Speaker
Description
Silicon photonic ring modulators are key components for future silicon photonic transmitter systems in particle detectors. It has been shown that standard designs can retain acceptable performance up to a total ionizing dose of approx. 3 MGy, and up to more than 7 MGy when employing annealing techniques. We present a modified design which is expected to increase radiation tolerance significantly beyond those levels by altering the ring geometry, especially of the p-doped parts, with minimal impact on device performance.
Summary (500 words)
Standard silicon photonic ring modulators (RM) consist of a ring waveguide with a loosely coupled bus waveguide. More advanced designs add a drop waveguide, also loosely coupled, which enables on-chip closed-loop working point control. Modulation of the resonance wavelengths is achieved by changing the optical pathlength in the ring by adding a pn-junction in the ring waveguide and using the plasma dispersion effect for changing the refractive index. To control the pn-junction, electrical connections through thin slabs are added to the ring waveguide, partly doped similarly to the waveguide pn-junction’s doping concentrations (see Figure a)). In harsh environments with high radiation doses, a pinch-off might occur on the p-side through irradiation induced collection of positive charges on the interface between the silicon slabs and the surrounding silicon-dioxide. A recent refined analysis of our measurements performed with a test chip produced in Imec’s iSiPP50G process shows that standard RMs retain acceptable performance up to 3 MGy without and beyond 7 MGy with forward bias annealing. But each annealing step lowers the additional dose until the next device breakdown with a pinched-off slab.
Thickening both slabs to mitigate the effect, which is already proven to be effective in Mach-Zehnder modulators, is not a viable option here since a strong mode confinement is required to keep the light in a waveguide ring with 10-15 µm diameter. However, this only applies to the outside of the ring and only the p-side is affected by the pinch-off. Therefore, we positioned the p-side of the ring modulator on the inside of the ring and increased only the thickness of its corresponding slab (Figure b)). This can be done because confinement to the inside can be much weaker than to the outside. To our knowledge, slab thickness increase is not yet employed in ring modulators to improve radiation hardness. Due to the changed mode confinement, the ring waveguide width was also modified to keep good phase matching in the coupling regions.
We will present extensive simulations of the new structure, as well as a chip design with these modulators, currently in production at Imec and expected to be delivered towards the end of 2026. Simulations show a slight impact of the new structure on ring modulator performance. Additional optimization of the geometry is currently ongoing, which is expected to reduce or eliminate this performance impact. For the modulator design, a self-developed tool was used, which will soon be published as open source software, and which will also be presented.