In this dissertation, the grain growth mechanism and performance of TFTs device based on
Ni-metal induced lateral crystallization (NILC) method has been studied. Moreover, combine
the ELA method to reduce the intra-grain defect density of NILC poly-Si films, to produce
high performance TFTs device. In the last part of this work, have using gettering method
which purposed to reduce the Ni contamination within NILC poly-Si films.
In chapter 2, effects of tensile stress on the growth of Ni-metal induced lateral
crystallization (NILC) of α-Si were investigated. It was found that tensile stress did not affect
the morphologies of needlelike Si grains, but enhanced the NILC growth rate. A series of
two-step annealing processes were introduced to study the effects of tensile stress on three
stages of the NILC process. Base on the results of two-step annealing process, it was found
that tensile stress did not enhance NiSi2 formation and c-Si nucleation stages, but enhanced
the c-Si growth stage.
In chapter 3, LTPS TFTs fabricated using <111> and <112> NILC needlelike Si grains
were investigated. 111-poly-Si grains were fabricated by traditional NILC, whereas
108
112-poly-Si grains were fabricated by Ni-metal imprint-induced crystallization. It is found the
performance of 112-TFT was far superior to that of 111-TFT. Compared with 111-TFT, the
field-effect mobility (µFE) was higher by a factor of 2.6, 44 compared with 117 cm2/V-s, and
the on/off current ratio (ION/IOFF) was higher by a factor of 4, 2.29×105 compared with
9.23×105 and the leakage current (IOFF) was lower, 13 compared with 5.47 pA/µm. These
improvements are realized 112-poly-Si has fewer branch grains and less un-crystallized α-Si
than 111-poly-Si. Also, all 112-poly-Si grains and their boundaries were parallel to each other
with the exception of very few branch grains between them.
In chapter 4, high-performance LTPS TFT fabricated by IMPRINT-ELA was investigated.
In this process, α-Si was first transformed to poly-Si using a Ni-imprint method at 550°C for
24 h, and then annealed using a KrF excimer laser. As mention in chapter 3, most IMPRINT
Si grains were parallel to each other. Laser-annealing at an energy density of 345mJ/cm2
greatly increased the width of the needle grains from 50nm to 250nm by geometrical
coalescence. IMPRINT-ELA-TFT markedly outperformed the IMPRINT-TFT because the
IMPRINT-ELA poly-Si film had larger grains and fewer intra-grain defects than the
IMPRINT poly-Si film. The IMPRINT-ELA-TFT has a lower threshold voltage, and a higher
ION/IOFF current ratio than the IMPRINT-TFT. The mobility of IMPRINT-ELA-TFT was 413
cm2/V-s, which was 31.7 times higher than that of the IMPRINT-TFT. The on/off current
ratio of the IMPRINT-ELA-TFT was 4.24×106, which was higher by two orders magnitude
109 than that of the IMPRINT-TFT.
In chapter 5, for the first section, an investigation of the relationship between Ni-gettering
layers (α-Si / SiNx films) and Ni-metal impurity within the NILC poly-Si film has led to the
development of a simple, effective, Ni-gettering process for large area NILC poly-Si films. To
form the GETR poly-Si film, a SiNx layer was capped on the top of NILC poly-Si film, and
then α-Si layer was deposited on the top of SiNx film. The SiNx layer was used as an etching
stop layer, while the α-Si layer served as a gettering layer. It was found that the
silicide-etching holes at the NILC poly-Si grain boundaries were greatly reduced after
samples were annealed at 550°C for 90 h. This is because the concentration gradient acts as
a driving force for transport of Ni from the NILC poly-Si through the SiNx layer to the α-Si
layer. It was also found that the gettering layer (α-Si) was transferred to the poly-Si by NILC
mechanism. The NILC fraction in the gettering layer increased with the increase of the
110
concentration within the NILC POLY film was reduced to 1/10 after the Ni-gettering process.
Both sides of the α-Si films were transformed into the poly-Si by NILC mechanism. The
quartz wafer also achieved the reduction of silicide-etching holes on the NILC POLY film.
NILC poly-Si grains were only located on the inner-side Si film of the quartz Ni-gettering
substrate. No NILC poly-Si grains were found on the outer side due to the quartz diffusion
barrier. These improvements increased with annealing time and α-Si thickness.
According to the results in this thesis, there are some interesting topics that are valuable for
the future research:
(1) Surface roughness of the NILC-ELA or ELA poly-Si films.
The surface roughness degrades the electrical characteristics and reliability of TFTs.
Therefore, to develop a smooth surface is required. Some approaches can be taken into
account to reduce the surface roughness, including the decrease the cooling rate of laser
crystallization process and surface planarization technique.
(2) Ni contamination within the NILC poly-Si films.
The Ni contamination within the NILC poly-Si films can degrade the performance of TFT. In
this study, a silicon nitride and α-Si were used as a gettering material and annealed at 550°C
for 90h. To consider the production cost and yield, must to decrease the annealing time. One
of the solutions is to increase the annealing temperature but can’t damage the glass substrate,
111
such as rapid thermal annealing technique. Another way is to search a new diffusion layer
with higher diffusivity for Ni atom, to replace the silicon nitride layer.