Work-function metal in transistors and method forming same

Abstract

A method includes forming a dummy gate stack over a semiconductor region, forming a source/drain region on a side of the dummy gate stack, removing the dummy gate stack to form a trench, forming a gate dielectric layer extending into the trench and on the semiconductor region, and depositing a first work-function layer over the gate dielectric layer. The first work-function layer comprises a metal selected from the group consisting of ruthenium, molybdenum, and combinations thereof. The method further includes depositing a conductive filling layer over the first work-function layer, and performing a planarization process to remove excess portions of the conductive filling layer, the first work-function layer, and the gate dielectric layer to form a gate stack.