Robert J. Tench

Tip-radius-induced artifacts in AFM images of protamine-complexed DNA fibers

Isolated DNA fibers complexed with protamine (the chromosomal protein that packages DNA in mammalian sperm) have been produced by partially decondensing the highly compacted mouse sperm chromatin particle on a glass coverslip. These DNA fibers were then scanned with the atomic force microscope (AFM). While the smallest of the fibers appear in AFM images as ribbon-like structures 250-350 A wide and 10-25 A high, experiments indicate that these images are the result of a convolution of the imaging-tips shape with the objects actual shape. In such convolutions the height of the object is affected only by the compressibility of the object, while the width is affected in addition by the sharpness of the tip. Images of polyamidoamine particles also appear to show this artifact. We have also deduced the tips radius of curvature from images of sharp steps and attempt to demonstrate the artifacts associated with a relatively large imaging tip.

Deposition of SiC films by pulsed excimer laser ablation

Thin films of β‐SiC were grown on Si substrates by excimer laser pulse ablation of bulk SiC. The films were examined by Auger electron, x‐ray, and photoelectron spectroscopies and laser ionization mass analysis techniques. The film was smooth as monitored by scanning electron microscopy. Scanning electron and scanning tunneling microscopy (STM) showed inclusions in the deposited SiC films, and laser ionization mass analysis detected SiC dimers in the vapor plume emitted from the target.

A comparison of nodular defect seed geometeries from different deposition techniques

A focused ion-beam milling instrument, commonly utilized in the semiconductor industry for failure analysis and IC repair, is capable of cross-sectioning nodular defects. Utilizing the instruments scanning on beam, high-resolution imaging of the seeds that initiate nodular defect growth is possible. In an attempt to understand the origins of these seeds, HfO2/SiO2 and Ta2O5/SiO2 coatings were prepared by a variety of coating vendors and different deposition processes including e-beam, magnetron sputtering, and ion beam sputtering. By studying the shape, depth, and composition of the seed, inferences of its origin can be drawn. The boundaries between the nodule and thin film provide insight into the mechanical stability of the nodule. Significant differences in the seed composition, geometry of nodular growth and mechanical stability of the defects for sputtered versus e-beam coatings are reported. Differences in seed shape were also observed from different coating vendors using e-beam deposition of HfO2/SiO2 coatings.

Clusters formed in laser‐induced ablation of Si, SiC, Pt, UO2 and evaporation of UO2 observed by laser ionization time‐of‐flight mass spectrometry and scanning tunneling microscopy

Cluster formation is traditionally observed by mass spectrometry, which has the disadvantage that the detection sensitivity often decreases with increasing mass. Alternatively, one may collect the clusters onto an atomically flat substrate and identify them by scanning tunneling microscopy (STM). Both techniques were used here. For the first technique, a Nd:YAG laser (frequency quadrupled to 266 nm, 5 ns pulse width) focused onto spots of 4–100 μm diameter was used to ablate refractory materials, and a reflectron time‐of‐flight tube served to mass‐analyze the plumes. The observed mass spectra for Si, Pt, SiC, and UO2 varied in the distribution of ablation products among atoms, molecules, and clusters, depending on laser power density and target material. For the second technique, cleaved surfaces of highly oriented pyrolytic graphite were positioned either 10 cm away from materials ablated at 10−5 Torr by 1–3 excimer laser (308 nm) pulses of 20 ns duration, or 1 m away from materials vaporized at 10−8 Tor...

Surface site specificity on the basal plane of graphite: 1.06 μm laser damage threshold and reactivity with oxygen between 350 and 2300 K

Scanning tunneling microscopy (STM) has been used to document changes in the nanometer‐scale morphology of the basal plane of highly oriented pyrolytic graphite after in situ exposure to 7 ns, 1064 nm Nd:YAG, and to ms Nd:Glass laser pulses producing surface temperatures up to 2300 K, and after ex situ exposure to temperatures as low as 350 K in air. Laser damage produced by the ns pulses was visible by STM at fluences far below those that produce melting and effects visible by other imaging techniques. Damage appears first on step edges and consists of exfoliation of graphite layers and recession of steps through removal of mono‐ or multilayer patches. ms and long‐term heating leads to reaction with oxygen, which proceeds by etching of both crystalline boundaries and step edges. In addition a high concentration of flat‐bottom shallow pits (two monolayers deep) and a smaller number of conical pits (greater than 30 A deep) can clearly be identified. The results give for the reaction at the grain boundaries...

