Nanoscale pattern formation from laser induced thin film instabilities: Role of internal and external effects

Pulsed laser assisted pattern formation in single and bilayer metal films was investigated in this dissertation. The overall goals were: (1) to overcome limitations in conventional pulsed laser dewetting techniques, (2) to better understand the role of effects such as thermal gradients, dispersion forces, pressure gradients, and electric fields on pattern formation, and (3) to investigate nanostructure morphology and its progression in the dewetting of bilayer metal films. This study was divided into two parts. In the first part, pulsed laser-induced instabilities of single layer metal films was discussed. The spinodal dewetting of Au films, a novel Rayleigh-Taylor instability induced by pressure gradients, and the role of DC electric field on pattern formation is presented. In pulsed laser dewetting of Au, the trend in particle spacing and diameter was consistent with the predictions of classical spinodal dewetting theory. The early stage dewetting morphology changed from bicontinuous structures to hole like, and thermal gradient forces were found to be significantly weaker than dispersive forces in Au. Next, we showed through experiment and theory that nanoscale Rayleigh-Taylor instabilities can be seen in thin metal films. This instability was a result of pressure gradients developed when Au films were melted inside a bulk fluid like glycerol. One of the primary findings in this pattern formation was that the spacing of the nanoparticles was independent of the film thickness and could be tuned by the magnitude of the pressure gradients. Finally, we concluded this part by presenting the discovery of phase array self assembly of metallic nanoparticles under the application of a DC electric (E) field. In the second part, the morphology evolution under pulsed laser dewetting of a bilayer of the immiscible metals Ag and Co was investigated. We found multiple transitions in morphology for bilayers and correlated these transitions with an experimentally constructed dewetting morphology phase diagram. Finally, the role of thermal gradients was assessed in the formation of a variety of bimetal nanostructure. Work by our collaborator using computational non-linear dynamics showed that different nanoscale morphologies such as core-shell, embedded, or stacked cases could be formed in the Co-Ag system

Robust Microfabrication of Highly Parallelized Three-Dimensional Microfluidics on Silicon

AbstractWe present a new, robust three dimensional microfabrication method for highly parallel microfluidics, to improve the throughput of on-chip material synthesis by allowing parallel and simultaneous operation of many replicate devices on a single chip. Recently, parallelized microfluidic chips fabricated in Silicon and glass have been developed to increase the throughput of microfluidic materials synthesis to an industrially relevant scale. These parallelized microfluidic chips require large arrays (&gt; 10,000) of Through Silicon Vias (TSVs) to deliver fluid from delivery channels to the parallelized devices. Ideally, these TSVs should have a small footprint to allow a high density of features to be packed into a single chip, have channels on both sides of the wafer, and at the same time minimize debris generation and wafer warping to enable permanent bonding of the device to glass. Because of these requirements and challenges, previous approaches cannot be easily applied to produce three dimensional microfluidic chips with a large array of TSVs. To address these issues, in this paper we report a fabrication strategy for the robust fabrication of three-dimensional Silicon microfluidic chips consisting of a dense array of TSVs, designed specifically for highly parallelized microfluidics. In particular, we have developed a two-layer TSV design that allows small diameter vias (d &lt; 20 µm) without sacrificing the mechanical stability of the chip and a patterned SiO2 etch-stop layer to replace the use of carrier wafers in Deep Reactive Ion Etching (DRIE). Our microfabrication strategy allows &gt;50,000 (d = 15 µm) TSVs to be fabricated on a single 4” wafer, using only conventional semiconductor fabrication equipment, with 100% yield (M = 16 chips) compared to 30% using previous approaches. We demonstrated the utility of these fabrication strategies by developing a chip that incorporates 20,160 flow focusing droplet generators onto a single 4” Silicon wafer, representing a 100% increase in the total number of droplet generators than previously reported. To demonstrate the utility of this chip for generating pharmaceutical microparticle formulations, we generated 5–9 µm polycaprolactone particles with a CV &lt;5% at a rate as high as 60 g/hr (&gt; 1 trillion particles / hour).</jats:p

Robust Microfabrication of Highly Parallelized Three-Dimensional Microfluidics on Silicon

