Improved Uniformity and Electrical Performance of Continuous-Wave Laser-Crystallized TFTs Using Metal-Induced Laterally Crystallized Si Film

Continuous-wave (CW) laser crystallization (CLC) of amorphous Si (α-Si) has previously been employed to fabricate high-performance low-temperature polycrystalline silicon (poly-Si) thin-film transistors (TFTs). Unfortunately, their uniformity was poor because the shape of the beam profiles was Gaussian. In this study, α-Si film was replaced by Ni-metal-induced laterally crystallized Si (MILC-Si). MILCLC-Si was MILC-Si irradiated by a CW laser (λ ≈ 532 nm and power ≈ 3.8 W). It was found that the performance and uniformity of the metal-induced laterally crystallized continuous-wave laser crystallization - thin film transistors (MILCLC-TFTs) were much better than those of the CLC-TFTs. Therefore, the MILCLC-TFT is suitable for application in systems on panels.     

Crystallization of hydrogenated amorphous silicon films by exposure to femtosecond pulsed laser radiation

To crystallize hydrogenated amorphous silicon films on glass substrates, pulsed Ti-sapphire laser radiation is used, with a pulse duration less than 30 fs. The initial films are grown by plasma-enhanced chemical-vapor deposition at the temperatures 200 and 250°C. The structural properties of the initial films and films treated with laser radiation pulses are studied by Raman spectroscopy. The conditions for complete crystallization of the films grown on glass substrates to thicknesses of up to 100 nm and hydrogen content of up to 20 at % are established. The conditions provide the fabrication of highly homogeneous films by scanning laser treatments. It is found that, if the hydrogen content in the film is 30–40 at %, the crystallization is an inhomogeneous process and laser ablation is observed in some areas of the films.     

Low temperature crystallization of amorphous silicon using an excimer laser

Low temperature processing is a prerequisite for compatible technologies involving combined a-Si and poly-silicon devices or for fabricating these devices on glass substrates. This paper describes excimer-laser-induced crystallization of thin amorphous silicon films deposited by plasma CVD (a-Si:H) and LPCVD (a-Si). The intense, pulsed UV produced by the laser is highly absorbed by the thin amorphous material, but the average temperature is compatible with low temperature processing. The process produces crystallites whose structure and electrical characteristics vary according to starting material and laser scan parameters. The crystallized films have been principally characterized using x-ray diffraction, TEM, and transport measurements. The results indicate that crystallites nucleate in the surface region and are randomly oriented. The degree of crystallization near the surface increases as the doping level and/or deposited laser energy density is increased. The crystallite size increases with a power law dependence on deposited energy, while the conductivity increases exponentially above threshold for unintentionally doped PECVD films. The magnitude of the Hall mobility of the highly crystallized samples is increased by two orders of magnitude over that of the amorphous starting material.     

Ageing of laser crystallized and unhydrogenated polysilicon thin film transistors

Ageing of low temperature polysilicon Thin Film Transistors (TFTs) is reported in this study. The active layer of these high performances transistors is amorphous deposited using Low Pressure Chemical Vapor Deposition (LPCVD) technique and then laser crystallized using a single shot ECL (SSECL of SOPRA) with very large excimer laser. The drain and source regions are in-situ doped during the LPCVD deposition by using phosphine or diborane to fabricate n-type or p-type transistors respectively.These laser crystallized TFT's show poorer reliability properties than solid-phase crystallized TFT's. This poor stability is explained to originate from the high surface roughness produced by the laser crystallization, which is highlighted from Atomic Force Microscopy observations.Moreover to this conclusion, the behaviour of the threshold voltage shift ΔV_T during positive and negative stresses is checked to the light of a stretched exponential law that is, as supposed, a federative law. This law is explained in hydrogenated amorphous silicon TFT's by a dispersive diffusion coefficient of hydrogen in the disordered material. Taking into account that such relation appears as sufficiently general and, particularly, can describe the behaviour of monocrystalline silicon MOSFET and un-hydrogenated polysilicon TFT's where the hydrogen cannot involved, it can be supposed that it deals with disordered materials and disordered regions in crystalline materials (interface, grain boundary, …..).     

