Patrick M. Smith

Excimer laser crystallization and doping of silicon films on plastic substrates

We report the pulsed laser recrystallization and doping of thin film amorphous silicon deposited on oxide-coated polyester substrates. Although our heat-flow simulation of the laser recrystallization process indicates that the plastic is briefly subjected to temperatures above its softening point, we see no evidence of damage to the plastic or film delamination from the substrate. Film grain size is found to vary up to ∼0.1 μm. Electrical characteristics obtained from simple strip line resistors and thin film transistors indicate that device-quality silicon films have been produced on an inexpensive flexible plastic substrate.

Polysilicon thin film transistors fabricated on low temperature plastic substrates

We present device results from polysilicon thin film transistors (TFTs) fabricated at a maximum temperature of 100 °C on polyester substrates. Critical to our success has been the development of a processing cluster tool containing chambers dedicated to laser crystallization, dopant deposition, and gate oxidation. Our TFT fabrication process integrates multiple steps in this tool, and uses the laser to crystallize deposited amorphous silicon as well as create heavily doped TFT source/drain regions. By combining laser crystallization and doping, a plasma enhanced chemical vapor deposition SiO2 layer for the gate dielectric, and postfabrication annealing at 150 °C, we have succeeded in fabricating TFTs with ION/IOFF ratios >5×105 and electron mobilities >40 cm2/V s on polyester substrates.

Polysilicon thin film transistors fabricated at 100/spl deg/C on a flexible plastic substrate

We present device results from polysilicon thin film transistors (TFTs) fabricated at a maximum temperature of 100/spl deg/C on plastic (PET) substrates. A XeCl excimer laser has been used both to crystallize sputtered a-Si and to heavily dope the TFT source/drain regions. Using a PECVD SiO/sub 2/ layer for the gate dielectric, and a post-fabrication anneal at 150/spl deg/C, we have succeeded in fabricating TFTs with I/sub ON//I/sub OFF/ ratios >5/spl times/10/sup 5/ and electron mobilities >60 cm/sup 2//V-s on polyester substrates.

Laser crystallization and structural characterization of hydrogenated amorphous silicon thin films

The influence of hydrogen and film thickness on the excimer laser-induced crystallization of plasma-enhanced chemical-vapor deposition-grown hydrogenated amorphous silicon thin films has been studied. Films with hydrogen concentrations varying from 1 to 10 at. % have been crystallized using short pulses from a XeCl excimer laser, at fluences sufficient to induce melting throughout the thickness of the films. Dehydrogenation prior to laser exposure was necessary only for films having initial hydrogen concentrations larger than 5 at. %. The structural properties of the laser-crystallized materials were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM measurements revealed that films requiring no dehydrogenation are smoother after laser crystallization than those requiring laser dehydrogenation, and that the roughness of these films increases as a function of film thickness, although it could be reduced by multiple laser exposure. Smoother films (root-mean-square r...

Laser-assisted transfer of silicon by explosive hydrogen release

We present a technique for the transfer of silicon thin films. This transfer is effected by irradiating a hydrogenated amorphous silicon film deposited on a quartz substrate with an excimer laser pulse. The resulting release and accumulation of hydrogen at the film/substrate interface generates pressures sufficient to propel the silicon onto an adjacent glass receptor wafer. Transient optical transmission measurements indicate that the amorphous film is melted by the laser pulse and breaks into droplets during ejection. For fluences above 400 mJ/cm2, the transferred films adhere well to the receptors and can be smoothed using a second laser irradiation.

Experimental and numerical investigations of a hydrogen-assisted laser-induced materials transfer procedure

We present investigations of the mechanisms of a laser-induced transfer technique, which can be used for the spatially selective deposition of materials such as Si. This transfer is effected by irradiating the backside of a hydrogenated amorphous silicon film, deposited on a transparent substrate with an excimer laser pulse. The resulting release and accumulation of hydrogen at the film/substrate interface propels the silicon onto an adjacent receptor wafer. Time-resolved infrared transmission measurements indicate that the amorphous film is melted by the laser pulse and breaks into droplets during ejection. These droplets travel towards the receptor substrate and coalesce upon arrival. The transfer velocity increases as a function of fluence, the rate of increase dropping noticeably around the full melt threshold of the film. At this fluence, the transfer velocity reaches values of around 1000 m/s for typical films. Atomic force microscopy reveals that films transferred below the full melt threshold only...

