Joel P. McDonald

Femtosecond pulsed laser direct write production of nano- and microfluidic channels

Nano- and microfluidic channels were produced by selectively delaminating 1200nm thermally grown oxide films (SiO2) films from Si(100) substrates using a femtosecond pulsed laser. Single pass channels exhibiting bell-like cross sections with widths of 24μm and heights of 355nm were directly written at a speed of 1cm∕s, while larger channels (320μm in width and ∼15μm in height) were produced by laterally overlapping single pass channels. The results of an investigation of the interior surfaces of the channels via atomic force microscopy and scanning electron microscopy are presented.

Femtosecond-laser-induced delamination and blister formation in thermal oxide films on silicon (100)

Silicon (100) substrates with thermal oxide films of varying thickness were irradiated with single and multiple 150 fs laser pulses at normal and non-normal incidences. A range of laser fluence was found in which a blister or domelike feature was produced where the oxide film was delaminated from the substrate. At normal and non-normal incidences blister features were observed for samples with 54, 147, and 1200 nm of thermal oxide. The blister features were analyzed with optical and atomic force microscopy. In addition, the time frame for blister growth was obtained using pump-probe imaging techniques.

Femtosecond pulsed laser ablation dynamics and ablation morphology of nickel based superalloy CMSX-4

Pump-probe shadowgraphic imaging of single pulse femtosecond laser ablation was performed to investigate the dynamics of material removal during femtosecond laser machining of the intermetallic superalloy CMSX-4. Time-resolved shadowgraphic images were collected, which showed the presence of an expanding shock wave in the air in front of the target, following the onset of laser ablation. The dimensions of the shock wave were measured as a function of time (0–10.3ns), following the onset of ablation. The energy release associated with the observed shock wave and the pressure at the shock wave front versus time as a function of incident laser fluence (1.27–62.8J∕cm2) were inferred from the shock dynamics. The measured shock wave dynamics and inferred shock energy release are discussed in light of the evolving ablation morphology and ablated crater depth as a function of incident laser fluence.

Pump-probe imaging of femtosecond pulsed laser ablation of silicon with thermally grown oxide films

Femtosecond pulsed laser ablation of silicon substrates with thin thermally grown oxide films (20–1200 nm) was studied using pump-probe microscopy techniques. Images from both the front and side of the ablation event produced at a laser fluence of 1.3 J/cm2 were obtained, and results from the two imaging geometries were compared yielding the optical properties of the ablated material. Ablation dynamics were studied over the time scale from 0 to 10.35 ns following the onset of ablation, and ablated material velocities ranging from 200±20 to −3010±360 m/s were determined depending on the thermal oxide film thickness.

Rare-earth transition-metal intermetallic compounds produced via self-propagating, high-temperature synthesis

Several binary intermetallic compounds—each containing a rare-earth (RE) element paired with a transition metal (TM)—were prepared by self-propagating, high-temperature synthesis (SHS). Thin multilayers, composed of alternating Sc or Y (RE element) and Ag, Cu, or Au (TM), were first deposited by direct current magnetron sputtering. Once the initially distinct layers were stimulated and caused to mix, exothermic reactions propagated to completion. X-ray diffraction revealed that Sc/Au, Sc/Cu, Y/Au, and Y/Cu multilayers react in vacuum to form single-phase, cubic B2 structures. Multilayers containing Ag and a RE metal formed cubic B2 (RE)Ag and a minority (RE)Ag 2 phase. The influence of an oxygen-containing environment on the reaction dynamics and the formation of phase were investigated, providing evidence for the participation of secondary combustion reactions during metal-metal SHS. High-speed photography demonstrated reaction propagation speeds that ranged from 0.1–40.0 m/s (dependent on material system and foil design). Both steady and spin-like reaction modes were observed.

Femtosecond pulsed laser patterning of poly(3,4-ethylene dioxythiophene)-poly(styrenesulfonate) thin films on gold/palladium substrates

Femtosecond pulsed laser damage studies were performed on poly(3,4-ethylene dioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) films of varying thickness on Au∕Pd substrates. The femtosecond laser induced damage thresholds of the PEDOT-PSS films were determined (0.007–0.03J∕cm2) and were found to behave similarly to metallic conductors as a function of film thickness. Femtosecond laser patterning of the PEDOT-PSS films was also performed, with minimum wire widths of 1.4μm produced at speeds of 1mm∕s. Discrete removal of the PEDOT-PSS films from the underlying substrate was also demonstrated, with the film cleared down to the depth of the substrate in linear channels as narrow as 4μm while maintaining the integrity of the substrate.

Role of a native oxide on femtosecond laser interaction with silicon (100) near the damage threshold

Si (100) with and without a 14–25A thick native oxide was laser machined at grazing incidence using a Ti:sapphire femtosecond pulsed laser under ultrahigh vacuum conditions. The resulting damage feature size and morphology indicate that the presence or absence of the native oxide significantly affects the mechanism for femtosecond laser-induced damage. We propose that a fluence-dependent modification of the oxide by the incident laser pulse must be considered when studying femtosecond laser damage of Si (100) with a native oxide. Data are also presented that are consistent with a dose-dependent phase transformation in the amorphous oxide. The implications of the native oxide, including relative damage thresholds of the underlying Si (100) and the role of the oxide in damage morphology are addressed.

Effects of oxidation on reaction front instabilities and average propagation speed in Ni/Ti multilayer foils

Vapor-deposited, equiatomic Ni/Ti multilayer foils exhibit low-speed, self-propagating formation reactions that are characterized by a spin-like reaction front instability. In addition to the intermetallic reaction between Ni and Ti, reactions performed in air can also exhibit a discrete combustion wave associated with the oxidation of Ti. In general, the oxidation wave trails the complex intermetallic reaction front. Multilayers that have a large reactant layer periodicity (≥200 nm) exhibit a decrease in net reaction speed as air pressure is reduced. Oxidation has a much smaller effect on the net propagation speed of multilayers with small layer periodicity (<100 nm). The net propagation speed of the multilayers is increased when air is present, due to the added energy release of Ti oxidation. High-speed optical microscopy shows that the increased front speed is associated with an increased nucleation rate of the reaction bands that typify the spinning reaction instability of the Ni/Ti system.

Heat transfer in reactive Co/Al nanolaminates.

Reaction front propagation rates of free standing multilayer thin foils of Co/Al have been determined using a diffusion limited reaction model by means of a method-of-lines code with a stiff solver and adaptive gridding. Predicted and measured reaction front speed variations with bilayer thickness, tb, can be separated into three regimes. The three regimes are delineated by the critical bilayer thickness, tb,c, and the bilayer thickness that produces the maximum front velocity, tb,max. The critical bilayer is composed predominately of premixed or fully reacted CoxAly with a thickness of ~2.7 nm. The front velocity in the three regimes 1) is zero when 0 ≤ tb ≤ tb,c since there are no reactants 2) increases when tb,c < tb < tb,max since the reactant concentration increases with tb and 3) decreases when tb,max < tb since the diffusive resistance increases with tb. The sensitivity of front velocity to property variation is discussed. Steady and oscillatory combustion are predicted for this material pair.

Noise Characterization of Polycrystalline Silicon Thin Film Transistors for X-ray Imagers Based on Active Pixel Architectures

An examination of the noise of polycrystalline silicon thin film transistors, in the context of flat panel x-ray imager development, is reported. The study was conducted in the spirit of exploring how the 1/f, shot and thermal noise components of poly-Si TFTs, determined from current noise power spectral density measurements, as well as through calculation, can be used to assist in the development of imagers incorporating pixel amplification circuits based on such transistors.