## 2 Current CORNERSTONE Platforms
### Passive Devices  
The passive devices discussed in this section refer to photonic devices, which do not generate, amplify, modulate or detect light. Dielectric optical waveguides [33] based on total internal reflection are the most common light guiding structures for long distance communications, i.e., the optical fiber. Yet unlike optical fibers, Si photonic waveguides are rectangular owing to the planar substrate and the fabrication process. Si waveguides can be partially etched, so-called rib waveguides, or fully etched, so-called strip waveguides, both of which are continuous along the propagation direction. A cross section of these two types of waveguides are schematized in Figure 1a,b, respectively. Optical mode profiles of the fundamental transverse electric (TE) mode are overlapped on the cross sections, which are denoted by the electric field intensity. Theoretically, the rib waveguide has weaker optical confinement than the strip waveguide with the same width and height. Usually, a rib waveguide has more light in the slab layer and therefore has lower propagation loss than a strip waveguide but requires a larger bending radius due to the weaker confinement. When working at telecommunication wavelengths, the propagation loss mostly generates from radiation loss and scattering loss. The radiation loss is \begin{table}
\begin{tabular}{c c} \hline \hline
**Platform** & **Availability** \\ \hline
220 nm silicon-on-insulator (SOI) & 2 MPW calls per year for active and/or passive devices \\
340 nm SOI & 1 MPW call per year for passive devices \\
500 nm SOI & 1 MPW call per year for passive devices \\ Silicon nitride & 2 MPW calls per year for passive devices from summer 2021 \\ Germanium-on-silicon & 2 MPW calls per year for passive devices from summer 2021 \\ Suspended silicon & 2 MPW calls per year for passive devices from summer 2021 \\ Electronic-phototic integration via bonding to SOI & 2 MPW calls per year, aligned to SOI calls from summer 2021 \\ Light source integration to SOI via pick-and-place & 2 MPW calls per year, aligned to SOI calls from summer 2021 \\ Deep-UV lithography service & On demand \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of CORNERSTONE rapid prototyping capabilities with approximate availability, which is subject to change. Bespoke fabrication batches are also available for all platforms on demand. determined by the waveguide dimensions, whilst the scattering loss is mainly caused by waveguide sidewall roughness [34], which is determined by both the lithography process used to define the waveguide in the horizontal and propagation direction, and the etch process used to define the waveguide in the vertical direction. A typical standard single mode (SM) waveguide width is 450 nm for \(\lambda\) = 1550 nm and 400 nm for \(\lambda\) = 1310 nm, although this is dependent on the Si thickness. Typical insertion loss values of standard CORNERSTONE components for all platforms, which are available in an open-source, license free PDK, are shown in Table 2. As mentioned above, through the use of e-beam lithography CORNERSTONE users have the capability of fabricating high resolution devices, for example, subwavelength grating waveguides [35], slot waveguides [36] and photonic crystal waveguides [37]. By way of comparison with other major foundries from around the world, Table 3 compares some publicly available data for various basic building blocks. Note that all of the compared foundries utilize 193 nm DUV lithography, which is a higher resolution, but more expensive technique.