本征非晶硅薄膜的准分子激光晶化

为了减低非晶硅薄膜太阳能电池的光致衰减效应和提高其光电转换效率,用等离子体化学气相沉积系统制备了本征非晶硅薄膜,用波长为248nm的KrF准分子激光器激光晶化了非晶硅表层,用共焦显微喇曼测试技术研究了非晶硅薄膜在不同的激光能量密度和不同的频率下的晶化状态,并用扫描电子显微镜测试晶化前后薄膜的形貌。结果表明,随着激光能量密度的增大,薄膜晶化效果越来越好,能量密度达到268.54mJ/cm2时晶化效果最好,此时结晶比约为76.34%;最佳的激光能量密度范围是204.99mJ/cm2~268.54mJ/cm2,这时薄膜表面晶化良好;在1Hz~10Hz范围内,激光频率越大晶化效果越好;晶化后薄膜明显出现微晶和多晶颗粒,从而达到了良好的晶化效果。     

Single-crystal silicon transistors in laser-crystallized thin films on bulk glass

High-performance thin-film transistors (TFT) have been fabricated in single-crystal silicon thin films on bulk fused silica. Deposited films of polycrystalline silicon were patterned to control nucleation and growth of single-crystal material in pre-selected areas and encapsulated with a dielectric layer (e.g., SiO2) in preparation for laser crystallization. Patterning also minimized microcracking during crystallization. The patterned silicon layer was crystallized with a scanning CO2laser, which produced islands with preferred crystal orientation. The single crystallinity of the islands was established with transmission electron microscopy after transistor evaluation. The silicon islands were processed with conventional microelectronic techniques to form metal-oxide-semiconductor-field-effect transistors operating in the n-channel enhancement mode. The devices display exceptional electrical characteristics with "low-field" channel mobilities > 1000 cm²/V sec and leakage currents < 10 pA, for a Channel length of 12 µm and width of 20 µm. Achievement of high-performance TFT's with the combined features of microcrack suppression, preferred orientation, and selected-area crystallization render CO2- laser processing of silicon films a viable and versatile basis for a silicon-on-insulator technology.     

非晶硅薄膜YAG激光微晶化晶粒生长研究

以单晶硅(111)为衬底,以等离子体增强化学气相沉积技术制备的非晶硅薄膜为前驱物,采用YAG激光晶化技术实现从非晶硅薄膜到纳米晶硅薄膜的相变过程。采用X射线衍射仪和原子力显微镜对YAG激光晶化薄膜进行了表征与分析。结果表明:薄膜的晶粒尺寸在纳米级;随着激光脉冲频率的增加,晶粒尺寸先变大后变小,其最佳结晶频率区间为10～12 Hz。     

Thin-film transistors fabricated with poly-Si films crystallized atlow temperature by microwave annealing

Solid phase crystallization of amorphous silicon films for poly-Si thin film transistors (TFTs) has advantages of low cost and excellent uniformity, but the crystallization temperature is too high. Using a microwave annealing method, we lowered the crystallization temperature and shortened the crystallization time. The complete crystallization time at 550°C was within 2 h. The device parameters of TFTs with the poly-Si films crystallized by microwave annealing were similar to those of TFTs with the poly-Si films crystallized by conventional furnace annealing. The new crystallization method seems attractive because of low crystallization temperature, short crystallization time, and comparable film properties     

氢等离子体辅助铝诱导晶化非晶硅的研究

本文中提出了一种通过氢等离子体改进和加速铝诱导晶化的工艺方法。通过拉曼散射谱、SIMS测试晶化多晶硅,结果说明了氢等离子体的作用缩短了铝诱导晶化的时间。这项技术使退火时间由10小时缩短到4小时同时使霍尔迁移率从22.1 cm2/V增加到42.5 cm2/V。另外也对氢等离子体辅助铝诱导晶化的可能机理做了讨论。     

连续氩氪离子激光晶化非晶硅薄膜的研究

为了研究连续激光晶化非晶硅薄膜中激光功率密度对晶化效果的影响,利用磁控溅射法制备非晶硅薄膜,采用连续氩氪混合离子激光器对薄膜进行退火晶化,用显微喇曼光谱测试技术和场发射扫描电子显微镜研究了薄膜在5ms固定时间下不同激光功率密度对晶化效果的影响,并对比了普通玻璃片和石英玻璃两种衬底上薄膜晶化过程的差异。结果表明,在一定激光功率密度范围内(0kW/cm2~27.1kW/cm2),当激光功率密度大于15.1kW/cm2时,普通玻璃衬底沉积的非晶硅薄膜开始实现晶化;随着激光功率密度的增大,晶化效果先逐渐变好,之后变差;激光功率密度增大到24.9kW/cm2时,薄膜表面呈现大面积散落的苹果状多晶硅颗粒,晶粒截面尺寸高达478nm ;激光功率密度存在一个中间值,使得晶化效果达到最佳;石英衬底上沉积的非晶硅薄膜则呈现与前者不同的结晶生长过程,当激光功率密度为19.7kW/cm2时,薄膜表面呈现大晶粒尺寸的球形多晶硅颗粒,并且晶粒尺寸随着激光功率密度的增大而增大,在 27.1kW/cm2处晶粒尺寸达到最大5.38m。研究结果对用连续激光晶化法制备多晶硅薄膜的研究具有积极意义。     

