James S. Im

Phase transformation mechanisms involved in excimer laser crystallization of amorphous silicon films

We have investigated the phase transformation mechanisms and the resulting microstructures of excimer laser‐induced crystallization of amorphous Si films on SiO2. It is shown that the process can be characterized into two major regimes, based on the dependence of the grain size and the melt duration as a function of the incident energy density. It is found that at the transition between the two regimes, exceedingly large grain‐sized polycrystalline films can be obtained. We call this the super lateral growth phenomenon, and propose a model based on liquid‐phase regrowth from the residual solid seeds when near‐complete melting of the Si film occurs.

Sequential lateral solidification of thin silicon films on SiO2

We report on a low‐temperature excimer‐laser‐crystallization process that produces a previously unattainable directionally solidified microstructure in thin Si films. The process involves (1) inducing complete melting of selected regions of the film via irradiation through a patterned mask, and (2) precisely controlled between‐pulse microtranslation of the sample with respect to the mask over a distance shorter than the single‐pulse lateral solidification distance, so that lateral growth can be extended over a number of iterative steps. Grains up to 200 μm in length were demonstrated; in principle, grains of unlimited length can be produced. We discuss how the technique can be extended to produce large single‐crystal regions on glass substrates.

ON THE SUPER LATERAL GROWTH PHENOMENON OBSERVED IN EXCIMER LASER-INDUCED CRYSTALLIZATION OF THIN SI FILMS

This letter reports on the experimental findings, and provides a theoretical description of the super lateral growth (SLG) phenomenon observed in the pulsed laser‐induced solidification of amorphous thin Si films on SiO2. Experimentally, we report and elaborate on the isolated single‐crystal disk structure that is observed at the upper threshold of the SLG regime; the structure is revealed as an important microstructural feature for understanding the phenomenon. A theoretical discussion of the SLG phenomenon is provided in terms of the key factors that are suggested by our model—the interface response function of the solid, the nucleation kinetics of the solid, and a highly transient lateral‐thermal profile near the solid‐melt interface. Our model and analysis point out the important inadequacies associated with the vertical solidification rate/temperature gradient model, which is currently being utilized to explain the excimer laser crystallization of thin Si films on SiO2.

Low-temperature single-crystal Si TFTs fabricated on Si films processed via sequential lateral solidification

Nonhydrogenated, n-channel, low-temperature-processed, single-crystal Si thin-film transistors (TFTs) have been fabricated on Si thin films prepared via sequential lateral solidification (SLS). The device characteristics of the resulting SLS TFTs exhibit properties and a level of performance that are superior to polycrystalline Si-based TFTs and are comparable to similar devices fabricated on silicon-on-insulator (SOI) substrates or bulk-Si wafers. We attribute these high-performance device characteristics to the absence of high-angle grain-boundaries within the active channel portion of the TFTs.

Single-crystal Si films for thin-film transistor devices

The fact that single-crystal Si would make an ideal material for thin-film transistor devices has long been recognized. Despite this awareness, a viable method by which such a material could be directly produced on a glass substrate has never been formulated. In this letter, it is shown experimentally that location-controlled single-crystal Si regions on a SiO2 surface can be obtained in a glass-substrate compatible manner, via excimer-laser-based sequential lateral solidification of thin Si films using a beamlet shape that self-selects and extends a single grain over an arbitrarily large area. This is accomplished by controlling the locations, shape, and extent of melting induced by the incident excimer-laser pulses, in such a manner as to induce interface-contour-affected sequential super-lateral growth of crystals, during which the tendency of grain boundaries to align approximately orthogonal to the solidifying interface is systematically exploited.

New excimer‐laser‐crystallization method for producing large‐grained and grain boundary‐location‐controlled Si films for thin film transistors

Based on the previously elucidated super lateral growth phenomenon, we have developed an excimer‐laser‐crystallization method that produces large‐grained and grain‐boundary‐ location‐controlled Si films on SiO2  and which possesses a wide processing window. For the set of experiments reported in this letter, a patterned SiO2 capping layer on top of Si films is utilized as an anti‐reflective coating in order to induce artificially controlled super‐lateral growth in the film upon being irradiated with a single excimer laser pulse. For a simple SiO2  stripe pattern, the occlusion among the laterally and directionally solidifying grains permits the eventual development of elongated parallel grains with a single perpendicular grain boundary which is localized in the middle of the completely melted regions, provided that the width of the completely molten region is sufficiently narrow so as to avoid the nucleation of solids in the supercooled liquid.

Effect of process parameters on the structural characteristics of laterally grown, laser-annealed polycrystalline silicon films

In this work, we have conducted a systematic study aiming at assessing the effects of process parameters on the microstructural characteristics of laterally grown polycrystalline silicon (poly-Si) films. Poly-Si films were formed by the sequential lateral solidification (SLS) method. The Si film thickness was found to affect significantly the quality of the poly-Si microstructure, manifested by a decreased crystal-growth defect density and increased subboundary spacing in thicker films. A weak (100) texture was observed in the lateral growth direction, except for very thin films (<30 nm) where (110) texture was observed. No specific texture was identified in the normal and transverse directions. Lateral crystallization proceeds by seeded, lateral epitaxial growth at an advancing pitch. We investigated the quality of lateral growth as a function of the advancing, substrate pitch. We found that an optimum pitch range exists, bound on the low end by the detailed shape of the beam-edge profile and on the high...

