Thesis Organization

In the first chapter of this thesis, we briefly review the revolution of Si-based TFTs, TFT structure, methods of preparation of poly-Si thin films. For realizing SOP applications, the advantage and necessity of artificially-controlled lateral grain growth by laser crystallization technology are discussed. In addition, we also suggest that self-aligned devices and double-gate structure can further improve the device characteristics.

In chapter 2, excimer laser crystallization (ELC) of a-Si thin films with bottom-gate (BG) structure was studied for the application of high-performance LTPS TFTs in detail. The microstructure of poly-Si thin film with bottom-gate structure was analyzed by several material analyses, including SEM, AFM, TEM, and the factors that affected the final lateral crystallization microstructure were also investigated, including thickness of a-Si thin film, thickness of gate dielectric, thickness of gate electrode, laser shot number, and laser energy density. The lateral grain growth mechanism was identified and the electrical characteristics of ELC LTPS TFTs with bottom-gate structure were then discussed. Moreover, the breakdown voltage and reliability are analyzed.

In chapter 3, a self–aligned (SA) bottom-gate TFT with appropriate channel length was demonstrated by the simple ELC and backside exposure. A self-aligned photolithography using the bottom-gate as an opaque mask is applied by backside exposure through the quartz substrate. From the optical microscope (OM) and SEM micrographs, the photo-resist is perfectly self-aligned to the bottom-gate regions. Electrical characteristics of the resulting devices were reviewed and placed emphasis on the improvement of device performance accompanying the symmetrical electrical characteristics and the all advantages of BG LTPS-TFTs with lateral silicon grains.

In chapter 4, we report the process and characteristics of ultra high-performance double-gate (DG) LTPS TFTs. The same advantage of high quality poly-Si films owing to the lateral grain growth in the channel region as the bottom-gate TFTs is obtained. The microstructure of poly-Si films and the completed device structure were analyzed by an analytical transmission electron microscopy (TEM). Both n-channel and p-channel LTPS TFTs were fabricated to investigate the relation between double gate operation conditions and resulting LTPS TFT performance and uniformity.

In chapter 5, a new and simple crystallization method to control lateral grain growth in the device channel region using excimer laser irradiation with a-Si spacer structure was proposed. The crystallization mechanism was presented. The results of crystallized poly-Si thin films were also analyzed by SEM and Raman spectrum and. The experimental results display that the resulting LTPS TFTs exhibit higher performance and better uniformity by using the new crystallization method.

In chapter 6, a novel crystallization technology for producing two-dimensional lateral grain growth, aiming at single-grain TFT, was demonstrated by excimer laser irradiation relying on spatially temperature distribution at artificially sites. The microstructure and quality of silicon grains were analyzed by SEM and TEM. The crystallization mechanism and grain boundary trap density were then presented. Not only high-performance poly-Si TFTs

but also excellent device uniformity was achieved owing to the artificially-controlled lateral grain growth.

In chapter 7, a novel and simple diode-pumped solid-state continuous-wave (CW) laser crystallization was proposed to produce lateral grain growth for ultra high-performance LTPS TFTs. A comparison of the efficiency of dopant activation among various activation methods is studied. In addition, the CW laser-crystallized poly-Si thin film was analyzed by several material analyses, including SEM, Raman, AFM, TEM, and the factors that affected the final lateral crystallization microstructure were also investigated, including, laser scanning speed, laser power, and the ambient. By analyzing the microstructure of poly-Si films, CW laser crystallization mechanism of a-Si films are presented and the electrical characteristics of resulting LTPS TFTs were then discussed.

Finally, important summary and conclusions are given in chapter 8. Future works worthy of further research are recommended in chapter 9.

Table 1-1. The SOP technology roadmap where LTPS TFT performances and related processes are going on in the features.

Figure 1-1. (a) The schematic illustration of the excimer laser crystallization mechanism of a-Si thin films in partial-melting regime. (b) The plane-view scanning electron micrograph (SEM) micrograph and (c) the cross-sectional transmission electron micrograph (TEM) of excimer-laser-crystallized poly-Si thin films in the partial-melting regime, respectively.

Figure 1-2. (a) The schematic illustration of the excimer laser crystallization mechanism of a-Si thin films in complete-melting regime. (b) The plane-view SEM micrograph and (c) the cross-sectional TEM of excimer-laser-crystallized poly-Si thin films in the complete-melting regime, respectively.

