  The research and development of silicon-based X-ray fluorescence detectors
achieved its submicron sensitivity. Its initial use is intended for in-situ
beam monitoring at advanced light-source facilities. The effectively
functioning prototype fully leveraged technologies and techniques from a wide
array of scientific disciplines: X-ray fluorescence technique, photon
scattering and spectroscopy, astronomical photometry, semiconductor physics,
materials science, microelectronics, analytical and numerical modelling, and
high-performance computing. In the design stage, the systematic two-track
approach was taken with the aim of attaining its submicron sensitivity:
Firstly, the novel parametric method, devised for system-wide full
optimisation, led to a considerable increase in detector's total solid angle
(0.9 steradian), or integrated field-of-view (~ 3000 deg$^{2}$), thus in turn
yielding a substantial enhancement of its photon-detection efficiency.
Secondly, the minimisation of all types of limiting noise sources identified
resulted in a boost to detector's signal-to-noise ratio, thereby achieving its
targeted range of sensitivity. The subsequent synchrotron-radiation experiment
with this X-ray detector demonstrated its capability to respond to 8-keV photon
beams with 600-nanometre sensitivity. This Article reports on the innovative
and effective design methods formulated for systematising the process of
custom-building ultrasensitive photodetectors, as well as future directions.
