Dependence of thin film transistor characteristics on thedeposition conditions of silicon nitride and amorphous silicon

The systematic relation between thin film transistors' (TFT's) characteristics and the deposition conditions of amorphous silicon nitride (a-SiN) films and hydrogenated amorphous silicon (a-Si:H) films is investigated. It is observed that field effect mobility μ_FE and threshold voltage V_th of the TFT's strongly depend on the deposition conditions of these films. The maximum μ_FE of 0.88 cm²/V·s is obtained for the TFT of which a-SiN film is deposited at a pressure of 85 Pa. This phenomenon is due to the variation of the interface states density between a-Si:H film and a-SiN film     

Dependence of Threshold Voltage of a-Si:H TFT on a-SiNx:H Film^①

The relation between threshold voltage for hydrogenated amorphous silicon thin film transistors(a-Si:HTFTs)and deposition conditions for hydrogenated amorphous silicon nitride(a-SiNx:H)films is investigated.It is observed that the threshold voltage,Vth,of a-Si:HTFT increases with the increase of the thickness of a-SiNx:H film,and the threshold voltage is reduced apparently with the increase of NH3/SiH4 gas flow rate ratio.     

Dependence of Threshold Voltage of a-Si:H TFT on a-SiN_x:H Film

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Amorphous-SiC thin-film p-i-n light-emitting diode usingamorphous-SiN hot-carrier tunneling injection layers

An approach to improve the luminosity of hydrogenated amorphous silicon carbide (a-SiC:H)-based thin-film visible light-emitting diodes is discussed. High bandgap near-stoichiometric hydrogenated amorphous silicon nitride (a-SiN:H) is utilized as a hot-carrier tunneling injection layer. An improvement of both carrier injection efficiency and luminosity is observed. Technical data on the new approach for carrier injection and on the recombination mechanism are presented. Preliminary results are also presented on the photoluminescence and electroluminescence properties of a-SiC:H/a-SiN:H multilayers     

Investigation of optical and chemical bond properties of hydrogenated amorphous silicon nitride for optoelectronics applications

The aim of this work is to determine optimal deposition parameters of silicon nitride for optical applications. The authors present the investigation of hydrogenated amorphous silicon nitride SiN_x:H deposited by the low temperature PECVD method in high frequency reactors. The study of hydrogen bonds in the SiN_x:H thin films were detailed. The impact of NH₃, SiH₄ and N₂ flow ratio and radio frequency power on optical coefficients in relation to chemical composition and roughness of the film is studied. The correlation between chemical bonds (N–H, Si–H) and refractive index and extinction coefficients is systematically verified. The experimental results show that the films with high refractive indexes superior to 2.05 and low roughness of about 0.35 nm can be achieved for optoelectronics applications by tuning the flow ratio or decreasing the RF power. A variety of processes have been suggested as compatible with low thermal budget (under 350 °C) in order to integrate optical waveguides with lower loss. In particular, the incorporation of N₂ as dilution gas is suited to the fabrication of SiN_x:H films optical waveguide requiring low N–H bonds, low concentration of hydrogen [H] and high refractive index.     

Industrial high‐rate (∼5 nm/s) deposited silicon nitride yielding high‐quality bulk and surface passivation under optimum anti‐reflection coating conditions

High‐quality surface and bulk passivation of crystalline silicon solar cells has been obtained under optimum anti‐reflection coating properties by silicon nitride (a‐SiN_x:H) deposited at very high deposition rates of ∼5 nm/s. These a‐SiN_x:H films were deposited using the expanding thermal plasma (ETP) technology under regular processing conditions in an inline industrial‐type reactor with a nominal throughput of 960 solar cells/hour. The low surface recombination velocities (50–70 cm/s) were obtained on p‐type silicon substrates (8·4 Ω cm resistivity) for as‐deposited and annealed films within the broad refractive index range of 1·9–2·4, which covers the optimum bulk passivation and anti‐reflection coating performance reached at a refractive index of ∼2·1. Copyright © 2005 John Wiley & Sons, Ltd.     

