Thesis Outline

This thesis was organized into the following manner.

In chapter 1, a brief overview of polysilicon thin-film transistors technology for various kinds of applications was introduced. Then we would describe several popular

laser crystallization technologies because they seem to be very promising to fulfill low temperature polycrystalline silicon (LTPS). Finally, the motivation of this work would be expressed.

In the beginning of chapter 2, various laser annealing system were given in detail in four kinds of methods. In addition, several thin film characterizing techniques containing material and electrical aspects were described. We also showed our device fabrication and process flow. Then we introduced the methods to measure or extract the typical parameters including threshold voltage, subthreshold swing, on/off current ratio and field-effect mobility.

In chapter 3, first we analyzed the SEM and AFM images of various crystallization technologies poly-Si TFTs thin films. And we discussed with the device performances of various crystallization technologies poly-Si TFTs. Then we from the I-V measurement to analyze the deep state density, tail state density and the interface traps density. Finally by means of the C-V measurement under different frequency, we tried to extract the effective interface capacitance by equivalent circuit.

In chapter 4 and chapter 5, the conclusions and future work were also given.

Chapter 2

Fabrication process and device characterization

2.1 Laser annealing system

2.1.1 Heat transport and grain growth mechanism of Solid-Phase Crystallization (SPC) of amorphous-silicon

Solid-phase crystallization (SPC) of amorphous silicon (a-Si) is a simple and effective method to acquire poly-Si thin film with large grains. In the SPC furnace annealing, the a-Si thin film is annealed in a furnace at temperature as high as 600℃ for 24 hours. Amorphous silicon is a thermodynamically metastable phase, processing a driving force for transformation to polycrystalline phase given sufficient energy to overcome the initial energy barrier.

Unfortunately, SPC using low-temperature furnace annealing requires very long anneal times and hence suffers from a substantial trade-off between performance and throughput [37]. The crystallization from an amorphous phase to a polycrystalline phase occurs through two processes—nucleation and grain growth [37]. Both of them have characteristic activation energies. The nucleation activation energy is extracted from the time to onset of crystallization, while the grain growth rate is extracted from grain progression data of about 5 eV. The rate of the crystal growth has an activation energy of about 2.7 eV. The nucleation activation energy is larger than the grain growth activation energy for the SPC system [38]. To achieve the largest possible grains, it is desirable to suppress nucleation relative to grain growth. Therefore, SPC is typically done at a low temperature. This results in a reduction in throughput through an increase

in the incubation time and a decrease in the grain growth rate. Higher temperatures increase throughput oppositely. However, excessive nucleation results in smaller grains and hence poorer performance. Deposition rate also affects the structural order of the as-deposited film. Amorphous-Si thin films deposited at higher rates have higher structural disorder which results in lower nucleation rate during crystallization and thus larger grain size. Therefore, crystallization of a-Si thin films deposited by thermal decomposition of disilane yield very large grain size.

Thus SPC can be accomplished within a wide annealing temperature range that requires a similar wide range of annealing time (i.e. time required for complete transformation of the precursor-Si film to poly-Si). The relationship between annealing temperature and annealing time is not unique. Based on the above transformation method, large grain size relates to longer crystallization time. For practical applications, the crystallization time corresponding to average grain size exceeding 0.5-1 µm may be prohibitingly long. The typical SPC poly-Si microstructure is characterized by a large density of structural defects. The result of this high grain-defect density is a saturation in the electrical performance of poly-Si TFTs, fabricated with such poly-Si films with grain size larger than approximately 0.3-0.5 µm. Therefore, standard SPC technology can only produce poly-Si TFTs of mediocre performance. This translated to a mobility range 20 - 40 cm² / Vs and the threshold voltage ranges from 3 to 6 V.

Despite of the mediocre performance, solid-phase crystallization (SPC) is a promising technique due to its simplicity, low cost, and excellent uniformity [39].

2.1.2 Heat transport and grain growth mechanism of Excimer Laser Annealing (ELA) of amorphous-silicon

Excimer laser crystallization (ELC) of a-Si thin film is presently widely used

method for preparing poly-Si thin film on foreign substrates. Excimer lasers emit in the UV region (output wavelengths 193, 248, and 308 nm for ArF, KrF and XeCl gas mixtures, respectively) with a short pulse duration (10-30 ns). The combination of strong optical absorption of the UV light in silicon ( α > 106 cm^-1) and small heat diffusion length during the laser pulse ( ~ 100 nm) implies that high temperature can be produced in the silicon surface region, causing melting, without appreciable heating of the substrate. This makes the ELC process compatible with glass or plastic substrate, one of the major advantages of this technique.

