We studied the grain growth in Ni-mediated crystallization of amorphous silicon ͑a-Si͒ films with various thicknesses. The Ni particles with density of 3.39 ϫ 10 13 cm Ϫ2 was deposited on a-Si and this was annealed at 580°C for 15 min. The grain size of the crystallized poly-Si increases from 25 to 104 m as the thickness of a-Si increases from 20 to 300 nm. The grain size has a linear relationship with the a-Si thickness. This can be understood on the basis of the cylindrical seed formation and subsequent cylindrical, lateral grain growth. A cylindrical grain was found in atomic force microscope image of a partially crystallized a-Si after the Secco etch of surrounding a-Si phase.Polycrystalline silicon thin-film transistors ͑poly-Si TFTs͒ fabricated on glass are of increasing interest for use in active-matrix liquid crystal display ͑AMLCD͒ and active-matrix organic light emitting diode ͑AMOLED͒. Recently, reducing the process time and lowering the process temperature for the poly-Si TFTs have become the main issues to increasing the throughput of TFT array production and also for use of glass substrate. Among many crystallization methods exist for low-temperature poly-Si, Ni silicide-mediated crystallization ͑SMC͒ has become of increasing interest because it does not use the laser and the TFT performance with the SMC poly-Si is almost the same as that of a good poly-Si by excimer laser annealing. 1 Therefore, low-cost production of low-temperature poly-Si TFT is possible using SMC.When an ultrathin metal is deposited onto amorphous silicon ͑a-Si͒, the a-Si crystallizes at a lower temperature than its solid phase crystallization ͑SPC͒ temperature. Among many metals used for crystallization, Ni is used mainly because the resulting poly-Si has a good material property. 2-4 The precipitates of Ni disilicide, formed at Ͻ400°C, act as seeds for crystallization, which then leads to lowering crystallization temperature. 5 Hayzelden et al. demonstrated the grain growth from the isolated nickel disilicide precipitates in Ni-implanted a-Si. 6 The performance of a TFT can be improved by reducing the a-Si thickness in metal induced lateral crystallization ͑MILC͒. 7,8 However, the grain size is independent of the film thickness for the MILC poly-Si. 9 Note that the poly-Si thickness for TFTs is ϳ50 nm, which is widely used also in excimer laser crystallization of a-Si. There is no literature indicating the relationship between the a-Si thickness and grain size for SMC and MILC, but the performance of TFT made by MILC poly-Si depends on a-Si thickness. 7,8,10