Determination of local stress in PECVD nitride films

Silicon nitride and silicon oxynitride films grown on silicon substrates by plasma-enhanced chemical vapour deposition (PECVD) are stressed and the correlation of the film stress to its refractive index is presented. It is shown that it is possible to determine the stress of the PECVD nitride films by measuring their refractive indices, which can be done locally, but that the method is not applicable to the PECVD oxynitride films.     

Low-Temperature Materials and Thin Film Transistors for Flexible Electronics

This paper addresses the low-temperature deposition processes and electronic properties of silicon based thin film semiconductors and dielectrics to enable the fabrication of mechanically flexible electronic devices on plastic substrates. Device quality amorphous hydrogenated silicon (a-Si:H), nanocrystalline silicon (nc-Si), and amorphous silicon nitride (a-SiN/sub x/) films and thin film transistors (TFTs) were made using existing industrial plasma deposition equipment at the process temperatures as low as 75/spl deg/C and 120/spl deg/C. The a-Si:H TFTs fabricated at 120/spl deg/C demonstrate performance similar to their high-temperature counterparts, including the field effect mobility (/spl mu//sub FE/) of 0.8 cm/sup 2/V/sup -1/s/sup -1/, the threshold voltage (V/sub T/) of 4.5 V, and the subthreshold slope of 0.5 V/dec, and can be used in active matrix (AM) displays including organic light emitting diode (OLED) displays. The a-Si:H TFTs fabricated at 75/spl deg/C exhibit /spl mu//sub FE/ of 0.6 cm/sup 2/V/sup -1/s/sup -1/, and V/sub T/ of 4 V. It is shown that further improvement in TFT performance can be achieved by using n/sup +/ nc-Si contact layers and plasma treatments of the interface between the gate dielectric and the channel layer. The results demonstrate that with appropriate process optimization, the large area thin film Si technology suits well the fabrication of electronic devices on low-cost plastic substrates.     

Effect of microwave power and reactive gas ratio on the properties of silicon nitride thin films deposited by ECR PECVD

Silicon nitride films were deposited at room temperature on a single crystal silicon substrate by ECR PECVD (electron cyclotron resonance plasma enhanced chemical vapor deposition). Effects of the microwave power and the reactive gas ratio (SiH₄/N₂) on the film properties, such as, refractive index and breakdown field were investigated. It turned out that the microwave power was closely related to the change in refractive index of the silicon nitride films, while breakdown field did not change much from 6 MV/cm. It was also found that the deposition rate, refractive index, and breakdown field were changed in a certain way with respect to the change in SiH₄/N₂ ratio, which could be explained in terms of the activated chemical species concentrations in the plasma during deposition.     

真空退火对低频PECVD氮化硅薄膜性能的影响

研究了真空退火温度对不同流量比工艺参数下PECVD氮化硅薄膜性能的影响,测试了退火后氮化硅薄膜厚度、折射率以及在氢氟酸中的腐蚀速率。结果表明,退火后氮化硅薄膜厚度及折射率变化与薄膜沉积工艺条件有关,而薄膜在氢氟酸中的腐蚀速率在退火后大大降低。结合退火前后氮化硅薄膜的红外透射谱对以上测试结果进行了讨论。     

a-Si:C:H and a-Si:N:H Thin Films Obtained by PECVD for Applications in Silicon Solar Cells

Hydrogenated amorphous silicon-carbon (a-Si:C:H) and hydrogenated silicon-nitrogen (a-Si:N:H) antireflective films were deposited by plasma-enhanced chemical vapor deposition (PECVD) at 13.56 MHz in SiH₄ + CH₄ and SiH₄ + NH₃ gaseous mixtures of various compositions. The silicon and glass samples were investigated by optical spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). A correlation between film properties and process parameters was found. The refractive index decreased and the energy gap increased with an increase of carbon and nitrogen in the films. For some process parameters, it was possible to obtain smooth, hydrogen rich, and homogeneous films of low reflectivity. The silicon solar cells with antireflective coatings revealed an increase in efficiency.     

