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. At 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 deg2), 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, and future directions.