Relative merits of Cl2 and CO/NH3 plasma chemistries for dry etching of magnetic random access memory device elements

A typical magnetic random access memory stack consists of NiFe/Cu/NiFeCo multilayers, sandwiched by contact and antioxidation layers. For patterning of submicron features without redeposition on the sidewalls, it is desirable to develop plasma etch processes with a significant chlorinated etch component in addition to simple physical sputtering. Under conventional reactive ion etch conditions with Cl-2-based plasmas, the magnetic layers do not etch because of the relatively involatile nature of the chlorinated reaction products. However, in high ion density plasmas, such as inductively coupled plasma, etch rates for NiFe and NiFeCo up to similar to 700 Angstrom min(-1) are achievable. The main disadvantage of the process is residual chlorine on the feature sidewalls, which can lead to corrosion. We have explored several options for avoiding this problem, including use of in situ and ex situ cleaning processes after the Cl-2-etching, or by use of a noncorrosive plasma chemistry, namely CO/NH3. In the former case, removal of the chlorine residues with in situ H-2 plasma cleaning (to form volatile HCl that is pumped away!, followed by ex situ solvent rinsing, appears effective in preventing corrosion. In the latter case, the CO/NH3 plasma chemistry produces metal carbonyl etch products, that are desorbed in the simultaneous presence of an ion flux. The etch rates with CO/NH3 are much lower than with Cl-2 over a broad range of source powers (0-1500 W), radio frequency chuck powers (50-450 W), pressures (1-30 mTorr) and plasma compositions. We have tried substitution of CO2 for CO, and addition of Ar to produce faster etch rates, without success. Maximum rates of similar to 300 Angstrom min(-1) for NiFe and NiFeCo were obtained with CO/NH3 under optimum conditions. The etched sidewalls tend to be sloped because of mask erosion during plasma exposure, in contrast to the case of Cl-2-based chemistries where the sidewalls are vertical. (C) 1999 American Institute of Physics. [S0021-8979(99)30308-X].