There are many discussions in the literature of the absorption mechanism of intense laser radiation in solids. Among them, four absorption mechanisms in semiconductors seem to be important. They are:

(1) Direct excitation of lattice vibrations by absorption of light with photon energy (hν) well below the band-gap energy (Eg).

(2) Excitation free or nearly free carriers by absorption of light with hν < Eg; such carriers will always be present as a result of finite temperatures and/or doping.

(3) An induced metallic mechanism due to free carriers generated by the laser light itself.

(4) Electron-hole excitation by light with hν < Eg.

Excimer laser annealing (ELA) systems use Long beam homogenizer to make excimer laser beam’s energy duration the amorphous silicon film for uniform annealing results. Excimer lasers have typically a rectangular beam profile, whereas the long axis has a top head and the short axis has a Gaussian like shape [40].

In LPTS-TFT laser annealing process the laser beam is formed to a line shape. This modified beam with an adjustable width between 0.1 and 1.0 mm and a length of up to 370 mm is scanned over the amorphous silicon (see Fig. 2-1). For increasing display size lasers with higher output power are required in order to achieve sufficient laser

energy density on the target. The recent high power excimer lasers for silicon annealing deliver 300 W (i.e. 1 Joule at 300 Hz) [40].

Kuriyama et al. used thermal analysis to calculate the recrystallization process during ELC [41], [42]. The solidification velocity of molten Si during laser annealing can be controlled by three factors: laser pulse width, laser energy density, and substrate temperature during ELC. According to their papers, the substrate temperature is most effective among the three factors. The melting duration increases with substrate temperature. The solidification velocity can be reduced to about one-third by substrate heating.

It is becoming increasing clear that the excimer laser-induced phase and structural modifications of thin Si films involve several melt-mediated and far-from equilibrium transformation process. J. S. Im et al. identified that excimer laser crystallization of a-Si thin film on foreign substrates can divide into three transformation regimes with respect to the applied laser energy densities [43], [44]. They are partial melting regime, fully melting regime, and near complete melting regime as shown in Fig. 2-2 ~ Fig. 2-4 respectively.

Partial-melting regime (low energy density regime)

In the partial melting regime, the energy density of incident laser pulse is above the surface melting threshold but below the complete melt-through energy density (i.e., melting depth < film thickness). The a-Si thin film can be partially melted and subsequently can be crystallized. Explosive crystallization of a-Si thin film occurs at the onset of the transformation and follows by vertical regrowth. The early trigger of explosive crystallization may be attributed either to the presence of microcrystalline clusters – which was confirmed by analyzing the solid-phase crystallization behavior

and is absent in high-dose ion irradiated a-Si thin films – and/or to the possible presence of impurities such as hydrogen.

In this regime, there is an increase in the grain size with increases in the laser energy density. This occurs up to the point at which the average grain radius is approximately equal to the film thickness.

Complete melting regime (high energy density regime)

In the complete melting regime, the energy density of incident laser pulse is sufficient high to lead to a complete melting of the a-Si thin film and no unmelted Si remains. A sudden increase in the melt duration, which is observed at the transition from the low to high energy density regime, is strong indicative of the transition from partial melting and regrowth to complete melting. The complete-melting Si thin film is then followed by significant supercooling of the liquid before the occurrence of the transformation to the solid phase. In this regime, the final microstructure is insensitive to large variations in laser energy densities. For low substrate temperature, fine-grained and small-grained poly-Si thin films are observed. In addition, amorphization of the poly-Si thin film is found for thinner film thickness.

Near complete melting regime (super lateral growth regime)

In the near complete melting regime, the energy density of the incident laser pulse leads to an unmelted a-Si thin film composed of discrete islands (i.e. melting depth ≅ film thickness). At this point, with a small increase in the energy density, an extremely sharp increase in the grain size occurs. Due to the technological significance of large-grained poly-Si thin film and its dramatic nature, this regime is also referred as the

super lateral growth (SLG) regime. With further slight increases in laser energy density, a dramatic reversal in the microstructural trend is observed in that fine-grained poly-Si thin film is obtained. This transition marks the end of the low energy density regime and the beginning of the high energy density regime.

