Methods of extracting mechanical stress in nano-scaled MOSFET and measuring threshold voltage variation along the channel length direction due to electrical stress

Abstract

In this thesis, a method of measuring the mechanical stress σ in the nano-scaled MOSFET is investigated. This method measures the doping concentration Nd of the body and the onset voltage Vhl for the high-level injection of the drain-body junction, uses Nd, an ideality factor η, and the Fermi potential ϕf ≈ Vhl/(2η) to calculate the intrinsic carrier concentration ni of the Si body, uses the calculated ni to obtain the bandgap energy Eg of the Si body, then uses deformation potential theory to estimate σ from Eg. The estimates of σ agree well with those obtained using previous methods. The proposed method requires one MOSFET, whereas the others require at least two MOSFETs. So, the proposed method can give an absolute measurement of σ on the MOSFET, whereas the previously reported methods can only find the σ difference between the two MOSFETs. A method to measure the threshold voltage degradation ΔVth along the channel direction due to electrical stress is also proposed. This method uses the ΔVths measured after electrical stress at different drain bias Vds, and calculated the depletion length Ldep into the channel for each condition under which ΔVth is measured. By substituting the measured ΔVth and the calculated Ldep into the proposed equation, the amount of degradation generated in each region of the MOSFET channel can be calculated. Then, this ΔVth profile along the channel is used to derive a model to predict the channel-length dependence of ΔVth due to negative bias temperature instability (NBTI) stress of pMOSFETs. The ΔVth calculated by using proposed model agrees well with the measured ΔVth.