Characterization of Plasma Process-Induced Low-Density Defect Creation by Lateral Junction Leakage

Abstract

In the design of ultralow leakage devices such as image sensors, it is necessary to understand the influence of low-density defects during plasma processing—plasma-induced physical damage (PPD)—on device performance. Defects created by plasma exposure act as carrier conduction sites and induce an increase in leakage (e.g., dark) current. This study proposes a PPD evaluation scheme for low-density defect assessments, specifically defects created in the lateral direction due to lateral stochastic straggling (lateral PPD). Two test structures were designed: a single device with a leakage current ( ${I} _{\mathrm{ leak}}$ ) and a complementary metal-oxide-semiconductor image sensor (CIS) circuit with a dark current ( ${I} _{\mathrm{ dark}}$ ). The energy level and density of defects distributed in the lateral direction were estimated using the Shockley–Read–Hall (SRH) model. The energy level ( ${E} _{\mathrm{ t}}$ ) was derived using the SRH model from the temperature dependence of ${I} _{\mathrm{ leak}}$ and ${I} _{\mathrm{ dark}}$ defined as the activation energy ${E} _{\mathrm{ a}}$ . The trap density ( ${N} _{\mathrm{ t}}$ ) was also determined. A comprehensive comparison of these parameters was conducted. Both ${I} _{\mathrm{ leak}}$ and ${I} _{\mathrm{ dark}}$ are dependent on the contact opening diameter after plasma exposure, which implies the presence of defects in the lateral direction via lateral PPD. From the analysis of the temperature dependence of ${I} _{\mathrm{ leak}}$ and ${I} _{\mathrm{ dark}}$ , the lateral PPD influenced the mean value of ${E} _{\mathrm{ a}}$ . Moreover, we confirmed that an increase in ${I} _{\mathrm{ dark}}$ indicates an increase in the number of trap sites, and more specifically, an increase in defects at shallow levels. The derived trap site density in the CIS circuit was consistent with that of a single device. The proposed evaluation scheme is useful for PPD evaluation in the presence of low-density defects, and is critical for the design of future low-leakage devices.

Keywords

• P–n junction
• leakage current
• CMOS image sensor
• dark current
• plasma materials processing
• MOSFET