Figure 1-3. (a) The schematic illustration of the excimer laser crystallization mechanism of a-Si thin films in near-complete-melting regime. (b) The plane-view SEM micrograph and (c) the cross-sectional TEM of excimer-laser-crystallized poly-Si thin films in the

Chapter 2

High-Performance Low Temperature Polycrystalline Silicon Thin-Film Transistor Crystallized by Excimer Laser Irradiation with

Bottom-Gate Structure

2.1 Introduction

Low-temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) have been extensively studied for high-definition active matrix liquid crystal displays (AMLCDs), active matrix organic light emitting displays (AMOLEDs), and great potential for flexible electronics and 3-dimensional integrated circuits (3D-ICs) applications owing to their superior mobility performance [2.1] - [2.22]. As compared to amorphous silicon (a-Si) TFTs and organic TFTs (OTFTs), LTPS TFTs have higher mobility, better thermal reliability, and higher immunity to water and oxygen ambient. Therefore, LTPS TFTs have been attracted much attention for integration of both the the peripheral driving circuitry and active matrix pixel switching elements onto the single substrate to reduce the fabrication cost and improve the system reliability. Although passivation of the defects in poly-Si film is the common method to improve further device performance, the essential solution to ultra-high-performance LTPS TFTs is to produce high-quality poly-Si thin film by elaborating the crystallization process [2.23] - [2.29].

One common and simple approach to crystallize amorphous Si (a-Si) into poly-Si

structure is solid phase crystallization (SPC) [2.30]. However, the conventional SPC suffer from long processing times of several tens of hours at temperature of 600°C and large defect density in the crystallized poly-Si thin films, which exclude it from high-performance TFT applications on glass or plastic substrates. Recently, metal-induced-crystallization (MIC) has been proved to produce large and uniform silicon grains with lower thermal budget as compared to SPC [2.31]-[2.36]. In spite of low crystallization temperature and high growth rate, metal contamination incorporated into the crystallized poly-Si thin films is a serious problem which resulted in poor TFT performance, such as high leakage current, large kink current, worse subthreshold swing, and poor device stability. As a result, among various crystallization technologies for preparing poly-Si thin films, laser crystallization (LC) are the most promising technology to produce high quality poly-Si thin films on foreign substrates at low temperature. Therefore, many researches of laser crystallization of amorphous silicon films for the preparation of poly-Si films for LTPS TFTs have been studied using various kinds of lasers techniques, such as CO2, Ar, Nd:YAG, Nd:YVO4, excimer, femosecond lasers, and etc [2.37]-[2.47]. Among these laser techniques, excimer laser crystallization (ELC), to date, is the widely used method to prepare poly-Si thin films because of its high pulsed-laser power for large area glass substrate and the large absorption coefficient for a-Si in the UV light region (optical absorption coefficient > 10⁶ cm^-1) for no damage to glass substrate.

According to the mixture gas used in the laser tube, excimer laser radiation of output wavelengths between 157 - 351 nm (157, 193, 248, 308 and 351 nm for F2, ArF, KrF, XeCl and XeF laser, respectively) by the transient high voltage discharge with a short pulse duration (full width of half maximum ~ tens of nanoseconds). The basic principle of excimer laser crystallization is the phase transformation of silicon thin film from amorphous to single-crystal material via melting the Si thin film within a very short time. Actually, the a-Si thin film is heated to the temperature of about 1200°C during laser irradiation. However, the

consequence, the introduction of thermal damage to the glass substrate and the thermal

compaction problem are relaxed, which are serious issues in the solid phase crystallization.

Another unique advantage of excimer lasers is the strong optical absorption of UV light in silicon. As a result, most of the incident laser energy is absorbed close to the surface of the thin film without causing severe thermal strain on the substrate. The unique advantages of strong optical absorption of the UV light in silicon and short pulse duration of the excimer laser imply that high temperature can be produced in the silicon surface region, causing rapidly melting and solidifying quickly, without significant heating the substrate and impurities contamination from the substrate diffusion into the silicon thin film. This technology yield high quality and large-grained poly-Si thin film for high-performance LTPS TFTs on glass or plastic substrate with high throughput.

Although high-performance LTPS TFTs have been fabricated by ELC, the average grain size is smaller than 1 µm, which results in an upper limitation of the field-effect mobility of LTPS TFTs [2.48]-[2.49]. In addition, the narrow laser process window and the non-uniform grain size distribution are noteworthy issues for ELC technology [2.50]-[2.51].

