Modeling of CMOS Image Sensors: Four Pixel Simulation Template

Figure 1: Four-pixel CIS structure with calculated optical generation rate field under red light illumination is shown. The individual pixels extracted from the structure and used for the electrical simulation are also shown. With the "red" pixel a red optical filter is used, and for the "green" pixel the red wavelength is absorbed by the totally absorbing filter and only light scattered due to the optical cross talk reaches to the active area of the "green" pixel.

Figure 2: Electrical response of the CMOS Image Sensor as simulated with Sentaurus Device. Time dependency of gate voltage, electrostatic potential, and electron density obtained in the transient simulation: solid lines correspond to pixels transmitting the incident light ("red" pixel); dashed lines correspond to one of the absorbing pixels ("green" pixel).

Overview
CMOS Image Sensor (CIS) is a device which inherently requires a 3D modeling capability for predictive simulation of device characteristics. Sentaurus TCAD allows for a seamless integration of process, optical and device simulation tools making the 3D simulation of CIS possible. Although many characteristics of the device can be simulated with a single pixel structure, modeling of the optical and spectral crosstalk typically requires considering a structure consisting of multiple pixels.

This month's application note considers simulation of a four-pixel structure. In the presented simulation flow, process simulation is done for a one-pixel structure and the four-pixel structure is obtained from the one-pixel device using geometrical symmetry operations. The optical calculation is done with this structure but the electrical simulation is conducted with the individual single pixels. While this method allows for a considerable savings of the CPU time, the optical crosstalk can be evaluated. Figure 1 shows part of the four pixel structure for which the optical generation rate is saved. The individual pixels used further for the electrical simulation (presented in Figure 2) are also shown.

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