Increasing the process temperature in CO/NH3 plasma etching has been investigated to suppress an etch stop in high-density magnetic tunnel junction stack (MTJ stack) patterning with a Ta mask. In a previous study, the occurrence of an etch stop was observed when using a 100 nm space pattern; specifically, the etching depth was unchanged with the 100 nm space pattern when the etching time was increased, although when wider spaces were used, the etching depth increased proportionally. In this study, differences in the etch stop depth with 100 nm space patterns were examined by changing the electrode temperature from 120 to 300 °C. The etch stop depth became deeper as the electrode temperature was increased and it was found that a high-temperature CO/NH3 process was an effective way to prevent the etch stop effect. However, at 300 °C, the MTJ stack's thickness was observed to expand, which may induce a deterioration of the junction's magnetic properties. A scanning transmission electron microscope image and an energy dispersive x-ray spectroscopy image of the MTJ stack revealed that nitridation of the MTJ stack was the reason of its expansion in thickness. Moreover, pure N2 plasma irradiation of the MTJ stack clearly indicated that this thickness expansion occurred for an electrode temperature of over 265 °C. Finally, the results demonstrate that, for a CO/NH3 process with an electrode temperature of 250 °C, it is possible to etch a 52-nm-thick MTJ stack without either etch stop or film thickness expansion occurring.