Large Grain creation And Destruction in Excimer Laser Crystallized Amorphous Silicon
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For fast-pulse laser-crystallized thin-film Si on non-crystalline substrates, the average grain size exhibits a peak as a function excimer laser energy density at a characteristic laser fluence Fm. The average grain size increases with increasing laser fluence and can reach a maximum value on the order of 10 pm or about 100 times the film thickness. The grain size then decreases with further increases in fluence. This peak in grain size is accompanied by a similar peak in the Hall electron mobility and x-ray scattering intensity. Our experiments have investigated as-deposited and ion-implanted samples, using a double-scan laser crystallization process. Devices have also been fabricated and studied. The results are consistent with the increase in grain size occurring because of the destruction of nucleation sites with increasing laser fluence (i.e., increased heating and complete melting). But substrate damage occurs in the vicinity of Fm, creating nucleation sites which give rise to small grain sizes in the solidified film. The disruption of the interface causes substantial current leakage through the dielectric of bottom-gate transistors, implying that devices should be laser fabricated below Fm. INTRODUCTION Fast-pulse excimer-laser crystallization of amorphous silicon on non-crystalline substrates is an important processing technique for large-area electronic devices [111]. Due to its short pulse length (- 10 ns) and small optical absorption length (- 7 nm for 308 nm laser wavelength), the silicon is melted and solidified in times of the order of 100 ns. This fast heating into the melt and cooling has the advantage that it produces polycrystalline silicon material of quality comparable to that produced by standard techniques at high temperatures (> 600oC) while keeping the average substrate temperature low (< 600oC). As a result, the process is compatible with low-temperature glass substrates as well as other temperature sensitive materials and can be used to produce hybrid amorphous and polycrystalline silicon material and/or devices in neighboring regions of the same substrate [12-13]. For this laser processing, the sought-after beam profile is a top-hat or a mesa with steep slopes. But for currently available excimer lasers and optical systems, the beam is inhomogeneous to varying degrees. One method used to diminish the inhomogeneity is to expose each point of the amorphous silicon to multiple shots while the beam is scanned over the film in steps that are smaller than the beam size. This multiple-shot crystallization provides a partial averaging of the beam inhomogeneities, but the final pulse at each point might be expected to imprint any beam inhomogeneities into the polysilicon. The step-size-to-beam-size ratio is adjusted to arrive at a compromise between averaging and throughput. Under scanned, multiple-shot conditions, we have observed that a narrow peak exists in the Si (111) x-ray peak intensity, the average grain size, and the electron mobilities at a particular laser fluence, Fm, for a given subst
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