Electrical Effect of Nitrogen Implanted Into LDD of MOSFETs

Abstract

The motivation of this study was to solve the high $\rm I_{D,off}$ problem in 8 Volt N-channel MOSFET. We experimented with implanting nitrogen into LDD at various doses. As a result, $\rm I_{D,off}$ increases and $\rm BV_{DSS}$ decreases as the dose increases. When it exceeds 1.0E15 cm $^{-2}$ , the occurrence of tail-type $\rm I_{D,off}$ and $\rm BV_{DSS}$ that deviate from the normal distribution increases. Implanted nitrogen enhances the diffusion of dopants in the LDD bulk but suppresses it on the silicon surface. As a result, the depletion curvature at the LDD edge becomes a negative shape and increases the electric field. We performed the same experiment on logic MOSFETs to comprehensively analyze other electrical effects. Nitrogen improves the HCI immunity of MOSFETs but degrades for 2.5 Volt and 8 Volt MOSFETs when the dose is above 1.0E15 cm $^{-2}$ . The short-channel effect of 2.5 Volt MOSFET is insensitive to nitrogen but is suppressed in CORE MOSFET when the dose is over 1.3E15 cm $^{-2}$ . Nitrogen changes $\rm I_{D,sat}$ through interactions with co-implanted species and nitrogen dose. As a result, nitrogen co-implanted with phosphorus shows a parabolic-like $\rm I_{D,sat}$ trend. However, in the case of CORE MOSFET implanted with arsenic, $\rm I_{D,sat}$ does not show a parabolic-like trend but increases continuously. This experiment did not find much benefit from nitrogen implantation for 2.5 Volt and 8 Volt MOSFETs. For all MOSFETs, it is recommended that the nitrogen dosage not exceed 1.0E15 cm $^{-2}$ .