Effects of Silicon Nitride Passivation Layer on Mean Dark Current and Quantum Efficiency of CMOS Active Pixel Sensors

In the present study, we have analyzed the influence of the silicon nitride (SiN) passivation layer properties on the mean dark current and the quantum efficiency of CMOS APS (Active Pixel Sensor) through electrical characterization of lot wafers processed with three different SiN passivation films. The SiN layers were characterized by Spectroscopic Ellipsometry (SE) and Fourier Transform Infra Red (FTIR) spectroscopy to get the optical indices and the hydrogen content of the films, respectively. Hydrogen desorption was also studied by Thermal Desorption Spectrometry (TDS) experiments.The different chemical bond concentrations enable to explain the device performance. It is shown that high [Si-H] and low [Si-N] bonds concentrations lead to high hydrogen desorption from the SiN films. Thus, the lowest dark current values have also been obtained with such silicon nitride passivation layers. Consequently, results are in agreement with the hydrogen passivation of defects being responsible of thermally generated carriers. Concerning the quantum efficiency (QE), it is highly influenced by the optical indices of the SiN passivation layer.Actually, the thickness and complex refractive index of the SiN layer affect the light reflection, while the light absorption, in the visible range, is controlled by the [Si-Si] bonds concentration. There are also competing requirements to minimize dark current and to maximize quantum efficiency. In fact, high [Si-H] and low [Si-N] bonds concentrations (for good passivation) are generally observed in silicon-rich SiN films, which also contain high amount of [Si-Si] bonds (inducing high absorption, thus low QE). Increasing simultaneously the [Si-H] and [N-H] bonds concentrations in the passivation layer can be a way to have high hydrogen desorption during passivation anneal just preserving a transparent layer.