Thin stoichiometric silicon nitride prepared by r.f. reactive sputtering

Thin silicon nitride films were prepared on silicon wafers at deposition rates of 4 nm/min to 56 nm/min by r.f. reactive sputtering. Various mechanical, chemical and optical properties were investigated as a function of r.f. power, gas composition and temperature by means of AFM, RBS, XPS and an ellipsometer. Chemical stoichiometric films, with N-to-Si atomic ratio of 4:3 and refractive index of 2.0 were achieved even at room temperature. An interference filter for a UV detector with central wavelength of 254 nm was manufactured based on sputtered nitride, aluminum and silicon dioxide.     

Low-Temperature deposition of dielectric films by microwave plasma enhanced decomposition of hexamethyldisilazane

Hexamethyldisilazane (HMDS) has been used as an organosilicon source for the deposition of dielectric films at low temperatures (200-250° C) by microwave plasma enhanced CVD technique. Hydrogenated films of variable composition of silicon carbonitride, silicon oxynitride and silicon dioxide have been deposited by decomposition of HMDS in the presence of additive gases like NH₃, O₂ and H₂ under different process conditions. Deposited films have been characterized by the measurement of refractive index and buffered HF etch rate, and by the analysis of XPS and infrared transmission spectra. An increase in HMDS partial pressure generally results in the decrease of refractive index. The films show stable C-V characteristics of metal-insulator-semicon-ductor (MIS) capacitors with positive insulator charge density.     

Room temperature deposition of silicon nitride films using very low frequency (50Hz) plasma CVD

Silicon nitride films have been deposited by low frequency 50Hz plasma CVD using a nitrogen and silane mixture at room temperature. To deposit high quality silicon nitride, the silane fraction in the nitrogen and silane mixture has to be less than 5 %. The refractive index, breakdown field strength and resistivity of the obtained silicon nitride film were 2.0, 1.2x10⁷ V/cm and 6x10¹⁵ Ωcm, respectively. Mechanism of the deposition of high quality silicon nitride is discussed on the basis of the experimentally observed light emission spectrum from the plasma and of the electron energy distribution function in the plasma theoretically calculated by the Boltzmann equation method.     

PECVD法氮化硅薄膜制备工艺的研究

采用等离子体增强化学气相沉积法（PECVD）在单晶硅衬底上制备了氮化硅薄膜,分别使用膜厚仪、椭圆偏振仪等手段对薄膜的厚度、折射率等参数进行了表征。研究了硅烷氨气流量比、极板间距等工艺参数对氮化硅薄膜性能的影响,发现当硅烷氨气流量比增加时,薄膜厚度和折射率均随之增加,并发现退火工艺可以有效降低氮化硅薄膜的氢氟酸腐蚀速率。     

PECVD法氮化硅薄膜生长工艺的研究

采用等离子体增强化学气相沉积法(PECVD),在单晶硅衬底(100)上成功制备了不同生长工艺条件下的氮化硅薄膜。分别采用XP-2台阶仪、椭圆偏振仪等手段测试了薄膜的厚度、折射率、生长速率等参数。并采用原子力显微镜(AFM)研究了薄膜的表面形貌。结果表明,温度和射频功率是影响薄膜生长速率的主要因素,生长速率变化幅度可以达到230nm/min甚至更高。对于薄膜折射率和成分影响最大的是NH3流量,折射率变化范围可以达到2.7～1.86。分析得出受工艺参数调控的薄膜生长速率对薄膜的性质有重要影响。     

Optimised antireflection coatings for planar silicon solar cells using remote PECVD silicon nitride and porous silicon dioxide