In view of the above interpretations imposed on the low and high energy density regimes, it can be argued that the large-grained poly-Si thin film obtained in the SLG regime is a consequence of the liquid phase regrowth from the discontinuous and small solid seeds, which are never fully melted. In other word, the SLG regime corresponds to the condition at which point near-complete melting of the film occurs to the extent that the unmelted a-Si thin film no longer forms a continuous layer; instead, the residual Si is composed of discrete island. Hence, as the temperature begins to drop, growth from these clusters can proceed. Depending on the separation distance between these seeds, it is possible for significant lateral growth to take place before the impingement of the grain occurs. However, there is a limit to the maximum lateral growth distance, which can be achieved as continuous cooling of the liquid layer via conduction to the substrate eventually would lead to copious nucleation of solids in bulk liquid ahead of the interface. High substrate temperature lead to lower quenching rates, which in turn provides more time for lateral growth to take place before bulk nucleation intervenes. In addition, the SLG distance will also increase with increasing film thickness, decreasing thermal conductive of the substrate, and increasing the laser pulse duration [44].

2.1.3 Heat transport and grain growth mechanism of Solid State Laser annealing (SSL) of amorphous-silicon

Compared to the ELA technology, solid state laser annealing (SSL) with Nd:YAG laser source have advantages such as stable laser power and low maintenance fee on

AMLCD mass production applications. Many researches have been proposed to use the 2^nd harmonic wave of Nd:YAG laser (532 nm wavelength) to crystallize the a-Si film [45-47]. Most papers discuss the large lateral grains produced by low-power continuous-wave Nd:YAG laser [45-47]. However, rectangular grain size causes device deviation when different current flow directions are designed on the same substrate. To overcome these problems, high power Nd-YAG pulsed laser is proposed.

SSL with high power (up to 175 W ) Nd:YAG (2ω) laser source, long line beam with beam length as 105 mm can be produced. In the sort-axis direction, the beam profile had Gaussian-like energy distribution with the Full Width at Half Maximum (FWHM) as 40 µm. Its Gaussian-like beam profile was a great advantage because it was much easier to induce lateral crystallization compared to typical ELA system. Besides, the scan pitch can be varied from 1 µm (overlap 97.5%) to 30 µm with repetition rate as 4 kHz. Although the beam size of SSL was much smaller than ELA, its high repetition rate can make the throughput to be comparable to ELA in mass production.

When mentioned the crystallization mechanism, we can propose three regions in the crystalline one. As illustrated in Fig. 2-5, three regions named vertical crystallization (VC), super lateral crystallization (SLG) and fine grain (FG) regions are divided by two specific laser energy. The first is the critical energy (Ec) that stands for the laser energy to fully melt the a-Si film. The second is the fine-grain energy (Efg) that stands for the minimum energy to cause the fine grain structure. Three regions will be further described as follows.

In regionⅠ, the laser energy is not high enough to completely melt the a-Si film.

As a result, the nucleation sites are located at the boundary between melted Si film and the bulk a-Si film. Vertical growth dominates the crystallization mechanism and gives rise to small grain structure. The grain size increases as the melted zone increases along with increasing laser energy density. The scan pitch also influences the grain size in this

VC region. When the scan pitch decreases, the grain size becomes larger. This is due to the increase of laser beam overlap and also the increase of melted Si zone. In other words, the absorbed energy per unit area of the Si film is increased when the scan pitch decreases.

In region Ⅱ, named the SLG region, grain size becomes large when the laser energy exceeds the critical energy (Ec). In this region, a-Si film is fully melted.

Nucleation happens at the cooling boundary produced by the Gaussian-distributed laser beam profile. Super lateral crystallization dominates the crystal mechanism. In this region, the grain size is not dependent on the laser energy obviously. It is plausible that reducing the beam profile slope can further increase the grain size. However the verification is still beyond our facility. This corresponds to the proposed critical energy concept that causes the a-Si film fully-melted.

In region Ⅲ, the laser energy exceeds the second specific laser energy which is the fine grain energy (Efg). Fine grain areas next to the large grain areas start to appear. We will see in the later discussion about device performance that the fine grain areas degrade device performance tremendously whereas the existence of large grain areas.

Since the slope of the Gaussian profile increases when we enlarge the peak laser energy.

It is plausible that the fine grain areas are caused by the large cooling rate. Large cooling rate produces areas where temperature variation is very large. In these specific areas, many nucleation sites appear simultaneously and form the fine grain structure.

