Optimization of Sequential Lateral Solidification Processes for High Performance TFT Applications

B. A. Turk, P. Ch. Van der Wilt, R. S. Sposili, A. B. Limanov, and James S. Im

Program in Materials Science and Engineering, Dept. of Applied Physics and Applied Mathematics, Columbia

University, New York, New York 10027

Sequential lateral solidification (SLS) is a pulsed-laser based thin-film crystallization process that can be used to manipulate the microstructure of thin silicon films on glass substrates. A variety of non-random microstructures, including large grained and grain-boundary-location-controlled polycrystalline films, directionally solidified microstructures, and location-controlled single-crystal regions, can be obtained by tailoring the details of the process (i.e., the shape of the patterned beamlets and the microtranslation sequence between the laser pulses). Microstructures resulting from various SLS crystallization techniques are shown (Figs. 1,2,3). The versatility of the process, along with its ability to produce highly uniform and low defect density materials with good throughput rates, lends itself to be a promising method for crystallization of high performance polysilicon thin film transistors (TFTs).

Fabrication of TFTs onto SLS processed silicon films has been demonstrated (fig. 4). Very good device characteristics can be achieved, with the level of performance depending on the chosen SLS implementation. While the best device results approach the single crystal upper limits, even SLS devices produced with the “worst case” scenario of grain-boundary location-controlled polycrystalline material are commensurate with devices fabricated by other low temperature compatible crystallization technologies. The results show that performance and/or uniformity of TFTs made in these films depends on the relationships between device size, shape and relative misorientation with respect to the active channel microstructure. Because of its flexibility, explicit control of microstructure and device geometry can be engineered for desired performance.

In this paper, we will discuss the various degrees of microstructural control and TFT performance that SLS techniques can produce, and show how the engineering tasks of device design and crystallization methodology are convoluted and should be considered simultaneously for a specific application. Given channel dimensions and degree of misalignment, an optimal matching condition can be found for a certain SLS microstructure. For example, directionally solidified material (fig. 2) demonstrated higher mobility than grain-boundary location-controlled material (fig. 1) because the device channel length was several times larger than the periodicity of grain boundaries. If the channel length is reduced, or similarly, periodicity of manipulated boundaries increased, the performance of the device would be improved. Furthermore, by incorporating a misorientation angle between the periodic grain boundaries and the channel, the uniformity of devices can be improved when location controlling of devices is not applicable. The channel can be engineered to contain the same number of boundaries within it, no matter where it lies within the film. We will illustrate how proper control of the microstructure can lead to increased performance as well as increased uniformity of device characteristics. Technical details of each of the processes, such as throughput considerations, energy process window, and necessary equipment will also be discussed.

Fig. 1. Grain boundary location controlled polycrystalline silicon film.

Fig. 2. SLS Directionally solidified microstructure.

Fig. 3. Location controlled single crystal region.

Fig. 4. Performance of SLS devices produced on films of various microstructural qualities.