3D Camera Based on Gain-Modulated CMOS Avalanche Photodiodes

In the last several years, both scientific and industrial community have shown an increasing interest in range imaging due to its potential use in various application domains such as robotics, vehicle safety, gaming, mobile applications as well as many others. Among the diversity of techniques available for range detection, Time-of-Flight (TOF) offers advantages in terms of compact system realization, good performance and low required computational power. Recent works have shown a trend towards higher resolutions, with a consequent reduction of pixel size, higher modulation frequencies and demodulation contrast to allow a higher distance precision. In this thesis we propose a new concept of range camera exploiting linear-mode avalanche photodiodes as in-pixel demodulating detectors. Due to photocurrent gain modulation, avalanche photodiodes can combine optical sensing and light signal demodulation in a single device. The main advantage of the avalanche photodiode implementation is the possibility to operate at high frequencies due to its very wide bandwidth that, in turn, influences the precision in distance measurement. In a first stage, the concept was experimentally validated on single pixel structures. These measurement results encouraged the implementation of a time-of-flight image sensor. A 64x64 pixel array has been designed and fabricated in a 0.35um standard CMOS technology. The pixel pitch is 30um with a fill-factor of 25.7%. Demodulation contrast exceeds 85% at 25MHz modulation frequency. A 3D camera system demonstrates best precision of 1.9cm and a 3D frame rate of 200fps. Additional tests performed on single pixels have shown demodulation contrast as high as 80% measured at 200MHz modulation frequency.