非晶硅薄膜的YAG激光晶化工艺研究

应用YAG激光器在不同工艺条件(激光脉冲频率及脉宽)下对非晶硅薄膜进行了微晶化处理。采用XRD和AFM对所制薄膜的物相结构和表面形貌进行了分析,并探索了激光脉冲占空比对非晶硅薄膜晶化的影响。结果表明,非晶硅薄膜在不同激光脉冲占空比情况下的结晶变化趋势均为多晶硅衬底表层先非晶化,后与非晶硅薄膜一起结晶,而利于其结晶的最佳占空比为1/25。已晶化硅薄膜的晶粒尺寸随占空比的增加先变大后变小。     

Sequential lateral solidification processing for polycrystalline Si TFTs

The sequential lateral solidification (SLS) process is an excimer-laser projection-based scheme for crystallization of thin films on amorphous substrates. This method can be used to readily produce a wide range of microstructures through manipulation of grain boundary placement within the crystallized material. In this paper, we focus on the 2-shot SLS process for crystallization of thin Si films for thin-film transistor (TFT) applications. We have investigated the effect of process parameter variation on the resulting microstructure, as well as on the performance of TFTs fabricated on the material. The 2-shot SLS microstructure was further engineered to reduce anisotropy of the TFT performance relative to the lateral growth direction using additional laser scans. Through this method, we were able to improve the mobility directionality ratio between devices with majority carrier flow parallel and perpendicular to the lateral growth direction, respectively, from 0.3 to over 0.7. Post-SLS process thinning and planarization of the Si surface was used to improve the uniformity and performance of the TFT devices.     

Effects of crystallization on trap state densities at grain boundaries in polycrystalline silicon

The relationship between crystallization processes in the formation of polycrystalline-silicon (poly-Si) films and trap state densities at grain boundaries is described. Three different crystallization techniques were used to obtain poly-Si films: 1) LPCVD, 2) solid-phase crystallization, and 3) laser recrystallization. Trap state densities in laser-recrystallized poly-Si are 9.2-9.6 × 10¹¹cm^-2, regardless of grain size. These values are half of those in LPCVD and solid-phase crystallized poly-Si. It is indicated quantitatively that laser-induced melting and the subsequent solidification process exert a significant influence on the electrical activity of silicon grain boundaries.     

The effects of hydrogen on aluminum-induced crystallization of sputtered hydrogenated amorphous silicon

The effects of hydrogen on aluminum-induced crystallization (AIC) of sputtered hydrogenated amorphous silicon (a-Si:H) were investigated by controlling the hydrogen content of a-SiH films. Nonhydrogenated (a-Si) and hydrogenated (a-Si:H) samples were deposited by sputtering and plasma-enhanced chemical vapor deposition (PECVD). All aluminum films were deposited by sputtering. Hydrogen was introduced into the sputter-deposited a-Si films during the deposition. After deposition, the samples were annealed at temperatures from 200°C to 400°C for different periods of time. X-ray diffraction (XRD) patterns were used to confirm the presence and degree of crystallization in the a-Si:H films. For nonhydrogenated films, crystallization initiates at a temperature of 350°C. The crystallization of sputter-deposited a-Si:H initiates at 225°C when 14% hydrogen is present in the film. As the hydrogen content is decreased, the crystallization temperature increases. On the other hand, the crystallization initiation temperature for PECVD a-Si:H containing 11at.%H is 200°C. Further study revealed that the crystallization initiation temperature is a function, not only of the total atomic percent hydrogen in the film, but also a function of the way in which the hydrogen is bonded in the film. Models are developed for crystallization initiation temperature dependence on hydrogen concentration in a-Si:H thin films.     

