The Cornell NanoScale Science and Technology Facility (CNF), a leading university research facility located at Cornell University, Ithaca, NY, and Oxford Instruments Plasma Technology (OIPT), UK have collaborated to develop a novel etching process targeted specifically at magnetic random-access memory MRAM based device fabrication.
These results, obtained at CNF, add a significant contribution to OIPT’s large portfolio of etching processes. MRAM is a high performance, low power, low degradation, non-volatile data storage technology that some suggest gives it the potential to become a “universal memory”, able to replace SRAM, DRAM, EEPROM and flash. Etching of magnetic based materials for the development and scaling of MRAM and spintronic devices is therefore of keen interest to several leading research groups using the CNF.
Recently, several research groups have shown that chemical etching of Co, Fe, and Ni based alloys can be achieved using plasmas formed from methanol (CH3OH) and argon. The new CNF/OIPT process is a result of a Design of Experiment (DOE) in which the level of CH3OH in Ar varied, along with variations in the ICP power, bias power, and pressure.
CNF was pleased to announce the full facilitation of the new PlasmaPro 100 Cobra ICP etch system from Oxford Instruments Plasma Technology (OIPT) in 2015. This inductively coupled plasma (ICP) based reactive ion etch platform is configured for state of the art nanoscale etching vital to the research work of CNF.
The system includes many extras that make for a highly flexible and powerful etch research tool. These include a wide range temperature (-150°C to +400°C) electrode, which greatly enhances the spectrum of materials that can be etched with volatile chemistries, low frequency electrode biasing and a vapour delivery system for methanol (CH3OH).
This advanced methanol-based etch capability for magnetic materials is an enabling process that is now available to the researchers at CNF and to the newly formed National Nanotechnology Coordinated Infrastructure Network (NNCI)
An element of MRAM consists of a magnetic tunnel junction (MTJ) and a CMOS transistor. One of the most challenging steps in MRAM fabrication is the etching of the MTJ stack. The stack typically contains a non-magnetic seed layer to promote proper crystalline growth (e.g. Ta), an antiferromagnet such as PtMn or IrMn, a stack of alloy pinned layers (CoFeB), a tunnelling barrier such as MgO, metals such as Ru and/or Pt, and a suitable hard mask such as TiN or Ta.
The problem is that magnetic materials have difficulty reacting with most chemically active plasma species to form volatile etch products, so users often have to resort to purely physical ion milling processes. However, ion milling suffers from low etch rates, low selectivity, undesirable sidewall redeposition especially for nanoscale features, and damage to the device structure itself.
Methanol, as the principal plasma reactant, forms volatile carbonyl compounds (e.g. Ni(CO)4, Fe (CO)5, and Co2(CO)8) at room temperature. This chemistry-based process avoids the disadvantages of purely physical milling. The antiferromagnet IrMn also etches in a methanol plasma. In addition, the selectivity over common mask materials such as Al2O3, Ta, Ti, TaN, and TiN is high, while leaving no residue on the etched devices.
We demonstrated successful etching of a 41nm thick magnetic tunnel junction stack stopping on the tantalum under layer (see figure). High selectivity (>10:1) over both the Ta mask and under layer is achieved through the formation of tantalum carbide in the methanol process.
Vincent J. Genova -Research Staff Member, CNF
This research work at CNF adds a significant contribution to Oxford Instruments Plasma Technology’s extensive portfolio of etching processes, enabled through the use of our state of the art PlasmaPro 100 Cobra ICP etch system.
We are delighted that our technology is assisting such a prestigious research centre achieve its fundamental research goals.
David Haynes - Global Field Sales Director,Oxford Instruments Plasma Technology
For further technical information, please contact Vincent Genova at [email protected].cornell.edu. or Colin Welch at [email protected]