Excimer Laser Annealing Process Research Articles

62‐4: Late‐News Paper: Evaluation of polycrystalline silicon after Excimer laser annealing by Retardation measurement method

The excimer laser annealing (ELA) process is one of the important processes in Low Temperature Polycrystalline Silicon (LTPS) and the optimization of laser intensity and scan pitch in ELA process are the key technology to make good performance in OLED panel. However, the visual inspection through human eye has been used to find the Optimum ELA energy density (OPED) because of the fast inspection speed and accuracy. As the size of glass in LTPS is increasing, the need of a new inspection method has been raised. So the measurement of retardation which has been used in liquid crystals’ cell gap measuring was applied to evaluate the degree of crystallization in polycrystalline silicon(p‐Si) made from the ELA process for the first time. The origin of the retardation is estimated as a Laser‐induced periodic surface structure (LIPSS).

Post-annealing effect on the optical property of indium tin oxide sputtered films

By using the spectroscopic ellipsometric technique, we investigated the excimer laser annealing (ELA) effect on the electronic properties of indium tin oxides (ITO) films fabricated by the DC-sputtering method, which is the one of the most commonly known methods for the commercial ITO films. We found that while the ELA process was helpful for enhancing the electronic property of the sputtered films, the degree of the enhancement was not so sizable as the case of the sol-gel films. This result appeared to originate from the difference in the physical properties of the sol-gel and sputtered amorphous films. We also examined the thermal annealing (TA) effect on the sputtered amorphous films in various ambient conditions, and compared their physical properties with those of the TA sol-gel films.

Study on excimer laser irradiation for controlled dehydrogenation and crystallization of boron doped hydrogenated amorphous/nanocrystalline silicon multilayers

We report on the excimer laser annealing (ELA) induced temperature gradients, allowing controlled crystallization and dehydrogenation of boron-doped a-Si:H/nc-Si:H multilayers. Depth of the dehydrogenation and crystallization process has been studied numerically and experimentally, showing that temperatures below the monohydride decomposition can be used and that significant changes of the doping profile can be avoided. Calculation of temperature profiles has been achieved through numerical modeling of the heat conduction differential equation. Increase in the amount of nano-crystals, but not in their size, has been demonstrated by Raman spectroscopy. Effective dehydrogenation and shape of the boron profile have been studied by time of flight secondary ion mass spectroscopy. The relatively low temperature threshold for dehydrogenation, below the monohydride decomposition temperature, has been attributed to both, the large hydrogen content of the original films and the partial crystallization during the ELA process. The results of this study show that UV-laser irradiation is an effective tool to improve crystallinity and dopant activation in p+-nc-Si:H films without damaging the substrate.

B-doping in Ge by excimer laser annealing

An experimental and theoretical study of the effect of excimer laser annealing (ELA) on B redistribution and electrical activation in Ge is reported. We performed detailed structural, chemical, and electrical characterizations of Ge samples implanted with B (20 keV, 1 × 1015, or 1 × 1016 B/cm2) and processed by ELA (λ = 308 nm) with multiple pulses (1, 3, or 10). We also developed a diffusion model, in order to simulate the B redistribution induced by the ELA process. We found an anomalous impurity redistribution in the molten phase, which causes a dopant incorporation during the melt-growth at the maximum melt depth. The investigated samples showed a partial electrical activation of the B dopant. The inactivation of B in the samples implanted with 1 × 1015 B/cm2 was correlated to an oxygen contamination, while the poor electrical activation of B in the samples implanted with 1 × 1016 B/cm2 was related to the precipitation of the dopant, in good agreement with the experimental and theoretical results.

Nickel silicide contacts formed by excimer laser annealing for high efficiency solar cells

ABSTRACTDriven by the relatively high cost of silver (Ag), interest has grown in the photovoltaic (PV) industry to substitute conventional screen printed (SP) Ag front contacts with copper (Cu) plated contacts. The approach chosen here applies selective laser ablation of the front anti‐reflection coating (ARC), then forming self‐aligned nickel silicides (NiSix) contacts, and thickening the lines by Cu plating to achieve the desired line conductivity. A successful implementation of this scheme requires annealing to form NiSix with low contact resistance. However, it has been shown that industrial shallow emitters can be damaged severely upon conventional annealing of nickel. In this paper, we show that by using large area excimer laser annealing (ELA), NiSix contacts can be formed on industrial shallow emitters without the associated junction degradation. On the basis of sheet resistance, transmission electron microscopy, and lifetime measurements, we demonstrate that NiSix formation by ELA can be achieved in narrow contact openings without damaging the passivation and reflectance properties of the neighboring ARC. In addition, the thresholds for NiSix formation for different Ni thicknesses are quantified by rigorous finite element simulations and compared with experimental data. Finally, high efficiency passivated emitter and rear cell type solar cells featuring a shallow 85 Ω/sq emitter have been processed on large area CZ–Si using laser ablation of the ARC and subsequent NiSix formation by ELA. These cells show an average efficiency gain of 0.4%abs compared with cells processed with reference SP contacts. In this work, the best performing cell with the ELA process reached 20.0% energy conversion efficiency. Copyright © 2013 John Wiley & Sons, Ltd.

