Paul C. Van Der Wilt

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

P-59: Thin-beam Crystallization Method for Fabrication of LTPS

Thin film transistors fabricated using Low Temperature Polycrystalline Silicon (LTPS) have several well-known advantages over those made with amorphous Silicon: higher electron mobility, smaller size, higher speed. However, LTPS has not been widely adopted due to the relatively high cost and low yield of the process relative to a-Si. We have investigated a laser annealing process that offers an improvement for both throughput and yield when compared to existing processes. The process uses a very narrow laser beam (∼5 microns) at high repetition rates to create large poly-Si crystals via lateral growth. The beam length is long enough to allow the substrate to be scanned in a single pass, resulting in better uniformity without any seams seen in multi-pass techniques. In this presentation, we will present results from a prototype system used to validate the critical elements of the process.

Thin-film polycrystalline silicon solar cells with low intragrain defect density made via laser crystallization and epitaxial growth

The case for thin-film polycrystalline silicon (pc-Si) solar cells is strong as it combines the cost benefit of thin-films and the quality potential of crystalline Si technology. The challenge is in making high-quality pc-Si layers on non-Si substrates. By studying layers based on aluminum-induced crystallization (AIC) we previously showed that electrically active intragrain defects are a major limiting factor for thin-film polycrystalline silicon solar cells. This paper investigates the use of a recently proposed novel scanning-laser based mixed-phase solidification (MPS) process which results in large grains with a low intragrain defect density, as well as a narrow grain size distribution and strong surface crystallographic texture [6]. Through subsequent epitaxial growth, absorber layers with the desired doping and thickness can be obtained. Defect etching and TEM measurements demonstrate the drastically decreased intragrain defect density compared to the AIC-based samples. The most efficient solar cell so far has an energy conversion efficiency of 5.4% and open circuit voltage (Voc) of around 500mV. From the preliminary results obtained, we conclude that mixed phase solidification is an attractive technique to crystallize seed layers for thin-film silicon solar cells.

34.4: High Performance CMOS‐on‐Plastic Circuits using Sequential Laterally Solidified Silicon TFTs

We developed a CMOS-on-plastic technology using sequential lateral solidification to form LTPS TFTs. We have achieved unity-gain frequencies ft greater than 250 MHz, with CMOS ring oscillators operating at 100 MHz. To our knowledge these are the highest frequency transistors and circuits ever fabricated directly on plastic.