Polycrystalline silicon thin film transistors fabricated in various solid phase crystallized films deposited on glass substrates

In this work, we have characterized various types of polysilicon films, crystallized upon thermal annealing from films deposited by low pressure chemical vapor deposition in the amorphous phase and a mixed phase using silane or in the amorphous phase using disilane. Polysilicon thin film transistors (TFTs) were fabricated, at low processing temperatures, in these three types of films on high strain point Corning Code 1734 and 1735 glass substrates. Double layer films, with the bottom layer deposited in a mixed phase and the top in the amorphous phase, allowed TFT fabrication at a drastically reduced thermal budget; optimum values of thicknesses and deposition rates of the layers are reported for reducing the crystallization time and improving film quality. Optimum deposition conditions for TFT fabrication were also obtained for films deposited using disilane. The grain size distribution for all types of films was shown to be wider for a larger grain size. Fabricated TFTs exhibited field effect electron mobility values in the range of 20 to 50 cm²/V·s, subthreshold swings of about 0.5–1.5 V/dec and threshold voltage values of 2–4 V.     

Formation and crystallization of silicon nanoclusters in SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:H films using femtosecond pulsed laser annealings

SiN _x :H films of different compositions grown on glass and silicon substrates using plasma-chemical vapor deposition at a temperature of 380°C have been subjected to pulsed laser annealings. The treatments are performed using titanium-sapphire laser radiation with a wavelength of 800 nm and a pulse duration of 30 fs. Structural changes in the films are studied using Raman spectroscopy. Amorphous silicon nanoclusters are detected in as-grown films with molar fractions of excess silicon of ∼1/5 and larger. Conditions required for pulsed crystallization of nanoclusters were determined. According to the Raman data, no silicon clusters were detected in as-grown films with a small amount of excess silicon (x > 1.25). Pulsed treatments resulted in the formation of silicon nanoclusters 1–2 nm in size in these films.     

激光脉冲频率对不同衬底上非晶硅薄膜晶化的影响

应用不同频率的YAG激光分别对单晶硅及多晶硅衬底上的非晶硅薄膜进行了退火处理。晶化后的非晶硅薄膜的物相结构和表面形貌用XRD和AFM进行分析。XRD测试结果表明:随着激光频率的增加,两种衬底上的非晶硅薄膜晶化晶粒尺寸均出现了先增加后降低的现象。所有非晶硅样品的衍射峰位与衬底一致,说明非晶硅薄膜的晶粒生长是外延生长。从多晶硅衬底样品的XRD可以看出,随着激光频率的增加,激光首先融化衬底表面,然后衬底表层与非晶硅薄膜一起晶化。非晶硅薄膜最佳晶化激光频率分别为:多晶硅衬底20Hz,单晶硅衬底10Hz。     

Single Pulse Laser‐Induced Phase Transitions of PLD‐Deposited Ge2Sb2Te5 Films

Phase transformations between amorphous and crystallized states are induced by irradiation with a single nanosecond laser pulse in Ge₂Sb₂Te₅ films grown by pulsed laser deposition. By adjusting the laser fluence, the two different phases are obtained and can be distinguished by their different optical reflectivity. The effect of laser fluence on the crystalline nature of the films is studied in detail. Large structural differences between the laser‐irradiated and thermally annealed films are revealed, due to the high heating rate and short duration of the laser pulse. X‐ray reflectivity measurements show a density increase of 3.58% upon laser‐induced crystallization.     

Excimer laser-assisted planarization of thick diamond films

The planarization of polycrystalline diamond films is critical for a large number of industrial applications. We have investigated a laser-assisted method for planarization of thick diamond films. This method is based on the application of excimer laser combined with simultaneous rotation of the sample. Thick diamond films (average surface roughness: ∼20 μm and thickness ∼500 μm) were fabricated by plasma jet chemical vapor deposition process. The planarization of diamond films was found to be critically dependent on the angle of incidence of laser beam. Smoother surfaces were obtained at higher incidence angles (θ = 80°). However, by combination of sample rotation with laser irradiation at higher incidence angles (θ = 80°), maximum surface planarization was achieved. Under optimum conditions, the surface roughness of the samples were reduced from 20 to 0.1 μm. The mechanisms for surface planarization of thick diamond films are discussed.     

Characterization and simulation analysis of laser-induced crystallization of amorphous silicon thin films

The effect of laser energy density on the crystallization of hydrogenated amorphous silicon (a-Si:H) thin films was studied theoretically and experimentally. The thin films were irritated with a frequency-doubled (λ=532 nm) Nd:YAG pulsed nanosecond laser. An effective finite element model was built to predict the melting threshold and the optimized laser energy density for crystallization of intrinsic amorphous silicon. Simulation analysis revealed variations in the temperature distribution with time and melting depth. The highest crystalline fraction measured by Raman spectroscopy (84.5%) agrees well with the optimized laser energy density (1000 mJ/cm²) in the transient-state simulation. The surface morphology of the thin films observed by optical microscopy is in fairly good agreement with the temperature distribution in the steady-state simulation.