Scanning acousto optic filter for measuring VLSI dynamic random access memory vertical capacitor cells

A new trench depth measurement system for VLSI dynamic random access memory vertical capacitor cells is presented. The proposed system is characterized by two features: (1) use of a TeO(2) acoustooptic tunable filter and (2) symmetric setup of the test and reference wafers. The former feature leads to high speed signal averaging and to stable performance without mechanical noise. With the symmetrical setup configuration, noise due to light power fluctuation can be effectively minimized by simultaneously detecting two light beams, one reflected from the test wafer and the other from the reference wafer without trenches. As a result, the proposed system is found to be entirely promising for a trench depth monitoring system used in the in-line process, where ultimately high trench depth detection sensitivity is required.