Line scan sequential lateral solidification of thin films

Sequential lateral solidification (SLS) is a recently demonstrated low-temperature pulsed-laser crystallization method that can produce polycrystalline and single-crystal microstructures in thinSi films for thin-film transistor (TFT) and other applications. In this paper, we show that the process can be accomplished using a focused line beam, produced by simple cylindrical optics, so long as two essential requirements associated with the SLS process are satisfied: (1) Complete melting is induced in an irradiated region of the film, whose location is reproducibly controlled, and (2) the film is translated precisely over a distance shorter than the single-pulse-induced lateral growth distance. Implemented in this way, the technique produces a directionally solidified microstructure over a large area, being that the length of the beam can span several centimeters, with grains whose length is limited only by the total distance over which the film is translated. The results demonstrate that SLS is a flexible method that can potentially be carried out using various technological approaches that lead to spatially localized melting of the film. We discuss and demonstrate how modifying the shape of the beam influences the microstructure, and how such modifications can be used in order to produce a directionally solidified microstructure where the high-angle grain boundaries are at precisely defined locations and are perfectly parallel to one another. PACS: 64.70.Dv; 81.10.-h; 85.40.Hp Sequential lateral solidification (SLS) is a pulsed-laser crystallization process that can produce near-SOI-quality crystallineSi films onSiO2-coated substrates, including those that are intolerant to high processing temperatures, such as glass or plastics [1–3]. It utilizes spatially controlled manipulation of melting and the ensuing lateral solidification to convert initially amorphous or polycrystallineSi thin films into either a directionally solidified lateral columnar microstructure [2], or location-controlled, large single-crystal regions [3]. Such microstructures are known to be better suited for various thin ∗ Corresponding author Si film-based electronic applications than either the amorphous [4] or polycrystallineSi films [5] that can be produced using various deposition and crystallization methods. The effectiveness of the SLS-processed material was demonstrated recently when we fabricated a first set of low-temperature single-crystal TFTs on SLS-processed Si films [6], using low-temperature device-processing methods [7]. The device characteristics of the non-hydrogenated, n-channel TFTs were found to exhibit properties and a level of performance comparable to similar devices fabricated on silicon-on-insulator (SOI) substrates or bulk Si wafers [8] (for example, mobilities as high as 560 cm2/Vs, Ion/Ioff > 106 when measured at Vds= 1.0 V, subthreshold swing of105 V/decade). According to our model of the process, the quintessential requirements for the SLS process are: (1) that each laser pulse result in the film being completely melted in an irradiated region(s) of controlled dimensions and location(s), and (2) that between pulses, the film be translated relative to the position of the beam over a distance smaller than the lateral crystal growth resulting from the previous irradiation pulse [1]. The above requirements can be satisfied through various technical approaches, using various combinations of pulsed lasers and beam-shaping schemes. In the previous works, controlled melting was accomplished using projection-irradiation of an excimer-laser beam through a patterned mask [2, 3]. This particular approach was utilized because (1) the technique has been welldeveloped for micromachining and microlithography applications, (2) the projected beam profiles can be imaged at high resolution, and (3) it permits division of the original beam into a large number of multiple beamlets whose shape and location can be tailored (using an appropriately designed mask) so as to enable the creation, in a parallel fashion, of numerous single-crystal regions at those positions where the devices are to be fabricated. In this paper, we demonstrate the flexibility associated with the SLS process by carrying out the process using an alternative configuration for inducing controlled complete melting of the films. (We refer to this process as line-scan sequential lateral solidification (LS-SLS)). As implemented in

Stochastic modeling of solid nucleation in supercooled liquids

We have formulated a nucleation model that can simulate nucleation of solids in supercooled liquid in a manner that is consistent with the stochastic nature and kinetic aspect of the phenomenon. This is accomplished, even under highly transient and nonuniform thermal conditions, by (1) calculating the probability of nucleation in each and every liquid node during each time step using the Poisson expression, and (2) triggering nucleation if and only when the random number assigned to a node for the time step is less than the calculated nucleation probability. In contrast with previous models, no empirical or deterministic conditions for nucleation are imposed. We demonstrate the effectiveness of the model by analyzing nucleation-initiated solidification as encountered in pulsed laser-induced irradiation of thin Si films, and discuss the generality and utility of the model.

Single‐crystal Si islands on SiO2 obtained via excimer‐laser irradiation of a patterned Si film

We have developed a single‐pulse based excimer‐laser crystallization technique that transforms photolithographically patterned and SiO2‐encapsulated a‐Si regions into single‐crystal islands on SiO2. The method utilizes the substantial superlateral growth distances that are attainable at high substrate temperatures in combination with precise manipulation of the evolution of the solidification front in order to allow only one of the several grains that originate from the intentionally incompletely melted (i.e., the seed) portion of an island to pass through a constriction and propagate into the rest of the island. This converts the main portion of the island, which extends tens of microns in the lateral dimensions, into a high‐angle‐grain boundary free crystalline material, in so far as the size of the island is commensurate with achievable superlateral growth distances.