AbstractWe present a new, robust three dimensional microfabrication method for highly parallel microfluidics, to improve the throughput of on-chip material synthesis by allowing parallel and simultaneous operation of many replicate devices on a single chip. Recently, parallelized microfluidic chips fabricated in Silicon and glass have been developed to increase the throughput of microfluidic materials synthesis to an industrially relevant scale. These parallelized microfluidic chips require large arrays (&gt;10,000) of Through Silicon Vias (TSVs) to deliver fluid from delivery channels to the parallelized devices. Ideally, these TSVs should have a small footprint to allow a high density of features to be packed into a single chip, have channels on both sides of the wafer, and at the same time minimize debris generation and wafer warping to enable permanent bonding of the device to glass. Because of these requirements and challenges, previous approaches cannot be easily applied to produce three dimensional microfluidic chips with a large array of TSVs. To address these issues, in this paper we report a fabrication strategy for the robust fabrication of three-dimensional Silicon microfluidic chips consisting of a dense array of TSVs, designed specifically for highly parallelized microfluidics. In particular, we have developed a two-layer TSV design that allows small diameter vias (d &lt; 20 µm) without sacrificing the mechanical stability of the chip and a patterned SiO2 etch-stop layer to replace the use of carrier wafers in Deep Reactive Ion Etching (DRIE). Our microfabrication strategy allows &gt;50,000 (d = 15 µm) TSVs to be fabricated on a single 4” wafer, using only conventional semiconductor fabrication equipment, with 100% yield (M = 16 chips) compared to 30% using previous approaches. We demonstrated the utility of these fabrication strategies by developing a chip that incorporates 20,160 flow focusing droplet generators onto a single 4” Silicon wafer, representing a 100% increase in the total number of droplet generators than previously reported. To demonstrate the utility of this chip for generating pharmaceutical microparticle formulations, we generated 5–9 µm polycaprolactone particles with a CV &lt; 5% at a rate as high as 60 g/hr (&gt;1 trillion particles/hour).</jats:p

Formation of organized nanostructures from unstable bilayers of thin metallic liquids

Dewetting of pulsed-laser irradiated, thin (\u3c 20 nm), optically reflective metallic bilayers on an optically transparent substrate with a reflective support layer is studied within the lubrication equations model. A steady-state bilayer film thickness (h) dependent temperature profile is derived based on the mean substrate temperature estimated from the elaborate thermal model of transient heating and melting/freezing. Large thermocapillary forces are observed along the plane of the liquid-liquid and liquid-gas interfaces due to this h-dependent temperature, which, in turn, is strongly influenced by the h-dependent laser light reflection and absorption. Consequently the dewetting is a result of the competition between thermocapillary and intermolecular forces. A linear analysis of the dewetting length scales established that the non-isothermal calculations better predict the experimental results as compared to the isothermal case within the bounding Hamaker coefficients. Subsequently, a computational non-linear dynamics study of the dewetting pathway was performed for Ag/Co and Co/Ag bilayer systems to predict the morphology evolution. We found that the systems evolve towards formation of different morphologies, including core-shell, embedded, or stacked nanostructure morphologies

Controlling nanoparticles formation in molten metallic bilayers by pulsed-laser interference heating

The impacts of the two-beam interference heating on the number of core-shell and embedded nanoparticles and on nanostructure coarsening are studied numerically based on the non-linear dynamical model for dewetting of the pulsed-laser irradiated, thin (\u3c 20 nm) metallic bilayers. The model incorporates thermocapillary forces and disjoining pressures, and assumes dewetting from the optically transparent substrate atop of the reflective support layer, which results in the complicated dependence of light reflectivity and absorption on the thicknesses of the layers. Stabilizing thermocapillary effect is due to the local thickness-dependent, steady- state temperature profile in the liquid, which is derived based on the mean substrate temperature estimated from the elaborate thermal model of transient heating and melting/freezing. Linear stability analysis of the model equations set for Ag/Co bilayer predicts the dewetting length scales in the qualitative agreement with experiment

Ultra-high throughput detection (1 million droplets per second) of fluorescent droplets using a cell phone camera and time domain encoded optofluidics

The microdroplet megascale detector (μMD) generates and detects the fluorescence of millions of droplets per second using a cellphone camera.</p

Liter-scale production of uniform gas bubbles via parallelization of flow-focusing generators

A parallelized microfluidic device is used to generate highly monodisperse gas bubbles at a production rate of ∼1 L h−1.</p