Process to fabricate bottom electrode for MRAM device

A process for forming a bottom electrode for an MTJ device on a silicon nitride substrate having a top surface, comprising:

• depositing a tantalum containing layer on said top surface;
• depositing a layer of NiCr on said tantalum containing layer;
• depositing a layer of ruthenium on said NiCr layer;
• depositing a layer of alpha tantalum on said ruthenium layer, thereby forming a base layer having an upper surface;
• forming said MTJ device on said upper surface;
• covering said MTJ device and said upper surface with a protective layer in the form of a conformal continuous layer of a material known to have a lower etch rate than alpha tantalum during a first reactive ion etching process that comprises an etchant selected from the group consisting of gaseous compounds of carbon and fluorine;
• forming on said conformal continuous layer a photoresist mask that defines multiple electrodes;
• then performing said first reactive ion etching process whereby all unprotected areas are etched at an etch rate that is reduced by a factor of about 10 when said ruthenium layer becomes exposed, at which point terminating said first reactive ion etching process;
• then removing all remaining photoresist;
• and
• then performing second reactive ion etching by means of an etchant selected from the group consisting of gaseous compounds of carbon, oxygen, and hydrogen whereby all exposed portions of said upper surface are etched at an etch rate that is reduced by a factor of about two thirds when said silicon nitride substrate becomes exposed, at which point terminating said second reactive ion etching process with minimal penetration of said silicon nitride substrate and with a non-zero thickness of said conformal continuous layer still present.