Effect of Bias Temperature Stress on the Anti-Reflection HfO2 Layer in Complementary Metal Oxide Semiconductor Image Sensors

Abstract

The effects of various electrical characteristics of HfO2 in CMOS image sensors on bias-thermal stress instability were evaluated. In this work, the HfO2 dielectric layer was used as the anti-reflection layer of image sensors because it had negative charges and could electrically form a p+ layer on a silicon photodiode surface. After the HfO2 layer was stressed with electric field 0.5 MV/cm, 200 °C, and 10 min, there was severe electrical degradation such as +18.8 V flatband voltage shift. In order to investigate this degradation, the oxide trap charges and border trap charges of the HfO2 layer were measured and calculated. Based on these results, the interface trap density and minority carrier generation lifetime, which are directly related to the dark current in CMOS image sensors, were measured. The interface trap density degraded from 4.5×1011 to 1.0×1012 cm-1 eV-1 and the generation lifetime also degraded from 983 to 17 µs after stress application. This trap generated degradation model is suggested for CMOS image sensors. Therefore, pre-stabilization of bias-thermal stress should be implemented to use the HfO2 layer in modern CMOS image sensors.