Nitrogen-Vacancy (NV) color centers in diamond are a promising and powerful tech- nology for sensing small magnetic fields in the order of μT. One of its applications is the use in magnetic biosensing to identify cells marked with magnetic nanoparticles. These nanoparticles can target cells linked to malignant tissue, which can subsequently be identified with great spatial resolution using the NV centers. Therefore promising a less invasive procedure and more accurate identification. Such a system using NV cen- ters as detectors requires small and effective photon detection hardware in order to be used in practice. In this bachelor graduation project Single Photon Avalanche Diodes (SPADs) are characterized, simulated and validated, to be integrated in a SPAD imaging sensor designed for magnetic bioimaging using NV centers.
A 3D model of the SPAD is created in TCAD Sentaurus to verify whether the designed SPAD performs as intended. To this end, the I-V and C-V curves are simulated. These graphs show that a proposed SPAD design works as intended whilst the previously im- plemented design does not. Furthermore, the simulation results can later be compared to the behavior of a physical SPAD. During the physical analysis of the stray SPADs on a previous IC, a design flaw was identified. To verify this a test circuit was built and the device was retested. The results confirm an error in the SPAD design. The SPAD is missing an N-well resulting in standard diode behavior and preventing it from detecting photons. Furthermore, to enhance single-photon detection, we explored a new analog- to-digital converter (ADC) circuit that employs a Schmitt trigger in place of a standard comparator. Our results show that this ADC delivers predictable behavior and achieves a more precise and efficient conversion to a digital signal.
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