Effect of ALD Processes on Physical and Electrical Properties of HfO2Dielectrics 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 (Qf) and interface trap density ( Dit). The as-synthesized samples show high Dit and Dit 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 Dit and increase in carrier lifetime. The H2 ambient FGA samples showed a remarkable decrease in the Dit values. To improve the overall performance of the device after passivation, we employed an Al2O3/HfO2 bilayer structure, which showed a low Dit 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.