Junlan Wang

A parametric study of laser induced thin film spallation

We report parametric studies of elastic wave generation by a pulsed laser and associated spalling of thin surface films by the corresponding high stresses. Two different substrate materials, single crystal Si (100) and fused silica, are considered. Spallation behavior of Al thin films is investigated as a function of substrate thickness, film thickness, laser energy, and various parameters governing the source. Surface displacement due to the stress wave is measured by Michaelson interferometry and used to infer the stresses on the film interface. Consistent with previous studies, the maximum stress in the substrate and at the film/substrate interface increases with increasing laser fluence. For many of the conditions tested, the substrate stress is large enough to damage the Si. Moreover, the maximum interface stress is found to increase with increasing film thickness, but decrease with increasing substrate thickness due to geometric attenuation. Of particular significance is the development of a decompression shock in the fused sillica substrates, which results in very high tensile stresses at the interface. This shock enhances the failure of thin film interfaces, especially in thicker samples.

Influence of grain size on transition temperature of thermochromic VO2

Vanadium(IV) oxide (VO2) is a unique material that undergoes a reversible phase transformation around 68 °C. The material could potentially be used as an energy-efficient coating for windows since its reflectance in the infrared (IR) increases significantly more than in the visible region. Currently, VO2 is limited by a transition temperature (τc) that is too high, luminous transmittance that is too low or both. In this study, a transition temperature of 45 °C is achieved for a reactively sputtered, undoped film by restricting grain size to approximately 30 nm. It is concluded that a higher density of grain boundaries (smaller grain size) provides a greater number of nucleating defects which in turn reduces τc. Similarly, a higher density of grain boundaries may reduce the hysteresis width (difference between transition temperatures in heating and cooling). Also in this study, a new set of optical performance metrics is proposed in which the solar spectrum is divided into the ultraviolet (UV), visible and...

Laser-induced decompression shock development in fused silica

Laser-induced weak shock formation in fused silica is studied using standard wave mechanics and applied to thin-film laser spallation experiments. Due to the negative nonlinear elasticity of fused silica, a laser-induced Gaussian stress pulse evolves into a shock after traveling a certain distance in a fused silica substrate. Experimental observations confirm theoretical predictions of shock development. A decompression shock forms and greatly enhances interfacial failure of a thin film deposited on the substrate. The effects of laser fluence and substrate thickness (attenuation) on shock development are also investigated.

Mixed-mode failure of thin films using laser-generated shear waves

A new test method is developed for studying mixed-mode interfacial failure of thin films using laser generated stress waves. Guided by recent parametric studies of laser-induced tensile spallation, we successfully extend this technique to achieve mixed-mode loading conditions. By allowing an initial longitudinal wave to mode convert at an oblique surface, a high amplitude shear wave is generated in a fused silica substrate and propagated toward the thin-film surface. A shear wave is obtained with amplitude large enough to fail an Al film/fused silica interface and the corresponding shear stress calculated from high-speed interferometric displacement measurements. Examination of the interfaces failed under mixed-mode conditions reveals significant wrinkling and tearing of the film, in great contrast to blister patterns observed in similar Al films failed under tensile loading.

Thermal fracture of oxidized polydimethylsiloxane during soft lithography of nanopost arrays

During the fabrication of nanopost arrays for measuring cellular forces, we have observed surface cracks in the negative molds used to replicate the arrays from a silicon master. These cracks become more numerous and severe with each replication such that repeated castings lead to arrays with missing or broken posts. This loss in pattern fidelity from the silicon master undermines the spatial resolution of the nanopost arrays in measuring cellular forces. We hypothesized that these cracks are formed because of a mismatch in the coefficient of thermal expansion (CTE) of polydimethylsiloxane (PDMS) and its oxidized surface layer. To study the fracture of PDMS due to thermal effects, we treated circular test samples of PDMS with oxidizing plasma and then heated them to cause surface cracks. These cracks were found to be more abundant at 180 °C than at lower temperatures. Finite element analysis of a bilayer material with a CTE mismatch was used to validate that thermal stresses are sufficient to overcome the fracture toughness of oxidized PDMS. As a consequence, we have ascertained that elevated temperatures are a significant detriment to the reproducibility of nanoscale features in PDMS during replica molding.

