Excimer-laser Crystallization Method Research Articles

Microsecond Crystallization of Amorphous Silicon Films on Glass Substrates by Joule Heating

Large area crystallization of amorphous silicon (a-Si) thin-films on glass substrates is one of the key technologies for manufacturing flat displays on commercial scales. Low-temperature polycrystalline silicon (LTPS) is a crucial active-layer material for polycrystalline silicon thin-film-transistors (poly-Si TFTs) mounted on thermally susceptible glass substrates. LTPS-TFT backplanes are required for fabricating flat panel displays such as active-matrix liquid-crystal-displays and active-matrix organic-light-emitting diodes (AMOLEDs). The LTPS-TFT backplanes are strongly preferred over a-Si TFTs since several devices, AMOLEDs in particular, operate in a current-driven mode. The crystallization methods used for producing LTPS include solid phase crystallization, metal induced crystallization, and excimer laser crystallization (ELC). The ELC method is currently used for crystallizing the a-Si films in mass production lines. Recently, we developed a novel and highly promising crystallization method named Joule-heating induced crystallization (JIC) by which an entire a-Si film could be crystallized on a glass backplane within tens of microseconds. However, arc instability was observed during the crystallization, which is attributed to the dielectric breakdown in the conductor/insulator/a-Si sandwich-structures at high temperatures during electrical pulsing for crystallization. We devised a method for crystallizing the a-Si films while preventing arc generation; in this method an a-Si active layer was pre-patterned into islands followed by the deposition of a gate oxide and gate electrode. An electric pulse was then applied to the gate electrode formed using a Mo layer. The Mo-layer was used as a Joule-heat source for the crystallization of the pre-patterned active islands of the a-Si films located below the Mo-layer. The JIC-processed poly-Si TFTs were fabricated successfully and the proposed method was found to be compatible with the standard processing of coplanar top-gate poly-Si TFTs. In this report, we will discuss some critical issues concerning the arc generation, glass backplane deformation, and phase-transformation phenomena observed during the JIC process on the basis of theoretical investigations as well as in-situ and ex-situ experiments. Finally, we will discuss the procedure for fabricating stable JIC poly-Si TFTs and optimize the process flow and processing conditions such as the power density and pulsing time for fabricating the JIC poly-Si TFTS. Figure 1

High-performance vertically stacked bottom-gate and top-gate polycrystalline silicon thin-film transistors for three-dimensional integrated circuits

The three-dimensional CMOS inverter with top-gate (TG) poly-Si thin film transistors (TFTs) vertically stacked on the bottom-gate (BG) poly-Si TFTs have been proposed to achieve high-performance characteristics via excimer laser crystallization (ELC) for the first time. Under an appropriate laser irradiation energy density, the silicon grain growth could be controlled from the sidewalls of the bottom-gate structure and thus the high-quality laterally grown poly-Si film with single perpendicular grain boundary in the channel would be formed for the BG TFTs. In addition, a simple ELC method was also utilized to the top-layered poly-Si film for TG TFTs as compared with solid-state-crystallized (SPC) ones. As a result, the field-effect mobilities of the proposed n-type BG and p-type TG TFTs could be significantly increased to be 390 and 131cm2/Vs, respectively, in contrast to 32.3 and 14.7cm2/Vs for the SPC ones, accordingly. Furthermore, such three-dimensional (3-D) TFT have also been employed to demonstrate the inverter devices and is suitable for future 3-D ICs as well as system-on-panel applications.

High-Performance Excimer-Laser-Crystallized Polycrystalline Silicon Thin-Film Transistors with the Pre-Patterned Recessed Channel

High-performance polycrystalline silicon (poly-Si) thin-film transistors (TFTs) have been achieved using an excimer laser crystallization method on a prepatterned recessed channel (RC). Such a prepatterned RC TFT exhibited an excellent field-effect mobility of 412 cm2 V-1 s-1 and an on/off current ratio higher than 1.1×108 as compared with 125 cm2 V-1 s-1 and 2.2×107 for the conventional ones, respectively. This is attributed to the two-dimensional control of the grain boundary location and the resultant cross-shaped grain boundary structures within the recessed regions. Therefore, the prepatterned RC TFTs with only one grain boundary parallel to the channel direction as the TFTs could be fabricated to avoid the perpendicular grain boundary. This technology is thus promising for the future applications of poly-Si TFTs in the system-on-panel (SOP) and three-dimensional integrated circuits (3D-ICs).