2.1.4 Femtosecond Laser Annealing (FLA) system

Femtosecond laser pulses impart extremely high intensities and provide precise laser-ablation thresholds at substantially reduced laser energy densities. Femtosecond laser induced phase transitions in covalently bonded semiconductors have been

extensively studied during the past decade.

On excitation with a femtosecond pulse, a semiconductor undergoes several stages of relaxation before returning to equilibrium. The energy is transferred first to the electrons and then to the lattice. The interaction includes several regimes of carrier excitation and relaxation. We can distinguish the following four regimes: (1) carrier excitation, (2) thermalization, (3) carrier removal and (4) thermal and structural effects [48]. These regimes and the timescales for the corresponding processes are shown in Fig.

2-6.The triangles at the top of the figure mark the current state-of-the-art in the generation of ultrashort pulses of various wavelengths. In the visible region, pulses as short as 5 fs allow direct probing of carrier dynamics down to the shortest timescales (triangle 1). Diffraction of hard X-ray pulses of 200 fs duration (triangle 2) permit observation of structural and atomic rearrangements in the bulk of materials, but not the carrier excitation and carrier–lattice interaction processes that precede the structural dynamics, because X-rays cannot see electrons. The shortest pulses obtained to date, of 800 as duration (triangle 3), are in the soft X-ray region, and are limited to probing corelevel transitions in excited atoms [30].

For picosecond and subpicosecond laser pulses, however, ample experimental evidence exists from Shank and co-workers [49] and other groups, that nonthermal structural changes can be driven directly by electronic excitation [50]. According to the so-called non-thermal plasma model, the lattice is disordered by direct excitation of the electronic system, while the lattice modes remain vibrationally cold. Absorption of photons creates a free carrier plasma and, when about 10% of valence electrons are removed from bonding orbitals, the lattice is weakened. Photoexcitation can thus give the atoms enhanced mobility without increasing their thermal energy. The non-thermal model assumes that the rate of phonon emission by the excited electronic system is slow compared with the laser pulse duration. When this assumption is satisfied, and a large

enough fraction of the valence electrons is excited, structural change can occur while the electronic system and the lattice are not in thermal equilibrium with each other, although each of these systems may internally be in quasi-equilibrium [48].

Mechanisms of femtosecond laser-induced ablation on crystalline silicon are investigated by time-resolved pump-and-probe microscopy in normal imaging and shadowgraph arrangements. The imaging with the time-delayed frequency-doubled probe beam had a temporal resolution of 100 fs. The high electron temperatures and dense solid state plasma formation suggest a non-equilibrium phase transition process.

Thermionic emission and photoemission of electrons can initiate air plasma in the proximity of the surface. Time-resolved shadowgraphs of the shock wave propagation show that the ultrafast initial plasma becomes visible at around 10 ps and is followed by a slower ‘‘thermal’’ contribution in the time scale of 30 ns. The instantaneous energy released upon initiation of explosion is estimated to be 10%–17% of the absorbed laser energy depending on the incident laser energy. The pressure of the released shock wave was calculated to be about a few hundreds of atm [33].

The intense femtosecond laser pulses lead to efficient nonlinear photon adsorption in irradiated materials, enabling a melting of amorphous silicon films. Femtosecond laser annealing (FLA) assisted by a scanning of the laser beam efficiently crystallizes amorphous silicon films with large grains, using a total laser energy as low as ~ 0.9 J /cm² [36]. The results of near-infrared femtosecond laser annealing are described in section §3.2. Then the comparisons between near-infrared femtosecond laser annealing and solid-phase crystallization is also given.

Up to now, a clear distinction of the transition between the ultrafast liquid phase and the electron-hole plasma state is hard. However, it is undoubted that the classical thermal model of crystallization mediated cannot explain the subpicosecond rapid phase transition satisfactorily.

2.2 Device fabrication

In this experiment, we fabricate the poly-Si TFTs with a typical structure of top-gate, coplanar self-aligned TFT. The process flow and the schematic cross sectional view of the devices were shown in Fig. 2-7 and Fig. 2-8. The fabrication procedure is

In this experiment, we fabricate the poly-Si TFTs with a typical structure of top-gate, coplanar self-aligned TFT. The process flow and the schematic cross sectional view of the devices were shown in Fig. 2-7 and Fig. 2-8. The fabrication procedure is