On‐Wafer Crystallization of Ultralow‐κ Pure Silica Zeolite Films

A higher goal: An on-wafer crystallization process to prepare pure silica zeolite (PSZ) MEL-type films that is superior to the previously used hydrothermal process is reported. These striation-free MEL-type films (right, see picture) outperform the traditional spin-on films (left) in terms of the kappa value, mechanical properties, surface roughness, mesopore size, and size distribution.

Cell adhesion measurement by laser-induced stress waves

Cell adhesion is a fundamental property of living cells and influences cell morphology, proliferation, and differentiation. The affinity of cells to relevant substrates plays an important role in tissue response to implanted devices and tissue regeneration. Directivity and precisely quantifying cell adhesion are paramount to the successful development of biomedical and hybrid devices. In this work, a laser-induced stress wave technique previously developed for thin solid film adhesion measurement is modified to investigate the cell-substrate adhesion. High-amplitude short-duration stress wave pulses induced by laser pulse absorption were used to detach cells from the substrate. The results obtained in this work proved the laser-induced stress wave technique to be an effective means for investigating the adhesion between biological cells and inorganic substrates.

LARGE MAGNETIC HALL EFFECT IN FERROMAGNETIC FEXPT100-X THIN FILMS

We have observed a very large extraordinary Hall effect (EHE) in a series of Fe–Pt thin films with various Fe contents. The origin of this remarkable EHE is the large spin–orbit interaction in the Fe–Pt alloys. At certain Fe content, the Hall resistivity can be saturated with a magnetic field less than 2 kG. The large EHE persists to room temperature with little change in magnitude. The EHE, which to our knowledge is the largest among magnetic transition metals, may find potential applications in magnetic sensors and nonvolatile magnetic random access memories. We will present structural analysis of the Fe–Pt films.

Pressure and temperature effects on stoichiometry and microstructure of nitrogen-rich TiN thin films synthesized via reactive magnetron DC-sputtering

Nitrogen-rich titanium nitride (TiN) thin films containing excess nitrogen up to 87.0 at.% were produced on (100) Si substrates via the reactive magnetron DC-sputtering of a commercially available 99.995 at.% pure Ti target within an argon-nitrogen (Ar-N 2) atmosphere with a 20-to-1 gas ratio. The process pressure (PP) and substrate temperature (TS) at which deposition occurred were varied systematically between 0.26 Pa-1.60 Pa and between 15.0°C-600°C, respectively, and their effects on the chemical composition, surface morphology, and preferred orientation were characterized by energy dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD). The EDS analysis confirms increasing nitrogen content with increasing PP and TS. The SEM images reveal a uniform and crystallized surface morphology as well as a closely packed cross-sectional morphology for all crystalline films and a loosely packed cross-sectional morphology for amorphous films. Films produced at lower PP and TS have a pyramidal surface morphology which transitions to a columnar and stratified structure as PP and TS increase. The XRD analysis confirms the existence of only the δ-TiN phase and the absence of other nitrides, oxides, and/or sillicides in all cases. It also indicates that at lower PP and TS, the preferred orientation relative to the substrate is along the (111) planes, and that it transitions to a random orientation along the (200), (220), and (311) planes as PP and TS increase and these results correlate with and qualify those observed by SEM.

Hydrofluoric-Acid-Resistant and Hydrophobic Pure-Silica-Zeolite MEL Low-Dielectric-Constant Films

A new technique for the silylation of pure-silica-zeolite MEL low-k films has been developed in which the spin-on films are calcined directly in trimethylchlorosilane or 1,1,1,3,3,3-hexamethyldisilazane (HMDS) in order to protect the films against corrosive wet etch chemicals and ambient moisture adsorption. In an alternative procedure, HMDS is also added to the zeolite suspension before film preparation. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, water-soak tests, and HF etch tests are performed to characterize the films. The dielectric constant is as low as 1.51, and the films resist HF attack up to 5.5 min. These properties are highly desirable by the semiconductor industry for next-generation microprocessors.