R. Q. Hwang

Identifying the forces responsible for self-organization of nanostructures at crystal surfaces

The spontaneous formation of organized surface structures at nanometre scales, has the potential to augment or surpass standard materials patterning technologies. Many observations of self-organization of nanoscale clusters at surfaces have been reported, but the fundamental mechanisms underlying such behaviour — and in particular, the nature of the forces leading to and stabilizing self-organization — are not well understood. The forces between the many-atom units in these structures, with characteristic dimensions of one to tens of nanometres, must extend far beyond the range of typical interatomic interactions. One commonly accepted source of such mesoscale forces is the stress field in the substrate around each unit,. This, however, has not been confirmed, nor have such interactions been measured directly. Here we identify and measure the ordering forces in a nearly perfect triangular lattice of nanometre-sized vacancy islands that forms when a single monolayer of silver on the ruthenium (0001) surface is exposed to sulphur at room temperature. By using time-resolved scanning tunnelling microscopy to monitor the thermal fluctuations of the centres of mass of the vacancy islands around their final positions in the self-organized lattice, we obtain the elastic constants of the lattice and show that the weak forces responsible for its stability can be quantified. Our results are consistent with general theories of strain-mediated interactions between surface defects in strained films.

Adhesion and fracture of tantalum nitride films

Indentation fracture and continuous nanoscratch testing were used in this study to determine the effects of compressive residual stresses on the fracture of thin sputter-deposited tantalum nitride films. Some films were tested in the as-deposited condition while others were vacuum annealed at 300 C. The only discernible change in structure was a surface rearrangement of atoms into parallel arrays of striations on the vacuum annealed samples revealing a high compressive residual stress state after deposition. Comparison of results and application of mechanics-based models showed that these stresses had a strong effect on the fracture of the as-deposited films. The models also provided a good measure of the residual stress levels and interfacial strain energy release rates.

Determining fracture toughness of vitreous silica glass

This study demonstrates that the Chevron-notched short rod (CNSR) test technique is well suited for accurate determination of the fracture toughness for vitreous silica. A manifestation of the accuracy of this fracture toughness test technique is evident by low standard deviation of values observed from statistically significant sample sizes. Findings of this study suggest that the CNSR test method could be useful for determination of subtle changes in fracture toughness of glass caused by aggressive service environments, such as, hydrogen and/or ionizing radiation. Fracture surface morphological features of vitreous silica were revealed by atomic force microscope imaging. AFM enabled the examination of the fracture surface morphology locally with nanometer scale resolution. Short range, nanofracture mechanisms can be discerned for glass by atomic force microscopy.

Spectroscopic light scattering for real-time measurements of thin film and surface evolution

We describe a light scattering technique for measuring the real-time evolution of thin film and surface morphology. By using spectroscopic detection, the technique requires no motion of the sample during the measurement, which makes it compatible with many processing geometries. Results from the growth of strained heteroepitaxial layers of SixGe1−x on Si(001) are presented to demonstrate the technique.

In-situ STM studies of strain-stabilized thin-film dislocation networks under applied stress

Abstract The effect of uniaxial applied stress on dislocation networks present in the atomic surface layer of Au(111) was studied. Themeasurements were made using a novel instrument combining ultrahigh vacuum scanned-probe microscopy with an in-situstress-strain testing machine. The technique provides microscopic information, up to atomic resolution, about the large scaleplasticity of surface layers under applied loads. The herringbone reconstruction of the Au(111) surface is a classic example of astrain stabilized dislocation network. We find that under 0.5% uniaxially applied compressive strain a dramatic restructuring ofthe network takes place. The three-fold orientational degeneracy of the system is removed and threading edge dislocations areannihilated.

Surface Self-Organization Caused by Dislocation Networks

We report a new mechanism of self-organization that can lead to robust surface ordering. We have quantitatively analyzed the thermal motion of holes created by sulfur atoms in a silver monolayer on a ruthenium surface, which we observed in real time with scanning tunneling microscopy. We find that the stability of the array of holes is determined by the arrangement and structure of misfit dislocations in the film.

Characterization of chemically amplified resists for soft x‐ray projection lithography

Sensitivity, lithographic performance, photoabsorption, and photodesorption of chemically amplified novolac‐based resists have been studied at an exposure wavelength of 140 A and are compared to poly(methylmethacrylate) (PMMA). Monochromatic exposures of the resists AZ PF514, AZ PN114, and SAL 601 yielded D0.9 values of 2.5–3 mJ/cm2 for 0.25 μm thick films. Contrast values ranged from 3 for AZ PN114 to 5 for SAL 601. Photoabsorption measurements of supported AZ PN114 films at 140 A yield an absorption coefficient of 4.4±0.1 μm−1. Photodesorption of fragment ions induced by 140 A radiation has been studied in PMMA and AZ PN114 using time‐of‐flight mass spectrometry. It is found that H+, CH2+, CH3+, H2O+, CHO+, C3H5+, and COOCH3+ dominate the ion mass spectra photodesorbed from PMMA, while H+, CH3+, H2O+, and CHO+ dominate the ion mass spectra for AZ PN114. The mass‐integrated ion desorption yield from AZ PN114 is three times less than that measured for PMMA per photon or 90 times less when expressed per ex...

Interplay between gas adsorption and dislocation structure on a metal surface

The influence of oxygen and sulfur on the dislocation patterns of the strained two monolayer Cu film on Ru was observed by scanning tunneling microscopy. Both oxygen and sulfur adsorption lead to the formation of vacancies that aggregate over existing dislocations in the film, thereby modifying the dislocation structure. With increasing adsorbate coverage the overall dislocation structure and pattern are transformed. The atomic mechanisms and general nature of this transformation can be explained in terms of generic dislocation reactions. This interpretation is also supported by atomistic simulations.

Linking dislocation dynamics and chemical reactivity on strained metal films

The interaction of a model strained metal film with oxygen is studied by scanning tunneling microscopy. Two monolayers of copper on Ru(0001) present a well-defined dislocation network composed of threading dislocations and Shockley partial dislocations separating areas of fcc and hcp stacking. We find that oxygen first reacts with these threading dislocations. Then, for exposures up to ~0.4 L O 2 , new threading dislocation arrays appear on the surface. With the addition of more oxygen, the mesoscopic structure of the film changes from a striped array of Shockley partials to a disordered array of triangular fcc regions bounded by dislocations, as oxygen proceeds to etch away the hcp areas of the copper film.

STM study of Au growth on S-modified Ru(0001)

Abstract We present results of a detailed scanning tunneling microscopy (STM) study of submonolayer nucleation and growth of two-dimensional Au islands during deposition on clean versus S-precovered Ru(0001), at room temperature. In the presence of tiny amounts of S, a dramatic increase in the mean island densities is observed, corresponding to a decrease in the effective mobility of Au on Ru(0001). As imaged also by STM, the structure of the S adlayer changes during Au deposition, from a dilute lattice-gas, at low Au coverages, to an increasingly denser sequence of ordered superstructure p(2 × 2), ( 3 × 3 - R 30° ) and c(2 × 4) domains, with increasing Au coverage. These findings suggest that the deposited Au atoms displace S adatoms, compressing S domains, a phenomenon consistent with a net repulsive interaction between neighboring Au and S mediated by the Ru substrate.