Mechanism of premature etch stop in high-density magnetic-tunnel-junction patterning using CO/NH3 plasma with Ta mask

We investigated the mechanism of premature etch stop in magnetic tunnel junction (MTJ) patterning using CO/NH3 plasma with a Ta mask to clarify the cause of etch-stop problem in high-density patterning due to mask deposition to a bottom space of a pattern. CO/NH3 plasmas consist of carbon monoxide, nitrogen, and hydrogen plasmas. To clarify the role of gas species in inducing this premature etch stop, the etching-depth difference of an MTJ with a Ru/CoFeB/MgO/CoFeB stack film was compared with CO/N2/H2, N2/H2, and N2/He plasmas at an electrode temperature of 200 °C. Premature etch stop was observed in the CO/N2/H2 plasma but not in the N2/H2 plasma. It was also observed in N2/He plasma, the position of which was found to be almost at the same level as the MgO layer. This etch stop is caused by the high etching durability of deposited Ta on the bottom space of the pattern, and oxidation of Ta enhances etching durability. Thus, the main reason for this type of etch stop is the oxidation of deposited Ta on the bottom space, and this oxidation is enhanced by two types of supplied oxygen, i.e., that from plasma when CO gas is added and that from MgO when the MgO surface is exposed by increasing the etching depth of an MTJ. Electrode temperature dependence in N2/H2 plasma as a function of the H2-mixing ratio indicates that this premature etch stop is suppressed by increasing the hydrogen content or the electrode temperature. Therefore, reducing oxygen by using hydrogen-including plasma was found to be effective in preventing the premature etch stop caused by oxidation of Ta, which is deposited to the bottom space of the pattern from the mask.