Demands for polycrystalline Si (poly-Si) thin films are increasing for the application to electronic devices like thin film transistors (TFTs), solar cells, and image sensors. Poly-Si thin films are generally fabricated by crystallizing amorphous Si (a-Si) thin films because it can render larger grains than those of directly deposited poly-Si films. But the solid phase crystallization (SPC) generally takes tens of hours to crystallize a-Si films at 600℃, which is too high temperature for large area glass substrates. The SPC temperature has been lowered by annealing a-Si films in contact with metals on the surface, called metal-induced crystallization (MIC) or silicide-mediated crystallization (SMC). Metals can be supplied from metal solutions as well as vacuum deposition methods. But conventional methods using dilute acid solutions have some problems. Limited kinds of metals of which electronegativity is larger than Si such as Cu and Au can be used and the solution on a-Si films exists in uncontrollable forms of droplets because the Si surface is hydrophobic, resulting in non-uniform and irreproducible deposition.
Another method to enhance crystallization, even without metal application, is annealing a-Si films using microwave heating. Lee first reported enhanced crystallization and dehydrogenation of PECVD a-Si films without metal supply. A revised form of metal induced crystallization of a-Si films is metal induced lateral crystallization (MILC) more specifically silicide mediated lateral crystallization (SMLC). In MILC, metal is supplied on a part of a-Si film. During annealing, the crystallization originated from a-Si region with metal supply propagates to a-Si region without metal supply. But even while many studies about MILC have been focused on its application to TFT`s and methods such as rapid thermal annealing and annealing with DC bias have been studied to enhance MILC, microstructure and crystallization mechanism of MILC poly-Si films have not been well-understood.
In this study, to overcome the limitations of methods using metal solutions, viscous metal solution was proposed. A viscous Ni solution was prepared by dissolving $NiCl_2$ in 1N HCl and mixing it with propylene glycol. $NiCl_2$ was uniformly coated after spin-coating and drying the viscous Ni solution. The uniform coating of $NiCl_2$ resulted in uniform crystallization of a-Si films making the crystallization also reproducible. Crystallization was enhanced with the viscous Ni solution that the crystallization completed in 5h at 500℃ using 0.1M solution. Intrinsic a-Si films without metal supply was fully crystallized in 30h at 600℃ and the crystallization was hardly enhanced with conventional metal solution method. The crystallization was mediated by $NiSi_2$ precipitates, the same process as the case using Ni metal deposition, even if $NiCl_2$ was coated.
The crystallization was further enhanced using microwave heating. The crystallization started in 1h and completed in 7h using microwave heating while it started in 2h and completed in 15h using furnace heating. Kinetic parameters explaining the crystallization behavior was extracted using Avrami-John-Mehl equation fitting and compared between microwave and furnace heating. Incubation time which is inverse proportional to the nucleation velocity and its activation energy was reduced with microwave heating. Crystallization time which depends on both nucleation and growth was further reduced with microwave heating. This indicated that the growth also was enhanced with microwave heating.
Microstructure of laterally crystallized poly-Si films was investigated. Growth of lateral crystallization was also mediated by $NiSi_2$ precipitates. But the needle-like grains at the growth front was aligned to a specific orientation. The grains not only was oriented with [011] zone axis but also had the same crystallization direction. The laterally grown poly-Si consisted of several μm long and tens of nm wide grains aligned to one direction while SMC poly-Si consisted of at most a μm long and tens of nm wide grains with random orientation.
SMLC also was enhanced with microwave heating. The SMLC started earlier and grew faster with microwave heating than furnace heating. The incubation time and velocity was extracted from the intersect of the linear growth behavior of SMLC with time axis and from the slope of the behavior. The incubation time was decreased and the activation energy was reduced from 3.08 eV to 2.14 eV. The velocity increased but the activation energy was reduced from 2.12 eV to 1.55 eV.
TFT`s were fabricated using intrinsic, SMC and SMLC poly-Si films. Among characteristics of the TFT`s the increase of field effect mobility of SMLC TFT was the most remarkable. The field effect mobility increased to 21㎠/Vsec from 2.6 and 2.7㎠/Vsec of intrinsic and SMC TFT. The increase of field effect mobility could be explained by the difference of microstructure between SMLC and SMC or intrinsic poly-Si films.
