Fabrication of Magnetic Nanostructures for MRAM using Electron Beam and Focused Ion Beam Exposure of HSQ
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Fabrication of Magnetic Nanostructures for MRAM using Electron Beam and Focused Ion Beam Exposure of HSQ Chen Chen1, Michael J. Cabral1, Robert Hull2, and Lloyd R. Harriott1 1 Electrical and computer engineering, University of Virginia, 351 Mccormick road, Charlottesville, VA, 22904 2 Material Science and Engineering, University of Virginia, Charlottesville, VA, 22903 Material names: hydrogen silsesquioxane (HSQ) tetramethyl ammonium hydroxide (TMAH) methyl isobutyl ketone (MIBK) nickel, iron, cobalt, copper, manganese, gallium Giant magnetoresistive (GMR) materials-based magnetic random access memory (MRAM) has become attractive due to non-volatility, speed and density1. The vertical MRAM (VMRAM) design model shows good signal level and high speed and density potential. The memory cell for the VMRAM model is a ring shaped magnetic material multilayer, which ensures high repeatability and low switching energy. This GMR structure, however, is difficult to pattern as it contains materials such as Ni, Fe, Co, and Cu, which are difficult to dry etch because they lack volatile etch products. This work shows that it is possible to overcome the difficulties associated with etching GMR a)
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materials by using hydrogen silsesquioxane (HSQ) as an etch mask. We have used HSQ in a direct-write electron-beam lithography system with a dose of 600 µC/cm2, and in a Ga+ ion focused ion beam (FIB) system with a dose of 12 µC/cm2. Both are followed by development and an argon plasma etch at 10mTorr and 100W RIE power. The HSQ layer provides high resolution as well as good etching resistance. Electron-beam exposed HSQ shows a 1:1.5 selectivity over the GMR film stack and the FIB exposed HSQ showed an improved etch selectivity of 1:1. Ring shaped GMR structures with a 75/225 nm (ID/OD) have been fabricated, which corresponds to a memory density of 4Gb/in2.
I. INTRODUCTION Magnetoresistive Random Access Memory (MRAM) uses the direction of the magnetic moment rather than the electrical charge to store data1. It is more robust and stable, and has the potential of higher switching speed than DRAM. If they can be made dense, fast and cost effective, it is attractive not only to applications which require radiation hardness, but is a viable candidate to replace DRAM, so that the entire memory system can be made on the same chip as a microprocessor. Device modeling work showed that vertical ring shaped memory cells (VMRAM) offer extraordinary promise for ultra high density MRAM2,3. The ring structure forms a circular magnetic flux, which eliminates the problems associated with irregular ends in the linear magnetic flux case. The model suggests that while a disc shape also reduces the effects caused by irregular ends, the center of the disc requires a large amount of energy for switching2. So, by using a ring structure, not only are the irregular ends problem eliminated, but it also reduces the switching energy required, which reduces the power consumption.
The density of ring sha
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