Analysis of the transmission spectrum of the flat-cutoff sensors on wafers with metal layer

A flat-cutoff sensor installed on the chamber wall or chuck allows precise measurement of the plasma density in real time, even with a wafer placed on the sensor. However, a few studies have been conducted on the measurement characteristics in an environment where a wafer containing a metal layer is placed on a sensor. In this study, we investigated the effect of wafers containing metal layers on flat-cutoff sensor measurements using circuit models and experiments. The metal layer in the wafer shifts the cutoff frequency up to four times higher, and the degree of shift depends on the plasma density. The relationship between the shift in the cutoff frequency and plasma density can be interpreted as the ratio of the plasma inductance to that of the metal layer from the circuit model of the flat-cutoff sensor. The calculation results were verified experimentally using wafers containing Al and Ti metal layers. As a result, measurement was found to be possible even when a wafer containing a metal layer was placed on a flat-cutoff sensor, and these results can improve the measurement accuracy of the flat-cutoff sensor for the real-time plasma measurement.