Nickel Silicide Formation Using Excimer Laser Annealing

In this work, we report on a self-aligned nickel silicide formation technique based on excimer laser annealing (ELA). We evaluate this process for the front contact formation of industrial PERC type solar cells on random pyramid textured Si surfaces where damage to surface texture, emitter passivation, or to the shallow junction should be avoided or minimized. PERC type solar cells obtained by POCl3 diffusion were processed on large area (12.5x12.5cm2) CZ-Si. Self-aligned litho-free Ni/Cu contacts defined by ps-laser ablation of the SiO2/SiNx anti-reflective coating (ARC) and subsequent ELA of the Ni layer were compared to conventional Ag screen printed contacts.The novel ELA process results in an absolute gain in Jsc of 0.8mA/cm2 as well as a drop of 0.3Ω.cm2 in series resistance (Rs) compared to SP Ag contacts due to reduced shading and resistance losses. This leads to 0.5% absolute increase in efficiency from 19.3% to 19.7% since other characteristics (Voc, pFF) could be maintained to the same level. In this work, the best performing cell with the ELA process reached an outstanding 20.0% energy conversion efficiency with Jsc=39.3mA/cm2, Voc=649.8mV, and FF=78.3%.

Ultra-shallow Junction Formation Using Plasma Doping and ExcimerLaser Annealing for Nano-technology CMOS Applications

A plasma-doping (PLAD) process utilizing a PH3 plasma is presented to fabricate a n /p ultrashallow junction at room temperature. When PLAD is completed, an excimer laser annealing (ELA) process is performed using an ArF (193 nm) excimer laser with pre-annealing. In our case, the preannealing is conducted for 5 min at 500 ◦C in a thermal furnace. The energy density of the excimer laser used for the ELA process ranged from 400 to 500 mJ/cm. Transmission electron microscope (TEM) and double-crystal X-ray diffraction (DXRD) measurements were employed to investigate defects on the plasma-doped samples. Some point defects were found in the DXRD analysis while TEM images reveal no crystalline defects. The result of a secondary ion mass spectrometry (SIMS) analysis on the formed shallow n/p junction revealed that the junction depth was thinner than 40 nm. We found that the drive-in process of phosphorous occurred only when the energy density of the laser was larger than 460 mJ/cm, and an increased laser energy density resulted in a decreased sheet resistance. With an energy density of 460 mJ/cm, a junction depth of 30 nm and a sheet resistance of 151.6 Ω/ were obtained. The leakage current for the fabricated n/p junction was indirectly investigated by measuring the I-V characteristics of the diode fabricated using PLAD, followed by the ELA processes.

Effects of Thermal Annealing on ZnO Thin-Film Transistor Characteristics and the Application of Excimer Laser Annealing in Plastic-Based ZnO Thin-Film Transistors

In this paper, we present the effects of thermal annealing and excimer laser annealing (ELA) on the characteristics of zinc-oxide (ZnO) thin-film transistors (TFTs) fabricated by magnetron sputtering at room temperature. The transfer characteristics of the ZnO-TFTs are found to improve with increasing thermal annealing temperature. Results of in situ high-temperature X-ray diffraction and electron back scattered diffraction pattern analyses indicate that this phenomenon arises from the crystallization of a low-crystallinity region present in as-deposited ZnO films. Almost all low-crystallinity regions crystallize at temperatures above 500 °C. Furthermore, to achieve such high-performance TFTs on flexible plastic substrates, ZnO-TFTs on a plastic substrate were irradiated with an excimer laser to raise the temperature of the ZnO films. Because of the extremely short pulse width of the laser (25 ns), it is possible to selectively increase the temperature of the ZnO with negligible thermal damage to the plastic substrate. The ELA process has enabled us to produce flexible high-performance ZnO-TFTs with a field-effect mobility of 12 cm2 V-1 s-1.

