research groups. They are divided into two categories;
an imaging type and a scanning type. In the imaging type, an x-ray image of
a sample to be observed is formed on a detector. In the scanning type,
x-ray micro-beam is scanned on a sample and the image is constructed. The
main purposes of an x-ray microscope are the observation of biological
specimens and the local area analysis. In the wavelength region from 2.33nm
(O K-edge) to 4.37nm (C K-edge), which is called "water window" region, the
difference of absorption coefficients between water and protein is
relatively large. Using this wavelength region, wet biological specimens,
which can not be observed by an electron microscope, can be observed with
higher resolution than that of an optical microscope. The local area
analysis is mainly performed using a scanning type. Photoelectrons or
fluorescent x-rays generated by incident x-rays are analyzed. 

       We are developing multilayer mirrors using an ion-beam sputtering
apparatus. To confirm the applicability of the multilayers which we
fabricated to practical x-ray optics, we made a Schwarzschild objective and
performed imaging experiments. The objective was tuned to the wavelength of
4.48nm, which was easy to be generated using carbon Ka radiation and was
slightly longer than "water window" region.

       A Schwarzschild objective comprises two concentric spherical
mirrors. In our objective, the magnification, the total length from object
to image and the numerical aperture (NA) were x32, 1300mm and 0.2,
respectively. We employed 50 layer pairs of NiCr(80:20 wt.%)/C multilayers
with the thickness period of 2.25nm. Thin nickel layers tend to form an
island structure, which causes interface roughness. The addition of
chromium to nickel has a marked effect on preventing island formation.
These multilayers were deposited using appropriately shaped masks to
control the thickness distribution. Though the calculated reflectivity of
these multilayers without interface roughness was about 20%, the actual
reflectivity were about 4%. 

       First, we performed imaging experiments using CKa radiation. A
commercial fine-focus x-ray source, operated at 10kV acceleration voltage
and 10 microamperes target current, with a 2.2 microns thickness of carbon
film target was employed as an x-ray source of 4.48nm radiation. The sample
was placed at a distance of 0.5mm from the target and a carbon filter was
employed to screen visible light. The magnified image of 1000 lines/mm of
gold transmission grating was taken on Fuji NEOPAN SS film with exposure
time of 2 hours. 0.5 micron-L&S patterns were resolved.  

       Next, imaging experiments using a laser-plasma x-ray source were
performed. The radiation from high density plasma produced by the
irradiation of a high peak intensity pulse laser on a solid target is
utilized in a laser-plasma x-ray source. We employed tantalum tape target
with the thickness of 15 microns and Q-SW YAG laser with the pulse width
and the repetition rate of 8nsec and 10Hz, respectively. The laser
intensity on the target was 3x1012W/cm2. To screen visible light and debris
emitted from a plasma, a carbon filter was employed. An ellipsoidal
condenser mirror coated with 100 layer pairs of Cr/C multilayers was
employed. Incidence angle of the condenser mirror was 65 degrees and the NA
was 0.022. Because the NA of the condenser mirror was smaller than that of
the objective, a portion of pupil of the objective was utilized. Using 1000
lines/mm gold transmission grating as a sample, 0.5 micron-L&S patterns
were clearly resolved on Fuji NEOPAN SS film after 8000 shots of exposure,
which was equivalent to the exposure time of 13.3min. The exposure time was
much shorter than that in the experiments using CKa radiation. Furthermore,
the 0.6 micron-scale structure of a diatom sample could be observed.

       Although the resolution was restricted to around 0.5 microns due to
the granularity of films in these experiments, 0.1 micron-L&S patterns were
delineated in the reduction lithography experiments using the same
Schwarzschild objective. Normal incidence reflectivity of present
multilayers at "water window" region is much lower than the theoretical
value owing to their interface roughness. I eagerly expect that the
optimization of materials and the improvement in deposition technology will
increase the reflectivity of x-ray multilayer mirrors.