  Amorphous boron carbide (B4C) thin films are by far the most popular form for
the neutron converting layers in the 10B-based neutron detectors, which are a
rising trend in detector technologies in response to the increasing scarcity
and price of 3He, the standard material for neutron detection. The
microstructure of the B4C films is closely related to the important properties,
e.g. density and adhesion, for the converting layers, which eventually affect
the detection efficiency and the long-term stability of the detectors. To study
the influence from substrates of different roughness and materials, the B4C
films were deposited on polished Si substrates with Al, Ti, and Cu buffer
layers and unpolished Si, Al, Ti, and Cu substrates by direct current magnetron
sputtering at a substrate temperature of 623 K. The tapered columnar grains and
nodular defects, generally observed in SEM images, indicated a strong shadowing
effect where voids were introduced around the grains. The change in the grain
size did not show a direct dependence to the substrate roughness, acquired from
the surface profile, nor to the mass density of the films, obtained from
reflectivity patterns. However, films with non-uniform size of columnar grains
were deposited on substrates with high skewness, leading to a drop of mass
density from ~95 % down to ~70 % of tabulated bulk density. On the other hand,
similar microstructures and mass density were obtained from the films deposited
on Al, Ti, and Cu of different roughness and good adhesion were observed from
cross-cut adhesion tests, showing the reliability of sputtering deposited B4C
films on common structural materials in neutron detectors.
