Effect of ALD Processes on Physical and Electrical Properties of HfO₂ Dielectrics for the Surface Passivation of a CMOS Image Sensor Application

Abstract

The surface passivation of a CMOS image sensor (CIS) is highly beneficial for the overall improvement of a device performance. We employed the thermal atomic layer deposition (T-ALD) and plasma enhanced (PE-ALD) techniques for the deposition of 20 nm HfO₂ as well as stacked with 3 and 5 nm Al₂O₃ thin films. The HfO₂/Si and Al₂O₃/HfO₂/Si metal-oxide-semiconductor structures were used to analyze the fixed charge density (Q_f) and interface trap density (D_it). The as-synthesized samples show high D_it and Q_f values (10¹² cm^-2eV^-1) and a minority carrier lifetime of 15-300 μs. The finite-difference time-domain simulation of high-k dielectrics confirmed that the Al₂O₃ (top)/HfO₂ stacked structures expected higher quantum efficiency for CIS application. The effect of vacuum annealing (VA) and forming gas annealing (FGA) treatments succeeded with the decomposition of the D_it and increase in carrier lifetime. The H₂ ambient FGA samples showed a remarkable decrease in the D_it values. To improve the overall performance of the device after passivation, we employed an Al₂O₃/HfO₂ bilayer structure, which showed a low D_it of 10¹¹ cm^-2eV^-1 and a minority carrier lifetime of ~3,700 μs after 400 °C and 30 min FGA. We believe that this surface passivation strategy will pave way for future CIS technology regarding the development of lower defective surface and superior performance.