Fabrication of nanoscale MOSFET with high injection velocity and investigation on their characteristic

Abstract

The injection velocity $(v_{inj})$ in a MOSFET at the top of the barrier (TOB) between the source and channel is a key device performance metric in a quasi-ballistic transport regime. $v_{inj}$ is strongly limited by the thermal velocity in a conventional pn junction based MOSFET. From this point of view, a Schottky barrier (SB) MOSFET is a strong candidate for high performance devices in the quasi-ballistic transport regime, because of the hot carrier injection from the Schottky source. First of all, carrier transports in nanoscale device is discussed by scattering theory established by Natori and Lundstorm. According to scattering theory, injection velocity cannot be exceeding the thermal velocity at the beginning of the channel, and is decreased by the scattering event at TOB. Reducing the gate length and effective mass of the channel carrier is effective in increasing the injection velocity. Therefore the intuitive solution to enhance the device performance is to use alternative channel material instead of silicon channel, recent research progress was also shown in the point of view of injection velocity. In the hot carrier injection point of view, the principle of Schottky barrier (SB) MOSFET operation was explained and the several research results were introduced to explain how the hot carrier injection of SB MOSFET is possible. And next part, the detailed fabrication of the SB MOSFET on ultra-thin body (UTB) is shown. At first, conventional pn junction MOSFET was tried on UTB strained SOI substrate to fabricate nano-scale MOSFETs, and the one of the world record peak mobility was obtained. However we could not solve the extremely high parasitic resistance due to the lack of recrystallization and activation of dopant in short channel MOSFETs. In order to reduce the parasitic resistance, SB MOSFET with CoSix on the source and drain was fabricated. The external parasitic resistance was reduced 1000 times than that of implanted UTB layer and the on state current was drastically improved. A remarkable injection velocity of 2.37 x $10^7$ cm/s, beyond the thermal velocity, is experimentally demonstrated in the SB MOSFET with W of 6.5 nm and LG of 35 nm, which was fabricated on an ultra-thin body (UTB) SOI wafer. The well-known MIT virtual source (MVS) model was used to extract injection velocity for the first time in SB MOSFET. The narrower SiNW results in, the sharper band bending and the higher lateral electric field in the SB MOSFET is. Therefore these features enable the faster velocity than the thermal velocity. That speed is expected to exceed over 2.37 x $10^7$ cm/s by the combination of the SB MOSFET with the lowered SBH and a shorter gate length. Based on the results, SB MOSFET with wide and thin nano-sheet channel is proposed as a next generation of FinFET device. In the fourth part, the temperature dependence on the characteristics of the SB MOSFET from very low temperature to high temperature is shown. Thermionic emission (TE) current at the SB MOSFET was suppressed at the most and we can observe the transition region of current mechanism from TE to tunneling (TU). Because the SB width is not thin enough to tunnel at the current transition region, the current is inferior to that of conventional pn junction based MOSFET. The peak effective mobility of the SB MOSFET is also smaller than that of conventional MOSFET with same reason. Additionally the current in the linear mode at the short channel length of 35 nm was increased with temperature. Although the dominant on-state current flow mechanism of SB MOSFET is TU current, on-state current of SB MOSFET was decreased with increase of device temperature. However the decrease ratio of the current was lowered by the reduction of channel length at which the injected carrier is supposed to be scattered. In addition the linear current was increased at the channel length of 35 nm owing to the increase of the electron to tunnel the SB, and this was demonstrated by the carrier transport simulation. In fifth part, we present the results of radio-frequency (RF) modeling based on parameter extraction of our fabricated SB MOSFETs. An analytical extraction of parameters has been performed through Y-parameter analyses on the non-quasi-static (NQS) small-signal equivalent circuit. S-parameter was measured to obtain the Y-parameters of our fabricated SB MOSFETs. The parasitic components of pad structure were removed by the well-known de-embedding procedure, because the measured S-parameter included the parasitic components. The capacitance in short channel was directly obtained by scattering parameters (S-parameter measurement) and we could verify the injection velocity of our SB MOSFETs which had been extracted by the MVS model. Finally figure of merit of RF performance such as cut-off frequency and maximum oscillation frequency was also analyzed. In this study, a remarkable injection velocity beyond the thermal velocity was experimentally demonstrated in a SB MOSFET, which was fabricated on an ultra-thin body (UTB) SOI wafer. As the gate controllability improved with the decrease of the SiNW size, the extracted $v_{inj}$ was increased as high as 2.37x$10^7$ cm/s, which is beyond the thermal velocity. Such high ballisticity defined as $v_{inj}/v_T$ larger than unity, is arisen from energetic hot electrons accelerated from the aforementioned sharp energy band bending. Therefore the combination of the SB MOSFET structure and SiNW scaling can overcome the ideal performance limit and make it possible to continue the device scaling down with further performance enhancement without taking a risk to use a new material.