An Excess bias voltage monitoring system for a time-of-flight sensor using SPADs

Abstract

The main goal of this work was to develop a system for monitoring and regulating the excess bias voltage of single-photon avalanche diodes (SPADs) for a Time-of-Flight (TOF) sensor system in order to achieve a wide operating temperature range despite the strong temperature dependence of the SPADs breakdown voltage. This excess bias voltage monitoring system (EBVMS) was developed for the TOF sensor chip realized in 55nm high-voltage (HV) CMOS technology. The compact package of the TOF sensor contains a VCSEL and a TOF chip integrated together with corresponding optical filters and lenses. It was important to generate the high reverse voltage for the SPADs with an integrated charge pump on the sensor chip. Another essential aspect for the sensor and EBVMS was a low power consumption for mobile applications. Furthermore, the necessary chip area should be minimized. Two test chips were developed, and two different excess bias voltage monitoring circuits (EBVMCs) were tested. The whole EBVMS system is described in this work, as also the improvements from which some should be tested for future sensor chip generations. During the design different approaches were developed. The first test chip used a so-called EBVMC1 circuit noted in this work, which is the first no-static power consumption circuit of this kind according to the knowledge of the author of this thesis. The second test chip used a so-called EBVMC2 circuit noted in this work, which uses binary search and requires an order of magnitude less time for finding a predefined excess bias voltage value. During development, the high parasitic capacitances of the high-voltage NMOS devices used as flying capacitors for a charge pump, affected the efficiency of the charge pump pretty much. Based on this fact, the author investigated possibilities of efficiency improvements and developed a new approach for enhancing the efficiency of the charge pump for the case of triple well technologies. During this work the SPAD model, reference circuit, quenching circuits, digital-to-analog converters, EBVMC circuits, different types of comparators and operational amplifiers were developed to meet the requirements for defined specification. The distance between the EBVMC and the measurement SPAD array on the silicon chip was investigated. The designed EBVMS was tested and measured inside the ams AG company by the author himself. The results showed a proper functionality of the designed circuits.