Bridge-contact resistance method for precise evaluation of electrical contacts of nano-scale semiconductor devices

Source/drain electrical contact resistance has become a significant parasitic component that should be considered in scaled-down semiconductor devices fabricated with nano-structured channel layers. It is therefore crucial to evaluate the electrical contacts between electrodes and nano-scale thin semiconductor layers precisely. The conventional method for evaluating contacts is based on the transmission-line model (TLM), which extracts the contact parameters (specific contact resistance and transfer length) by assuming that the electrical properties of the semiconductor layer under the electrode are the same as the channel region between the electrodes. However, it is difficult to apply this method directly to modern scaled devices because the electrical properties of ultrathin semiconductor layers under the electrode are altered after metal contact formation. Here, we propose a bridge-contact resistance method that can be used for precise evaluation of the intrinsic contact parameters and the altered sheet resistance under electrodes by accounting for the change in electrical properties of an ultrathin semiconductor layer after contact formation. In this method, the intrinsic electrical contacts are accurately evaluated by analyzing the current distribution through an auxiliary electrically-floated electrode formed on the channel between the two contact electrodes. The effectiveness of the proposed characterization method was verified by evaluating electrical contacts on an ultrathin silicon layer (12 nm thickness). The results indicated that the specific contact resistance and transfer length were extracted to be approximately 20 % lower than those obtained using the conventional TLM method, which was due to the increased sheet resistance under the electrode after contact formation.