R. E. Mulder, Silicon Labs, Austin, TX; Y. Tsang, Freescale Semiconductor, Inc., Austin, TX

Summary: The relationship between blocked LDD (lightly doped drain) implants and Vt shifts resulting in suppressed drive current have been thoroughly investigated and characterized through nano-probe analysis. The results have been presented in this forum over the last several years [1], [3]. The blocked LLD implant results in reduced (or missing) dopant in one of the LDDs of the affected transistor. The reduced implant causes an increase in resistance on one side of the transistor. The increased resistance of the defective LDD requires an additional voltage drop that increases the Vt and decreases the drain current through the transistor. The transistor is also no longer symmetrical in regards to its electrical performance due to the fact that one LDD is more resistive than the LDD on the other side of the transistor. This electrical signature is easily identifiable by nano-probe analysis which shows a family of curves that are asymmetrical when the bias for the drain is changed from one side of the transistor to the other side. Recently, a second Vt shift failure mechanism has been identified. This failure mechanism also produces a Vt shift and suppressed drive current. However, the electrical signature that identifies this failure mechanism is the symmetry of the family of curves when the biasing for the drain is changed from one side of the transistor to the other side. In this paper review the proper technique for characterizing Vt shift failures will be presented as well as a brief review of the LDD Vt shift failure. A case study will then be presented showing the characterization, identification and the physical analysis results for the symmetrical Vt shift failure mechanism.