DC and RF characterization of NiSi Schottky barrier MOSFETs with dopant segregation

摘要

The continuous downscaling of the Si-based microelectronics, which is the fundament of today's information technology, requires novel concepts for the source/drain (S/D) architecture of metal-oxide-semiconductor field-effect transistors (MOSFETs). The improvement of the carrier injection is of prime importance because of the increasing impact of parasitic resistances which strongly limit the performance of ultimately scaled transistors. Moreover, steeper junctions at the contact/channel interfaces become more and more crucial for nanoscale devices. In this context, Schottky-barrier (SB) MOSFETs with metallic S/D are promising performance boosters since they offer low extrinsic resistances and atomically abrupt junctions formed at the metal/silicon interface. However, a drawback of these devices is their performance which is inferior to conventional MOSFETs due to the relatively high Schottky barrier. Recently, dopant segregation has attracted much interest since the highly doped layer formed at the silicide/silicon interface during silicidation strongly improves the tunneling probability of carriers through Schottky contacts. The present thesis studies the integration of NiSi with dopant segregation in SB-MOSFETs on thin-body silicon-on-insulator experimentally. The objective of the detailed direct-current (DC) and radio-frequency (RF) characterization is to gain a better insight into the physics of these devices. The modeling of NiSi/p-Si Schottky contacts using a numerical model which combines the thermionic emission theory with image-force induced barrier lowering and quantum-mechanical tunneling provides a solid understanding of the carrier injection of Schottky contacts. The characterization of Schottky diodes with silicidation induced dopant segregation using boron, arsenic and antimony reveals effective Schottky barrier heights in the 0.1eV regime depending on the implantation dose. Below this value SB-MOSFETs are capable of outperforming conventional MOSFETs. Successfully fabricated long- and short-channel p- and n-type SB-MOSFETs with and without dopant segregation are characterized performing direct-current (DC) measurements. Transistors with 80nm channel length reveal on-currents as high as 427µA/µm for p-type and 1150µA/µm for n-type devices, respectively, which compete well with state-of-the-art SB-MOSFETs. For the first time, the extrinsic and intrinsic device parameters of short-channel n- and p-type NiSi SB-MOSFETs are extracted using scattering parameter measurements. The radio-frequency (RF) investigation of the devices reveals a perfectly linear scaling and high cut-off frequencies of 140GHz for n-type and 63GHz for p-type SB-MOSFETs with a gate length of 80nm. The optimization of the reproducible fabrication process improves the S/D resistances by 30% and yields a value of 508 Ohm µm for devices fabricated on 20nm thick silicon-on-insulator. Although, the DC performance of SB-MOSFETs is strongly deteriorated by high Schottky barriers, it has only a small impact on the cut-off frequency. This can be explained by the similar behavior of the transconductance and the total gate capacitance when the implantation dose and therefore the Schottky barrier height is changed. The benchmarking of the obtained cut-off frequencies with state-of-the-art MOSFETs demonstrates the superior RF performance of the fabricated devices and predicts an impressive performance increase for further downscaling. RF performance limiting parameters are identified and the origin of the RF variability of SB-MOSFETs is discussed. In summary, the results of this work demonstrate a high potential of NiSi S/D SB-MOSFETs with dopant segregation for a use in ultimately scaled microelectronics.