Doping extraction in FinFETs

In the past decades the minimum transistor size has been down-scaled according to Moore’s law. However, scaling of conventional MOSFET devices is limited due to short channel effects, gate insulator tunneling and limited control of doping concentrations. FinFETs are the most promising device structures in order to overcome these negative effects. The gate in a FinFET is wrapped around a thin silicon fin to exercise more control over the conducting channel. The objective is to try to find a unique doping profile in and near the channel region such that its electrical subthreshold behavior, obtained through device simulations, matches its experimentally determined counterpart, in order to understand which device parameters influence the electrical behavior the most. An advantages of this technique, also known as inverse modeling, is that it is nondestructive. A (quasi-2D) theoretical model for the subthreshold I-V behavior is deduced, which takes into account the Subthreshold Slope (SS) and the threshold voltage. The device parameters that influences the electrical characteristics the most are the doping profile in the fin, and hence electrical channel length, the oxide thickness, the dielectric constant of the oxide and gate work function. The model is accurate at low and high drain-source voltages for long and short channel devices. A manual routine is developed to easily extract various device parameters and give insight into the importance of these parameters using device simulations. An initial attempt on automating this routine shows promising results. The routine is verified by extracting device parameters of FinFETs fabricated by IMEC/NXP in Leuven (Salsa 2). The simulation results fit well with its measured counterpart. Only for very short channel devices (≤ 35nm) the doping profile estimation has to be improved. The results show that the electric behavior of FinFETs cannot be described with 2D simulations only. Nevertheless, it is questionable whether a unique doping profile in and near the channel region can be obtained, because some device parameters are derived based on specifications given by IMEC, such as equivalent dielectric layer thickness, fin dimensions and the doping of the device. When such a parameter is different in reality, a different combination of other device parameters would give similar simulated electrical behavior, such that it still fits nicely with its measured counterpart. Moreover, possibly another combination of lateral and vertical doping profile can be obtained. In order to determine the doping profiles in and near the channel region accurately, especially across the height of the fin, more information is needed.