Performance Improvement of CMOS APS Pixels using Photodiode Peripheral Utilization Method

Abstract

Charge-coupled device (CCD) technology had been leading the field of solid-state imaging for over two decades, in terms of production yield and performance until a relatively new image sensor technology called active pixel sensor (APS) (Fossum, 1993), using existing CMOS facilities and processes, emerged as a potential replacement in the early 1990s. While CMOS APS technology was originally considered inferior, continuous improvements in cost, power consumption (Cho et al., 2000), dynamic range (Gonzo et al., 2002), blooming threshold, readout scheme and speed (Krymsky et al.,1999), low supply voltage operation (Cho et al., 2000), large array size (Meynants, 2005), radiation hardness (Eid et al., 2001), and smartness have achieved performance equal to or better than CCD technology (Agranov et al., 2005; Krymsky et al., 2003). Electro-optical performance of a photodiode (PD) type APS pixel is directly related to physical properties of photodiode diffusion layer. Doping concentration, junction depth, junction grading, biasing conditions, and physical shape of the photodiode diffusion layer determine the pixel full-well capacity, which is one of the main performance benchmarks of the PD-APS pixel. Pixel full-well capacity is related to sensitivity, charge capacity, charge saturation, dynamic range, noise performance, and the spectral response of the pixel (Theuwissen, 1995). Pixel dynamic range versus full well capacity for different pixel noise levels could be plotted as shown on Fig. 1. Thus, increasing full well capacity is desirable. In this chapter, so called photodiode peripheral utilization method (PPUM) is introduced addressing performance improvement of photodiode type CMOS APS pixels, (Ay, 2008). PPUM addresses the improvement of the metrics full well capacity and spectral response especially in blue spectrum (short wavelength). First, identification of junction and circuit parasitics and their use in improving the full-well capacity of a three-transistor (3T) PD-APS pixel through photodiode peripheral capacitance utilization is discussed. Next, spectral response improvement of PD-APS pixels by utilizing the lateral collection efficiency of the photodiode junction through PPUM is discussed. The PPUM method and its proposed benefits were proven on silicon by designing a multiple-test-pixel imager in a 0.5μm, 5V, 2P3M CMOS process. Measurement results and discussions are presented at the end of the chapter.