Dooho Choi

Phase, grain structure, stress, and resistivity of sputter-deposited tungsten films

Sputter-deposited W films with nominal thicknesses between 5 and 180 nm were prepared by varying the base pressure prior to film deposition and by including or not including sputtered SiO2 encapsulation layers. X-ray and electron diffraction studies showed that single phase, polycrystalline α-W could be achieved in as-deposited films as thin as 5 nm. The stress state in the as-deposited films was found to be inhomogeneous. Annealing resulted in stress relaxation and reduction of resistivity for all films, except the thinnest, unencapsulated film, which agglomerated. In-plane film grain sizes measured for a subset of the annealed films with thicknesses between 5 and 180 nm surprisingly showed a near constant value (101–116 nm), independent of film thickness. Thick-film (≥120 nm) resistivity values as low as 8.6 μΩ cm at 301 K were obtained after annealing at 850 °C for 2 h. Film resistivities were found to increase with decreasing film thicknesses below 120 nm, even for films which are fully A2 α-W with no metastable, A15 β-W evident. Sputter-deposited W films with nominal thicknesses between 5 and 180 nm were prepared by varying the base pressure prior to film deposition and by including or not including sputtered SiO2 encapsulation layers. X-ray and electron diffraction studies showed that single phase, polycrystalline α-W could be achieved in as-deposited films as thin as 5 nm. The stress state in the as-deposited films was found to be inhomogeneous. Annealing resulted in stress relaxation and reduction of resistivity for all films, except the thinnest, unencapsulated film, which agglomerated. In-plane film grain sizes measured for a subset of the annealed films with thicknesses between 5 and 180 nm surprisingly showed a near constant value (101–116 nm), independent of film thickness. Thick-film (≥120 nm) resistivity values as low as 8.6 μΩ cm at 301 K were obtained after annealing at 850 °C for 2 h. Film resistivities were found to increase with decreasing film thicknesses below 120 nm, even for films which are fully A2 α-W with no...

Failure of semiclassical models to describe resistivity of nanometric, polycrystalline tungsten films

The impact of electron scattering at surfaces and grain boundaries in nanometric polycrystalline tungsten (W) films was studied. A series of polycrystalline W films ranging in thickness from 10 to 310 nm and lateral grain size from 74 to 133 nm were prepared on thermally oxidized Si. The Fuchs-Sondheimer surface-scattering model and Mayadas-Shatzkes grain-boundary scattering model were employed for quantitative analyses. Predictions from the theoretical models were found to deviate systematically from the experimental data. Possible reasons for the failure of the theoretical models to describe the experimental data are explored. Finally, a discussion of the crucial features lacking from existing models is presented, along with possible avenues for improving the models to result in better agreement with experimental data.

Crystallographic anisotropy of the resistivity size effect in single crystal tungsten nanowires.

This work demonstrates an anisotropic increase in resistivity with decreasing width in single crystal tungsten (W) nanowires having a height of 21 nm. Nanowire-widths were in the range of 15–451 nm, with the anisotropy observed for widths below 50 nm. The longitudinal directions of the nanowires coincided with the <100>, <110> and <111> orientations of the body centered cubic phase of W. The resistivity increase was observed to be minimized for the <111>-oriented single crystal nanowires, exhibiting a factor of two lower increase in resistivity at a width of ~15 nm, relative to the thin film resistivity (i.e., an infinitely wide wire). The observed anisotropy is attributed to crystallographic anisotropy of the Fermi velocity and the resultant anisotropy of the electron mean free path in W, and underscores the critical role of crystallographic orientation in nanoscale metallic conduction.

Surface roughness and magnetic properties of L10 FePt films on MgO/CrRu/TiN

Processes for making well ordered L10 FePt thin films with root-mean-square surface roughness close to 0.4 nm, perpendicular anisotropy greater than 5 kOe and perpendicular squareness near 1 using a deposition temperature of 390 °C have been developed. In this study, we focused on reducing the ordering temperature and smoothness of L10 FePt films, while achieving good perpendicular magnetic properties by using MgO/CrRu/TiN or MgO/CrRu/Pt seed layers on Si/SiO2 substrates. It was found that the chemical ordering of L10 FePt films is strongly affected by the insertion of a sputtered MgO seed layer, and the CrRu (002) texture on a (002) textured MgO seed layer is substantially improved at high DC power. In order to prevent Cr diffusion, TiN and Pt films were inserted between the CrRu and FePt films. It was found that the smoothness, L10 FePt ordering, and perpendicular magnetic properties were strongly improved by the TiN barrier layer compared to the Pt barrier layer. Increased TiN thickness improved the pe...

On twin density and resistivity of nanometric Cu thin films

Crystal orientation mapping in the transmission electron microscope was used to quantify the twin boundary length fraction per unit area for five Ta38Si14N48/SiO2 encapsulated Cu films with thicknesses in the range of 26–111 nm. The length fraction was found to be higher for a given twin-excluded grain size for these films compared with previously investigated SiO2 and Ta/SiO2 encapsulated films. The quantification of the twin length fraction per unit area allowed the contribution of the twin boundaries to the size effect resistivity to be assessed. It is shown that the increased resistivity of the Ta38Si14N48 encapsulated Cu films compared with the SiO2 and Ta/SiO2 encapsulated films is not a result of increased surface scattering, but it is a result of the increase in the density of twin boundaries. With twin boundaries included in the determination of grain size as a mean-intercept length, the resistivity data are well described by 2-parameter Matthiessens rule summation of the Fuchs-Sondheimer and Ma...

Tunneling magnetoresistance of perpendicular magnetic tunnel junction using L10 FePt electrodes on MgO/CrRu/TiN under-layers

A perpendicular magnetic tunnel junction (pMTJ) device was fabricated using L10 ordered FePt electrodes, which were deposited on MgO(8 nm)/CrRu(10 nm)/TiN(4 nm) under-layers. It was found that the MgO/CrRu/TiN under-layer helps lower the required FePt deposition temperature to below 400 °C, and provides a well-ordered bottom L10 FePt electrode with root-mean-square (RMS) surface roughness close to 0.4 nm. Magnetoresistance (MR) ratio and resistance-area (RA) were measured at room temperature by the current-in-plane tunneling (CIPT) method from a lithographically unpatterned PMTJ sample and 138% and 6.4 kΩ μm2 were obtained, respectively. A PMTJ test pattern, with a junction size of 80 × 40 μm2, was also fabricated and showed a MR ratio and RA product of 108% and 4 ∼ 6 kΩ μm2, respectively, in good agreement with the CIPT measurements.