Polycrystalline silicon thin-film transistors with location-controlled crystal grains fabricated by excimer laser crystallization

In this paper, location-controlled silicon crystal grains are fabricated by the excimer laser crystallization method which employs amorphous silicon spacer structure and prepatterned thin films. The amorphous silicon spacer in nanometer-sized width formed using spacer technology is served as seed crystal to artificially control superlateral growth phenomenon during excimer laser irradiation. An array of 1.8-μm-sized disklike silicon grains is formed, and the n-channel thin-film transistors whose channels located inside the artificially-controlled crystal grains exhibit higher performance of field-effect-mobility reaching 308cm2∕Vs as compared with the conventional ones. This position-manipulated silicon grains are essential to high-performance and good uniformity devices.

Cw argon laser crystallization of silicon films: Structural properties

This paper deals with the structural characterization of amorphous silicon films deposited on glass in the amorphous state and then post-crystallized using a continuous wave argon laser. In opposite to the excimer laser crystallization method, the processing window is wider. Due to the low cooling rate induced by the continuous irradiation, very large grains are obtained. With an epitaxial growth induced by an adequate overlapping of the laser traces, grains as large as 100μm can be reached. Electron back-scattered diffraction analysis highlights the single crystalline character of the large size grains crystallized with this kind of laser. The technique is able to produce large area single crystalline regions, suitable to fabricate high speed circuits.

Fabrication of Location-Controlled Silicon Crystal Grains by Combining Excimer Laser Irradiation with Nanometer-sized A-Si

AbstractIn this paper, location-controlled Silicon crystal grains are fabricated by a novel excimer laser crystallization method. An array of 1.8-μm-sized disk-liked grains are formed by this method, and the high-performance n-channel LTPS TFTs with field-effect-mobility reaching 308 cm2/Vs can be fabricated owing to the artificially-controlled lateral grain growth. This position-manipulated Silicon grains are essential to high performance and good uniformity thin film transistors.

Novel Phase Modulator for ELA-Based Lateral Growth of Si

Phase-modulated excimer laser annealing (ELA) is an advanced excimer-laser crystallization method characterized by the intensity modulation of irradiated light by a phase modulator. In this method, a temperature gradient is formed in melted Si and large crystal grains are laterally grown at predetermined positions. In order to form grains with a high packing efficiency, a periodic “V-shaped” form of the light intensity distribution is desired. In the present study, a novel duty phase modulator is developed for projection-type PMELA. The light intensity distribution on the sample surface can be freely controlled and its design method is simple. We confirmed that a V-shaped light intensity distribution could be achieved by preparing a prototype duty phase modulator. In addition, crystallization was carried out with this duty phase modulator and -long crystal grains with a high packing efficiency were successfully grown.

A proposed single grain-boundary thin-film transistor

A new Si thin-film transistor (TFT) has been proposed where only one grain-boundary exists at the center of channel, and the source and drain are within single grains with good crystallinity. The device fabricated by an excimer-laser crystallization method at the maximum temperature of 500/spl deg/C, had the on-off current ratio /spl cong/10/sup 6/, the field-effect mobility /spl cong/330 cm/sup 2//Vs and the subthreshold swing /spl cong/1.1 V/dec, respectively, For the device processed at 800/spl deg/C, they are >10/sup 6/, >450 cm/sup 2//Vs and /spl cong/0.51 V/dec, respectively.

Characteristics of Amorphous Si Films Fabricated by Mesh-type PECVD and Their Crystallization Behavior Using Excimer Laser

It is increasingly necessary to use poly-Si n's as high resolution and integration of Tn for LCD. Excimer Laser Crystallization (ELC) of a-Si is mainly used as a low temperature process. But the ELC method for the fabrication of poly-Si has the eruption problems associated with hydrogen in the a-Si film. So we need a dehydro-genation process additionally. Hydrogen in a-Si film can degrade the quality of poly-Si film and electrical properties of device due to the hydrogen eruption and voids which occur during the excimer laser annealing. In this study, we propose mesh-type PECVD as the a-Si film deposition method for achieving the low concentration hydrogen. Mesh-type PECVD was found to reduce the hydrogen content substantially. We could obtain a as-deposited a-Si film with hydrogen contents less than at . We also investigated the behavior by XeCl excimer laser annealing of a-Si fabricated by mesh-type PECVB. As a result, we were able to confirm the broad process window in contrast to the narrow process range typically obtained in ELC. Hydrogen eruption was not observed in poly-Si films after ELC These results suggests that mesh-type PECVD is a viable method to achieve the low hydrogen content a-Si and improve the process windows for ELC.

A Novel Excimer Laser Crystallization Method of Poly-Si Thin Film by Grid Line Electron Beam Irradiation

AbstractExcimer laser annealing technique is proposed to increase the grain size and controlling the microstructure of polycrystalline silicon (poly-Si) thin film. Our method is based on the lateral grain growth during laser annealing. Our specific grid ion beam irradiation method was designed to maximize the lateral growth effect and arrange the location of grain boundaries. We observed well-arranged poly-Si grains up to micrometer order by transmission electron microscopy (TEM).

