High-Sensitivity Image Sensor with Stacked Structure comprising Crystalline Selenium Photoconductor, Crystalline OS FET, and CMOS FET

To realize a high-sensitivity CMOS image sensor using avalanche multiplication, we investigate the feasibility of a pixel structure that uses a crystalline selenium-based photoelectric conversion element and a crystalline oxide semiconductor (OS)-based FET. The OS FET is shown to have a withstand voltage of over 20 V, with which the photoelectric conversion element can cause avalanche multiplication. An amorphous selenium-based photoelectric conversion element in which holes are mainly travelling carriers and which is formed on a pixel electrode on a pseudo OS FET/CMOS FET substrate exhibits favorable photoconductivity, indicating compatibility with an OS FET/CMOS FET hybrid process. This shows the potential for realizing an OS/CMOS image sensor using avalanche multiplication in a crystalline selenium-based photoelectric conversion element. Introduction A configuration for a CMOS image sensor with a photoelectric conversion element instead of a silicon photodiode has been proposed in [1, 2], incorporating a high-gain avalanche rushing amorphous photoconductor (HARP) stacked onto a transistor with a high withstand voltage through a micro-bump. Unfortunately, this configuration requires a high process cost, and poses problems such as variations in device properties and mechanical stress. Furthermore, a crystalline selenium film has been reported to cause avalanche multiplication and thus have a quantum efficiency greater than 100% with over 20 V bias [3], and an image sensor having a photoelectric conversion element using the crystalline selenium film formed on a CMOS circuit has been proposed [4]. A high-sensitivity CMOS image sensor utilizing avalanche multiplication can be achieved with a hole-readout structure, a lower multiplication voltage, a considerably lower dark current, and a higher multiplication factor. A crystalline oxide semiconductor (OS), typically c-axis aligned crystalline In-Ga-Zn oxide (CAAC-IGZO), is a wide bandgap semiconductor. An FET containing the OS exhibits ultra-low off-state current and has a high withstand voltage [5]. Image sensors utilizing these features have been proposed, including an OS FET as a pixel transistor [6–8]. This paper proposes a CMOS image sensor using a crystalline selenium film for a photoelectric conversion element and an OS FET in a pixel. It is shown that the OS FET has a high withstand voltage and that a selenium-based photoelectric conversion element is compatible with the OS FET/CMOS FET hybrid process in terms of electrode structure. Furthermore, this paper examines the feasibility of the avalanche-mode CMOS image sensor with the crystalline selenium film. Device and Circuit Configurations A. Device Configuration Fig. 1 illustrates the configuration of the device proposed in this paper. A pixel electrode, a photoelectric conversion element, and a common electrode are stacked on an OS FET/CMOS FET substrate. We aim for a photoelectric conversion element using a crystalline selenium film. This study will examine the withstand voltage of an OS FET and, as a first step of investigating the feasibility of the novel configuration, process compatibility by evaluating a sample that is a selenium-based photoelectric conversion element shown in Fig. 2. Fig. 1. Device configuration of a stacked structure comprising an avalanche-mode selenium-based photoconductor, a crystalline OS FET, and a CMOS FET. Pixel Electrode Photoelectric Conversion Element