Monolithic Integration of Thin-Film Photodiodes on Silicon CMOS for Multi-spectral 2D and High-speed 3D Image Sensors

Abstract

Over the past decades, CMOS Image Sensors (CISs) have evolved significantly, expanding their capabilities from 2D imaging to 3D sensing and their spectral range from the visible (VIS) to the near- infrared (NIR) spectrum. This progression has enabled a diverse range of applications, from consumer electronics to autonomous systems. However, conventional CIS relies on silicon (Si) photodiodes for photodetection. The inherent material properties of silicon, particularly its indirect bandgap, result in a low absorption coefficient and a fundamental spectral limitation, especially in the long-wavelength region. These factors create an urgent need for beyond-silicon thin-film photodiodes (TFPDs) to overcome these limitations. To date, research on TFPDs has primarily focused on the material and discrete device level. The critical next step, the heterogeneous integration of high-performance TFPDs directly onto silicon read-out integrated circuits (ROICs), remains a significant, under-explored challenge. This hybrid integration represents the next frontier for CIS technology. This dissertation first provides a comprehensive overview of this landscape and then presents the design, fabrication, and characterization of two novel beyond-silicon hybrid image sensors: A VIS-NIR dual image sensor based on an organic photodiode and an indirect time-of-flight (iToF) sensor based on a halide perovskite photodiode. The first sensor achieves VIS-NIR dual-mode operation by integrating a solution-processed organic photodiode (OPD), which is transparent to visible light but highly absorbing in the NIR spectrum. This OPD is vertically stacked and isolated within the same pixel as the underlying SiPD, enabling simultaneous and independent capture of VIS data (via SiPD) and NIR data (via OPD). The sensor featuring a 120 × 64 pixel array was fabricated in a 110-nm 1P6M CIS frontside illumination (FSI) process, with the OPD monolithically integrated via low-temperature spin-coating. The device was fully characterized, successfully demonstrating VIS-NIR dual imaging and investigating the influence of the interconnection node pitch on the pixel response additionally. The second sensor, a high-resolution iToF imager, utilizes a fast-response (FA, Pb, I-based) halide perovskite photodiode (PePD). A novel demodulation scheme is introduced, achieving ‘shuttering’ of the PePD by modulating its top electrode (ITO), thereby eliminating the necessity for a conventional transfer gate (TX). The sensor with a 512 × 768 resolution was fabricated in a 130-nm 1P4M FSI process and integrated Pe PD by spin-coating. The sensor with a 5-𝜇𝜇m pitch successfully demonstrated high-quality 2D imaging, exhibited accurate phase detection with non-linearity of <1%, and achieved successful 3D depth imaging. The results presented in this dissertation validate the hybrid TFPD-on-Si-ROIC approach as a powerful and viable pathway for realizing next-generation, high-performance image sensors with functionalities beyond the reach of conventional silicon technology.