As many previous researches have been reported, the grain size of the ELC poly-Si thin film is significantly dependent on the laser energy density [2.52]-[2.53]. The fluctuation of pulse-to-pulse laser energy density, non-uniform laser beam profile, and non-uniformity of a-Si thin film thickness will result in a very non-uniform grain distribution as the laser energy density is controlled in the SLG regime. It is very undesirable for devices and circuits applications due to the device degradation and poor device-to-device uniformity. In addition, to realize system-on-panel (SOP), TFTs with high performance and good uniformity are essential to integrate the memory and controller with driver circuits on a single substrate [2.54]-[2.58]. Thus, many laser crystallization methods have been proposed to solve the above problems by producing large grains with uniformly grain size distribution, including sequential lateral solidification (SLS) [2.59]-[2.72], grain filters (µ-Czochralski) method

[2.73]- [2.80], thin-beam direction crystallization [2.81]-[2.82], phase-modulated ELC [2.83]-[2.91], dual beam ELA [2.92]-[2.93], double–pulsed laser annealing [2.94]-[2.96], capping reflective or anti-reflective layer [2.97]-[2.102], ELC of pre-patterned a-Si thin film [2.103]-[2.107], near-infrared femtosecond laser crystallization [2.43]-[2.45], comb-shaped beam ZMR-ELA [2.108]-[2.109], ELC of selectively floating a-Si active layer [2.110]-[2.115], heat retaining enhanced crystallization [2.116]-[2.117], CLC method using the diode-pumped solid-state continuous wave laser [2.118]-[2.134], and selectively enlarging laser crystallization (SELAX) [2.135]-[2.140], excimer laser crystallization with recessed channel [2.141]-[2.142], and so on [2.143]-[2.144]. Although large-grain poly-Si thin films can be produced by the above mentioned methods, some of them need additional masks or processes, some of them are problematic for circuit layout due to the anisotropy of the grain boundary spacing, and others are not readily to the existing excimer laser annealing systems, which result in the increase of the fabrication cost.

The bottom-gate structure, the gate electrode located below the semiconductor layer, is the most common configuration for a-Si TFTs due to the clean interface. As a result, if the bottom-gate is used for poly-Si TFTs, it offers some benefits over the top-gate structure for AMLCD applications. First, clean interface control can be easily achieved due to the ability to deposit the gate dielectric and silicon films sequentially in a single system without breaking vacuum. Second, the plasma hydrogenation diffusion rate in the bottom-gate TFT structure is significantly higher than that in top-gate TFT structure, because the channel thin film is not blocked by the gate-electrode thin films during hydrogenation passivation. Therefore, in the early stage of the development of LTPS TFTs, bottom-gate (BG) TFT structure was very attractive because the excimer laser annealing was thought as an additional process step to the a-Si TFTs. However, bottom-gate TFTs suffered from worse electrical performance than top-gate (TG) TFTs. The effective carrier mobility of bottom-gate TFTs is generally much

grain quality produced resulting from the bottom-gate metal acting as a heat sink during excimer laser crystallization [2.145]-[2.146]. As a result, only a few studies have been conducted for bottom-gate TFTs with short channel length and top-gate TFTs have been widely adopted in AMLCDs for the last decade.

In this chapter, a novel and simple lateral grain growth method has been proposed using the conventional fabrication process of bottom-gate a-Si TFTs. In this method, the a-Si thin film with two kinds of thicknesses in a local region was formed by the deposition of a-Si on the plateau structure. When the excimer laser irradiation is applied on the a-Si thin film, the applied laser energy density is controlled to completely melt the thin region of a-Si film in the channel region but partially melt the thicker region of a-Si films near the edges of bottom gate.

Therefore, a lot of un-melting solid seeds remain near the edges of bottom gate electrode and a lateral temperature gradient can be produced between the local thin and thick regions of a-Si film, and the lateral grain growth started from the un-melted silicon solid seed at the base neighbor to the bottom-gate corner, and extended toward the completely melted region until the solid-melt interface from opposite direction impinges. Consequently, large and uniform longitudinal grains could be formed in the device channel regions which lead to the improved TFT performance and uniformity. Moreover, ideally a single laser pulse is sufficient to induce the lateral grain growth and a wide laser process window is also shown in this method.

In this chapter, the concept of controlled lateral grain growth is first discussed. Then, the experimental details are described in detail. Next, the microstructure of ELC poly-Si thin film with bottom-gate structure is analyzed and the factors that affected the final lateral crystallization microstructure were also investigated, including thickness of a-Si thin film, thickness of gate dielectric, thickness of gate electrode, laser shot number, and laser energy density. The results of ELC BG LTPS TFT performance are presented and analyzed, demonstrating the performance and uniformity enhancement achieved by using the new crystallization method. Moreover, the breakdown voltage and reliability of BG TFTs are

analyzed.

