## 1 Introduction  
Silicon photonics is the manipulation of light (photons) in Si based substrates in a comparable manner to electronics is the manipulation of electrons in Si based substrates [1; 2]. Using light to transfer and manipulate data has many key advantages, most notably bandwidth and power. In addition, using Si as the substrate enables integration with electronics technology using the existing state-of-the-art complementary metal oxide semiconductor (CMOS) infrastructure. Si photonics research and commercial markets have grown exponentially in the last decade. The Si photonic integrated circuit (PIC) industry is predicted to be worth $3.9 B by 2025, with $3.6 B of this market expected to be data center transceivers [3]. However, in recent years Si photonics has expanded into new application areas including sensing for healthcare and environmental applications, quantum photonics for computing and communication applications, imaging for light detection and ranging (LIDAR) and spectroscopy applications and artificial intelligence (AI) for agriculture and big data applications, to name but a few. Mature Si PIC fabrication requires a huge investment of time and development of infrastructure and expertise in the multiple fabrication processes, which are required to build a Si PIC. Only with technological maturity of the processes are complex integrated systems enabled, and this is only achieved with optimized, well controlled fabrication processes. Fortunately, the Si photonics industry can exploit the vast knowledge and infrastructure developed in the electronics industry for the past several decades to achieve the required technological maturity. Over the past several years, many foundries have developed a Si photonics process line and offered access via a multi-project-wafer (MPW) service whereby many users' designs are fabricated on a single wafer, which is then diced into individual chips once fabrication is complete, so that each user only receives their own designs on their chips. This means users can share the fabrication costs thereby lowering the barrier to entry for researchers and fabless companies alike. This foundry model has majorly contributed to the growth and success of Si photonics in recent years [4; 5]. There are three categories of Si photonics foundries that have emerged: (1) rapid prototyping foundries typically based in university cleanrooms using e-beam lithography (e.g., AMO [6], Applied Nanotools [7], Australian Si Photonics [8] and INPHOTEC [9]); (2) research foundries typically based on deep-UV (DUV) projection lithography (e.g., AIM Photonics [10], AMF [11], CEA-Leti [12], IHP [13], imec [14], IMECAS [15], Sandia National Laboratories [16] and VTT [17]) and (3) high volume foundries capable of mass production (e.g., CompoundTek [18], Global Foundries [19] and Tower Semiconductors [20]). The core technology offered by these foundries is silicon-on-insulator (SOI), although there is no standardization of the top Si layer thickness [21]. In addition to SOI based technology, several foundries are now offering Si nitride technology, either as a standalone platform or integrated with SOI to address different markets including those that utilize visible wavelengths (e.g., AIM [10], AMF [11], CNM [22], CompoundTek [18], imec [14], Ligentec [23] and LioniX International [24]). CORNERSTONE [25], a collaboration between the Universities of Southampton and Glasgow, bridges the gap between the e-beam based rapid prototyping foundries, which offer a rapid turn-around using a non-scalable process and the research foundries, which offer scalable processes but typically lack process flexibility, which nurtures device level innovation. CORNERSTONE achieves this by offering an MPW+ service whereby users can customize certain steps within the MPW, with a technology based on scalable deep-UV lithography.