Engineering systems based on integrated circuits (IC) and microsystem technologies (MST) increasingly rely on materials whose behaviour is governed by coupled thermal, mechanical, and transport processes. Among these materials, borophosphosilicate glass (BPSG) thin films deposited by chemical vapor deposition (CVD) exhibit distinctive low-temperature flow characteristics that critically influence device-level performance. This study aims to provide a physically grounded synthesis of the mechanisms governing the flow behaviour of low-temperature BPSG thin films and to examine their functional roles across IC, MST, and optical device technologies. The analysis integrates reported experimental observations and process data to interpret BPSG behaviour in terms of thermo-viscous flow, compositional dependence, and surface evolution under thermal treatment. The results show that the reduced glass transition temperature induced by boron and phosphorus incorporation enables controlled viscous flow at temperatures as low as approximately 700–800 ℃, leading to effective surface planarization, void elimination, and geometry reconfiguration in complex device reliefs. The interaction between thermal activation, film composition, and structural constraints governs key performance outcomes, including planarization efficiency, gap-filling capability, and stress evolution. In MST and optical applications, the same flow mechanisms enable the formation of sealed cavities, microfluidic channels, and optically functional structures such as microlenses and waveguide cladding layers. It is concluded that the engineering functionality of BPSG films arises from the coupled interaction between thermal processes, material composition, and geometrical confinement. This work provides a unified interpretation of these mechanisms and highlights their implications for process optimisation and device design in integrated and multiphysics engineering systems.
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