Cell Shape and Patterning Considerations for Magnetic Random Access Memory (MRAM) Fabrication

Industrial and academic development laboratories worldwide are working to perfect the circuit designs, fabrication methods and integration schemes required for successful commercial production of Magnetic Random Access Memory (MRAM) devices, a new kind of nonvolatile memory technology that some forecast to be a “universal” memory replacement for DRAM,SRAM and flash.Among the more important issues for MRAM cell design and fabrication are the basic configuration of the magnetic memory element (pseudospin-valve or magnetic tunneling junction, for example), the material set used to fabricate the magnetic memory element, the shape of the magnetic memory element and the patterning techniques used to fabricate the cell. Two important attributes of the MRAM cell may be unfamiliar to those with experience in other IC fabrication processes. These are the special considerations that must be taken when designing the physical shape of the magnetic memory storage element and the specific fabrication techniques that need to be applied to pattern the many layers of alloys (NiFe, for example) and metals (ruthenium, cobalt) found in the MRAM stack. Ion milling has been the historically- important method of record for fabricating low-density MRAM products for applications with limited production volumes. Now that large IDMs like IBM and Infineon, Motorola, Philips, STMicroelectronics and NEC are approaching MRAM processing in earnest, the perceived manufacturing limitations of ion milling have motivated development teams to consider other methods, including wet etch, plasma etch and damascene for patterning MRAM stacks. We review here the theory of MRAM operation, discuss the interaction between the physical shape of the MRAM cell and its ability to store binary information, present the various options for patterning MRAM stacks as championed by the major IDMs with public MRAM programs, and summarize some of our own work on plasma etching MRAM devices.