The Density of States of hole-doped Manganites: A Scanning Tunneling Microscopy/Spectroscopy study

Variable temperature scanning tunneling microscopy/spectroscopy studies on single crystals and epitaxial thin films of hole-doped manganites, which show colossal magnetoresistance, have been done. We have investigated the variation of the density of states, at and near the Fermi energy ($E_f$), as a function of temperature. Simple calculations have been carried out, to find out the effect of temperature on the tunneling spectra and extract the variation of density of states with temperature, from the observed data. We also report here, atomic resolution images, on the single crystals and larger range images showing the growth patterns on thin films. Our investigation shows unambiguously that there is a rapid variation in density of states for temperatures near the Curie temperature ($T_c$). While for temperatures below $T_c$, a finite DOS is observed at $E_f$, for temperatures near $T_c$ a hard gap opens up in the density of states near $E_f$. For temperatures much higher than $T_c$, this gap most likely gives way to a soft gap. The observed hard gap for temperatures near $T_c$, is somewhat higher than the transport gap for all the materials. For different materials, we find that the magnitude of the hard gap decreases as the $T_c$ of the material increases and eventually, for materials with a $T_c$ close to 400 K, the value of the gap approaches zero.Comment: 9 pages RevTeX, 12 postscript figures, 1 table included in text, submitted to Physical Review

Surface effects in simulations of scanning electron microscopy images

We have investigated the contributions of surface effects to Monte Carlo simulations of top-down scanning electron microscopy (SEM) images. The elastic and inelastic scattering mechanisms in typical simulations assume that the electron is deep in the bulk of the material. In this work, we correct the inelastic model for surface effects. We use a model for infinite flat surfaces, and apply it to non-flat, but smooth, geometries. Though this is a simplification, it captures most qualitative differences to the bulk model, including coupling to surface plasmons. We find that this correction leads to an increased SE signal near a feature's sidewall in low-voltage critical dimension SEM (CD-SEM). The effect is strongest for low beam energies. Due to some of the assumptions in our model, we are unable to quantitatively predict the extent by which the signal from the sidewall is enhanced. The enhancement of signal from the sidewall may be large enough to cause the measured edge position to shift significantly.</p

Simulation of shotnoise induced side-wall roughness in electron lithography

We have developed a fast three dimensional Monte-Carlo framework for the investigation of shotnoise induced side-wall roughness (SWR) formation. The calculation outline is demonstrated by an example for an exposure of a 100nm thick layer of negative tone resist (NTR) resist on top of an infinitely thick silicon substrate. We use our home built Monte-Carlo simulator for electron-matter interaction for the purpose of lithography. A pattern of an isolated line is written into the resist layer by scanning a beam with 20 keV electrons over an area of 32nm×1μm (width and length). During the exposure, we use a spot size of 20 nm, beam step size of 4nm and a Poisson distributed exposure dose of 80 μC/cm2, 60 μC/cm2 and 40 μC/cm2. During the exposure of the sample, we record the locations of the inelastic events within the resist layer. The distribution of released acids is determined under the simplified assumption that every inelastic event corresponds to a release. We now construct a three dimensional image of the (in)solubility of the resist layer within a cuboid of 128 nm(256px) wide, 800 nm(1024px) in length and 100 nm(128px) in height. It is obtained by summing the contribution of all acids to every voxel in the three dimensional image. We have used a three dimensional Gaussian with σx,y,z = rd =5nm for the diffusion of the acid. The boundary between exposed and unexposed resist is determined by a threshold. The resulting image of the (in)solubility is analyzed in different ways by considering slices and three dimensional views of the border. The average line edge roughness (LER) is obtained by calculating the standard deviation (one-sigma) of the left and right border from yz-slices. By considering all slices, ranging from the top of the resist layer to the bottom of the substrate, the average LER as a function of the depth from the top surface of the resist layer is obtained. Shotnoise effects are observed as we decrease the exposure dose. An increased effect of shotnoise is observed near the vacuum and substrate interface. One contribution relates to the actual number of acids, which due to the scattering is less near the interface than away from the interface. Another contribution stems from the fact that no acids are found on the vacuum side nor on the substrate side.</p

First results from the large dynamic range atomic force microscope for overlay metrology

TNO is developing a novel Large Dynamic Range Atomic Force Microscope (LDR-AFM), primarily but not exclusively designed for sub-nm accurate overlay metrology. The LDR-AFM combines an AFM with a 6 degrees- of-freedom interferometric positioning stage, thereby enabling measurements of sub-nm features on a wafer over multiple millimeters marker-to-feature distances. The current work provides an overview of recent developments and presents the first results obtained after final integration of the complete system. This includes results on the AFM head development, the validated positioning stage performance, the first AFM images, and long-term stability measurements