2.2 The Concept of Controlled Lateral Grain Growth with Bottom-Gate Structure

Large-grained poly-Si thin films always result in high-performance poly-Si TFTs due to the reduction of defect traps in the grain boundaries. Hence, enlarging grain size is the most effective method for improving the device performance. For realizing the SOP applications, it is essential not only to produce large silicon grains but also to control the locations of the grains and the grain boundaries, since it enables the realization of the high-performance LTPS TFTs with excellent device-to-device uniformity by precisely controlling the number and direction of grain boundaries in TFTs. For the formation of silicon grains at the desired position, it can be achieved by controlling the Si nucleation site at the selected region.

In order to induce lateral grain growth, a lateral temperature gradient must be created between the adjacent areas and there must be un-melted solid silicon seeds to act as the nucleation sites for lateral grain growth. If the laser fluence gradient is performed, the a-Si thin film is completely melted at the areas exposed to higher laser fluence and partially melted at the adjacent areas exposed to lower laser fluence. As a result, a large lateral temperature gradient will exist between the complete melting liquid-phase regions and un-melting solid-phase silicon seeds, and grains will grow laterally towards the complete melting regions from the un-melting solid silicon seeds. The lateral grain growth will eventually be arrested by either colliding with lateral grains grown from the other side or by spontaneous nucleation launched in the severely super-cooling molten silicon. Evidently, higher laser fluence gradient makes steeper temperature gradient resulting in a longer lateral grain growth. Because it takes

spontaneous nucleation, the lateral grain growth can go on for a longer distance [2.147]- [2.148].

In this work, the a-Si thin film with two kinds of thicknesses in a local region was adopted to produce a local temperature gradient during the excimer laser irradiation. A schematic illustration of lateral grain growth mechanism using bottom-gate structure of a-Si thin film is displayed in Fig. 2-1. As the excimer laser irradiation is performed on the a-Si thin film, the applied laser energy density is controlled to completely melt the thin region of a-Si film but partially melt the thicker region of a-Si films near the edges of bottom gate.

Therefore, a lot of un-melting solid seeds remain near the edges of bottom gate electrode and a lateral temperature gradient can be produced between the complete melting liquid-phase regions and un-melting solid-phase silicon seeds. The lateral grain growth started from the un-melted silicon solid seeds at the base neighbor to the bottom-gate corner, and extended toward the completely melted region until the solid-melt interface from opposite direction impinges. As the bottom-gate structure of a-Si thin film is formed artificially and the channel are properly designed, the lateral grain growth can be artificially controlled in the desired local region and the grain boundary perpendicular to the current flow in the channel region can be reduced. LTPS TFTs made by such crystallization method will exhibit higher performance and better uniformity.

2.3 Experiments

2.3.1 The Setup of Excimer Laser Crystallization System

Figure 2-2 shows the schematic illustration of excimer laser crystallization system. The

laser light source of this system is KrF excimer laser, which output wavelength is 248 nm (Lambda Physik, LPX 210i series). The maximum laser output peak density is about 670 J/cm², the maximum frequency is 100 Hz, and the FWHM of the laser pulse is approximately 30 ns. In this work, the pulse laser is operated at a frequency of 10 Hz and the peak energy density at the substrate stage is below 600 mJ/cm². The long axis optical homogenizer and optical condensers are used to transform the original rectangular laser beam profile into the 22.5 mm × 2.0 mm laser beam spot with a spatial uniform top-hat profile for the long axis and a semi-Gaussian profile for the short axis at the working stage.

The sample is crystallized and scanned via laser beam overlapping on the x-y translation stage in a vacuum chamber. The overlap helps to improve the uniformity of laser-crystallized poly-Si thin films because the crystallinity of poly-Si film in the middle of the laser pulse is better than that in the edges of laser pulse. Crystallization of large area is achieved by moving the sample beneath the laser beam by controlling the movement of the x-y translation stage.

The scanning direction is in the short axis direction. The velocity of the x-y translation stage during crystallization process can be adjusted depending on the laser shot number per unit area under determined operating laser frequency. In addition, the crystallization experiments can be performed at either room temperature or 400°C.

2.3.2 Sample Preparation for Material Analysis

Figure 2-3 shows the process procedures for the preparation of poly-Si thin films with bottom-gate structure crystallized by ELC. In order to prepare the sample for material analysis, the silicon dioxide (SiO₂) of 1 µm-thick was thermally growth on bare silicon wafers by

Figure 2-3 shows the process procedures for the preparation of poly-Si thin films with bottom-gate structure crystallized by ELC. In order to prepare the sample for material analysis, the silicon dioxide (SiO₂) of 1 µm-thick was thermally growth on bare silicon wafers by