Pixel Technology for Improving IR Quantum Efficiency of Backside- illuminated CMOS Image Sensor

In this paper, we present CMOS image sensor containing suitable pixel structure for IR light harvesting. In order to enhance the IR sensitivity, several pixel process technologies have been applied such as silicon thickness increment, antireflecting layer (ARL) modification, and introduction of backside scattering technology (BST). The effect of each technology on the infrared quantum efficiency (IRQE) of image sensor has been demonstrated. The BST patterns were formed at silicon surface for the light scattering effect. In order to investigate the effect of light scattering, the quantum efficiencies and the optical crosstalk between the neighboring pixels based on various BST shapes were measured. By using the above technologies, IRQE of the image sensor has dramatically increased up to 43% at 940nm wavelength, which is nearly 400% enhancement compared to the conventional image sensor. Introduction Recently, according to the demand of image sensor applications such as iris scanner, security camera and Time-of-Flight (ToF) sensor, the importance of the light sensing ability at near infra-red (IR) range has raised a lot. However, the IR sensitivity of conventional image sensor is insufficient for such applications. To enhance the IR sensing ability, appropriate pixel structure is needed. In this paper, we have developed CMOS image sensor containing optimized pixel structure for effective IR light harvesting. For the development of IR sensor, we utilized 2.1um backside-illuminated image sensor containing deep trench isolation (DTI) [1][2][3] and microlens arrays [4]. Results and discussion The conventional pixel structure of back-side illumination (BSI) image sensor and the concept of pixel design for IRQE enhancement are described in Figure 1. Figure 1. Schematic images of design concept for image sensor having IR sensitivity For the conventional BSI pixel, an epitaxial silicon (Si) layer which contains the photodiodes is located above the metal-lines and transistors. [5] Due to the Si band gap energy of ~1.1eV, the photodiodes based on Si can absorb light effectively at the wavelength range of 4001100nm. Between the photodiodes, deep trench isolation (DTI) is existed in order to suppress the optical/electrical crosstalk [1] and on the surface of Si, anti-reflection layer (ARL) which consists of various transparent oxide layers are deposited for the transmittance enhancement and dark current suppression. [6] Finally, planarization R14