Anomalous Diffusion of Fe in Liquid Al Measured by the Pulsed Laser Technique

The diffiisivity of Fe and Cu in liquid Al was measured by using a nanosecond-duration pulsed laser to melt thin Al films ion implanted with solute. The thin film geometry eliminates convection in the melt during the experiment. The time-dependent electrical conductance and optical reflectance of the Al film during melting were measured to determine the melt duration, allowing the diffusivity to be calculated based on the one-dimensional broadening of an ion-implanted solute depth profile. The measured diffusivity of Cu is about three times that of Fe, which is consistent with Turnbull’s cluster model for liquid Al-Fe. The diffusion coefficients measured for both Fe and Cu changed very little as the peak concentration decreased with time, implying little or no concentration dependence. The temperature dependence of the diffusivity was examined by using heat-flow simulations to extract temperature information from the transient conductance data. Our results for Fe diffusion in liquid Al are consistent, within experimental uncertainties, with extrapolations of Ejima’s data to lower temperatures, but we observe Cu diffusivities approximately twice as large as would be expected from extrapolations of Ejima’s data.

Formation of bands of ultrafine beryllium particles during rapid solidification of Al-Be alloys: Modeling and direct observations

Abstract Rapid solidification of dilute hyper-eutectic and monotectic alloys sometimes produces a dispersion of ultrafine randomly-oriented particles that lie in arrays parallel to the advancing solidification front. We characterize this effect in AlBe where Be-rich particles with diameters on the order of 10 nm form in arrays spaced approximately 25 nm apart, and we present a model of macroscopically steady state but microscopically oscillatory motion of the solidification front to explain this unusual microstructure. The proposed mechanism involves; (i) the build-up of rejected solute in a diffusional boundary layer which slows down the growing crystal matrix, (ii) the boundary layer composition entering a metastable liquid miscibility gap, (iii) homogeneous nucleation of solute rich liquid droplets in the boundary layer, and crystallization of these droplets, and (iv) growth of the matrix past the droplets and its reformation into a planar interface. The size of the Be-rich particles is limited by the beryllium supersaturation in the diffusional boundary layer. A numerical model was developed to investigate this solidification mechanism, and the results of the model are in good agreement with experimental observations of rapidly solidified Al-5 at. % Be.

Measurement of the density of liquid aluminum alloys by an x-ray attenuation technique

Aluminum alloy casting is becoming an increasingly important field of study, due to its widespread demand, especially in the transportation industries. As more powerful and inexpensive computers have become available to simulate the casting process, more cast aluminum components are being designed and optimized by computer simulation. However, simulating the casting process is limited, because the important physical properties of the materials (e.g., liquid density) are not always known. This study presents the construction of a relatively simple apparatus for measuring the density of aluminum-based alloys in the solid and liquid states, at temperatures up to 900 C. The authors measured one of the more important physical properties of a casting alloy, the solidification shrinkage, for a commercial aluminum alloy, Al-319. They find that while the thermal expansion of the alloy in both the solid and liquid states is similar to that of pure aluminum, the density of the liquid alloy is lower than one would estimate by averaging the atomic volumes of the pure liquid components.

Thin Film Transistors Fabricated in Printed Silicon

We report the fabrication of thin film transistors (TFTs) from printed silicon. The printing is performed by irradiating a hydrogenated amorphous silicon-coated quartz wafer facing a glass substrate with a high-energy laser pulse. The ensuing explosive effusion of hydrogen from the layer results in transfer of the silicon onto the glass substrate. Adhesion and smoothing of the transferred film is ensured by high-energy laser annealing. Top-gate TFTs were fabricated in this material using standard photolithographic processing and ion implantation. These transistors, which have reasonable electrical characteristics, are the first step towards the fabrication of directly printed electronic devices.