T. W. Sigmon

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.

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.u200a% 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.u200a%. 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 400u200amJ/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...

Ex situ ellipsometry characterization of excimer laser annealed amorphous silicon thin films grown by low pressure chemical vapor deposition

Spectroscopic ellipsometry was used to monitor excimer laser annealed thin (∼100u2009nm) amorphous silicon (a-Si) films grown on quartz substrates by low pressure chemical vapor deposition (LPCVD). The peak position of the imaginary part of the complex dielectric function e2 was used to determine the degree of crystallization of the a-Si. The amplitude of e2 at the Si E1 transition energy is found to be a good indicator of the polycrystalline silicon (poly-Si) grain size after laser annealing with good correlation between ex situ ellipsometric data and poly-Si grain sizes being observed. Spectroscopic ellipsometry provides a contactless, nondestructive, and simple technique for monitoring laser annealing both in situ during the annealing process or ex situ after annealing.

Spatially selective materials deposition by hydrogen-assisted laser-induced transfer

Si and Al lines were deposited on glass substrates using a transfer technique based on the explosive release of hydrogen from a hydrogenated amorphous Si film melted by a laser pulse. The Si lines have a minimum width of 4.5 μm and are well defined, while the Al lines are wider and less uniform. Analysis of time-resolved infrared transmission signals reveals that the lines do not break into droplets upon ejection, in contrast to the behavior of unpatterned films. This difference is attributed to the escape of hydrogen through the sides of the molten lines into the adjacent material.

Polycrystalline thin-film transistors on plastic substrates

Flat panel displays made on plastic substrates are envisioned for use in certain commercial and military systems because they are more rugged and lightweight than displays made on glass substrates. High information content can be attained for such displays using an active matrix array of thin film transistors (TFTs) for the pixels and high current TFTs for the drivers. In this work the fabrication of high performance polysilicon TFTs on flexible plastic substrates is presented along with corresponding electrical characteristics. Plastic substrates pose severe temperature constraints on the fabrication process. To overcome electrical characteristics. Plastic substrates pose sever temperature constraints on the fabrication process. To overcome these constraints, our group at LLNL has used low temperature silicon, oxide, and aluminum thin film deposition steps and pulsed excimer laser processing to perform the TFT channel crystallization and the source/drain doping. Sheet resistance values below 1k(Omega) /

Structural Investigations of Laser-Crystallized Hydrogenated Amorphous Silicon

DAL are obtained using our laser doping technique for 900 angstrom thick polysilicon films. Our n-channel polysilicon TFT electrical performance on plastic shows mobilities up to 50 cm2/V-sec and ON current to OFF current ratios of up to 1 X 106 for gate voltages from -1 to +35 V.

Uniform, High Performance Poly-Si TFTs Fabricated by Laser- Crystallization of PECVD-Grown a-SI:H

We investigated the structure of hydrogenated amorphous silicon, thin films crystallized by short pulses from a XeCl excimer laser at fluences for which total melting of the films occurs. Atomic force microscopy revealed that films prepared using optimized process conditions, leading to hydrogen contents ≤ 5 at.%, are smoother after laser crystallization than those prepared by laser-dehydrogenation. The roughness of the laser-crystallized films increases with their thickness, and can be reduced by multiple exposure. A better smoothing is obtained by partially remelting the films after the first irradiation. Transmission electron microscopy shows that the grains in the laser-crystallized films have sizes that are comparable to the film thickness.

Applications of pulsed lasers in low-temperature thin film electronics fabrication

Polycrystalline silicon thin film transistors (TFTs) were fabricated using laser crystallization of thin amorphous Si films grown by plasma-enhanced chemical vapor deposition. The films were exposed to a scanned XeCl excimer laser beam at 350 mJ/cm 2 . At this fluence the Si film com- pletely melted and crystallized in the form of uniformly distributed grains with an average size of 39 nm. One of the films was then subjected to a low fluence laser scan (250 mJ/cm 2 ), which re- sulted in the melting of the top part of the film and lead to an increase in grain size. The TFTs fabricated without the partial melt method had good electrical properties and uniformities. The partial melt method lead to substantial improvements in most device characteristics, while the uniformity remained good.