Excimer-Laser-Induced Lateral-Growth of Silicon Thin-Films

A new excimer-laser crystallization method called the “gradient method", has been developed for large-grain growth of Si thin-films on glass. The method is based on a spatial modulation of an incident light intensity, which triggers the lateral grain growth. Grains of size as large as 5 µm were grown by a single shot irradiation at a substrate temperature of 500°C. By combining a step motion of the sample and the proposed method, the grain could be enlarged drastically.

Poly-Si/poly-SiC/sub x/ heterojunction thin-film transistors

We have proposed heterojunction thin-film transistors having a stacked structure of poly-crystal silicon-carbon (SiC/sub x/) and Si thin films, both of which are prepared by an excimer-laser crystallization method. A Si/SiC/sub x/ interface after intense excimer-laser irradiation for crystallization, was as abrupt as that of the as-deposited and amorphous structure. The device had a relatively high mobility of about 4 cm/sup 2//Vs and a sufficiently low leakage current of an order of 10/sup -14/ A//spl mu/m even under intense light illumination conditions.

Excimer-Laser-Produced Single-Crystal Silicon Thin-Film Transistors

Single-crystal silicon thin-film transistors (TFTs) were fabricated within an ultra-large grain thin film on a glassy substrate which was formed by an excimer-laser crystallization method. The field-effect mobility of the TFTs was 460 cm2/Vs for electrons. The off-current was less than 3 ×10-13 A/µm per unit channel width for a wide range of gate voltages.

Amorphous-Silicon Thin-Film Transistors Matched to Ultra-Large Panels

ABSTRACTTime-delay problems will become extremely severe in active-matrix-addressed ultra-large liquid-crystal display-panels. The problem caused by large crossover capacitance can be solved by moving the signal bus-line to the facing glass plate. The problem caused by large gate capacitance can be solved only by reducing the TFT channel much shorter than the conventional value. This requirement can be satisfied by a novel vertical TFT fabricated by using a selective excimer-laser crystallization method. Switching characteristics have been evaluated for the prototype devices, and good on-characteristics have been confirmed for the O.5μm-long gate device. Improved device structures are also presented.

A Novel Double-Pulse Excimer-Laser Crystallization Method of Silicon Thin-Films

We propose a novel double-pulse excimer-laser crystallization method, where two laser light pulses successively irradiate the surface of thin silicon (Si) films. The first light pulse supplies thermal energy near the Si/substrate interface, and this energy reduces the heat removal rate in the melt-regrowth phase triggered by the second light pulse, resulting in large-grain growth of the Si film. The average grain size was enlarged up to 0.8 µm, i.e., more than 10 times larger than that obtained by the conventional method. Solidification characteristics were also investigated numerically.

High-mobility poly-Si thin-film transistors fabricated by a novel excimer laser crystallization method

High-mobility poly-Si thin-film transistors (TFTs) were fabricated by a novel excimer laser crystallization method based on dual-beam irradiation. The new method can reduce the solidification velocity of the top Si layer by heating the bottom Si layer of the Si/SiO/sub 2//Si/glass substrate structure by means of laser irradiation not only from the front side but also from the back side. The grain size of poly-Si film was enlarged up to 2 mu m. The field-effect mobilities of the TFT exceeded 380 cm/sup 2//V-s for electrons and 100 cm/sup 2//V-s for holes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Drastic Enlargement of Grain Size of Excimer-Laser-Crystallized Polysilicon Films

Polysilicon thin films with extremely large grains (grain size more than 50 µm, i.e., three orders of magnitude more than the typical value) have been formed using an excimer-laser crystallization method for the first time. This drastic enlargement was achieved by the reduction of the solidification rate of molten silicon by the small heat capacitance effect of the thin silicon-dioxide membrane used as a substrate. Crystallinity and film properties have been evaluated from Raman spectroscopy, resistivity, Hall effect and transistor characteristics. The Hall mobility of electrons was as high as 610 cm2/Vs.

High-mobility poly-Si TFT's fabricated by a novel excimer laser crystallization method

Summary form only given. High-mobility poly-Si TFTs (thin-film transistors) were fabricated by a novel excimer-laser crystallization method using dual-beam irradiation. The field effect mobility of the TFT was as high as 380 cm/sup 2//V-s. The origin of high mobility was the large grain size (2 mu m) of poly-Si film obtained by the method used. Si(50 nm)/SiO/sub 2/(80 nm)/Si(100 nm) structure was deposited over a quartz substrate and irradiated by an ArF excimer laser at 300 mJ/cm/sup 2/ on both front and back surfaces at the same time. Bottom-gate TFTs were fabricated using the top Si layers as active layer; SiO/sub 2/ and bottom Si layers were used as gate insulator and gate electrode, respectively; aluminum as source/drain electrodes were directly deposited over the active layer. >