A Method for Effective Work Function Monitoring

Abstract

The effective work function (EWF) is a critical gate parameter dependent on metal, dielectric, and interaction between them. As such, it requires stringent control, both in R&D stage and in manufacturing of advanced MOSFET’s. In this work we present non-contact charge-based EWF monitoring that brings advantages of cost-effectiveness and fast data feedback. The method extends our micro corona-Kelvin testing of dielectrics and interfaces in scribe-line boxes to EWF suitable configuration with a thin metal layer on a top of a dielectric. Two key elements are used in the present method: 1) corona charge deposition on metal surface and 2) surface voltage measurement by Kelvin force microscopy. In a dynamic version of the flatband voltage determination, the surface voltage decay after corona charging is measured under modulated light conditions enabling parameter free extraction of VFB. Results for n- and p-type metals show sensitivity of VFBto EWF and agreement with results at end of the line CV measurements. The micro corona-Kelvin method allows quantification of two more work function sensitive parameters: the silicon surface barrier, Vsb, and the tunneling barrier related to the band offset at the metal / high-k dielectric interface. Results for n- and p-type metals correlate with the expected behavior. Measurement of the silicon surface barrier, Vsb, can be especially attractive since it can be done on a blanket wafer with a thin metal layer over the high-k dielectric, providing means for high resolution mapping. The third EWF sensitive parameter is obtained from the analysis of the tunneling current across the dielectric under negative bias condition, i.e. tunneling of electrons from the metal gate through the dielectric. Measured shifts of the tunneling barrier were consistent with EWF.