Flexible High-Performance Amorphous InGaZnO4 Thin-Film Transistors Utilizing Excimer Laser Annealing

We have investigated an excimer laser annealing (ELA) process for use in fabricating high-performance amorphous InGaZnO4 (IGZO) thin-film transistors (TFTs) on flexible plastic substrates. We numerically estimate the temperature increase of the IGZO film and substrate as a function of laser energy density. This is one of the most important measures for optimizing ELA conditions in order to apply plastic-based TFT fabrication. Because the optical absorption coefficient of IGZO film is three orders of magnitude higher than that of plastic substrates with respect to 308-nm laser light, it is possible to selectively increase the temperature of the IGZO film. The temperature of the IGZO film is estimated to increase to approximately 1500 °C at typical laser energy levels. Furthermore, incorporating a SiO2 buffer layer (some hundreds of nm) between the IGZO film and the plastic substrate is found to effectively suppress thermal damage to the substrate. We have experimentally investigated the properties of IGZO films irradiated with various excimer-laser energy densities. X-ray diffraction patterns and carrier densities of the IGZO films are found to significantly vary with laser energy density. We have used calculations and experimental results to optimize the ELA process; this has enabled us to produce high-performance IGZO-TFTs having a field-effect mobility of 15.6 cm2 V-1 s-1.

53.4: Treatments in ELA Process to Improve the Image Quality in AMOLED Displays

Supplementary treatments combined with the standard Excimer Laser Annealing (ELA) process are introduced to poly-Si recrystallization. The surface condition of the poly silicon film becomes more uniform and smoother after these treatments. The variation of the current that drives AMOLED is reduced from 30% to 12%. The streak mura caused by ELA process has been greatly reduced as well. Finally, the image quality of AMOLED is highly improved.

Excimer Laser annealing for shallow junction formation in Si power MOS devices

Integration of excimer laser annealing (ELA) process for power MOSFET devices is currently studied and evaluated within the frame of the IST project FLASH (Fundamentals and applications of laser processing for highly innovative MOS technology), funded by the European Commission. Final goal of the project is to show that ELA can be industrialized in the context of semiconductor device fabrication. In this work, the recent advancements of the FLASH project are described, including the formation of n +/p source junctions in both one and two dimensions. The experimental results are analysed by using a two-dimensional simulation program, specifically developed by the consortium. The simulation tool allowed a deeper insight of the ELA process and has been successfully used to optimise the process parameters. Finally, data on electrical integrity of the test devices are also presented, showing a yield for optimum ELA conditions up to 84%.

A 3.5 in. QVGA poly-Si TFT-LCD with integrated driver including new 6-bit DAC

By the SOP (system-on-panel) technology with laser crystallization process, all the driver circuits required for 6-bit color QVGA (320 × RGB × 240) LCD are integrated on panel. The integrated data driver comprises a new ramp-driven 6-bit DAC. The characteristics and uniformity of the LTPS (low temperature polycrystalline silicon) has been remarkably enhanced by applying multi-shot ELA (excimer laser annealing) process and surface treatment. The characteristics of the poly-Si TFTs for integration of driver circuits are following: (1) in NMOS, V th ∼ 1 V and mobility ∼ 240 cm 2/V s, (2) in PMOS, V th ∼ −1.8 V and mobility ∼ 100 cm 2/V s.

High-Performance Polycrystalline Silicon Thin-Film Transistors Fabricated by High-Temperature Process with Excimer Laser Annealing

Polycrystalline silicon (p-Si) thin-film transistors (TFTs) were fabricated using a high-temperature process that included solid-phase crystallization (SPC) and dry thermal oxidation with excimer laser annealing (ELA). X-ray diffraction and transmission electron microscopy analyses showed that the ELA process improved the quality of p-Si films markedly. The p-Si TFTs exhibited a higher performance than the SPC and a-Si+ELA p-Si TFTs. The field-effect mobility for n-type self-aligned TFTs was 251 cm2·V-1·s-1. The longitudinal junction diffusion length of the p-Si TFTs was shorter than that of the SPC p-Si TFTs. This is favorable for fine design rules. This fabrication process is consistent with the high-temperature-processed p-Si TFT development trend towards using large substrates, low temperatures, and fine design rules.

