Effects of high energy boron ions implanted in MOSFETs

Abstract

In recent years, high energy ion implantation has been found to be very attractive for device fabrication because of its ability to place dopants more than 1 μm beneath the substrate's surface. Many new applications and structures are made possible with this feature. Examples are the replacement of epilayer growth by high energy ion implant to form bipolar transistor subcollectors [1] and the construction of retrograded wells to reduce latchup and soft-error in CMOS transistors [2,3]. In this work, we will investigate the feasibility of customizing integrated circuits by threshold adjustment implant as a late processing step. Different doses of boron ions with energies ranging from 0.75 to 0.90 MeV are implanted into n-MOSFETs after second-level poly-Si deposition. Rapid thermal annealing is used for postimplant annealing and dopant activation. For low-dose implants, full activation can be realized at about 750°C for 15 s. For high-dose implants, however, no full activation is seen even at 1000°C. I-V measurements are used to investigate the high-energy implantation effects on the threshold voltage, the channel leakage current, and the subthreshold slope of the MOSFET transistors. It is found that the threshold voltage shifts are no longer linearly proportional to the implant doses. A new relationship is presented. After annealing, the leakage current for the implanted transistors can be reduced to the same level as that of unimplanted devices. However, the subthreshold slope is larger for an implanted transistor compared to an unimplanted transistor.