Sputter deposited beryllium fuel capsules for NIF Page: 4 of 11
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pan allowed the application of both a high frequency oscillation for randomly
bouncing the microspheres and a negative voltage for bias sputtering. The
deposition process was monitored remotely using a high magnification
video camera mounted on a viewport.
Our studies can be roughly divided into three categories. First, there are
those in which the effects of substrate biasing have been investigated.
This includes varying the substrate voltage from 0 to 120 V and applying
an intermittent bias. Next, there are studies of Be combined with boron, a
non-soluble dopant. Because of it's low Z this dopant is of particular
interest for x-ray related applications. Finally, there are experiments
in which pulses of nitrogen are admitted to the vacuum chamber during
deposition. The layers of nitride formed tended to disrupt the growth of
Be grains, leading to a more fine-grained microstructure. For all these
studies, we have most often used hollow plastic spheres for our substrate
material. However, there have been some samples deposited on glass
spheres or silicon flats.
The application of a voltage bias to the substrate had a significant effect
on the morphology of the Be films. For 8- to 10-pm-thick films deposited on
plastic spheres, the rms. roughness decreased from -150 nm with no bias
(Fig.1) to -40 nm with a 120 V bias (Fig.2). At the same time, the grain
size was reduced, and the film density increased (reflecting the elimination
of voids). X-ray fluorescence measurements detected the presence of
implanted argon in 120 V bias films, but not at 80 V bias or below.
This observation would be relevant for applications sensitive to the x-ray
transparency of the film.
Two experiments were conducted using intermittent biasing: 1 min at 100
V/10 min at 0 V and 1 min at 200 V/10 min at 0 V. In both cases, the
biasing appeared to have no significant effect.
The studies involving boron doping have just begun, so there are few
results. We have observed that films with about 3 atom % boron display a
distinctly different morphology. Figure 3. shows an outer surface and
cross section of a 1 mm diameter microsphere with no boron doping while
Figure 4. shows the same sized microsphere with finer grain structure
and reduced surface roughness doped with 3 atom % boron. We have
also noted that films deposited with 18 atom % boron are quite smooth
with -1 nanometer surface roughness. We hope to perform x-ray
diffraction measurements on such films to determine how the crystal
structure differs from pure beryllium.
Exposing the films to pulses of nitrogen gas during deposition also had a
significant effect. In these experiments, a 0.8 second pulse of N2 was
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Alford, C.S. Sputter deposited beryllium fuel capsules for NIF, report, February 12, 1998; California. (https://digital.library.unt.edu/ark:/67531/metadc674137/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.