A poly-Si thin film transistor fabricated by new excimer laser recrystallization employing floating active structure

Abstract We fabricated a high-performance polycrystalline silicon (poly-Si) thin film transistor (TFT) by new excimer laser annealing (ELA) method employing floating active structure. The new simple ELA method produces large lateral grains exceeding 4 μm. A novel poly-Si TFT that exhibits very high mobility (μFE=331 cm2/V s) and low leakage current has been successfully fabricated by single laser irradiation on selectively floating a-Si layer. Uniform lateral grains have been obtained with wide ELA process window.

The Elimination of Ion Implantation Damage at the Source/Drain Junction of Poly-Si TFTs

AbstractWe report a new poly-Si TFT, of which the troublesome implantation damage near the source/drain junction was eliminated, by performing source/drain ion implantation as well as excimer laser annealing (ELA) prior to the gate formation. The recrystallization of a-Si in the channel region and the dopant activation in the source/drain region were simultaneously accomplished by single ELA process step so that the implantation damage, which results in large leakage current of TFT, has been successfully eliminated. TEM image verifies that large poly-Si grains were successfully grown from the ion implanted source/drain region to the channel region. The proposed poly-Si TFT, of which mobility is 171 cm2/Vs, exhibits a large on/off current ratio exceeding 4.1 × 107 without LDD or offset structure while those of the conventional TFT, which has been fabricated simultaneously for the comparison purpose, are 86 cm2/Vs and 6.1 × 106.

Excimer laser processing for a-Si and poly-Si thin film transistors for imager applications

Pulsed excimer laser annealing (ELA) has become an important technology by which to produce high performance thin film transistors (TFTs) for large area electronics. The applications of these advanced TFTs in flat-panel displays and flat-panel imagers for two-dimensional x-ray imaging have attracted much interest. TFTs made from excimer laser crystallized poly-Si thin films with mobilities higher than 100 cm2/V s and off-state leakage currents lower than 2 fA/μm at drain voltage VD=5 V have recently been achieved. The improved off-state leakage current enables one to consider making flat-panel imagers based on poly-Si TFT technology. Laser doping or dopant activation is another important application of the ELA process. Using a laser doping process, we have fabricated a-Si TFTs with self-aligned poly-Si source/drain contacts. These new devices have reduced source/drain parasitic capacitance and their channel length can easily be scaled down. In this article, we will review the excimer laser processing technology for both the poly-Si TFT technology and self-aligned a-Si:H TFT technology and report the current status of developing flat-panel imagers using these new TFT technologies.

Poly-Silicon TFT Annealing with XeCl Excimer Laser.

Liquid-crystal displays (LCDs) with thin film transistors (TFTs) are common in digital and video cameras. An Excimer-Laser Annealing (ELA) method is now commonly integrated into a low-temperature poly-silicon (p-Si) TFT process. Since the ELA process can be performed below 400°C, it is possible to use a low-cost standard glass instead of a quartz glass as a TFT substrate. Our ELA system with a XeCl excimer laser has a line beam optic system, handling the beam length of up to 275mm. The line beam scans an amorphous-silicon (a-Si) thin film on the substrate. As a result, the a-Si film changes into the p-Si film immediately. We demonstrated crystallization of a-Si thin films on different annealing conditions such as various laser energy levels and various substrate temperatures. It was found that the crystal-grain size of the p-Si film strongly depends on both the energy density and the substrate temperature.

Excimer laser annealing of amorphous and solid-phase-crystallized silicon films

Very thin (25–50-nm-thick) amorphous silicon (a-Si) films were crystallized into polycrystalline silicon (polysilicon) films by the combination of low temperature solid phase crystallization (SPC) and subsequent excimer laser annealing (ELA). These films are, then, subjected to a standard low temperature process (<600 °C) of thin film transistor (TFT) fabrication. The performance of resultant TFTs was compared to those fabricated on polysilicon films obtained by simple excimer laser annealing of amorphous silicon films. The electrical characteristics of the TFTs were correlated with the structural characteristics of the polysilicon films, using transmission electron microscopy and x-ray diffraction as analytical tools. The polysilicon films obtained by the SPC process consist of large and heavily defected crystalline grains. These defects, however, could be eliminated by melting and solidifying the polysilicon films during the ELA process. As a result, the electrical properties of the 50-nm-thick polysilicon film subjected to SPC+ELA process improved dramatically when the laser energy density was between 280 and 360 mJ cm−2. The SPC+ELA method did not work successfully for the 25-nm-thick films due to the instability of the laser energy density. Hillock formation at the grain boundaries during the ELA process is also discussed in this article.