Single Photon Avalanche Diodes (SPADs) are a key technology in the detection of light and the coupling between electrical and optical systems. Their design has developed rapidly in the past 20 years, leading them to become commonplace in consumer technology such as LIDAR, Time-of-Flight (ToF) Sensing and optical communication. They can detect low energy signals (as low as a singular photon) with great temporal accuracy (on the order of 10-10 seconds and better). 

The STMicroelectronics Imaging Division, in Edinburgh, designs imaging and sensing products in Complementary Metal Oxide Semiconductor (CMOS) technology for a wide range of markets. The SPAD Pixel Team, embedded within this division, works with the Research and Development team in the manufacturing organisation on the definition and development of SPAD technology, using the group’s expertise in design and characterization to achieve this. The main function of the team is to provide device design to support the technology development.  

The team are currently working on making SPADs faster, smaller and cheaper. This is done while maintaining the device’s quality by ensuring Figures of Merit (FoM) such as Afterpulsing, Jitter, Dark Count Rate (DCR) are kept to an appropriate level. As there is no definitive ‘best’ design for a SPAD, the pixel team trials many new designs. To be able to develop SPAD technology and see which designs operate optimally, it is imperative to fully simulate any new device before putting it into production at ST’s fabrication facilities so that its operation can be fully understood. Designs with unwanted characteristics can be discarded and other potential problems that may arise as a result of a design can be fixed before being fabricated, saving silicon cycles and hence money. 

This project will aim to validate ST’s current simulation tools and improve them, resulting in the pixel team having a tool that is closer to being fully predictive. This will be done by analysing characterisation results and comparing them with extracted FoM’s from the current simulations. The simulation tools will then be developed and improved using these comparisons. The project also aims to use the simulation tools to identify relationships between design parameters and resulting FoM’s, which in the future may help guide SPAD design decisions. 

The aims and the objectives are:  

• To validate and improve the current simulation tools used by the STMicroelectronic (ST)’s pixel team for Single Photon Avalanche Diodes (SPAD) devices by comparing simulation results with characterization results, using the outcomes to feed into the improvement and development of the simulation tools, with the overall aim of making the tools fully predictive of device performance prior to manufacturing, hence reducing development costs for ST. 

• To use ST’s simulation tools to identify relationships between SPAD design parameters and resulting Figures of Merit (FoM), which in the future may help guide design decisions. 

• To become familiar with the TCAD tools used in optical device & SPAD simulation and have a greater understanding of the simulation process to be able to improve the tools.