Analysis of adenine and thymine adsorbed on graphite by scanning tunneling and atomic force microscopy

Abstract Analysis by AFM of adenine and thymine adsorbed onto a hot graphite surface shows that these molecules are adsorbed in patches typically 50 nm wide and 5 nm high, but does not provide any discernable atomic structure. In contrast, STM image scans that contain both parts of the graphite substrate and of the adsorbate would mislead one to believe that the adsorbates consist of just one ordered monolayer of either adenine of thymine (like alkylcyanobiphenyl on graphite [D.P.E. Smith et al., Nature 344 (1990) 641]). From these STM images the lattice dimensions, structural periodicities, and the epitaxy of the adsorbed molecules with respect to the basal plane of graphite can be determined. The aromatic regions are strongly detected with near-atomic resolution in both molecules, while the various sidegroups are not well resolved. Thus STM can discriminate between purines and pyrimidines - if they are held in thick patches.

Investigation of the microstructure of coatings for high-power lasers by nonoptical techniques

The microstructure of optical coatings strongly influence their resistance to high fluence laser, scatter properties, as well as their mechanical and environmental stability. The relative merits of nonoptical techniques such as scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and focused ion beam are discussed as they apply to optical multilayer coatings. The combination of these techniques provides a unique method to analyze defects in coatings. the long-term objective of this work is to understand the initiation and growth mechanisms of defects in optical coatings, investigate failure mechanisms of laser coatings, and suggest methods for reducing the number of defects during the deposition process. \to date, our defect analysis using nonoptical techniques has focused on hafnia/silica multilayers for high-power lasers. In summary, the information we have compiled about the defect seeds indicates that (1) seed size has an influence upon the mechanical stability of the whole defect, (2) seed shape and chemical composition reveal potential seed sources in the coating systems, and (3) defects can be initiated either as a single event or continuously during the deposition process. Also, it is shown that different vendors have characteristic defects and seeds.

Characterization of defect geometries in multilayer optical coatings

Laser-induced damage in optical coatings is generally associated with micrometer-scale defects. A simple geometric model for nodule-shaped defects is commonly used to describe defects in optical coatings. No systematic study has been done, however, to prove the applicability of that model to standard optical coating deposition. Some defects are known not to have a classical nodule geometry. The present study uses atomic force microscopy (AFM) and scanning electron microscopy to characterize the topography of coatings defects in a HfO2/SiO2 multilayer mirror system. Focused ion-beam cross-sectioning is then used to study the underlying defect structure. This work develops a model for defect shape such that the overall geometry of a coating defect, particularly seed size and depth, can be inferred from non-destructive evaluation measurements such as AFM. The relative mechanical stabilities of nodular defects can be deduced based on the nodules geometry. Auger analysis showed that the seed material that causes nodular defects in HfO2/SiO2 multilayers is a hafnia oxide. Such characterization capabilities are needed for understanding the enhanced susceptibility of particular defects to laser damage and for developing improved techniques for depositing low-defect density coatings.

A pulse‐deposition method for scanning tunneling microscopy of deoxyribonucleic acid on graphite

Images of three different synthetic DNAs have been obtained by scanning tunneling microscopy (STM) following their deposition on graphite using a voltaic pulse. DNA applied to the STM tip is desorbed during a 4 V/10 μs pulse and deposited intact onto the surface of highly oriented pyrolytic graphite. Images of 22, 47, and 100 base‐pair molecules show that deposition occurs in close proximity to the tunneling tip and that the molecules appear to deposit singly or in highly oriented groups.

SiC film deposited by pulsed excimer laser ablation

Thin films of {Beta} - SiC were grown on Si substrates by excimer laser pulse ablation of bulk SiC. The films were examined by Auger electron, x-ray, and photoelectron spectroscopies. The film was smooth as monitored by scanning electron microscopy. Scanning electron and scanning tunneling microscopy showed inclusions in the deposited SiC film and laser ionization mass analysis detected SiC dimers in the vapor plume emitted from the target. 13 refs., 4 figs.