Silicon nitride (SiN) films fabricated by remote plasma‐enhanced chemical vapour deposition (RPECVD) have recently been shown to provide an excellent electronic passivation of silicon surfaces. This property, in combination with its large refractive index, makes RPECVD SiN an ideal candidate for a surface‐passivating antireflection coating on silicon solar cells. A major problem of these films, however, is the fact that the extinction coefficient increases with increasing refractive index. Hence, a careful optimisation of RPECVD SiN based antireflection coatings on silicon solar cells must consider the light absorption within the films. Optimal optical performance of silicon solar cells in air is obtained if the RPECVD SiN films are combined with a medium with a refractive index below 1·46, such as porous SiO₂. In this study, the dispersion of the refractive indices and the extinction coefficients of RPECVD SiN, porous SiO₂, and several other relevant materials (MgF₂, TiO_x, ZnS, B270 crown glass, soda lime glass, ethylene vinyl acetate and resin as used in commercial photovoltaic modules) are experimentally determined. Based on these data, the short‐circuit currents of planar silicon solar cells covered by RPECVD SiN and/or porous SiO₂ single‐ and multi‐layer antireflection coatings are numerically maximised for glass‐encapsulated as well as non‐encapsulated operating conditions. The porous SiO₂/RPECVD SiN‐based antireflection coatings optimised for these applications are shown to be universally suited for silicon solar cells, regardless of the internal blue or red response of the cells. Copyright © 1999 John Wiley & Sons, Ltd.     

原子层沉积超薄氮化铝薄膜的椭圆偏振光谱法表征

采用原子层沉积(ALD)工艺在硅衬底上生长了35 nm以下不同厚度的超薄氮化铝(AlN)晶态薄膜。利用椭圆偏振光谱法在波长275~900 nm内测量并拟合薄膜的厚度及折射率和消光系数等光学参数。利用原子力显微镜(AFM)表征AlN晶粒尺寸随生长循环次数的变化,计算得到薄膜表面粗糙度并用于辅助椭偏模型拟合。针对ALD工艺特点建立合适的椭偏模型,可获得AlN超薄膜的生长速率为0.0535 nm/cycle,AlN超薄膜的折射率随着生长循环次数的增加而增大,并逐渐趋于稳定,薄膜厚度为6.88 nm时,其折射率为1.6535,薄膜厚度为33.01 nm时,其折射率为1.8731。该模型为超薄介质薄膜提供了稳定、可靠的椭圆偏振光谱法表征。     

Silicon Nitride Films Deposited by RF Sputtering for Microstructure Fabrication in MEMS

In the present work, we report silicon nitride films deposited by a radio- frequency (RF) sputtering process at relatively low temperatures (<260°C) for microelectromechanical system (MEMS) applications. The films were prepared by RF diode sputtering using a 3-inch-diameter Si₃N₄ target in an argon ambient at 5 mTorr to 20 mTorr pressure and an RF power of 100 W to 300 W. The influence of the film deposition parameters, such as RF power and sputtering pressure, on deposition rate, Si-N bonding, surface roughness, etch rate, and stress in the films was investigated. The films were deposited on single/double-side polished silicon wafers and transparent fused-quartz substrates. To explore the RF-sputtered silicon nitride film as a structural material in MEMS, microcantilever beams of silicon nitride were fabricated by bulk, surface, and surface-bulk micromachining technology. An RF-sputtered phosphosilicate glass film was used as a sacrificial layer with RF-sputtered silicon nitride. Other applications of sputtered silicon nitride films, such as in the local oxidation of silicon (LOCOS) process, were also investigated.     

SiH_4-NH_3-N_2体系LPCVD氮化硅薄膜的研究

通过傅立叶红外光谱(FTIR)和X光电子能谱(XPS)研究了SiH4-NH3-N2体系在不同气体原料比情况下,低压化学气相沉积(LPCVD)SiNx薄膜的化学组成,利用原子力显微镜观察了SiNx薄膜的微观形貌,借助椭圆偏振仪研究了薄膜的折射率。结果表明:当原料气中氨气与硅烷的流量之比(R)较小时(R<2),获得富Si的SiNx薄膜(x<1.33),折射率较高。当氨气远远过量时(R >4),获得近化学计量的的SiNx薄膜(x = 1.33),折射率处于1.95~2.00之间。在适当的工艺条件下,获得的SiNx薄膜H、O含量很低,薄膜表面均匀、平整。