Optimization of lightly-doped-drain (LDD) structure for sub-quarter-μm devices using statistical design and response surface methodology

A Lightly-Doped-Drain (LDD) structure was optimized by extensive process and device simulations with advanced mobility and impact ionization models for sub-quarter-μm devices. A quadratic response-surface-model (RSM) was derived from Design-of-Experiment (DOE) with input variables related to the drain structure. The saturation current (I_sat), the off-state current (I_off), and the maximum substrate current (I_sub) were treated as figure-of-merits (FOM´s) for tradeoffs between device performance, device short channel effect and device reliability. The source/drain junction depth had little effect on the three FOM´s. This enhanced the concept of drain-decoupling by separating the drain contact to channel and the drain contact to metal silicide. Even though the LDD tab does not necessarily improve the device reliability, it does provide a practical way of forming ultra-shallow junctions to meet the scaling requirement. The tab length has to be reduced to improve the driving capability, while the tab dose has to be increased to minimize the sensitivity of the driving capability to spacer length variations. Devices with optimized LDD drain structure were fabricated to verify the RSM prediction, and excellent device characteristics in agreement with simulation have been achieved