F. Sharifi

Fabrication of nanometer-sized magnetic structures using e-beam patterned deposition masks

We report on the development of a novel technique designed to study dimensional effects in sputter-deposited magnetic materials and multilayer nanostructures without the need for post-deposition patterning. Wires with widths ranging from 40 to 1000 nm have been deposited on Si wafer substrates through a deposition mask fabricated by e-beam lithography. A bilevel resist masking scheme allows smooth, near vertical sidewall profiles, and the resulting structures can be exposed using a simple lift-off process. This process was used to fabricate test structures of Fe, Cu, and Co which were characterized by scanning electron microscopy, atomic force microscopy, and ac resistivity measurements from 3 to 300 K. While the wire structures are geometrically well defined, transport measurements reveal high crystalline defect densities which must be eliminated to fabricate low-dimensional magnetoresistive structures.We report on the development of a novel technique designed to study dimensional effects in sputter-deposited magnetic materials and multilayer nanostructures without the need for post-deposition patterning. Wires with widths ranging from 40 to 1000 nm have been deposited on Si wafer substrates through a deposition mask fabricated by e-beam lithography. A bilevel resist masking scheme allows smooth, near vertical sidewall profiles, and the resulting structures can be exposed using a simple lift-off process. This process was used to fabricate test structures of Fe, Cu, and Co which were characterized by scanning electron microscopy, atomic force microscopy, and ac resistivity measurements from 3 to 300 K. While the wire structures are geometrically well defined, transport measurements reveal high crystalline defect densities which must be eliminated to fabricate low-dimensional magnetoresistive structures.

Corrosion-free dry etch patterning of magnetic random access memory stacks: Effects of ultraviolet illumination

Individual layers of Ni0.8Fe0.2, Ni0.8Fe0.13Co0.07, TaN, and CrSi, and a full magnetic random access memory stack consisting of NiFeCo/CoFe/Cu/CoFe/NiFeCo/TaN/CrSi multilayers, were etched in high density Cl2/Ar plasmas either with or without concurrent ultraviolet (UV) illumination. Under optimized conditions, etch rate enhancement up to a factor of 5 was obtained for NiFeCo, TaN, and CrSi with UV illumination; whereas the etch rate of NiFe was retarded. Post-etch cleaning with H2 or SF6 plasmas or H2O rinsing was necessary in all cases in order to prevent corrosion of the metal layers from chlorinated etch residues.

Dry Etching of MRAM Structures

A wide variety of GMR and CMR materials have been patterned by high density plasma etching in both corrosive (Cl 2 -based) and non-corrosive (CO/NH 3 ) plasma chemistries. The former produce much higher etch rates but require careful in-situ or ex-situ , post-etch cleaning to prevent corrosion of the metallic multilayers. The former may have application for shallow etching of NiFe-based structures, but there is little chemical contribution to the etch mechanism and mask erosion can be a problem. The magnetic performance of patterned MRAM elements is stable over long periods (>1 year) after etching in Cl 2 plasmas, provided a suitable cleaning protocol is followed. It is also clear that high ion energies during patterning of magnetic materials can have a significant influence on their coercivity. The effects of ion energy, ion flux and process temperature are discussed.

Epitaxial Growth of Dilute Magnetic Semiconductors: GaMnN and GaMnP

Abstract : Epitaxial growth of the dilute magnetic semiconductors GaMnP and GaMnN has been investigated by Gas Source Molecular Beam Epitaxy (GSMBE). GaMnP films grown with < 4.5% Mn show the preferential formation of the second phases MnP and Mn(5.64)P3, resulting in only a slight deviation from purely diamagnetic behavior. GaMnN films grown on both Al2O3 and Metal-Organic Chemical Vapor Deposition (MOCVD) derived GaN surfaces show strong ferromagnetism when grown with either C codoping or at elevated temperatures to raise the concentration of n-type carriers. Comparable GaMnN films grown under conditions which produce highly resistive material show only paramagnetism, indicating the importance of carrier concentration on the resulting magnetic behavior. The formation of second phases was not observed in the GaMnN material for Mn concentrations less than 9%.