Silicon photonics is the manipulation of light (photons) in silicon-based substrates, analogous to electronics, which is the manipulation of electrons. The development cycle of a silicon photonics device consists of three stages: design, fabrication, and characterisation. Whilst design and characterisation can readily be done by research groups around the country, the fabrication of silicon photonics devices, circuits and systems requires large scale investments and capital equipment such as cleanrooms, lithography, etching equipment etc. Based at the Universities of Southampton and Glasgow, CORNERSTONE 2.5 will provide world-leading fabrication capability to silicon photonics researchers and the wider science community. Whilst silicon photonics is the focus of CORNERSTONE 2.5, it will also support other technologies that utilise similar fabrication processes, such as MEMS or microfluidics, and the integration of light sources with silicon photonics integrated circuits, as well as supporting any research area that requires high-resolution lithography. The new specialised capabilities available to researchers to support emerging applications in silicon photonics are: 1) quantum photonics based on silicon-on-insulator (SOI) wafers; 2) programmable photonics; 3) all-silicon photodetection; 4) high efficiency grating couplers for low energy, power sensitive systems; 5) enhanced sensing platforms; and 6) light source integration to the silicon nitride platform. Access will be facilitated via a multi-project-wafer (MPW) mechanism whereby multiple users' designs will be fabricated in parallel on the same wafer. This is enabled by the 8" wafer-scale processing capability centred around a deep-UV projection lithography scanner installed at the University of Southampton. The value of CORNERSTONE 2.5 to researchers who wish to use it is enhanced by a network of supporting companies, each providing significant expertise and added value to users. Supporting companies include process-design-kit (PDK) software specialists (Luceda Photonics), reticle suppliers (Compugraphics, Photronics), packaging facilities (Tyndall National Institute, Bay Photonics, Alter Technologies), a mass production silicon photonics foundry (CompoundTek), an epitaxy partner for germanium-on-silicon growth (IQE), fabrication processing support (Oxford Instruments), an MPW broker (EUROPRACTICE), a III-V die supplier (Sivers Semiconductors) and promotion and outreach partners (Photonics Leadership Group, EPIC, CSA Catapult, CPI, Anchored In). Access to the new capabilities will be free-of-charge to UK academics in months 13-18 of the project, and 75% subsidised by the grant in months 19-24. During the 2-year project, we will also canvas UK demand for the capability to continue to operate as an EPSRC National Research Facility, and if so, to establish a Statement of Need.